Enzymatic etching used to build nano- and microscale surface topologies.

In
living systems, complex nano- and microscale structures perform a host
of physical and biological functions. While two-dimensional patterns can
be recreated fairly well with techniques like microlithography,
three-dimensional structures represent a big challenge. In the journal
Angewandte Chemie, researchers have now reported a new method
for the lithography-free etching of complex surface motifs with the use
of biodegradable polymers and enzymes. Starting with structured
microchannels, they have built an assembly for the isolation and
concentration of cells from whole blood.

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.

Enzymatic Sculpting of Nanoscale and Microscale Surface Topographies

Source: Wiley