Many robotic designs take nature as their
muse: sticking to walls like geckos, swimming through water like tuna, sprinting
across terrain like cheetahs. Such designs borrow properties from nature, using
engineered materials and hardware to mimic animals’ behavior.
Now, scientists at Massachusetts Institute
of Technology (MIT) and the University of Pennsylvania are taking more than
inspiration from nature—they’re taking ingredients. The group has genetically
engineered muscle cells to flex in response to light, and is using the
light-sensitive tissue to build highly articulated robots. This “bio-integrated” approach, as they call it, may one day enable robotic animals
that move with the strength and flexibility of their living counterparts.
The researchers’ approach will appear in Lab
on a Chip.
Harry Asada, the Ford Professor of
Engineering in MIT’s Department of Mechanical Engineering, says the group’s
design effectively blurs the boundary between nature and machines.
“With bio-inspired designs, biology is a
metaphor, and robotics is the tool to make it happen,” says Asada, who is a
co-author on the paper. “With bio-integrated designs, biology provides the
materials, not just the metaphor. This is a new direction we’re pushing in
biorobotics.”
Seeing the light
Asada and MIT postdoctoral researcher Mahmut Selman Sakar collaborated with
Roger Kamm, the Cecil and Ida Green Distinguished Professor of Biological and
Mechanical Engineering, to develop the new approach. In deciding which bodily
tissue to use in their robotic design, the researchers set upon skeletal
muscle—a stronger, more powerful tissue than cardiac or smooth muscle. But
unlike cardiac tissue, which beats involuntarily, skeletal muscles—those
involved in running, walking, and other physical motions—need external stimuli
to flex.
Normally, neurons act to excite muscles,
sending electrical impulses that cause a muscle to contract. In the laboratory,
researchers have employed electrodes to stimulate muscle fibers with small
amounts of current. But Asada says such a technique, while effective, is
unwieldy. Moreover, he says, electrodes, along with their power supply, would likely
bog down a small robot.
Instead, Asada and his colleagues looked to
a relatively new field called optogenetics, invented in 2005 by MIT’s Ed Boyden
and Karl Deisseroth from Stanford University, who genetically modified neurons
to respond to short laser pulses. Since then, researchers have used the
technique to stimulate cardiac cells to twitch.
Asada’s team looked for ways to do the same
with skeletal muscle cells. The researchers cultured such cells, or myoblasts,
genetically modifying them to express a light-activated protein. The group
fused myoblasts into long muscle fibers, then shone 20-msec pulses of blue
light into the dish. They found that the genetically altered fibers responded
in spatially specific ways: Small beams of light shone on just one fiber caused
only that fiber to contract, while larger beams covering multiple fibers
stimulated all those fibers to contract.
A light workout
The group is the first to successfully stimulate skeletal muscle using light,
providing a new “wireless” way to control muscles. Going a step further, Asada
grew muscle fibers with a mixture of hydrogel to form a 3D muscle tissue, and
again stimulated the tissue with light—finding that the 3D muscle responded in
much the same way as individual muscle fibers, bending and twisting in areas
exposed to beams of light.
The researchers tested the strength of the
engineered tissue using a small micromechanical chip—designed by Christopher
Chen at Penn—that contains multiple wells, each housing two flexible posts. The
group attached muscle strips to each post, then stimulated the tissue with
light. As the muscle contracts, it pulls the posts inward; because the
stiffness of each post is known, the group can calculate the muscle’s force using
each post’s bent angle.
Asada says the device also serves as a
training center for engineered muscle, providing a workout of sorts to
strengthen the tissue. “Like bedridden people, its muscle tone goes down very
quickly without exercise,” Asada says.
The light-sensitive muscle tissue exhibits
a wide range of motions, which may enable highly articulated, flexible robots—a
goal the group is now working toward. One potential robotic device may involve
endoscopy, a procedure in which a camera is threaded through the body to
illuminate tissue or organs. Asada says a robot made of light-sensitive muscle
may be small and nimble enough to navigate tight spaces—even within the body’s
vasculature. While it will be some time before such a device can be engineered,
Asada says the group’s results are a promising start.
“We can put 10 degrees of freedom in a
limited space, less than one millimeter,” Asada says. “There’s no actuator that
can do that kind of job right now.”
In the meantime, there may be a more
immediate application for both the engineered muscles and the microchip: Asada
says the setup may be used to screen drugs for motor-related diseases.
Scientists may grow light-sensitive muscle strips in multiple wells, and
monitor their reaction—and the force of their contractions—in response to various
drugs.
