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New polyelectrolyte inks create fine-scale structures through direct writing
James E. Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@illinois.edu
3/24/04
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CHAMPAIGN,
Ill. — Like spiders spinning webs, researchers at the University
of Illinois at Urbana-Champaign are creating complex, three-dimensional
structures with micron-size features using a robotic deposition process
called direct-write assembly.
As reported in the March 25 issue of the journal Nature, Jennifer Lewis
and her research team have developed novel inks that readily flow through
micro-capillary nozzles and then rapidly solidify to retain their shape.
Patterning such fine structures could be useful in applications such
as drug-delivery, micro-fluidics, photonics and tissue engineering.
“This research builds upon our previous work with colloid-based
inks, and represents a major step forward in both ink design and pattern
resolution,” said Lewis, a Willett Faculty Scholar and a professor
of materials science and
engineering and of chemical
and biomolecular engineering at Illinois. “Because this new
ink is based solely on polyelectrolyte mixtures rather than colloidal
particles, we are able to produce three-dimensional periodic structures
with feature sizes that are 100 times smaller than before.”
The smallest feature size Lewis’ team has demonstrated so far
is 500 nanometers. An entire three-dimensional structure can fit in
the diameter of a human hair.
“The new inks are low-viscosity fluids created by combining oppositely
charged polyelectrolyte complexes,” Lewis said. “The ink
viscosity can be tailored over a wide range to control its flow through
fine deposition nozzles of varying diameter.”
A spider creates its web by secreting a concentrated protein solution
through its spinneret to produce fine silk filaments. “We drew
inspiration from this natural process to guide our ink design,”
said graduate student Gregory Gratson, who helped develop the inks.
“For example, we worked in a similar concentration range as spider-spinning
dope, though our ink structure is significantly less complicated.”
To produce the desired three-dimensional structure, the extruded ink
is deposited into a coagulation reservoir containing deionized water
and isopropyl alcohol, Gratson said. Solidification of the ink is induced
by electrostatic interactions in a water-rich reservoir or by solvent-quality
effects in an alcohol-rich reservoir. A careful balance is needed so
that the extruded ink filament is elastic enough to maintain its shape
while spanning the structure, but flexible enough to adhere to other
filaments.
Dispensed from a syringe by a computer-controlled, three-axis micropositioner,
the polyelectrolyte ink exits the nozzle as a continuous filament that
is deposited into a reservoir on the substrate surface, yielding a two-dimensional
pattern. After the first layer is generated, the nozzle is raised and
another layer is deposited. This process is repeated until the desired
three-dimensional structure is produced.
The precisely patterned parts could be used as bio-scaffolds, micro-fluidic
networks, sensor arrays or templates for photonic materials.
Graduate student Mingjie Xu is also a co-author of the paper. The U.S.
Department of Energy and the U.S. Army Research Office MURI program
funded this work.
