Polycarbonate Heat Molding for Soft Lithography

Significance 

Soft lithography refers to a family of techniques used for fabricating/replicating structures using elastomeric stamps, molds and conformable photomasks. Ideally, it is a prominent technique for rapidly fabricating miniaturized devices with elastomeric materials. Technically, fabrication of these devices conventionally requires casting of precured elastomers on rigid silicon–photoresist (Si–Pr) composite master molds in order to replicate the microfeatures on the Si–Pr master mold. This strategy then is frequently used with polydimethylsiloxane (PDMS) for implementing different microfabrication techniques such as replica molding and microcontact printing. This technique has enjoyed success over the past two decades; unfortunately, shortfalls such as the development of novel microtechnological innovations, associated with master mold fabrication has limited continued advancement. Various researchers have attempted to address this issue by proposing various techniques to copy the existing master molds instead of using time consuming, expensive, and cumbersome master mold fabrication methods to refabricate the damaged master molds or to scale up the fabrication. Regardless of these advances, alternate approaches that could help circumvent inherent drawbacks would be highly welcome.

In general, soft lithography enables rapid microfabrication of many types of microsystems by replica molding elastomers into master molds. However, master molds can be very costly, hard to fabricate, vulnerable to damage, and have limited casting life. In light of this, researchers from the Department of Mechanical Engineering at Carnegie Mellon University: Utku Sonmez (PhD candidate), Stephen Coyle (PhD candidate), Professor Rebecca E. Taylor and Professor Philip R. LeDuc, proposed a new approach to clone master molds for soft lithography. Specifically, their approach would enable the multiplication of master molds into monolithic thermoplastic sheets for further soft lithographic fabrication. Their work is currently published in the research journal, Small.

The proposed technique encompassed the use of polycarbonate thermoplastic sheets as a starting material and melted them on PDMS molds by heating them above the glass transition temperature without applying any external force. The team then applied the polycarbonate heat molding (PCH molding) technique to copy various types of master molds that were fabricated through photolithography, mechanical micromilling, and 3D printing to obtain a variety of microstructures at multiple scales in monolithic PC copy molds. Further, the technique was used to combine master molds fabricated through different methods into a single monolithic PC copy mold for the fabrication of multilayered complex microdevices.

The research team reported that when tested with master molds fabricated through photolithography, mechanical micromilling as well as 3D printing, their technique successfully copied the microstructures with submicron feature sizes and high aspect ratios. The technique presented was fast, reliable, and economical for copying molds without the need of sophisticated equipment or specialized facilities; attributes that could allow researchers to more easily scale up their fabrication and share their master mold geometries with each other fostering interdisciplinary collaborations.

In summary, the study presented a new PCH molding technique to fabricate alternative master molds for soft lithographic applications. Remarkably, the presented technique did not require any hazardous or cytotoxic solvents, costly polymers, or any specialized equipment, which made it useful to almost any laboratory that has a PC sheet and an oven, which are easy to obtain. In a statement to Advances in Engineering, they highlighted that their microfabrication technique can be performed outside the cleanroom without using any sophisticated equipment, suggesting a simple way for high throughput rigid monolithic mold fabrication that can be used in analytical chemistry studies, biomedical research, and microelectromechanical systems.

About the author

Mr. Utku M. Sonmez is currently a Ph.D. candidate in the Department of Mechanical Engineering, Carnegie Mellon University. He received his Bachelor’s degree from Istanbul Technical University and Master’s degree in Biomedical Engineering from Carnegie Mellon University.

His research is mainly focused on developing microtechnological tools for studying mechanosensitive biological processes.

About the author

Mr. Stephen Coyle is a Ph.D. candidate in Mechanical Engineering at Carnegie Mellon University. He received his Master’s degree in Mechanical Engineering at Carnegie Mellon University and his Bachelor’s degree in Applied Mathematics at Claflin University. His research is focused on investigating mechanisms that impact cell alignment within the field of tissue engineering.

About the author

Dr. Rebecca E. Taylor is currently an assistant professor of Mechanical Engineering, and, by courtesy, of Biomedical Engineering and Electrical and Computer Engineering at Carnegie Mellon University (CMU). Dr. Taylor earned her Ph.D. with Prof. Beth Pruitt in Mechanical Engineering with a Ph.D. Minor in Bioengineering from Stanford University in 2013. She received her B.S.E. in Mechanical Engineering and a Certificate in Robotics and Intelligent Systems from Princeton University in 2001. During her Ph.D. studies she developed microfabricated sensors to characterize the electrical and mechanical properties of developing stem cell derived cardiomyocytes. For her postdoctoral studies she turned her focus to the nanoscale, joining Prof. James Spudich’s molecular motors lab in the Biochemistry Department at the Stanford University School of Medicine.

She now combines both microfabrication and nanofabrication to create hybrid top-down/bottom-up fabricated sensors and actuators for nanobiosensing, robotics, advanced manufacturing applications. Prof. Taylor is the recipient of a NIH F32 NRSA postdoctoral fellowship award, the Air Force Office of Scientific Research Young Investigator Program award, and the NSF CAREER award.

About the author

Dr. Philip R. LeDuc (Ph.D. Johns Hopkins University; post-doctoral fellow, Children’s Hospital and Harvard Medical School) is the William J. Brown Professor in the Mechanical Engineering Department at Carnegie Mellon University with appointments in Biomedical Engineering, Computational Biology, and Biological Sciences. He has received the National Science Foundation CAREER Award, the Beckman Foundation Young Investigators Award, while also being selected as a faculty member for the Sloan Foundation minority Ph.D. Program. He has also been funded by other organizations including the Bill & Melinda Gates Foundation, Office of Naval Research, Department of Energy, National Institute of Health, and Keck Foundation. During his career, he has published articles in many journals, including Proceedings of the National Academy of Sciences, Nature Nanotechnology, PLoS ONE, JACS, Applied Physics Letters, Methods in Cell Biology, Advanced Materials, Nano Letters, Nature Protocols, and Nature and has given seminars across the world including South Africa, India, and Brazil. He has been on and helped organize many scientific meetings including for the National Academy of Engineering, the National Academy of Sciences, the Institute of Medicine, and the United States Congress as well as being elected to the Science Advisory Council of the Beckman Foundation, and the Board of Directors for the Biomedical Engineering Society and American Institute for Medical & Biological Engineering.

He is also a Fellow of the Biomedical Engineering Society, the American Society of Mechanical Engineers, and the American Institute for Medical & Biological Engineering. He has filed numerous patents, has started companies, and has consulted for a diversity of companies. His wife, Rachel, and Philip have a daughter and two sons. He has also been involved with many philanthropic organizations including raising money for non-profit organizations and mission trips to Africa and Armenia.

Reference

Utku M. Sonmez, Stephen Coyle, Rebecca E. Taylor, and Philip R. LeDuc. Polycarbonate Heat Molding for Soft Lithography. Small 2020, volume 16, 2000241.

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