April 20, 2016      

Robots aren’t usually thought of as mushy, squishy things. Yet this perception may soon change as soft robotics begin finding applications in fields ranging from healthcare to undersea repairs and maintenance.

Most robots have rigid metallic structures featuring joints that use conventional bearings. While such robots often possess limb-like structures similar to those found on animals, they also feature mobility technologies, such as wheels and treads, never found in nature.

Researchers worldwide are now looking to nature and other inspirations for soft robots that are more configurable, agile, and adaptable and that can be easily controlled from a distance without wires or cables.

Soft robotics, but with rigidity

Soft robots generally move slowly, particularly when working autonomously. Engineers at Harvard University and the University of California, San Diego, have developed a technology that will enable rigid components to be more tightly integrated within soft robots, an approach that promises to help the systems move faster without endangering the humans who would work with them.

The researchers recently introduced the first robot with both a rigid core and a soft exterior. The robot has a 3D-printed body and is powered by a mixture of butane and oxygen. It is capable of completing over 30 untethered jumps, the researchers claim.

“We believe that bringing together soft and rigid materials will help create a new generation of fast, agile robots that are more robust and adaptable than their predecessors and can safely work side by side with humans,” said Michael Tolley, an assistant professor of mechanical engineering at UC San Diego.

Tolley collaborated on the project with Nicholas Bartlett, a doctoral student at Harvard’s Wyss Institute. Bartlett and Tolley jointly designed, manufactured, and tested the robot.

The robot consists of a pair of hemispheres. The upper hemisphere, 3D-printed in a single piece, is shaped like a half shell and features nine layers of stiffness, creating a structure that transitions from rubber-like flexibility on the exterior to full rigidity close to the core.

University researchers use 3D printing to tightly integrate soft robot components.

New technology allows rigid components to be more tightly integrated within soft robots. Credit: UC San Diego/Harvard University

The researchers evaluated several design versions before deciding that while a fully rigid top would enable higher jumps, a more flexible top was more likely to survive impacts on landing, allowing the robot to be reused.

According to Tolley, the rigid layers provide an excellent interface for the robot’s electronics and power sources. The soft layers make the robot less likely to suffer damage as it lands after jumping.

The flexible bottom half of the robot incorporates a small chamber where oxygen and butane are injected prior to the jumps. After the gases are ignited, the bottom half of the robot behaves like a basketball that’s inflated almost instantaneously, propelling the robot. After the chemical charge is exhausted, the bottom hemisphere returns to its original shape.

The dual hemispheres surround a rigid core module that includes a custom circuit board, a high-voltage power source, a battery, a butane fuel cell, a miniature air compressor, and several other components.

In a series of tests, the researchers were able to get the robot to leap two and a half feet (0.75 m) in height and half a foot (0.15m) in distance. Cumulatively, the robot jumped a total of over 100 times and survived 35 drops from a height of nearly four feet (1.2 meters).

One of the project’s most challenging aspects was designing and 3D-printing the robot using only off-the-shelf materials. According to Tolley, the specifications provided by the material manufacturers weren’t sufficiently detailed, so the researchers were forced to print test samples for every material used.

While it was a time-consuming process, the data collected enabled the team to calculate the precise stiffness of the nine unique layers used in the robot’s upper hemisphere. The data was then used to simulate the robot’s anticipated performance and to speed up development.

The hopping robot could be useful for energy-efficient exploration of varied terrain on Earth or Mars.

Soft robotics shape shifters

Imagine a robot that’s able to morph into a variety of different shapes, allowing it to travel across land, propel itself through water, or even fly through that air. Cornell University engineering professor Rob Shepherd and his research team are hoping to bring such a vehicle, and an array of other shape-shifting robots, into reality.

The key to creating a shape shifting robot is a new hybrid material developed by the researchers incorporating both stiff metal and a soft, porous rubber foam that has stiff properties when required and elasticity when a shape change is necessary. The material also has unique the ability to self-heal following a puncture, dent, or other form of minor damage.

The technology aims to blends the rigidity and load-bearing capacity of flesh and bone mammals with the ability to significantly alter shape, like an octopus.

“That’s what this idea is about, to have a skeleton when you need it, melt it away when you don’t, and then reform it,” Shepherd said.

This hybrid material blends a soft alloy, “Field’s metal,” with porous silicone foam. Besides providing a low melting point of 144 degrees Fahrenheit, Field’s metal, unlike similar alloys, contains no lead.

The elastomer foam is dipped into the molten Field’s metal and then placed in a vacuum. This allows all of the air in the foam’s pores to be evacuated and replaced by the alloy. The foam can then be tuned to create a stiffer or a more flexible material.

In strength and elasticity tests, the material exhibited an ability to deform when heated above 144 degrees, regain its rigidity when cooled, and then return to its original shape and strength when reheated.

“Sometimes you want a robot, or any machine, to be stiff,” Shepherd said. “But when you make them stiff, they can’t morph their shape very well.”

Disney creates soft robotic skin

A new soft robot skin developed by Disney Research, a network of research laboratories supporting The Walt Disney Co. in Pittsburgh, uses air-filled spaces to cushion collisions and to supply the pressure feedback needed to grasp delicate, fragile objects.

By monitoring pressure changes that occur when the airtight but flexible chamber is deformed, the air-filled skin modules can absorb different types of impacts.

The chamber can also serve as a contact sensor, providing feedback for touching, grasping, and manipulating.

Soft robotic skin at Disney Research

Soft robotics skin gives robots the feedback needed to grasp delicate, fragile objects. Credit: Disney Research Pittsburgh

The researchers created cylindrical soft-skin modules with hemispheric ends, measuring slightly less than 5 in. long and 2.5 in. in diameter.

In addition to the air-filled outer skin, each module features a rigid link at the center, enabling the units to assume a wide range of material properties, from flexible to rigid.

In tests conducted with the inflated skins removed, using only the rigid link, the modules were able to grip a disposable cup. Yet lacking the pressure feedback supplied by the soft skin, the cup was quickly crushed.

With the soft skins reattached, sufficient pressure feedback enabled the modules to grasp the cup, as well as other delicate objects, without damaging them.

Collision tests showed that the inflatable modules reduced the peak force of frontal impacts by 32 to 52 percent and side impacts by 26 to 37 percent.

“Humans interacting with robots in everyday environments is no longer just science fiction,” said Joohyung Kim, an associate research scientist. “Making them soft is particularly important for robots that will interact with children, the elderly, or with patients.”

Magnetic nanoparticles to control soft robotics

North Carolina State University researchers are using chains of magnetic nanoparticles to manipulate elastic polymers in three dimensions, a technique that could someday be used to remotely control soft robots.

Led by Joe Tracy, an associate professor of materials science and engineering, the researchers have developed a method of embedding long chains of nanoscale magnetite particles into sheets of elastic polymer to form a magnetic polymer nanocomposite.

Magnetic nanoparticles enable this soft robot to move.

A horizontal uniform magnetic field allows for selective actuation of the side arms of a soft robot. Credit: Sumeet Mishra, North Carolina State University

By applying a magnetic field, the researchers can control the way the material bends — turning it into a basic soft robot.

Magnetic field control allows soft robot movements to be accomplished remotely with no physical contract or connection to the polymer.

Another benefit is that magnetic fields can be easily obtained from permanent magnets and electromagnets.

The elastic polymer is developed by dispersing nanoparticles of an iron oxide — magnetite –into a solvent. A polymer is then dissolved into the mixture, which is poured into a mold to form the desired shape.

A magnetic field is then applied, forcing the magnetite nanoparticles to arrange themselves into parallel chains.

In the final step, the solution is dried, locking the chains into place, and the finished nanocomposite can be cut to further refine its shape. The technique is both inexpensive and uses widely available materials, Tracy noted. The process is also relatively simple and easy to follow.

The researchers believe that their control approach is particularly suitable for biomedical use, as well as other applications where electrical control may pose a health or safety issue.

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