Artificial skin for prosthetic hands

Electronics-laden glove made up of layers of materials with stretchable gold and silicon sensors. Credit: Seoul National University

A piece of the prototype smart skin with integrated sensors is shown being stretched by 20 percent. Credit: Seoul National University

Several progresses have been made since the turn of the century in embedding electronics in flexible materials thus making it possibile to embed sensors in many types of surfaces.

At the Seoul National University, SNU, a team of researchers have created an artificial skin with embedded sensors that can be used to provide a sense of touch to a prosthetic hand.

The progress in prosthetic limbs is such that today people can control their prosthetics as they are controlling the real thing, using their nerves signals. Hence you can pick up an object with a prosthetic hand just "thinking" about doing it. The signals from the brain move through the nerves and a transducer pick up these signals and a computer convert them into the mechanical operation of the hand.

However, it has not been possibile, so far, to activate the reverse channel, to transfer sensation from the touching of an object, like pressure, heat, humidity ....

This was the goal of the researchers at SNU. In order to do that the first step is to have a sensitive surface on the prosthetic hand. The surface has to be able to stretch, bend, as our skin does. The researchers at SNU in cooperation with a team at Case Western Reserve University in the US have created a flexible polymer and infused in it  a dense network of sensors made in ultra thin gold and silicon. The silicon is shaped in serpentine shapes to sustain stretching.

And it is really "dense": the artificial skin contains up to 400 sensors per square mm. That is much more than the 2,500 receptors we have on our fingertips per square cm (although these are, at least for now, more sophisticated in terms of the nuances they can detect).

In our hands the distribution and sensitivity of sensors changes based on the location and the researchers mimicked that by capturing through a video camera the actual movements of a human hand and replicated that in terms of sensors stretchability depending on the location. Additionally, the embedded thermo-actuators that can worm up the prosthetic hand surface to the normal hand temperature.

The next step is clearly to connect the signals generated by the prosthetic hand sensors to the nerves so that they can be brought to the brain. 

The researchers have shown that this connection is possibile demonstrating it on a mouse: they connected the sensory output to nerves of a mouse leg and showed reactions at the mouse brain level.  However, we are still far from saying that we can seamlessly recreate sensations through a prosthetic hand. 
A prosthetic hand was implanted on a patient in Cleveland and doctors have been able to connect some 20 pressure sensors in the prosthetic hand to the nerves allowing the person to feel if the object is hard or soft, which is important if you pick up a cherry and don't want to crush it!

All in all, we are learning and keep making progress. It is a matter of "when", not of "if".

Author - Roberto Saracco

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