Devices utilizing self-assembled gel and method of manufacture

[0001] This application claims priority from provisional application Serial No. 60/328,595, filed on Oct. 11, 2001, which is incorporated herein by reference.

[0003] Devices containing gels, such as microfluidic devices, find use in many analytical detection techniques. For example, capillary electrophoresis is used for DNA profiling, analysis of pharmaceuticals, and detection and analysis of proteins and peptides.

[0004] A typical capillary for capillary electrophoresis is made of glass. Also used are polymeric materials, such as poly(dimethylsiloxane) (PDMS) silicone elastomer. The use and fabrication of PDMS devices are described by Effenhauser et al., Integrated Capillary Electrophoresis on Flexible Silicone Microdevices: Analysis of DNA Restriction Fragments and Detection of Single DNA Molecules on Microchips, Anal. Chem. (1997) 69, 3451-3457 which is incorporated herein by reference. An advantage of polymeric materials is fabrication costs are less than the fabrication costs with glass, and thus arrays of capillaries in a detection device or on a chip can be fabricated at lower cost with elastomers than glass. However, a limitation on use of PDMS and other polymeric material capillaries is they have less structural strength than glass.

[0005] Because of the lower structural strength of some polymeric capillaries, they cannot be used with high viscosity gels, because high pressures are required to introduce the gel into a microchannel. Thus many polymeric capillaries are limited to use with low viscosity gels or linear polymers as the separation fluid. High viscosity gels have advantages in some applications, particularly where high resolution is needed. For example, low viscosity gels are not effective for DNA sequencing on microchips, because high base-pair resolution is needed with relatively short separation channels, which requires a high viscosity gel.

[0006] Attempts have been made to form high viscosity gels in situ, starting with a low viscosity liquid. For example, chemically cross-linked gels have been used in capillary electrophoresis. However, in situ chemically cross-linked gels are difficult to fabricate inside capillaries, partly due to the shrinkage that occurs during the cross-linking reaction. In addition, the cross-linking chemistry includes an additional step in fabrication of the device, which increases cost. Furthermore, the gelation is irreversible, i.e., a matrix cannot be replaced after the formation of the gel.

[0007] An attempt was made to use temperature control to form a high viscosity gel in situ from a gel precursor with low viscosity. The gel precursor was designed so that a temperature change yields a high viscosity gel. See, Wu, C. H. et al. (1998), Viscosity-Adjustable Block Copolymer for DNA Separation by Capillary Electrophoresis. Electrophoresis 19(2):231-241. However, this techniques requires temperature control during the fabrication process, and results in a limited temperature range in which the device can be used. If the temperature changes, the gel can change properties and even become very fluid.

[0008] Accordingly, there is a need for an easy method of fabricating devices utilizing a high viscosity gel, without the need for the fabrication pressures associated with high viscosity gels, and where the device does not have a narrow temperature range in use.

[0009] The present invention satisfies this need with a unique low viscosity gel precursor that can easily be converted to a high viscosity self-assembled gel without any temperature change required. The gel precursor is formed from a water soluble polymer having hydrophobic moieties, typically hydrophobic end groups. In its low viscosity gel precursor state, the water soluble polymer is in a solvent mixture comprising water and an organic solvent, where in the absence of the organic solvent, the polymer forms a self-assembled gel. The gel precursor contacts a substrate that comprises a permeable material that has higher permeability for the organic solvent than water. The organic solvent diffuses through the permeable material, resulting in formation of the self-assembled gel. A preferred permeable material is PDMS silicone elastomer.

[0010] In one version of the invention, a microfluidic device such as a capillary for capillary electrophoresis can be made at least partly of the permeable material, and when the self-assembling gel is introduced into the capillary, the organic solvent diffuses through the permeable material, and the self-assembled gel forms.

[0011] In another version of the invention, the self-assembling gel precursor is placed on a first substrate which is substantially hydrophobic which can be substantially non-permeable. The precursor is then contacted with a second substrate comprising a permeable material such as PDMS, resulting in the formation of the self-assembled gel on the first substrate. If the second substrate has a plurality of holes or voids in it, the resulting product can be an array of separated regions containing the self-assembled gel formed on the first substrate.

[0012] A reagent, such as a biopolymer, can be immobilized in the gel. The reagent can be an enzyme so that gel can be used for enzymatic reactions, or an antibody for detecting a particular antigen.

[0013] Thus, according to the present invention, devices having a high viscosity gel, with or without an immobilized reagent, can easily be fabricated because a low viscosity gel precursor is used.

[0014] These and other features, aspects and advantages of the present invention will become better understood with reference to the following description, appended claims, and accompanying drawings where FIGS. 1-3 schematically show the formation of self-assembled gel regions on a substrate.

[0015] The present invention utilizes gel precursors, also known as self-assembling gels, that upon a change in solvent composition, form a self-assembled gel.

[0016] The gel precursor is made of a water-soluble hydrophilic polymer modified with hydrophobic groups. Suitable gels are described by Menchen et al., Synthesis, Characterization, and Rheological Behavior of Polyethyelene Glycols End-Capped with Fluorocarbon Hydrophobes, (1997), 13, 2447-2456, which is incorporated herein by reference. The method taught by Menchen et al. can be used to form the gel precursor.

[0017] Exemplery of suitable water-soluble hydrophilic polymers and poly(ethylene glycol) (PEG), poly(vinyl alcohol), poly(vinyl pyrrolidone), are poly(acrylamide).

[0018] The molecular weight of the polymer is sufficiently high to form a gel, but sufficiently low that the gel precursor is a low viscosity fluid. The polymer before modification with a hydrophobic group generally has a molecular weight of at least 2,000, and generally up to about 100,000.

[0019] The hydrophobic moiety is usually attached to the hydrophilic polymer as an end group, but it need not be. Preferably there is a hydrophobic group at each end of the hydrophilic polymer.

[0020] The hydrophobic moiety can be an alkyl group, which preferably is fluorinated, and most preferably is fully fluorinated. Exemplary of suitable compounds for attachment to PEG to yield an attached hydrophobic moiety are branched C_nF_2n1C_mH_2mO; C_mH_2m1O); or short blocks of a hydrophobic polymer such as polystyrene polyisoprene, polybutadiene or their hydrogenated or fluorinated analogues. Typically n can be 6 and m can be 2.

[0021] Generally only one type of hydrophobic group is used, although in some circumstances, different hydrophobic groups can be used with the hydrophilic polymer.

[0022] The solvent mixture used contains water and at least one organic solvent that is miscible with water and is capable of dissolving the end group. Generally, a polar solvent is used. Solvents that can be used include methanol, ethanol, N-methylpyrollidone, and tetrahydrofuran. Generally, only one organic solvent is used, but more than one can be used.

[0023] To prevent premature gelation and to have a low viscosity, preferably the concentration of the modified polymer in the solvent is less than 20%, and it can be as low as 1% by weight.

[0024] There needs to be sufficient organic solvent in the solvent mixture to prevent premature gelation. It is found that a 50%/50% by weight mixture is satisfactory.

[0025] By removal of the organic solvent, a self-assembled, high viscosity gel is formed. The self-assembled gel has a viscosity of at least about 20 Pa s at low shear rates. This occurs when about 90% of the organic solvent is removed. Generally, it is preferred that substantially all of the organic solvent is removed to form a homogeneous hydrogel. Removal is effected with a permeable material that is substantially more permeable to the organic solvent than to water. The permeable material selectively removes the organic solvent(s) from the solvent mixture. Preferably, the permeability of the organic solvent through the permeable material is at least five times greater than the permeability of water through the permeable material. A suitable permeable material is PDMS silicone elastomer. Other suitable materials are polymethylmethacrylate and polycarbonate. A polyolefin elastomer, a polyurethane, a Kraton-type thermoplastic elastomer, or polynorbornene can be suitable, particularly if the organic solvent is non-polar.

[0026] Thus, by contacting the gel precursor with PDMS silicone elastomer, a self-assembled gel is formed. This occurs quickly and can occur in just a few minutes at room temperature and ambient pressure within gaps, channels or voids.

[0027] Preferably the gel precursor contains no constituents that interfere with the performance of the formed gel. For example, for capillary electrophoresis, it is desirable to have a homogenous gel, and thus preferably the gel precursor contains no electrolytes that are attached to the polymer that can interfere with the performance of the gel as a separation medium for capillary electrophoresis.

[0028] Different types of articles can be made utilizing the gel precursor. In one type of article, the permeable material forms part of the finished article. In another type of article, the permeable material is not part of the finished article, but is only used to manufacture the article.

[0029] Exemplary of the first type are microfluidic devices, such as capillaries. The term capillaries is used to refer to a structure having at least one dimension smaller than about 200 microns. Capillaries typically have diameters of from 10 to about 200 microns, and can have a length as long as 50 cm. Exemplary of such devices are flexible silicone microdevices described in the aforementioned Effenhauser et al. article. The gel precursor is introduced, such as by a syringe, into the channels of the device. The organic solvent diffuses out through the permeable material, resulting in formation of the self-assembled hydrogel.

[0030] It is not necessary that the entire device be formed of the permeable material. All that is necessary is that there may be some permeable material in contact with the gel precursor, so that the organic solvent can diffuse out of solution.

[0031] In the second type of device, the substrate on which the gel is formed need not be a permeable material. For example, the precursor gel can be placed on a first substrate made of substantially non-permeable hydrophobic material, such as a glass plate. Then a second substrate comprising a permeable material is placed in contact with the gel precursor, resulting in the formation of the gel. For example, a stamp having voids therein can be used to print patterns of hydrogel on a surface of the first substrate. The stamp can be made of PDMS, and can be pressed down on the glass surface having the gel precursor. The gel precursor is pressed into the voids, and as the solvent diffuses through the PDMS, gel is formed in the voids. This results in the first substrate having multiple separated regions of gel thereon. The regions can be in the form of an array.

[0032] For example, as shown in FIGS. 1-3, a substrate 10 such as glass that has been silanized has a gel precursor 12 placed thereon, and is then pressed with a stamp 14 having a plurality of voids 16 or holes therein. The stamp 14 is made of PDMS. The organic solvent in the gel precursor diffuses from the gel precursor solution, resulting in the formation of gel regions 18 where the voids 16 are located.

[0033] Biopolymers can be incorporated into the gel. The term biopolymers is meant to include all molecules of biological interest, such as catalysts; drugs; DNA; RNA; and peptides and proteins, including antibodies, ligands, receptors and enzymes.

[0034] A biopolymer can be incorporated by physical entrapment or modified for incorporation into the gel through pegylation reactions such as those described by Veronese, F. N. (2001) Peptide and Protein PEGylation: A review of Problems and Solutions, Biomaterials, 22(5): 405-417; and Zalipsky, S. (1995), Chemistry of Polyethylene-Glycol Conjugates with Biologically-Active Molecules, Advanced Drug Delivery Reviews 16(2-3): 157-182, both of which documents are incorporated herein by reference.

[0035] A hydrophilic polymer coupled to at least one hydrophobe may be coupled to one or more biopolymers. The resulting conjugate comprising (i) at least one hydrophilic polymer, (ii) at least one hydrophobic moiety group, and (iii) at least one biopolymer, can be linked and immobilized to the hydrogel network by the aggregation of the attached hydrophobic moiety. One route to an exemplary biopolymer conjugate is as follows. When a biopolymer is to be incorporated into the network, a reagent comprising a hydrophilic polymer with a single hydrophobe can be obtained by reacting a hydrophilic polymer with an amount of hydrophobic moiety in the order of about equivalent with respect to the mount of hydrophilic polymer chains, so that some of the hydrophilic polymer chains have only one hydrophobic end group. By a fractionation process as exemplified in the Examples section below, the polymer is divided into three fractions, those with two hydrophobic end groups, those with one hydrophobic end group, and those with no hydrophobic end groups. The fraction with no hydrophobic end groups is discarded. The fraction with one hydrophobic end group is bound to a biopolymer.

[0036] The immobilized biopolymer species can serve many functions, depending on the particular biopolymer. For example, if the biopolymer is an enzyme, the gel can serve as a microreactor for in column digestion. If the biopolymer is an antibody, it can be used for specific binding of a particular antigen. If it is a ligand it can be used for chromatography for a particular receptor. If it is a receptor, it can be used to screen potential drugs for their binding affinity.

[0037] The present invention has significant advantages. For example, it is possible to fabricate a microfluidic device inexpensively due to the low viscosity nature of the gel precursor, and its ability to be used with polymeric substances. The invention provides an inexpensive way of forming an array on a chip. Moreover, the self-assembled gel can easily be removed from devices, or the configuration of the gel can be changed, merely by introducing the organic solvent, resulting in the self assembled gel reforming into a gel precursor having a low viscosity state.

[0038] Synthesis of Hydrophobic End-Functionalized PEG.

[0039] This procedure is similar to ones reported in literature; namely Xu, L. Li, A. Yekta, Z. Masoumi, S. Kanagalingam, M. A. Winnik, K. Zhang, P. M. Macdonald, S. Menchen, Langmuir (1997) 13, 2447, which is incorporated herein by reference. The general reaction scheme for synthesizing PEG with fluoralkyl groups on both chain ends is as follows:

[0040] Perfluoroalcohols were reacted with an excess of isophorone diisocyanate (IPDI) linker. The excess IPDI was distilled of at 130 C. under vacuum. Then the hydrophobes were recrystallized four times in hexanes at 20 C.

[0041] For the synthesis of the hydrophobic end-functionalized PEG, an excess of hydrophobes were reacted with the PEG (MW6,000 to 35,000), so that almost complete substitution of the PEG end-groups was obtained (both end groups modified with a hydrophobe). For the synthesis of a mono-functional hydrophobically modified PEG, the hydrophobes were reacted with PEG (MW6, 000 in a 1:1 ratio (1 hydrophobe per PEG chain). The reaction was performed at 75 C. in ethyleneglycol dimethylether or tetrathydrofuran, with the addition of a few drops of dibutyltinlaureate as a catalyst. The product was precipitated in cold diethylether.

[0042] Typically the reaction for synthesizing the PEG substituted with a single hydrophobe gave a mixture of unmodified PEG, PEG with a single hydrophobe, and PEG with two hydrophobes. The product can be analyzed by HPLC (conditions: linear gradient: H₂O/CH₃CN; 70/30 to 0/100 in 70 minutes. 20 microliter injected, C18 column, evaporative mass detector).

[0043] The product of interest was isolated by a fractionation procedure as follows. First the product was dissolved in ethyl acetate by slightly heating. Then it was allowed to recrystallize at room temperature for two days. The precipitate (unreacted PEG) was filtered off and the solution was stored at 4 C. overnight. A small amount of precipitate formed (PEG with single hydrophobe) which was filtered from the solution and dried in vacuum. Since only a small amount of product can be recovered each cycle by this procedure, it was repeated several times, until the desired amount of product has been recovered. The product was analyzed by HPLC.

[0044] Activation of the PEG with Single Hydrophobe.

[0045] The product of Example 1 that has one end of the PEG bearing the hydrophobe of interest, with the other end being alcohol is being used for activation. Activation of the alcohol end can be done by a variety of routes that have been described in the literature; for example, by using procedures described by F. M. Veronese, Biomaterials (2001), 22, 405, which is incorporated herein by reference. The general reaction scheme used is as follows:

[0046] The activation rendered the end group of PEG reactive towards amines. This molecule was suitable for PEGylation reactions with biopolymers.

[0047] Fabrication of Microchannels.

[0048] A microfluidic device having microchannels was fabricated by pouring prepolymer on a silicon wafer mold containing positive-relief channels patterned in photoresist, which was then cured. The channels were fully encapsulated by curing the patterned polymer matrix material on a coverslip coated with a thin layer of polymer matrix material and bonding the two layers together through an additional cure. The procedure is similar to that described in the literature for multilayer soft lithography, namely M. A. Unger, H. P. Chou, T. Thorsen, A. Scherer, S. R. Quake, Science (2000), 288, 133, which is incorporated herein by reference.

[0049] Filling of Microchannels with Hydrogel.

[0050] The hydrogel forming material from Example 1 with two hydrophobes on each PEG chain (e.g. a 10,000 PEG chain modified with two C₈F₁₇ end groups) was dissolved in a H₂O/Alcohol (either methanol or ethanol) mixture. Already at fairly low alcohol fractions approximately 10% alcohol or more, the solution was liquid-like, and could be injected with a syringe into the microchannels. Gelation was complete as soon as the alcohol diffused out of the microchannels, which took a few minutes.

[0051] Fabrication of Stamps.

[0052] Stamps were prepared by replica molding by casting the liquid prepolymer of an elastomer against a master that had a patterned relief structure in its surface, according to the technique of Y. Xia, G. M. Whitesides, Angew. Chem. Int. Ed. 1998, 37, 550, which is incorporated herein by reference. The elastomer used was poly(dimethylsiloxane) (PDMS).

[0053] Hydrogel Array Manufacture.

[0054] An array was formed as shown in FIGS. 1-3. Hydrogel forming material prepared by the method of Example 1 (PEG end-modified with two hydrophobes per PEG chain, e.g. PEG MW 6,000 with two C₁₀F₂₁ ends) was dissolved in an H₂O/Alcohol (ethanol or methanol) mixture (1/1) at approximately 0.5-2 wt %.

[0055] A drop of the solution, consisting of the hydrogel forming material in a H₂O/alcohol mixture, was placed on a glass slide or silicon wafer that was previously silanized with a perfluoro silane e.g. trichloro (1H, 1H, 2H, 2H perfluorooctyl) silane. After placing a drop of the polymer solution onto the substrate, the stamp of Example 5 was placed onto the drop. After leaving the stamp and substrate in contact overnight, the stamp was removed from the substrate, leaving the pattern of the hydrogel forming material behind in an array.

[0056] All features disclosed in the specification, including the claims, abstracts, and drawings, and all the steps in any method or process disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in the specification, including the claims, abstract, and drawings, can be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0057] Any element in a claim that does not explicitly state means for performing a specified function or step for performing a specified function, should not be interpreted as a means or step clause as specified in 35 U.S.C. 112.

[0058] Although the present invention has been described in considerable detail with reference to the preferred versions thereof, other versions are possible, Therefore the scope of the appended claims should not be limited to the descriptions of the preferred versions contained therein.