Surface Infusion Micropatterning of Elastomeric Substrates: A Novel Processing Method for Microfluidic Device Fabrication

The last decade has witnessed revolutionary advances in the fields of microfabrication and microfluidics because of a number of attractive attributes of miniaturization, such as conservation of material in biological analyses.  Microfluidic devices have found applications in a variety of fields including cell and molecular biology, medicine, synthesis of novel materials, biosensors and food safety.  In recent years, microfabrication procedures have shifted from materials such as silicon or glass toward inexpensive polymers, which may be more amenable to low-cost mass production of disposable devices. The development of accessible processing methods for microscale patterning of soft elastomeric materials such as poly(dimethylsiloxane) (PDMS) has led to an explosive growth in research in microfluidic devices and microcontact printing devices.

Current microfabrication processes for polymers include photolithography, casting, injection molding, microthermoforming, and hot embossing. ‘Soft lithography' has revolutionized research in academic laboratories, where fabrication of PDMS devices by simple casting protocols proves cost effective.  On the other hand, injection molding, microthermoforming, and hot embossing are more adaptable to high-throughput manufacturing.  However, these processes are arguably less attractive to labs engaged in rapid prototyping of new devices because of the prohibitive costs associated with repeated mold tooling.  As a result, scale-up of new devices from the prototype stage to the mass production stage can require a change in both materials and equipment, a hindrance to commercialization.  A single process that allows rapid, cost-efficient production of micropatterned polymer devices at both the lab scale and the industrial scale could prove to be a critical advance in microfabrication science and industry.

Surface Infusion Micropatterning (SIM) is a new microfabrication process for surface relief patterning of elastomeric substrates, including both chemically crosslinked or vulcanized materials (e.g. polydimethylsiloxane, PDMS) and thermoplastic elastomers (e.g., thermoplastic polyurethanes, TPUs).  The starting material (substrate) is an optically transparent elastomeric material, which can be formed into flat sheets by standard processing techniques prior to SIM.  The substrate may be either free-standing or mounted as a surface layer on glass or another rigid material.  Patterning of the substrate by SIM is a three-step process: 1) monomer infusion, 2) photopatterning, and 3) drying.  For the first time, SIM has been demonstrated with two elastomeric materials, a crosslinked polydimethylsiloxane (PDMS) and a thermoplastic polyurethane (TPU), to create micropatterned surface relief features including a "checkerboard" well pattern and a microfluidic channel system.  Surface micropatterns were characterized by scanning electron microscopy, optical microscopy, and optical profilometry to examine channel depth and shape.  High-resolution surface features of dimensions as small as 10 micrometers can be obtained using an uncollimated, broadband ultraviolet light source, making the technique amenable to low-cost lab production.  SIM is most suitable for producing shallow (low aspect-ratio) microfluidic channels in soft elastomeric substrates.  After considering process advantages and limitations, the potential for SIM to impact emerging microfluidic manufacturing technologies will be discussed.