Tech Briefs Magazine - February 2022 - 22

Manufacturing &
Prototyping
3D Printing of Polymers
A 3D-printable elastomer yields soft, elastic objects that feel like human tissue.
University of California, Santa Barbara, CA
esearchers have developed the first 3Dprintable
" bottlebrush " elasto mer that
results in printed objects with unusual softness
and elasticity - mechan ical properties
that closely resemble those of human
tissue.
R
Conventional elastomers, such as rubbers,
are stiffer than many biological tissues
due to the size and shape of their constituent
polymers, which are long, linear
molecules that easily entangle like cooked
spaghetti. In contrast, bottlebrush polymers
have additional polymers attached to
the linear backbone, leading to a structure
more akin to a bottle brush you might find
in a kitchen. The bottlebrush polymer
structure im parts the ability to form
extremely soft elastomers.
The ability to 3D print bottlebrush
elastomers makes it possible to leverage
these unique mechanical properties in
applications that require careful control
over the dimensions of objects ranging
from biomimetic tissue to high-sensitivity
electronic devices such as touch pads,
sensors, and actuators.
The bottlebrush polymer material is categorized
as a yield-stress fluid, meaning it
LED
From left: The unlinked polymer ink, infrared light being applied to activate the crosslinks, and the
final product - a super-soft, super-elastic crosslinked elastomer. (Photo: Isabelle Chabinyc)
begins as a semi-soft solid that holds its
shape, like butter or toothpaste, but when
sufficient pressure is applied, it liquefies
and can be squeezed through a syringe.
The researchers can tune the material to
flow under various amounts of pressure to
match the desired processing conditions.
Once the object is printed, UV light is
shined onto it to activate crosslinkers
that were synthesized and included as a
part of the ink formulation. The
crosslinkers can link up nearby bottlebrush
polymers, resulting in a super-soft
elastomer. At that point, the material
becomes a permanent solid - it will no
longer liquefy under pressure - and
exhibits extraordinary properties.
The softness of a material is measured
in terms of its modulus and for most elastomers,
it is rather high, meaning their
stiffness and elasticity are similar to those
of a rubber band. The modulus of the new
material is 1,000 times smaller than that of
a rubber band. The material can also
stretch about three to four times its length.
The super-soft elastomers might be
applicable as implants; for example, to
reduce inflammation and rejection by the
body if the mechanical properties of an
implant match native tissue. Another
important element of the material is that it
is pure polymer - there is no water or
other solvent to artificially make it softer.
For more information, contact James
Badham at jwbadham@ucsb.edu.
3D-Printed Objects Sense How a User is Interacting with Them
Sensing is incorporated directly into an object's material, with applications for assistive
technology and " intelligent " furniture.
Massachusetts Institute of Technology, Cambridge, MA
R
esearchers have developed a new
method to 3D print mechanisms that
detect how force is being applied to an
object. The structures are made from a
single piece of material, so they can be
rapidly prototyped. A designer could use
this method to 3D print interactive input
devices, like a joystick, switch, or handheld
controller, in one pass.
To accomplish this, the researchers
integrated electrodes into structures
made from metamaterials, which are
22
Cov
materials divided into a grid of repeating
cells. They also created editing software
that helps users build these interactive
devices.
Sensing can be integrated directly
into the material structure of objects,
enabling intelligent environments in
which objects can sense each interaction
with them. For instance, a chair or
couch made from the smart material
could detect the user's body when the
user sits on it and either use it to query
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particular functions (such as turning on
the light or TV) or collect data for later
analysis (such as detecting and correcting
body posture).
Because metamaterials are made from
a grid of cells, when the user applies
force to a metamaterial object, some of
the flexible, interior cells stretch or compress.
The researchers took advantage of
this by creating " conductive shear cells "
- flexible cells that have two opposing
walls made from conductive filament
Tech Briefs, February 2022
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Tech Briefs Magazine - February 2022

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