WO2025090560A1 - Improved silicone prosthetic liners and methods of producing same - Google Patents

Abstract

Methods for selective matting of some, but not all, surfaces of a silicone article of manufacture are disclosed. By use of a silicone matting additive and manipulation of process variables, a silicone article of manufacture may be produced that comprises some surface areas that exhibit silicone's inherent tacky texture and other surface areas that exhibit a matted texture. In an exemplary embodiment, a silicone prosthetic liner may be produced where an interior surface is taky and an exterior surface is matted.

Description

IMPROVED SILICONE PROSTHETIC LINERS AND METHODS FOR

PRODUCING SAME

I. PRIORITY CLAIM

[0001] The present invention claims priority from United States Provisional Application No. 63/592,280, filed October 23, 2023, the disclosure of which is hereby incorporated by this reference in its entireties.

II. FIELD OF THE INVENTION

[0002] The inventions of this disclosure comprise improvements to silicone prosthetic liners and methods for producing silicone prosthetic liners.

III. BACKGROUND

[0003] Silicone prosthetic liners are used to cover a residual limb and serve as a layer between the residual limb and a prosthetic device. Figure 1 contains a conceptual drawing of a residual limb 101 with a prosthetic liner 102 for insertion into a prosthetic device 103. A well-fitting prosthetic liner improves comfort of the prosthesis and protects skin that would otherwise be in direct contact with the prosthesis. When fabricated by conventional processes, silicone prosthetic liners retain a slightly sticking feel — i.e., a “tacky” feeling. While it is advantageous for the inner surface of a prosthetic liner to be tacky, it is also advantageous for the outer surface to be low tack.

[0004] A tacky inner surface improves the grip of the liner against the skin and helps reduce slippage. A low-tack outer surface, by contrast, makes it easier to roll the liner on and to insert the liner-covered limb into prosthetics. Current prosthetic liners on the market often use fabric or other coating methods on the outer surface to achieve this effect.

[0005] Silicone rubber is inherently tacky, in that it sticks to surfaces, especially itself, with minimal pressure, and is not able to slide past another silicone surface. This effect tends to become more pronounced the softer the material is. A softer silicone more easily conforms to an adjoining surface and thereby maximizes surface contact. Liner silicone is relatively soft and is therefore quite tacky. Tackiness may be caused by chemical adhesion, and there is some of such an effect in silicones, but applicant conceptualizes silicone tackiness it as static/sliding friction against its own surface.

[0006] Additive finishes to reduce tackiness in silicones are known. For example, Nusil MED 10-6670 is a low coefficient of friction silicone coating that may be applied by as a sprayable coating. Such substances are not ideal as they have difficulty bonding to the silicone substrate and will often delaminate when the silicone deforms or strain is otherwise applied to the silicone substrate.

[0007] The invention of this disclosure solves the foregoing problems by providing a silicone liner wherein a first surface — for example, the inner surface — retains the tackiness that is inherent in silicone, and a second surface — for example, the outer surface — is not tacky or has reduced tackiness without the need for a fabric coating or low friction treatment. IV. BRIEF SUMMARY OF THE INVENTION

[0008] By use of a silicone matting additive and manipulation of process variables, a silicone article of manufacture may be produced that comprises some surface areas that exhibit silicone’s inherent tacky texture and other surface areas that exhibit a matted texture. The invention of this disclosure entails modifying a first surface of a silicone liner to create a low-friction surface, which should serve as a low tack rubber while retaining the silicone’s inherent tackiness on a second surface.

[0009] In the instant application, applicant discloses a method and approach based on surface “matting,” which entails pre-mixing additives with the silicone that will develop a low-friction surface on the final part during subsequent material treatment steps. [0010] The pre-mixed additive becomes part of the material surface and cannot be unbonded or delaminated and does not require re-application. It is believed that these additives modify the silicone surface roughness.

[0011] In applicant’s method, a first surface of a silicone rubber piece with a premixed matting substance added is masked while a second surface of the silicone rubber piece is exposed to vented air and the entire piece is heated for a duration. After heating and unmasking, the first surface of the resulting silicone piece is non-matte and the second surface of the resulting silicone piece is matted.

V. BRIEF DESCRIPTIONS OF THE DRAWINGS

[0012] Figure 1 depicts a prosthetic liner forming an interface between a residual limb and a prosthesis.

[0013] Figure 2 depicts a flow diagram of an exemplary embodiment of the invention.

[0014] Figure 3A shows silicone piece samples processed under various combinations of process variables without a pre-venting step

[0015] Figure 3B shows silicone piece samples processed under various combination of process variables with a pre-venting step.

VI. DESCRIPTION OF THE INVENTION

[0016] Applicant’s method of selective matting uses a friction-lowering or matting additive in the silicone from which a piece is to be manufactured. In conventional processes, such an additive creates low friction on all surfaces of the resulting silicone piece. In applicant’s method, however, process variables are manipulated to preserve the effect of the low-friction pre-mixing additive on some, but not all, surfaces.

[0017] Applicant’s method may be applied to two part, or addition curing silicones, in which a liquid comprising a silicone polymer and a catalyst (Part A) is mixed with a liquid comprising a cross-linker or curing agent (Part B). In two part silicone, addition of Part A and

Part B in the correct proportions causes a reaction that creates a solid silicone rubber.

[0018] Matting solutions for silicone are known and commercially available. For example, SLIDE™ STD Liquid Surface Tension Diffuser from Smooth-On, Inc. is an additive for platinum cure silicone rubber that greatly reduces surface tension. Cured silicone treated with SLIDE™ STD Liquid Surface Tension Diffuser slides across itself and other surfaces. QSil Matting Agent from CHT Germany GmbH is another additive which reduces the coefficient of friction and results in a matte finish on select silicone elastomer.

[0019] Commercial matting solutions are generally added to Part B of two-part silicone, for example, in levels up to 1.5% (SLIDE) or between 0.5% and 1.0% (Qsil). The desired surface properties develop after a period of time, for example after 24 hours (SLIDE) or between 24 and 48 hours (Qsil). Matting additives lengthen the cure process of the silicone to which they are added.

[0020] In working with matting additives, applicant observed that silicone parts with matting additives which are not exposed to air after curing did not matte, even weeks later. While such additives may leave a distinctive smell, and old silicone parts left in a box would smell strongly even after months, old parts left open air would eventually lose most of the smell associated with the additive. In conventional processes, such as those recommended by the matting additive manufacturers, a silicone piece is simply exposed to the air. However, forced airflow during cure tends to cause a faster and greater degree of matting effect, in comparison to simply exposing the parts to air.

[0021] In addition, surfaces that are not exposed to air remain unmatted, but may become matted when later exposed to air. For example, in some tests it was verified that adding SLIDE to a silicone formulation resulted in matted exposed surfaces as expected. After approximately one week sitting exposed to air on a desk, unmasking the of the piece (i.e., the piece was pried off the plate on which it was curing) revealed a normal tacky silicone surface underneath, which then matted.

[0022] Applicant concluded that matting additives appear to have a reaction life, which can be manipulated and exploited to selectively matte different surfaces of the same part. Applicant investigated how a number of process variables affect the action of commercial matting additives. For example: whether additives require volatilization; whether require a silicone rubber that is still curing to act upon (i.e., demoldable but not fully cured); whether additives are heat sensitive, either to volatilize more quickly, to act more quickly, or to cure the rubber more quickly; and whether they “run out” — i.e., whether one can induce non-matting in a surface that should still be able to matte (still curing) if all of the additive matting on another surface has been “consumed” or used up.

[0023] Figure 2 shows a flow diagram of an exemplary embodiment of the method disclosed herein. In step 201, a matting additive is pre-mixed with Part B of a two-part silicone. In step 202, Parts A and B of the two-part silicone are mixed in a predetermined proportion to create a silicone piece. In step 203, a first surface of the silicone piece is masked. In step 204, heat is applied to the entire silicone piece for a predetermined amount of time in a vented environment.

[0024] The inventive method disclosed herein can be applied in various methods of silicone manufacturing. For example, applicant designs and manufactures machines for additive manufacturing of silicone (among other substances) wherein silicone is 3D printed within a support gel, a process known as rapid liquid printing, and the teachings of this disclosure are suitable for application in that process. However, the disclosed selective matting approach can be applied following any silicone manufacturing process in which the masking, heating, and venting steps are not precluded. A. Early Tests

[0025] Applicant printed a silicone prosthetic liner on one of its 3D printers with a commercial matting additive mixed in per manufacturer’s instructions. The inside surface of the piece was masked by a mixing tub. The piece was left in a vent box. The outer surface matted as expected, while the inner, masked surface had identical (i.e., tacky) feel to silicone without a matting additive. However, some days later, the inner surface began to feel less tacky. Applicant concluded that matting can be directed to one surface over another, but subsequent matting action on the non-matte surface needs to be inhibited or prevented thereafter.

[0026] Applicant tested baking, covering, and venting as potential methods to selectively prevent and/or inhibit subsequent matting effect. In this investigation, applicant used Dragon Skin 10 FAST platinum cure liquid silicone loaded with 1% by weight SLIDE (resulting in 0.5% by weight in the final mix) and pigment. A series of test pieces was processed with various combinations of the following processes (1) baking (75° C on a hot plate), (2) covering (in a closed or sealed container), and (3) venting (placed in glovebox with about 100 ft/min airflow).

B. Initial Results

[0027] Figures 3 A and 3B show silicone samples treated with various combinations of process variables. In Figure 3 A: sample 301 was baked, covered, and not vented, which yielded no matte; sample 302 was not baked, covered, and not vented, which yielded no matte and a very shiny surface; sample 303 was baked, not covered, and not vented, which yielded no matte; and sample 304 was not baked, not covered, and not vented, which yielded a matted surface.

[0028] In Figure 3B: sample 305 was baked, covered, and vented, which yielded no matte and a strong smell; sample 306 was not baked, covered, and vented, which yielded no matte; sample 307 was baked, not covered, and vented, which yielded no matte; and sample

308 was not baked, not covered, and vented, which yielded a surface that matted slightly over a longer period of time and a stronger smell.

[0029] Table 1 also shows and summarizes the various process variable combinations applied to the silicone samples.

Table. 1 Process variable combinations [0030] The pre-venting process consisted of exposing samples 5-8 to venting for approximately eight hours before other processing. For samples 5-8, all pieces were mixed and smeared on a plate such that the top surface is exposed and the bottom surface remained masked by the plate. After leaving in a vent box for 8 hours, all pieces matted on top surface. Samples 1-4 were not pre-vented in this way. Where baking is indicated, samples were baked at 75° C. Samples 1-4 were baked with a fan blowing over the exposed surface. Samples 5-8 were baked for approximately fifteen hours.

[0031] For sample 5, the piece was moved to 75° C hot plate and covered for approximately 15 hours. The piece was then removed from heat and covering, and the masked surface was then exposed to a vent box overnight. This surface did not matte. Anecdotally, this piece seemed to smell more strongly.

[0032] For sample 6, the piece was covered for approximately 15 hours. The piece was removed from covering, and the masked surface was then exposed to a vent box overnight. This did not matte.

[0033] For sample 7, the piece was moved to a 75° C hot plate for approximately 15 hours. The piece was then removed from heat, and the masked surface was exposed to a vent box overnight. This surface did not matte.

[0034] For sample 8, the masked surface was exposed to a vent box overnight. There was little visible difference overnight, but eventually matted slightly after a weekend in the vent box.

C. Initial Conclusions

[0035] At least some low friction additives, such as SLIDE, require an actively curing surface to provide a matting property. This initial conclusion is reinforced by the test run on sample 2 — consisting of no heat and minimized volatilization — which did not matte. The matting effect is prevented if the piece is heated. This may be due to heat accelerating the effect on one surface (and “running out” before the second), or potentially accelerating the rubber cure such that the later exposed surface is no longer “active.” It is also possible that heat is volatilizing the reagent more quickly so that it “runs out” faster, but after the test run on sample 2, application concluded that volatilization is likely not important to the action, only to the odor of the part.

[0036] It is not entirely clear why pieces that were covered, but not heated or baked, did not matte. Samples 6 and 8 both should have mostly fully cured surfaces, but sample 8 should have volatilized more and matted slightly, while sample 6 did not matte. Applicant hypothesizes that volatizing encourages diffusive movement of the matting additive active agents towards the exposed surfaces of the rubber. It is possible that covered pieces have more active agents in the body of the rubber due to lower volatilization, but not at the surface, so matting does not occur.

D. Follow up Tests

[0037] In a first follow up test, applicant prepared a silicone sample using Dragon Skin 10 FAST platinum cure liquid silicone loaded with 1% by weight SLIDE. Exposed surfaces matted overnight and the sample was then baked for approximately six hours on a hot plate at 75° C uncovered. The second surface was removed from the plate and a section of foil was applied to part of the second surface, thereby exposing some, but not all of the second surface, to air. The assembly was moved to vent box overnight. No matting was observed on any of the second surface, and the foil left no impression — i.e., there was zero matting on second surface at all, whether exposed or unexposed.

[0038] In a second follow up test, a silicone sample was prepared in a similar way as the first test except that a cure slowing agent (SLO-JO™ Platinum Silicone Cure Retarder) was added to 1% by weight. The sample was processed by the same procedure as the first test and no matting observed on second surface. E. Conclusion

[0039] Masking a silicone rubber surface will prevent the matting effect. However, a masked surface can still matte if later unmasked and exposed to air. Heating a silicone piece at 75° C for after demolding is sufficient to prevent matting, even on exposed surfaces.

[0040] A process for selective matting of a silicone piece therefore comprises:

(1) addition of a matting additive to a silicone rubber preparation; (2) forming a silicone piece with the silicone rubber preparation; (3) masking a first surface of the silicone piece;

(4) exposing the silicone piece to vented air while the piece is baked for a duration; and

(5) removing the mask from the first surface. In an exemplary process, baking may take place at a temperature of 75° C and baking duration may be approximately six hours. The resulting silicone piece will have a first surface that retains silicone’s inherent tackiness, while other surfaces have a matted texture.

[0041] This process may be employed in silicone pieces that comprise a plurality of surfaces and the sections of the piece to be matted or non-matte comprise a plurality of surfaces or simply portions of one or more surfaces. Any combination of masking a portion or all of one or more surfaces where tackiness is desired may be accomplished. The choice of temperature for baking may be substantially 75° C or may be higher or lower than 75° C. The choice of duration for baking may be substantially six hours or may be shorter or longer than six hours.

[0042] Applying the inventive process to a prosthetic liner comprises, for example, fabrication of the liner via an additive manufacturing process, masking the interior surface of the liner that is intended to be in contact with the skin of a residual limb, exposing the prosthetic liner to a baking and venting process, which results in matting on the exterior surface and tackiness on the interior surface.

* * * [0043] The foregoing exemplary descriptions and the illustrative embodiments of the present disclosure have been explained in the drawings and described in detail, with varying modifications and alternative embodiments being taught. While the disclosure has been so shown, described and illustrated, it should be understood by those skilled in the art that equivalent changes in form and detail may be made therein without departing from the true spirit and scope of the disclosure.

Claims

1. A method for producing a silicone object with one or more tacky surfaces and one or more matte surfaces comprising the steps of: adding a silicone matting additive to a silicone preparation; forming a silicone object with said silicone preparation wherein the silicone object comprises one or more surfaces; applying one or more masks to one or more portions of said one or more surfaces; exposing the silicone object to vented air; baking the silicone object for a duration; and removing the one or more masks from the one or more portions of one or more surfaces.

2. The method of claim 1 wherein the silicone preparation comprises a two part curing silicone.

3. The method of claim 2 wherein the silicone matting additive is added to a first part of the two part curing silicone prior to mixing.

4. The method of claim 1 wherein the step of forming a silicone object is accomplished via an additive manufacturing process.

5. The method of claim 4 wherein the additive manufacturing process comprises three- dimensional printing of silicone within a support gel.

6. The method of claim 1 wherein the step of applying one or more masks to one or more portions of said one or more surfaces comprises applying a single mask to the entirety of a single surface of the silicone object.

7. The method of claim 1 wherein the step of applying one or more masks to one or more portions of said one or more surfaces comprises applying a single mask to a portion of a single surface of the silicone object.

8. The method of claim 1 wherein the step of applying one or more masks to one or more portions of said one or more surfaces comprises applying a plurality of masks to one or more surfaces of the silicone object.

9. The method of claim 1 wherein the step of baking the silicone object for a duration comprises exposing the silicone object to a temperature of substantially 75 degrees Celsius.

10. The method of claim 1 wherein the step of baking the silicone object for a duration comprises exposing the silicone object to a set temperature for approximately six hours.

11. The method of claim 1 wherein the silicone object comprises a prosthetic liner comprising an interior surface for contact with a residual limb and an exterior surface for contact with a prosthetic device.

12. The method of claim 11 wherein the step of applying one or more masks to one or more portions of said one or more surfaces comprises masking the interior surface of the prosthetic liner.