9.2.1.1 Mimicking the SA Receptors – Static Force Transduction

Sensors are made up of a variety of materials such as polyurethane, polydimethylsiloxane

(PDMS), and poly (styrene-butadiene-styrene). The working of these sensors is based on

two phenomena, piezocapacitive, and piezoresistive. In capacitance sensors, a change in

capacitance occurs under the pressure because of the compression of the dielectric film.

The films can be the air gap, elastomer, and microfluidic channel [5]. The sensitivity of a

sensor has been reportedly improved by using the array of PDMS film as a dielectric

layer. The shaped and structured layer deformed under the pressure as compared to its

shapeless design. The structured film detected the pressure up to 3 Pa with enhanced

sensitivity of 0.55–1 kPa. The microstructuring of the biocompatible materials as sensor

show quick response time and excellent sensitivity. In piezoelectric sensors, resistance

change under pressure takes place due to two mechanisms. In the first, the gap between

conductive fillers changes by applying the pressure that results from the increase of the

number of conductive pathways. In the second mechanism, the microstructured dielectric

film contains an additional deposited film of a conductive layer that interfaces the elec­

trode. An applied pressure deforms the microstructure hence leading to the increase of

contact area and decrease of contact resistance [6]. These resistive-based sensors are

simple, sensitive, and require only a readout circuit. The piezoelectric sensor made up of

poly(ethylene dioxythiophene)–poly(styrene sulfonate) with an aqueous polyurethane

dispersion elastomer layered on the micro-pyramid array shows a sensitivity up to

4.88 kPa⁻¹ [7]. This sensor can detect an arterial pulse. A variety of microstructures such

as micropillars, microstructures, and interlocked structures has improved the sensitivity

and linearity of resistive-based sensors. In piezocapacitive sensors and piezoelectric

sensors, mechanical signals are transduced into electrical signals, while pressure values

correspond to the capacitance and resistance, respectively [8].

9.2.1.2 Mimicking the Rapid Adapting (RA) Receptors – Dynamic Force Transduction

RA receptors are more responsive toward the stimuli to detect the vibrations and move­

ment of the body while triboelectric and piezoelectric sensors are more sensitive to rapid

dynamic motion. The piezoelectric sensor deals with the intrinsic properties of the material

in which applied force changes the lattice of the materials. A variety of inorganic and soft

materials has been used in energy systems and pressure sensors to enhance the response

time and sensitivity. In triboelectric sensors, the loss of electrons takes place in materials

through triboelectrification and electrostatic induction. In electrification, two surfaces

contact and separate by periodic force; hence, charges separate after induction and current

flows. The triboelectric sensor responds to the pressure, in which frequency, force, and

separation distance affect the output. Electrostatic induction can be improved by the ad­

dition of ionic liquid, microstructural designs, and surface treatment of the materials [9].

Self-powered flexible devices developed by triboelectric effect can be used in wearable

electronics for the next generation. Recently, various kinds of tactile sensors as flexible

devices (Table 9.1) have been developed that demonstrate better sensing to human skin.

9.2.1.3 Biomimetic Sensors

Biomimetic sensors are kind of sensors that uses designated biomaterials and biomimetic

approaches similar to those of biological system. These sensors consist of voltammetric,

potentiometric, and impedance phenomena. The electronic skin, taste sensors (sweetness,

bitterness, sourness, saltiness, umami), odor sensor, cochlear amplifier, and cochlear

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Bioelectronics