^ NONPROVISIONAL APPLICATION PHOTOPLETHSYMOGRAPHY INDEPENDENT OF ONE OR MORE SKIN VARIABLES Cross-Reference to Related Applications [0001] This application claims the benefit of U.S. Provisional Application Serial No.63/606,667, filed 6 December 2023, entitled “A NOVEL OPTICAL METHOD FOR ACCURATE BLOOD OXYGENATION MEASUREMENTS INDEPENDENT OF SKIN TONE AND OTHER SKIN CONTRIBUTIONS”, U.S. Provisional Application Serial No.63/642,428, filed 3 May 2024, entitled “INVESTIGATING THE USE OF A DUAL-WAVELENGTH, POLARIZATION-SENSITIVE WEARABLE PHOTOPLETHYSMOGRAPHIC SENSOR ACROSS VARYING SKIN TONES”, and U.S. Provisional Application Serial No.63/689,650, filed 31 August 2024, entitled “PHOTOPLETHYSMOGRAPHIC SENSOR INDEPENDENT OF SKIN TONE”. The entirety of these applications is incorporated by reference for all purposes. Technical Field [0002] The present disclosure relates generally to photoplethysmography and, more specifically, to systems and methods for photoplethysmography that accounts for one or more skin variables (e.g., skin tones, motion, and the like) such that one or more photoplethysmography-based measurements are independent of the one or more skin properties. Background [0003] Pulse oximeters, commonly used in both hospitals and wearable devices, generally use photoplethysmography (PPG) to estimate a subject’s peripheral oxygen saturation (SpO2). PPG is a non-invasive optical technique that can detect volumetric changes in peripheral blood circulation. However, traditional PPG used in pulse oximeters cannot account for certain skin related variables, including differences in skin tones, motion (e.g., changes in detection/emission distances, etc.), and the like. Notably, traditional pulse oximeters have been shown to provide inaccurate results for populations with increased melanin in their skin (specifically the Brown and Black communities) such that members of the Brown and ^
^ Black communities are almost three times more likely to have inaccurate PPG-based measurements. These inaccuracies can cause patients from Brown and Black communities to silently suffer worsening conditions (including silent hypoxia) without being offered timely, necessary medical interventions. Another less prejudicial concern is that pulse oximeters that are used incorrectly (e.g., attached too loosely, not properly timed with heart rate, etc.) their measurements can be subject to motion artifacts. Motion artifacts can also negatively affect traditional PPG-based measurements, leading users and/or monitoring medical professionals to believe the user is better oxygenated than the user actually is (missing important treatment time), and/or less oxygenated than the user actually is (leading to unnecessary concern and/or treatments). Summary [0004] The present disclosure illustrates improved systems and methods for photoplethysmography (PPG) that account for one or more skin variables (e.g., skin tones due to varying levels of melanin, motion, and other currently confounding variables) for improved oxygen saturation measurement accuracy. From the resulting improved photoplethysmographs, estimates of a user’s heart rate, blood pressure, blood oxygen concentration (SpO2), and respiration rate can be derived – for monitoring user health in both hospital and home settings. [0005] One aspect of the present disclosure is a wearable device for determining a photoplethysmography (PPG) signal and/or peripheral blood oxygen saturation (SpO
2) that accounts for motion and/or confounding variables of skin of a wearer, including melanin quantity. The wearable device includes at least two light emitters, a detector, and a controller in communication with the two light emitters and the detector. A first emitter can be configured to emit a first polarized light signal towards the skin of the wearer of the wearable device. The first emitter can include a first light emitting diode (LED), which can emit a first light signal at a first wavelength, and a first polarizer positioned between the first LED and the skin of the wearer, which can polarize the first light signal. The second emitter can be configured to emit a second polarized light signal towards the skin of the wearer. The second emitter can include a second LED, which can emit a second light signal at a second wavelength, and a second polarizer positioned between the second LED and the skin of the wearer, which can polarize the second light signal. The detector can ^
^ detect light signals coming off from the skin of the wearer in response to application of the first polarized light signal and the second polarized light signal. The detector can include a co-polarization channel, which can detect a portion of the light signals coming from the skin comprising superficial components and deep components, and a cross-polarization channel, which can detect a portion of the light signals coming from the skin comprising the deep components. The co-polarization channel can include at least two first photodetectors and at least two first analyzers that can be aligned parallel, or near parallel, to the direction of the at least two first polarizers. The at least two first analyzers can be paired with the at least two first photodetectors such that one of the at least two first analyzers is positioned between one of the at least two first photodetectors and the skin for each pair. The cross- polarization channel can include at least two second photodetectors and at least two second analyzers that can be aligned perpendicular, or near perpendicular, to the direction of the at least two second polarizers. The at least two second analyzers can be paired with the at least two second photodetectors such that one of the at least two second analyzers is positioned between one of the at least two second photodetectors and the skin for each pair. The controller, which can include a memory and a processor, can determine the PPG signal and/or a SpO2 that accounts for motion and/or confounding variables of the skin based at least in part on the light signals from the skin and detected by the co-polarization channel and/or the cross-polarization channel. [0006] Another aspect of the present disclosure is a method for determining a PPG signal and/or SpO2 that accounts for motion and/or confounding variables of skin of a wearer, including melanin quantity. At least two polarized light signals having different wavelengths can be applied to the skin of the wearer by at least two light emitters. The at least two polarized light signals can be at least one of reflected, absorbed, or transmitted by the skin. At least a portion of light signals caused by the skin interacting with the at least two polarized light signals can be detected by a first channel (of a detector) with a first polarization. The portion of the light signals can include superficial components and deep components. Another portion of the light signals caused by the skin interacting with the at least two polarized light signals can be detected by a second channel (of a detector) with a second polarization. The other portion of the light signals can include superficial components. A PPG signal can be determined based on the superficial components and the deep components ^^^^ ^
^ detected by the first channel with the first polarization and the second channel with the second polarization. The PPG signal and/or SpO2 can account for motion artifacts and/or confounding skin contributions. [0007] A further aspect of the present disclosure is another wearable device for determining a PPG signal and/or a SpO2 that accounts for motion and/or confounding variables of the skin, including melanin quantity. The wearable device can include an emission component, a detection component, and a controller in communication with the emission component and the detection component. The emission component can emit at least two polarized light signals with different wavelengths towards skin of a wearer of the wearable device. The detection component can detect light signals from the skin of the wearer in response to the at least two polarized light signals with different wavelengths interacting with the skin of the wearer. The detection component can include at least two polarization channels that each can include at least two photodetectors and at least two analyzers. The at least two analyzers can be aligned relative to a polarization alignment of a portion of the emission component. The at least two photodetectors and the at least two analyzers can be paired such that one of the at least two analyzers can be positioned between one of the at least two photodetectors and the skin for each pair. The at least two analyzers of each of the at least two polarization channels can be aligned differently relative to the alignment of the at least two analyzers of another of the at least two polarization channels to detect different portions of the light signals reflected by the skin. The different portions of the light signals reflected by the skin can include at least one of a superficial component of the reflected light signal and a deep component of the reflected light signals. The controller can include a memory and a processor that can determine the PPG signal and/or the SpO2 that accounts for motion and/or confounding variables of the skin based at least in part on the light signals from the skin and detected by the at least two polarization channels. Brief Description of the Drawings [0008] The foregoing and other features of the present disclosure will become apparent to those skilled in the art to which the present disclosure relates upon reading the following description with reference to the accompanying drawings, in which: ^
^ [0009] FIG.1 shows a system for determining at least photoplethysmography (PPG) of a patient that accounts for one or more skin variables (e.g., skin tones due to varying levels of melanin, motion, and other currently confounding variables)^ [0010] FIG.2 shows an example ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ [0011] FIG.3 shows an example ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ [0012] FIG.4 shows an example controller and output portion of the system of ^^^^^^^ [0013] FIG.5 is an illustration showing different examples of wearable devices that can be worn ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ [0014] FIG.6 shows a skin facing side of an example ^^^^^^^^^^^^^^^^^^^^^^^^^^ [0015] FIG.7 shows emission and detection portions of the example wearable ^^^^^^^^^^^^^^^^^^^^^^ ^ [0016] FIG.8 shows an example of a system of FIG.1 having co-polarization and cross-^^^^^^!^^^^^^^^^^^^^^^ [0017] FIG.9 shows another example of a system for performing at least PPG ^^^^^^"^ [0018] FIG.10 shows a portion of the example of the ^^^^^^^^^^^^^^^#^ [0019] FIGS.11-13 are process flow diagrams that show methods for determining at least a PPG recording that accounts for one or more skin variables (e.g., skin tones due to varying levels of melanin, motion, and other currently confounding variables)^ [0020] FIGS.14-18 are example illustrations related to the mechanics of polarization gating^ [0021] FIG 19 shows pictures and illustrations of an ^$^^^^^^^^^^^^^^^^^ [0022] ^^^^^%&^^^^^^^^^^^^^^^^^^^^^^^^^^^^'^^^^^^^^^^^^^(^^^^^^^^^^^^^^^^^ [0023] FIG.21 shows graphical representations of results from the ^^^^^^^^^^^ [0024] FIG.22 shows graphical representations of signal to noise (SNR) ratio vs ^^^^^^^^^^^^^^^^^"^^^^"^^^)^*+,^ [0025] FIG.23 shows graphical representations of time series data for different ^^^^^^^"^^^^^^^^^^^^^!^^^^^^"^^^^"^^^^^^^^^^^^^^ [0026] FIG.24 shows graphical representations of the power spectrum of a --^^^^"^^^^ [0027] FIG.25 shows a graphical representation of the degree of polarization in arterial tissue (prior art)^ ^^^^ ^
^ [0028] FIG.26 shows a representation of a simulation of depth penetration in a scattering medium (prior art)^ [0029] FIG.27 shows an illustration of the difference in optical scattering between lighter and darker skin tones (prior art)^ [0030] FIG.28 shows another example experimental setup and representations ^^^^^^^^^^^ [0031] FIG.29 shows another ^$^^^^^^^$^^^^^^^^^^^^^^^^^^^^^ [0032] FIG.30 shows a volunteer’s finger and graphical representations of results. Detailed Description I. Definitions [0033] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present disclosure pertains. [0034] As used herein, the singular forms “a,” “an,” and “the” can also include the plural forms, unless the context clearly indicates otherwise. [0035] As used herein, the terms “comprises” and/or “comprising,” can specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups. [0036] As used herein, the term “and/or” can include any and all combinations of one or more of the associated listed items. [0037] As used herein, the terms “first,” “second,” etc. should not limit the elements being described by these terms. These terms are only used to distinguish one element from another. Thus, a “first” element discussed below could also be termed a “second” element without departing from the teachings of the present disclosure. The sequence of operations (or acts/steps) is not limited to the order presented in the claims or figures unless specifically indicated otherwise. [0038] As used herein, the term “skin” refers to a natural exterior covering of a body of a subject which comprises (from outside in) the epidermis, dermis, and hypodermis. The epidermis includes melanin which can affect skin tone – the more melanin the darker the skin tone. The dermis makes up 90% of skin’s thickness and ^
^ includes the most peripheral blood vessels and nerves, proteins such as collagen and elastin, nerves, and oil and sweat glands. The hypodermis is the fatty layer of the skin and includes fat, connective tissue, and the generally larger portions of the nerves and blood vessels that run through the dermis. [0039] As used herein, the term “confounding variables” (of the skin) refers at least one traditionally unmeasured variable that influences photoplethysmography measurements and/or estimates. Confounding variables of the skin can include chromophores such as melanin, relative motion (too loose, heartbeat, etc.), and the like. The terms confounding contributions of the skin and/or confounding variables of the skin can be used interchangeably. [0040] As used herein, the term “melanin” refers to a chromophore in a patient’s body that produces hair, eye, and skin pigmentation. The more melanin produced, the darker coloration of the hair, eyes, and skin. The darker coloration of the hair, skin, and/or eyes, the more melanin produced by the patient. [0041] As used herein, the term “cardiovascular variability parameter” refers to a parameter related to blood flow and/or transportation of substances in blood. Examples of cardiovascular variability parameters include heart rate, respiratory rate, estimated oxygen saturation (SpO2), tissue oxygenation (StO2), atrial blood pressure, blood vessel stiffness, microvascular blood flow, tissue viability, vasomotor function, thermoregulation, etc. [0042] As used herein, a “photoplethysmography" also referred to as a “PPG” refers to a simple, low cost, and non-invasive procedure and device (also called a photoplethysmograph) used in a variety of commercially available medical and fitness devices for optical-physiological monitoring of one or more cardiovascular variability parameters. It should be noted that the recorded output of a PPG device over time can also be called a photoplethysmograph. Generally, a PPG can include at least one light source for illuminating skin of a patient and a detector for measuring light signals transmitted and/or reflected from the skin of the patient. [0043] As used herein, a “photodetector” refers to a device or circuit that can detect light incident on it. Examples of photodetectors can include one or more CCD cameras, one or more CMOS cameras, one or more photodiodes, one or more photoconductors, one or more polarimeters, one or more thermal-detectors, one or more photomultiplier tube (PMT) balanced detectors, or the like. ^
^ [0044] As used herein, the term “polarize” refers to restricting vibrations of a light wave wholly or partially to one direction. [0045] As used herein, the term “polarizer” refers to an optical device that can convert a beam of unpolarized light into one that is polarized into a polarization state. As used herein a polarizer refers to polarizing a light signal that is being output from a light emitter. [0046] As used herein, the term “analyzer” refers to an optical device that can receive a beam of polarized light based on a polarization state of the analyzer and the polarization state of the beam of polarized light. As used herein an analyzer refers to a reception device. [0047] As used herein, the term “polarization state” refers to a state of a light wave that has been polarized. There are various kinds of polarization states of light, including but not limited to, linear, circular, elliptical, radial, and azimuthal. Light can include one or more than one polarization state at a time. When multiple linear polarization states are compared, they can be referred to as co-, cross-, parallel, perpendicular, or the like depending on the angle between the two linear states. For example, light can be polarized to have an inhomogeneous optical polarization wavefront. [0048] As used herein, the term “spectrophotometer” refers to a device that measures the intensity of light in a part of the spectrum, especially as transmitted or emitted by particular substances. One example spectrophotometer that has a narrow band of operation at the RGB channels is a colorimeter. [0049] As used herein, the term “colorimeter” refers to a device that measures and quantifies emitted light from a sample, in this case skin. A colorimeter can provide an input, based on the light emitted from the skin when the at least two polarized light signals are applied to the skin, to a controller to determine an individual typology angle (ITA) of the skin. [0050] As used herein, the term “individual typology angle” (ITA) refers to a measure of constitutive pigmentation (e.g., melanin) that can be categorized into different distinct skin tones based on the angle. [0051] As used herein, the term “wearer” refers to any warm-blooded organism that can have a wearable device removably attached to their skin, including, but not limited to, a human being, a pig, a rat, a mouse, a dog, a cat, a goat, a sheep, a ^
^ horse, a monkey, an ape, a rabbit, a cow, etc. The terms wearer, user, subject, and patient can be used interchangeably. II. Overview [0052] Photoplethysmography (PPG) can be used to determine peripheral blood oxygen saturation (SpO2), which can be used to calculate other cardiovascular variability parameters that are critical measurements of health. PPG estimates arterial oxygen saturation (SaO2) using variations of optical intensity resulting from blood volumetric changes, the resultant PPG signal recordings are then used to derive cardiovascular variability parameters such as SpO2, heart rate, and respiration rate. However, PPG estimates from traditional pulse oximeters are often inaccurate for patients with higher levels of melanin in their skin. For instance, researchers found inaccurate blood-oxygen readouts from Hispanic and Black patients caused patients in these groups to be 25% less likely to be recognized for COVID-19 treatments compared to groups with lighter skin tones. In another instance, it was determined that 11 popular fingertip pulse oximeters that met federal and international regulations performed worse in subjects with more melanin in their skin. In fact, traditional pulse oximeters have been found to be inaccurate in three times as many cases for African American patients as for white patients. Inaccurate diagnoses can cause patients to receive improper medical care, or no medical care, when it is needed, and can lead to distrust of the entire medical community. Accordingly, there is a critical need to improve pulse oximetry measurements, particularly PPG measurements from which pulse oximetry measurements are derived. [0053] The optics for PPG sensors are straightforward and include illuminating perfused skin and measuring either reflection (most common for wearable devices, particularly for home use) or transmission. A traditional pulse oximeter can utilize two lights at different wavelengths (e.g., red and infrared) to differentiate the absorption of chromophores in oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb). However, the traditional methods do not account for the effect of melanin at different wavelengths, motion within the sensing site, or other confounding variables. The systems described herein account for confounding variable such as melanin levels, motion in the system, and the like. These systems employ multiple types of polarized ^
^ light of at least two different wavelengths, and in some instances a colorimeter for real-time calibration, to improve signal fidelity against motion artifacts and variations in skin tone. III. Systems [0054] Shown in FIG.1 is a system 100 for determining at least photoplethysmography (PPG) of a patient that accounts for one or more skin variables (e.g., skin tones due to varying levels of melanin, motion, and other currently confounding variables). Pulse oximetry via photoplethysmography (PPG) targets time-varying arterial blood volume changes by correlating the relative change in optical intensity per wavelength with the cardiac cycle in the form of ratiometric measurements of pulsatile (AC) to non-pulsatile (DC) signal to allow for convenient, continuous monitoring of peripheral blood oxygen saturation (SpO2) in peripheral blood. The system 100 and those that follow incorporate multi-channel polarization gating with multi-wavelength illumination to introduce tissue layer selectivity, depth penetration, and tissue responsivity into pulse oximetry. The incorporation of polarization gating can at least partially account for complex volumetric effects within the skin (e.g., of a finger, wrist, toe, ear, or the like) and can improve pulse oximetry accuracy across all skin tones. For instance, melanin’s contribution to absorption in the epidermis, as well as other surface effects such as skin topology, tattoo ink, and the like can be at least partially reduced and/or accounted for in the readings output to users and/or medical professionals. It should be understood that while individual portions of the systems herein are described with reference to one or more figures that any of the elements from each system/wearable device can be included in other versions of the system/wearable device. [0055] Without wishing to be bound by theory, polarization gating leverages the fact that specular-reflected light intensity and light intensity resulting from multiple scattering within the tissue volume (including epidermis, subcutaneous layers, and arteries of the skin) have different polarization signatures that can be separated. In this way, the system can include at least partial layer selectivity and depth discrimination within the skin tissue. Polarization gating can also reduce the effects of motion artifacts (e.g., from movement of user, movement of the wearable device, etc.) compared to non-polarized illumination. Obliquely incident polarized light that ^
^ impinges at the interface between two-different non-conducting (dielectric) media (such as the skin) can be separated into at least two components: a specular- reflected component that can retain the incident polarization state and a multiple scattered component, which generally can have a different polarization state in the presence of the turbidity common in human tissue. For linearly polarized illumination, co-polarized (parallel aligned) and cross-polarized (orthogonally or perpendicularly aligned) detection can permit separation (aka gating) of surface components of reflected light (e.g., the specular light) from deep components of reflected light (e.g., multiple scattered light), respectively. It is noted than any weakly scattered light (e.g., from 200 microns or less from the skin surface) can also maintain the initial polarization and can thus be detected similarly to specular light. Positioning a photodetector at an angle that is not an angle of reflection with respect to the obliquely incident light can further enhance the sensitivity. [0056] Polarization gating can use linearly polarized illumination, circularly polarized illumination, and/or elliptically polarized illumination for a variety of effects. For instance, polarization gating using circularly polarized illumination and/or elliptically polarized light can cause a reflected specular light to undergo a ± 180° phase change between the eigen polarizations to flip the helicity (e.g., a right-handed circularly polarized light can become a left-handed circularly polarized light). Therefore, circular polarization can separate mirror-reflected photons (e.g., those with the helicity flipped) from weakly scattered photons (although multiple scattered photons can still be present). If combined with linearly polarized light, then weakly scattered photons can be uniquely extracted from both surface components of the reflected light (e.g., specularly-reflected and/or weakly scattered light) and deep components of the reflected light (e.g., multiple scattered light). [0057] Additionally, still not wishing to be bound by theory, a second property of polarization is that linear polarizations and circular and/or elliptical polarization states can have different penetration depths in mediums (e.g., skin) and sensitivities to depolarization depending on properties of the mediums (e.g., skin properties and/or components). For instance, blood can exhibit a preferential depolarization of linear polarization states over circular polarization states and arterial tissue can more strongly depolarize circular polarization compared to linear polarization. The degree of linear polarization can be more sensitive to scattering, while the degree of circular polarization can be sensitive to both scattering and intrinsic birefringence (all ^
^ depending on the dominant scattering mechanisms in the tissue, e.g., Rayleigh, Mei, or Rayleigh-Mie transition regime). [0058] The combination of light-emitters and photodetectors, across various visible and infrared wavelengths with distinct polarization states, plays a crucial role in isolating and analyzing the photoplethysmography (PPG) signal. Each channel of the detector system is designed to capture light that has interacted with unique layers and depths of the skin. Specifically, light that is reflected or scattered superficially, from the epidermis or upper dermis, tends to maintain unique polarization characteristics, while light that penetrates deeper into the tissue and interacts with blood vessels undergoes increased depolarization and a change in polarization state due to multiple scattering events. By employing sources with different polarization input polarizations and analyzers with different polarization alignments, selectivity to different layers within tissues, such as superficial vs. deep components, can be achieved. For example, one polarization channel primarily detects co-polarized light, which corresponds to light that has undergone lower amounts of scattering and primarily reflects from the superficial layers. In contrast, the cross-polarized channel captures depolarized light, which includes more isolated information from deeper tissue layers where hemoglobin concentration and blood flow dynamics dominate. The use of multiple polarization-sensitive channels (sources plus detector combinations) allows the system to mathematically decompose the detected signals into their points of origin within the tissue. Furthermore, synchronizing the polarized signal extraction with the cardiac cycle, ensures that the PPG measurements are more sensitive to hemodynamic-based optical intensity modulation from the target regions deep within tissue while minimizing superficial signals such as from the epidermal region. [0059] As shown in FIG.1, the system 100 can be embodied entirely as a wearable device and/or can be partially embodied as a wearable device and at least one other device (e.g., a smartphone, tablet, computer, dedicated device, etc.), where the at least one other device can be in electrical communication with the wearable device. Unless otherwise mentioned the system 100 will be described as only a wearable device for ease of illustration. The wearable device (e.g., system 100) can be removable attached over skin of a user that contains at least one peripheral blood vessel (e.g., skin of a wrist, finger, ear, toe, forehead, or the like). The system 100 can optically determine one or more cardiovascular variability ^
^ parameters via a PPG signal. The one or more cardiovascular variability parameters can include, but are not limited to, oxygen saturation values (arterial and/or peripheral), a heart rate value, a respiratory rate value, a tissue oxygenation value, an arterial blood pressure value, a blood vessel stiffness value, a vascular assessment value, a microvascular blood flow value, a tissue viability value, a vasomotor function value, a thermoregulation value, an orthostasis value, a neurology value, or the like. While generally, only the PPG signal and/or the SpO2 will be described, this is only for ease of illustration and discussion and a person of ordinary skill in the art would understand how to apply further calculations to derive other cardiovascular variability parameters from the PPG signal. [0060] The system 100 can include a controller 10, at least two emitters (e.g., emitter 1 through emitter N) 12(1)-12(N) and a detector 14. In some instances, the system 100 can include a colorimeter 16 and/or a communication interface 18. The at least two emitters 12(1)-12(N) can each emit a polarized light signal having a different wavelength towards the skin of the user. The at least two emitters 12(1)- 12(N) can emit the polarized light signals concurrently, consecutively, and/or in an overlapping manner. Each of the polarized light signals can interact with the skin (e.g., be at least partially reflected, transmitted, absorbed, and/or the like). At least a portion of each of the polarized light signals applied by the at least two emitters 12(1)-12(N) can be reflected from the skin and detected by the detector 14. Information from the detected light signals can be transmitted from the detector 14 to the controller 10 where at least a PPG signal can be determined. In some instances, the colorimeter 16 can be used to determine an individual typology angle (ITA) of the skin and the controller 10 can adjust one or more of the polarized light signals based on the ITA to help account for skin tone (e.g., adjust the polarization, the wavelength, the incidence angle, or the like). In another instance, the controller 10 can output data related to the PPG signal and/or one or more cardiovascular variability parameters to the communication interface 18 to display a representation of the PPG signal and/or one or more cardiovascular variability parameters, send the user a notification and/or an alert, send a medical professional associated with the user a notification and/or an alert, or the like. It should be understood that any additional basic components such as battery, power source, circuitry, and/or wireless transmitters required for system 100 to function are included but not shown for ease of illustration. ^
^ [0061] The controller 10 can be in electrical communication with the at least two emitters 12(1)-12(N) (at least unidirectional) to control illumination from the at least two emitters. In some instances, the controller 10 can be in bi-directional electrical communication with the detector 14 to at least send control information to and receive recordings from the detector. The controller 10 can determine a PPG signal and/or a SpO
2 that accounts for one or more confounding variables of the skin (e.g., motion, motion relative to the skin, tattoos, birthmarks, or the like) based on the recordings received from the detector 14. In some instances, the controller 10 can be in electrical communication (which can be uni-or bi-directional) with the communication interface 18, that can act as an audio, visual, and/or tactile display and/or a user interface. In other instances, the controller 10 can be in bi-directional communication with a colorimeter 16 to detect a measurement of the user’s skin tone and/or other surface effects (e.g., tattoos, birthmarks, or the like). In some instances, not shown, the controller 10 can be more than one controller embodied in one or more separate devices where the functions of the controller can be split between each of the one or more controllers (e.g., one controller can be in the wearable device and another controller can be in another device such as a smartphone or the like, which may include a communication interface 18. FIGS.2-4 illustrate details about the structures, functions, and/or use of components of system 100. It should be understood that any components of system 100 not shown in FIGS.2-4 still exist but are not shown for ease of illustration. [0062] FIG.2 shows a first portion of the system 100 (labeled (1))for illuminating the skin in greater detail. The at least two emitters 12(1)-12(N) can each include an LED 22(1)-22(N) and a polarizer 24(1)-24(N) positioned between the LED and the skin. It should be noted that while an LED is shown and described herein, a laser diode could be used instead. Each LED 22(1)-22(N) can emit a light signal at a wavelength towards a polarizer 24(1)-24(N). Each LED 22(1)-22(N) can emit light having a different wavelength. For example, each wavelength can be one of a red wavelength (e.g., between about 620 nm and 750 nm), an infrared wavelength (e.g., between about 780 nm and 1000 nm), a green wavelength (e.g., between about 490 nm and 580 nm), a blue wavelength (e.g., between about 450 nm and 595 nm), and the like. The polarizers 24(1)-24(N) can each polarize a light signal (e.g., let light waves of a specific polarization pass through while blocking light waves of other polarizations). The polarizers 24(1)-24(N) can have, for instance, a linear polarization ^
^ state at any angle . relative to the x-direction, a right-handed circular polarization state, a left-handed circular polarization state, a right-handed elliptical polarization state, or a left-handed elliptical polarization state. Unless noted otherwise each of the polarizers 24(1)-24(N) has a same polarization state, although each of the polarizers 24(1)-24(N) can in some instances have one or more different polarization states. In some instances (not shown for ease of illustration) each of the at least two emitters 12(1)-12(N) can also include a lens (e.g., polydimethylsiloxane (PDMS) or metalens) that can direct and/or focus the emission of the light signal(s) and may be manually and/or automatically controllable (e.g., motorized and/or electrical methods to change directions and/or focal lengths). [0063] The controller 10 can control a frequency at which the emitters 12(1)- 12(N) can emit the light signals and/or one or more other parameters related to the light signals, including which emitters of emitters 12(1)-12(N) can be active at a given time. In some instances, the controller 10 can be pre-programmed (e.g., frequency of light emission is constant). For example, the emitters 12(1) – 12(N) can emit light at frequencies similar to traditional pulse oximeters. In other instance, the controller 10 can be at least partially manually controller (e.g., user and/or medical professional can input a manual command to sample at a specific time, change frequency, change emitters, etc.). In a further instance, the controller 10 can be at least partially automatically controlled by feedback from one or more sensors, such as colorimeter 16 (or other type of spectrophotometer). Other sensors can include, but are not limited to a heart rate sensor, a motion sensor, a contact sensor, or the like that can alert to a change in one or more additional confounding variables related to the skin. A colorimeter 16 can determine an ITA of the skin that can be classified in different categories based on skin darkness. The colorimeter can be, for example a tristimulus colorimeter. The ITA can be based on CIELAB color space (e.g., color parameters L* (lightness/luminance) and b*(yellow/blue). For an individual with a lighter skin color, the ITA can be expected to be a higher positive value than that of an individual with a darker skin color. The controller 10 can adjust one or more of the light signals based on the ITA to account for different skin tones (e.g., adjust at least one parameter of the one or more light signals, adjust an angle of emission of the one or more light signals, adjust which light signals are emitted, or the like). Another metric for determining melanin levels that can be used alternatively and/or in addition to ITA, is ^
^ the melanin index where the level of melanin in skin is related to the amount of red reflected from the skin. [0064] FIG.3 shows a second portion of the system 100 (labeled (2)) that can detect light from the skin after the light was applied as described with respect to FIG. 2. The detector 14 can detect light signal from (e.g., at least partially reflected from) the skin in response to application of each of the polarized light signals. The detector 14 can include at least two polarization channels 26(1)-26(R). Each polarization channel 26(1)-26(R) can include at least two photodetectors (e.g., at least two first photodetectors 28(1)-28(M) in a first polarization channel 26(1) and at least two other photodetectors 32(1)-32(P) in each other polarization channel 26(R)) and at least two analyzers (e.g., at least two first analyzers 30(1)-30(M) in a first polarization channel 26(1) and at least two other analyzers 34(1)-34(P) in each other polarization channel 26(R)). For each polarization channel, each of the at least two photodetectors can be paired with each of the at least two analyzers (e.g., 28(1)- 28(M) with 30(1)-30(M) and 32(1)-32(P) with 34(1)-34(P)) such that one of the at least two analyzers is positioned between one of the at least two photodetectors and the skin. [0065] In some instances, the photodetectors (e.g., 28(1)-28(M) and 32(1)- 32(P)) for each of the polarization channels 26(1)-26(R) can be wavelength dependent. As a non-limiting example, in the first polarization channel 26(1) the at least two first photodetectors 28(1)-28(M) can include a first photodetector 28(1) and another first photodetector 28(2) and in the second polarization channel 26(2) the at least two second photodetectors 32(1-P) can include a second photodetector 32(1) and another second photodetector 32(2). The first photodetector 28(1) and the second photodetector 32(1) can each detect the light signals from the skin of the wearer in response to application of the first polarized light signal (e.g., from emitter 12(1)). The other first photodetector 28(2) and the other second photodetector 32(2) can each detect the light signals from the skin of the wearer in response to application of the second polarized light signal (e.g., from emitter 12(2)). [0066] Each polarization channel 26(1)-26(R) can detect at least a portion of the reflected light from the light signals 1 – N depending on the configurations of the photodetectors (e.g., 28(1)-28(M) and 32(1)-32(P)) and the polarization state of each the light signals 1-N and the polarization state of each of the analyzers (e.g., 30(1)- 30(M) and 34(1)-34(P)). For instances the at least two first analyzers 30(1)-30(M) ^
^ can be aligned at a relative angle . to a direction or a handed-ness (depending on linear or circular/elliptical polarization state) relative to a polarization state of the polarizers (e.g., polarizers 24(1)-24(N)). In a main instance, the polarizers (e.g., polarizers 24(1)-24(N)) can all have a same and/or similar (e.g., within e.g., within ±5°, within ±10°, within ±20° or the like tolerance) polarization state. For instance, if the polarizers (e.g., polarizers 24(1)-24(N)) are linearly polarized (e.g., at an angle . relative to the x direction), then a first polarization channel 26(1) can be a co- polarization channel and a second polarization channel 26(2) can be a cross- polarization channel. [0067] In a co-polarization channel, each of the at least two first analyzers 30(1)-30(M) can be aligned parallel or near parallel (e.g., within ±5°, within ±10°, within ±20° or the like) to the angle . relative to the x direction of the polarizers (e.g., polarizers 24(1)-24(N)). In a cross-polarization channel, each of the at least two second analyzers 34(1)-34(M) can be aligned perpendicular (and/or orthogonal in 3D) or near perpendicular (and/or orthogonal in 3D) (e.g., within ±5°, within ±10°, within ±20° or the like) to the angle . relative to the x direction of the polarizers (e.g., polarizers 24(1)-24(N)). The co-polarization channel can, for instance, detect a portion of the light signal from (e.g., reflected from the skin) comprising superficial components (e.g., specularly-scattered and/or weakly scattered light from at or near the surface of the skin) and deep components (e.g., multiple scattered light from deeper components of the skin). The superficial skin regions can be about 200 micrometers or less from a surface of the skin and the deep skin regions can be more than about 200 micrometers from the surface of the skin. The cross- polarization channel can, for instance, detect a portion of the light signals from the skin comprising the deep components (e.g., multiple scattered light from deeper components of the skin) because any superficial components are filtered out by the at least two second analyzer(s) 34(1)-34(M) in the cross-polarization alignment. Additionally, the co-polarization channel and the cross-polarization channel can each be configured to receive and separate inputs resulting from each of the polarized light signals (e.g., based on wavelength dependency, or the like). For instance, the co-polarization channel can detect a first co-polarized input based on a first polarized light signal and a second co-polarized input based on a second polarized light signal, and the cross-polarization channel can detect a first cross-polarized input based on ^
^ the first polarized light signal and a second co-polarized input based on the second polarized light signal. [0068] The controller 10 can receive data related to the detection of the polarized light signals (1-N) from the skin from each polarization channel 26(1)- 26(R). The controller 10 can then calculate at least a PPG signal that accounts for melanin in the skin, motion relative to the skin, or the like, at least in part on the light signals from the skin. The controller 10 can also send one or more instructions to the detector 14 and/or specifically to the polarization channels 26(1)-26(R). [0069] FIG.4 describes a third portion of the system 100 (labeled (3)) focusing on the controller 10 and the optional communication interface 18 in greater detail. The controller 10 can include a non-transitory memory (e.g., memory) 36 that can store instructions and a processor 38 that can execute the instructions. The memory 36 and the processor 38 can be embodied separately or in a single device. For instance, the controller can be a microcontroller with a combined memory 36 and processor 38. The communication interface 18 can include at least one information outputting feature such an audio, visual, and tactile mechanism for providing information to a user and/or a medical professional and/or at least one information inputting feature such as a touchscreen, keyboard, mouse, buttons, or the like for manually inputting information into the system 100. [0070] In some instances, the controller 10 and the communication interface 18 can both be entirely embodied within the wearable device. In another instance, the controller 10 can be more than one controller and can be at least partially embodied in the wearable device and at least partially embodied in another device. In this instance, the communication interface 18 can be a single communication interface embodied in either the wearable device or the other device or the communication interface can be multiple communication interfaces with at least one embodied in the wearable device and another embodied in the other device. The other device can be, for example a smartphone, a tablet, a computer, a dedicated medical device, or the like. The other device can be one or more other devices, such as one other device used by the user and a second other device used by a medical professional (often at another location). [0071] The controller 10 can control one or more functional aspects of the system 100 via the instructions executed by the processor 38. For instance, the On/Off/Start 40 functionality can be controlled by the controller 10 – e.g., turning the ^
^ system on and off, a timed and/or manual request to start PPG and monitoring at least one cardiovascular variability parameter, or the like. Turning the system 100 on and/or a manual/automatic request to begin PPG can begin application of the polarized light signals 1-N by the emitters 12(1)-12(N). The controller 10 can receive 42 the data related to the detected polarized light signals from the skin from the detector 14 and/or data from the colorimeter 16. For instance, the controller 10 can set a frequency of sampling the detector 14, the individual polarization channels 26(1)-26(N), and/or the colorimeter 16. If the system 100 includes a colorimeter 16, then the controller 10 can perform color feedback 44 to determine if one or more parameters related to the application of the polarized light signals should be adjusted based on skin tone of the user (e.g., lighter or darker skin tone based on the ITA from the colorimeter 16), what the adjustment should be (e.g., change to one or more wavelengths, intensities, powers, or the like of at least one light signal, change of a polarization state of at least one polarizer 24(1)-24(2) and/or analyzer 30(1-M) or 34(1-P), adjust an angle of emission at least one light signal, or the like), and cause the adjustment in the system 100. [0072] The controller 10 can analyze 46 the data related to the detected polarized light signals, account for 48 one or more skin variables (e.g., melanin level, motion relative to the skin, tattoo pigment, or the like), and determine 50 at least a PPG signal and/or one or more cardiovascular variability parameters, such as SpO2. For instance, the controller 10 can analyze the different superficial and deep components detected to remove the majority of the effect of at least the superficial components (e.g., including the melanin, the movement, and any tattoo pigments). For example, the controller 10 can analyze the detected portions (e.g., the portion and the other portion) of the at least two light signals after at least reflection by the skin using weighting techniques. The portion of the light signal and the other portion of the light signals can be transformed into at least values representative of an intensity of the superficial components and at least values representative of an intensity of the deep components. The superficial components can correspond to the light signals from superficial skin regions in response to application of the first and second polarized light signals. The deep components can correspond to the light signals from deep skin regions in response to application of the first and second polarized light signals. The at least the values representative of the intensity of the superficial components and the at least the values representative of the intensity of ^
^ the deep components can be weighted. For example, the weighting can minimize the contributions from the superficial components and/or emphasize contributions from the deep components. In some instance, the weighting can be based on accounting for at least one of motion, skin tone, skin composition, skin color, a distance to a vein and/or an artery. These can be determined for instance, at least partially based on the type of wearable device being used (e.g., motion, distance to a vein and/or artery, skin composition, etc., can be predetermined based on how secure the device can be attached and/or where the device is attached on the body) and feedback data such as the ITA. [0073] The PPG signal and/or one or more cardiovascular variability parameters, such as SpO2, and/or a representation of the PPG signal and/or one or more cardiovascular variability parameters, such as SpO2, can be output 52 to the communication interface 18. These steps can happen at a given sampling frequency for continuous monitoring of the one or more cardiovascular variability parameters. If the one or more cardiovascular variability parameters fall below one or more given limits the communication interface can at least one of alert and/or notify (e.g., audio, visual, and/or haptic) the user and/or a medical professional, and/or alert emergency services depending on the severity and/or length of time the one or more cardiovascular variability parameters have fallen below one or more given limits. In other instances, for example in a hospital setting if the one or more cardiovascular variability parameters has fallen below one or more given limits for a predetermined length of time at least one medical device (not shown) in communication with the system 100 may be triggered (e.g., provided supplemental oxygen or increase supplemental oxygen to a patient with a low SpO2, or the like). [0074] FIG.5 shows examples of wearable devices 200(A, B, and C) that can embody and/or at least partially embody the system 100 as described above. The wearable devices 200(A, B, and C) are shown as wearable on a portion of an upper limb of the user including at least a portion of a hand with fingers and wrist of the user. However, it should be understood that FIG.5 shows only a few examples of where the wearable devices 200 (A, B, and C) can be worn for illustration purposes and are not intended to be limiting. Each of the wearable devices 200(A, B, and C) can include an attachment mechanism 254 (shown with respect to wearable device A 200A) that can reversibly attach the wearable device to the skin of the user. For instance, the attachment mechanism 254 can include a skin safe adhesive, a band ^
^ with a clasp, a buckle, a clip, or the like, a stretchable/compressive band, a suction mechanism, a clip-on mechanism, or the like. For instance, wearable device A 200A can be wearable on a dorsal side of a wrist of the user. The wearable device A 200A can be, for example, worn like a bracelet or watch. In another instance the wearable device can be wearable on a finger of a user such as around a length of the finger, like wearable device B 200B (e.g., like a ring), or on a fingertip, like wearable device C 200C (e.g., like a finger clip used in current medical technology). While not shown, it should be understood that a wearable device can additionally and/or alternatively be worn and reversibly attached to a toe, an ankle, an ear, a forehead, or the like as is known in the art of medical devices for at least pulse oximetry. [0075] FIG.6 shows a skin facing side of a wearable device 300 that can be, for instance, any of the wearable devices 200(A, B, or C) from FIG.5. The wearable device 300 can include a housing 370 that can at least partially encompass and/or include at least a portion of the components of the system (e.g., system 100). The housing 370 can be any configuration, shape, size, and/or material that can be used generally for pulse oximetry wearables and depending on the intended location for wearing the wearable device 300. The skin facing side of the wearable device 300 can include at least two emitting windows 366(1)-366(N) in the housing 370 and at least four receiving windows including at least two first receiving windows 361(1)- 362(M) and at least two other (X) receiving windows 364(1)-362(N) in the housing. The at least two emitting windows 366(a)-366(N) can expose the skin to the light signals from the at least two LEDs (e.g., LEDs 22(1)-22(N) of system 100). The at least four emitting windows (e.g., the at least two first receiving windows 361(1)- 362(M) and the at least two other (X) receiving windows 364(1)-362(N)) can expose the at least two first photodetectors (e.g., first photodetectors 28(1)-28(M) of system 100) and the at least two other photodetectors (e.g., other (X) photodetectors 28(1)- 28(P)) to the skin. [0076] FIG.7, element A shows the emitting windows 366(1)-366(N) with respect to the other emission components of the system 100 in wearable device 300. The at least two emitting windows 366(1)-366(N) can each include a polarizer 324(1)-324(N). Thus, each emitter 312(1)-312(N) can include an LED 322(1)-322(N) that can emit a light signal having a wavelength towards an emitting window 366(1)- 366(N) that can include a polarizer 324(1)-324(N) that can emit the polarized light signals 1-N to the skin. FIG.7, element B shows the receiving windows, including the ^
^ at least two first receiving windows 362(1)-362(M) and the at least two other (X) receiving windows 364(1)-364(P), with respect to the detection components of eth system 100 in wearable device 300. The at least four receiving windows can include the at least two first analyzers and the at least two other analyzers such that the at least two first receiving windows 362(1)-362(M) can include the at least two first analyzers 330(1)-330(M) and the at least two other (X) receiving windows 364(1)- 362(P) can include the at least two other analyzers 334(1)-334(P) for each of the polarization channels 326(1)-326(R). For each polarization channel 326(1)-326(R) of the detector 314, the polarized light signals 1-N from the skin can each be separately detected through a receiving window (e.g., 362(1)-362(M) or 364(1)-364(P)) including at least two analyzers (e.g., at least two first analyzers 330(1)-330(M) or at least two other analyzers 334(1)-334(P)) then the at least two photodetectors (e.g., at least two first photodetectors 328(1)-328(M) or the at least two other photodetectors 332(1)-332(P)) can detect the filtered light signals 1-N. [0077] FIG.8 shows a system 400 with a co-polarization channel 480 and a cross-polarization channel 482. It should be understood that system 400 is a specific example of system 100 and all of the descriptions related to system 100 apply to system 400 as well. Note the detector 412 is not explicitly shown for ease of illustration but should be understood to include the entirety of the co-polarization channel 480 and the cross-polarization channel 482. The controller 410 can be in electrical communication with at least a portion of each of a first emitter 412(1) and a second emitter 412(2), the co-polarization channel 480 and the cross-polarization channel 482 of the detector 414, and, optionally, the communication interface 418. The first emitter 412(1) can emit a first polarized light signal 1 (the full line) towards skin of a wearer (aka a user) of the wearable device (that embodies system 400). The first emitter 412(1) can include a first LED 422(1), which can emit a first light signal at a first wavelength, and a first polarizer 424(1), which can be positioned between the first LED and the skin and can polarize the first light signal to form the first polarized light signal 1. The first polarizer 424(1) can be within a first emitting window 466(1). The second emitter 412(2) can emit a second polarized light signal 2 (the dotted line) towards the skin. The second emitter 412(2) can include a second LED 422(2), which can emit a second light signal at a second wavelength, and a second polarizer 424(2), which can be positioned between the second LED and the skin and can polarize the second light signal to form the second polarized light signal ^
^ 2. The second polarizer 424(2) can be within a second emitting window 466(2). The first polarizer 424(1) and the second polarizer 424(2) can have the same polarization state. For instance, the first polarizer 424(1) and the second polarizer 424(2) can each have a linear polarization state at angle . relative to the X direction. [0078] The detector 414 (not shown in FIG.8) can detect light signals from the skin in response to the application of the first polarized light signal 1 and the second polarized light signal 2. The detector 414 can include the co-polarization channel 480 and the cross-polarization channel 482 that can each detect portions of light signals from the skin based on the first polarized light signal 1 and the second polarized light signal 2. [0079] The co-polarization channel 480 can include at least two first photodetectors (shown as two) 428(1) and 428(2) and at least two first analyzers (shown as two) 430(1) and 430(2). Each of the at least two first analyzers 430(1) and 430(2) can be within a first receiving window such that a first first analyzer 430(1) can be within a first first receiving window 462(1) and a second first analyzer 430(2) can be within a second first receiving window 462(2). Each of the at least two first analyzers (shown as two) 430(1) and 430(2) can be aligned parallel or near parallel (e.g., within 1° of ., within 5° of ., within 10° of ., within 20° of ., or the like) to the direction of the first polarizer 424(1). The at least two first photodetectors (e.g., 428(1) and 428(2)) and the at least two first analyzers (e.g., 430(1) and (430(2)) can be paired such that one of the at least two first analyzers can be positioned between one of the at least two first photodetectors and the skin. As shown, the first first analyzer 430(1) within first first receiving window 462(1) can be between the first first photodetector 428(1) and the skin and the second first analyzer 430(2) within second first receiving window 462(2) can be between the second first photodetector 428(2) and the skin. Each of the first first analyzer 430(1) and the second first analyzer 430(2) can receive portions of light signals from the skin that originated as the first polarized light signal 1 and the second polarized light signal 2. Based on the polarization gating (e.g., the polarization state of the analyzers 430(1) and 430(2) relative to the polarization state of the polarizers 424(1) and 424(2)) of the co- polarization channel 480, the co-polarization channel can detect a portion of the light signals from the skin (e.g., from the first and second polarized light signals 1 and 2) comprising superficial components (e.g., specularly reflected light from the surface of the skin and weakly scattered light from near the surface of the skin) and deep ^
^ components (e.g., multiple scattered light from deeper components of the skin). The superficial skin regions can be about 200 micrometers or less from a surface of the skin and the deep skin regions can be more than about 200 micrometers from the surface of the skin. [0080] The cross-polarization channel 482 can include at least two second photodetectors (shown as two) 432(1) and 432(2) and at least two second analyzers (shown as two) 434(1) and 432(2). Each of the at least two second analyzers 434(1) and 434(2) can be within a second receiving window such that first second analyzer 434(1) can be within a first second receiving window 464(1) and a second second analyzer 434(2) can be within a second second receiving window 464(2). Each of the at least two second analyzers (shown as two) 434(1) and 434(2) can be aligned perpendicular (in 2D)/orthogonal (in 3D) or near perpendicular/orthogonal (e.g., within 1° of ., within 5° of ., within 10° of ., within 20° of ., or the like of perpendicular/orthogonal) to the direction of the second polarizer 424(2). The at least two second photodetectors (e.g., 432(1) and 432(2)) and the at least two second analyzers (e.g., 434(1) and (434(2)) can be paired such that one of the at least two second analyzers can be positioned between one of the at least two second photodetectors and the skin. As shown, the first second analyzer 434(1) within first second receiving window 464(1) can be between the first second photodetector 432(1) and the skin and the second second analyzer 434(2) within second second receiving window 464(2) can be between the second second photodetector 432(2) and the skin. Each of the first second analyzer 434(1) and the second second analyzer 434(2) can receive portions of light signals from the skin that originated as the first polarized light signal 1 and the second polarized light signal 2. Based on the polarization gating (e.g., the polarization state of the analyzers 434(1) and 434(2) relative to the polarization state of the polarizers 424(1) and 424(2)) of the cross- polarization channel 482, the cross-polarization channel can detect a portion of the light signals from the skin (e.g., from the first and second polarized light signals 1 and 2) comprising deep components (e.g., multiple scattered light from deeper components of the skin). [0081] The controller 410 can include a non-transitory memory and processor (not shown in FIG.8) and can at least determine a PPG signal and/or one or more cardiovascular variability parameters, such as SpO2, that can account for motion, motion relative to the skin, melanin, tattoo pigmentation, and/or other confounding ^
^ variables of the skin based at least in part on the light signals from the skin detected by the co-polarization channel 480 and/or the cross-polarization channel 482. For instance, the controller 410 can analyze the different superficial and deep components detected to remove the majority of the effect of at least the superficial components (e.g., including the melanin, the movement, and any tattoo pigments). The controller 410 can send the PPG signal and/or the one or more cardiovascular variability parameters, and/or representations thereof, to the communication interface 418. The communication interface 418 can, for instance, visually display or audibly announce the PPG signal and/or the one or more cardiovascular variability parameters, and/or representations thereof, at a time or over a period of time (e.g., values, graphs, etc.). In another instance, the communication interface 418 can classify the PPG signal and/or the one or more cardiovascular variability parameters based on severity and/or alert/notify (e.g., visually, audibly, and/or tactile) the user and/or a medical professional if below one or more given thresholds. [0082] FIG.9 shows another system that can be at least partially embodied as a wearable device 500 for determining a PPG signal and/or one or more cardiovascular variability parameters that can account for motion, motion relative to the skin, melanin, tattoo pigmentation, and/or other confounding variables of the skin. The wearable device 500 can include an emission component 502, a detection component 504, and a controller 510 that can be in communication (wired and/or wireless) with detection component and the emission component. The wearable device 500 can optionally include a communication interface 518(a) and/or a communication interface 518(b) can be a portion of another device (not shown, that can include at least a processor) that can either or both be in communication (wired and/or wireless) with the controller 510. The emission component 502 can emit at least two polarized light signals with different wavelengths towards skin of a wearer of the wearable device 500. The emission component 502 can include at least two light emitters 506(1)-506(F), which can each emit a light signal at a different wavelength, and at least two polarizers 508(1)-508(G), which can be positioned between each of the at least two light emitters and the skin and can polarize each of the at least two light signals having different wavelengths, respectively. [0083] The detection component 504 can detect light signals from the skin of the wearer in response to the at least two polarized light signals with different wavelengths hitting the skin of the wearer and interacting (e.g., at least partially ^
^ reflecting). The detection component 504 can include at least two polarization channels 526(1)-526(A). Each of the at least two polarization channels can include at least two photodetectors and at least two analyzers. As shown, a first polarization channel 526(1) can include at least two photodetectors 528(1)-528(B) and at least two first analyzers 530(1)-530(C) and the at least the second polarization channel 526(A) can include at least two other photodetectors 532(1)-532(D) and at least other two analyzers 534(1)-534(E). For each of the at least two polarization channels 526(1)-526(A) the at least two photodetectors (e.g., 528(1)-528(B) and 532(1)- 532(D)) can detect light signals from the skin of the wearer in response to each of the at least two polarized light signals with different wavelengths hitting and interacting with the skin of the wearer, after the light signals have passed through each of the at least two analyzers (e.g., 530(1)-530(C) and 534(1)-534(E)). The at least two photodetectors (e.g., 528(1)-528(B) and 532(1)-532(D)) and the at least two analyzers (e.g., 530(1)-530(C) and 534(1)-534(E)) can be paired such that one of the at least two analyzers is positioned between one of the at least two photodetectors and the skin. [0084] The at least two analyzers (e.g., 530(1)-530(C) and 534(1)-534(E)) can be aligned relative to a polarization alignment of a portion of the emission component 502 (e.g., the at least two polarizers 508(1)-508(G)). The at least two analyzers 530(1)-530(C) of each of the at least two polarization channels 526(1)-526(A) can be aligned differently relative to an alignment of the at least two analyzers 534(1)- 534(E) of another of the at least two polarization channels 526(1)-526(A) to detect different portions of the light signals reflected by the skin comprising at least one of a superficial component of the reflected light signal and a deep component of the reflected light signal. The superficial skin regions can be about 200 micrometers or less from a surface of the skin and the deep skin regions can be more than about 200 micrometers from the surface of the skin. The at least two different alignments can be at least two of circular, elliptical, horizontal, or vertical. In one instance the at least two polarization channels can include at least a co-polarization channel and a cross-polarization channel. In the co-polarization channel, the at least two analyzers (e.g., 530(1)-530(C)) can be arranged in parallel or near parallel (e.g., within 1° of ., within 5° of ., within 10° of ., within 20° of ., or the like) with a polarization state direction of angle . of the at least two polarizers 508(1)-508(G). In the cross- polarization channel, the at least two analyzers (e.g., 534(1)-543(E)) can be ^
^ arranged in perpendicular (in 2D)/orthogonal (in 3D) or nearly (e.g., within 1° of ., within 5° of ., within 10° of ., within 20° of ., or the like) with a polarization state direction of angle . of the at least two polarizers 508(1)-508(G). [0085] The controller 510 can include a non-transitory memory (e.g., memory 536) and processor 538 and can at least determine a PPG signal and/or one or more cardiovascular variability parameters, such as SpO
2, that can account for motion, motion relative to the skin, melanin, tattoo pigmentation, and/or other confounding variables of the skin based at least in part on the light signals from the skin detected by the at least two polarization channels 526(1)-526(A). For instance, the controller 510 can analyze the different superficial and deep components detected to remove the majority of the effect of at least the superficial components (e.g., including the melanin, the movement, and any tattoo pigments). The controller 510 can send the PPG signal and/or the one or more cardiovascular variability parameters, and/or representations thereof, to the communication interface 518. The communication interface 518 can, for instance, visually display or audibly announce the PPG signal and/or the one or more cardiovascular variability parameters, and/or representations thereof, at a time or over a period of time (e.g., values, graphs, etc.). In another instance, the communication interface 518 can classify the PPG signal and/or the one or more cardiovascular variability parameters based on severity and/or alert/notify (e.g., visually, audibly, and/or tactile) the user and/or a medical professional if below one or more given thresholds. [0086] FIG.10 shows a portion of wearable device 500 in another view to illustrate the distances d (as shown in FIG.9 – the distance along the skin facing surface) between the emission component 502 (e.g., at least one emitter of emission component 502) and at least a portion of the detection component 504 (e.g., the at least two photodetectors of each polarization channel). The distance between the emission component 502 and at least a portion of the detection component 504 can be manually variable. For instance, three photodetectors 528(1), 528(2), and 528(3) of a first polarization channel 526(1) are shown in FIG.10 relative to the emission component 502. Each of the photodetectors 528(1), 528(2), and 528(3) is a variable distance, d1, d2, and d3, from the emission component 502, respectively. In some instance the controller 510 can choose which of the photodetectors 528(1), 528(2), and 528(3) of the polarization channel 526(1) has the best signal strength. The controller can make this choice for each polarization channel 526(1)-526(A). As an ^
^ example, the signal strength determination can be completed for each re-attachment of the wearable device 500, at a timed interval, at a manual input from a user and/or medical professional, or the like. IV. Methods [0087] Another aspect of the present disclosure can include methods 600, 700, and 800 (FIGS.11, 12, and 13) for at least determining a photoplethysmography (PPG) signal and/or one or more cardiovascular variability parameters, such as peripheral oxygen saturation (SpO2), that can account for motion, motion relative to the skin, melanin, tattoo pigmentation, and/or other confounding variables of the skin. The methods can integrate polarization gating into a PPG-based device, such as a pulse oximeter. Cardiovascular variability parameters can include, for example, an oxygen saturation value (peripheral or arterial), a heart rate value, a respiratory rate value, a tissue oxygenation value, an arterial blood pressure value, a blood vessel stiffness value, a vascular assessment value, a microvascular blood flow value, a tissue viability value, a vasomotor function value, a thermoregulation value, an orthostasis value, or a neurology value. [0088] The methods 600, 700, and 800 can be executed using the system 100 and/or the wearable devices 200, 300, 400, and 500 as shown in FIGS.1-10. For purposes of simplicity, the methods 600, 700, and 800 are shown and described as ^^^^"^^$^^^^^^^^^^^^^^^^^^^^^ver, it is to be understood and appreciated that the present disclosure is not limited by the illustrated order as some steps could occur in different orders and/or concurrently with other steps shown and described herein. Moreover, not all illustrated aspects may be required to implement the methods 600, 700, and 800, nor are methods 600, 700, and 800, limited to the illustrated aspects. [0089] Referring now to FIG.11, method 600 illustrates determining a PPG signal and/or one or more cardiovascular variability parameters that account for motion artifacts and/or confounding skin contributions such as melanin levels and/or tattoo pigment. The method 600 (as well as methods 700 and 800) can be performed by a system comprising at least a processor (e.g., system 100 or a wearable device 500) that can perform photoplethysmography with integrated polarization gating. For instance, such a system can include an emission component (e.g., 502), which includes at least two emitters (e.g., 12(1)-12(N) or 506(1)-506(F)), a detection ^
^ component (e.g., 504, otherwise called a detector 14), and a controller (e.g., 10, 510, etc.) that includes the processor (e.g., 38, 538, etc.) and a non-transitory memory (e.g. memory 36, 536, etc.). The wearable device (e.g., 200, 500, etc.) can be, for example, reversibly attached to a dorsal side of a wrist (e.g., via a band or an adhesive), clipped on a fingertip, worn on a finger, worn on a wrist, worn on an ear, worn on a toe, worn on an ankle, or the like. [0090] At 602, at least two polarized light signals having different wavelengths can be applied to skin of a wearer of a wearable device at least partially embodying the system comprising the processor. The processor can execute instructions to control two or more emitters to emit the at least two polarized light signals. The at least two polarized light signals can be applied simultaneously and/or in an alternating fashion, the controller can control the timing and/or frequency of light application. In some instances, the at least two polarized light signals can be applied at a frequency that is less than or equal to a heart rate of the user. Each of the two or more emitters can include a light source such as an LED (e.g., one of 22(1)-22(N), etc.) or laser diode that can emit a light signal with a wavelength (e.g., at least two of red, infrared, blue, green, or the like) towards a polarizer (e.g., one of 28(1)-28(M), 32(1)-32(R), etc.) having a given polarization state (e.g., linear at an angle . relative to an x or y direction, right handed or left handed circular, or right handed or left handed elliptical). The at least two polarized light signals having different wavelengths can then be emitted towards the skin of the wearer. In some instances, the polarizers can be within windows (e.g., emission windows 366(1)-366(N)) in a housing of the wearable device. The windows can expose at least a portion of the interior components of the wearable device to the skin so light can emit. The at least two polarized light signals having different wavelength can then be at least one of reflected, absorbed, or transmitted by various layers and components of the skin depending on, for instance, the wavelengths, intensities, and polarizations. Additionally, confounding variables of the skin such as melanin levels, tattoo pigments, and/or changing distance from the emission point to the skin (e.g., worn too loosely, not in synch with heartbeat, etc.) can affect how the light signals interact with the skin. In some instance, the wearable device can include a colorimeter (e.g., 16) that can classify the melanin content of the user’s skin into an individual typology angle (ITA) that the processor can use to adjust the application of the at least two ^
^ polarized light signals (e.g., wavelength, intensity, polarization, angle of emission, etc.). [0091] At 604, at least a portion of the at least two polarized light signals at least reflecting from the skin can be detected (e.g., by at least a portion of detection component 504 and/or detector 14). For example, the detection component can include at least two polarization channels (e.g., 26(1)-26(R), 526(1)-526(A), etc.). Each of the at least two polarization channels can include at least two photodetectors and at least two analyzers. For instance, a first polarization channel can include at least two photodetectors (e.g., 28(1)-28(M), 528(1)-528(B), etc.) each paired with at least two first analyzers (e.g., 30(1)-30(M), 530(1)-530(C), etc.) and the at least the second polarization channel can include at least two other photodetectors (e.g., 32(1)-32(P), 532(1)-532(D), etc.) each paired with at least one of at least two other analyzers (e.g., 34(1)-34(P), 534(1)-534(E), etc.). For each of the at least two polarization channels the at least two photodetectors can detect the light signals from the skin of the wearer in response to each of the at least two polarized light signals with different wavelengths hitting and interacting with the skin of the wearer, after the light signals have passed through each of the at least two analyzers. Depending on the alignment of the analyzer the portion of each of the at least two light signals can include superficial components (e.g., from specularly- reflected light from the skin surface and/or weakly scatter light from the nearer portions of the skin) and/or deep components (e.g., from multiple scattered light from deeper layers and/or components of the skin). For instance, superficial and deep components can be detected if the analyzer is co-polarized with the polarizers on the light sources, then the alignment can be parallel or nearly parallel (e.g., within 1° of ., within 5° of ., within 10° of ., within 20° of ., or the like) with a polarization state direction of angle . of the at least two polarizers (e.g., 508(1)-508(G)). [0092] At 606, at least another portion of the light signals from the skin can be detected (e.g., by the at least the other polarization channel of the detection component). The at least another portion of the light signals can include superficial components if, for instance, the at least the other polarization channel is a cross- polarization channel. In the cross-polarization channel, the at least two analyzers (e.g., 534(1)-543(E)) can be arranged in perpendicular (in 2D)/orthogonal (in 3D) or nearly (e.g., within 1° of ., within 5° of ., within 10° of ., within 20° of ., or the like) with a polarization state direction of angle . of the at least two polarizers (e.g., ^
^ 508(1)-508(G)). It should be understood that while only two detection steps are described the number of detections is based on the configurations and number of polarization channels and the configurations and number of photodetector/analyzer pairs therein so additional detection steps can occur. [0093] At 608, a photoplethysmography (PPG) signal can be determined (e.g., by a processor) based on the superficial components and the deep components detected by the co-polarized channel and the cross-polarized channel. The use of multiple polarization-sensitive channels (in combination with light sources with multiple polarizations) can allow the system to mathematically decompose the detected signals into their points of origin within the tissue. Furthermore, synchronizing the polarized signal extraction with the cardiac cycle, ensures that the PPG measurements are more sensitive to hemodynamic-based optical intensity modulation from the target regions deep within tissue while minimizing superficial signals such as from the epidermal region. The PPG signal can account for motion artifacts and/or confounding skin contributions (e.g., melanin levels, tattoo pigmentation, etc.) by analyzing the different superficial and deep components detected to remove the majority of the affect of at least the superficial components. One or more cardiovascular variability parameters can then be determined based on the PPG signal. The PPG signal and/or the one or more cardiovascular variability parameters can be displayed to the user and/or a medical professional and/or the user and/or the medical professional can be alerted when the PPG signal and/or the one or more cardiovascular variability parameters fall below a safe level for a predetermined length of time. [0094] It should be understood that each polarization channel can capture light that has interacted with unique layers and depths of the skin. Specifically, light that is reflected or scattered superficially, from the epidermis or upper dermis, tends to maintain unique polarization characteristics, while light that penetrates deeper into the tissue and interacts with blood vessels undergoes increased depolarization and a change in polarization state due to multiple scattering events. By employing sources with different polarization input polarizations and analyzers with different polarization alignments, selectivity to different layers within tissues, such as superficial vs. deep components, can be achieved. One polarization channel can primarily detect co- polarized light, which corresponds to light that has undergone lower amounts of scattering and primarily reflects from the superficial layers, and another polarization ^
^ channel can primarily detect cross-polarized (or depolarized light), which includes more isolated information from deeper tissue layers where hemoglobin concentration and blood flow dynamics dominate. [0095] Referring now to FIG.12, method 700 shows further steps taken by the processor to analyze the detected portions (e.g., the portion and the other portion) of the at least two light signals after at least reflection by the skin. At 702, the portion of the light signal and the other portion of the light signals can be transformed into at least values representative of an intensity of the superficial components and at least values representative of an intensity of the deep components. The superficial components can correspond to the light signals from superficial skin regions in response to application of the first and second polarized light signals. The deep components can correspond to the light signals from deep skin regions in response to application of the first and second polarized light signals. The superficial skin regions can be about 200 micrometers or less from a surface of the skin and the deep skin regions can be more than about 200 micrometers from the surface of the skin. At 704, the at least the values representative of the intensity of the superficial components and the at least the values representative of the intensity of the deep components can be weighted. For example, at 706, the weighting can minimize the contributions from the superficial components and/or emphasize contributions from the deep components. In some instance, the weighting can be based on accounting for at least one of motion, skin tone, skin composition, skin color, a distance to a vein and/or an artery. These can be determined for instance, at least partially based on the type of wearable device being used (e.g., motion, distance to a vein and/or artery, skin composition can be predetermined based on how secure the device can be attached and where the device is attached on the body). A colorimeter can be used to find an ITA and determine a skin tone and/or skin color. At 708, a PPG signal can be determined based on the analysis and the weighting of the superficial and deep components that accounts for motion artifacts and confounding skin variables/contribution. From the PPG signal one or more cardiovascular variability parameters can then be determined as is known in the art. For example, SpO2 can be determined based on a at least the PPG signal with known methods but can now account for the user’s ITA to improve measurement accuracy. [0096] Referring now to FIG.13, method 800 is an example of how to determine SpO2 while accounting for the user’s ITA. It should be noted that this method can ^
^ also be used with other metrics of melanin levels such as the Melanin Index, that may be output by other types of spectrophotometer besides a colorimeter. At 802, a portion of each of the light signals from the skin can be detected (e.g., by a first polarization channel) after the application of each of the at least two polarized light signals having different wavelengths. The portion of each of the light signals can be detected by a co-polarization channel and can include superficial components and deep components. At 804 another portion of the light signals from the skin can be detected (e.g., by a second polarization channel) after the application of each of the at least two polarized light signals having different wavelengths. The other portion of each of the light signals can be detected by a cross-polarization channel and can include superficial components. At 806, the ITA of the skin can be detected by the colorimeter. Optionally, one or more aspects of the application of the at least two polarized light signals having different wavelengths can be adjusted based on the ITA (e.g., intensity, wavelength, frequency, angle of emission, polarization, etc.). Then the detecting steps 802 and 804 can be re-done. At 808, a PPG signal can be determined based on the superficial components and the deep components that can account for motion artifacts and confounding skin contributions. From the PPG signal and the ITA a SpO2 of the user can be determined at 810. V. Polarization Gating Examples [0097] The following examples show how polarization gating can be used to at least partially reduce contributions from the surface of the skin. Specularly-reflected (reflected light from the surface layer of the skin) and weakly scattered light retain the same polarization as the input light, while multiple scatter light (reflected light from deeper layers of the skin) will depolarize. These different polarization signatures can be recorded and separated by polarization optics in the analyzers as described herein. These examples show only a single light emitter (e.g., LED or laser diode) emitting light of a single wavelength and with a single polarization state for ease of illustration only. It should be understood that additional light emitters emitting light of different wavelengths and with different polarization states (e.g., different polarizer optics) can be combined to allow synchronization between the measurements of one or more photodetector/analyzer combinations. It should also be understood that while only co-polarization and cross-polarization analyzer channels are shown for ^
^ ease of illustration, other analyzer channels with different relative polarization optics may be used as well as additional photodetector/analyzer combinations. [0098] FIG.14 elements A-E each define a different polarization state of a light ^
as a 2-element Jones vector, ^ ^ ^ ^ ^ ^^^, which defines the state of polarization of the light with Ax and Ay representing the x- and y- components of the electric field vector, respectively^ and a graphical representation. FIG.14, element A shows representations of linear polarization in the x-direction and FIG.14, element B shows representations of linear polarization at an angle . more generally, where . can be between 0° and 360°. FIG.14, element C shows representations of right-hand circular polarization and FIG.14, element D shows representations of left-hand circular polarization. FIG.14, element E shows representations of elliptical polarization where Γ is the phase retardation. While only four examples are described below, it should be understood that each of these types of polarization, linear at any angle . from the x or y axis, right or left circular, and/or elliptical, can be used additionally and/or alternatively.
^ ^ ^^ ^ ^ ^^, which defines the action of the polarization component can operate on the 2-element Jones vectors J such that a polarizer does something to the state of polarization of light.2x2 Jones matrices are used to give a representation of how the optical elements of the analyzers each affects the state of polarized light after it comes off the surface of the skin in FIGS.
15-18. Generally, ^ ^ ^ ^^ ^ ^ ^^ is the Jones matrix for a linear polarizer along the x direction, ^ ^ ^ ^^ ^ ^ ^^ is the Jones matrix for a linear polarizer along the y direction,
axis making an angle . with respect to the x-axis. The Jones matrix for wave
retarder can be represented with ^
where wave retarder has a fast axis along x direction and Γ is the phase retardation. Another example is ^ ^ for
a quarter-wave retarder where Γ = //2. ^
^ [00100] FIG.15-18 shows examples of a light source and polarizer combination producing a polarized light that impinges on the surface of the skin. For the various example, J is shown for different input polarization states. Upon reflection from the skin the light first encounters analyzers with polarization elements with representative T before the light is detected by a photodetector. In the examples shown T can be co-polarized (e.g., parallel when discussing linear polarization) or cross-polarized (e.g., perpendicular when discussing linear polarization). However, the analyzers can include any combination of polarization elements (with other, non- limiting example Ts discussed above). For each of the examples a correspond T for what the polarization element should be to obtain the co or cross-polarized signal is shown within the analyzers and the detected intensity is shown (maximum for co- polarized channel and minimum for cross-polarized channel), in some instance with a graphical representation of the output polarization state. FIG.15 shows an example with horizontal input polarization, FIG.16 shows an example with diagonal input polarization, FIG.17 shows an example with an arbitrary input linear polarization, and FIG.18 shows an example with a right-circular input polarization. VI. Experimental [00101] The following experiments present a flexible, wireless, dual-wavelength wearable photoplethysmography (PPG) device with two polarization channels integrated in a single device to improve the signal fidelity against motion artifacts and variations in skin tone and between users. The first experiment investigated the use of two-polarization channels, co-polarization and cross-polarization, integrated into the single PPG device on four healthy volunteers with varying skin tones. A handheld tristimulus colorimeter was used to determine each volunteer’s individual typology angle for comparison and calculation purposes. The second experiment investigated the impact of reducing epidermal melanin and surface impacts on pulse oximetry and the optimum polarization contrast between oxygenated hemoglobin and hemoglobin. A. Experiment 1 [00102] This experiment investigated construction of a PPG dual wavelength, wireless, biosensing wearable designed to be worn on the dorsal side of the wrist with two-polarization channels, co-polarization and cross-polarization, integrated into the single PPG device (four healthy volunteers with varying skin tones). The SNR of the measured co-polarized signal dropped by 3.70 dB and 3.57 dB at 655 nm and ^
^ 940 nm, respectively, and with increasing negative ITA, corresponding to increasingly darker skin tone. In general, while the SNR for the cross-polarized signal also dropped with increasing darker skin tone, it remains consistently higher than the co-polarized response for all ITA angles, with the cross-polarized channel exhibiting a 12% larger SNR for the darkest skin tone (as the melanin in the skin increases). However, the SNR for the cross-polarized channel remains consistently higher than the co-polarized response for all skin tones, with the cross-polarized channel exhibiting a 12% larger SNR for the darkest skin tone. Additionally, the device provided partial mitigation to motion artifacts. [00103] 1. Materials and Methods [00104] a. Device Design [00105] The device is built on a flexible printed circuit board (fPCB), which is composed of a polyimide interlayer (100 µm) and two patterned copper layers (18 µm) on the top and bottom sides. The electronic system mainly includes: (1) a pair of multi-color LEDs (SFH 7016, OSRAM), (2) four silicon PIN photodiodes (VEMD 5080, Vishay Semiconductors), (3) a Bluetooth® low-energy (BLE) system on a chip )^^0,^^^^^^^^)^^-^1&2^^^^^^"^^^^^-,3^)4,^^^^^^^^^^^^^"^^^^^^^0^)^-5^^&&3^ Nordic Semiconductor), (5) an optical analog front-end (MAX86141, Analog Devices), (6) tuned copper coil and (7) relevant passive components. [00106] During measurement, the red (655nm) LED die and infrared (940nm) LED die are driven by a current of 14.53 mA, yielding irradiance of 3.78 mW and 2.83 mW respectively. One set of photodiodes (PDs) is distributed on each side of the LEDs, with a horizontal distance of 9mm from the centerline. The output of PDs is processed by a 19-bit charge integrating analog-digital converter with a sampling rate of 25 sps and sent to the microcontroller, followed by the data transmission through BLE protocol. A 60 mAh lithium-polymer battery allows continuous measurement for 24 hours, while the operation duration can be further extended by adjusting the power supplied to the light source. [00107] To fabricate the outer shell, transparent silicone rubber (Ecoflex 00-31 Near Clear, Smooth-On) is placed in a vacuum chamber (KQ-1K, VEVOR) and subjected to a negative pressure of -80 kPa for 10 minutes to expel the dispersed bubbles. Black pigment (Silc Pig) is interfused into the silicone to mitigate parasitic optical responses caused by ambient light. An encapsulation layer with a thickness of 1.2 mm is cured subsequently on a PMMA-coated glass slide for 15 minutes on a ^
^ hotplate (RT2 Advanced Hotplate Stirrer, Thermo Scientific) at 83°C. A pair of 2 mm x 2 mm windows and two sets of 5mm x 4mm windows are exposed for the LEDs and PDs respectively, on which commercially available polarizers (diffuse reflective film polarizer for near-infrared light, LPNIRE2X2, Thorlabs Inc.) are tailored and sealed to introduce polarizing gating. FIG.19, elements a and b, showcase the setup of the optical components in a dual-wavelength, polarization-sensitive device. Using a parallel orientation to the polarizer on the LEDs, the second polarizers (labeled as analyzers) on the left group of PDs let through the co-polarized light, while the perpendicularly configured analyzers on the right side block the specularly reflected light, thereby isolating light from deeper layers. FIG.19, element a, shows the setup with LED light sources and photodiode detectors, each coupled with polarizers and analyzers, respectively. FIG.19, element b shows a transverse section of the skin at the measurement site reveals how unpolarized light emitted by the LEDs undergoes polarization and penetrates the skin. The light's polarization state gradually becomes more depolarized as it traverses through the skin layers where the 940 nm light penetrates deeper than the 655 nm light. Reflected light from the arterial line passes through analyzers oriented in both parallel (co-polarization) and perpendicular (cross-polarization) configurations relative to the polarizers coupled with the LEDs. In the co-polarized state, superficially and deeply reflected light is permitted to pass through, whereas in the cross-polarized state, superficially reflected light is filtered out, allowing only deeply reflected light to pass. [00108] Adjacent blocks of the fPCB are folded during packaging to reduce the overall size of the device (6.2 cm x 2.4 cm x 0.8 cm). The miniaturized, flexible design allows close-knit deployment on the curved skin via medical-grade adhesives, thus enabling real-time monitoring of PPG signals, as shown in FIG.19, element c, shows the device pre-encapsulation and FIG.19, element d, shows the encapsulated device worn by a participant. [00109] b. Experimental Procedure [00110] Note that for this study, a waiver for IRB approval was received because the work focused on calibrating the device, thus did not meet the federal definition of generalizability. Prior to carrying out measurements, informed consent was given by the volunteers. Before conducting PPG measurements on the volunteers, colorimetry measurements were performed at the measurement site to quantify the skin tone of each of the volunteers. In colorimetry, the most widely adopted approach is the ^
^ Individual Topology Angle (ITA) which is based on CIELAB color space, more specifically the color parameters L* (lightness/luminance), b* (yellow/blue). The resulting calculated ITA values can be classified into different categories. For an individual with a lighter skin color, the ITA° can be expected to be a higher positive value than that of an individual with a darker skin phototype. Ten sequential measurements were performed at the measurement site using tristimulus colorimetry. [00111] To ensure a firm adhesion at the skin-sensor interface, the device was attached to participants’ wrists using medical-grade transparent film dressing (Tegaderm, 3M). Measurements are initiated with 5 minutes of baseline recording. After the readings are stabilized, participants are requested to move their wrists periodically for one minute, with the angle varying between -20° and +20°. The frequency of motion is maintained around 0.5 Hz to distinguish it from heartbeats. The measurements are repeated two additional times with 2 minutes of rest between sets in case of fatigue-induced motion retardation. [00112] c. Data Analysis [00113] The Fourier transform was applied to the raw time-series signals using OriginPro. The SNR was then calculated based on the root-mean-square values of signal and noise using MATLAB (R2023b), as follows:
higher, and ^
5 represents the sample size of the signal, which is within the frequency range of 0.5 to 7 Hz. [00116] 2. Results [00117] FIG.20 shows the results of the skin tone analysis. Based on recommendations by the FDA it is crucial to have participants representing a range of skin tones. In the study, the range of skin tones of the volunteers is shown in FIG. 20, element a, while the quantitative skin tone assessment is shown in FIG.20, element b. FIG.20, element c, shows the L-b plane with uniform skin phenotyping lines. ^
^ [00118] FIG.21, element a, presents the PPG signals collected from a participant with an ITA of -15°. During the resting period, pulsatile signals with a frequency at 1.4 Hz, which corresponds to the heartbeat, are captured in both 655 nm wavelength and 940 nm wavelength channels, with no significant amplitude difference between co-polarized light and cross-polarized light. However, after wrist movement is induced, light with the co state of polarization is severely disturbed by motion artifacts, which drown out the heartbeat and its harmonic signals amid the 0.6 Hz oscillations originating from periodic motions. In contrast, with a deeper probing depth, cross-polarized light is less affected by the property changes in superficial tissue and thus less vulnerable to the motion artifacts, facilitating the distinction of the heart rate (HR) signals. [00119] Time series data were recorded subsequently on the other three participants following the same protocol, as illustrated in FIG.21, elements b, c, and d. For presentation purposes, only the PPG signals at the resting stage are listed. When given the same polarization state, signals acquired for every individual using infrared light consistently exceed those generated by red light, while a decrease in the signal intensity is observed in all channels when the device is deployed on participants with deeper skin tone. [00120] FIG.22, elements a and b, show that compared to waveforms elicited by co-polarized light, waveforms elicited by cross-polarized light demonstrate an enhanced clarity, achieving a higher SNR that reaches a maximum of 12.80 dB at 940 nm and 12.53 dB at 655 nm, both for an ITA of 30°. As ITA declines from 30° to -10°, an anticipated drop of signal quality occurs, leading to SNRs falling below 10 dB for co-polarized light and 8.5 dB for cross-polarized light. It is worth noting that light with cross state polarization exhibits lower susceptibility to the interference of melanin. Specifically, the SNR of cross-polarized signal reduces by 2.95 dB at 940nm and 3.04 dB at 655 nm, whereas the SNR of co-polarized light degrades by 3.57 dB at 940nm and 3.70 dB at 655 nm respectively. Additionally, FIG.23 shows time series data collected from the four participants. The output of the analog-to- digital converter was recorded for 30 seconds on the four participants and wrist movement was induced at 15 seconds. An expected drop of signal intensity was observed with a decreasing ITA, and the suppression of motion artifacts can be identified in the data of co-polarized light. FIG.24 shows the power spectrum of the PPG signal at an ITA of -15°. The distribution of the normalized amplitude in the ^
^ frequency domain was calculated via Fourier transform. At the resting stage, a peak at the frequency of 1.4 Hz can be distinguished, corresponding to the HR signals, whereas another peak at the frequency of 0.7 Hz occurs after the wrist movement is introduced. [00121] 3. Importance of Real-time Monitoring [00122] Real-time telemetry of the PI at two wavelengths is important as monitoring in real time allows the user of the device to assess the adequacy of the tissue perfusion. A sudden drop in perfusion may indicate changes in the patient’s cardiovascular state or the perfusion to a particular part of the body. Real-time monitoring facilitates early intervention and treatment plans before escalation. The results show a noticeable improvement in signal quality across all skin tones by simply incorporating polarization gating. In the cross-polarized state, the specular and superficial reflected light are filtered out, thereby allowing only the light reflected from the deep layers to pass through. However, in the co-polarized state, both reflect components are allowed to pass through the analyzer to the detector, thereby allowing potential confounders to strongly contribute to the signal. It is noted that the state of polarization used is simply linear and not necessarily the optimal state for PPG analysis. Incorporating a state that exhibits a coherent polarization spatial- spectrum, such as a vector beam, into a wearable could offer much more insight to optimizing the PPG response, and by extension pulse oximetry. B. Experiment 2 [00123] In this experiment a study leveraging several unique features of optical polarization is described. [00124] First, polarization gating was used to enable (limited) depth discrimination with the tissue (the finger). Obliquely incident polarized light impinging at the interface between two different non-conducting (dielectric) media can be separated into at least two components: a specular-reflected component that retains the incident polarization state and a multiple scattered component, which generally will have a different polarization state in the presence of the turbidity common in human tissue. For linearly polarized illumination, using co-polarized (parallel aligned) and cross-polarized (orthogonally aligned) detection permits separation (or gating) of surface reflection (specular light) from multiple scattered (deep within the tissue) light, respectively. Note that generally any weakly scattered light (below the surface) will also maintain its initial polarization and thus will be detected similarly as the ^
^ specular light. Moving a detector or camera to be in a position that is not at the angle of reflection (with respect to obliquely incident light) further enhances this sensitivity. [00125] Polarization gating using circularly polarized illumination had a somewhat different outcome. The reflected specular light undergoes a +/- 180-degree phase change between the eigenpolarizations, and thus its helicity is flipped, i.e., right- handed circularly polarized light becomes left-handed circularly polarized light. Therefore, circular polarization can separate mirror-reflected photons (helicity flipped) from weakly scattered photons, although multiple scattered photons will be present. Elliptically polarized light has been shown to have similar features. If combined with linearly polarized light (which is traditionally done sequentially) then weakly scattered photons can be uniquely extracted from both surface reflections and multiple scattered light. Therefore, these features of polarization gating were harnessed with the parallelization properties of vector field illumination to permit single-shot polarization gating. A variety of vector field states can be synthesized that can evolve from all linear to hybrid circular and elliptical to fully circular using methods described elsewhere. [00126] A second property of polarization that was used is that linear and circular (or elliptical) polarization states have different penetration depths in media as well as sensitivities to depolarization, depending on the properties of the medium. For example, it has been shown that blood exhibits a preferential depolarization of linear over circular polarization states, whereas arterial tissue has been shown to more strongly depolarize circular polarization over linear. FIG.25 shows an example for porcine arterial tissue for both linearly (dark circles) and circularly (open circles) polarized light. It is believed that the degree of linear polarization is more sensitive to scattering, while the degree of circular polarization is sensitive to both scattering and intrinsic birefringence. However, the caveat is that this depends on the dominant scattering mechanisms in the tissue, i.e., Rayleigh, Mie, or Rayleigh-Mie transition regime. An example of the increased penetration depth for circular over linear polarization is shown in FIG.26, which is a result of a Monte Carlo simulation of a scattering medium (refer to for details of the simulation). [00127] Thus, using spectrally swept, vector beam illumination for single-shot probing, polarization-based layer selectivity, depth penetration, and tissue responsivity can be incorporated into pulse oximetry. Compared to traditional pulse oximetry, the contribution to the PPG signal of only epidermal absorption and surface ^
^ effects can be removed and the detected signal can be weighted more on the deep layers. [00128] 1. Ultimate Goal – Accurately Recording Oxygen Saturation in Atrial Blood in ALL Patients [00129] Oxygen saturation in arterial blood is a critical measure of the efficiency of the lungs’ ability to transmit oxygen to blood vessels through capillaries. This can be measured using arterial blood gas analysis, which is the most accurate and precise technique, with the tradeoff of being invasive and time intensive. A common noninvasive approach is based on photoplethysmography (PPG), an optical technique used to measure blood volume changes in the microvascular bed of tissue. Typically implemented through pulse oximetry, the level of peripheral blood
oxygen saturation, defined as %XY^ ^ Z[D\Y^]_ [D\Y^] ^ [D\] ` ^a ^^^^b^, i.e., the percentage of oxygenated hemoglobin to total hemoglobin, is experimentally determined through a ratiometric comparison R of light absorption at two wavelengths. [00130] To ensure that the measured intensity is an accurate estimate of oxygen saturation in arterial blood (SaO2), the measured intensity is synchronized with the cardiac cycle such that there is a periodic PPG or pulsatile, AC signal, which is offset by a constant DC, non-pulsatile signal (resulting from blood volume elsewhere such
as in veins, capillaries, skin, and fat). It is from there that R is given as [^^c^_ ^^c^]d [^^c^_ ^^c^] , where the wavelengths e^ and e( are typically taken as red (e.g., 660 nm) over infrared (e.g., 940 nm), corresponding to peak absorptions for Hb and HbO2, respectively. To calibrate R, controlled desaturation studies are conducted where the oxygen levels of healthy volunteers are brought down from 100% to 70% SaO2 (usually in 5% intervals). The most general non-linear expression
is given by %XY ^ f'^ ^ ^ \' ^ ^, where a, b and c are device-specific constants determined by regression methods that are set to fit to a second or first order curve from the calibration data. SpO2 readings above approximately 95% are considered normal, while those in the range 90-94% imply the possibility of a disease process in ^^^^^^^^^^^^^^^^^^^"^^^^^^^^#&6^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ requiring immediate intervention or chronic heart-lung conditions requiring long term interventions.^ ^
^ [00131] Current approaches using state-of-the-art plethysmographs do not account for wavelength-dependent subsurface scattering due to, for example, the melanosomes within melanocytes (melanin-^^^^^^^^"^^^^^^,^^^^^^^^^^^^^^^^^^^^^^^^ how these organelles are larger and denser in dark skin. FIG.27 schematically represents this. The potential dependence of this scattering phenomenon with the cardiac cycle is also not known. [00132] 2. Reducing epidermis surface effects [00133] a. Rationale [00134] Polarization gating can be used to remove the surface contribution in dual-wavelength pulse oximetry. For simple linear polarization, specular-reflected light and weakly scattered light retains the polarization state of the input light. Thus, a polarization analyzer oriented 90-degrees with respect to the input polarization will block these signals and only permit the deeply, multiple scattered light to be detected. To be more sensitive to this effect, the detector can be moved to be normal to the surface. While this approach removes contributions from the surface, any absorption effects, e.g., by melanin, in the epidermis will still be present in the multiple scattered signal. To address this, one requires the data from both channels, i.e. both the co- (g
h1i) and cross-polarization
intensity signals. It has previously
been shown that gh1i ^ g4^j62O'5 ^ 'k_ ( V and gh6i ^ g4^j62 'k_ ( , where g4^is the initial intensity, ^j62^is the transmission coefficient describing absorption by melanin, '5 is the intensity of the reflected light from the superficial layer, and 'k is the intensity of the deeply reflected light. A polarization ratio Pol can then be determined
as l^) ^ Ogh1i ^ gh6iV_ Ogh1i ^ gh6iV ^ O'5_ '5 ^ 'kV . Thus, this ratiometric combination of signals eliminates ^
j62B Adapting this approach to standard pulse oximetry that requires synchronizing measured intensity with the cardiac cycle, means recording a pulsatile (AC) signal for co- and cross-polarizations and at each wavelength, simultaneously. As shown previously, illuminating with a vector beam such as radially polarized light, eliminates the requirement for sequential polarization measurements. This aim uses dual-wavelength illumination that simultaneously illuminates with two vector beams, one at 660 nm and the other at 940 nm. A camera was split into two quadrants to detect the contributions from each wavelength. [00135] b. Experimental Approach ^
^ [00136] Light from two laser diodes, one at 660-nm wavelength and the other at 940 nm, was collimated and each sent to a vector-field generator, which generally can be a combination of a vortex waveplate sandwiched between a half-wave plate (HWP) and quarter-wave plate (QWP). Each output vector field can then be focused onto the finger where the non-specular-reflected components are each directed to a different region on a camera after going through a fixed polarization oriented parallel to the input polarization. [00137] c. Findings [00138] A polarization pulse oximeter enabled by two eigenpolarization states encoded along the wavefront of a monochromatic infrared vortex beam. An optical setup is shown in FIG.18, element A comprised of a coherent 780 nm source in combination with a beam expander and zero-order vortex waveplate (VWP), a polarization-sensitive imaging system (8-bit, monochrome), and a polarization gating algorithm to estimate oxygen saturation. Large-area regions of interest (ROI) are denoted in one false colored image heatmap in FIG.28, element B of the time lapsed image sequence by co- (blue) and cross- (orange) linear states of polarization (SOP). Mean intensities of each ROI, per image frame are used in a polarization- gating algorithm. The co- and cross- linear polarization PPG (i.e. AC-components) are shown in FIG.28, element C over a 15 second window. A much larger DC component was observed for the co-polarized signal than the cross-polarized signal, as the former has a significant contribution from superficial layer scattering. [00139] 3. Optimizing polarization contrast between Hb and HbO2 [00140] a. Rationale [00141] Unlike conventional pulse oximetry, which relies only on relative absorption at two wavelengths, i.e., spectral contrast, the proposed polarization model relies on polarization contrast (between Hb and HbO2). However, the refractive index of tissues ^^is in general a complex function, comprising a real and imaginary component, where the former dictates propagation (wave vector and speed) and the latter corresponds to absorption (and hence dispersion). Moreover, biological tissues also have anisotropic properties, and thus the propagation of light in such materials is a function of the input polarization. Therefore, in order to adequately maximize polarization contrast, it is important to account for wavelength- dependent wave propagation in the medium. The experiments described in this specific aim are to determine the maximum polarization contrast achievable using ^
^ polarization gating as this maps directly to the dynamic range for SpO2 measurements. Given that light in the near-infrared region has a mean-free path
(average distance photons travels before encountering a scattering event) of ~ 100 mm and a transport mean-free path (average distance photons travels before becoming completely diffuse) of ~ 1 mm, the wavelengths that will be selected to probe finger tissue will range from ~700 nm– 1000 nm. The polarization model for pulse oximetry permits cancelation of skin surface effects and epidermal absorption. Typically, the thickness of the epidermis is ~ 100 mm and the dermis thickness is ~ 1- 5 mm, and thus the wavelength range selected will have sufficient coverage of the skin tissue. Furthermore, the spectroscopic response of the tissue in this regime can be obtained simultaneously, which will also facilitate quantitation of the skin-tone of the volunteers rather than relying on more subjective scales such as Fitzpatrick and Martin-Massey. Bandpass filters can be used to sub-select spectral regions for which to optimize the wavelength-dependent polarization contrast. [00142] b. Experimental Approach [00143] The proposed experimental setup is shown in FIG.29. A femtosecond- pulsed source, spectrally centered at 900-nm, can be used with a photonic crystal fiber to generate a supercontinuum. Note that only a relatively small spectral region is required for the work, although there are opportunities to use the full bandwidth. The output of the continuum can be collimated and sent to a vector-field generator, which generally will be a combination of a vortex waveplate sandwiched between a half-wave plate (HWP) and quarter-wave plate (QWP). The output multicolor vector field can then be focused onto the finger where the specular-reflected component is directed onto a spectrometer, and the diffuse component can be spectrally filtered (for selection of wavelength-range of interest) and then directed to a camera for polarization analysis. Initially, a single, fixed polarization analyzer can be placed after the finger and oriented parallel to the input polarization. [00144] c. Findings [00145] FIG.30 summarizes results. FIG.30, element A shows the finger of a volunteer illuminated by a white-light LED source. The pulsatile (PPG) time trace shown in FIG.30, element B was obtained using a fiber-coupled spectrometer. The data shown in this case is for 633 nm and 700 nm. Additionally, sequential polarimetric analysis (no vector beam illumination) of the region comprising the mole (as is highlighted in FIG.30, element C) was carried out. In this case, the analysis ^
^ included the degree-of-polarization (DOP) for linear and circular polarization, the total depolarization, and the total dichroism (relative absorption of one polarization state over the other). Taken as is, the polarization data was not correlated with the cardiac cycle as the measurements were sequential. The integration of a broadband vector field illumination can allow the polarization and pulsatile response to be synchronized. [00146] From the above description, those skilled in the art will perceive improvements, changes, and modifications. Such improvements, changes and modifications are within the skill of one in the art and are intended to be covered by the appended claims. ^
^ ^ ^^ ^ ^ ^^, which defines the action of the polarization component can operate on the 2-element Jones vectors J such that a polarizer does something to the state of polarization of light.2x2 Jones matrices are used to give a representation of how the optical elements of the analyzers each affects the state of polarized light after it comes off the surface of the skin in FIGS. 15-18. Generally, ^ ^ ^ ^^ ^ ^ ^^ is the Jones matrix for a linear polarizer along the x direction, ^ ^ ^ ^^ ^ ^ ^^ is the Jones matrix for a linear polarizer along the y direction,
axis making an angle . with respect to the x-axis. The Jones matrix for wave retarder can be represented with ^
where wave retarder has a fast axis along x direction and Γ is the phase retardation. Another example is ^ ^ for
intensity signals. It has previously been shown that gh1i ^ g4^j62O'5 ^ 'k_ ( V and gh6i ^ g4^j62 'k_ ( , where g4^is the initial intensity, ^j62^is the transmission coefficient describing absorption by melanin, '5 is the intensity of the reflected light from the superficial layer, and 'k is the intensity of the deeply reflected light. A polarization ratio Pol can then be determined as l^) ^ Ogh1i ^ gh6iV_ Ogh1i ^ gh6iV ^ O'5_ '5 ^ 'kV . Thus, this ratiometric combination of signals eliminates ^j62B Adapting this approach to standard pulse oximetry that requires synchronizing measured intensity with the cardiac cycle, means recording a pulsatile (AC) signal for co- and cross-polarizations and at each wavelength, simultaneously. As shown previously, illuminating with a vector beam such as radially polarized light, eliminates the requirement for sequential polarization measurements. This aim uses dual-wavelength illumination that simultaneously illuminates with two vector beams, one at 660 nm and the other at 940 nm. A camera was split into two quadrants to detect the contributions from each wavelength. [00135] b. Experimental Approach ^
^ [00136] Light from two laser diodes, one at 660-nm wavelength and the other at 940 nm, was collimated and each sent to a vector-field generator, which generally can be a combination of a vortex waveplate sandwiched between a half-wave plate (HWP) and quarter-wave plate (QWP). Each output vector field can then be focused onto the finger where the non-specular-reflected components are each directed to a different region on a camera after going through a fixed polarization oriented parallel to the input polarization. [00137] c. Findings [00138] A polarization pulse oximeter enabled by two eigenpolarization states encoded along the wavefront of a monochromatic infrared vortex beam. An optical setup is shown in FIG.18, element A comprised of a coherent 780 nm source in combination with a beam expander and zero-order vortex waveplate (VWP), a polarization-sensitive imaging system (8-bit, monochrome), and a polarization gating algorithm to estimate oxygen saturation. Large-area regions of interest (ROI) are denoted in one false colored image heatmap in FIG.28, element B of the time lapsed image sequence by co- (blue) and cross- (orange) linear states of polarization (SOP). Mean intensities of each ROI, per image frame are used in a polarization- gating algorithm. The co- and cross- linear polarization PPG (i.e. AC-components) are shown in FIG.28, element C over a 15 second window. A much larger DC component was observed for the co-polarized signal than the cross-polarized signal, as the former has a significant contribution from superficial layer scattering. [00139] 3. Optimizing polarization contrast between Hb and HbO2 [00140] a. Rationale [00141] Unlike conventional pulse oximetry, which relies only on relative absorption at two wavelengths, i.e., spectral contrast, the proposed polarization model relies on polarization contrast (between Hb and HbO2). However, the refractive index of tissues ^^is in general a complex function, comprising a real and imaginary component, where the former dictates propagation (wave vector and speed) and the latter corresponds to absorption (and hence dispersion). Moreover, biological tissues also have anisotropic properties, and thus the propagation of light in such materials is a function of the input polarization. Therefore, in order to adequately maximize polarization contrast, it is important to account for wavelength- dependent wave propagation in the medium. The experiments described in this specific aim are to determine the maximum polarization contrast achievable using ^
^ polarization gating as this maps directly to the dynamic range for SpO2 measurements. Given that light in the near-infrared region has a mean-free path (average distance photons travels before encountering a scattering event) of ~ 100 mm and a transport mean-free path (average distance photons travels before becoming completely diffuse) of ~ 1 mm, the wavelengths that will be selected to probe finger tissue will range from ~700 nm– 1000 nm. The polarization model for pulse oximetry permits cancelation of skin surface effects and epidermal absorption. Typically, the thickness of the epidermis is ~ 100 mm and the dermis thickness is ~ 1- 5 mm, and thus the wavelength range selected will have sufficient coverage of the skin tissue. Furthermore, the spectroscopic response of the tissue in this regime can be obtained simultaneously, which will also facilitate quantitation of the skin-tone of the volunteers rather than relying on more subjective scales such as Fitzpatrick and Martin-Massey. Bandpass filters can be used to sub-select spectral regions for which to optimize the wavelength-dependent polarization contrast. [00142] b. Experimental Approach [00143] The proposed experimental setup is shown in FIG.29. A femtosecond- pulsed source, spectrally centered at 900-nm, can be used with a photonic crystal fiber to generate a supercontinuum. Note that only a relatively small spectral region is required for the work, although there are opportunities to use the full bandwidth. The output of the continuum can be collimated and sent to a vector-field generator, which generally will be a combination of a vortex waveplate sandwiched between a half-wave plate (HWP) and quarter-wave plate (QWP). The output multicolor vector field can then be focused onto the finger where the specular-reflected component is directed onto a spectrometer, and the diffuse component can be spectrally filtered (for selection of wavelength-range of interest) and then directed to a camera for polarization analysis. Initially, a single, fixed polarization analyzer can be placed after the finger and oriented parallel to the input polarization. [00144] c. Findings [00145] FIG.30 summarizes results. FIG.30, element A shows the finger of a volunteer illuminated by a white-light LED source. The pulsatile (PPG) time trace shown in FIG.30, element B was obtained using a fiber-coupled spectrometer. The data shown in this case is for 633 nm and 700 nm. Additionally, sequential polarimetric analysis (no vector beam illumination) of the region comprising the mole (as is highlighted in FIG.30, element C) was carried out. In this case, the analysis ^
^ included the degree-of-polarization (DOP) for linear and circular polarization, the total depolarization, and the total dichroism (relative absorption of one polarization state over the other). Taken as is, the polarization data was not correlated with the cardiac cycle as the measurements were sequential. The integration of a broadband vector field illumination can allow the polarization and pulsatile response to be synchronized. [00146] From the above description, those skilled in the art will perceive improvements, changes, and modifications. Such improvements, changes and modifications are within the skill of one in the art and are intended to be covered by the appended claims. ^