WO2022191829A1

WO2022191829A1 - Microfluidic valves

Info

Publication number: WO2022191829A1
Application number: PCT/US2021/021662
Authority: WO
Inventors: Viktor Shkolnikov; Alexander Govyadinov; Pavel Kornilovich
Original assignee: Hewlett Packard Development Co LP
Current assignee: Hewlett Packard Development Co LP
Priority date: 2021-03-10
Filing date: 2021-03-10
Publication date: 2022-09-15
Anticipated expiration: 2023-09-10

Abstract

The present disclosure is drawn to microfluidic valves and microfluidic devices that include the microfluidic valves. In one example, a microfluidic valve can include a microfluidic channel, a solid gas-generating material in the microfluidic channel, and an electric initiator adjacent to the solid gas-generating material. The solid gas-generating material can be chemically reactive to form a gas. The electric initiator can be configured to initiate a chemical reaction of the solid gas-generating material to form the gas when an electric current is applied to the electric initiator.

Description

MICROFLUIDIC VALVES BACKGROUND [0001]Microfluidics relates to the behavior, precise control and manipulation of fluids that are geometrically constrained to a small, typically sub- millimeter, scale. Numerous applications employ passive fluid control techniques such as capillary forces. In some applications, external actuation techniques are employed for a directed transport of fluid. A variety of applications for microfluidics exist, with various applications involving differing controls over fluid flow, mixing, temperature, evaporation, and so on. BRIEF DESCRIPTION OF THE DRAWINGS [0002]Additional features of the disclosure will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the present technology. [0003]FIGs.1A-1B are schematic cross-sectional side views of an example microfluidic valve in accordance with the present disclosure; [0004]FIGs.2A-2C are schematic cross-sectional side views of another example microfluidic valve in accordance with the present disclosure; [0005]FIGs.3A-3C are schematic cross-sectional side views of another example microfluidic valve in accordance with the present disclosure; [0006]FIGs.4A-4B are schematic cross-sectional top views of an example microfluidic valve in accordance with the present disclosure [0007]FIGs.5A-5H are schematic cross-sectional top views of additional example microfluidic valves in accordance with the present disclosure; [0008]FIGs.6A 6D are schematic cross sectional top views of still more example microfluidic valves in accordance with the present disclosure [0009] FIGs.7A-7B are schematic cross-sectional side views of another example microfluidic valve in accordance with the present disclosure; [0010]FIG.8 is a schematic cross-sectional top view of an example microfluidic device in accordance with the present disclosure; and [0011]FIG.9 is a flowchart illustrating an example method of making a microfluidic valve in accordance with the present disclosure. [0012]Reference will now be made to several examples that are illustrated herein, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended. DETAILED DESCRIPTION [0013]The present disclosure is drawn to microfluidic valves and microfluidic devices that include the microfluidic valves. In one example, a microfluidic valve includes a microfluidic channel, a solid gas-generating material in the microfluidic channel, and an electric initiator adjacent to the solid gas- generating material. The solid gas-generating material is chemically reactive to form a gas. The electric initiator is configured to initiate a chemical reaction of the solid gas-generating material to form the gas when an electric current is applied to the electric initiator. In certain examples, the electric initiator can include a thermal resistor or a spark gap. In further examples, the electric initiator can include an electrically conductive layer formed at a wall of the microfluidic channel and the solid gas-generating material can be formed as a layer over the electrically conductive layer. In some examples, the solid gas-generating material can be positioned in the microfluidic channel so that the solid gas-generating material does not block the microfluidic channel. In further examples, the solid gas-generating material can be chemically reactive to form a gas by a thermal decomposition reaction, or by a combustion reaction, or by a chemical reaction with a fluid in the microfluidic channel. In certain examples, the solid gas- generating material can include an Azobis compound, a peroxide, a carbonate, a nitrate, a nitrite, an azide, nitrocellulose, or a combination thereof. In further examples, a barrier layer can cover the solid gas-generating material to protect the solid gas-generating material from fluid in the microfluidic channel. This barrier layer can be degradable by the electric initiator. In some examples, the microfluidic valve can also include a solid gas-absorbing material in the microfluidic channel adjacent to the solid gas-generating material. In other examples, the microfluidic channel can include a gas vent downstream or upstream of the solid gas-generating material to allow the gas to escape from the microfluidic channel. In still further examples, the microfluidic channel can include a pinch point having a reduced width compared to a width of an adjacent portion of the microfluidic channel. The pinch point can be positioned to constrain a gas bubble generated by the solid gas-generating material. [0014]The present disclosure also describes microfluidic devices. In one example, a microfluidic device includes a fluid reservoir, a microfluidic channel connected to the fluid reservoir, a solid gas-generating material in the microfluidic channel, and an electric initiator adjacent to the solid gas-generating material, e.g., a solid material capable of forming a gas. The electric initiator in this example causes the solid gas-generating material to form the gas when an electric current is applied to the electric initiator. In further examples, the microfluidic device can also include an electric power source connected to the electric initiator. The electric initiator can include a thermal resistor or a spark gap. In some examples, the fluid reservoir and the microfluidic channel can contain an aqueous liquid. [0015]The present disclosure also describes methods of making microfluidic valves. In one example, a method of making a microfluidic valve includes forming a microfluidic channel enclosed by channels walls. An electric initiator is formed, where the electric initiator includes an electrically conductive layer at a channel wall of the microfluidic channel. A solid gas-generating material is placed over the electrically conductive layer such that the solid gas-generating material is in the microfluidic channel but not blocking the microfluidic channel. The solid gas-generating material is chemically reactive to form a gas, and the electric initiator is configured to initiate a chemical reaction of the solid gas- generating material to form the gas when an electric current is applied to the electric initiator. In other examples, the method can also include placing a solid gas-absorbing material in the microfluidic channel adjacent to the solid gasgenerating material. in still further examples, forming the microfluidic channel can include forming a pinch point having a reduced width compared to a width of an adjacent portion of the microfluidic channel. The pinch point can be positioned to constrain a gas bubble generated by the solid gas-generating material.

[0016] The microfluidic valves described herein can provide reliable valves for microfluidic channels, without complex mechanical components. Providing valves for small microfluidic channels can be difficult. Microfluidic channels can have a width on the order of 1 mm or less. Many microfluidic channels can be considerably smaller, such as having widths of 500 μm or iess, or 100 μm or less, 50 μm or less, or even 35 μm or iess. Making miniature mechanical valves for such small channels can be difficult and expensive, if possible at all. Some types of valves that have been used in microfluidic devices include pneumatic valves, which often have complicated designs and bulky external drivers. At very small scales, mechanical valves are also subject to failure caused by random mechanical, interfacial, and electrostatic forces. This makes miniaturized mechanical valves iess reliable. Certain types of dynamic valves have been used, but these valves can be leaky and may not work against a significant pressure head. Many valves used in microfluidic devices have slow response times. Many valves also occupy a large space in the microfluidic device, which results in a large dead volume where fluid filling the dead volume is not able to otherwise participate in the intended function of the microfluidic device. This can be particularly detrimental because microfluidic devices often operate with very small volumes of fluids.

[0017] Microfluidic valves can be used In microfluidic devices for a variety of fluid flow control purposes. Some microfluidic devices can include one-way valves, which can be used as check valves to ensure that fluid can flow in one direction while stopping flow in the opposite direction. These valves are often used in combination with pumps to ensure that the net flow of fluid produced by the pump is in a particular direction. Active valves are another type of valve, which can be opened and/or closed on command. Active valves that are designed to fully open or fully closed can be used in any situation where fluid flow through a microfluidic channel is to be started or stopped at a specific time. Active valves can also be designed to be partially opened, to allow for finer control over flow rate of fluid through the active valve. Such valves can be used to direct flow of fluids into mixers, reactors, outlets, and other components in a microfluidic device. For example, an active valve can be connected to a reservoir of a particular reagent and the active valve can be opened at a desired time to allow the reagent to mix with other reagents. Active valves can be particularly difficult to make and use in microfluidic devices, which can lead to the limitations of the types of microfluidic valves described above. [0018] In contrast to the other types of microfluidic valves described above, the microfluidic valves described herein have no moving parts other than the fluid in the microfluidic channel. The valves are small, possibly having approximately the same width as the microfluidic channel itself. No external components are used to actuate the valves, other than a source of electric current to be supplied to the electric initiator of the valve. In a basic example, the microfluidic valves described herein can include a solid gas-generating material placed in the microfluidic channel and an electric initiator adjacent to the solid gas-generating material. The solid gas generating material can be placed in a way that does not block the flow of fluid through the microfluidic channel. For example, the solid gas-generating material may be formed as a thin film of material on one of the walls of the microfluidic channel. In some examples, the electric initiator can be a thermal resistor or a spark gap that is located just next to or under the solid gas- generating material. The electric initiator can provide heat or a spark that can, depending on the specific type of gas-generating material, initiate a chemical reaction the converts the gas-generating material to a gas. The gas can form a bubble in the microfluidic channel that can block fluid flow through the microfluidic channel. Thus, the valve can be in an open state at first, and then the valve can switch to a closed state when the electric initiator initiates the chemical reaction of the gas-generating material to form the gas bubble. [0019]The microfluidic valves can also include a variety of features that allow the valves to be reversible. In other words, some microfluidic valves described herein can be closed and then opened again. Mechanisms to allow re- opening of the valve can include venting the gas bubble out of the microfluidic channel, or absorbing the gas bubble using a gas absorbing material, or allowing the gas bubble to dissolve in the fluid in the microfluidic channel. The microfluidic valves can also include features to allow the valves to be used multiple times, such as by including sufficient solid gas-generating material to form multiple gas bubbles. These and other features of the valves are described in more detail below. [0020]FIG.1A is a cross-sectional side view of one example microfluidic valve 100 in accordance with the present disclosure. The microfluidic valve includes a microfluidic channel 110 enclosed by channel walls 112, a solid gas- generating material 120 in the microfluidic channel, and an electric initiator 130 under the solid gas-generating material. FIG.1A shows the microfluidic valve in an initial open state. The solid gas-generating material does not block the microfluidic channel. Therefore, fluid can flow freely through the microfluidic channel in this open state. [0021]FIG.1B shows the microfluidic valve 100 in a closed state. This figure shows the valve after the electric initiator 130 has been used to cause a chemical reaction of the solid gas-generating material. The solid gas-generating material has reacted to form a gas bubble 122. The gas bubble blocks the microfluidic channel 110 so that fluid cannot flow freely through the microfluidic channel. Before initiating the chemical reaction to form the gas bubble, the microfluidic channel is filled with liquid 102. Without liquid in the microfluidic channel, the gas would merely dissipate without forming a bubble. [0022] In various examples, the electric initiator can be configured to initiate a chemical reaction of the solid gas-generating material to cause the solid gas-generating material to form a gas when an electric current is applied to the electric initiator. In some examples, the chemical reaction can be initiated by heat. In such examples, the electric initiator can include a thermal resistor to generate heat when electric current is applied to the thermal resistor. The thermal resistor can include a heating element made of a resistive material such as metal, metal alloys, metal nitrides, metal oxides, or others. In certain examples, thermal resistors can include metals such as aluminum, tantalum, nickel, tungsten, copper, chromium, tin, and alloys thereof. The thermal resistor can be formed by thin film deposition processes, in some examples. In certain examples, the thermal resistor can be similar to a thermal inkjet resistor, and may be formed using similar techniques. The size of the thermal resistor can be suitable for initiating the chemical reaction of the solid gas-generating material. In some examples, the thermal resistor can have a width that is about equal to or less than a width of the microfluidic channel. In further examples, the thermal resistor can have a width or a length that is from 1 ^m to 500 ^m, or from 1 ^m to 100 ^m, or from 1 ^m to 50 ^m, or from 1 ^m to 35 ^m. A thin film thermal resistor can have a relatively small thickness, such as from 1 nm to 5 ^m, or from 1 nm to 1 ^m, or from 1 nm to 500 nm. Thermal resistors can also be formed using other techniques, such as thick film resistors. In such examples, the thickness can be larger, such as from 1 ^m to 100 ^m, or from 1 ^m to 50 ^m, or from 1 ^m to 20 ^m. [0023]Some types of solid gas-generating material can be combustible, and a combustion reaction can be initiated to generate gas from the solid gas- generating material. Combustion can be initiated using an electric initiator that includes a thermal resistor, as described above, or a spark gap. A spark gap, or spark plug, can include two electrodes separated by a gap. When a sufficient voltage difference is applied between the two electrodes, a spark or arc can form between the electrodes. This spark can ignite the solid gas-generating material. In some examples, the electrodes of the spark gap can also be formed by thin film deposition processes. The electrodes can be made of a metal such as aluminum, tantalum, nickel, tungsten, copper, chromium, tin, gold, silver, or alloys thereof. In further examples, the electrodes of the spark gap can be separated by a distance from 1 ^m to 100 ^m, or from 1 ^m to 50 ^m, or from 1 ^m to 35 ^m, or from 1 ^m to 20 ^m, or from 1 ^m to 10 ^m. The solid gas-generating material, or a portion thereof, can be located at or near the area between the two electrodes so that the solid gas-generating material can be ignited by the spark between the electrodes. In further examples, the electrodes can be formed using similar processes to the thermal resistors described above. The electrodes can also have a similar width, length, and thickness to the thermal resistors described above. Therefore, the dimensions of the thermal resistors described above also apply to the electrodes of spark gaps. [0024]The electric initiator can be located adjacent to the solid gas generating material. The term “adjacent” as used herein regarding the electric initiators can mean that the electric initiator is either in direct physical contact with the solid gas-generating material or sufficiently proximate to the solid gas- generating material that applying an electric current to the electric initiator can cause the solid gas-generating material to react and form a gas. For example, a thermal resistor can be placed in direct contact with the solid gas-generating material or there may be other materials between the thermal resistor and the solid gas-generating material, provided that sufficient heat from the thermal resistor can be conducted to the gas-generating material to initiate a chemical reaction. In certain examples, the thermal resistor can be separated from the solid gas-generating material by a wall of the microfluidic channel. In other examples, the thermal resistor can be formed as a thin layer on an interior wall of the microfluidic channel and the solid gas-generating material can be formed as a layer directly over the thermal resistor. [0025] In examples that utilize a spark gap as the electric initiator, the spark gap electrodes can be in direct contact with the solid gas-generating material or proximate to the solid gas-generating material so that a spark between the electrodes will ignite the gas-generating material. In some examples, the solid gas-generating material, or a portion of the solid gas-generating material, can be located directly between the electrodes. In other examples, the electrodes can both be formed on an interior wall surface of the microfluidic channel and the solid gas-generating material can be formed as a layer over the electrodes, or over an area between the electrodes. [0026]The solid gas-generating material can include a variety of chemical compounds that are capable of producing a gas through a chemical reaction. It is noted that the solid gas-generating material produces gas through a chemical reaction and not a physical state change, such as evaporation. In some examples, the solid gas-generating material can form a gas through a thermal decomposition reaction, or a combustion reaction, or a chemical reaction between the solid gas-generating material and a fluid in the microfluidic channel. Thermal decomposition reactions can refer to a reaction in which a compound breaks down into two or more simpler compounds. This reaction can be initiated by heat supplied by a thermal resistor as described above. Combustion reactions can involve a fuel and oxidizer mixture that is ignited. In the microfluidic valves described herein, the microfluidic channel will often be filled with a liquid such as water or an aqueous solution. Therefore, oxygen gas will often not be available as an oxidizer. Therefore, in some examples the solid gas-generating material itself can include an oxidizer, such as a nitro group, a peroxide, ammonium nitrate, or others. In certain examples, the solid gas-generating material can include a mixture of a solid fuel compound and a solid oxidizing compound, such as a mixture of cellulose and ammonium nitrate. In other examples, the solid gas- generating material can include a compound that can act as fuel and oxidizer together, such as nitrocellulose. Further, in examples where the solid gas- generating material is reactive with the fluid in the microfluidic channel, a barrier layer can be included to protect the solid gas-generating material from the fluid. In some such examples, the electric initiator can degrade the barrier layer, such as by melting or combusting the barrier layer, and then the solid gas-generating material can react with the fluid in the microfluidic channel to form a gas. These various examples are describes in more detail below. [0027]Some examples of compounds that undergo a thermal decomposition reaction can include Azobis compounds. Specific examples can include: Azobisisobutyronitrile, which decomposes to produce nitrogen gas at a decomposition temperature of 90 °C to 107 °C; 2-2’-Azobis(2,4- dimethylvaleronitrile), which decomposes to produce nitrogen gas at a temperature of 50 °C to 60 °C; 1,1’-Azobis(cyanocyclohexane), which decomposes to produce nitrogen gas at a temperature of 114 °C to 118 °C; 2,2’- Azobis(2-methylbutyronitrile); and other Azobis compounds. [0028]Additional compounds that can decompose to form a gas can include organic peroxides. Organic peroxides can decompose to produce oxygen gas. If combusted, organic peroxides can also produce carbon dioxide gas. Types of organic peroxides that can be included in the gas-generating material include: dialkyl peroxides, diacyl peroxides, hydroperoxides, peroxyacids, peroxyesters, peroxyketals, peroxycarbonates, peroxydicarbonates, and ketone peroxides. Some specific organic peroxides that can be included in the gas- generating material can include: benzoyl peroxide, which can decompose at a temperature of 105 C to 140 C; tert butyl peroxy 3,5,5 trimethylhexanoate, which can decompose at a temperature of 114 °C; dicumyl peroxide, which can decompose at a temperature of 143 °C; tert-butyl peroxy-2-ethylhexanoate, which can decompose at a temperature of 96 °C; tert-butyl peroxy-2- ethylhexylcarbonate, which can decompose at a temperature of 125 °C; 2,5- dimethyl-2,5-di(tert-butylperoxy)hexane, which can decompose at a temperature of 148 °C; tert-butyl peroxypivalate, which can decompose at a temperature of 85 °C; di-(2-ethylhexyl) peroxydicarbonate, which can decompose at a temperature of 65 °C; tert-amyl peroxy-2-ethylhexanoate, which can decompose at a temperature of 98 °C; di-tert-butyl peroxide, which can decompose at a temperature of 153 °C; di-tert-amyl peroxide, which can decompose at a temperature of 149 °C; dilauroyl peroxide, which can decompose at a temperature of 86 °C; tert-butyl peroxybenzoate, which can decompose at a temperature of 121 °C; tert-amyl hydroperoxide, which can decompose at a temperature of 143 °C; tert-butyl hydroperoxide, which can decompose at a temperature of 91 °C; tert-butyl cumyl peroxide, which can decompose at a temperature of 147 °C; 2,5-dimethyl-2,5-dihydroperoxyhexane, which can decompose at a temperature of 127 °C; 1,1-di-(tert-butylperoxy)-3,3,5- trimethylcyclohexane, which can decompose at a temperature of 120 °C; 1,1-di- (tert-butyl peroxy) cyclohexane, which can decompose at a temperature of 121 °C; tert-amyl peroxy-2-ethylhexyl carbonate, which can decompose at a temperature of 123 °C; ethyl-3,3-di-(tert-amyl peroxy) butyrate, which can decompose at a temperature of 140 °C; tert-amyl peroxy-3,5,5- trimethylhexanoate, which can decompose at a temperature of 118 °C; tert-butyl peroxyisopropylcarbonate, which can decompose at a temperature of 127 °C; di- n-propyl peroxydicarbonate, which can decompose at a temperature of 53 °C; di- (3,5,5-trimethylhexanoyl) peroxide, which can decompose at a temperature of 87 °C; didecanoyl peroxide, which can decompose at a temperature of 88 °C; 2,2-di- (tert-butyl peroxy) butane, which can decompose at a temperature of 137 °C; 2,5- dimethyl-2,5-di-(2-ethylhexanoyl peroxy) hexane, which can decompose at a temperature of 95 °C; 1,1-di-(tert-amyl peroxy) cyclohexane, which can decompose at a temperature of 121 °C; tert-butyl peracetate, which can decompose at a temperature of 129 °C; 2,5-di(tert-butylperoxy)-2,5-dimethyl-3- hexyne, which can decompose at a temperature of 144 C; di (4 tert butylcyclohexyl) peroxydicarbonate, which can decompose at a temperature of 85 °C; dicetyl peroxydicarbonate, which can decompose at a temperature of 63 °C; dimyristyl peroxydicarbonate, which can decompose at a temperature of 60 °C; 1,1,3,3-tetramethylbutyl peroxy-2-ethylhexanoate, which can decompose at a temperature of 92 °C; tert-butyl peroxydiethylacetate, which can decompose at a temperature of 93 °C; 1,1,3,3-tetramethylbutyl hydroperoxide, which can decompose at a temperature of 127 °C; 3,3,5,7,7-pentamethyl-1,2,4-trioxepane, which can decompose at a temperature of 180 °C; dicetyl peroxydicarbonate; tert-amyl-peroxybenzoate; and others. [0029]Other compounds that can decompose to form a gas include carbonates. Many carbonates can decompose to produce carbon dioxide gas. Some specific examples of carbonates include: magnesium carbonate, calcium carbonate, strontium carbonate, barium carbonate, lithium carbonate, sodium carbonate, potassium carbonate, rubidium carbonate, and cesium carbonate. [0030]Additional compounds that can be included in the solid gas- generating material can include: nitrocellulose, which can combust to form carbon dioxide and nitrogen gas; ammonium nitrite, which can decompose to form water and nitrogen gas; ammonium nitrate mixed with cellulose, which can combust to form carbon dioxide, nitrogen gas, and water; sodium nitrate, which can decompose to form sodium nitrite and oxygen gas; azides such as sodium azide, barium azide, or others, which can decompose to produce nitrogen gas. [0031] In various examples, gases that can be formed by the gas- generating material can include acetylene, ammonia, bromine, carbon dioxide, carbon monoxide, chlorine, ethane, ethylene, hydrogen, hydrogen sulfide, methane, nitrogen, oxygen, sulfur dioxide, and others. Water vapor can also be formed by some gas-generating materials. However, when water or an aqueous liquid is present in the microfluidic channel, the water vapor can often condense into the liquid phase. Additionally, some gases produced by the gas-generating material may be soluble in water. These gases can gradually dissolve into the liquid in the microfluidic channel. Therefore, the gas bubble formed by these gases may be temporary. In some examples, this effect can be utilized to provide valves that close temporarily and then open again when the gas bubble dissolves. [0032]Some of the compounds that can decompose or combust to generate gas may be unstable at normal conditions, such as room temperature and pressure. In some cases, microfluidic devices incorporating these materials can be stored at low temperatures, such as under refrigeration, in order to preserve the gas-generating material. In other examples, a reactive gas- generating compound can be mixed with an inert material to stabilize the material. For example, some of the compounds described above can be mixed with an inert material such as a polymer. In a particular example, the gas- generating compound can be mixed with an epoxy paste such as SU-8. Thus, the solid gas-generating material can be a mixture of a reactive gas-generating compound and an inert material in some examples. [0033]The solid gas-generating material can be deposited in a microfluidic channel using methods such as lyophilization, drop deposition, and so on. In one example, the gas-generating material can be dissolved in a solvent such as hexane, acetone, ethanol, water, or others. A drop of this solution can then be deposited in the desired location for the solid gas-generating material. The solvent can evaporate as the solution dried, leaving behind the dry solid gas- generating material. In another example, the solid gas-generating material can be powder. The powder can be mixed with a binder such as nitrocellulose, epoxy nitrate, or others. The mixture can be allowed to dry to form the solid gas- generating material in the microfluidic channel. In some examples, the solid gas- generating material can be deposited as a layer having a thickness from 10 nm to 5 ^m, or from 100 nm to 5 ^m, or from 1 ^m to 5 ^m, or from 100 nm to 1 ^m. Other dimensions of the solid gas-generating material can be selected to provide a sufficient amount of the material to produce a gas bubble that can block the microfluidic channel. In certain examples, a sufficient amount of the solid gas- generating material can be deposited to allow for multiple gas bubbles to be formed so that the valve can be used multiple times. Additionally, in some examples the solid gas-generating material can be deposited in a recess on the floor of the channel or one a side wall of the channel, and the dimensions of the solid gas-generating material can be defined by the dimensions of the recess. In various examples, the solid gas generating material can have a width or a length that is from 1 ^m to 5mm, or from 1 ^m to 1 mm, or from 1 ^m to 500 ^m, or from 1 ^m to 100 ^m, or from 1 ^m to 50 ^m, or from 1 ^m to 35 ^m. [0034] It is noted that any of the types of solid gas-generating materials and electric initiators described herein can be used in any of the example microfluidic valves and microfluidic devices described herein. Thus, any of the specific examples described herein or illustrated in the figures can include any of the solid gas-generating materials or any of the types of electric initiators described above. [0035]Another example microfluidic valve 100 is shown in FIG.2A. This example includes a microfluidic channel 110 having channel walls 112, a solid gas-generating material 120 in the microfluidic channel, and an electric initiator 130 adjacent to the solid gas-generating material. This example also includes a gas vent 114, which is an opening in the channel wall downstream of the solid gas-generating material. A push resistor 140 is formed in the channel wall upstream of the solid gas-generating material. [0036]FIG.2B shows the microfluidic valve 100 after the microfluidic channel 110 has been filled with a liquid 102 and the solid gas-generating material has been converted into a gas bubble 122. The gas bubble can block the microfluidic channel. Thus, this figure shows the valve in the closed state. [0037]The push resistor 140 and gas vent 114 can allow this microfluidic valve to be re-opened after the valve has been closed. The push resistor can be used as a type of pump to push the gas bubble 122 toward the gas vent 114. In particular, the push resistor can generate heat to form a vapor bubble in the liquid 102 in the microfluidic channel. The vapor bubble can momentarily displace a volume of the liquid, which can push the gas bubble downstream toward the gas vent. When the gas bubble reaches the gas vent, the gas bubble can escape from the microfluidic channel through the gas vent. FIG.2C shows the gas bubble in the process of escaping through the gas vent. After the entire gas bubble has escaped through the gas vent, the liquid can flow freely through the microfluidic channel. Thus, the valve is again in the open state. This type of microfluidic valve can be useful in applications where it is desired to begin with the valve in an open state, and then close the valve, and then re-open the valve on command. In particular, the valve can be closed on command by sending an electric current to the electric initiator to cause the solid gas-generating material to react. The valve can be re-opened on command by sending an electric current to the push resistor to generate a vapor bubble to push the gas bubble downstream to the gas vent. [0038]As used herein, “downstream” is used to refer to the direction that fluid and the gas bubble travel when the push resistor generates a vapor bubble. When the vapor bubble forms, the gas bubble and fluid caught between the gas bubble and the vapor bubble are both pushed downstream. As described above, a gas vent can be positioned downstream from the original location of the gas bubble. Therefore, the push resistor can push the gas bubble downstream to the gas vent, where the gas bubble can be vented outside the microfluidic channel. As used herein, “upstream” refers to the opposite direction from downstream. [0039] In various examples, the push resistor can be formed from the same materials as a thermal resistor-type electric initiator described above. The push resistor can also have similar dimensions, such as dimensions in the same ranges described above for the thermal resistor-type electric initiators. In some examples, the push resistor can be similar to a resistor used in an inkjet printhead. A sufficient amount of electric current can be supplied to the push resistor to form a vapor bubble in the liquid in the microfluidic channel. In some examples, the amount of current supplied to the push resistor can be the same as, or less than, or more than the amount of current that is supplied to the electric initiator to initiate the chemical reaction of the solid gas-generating material. [0040] It is noted that the vapor bubble formed by the push resistor can be temporary. The vapor bubble can contain vapor of the liquid in the microfluidic channel. For example, if the liquid in the microfluidic channel is water or an aqueous liquid, then the vapor bubble can contain water vapor. In further examples, the liquid in the microfluidic channel can include an organic solvent and the vapor bubble may contain a vapor of the organic solvent. In any case, the push resistor can generate heat momentarily to form a vapor bubble. The push resistor can then be turned off. Without the heat from the push resistor, the vapor can condense back into the liquid in the microfluidic channel. Therefore, the vapor bubble does not remain in the microfluidic channel longer than the time taken for the gas bubble to be pushed downstream to the gas vent. In contrast, the gas bubble that is formed from the solid gas-generating material can remain in the microfluidic channel for a longer time. If the gas in the gas bubble is not soluble in the liquid in the microfluidic channel, then the gas bubble can remain indefinitely. [0041]Another mechanism for causing the gas bubble escape through a gas vent can include using a thermal resistor, such as the push resistor or a thermal resistor-type electric initiator, to heat the gas bubble. Heating the gas bubble can cause the gas to expand. If the gas bubble expands to a large enough volume that the gas bubble reaches the gas vent, then the gas bubble can escape through the gas vent. The temperature of the fluid in the microfluidic channel can then be cooled back to normal operating temperature. [0042]The gas vent can include an opening or another feature that allows gas to pass out of the microfluidic channel, but which retains liquid inside the microfluidic channel. In some examples, the gas vent can include a small opening and a hydrophobic surface. In such examples, the opening can be sufficiently small and sufficiently hydrophobic that water or aqueous liquids will not flow out through the opening. In certain examples, a hydrophobic coating can be applied to the surfaces of the channel wall near and within the opening. However, gases can freely flow through the opening. In other examples, the gas vent can include a gas-permeable membrane that can retain the liquid inside the microfluidic channel. In various examples, the vent can have a width or diameter from 500 nm to 50 ^m, 1 ^m to 30 ^m, or 1 ^m to 20 ^m in some cases. In further examples, the vent can have a width that is from 1% to 99% the width of the microfluidic channel, 5% to 50% the width of the microfluidic channel, or 5% to 25% the width of the microfluidic channel. The shape of the vent is not particularly limited. In some examples, the vent can be circular, square, rectangular, or another shape. [0043]FIG.3A shows another example microfluidic valve 100 that can be reopened by absorbing the gas in the gas bubble. This microfluidic valve includes a microfluidic channel 110 enclosed by channel walls 112. An electric initiator 130 is formed on a channel wall, and a solid gas-generating material 120 is deposited over the electric initiator. Additionally, a solid gas-absorbing material 150, such as iron metal that can absorb oxygen gas to form iron oxide, or a porous metal that can adsorb a variety of gasses within pores, is deposited adjacent to the solid gas-generating material. In the example shown in the figure, the solid gas- absorbing material is not in direct contact with the solid gas-generating material, although in other examples these materials may be in direct contact. The term “adjacent” when used with respect to the gas-absorbing material means that the gas absorbing material is sufficiently near the gas-generating material that the gas bubble generated by the gas-generating material will be in direct contact with the gas-absorbing material. This microfluidic valve also includes a heater 142 under the solid gas-absorbing material. In some examples, the gas-absorbing material can be reactive with the gas in the gas bubble such that the gas is absorbed through a reaction with the gas-absorbing material. The reactivity of the gas-absorbing material can be increase by heating. Thus, the heater can be activated to cause the gas-absorbing material to quickly absorb the gas bubble. In this way, the valve can be re-opened on command. [0044]FIG.3B shows the microfluidic valve 100 after the gas-generating material has been converted to a gas bubble 122. The gas bubble is in direct contact with the solid gas-absorbing material 150. In this figure, the valve is in a closed state because the gas bubble blocks flow through the microfluidic channel 110. In some examples, the gas-absorbing material can have a low reactivity when not heated, so that the gas bubble can remain in the microfluidic channel for a long time. [0045]FIG.3C shows the microfluidic valve 100 as the heater 142 is being used to heat the solid gas-absorbing material 150. The gas bubble 122 is in the process of being absorbed by the solid gas-absorbing material. When the gas bubble has been fully absorbed, the valve will be fully open again to allow fluid to flow through the microfluidic channel. [0046]Gas-absorbing materials can include oxidizable materials that can be used to absorb oxygen gas. In one example, the gas-absorbing material can include iron metal, which can react with oxygen gas to form iron oxides. This reaction can proceed significantly more quickly at elevated temperatures. Therefore, a bubble of oxygen gas can be absorbed on command by heating an iron-containing gas-absorbing material. Other examples of gas-absorbing materials can include cobalt, aluminum, zirconium, vanadium, and alloys thereof. In a particular example, an alloy of 84 wt% zirconium and 16 wt% aluminum can be used. Some gas-absorbing materials can have a passivation layer at room temperature, but the passivation layer can be removed by heating. In further examples, the gas-absorbing material can have a roughened surface or a surface with micropores or nanopores in order to increase the surface area that can absorb gas. In certain examples, the gas-absorbing material can be a porous material formed from sintered powder particles. [0047] In the example shown in FIGs.3A-3C, the solid gas-generating material is fully consumed to form a gas bubble. Therefore, this example is a one- time-use valve. However, in other examples, a sufficient amount of solid gas- generating material can be used to form gas bubbles multiple times. Alternatively, multiple bodies of solid gas-generating material can be included, with multiple electric initiators. In such examples, the valve can be closed multiple times by generating multiple gas bubbles. The gas-absorbing material can then be used to absorb the gas bubbles, causing the valve to open again. Thus, other examples can include microfluidic valves that can be used multiple times. [0048] In further examples, the microfluidic channel can include a pinch point that has a reduced width compared to the width of adjacent portions of the microfluidic channel. This pinch point can be used to constrain the gas bubble that is generated by the solid gas-generating material. As used herein, the “width” of the microfluidic channel can refer to any dimension of the channel that is perpendicular to the longitudinal or length dimension. In some examples, two dimensions of the microfluidic channel can be constricted at the pinch point. In other examples, one dimension can be constricted. In certain examples, the microfluidic channel can be manufactured using a method that utilizes layers of materials that can be pattern using photolithography. With some manufacturing methods, it can be easier to form a microfluidic channel have a flat floor and ceiling, whereas the side walls can be patterns to have various shapes such as constricted pinch points. Accordingly, in some examples the microfluidic channel can have a pinch point including a reduced width between the sidewalls, but the floor and ceiling of the microfluidic channel can be flat. [0049]FIG.4A shows top-down view of a microfluidic valve 100 that includes a microfluidic channel 110 enclosed by side walls 116. An electric initiator 130 is formed on the floor of the microfluidic channel. A solid gas generating material 120 is deposited over the electric initiator. Pinch points 118 are formed in the side walls. The pinch points are located on either side of the solid gas-generating material. It is noted that the top-down view shown in this figure is different from the side cross-sectional views shown in the previous figures. Accordingly, the channel walls shown in the previous figures can also be referred to as a channel floor and channel ceiling, whereas the channel walls shown in FIG.4A are side walls. [0050]FIG.4B shows the microfluidic valve 100 after the microfluidic channel 110 has been filled with liquid 102 and the solid gas-generating material has reacted to form a gas bubble 122. The gas bubble is constrained in the volume of the microfluidic channel that is between the pinch points 118. The pinch points are able to constrain the gas bubble in this way because of capillary forces, which are relatively larger in the narrowed portion of the pinch points. The gas bubble can be maintained between the pinch points even if pressure is applied to the liquid in the microfluidic channel from one direction or the other. Without the pinch points, the gas bubble could be subject to being pushed along the microfluidic channel if any pressure is applied to the liquid. [0051] In some examples, the microfluidic channel can include a pinch point that has a reduced width that is less than the width of an adjacent portion of the microfluidic channel. In some examples, the width of the pinch point can be less than the width of the microfluidic channel on both sides of the pinch point. In other examples, the width of the pinch point can be less than the width of the microfluidic channel on one side of the pinch point. [0052]FIGs.5A-5H show top-down views of several different microfluidic valves 100 having various designs. These designs include a microfluidic channel 110 that includes a gas bubble chamber 124. Liquid can flow through the gas bubble chamber through pinch points 118. The pinch points have a reduced width compared to the width of the gas bubble chamber, and/or portions of the microfluidic channel outside of the gas bubble chamber. An electric initiator 130 and a solid gas-generating material 120 are located within the gas bubble chamber. In further examples, these designs can also include other features described herein, such as gas vents, push resistors, gas absorbing materials, and so on. [0053] It is noted that the microfluidic valves described herein can include microfluidic channels having a variety of shapes. In some examples, the microfluidic channel can have multiple branches. These multiple branches can join at or proximate to the solid gas-generating material so that the multiple branches can be blocked by the gas bubble formed from the solid gas-generating material. FIG.5G shows one such example microfluidic valve 100. In this example, the microfluidic channel 110 has three branches that meet at a gas bubble chamber 124. As in the previous examples, the branches are narrower than the gas bubble chamber, so that transitions between the gas bubble chamber and the branches act as pinch points 118 to constrain the gas bubble. A solid gas-generating material 120 and an electric initiator 130 are positioned in the gas bubble chamber. FIG.5H shows a similar example that has a microfluidic channel with four branches instead of three. Additionally, any other example microfluidic valves described herein can be modified to include microfluidic channels having multiple branches. [0054]As mentioned above, in some examples the electric initiator can include a spark gap. The spark gap can be made up of two electrodes that are separated by a gap. When a sufficient voltage is applied between the electrodes, a spark can arc between the electrodes. The spark can ignite a combustible gas- generating material, such as nitrocellulose. FIGs.6A-6D show top-down views of microfluidic valves 100 that include several different designs of spark gaps. The spark gaps are made up of electrodes 132 separated by a gap. In some examples, the electrodes can be formed by depositing metal or another conductive material on the floor of the microfluidic channel 110. A solid gas- generating material 120 is deposited over the electrodes so that a spark between the electrodes can ignite the solid gas-generating material. In these figures, the solid gas-generating material is outline with a dashed line so that the shape of the underlying electrodes can be seen. [0055]Some solid gas-generating materials can be insoluble in the liquid in the microfluidic channel. For example, some solid gas-generating materials can be insoluble in water. Additionally, in some examples the solid gas- generating material can be non reactive with the liquid in the microfluidic channel. For example, the solid gas-generating material can be non-reactive with water. However, some solid gas-generating materials may be soluble in the liquid, or reactive with the liquid, or both. In these cases, it can be useful to include a barrier layer that protects the solid gas-generating material from the liquid in the microfluidic channel. The barrier layer can be a coating that covers the solid gas- generating material. Additionally, the barrier layer can be degradable by the electric initiator. In some examples, the barrier layer can be made of a material that melts when heated by a thermal resistor. Meltable materials can include wax, petroleum wax, paraffin wax, microcrystalline wax, or others. In other examples, the barrier layer can be made of a combustible material that can be ignited by a spark gap. The combustible barrier material can include nitrocellulose, or any of the other combustible materials described above with respect to the gas generating materials. [0056]FIG.7A shows a side cross-sectional view of an example microfluidic valve 100 that includes a barrier layer 160, such as a wax layer as mentioned above, which can protect the solid gas-generating material 120 from reacting with or dissolving in fluid in the microfluidic channel 110 as mentioned above. The barrier layer is positioned over the solid gas-generating material. An electric initiator 130 is under the solid gas-generating material and the barrier layer. In this example, the barrier layer is made of a meltable material that can be melted by heat produced by the electric initiator. The barrier layer and the solid gas-generating material are in the microfluidic channel, but do not block the microfluidic channel. The microfluidic channel is enclosed by channel walls 112. It is noted that this view is a side cross-sectional view. Therefore, the channel walls visible in the figure can also be referred to as a floor and a ceiling of the microfluidic channel. [0057]FIG.7B shows the example microfluidic valve 100 after the microfluidic channel 110 has been filled with liquid 102 and the barrier layer has been removed by the electric initiator 130. The solid gas-generating material has reacted to form a gas bubble 122. In further examples, this design can also incorporate push resistors, gas vents, gas-absorbing materials, pinch points, or any other features described herein. [0058] In one particular example, a microfluidic valve can include calcium carbide as the solid gas-generating material. Calcium carbide can react with water to produce acetylene gas and calcium hydroxide. Therefore, the calcium carbide can be protected by a barrier layer. The barrier can include wax or nitrocellulose in some specific examples. When the valve is used, the barrier layer can be removed by an electric initiator. This can allow the calcium carbide to react with water in the microfluidic channel. In further examples, a buffering reagent can be mixed with the calcium carbide to control the pH of the water in the microfluidic channel. In some examples, the buffering reagent can include citric acid, phosphoric acid, phosphate buffered saline, HEPES buffer, TRIS buffer, and so on. [0059]The microfluidic valves described herein can be included in any type of microfluidic device that can utilize a valve that can be closed on command. In some examples, a microfluidic device can include a microfluidic valve as described above, including a microfluidic channel with a solid gas- generating material and an electric initiator as described above. Additionally, the microfluidic device can include a fluid reservoir that is connected to the microfluidic channel. In certain examples, fluid from the fluid reservoir can flow through the microfluidic channel to fill the microfluidic channel. Whenever it is desired to close the microfluidic valve, the electric initiator can be activated to cause the solid gas-generating material to react and form a gas bubble. [0060]FIG.8 shows a top-down view of an example microfluidic device 200. The microfluidic device includes a fluid reservoir 210. This fluid reservoir can be filled with a fluid that will flow through a microfluidic channel 110 connected to the fluid reservoir. A solid gas-generating material 120 is formed in the microfluidic channel but not blocking the microfluidic channel. As explained above, the solid gas-generating material can be chemically reactive to form a gas. An electric initiator 130 is located under the solid gas-generating material. Although not shown in the figure, a gas vent can also be included, such as an opening formed in the ceiling of the microfluidic channel. The electric initiator can be configured to initiate the chemical reaction of the solid gas-generating material that forms the gas. This example microfluidic device also includes an electric power source 230 connected to the electric initiator. The electric initiator can be a thermal resistor or a spark gap. In either case, electric current supplied by the electric power source can cause the electric initiator to initiate the chemical reaction of the solid gas-generating material. In further examples, the microfluidic device can also include a controller to direct electric current from the power source to the electric initiator and any other electric components such as push resistors, heaters, and so on. [0061]The present disclosure also describes methods of making microfluidic valves. FIG.9 is a flowchart illustrating a particular example method 300 of making a microfluidic valve. This method includes: forming a microfluidic channel enclosed by channel walls 310; forming an electric initiator including an electrically conductive layer at a channel wall of the microfluidic channel 320; and placing a solid gas-generating material over the electrically conductive layer such that the solid gas-generating material is in the microfluidic channel but not blocking the microfluidic channel, wherein the solid gas-generating material is chemically reactive to form a gas, and wherein the electric initiator is configured to initiate a chemical reaction of the solid gas-generating material to form the gas when an electric current is applied to the electric initiator 330. [0062] In further examples, methods of making microfluidic valves can include forming any of the features described above, such as push resistors, gas vents, gas absorbing materials, heaters, pinch points, barrier layers, and so on. Additionally, the features of the microfluidic valves can be formed using any techniques described herein. [0063]The microfluidic devices described are not limited to being formed by any particular process. However, in some examples, any of the microfluidic devices described herein can be formed from multiple layers of material. In certain examples, the one or multiple of the layers can be formed photolithographically using a photoresist. In one such example, the layers can be formed from an epoxy-based photoresist, such as SU-8 or SU-82000 photoresist, which are epoxy-based negative photoresists. Specifically, SU-8 and SU-82000 are Bisphenol A Novolac epoxy-based photoresists that are available from various sources, including MicroChem Corp. These materials can be exposed to UV light to become crosslinked, while portions that are unexposed can remain soluble in a solvent and can be washed away to leave voids. [0064] In some examples, microfluidic devices can be formed on a substrate formed of a silicon material. For example, the substrate can be formed of single crystalline silicon, polycrystalline silicon, gallium arsenide, glass, silica, ceramics, plastic, epoxy, a printed circuit board, or a semiconducting material. In a particular example, the substrate can have a thickness from about 200 ^m to about 1200 ^m. In certain examples, channels or holes can be formed in the silicon substrate by laser machining and/or chemical etching. [0065] In further examples, a layer of photoresist can be formed or placed on the substrate and patterns to form the microfluidic channel and other microfluidic features such as the fluid reservoir described above. For example, a layer of photoresist can be exposed to a pattern of UV light that defines the microfluidic channel walls and walls of the fluid reservoir. If the microfluidic channel includes pinch points, gas bubble chambers, or other such features then these features can be a part of the pattern. After exposure, any unexposed photoresist can be washed away. In some examples, this layer of photoresist can have a thickness from about 2 ^m to 100 ^m. The microfluidic channels can be formed having a width from about 2 ^m to about 200 ^m, from about 10 ^m to about 50 ^m, or from about 20 ^m to about 35 ^m in some examples. [0066] In further examples, a top layer can be formed over the layer defining the microfluidic channels. This top layer can form the ceiling of the microfluidic channels. In some examples, the top layer can be formed by laminating a dry film photoresist over the microfluidic channel layer and exposing the dry film photoresist with a UV pattern defining any features of the top layer. For example, gas vents can be formed by using a pattern that leaves a small opening for the gas vents uncured. In other examples, the top layer can be a substantially solid layer without any openings. The top layer can have a thickness from about 2 ^m to about 200 ^m. [0067]Some other methods of forming the top layer, such as using a lost wax method, can utilize additional ports in the top layer. For example, in a lost wax method, the microfluidic channels can be filled with a wax before applying the top layer. The wax can then be removed by chemical etching from the microfluidic channels. However, in some cases wax can be removed up to a finite distance away from a port. Therefore, multiple ports in the top layer may be used so that all of the wax can be removed. [0068] It is to be understood that this disclosure is not limited to the particular process operations and materials disclosed herein because such process operations and materials may vary somewhat. It is also to be understood that the terminology used herein is used for the purpose of describing particular examples. The terms are not intended to be limiting because the scope of the present disclosure is intended to be limited by the appended claims and equivalents thereof. [0069] It is noted that, as used in this specification and the appended claims, the singular forms ”a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. [0070]As used herein, the term “substantial” or “substantially” when used in reference to a quantity or amount of a material, or a specific characteristic thereof, refers to an amount that is sufficient to provide an effect that the material or characteristic was intended to provide. The exact degree of deviation allowable may in some cases depend on the specific context. [0071]As used herein, the term “about” is used to provide flexibility to a numerical range endpoint by providing that a given value may be “a little above” or “a little below” the endpoint. The degree of flexibility of this term can be dictated by the particular variable and determined based on the associated description herein. [0072]As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though members of the list are individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. [0073]Concentrations, amounts, and other numerical data may be expressed or presented herein in a range format. It is to be understood that such a range format is used merely for convenience and brevity and thus should be interpreted flexibly to include the numerical values explicitly recited as the limits of the range, and also to include individual numerical values or sub ranges encompassed within that range as if the numerical values and sub-ranges are explicitly recited. As an illustration, a numerical range of “about 1 wt% to about 5 wt%” should be interpreted to include the explicitly recited values of about 1 wt% to about 5 wt%, and also include individual values and sub-ranges within the indicated range. Thus, included in this numerical range are individual values such as 2, 3.5, and 4 and sub-ranges such as from 1-3, from 2-4, and from 3-5, etc. This same principle applies to ranges reciting one numerical value. Furthermore, such an interpretation should apply regardless of the breadth of the range or the characteristics being described. EXAMPLE [0074]The following illustrates an example of the present disclosure. However, it is to be understood that the following is merely illustrative of the application of the principles of the present disclosure. Numerous modifications and alternative compositions, methods, and systems may be devised without departing from the spirit and scope of the present disclosure. The appended claims are intended to cover such modifications and arrangements. Microfluidic Valve [0075]A microfluidic valve is constructed similar to the design shown in FIG.2A. First, a silicon substrate is used as a bottom layer. Two thermal resistors are formed by depositing layers of metal on the silicon substrate. The layers of metal have a length and width of 30 ^m and a thickness of 0.1 ^m. Electric connections are formed to the thermal resistors, and the electric connections are connected to a power source that can supply electric current to the thermal resistors. A primer layer of SU-8 photoresist is then spin coated onto the substrate, with a thickness of about 4 ^m. The primer layer is removed from over the thermal resistors. A microfluidic layer is formed on the primer layer. First, a 17 ^m thick layer of SU-8 is spin coated onto the primer layer. Next, a 14 ^m thick dry photoresist layer is laminated onto the previous layer. The dry layer is exposed to a UV pattern that includes a microfluidic channel with a width of 50 m. The microfluidic channel is oriented so that the thermal resistors are on a floor of the microfluidic channel. The photoresist is then developed by dissolving unexposed portions of the photoresist. A layer of solid gas-generating material is then deposited over one of the thermal resistors. In this example, the solid gas- generating material is benzoyl peroxide mixed with epoxy paste. The thermal resistor can act as an electric initiator to cause a decomposition reaction of the benzoyl peroxide, forming oxygen gas. The other thermal resistor can act as a push resistor to push the gas bubble toward a gas vent when it is desired to re- open the valve. A top layer is then formed by laminating a 14 ^m thick dry photoresist layer over the microfluidic layer. The top layer is exposed to a UV- light pattern defining the gas vent opening. The top layer is then developed by dissolving the unexposed portions. [0076]While the present technology has been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the disclosure. It is intended, therefore, that the disclosure be limited by the scope of the following claims.

Claims

CLAIMS What is claimed is: 1. A microfluidic valve comprising: a microfluidic channel; a solid gas-generating material in the microfluidic channel, wherein the solid gas-generating material is chemically reactive to form a gas; and an electric initiator adjacent to the solid gas-generating material and configured to initiate a chemical reaction of the solid gas-generating material to form the gas when an electric current is applied to the electric initiator.

2. The microfluidic valve of claim 1, wherein the electric initiator comprises a thermal resistor or a spark gap.

3. The microfluidic valve of claim 1, wherein the electric initiator comprises an electrically conductive layer formed at a wall of the microfluidic channel and wherein the solid gas-generating material is formed as a layer over the electrically conductive layer.

4. The microfluidic valve of claim 1, wherein the solid gas-generating material is chemically reactive to form a gas by a thermal decomposition reaction, or by a combustion reaction, or by a chemical reaction with a fluid in the microfluidic channel.

5. The microfluidic valve of claim 1, wherein the solid gas-generating material comprises an Azobis compound, a peroxide, a carbonate, a nitrate, a nitrite, an azide, nitrocellulose, or a combination thereof.

6. The microfluidic valve of claim 1, further comprising a barrier layer covering the solid gas-generating material to protect the solid gas-generating material from fluid in the microfluidic channel, wherein the barrier layer is degradable by the electric initiator.

7. The microfluidic valve of claim 1, further comprising a solid gas- absorbing material in the microfluidic channel adjacent to the solid gas- generating material.

8. The microfluidic valve of claim 1, wherein the microfluidic channel comprises a gas vent downstream or upstream of the solid gas-generating material to allow the gas to escape from the microfluidic channel.

9. The microfluidic valve of claim 1, wherein the microfluidic channel comprises a pinch point having a reduced width compared to a width of an adjacent portion of the microfluidic channel, wherein the pinch point is positioned to constrain a gas bubble generated by the solid gas-generating material.

10. A microfluidic device comprising: a fluid reservoir; a microfluidic channel connected to the fluid reservoir; a solid gas-generating material in the microfluidic channel; and an electric initiator adjacent to the solid gas-generating material and configured to cause the solid gas-generating material to form the gas when an electric current is applied to the electric initiator.

11. The microfluidic device of claim 10, further comprising an electric power source connected to the electric initiator, wherein the electric initiator comprises a thermal resistor or a spark gap.

12. The microfluidic device of claim 10, wherein the fluid reservoir and the microfluidic channel contain an aqueous liquid.

13. A method of making a microfluidic valve comprising: forming a microfluidic channel enclosed by channels walls; forming an electric initiator comprising an electrically conductive layer at a channel wall of the microfluidic channel; and placing a solid gas generating material over the electrically conductive layer such that the solid gas-generating material is in the microfluidic channel but not blocking the microfluidic channel, wherein the solid gas-generating material is chemically reactive to form a gas, and wherein the electric initiator is configured to initiate a chemical reaction of the solid gas-generating material to form the gas when an electric current is applied to the electric initiator.

14. The method of claim 13, further comprising placing a solid gas- absorbing material in the microfluidic channel adjacent to the solid gas- generating material.

15. The method of claim 13, wherein forming the microfluidic channel comprises forming a pinch point having a reduced width compared to a width of an adjacent portion of the microfluidic channel, wherein the pinch point is positioned to constrain a gas bubble generated by the solid gas-generating material.