Artificial skin breakthrough! Compliant prosthetic or robot assisted sensor having real feeling
Slow to adapt in human skin (SA) receptors, SA-I (Merkel cell) and SA-II (Ruffini organ) play a key role in compliance feeling. The former is applied to the skin on the static pressure measurement at high resolution, while the latter can be detected stretch the skin. With the development of stretchable material and microfabrication technology, it is reported the flexible sensor capable of detecting temperature and static and dynamic forces. Since the compliance of the sensor is an important sense block, and therefore need to be developed and integrated into the artificial skin, the feeling of the prosthetic or robotic systems that provide human-like. However, there are four sensing mechanisms (digital read into an electrical signal) of the sensor external components (measurement precision optical components, etc.) are bulky, thus these sensors for applications demand compact dimensions continue to face enormous challenges. In addition, the sensing device is complicated compliance since it requires simultaneous measurement of pressure and deformation information is applied to detect two parameters compliant object. In the case where no coupling effect, the strain sensor integration and pressure is also a challenge. To develop an artificial skin or may be integrated into the robot system in compliance of the sensor, it is necessary to meet the following requirements: 1) It should be easily integrated with compact size; 2) no need for large external components or integrated major structural changes in the system; 3) should have a decoupling sensor reliability. 
Based on this, Professor Bao Zhenan Stanford (corresponding author) team reported a bionic, thin, conformable sensor. The sensor can detect the use of human skin similar to SA-I and SA-II strain and pressure, without the need for any bulky external components and does not occupy substantial volume. To simulate SA-I and SA-II tensile and pressure sensing capabilities, researchers based coupling film strain sensor (MBSS) to the pressure sensor, to identify contact material. The experimental results confirmed that the hybrid sensor may capture a pressure deformable surface of the contact material and applied simultaneously. By using resistance and capacitance-based sensor, as MBSS researchers developed two different sensing methods. For example, when tested separately modulus of 75 GPa and 20 kPa material, the sensitivity of the sensor resistance of 11 Ω / N and 104 Ω / N. Also, for a similar material, the sensitivity of the capacitive sensor were 80 fF / N (femtofarad Newton) and 1280 fF / N. At the same time, also it shows the sensitivity of the sensor can be adjusted by reducing the thickness of the film, especially when higher resolution is required. Further, the sensor thin and small size so that it can be used for different applications. First, the sensor is integrated into the manipulator fingers, and to determine the compliance of the gripped object. Secondly, by building the surface of the array sensor of FIG able to plot an object made of different materials, useful for detecting irregular objects inside the tissue (e.g. tumor).
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Researchers propose a double sensing method, in order to achieve such a compact sensor compliance. Wherein the first layer is composed of a stretchable film is deformed to detect the surface of the touch material, a second layer composed of pressure sensors. The sensor array may be fabricated by aligning and laminating the flexible layer. Strain sensors (MBSS) with respect to the circular opening in the columnar structure aligned with the strain sensor based on capacitive or resistive composition. MBSS when in contact with the material, it will deform to increase the contact pressure. At the same time, measuring the pressure applied by the pressure sensor. Binding MBSS and pressure sensor output may be calculated for each object the sensitivity (S, the strain ratio of the response in response to the pressure), and S can distinguish between different compliant material (i.e. material having a higher S larger compliant represented) .
Researchers have developed finite element (FE) model to determine the sensor geometry and material characteristics and importance of the different compliance responsive material. When the radius increases from 0.5 mm 2 mm, MBSS deflection increased more than 4-fold. By modulus MBSS when changed from 0.25 MPa to 2 MPa, there was no significant difference in displacement. When a radius of 1 mm and a thickness of 50 μm polydimethyl siloxane (PDMS) film identified modulus material 0.25 and 1 MPa, its sensitivity is increased by nearly twice the diameter of the material 10 MPa strain almost no response. In the deformed state small, the deformation is increased to increase the curvature of the film, resulting in an increase in capacitance. The film is further deformed due to the stretching gap between the electrodes is increased, and led curvature effect, the capacitance starts to decrease.
Researchers use PDMS elastomer to produce materials having different moduli. At the same time, were also tested for three different proportions of PDMS, i.e. PDMS (10: 1), PDMS (25: 1) and PDMS (50: 1), have a thickness of 3 mm, a test to determine the Young\’s modulus by uniaxial compression were 2.02 ± 0.18,0.39 ± 0.038 and 0.0247 ± 0.0017 MPa. When the sensor is in contact with the more compliant materials, higher sensitivity of the sensor response. For PDMS (50: 1), PDMS (25: 1), PDMS (10: 1) and S glass measured values were 104 ± 7.8,75 ± 6.1,47 ± 2.4 and 11 ± 0.94 Ω / N. Sensor measuring compliance of different materials, a material found to be more compliant produced higher S. By various test objects of the same thickness (3 mm), and these objects are supported on a rigid substrate, and a Young\’s modulus of the material in accordance can exhibit a significant difference in S. Thus, in the case where the size of an unknown material, may be used for sensor output S classified according to their flexibility.
Finally, a separate sensing unit researchers prepared, consisting of one area of RMB sensor 1 × 1 cm2 of the composition, and integrated in the side of the robot fingers. Placing different materials between the robot finger, it was found when the capacitance reaches the maximum limit, the robot fingers will stop and restart the movement in the opposite direction, to release the captured material. For compliant material, under similar force, the maximum resistance value increases. The results demonstrate the ability of compliant sensors on the sensor can be used as a robot finger. In addition, the development of two different compliance map means, to show the applicability of the sensor in the prosthetic applications. For both tests, a more compliant material in contact with a pixel having a relatively high S-value. For both cases, the sensor can be classified compliance of the compliant material, thereby demonstrating that the prosthetic device having a function of potential sensor.
The full text link: https: //www.pnas.org/content/early/2020/05/07/1909532117
