1.  Field of the Invention
Embodiments of the invention generally pertain to the field of microfluidics and, more particularly, to a laminated, polymeric microfluidic structure and to a method for making a laminated, polymeric microfluidic structure. 
2.  Description of Related Art
The technology of manipulating minute volumes of biological and chemical fluids is widely referred to as microfluidics.  The realized and potential applications of microfluidics include disease diagnosis, life science research, biological and/or chemical sensor development, and others appreciated by those skilled in the art. 
A microfluidic structure including a substrate having one or more microfluidic channels or pathways and a cover plate or a second or more substrates with fluid pathways that may or may not be interconnected, may commonly be referred to as a microfluidic chip.  Highly integrated microfluidic chips are sometimes called ‘labs on a chip’.  Inorganic microfluidic chips having substrates made of glass, quartz or silicon have advantageous organic solvent compatibilities, high thermal and dimensional stability and excellent feature accuracy.  These chips are typically fabricated using well-established microfabrication technologies developed for the semiconductor industry.  However, the material and production costs of the inorganic chips may become prohibitively high especially when the fluidic pathway(s) requires significant area or the chip has to be disposable.  In addition, many established biological assays were developed utilizing the surface properties of polymeric substrates.  The research effort required to redevelop these assays on inorganic surfaces would require significant time and resource investments. 
As an alternative to inorganic microfluidic structures such as those referred to immediately above, microfluidic structures or devices can also be made from polymeric materials.  Polymeric microfluidic structures have advantageous low material costs and the potential for mass production.  However, the fabrication of polymeric microfluidic chips presents a variety of challenges.  For example, microfluidic chips may contain sealed microstructures.  They can be formed by enclosing a substrate having a pre-fabricated fluid pathway or other microfeatures with a thin cover plate, or with one or more additional substrates to form a three-dimensional fluid network.  The pathways or other microstructures have typical dimensions in the range of micrometers to millimeters.  This multilayer microfluidic structure is integrated, or joined together, by various conventional techniques.  These techniques include thermal, ultrasonic and solvent bonding.  Unfortunately, these techniques often significantly alter the mated surfaces and yield distorted or completely blocked microfluidic pathways due, for example, to the low dimensional rigidity of polymeric materials under the aforementioned bonding conditions. 
The use of adhesive lamination may circumvent some of these potential difficulties by avoiding the use of excessive thermal energy or a strong organic solvent.  However, the introduction of an adhesive layer to a wall surface of an enclosed fluid pathway can cause other fabrication and/or application problems.  Commercially available adhesives tend to be conforming materials with typical applied thicknesses of 12-100 micrometers.  The compressive force required to produce a uniform seal between component layers will often extrude the adhesive into the fluid pathways resulting in microchannel dimensional alteration or obstruction.  An additional potential problem with using adhesives is the formation of an adhesive wall within the enclosed microstructure.  The presence of this dissimilar material makes uniform surface modification of the microstructure difficult.  Furthermore, the manipulation or patterning of an adhesive layer is difficult, limiting the use of the adhesives to uniform continuous sheets or layers between two opposing planer surfaces.  This restricts fluidic communication through a network to one planer surface, as the fluid cannot flow through the adhesive layer, preventing the use of a more versatile three-dimensional space. 
The use of a strong organic solvent to join two or more discrete plastic parts is a well known practice in the art.  In solvent welding, as this process is referred to, lamination solvents work by aggressively penetrating the macromolecular matrix of the polymeric component.  This loosens the macromolecule-to-macromolecule bonds, uncoiling or releasing them from their polymer network to generate a softened surface.  When two opposing softened surfaces are brought into close proximity, new macromolecular interactions are established.  After the solvent evaporates there is a newly formed macromolecular network at the bonded interface with mechanical strength defined by the force of the macromolecular interaction.  Exemplary strong organic solvents used for plastic lamination include ketones (acetone, methylethyl ketone or MEK), halogenated hydrocarbons (dichloromethane, chloroform, 1,2-dichloroethane), ether (tetrahydrofurane or THF) or aromatic molecules (xylene, toluene) and others known by those skilled in the art. 
The use of the aforementioned strong solvents for bonding microfluidic chips with substrates composed of polystyrene, polycarbonate or acrylic is problematic.  All of the solvents known to be used in the field of solvent bonding are “strong” (as defined by their ability to dissolve the polymeric substrate) organic solvents.  That is, these solvents tend to over-soften or dissolve the surface of the substrates during the bonding process.  This may damage the microfluidic structure by completely erasing, blocking or destroying the fluid pathways when the substrates are laminated.  Acetone, dichloromethane or xylene, for example, begin to dissolve a polystyrene sheet within seconds of application at room temperature.  Although it is possible to weaken the solvent strength by mixing the solvent with “inert” solvents such as methanol or ethanol, the resulting bond often does not provide a satisfactory result. 
The contemporary patent literature discloses using thermal bonding, thermal-melting adhesive, liquid curable adhesive, and elastomeric adhesive approaches to enclose two opposing microfluidic structure surfaces of the same or different materials.  It is suggested that these methods are applicable to the fabrication of microchannels of various shapes and dimensions.  It is apparent, however, that these approaches rely on stringent control of the fabrication and process conditions, which may result in unacceptable fabrication throughput and production yield. 
Another reported technique suggests that the quality of a thermally laminated polymeric microchannel can be drastically improved if the opposing substrates have different glass transition temperatures.  While this approach may provide a way to retain microstructural integrity during thermal bonding, the success rate will rely on precise process control.  Consequently, its application to microfluidic chip manufacturing is restricted. 
A recent publication describes a method of creating a plurality of relief structures along the length of a microfluidic channel wall, projecting from the opposing surface in the non-functional area of the substrate.  Subsequent deposition of a bonding material fills this relief structure, completing the bond.  This method allegedly can increase the manufacturing yield of adhesive bonded microfluidic devices.  The significant challenge of dispensing the correct volume of bonding material into the relief structures is not addressed.  The necessary control of the small volume of bonding material does not lend itself to high production yields. 
In view of the foregoing, the inventors have recognized that a simple, reproducible, high yield method for enclosing polymeric microstructures is needed.  Such a method would be particularly valuable for the fabrication of microfluidic chips from polystyrene, which is the most widely used material for biochemical, cellular and biological assays, acrylics and polymeric materials.  It would also be desirable to have a method for microfluidic chip fabrication that is amenable to both laboratory use and manufacturing environments.  Such a method would further be useful if it were applicable to the production of prototype devices, as well as being substantially directly transferable to large-scale production.  Microfluidic structures made according to the envisioned methods would also be desirable for their economy and ease of production.  Accordingly, embodiments of the invention are directed to microfluidic structures and fabrication methods that address the recognized shortcomings of the current state of technology, and which provide further benefits and advantages as those persons skilled in the art will appreciate.