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Y have its function changed by chang-ing how it is wrapped around a flexi-ble core. With actuators aligned along the length of a flexible rod of foam or an inflatable, they can force a bending motion. Two or three close together become grippers. Rotate them 90°, and the actuators let the rod move

IN A LABORATORY at Yale Univer-sity, a soft toy horse with pros-thetic coverings around its foam-stuffed legs has taken its first tentative steps. De-

spite its stiff and not entirely coor-dinated gait, the toy demonstration may point the way toward helping space agencies put lighter, more ver-satile robots into space.

Rebecca Kramer-Bottiglio, assistant professor at the Yale School of Engineer-ing & Applied Science, says she was wres-tling with the problem of how to allow robots to handle a wider variety of jobs than current approaches, which often focus on performing a single function well, when the U.S. National Aeronau-tics and Space Administration (NASA) issued a request for novel robot designs based on lighter, plastic approaches.

Rather than attempt to lift many single-task robots into orbit, the space agency wants a single reconfig-urable machine to be able to handle different tasks and, occasionally, to act as prosthetics for human astro-nauts. “You may need to make an exploratory locomotion robot that can go out and collect data from an unknown environment. At the same time, you may need suits to promote

blood flow in the astronauts who are onboard,” she says. And NASA wants to avoid the weight of bulky, metal-framed robots.

“The idea I had was to have a robot-ic skin,” she says.

Armed with sensors and pneu-matic actuators, the artificial skin can

Soft Robots Look to New EnvironmentsThese non-standard automatons appear best suited for applications under water and in space.

Science | DOI:10.1145/3311719 Chris Edwards

“Robotic Skins” technology developed by Rebecca Kramer-Bottiglio and colleagues at the Yale School of Engineering & Applied Science enables novel designs for robots that can move more freely.

10 COMMUNICATIONS OF THE ACM | APRIL 2019 | VOL. 62 | NO. 4

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around like a worm. The smart skin may massage the legs of an astronaut, or act as exoskeletons to help with resistance exercises; the function changes as the skin is peeled off, ro-tated, and replaced.

Kramer-Bottiglio’s ultimate hope is that with a sufficiently malleable in-terior, the skins could have two types of actuator. One would mold the inte-rior to form appendages, while anoth-er type would move those appendages around; in effect, forming a robot that morphs based on the job it is asked to perform. “It is a vision that we are quite far from today,” she concedes.

Like other researchers into ro-botics, Kramer-Bottiglio faces two key problems: force and control. Al-though the soft robots are made of lighter materials than traditional ro-bots and should be easier to move, it is difficult to deliver large amounts of power to the artificial muscles. En-gineers are still many years from be-ing able to emulate the high power-weight ratio of organic musculature. Yale’s robotized toy horse makes slow progress because the pneumatic ac-tuators find it hard to bend its foam-

stuffed legs, and the use of open-loop control leads to motion that is far less coordinated than that of a real horse. Not only that, most of these robots need to be tethered to electronic and pneumatic or hydraulic power sourc-es, which limits their freedom.

A shift away from traditional elec-tronic robot design and construction could liberate soft automatons from their tethers and help them move more

freely. Several years ago, Jennifer Lewis and colleagues at Harvard University were asked whether it was possible to make a fully autonomous soft robot. In attempting to design one, they moved away from electronics, batteries, and motors to a structure that could be controlled by microfluidics.

The Octobot they produced pro-vided the mechanics and core struc-ture of an octopus-like robot made from a sandwich of materials that are not very different from the silicone caulk used to line bathroom sinks. However, the construction is much more complex. As an experimentalist with long-term involvement in three-dimensional (3D) printing, Lewis and her colleagues took advantage of the technology’s ability to build complex structures in layers. As it forms each layer, the 3D printer works around the voids that will become microfluidic channels and pneumatic pipes used to fuel and distort the limb’s shape.

Motive power for the Octobot’s limbs comes from a supply of hydrogen peroxide in the robot’s body. Micro- fluidic channels controlled by a net-work of tiny structures analogous

A shift away from traditional electronic robot design and construction could liberate soft automatons from their tethers and help them move more freely.

“Small, thin, and sleek” has long been the mantra for designing smartphones, smartwatches, tablets, and laptops. A focus on usability has driven advances in form factors. Yet, one thing has stayed the same: virtually all computing devices remain rigid.

It is a challenge that has vexed researchers struggling to develop electronic displays that can bend, fold, flex, and roll—while containing batteries, circuits, and other components. Ultimately, every advance has led to the same dead end: a display that cannot stand up to the rigors of everyday use.

However, that situation is about to change. After decades of research and false starts, manufacturers are introducing products with flexible displays.

Royole Corp. recently unveiled a smartphone with a flexible screen that allows the device to be folded like a billfold; the product has been available in China and

the U.S. since December 2018.Meanwhile, Samsung plans

to introduce a smartphone with a flexible display this year; others are incorporating flexible designs into products as well.

Says Vladimir Bulovic, a professor of electrical engineering at the Massachusetts Institute of Technology (MIT), “Flexible formats can be applied to many devices.”

BEND, DON’T BREAKNew materials, better production methods, and other advances in technology have raised hopes of viable flexible products. The underlying OLED technology is now at a point where it works well, but encasing the displays in plastic or ultra-thin glass remains a challenge. Samsung’s foldable smartphone, for example, features an interior screen that uses a composite polymer transparent material to encase a bendable AMOLED display.

The manufacturer claims the Samsung Infinity Flex Display can open and close 300,000 times without suffering damage.

Other companies are also moving flexible products from the research lab to production. Royole’s FlexPai device features a 7.8-inch 1440p AMOLED display supported by a hinge that allows the device to flex to almost any desired angle. Royole also has partnered with Airbus to produce flexible electronics for aircraft, and plans to produce clothing with display technology.

Gregory Raupp, Foundation Professor of Chemical Engineering and founding director of the Flexible Display Center at Arizona State University, says flexible technology could impact fitness trackers, smart watches, Internet of Things devices, consumer electronics, and industrial control systems.

Yet, some questions remain. For one, “How do you deploy

that flimsy, plastic display into a product that will be robust and that the user won’t damage by flexing it too much?” asks Raupp.

For another, “No one has attempted to produce flexible displays on a larger scale. Mass production creates distinct difficulties,” says William Stofega, program director for Mobile Device Technology and Trends at IDC.

Finally, and perhaps most importantly, will the public desire flexible devices? “The social response to this technology is a complete unknown,” Bulovic says.

Concludes Raupp, “We’re now at a point where the manufacturing problems have largely been overcome and it is a question of innovating and integrating flexible displays in new, unique, and novel ways. It’s up to the design community to transform ideas into reality.”

—Samuel Greengard is an author and journalist based

in West Linn, OR, USA.

ACM News

Flexible Displays Enter the Picture

APRIL 2019 | VOL. 62 | NO. 4 | COMMUNICATIONS OF THE ACM 11

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tendrils, swapping hard for soft ver-sions, the FAU team found there was an optimum combination for thrust: with both top and bottom being made of a material with floppiness similar to that of a mouse pad, being pulsed a little less than once a second.

In addition to the challenges roboti-cists face with materials and power de-livery, Josh Bongard, associate professor at the University of Vermont, says con-trol presents further problems. “Exploit-ing the capabilities of soft robots is a very non-intuitive thing for human engi-neers to do. The mathematics that we’ve developed over decades for designing and controlling traditional robots made up of rigid links simply doesn’t apply to [these] systems. In short: it’s hard to de-sign and control moving blobs.”

In contrast to the inverse kinematics and closed-loop control that dominate fixed-function robots, Bongard propos-es harnessing evolutionary program-ming coupled with machine learning to develop novel control methods for producing movement that take into ac-count how plastic materials bend and compress under force.

In these simulations, the design starts with a basic shape made from blocks with different levels of stiff-ness and mobility. Evolutionary algorithms gradually change the properties of different blocks until the robot is able to move. The algo-rithms tune their response to the way materials flex under strain in different positions using machine-learning algorithms such as neural

networks. Sometimes, the simulat-ed machines use the equivalent of body fat to help leverage the effects of whatever type of motion the robot adopts. Bongard says, “Our evolution-ary algorithms often find very non-in-tuitive designs, such as one that has a hump on its back and uses it to throw its weight forward to make movement more efficient.”

The work tends to agree with re-sults such as those from FAU: softer robots perform better under water. Those simulated on land tended to re-quire stiffer structures. Bongard now plans to take the work to physical ro-bots in a collaboration with Kramer-Bottiglio and her team on a project backed by the National Science Foun-dation. The simulations will help create configurations for the robotic skins and the underlying shapes the skins are wrapped around.

Although soft robots have a long way to go before becoming autono-mous enough to deliver on the prom-ise of lighter, more functional ma-chines, research and development is gradually bringing together the types of control and materials science that will make them work well on Earth and beyond.

Further Reading

Rus, D., and Tolley, M.T.Design, Fabrication and Control of Soft RobotsNature 521, Number 7553, pp467-475 (2015)

Booth, J.W, Shah, D., Case, J.C., White, E.L., Yuen, M.C., Cyr-Choiniere, O., and Kramer-Bottiglio, R.OmniSkins: Robotic Skins That Turn Inanimate Objects into Multifunctional Robots,Science Robotics, Vol 3, Issue 22, eaat1853 (2018)

Frame, J., Lopez, N., Curet, O., and Engeberg, E.D.Thrust Force Characterization of Free-Swimming Soft Robotic Jellyfish,Bioinspiration & Biomimetics, Volume 13, Number 6 (2018)

Corucci, F., Cheney, N., Giorgio-Serchi, F., Bongard, J., and Laschi, C.Evolving Soft Locomotion in Aquatic and Terrestrial Environments: Effects of Material Properties and Environmental Transitions, Soft Robotics, 5(4): 475-495 (2018)

Chris Edwards is a Surrey, U.K.-based writer who reports on electronics, IT, and synthetic biology.

© 2019 ACM 0001-0782/19/4 $15.00

to electronic logic gates to imple-ment components such as oscilla-tors convey the peroxide to reaction chambers. Each chamber contains a platinum catalyst, which splits the molecule into water and oxygen. The resulting gas drives pneumatic actua-tors that move the limbs, although they can do little more than twitch.

Soft robots may overcome the power problem by operating in environments where gravity is not as big a problem as it is on land. The microgravity of space may be one obvious habitat for them, but robots made from elastomers and pumps already find the going much easier under water.

Former Florida Atlantic University (FAU) student Jennifer Frame chose the jellyfish as the biological model for her thesis. Named JenniFish, the robot uses stubby plastic tentacles harnessed to hydraulic pumps pow-ered by a battery to mimic the puls-ing action of the invertebrate’s body as it moves. Able to swim untethered in the ocean, it is flexible enough to squeeze through narrow orifices. In experiments, the robot would collide with the edges of a hole, then gener-ate enough thrust to push its append-ages to its side and squeeze through.

Another situation in which soft ro-bots could perform well is in a different fluid environment: inside the human body. In work performed in Europe before moving to Stanford University in 2016, Stanford postdoctoral fellow Giada Gerboni (who works in surgical robotics) developed a soft endoscopic camera for surgery. Now Gerboni is fo-cusing on soft robots that can enter the body, move around, and perform mi-crosurgery operations without direct in-tervention from human surgeons. She describes it as a very flexible needle that can steer around parts of the body and into organs with minimal disruption.

In further work by the group led by FAU associate professor Erik Engeberg, the JenniFish has helped demonstrate how material choices form a key part of design for liquid environments. The algorithm needed to make the robot swim in different directions is rela-tively simple: it takes advantage of the way the polymer limbs are shaped to bend in on themselves when flexed. By changing the plastics used for the top and bottom surface of the JenniFish

Gerboni is focusing on soft robots that can enter the body, move around, and perform microsurgery operations without direct intervention from human surgeons.