WO2009029229A2

WO2009029229A2 - Ferrofluid emulsions, particles, and systems and methods for making and using the same

Info

Publication number: WO2009029229A2
Application number: PCT/US2008/010020
Authority: WO
Inventors: Enric Santanach Carreras; Jerome Bibette; David A. Weitz
Original assignee: President And Fellows Of Harvard College
Priority date: 2007-08-24
Filing date: 2008-08-22
Publication date: 2009-03-05
Also published as: WO2009029229A3

Abstract

The present invention generally relates to ferrofluid emulsions and/or magnetically susceptible particles. In some cases, such emulsions and particles can be produced in a microfluidic system. In one aspect, an emulsion containing a ferrofluid can be produced, and in certain cases, the emulsion is substantially monodisperse. In some embodiments, the emulsion also contains a precursor that can be solidified, for example, a polymer precursor. The emulsion may also contain an emulsion stabilizing agent and/or a viscous agent. In another aspect of the present invention, the ferrofluid emulsion may be hardened to form particles, for example, by removing a fluid from the emulsion and/or by solidifying a precursor, such as a polymer precursor, around the emulsion. For example, the ferrofluid may be formed from an organic solvent containing nanoparticles, and the organic solvent removed such that the nanoparticles form magnetically susceptible particles. The particles may also be further manipulated, for example, using a magnet. For instance, in one set of embodiments, the particles may be moved from one fluid stream to another fluid stream using the magnet, for example, for further processing, reaction, analysis, etc., of the particles. In some cases, the particles may be formed 'on demand,' e.g., for a variety of different applications.

Description

FERROFLUID EMULSIONS, PARTICLES. AND SYSTEMS AND METHODS FOR MAKING AND USING THE SAME

RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Patent Application Serial

No. 60/966,044, filed August 24, 2007, entitled "Ferrofluid Emulsions, Magnetic Particles, and Systems and Methods for Making and Using the Same," by E. Santanach Carreras, et al. , incorporated herein by reference.

FIELD OF INVENTION The present invention generally relates to ferrofluid emulsions and/or magnetically susceptible particles. In some cases, such emulsions and particles can be produced in a microfluidic system.

BACKGROUND The manipulation of fluids to form fluid streams of desired configuration, discontinuous fluid streams, dispersions, etc., for purposes of fluid delivery, product manufacture, analysis, and the like, is a relatively well-studied art. For example, highly monodisperse gas bubbles, less than 100 micrometers in diameter, have been produced using a technique referred to as capillary flow focusing. In this technique, gas is forced out of a capillary tube into a bath of liquid, the tube is positioned above a small orifice, and the contraction of flow of the external liquid through this orifice focuses the gas into a thin jet which subsequently breaks into bubbles via capillary instability. In a related technique, a similar arrangement can be used to produce liquid droplets in air.

Microfluidics is an area of technology involving the control of fluid flow at a very small scale. Microfluidic devices typically include very small channels, within which fluid flows, which can be branched or otherwise arranged to allow fluids to be combined with each other, to divert fluids to different locations, to cause laminar flow between fluids, to dilute fluids, or the like. Significant effort has been directed toward "lab-on-a-chip" microfluidic technology, in which researchers seek to carry out known chemical or biological reactions on a very small scale on a "chip," or a microfluidic device. Additionally, new techniques, not necessarily known on the macro scale, are being developed using microfluidics. Examples of techniques being investigated or developed at the microfluidic scale include high-throughput screening, drug delivery, chemical kinetics measurements, combinatorial chemistry (where rapid testing of chemical reactions, chemical affinity, or microstructure formation are desired), as well as the study of fundamental questions in the fields of physics, chemistry, and engineering. Microfluidics also show promising applications in fields such as combinatorial chemistry or the rapid screening of catalysts. Rapid mass transfer may lead to enhanced efficiency of existing chemical reactions, and may allow one to explore new reaction pathways that would be difficult in conventional reactors.

The formation of particles can be carried out in equipment including moving parts (e.g., a blender or device similarly designed to break up material), which can be prone to failure and, in many cases, is not suitable for control of very small dispersed phase droplets. Specifically, traditional industrial processes typically involve manufacturing equipment built to operate on size scales generally unsuitable for precise control. Membrane emulsification is one small scale technique using micrometer-sized pores to form emulsions. However, the polydispersity of the dispersed phase can in some cases be limited by the pore sizes of the membrane.

SUMMARY OF THE INVENTION

The present invention generally relates to ferrofluid emulsions and/or magnetically susceptible particles. The subject matter of the present invention involves, in some cases, interrelated products, alternative solutions to a particular problem, and/or a plurality of different uses of one or more systems and/or articles.

In one aspect, the invention is directed to a method. In one set of embodiments, the method includes acts of providing a first fluid stream comprising a liquid containing magnetically susceptible nanoparticles, surrounding the first fluid stream with a second fluid stream, causing the first fluid stream to form a plurality of droplets, and removing at least a portion of the liquid from the plurality of droplets.

The method, in another set of embodiments, includes acts of providing a first fluid stream comprising a liquid containing particles formed from a plurality of magnetically susceptible nanoparticles, and moving the particles from the first microfluidic stream to a second fiuidic stream using a magnet. The invention, an in another aspect, is directed to a collection of articles comprising a plurality of particles. In one set of embodiments, the particles have an average dimension of less than about 500 micrometers and a distribution of dimensions such that no more than about 5% of the particles have a dimension greater than about 10% of the average dimension. In some cases, at least some of the plurality of particles comprise uniformly distributed magnetically susceptible nanoparticles.

In another aspect, the present invention is directed to a method of making one or more of the embodiments described herein, for example, a ferrofluid emulsion. In another aspect, the present invention is directed to a method of using one or more of the embodiments described herein, for example, a ferrofluid emulsion.

Other advantages and novel features of the present invention will become apparent from the following detailed description of various non-limiting embodiments of the invention when considered in conjunction with the accompanying figures. In cases where the present specification and a document incorporated by reference include conflicting and/or inconsistent disclosure, the present specification shall control. If two or more documents incorporated by reference include conflicting and/or inconsistent disclosure with respect to each other, then the document having the later effective date shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. In the figures:

Fig. 1 illustrates the production of fluidic droplets, in accordance with one embodiment of the invention;

Fig. 2 illustrates the manipulation of a particle or a fluidic droplet using a magnet; Fig. 3 illustrates a plurality of units in a device according to one embodiment of the present invention, useful in the production of particles; and

Figs. 4A-4B illustrate an example of a device of one embodiment of the invention. DETAILED DESCRIPTION

The present invention generally relates to ferrofluid emulsions and/or magnetically susceptible particles. In some cases, such emulsions and particles can be produced in a microfluidic system. In one aspect, an emulsion containing a ferrofluid can be produced, and in certain cases, the emulsion is substantially monodisperse. In some embodiments, the emulsion also contains a precursor that can be solidified, for example, a polymer precursor. The emulsion may also contain an emulsion stabilizing agent and/or a viscous agent. In another aspect of the present invention, the ferrofluid emulsion may be hardened to form particles, for example, by removing a fluid from the emulsion and/or by solidifying a precursor, such as a polymer precursor, around the emulsion. For example, the ferrofluid may be formed from an organic solvent containing nanoparticles, and the organic solvent removed such that the nanoparticles form magnetically susceptible particles. The particles may also be further manipulated, for example, using a magnet. For instance, in one set of embodiments, the particles may be moved from one fluid stream to another fluid stream using the magnet, for example, for further processing, reaction, analysis, etc., of the particles. In some cases, the particles may be formed "on demand," e.g., for a variety of different applications.

Fields in which such particles may prove useful include, as non-limiting examples, food, beverage, health and beauty aids, paints and coatings, or drugs and drug delivery. For instance, a precise quantity of a drug, pharmaceutical, or other agent can be contained within a particle, and in some cases, produced as needed. Other species that can be contained within a particle include, for example, biochemical species such as nucleic acids such as siRNA, RNAi and DNA, proteins, peptides, or enzymes. Additional species that can be contained within a particle include, but are not limited to, nanoparticles, quantum dots, fragrances, proteins, indicators, dyes, fluorescent species, chemicals, or the like.

The following are each incorporated herein by reference: U.S. Patent Application Serial No. 11/246,911, filed October 7, 2005, entitled "Formation and Control of Fluidic Species," published as U.S. Patent Application Publication No. 2006/0163385 on July 27, 2006; U.S. Patent Application Serial No. 11/024,228, filed December 28, 2004, entitled "Method and Apparatus for Fluid Dispersion," published as U.S. Patent Application Publication No. 2005/0172476 on August 11 , 2005; U.S. Patent Application Serial No. 11/360,845, filed February 23, 2006, entitled "Electronic Control of Fluidic Species," published as U.S. Patent Application Publication No. 2007/000342 on January 4, 2007; International Patent Application No. PCT/US2006/007772, filed March 3, 2006, entitled "Method and Apparatus for Forming Multiple Emulsions," published as WO 2006/096571 on September 14, 2006; U.S. Patent Application Serial No. 11/368,263, filed March 3, 2006, entitled "Systems and Methods of Forming Particles," published as U.S. Patent Application Publication No. 2007/0054119 on March 8, 2007; U.S. Provisional Patent Application Serial No. 60/920,574, filed March 28, 2007, entitled "Multiple Emulsions and Techniques for Formation"; and International Patent Application No. PCT/US2006/001938, filed January 20, 2006, entitled "Systems and Methods for Forming Fluidic Droplets Encapsulated in Particles Such as Colloidal Particles," published as WO 2006/078841 on July 27, 2006. Also incorporated herein by reference is U.S. Provisional Patent Application Serial No. 60/966,044, filed August 24, 2007, entitled "Ferrofluid Emulsions, Magnetic Particles, and Systems and Methods for Making and Using the Same," by E. Santanach Carreras, et al.

One aspect of the invention relates to systems and methods for producing droplets of fluid surrounded by a liquid, and solidifying the fluidic droplet, or at least a portion thereof, into a solid. These fluids can be selected among essentially any fluids by those of ordinary skill in the art by considering the relationship between the fluids. The fluidic droplets may also contain other species in some cases, for example, certain molecular species (e.g., monomers, polymers, metals, magnetizable materials, porogens, etc.), cells, particles, other fluids, or the like. In one particular set of embodiments, the fluidic droplets contain magnetically susceptible nanoparticles, and in some cases, the fluidic droplets may contain a ferrofluid. In some cases, the fluid and the liquid may be selected to be immiscible within the time frame of the formation of the fluidic droplets. The fluid and the liquid may be essentially immiscible, i.e., immiscible on a time scale of interest (e.g., the time it takes a fluidic droplet to be transported through a particular system or device). In certain cases, the droplets may each be substantially the same shape and/or size.

As used herein, the term "fluid" generally refers to a substance that tends to flow and to conform to the outline of its container, i.e., a liquid, a gas, a viscoelastic fluid, etc. Typically, fluids are materials that are unable to withstand a static shear stress, and when a shear stress is applied, the fluid experiences a continuing and permanent distortion. The fluid may have any suitable viscosity that permits flow. If two or more fluids are present, each fluid may be independently selected among essentially any fluids (liquids, gases, and the like) by those of ordinary skill in the art, by considering the relationship between the fluids. The fluids may each be miscible or immiscible. Where the portions remain liquid for a significant period of time, then the fluids may be chosen to be at least essentially immiscible. Where, after contact and/or formation of solid particles from fluidic droplets hardened by polymerization or the like, the fluids need not be as immiscible. Those of ordinary skill in the art can select suitable miscible or immiscible fluids, using contact angle measurements or the like, to carry out the techniques of the invention. In some embodiments, two fluids are immiscible, or not miscible, with each other when one is not soluble in the other to a level of at least 10% by weight at the temperature and under the conditions at which the emulsion is produced. For instance, the fluid and the liquid may be selected to be immiscible within the time frame of the formation of the fluidic droplets.

A "fluidic droplet" or a "droplet," as used herein, is an isolated portion of a first fluid that is surrounded by a second fluid. It is to be noted that a fluidic droplet is not necessarily spherical, but may assume other shapes as well, for example, depending on the external environment, the dimensions of the channel or other container that the fluidic droplet is contained within, etc. Examples of a fluidic droplet contained within a liquid include, but are not limited to, a hydrophilic liquid suspended in a hydrophobic liquid, a hydrophobic liquid suspended in a hydrophilic liquid, a gas bubble suspended in a liquid, etc. Typically, a hydrophobic liquid and a hydrophilic liquid are essentially immiscible with respect to each other, where the hydrophilic liquid has a relatively greater affinity to water than does the hydrophobic liquid. Examples of hydrophilic liquids include, but are not limited to, water and other aqueous solutions comprising water, such as cell or biological media, salt solutions, etc., as well as other hydrophilic liquids such as ethanol. Examples of hydrophobic liquids include, but are not limited to, oils such as hydrocarbons, silicone oils, mineral oils, fluorocarbon oils, organic solvents, etc. In certain cases, a fluidic stream and/or the fluidic droplets may be produced on the microscale, for example, in a microchannel. Thus, in some, but not all embodiments, at least some of the components of the systems and methods described herein using terms such as "microfluidic" or "microscale." As used herein, "microfluidic," "microscopic," "microscale," the "micro-" prefix (for example, as in "microchannel"), and the like generally refers to elements or articles having widths or diameters of less than about 1 mm, and less than about 100 microns (micrometers) in some cases. In some cases, the element or article includes a channel through which a fluid can flow. In all embodiments, specified widths can be a smallest width (i.e. a width as specified where, at that location, the article can have a larger width in a different dimension), or a largest width (i.e. where, at that location, the article has a width that is no wider than as specified, but can have a length that is greater). Thus, for example, a fluidic stream may be produced on the microscale, e.g., using a microfluidic channel. For instance, the fluidic stream may have an average cross-sectional dimension (e.g., perpendicular to the direction of flow of the fluidic stream) of less than about 1 mm, less than about 500 microns, less than about 300 microns, or less than about 100 microns. In some cases, the fluidic stream may have an average diameter of less than about 60 microns, less than about 50 microns, less than about 40 microns, less than about 30 microns, less than about 25 microns, less than about 10 microns, less than about 5 microns, less than about 3 microns, or less than about 1 micron.

A "channel," as used herein, means a feature on or in an article (e.g., a substrate) that at least partially directs the flow of a fluid. In some cases, the channel may be formed, at least in part, by a single component, e.g. an etched substrate or molded unit. The channel can have any cross-sectional shape, for example, circular, oval, triangular, irregular, square or rectangular (having any aspect ratio), or the like, and can be covered or uncovered (i.e., open to the external environment surrounding the channel). In embodiments where the channel is completely covered, at least one portion of the channel can have a cross-section that is completely enclosed, and/or the entire channel may be completely enclosed along its entire length with the exception of its inlet and outlet.

A channel may have an aspect ratio (length to average cross-sectional dimension) of at least about 2:1, more typically at least about 3:1, at least about 5:1, at least about 10:1, etc. As used herein, a "cross-sectional dimension," in reference to a fluidic or microfluidic channel, is measured in a direction generally perpendicular to fluid flow within the channel. An open channel generally will include characteristics that facilitate control over fluid transport, e.g., structural characteristics and/or physical or chemical characteristics (hydrophobicity vs. hydrophilicity) and/or other characteristics that can exert a force (e.g., a containing force) on a fluid. The fluid within the channel may partially or completely fill the channel. In some cases the fluid may be held or confined within the channel or a portion of the channel in some fashion, for example, using surface tension (e.g., such that the fluid is held within the channel within a meniscus, such as a concave or convex meniscus). In an article or substrate, some (or all) of the channels may be of a particular size or less, for example, having a largest dimension perpendicular to fluid flow of less than about 5 mm, less than about 2 mm, less than about 1 mm, less than about 500 microns, less than about 200 microns, less than about 100 microns, less than about 60 microns, less than about 50 microns, less than about 40 microns, less than about 30 microns, less than about 25 microns, less than about 10 microns, less than about 3 microns, less than about 1 micron, less than about 300 nm, less than about 100 nm, less than about 30 nm, or less than about 10 nm or less in some cases. In one embodiment, the channel is a capillary. Of course, in some cases, larger channels, tubes, compartments, etc. can be used to store fluids in bulk and/or deliver a fluid to a channel.

In some embodiments, the present invention generally relates to an emulsion. The emulsion may include droplets, such as those described above, and/or colloid particles, for example, nanoparticles such as those described below. As used herein, an "emulsion" is given its ordinary meaning as used in the art, i.e., a liquid dispersion. In some cases, the emulsion may be a "microemulsion" or a "nanoemulsion," i.e., an emulsion having a dispersant on the order of micrometers or nanometers, respectively. As one example, such an emulsion may be created by allowing fluidic droplets of the appropriate size or sizes (e.g., created as described herein) to enter into a solution that is immiscible with the fluidic droplets. In one set of embodiments, the fluidic droplet may contain a ferrofluid. A ferrofluid is a fluid that typically has a high magnetic susceptibility and often can be manipulated using a magnet. In one embodiment, the ferrofluid comprises a relatively stable suspension of nanoparticles (for example, of iron oxide, magnetite, hematite or some other compound containing iron), having average diameters of approximately 100 nm, approximately 50 nm, approximately 10 nm, etc., which may be present as a colloidal suspension. The nanoparticles may be magnetically susceptible in some cases, e.g., the nanoparticles may display paramagnetism or ferromagnetism. For instance, the nanoparticles may have a positive magnetic susceptibility, and/or a relative magnetic permeability greater than 1. As a specific example, a ferrofluid may include 5% or 10% nanoparticles (by volume) in a carrier fluid. The carrier fluid may be, for instance, water (optionally containing one or more salts) or another aqueous fluid, or an oil or other hydrophobic liquid such as octane, or an alkene such as hexane, decane, dodecane, hexadecane, or the like. Other examples of hydrophobic liquids that would be suitable include chloroform and dichloromethane. The ferrofluid optionally can include a surfactant, e.g., to prevent particle agglomeration. However, in some cases, a surfactant is not necessarily required, and other methods may be used to prevent particle agglomeration, for example, ionic charges on the particles. Non-limiting examples of potentially suitable surfactants include oleic acid, tetramethylammonium hydroxide, citric acid, or soy lecithin.

In some embodiments, the fluidic droplets may also contain additional entities, for example, other chemical, biochemical, or biological entities (which may be dissolved or suspended in the fluid in some cases), for example, monomers, polymers, metals, magnetizable materials, cells, gases, other fluids, or the like. Examples of species that may be contained within, or otherwise associated with, a fluidic droplet include, but are not limited to, pharmaceutical agents, drugs, DNA, RNA, proteins, fragrance, reactive agents, biocides, fungicides, preservatives, chemicals, or cells. As a specific example, the fluidic droplets may contain a monomer such as a silicon-containing monomer, which may be polymerized to form a polymer such as a polysiloxane. Other suitable polymers include any photocrosslinkable monomer and initiator combination such as 1 ,6-hexanediol diacylate as the monomer and Darocur 1173 or hydroxy-2-methyl-l- phenyl-1-propenone as the initiator. The monomer and the initiator may each be present in any suitable concentration, for example, about 2% initiator and about 98% monomer.

As another example, the fluidic droplet may contain a metal (including metal alloys) and/or a magnetizable material. Examples of potentially suitable metals and other materials, but are not limited to, iron, aluminum, nickel, cobalt, copper, titanium, neodymium, samarium, or the like. In some embodiments, one or more entities may be introduced by introducing the entities into a fluidic droplet before solidifying the fluidic droplet to form a particle. Such entities may be useful, for example, in forming magnetically susceptible particles, as discussed below.

In another set of embodiments, the fluidic droplets may contain a viscous agent, i.e., an agent that, when added to the fluid, increases the viscosity of the fluid relative to the fluid in the absence of the viscous agent. In some embodiments, the size of the fluidic droplet is at least partially determined by the viscosity of the fluids forming the fluidic droplet. For example, in one set of embodiments, the size of the fluidic droplet may be controlled by controlling one or more of the viscosity and/or the flow rate of the fluidic droplet and/or the fluid containing the fluidic droplet; the interfacial tension between the fluids; the diameter of the dimensional restriction (e.g., a nozzle) used to produce the fluidic droplet, or the like. For instance, the viscosity and/or the flow rate can be used to determine the amount of shear applied to the droplet, which may be used to determine the size of the fluidic droplets.

In yet another set of embodiments, the fluidic droplets may contain an emulsion stabilizing agent. The emulsion stabilizing agent may stabilize the emulsion, e.g., preventing the emulsion from phase separating during a time scale of interest (e.g., the time it takes a fluidic droplet to be transported through a particular system or device). In some cases, an agent may function both as an emulsion stabilizing agent and a viscous agent. A non-limiting example of such an agent is poly(vinyl alcohol), for instance, having a molecular weight of about 20,000. Other non-limiting examples include glycerol, alginate, sugars, or any polymer that is soluble in the continuous phase of the fluidic droplet.

In some embodiments, the fluidic droplets of the present invention may each be substantially the same shape and/or size ("monodisperse"). For example, the fluidic droplets may have a distribution of dimensions such that no more than about 10% of the fluidic droplets have a dimension greater than about 10% of the average dimension of the fluidic droplets, and in some cases, such that no more than about 8%, about 5%, about 3%, about 1%, about 0.3%, about 0.1%, about 0.03%, or about 0.01% have a dimension greater than about 10% of the average dimension of the fluidic droplets. In some cases, - l i ¬

no more than about 5% of the fluidic droplets have a dimension greater than about 5%, about 3%, about 1%, about 0.3%, about 0.1%, about 0.03%, or about 0.01% of the average dimension of the fluidic droplets.

The shape and/or size of the fluidic droplets can be determined, for example, by measuring the average diameter or other characteristic dimension of the droplets. The term "determining," as used herein, generally refers to the analysis or measurement of a species, for example, quantitatively or qualitatively, and/or the detection of the presence or absence of the species. "Determining" may also refer to the analysis or measurement of an interaction between two or more species, for example, quantitatively or qualitatively, or by detecting the presence or absence of the interaction. Examples of suitable techniques include, but are not limited to, spectroscopy such as infrared, absorption, fluorescence, UV/visible, FTIR ("Fourier Transform Infrared Spectroscopy"), or Raman; gravimetric techniques; ellipsometry; piezoelectric measurements; immunoassays; electrochemical measurements; optical measurements such as optical density measurements; circular dichroism; light scattering measurements such as quasielectric light scattering; polarimetry; refractometry; or turbidity measurements.

The "average diameter" of a plurality or series of droplets (or particles) is the arithmetic average of the average diameters of each of the droplets (or particles). Those of ordinary skill in the art will be able to determine the average diameter (or other characteristic dimension) of a plurality or series of droplets or particles, for example, using laser light scattering, microscopic examination, or other known techniques. The diameter of a droplet (or particle), in a non-spherical droplet, is the diameter of a perfect sphere having the same volume as the droplet (or particle). The average diameter of a droplet or particle (and/or of a plurality or series of droplets and/or particles) may be, for example, less than about 1 mm, less than about 500 micrometers, less than about 200 micrometers, less than about 100 micrometers, less than about 75 micrometers, less than about 50 micrometers, less than about 40 micrometers, less than about 25 micrometers, less than about 10 micrometers, less than about 5 micrometers, less than about 1 micrometer, less than about 0.3 micrometers, less than about 0.1 micrometers, less than about 0.03 micrometers, or less than about 0.01 micrometers in some cases. The average diameter of the droplet(s) and/or particle(s) may also be at least about 1 micrometer, at least about 2 micrometers, at least about 3 micrometers, at least about 5 micrometers, at least about 10 micrometers, at least about 15 micrometers, or at least about 20 micrometers in certain cases.

As mentioned, the fluid may be present within the liquid as one or more droplets. In some cases, the droplets may be formed in a device (e.g., a microfluidic device), which allows for the formation of fluidic droplets having a controlled size and/or size distribution. The device may be free of moving parts in some cases. That is, at the location or locations at which fluidic droplets of desired shape and/or size are formed, the device is free of components that move relative to the device as a whole to affect fluidic droplet formation. For example, where fluidic droplets of controlled shape and/or size are formed, the droplets are formed without parts that move relative to other parts of the device that define a channel within which the fluidic droplets flow. This can be referred to as "passive control," "passive breakup," or a "passive system."

In one example of a passive system, fluid may be urged through a dimensionally- restricted section of a channel of a fluidic device, which can cause the fluid to break up into a series of droplets within the channel. The dimensionally-restricted section can take any of a variety of forms. For example, it can be an annular orifice, elongate, ovoid, square, or the like. Preferably, it is shaped in any way that causes the surrounding liquid to surround and constrict the cross-sectional shape of the fluid being surrounded. The dimensionally-restricted section is non-valved in certain embodiments. That is, it is an orifice that cannot be switched between an open state and a closed state, and typically is of fixed size. One or more intermediate fluid channels can also be provided in some cases to provide an encapsulating fluid surrounding discontinuous portions of fluid being surrounded. Thus, in one embodiment, two intermediate fluid channels are provided, one on each side of a central fluid channel, each with an outlet near the central fluid channel. Control of the fluid flow rate, and ratio between the flow rates of the various fluids within the device, can be used to control the shape and/or size of the fluidic droplets, and/or the monodispersity of the fluidic droplets.

Some embodiments of the present invention involve formation of fluidic droplets in a liquid where the fluidic droplets have a mean cross-sectional dimension no smaller than the mean cross-sectional dimension of the dimensionally-restricted section. The invention, in such embodiments, may involve control over these mean cross-sectional dimensions by control of the flow rate of the fluid, liquid, or both, and/or control of the ratios of these flow rates. In other embodiments, the fluidic droplets have a mean cross- sectional dimension no smaller than about 90% of the mean cross-sectional dimension of the dimensionally-restricted section, and in still other embodiments, no smaller than about 80%, about 70%, about 60%, about 50%, about 40%, or about 30% of the mean cross-sectional dimension of the dimensionally-restricted section.

In another set of embodiments, droplets of fluid can be created in a channel from a fluid surrounded by a liquid by altering the channel dimensions in a manner that is able to induce the fluid to form individual droplets. The channel may, for example, be a channel that expands relative to the direction of flow, e.g., such that the fluid does not adhere to the channel walls and forms individual droplets instead, or a channel that narrows relative to the direction of flow, e.g., such that the fluid is forced to coalesce into individual droplets. In some embodiments, internal obstructions may also be used to cause droplet formation to occur. For instance, baffles, ridges, posts, or the like may be used to disrupt liquid flow in a manner that causes the fluid to coalesce into fluidic droplets. In some cases, the channel dimensions may be altered with respect to time (for example, mechanically, electromechanically, pneumatically, etc.) in such a manner as to cause the formation of individual fluidic droplets to occur. For example, the channel may be mechanically contracted ("squeezed") to cause droplet formation, or a fluid stream may be mechanically disrupted to cause droplet formation, for example, through the use of moving baffles, rotating blades, or the like.

As a non-limiting example, a schematic diagram of a device able to produce fluidic droplets is illustrated in Fig. 1. Briefly, a continuous liquid phase 12 is supplied from side channels 11 of the device, and a liquid stream 15 (e.g., containing a ferrofluid) is supplied from a center channel 14. In this geometry, the continuous liquid phase 12 surrounded the inner liquid stream 15; of course, in other embodiments, other arrangements are also possible. The resulting inner liquid stream has an unstable cylindrical morphology, and may break up within dimensional restriction 13 in a generally periodic manner to release fluidic droplets 19 contained within continuous liquid phase 12 into outlet channel 18.

Other techniques of producing droplets of fluid surrounded by a liquid are described in U.S. Patent Application Serial No. 11/024,228, filed December 28, 2004, entitled "Method and Apparatus for Fluid Dispersion," published as U.S. Patent Application Publication No. 2005/0172476 on August 11, 2005; U.S. Patent Application Serial No. 11/360,845, filed February 23, 2006, entitled "Electronic Control of Fluidic Species," published as U.S. Patent Application Publication No. 2007/000342 on January 4, 2007; or U.S. Patent Application Serial No. 11/368,263, filed March 3, 2006, entitled "Systems and Methods of Forming Particles," published as U.S. Patent Application Publication No. 2007/0054119 on March 8, 2007, each incorporated herein by reference. For example, in some embodiments, an electric charge may be created on a fluid surrounded by a liquid, which may cause the fluid to separate into individual droplets within the liquid.

In another aspect of the present invention, the fluidic droplets are solidified to form solid particles. Any technique able to solidify a fluidic droplet into a solid particle can be used. In one particular embodiment, a fluidic droplet containing a ferrofluid may be solidified to form a solid particle, which may be a magnetically susceptible particle in some cases. In other embodiments, a chemical reaction may be induced that causes the fluidic droplet to solidify (for example, a polymerization reaction, a reaction between two fluids that produces a solid product, etc.), a fluidic droplet may be cooled to a temperature below the melting point or glass transition temperature of a fluid within the fluidic droplet, or the like. Thus, according to one set of embodiments, a fluidic droplet containing a ferrofluid may be solidified to form a magnetically susceptible solid particle. For example, if the fluidic droplet contains a fluid containing nanoparticles, the fluid within the fluidic droplet may be partially or completely removed, and the nanoparticles contained therein may then form a solid particle, for instance, via agglomeration, and/or by forming a polymer particle containing the nanoparticles, as discussed below. The fluid containing the nanoparticles may be removed by any suitable technique, for example, physically and/or chemically. Non-limiting examples of suitable techniques include evaporation, diffusion, liquid extraction, or the like. The fluid may removed, for example, to concentrate any species that may be contained within the fluidic droplet. For instance, fluid can diffuse from or evaporate, concentrating any components of the droplet that do not substantially diffuse or evaporate. For example, the volume of a droplet can be reduced by more than about 50%, about 75%, about 90%, about 95%, about 99%, or about 99.9%. Thus, the radius of the droplet can be reduced by, for example, a factor of about 2, about 5, about 10, or more in some cases.

Other examples include deswelling or dialysis. For example, in one embodiment a fluidic droplet containing a first fluid (or other species) is placed in contact with another, essentially immiscible fluid that is not saturated with the first fluid. The first fluid within the fluidic droplet may then exit the fluidic droplet into the immiscible fluid, e.g., via processes such as osmosis or dialysis.

In some cases, the fluidic droplet may be solidified using a chemical reaction, such as a polymerization reaction. For example, a fluidic droplet containing nanoparticles may be caused to form a solid particle via a polymerization reaction, thereby resulting in a polymeric particle containing the nanoparticles. In some cases, the chemical reaction may occur before, during, or after other solidification techniques, for instance, techniques such as those described herein. As a specific example, a polymerization reaction may be initiated within a fluidic droplet using any suitable technique. For instance, the fluidic droplet may contain one or more monomer or oligomer precursors (e.g., dissolved and/or suspended within the fluidic droplet), which may polymerize to form a polymer that is solid. The polymerization reaction may occur spontaneously, or be initiated in some fashion, e.g., during formation of the fluidic droplet, or after the fluidic droplet has been formed. For instance, the polymerization reaction may be initiated by adding an initiator to the fluidic droplet, by applying light or other electromagnetic energy to the fluidic droplet (e.g., to initiate a photopolymerization reaction), by adding heat to the fluidic droplet, or the like.

Other examples of solidifying fluidic droplets are discussed in International Patent Application No. PCT/US2006/001938, filed January 20, 2006, entitled "Systems and Methods for Forming Fluidic Droplets Encapsulated in Particles Such as Colloidal Particles," published as WO 2006/078841 on July 27, 2006, incorporated herein by reference.

Certain embodiments of the invention are directed to the production of particles having substantially the same shape and/or size, as previously mentioned. For example, the particles may have a distribution of dimensions such that no more than about 5% of the particles have a dimension greater than about 10% of the average dimension of the particles, and in some cases, such that no more than about 3%, about 1%, about 0.3%, about 0.1%, about 0.03%, or about 0.01% have a dimension greater than about 10% of the average dimension of the particles. In some cases, no more than about 5% of the particles have a dimension greater than about 5%, about 3%, about 1%, about 0.3%, about 0.1%, about 0.03%, or about 0.01% of the average dimension of the particles, depending on the particular application. The average diameter of the particle may be, for instance, less than about 1 mm, less than about 500 micrometers, less than about 200 micrometers, less than about 100 micrometers, less than about 75 micrometers, less than about 50 micrometers, less than about 40 micrometers, less than about 25 micrometers, less than about 20 micrometers, less than about 10 micrometers, less than about 5 micrometers, less than about 1 micrometer, less than about 0.3 micrometers, less than about 0.1 micrometers, less than about 0.03 micrometers, or less than about 0.01 micrometers in some cases. The average diameter of the droplet(s) and/or particle(s) may also be at least about 1 micrometer, at least about 2 micrometers, at least about 3 micrometers, at least about 5 micrometers, at least about 10 micrometers, at least about 15 micrometers, or at least about 20 micrometers in certain cases.

Accordingly, the present invention relates, in one aspect, to the production of a plurality of solid particles, for example, magnetically susceptible particles such as those previously described. For instance, a fluid containing nanoparticles (e.g., a ferrofluid) may be formed into a plurality of fluid droplets, e.g., using a surrounding fluid, and at least a portion of the fluid containing the nanoparticles removed to cause the nanoparticles to form a solid particle contained within the surrounding fluid. As mentioned, the solid particle may be formed via agglomeration and/or a chemical reaction (e.g., a polymerization reaction) involving a precursor material contained within the fluid droplets. In certain embodiments of the invention, the droplets and/or particles formed from the droplets may be produced at relatively high frequencies. For example, the droplets and/or particles may be formed at frequencies between approximately 100 Hz and 5000 Hz. In some cases, the rate of production may be at least about 200 Hz, at least about 300 Hz, at least about 500 Hz, at least about 750 Hz, at least about 1,000 Hz, at least about 2,000 Hz, at least about 3,000 Hz, at least about 4,000 Hz, or at least about 5,000 Hz. In other embodiments, at least about 10 droplets and/or particles per second may be produced in some cases, and in other cases, at least about 20 droplets and/or particles per second, at least about 30 droplets and/or particles per second, at least about 100 droplets and/or particles per second, at least about 200 droplets and/or particles per second, at least about 300 droplets and/or particles per second, at least about 500 droplets and/or particles per second, at least about 750 droplets and/or particles per second, at least about 1000 droplets and/or particles per second, at least about 1500 droplets and/or particles per second, at least about 2000 droplets and/or particles per second, at least about 3000 droplets and/or particles per second, at least about 5000 droplets and/or particles per second, at least about 7500 droplets and/or particles per second, at least about 10,000 droplets and/or particles per second, at least about 15,000 droplets and/or particles per second, at least about 20,000 droplets and/or particles per second, at least about 30,000 droplets and/or particles per second, at least about 50,000 droplets and/or particles per second, at least about 75,000 droplets and/or particles per second, at least about 100,000 droplets and/or particles per second, at least about 150,000 droplets and/or particles per second, at least about 200,000 droplets and/or particles per second, at least about 300,000 droplets and/or particles per second, at least about 500,000 droplets and/or particles per second, at least about 750,000 droplets and/or particles per second, at least about 1,000,000 droplets and/or particles per second, at least about 1,500,000 droplets and/or particles per second, at least about 2,000,000 or more droplets and/or particles per second, or at least about 3,000,000 or more droplets and/or particles per second may be produced.

As mentioned, certain aspects of the invention are directed to the manipulation of ferrofluids and/or magnetically susceptible particles, e.g., contained within a fluidic device such as a microfluidic device. For example, a magnet may be used to manipulate the ferrofluids and/or the magnetically susceptible particle. The magnet may be any suitable magnet, for instance, a permanent magnet or an electromagnet. Non-limiting examples of magnets include magnetizable metals, for example, iron, nickel, or cobalt, or a magnetizable alloy such as neodymium-iron-boron, strontium ferrite, alnico, etc. The magnet may be used to control the movement of ferrofluid droplets and/or magnetically susceptible particles within a device. Following is a non-limiting example discussed with reference to Fig. 2. In this figure, within unit 20, a first fluidic stream 22, which may be a microfluidic stream, contains one or more ferrofluid droplets and/or magnetically susceptible particles 25. The first fluidic stream 22 may contact a second fluidic stream 23, and in some cases, there may be relatively little mixing between the first and second fluidic streams, for instance, if one or both streams are substantially laminar. As shown in Fig. 2, the first and second fluidic streams flow cocurrently, although other arrangements are also possible. A magnet 28 (e.g., a permanent magnet, an electromagnet, etc.) may be used to cause the ferrofluid droplets and/or magnetically susceptible particles 25 to move from first fluidic stream 22 to second fluidic stream 23. As shown here, the magnet is used in an attractive fashion, although in other embodiments, the magnet may also be used in a repulsive fashion, for instance, if the magnet and the droplets or particles had the same magnetic polarity. The ferrofluid droplets and/or magnetically susceptible particles 25 then exit unit 20 via second fluidic stream 23.

A device of the present invention, in some embodiments, may contain more than one of the systems shown in Fig. 2 and/or other systems able to manipulate fluidic droplets and/or particles. A specific, non-limiting example of this is shown in Fig. 3. In this figure, unit 20 is contained within device 30. A device 31 such as that shown in Fig. 1 may be used to create droplets of a fluid (e.g., a ferrofluid, or other fluid containing particles such as magnetically susceptible nanoparticles) within first fluidic stream 22. System 20 may then be used to direct the droplets from first fluidic stream 22 to second fluidic stream 23. Within second fluidic stream 23, the fluid may be solidified to form magnetically susceptible particles, for instance, as follows. In unit 33, the fluid containing the nanoparticles may be removed from the fluidic droplets, e.g., as previously described; in unit 34, a precursor within the fluidic droplets may be polymerized, thereby forming solid particles containing the magnetically susceptible nanoparticles, e.g., as described above. The magnetically susceptible particles may also be manipulated using a magnet.

For instance, in unit 35, similar to unit 20, the magnetically susceptible particles may be moved from second fluidic stream 23 to third fluidic stream 39 using a magnet 38. Similarly, the particles may be moved into a fourth stream 41 in unit 40 using magnet 48, etc., as necessary, depending on the particular application. In some cases, for example, the streams, fluids, and/or particles may also be subjected to other operations, for instance, characterization of a property of the stream and/or particle using a sensor in unit 45. Other unit operations may, for example, be used to sort droplets and/or particles, separate the droplets and/or particles (e.g., into a first population, such as an "acceptable" population and a second population, such as a "reject" population"), perform other chemical reactions using the droplets and/or particles, or the like. Non-limiting examples of potentially suitable operations are disclosed in, for example, U.S. Patent Application Serial No. 11/360,845, filed February 23, 2006, entitled "Electronic Control of Fluidic Species," published as U.S. Patent Application Publication No. 2007/000342 on January 4, 2007; U.S. Patent Application Serial No. 11/024,228, filed December 28, 2004, entitled "Method and Apparatus for Fluid Dispersion," published as U.S. Patent Application Publication No. 2005/0172476 on August 11, 2005; or U.S. Patent Application Serial No. 11/246,911 , filed October 7, 2005, entitled "Formation and Control of Fluidic Species," published as U.S. Patent Application Publication No. 2006/0163385 on July 27, 2006, each incorporated herein by reference.

In some aspects, the fluidic droplet or particle may be reacted with the second fluidic stream, and/or an agent contained within the second fluidic stream. For example, the second fluidic stream may contain an agent able to diffuse into the droplet or particle, and/or an agent that can be bound to the surface of a particle, depending on the particular application. Non-limiting examples of potentially useful agents include biochemical species such as nucleic acids such as siRNA, RNAi and DNA, proteins, peptides, enzymes, proteins, indicators, dyes, fluorescent species, antibodies, polymers, or the like. In another aspect, a property of a fluidic droplet and/or a particle may be determined, for example as illustrated in Fig. 3. In certain embodiments of the invention, one or more sensors are provided that can sense and/or determine one or more characteristics of the fluidic droplets or particles, and/or a characteristic of a portion of the fluidic system containing the droplets or particles (e.g., a liquid surrounding the fluidic droplet) in such a manner as to allow the determination of one or more characteristics of the droplets or particles. In one set of embodiments, the sensor may be used to control formation or production of the droplets and/or particles. For instance, information about a characteristic of a particle or droplet may be used to control the formation process of the particle or droplet, for example, by altering a flowrate, a concentration, the temperature, of the device, or of a fluid or other component of the device. Characteristics determinable with respect to the droplet and usable in the invention can be identified by those of ordinary skill in the art. Non-limiting examples of such characteristics include fluorescence, spectroscopy (e.g., optical, infrared, ultraviolet, etc.), radioactivity, mass, volume, density, temperature, viscosity, pH, concentration of a substance, such as a biological substance (e.g., a protein, a nucleic acid, etc.), or the like. Other non-limiting examples of such sensors include sensors able to determine electrical characteristics or magnetic characteristics.

Non-limiting examples of sensors useful in the invention include optical or electromagnetically-based systems. For example, the sensor may be a fluorescence sensor, a microscopy system (which may include a camera or other recording device), or the like. As another example, the sensor may be an electronic sensor, e.g., a sensor able to determine an electric field or other electrical characteristic. For example, the sensor may detect capacitance, inductance, etc., of a fluidic droplet or particle and/or the portion of the fluidic system containing the fluidic droplet or particle. Any suitable property may be sensed or otherwise determined. The property may be a physical property, such as size, density, color (e.g., fluorescence or opacity), temperature, etc., and/or a chemical or a biological property. For instance, the fluidic droplet may contain a sensing entity which can be determined in some fashion, e.g., optically or spectrally. A sensing entity may be one that can interact with another entity such as an analyte (e.g., a chemical, biochemical, and/or biological species) in such a manner to cause a determinable change in a property of the sensing entity. As an example, a sensing entity may fluoresce if a certain analyte is present within the fluidic droplet. For instance, the sensing entity may comprise a binding partner to which the analyte binds. Specific examples include antibody/antigen, antibody/hapten, enzyme/substrate, enzyme/inhibitor, enzyme/cofactor, binding protein/substrate, carrier protein/substrate, lectin/carbohydrate, receptor/hormone, receptor/effector, complementary strands of nucleic acid, protein/nucleic acid repressor/inducer, ligand/cell surface receptor, virus/ligand, etc. The sensing entity, when it comprises a binding partner, can comprise a specific binding partner of an analyte. For example, the binding partner entity may be a nucleic acid, an antibody, a sugar, a carbohydrate, a protein, an enzyme, etc. In one aspect, particles may be produced using various devices of the invention "on demand." For instance, the device may be capable of producing a plurality of different particles, and a user may determine which particles the device produces. The device may have and/or be connected to one or more reservoirs, which can contain agents, fluidic species, etc., which can be reacted by the device to produce the desired particle. In some cases, a control panel, or a computer may be used to control production of the particles. For instance, the user may input a desired particle, and/or reaction conditions or reagents used to produce the desired particle, and the computer, running a suitable program (e.g., on a suitable computer readable medium or memory storage device), may then operate the device to produce the desired particle. Those of ordinary skill in the art will be able to program and operate a computer to perform tasks such as those described herein.

A variety of materials and methods, according to certain aspects of the invention, can be used to form the fluidic or microfluidic system. For example, various components of the invention can be formed from solid materials, in which the channels can be formed via micromachining, film deposition processes such as spin coating and chemical vapor deposition, laser fabrication, photolithographic techniques, etching methods including wet chemical or plasma processes, and the like. See, for example, Scientific American, 248:44-55, 1983 (Angell, et at). In one set of embodiments, at least a portion of the fluidic system is formed of silicon by etching features in a silicon chip. Technologies for precise and efficient fabrication of various fluidic systems and devices of the invention from silicon are known. In another embodiment, various components of the systems and devices of the invention can be formed of a polymer, for example, an elastomeric polymer such as polydimethylsiloxane ("PDMS"), polytetrafluoroethylene ("PTFE" or Teflon^®), or the like. For instance, according to one embodiment, system 10 shown in Fig. 1 may be implemented by fabricating the fluidic system separately using PDMS or other soft lithography techniques (details of soft lithography techniques suitable for this embodiment are discussed in the references entitled "Soft Lithography," by Younan Xia and George M. Whitesides, published in the Annual Review of Material Science, 1998, Vol. 28, pages 153-184, and "Soft Lithography in Biology and Biochemistry," by George M. Whitesides, Emanuele Ostuni, Shuichi Takayama, Xingyu Jiang and Donald E. Ingber, published in the Annual Review of Biomedical Engineering, 2001, Vol. 3, pages 335-373; each of these references is incorporated herein by reference).

Different components can be fabricated of different materials. For example, a base portion including a bottom wall and side walls can be fabricated from an opaque material such as silicon or PDMS, and a top portion can be fabricated from a transparent or at least partially transparent material, such as glass or a transparent polymer, for observation and/or control of the fluidic process. Components can be coated so as to expose a desired chemical functionality to fluids that contact interior channel walls, where the base supporting material does not have a precise, desired functionality. For example, components can be fabricated as illustrated, with interior channel walls coated with another material. Material used to fabricate various components of the systems and devices of the invention, e.g., materials used to coat interior walls of fluid channels, may desirably be selected from among those materials that will not adversely affect or be affected by fluid flowing through the fluidic system, e.g., material(s) that is chemically inert in the presence of fluids to be used within the device.

In some embodiments, various components of the invention are fabricated from polymeric and/or flexible and/or elastomeric materials, and can be conveniently formed of a hardenable fluid, facilitating fabrication via molding (e.g. replica molding, injection molding, cast molding, etc.). The hardenable fluid can be essentially any fluid that can be induced to solidify, or that spontaneously solidifies, into a solid capable of containing and/or transporting fluids contemplated for use in and with the fluidic network. In one embodiment, the hardenable fluid comprises a polymeric liquid or a liquid polymeric precursor (i.e. a "prepolymer"). Suitable polymeric liquids can include, for example, thermoplastic polymers, thermoset polymers, or mixture of such polymers heated above their melting point. As another example, a suitable polymeric liquid may include a solution of one or more polymers in a suitable solvent, which solution forms a solid polymeric material upon removal of the solvent, for example, by evaporation. Such polymeric materials, which can be solidified from, for example, a melt state or by solvent evaporation, are well known to those of ordinary skill in the art. A variety of polymeric materials, many of which are elastomeric, are suitable, and are also suitable for forming molds or mold masters, for embodiments where one or both of the mold masters is composed of an elastomeric material. A non-limiting list of examples of such polymers includes polymers of the general classes of silicone polymers, epoxy polymers, and acrylate polymers. Epoxy polymers are characterized by the presence of a three- membered cyclic ether group commonly referred to as an epoxy group, 1 ,2-epoxide, or oxirane. For example, diglycidyl ethers of bisphenol A can be used, in addition to compounds based on aromatic amine, triazine, and cycloaliphatic backbones. Another example includes the well-known Novolac polymers. Non-limiting examples of silicone elastomers suitable for use according to the invention include those formed from precursors including the chlorosilanes such as methylchlorosilanes, ethylchlorosilanes, phenylchlorosilanes, etc. Silicone polymers are used in certain embodiments, for example, the silicone elastomer polydimethylsiloxane. Non-limiting examples of PDMS polymers include those sold under the trademark Sylgard by Dow Chemical Co., Midland, MI, and particularly Sylgard 182, Sylgard 184, and Sylgard 186. Silicone polymers including PDMS have several beneficial properties simplifying fabrication of the microfluidic structures of the invention. For instance, such materials are inexpensive, readily available, and can be solidified from a prepolymeric liquid via curing with heat. For example, PDMSs are typically curable by exposure of the prepolymeric liquid to temperatures of about, for example, about 65 ⁰C to about 75 ⁰C for exposure times of, for example, about an hour. Also, silicone polymers, such as PDMS, can be elastomeric and thus may be useful for forming very small features with relatively high aspect ratios, necessary in certain embodiments of the invention. Flexible (e.g., elastomeric) molds or masters can be advantageous in this regard.

One advantage of forming structures such as microfluidic structures of the invention from silicone polymers, such as PDMS, is the ability of such polymers to be oxidized, for example by exposure to an oxygen-containing plasma such as an air plasma, so that the oxidized structures contain, at their surface, chemical groups capable of cross-linking to other oxidized silicone polymer surfaces or to the oxidized surfaces of a variety of other polymeric and non-polymeric materials. Thus, components can be fabricated and then oxidized and essentially irreversibly sealed to other silicone polymer surfaces, or to the surfaces of other substrates reactive with the oxidized silicone polymer surfaces, without the need for separate adhesives or other sealing means. In most cases, sealing can be completed simply by contacting an oxidized silicone surface to another surface without the need to apply auxiliary pressure to form the seal. That is, the pre- oxidized silicone surface acts as a contact adhesive against suitable mating surfaces. Specifically, in addition to being irreversibly sealable to itself, oxidized silicone such as oxidized PDMS can also be sealed irreversibly to a range of oxidized materials other than itself including, for example, glass, silicon, silicon oxide, quartz, silicon nitride, polyethylene, polystyrene, glassy carbon, and epoxy polymers, which have been oxidized in a similar fashion to the PDMS surface (for example, via exposure to an oxygen-containing plasma). Oxidation and sealing methods useful in the context of the present invention, as well as overall molding techniques, are described in the art, for example, in an article entitled "Rapid Prototyping of Microfluidic Systems and

Polydimethylsiloxane," Anal. Chem., 70:474-480, 1998 (Duffy et al.), incorporated herein by reference.

Another advantage to forming microfluidic structures of the invention (or interior, fluid-contacting surfaces) from oxidized silicone polymers is that these surfaces can be much more hydrophilic than the surfaces of typical elastomeric polymers (where a hydrophilic interior surface is desired). Such hydrophilic channel surfaces can thus be more easily filled and wetted with aqueous solutions than can structures comprised of typical, unoxidized elastomeric polymers or other hydrophobic materials.

In one embodiment, a bottom wall is formed of a material different from one or more side walls or a top wall, or other components. For example, the interior surface of a bottom wall can comprise the surface of a silicon wafer or microchip, or other substrate. Other components can, as described above, be sealed to such alternative substrates. Where it is desired to seal a component comprising a silicone polymer (e.g. PDMS) to a substrate (bottom wall) of different material, the substrate may be selected from the group of materials to which oxidized silicone polymer is able to irreversibly seal (e.g., glass, silicon, silicon oxide, quartz, silicon nitride, polyethylene, polystyrene, epoxy polymers, and glassy carbon surfaces which have been oxidized). Alternatively, other sealing techniques can be used, as would be apparent to those of ordinary skill in the art, including, but not limited to, the use of separate adhesives, thermal bonding, solvent bonding, ultrasonic welding, etc.

The following examples are intended to illustrate certain embodiments of the present invention, but do not exemplify the full scope of the invention. EXAMPLE 1

The devices in this example are based on the devices illustrated in International Patent Application No. PCT/US2006/007772, filed March 3, 2006, entitled "Method and Apparatus for Forming Multiple Emulsions," published as WO 2006/096571 on September 14, 2006, incorporated herein by reference. This device may include a round capillary with a nozzle for an inner fluid. This capillary is inserted into a collection tube where the droplets are formed. Both capillaries may be centered in a square capillary. This square capillary can be also used to direct the continuous phase (outer fluid).

An example of this device is shown in Fig. 4 (Fig. 4A shows a side view, while Fig. 4B shows a top view). In this figure, device 50 includes a first inlet 51 (e.g., a nozzle) for the fluid used to form fluidic droplets (the "dispersed" phase), and a second inlet 52 for a surrounding fluid (the "continuous" phase). In this figure, inlet 51 is fluidically connected to a nozzle 55, which extrudes the dispersed phase. The nozzle is placed inside a collection tube 57. The nozzle may have a diameter down to 1 micron. Collection tube 57, in this example, is a capillary with a diameter of about 100 micrometers to about 200 micrometers, and can be pulled and tapered at the end where the nozzle is inserted. The diameter of the collection tube where the nozzle sits is about 50 to 70 micrometers. This dimension is not critical but can be used to determine the flow rate to obtain a specific drop size for one given continuous phase fluid, as shown by droplets 61 contained within fluid 63.

In addition, a purge valve 53 may be placed at one end of a square capillary 58 used for centering nozzle 55 and collection tube 57. This purge valve, closed during regular operation of the device, can allow air to be removed from the device during the filling process. It also allows the device to be cleaned in the event of minor clogging in the collection tube. A second purge valve (not shown) can also be placed just upstream of the nozzle capillary. This valve may be used to relieve the air in the tubing as the fluid fills the device. The valve remains open until fluid starts flowing from it. This valve may allow a smooth filling process.

In one embodiment, the emulsion may be collected as follows. A vial with an inlet and outlet can be connected to the collection tube to concentrate the resulting emulsion. The vial has an inlet that brings in the emulsion and an outlet that expels only continuous phase due to differences in density between the continuous and the dispersed phase. The positions of the inlet and the outlet depend on the density difference between the two phases.

A non-limiting example of operation of this particular device follows (of course, in other embodiments, other procedures may be used). Here, the inner fluid channel can be filled first, which allows the removal all air from the nozzle. At first, all the purge valves are open. Once inner fluid comes out of the second purge valve, this purge valve is closed and the flow rate is reduced to about 100 microliters/hr. Once inner fluid starts coming out of the nozzle, the flow rate is lowered to microliters/hr and the outer flow rate is increased to 5000 microliters/hr. After all of the air has been escaped the device, the first purge valve is closed. The working flow rates can then be set, depending on the desired droplet size, and the system can be allowed to reach steady state.

EXAMPLE 2

This example illustrates particle fabrication to produce magnetically susceptible particles on demand using a microfluidic device. It includes a series of modules or units, shown schematically in Fig. 3. The particles are produced as an emulsion made from a ferrofluid, using a capillary microfluidic device. The emulsion drops then go to a deswelling module, where excess internal phase oil is removed, leaving a solid, spherical particle of close-packed magnetic grains. The size of these particles is completely adjustable, and they will be very monodisperse, in size and/or in magnetic content. Once formed, chemistry can be performed on the particle core, and/or on its surface, using a chemistry module. This module includes a second entrance to the channel to bring a new stream of fluid (e.g., water), containing suitable reactants. The laminar flow characteristic of microfluidic devices will maintain this stream separate from the stream bearing the particles. A magnetic field will be used to pull the particles from the original stream into the new stream, as shown in Fig. 2. This is a module that can be used for any sort of chemistry; moreover, the reactants can be changed by having a fluidic switch upstream to bring different materials into the reactant stream, as necessary. This can thus provide great selectivity and flexibility of this device. Following this chemical reaction, a similar module can be used as a wash step, if necessary. Additional chemical reaction modules can be used if more complex chemistry is desired, depending on the particular application. The types of chemistry that can be used with this device are many. For example, chemistry on the particle can be accomplished by adding some reactants that diffuse into the core, or monomers introduced with the original ferrofluid may be polymerized to create a polymeric particle. Non-limiting examples of such monomers include Si monomers to create a Si particle with embedded magnetic nanoparticles.

As another example, surface chemistry may be used in biotechnological applications involving these particles. For instance, specific biomolecular adsorbates can be introduced in the reactant stream of a chemistry module and then grafted or otherwise bonded onto the surface of the particles. The advantage of this microfluidic method is that by changing the contents of the reactant stream, new types of particles can be produced, each having distinct surface-grafted biomolecules. As another example, polymers could be grafted or otherwise bonded onto the surface of the particles, e.g., to minimize non-specific binding of proteins to the beads. After each grafting step, a similar chemistry module can be incorporated to wash the particles, if necessary. This strategy is very flexible and controllable. It allows essentially all currently- used batch processes to be used with this device. Control of the particles may be facilitated by allowing a magnetic field to be used to divert them from one fluidic stream to another, as necessary.

Further downstream, an optional sensor module may optionally be used. For instance, the sensor module may perform an assay for which the particles are being made. This assay can utilize any suitable detection, for instance, optical detection, and the results of this module can, for instance, be used to provide direct feedback to control the input of the chemistry module, or of the production of the particles. In some cases, this may be used to optimize the performance of the particles in the test for which they are designed.

This magnetic bead production line shown in this example represents a generic way of producing magnetic colloidal particles from fluid precursors. These particles can be optimized for a wide variety of different applications, in biotechnology, chemistry, etc. The particles can all be produced to be highly uniform and identical. This method can also be adapted to produce particles only when needed, and/or the particles can be changed rapidly to meet different requirements. While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the functions and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings of the present invention is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the scope of the present invention. All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms.

The indefinite articles "a" and "an," as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean "at least one."

The phrase "and/or," as used herein in the specification and in the claims, should be understood to mean "either or both" of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with "and/or" should be construed in the same fashion, i.e., "one or more" of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the "and/or" clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to "A and/or B", when used in conjunction with open-ended language such as "comprising" can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

As used herein in the specification and in the claims, "or" should be understood to have the same meaning as "and/or" as defined above. For example, when separating items in a list, "or" or "and/or" shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as "only one of or "exactly one of," or, when used in the claims, "consisting of," will refer to the inclusion of exactly one element of a number or list of elements. In general, the term "or" as used herein shall only be interpreted as indicating exclusive alternatives (i.e. "one or the other but not both") when preceded by terms of exclusivity, such as "either," "one of," "only one of," or "exactly one of." "Consisting essentially of," when used in the claims, shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase "at least one," in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase "at least one" refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, "at least one of A and B" (or, equivalently, "at least one of A or B," or, equivalently "at least one of A and/or B") can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. It should also be understood that, unless clearly indicated to the contrary, in any methods claimed herein that include more than one step or act, the order of the steps or acts of the method is not necessarily limited to the order in which the steps or acts of the method are recited. In the claims, as well as in the specification above, all transitional phrases such as

"comprising," "including," "carrying," "having," "containing," "involving," "holding," "composed of," and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases "consisting of and "consisting essentially of shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.

What is claimed is:

Claims

1. A method, comprising: providing a first fluid stream comprising a liquid containing magnetically susceptible nanoparticles; surrounding the first fluid stream with a second fluid stream; causing the first fluid stream to form a plurality of droplets; and removing at least a portion of the liquid from the plurality of droplets.

2. The method of claim 1, further comprising causing the magnetically susceptible nanoparticles within each droplet to form a solid particle.

3. The method of claim 1, wherein the liquid of the first fluid stream is essentially immiscible in water.

4. The method of claim 1, wherein the second fluid stream is substantially miscible in water.

5. The method of claim 1 , wherein no more than about 5% of the particles have a dimension greater than about 10% of the average dimension.

6. The method of claim 1, wherein no more than about 5% of the droplets have a diameter greater than about 1% of the average diameter.

7. The method of claim 1, wherein no more than about 5% of the droplets have a diameter greater than about 0.1% of the average diameter.

8. The method of claim 1, wherein no more than about 5% of the droplets have a diameter greater than about 0.01% of the average diameter.

9. The method of claim 1, wherein no more than about 1% of the droplets have a diameter greater than about 10% of the average diameter.

10. The method of claim 1, wherein no more than about 0.1% of the droplets have a diameter greater than about 10% of the average diameter.

11. The method of claim 1 , wherein no more than about 0.01 % of the droplets have a diameter greater than about 10% of the average diameter.

12. The method of claim 1, wherein the solid particles have an average dimension of less than about 50 micrometers.

13. The method of claim 1 , wherein the solid particles have an average dimension of less than about 20 micrometers.

14. The method of claim 1, wherein the solid particles have an average dimension of between about 1 micrometer and about 20 micrometers.

15. The method of claim 1 , wherein the liquid of the first fluid stream is removed through evaporation or diffusion.

16. The method of claim 1 , further comprising manipulating at least some of the solid particles using a magnet.

17. The method of claim 1, wherein the first fluid further comprises a polymer.

18. The method of claim 1 , the first fluid further comprising a precursor material, the method further comprising the act of causing the precursor material to solidify.

19. The method of claim 18, wherein the act of causing the precursor material to solidify comprises exposing the precursor to ultraviolet light.

20. The method of claim 18, wherein the act of causing the precursor material to solidify comprises exposing the precursor to heat.

21. The method of claim 1 , wherein the first fluid stream further comprises an emulsion stabilizing agent.

22. The method of claim 1, wherein the first fluid stream further comprises a viscous agent.

23. The method of claim 1, wherein the first fluid stream further comprises poly( vinyl alcohol).

24. The method of claim 1, comprising producing the plurality of droplets at a rate of at least about 5,000 droplets per second.

25. The method of claim 1 , comprising producing the plurality of droplets at a rate of at least about 10,000 droplets per second.

26. A collection of articles comprising a plurality of particles, the particles have an average dimension of less than about 500 micrometers and a distribution of dimensions such that no more than about 5% of the particles have a dimension greater than about 10% of the average dimension, wherein at least some of the plurality of particles comprise uniformly distributed magnetically susceptible nanoparticles.

27. The collection of articles of claim 26, wherein no more than about 5% of the particles have a dimension greater than about 10% of the average dimension.

28. The collection of articles of claim 26, wherein no more than about 5% of the particles have a diameter greater than about 1% of the average diameter.

29. The collection of articles of claim 26, wherein no more than about 5% of the particles have a diameter greater than about 0.1% of the average diameter.

30. The collection of articles of claim 26, wherein no more than about 5% of the particles have a diameter greater than about 0.01% of the average diameter.

31. The collection of articles of claim 26, wherein no more than about 1 % of the particles have a diameter greater than about 10% of the average diameter.

32. The collection of articles of claim 26, wherein no more than about 0.1 % of the particles have a diameter greater than about 10% of the average diameter.

33. The collection of articles of claim 26, wherein no more than about 0.01% of the particles have a diameter greater than about 10% of the average diameter.

34. The collection of articles of claim 26, wherein at least some of the particles each comprises a polymer.

35. The collection of articles of claim 26, wherein at least some of the particles each comprises poly(vinyl alcohol).

36. A method, comprising: providing a first fluid stream comprising a liquid containing particles formed from a plurality of magnetically susceptible nanoparticles; and moving the particles from the first microfluidic stream to a second fluidic stream using a magnet.

37. The method of claim 36, wherein the particles are formed by removing a fluid from a ferrofluid comprising magnetically susceptible nanoparticles.

38. The method of claim 36, further comprising reacting the particles in the second fluidic stream with an agent in the second fluidic stream.

39. The method of claim 38, comprising binding the agent to the surfaces of the particles.

40. The method of claim 36, further comprising polymerizing a precursor contained within the particles.

41. The method of claim 36, comprising polymerizing the precursor to form a siloxane.

42. The method of claim 36, wherein the first fluid stream and the second fluid stream are each substantially laminar.

43. The method of claim 36, wherein the first fluid stream and the second fluid stream are each microfluidic.

44. The method of claim 36, wherein the first fluid stream and the second fluid stream flow cocurrently.

45. The method of claim 36, wherein the magnet is an electromagnet.

46. The method of claim 36, wherein the magnet is a permanent magnet.

47. The method of claim 36, further comprising determining the particles in the second fluidic stream.

48. The method of claim 47, comprising optically determining the particles.

49. The method of claim 47, further comprising controlling production of the particles contained within the first fluid stream based on the determination of the particles in the second fluidic stream.