Citation: Nguyen, T.-D.; Lee, J.S.
Recent Development of Flexible
Tactile Sensors and Their
Applications. Sensors 2022, 22, 50.
https://doi.org/10.3390/s22010050
Academic Editor: Nunzio Cennamo
Received: 10 November 2021
Accepted: 20 December 2021
Published: 22 December 2021
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Review
Recent Development of Flexible Tactile Sensors and
Their Applications
Trong-Danh Nguyen and Jun Seop Lee *
Department of Materials Science and Engineering, College of Engineering, Gachon University,
Seongnam 13120, Korea; ntdanh041@gachon.ac.kr
* Correspondence: junseop@gachon.ac.kr; Tel.: +82-31-750-5814; Fax: +82-31-750-5389
Abstract:
With the rapid development of society in recent decades, the wearable sensor has attracted
attention for motion-based health care and artificial applications. However, there are still many
limitations to applying them in real life, particularly the inconvenience that comes from their large
size and non-flexible systems. To solve these problems, flexible small-sized sensors that use body
motion as a stimulus are studied to directly collect more accurate and diverse signals. In particular,
tactile sensors are applied directly on the skin and provide input signals of motion change for the
flexible reading device. This review provides information about different types of tactile sensors and
their working mechanisms that are piezoresistive, piezocapacitive, piezoelectric, and triboelectric.
Moreover, this review presents not only the applications of the tactile sensor in motion sensing and
health care monitoring, but also their contributions in the field of artificial intelligence in recent years.
Other applications, such as human behavior studies, are also suggested.
Keywords: tactile sensor; flexible; piezoresistive; piezocapacitive; piezoelectric; triboelectric
1. Introduction
Tactile sensation, one of the five senses defined by Aristotle, is caused by the excitement
of a certain sensitivity in the skin [
1
,
2
]. Tactile sensing occurs when biological tissue
movements, such as hair movement, or deformation or twisting of the mucous membrane
of the skin, occur [
3
–
6
]. Specifically, when stimulation with mechanical energy is applied,
the shape of the rear container located in the cell membrane changes, opening the ion
passage. After that, depolarization occurs as the voltages inside and outside the cell change.
This depolarization creates an activity potential, and the generated activity potential is
transmitted as an electrical signal to the cerebrum along the axon of the nerve tax mark.
With the development of technology, sensors that simulate the tactile transmission process
have been developed due to the increase in demand for devices that can sensitively react
to external stimuli [
7
,
8
]. The tactile sensor is a device that detects mechanical external
stimuli (e.g., strain, pressure, humidity, sound, and temperature), and transmits an electrical
signal [
9
,
10
]. The signal shows not only the relationship between the stimulus and the
device, but also the properties of the stimulus. In detail, it provides data on the magnitude,
shape, position, and distribution of forces derived from the tactile sense. Nowadays,
scientists tend to develop their tactile sensors into multiarray devices. The structure of
these devices usually contains many pixels, with the pixel size becoming smaller and
smaller with the new technologies [
11
,
12
]. Therefore the sensor became more flexible with
higher resolution. On the other side, sensitivity and the range of measurement also attached
much attention [13].
To improve the performance of tactile sensors, many researchers have studied sensing
materials acting as receptors [
14
–
18
]. For example, polarizable materials, such as barium
titanate, have not only been made into nanostructures of various shapes, but have also
been made into composite materials with other materials to improve the performance of
sensing material. In addition, research efforts are being conducted to simplify the existing
Sensors 2022, 22, 50. https://doi.org/10.3390/s22010050 https://www.mdpi.com/journal/sensors
