Japanese researchers use human skin to cover a robot's finger
The skin regenerated after collagen was applied to a cut with a surgical knife.
The skin regenerated after collagen was applied to a cut with a surgical knife.Shoji Takeuchi

It is just a finger and the tissues were corrupted after a short time, but that a group of Japanese researchers manage to coat a robot with human cells is quite an achievement. This kind of living skin fit the artificial limb perfectly and, like real wounds, had the ability to heal from a cut. There are still several problems to solve, to provide it with sensitivity or the ability to sweat, but the scientists who have done it believe it is feasible to dress an entire robot with human skin.

For different reasons, demographic, economic and cultural, in Japan they have a special fondness for humanoid robots. In other latitudes they do not believe it is necessary to give a robot a human form in order for it to do its job, but most humanoids are made in Japan. In an article in the scientific journal Matter, published this week, scientists from the University of Tokyo argue that, in their interactions with humans, robots require a human appearance to improve the efficiency of information exchange and elicit sympathy. Above the many artificial materials that are being tested, nothing better than human skin itself to achieve that appearance.

These researchers chose a finger to see if their idea worked. In addition to its small size, its artificial index finger was three-jointed like the real one. This way they could study the elasticity and resistance of the skin with which they were going to dress it. The first thing they created was its innermost part, the dermis. Other scientists have used various derivatives of silicone, but without achieving the realism of the skin. The human dermis is based on an extracellular matrix made up of collagen, a protein that acts as a scaffold for connective tissue cells. These cells are essentially fibroblasts. In their work, they covered the finger with collagen and left it in a culture medium.

“Cells have contact inhibition, which means that once a certain number is reached, they don’t proliferate anymore”

Shoji Takeuchi, professor at the University of Tokyo

When introducing the first fibroblasts, they observed how they proliferated and occupied the entire space until, around seven days later, they stopped dividing and multiplying. How? Explains Shoji Takeuchi, professor at the University of Tokyo and main author of the study: “Cells have contact inhibition, which means that once a certain number is reached, they do not proliferate anymore.” Next was the epidermis. Here they used keratinocytes. These cells represent up to 90% of the outermost part of the skin and fulfill two essential functions. On the one hand, they produce keratin, the main protein that gives the epidermis its structure. On the other, they serve as insulation, especially against water. Histological analysis showed that in the robotic finger they continued to perform the same functions.

So far the easy part of the story. Like human skin, this skin is made up of several layers and it was the biggest challenge, according to Takeuchi. “Previous methods of making live skins could only achieve flat sheets of skin. It would be very difficult to use these layers to cover 3D objects with irregular surfaces like the body of robots; you would have to have the hands of a craftsman who knew how to deftly cut and sew,” he says. What they did was devise an automated system “to directly mold the tissue surrounding the robot, resulting in a seamless covering on a robotic finger with an irregular surface,” he adds.

Once the suit was put on, it was necessary to check if it was comfortable. The researchers subjected the skin of the robotic finger to various resistance tests. The movement of the joints did not damage the tissue, which showed an elasticity similar to that of human skin. However, the resistance was much less. The tensile strength of the robot dermis was more than 1,000 times lower than that of the human dermis (5.6 kilopascals vs. 10-30 megapascals). They are convinced that by increasing the concentration of collagen, the tension would improve.

What they did achieve was to replicate one of the essential functions of fibroblasts, their ability to close wounds. They verified this by cutting the skin over one of the finger joints. Inspired by the treatment of severe burns, they applied a collagen gel to the area. In a few days, the replication of the cells had made it possible to seal the cut, offering the area a resistance and elasticity similar to uninjured areas.

But, once out of the culture medium, the skin was corrupted. He gave them time to carry out the experiments and little else. The differences in temperature and humidity caused the robotic finger to dry out until the dermis and epidermis lost their appearance and properties. Without the supply of nutrients, blood and oxygen they were doomed. “We plan to develop a vascular structure inside in the future so that it can be maintained for a long period of time, even in contact with air,” Takeuchi concludes.

Getting a skin made of human cells to last on the surface of a robot is not going to be easy. But it would have many advantages. Some are highlighted by José Barreiros, a robotics researcher, specifically in electronic skins, at Cornell University (United States). “Biological leathers have phenomenal mechanical characteristics that are difficult to replicate with synthetic materials, they can be stretched a lot and through many cycles, they are waterproof, they are durable, they are soft, among others,” he says. In addition, from the point of view of human-robot interaction, “the fact that the robot has biological skin makes it friendlier and safer, since being soft is more comfortable to the touch, avoiding strong impacts,” he adds.

“Imagine a humanoid robot that can feel a hug from a person, or that can feel itself and understand what its body is and what it isn’t.”

José Barreiros, robotics researcher, specifically in electronic skins, at Cornell University, United States

Barreiros works in a field that, perhaps in the future, will intersect with that of researchers at the University of Tokyo, endowing these skins of human origin with the ability to feel. A few days ago he published, together with colleagues from Cornell, his latest research on a kind of electronic meat made up of elastic compounds (elastomers) and crossed by tiny fiber optic cables that acted as a nervous system. Thus, elastomers can feel touch, temperature and even damage. “The robotic meat has several layers that encode the haptic (touch) stimulus in light: mechanical deformation is encoded in changes in light intensity and temperature is encoded in changes in the color of light,” explains the Ecuadorian scientist.

Thus, “when a section of robotic skin is pressed, more light rays are directed towards the fibers near the point of contact”, thus perceiving the tactile sensation. For the heat one, when a part of this electronic skin is heated, “one of its layers that contains nanoparticles changes color and, therefore, the light inside the skin does too, due to the principle of reflection,” he explains. Sweepers. That change is interpreted by the system as an increase or decrease in temperature. The same fiber optic network allows to detect lesions. For example, an incision by a knife affects some of these fibers, creating shadows that the flesh interprets as damage.

The system devised at Cornell could be applied to any robotic system, human-shaped or not, but would fit into approaches such as the human skin approach of Japanese scientists. “In fact, my work is motivated by applications for physical artificial agents, imagine a humanoid robot that can feel a hug from a person, or that can feel itself and understand what its body is and what it is not, and that can realizing that he had damage to his skin, which would help him learn which objects are safe and which are not, and how to operate them,” says Barreiros.

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