Versatile new material family could build realistic prosthetics,
futuristic army platforms
Researchers have developed a new family of polymers that can self-heal,
have shape memory and are recyclable

Date:
August 14, 2020
Source:
Texas A&M University
Summary:
Nature's blueprint for the human limb is a carefully layered
structure with stiff bone wrapped in layers of different
soft tissue, like muscle and skin, all bound to each other
perfectly. Achieving this kind of sophistication using synthetic
materials to build biologically inspired robotic parts or
multicomponent, complex machines has been an engineering challenge.



FULL STORY ========================================================================== Nature's blueprint for the human limb is a carefully layered structure
with stiff bone wrapped in layers of different soft tissue, like muscle
and skin, all bound to each other perfectly. Achieving this kind of sophistication using synthetic materials to build biologically inspired
robotic parts or multicomponent, complex machines has been an engineering challenge.


==========================================================================
By tweaking the chemistry of a single polymer, researchers at Texas A&M University and the U.S. Army Combat Capabilities Development Command Army Research Laboratory have created a whole family of synthetic materials
that range in texture from ultra-soft to extremely rigid. The researchers
said their materials are 3D printable, self-healing, recyclable and they naturally adhere to each other in air or underwater.

Their findings are detailed in the May issue of the journal Advanced
Functional Materials.

"We have made an exciting group of materials whose properties can be
fine-tuned to get either the softness of rubber or the strength of
load-bearing plastics," said Dr. Svetlana Sukhishvili, professor in the Department of Materials Science and Engineering and a corresponding author
on the study. "Their other desirable characteristics, like 3D printability
and the ability to self-heal within seconds, make them suited for not
just more realistic prosthetics and soft robotics, but also ideal for
broad military applications such as agile platforms for air vehicles
and futuristic self-healing aircraft wings." Synthetic polymers are
made up of long strings of repeating molecular motifs, like beads on
a chain. In elastomeric polymers, or elastomers, these long chains are
lightly crosslinked, giving the materials a rubbery quality.

However, these crosslinks can also be used to make the elastomers more
rigid by increasing the number of crosslinks.

Although previous studies have manipulated the density of crosslinks
to make elastomers stiffer, the resulting change in mechanical strength
was generally permanent.



========================================================================== "Crosslinks are like stitches in a piece of cloth, the more stitches
you have, the stiffer the material gets and vice versa," said
Sukhishvili. "But instead of having these 'stitches' be permanent,
we wanted to achieve dynamic and reversible crosslinking so that we can
create materials that are recyclable." So, the researchers focused their attention on the molecules involved in the crosslinking. First, they
chose a parent polymer, called prepolymer, and then chemically studded
these prepolymer chains with two types of small crosslinking molecules
-- furan and maleimide. By increasing the number of these molecules in
the prepolymer, they found that they could create materials stiffer. In
this way, the hardest material they created was 1,000 times stronger
than the softest.

However, these crosslinks are also reversible. Furan and maleimide
participate in a type of reversible chemical bonding. Put simply,
in this reaction, furan and maleimide pairs can "click" and "unclick"
depending on temperature. When the temperature is high enough, these
molecules come apart from the polymer chains and the materials soften. At
room temperature, the materials harden since the molecules quickly click
back together, once again forming crosslinks.

Thus, if there is any tear in these materials at ambient temperatures,
the researchers showed that furan and maleimide automatically re-click,
healing the gap within a few seconds.

The researchers noted that the temperatures at which the crosslinkers dissociate or unclick from the prepolymer chains are relatively the
same for different stiffness levels. This property is useful for 3D
printing with these materials. Regardless of whether they are soft or
hard, the materials can be melted at the same temperature and then used
as printing ink.

"By modifying the hardware and processing parameters in a standard
3D printer, we were able to use our materials to print complex 3D
objects layer by layer," said Dr. Frank Gardea, research engineer in
the United States Army Research Laboratory and a corresponding author
on the study. "The unique advantage of our materials is that the layers
that make up the 3D part can be of vastly different stiffness." As the
3D part cools to room temperature, he added that the different layers
join seamlessly, precluding the need for curing or any other chemical processing. Consequently, the 3D-printed parts can easily be melted using
high heat and then recycled as printing ink. The researchers also noted
that their materials are reprogrammable. In other words, after being
set into one shape, they can be made to change into a different shape
using just heat.

In the future, the researchers plan to increase the functionality of
their new materials by amplifying its multifaceted properties outlined
in the current study.

"Right now, we can easily achieve around 80% self-healing at room
temperature, but we would like to reach 100%. Also, we want to make our materials responsive to other stimuli other than temperature, like light,"
said Gardea. "Further down the road, we'd like to explore introducing
some low-level intelligence so that these materials know to autonomously
adapt without needing a user to initiate the process."

========================================================================== Story Source: Materials provided by Texas_A&M_University. Original written
by Vandana Suresh and Dharmesh Patel. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Qing Zhou, Frank Gardea, Zhen Sang, Seunghyun Lee, Matt Pharr,
Svetlana
A. Sukhishvili. A Tailorable Family of
Elastomeric‐to‐Rigid, 3D Printable, Interbonding
Polymer Networks. Advanced Functional Materials, 2020; 30 (30):
2002374 DOI: 10.1002/adfm.202002374 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200814163309.htm

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