ADVANCES
20 Scientific American, December 2019
INDIAN INSTITUTE OF TECHNOLOGY GUWAHATIMATERIALS SCIENCE
Silky Tissue
Wild silkworms generate proteins
ready for 3-D bioprinting
Many research groups are testing “ink”
made from silk proteins to print human tis-
sues, implants and perhaps even organs. The
process is a less costly alternative to conven-
tional 3-D printing with collagen, a key pro-
tein in the body’s natural scaffolding. Re
searchers in Assam, a state in India, are
investigating using local silkworm species for
the task—they recently submitted a patent
for bioinks using a combination of proteins
extracted from local species Antheraea assa-
mensis and Samia ricini, as well as the com-
monly used Bombyx mori. The scientists have
woven them into synthetic structures rang-
ing from blood vessels to liver lobes; in a
paper published in September in ACS Applied
Materials & Interfaces, they described mim-
icking the cartilage of an entire ear.
Silk is a natural polymer, a substance
with long, repeating molecular chains. It is
mechanically strong and completely bio-
degradable, well suited for applications in
tissue engineering. To use it, researchers
draw liquid silk from the silkworm’s glands
or dissolve silk fibers in solvents. They
carefully mix the gelatinous liquid with a
patient’s stem cells, then build structures
layer by layer with a 3-D printer. After
implantation, the cells grow and replace
the silken scaffold, which eventually
degenerates into amino acids.
Extracting and purifying collagen
from animal remains, a common medical
source, is complex and expensive. “Com-
pared with collagen, silks have an immense
advantage in terms of supply and process-
ing. Local sourcing is also a clear plus in
their use in India,” says David Kaplan, who
heads the department of biomedical engi-
neering at Tufts University and is not in -
volved in the new research. Silk fromdomesticated silkworms has been used
widely in bioprinting, but Biman B. Man-
dal’s laboratory at the Indian Institute of
Technology Guwahati in Assam is among
the first to incorporate wild silks.
These silks are ideal candidates for bio-
inks because they can be combined to
build strong and resilient scaffolds, says
Mandal, the lab’s principal investigator.
“This is important, for example, when
making bone tissue,” he adds.
Researchers commonly use chemicals to
cross-link silk polymer chains, which helps
to maintain a 3-D structure, but Mandal’s
group found a blend of silks and gelatin that
works without many of those chemicals.
Also, the wild silk has spots that cells natural-
ly attach to, he says: “For other silks, they
have to be decorated with chemicals that
promote adherence. This can be complicat-
ed, expensive and potentially toxic.” Kaplan
agrees, adding that these binding spots allow
cells to adhere rapidly to the silk matrix.
Mandal and his collaborators have al -
ready created prototype structures, includ-
ing bone and soft tissues such as those of
the heart and liver. Reconstructing a human
knee meniscus and the complex tissue at the
ends of a bone will be next. — Harini BarathT E C H
Fusing
Ceramics
A new laser technique
could pave the way for
tougher electronics
Ceramics are hard and durable; they
resist scratches better than glass and stand
up to high heat better than most metals.
They could protect electronic devices
from challenging conditions found in space
or in the human body—but their very
toughness makes them hard to manipulate.
Joining two ceramic slabs with an airtight
seal requires heating them to about 2,000
degrees Celsius, which would typically
destroy embedded electronics. Now, how-
ever, researchers have developed a welding
technique that spot heats the ceramics with
lasers, as described in August in Science.
Lasers have already fused glass: pulsing
a specially tuned beam about a trillion times
a second can melt a targeted spot. But
unlike glass, a ceramic scatters this light
instead of absorbing it. “When you think
of a ceramic, you think of a coffee cup or
a bowl,” says principal investigator Javier E.
Garay, a mechanical and aerospace engi-
neer at the University of California, San
Diego. Such items are opaque because they
contain tiny light-scattering pores, Garay
explains. Adjusting the manufacturing pro-
cess to reduce the pores’ size and number,an idea pioneered by ceramic scientist Rob-
ert Coble 60 years ago, can make the mate-
rial translucent or transparent.
Working with a transparent version of
a common ceramic and a laser technique
similar to the one used for glass, the re -
searchers successfully welded cylindrical
containers. The resulting seam was tight
enough to hold a vacuum with little air
leakage, qualifying it for use in harsh envi-
ronments such as space. Because these
ceramics do not react with living tissue,
they could also encase electronic devices
implanted in the human body.
“It’s a major engineering achievement,”
says Himanshu Jain, a materials scientist
at Lehigh University, who was not involved
in the new study. Although previous re -
search has used lasers to melt ceramics, he
notes, this is the first time a laser has weld-
ed ceramic pieces together. “The hardest
part is to get the proof of principle,” he
says. “Now, to go into detail and under-
stand the science behind it, why it works
and how it works—all those things are yet
to be done.” — Sophie BushwickWild silkworm species Antheraea assamensis© 2019 Scientific American