Robot developers and physicians have been working for a long time on the idea of electronically reproducing the functions of human skin. New sensors that can distinguish between warm and cold touch, among other things, may simplify their work somewhat in the future.
It should not be applied too heavily and should be flexible – the future electronic skin on prostheses and robots. It is a task that is not so simple because this skin will have to accommodate a huge range of different sensors. Exceptional multi-talented devices from the University of Linköping in Sweden could turn out to be the space-saving solution. The university’s artificial skin receptors can act much like our flesh and blood, drawing distinctions between body temperature and the warmth of sunlight as well as between the touch of a warm finger and a cold object. The key: special combinations of material and several physical phenomena.
The physics of combination sensors
During pyroelectric effects, certain piezoelectric crystals react to the smallest change in temperature by separating the charge on the surface. The result: an electric signal. This quality is used by infrared devices (motion detectors, smoke alarms), microwave detectors, temperature gauges and calorimeters.
The second fundamental principle is thermoelectric power generation produced by the Seebeck effect. You need two electric conductors that have dissimilar electronic heat capacity (Seebeck coefficients). Even though they are the same temperature, their electrons have disparate motion energies. If they come in contact with each other, a flow of higher energy electrons is generated in the direction of the conductor with the low energy electrons.
Thermoelements made of metal convert heat into electric energy in a very inefficient process. For this reason, they are used only to measure temperature. This process can be improved by using semiconductor materials as well as, more recently, electroconductive polymers.
Pyroelectric polymer, thermoelectric gel and plasmons
To profit from both physical principles, the Swedish scientists combined a pyroelectric polymer and a thermoelectric gel for the first time in their combination sensors. However, this is not enough: Nanoparticles made of gold placed on the sensor generate an additional electric signal when incident light hits them. It is energy produced during the excited collective oscillations of the free electrons (plasmons).
The sensor also reacts to “cold” pressure because pyroelectric polymers have piezoelectric properties as well. This may indeed be a much smaller consideration. But it can play a key role in certain applications. After all, future robots should be able to tell the difference between contact with a human being and contact with objects.