Soft robots, plastics play hardball

Soft polymers give scientists a break

Although our vision of robots has changed considerably over the last few decades, the future of robotics is undeniably soft robotics. To better understand this "concept", we must imagine a robot without joints whose movement would be based only on the deformation of its structure. Unlike traditional robotics, which relies primarily on the weight/rigidity ratio of materials to design robots that are stable and do not shake (imagine the potential damage caused by an unstable surgical arm), soft robotics seeks to play with the controlled deformation of the robot to make it complete the programmed task. In the world of robotics, there is talk of a paradigm shift and many scientists have embarked on this new and promising path. This new approach is based on a better knowledge of flexible polymer materials, on the ever-growing computing power of computers that allows for more precise modelling of robot movement, and on 3D printers capable of printing objects using flexible materials.

In industry, some robot arms are already equipped with deformable grippers instead of the usual clamps. Very briefly, this new hand consists of bagged plastic granules. When it is placed on the object to be gripped, the flexibility of the granules ensures that it naturally adapts to the shape of the object. All that remains is to create an air gap between the hand and the object so that the robot can grasp the object and handle it without any risk of damaging it.

This robot's flexible grippers, made from a soft polymer, enable it to grasp the most delicate objects without damaging them.

As another example, there are flexible grippers made from a food-grade polymer that can handle any type of food, including fragile foods such as cakes.

Polymers are not short of air

Exoskeletons also have a bright future ahead of them in the world of robotics. The only issues are their size and, to a lesser extent, their weight, which remains significant despite the use of composite materials. A soft structure could solve this problem. Some researchers, including those at the University of Lausanne in Switzerland, are in the process of developing an elastomer exoskeleton in the shape of a sock. It can be moved by injecting air under pressure. In concrete terms, this robot consists of a series of sealed elastomer pockets connected to a compressor and packed in a silicone envelope. The only problem is that these two materials have a particularly high stretching coefficient. To control their movement, they had to be constrained. This was done by wrapping them in a non-stretchable aramid-type polymer fibre.

As a concept, injecting air to move a robot is seeing a lot of uptake. At Temple University in the United States, researchers have successfully developed a small caterpillar able to move across all types of surfaces, both horizontally and vertically, and in water as well as on land. Like a caterpillar, it folds and unfolds and thus moves at a speed of 3 mm per minute. This may not seem like much, but at the stage research is at in this field it is already a great achievement. It consists of two layers of silicone that are sealed together to form a kind of small inflatable tube. By simply injecting air into the tube, the rear part bends forward and then releases the pressure to move the front part. Eventually, it is hoped that it will become a champion of underwater exploration because this small caterpillar is capable of carrying a load five times its weight.

This small caterpillar robot made of silicone is intended for underwater exploration.

Can its speed be improved? Certainly, as revealed by a university in North Carolina, which was inspired by the cheetah's running gait to create a small robot whose principle is based more or less on that of Temple University's little caterpillar that bends to move forward. Named LEAP (Leveraging Elastic instabilities for Amplified Performance), it takes advantage of the elasticity of the flexible polymer that composes it to improve its performance.

By reproducing the morphology of the cheetah, a small soft robot has beaten all speed records in its category.

This soft robot mimics the cheetah’s bistable spine which allows it to maintain its stability while running, regardless of the position of its legs (whether stretched or bent). This partly explains how the animal can reach such high speeds. This 7 mm long robot can cover 2.7 body lengths per second. In comparison, the previous fastest soft robot in the world moved at a speed of 0.8 body lengths per second. The Leap is therefore more than 3 times faster.

 In terms of applications, there is talk of developing a robotic hand able to grasp an object with both delicacy and speed.

A round of applause for polymers

The octopus, with its eight tentacles, has a prominent place in the bestiary of our imaginations. It is as fascinating as it is frightening. This is already reason enough to study it a little more closely and attempt to reproduce its astonishing abilities. A few months ago, academics at Harvard in the United States unveiled the Octobot, the first autonomous and completely soft robot. It too is made of gelled silicone. However, what sets it apart from its cousins at other universities is its ability to be autonomous without requiring a battery or hoses to inject air from the outside. Its secret lies in a chemical reaction between hydrogen peroxide and platinum, both placed in small tanks. When these two elements come into contact with each other, the reaction creates a gas that diffuses into the small octopus’ arms, causing the robot to move. Here too, research is in its infancy and the chemical reaction still needs to be controlled to better direct the animal.

The Octobot, a gelled silicon-based octopus robot, is one of the first almost completely autonomous soft robots.

At Harvard, they promise that adding sensors to the Octobot will enable it to interact with its environment. This small robot should be used mainly as a basis for more complex designs. This will lead to applications in fields such as ocean surveillance and the search for victims in underwater areas that are difficult to access.

Other researchers continue their research into the octopus and more precisely into its tentacles. It should be noted that two thirds of the octopus' neurons are entirely dedicated to its eight "arms". That is why each of its tentacles is independent and above all so agile. Researchers from Beihang University in China and the Harvard John A. Paulson School of Engineering and Applied Sciences in the United States have succeeded in developing a robotic arm that imitates an octopus’ tentacle. It is flexible and equipped with suction cups, and can thus move, grasp and carefully handle a wide range of objects. This is the first time ever that the characteristics of an octopus’ tentacles have been so precisely reproduced. The arm also works by injecting a stream of air into the structure. It is difficult to guess the nature of the polymer used to make it. Elastomer or silicone? The material remains confidential. In any case, the small robot is of interest to industrialists and should be marketed in the coming months.

The octopus tentacle’s power, suppleness and delicacy fascinate researchers. Reproducing a tentacle’s characteristics has been a choice mission for flexible polymers.

Plastics take on all colours

A few years ago, researchers at the Finnish University of Tampere succeeded in inventing a polymer gel capable of becoming liquid under the effect of light. Since then, they have developed a new liquid crystal polymer that can move and take on the colour of its surroundings. The plastic used is filled with liquid crystals that, under an influx of light, react by changing position, thus causing the robot to move. Today, research has given it a colour memory. Without going into detail, the liquid crystal polymer has been "educated" to recognise certain colours. This is not artificial intelligence but a simple reaction of the liquid crystals to the wavelength of each colour. An application that could be of interest to the agri-food industry because it is easy to imagine that a robotic arm equipped with this technology could more or less delicately grasp a fruit, whether it is a strawberry or an orange.

Soft robotics is likely to put good old tin cans like R2D2 out of business for good. This new discipline is inseparable from materials sciences. They are both progressing at high speed. Admittedly, many experiments are still in progress, but the applications are numerous. In the field of health too, soft robots could well revolutionize therapeutic approaches within a few years.