Development of Artificial Skin

These specific characteristics and functions make the human skin an interesting field of research for different, interdisciplinary specialty fields that try to adapt several skin characteristics for technology. And researchers all over the world are not lacking for ideas here either.

A biologically inspired adaptive sensor system

Based on the model of the human skin, a research team made it their mission to design an intermediary system between human beings and technical instruments. Properties like for instance flexibility, three-dimensional geometry and actuators (subsidiary network) that support the sensor system are optimally suited to develop an adaptive sensor system. But how can this work?

“The bionic implementation consists primarily so that we understand signal transmission and signal intake of important components of the human skin“, explains graduate engineer Irina Gavrilova, scientific research assistant for Biomechatronics at the Ilmenau University of Technology. “To be able to transform and transmit the induced tactile stimuli of different direction and dynamic into electrochemical membrane potential, biological sense organs possess non-nervous accompanying structures, which form the stimulus-conduction-apparatus with the functional components.“ Particularly the properties and the geometry of the individual dermal layers as well as the actuators, like blood vessels, belong to such components.



Learning from the natural dermal layers

But what is so special about the composition of the human skin? If you take a closer look at the dermis, also called true skin or corium, this middle layer of the epidermis delivers shape and form through firm collagen and elastic flexible fibres. The epidermis also does not just rest flat on the dermis. In fact it is separated by a sinuous and irregular border.

It is skin components like these - the characteristics, the geometry of dermal layers as well as the actuators in the form of blood vessels - which support the mechanoreceptors. Particularly the papillary boundary layer between epidermis and dermis influences the mechanical signal transmission from the skin surface all the way to the mechanoreceptors.

In the blood vessels of the dermis, the rigidity is tailor-made and adjusted for the individual layer. This then leads to a flexibility change of one of the layers. The multilayered composition, the boundary layer structure and the different characteristics of the layers were bionically derived from the model of human skin. “This interaction between actuators and sensor system for specific movements can be used in the operational system to adjust it to the changing measuring conditions“, explains Gavrilova. “Integrated sensors provide a feedback via local pressure- and power load. This data is transmitted to a central control unit, which specifically uses the integrated actuators to deform the compressed areas. “



In a technical sense, the papillary dermis of the human skin forms a large-scale sensor-actuator array, which in the bionic implementation then forms a multitude of homogenous artificial papillae. The Ilmenau research group researches the impact of layer characteristics and boundary layer profile on the conduction of mechanical stimuli from the surface to the sensors and analytically calculates the decomposition of the acting force in its normal and tangential components. Different models, for example the finite element method, simulate different stresses and strains, be that through vertical, horizontal, punctual or area-measured forces. Through this, the load distribution and deformations can be assessed and the profile of the boundary layers between two layers as well as an optimal array of sensors and actuators in the relative physical structure be analyzed. “We can thus implement flexible sensor areas to identify the stress – and pressure distribution on a surface. In the process, the embedded sensors detect the occurring forces or pressure“, Gavrilova elaborates.

These dynamic systems, which are meant to measure pressure and force, can for instance be used as an intermediary layer between the human body and the surface of technical instruments.

Abstraction of skin for decubitus prophylaxis

The sensory characteristics of the skin are also being analyzed for the development of an adaptive storage module for prevention and therapy. This especially applies to the design, the structure and the geometry of boundary layers because the storage module is supposed to produce an effective pressure adjustment and minimization of shear force well as an active and individual skin blood flow.

Scientist discovered that skin deformation behavior and strain distribution are significantly influenced by the geometry of the boundary layers and the rigidity differences in the individual layers. “In essence the human skin is being reproduced on a larger scale via sensory controlled flexibility and integrated actuators“, explains Gavrilova. The sensor system is supposed to locate places of pressure peaks, pressure distribution and several other surrounding area parameters, in which the actuators target-specifically relieve and stimulate the concerned areas. “This way in the future we very likely will be able to identify and relieve pressure areas at risk at an early stage and stimulate the tissue blood supply“, Gavrilova knows.


Safety systems: robots and floors

Sensors which for instance register touch in a laboratory environment to stop robots can be important for so-called artificial intelligence. Fraunhofer research scientists developed artificial skin for a robot, which consists of conducting foam and textiles, in which sensors are embedded. “There are two fundamental strategies to protect robots in such scenarios. For one you can apply prognostic sensors, which identify objects or people before a collision happens or you allow collisions, but outfit the machine with pressure-sensitive sensors and are thus able to restrict the interaction forces. We have chosen the second approach and developed a sensor system that is able to detect pressure distribution on a large scale“, Markus Fritzsche from the Fraunhofer IFF explains. As soon as the sensors register a touch by a person, the robot immediately stops. In addition, dampers that are embedded in the skin, absorb possible thrusts.

“A big advantage of our sensor technology is the fact that you can also attach it to curved or intricately formed surfaces. That’s why in the future even implants could be outfitted with the sensor system, “explains Fritzsche. Floors in nursing home could also be furnished with the sensor system in the future. Thanks to the sensors it would be possible to determine whether a patient fell out of a bed for example.

The future of touch-sensitive technologies

Many technical inventions are still pure research, but the international exploratory urge is unbroken. At the end of 2010, US scientists invented an artificial skin, a flexible surface of pressure sensors, which imitates the haptic characteristics of human skin and generated a response at a pressure of under one kilopascal.

Fritzsche knows: “Currently there is a lot of increased research in the area of tactile sensor systems. The developments keep going more and more towards imitating the human skin, on the one hand in its receptivity, on the other hand in its high spatial resolution.“

For the German scientists, the research on skin and its technical adaptation keeps going further and further. “At the moment we research sensor systems that can identify touch and are additionally combined with prognostic sensors. For applications where human beings and robots directly work together, safety can thus be further increased“, Fritzsche explains.
Ilmenau also designs and researches. “Papillary boundary layers made of different materials will come to play their own role in our technical development“, Professor Hartmut Witte from the Technical University of Ilmenau, reveals.

Despite all of these advances, nature however is still a practical step ahead in all kinds of developments. One example: The tactile sense-cells on the human skin are able to identify a surface structure whose unevenness measures 50 micrometers. For the time being, it remains one of the many technological challenges to be able to adapt this.

Diana Posth

(Translated by Elena O'Meara)