Flexible sensor encodes tactile stimuli into physiological signals

September 25, 2018 // By Julien Happich
In a paper titled "A skin-inspired tactile sensor for smart prosthetics" published in Science Robotics, researchers from the Ningbo Institute of Materials Technology and Engineering describe a novel membrane-based tactile sensor that couples giant magneto-impedance (GMI) effects with a specially-designed read-out circuitry whose output mimics the action potentials of real skin stimuli.

The idea here is to create an artificial skin for use on prosthetics which would directly encode tactile stimuli into physiological signals, those could readily and naturally be interpreted by nerve endings at the periphery of a missing limb. With such an approach, it would become possible to create smart neuroprosthetics offering a biomimetic tactile feedback. In its bibliography, the paper lists recents neuroprosthetic studies revealing that pressure sensation can be stimulated by injecting a train of pulses having different frequencies via cuff interfaces.

In their paper, the researchers propose the direct transduction of force stimuli into digital frequency signals thanks to the integration of a specially-designed tactile sensor with an LC oscillation circuit, where the output frequency is inversely proportional to the square root of the LC product.

Each sensor cell in the proposed sensing skin consists of a tiny cobalt-based amorphous wire (chosen as the giant magneto-impedance material) surrounded by a copper coil to form an inductance laminated by a micrometer-thick PDMS membrane on a flexible substrate. An air gap is encapsulated by a top polymer magnet layer consisting of PDMS and neodymium iron boron (NdFeB) magnetic powder. Several such pockets are replicated to form multiple tacsel (or tactile pixels) only a few millimetres in diameter.

As even minute pressure is applied to the top free-standing magnetic membrane, the displacement of the magnetic powder relative to the inductance changes the magnetic flux passing through the inductive sensing element, which decreases the impedance of the sensing element due to the GMI effect. Because the frequency of the output signal is inversely related to impedance, any decrease in impedance (when pressure is applied to the magnetic membrane) will increase the frequency of the LC oscillation circuit.

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