WO2022191832A1

WO2022191832A1 - Microfluidic sample compartment arrays

Info

Publication number: WO2022191832A1
Application number: PCT/US2021/021671
Authority: WO
Inventors: Viktor Shkolnikov; Alexander Govyadinov
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2021-03-10
Filing date: 2021-03-10
Publication date: 2022-09-15

Abstract

The present disclosure is drawn to microfluidic sample compartment arrays and digital nucleic acid amplification systems. In one example, a microfluidic sample compartment array can include a microfluidic channel, an array of gas-generating elements in the microfluidic channel, and an array of electric initiators, where individual electric initiators are adjacent to individual gas-generating elements. The gas-generating elements can include a solid gas-generating material in the microfluidic channel but not blocking the microfluidic channel. The solid gas-generating material can be chemically reactive to form a gas. The electric initiators 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 initiators.

Description

MICROFLUIDIC SAMPLE COMPARTMENT ARRAYS 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-1C are schematic cross-sectional side views of an example microfluidic sample compartment array in accordance with the present disclosure; [0004] FIGs.2A-2C are schematic cross-sectional top views of another example microfluidic sample compartment array in accordance with the present disclosure; [0005] FIGs.3A-3B are schematic cross-sectional top views of another example microfluidic sample compartment array in accordance with the present disclosure; [0006] FIGs.4A-4F are schematic cross-sectional top views of yet another example microfluidic sample compartment array in accordance with the present disclosure [0007] FIGs.5A-5F are schematic cross-sectional side views of additional example microfluidic sample compartment arrays in accordance with the present disclosure; [0008] FIG.6 is a schematic cross-sectional top view of an example digital nucleic acid amplification system in accordance with the present disclosure; [0009] FIG.7 is a schematic cross-sectional top view of another example digital nucleic acid amplification system in accordance with the present disclosure; [0010] FIG.8 is a schematic cross-sectional top view of yet another example digital nucleic acid amplification system in accordance with the present disclosure; and [0011] FIG.9 is a flowchart illustrating an example method of making a digital nucleic acid amplification system 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 sample compartment arrays and digital nucleic acid amplification systems that make use of such microfluidic sample compartment arrays. In one example, a microfluidic sample compartment array includes a microfluidic channel, an array of gas- generating elements in the microfluidic channel, and an array of electric initiators. The gas-generating elements include a solid gas-generating material in the microfluidic channel. The solid gas-generating material is chemically reactive to form a gas. The array of electric initiators includes individual electric initiators that are adjacent to individual gas-generating elements. The electric initiators are 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 initiators. In some examples, the solid gas-generating material can be positioned in the microfluidic channel such that the solid gas-generating material does not block the microfluidic channel. In further examples, the gas-generating elements can be spaced along a length of the microfluidic channel to form a 1-dimensional array of fluid compartments separated by gas barriers when the solid gas-generating material reacts to form the gas. In certain examples, the fluid compartments can have a volume from about 1 picoliter to about 1 microliter. In other examples, the gas-generating elements can be spaced along a length and a width of the microfluidic channel to form a 2-dimensional array of fluid compartments separated by gas barriers when the solid gas-generating material reacts to form the gas. In certain examples, the microfluidic channel can include pinch points between individual gas-generating components. The pinch points can have a reduced width and/or height compared to an adjacent portion of the microfluidic channel. The pinch points can be positioned to constrain gas bubbles generated by the solid gas-generating material. In further examples, the electric initiators can include a thermal resistor or a spark gap. In some 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. [0014] The present disclosure also describes digital nucleic acid amplification systems. In one example, a digital nucleic acid amplification system includes a microfluidic channel, a nucleic acid sample inlet into the microfluidic channel, an array of gas-generating elements in the microfluidic channel and an array of electric initiators. The gas-generating elements include a solid gas- generating material in the microfluidic channel but not blocking the microfluidic channel. The solid gas-generating material is chemically reactive to form a gas. The array of electric initiators includes individual electric initiators that are adjacent to individual gas-generating elements. The electric initiators are 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 initiators. The gas- generating elements are spaced apart such that the gas forms an array of fluid compartments of the nucleic acid sample separated by gas barriers. In some examples, the fluid compartments of the nucleic acid sample can have a volume from about 1 picoliter to about 1 microliter. In further examples, the array of fluid compartments of the nucleic acid sample can be a 1-dimensional array or a 2- dimensional array. In still further examples, the system can also include a primer immobilized on an interior surface of the microfluidic channel. In certain examples, the system can also include a second microfluidic channel, a second array of gas-generating elements in the second microchannel, and a second array of electric initiators adjacent to the second array of gas-generating elements, wherein a second primer is immobilized on an interior surface of the second microfluidic channel. [0015] The present disclosure also describes methods of making digital nucleic acid amplification systems. In one example, a method of making a digital nucleic acid amplification system includes forming a microfluidic channel enclosed by channel walls and having a nucleic acid sample inlet; forming an array of electric initiators including an electrically conductive layer at a channel wall of the microfluidic channel; and forming an array of gas-generating elements over the array of electric initiators, wherein the gas-generating elements include a solid gas-generating material 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 initiators are 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 initiators, and wherein the gas-generating elements are space apart such that the gas forms an array of fluid compartments of the nucleic acid sample separated by gas barriers. In further examples, the method can also include immobilizing a primer on an interior surface of the microfluidic channel. In still further examples, the channel walls can include pinch points between individual gas-generating components, wherein the pinch points have a reduced width and/or height compared to an adjacent portion of the microfluidic channel, wherein the pinch points are positioned to constrain gas bubbles generated by the solid gas-generating material. [0016] The microfluidic sample compartment arrays described herein can be useful for separating a small volume of sample fluid into a number of individual fluid compartments. Separating a sample into small volume fluid compartments in this way can be useful in a variety of applications. For example, the individual fluid compartments can act as reactors, in which some type of chemical reaction can take place. Separating a sample fluid into many individual fluid compartments can allow for reactions to take place in the fluid compartments individually, without any diffusion of mixing of materials between the individual fluid compartments. In further examples, separating a sample into many individual fluid compartments can be useful for counting a discrete number of a certain target in the sample fluid. [0017] A particular application of the technology described herein can involve using the microfluidic sample compartment arrays for nucleic acid amplification. In particular, digital nucleic acid amplification tests can be performed using the microfluidic sample compartment arrays. These tests can be capable of determining how many copies of a specific nucleic acid exist in a sample. Digital nucleic acid amplification tests can operate by diluting a sample containing a target nucleic acid and then dividing the sample into a number of small sample compartments. The dilution of the sample fluid and the volume of fluid in the individual sample compartments can be selected so that no more than a single copy of the target is expected to be in any one individual sample compartment (i.e., many of the sample compartments will not contain any copies of the target nucleic acid, and if a sample compartment does contain a copy of the target nucleic acid, then it will be statistically very unlikely for the sample compartment to contain more than one copy of the nucleic acid). A nucleic acid amplification process can be performed in all of the sample compartments. In various examples, the nucleic acid amplification process can include adding reagents such as master mix reagents and primers to the sample fluid. Various nucleic acid amplification process can also include specific temperature changes, such as holding at a particular temperature in isothermal nucleic acid amplification process, or cycling between different temperatures in temperature- cycling nucleic acid amplification process (such as PCR processes). After performed the nucleic acid amplification process, any individual sample compartments that originally contained a single copy of the target nucleic acid will now contain many copies of the target nucleic acid. These nucleic acids can be detected using various detections methods, such as adding fluorescent dyes that fluoresce in the presence of the nucleic acid. The number of individual sample compartments that contain the nucleic acid can be counted, providing a specific number of copies of the target nucleic acid that were present in the original sample. [0018] Multiple target nucleic acids can also be counted using a multiplexed system, such as a multiplex PCR system. Such a system can include multiple microfluidic sample compartment arrays as described herein. The individual sample compartment arrays can be configured to detect different target nucleic acids by mixing the nucleic acid sample with different primers in the individual sample compartment arrays. As described in more detail below, in some examples it can be convenient to immobilize primers in the microfluidic channel of the microfluidic sample compartment array before the nucleic acid sample fluid is introduced. Thus, the different primers used to amplify different target nucleic acids can be confined to their own individual microfluidic sample compartment arrays to eliminate risk of cross-contamination. [0019] In the context of the microfluidic sample compartment arrays described herein, the sample compartments are small volumes of sample fluid that are separated by gas barriers in a microfluidic channel. In one example, a sample fluid containing a number of copies of a target nucleic acid can be introduced into a microfluidic sample compartment array as described herein. The sample fluid can fill the microfluidic channel of the microfluidic sample compartment array. Then, the gas-generating elements mentioned above can be activated to form gas barriers separating individual compartments of sample fluid. The size of the fluid compartments and the dilution of the sample fluid can be selected so that the individual fluid compartments will contain no more than one copy of the target nucleic acid. The target nucleic acid can then be amplified using a suitable nucleic acid amplification technique, such as the PCR nucleic acid amplification process, or an isothermal nucleic acid amplification process. In some examples, reagents for performing the nucleic acid amplification technique can be added to the sample fluid before the sample fluid is loaded into the microfluidic sample compartment array. Alternatively, reagents used in nucleic acid amplification can be stored in the microfluidic sample compartment array before the sample fluid is loaded. As mentioned above, in some examples primers can be immobilized within the microfluidic channel before the sample fluid is introduced into the channel. Additionally, in some examples, master mix reagents can be mixed with the nucleic acid sample before introducing the sample into the channel. The nucleic acid sample itself can include purified nucleic acid provided by a separate sample purification system, in some examples. Thus, the nucleic acid and reagents, such as master mix reagents and primers, can all be present in a mixture within the microfluidic channel. This mixture can be divided into many small fluid compartments using gas barriers as described herein. Then, through the particular nucleic acid amplification process that has been selected, the mixture of nucleic acid and reagents can react to produce many more copies of the target nucleic acid. The target nucleic acid can then be detected using a fluorescence detector or other detection device. As mentioned above, the individual fluid compartments can be separate one from another. Therefore, the compartments containing a copy of the target nucleic acid from the sample fluid can be counted by detecting the amplified nucleic acids in these compartments. As mentioned above, the initial concentration of the nucleic acid in the sample fluid can be relatively small, so that no more than one copy of the nucleic acid is statistically likely to be present in any individual fluid compartment in the microfluidic sample compartment array. Again, the fluid compartments refer to the small, separate volumes of fluid that are separated by the gas barriers. When the sample fluid is loaded into the microfluidic sample compartment array and separated into many small fluid compartments, some number of the fluid compartments can initially contain a single copy of the nucleic acid. The remaining fluid compartments do not contain any copies of the nucleic acid. After the nucleic acid amplification process has been completed, the fluid compartments that initially contained a single copy of the nucleic acid will contain many copies of the nucleic acid. However, the remaining compartments will still not contain any copies of the nucleic acid. Therefore, the total number of copies of the nucleic that were present in the original sample can be determined by counting the number of fluid compartments that contain many copies of the nucleic acid. In some examples, the nucleic acids can be detected by including fluorescent dyes that increase in fluorescence in the presence of the nucleic acid. These dyes can be detected using an optical detector. This can be referred to as a “digital” nucleic acid amplification test because the test is capable of counting a discrete number of nucleic acids in the sample fluid. [0020] Other types of nucleic acid amplification tests can include digital droplet nucleic acid amplification, such as droplet PCR processes, and quantitative PCR, or “real-time PCR” processes. Digital droplet nucleic acid amplification involves forming many small droplets of the sample fluid. These droplets function as individual fluid compartments, similar to the fluid compartments in the microfluidic sample compartment arrays described herein. In quantitative PCR, the fluorescence of the sample (which includes fluorescent dye as described above) is measured during the nucleic acid amplification process, instead of at the end of the process. Observing the rate of increase in the fluorescence can allow the initial quantity of the target nucleic acid to be estimated. In comparison with these other process, the systems described herein can be easier to use, more accurate, simpler, and cheaper than these other processes that have been used previously. For example, in digital droplet nucleic acid amplification, generating a large number of small droplets and incorporating the appropriate reagents into the droplets is not trivial, and often represents a large part of the total cost of such systems. Furthermore, the systems described herein can provide a true digital count of the nucleic acids in the initial sample. This can be more accurate that quantitative PCR in some examples. [0021] The systems described herein can be simpler, and do not include droplet generators because fluid compartments are used instead of droplets. As mentioned above, a sample fluid is loaded into a microfluidic channel that includes an array of gas-generating elements. The gas-generating elements are then activated to form gas barriers that separate individual volumes of the sample fluid, which are referred to as fluid compartments. Any reagents that are used for nucleic acid amplification can be mixed into the sample fluid before loading the sample fluid into the microfluidic channel. Alternatively, reagents can be placed in the microfluidic channel (by immobilizing the reagents on an interior surface of the microfluidic channel, for example) before the sample fluid is loaded. A variety of reagents can be involved in the nucleic acid amplification process, including nucleic acid monomers, polymerase, fluorescent probes, buffering agents, and other reagents. In some examples, it can be easier to combine these reagents with the sample fluid using the systems described herein than ensuring that the appropriate reagents are present in the individual droplets formed using a droplet generator. [0022] In further detail, after the sample fluid has been loaded and separated into many individual fluid compartments, a nucleic acid amplification process such as a polymerase chain reaction (PCR) assay or another type of amplification process can be performed. PCR assays are processes that can rapidly copy millions to billions of copies of a very small nucleic acid sample, such as DNA or RNA. In the PCR process, nucleic acid monomers can react to form many copies of the target nucleic acid. Therefore, if a single copy of the target nucleic acid was originally in one of the fluid compartments, then the PCR process can create many more copies of that target nucleic acid in that particular fluid compartment. However, any fluid compartments that did not contain a nucleic acid will still contain no nucleic acids after the PCR process. The reagents can also include fluorescent probes that increase in fluorescence when they intercalate nucleic acids. An optical sensor can be used to detect the fluorescence of the fluorescent probes. [0023] If it is desired to detect multiple different target nucleic acids, then a multiplexed system with multiple microfluidic sample arrays can be used. The individual microfluidic sample arrays can be used to detect different target nucleic acids by mixing different primers with the sample fluid in the individual microfluidic sample arrays. Thus, the different target nucleic acids can be amplified and detected simultaneously by detecting the number of fluid compartments containing amplified nucleic acid in the individual arrays. [0024] In further examples, the microfluidic sample compartment arrays described herein can be used in other applications such as cell growth assays, cell counting, and others. [0025] With this description in mind, FIG.1A shows a side cross-sectional view of one example microfluidic sample compartment array 100. This example includes a microfluidic channel 110, an array of gas-generating elements 120 in the microfluidic channel, and an array of electric initiators 130. The gas- generating elements include a solid gas-generating material that is in the microfluidic channel. In this example, the solid gas generating material is positioned so that the microfluidic channel is not blocked. Therefore, a sample fluid can flow into the microfluidic channel past the gas-generating elements. The solid gas-generating material can be chemically reactive to form a gas. The individual electric initiators are adjacent to individual gas-generating elements. The electric initiators are 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 initiators. The individual electric initiators can be individually activated to cause the adjacent gas-generating element to generate gas. [0026] FIG.1B shows a cross-sectional top view of the same example microfluidic sample compartment array 100. The array of electric initiators 130 and the array of gas-generating elements 120 are seen on the floor of the microfluidic channel 110. [0027] FIG.1C shows the example microfluidic sample compartment array 100 after the microfluidic channel 110 has been filled with sample fluid 102 and the gas-generating elements have been activated. The gas generated by the gas- generating elements forms gas barriers 122 that separate small volumes of sample fluid one from another. The small volumes of sample fluid can be referred to as fluid compartments. In this example, the gas-generating elements are spaced along a length of the microfluidic channel. When the gas-generating elements are activated, gas barriers form. The gas barrier in this example extends across the entire width of the microfluidic channel so that the sample fluid in the fluid compartments is prevented from flowing to other compartments. The gas barriers also stop or drastically reduce diffusion of molecules from one fluid compartment to the next. When the gas-generating elements are lined up along one line as in this example, the array of fluid compartments that is formed can be a 1-dimensional array. [0028] The volume of the fluid compartments can be determined by size of the microfluidic channel and the distance between gas barriers. Various fluid compartment volumes can be useful for various applications. In some examples, the fluid compartments can have a volume of 1 picoliter to 1 microliter per fluid compartment. In further examples, the fluid compartments can have a volume of 1 picoliter to 100 nanoliters. In still further examples, the fluid compartments can have a volume of 10 picoliters to 10 nanoliters. [0029] A variety of example microfluidic sample compartment arrays having various designs are described herein. In these examples, gas barriers can be used to separate individual fluid compartments one from another within a microfluidic channel. As shown in FIGs.1A-1C, the gas barriers can be formed using gas-generating elements and electric initiators. In some examples, the gas- generating elements can include a thin layer of a solid gas-generating material deposited inside the microfluidic channel. In certain examples, the thin layer of solid gas-generating material can be deposited on a floor, ceiling, or side wall of the microfluidic channel. Electric initiators can be formed adjacent to the solid gas-generating material. In some examples, the electric initiators can be thermal resistors or spark gaps located just next to or under the solid gas-generating material. These electric initiators 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 gas bubble can act as a barrier to separate fluid compartments one from another. [0030] In various examples, the electric initiators 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 an electric initiator. The electric initiators can be individually addressable to allow individual gas-generating elements to be activated on command. In some examples, the chemical reaction of the solid gas-generating material 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, 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 μm to 500 μm, or from 1 μm to 100 μm, or from 1 μm to 50 μm, or from 1 μm to 35 μm. A thin film thermal resistor can have a relatively small thickness, such as from 1 nm to 5 μm, or from 1 nm to 1 μm, 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 μm to 100 μm, or from 1 μm to 50 μm, or from 1 μm to 20 μm. [0031] 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, 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 μm to 100 μm, or from 1 μm to 50 μm, or from 1 μm to 35 μm, or from 1 μm to 20 μm, or from 1 μm to 10 μm. 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. [0032] 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. [0033] 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. [0034] 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. 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, such as organic compounds, and oxidizer mixture that is ignited. 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. These various examples are described in more detail below. [0035] 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. [0036] 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. [0037] 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 caesium carbonate. [0038] 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. [0039] 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. However, in some examples, the gas bubbles can last long enough so that the microfluidic sample compartment array can be used for a desired process, such as a nucleic acid amplification process. [0040] 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 systems 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. [0041] 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 μm, or from 100 nm to 5 μm, or from 1 μm to 5 μm, or from 100 nm to 1 μm. Other dimensions of the solid gas-generating material can be selected to provide a sufficient amount of the material to produce a gas barrier that can separate fluid compartments one from another. In various examples, the solid gas-generating material can have a width or a length that is from 1 μm to 5mm, or from 1 μm to 1 mm, or from 1 μm to 500 μm, or from 1 μm to 100 μm, or from 1 μm to 50 μm, or from 1 μm to 35 μm. [0042] 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 sample compartment arrays and nucleic acid amplification systems 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. [0043] The microfluidic sample compartment arrays described herein can have a variety of designs, including 1-dimensional and 2-dimensional arrays. Another example microfluidic sample compartment array 100 is shown in FIG. 2A. This figure shows a top-down cross-sectional view of the array. The array in this example includes multiple microfluidic channels 110. Multiple gas-generating elements 120 and electric initiators 130 are spaced apart along the individual microfluidic channels. In this example, the electric initiators and gas generating elements are formed as thin layers on the floor of the microfluidic channels. [0044] FIG.2B shows the microfluidic sample compartment array 100 after fluid 102 has been loaded into an inlet end of the microfluidic channels 110. Additionally, a first set of gas-generating elements has been activated near the inlet end to form gas barriers 122 in the individual microfluidic channels. When the gas barriers form, a volume of sample fluid is displaced by the gas. Thus, a sufficient amount of fluid was loaded into the microfluidic channels to partially fill the microfluidic channels so that some empty volume is left in the microfluidic channels. The gas-generating elements can be activated one at a time, starting from the inlet end of the microfluidic channels. When a gas-generating element is activated, the sample fluid in the microfluidic channel can be pushed farther down the microfluidic channel, with the exception of the volume of fluid that is trapped in a compartment upstream of the gas barrier. FIG.2C shows the microfluidic sample compartment array at the end of the process, after all of the gas- generating elements have been activated to form gas barriers. This example forms a 2-dimensional array of fluid compartments, which are separated both by gas barriers and by microfluidic channel walls. [0045] In certain examples, a microfluidic array such as the example shown in FIGs.2A-2C can be used with a sample fluid that includes a target nucleic acid. The sample fluid can be mixed with reagents for a nucleic acid amplification process, and the mixture can be loaded into the microfluidic array as shown in FIG.2B. The fluid can then be partitioned into an array of small fluid compartments as shown in FIG.2C. The sample fluid can be diluted appropriately so that no more than one copy of the nucleic acid is likely to be in any individual fluid compartment. After partitioning the fluid compartments, a nucleic acid amplification process can be performed, after which any compartments that contained a nucleic acid will contain many copies of the nucleic acid. As explained above, the presence of the nucleic acids can be detected using fluorescent dyes that can fluoresce in the presence of the nucleic acids. Accordingly, the number of fluid compartments in which fluorescence is detected can correspond to the number of copies of the nucleic acid that were present in the original sample fluid. [0046] The gas barriers that separate fluid compartments can be bubbles of gas produced by the solid gas-generating material, as explained above. The gas barriers can fill a volume of the microfluidic channel so that fluid compartments on either side of the gas barrier are fluidically separated one from another. In other words, there is no fluid connecting the fluid compartments to one another and molecules do not diffuse from one fluid compartment to another in the liquid phase (a very small amount of material may diffuse through the gas phase across the gas barrier, but this is likely to be mostly small molecules such as water and not large molecules such as nucleic acids). In certain examples, the microfluidic channel can be designed with pinch points to help keep the gas barriers in a particular location. The pinch points can have a reduced width or height compared to the width or height of adjacent portions of the microfluidic channel. In some examples, both the width and height 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. However, other examples can include features formed on the floor or ceiling of the microfluidic channel that reduce the height of the microfluidic channel. Thus, pinch points can also be made in which the height of the microfluidic channel is reduced. [0047] FIG.3A shows one example microfluidic sample compartment array 100 that includes pinch points 112 to help constrain gas barriers. This example includes a single microfluidic channel 110 that has a serpentine shape. The microfluidic channel is designed having gas chambers 114 and liquid chambers 116 connected by narrower sections of microfluidic channel. Thus, the narrower sections act as pinch points. An array of electric initiators 130 and gas-generating elements 120 are formed as thin layers on the floor of the gas chambers. [0048] FIG.3B shows the same example microfluidic sample compartment array 100 after a fluid 102 has been loaded into the microfluidic channel 110 and the gas elements have been activated to form gas barriers 122. The gas elements can be activated one at a time, starting near the inlet end of the microfluidic channel. This can allow the individual gas bubbles formed from the gas-generating elements to displace the fluid downstream before the next gas bubble blocks the fluid. The pinch points can keep the gas barriers in place through capillary force. Capillary forces can be relatively larger in the narrowed pinch points. Therefore, the gas barriers can be maintained in place even if pressure is applied to the fluid in one direction or another. [0049] FIG.4A shows a top cross-sectional view of yet another example microfluidic sample compartment array 100. This example includes a single microfluidic channel 110 that is wide and flat. For example, the width of the microfluidic channel can be 10 times the height (not shown) or more. This microfluidic channel includes a pattern of gas-generating elements 120 and electric initiators 130 on the floor of the channel. This pattern is designed to form a network of gas barriers that can partition fluid in the microfluidic channel into a 2-dimensional array. [0050] FIG.4B shows the microfluidic sample compartment array 100 partially filled with sample fluid 102. Again, the microfluidic channel 110 is partially filled because the gas barriers will displace a volume of fluid. Therefore, some of the fluid will be pushed further downstream by the formation of the gas barriers. It is noted that it is also possible to fully fill the microfluidic channel with sample fluid before forming the gas barriers. If there is an outlet downstream then displaced sample fluid can flow out the outlet. However, this can waste a portion of the sample fluid. [0051] FIG.4C shows the example microfluidic sample compartment array 110 after a first set of gas-generating elements 120 have been activated to generate a gas barrier 122. The formation of the gas barrier displaces some of the sample fluid 102 and pushes the fluid farther downstream. In more detail, the first set of gas-generating elements can be divided into two subsets. First, a single row of gas-generating elements that are oriented across the width of the microfluidic channel can be activated to form a gas barrier that extends across the width of the microfluidic channel. Then, the gas generating elements that are oriented lengthwise along the length of the microfluidic channel can be activated. This can form gas barrier sections that extend lengthwise along the microfluidic channel. [0052] FIG.4D shows the microfluidic sample compartment array 100 after a second set of gas-generating elements 120 have been activated. This adds to the gas barrier 122. At this point, a row of fluid compartments have been completely isolated from the other sample fluid 102 in the microfluidic channel 110. Additionally, the sample fluid downstream of these fluid compartments is pushed farther downstream by the formation of the gas barriers. [0053] FIG.4E shows the example microfluidic sample compartment array 100 after a third set of gas-generating elements 120 have been activated. The gas-generating elements react to form gas bubbles that add to the gas barrier 122. The sample fluid downstream of the gas barrier is again pushed farther downstream. [0054] FIG.4F shows the example microfluidic sample compartment array after the remaining gas-generating elements have been activated. A 2- dimensional array of fluid compartments is formed, with gas barriers 122 separating the fluid compartments. This example differs from the example shown in FIG.2C because in that example, the fluid compartments were separated by microfluidic walls and by gas barriers. In contrast, in this example there are no microfluidic walls separating the fluid compartments one from another. Instead, the fluid compartments are separately solely by the gas barriers. [0055] In 2-dimensional arrays like the example shown in FIGs.4A-4F, it can be difficult to maintain the shape and placement of the gas barriers. For example, surface tension forces often tend to simply the gas barrier shape into a single large gas bubble instead of a network of gas barrier separating individual fluid compartments. In some examples, pinch points can be used to help constrain the gas barriers and the fluid compartments and keep the gas barriers and fluid compartments from moving out of place. In certain examples, the pinch points can be made by forming protrusions from the floor of the microfluidic channel and/or from the ceiling of the microfluidic channel. A protrusion from the floor or ceiling can create an area having a smaller height dimension compared with the rest of the microfluidic channel. In some examples, the area having the smaller height can tend to hold liquid within the area, because capillary forces in that area can be greater than in the surrounding areas having a greater height. Accordingly, pinch points formed using protrusions from the floor and/or ceiling can be used to help constrain the fluid compartments and gas barriers. [0056] FIG.5A shows a cross-sectional side view of an example microfluidic sample compartment array 100 that includes pinch points formed as protrusions 118 from the ceiling of the microfluidic channel 110. These pinch points constrain gas barriers 122 and the sample fluid 102 in individual fluid compartments. The cross-sectional view shown in FIG.5A represents a cross- section of a single row of fluid compartments. The electric initiators 130 (such as thermal resistors or spark gaps, for example) that were used to activate gas- generating elements to form the gas barriers are shown as a thin layer on the floor of the microfluidic channel. It is noted that the single row shown in FIG.5A can be repeated many times across the width of the array to make a 2- dimensional array similar to the example shown in FIGs.4A-4F. Thus, the pinch points illustrated in FIG.5A are one example of pinch points that can be used in a 2-dimensional array as in FIGs.4A-4F to constrain the gas barriers and fluid compartments in place. [0057] FIG.5B shows another example microfluidic sample compartment array 100 with a different pinch point design. In this example, the pinch points are formed as protrusions 118 from the floor of the microfluidic channel 110 instead of the ceiling. In this example and the example of FIG.5A, the protrusions can have the form or square or rectangular shapes that extend from the floor or ceiling. The fluid compartments can be located in the pinch points, while a gas barrier surrounds the pinch points in the areas of the microfluidic channel having a greater height. [0058] FIG.5C is a cross-sectional side view of another example microfluidic sample compartment array 100 having a different pinch point design. This example includes pinch points formed as convex curved protrusions 118 from the floor of the microfluidic channel 110. Another design in shown in FIG. 5D. This example includes pinch points formed as concave protrusions 118 from the floor of the microfluidic channel 110. It is noted that in other examples, such convex or concave protrusions can extend from the ceiling instead of the floor. In further examples, such protrusions can extend from both the ceiling and floor at the same time. [0059] FIG.5E shows yet another example microfluidic sample compartment array 100. In this example, pinch points are formed as triangular protrusions 118 on the floor of the microfluidic channel 110. This can be small individual pointed protrusions, or ridges having a triangular cross-section. For example, the fluid compartments can be surrounded by ridges having the triangular cross-section shown in FIG.5E. FIG.5F shows another example in which the pinch points are formed as protrusions 118 that are shaped as small walls having a rectangular cross-section. The walls can surround the fluid compartments. In this example, the walls protrude from both the floor and the ceiling. However, in other examples, the walls can be either on the floor or on the ceiling. [0060] In any of the above examples, the protrusions can have a variety of shapes and sizes. For example, the protrusions can be square, rectangular, triangular, circular, or another shape. The width and length of the protrusions can be selected depending on the desired spacing between gas barriers. In some examples, the length and/or width of the protrusions can be from 1 μm to 1 mm, or from 1 μm to 500 μm, or from 1 μm to 100 μm, or from 1 μm to 50 μm, or another distance. The height of the protrusions (i.e., the distance that the protrusions protrude down from the ceiling or up from the floor of the microfluidic channel) can be from 1% to 90% of the height of the microfluidic channel, or from 1% to 75% of the height of the microfluidic channel, or from 1% to 50% of the height of the microfluidic channel, or from 1% to 35% of the height of the microfluidic channel, or from 1% to 20% of the height of the microfluidic channel, in some examples. In further examples, the protrusions can have a height from 1 μm to 500 μm, or from 1 μm to 100 μm, or from 1 μm to 50 μm, or from 1 μm to 35 μm, or from 1 μm to 20 μm. [0061] The present disclosure also describes digital nucleic acid amplification systems. The digital nucleic acid amplification systems can include a microfluidic sample compartment array as described above. Any of the specific examples described above can be included in a digital nucleic acid amplification system. In some examples, a digital nucleic acid amplification system can include a microfluidic channel, a nucleic acid sample inlet into the microfluidic channel, an array of gas-generating elements in the microfluidic channel, and an array of electric initiators adjacent to the gas-generating elements. The gas-generating elements can include a solid gas-generating material in the microfluidic channel but not blocking the microfluidic channel. The solid gas-generating material can be chemically reactive to form a gas. The electric initiators 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 initiators. The gas-generating elements can be spaced apart such that the gas forms an array of fluid compartments of the nucleic acid sample separated by gas barriers. [0062] In some examples, the digital nucleic acid amplification system can be configured to perform a PCR process. For such processes, a sample of a nucleic acid can be mixed with PCR master mix reagents. PCR master mix reagents can include a mixture of multiple compounds that are used in a PCR assay. These compounds can include DNA polymerase, nucleoside triphosphate, deoxyribose nucleoside triphosphate, magnesium chloride, magnesium sulfate, template DNA, forward primer, reverse primer, tris hydrochloride, potassium chloride, and others. Examples of commercially available PCR master mixes include TITANIUM TAQ ECODRY™ premix, ADVANTAGE 2 ECODRY™ premix (available from Takara Bio, Inc. Japan); Lyophilized Ready-to-Use and Load PCR Master Mix (available from Kerafast, Inc., USA); MAXIMO™ Dry-Master Mix (available from GenEon Technologies, USA), and others. In some examples, a sample fluid containing a nucleic acid can be introduced into the system through the nucleic acid sample inlet and a separate master mix inlet can be used to introduce the master mix reagents. In certain examples, the sample fluid and the master mix reagents can mix in the microfluidic channel before the gas barriers are formed. In other examples, a sample fluid can be pre-mixed with master mix reagents and the mixture can be introduced into the system through the nucleic acid sample inlet. [0063] The PCR process can involve thermal cycling, in which the temperature of sample fluid is repeatedly raised and lowered. In the systems described herein, thermal cyclic can be accomplished using thermal resistors. In some examples, the electric initiators can be thermal resistors. After using the thermal resistors to cause the solid gas-generating material to form the gas barriers, the same thermal resistors can then be used to heat the sample fluid for thermal cycling. In other examples, an additional set of thermal resistors or another type of heater can be included specifically for thermal cycling. In some cases, the thermal cycling can cause the gas barrier bubbles to expand and contract due to the temperature dependency of the density of the gas. However, in some examples the microfluidic channel can be hermetically sealed so that the volume of the gas barrier bubbles cannot change. When the microfluidic channel is sealed in this way, the gas barriers can change in pressure with the changing temperature, but the volume of the gas barriers can remain constant. [0064] In other examples, the digital nucleic acid amplification system can be configured to perform an isothermal nucleic acid amplification process. In such processes, the system can be brought up to the desired temperature before activating the gas-generating elements to form the gas barriers. In some cases, the electric initiators can be thermal resistors, and these thermal resistors can be used to bring the sample fluid up to the desired temperature for isothermal nucleic acid amplification. In other examples, a separate set of thermal resistors or other heaters can be included to heat the sample fluid to the desired temperature. Thus, the gas barriers can be at a constant temperature and remain at a constant volume during the amplification process. Some examples of nucleic acid amplification processes that can be performed include Nucleic Acid Sequence-Based Amplification (NASBA), Exponential Strand Displacement Amplification (E-SDA), Exponential Rolling Circle Amplification (E-RCA), Loop- Mediated Isothermal Amplification (LAMP), Helicase- Dependent Amplification (HDA), Recombinase Polymerase Amplification (RPA), Exponential Amplification Reaction (EXPAR) and others. [0065] As mentioned above, in some examples the electric initiators can be thermal resistors. In some cases, the thermal resistors can be used as heaters to heat the system to the appropriate temperature for a nucleic acid amplification process. Thus, the thermal resistors can have a dual function as heaters for the system and also to initiate the chemical reaction of the solid gas-generating material. In other examples, the system can include additional heaters such as additional thermal resistors to maintain the temperature of the system during the nucleic acid amplification process. Temperature sensors can also be included to help control the temperature of the system accurately. In some examples, thermal resistors can be used both as heaters and as temperature sensors. [0066] In further examples, primers can be introduced into the system for use in the nucleic acid amplification process. Primers are short single-stranded nucleic acids used in nucleic acid synthesis. In PCR processes, a pair of custom primers can be used to direct elongation of a nucleic acid being formed from opposite ends of the specific nucleic acid sequence that is being amplified. The primers can code for specific sites at either end of the sequence that is being amplified. Thus, specific pairs of primers can be selected to amplify a specific target nucleic acid sequence. In some examples, primers can be mixed with the sample fluid before the sample fluid is loaded into the system. In other examples, primers can be mixed with the sample fluid inside the system. In certain examples, primers can be applied to interior surfaces of the microfluidic channel before the sample fluid is introduced into the system. For example, after the microfluidic channel has been manufactured, the microfluidic channel can be filled with a solution of the desired primers. The primers can then be lyophilized or immobilized on the interior surfaces of the microfluidic channel. Thus, the primers can be included in a dry state within the microfluidic channel at the time of manufacture. In a certain examples, the primers can be immobilized on an interior surface of the microfluidic channel using linker molecules. The linker molecule can be a thermally labile linker, meaning that the linker can degrade or release the primer molecules at a certain temperature. Some examples of thermally labile linkers can include esters; sulfur-containing linkages such as disulfides, sulfonate, 5-membered cyclic dithiocarbonates, trithiocarbonates, or sulfites; nitrogen-containing linkages such as acylhydrazones, alkoxyamines, azlactones, Schiff base, hindered ureas, aminals, or carbamates; orthoesters, carbonates, acetals, hemiacetals, olefinic bonds, vicinal tricarbonyls, peroxide bonds, and others. Thus, the primer molecules can remain immobilized until heat is applied, such as when the system is heated up to an appropriate temperature for a nucleic acid amplification process. In various examples, the temperature of the fluid in the system during a nucleic acid amplification process can be from 35 °C to 90 °C, or from 35 °C to 60 °C, or from 40 °C to 50 °C. [0067] In some cases it can be useful to perform nucleic acid amplification with multiple different primers or sets of primers. The digital nucleic acid amplification systems described herein can be multiplexed to use multiple sets of primers by using multiple separate microfluidic channels. The individual microfluidic channels can be pretreated with different sets of primers. A sample fluid can then be loaded into the system, and the sample fluid can fill all the separate microfluidic channels. The microfluidic channels can include arrays of gas-generating elements and electric initiators as described herein. Thus, digital nucleic acid amplification processes can be easily performed with multiple primer sets at one time. [0068] FIG.6 is a top cross-sectional view of an example digital nucleic acid amplification system 200. The system includes a nucleic acid sample inlet channel 210 and an outlet channel 220 at the opposite end. Between the inlet and the outlet is a microfluidic channel 110 that is divided into multiple subchannels. An array of electric initiators 130 and an array of gas-generating elements 120 are located on the floor of the microfluidic channel. The microfluidic channel can be filled or partially filled with a sample fluid through the inlet channel, and then the gas-generating elements can be activated to form gas barriers separating fluid compartments as explained above. In some examples, the gas-generating elements can be activated in sequence starting with the elements nearest to the inlet channel as explained above. In further examples, a nucleic acid amplification reaction can take place within the fluid compartments, and a fluorescence detector can be used to detect fluorescent dyes intercalating nucleic acids as mentioned above. [0069] FIG.7 is a top cross-sectional view of another example digital nucleic acid amplification system 200. This system includes a microfluidic channel 110 that is divided into multiple subchannels, similar to the system shown in FIG.6. The microfluidic channel also includes an array of electric initiators and an array of gas-generating elements, however these are not shown in the figure for the sake of clarity. This example includes a nucleic acid sample inlet channel 210 and a master mix reagent inlet channel 230. Both the nucleic acid sample inlet channel and the master mix reagent inlet channel have a micropump 240 for pumping fluid into the system. The system also includes an outlet channel 220, which leads to an outlet reservoir 222. The outlet channel also includes a micropump for pumping fluid into the outlet reservoir. [0070] The micropumps used in the digital nucleic acid amplification system can include any suitable type of micropump. In some examples, the micropump can include a thermal resistor. Electric current can be applied to the thermal resistor to generate heat. The heat can evaporate fluid in the channels to momentarily form a vapor bubble. The vapor bubble can displace a volume of fluid. Then when the vapor bubble collapses as the thermal resistor is turned off, fluid can flow into the volume to replace the vapor bubble. The thermal resistor can be turned on and off repeatedly to repeatedly form vapor bubbles in this way. If the thermal resistor is placed in such a way that resistance to fluid flow in the channel is less on one side, then a net flow fluid in one direction can occur. Thus, the thermal resistor can be used as a micropump to pump fluids through microfluidic channels in the system. [0071] FIG.8 shows yet another example digital nucleic acid amplification system 200. This example is a multiplexed system that includes multiple microfluidic channels 110 capable of forming an array of fluid compartments. A nucleic acid sample reservoir 212 is located on one side of the system, and a master mix reagent reservoir 232 is located on the opposite side of the system. Nucleic acid sample inlet channels 210 and master mix reagent inlet channels 230 are connected between the respective reservoirs and the microfluidic channels 110. The microfluidic channels in this example are the same type as shown in FIG.6 and FIG.7, which are divided into multiple subchannels. An array of electric initiators and gas-generating elements are present in the microfluidic channels, but these are not shown for the sake of clarity. Outlet channels 220 lead from the microfluidic channels to outlet ports 224. The system also includes micropumps located in the inlet channels and outlet channels. The multiple microfluidic channels can be pre-loaded with different primer sets as explained above. For example, the primers can be immobilized on interior surfaces of the microfluidic channels using thermally labile linker molecules that can release the primers when heated to a certain temperature. [0072] The present disclosure also describes methods of making digital nucleic acid amplification systems. FIG.9 shows a flowchart of one example method of making a digital nucleic acid amplification system 300. The method includes: forming a microfluidic channel enclosed by channel walls and having a nucleic acid sample inlet 310; forming an array of electric initiators including an electrically conductive layer at a channel wall of the microfluidic channel 320; and forming an array of gas-generating elements over the array of electric initiators, wherein the gas-generating elements include a solid gas-generating material 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 initiators are 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 initiators, and wherein the gas-generating elements are spaced apart such that the gas forms an array of fluid compartments of the nucleic acid sample separated by gas barriers 330. [0073] In further examples, methods of making digital nucleic acid amplification systems can include forming any of the features described above, such as microfluidic subchannels, pinch points, micropumps, reservoirs, and so on. Additionally, the features of the digital nucleic acid amplification systems and microfluidic sample compartment arrays can be formed using any techniques described herein. [0074] The systems described herein are not limited to being formed by any particular process. However, in some examples, any of the devices and systems 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. [0075] In some examples, the devices and systems described herein 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 or a semiconducting material. In a particular example, the substrate can have a thickness from about 500 μm to about 1200 μm. In certain examples, channels or holes can be formed in the silicon substrate by laser machining and/or chemical etching. [0076] 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 described above. For example, a layer of photoresist can be exposed to a pattern of UV light that defines the microfluidic channel walls. 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 μm to 100 μm. Single microfluidic channels can be formed having a width from about 2 μm to about 100 μm, from about 10 μm to about 50 μm, or from about 20 μm to about 35 μm in some examples. In examples having wide microfluidic channels configured for 2- dimensional arrays of fluid compartments, the width can be much larger. In some examples, the width of such microfluidic channels can be from 10 μm to 5 cm, or from 10 μm to 1 cm, or from 10 μm to 5 mm. [0077] 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 μm to about 200 μm. [0078] In further examples, photoresist material can be used to form a microfluidic channel floor or ceiling having protrusions to form pinch points as described above. This can be accomplished by applying photoresist material to the floor or ceiling and patterning the photoresist with a pattern including the protrusions, and then removing the uncured photoresist material. [0079] 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 from the microfluidic channels. [0080] 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. [0081] 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. [0082] 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. [0083] 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. [0084] 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 the members of the list are individually identified as separate and unique members. 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. [0085]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 not only the numerical values explicitly recited as the limits of the range, but 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 not only the explicitly recited values of about 1 wt% to about 5 wt%, but 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 only one numerical value. Furthermore, such an interpretation should apply regardless of the breadth of the range or the characteristics being described. EXAMPLE [0086] 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 scope of the present disclosure. The appended claims are intended to cover such modifications and arrangements. Digital Nucleic Acid Amplification System [0087] A digital nucleic acid amplification system is constructed having the design shown in FIG.7. First, a silicon substrate is used as a bottom layer. Thermal resistors are formed by depositing layers of metal on the silicon substrate. The layers of metal have a length and width of 30 μm and a thickness of 5 μm. 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. One thermal resistor is positioned to be used as micropump for pumping a nucleic acid sample through a nucleic acid sample inlet channel. A second thermal resistor is positioned to be used as a micropump for pumping master mix reagents through a master mix reagent inlet channel. A third thermal resistor is positioned to be used as a micropump for pumping waste into an outlet reservoir. Finally, a 2-dimensional array of thermal resistors is positioned to be used as electric initiators for gas-generating elements. A primer layer of SU-8 photoresist is then spin coated onto the substrate, with a thickness of about 4 μm. A microfluidic layer is formed on the primer layer. First, a 17 μm thick layer of SU- 8 is spin coated onto the primer layer. Next, a 14 μm thick dry photoresist layer is laminated onto the previous layer. The dry layer is exposed to a UV pattern that includes a nucleic acid sample inlet channel, a master mix reagent inlet channel, an outlet channel, an outlet reservoir, and a wide microfluidic channel having interior channel walls to divide the channel into multiple subchannels as shown in FIG.7. The photoresist is then developed by dissolving unexposed portions of the photoresist. A layer of solid gas-generating material is then deposited over the array of electric initiators. In this example, the solid gas-generating material is benzoyl peroxide mixed with epoxy paste. The thermal resistors can act as electric initiators to cause a decomposition reaction of the benzoyl peroxide, forming oxygen gas. A top layer is then formed by laminating a 14 μm thick dry photoresist layer over the microfluidic layer. A reservoir of nucleic acid sample fluid can be connected to the nucleic acid sample inlet channel. Similarly, a reservoir of master mix reagent can be connected to the master mix reagent inlet channel. [0088] 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 only by the scope of the following claims.

Claims

CLAIMS What is claimed is: 1. A microfluidic sample compartment array comprising: a microfluidic channel; an array of gas-generating elements in the microfluidic channel, wherein the gas-generating elements comprise a solid gas-generating material in the microfluidic channel, wherein the solid gas-generating material is chemically reactive to form a gas; and an array of electric initiators, wherein individual electric initiators are adjacent to individual gas-generating elements, wherein the electric initiators are 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 initiators.

2. The microfluidic sample compartment array of claim 1, wherein the gas- generating elements are spaced along a length of the microfluidic channel to form a 1-dimensional array of fluid compartments separated by gas barriers when the solid gas-generating material reacts to form the gas.

3. The microfluidic sample compartment array of claim 2, wherein the fluid compartments have a volume from about 1 picoliter to about 1 microliter.

4. The microfluidic sample compartment array of claim 1, wherein the gas- generating elements are spaced along a length and a width of the microfluidic channel to form a 2-dimensional array of fluid compartments separated by gas barriers when the solid gas-generating material reacts to form the gas.

5. The microfluidic sample compartment array of claim 1, wherein the microfluidic channel comprises pinch points between individual gas-generating components, wherein the pinch points have a reduced width and/or height compared to an adjacent portion of the microfluidic channel, wherein the pinch points are positioned to constrain gas bubbles generated by the solid gas generating material.

6. The microfluidic sample compartment array of claim 1, wherein the electric initiators comprise a thermal resistor or a spark gap.

7. The microfluidic sample compartment array 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.

8. A nucleic acid amplification system comprising: a microfluidic channel; a nucleic acid sample inlet into the microfluidic channel; an array of gas-generating elements in the microfluidic channel, wherein the gas-generating elements comprise a solid gas-generating material in the microfluidic channel; and an array of electric initiators configured to cause the gas-generating elements to form gas when an electric current is applied to the electric initiators, wherein the gas-generating elements are spaced apart such that the gas forms an array of fluid compartments of the nucleic acid sample separated by gas barriers.

9. The nucleic acid amplification system of claim 8, wherein the fluid compartments of the nucleic acid sample have a volume from about 1 picoliter to about 1 microliter.

10. The nucleic acid amplification system of claim 8, further comprising a master mix reagent inlet positioned to mix a master mix reagent with the nucleic acid sample.

11. The nucleic acid amplification system of claim 8, further comprising a primer immobilized on an interior surface of the microfluidic channel.

12. The nucleic acid amplification system of claim 11, further comprising a second microfluidic channel, a second array of gas-generating elements in the second microchannel, and a second array of electric initiators adjacent to the second array of gas-generating elements, wherein a second primer is immobilized on an interior surface of the second microfluidic channel.

13. A method of making a digital nucleic acid amplification system comprising: forming a microfluidic channel enclosed by channel walls and having a nucleic acid sample inlet; forming an array of electric initiators comprising an electrically conductive layer at a channel wall of the microfluidic channel; and forming an array of gas-generating elements over the array of electric initiators, wherein the gas-generating elements comprise a solid gas-generating material 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 initiators are 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 initiators, and wherein the gas-generating elements are spaced apart such that the gas forms an array of fluid compartments of the nucleic acid sample separated by gas barriers.

14. The method of claim 13, further comprising immobilizing a primer on an interior surface of the microfluidic channel.

15. The method of claim 13, wherein the channel walls comprise pinch points between individual gas-generating components, wherein the pinch points have a reduced width and/or height compared to an adjacent portion of the microfluidic channel, wherein the pinch points are positioned to constrain gas bubbles generated by the solid gas-generating material.