Patent Publication Number: US-11040347-B2

Title: Microfabricated droplet dispensor with immiscible fluid

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not applicable. 
     STATEMENT REGARDING MICROFICHE APPENDIX 
     Not applicable. 
     BACKGROUND 
     The present invention is directed to a system for the manipulation of particles and biological materials, and forming droplets containing these particles. 
     Biomedical researchers have for some time perceived the need to work with small quantities of fluid samples, and to identify compounds uniquely within these small volumes. These attributes allow large numbers of experiments to be carried out in parallel, saving time and money on equipment and reagents, and reducing the need of patients to produce large volume samples. 
     Indeed, the analysis of small fragments of nucleic acids and proteins suspended in small quantities of buffer fluid is an essential element of molecular biology. The ability to detect, discriminate, and utilize genetic and proteomic information allows sensitive and specific diagnostics, as well as the development of treatments. In particular, there is a need to unambiguously identify small quantities of biological material and analytes. 
     Most genetic and proteomic analysis requires labeling for detection of the analytes of interest. Such labelling may be referred to as “barcoding”, suggesting that the label is unique and correlated to some feature or identity. For example, in sequencing applications, nucleotides added to a template strand during sequencing-by-synthesis typically are labeled, or are intended to generate a label, upon incorporation into the growing strand. The presence of the label allows detection of the incorporated nucleotide. Effective labeling techniques are desirable in order to improve diagnostic and therapeutic results. 
     At the same time, precision manipulation of streams of fluids with microfluidic devices is revolutionizing many fluid-based technologies. Networks of small channels are a flexible platform for the precision manipulation of small amounts of fluids. The utility of such microfluidic devices depends critically on enabling technologies such as the microfluidic pumps and valves, electrokinetic pumping, dielectrophoretic pump or electrowetting driven flow. The assembly of such modules into complete systems provides a convenient and robust way to construct microfluidic devices. 
     However, virtually all microfluidic devices are based on flows of streams of fluids; this sets a limit on the smallest volume of reagent that can effectively be used because of the contaminating effects of diffusion and surface adsorption. As the dimensions of small volumes shrink, diffusion becomes the dominant mechanism for mixing leading to dispersion of reactants. This is a large and growing area of biomedical technology, as indicated by a growing number of issued patents in the field. 
     U.S. Pat. No. 9,440,232 describes microfluidic structures and methods for manipulating fluids and reactions. The structures and methods involve positioning fluid samples, e.g., in the form of droplets, in a carrier fluid (e.g., an oil, which may be immiscible with the fluid sample) in predetermined regions in a microfluidic network. In some embodiments, positioning of the droplets can take place in the order in which they are introduced into the microfluidic network (e.g., sequentially) without significant physical contact between the droplets. Because of the little or no contact between the droplets, there may be little or no coalescence between the droplets. Accordingly, in some such embodiments, surfactants are not required in either the fluid sample or the carrier fluid to prevent coalescence of the droplets. 
     U.S. Pat. No. 9,410,151 provides microfluidic devices and methods that are useful for performing high-throughput screening assays and combinatorial chemistry. This patent provides for aqueous based emulsions containing uniquely labeled cells, enzymes, nucleic acids, etc., wherein the emulsions further comprise primers, labels, probes, and other reactants. An oil based carrier-fluid envelopes the emulsion library on a microfluidic device. Such that a continuous channel provides for flow of the immiscible fluids, to accomplish pooling, coalescing, mixing, Sorting, detection, etc., of the emulsion library. 
     U.S. Pat. No. 9,399,797 relates to droplet based digital PCR and methods for analyzing a target nucleic acid using the same. In certain embodiments, a method for determining the nucleic acid make-up of a sample is provided. 
     U.S. Pat. No. 9,150,852 describes barcode libraries and methods of making and using them including obtaining a plurality of nucleic acid constructs in which each construct comprises a unique N-mer and a functional N-mer and segregating the constructs into a fluid compartments such that each compartment contains one or more copies of a unique construct 
     None of these references uses a small, micromechanical valving structure to control the volume of fluid surrounding the barcoded item, and to select the particle enclosed in the droplet. Accordingly, the droplets cannot be made “on demand”, and cannot be made to enclose a particle which is the object of the study. 
     SUMMARY 
     Accordingly, it was the object of the invention to provide a microfabricated system that can separate target particles from non-target material, also separate a labelled bead, and combine the two particles in a single droplet. In addition to the target particle and the bead, the droplet may comprise a first aqueous fluid, such as a saline or buffer fluid. The droplet may be dispensed into a stream of a second fluid, immiscible with the first fluid. Thus, the droplet may maintain its integrity as a single, discrete, well defined unit because the fluids are immiscible and the droplets do not touch or coalesce. 
     When the target particle is a biological material such as a cell, with antigens located on its outer surface, the target particle may become attached to the bead by conjugation of these antigens with antibodies disposed on the bead. The bead may further be labelled by an identifying fluorescent signature, which may be a plurality of fluorescent tags affixed to the bead. Accordingly, each target cell, now bound to an identifiable, labelled fluorescent bead, may be essentially barcoded for its own identification. This may allow a large number of experiments to be performed on a large population of such droplets, encased in the immiscible fluid, because the particles are all identifiable and distinguishable. 
     Accordingly, a microfabricated droplet dispensing structure is described, which may include a MEMS micromechanical fluidic valve, configured to open and close a microfluidic channel. The opening and closing of the valve may separate a target particle and/or a bead from a fluid sample stream, and direct these two particles into a single droplet. The droplet may then be encased in a sheath of an immiscible fluid and delivered to a downstream receptacle or exit. 
     The system may further comprise a fluid sample stream flowing in the microfluidic channel, wherein the fluid sample stream comprises target particles and non-target material, and an interrogation region in the microfluidic channel. Within the interrogation region, the target particle may be identified among non-target material, and the microfabricated MEMS fluidic valve may separate the target particle from the non-target material in response to a signal from the interrogation region, and direct the target particle into the droplet. 
     The system may also make use of a bead attached to a plurality of fluorescent tags, wherein the fluorescent tags specify the identity of the bead with a fluorescent signal, and wherein the microfabricated MEMS fluidic valve is configured to separate the bead and direct the bead into the droplet, wherein the bead and a target particle, are located within the same droplet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various exemplary details are described with reference to the following figures, wherein: 
         FIG. 1  is a schematic illustration of an embodiment of a microfabricated droplet dispenser with an immiscible fluid with the microfabricated MEMS fluidic valve in the closed position; 
         FIG. 2  is a schematic illustration of an embodiment of a microfabricated droplet dispenser with an immiscible fluid with the microfabricated MEMS fluidic valve in the open (sort) position; 
         FIG. 3  is a chart showing the functional dependence of the water droplet size on the duration that the microfabricated MEMS fluidic valve is open; 
         FIG. 4  is a schematic illustration of an embodiment of a microfabricated droplet dispenser with an immiscible fluid generating an empty droplet in oil; 
         FIG. 5  is a schematic illustration of an embodiment of a microfabricated droplet dispenser with an immiscible fluid generating a droplet, wherein the droplet contains both a particle and a bead; 
         FIG. 6  is a schematic illustration of an embodiment of a microfabricated droplet dispenser with an immiscible fluid in a butt junction; 
         FIG. 7  is a schematic illustration of an embodiment of a microfabricated droplet dispenser with a laser assisted droplet coalescence; and 
         FIG. 8  is a schematic illustration of an embodiment of a microfabricated droplet dispenser with a variable channel cross section. 
     
    
    
     It should be understood that the drawings are not necessarily to scale, and that like numbers may refer to like features. 
     DETAILED DESCRIPTION 
     The following discussion presents a plurality of exemplary embodiments of the novel microfabricated droplet dispensing system. The following reference numbers are used in the accompanying figures to refer to the following:
           110  microfabricated MEMS valve     120  fluid input channel     122  sort channel     140  waste channel     150  nozzle     170  interrogation region     145  non-sort flow     200  oil     220  oil input  1       240  oil input  2       260  oil flowing to outlet via     300  water droplet in oil     310  bead in water droplet     320  target particle in water droplet     400  laser heater     500  merging area       

     The system includes a microfabricated droplet dispenser that dispenses the droplets into an immiscible fluid. The system may be applied to a fluid sample stream, which may include target particles as well as non-target material. The target particles may be biological in nature, such as biological cells like T-cells, tumor cells, stem cells, for example. The non-target material might be plasma, platelets, buffer solutions, or nutrients, for example. 
     The microfabricated MEMS valve may be, for example, the device shown generally in  FIGS. 1 and 2 . It should be understood that this design is exemplary only, and that other sorts of MEMS valves may be used in place of that depicted in  FIGS. 1 and 2 . 
     In the figures discussed below, similar reference numbers are intended to refer to similar structures, and the structures are illustrated at various levels of detail to give a clear view of the important features of this novel device. It should be understood that these drawings do not necessarily depict the structures to scale, and that directional designations such as “top,” “bottom,” “upper,” “lower,” “left” and “right” are arbitrary, as the device may be constructed and operated in any particular orientation. In particular, it should be understood that the designations “sort” and “waste” are interchangeable, as they only refer to different populations of particles, and which population is called the “target” or “sort” population is arbitrary. 
       FIG. 1  is an plan view illustration of the novel microfabricated fluidic MEMS droplet dispensing device  10  in the quiescent (un-actuated) position. The MEMS droplet dispensing device  10  may include a microfabricated fluidic valve or movable member  110  and a number of microfabricated fluidic channels  120 ,  122  and  140 . The fluidic valve  110  and microfabricated fluidic channels  120 ,  122  and  140  may be formed in a suitable substrate, such as a silicon substrate, using MEMS lithographic fabrication techniques as described in greater detail below. The fabrication substrate may have a fabrication plane in which the device is formed and in which the movable member  110  moves. Details as to the fabrication of the valve  110  may be found in U.S. Pat. No. 9,372,144 (the &#39;144 patent) issued Jun. 21, 2016 and incorporated by reference in its entirety. 
     A fluid sample stream may be introduced to the microfabricated fluidic valve  110  by a sample inlet channel  120 . The sample stream may contain a mixture of particles, including at least one desired, target particle and a number of other undesired, nontarget materials. The particles may be suspended in a fluid, which is generally an aqueous fluid, such as saline. For the purposes of this discussion, this aqueous fluid may be the first fluid, and this first fluid may be immiscible in a second fluid, as described below. 
     The target particle may be a biological material such as a stem cell, a cancer cell, a zygote, a protein, a T-cell, a bacteria, a component of blood, a DNA fragment, for example, suspended in a buffer fluid such as saline. The fluid inlet channel  120  may be formed in the same fabrication plane as the valve  110 , such that the flow of the fluid is substantially in that plane. The motion of the valve  110  may also be within this fabrication plane. The decision to sort/save or dispose/waste a given particle may be based on any number of distinguishing signals. 
     In one embodiment, the fluid sample stream may pass through an interrogation region  170 , which may be a laser interrogation region, wherein an excitation laser excites fluorescent tag affixed to a target particle. The fluorescent tag may emit fluorescent radiation as a result of the excitation, and this radiation may be detected by a nearby detector, and thus a target particle or cell may be identified. Upon identification of the target particle or cell, the microfabricated MEMS valve may be actuated, as described below, and the flow directed from the nonsort (waste) channel  145  to the sort channel  122 , as illustrated in  FIG. 2 . The actuation means may be electromagnetic, for example. The analysis of the fluorescent signal, the decision to sort or discard a particle, and the actuation of the valve, may be under the control of a microprocessor or computer. 
     In some embodiments, the actuation may occur by energizing an external electromagnetic coil and core in the vicinity of the valve  110 . The valve  110  may include an inlaid magnetically permeable material, which is drawn into areas of changing magnetic flux density, wherein the flux is generated by the external electromagnetic coil and core. In other embodiments, other actuation mechanisms may be used, including electrostatic and piezoelectric. Additional details as to the construction and operation of such a valve may be found in the incorporated &#39;144 patent. 
     In one exemplary embodiment, the decision is based on a fluorescence signal emitted by the particle, based on a fluorescent tag affixed to the particle and excited by an illuminating laser. Accordingly, these fluorescent tags may be identifiers or a barcoding system. However, other sorts of distinguishing signals may be anticipated, including scattered light or side scattered light which may be based on the morphology of a particle, or any number of mechanical, chemical, electric or magnetic effects that can identify a particle as being either a target particle, and thus sorted or saved, or an nontarget particle and thus rejected or otherwise disposed of. 
     This system may also be used to sort the labelled or barcoded bead. Accordingly, the “target particle” may be either a cell and/or a labelled bead. 
     With the valve  110  in the position shown in  FIG. 1 , the microfabricated MEMS fluidic valve  110  is shown in the closed position, wherein the fluid sample stream, target particles and non-target materials flow directly in to the waste channel  140 . Accordingly, the input stream passes unimpeded to an output orifice and channel  140  which may be out of the plane of the inlet channel  120 , and thus out of the fabrication plane of the device  10 . That is, the flow is from the inlet channel  120  to the output orifice  140 , from which it flows substantially vertically, and thus orthogonally to the inlet channel  120 . This output orifice  140  leads to an out-of-plane channel that may be perpendicular to the plane of the paper showing  FIG. 1 . More generally, the output channel  140  is not parallel to the plane of the inlet channel  120  or sort channel  122 , or the fabrication plane of the movable member  110 . 
     The output orifice  140  may be a hole formed in the fabrication substrate, or in a covering substrate that is bonded to the fabrication substrate. Further, the valve  110  may have a curved diverting surface  112  which can redirect the flow of the input stream into a sort output stream, as described next with respect to  FIG. 2 . The contour of the orifice  140  may be such that it overlaps some, but not all, of the inlet channel  120  and sort channel  122 . By having the contour  140  overlap the inlet channel, and with relieved areas described above, a route exists for the input stream to flow directly into the waste orifice  140  when the movable member or valve  110  is in the un-actuated waste position. 
       FIG. 2  is a schematic illustration of an embodiment of a microfabricated droplet dispenser with an immiscible fluid with the microfabricated MEMS device  10 . In  FIG. 2 , the MEMS device  10  may include a MEMS fluidic valve  110  in the open (sort) position. In this open (sort) position, a target cell  5  as detected in the laser interrogation region  170  may be deflected into the sort channel  122 , along with a quantity of the suspending (buffering) fluid. 
     In this position, the movable member or valve  110  is deflected upward into the position shown in  FIG. 2 . The diverting surface  112  is a sorting contour which redirects the flow of the inlet channel  120  into the sort output channel  122 . The sort output channel  122  may lie in substantially the same plane as the inlet channel  120 , such that the flow within the sort channel  122  is also in substantially the same plane as the flow within the inlet channel  120 . Actuation of movable member  110  may arise from a force from force-generating apparatus (not shown). In some embodiments, force-generating apparatus may be an electromagnet, however, it should be understood that force-generating apparatus may also be electrostatic, piezoelectric, or some other means to exert a force on movable member  110 , causing it to move from a first position ( FIG. 1 ) to a second position ( FIG. 2 ). 
     More generally, the micromechanical particle manipulation device shown in  FIGS. 1 and 2  may be formed on a surface of a fabrication substrate, wherein the micromechanical particle manipulation device may include a microfabricated, movable member  110 , wherein the movable member  110  moves from a first position to a second position in response to a force applied to the movable member, wherein the motion is substantially in a plane parallel to the surface, a fluid sample inlet channel  120  formed in the substrate and through which a fluid flows, the fluid including at least one target particle and non-target material, wherein the flow in the fluid sample inlet channel is substantially parallel to the surface, and a plurality of output channels  122 ,  140  into which the microfabricated member diverts the fluid, and wherein the flow in at least one of the output channels  140  is not parallel to the plane, and wherein at least one output channel  140  is located directly below at least a portion of the movable member  110  over at least a portion of its motion. 
     It should be understood that although channel  122  is referred to as the “sort channel” and orifice  140  is referred to as the “waste orifice”, these terms can be interchanged such that the sort stream is directed into the waste orifice  140  and the waste stream is directed into channel  122 , without any loss of generality. Similarly, the “inlet channel”  120  and “sort channel”  122  may be reversed. The terms used to designate the three channels are arbitrary, but the inlet stream may be diverted by the valve  110  into either of two separate directions, at least one of which does not lie in the same plane as the other two. The term “substantially” when used in reference to an angular direction, i.e. substantially tangent or substantially vertical, should be understood to mean within 15 degrees of the referenced direction. For example, “substantially orthogonal” to a line should be understood to mean from about 75 degrees to about 105 degrees from the line. 
     When the valve is in the open or sort position shown in  FIG. 2 , the suspending aqueous fluid, along with at least one suspended particle, may flow into the sort channel  122 , and from there to the edge of the fabrication substrate. The fluid that was flowing in the fluid sample inlet channel  120  may then form a droplet at the edge of the fabrication substrate. Alternatively, the generated droplet might flow to and accumulate in the sort chamber. 
     Various structures may be used in this region to promote the formation of the droplet. These structures may be, for example, rounded corners or sharp edges which may influence or manipulate the strength or shape of the meniscus forces, wetting angle or surface tension of the first fluid droplet. These structures may be generally referred to as a “nozzle” indicating the region where the droplet is formed. At this nozzle point where the droplet is formed, an additional manifold may deliver an immiscible second fluid to the aqueous droplet, suspending the aqueous droplet in the fluid and preserving its general contours and boundary layers. 
     As mentioned, the valve  110  may be used to sort both a target cell and a bead. Laser induced fluorescence may be the distinguishing feature for either or both particles. These particles may both be delivered into a single droplet. These particles may be suspended in, and surrounded by, an aqueous first fluid, such as saline. Accordingly, the droplet may comprise primarily this first fluid, as well as the chosen particle(s), a target cell and/or a bead. The bead may be “barcoded”, that is, it may carry identifying markers. The droplet may then be surrounded by an immiscible second fluid that is provided by a source of the second fluid, These features are described further below, with respect to a number of embodiments. 
     Accordingly, because of the flow in the microfabricated channels, droplets may be formed at the intersection with the immiscible fluid. These droplets may be encased in an immiscible second fluid, such as a lepidic fluid or oil  200 , as shown in  FIGS. 1 and 2 . The oil  200  may be applied symmetrically by oil input  220  and oil input  240 . The immiscible fluid may serve to maintain the separation between droplets, so that they do not coalesce, and each droplet generally contains only one target particle and only one bead. The stream of oil may exit the sort outlet via  260 . The lipidic fluid may be a petroleum based lipidic fluid, or a vegetable based lipidic fluid, or an animal based lipidic fluid. 
     The pace, quality and rate of droplet formation may be controlled primarily by the dynamics of the MEMS valve  110 . That is, the quantity of fluid contained in the droplet, and thus the size of the droplet, may be a function of the amount of time that the MEMS valve  110  is in the open or sort position shown in  FIG. 2 . The functional dependence of the size of the droplet on the valve open time is illustrated in  FIG. 3 . As can be seen in  FIG. 3 , the diameter of the droplet is proportional to the valve open time, over a broad range of values. Only at exceedingly large droplets and long open times (greater than about 100 μsecs and 60 microns diameter) does the functional dependence vary from its linear behaviour. 
     Accordingly, the length of the sort pulse can determine the size of the generated droplet. If the pulse is too short, the oil meniscus may remain intact and no water droplet is formed. If the sort pulse is sufficiently long, a droplet may be formed at the exit and released into the stream of the second immiscible fluid. 
     If a target cell  5  is sorted within this time frame, the target cell  5  may be enclosed in the aqueous droplet. If the target particle is not sorted within this time frame, an empty aqueous droplet, that is, a droplet without an enclosed particle  5 , may be formed. The situation is shown in  FIG. 4 . 
     As mentioned above, the MEMS valve  110  may be made on the fabrication surface of at least one semiconductor substrate. More generally, a multi-substrate stack may be used to fabricate the MEMS valve  110 . As detailed in the &#39;144 patent, the multilayer stack may include at least one semiconductor substrate, such as a silicon substrate, and a transparent glass substrate. The transparent substrate may be required to allow the excitation laser to be applied in the laser interrogation region  170 . 
     The droplet  300  may be formed at the edge of the semiconductor substrate, or more particularly, at the edge of the multilayer stack. The droplet  300  may be formed at the exit of the sort channel  122  from this multilayer stack. In another embodiment, the droplet is not formed at the edge of the multilayer stack, but instead may be formed at the intersection of the sort flow and oil input, within the semiconductor substrate. At this location, a structure may be formed that promotes the formation of the droplet. This structure may include sharply rounded corners so as to manipulate surface tension forces, and the formation of meniscus and wetting angles. The structure designed to promote droplet formation may be referred to herein as a nozzle  150 , and the term “nozzle” may refer generally to the location at which the droplet may be formed. 
     In the structure shown in  FIG. 4 , downstream of the microfabricated MEMS valve, and in the vicinity of the nozzle structure  150 , there may be disposed a flow junction with the immiscible second fluid. In the sort channel, downstream of the valve, there may be a flow junction with oil (as a carrier for water droplets) flowing from the sides towards the sort channel  122 . This flow junction may have an inlet  220  and  240  on either end of the sort channel  122 , forming an oil stream  200  downstream of the nozzle  150  and sort channel  122 . 
     Sorting Strategy Using the Valve to Form a Droplet in Oil 
     The method for forming a droplet in oil may be as follows. A target cell is first detected in the laser interrogation region  170 . A computer or controller may monitor the signals from the laser interrogation region. Upon detecting a target particle in the region, the computer or controller may send a signal to open the MEMS valve  110  by energizing the electromagnet. Magnetic interactions then move the MEMS valve as shown in  FIG. 2 . In this open (sort) position, a target cell  5  may be deflected into the sort channel, along with a quantity of the suspended fluid. 
     A bead is then sorted to accompany the sorted cell as a unique barcode. A second sort pulse is long enough to cause an instability in the oil-water interface and form a water droplet in oil containing the cell and the bead. 
     When the valve is stationary and no sorting occurs, as depicted in  FIG. 1 , oil continues flowing towards the sort outlet via, blocking water flow in the sort. In fact however, because of the finite gaps between the moving edges of the MEMS valve  110  shown in  FIGS. 1 and 2 , a small but finite amount of the fluid sample stream fluid may continue to flow down the sort channel  122 . However, these leak flow rates through the valve gaps, are not sufficient to break the oil front and create a water droplet, in normal operation. 
     However, as oil may continue to flow, the effluent may be directed into a waste receptacle, until a target particle is detected. It may also be the case that continued leakage of the fluid sample stream through the gaps around the MEMS valve  110 , may eventually cause a water droplet to form. Because no target cell has been detected, and the MEMS valve  110  has not been opened, this aqueous droplet may be empty. 
     Accordingly,  FIG. 4  is a schematic illustration of an embodiment of a microfabricated droplet dispenser with an immiscible fluid generating an empty first fluid droplet  300  in oil  200 . This situation may occur if no target particle is present in the fluid sample stream. The MEMS valve  110  may leak slightly, causing an aqueous droplet to form but without an enclosed target particle. In this case, the droplet may be allowed to flow into a waste area of a holding receptacle. 
     In another embodiment, the MEMS valve  110  may sort both a target particle  5  (here, a target cell  320 ) and a bead  310 , as shown in  FIG. 5 . The bead may be a biologically inert material coated with a biologically active material, and additional compounds. The biologically active materials may be antibodies that can become conjugated to antigens appearing on a target cell surface  320 . In addition to the antigens and inert materials, the bead may further be coupled to a plurality of fluorescent tags, that is, compound which fluoresces when irradiated by an excitation laser of the proper wavelength and intensity. This plurality of fluorescent tags may be different for each bead  310 , and may therefore act as a signature or identifier for the bead. 
     When a bead  310  is in proximity to a target cell  320 , and the antibodies of the bead  310  may become conjugated with the antigens of the cell, the bead, along with its identifying fluorescent tags, may become affixed to the cell  320 . Thus, the bead  310  provides an identifying marker for the cell  320 , or a “barcode” which identifies the cell. A computer or controller may associate this particular barcode with the particular cell. Accordingly, a large number of such droplets may be placed in a small volume of fluid, each containing a target cell and identifying barcode and all within a field of view of a single detector. This may allow a very large number of biological assays or polymerase chain reactions, to be undertaken in parallel, and under a single detection system. 
       FIG. 5  is a schematic illustration of an embodiment of a microfabricated droplet dispenser with an immiscible fluid generating a droplet in oil, wherein the droplet contains both a particle or cell  320  and a bead  310 . Accordingly, the MEMS valve  110  may first sort a particle  320 , enclosing the particle  320  in an aqueous droplet as described above. The MEMS valve  110  may then also sort a barcoded bead  310 , and both particle  320  and the bead  310  may be enclosed in the same aqueous droplet, as shown in  FIG. 5 . 
       FIG. 6  is a schematic illustration of another embodiment of a microfabricated droplet dispenser with an immiscible fluid in a butt junction. In this embodiment, the application of the surrounding second immiscible fluid is asymmetrical. Instead of coming both from the right and the left of the nozzle region, the oil  200 , the oil junction is applied in parallel to the sort channel  122  and may exit downstream  260  of the sort channel  122 . The second immiscible fluid may flow from right to left. The aqueous fluid droplet may break the oil meniscus from the side channel, as shown. As before, each droplet  300  in oil  200  may contain both a target cell  320  and an identifying bead  310 . 
     Laser Assisted Droplet Formation 
       FIG. 7  is a schematic illustration of another embodiment of a microfabricated droplet dispenser with a laser assisted droplet coalescence. In this embodiment, the two particles the target cell  320  and the bead  310  are sorted separately and placed into two separate aqueous droplets in the oil stream  200 . For each event, the passage of the target cell  320  and the passage of the bead  310 , the sort pulse is long enough to cause an instability in the oil-water interface and form a water droplet in oil containing the cell. The two separate droplets are then merged by application of laser light  400  on to oil channel containing the aqueous droplets. 
     Any of a variety of pulsed or continuous wave lasers may be suitable for this application. For example, a pulsed CO 2  laser may be directed onto the channel as shown in  FIG. 7 , to heat the droplets. The application of energy causes the fluids to heat, which weakens meniscus and membrane forces, allowing the droplets to merge. 
     In  FIG. 7 , as in previous embodiments, the microfabricated droplet dispenser in  FIG. 7  may have a symmetric (or asymmetric) oil input configuration. In either configuration, the droplets  300  may be encased in an immiscible second fluid, such as a lepidic fluid or oil  200 . The oil  200  may be applied symmetrically by oil input  220  and oil input  240 . The stream of oil may exit the sort outlet via  260 . 
     The embodiment shown in  FIG. 7  may have a flow channel which is capable of sorting two aqueous droplets, and then merging them into a single larger droplet. In this embodiment, the sort pulse is long enough to cause an instability in the oil-water interface and form a water droplet in oil containing the cell. Then a bead is sorted and a separate droplet is formed. Accordingly, the first droplet may contain a target cell  320 , and the second aqueous droplet may contain a bead  310  as previously described. A merging area is a portion of the sort flow channel  122  wherein the laser  400  is directed. The laser light may be focused to increase its peak intensity. The applied laser light may heat the droplet as well as the surrounding fluid, and allow the two droplets to merge. The merging may be caused by the laser-induced heating and consequent weakening of surface tension of the fluid droplet. 
     Alternatively, the first droplet may contain the bead  310 , and the second aqueous droplet may contain the target cell  320 . In either case, the application of heat onto the channel in the laser  400  may serve to heat the fluids and allow the two droplets to merge. Accordingly, at the output of the microfabricated droplet dispenser may emerge an aqueous droplet encased in oil wherein the droplet contains both a target cell  320  and a bead  310 . The bead  310  may have a fluorescent barcode affixed to it, and the bead may be conjugated to the target cell  320 . 
     Geometry-Induced Flow Slowdown 
       FIG. 8  is a schematic illustration of an embodiment of a microfabricated droplet dispenser with a variable channel cross section. Like previous embodiments, the microfabricated droplet dispenser in  FIG. 8  may have a symmetric (or asymmetric) oil input configuration. In this configuration, the droplets may be encased in an immiscible second fluid, such as a lepidic fluid or oil  200 . The oil  200  may be applied symmetrically by oil input  220  and oil input  240 . The stream of oil may exit the sort outlet via  260 . 
     The embodiment shown in  FIG. 8  may have a flow channel which is capable of sorting two aqueous droplets, and then merging them into a single larger droplet. In this embodiment, the sort pulse is long enough to cause an instability in the oil-water interface and form a water droplet  300  in oil containing the cell. Then a bead  310  is sorted and a separate droplet is formed. Accordingly, the first droplet may contain a target cell  320 , and the second aqueous droplet may contain a bead  310  as previously described. A merging area  500  is a portion of the sort channel  122  having a variable cross section  500 . The sudden widening of the channel in the merging area  500  may serve to slow the flow down within the merging area, allowing the two droplets to merge. In other words, the sudden widening may produce geometry-induced flow slowdown, which allows the droplets to merge. 
     Alternatively, the first droplet may contain the bead  310 , and the second aqueous droplet may contain the target cell  320 . In either case, the sudden widening of the channel in the merging area  500  may serve to slow the flow down within the merging area, allowing the two droplets to merge. Accordingly, at the output of the microfabricated droplet dispenser may emerge an aqueous droplet  300  encased in oil  200  wherein the droplet  300  contains a target cell  320  and a bead  310 . The bead  310  may have a fluorescent barcode affixed to it, and the bead may be conjugated to the target cell  320 . 
     Accordingly, described here is a microfabricated droplet dispenser, comprising a microfluidic channel formed in a substrate and a fluid flowing in the microfluidic fluid channel; a microfabricated MEMS fluidic valve, configured to open and close the microfluidic channel, a droplet comprising a first fluid dispensed at an end of the microfluidic channel, wherein a dimension of the droplet is determined by a timing of opening and closing of the microfabricated microfluidic valve, and a source of a second fluid immiscible with the first fluid wherein the droplet is dispensed from the microfluidic channel into, and immersed in, the second immiscible fluid 
     The droplet dispenser may further comprise a fluid sample stream flowing in the microfluidic channel, wherein the fluid sample stream comprises target particles and non-target material, an interrogation region in the microfluidic channel, wherein a target particle is identified among non-target material; and wherein the microfabricated MEMS fluidic valve is configured to separate the target particle from the non-target material in response to a signal from the interrogation region, and direct the target particle into the droplet. It may also include a bead attached to a plurality of fluorescent tags, wherein the fluorescent tags specify the identity of the bead with a fluorescent signal, and wherein the microfabricated MEMS fluidic valve is configured to separate the bead and direct the bead into the droplet, wherein the bead and a target particle, are located within the same droplet. The bead may comprise a plurality of fluorescent tags, such that the bead has an identifying fluorescent signature. The bead may also have at least one antibody, that binds to an antigen on the target particle. 
     The microfabricated MEMS valve may move in a single plane when opening and closing, and wherein that plane is parallel to a surface of the substrate. The droplet may be dispensed at a nozzle structure formed in the microfluidic channel in the substrate. The source of immiscible fluid is disposed symmetrically about the nozzle. Surfactant may be added to the fluid stream. 
     The droplet dispenser may further comprise a laser focused on the microfluidic channel upstream of the nozzle, heating the droplet to assist in severing the droplet from the fluid in the microfluidic channel, or to heat the droplet to coalesce adjacent droplets in the microfluidic channel. The microfluidic channel may have a channel widened area, wherein the cross section of the channel increases and then decreases. The microchannel may intersect the source of immiscible fluid in a butt junction. The target particles are at least one of T-cells, stem cells, cancer cells, tumor cells, proteins and DNA strands. 
     A method for dispensing droplets is also described. The method may include method may include forming a microfluidic channel on a substrate, providing a fluid flowing in the microfluidic fluid channel, opening and closing a microfabricated MEMS fluidic valve, The method may further comprise opening and closing a microfabricated MEMS fluidic valve, to open and close the microfluidic channel, capturing at least one of a target particle and a bead with identifiers disposed thereon, providing a source of an immiscible second fluid, immiscible with the first fluid, and dispensing a droplet of the first, wherein a dimension of the droplet is determined by a timing of opening and closing of the microfabricated microfluidic valve, and wherein the droplet encloses at least one of the bead and the target particle. 
     The fluid flowing in the microfluidic channel may include target particles and non-target material. The method may further include identifying a target particle among non-target material in a laser interrogation region, opening and closing the microfabricated MEMS fluidic valve to separate the identified target particle from the non-target material in response to a signal from the interrogation region, and directing the target particle into the droplet. 
     The method may also include providing a bead attached to a plurality of fluorescent tags, wherein the fluorescent tags specify the identity of the bead with a fluorescent signal, separating the bead using the microfabricated MEMS fluidic valve, and directing the bead into the droplet, wherein the bead and the target particle, are located within the same droplet. 
     The droplet may be formed at a nozzle structure formed in the substrate. The method may further include heating the fluid with a laser focused just upstream of the nozzle. 
     While various details have been described in conjunction with the exemplary implementations outlined above, various alternatives, modifications, variations, improvements, and/or substantial equivalents, whether known or that are or may be presently unforeseen, may become apparent upon reviewing the foregoing disclosure. Accordingly, the exemplary implementations set forth above, are intended to be illustrative, not limiting.