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Inspired
by nature’s ability to shape a petal, and building on simple techniques
used in photolithography and printing, researchers at the University of
Massachusetts Amherst have developed a new tool for manufacturing
three-dimensional shapes easily and cheaply, to aid advances in
biomedicine, robotics and tunable micro-optics. Ryan Hayward, Christian
Santangelo and colleagues describe their new method of halftone gel
lithography for photo-patterning polymer gel sheets in the current issue
of Science.
They say the technique, among other applications, may someday help
biomedical researchers to direct cells cultured in a laboratory to grow
into the correct shape to form a blood vessel or a particular organ.
“We
wanted to develop a strategy that would allow us to pattern growth with
some of the same flexibility that nature does,” Hayward explains. Many
plants create curves, tubes and other shapes by varying growth in
adjacent areas. While some leaf or petal cells expand, other nearby
cells do not, and this contrast causes buckling into a variety of
shapes, including cones or curly edges. A lily petal’s curve, for
example, arises from patterned areas of elongation that define a
specific three-dimensional shape.
Building
on this concept, Hayward and colleagues developed a method for exposing
ultraviolet-sensitive thin polymer sheets to patterns of light. The
amount of light absorbed at each position on the sheet programs the
amount that this region will expand when placed in contact with water,
thus mimicking nature’s ability to direct certain cells to grow while
suppressing the growth of others. The technique involves spreading a
10-micrometer-thick layer (about 5 times thinner than a human hair) of
polymer onto a substrate before exposure.
Areas
of the gel exposed to light become crosslinked, restricting their
ability to expand, while nearby unexposed areas will swell like a sponge
as they absorb water. As in nature, this patterned growth causes the
gel to buckle into the desired shape. Unlike in nature, however, these
materials can be repeatedly flattened and re-shaped by drying out and
rehydrating the sheet.
To
date, the UMass Amherst researchers have made a variety of simple
shapes including spheres, saddles and cones, as well as more complex
shapes such as minimal surfaces. Creating the latter represents a
fundamental challenge that demonstrates basic principles of the method,
Hayward says.
He
adds, “Analogies to photography and printing are helpful here.” When
photographic film is exposed to patterns of light, a chemical pattern is
encoded within the film. Later, the film is developed using several
solvents that etch the exposed and unexposed regions differently to
provide the image we see on the photographic negative. A very similar
process is used by UMass Amherst researchers to pattern growth in gel
sheets.
Santangelo
and Hayward also borrowed an idea from the printing industry that
allows them to make complicated patterns in a very simple way. In
photolithography, just as in printing, it is expensive to print a
picture using different color shades because each shade requires a
different ink. Thus, most high-volume printing relies on “halftoning,”
in which only a few ink colors are used to print varied-sized dots.
Smaller dots take up less space and allow more white light to reflect
from the paper, so they appear as a lighter color shade than larger
dots.
An
important discovery by the UMass Amherst team is that this concept
applies equally well to patterning the growth of their gel sheets.
Rather than trying to make smooth patterns with many different levels of
growth, they were able to simply print dots of highly restricted growth
and vary the dot size to program a patterned shape.
“We’re
discovering new ways to plan or pattern growth in a soft polymer gel
that’s spread on a substrate to get any shape you want,” Santangelo
says. “By directly transferring the image onto the soft gel with
half-tones of light, we direct its growth.”
He
adds, “We aren’t sure yet how many shapes we can make this way, but for
now it’s exciting to explore and we’re focused on understanding the
process better. A model system like this helps us to watch how it
unfolds. For biomedicine or bioengineering, one of the questions has
been how to create tissues that could help to grow you a new blood
vessel or a new organ. We now know a little more about how to go from a
flat sheet of cells to a complex organism.”
Designing Responsive Buckled Surfaces by Halftone Gel Lithography
