Rubber film used to create touch-sensitive artificial skin

As we already wrote in our previous articles, scientists around the world are working to develop pressure sensors for artificial skin which could be applied on prosthetic limbs or robots. By sandwiching a precisely molded, highly elastic rubber layer between two parallel electrodes, the team from Stanford University created an electronic sensor that can detect the slightest touch.

“It detects pressures well below the pressure exerted by a 20-milligram bluebottle fly carcass we experimented with, and does so with unprecedented speed”, said lead researcher Zhenan Bao, an associate professor of chemical engineering. ”The key innovation in the new sensor is the use of a thin film of rubber molded into a grid of tiny pyramids.”

“We found that with a very thin continuous film, when you press on it, the material does not have room to expand”, said Stefan Mannsfeld, a former postdoctoral researcher in chemical engineering and a coauthor. “So the molecules in the continuous rubber film are forced closer together and become entangled. When pressure is released, they cannot go back to the original arrangement, so the sensor doesn’t work as well.”

The total thickness of the artificial skin, including the rubber layer and both electrodes, is less than one millimeter. The speed of compression and rebound of the rubber is critical for the sensor to be able to detect and distinguish between separate touches in quick succession. The thin rubber film between the two electrodes stores electrical charges, much like a battery. When pressure is exerted on the sensor, the rubber film compresses, which changes the amount of electrical charges the film can store.  That change is detected by the electrodes and is what enables the sensor to transmit what it is “feeling”.

The largest sheet of sensors that Bao’s group has produced to date measures about seven centimeters on a side. The sheet exhibited a great deal of flexibility, indicating it should perform well when wrapped around a surface mimicking the curvature of something such as a human hand or the sharp angles of a robotic arm. The researchers mentioned that just as real skin, molding the rubber in different shapes yields sensors that are responsive to different ranges of pressure and thus correlating to different sensitivity of skin on different limbs.

The sensors have from several hundred thousand up to 25 million pyramids per square centimeter. By altering the configuration of the microstructure or the density of the sensors, the researchers think the sensor can be refined to detect subtle details in the shape of an object.

That degree of sensitivity could make the sensors useful in a broad range of medical applications, including robotic surgery, or in bandages that could aid to doctors with data from the sensors to be sure the bandages were not too tight. The team also invented a new type of transistor in which they used the structured, flexible rubber film to replace a component that is normally rigid in a typical transistor. When pressure is applied to their new transistor, the pressure causes a change in the amount of current that the transistor puts out. The new, flexible transistors could also be used in making artificial skin.

For more information, read their research paper published in Nature Materials named: “Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers“.