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A Pressure and Strain Sensor Fabricated on Soft Artificial Skin
October 03, 2013

This article describes the design, fabrication, and calibration of a highly compliant artificial skin sensor. The sensor consists of multilayered mircochannels in an elastomer matrix filled with a conductive liquid, capable of detecting multiaxis strains and contact pressure. A novel manufacturing method comprised of layered molding and casting processes is demonstrated to fabricate the multilayered soft sensor circuit. Silicone rubber layers with channel patterns, cast with 3-D printed molds, are bonded to create embedded microchannels, and a conductive liquid is injected into the microchannels. The channel dimensions are 200 µm (width) x 300 µm (height). The size of the sensor is 25 mm x 25 mm, and the thickness is approximately 3.5 mm. The prototype is tested with a materials tester and showed linearity in strain sensing and nonlinearity in pressure sensing. The sensor signal is repeatable in both cases. The characteristic modulus of the skin prototype is approximately 63 kPa. The sensor is functional up to strains of approximately 250%.

The development of highly deformable artificial skin (Figure A) with contact force (or pressure) and strain sensing capabilities is a critical technology to the areas of wearable computing, haptic interfaces, and tactile sensing in robotics. With tactile sensing, robots are expected to work more autonomously and be more responsive to unexpected contacts by detecting contact forces during activities such as manipulation and assembly. Application areas include haptics, humanoid robotics, and medical robotics.

Different approaches for sensitive skin have been explored. One of the most widely used methods is to detect structural deformation with embedded strain sensors in an artificial skin. There have been stretchable skin-like sensors proposed using different methods. strains over 100%. In this paper, the authors present a highly deformable artificial robotic skin with multi-modal sensing capable of detecting strain and contact pressure simultaneously, designed and fabricated using the combined concept of hyperelastic strain and pressure sensors with embedded microchannels filled with eutectic gallium-indium (EGaIn)EGaIn, a conductive liquid material.

When the microchannels filled with EGaIn are deformed by either pressing or stretching, the electrical resistance of the microchannels increases due to their reduced cross-sectional areas, increased channel lengths, or both.

The overall design includes three soft sensor layers made of silicone rubber (Figure B) that is highly stretchable and compressible. Layers 1 and 2 have straight-line microchannels with a strain gauge pattern that results in directional sensitivity in axial directions as well as surface pressure sensitivity, and Layer 3 has circular patterned microchannels that are sensitive to surface pressure but are not directionally sensitive to strains along any axis. The layers are cast separately, bonded together, and the microchannels are filled with EGaIn using syringes. Wires are then inserted into the sensors and sealed.

A constant current generator is connected to the three sensors, in series. The resulting three voltages (one per sensor) are amplified and converted to digital signals. A microcontroller then processes these signals to produce x- and y- strain and pressure indications. The paper describes the strain and pressure responses of the resulting artificial skin sensor assembly.

The authors report that while the current design showed linear and repeatable responses in strain sensing, it showed a nonlinear response and a high hysteresis level in pressure sensing, as already modeled. The nonlinearity in pressure sensing is due to the nonlinear areal reduction rate of the rectangular microchannels when contact pressure is applied. Improvement on the linearity in pressure sensing is currently being investigated by changing the channel geometry. The hysteresis could be also reduced by increasing the aspect ratio of the microchannels. Improving linearity and repeatability in pressure sensing is an active ongoing research area.

During fabrication, EGaIn injection in the multi-layered structure through the inter-layer interconnects makes the fabrication process simple. However, it will not be practical for filling an extremely long channel and/or a higher number of channels. A new manufacturing method that does not require the EGaIn injection step is another ongoing effort to enable faster and higher volume production.

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