Mixing silk with polymers could lead to better biomedical implants


Date:
August 17, 2020
Source:
American Chemical Society
Summary:
Spun by spiders and silkworms, silk has mystified human engineers
who have yet to figure out how to artificially recreate it. But by
combining silk with synthetic compounds, researchers are getting
closer to developing new implantable composite materials with the
best properties of both. Potential applications include structures
that hold bone in place or replacements for cartilage.



FULL STORY ==========================================================================
Spun by spiders and silkworms, silk has mystified human engineers
who have yet to figure out how to artificially recreate this tough,
fine fiber. But by combining silk, which is safe for use in the human
body, with synthetic compounds, one research team is getting closer to developing new implantable composite materials with the best properties of both. Potential applications, which are still years away, could include structures that hold bone in place after surgery or replacements for
the cartilage cushions in the knee.


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The researchers will present their results today at the American Chemical Society (ACS) Fall 2020 Virtual Meeting & Expo.

"Silk has great potential for use in biomedical applications," says Juan
Guan, Ph.D., the project's principal investigator. "Silk is versatile,
and the human body tolerates it quite well, and can even degrade and
absorb it." Silk has a long history in medicine. Records of ancient
doctors stitching up patients with fibers spun by silkworms date back
nearly 2,000 years. And today, surgeons finish certain surgeries, such
as those on the eye, with silk sutures.

By combining silk and synthetic polymers, Guan and her colleagues at
Beihang University are seeking to develop versatile new materials for
use in medicine and, potentially, other fields as well. While other
researchers have already developed composite materials with silk,
they have typically worked with short fibers or the primary protein
in silk. Guan, however, focuses on silk fabric woven from a long,
single thread. Silkworms' cocoons can contain fibers nearly 5,000 feet
long, and when used whole in fabric, such a fiber can more effectively distribute mechanical stress than a series of shorter, discrete ones,
she says. In their studies, Guan's team uses silk from the common,
domesticated silkworm Bombyx mori, as well as tougher, more stretchy
fibers from the wild species Antheraea pernyi.

The researchers combine this fabric with a polymer matrix, often an
epoxy, which is used in adhesives. Together, the fabric and the polymer
form a laminate -- similar to the durable surface covering found on some furniture - - which can then be cut into the shapes the researchers need.

Guan and her colleagues say that the properties of these new materials
could make them a better match for the tissues within the human body
than what is being used today. For instance, they are collaborating with orthopedic doctors to devise structures resembling cages that temporarily
hold vertebrae in place as they fuse after surgery, a task currently accomplished mostly using metal.

The silk composites' hardness and stiffness is more compatible with bone, making them potentially more resilient yet more comfortable than metal structures, she says.

There are challenges, however. The inside of the human body is moist,
a potential problem because water can soften and weaken silk. In new experiments, Guan and her colleagues tested how silk-epoxy composite
materials hold up when exposed to humidity or immersed in water. For use alongside bone, they must maintain a certain stiffness. The experiments
showed that while this attribute decreased under wetter conditions,
the composites remained stiff enough to function as implants, she says.

While the epoxy attaches firmly to the silk fiber, it has a major
drawback: The body can't break down the epoxy and absorb it, meaning
it would not be suitable for implants intended to dissolve. So, Guan
recently began working with biopolymers that, like silk, the body can
break down and absorb. However, these composites have less internal
cohesion than those that contain an epoxy. "The key question is how
to make the interface between the biopolymer and the silk fabric more
robust," she says.

The scientists are also looking to supplement silk with other types
of fibers.

In a recent study, they added carbon fibers into the mix. "The notion
of hybridizing silk with other fibers makes it possible to produce a
rather nice spectrum of properties that you can optimize for a given application," says Robert O. Ritchie, Ph.D, an author of the carbon
fiber study. Potential uses for these new structural materials, he says,
could be anywhere: in the human body, or even in tennis rackets or on
airplane engines.


========================================================================== Story Source: Materials provided by American_Chemical_Society. Note:
Content may be edited for style and length.


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Link to news story: https://www.sciencedaily.com/releases/2020/08/200817104307.htm

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