Transduction Mechanisms, Micro-Structuring Techniques, and Applications of Electronic Skin Pressure Sensors: A Review of Recent Advances

Abstract

Electronic skin (e-skin), which is an electronic surrogate of human skin, aims to recreate the multifunctionality of skin by using sensing units to detect multiple stimuli, while keeping key features of skin such as low thickness, stretchability, flexibility, and conformability. One of the most important stimuli to be detected is pressure due to its relevance in a plethora of applications, from health monitoring to functional prosthesis, robotics, and human-machine-interfaces (HMI). The performance of these e-skin pressure sensors is tailored, typically through micro-structuring techniques (such as photolithography, unconventional molds, incorporation of naturally micro-structured materials, laser engraving, amongst others) to achieve high sensitivities (commonly above 1 kPa−1), which is mostly relevant for health monitoring applications, or to extend the linearity of the behavior over a larger pressure range (from few Pa to 100 kPa), an important feature for functional prosthesis. Hence, this review intends to give a generalized view over the most relevant highlights in the development and micro-structuring of e-skin pressure sensors, while contributing to update the field with the most recent research. A special emphasis is devoted to the most employed pressure transduction mechanisms, namely capacitance, piezoelectricity, piezoresistivity, and triboelectricity, as well as to materials and novel techniques more recently explored to innovate the field and bring it a step closer to general adoption by society.

Nanowire‐Based Soft Wearable Human–Machine Interfaces for Future Virtual and Augmented Reality Applications

“…Resistive and capacitive sensors are suitable for monitoring static or low‐frequency stimuli, [ 113 ] and piezoelectric and triboelectric sensors are appropriate for dynamic and high‐frequency stimuli such as vibrations. [ 114 ]…”

Review: Sensors for Biosignal/Health Monitoring in Electronic Skin

“…Therefore, to improve the sensor’s performance, the compressibility of the dielectric layer must be maximized. This can be achieved using foam-type or micro-structured dielectrics, which increase the sensor’s sensitivity and decrease its hysteresis and response time [ 44 , 49 ]. A microstructure is vulnerable to deformations because of the increased gap between the electrodes and the dielectric layer.…”

E‐Skin Piezoresistive Pressure Sensor Combining Laser Engraving and Shrinking Polymeric Films for Health Monitoring Applications

“…Human skin has attracted the attention of the scientific community due to its apparent simplicity, which conceals complex functionalities, ranging from protection from external aggressions to temperature regulation and detection of several mechanical stimuli, such as pressure, vibration, stretching, and bending, among others. [1][2][3] As of the 21st century, efforts have been made in the creation of an electronic human skin pressure, there is a greater shift in contact area, which leads to a higher output variation in a sensor and, consequently, a higher sensitivity. [14,17,28,60,70] The pressure range for which the sensor has a linear output can be tuned by introducing hierarchical micro-structures, [17] micro-structures of different heights, [71][72][73] or less compressible micro-structures, [62] so that when pressure increases, instead of an output saturation, new contact points are established between the micro-structures and the electrodes, allowing a linear output variation.…”

“…Different micro-structures may be achieved by changing the shape of the cavities, the material of the mold, laser power and speed, type of laser beam, and laser lens. [3,34,60,62,82,83] In order to produce highly conformal sensors, thus facilitating the acquisition of subtle signals such as the blood pressure wave at the wrist, small micro-structures are required (few micrometers to tens of micrometers). Consequently, the thickness of the polymeric film is low enough to reach conformality, yet high enough to provide structural support to the micro-structures.…”

“…Recently, other parameters have achieved higher relevance, namely biocompatibility, biodegradability, recyclability, self-healing, and self-powering. [3,[8][9][10][11] For sensing of external stimuli, e-skins are equipped with sensors, either sensitive to one particular stimulus or different stimuli simultaneously, from pressure to strain, humidity, temperature, chemical molecules, and even biochemical molecules. [8][9][10]12] From the said stimuli, pressure has a highlighted role in the sensing performance given its major relevance in a plethora of applications, particularly health monitoring (for the detection of the blood pressure wave at several body locations, [13][14][15][16][17][18][19][20] heartbeat, [21,22] breathing, [23][24][25][26][27] speech, [28][29][30][31] gait patterns, [32][33][34] and other general muscular movements [35][36][37][38] ), functional prosthesis, [39][40][41][42] robotics, [43][44][45][46] and human-machine-interfaces (HMI).…”