Implementing an obvious idea isn’t always obvious

Some biochemical experiments entail processing small batches of a precious solution under different, controlled conditions. To meet that need and others, scientists and engineers have developed a technology called microfluidics.

The most sophisticated microfluidic devices are known as labs-on-a-chip. They consist of networks of tiny pumps, pipes, and reaction vessels. Like a computer chip, a lab-on-a-chip is fabricated lithographically, usually from a single material. The ideal material is chemically inert, has a low coefficient of friction, and is clear so that you can use an optical microscope to monitor the progress of an experiment.

Polydimethylsiloxane (PDMS) is a popular material for microfluidic applications. It’s clear, chemically inert, and soft enough to form into different shapes. For biochemical applications, PDMS has the additional advantage of being nontoxic to cells, but it has serious disadvantages, too. Large biomolecules tend to stick to its surface, small molecules can seep in and out of its bulk, and it can’t be used with organic solvents.

Aware of those disadvantages, Hongkai Wu of the Hong Kong University of Science and Technology sought an alternative. His choice, Teflon, might seem obvious. The material—or, more accurately, the family of materials—is famously unsticky. The family members are also chemically inert; two of them, perfluoroalkoxy (PFA) and fluorinated ethylenepropylene (FEP) are clear.

But PFA and FEP aren’t easy to stamp or mold into microfluidic devices, in part because of their relatively high melting temperatures (around 260°C). To implement an obvious idea, Wu and his team had to come up with a nonobvious solution, which they described in a recent paper in the Proceedings of the National Academy of Sciences.

Nonobvious might even be an understatement. Ideally, you’d heat PFA or FEP above its melting temperature then stamp a pattern into it using a master made from an easily shaped plastic. At first glance, making the master out of PDMS might seem like a nonstarter. The material’s highest operating temperature is cited as just 150°C.

That upper limit, Wu realized, is not due to the melting temperature of PDMS, but to the out-gassing of small molecules from the bulk. By adjusting the ratio of the two precursors used to make PDMS, Wu found he could raise its operating temperature to at least 350°C—high enough to use as a master for PFA or FEP.

Wu and his team made and tested PFA microfluidic devices. Like PDMS devices, the PFA devices proved to be nontoxic to cells. Unlike PDMS devices, the PFA devices could be cleaned and reused, thanks to their nonstickiness. Wu concludes his paper with a hopeful note: