NIST Microreactor Could Lead to New Biodegradable Polymers

Using a microfluidic device consisting of a small block of aluminum with a tiny groove carved in it filled with tiny beads, a team of researchers from the National Institute of Standards and Technology (NIST; Gaithersburg, MD) and the Polytechnic Institute of New York University (Brooklyn, NY) is developing an improved "green chemistry" method for making biodegradable polymers. The technology should be of interest to designers and developers of medical device materials.

The group studied the synthesis of polycaprolactone (PCL), a biodegradable polyester used in medical device applications. PCL, according to NIST materials scientist Kathryn Beers, is usually synthesized using an organic tin-based catalyst to stitch the base chemical rings together into long polymer chains. The catalyst is highly toxic, however, and has to be disposed of.

Modern biochemistry has found a more environmentally friendly substitute in an enzyme produced by the yeast strain Candida antartica, Beers says. However, standard batch processes in which the raw material and tiny beads that carry the enzyme are stirred together in a vat, is too inefficient to be commercially competitive. It also leaves an enzyme residue that contaminates and degrades the product.

In contrast, the NIST microreactor is a continuous-flow process. The feedstock chemical flows through the narrow channel of the microfluidic device around the enzyme-coated beads and emerges out the other end in polymerized form. This arrangement allows precise control of temperature and reaction time, so that detailed data on the chemical kinetics of the process can be recorded to develop an accurate model in order to scale the process.

"We basically developed a microreactor that lets us monitor continuous polymerization using enzymes," Beers explains. "These enzymes are an alternate green technology for making these types of polymers." Although the technology is not industrially competitive yet, data from the microreactor show how the process of developing biodegradable polymers could be made much more efficient. The team believes that their technology is the first example of polymerization produced using a solid-supported enzyme in a microreactor.

"The small-scale flow reactor allows us to monitor polymerization and look at the performance recyclability and recovery of these enzymes," Beers says. "With this process-engineering approach, we've shown that continuous flow really benefits these reactors. Not only does it dramatically accelerate the rate of reaction, but it improves your ability to recover the enzyme and reduce contamination of the product."