Lithography-free etching of complex surface features is achieved by harnessing the enzyme proteinase K (PK), controlled by bovine serum albumin (BSA), to digest a biodegradable polymer. This bio-sculpting process is used to construct a membraneless filtration device for the size-based isolation and enrichment of cells from whole blood.
[Credit: Angewandte Chemie]
A team led by Victor M. Ugaz at Texas A&M University uses proteinase K (PK), a protein-cleaving enzyme that can also break down the bioplastic polylactic acid. First, the researchers apply a mask to a small block of polylactic acid, leaving only a narrow trail. The liquid containing PK is directed through this microchannel. Wherever the enzyme comes into contact with the polylactic acid, the latter is etched away.
Within microchannels, liquids can flow past each other without appreciable mixing. The researchers use this phenomenon to make structured channels. They direct PK solution through the channels on the left and right, while allowing a protein solution flowing through the middle to inhibit the etching process. This etches neighboring channels separated by a “weir” into the polymer. In the next step, a protein solution is directed through one of the etched channels and over the central weir, while the second channel is further etched with PK. This allows one channel to remain flat while the second is made deeper. Finally, all three “tracks” are made even deeper with PK. This causes the top of the weir to be lower than the outer edges of the double channel.
To make their device, the researchers bent one such channel into a hairpin turn and put a lid over it. They allow blood spiked with tumor cells to flow through the inner, flatter channel. A buffer solution flows through the deeper outer channel. In the curve, centrifugal forces push blood cells into the outer track with the buffer. However, the small space between the top of the weir and the lid over the system only allows small blood cells to pass through. The larger tumor cells do not fit through and become more concentrated in the inner channel as the red blood cells become less concentrated. The different depths of the channels enhances this process. Rare cells like freely circulating tumor cells can be detected much more rapidly and easily when blood samples are tested by this method than by conventional methods like membrane filtration.
Special thermal preparation allows for the targeted formation of crystalline regions in the polylactic acid. PK does not degrade these regions well. This allows for the formation of defined small obstacles within the channels, which could be useful in filtration or chromatography systems.
About the Author
Dr. Victor M. Ugaz is an Associate Professor and Kenneth R. Hall Development Professor in the Artie McFerrin Department of Chemical Engineering at Texas A&M University. His research focuses broadly on understanding and exploiting the unique characteristics of transport and flow at the microscale. He also serves as Chair of the interdisciplinary Professional Program in Biotechnology (PPiB) at Texas A&M.