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Title: Interfacial force mapping by artificial smart skins
Authors: Tao, XM 
Zhang, Z
Wang, F
Li, Q
Issue Date: 2016
Source: CIMTEC 2016 - 7th Forum on New Materials, includes the 5th International Conference “Smart and Multifunctional Materials, Structures and Systems” and the 11th International Conference “Medical Applications of Novel Biomaterials and Nanotechnology, Perugia, Italy, 5-9 Jun 2016 How to cite?
Abstract: With many upcoming aging societies, the need for surgery robots in hospitals, and the demand for automated assembly lines, touch-sensitive artificial skins have become imperative for interfacial force mapping in robotics, biomedicine and health-care, etc. These artificial smart skins entail soft pressure and shear force sensors and stretchable interconnects on a flexible and stretchable substrate. During the past decades, substantial effort has been made to create smart skins for interfacial force mapping. On one hand, such endeavours produced diverse tactile sensors with distinctive traits analogous to or even outperforming that of the human skin. Nevertheless, it remains challenging to produce tactile sensors with sufficiently high sensitivity, stretchability, softness and three-axial force mapping capability, especially in the low pressure regime (<10 kPa) and shear force. On the other hand, robust and stretchable circuit boards for sensor connections have long been a critical issue despite the great effort made. Therefore, we present two types of soft pressure/force sensors and one type of fabric circuit board for force mapping and sensor connection in artificial smart skins respectively. First, soft pressure/shear force sensor is based on elastomer composite integrated with polymer fibre Bragg gratings (PFBGs). A systematic study was therefore carried out on the PFBG formation in polymethyl methacrylate (PMMA) based fibres doped with trans-4-stilbenemethanol (TS). TS was chosen for the photoisomerisation, non-degradation, and thermally-stable cis state at room temperature. Firstly, step-index single-mode polymer optical fibres (POFs) were fabricated by perform-drawing method, and then inscribed using phase masked UV exposure at an optimized period of 5 min. For the first time, it was found that Bragg peak of such inscribed PFBGs demonstrated a relaxation process, which was induced by photoisomerization, because trans- and cis-isomers of 4-stilbenemethanol are different in geometry. Despite a synergetic sensitivity in temperature and humidity, the PFBGs at 1310 nm showed a high sensitivity of 1.2 pm·με-1 to longitudinal strains below 1%. After the study on formation mechanisms of PFBGs, a soft sensor was developed by imbedding two PFBGs into a silicone cube. The sensor was able to detect both normal and shear force simultaneously because the two PFBGs were not parallel: one was horizontal, and the other tilted. Two gaskets placed at two ends of each fibre, a release agent applied onto fibres ensures minimum adhesion between the optic fibers and the silicone. As revealed in a finite element analysis, strain distribution along fibre axis was non-uniform if the fibre was bonded into silicone matrix. Such soft PFBG sensors demonstrate excellent normal and shear force sensitivities at 0.82 and 1.33 nm·kPa-1 respectively. Besides, the sensor has similar Young’s modulus to that of the human skin. The second type is an all-elastomer pressure sensor array for 3-axial contact force measurement, the structure of which was inspired by the skin of a toad. It was fabricated by injecting polyethylene glycol (PEG) 400 between two PDMS layers with stencil printed carbon/silicone strain gauges, and silver/silicone conductive interconnects. Being composed exclusively of elastomeric materials and liquid, the sensor array exhibited a stretchability and elastic modulus approaching those of a human skin, which marks substantial progress of tactile sensors in this regard. The sensor was tested on an electromechanically coupled setup with a pulley to redirect loading. A sensitivity of 0.097 N-1 to normal forces below 1.62 N and an excellent sensitivity of 0.337 N-1 to shear forces below 1.3 N (with a normal force at 0.4 N) have been achieved, exceeding the best reported flexible 3D force sensors, especially for small shear forces below 0.5 N. Such soft sensor arrays can be used in gait analysis, slippage detection and dexterous manipulation of robots, etc, where friction measurement is of utmost importance. The last is fabric circuit boards (FCBs). As a new type of circuit boards, the FCBs are three-dimensionally deformable, highly stretchable, durable, and washable. They are fabricated by using computerized knitting technologies at ambient conditions. The FCBs exhibit outstanding electrical stability with less than 1% relative resistance change at up to 300% strain in unidirectional tensile test, and 150% membrane strain in three-dimensional ball punch test, an extraordinary fatigue life of over 1 000 000 loading cycles at 20% strain, and a 30-time washing capability. Theoretical analysis and numerical simulation illustrate that these excellent electromechanical properties are mostly attributed to the structural conversion of knitted fabrics, which effectively mitigates the strain in metal fibres. To the best of our knowledge, the new FCBs has far exceeded previously reported metal-coated elastomeric films or other organic materials in both mechanical and electrical properties, including changes in electrical resistance, stretchability, fatigue life, washing capability, and permeability. These features make the FCBs particularly suitable for next-to-skin electronic devices. Moreover, as a demonstration of potential applications, the FCBs have been integrated in smart protective apparel for in-situ strain measurement during ballistic impact. The FCBs provided effective electromechanical connection of strain sensor elements during ballistic impact, and reliable electrical signals were acquired from the sensor arrays. The above three technologies of fibre-structured electronics/photonics are suitable candidates for artificial skin use. They will lead to substantial advancement for interfacial force mapping by artificial smart skins.
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