Increasingly smaller and more intricate microsystems technology is gaining
ground. Complete systems for chemical analysis or
medical diagnostics, among
others, can now be reduced to the size of a thumbnail. However, production of
the necessary three-dimensional microstructures by conventional processes is
very expensive.
American researchers David LaVan and Paul George working with Robert Langer at
MIT have now developed a substantially simplified production method based on
time-delayed electrodeposition of the conducting polymer polypyrole or the metal
nickel.
In the first step of the process, conventional
photolithography is used to form
a two-dimensional structure, which acts as the base. This is done by depositing
photoresist (a light-sensitive plastic) onto a
silicon nitride coated silicon
wafer and exposing it to light through a mask containing a desired pattern.
Wherever the photoresist is exposed to light, it is changed such that it can be
selectively dissolved away. After a subsequent electron beam deposition process,
only the exposed surfaces are covered with gold. Once the remaining photoresist
is removed, the desired two-dimensional gold pattern is left behind. The novel
factor is the tiny gaps with which the researchers selectively separate
individual regions of the gold pattern. During the following electrodeposition
step, when a voltage is applied at a point on the gold pattern, the current only
affects a single region, which is surrounded by gaps. At this point, the
deposition of polypyrole -- or of nickel -- begins. As the material is
deposited, it grows in height as well as laterally over the gold surface. In
this way, the gaps are eventually bridged. Once a connection to a neighboring
region is established, the current can
flow there as well. Deposition thus
starts in the adjacent region until the next gap is bridged, and so on. Because
the growth of the material in the individual regions is staggered over time, the
resulting structures are staggered in height. The difference in height can be
controlled by changing the size of the gaps. The completed structure can then be
used as a mold for duplication.
The MIT team was thus able to build a mold for a branched microvascular system.
This technique may allow for the generation of scaffolds for the production of
blood vessels within artificial organs.