Center for Polymer Microfabrication (CPM): Tooling and Equipment

Develop fundamental methods for design of high throughput machines for micro-casting, micro-forging and micro-injection molding suitable for commercial scale production.

There are a variety of methods available for manufacturing polymeric substrates for microfluidic applications. These can be broadly separated into direct methods and replication methods. In direct fabrication methods, such as laser ablation, reactive ion etching and mechanical milling, individual polymer surfaces are serially “machined” to form desired microstructural features. While highly flexible, such methods are inherently slow and initial comparisons to net shape processes show potentially higher variability in the final product. On the other hand, replication methods such as injection molding, hot-forging/embossing, and softlithography, involve the use of a precision template or master-mold from which many identical polymer microstructures can be made.

The micro-injection molding, micro-forging, and micro-casting techniques for the elastomeric and polymeric materials are in their infancy and are not well understood or sufficiently developed for the mass production of cost-effective and reliable microfluidic devices. Many of the basic principles of these emerging microfabrication methods are, of course, conceptually not new, and are well known at the macroscale. However, recent laboratory scale research has shown that the application of these principles can be dramatically improved and extended to produce features in the nano-/micro-range by introducing advanced materials, chemistries, and processing techniques.

We explore micro-hot-forging/embossing (nanoimprint lithography), micro-casting (soft-lithography), and micro-injection molding for the mass production of cost-effective and reliable microfluidic devices, with an emphasis the former two. Each technique has advantages and disadvantages; one or a combination of theses processes will be used based on product design requirements. For example, microinjection molding is most appropriate for mass producing “interconnections” for interfacing the smaller features generated by embossing or soft lithography with the macro-world.

Micro-hot-forging/embossing (“nanoimprint lithography”). Chou and co-workers at Princeton have pioneered nanoimprint lithography, a new, reasonably fast, cost-effective nano/micro-forging technique that holds substantial promise for the high-volume manufacturing of dense microstructures in imprintable polymeric materials such as PMMA and PC. Typical capillary cross-sections that can be produced with fidelity by hot-embossing are expected to be in the 1- to 50-micron diameter range (or even smaller).

Micro-casting (“soft-lithography”). Soft lithography has become a highly popular replication process over the past five years, offering a rapid, flexible and cost-effective manufacturing route to the creation of micron-sized features on planar substrates in elastomeric siloxane polymers such as PDMS. These materials are easily molded, optically transparent (well into the UV), durable, cheap, chemically inert, non-toxic and stable over wide temperature ranges.

Micro-injection molding. Injection molding is a mature manufacturing method that can be used to fabricate a variety of structures from thermoplastic materials. It is widely used for making parts with macroscale feature, as well as compact discs with micron-sized features, and is beginning to be used to make microfluidic devices. Injection molding is an excellent process for plastic replication owing to its good dimension control, short cycle time (several seconds), and high productivity. Briefly, the polymer of choice is melted and then injected under high pressure into an evacuated cavity containing a precision master mold. The cavity is maintained at a temperature close to the melting point of the polymer to allow efficient fluid flow into all parts of the mold. The cavity is then cooled and the micro-structured part ejected. Typical capillary cross-sections that can be produced with fidelity by injection molding are in the 50- to 100- micron diameter range.

Some current research endeavors include:
(*) Production methods for nano/micro-scale features on master molds and replicas.
(*) Mold finishing and coating analysis and life estimation.
(*) Process mechanics for micro-casting, hot micro-embossing, micro-injection molding equipment design.
(*) Design of mechanical and thermal environment.
(*) Control systems for rapid and repeatable operation.
(*) Design of multilayer alignment and bonding equipment.
(*) Methods for feature-to-feature alignment with soft materials.