Citation: Liu, E.; Cai, Z.; Ye, Y.; Zhou,
M.; Liao, H.; Yi, Y. An Overview of
Flexible Sensors: Development,
Application, and Challenges. Sensors
2023, 23, 817. https://doi.org/
10.3390/s23020817
Academic Editors: Ki-Hyun Kim and
Deepak Kukkar
Received: 29 November 2022
Revised: 1 January 2023
Accepted: 4 January 2023
Published: 10 January 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Review
An Overview of Flexible Sensors: Development, Application,
and Challenges
Enze Liu
†
, Zhimin Cai
†
, Yawei Ye, Mingyue Zhou, Hui Liao and Ying Yi *
School of Mechanical Engineering and Electronic Information, China University of Geosciences,
Wuhan 430074, China
* Correspondence: yiying@cug.edu.cn
† These authors contributed equally to this work.
Abstract:
The emergence and advancement of flexible electronics have great potential to lead devel-
opment trends in many fields, such as “smart electronic skin” and wearable electronics. By acting
as intermediates to detect a variety of external stimuli or physiological parameters, flexible sensors
are regarded as a core component of flexible electronic systems and have been extensively studied.
Unlike conventional rigid sensors requiring costly instruments and complicated fabrication processes,
flexible sensors can be manufactured by simple procedures with excellent production efficiency, reli-
able output performance, and superior adaptability to the irregular surface of the surroundings where
they are applied. Here, recent studies on flexible sensors for sensing humidity and strain/pressure are
outlined, emphasizing their sensory materials, working mechanisms, structures, fabrication methods,
and particular applications. Furthermore, a conclusion, including future perspectives and a short
overview of the market share in this field, is given for further advancing this field of research.
Keywords: flexible sensor; strain sensor; humidity sensor; human machine interaction
1. Introduction
Recent progress in electronic systems has ignited promising applications in many
fields, including consumer electronics [
1
,
2
], human–computer interactions [
3
,
4
], augmented
reality devices [
5
,
6
], and electronic skins [
7
,
8
]. A sensor that can sense the physical world
is an essential part of these systems. Conventional sensors are generally fabricated from
semiconductors that have rigid substrates or scaffolds that become deformable once they
are thinned and oriented into nanostructures [
9
], thus limiting their applications such
as wearable “smart” devices [
10
], soft robots [
11
,
12
], body motion tracking [
13
,
14
], and
portable medical diagnostic devices [
15
,
16
]. The emergence of flexible sensors made of
inherently elastic materials, such as hydrogels and organic semiconductors, may allow the
revolutionary development of electronic systems because of their significant advantage of
allowing a high degree of design freedom. A flexible sensor can be folded into different
shapes and even trimmed down to different sizes, thus greatly enlarging its fields of
application. For example, the integration of wearable electronics could meet the softness
demands of clothing or fit the irregular surfaces of tissues and the body.
The manufacture of flexible sensors needs novel designs and appropriate materials,
which mainly include conductors and synthesized materials. Conductors are generally
classified into the carbon family and the metal oxides (and sulfides). For example, graphene
is one of the most popular two-dimensional (2D) nanostructure-based semiconductors in
the carbon family [
17
]. Non-transition-metal oxides such, as ZnO and SnO
2
, which exhibit
high sensitivity and favorable conductivity, are also widely used as sensing materials [
17
].
As a representative synthesized material, the emergence of MXene (e.g., Ti
3
C
2
Tx) has
greatly widened the selection of materials that are suitable for flexible sensors because of
its unique sandwich-like layer structure, its excellent electrical conductivity, and its large
area of hydrophilicity [
18
]. Furthermore, organic semiconductors with features such as
Sensors 2023, 23, 817. https://doi.org/10.3390/s23020817 https://www.mdpi.com/journal/sensors
