Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1997-06-18
2001-01-23
Lorin, Francis J. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S273500
Reexamination Certificate
active
06176962
ABSTRACT:
BACKGROUND
This invention relates to construction of microchannel structures for use in microfluidic manipulations.
Microchannel structures are of great interest for applications involving the manipulation of small fluid volumes, such as chemical and biochemical analysis. Various microchannel structures having channel dimensions on the order of one or a few millimeters have been used for chemical and biochemical assays.
These structures are typically produced by injection molding using various thermoplastic polymers. Injection molding is an economical process, and a variety of thermoplastics having good optical and mechanical properties can be processed by injection molding to form the desired structures. The injection molding process involves introducing a molten thermoplastic material into a mold cavity, and then cooling the cavity to solidify the resin. In the case of forming microchannel structures, a mold having the negative pattern of the desired channel structures must be created. Conventional tooling methods can be used to create molds for channels having dimensions as small as about 1 mm. Typically, enclosed microchannels are desired for the final structure. A common method for enclosing microchannel structures formed in plastics is to join a base and cover substrate using sonic welding. In addition, certain adhesives can also be used to join the base and cover substrates.
It has become desirable to create microchannel structures having capillary dimensions, i.e., having dimensions ranging from less than 1 micron to upwards of 1 mm. These structures are of interest for manipulating very small fluid volumes through the application of electric fields to perform electrofluidics, i.e., the movement of fluids in microchannels utilizing electrokinetic flow, that is, electrophoresis and/or electroosmotic flow (EOF). Electrophoresis is the movement of individual charged particles or molecules in response to the application of an electric field to an ionic solution. Electroosmotic flow is a bulk fluid flow (individual ions plus solvent molecules) that also results from the application of an electric field to an ionic solution. The extent of the bulk fluid flow is a function of the charge on the wall of the channel, as well as the viscosity of the solution. Both EOF and electrophoresis can be used to transport substances from one point to another within the microchannel device.
To create microchannels having capillary dimensions, photolithography in silicon or glass substrates has been employed. See, e.g., U.S. Pat. No. 4,908,112, U.S. Pat. No. 5,250,263. In the case of fused silica, these structures can be enclosed by anodic bonding of a base and cover substrate.
Although microchannel structures of such materials have been produced, it would be much more economical, and therefore desirable, to produce structures of capillary dimensions in polymeric materials or plastics. However, the conventional methods for forming and enclosing channels in plastic do not provide the accuracy and precision required for structures of capillary dimensions. For example, when using sonic welding, heating and deformation may occur in the channel regions. When the edges of a sonic weld are uneven, poor electrofluidic performance may result. Furthermore, sonic welding of highly defined intersections of capillary dimensions is not easily accomplished with adequate fidelity. Similarly, with conventional adhesive methods, the adhesive material may flow into and plug the channels.
Thus, there is interest in the development of new methods of fabricating polymeric microstructures, specifically in new methods of sealing the cover and base plates together, where such new methods do not result in deformation or filling in of the microchannels enclosed in the structure. Ideally, such methods should be simple and readily reproducible so as to be suitable for large scale manufacturing.
U.S. Pat. No. 5,376,252 to Eckstrom et al. describes a process for creating capillary size channels in plastic using elastomeric spacing layers. {umlaut over (O)}hman International Patent Publication WO 94/29400 describes a method for producing microchannel structures involving the application of a thin layer of a thermoplastic material to one or both of the surfaces to be joined, then joining the surfaces and heating the joined parts to melt the thermoplastic bonding layer.
SUMMARY OF THE INVENTION
Methods are provided for the fabrication of polymeric microchannel structures having enclosed microchannels of capillary dimension. The microchannel structures are constructed of a base plate and a cover, sealed together. Microchannel structures having walls of a plastic material are formed in a generally planar surface of at least the base plate. The cover has at least one generally planar surface, and the microchannel structures are enclosed by bonding the planar surfaces of the cover and the base plate together. The microchannel structures according to the invention find use in a variety of applications, particularly in electrofluidic applications.
Approaches to sealing the cover and base plate according to the invention include thermal bonding of the base plate and cover surfaces, and use of a bonding material between the base plate and cover surfaces. Suitable bonding materials include elastomeric adhesive materials, and activatable bonding materials, including liquid curable adhesive materials and thermo-melting adhesive materials.
The thermal bonding approach can be employed where the apposing planar surfaces of the base plate and the cover are made of similar polymeric materials. Generally, in this approach, the planar surfaces of the base plate and the cover are aligned and confined to a mechanical fixture, in which they are progressively heated under pressure to a temperature 2-5° C. above the glass transition temperature of the polymer. In this first step, small irregularities in the surfaces accommodate to each other, while maintaining the physical integrity of the channels. Then, the temperature is maintained above the glass transition temperature of the polymer for a time sufficient to allow the polymer molecules to interpenetrate the two surfaces and create a morphological bonding. Above the glass transition temperature the molecules have sufficient entropy to entangle and orient in the surfaces of the two plates. In a final step of the bonding process the temperature is slowly reduced in order to maintain a stress free interface that provides a stable assembled microchannel structure.
Bonding materials can be employed where the apposing planar surfaces of the base plate and the cover are made either of similar or of different materials.
In approaches employing thermo-melting adhesives, the adhesive formulation includes medium molecular weight components that upon heating melt and diffuse into the two apposed surfaces, interpenetrating the two surfaces and creating a stable interface for the assembled microchannel structure. Suitable thermo-melting adhesives are usually formulated with chemistries that provide secondary bond interactions (e.g., hydrogen bonding, Van der Waals forces, and hydrophobic forces) between the surfaces being bonded and the adhesive.
In approaches using liquid curable materials, one of the planar surfaces (usually of the cover), is coated with a layer or film of a liquid, curable adhesive material. The fluid layer is then rendered non-flowable, after which the coated surface is contacted with the apposing planar surface (usually of the base plate in which the microchannels have been formed). Then the curable adhesive material is cured to seal the surfaces together, forming the enclosed microchannel structure.
In approaches employing elastomeric bonding materials, the adhesive layer or film is a rubber or elastomer material (e.g., natural and synthetic rubbers, polyurethane, polysulfides and silicones). The elastomeric bonding material can be applied in solution, as an emulsion, or in formulations of two reactive components. Elastomeric bonding materials can be used in contact adhesive formulati
Amigo M. Goretty Alonso
Hooper Herbert H.
Soane David S.
Soane Zoya M.
ACLARA BioSciences, Inc.
Lorin Francis J.
Rae-Venter Law Group P.C.
Rowland Bertram
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