Recent advances of flexible sensors for biomedical applications Flexible and contactless sensors do not need to be attached to skin or organs or implanted in the body, avoiding safety issues faced by skin-attached or implantable devices. A comprehensive review is provided on the progress of flexible sensors for biomedical applications including their advantages, challenges, and future directions. Flexible sensors can be used for both skin-attached and implantable medical devices and have potential for applications like wearable electronics and electronic skin.

1. Recent advances of flexible sensors for biomedical
applications
Flexible and contactless sensors do not need to be attached to skin/ organ or to be imbedded in
human body, it avoids the safety issues faced to skin-attached or implantable biomedical devices
Guozhen Shen
2021

2. Synopsis
A review of the latest developments of flexible sensors for biomedical applications, including skin-attached devices and
implantable devices and their challenges and future research directions.
Guozhen Shen (2021) reports that Flexible electronics, which integrated microelectronics, materials science, mechanics, physics,
biology, etc. has great potential in various applications, such as wearable electronics, electronic eye cameras, electronic skins for
diagnosis and treatment.
A comprehensive review is provided on recent progress of flexible sensors for biomedical applications.
The main advantage of flexible electronics is the ability to fit onto arbitrary curved surfaces, such as the human skin and organs.
Flexible sensors, which usually have low modulus, low bending stiffness, and elastic response to strain deformation, can capture
target analytes more efficiently and generate higher quality signals.
The challenges and future research directions of flexible sensors for biomedical applications are discussed.
Some of the authors’ conclusions claim to substantiate what was previously known about this subject: “Hollow nanostructures
possess excellent intrinsic ductility and thin spherical shell.

3. Highlights
• Flexible electronics, which integrated microelectronics, materials science, mechanics, physics, biology, etc. has great potential
in various applications, such as wearable electronics, electronic eye cameras, electronic skins for diagnosis and treatment,
flexible biomedical devices, flexile displays, etc [1,2,3,4,5,6,7,8,9,10]
• A comprehensive review is provided on recent progress of flexible sensors for biomedical applications
• The researches focusing on flexible sensors for biomedical applications expended very fast in the past several years and had
shown some promising practical applications, there are still many problems and challenges to be solved
• Flexible and contactless sensors do not need to be attached to skin/ organ or to be imbedded in human body, it avoids the safety
issues faced to skin-attached or implantable biomedical devices
• The types of contactless sensors that can be used for medical applications are quite limited, and there are only several flexible
contactless humidity sensors and light-sensitive sensors for medical applications
• The results showed that the sensing device allowed for the short-term monitoring of vessel patency
• Till almost all of the reported biomedical devices with flexible sensors are with laboratory scale, which is not suitable for
practical applications

4. Introduction
Flexible electronics, which integrated microelectronics, materials science, mechanics, physics, biology, etc. has great potential in
various applications, such as wearable electronics, electronic eye cameras, electronic skins for diagnosis and treatment, flexible
biomedical devices, flexile displays, etc [1,2,3,4,5,6,7,8,9,10].
Has great potential in various applications, such as wearable electronics, electronic eye cameras, electronic skins for diagnosis and
treatment, flexible biomedical devices, flexile displays, etc [1,2,3,4,5,6,7,8,9,10].
The main advantage of flexible electronics is the ability to fit onto arbitrary curved surfaces, such as the human skin and organs,
without significant changes in their characteristics, which makes it possible to manufacture wearable or implantable devices for
human healthcare and monitoring [11,12,13,14,15].
Flexible sensors, which usually have low modulus, low bending stiffness, and elastic response to strain deformation, can capture
target analytes more efficiently and generate higher quality signals.
The challenges and future research directions of flexible sensors for biomedical applications are discussed

5. Functional sensing materials for flexible sensors
The sensing performance of flexible sensors is dominated by the used active sensing materials.
Flexible thin-film transistors have been fabricated with many organic semiconductors, which showed very good response to
external physical signals.
Based on the response properties of the flexible sensing materials to different stimuli, we can basically divide the sensing materials
into the following groups: mechanical-response materials, photo-response materials, chemo-response materials, thermo-response
materials and humidity-response materials [33,34,35,36].
Flexible mechanical-response materials are used to fabricate pressure or strain sensors with excellent response to various regimes
of pressure or strain at different positions on human body during different physical activities.
Similar to thermo-responsive materials, flexible humidity-response materials are used to real-time monitoring human health states.
It can be used to monitoring the environmental relative humidity and noninvasive human respiratory

6. Flexible sensors for skin-attached biomedical devices
Skin provides human the largest organ to interaction with the environment. It has a wide variety of important properties such as
stretchability, self-healing ability, high mechanical toughness, temperature and tactile sensing capability, etc.
Real-time monitoring pressure or strain of human body/organs provides the most widely skin-attached biomedical applications.
We proved that the introduce of hollow nanostructures into the sensing materials for flexible pressure sensors could greatly
improve the sensitivity (Fig. 1a) [36].
The sensor arrays can be conformal to the human skin for the applications in real-time monitoring of arterial blood pressure, heart
rate and body temperature, etc.
Soft, thin and flexible imaging technology led to the rapid growth in medical diagnostic measurement devices in recent years
They can be used to continuously monitor biological information such as pulse, blood pressure, and blood oxygen levels, etc
[53,54,55,56,57,58,59,60,61,62,63,64,65].
Using the same functional materials, the integrated system can be largely simplified to meet for practical applications

7. Flexible sensors for implantable biomedical devices
Implantable biomedical devices have received significant attention in recent years because they can provide unprecedented cures
for patients suffering from neurodegenerative disease, chronicpain and so on.
Compared with the skin-attached medical devices, the implantable biomedical devices are partially or fully introduced into the
human body, which provide a powerful way for a continuous monitoring, minimizing the pain and discomfort of the person
[73,74,75,76,77,78,79].
Given to the outstanding in vivo performance of the iTENG, the work proved that iTENG could be applied to power implantable
biomedical devices, and to fabricate a self-powered and wireless healthcare monitoring system

8. Findings
The results showed that the flexible visual memory system could realize the detection and memory process of UV light distribution
for a long-term retention and the stored image information could be reset by applying a negative voltage sweep.
The results showed that the sensing device allowed for the short-term monitoring of vessel patency

9. Conclusions and remarks
A comprehensive review is provided on recent progress of flexible sensors for biomedical applications.
The materials showing response to different stimuli were first briefly introduced, including mechanical-response materials, photo-
response materials, chemoresponse materials, thermo-response materials and humidity-response materials
Following to this part, the main stream applications of flexible sensors for skin-attached biomedical devices and implantable
biomedical devices were comprehensively reviewed.
As the biomedical devices are either attached to the skin or implanted in human body, many attentions should be paid to address the
device safety issue.
Flexible and contactless sensors do not need to be attached to skin/ organ or to be imbedded in human body, it avoids the safety
issues faced to skin-attached or implantable biomedical devices.
Compared with the development of flexible sensors for skin-attached applications, the researches on flexible sensors for
implantable applications are hindered because of many technical or safety issues.
More works should be further performed to promote this direction