Medicine in Novel Technology and Devices 16 (2022) 100195
Contents lists available at ScienceDirect
Medicine in Novel Technology and Devices
journal homepage: www.journals.elsevier.com/medicine-in-novel-technology-and-devices/
Triboelectric nanogenerators for clinical diagnosis and therapy: A report of
recent progress
Yichang Quan a, 1, Xujie Wu a, 1, Simian Zhu a, b, 1, Xiangyu Zeng a, b, Zhu Zeng a, b, **,
Qiang Zheng a, b, *
a
Key Laboratory of Infectious Immune and Antibody Engineering of Guizhou Province, Engineering Research Center of Cellular Immunotherapy of Guizhou Province,
School of Biology and Engineering/School of Basic Medical Sciences, Guizhou Medical University, Guiyang, 550025, China
b
Immune Cells and Antibody Engineering Research Center of Guizhou Province, Key Laboratory of Biology and Medical Engineering, Guizhou Medical University, Guiyang,
550025, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Triboelectric nanogenerators
Wearable and implantable medical devices
Self-powered medical services
Diagnostic and therapeutic applications
Triboelectric nanogenerators (TENGs) are considered as an ideal platform for power harvesting for living organisms, thanks to their unique characteristics like flexibility, conversion efficient, and manufacturing cost.
Recent advances in TENGs have brought innovative solutions for clinical healthcare. Particularly, TENGs offer
novel solutions of continues power supply for wearable and implantable medical devices with lightweight,
thinness, good biocompatibility, and excellent soft tissue conformability. In this review, we discuss (1) The
working principle and representative structure of TENGs, (2) the material selection of TENGs, (3) the recent
progression of application of TENG in the medical field of cardiovascular system, nervous system, respiratory
system, microbial inactivation, antibiofouling, disinfection, and tissue repair, (4) challenges and future perspectives of TENG-based medical devices. The emerging TENGs and their applications in medicine cannot simply
be seen as an alternative to conventional power supplies, it provides a revolutionary solution for wearable and
implantable medical devices, and they will surely change the paradigm of disease diagnosis and treatment in the
future.
1. Introduction
Advancements of new materials and their processing technology in
the field of microelectronics have promoted the development of
implantable and wearable medical devices toward further miniaturization, flexibilization, and intellectualization. Implantable and wearable
medical devices have shown strong potential in the diagnosis of treatment of various diseases [1–8]. However, the majority of implantable or
wearable medical devices still use battery power, while frequent batteries
recharging or replacement can largely reduce patient compliance, increase risk of surgical complications and increase patient financial
burden [9]. Discarded waste batteries also brings environmental risks. All
these issues could restrict further development of portable medical
electronic devices. Therefore, researchers across industry and academia
have been continuously seeking new, sustainable, self-powered, and
environmentally friendly power supply for implantable or wearable
medical electronic devices [10–15]. Energy harvesting devices and
self-powered sensors based on the idea of triboelectric nanogenerators
(TENGs) have drawn much attention in this field, especially for long-term
therapy and sensing monitoring purposes. To fulfill the unique requirements of human diseases diagnosis and treatment, higher standards
are proposed for the biological safety, transformation efficiency, and
sensitivity of TENGs for implantable or wearable medical electronic devices. TENG still faces key technological challenges such as flexibility,
miniaturization, integration, high output, and simple structure etc. Researchers designed a variety of novel self-powered devices based on the
basic principle of TENG, to solve and optimize the aforementioned issues,
and to adapt the actual clinical needs of diagnosis and therapy, thus
* Corresponding author. Key Laboratory of Infectious Immune and Antibody Engineering of Guizhou Province, Engineering Research Center of Cellular Immunotherapy of Guizhou Province, School of Biology and Engineering/School of Basic Medical Sciences, Guizhou Medical University, Guiyang, 550025, China.
** Corresponding author. Key Laboratory of Infectious Immune and Antibody Engineering of Guizhou Province, Engineering Research Center of Cellular Immunotherapy of Guizhou Province, School of Biology and Engineering/School of Basic Medical Sciences, Guizhou Medical University, Guiyang, 550025, China.
E-mail addresses: zengzhu@gmc.edu.cn (Z. Zeng), Zhengqiang@gmc.edu.cn (Q. Zheng).
1
The three authors contributed equally to this work.
https://doi.org/10.1016/j.medntd.2022.100195
Received 28 September 2022; Received in revised form 25 November 2022; Accepted 27 November 2022
2590-0935/© 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).
Y. Quan et al.
Medicine in Novel Technology and Devices 16 (2022) 100195
further expanded its potential in the field of biomedical applications and
provided new way of thinking for self-powered diagnosis and therapy
technology (see Table 1).
In this review, we first briefly introduce the TENG technology from
the aspects of principle, structure, and materials. Then, we summarize
the five-year progress of TENG applications in several biomedical fields
(Fig. 1). For implantable or wearable biomedical devices, TENG can be
utilized as a promising, complementary, and even alternative power
supply. However, this process still faces multiple great challenges and
difficulties. In the part of this paper, we appropriately summarized and
prospected the challenges and solutions of TENGs in the future.
2. Mechanism
TENG is a device that converts mechanical energy into electrical energy, first proposed and developed by Wang et al. [49]. The first TENG
uses the classic contact-separation mode (Fig. 2a), which still the basis for
most modern TENG designs [49,50]. The whole device consists of two
different friction materials with a back electrode and joint the external
load through the wire. The working principle of TENG is mainly based on
the coupling effect of triboelectric and electrostatic induction [49,51]. In
a full working cycle, the two different friction materials first come into
contact with each other, driven by an external force (such as heartbeat),
the charge will transfer from one material to the other, due to their
different electron affinities. This will create an equal amount of opposite
charge on the surfaces of the materials. When the external force is
released, the distance between the two friction materials increases,
creating a potential difference between the surfaces of the materials [50].
Driven by such potential difference, electrons flow between the two
electrodes, creating an electric current (Fig. 2c). The working cycle repeats and generates a continuous flow of Alternating current (AC) signals
from rhythmic mechanical movement and convert it into electrical energy [52]. With the ongoing advancement of TENG research in recent
years, its complicated theoretical model has gradually become clear. In
2017, Wang et al. proposed the Maxwell displacement current [53], and
since then, the nature of TENG's work has been gradually revealed [54,
55]. The Maxwell displacement current is defined as:
Table 1
Represented TENG-based biomedical devices with their working modes, electric
performance, and potential clinical applications.
Object
Mode
Output
voltage(V)
Function
Remark
Cardiovascular
Contactseparation
[16–25]
0.13–61.2
[16], 1.52
[17],10
[18],3.73
[19],4
[20],14
[21],10
[22],1.2 to
6.2 [23],
4.2 [24],
2.2 [25]
Sensor
[16–18,
21–25]
Power
source
[19,20]
HR monitoring
[16,17,21,22,
24,25]
Human activity
monitoring
[18]
First
implantation of
chest [19]
Correction of
sinus
arrhythmias
[20]
Breathing and
HR monitoring
[23]
Driving
muscles [26]
Dietary control
[27]
Reducing time
in AF [28]
Olfactory
simulation
[29]
Nerve
regeneration
and repair
[30–32]
Human
respiratory
monitoring
[33,34]
Distinction of
respiratory
states [37,35]
Judging drunk
driving [38,36]
Exhaled gas
concentration
detection [39,
40]
Algae removal
and
sterilization
[41,42]
Antibiofouling
activity [43]
Promote
MC3T3-E1
deposition [44,
45,46]
Electrical pulse
therapy [44,
45,46,48]
Endogenous
electric field
therapy [47]
Nerve
ContactSeparation
[26–29,30–32]
68 and 76
[26], 0.05
to 0.12
[27], 16.7
[28,29],
300 [30],
0 [31], 40
[32]
Sensor
[29]
Power
source
[26,30,32]
Electrical
stimulator
[27–29,
30,31]
Respiratory
Contactseparation [33,
34,35,36]
Single Electrode
[37]
Freestanding
[38]
27 [33],
2.4 [34],
200 to
1300 [37],
4.9 to
15.0,12.6,
5.9,1.6
[35],14 to
19 [38], 9
to 35 [36]
Sensor
[33–38]
Disinfection
ContactSeparation [41,
42]
Single-Electrode
[43]
210 [41],
50 [42],
200 to 300
[43]
ContactSeparation [44,
45,46,47]
Lateral Sliding
[48]
0.2 [44],
4.5 [45],
0.06 [46],
2.2 [48],
25 to 81
[47]
Power
source
[41,43]
Electrical
stimulator
[42,43]
Power
source
[44]
Electrical
stimulator
[45–48]
Tissue repair
JD ¼
∂D
∂ E ∂ Ps
¼ε þ
∂t
∂t
∂t
(1)
where D represents the displacement electric field, ε represents the
dielectric constant of the medium, E is the electric field, s is the polarization electric field caused by a surface polarized charge formed by
piezoelectric or triboelectric effects. The first term of the formula is a
time-varying electric field, related to the origin of the electromagnetic
wave, and the second term of the formula represents the contribution of
surface polarization, which is the origin of the nanogenerator. Specifically, the piezoelectric polarized charge produced by the applied stress is
responsible for this polarization in piezoelectric nanogenerator. In a
triboelectric nanogenerator, the external electrostatic charge caused by
tribological electricity constructs a time-varying surface polarization
during contact friction between two materials.
The basic model of triboelectric nanogenerator in contact-separation
mode is shown in Fig. 2b. The two back electrodes are jointed with the
external load, and the two layers of dielectric material are used to
generate electricity through contact friction. The dielectric surfaces can
be oppositely charged after contact friction, and the surface charge
density is σtribo. Surface charge density is saturated at the initial contact
cycle, independence of the gap z between dielectric layer materials. The
frictional charge creates an electrostatic field, driving the free electron
flow between the two electrodes through an external load. The transferred charge σtr accumulated on electrodes is a function of the gap z,
thus, the mechanical energy that changes the value of z is converted into
electrical energy. The corresponding displacement current can be
calculated as:
JD ¼
∂DZ ∂σ tr ðz; tÞ
¼
∂t
∂t
(2)
From another perspective, a pair of relatively charged and variablespaced surfaces can be regarded as a variable capacitor. Fig. 2b shows
the schematic diagram of TENG as a capacitance model, and its current
can be expressed as:
I¼
dQ
d σ tr
¼A
dt
dt
(3)
The result is equivalent to Equation (2), and it is verified that Maxwell
displacement current is the basis of the capacitance model. The output
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Medicine in Novel Technology and Devices 16 (2022) 100195
Fig. 1. (a) Clinical applications of triboelectric nanogenerators in the nervous system, cardiovascular system, respiratory system, and others. (b) Conventional form
factors and future trends of medical devices.
Fig. 2. Several working modes and principles of TENG.
working mode, applications in biomedical field are no exception. It is
suitable for harvesting the energy from most of the mechanical movements from moving parts like thoracic or limb joints, which is ideal for
contact-separation mode TENG that operate in sites where normal motion towards the contact surface dominates. However, the output power
of contact-separation mode TENG is highly depends on the facing surface
voltage of the corresponding triboelectric nanogenerator can be
expressed as:
V¼
1
Q þ VOC ðzÞ
CðzÞ
(4)
The contact-separation mode is the first and most widely used TENG
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Medicine in Novel Technology and Devices 16 (2022) 100195
material. The reason of using liquid metal mercury for quantitative test is
that its adaptability to shape maximizes the contact area with various
testing materials, thus effectively avoids the influence of external factors
like material roughness, humidity, and temperature etc. Besides, the
strong surface tension of mercury makes it easy to separate from the
testing material during the measurement. The parameter describing the
triboelectric capabilities of a material called the “triboelectric charge
density”, also known as the new “material gene”, could be systematically
characterized by this method (Fig. 3e). Polymer materials such as polytetrafluoroethylene (PTFE), polydimethylsiloxane (PDMS), polyimide
(PI) etc. have strong electron-donating capability thereby are widely used
as electronegative materials. Common metals such as copper, aluminum,
and iron are generally selected for electropositive materials. These materials are easily processed into film, and inexpensive.
Suitable material collocation can be chosen based on the triboelectric
charge density of the materials, to create high output and high performance TENG. To further enhance the output performance, there are
several ways to modify the surface of friction layer. The most common
method is to create superficial micro-nano structures, thus increase the
actual contact area and surface charge density. PTFE and PI polymer
films can be etched directly using inductively coupled plasma (ICP)
(Fig. 3c), while flexible PDMS can be modified through nano imprinting
the micro- and nanostructures of pre-built silicon wafer templates
(Fig. 3d) [44], and aluminum foil can be roughened by electrochemical
corrosion techniques (Fig. 3b) [62]. Apart from etching, imprinting, and
electrochemical corrosion, it is particularly important to develop a
fast-processing method that capable to create micro- or nanostructures in
large areas, which is the key factor for mass production of TENG. Zhao
et al. proposed a straightforward, quick, and inexpensive method for
surface modification of friction layer by sandpaper grinding [61]. According to the study, sandpaper grinding can polish the surface of
aluminum foil and PI film and create micro-channels that significantly
enhance the output performance of TENG.
Recently, some researchers have suggested a surface modification
method using low-energy ion radiation to adjust the chemical structure
and functional groups of polymer friction layer materials at the molecular
level [63]. The microstructure and mechanical flexibility of processed
polymer material are unaffected, but its surface charge density is
enhanced, thereby improving the triboelectric properties of the friction
layer made from it (Fig. 3a). This method has excellent processing efficiency and long-term stability.
For biomedical applications, the selection of materials is highly
dependent on the working scenario of the device. The TENG can be used
as a backup or emergency plug-in power supply for most conventional
biomedical devices. For this case, the key consideration will be the output
capability while flexibility and biocompatibility are not so important, the
selection of materials is mainly to ensure high output, rigid metal with
high triboelectric charge density is acceptable. However, for wearable
biomedical devices, the unbalanced interfaces between soft tissue and
rigid electric components bring a series of reliability problems, thus
flexibility becomes the primary consideration and many polymer materials commonly used in flexible electronics become the first choice of
such devices. Ultimately implantable biomedical devices have the
strictest material restrictions, developer must balance between output
capability, flexibility, and biocompatibility. Biocompatible hydrogels,
natural products and their extracts, and bio-organic packaging materials
have become popular choices, while nanostructure modifications or
doping are being used for improving the triboelectric charge density of
those materials.
area of friction layers, which can be compensated by stacking multiple
friction layers, but the cost will be the overall thickness and flexibility of
the devices. Apart from classic contact-separation mode, there are three
other operating modes of TENG in practice, which are briefly described
as follows:
2.1. Lateral sliding mode
The lateral sliding mode has a similar structure to the vertical contactseparation mode, but in this mode, the friction layer moves horizontally.
When the two friction layers slide laterally and come into contact, the
two friction layers will carry opposite charges. With the lateral sliding of
the friction layer, the charge distribution on the dislocation region
changes accordingly, resulting in an electric field. Electrons flow directionally in the electric field and produce electric current [56]. If the
motion is periodic, the alternating flow of electrons can produce a
continuous AC output between the two electrodes (Fig. 2d). Comparing
to contact-separation mode, the lateral sliding mode TENGs can produce
higher output thanks to its much higher degrees of freedom for horizontal displacement, which is suitable for external power supply for
high-load or high-frequency applications like multi-parameter monitoring or electrical stimulation treatment, and innovative structural designs like rotary mechanism or grating electrodes are usually needed to
maximize the output efficiency of contact-separation mode TENGs.
2.2. Single-electrode mode
Single-electrode TENG consists of two electrode layers, one of which
is grounded. When the non-ground electrode layer moves and the distance between the two electrodes changes, the local electric field distribution changes, and an electrostatic potential is generated between the
electrode layers [57], thereby generating a current. The TENG produces a
continuous output as the ungrounded electrode layer keeps moving
(Fig. 2e). It is challenging to apply a single-electrode mode TENG to the
biomedical applications, the reference ground need to be carefully
selected, and the output power of the TENG are substantially halved. But
the benefits are also obvious, the electrical circuit design can be simplified, thereby improve the flexibility and wearability of the device.
2.3. Freestanding mode
The freestanding mode is upgraded from the single electrode mode,
which consists of two symmetrical stationary electrode layers and one
moving electrode layer, and the two stationary electrode layers are
connected by a load circuit [58]. When the moving electrode layer is
close to or far away from the one of the stationary electrode layer,
electrostatic induction occurs in the dielectric material, and the charge
distribution of the material surfaces becomes asymmetric, which makes
the electrons move from one stationary electrode to another, thus
generating a current [59]. The TENG produces a continuous output as the
moving electrode layer continues to move (Fig. 2f).
3. Triboelectric nanogenerator material
The materials for the friction layers of TENG come from various
sources. Objectively speaking, almost all materials exhibit triboelectric
phenomenon. Metals, inorganic nonmetallic materials, organic polymers,
and composite materials can be used as raw materials for the creation of
TENG. However, for better output performance, the difference of electron
affinities of the two friction layer materials must be maximized, which
means, one material should have the greatest potential for electron gain,
while the other has the greatest potential for electron donating. To
accurately quantify triboelectric capabilities of different materials, researchers have systematically measured the triboelectric charge density
of selected materials [60]. Such measurement utilizes the liquid metal
mercury to perform a contact-and-separate motion with each test
4. TENGs for the cardiovascular system
4.1. Wearable cardiovascular electronic devices
Wearable devices can either be attached to the human body or integrated with other functional devices on clothing or accessories. Its
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Medicine in Novel Technology and Devices 16 (2022) 100195
Fig. 3. (a) Concept sketch of Zhao et al. [61], low-energy ion radiation surface modification of triboelectric film materials. (b) Chemically etched aluminum foil with
nanostructure. (c) ICP etched Polymer films with nanostructures. (d) Nanostructured flexible PDMS using nano imprinting technique. (e) Triboelectric charge density
of commonly used TENG materials.
self-powered pulse wave sensor for accurate heart rate (HR) measurement and monitoring of cardiovascular system health. In addition, the
device can be attached to the throat as a microphone, picking up and
restoring voice signal without external power. BMS has great potential in
wearable medical devices and biometric applications owing to its wearable, small size, low cost, and self-powered nature (Fig. 4a).
Ouyang et al. developed a wearable self-powered pulse sensor (SUPS)
based on TENG. This SUPS also operates in a vertical contact-separation
mode [17]. The SUPS typically produces voltages and currents of up to
1.09 V and 2.97 μA at the normal output, and when used to measure the
human radial artery, the SUPS produces effective output voltages and
currents of up to 1.52 V and 5.4 nA. The high linearity of the R-R and P–P
intervals captured by the SUPS can be used directly for characteristic
index analysis, and the SUPS also operates in a vertical
structure plasticity and portability make it one of the current research
hot-spots in the field of electronics. Wearable devices have been widely
used for cardiovascular signal acquisition such as heart rate, blood
pressure, and electrocardiogram (ECG). In recent years, much attention
has been paid to TENGs as energy harvesting power sources or active
cardiovascular signal sensors in wearable application.
Yang et al. designed a wearable self-powered bionic membrane sensor
(BMS) based on the coupling effects of the contact electrification and
vertical contact-separation mode TENGs [16]. The PTFE layer of BMS can
respond to mechanical vibrations through contact electrical effects, and
the sensitivity of such responds can be as high as 51 mVPa1 with a very
fast response time and low limit-of-detection of pressure, down to 2.5 Pa.
After 40,000 loading-unloading cycles, the BMS exhibits an excellent
stability and durability. By analyzing pulse data, the BMS can be used as a
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Medicine in Novel Technology and Devices 16 (2022) 100195
Fig. 4. (a) BMS that can restore voice signals and arterial signals designed by Yang et al. [16]. (b) SUPS that can capture arterial signals designed by Ouyang et al.
[17]. (c) FS-TENG can output stably under elastic stretch designed by Fu et al. [18].
rhythm, respiratory rate, and blood pressure etc.) and therapeutic
capability (heart failure and atrial fibrillation etc.) (Fig. 1). However,
conventional implantable devices face issues of battery life, which increases the risk of device failure, causes uncertainty among patients, and
also limits the further development of these device to miniaturization and
intelligence. The TENG can be used as both a power source that last long
and an active sensor that collect physiological signals. In combing with
virous biocompatible materials, the TENG can significantly improve the
performance of implantable devices and achieve the goal of long-term
operation of the cardiovascular system in vivo.
contact-separation mode. The SUPS is capable of outputting voltages and
currents of up to 1.09 V and 2.97 μA under typical conditions and producing effective output voltages and currents of up to 1.52 V and 5.4 nA
when used for measuring the human radial artery pulses. Moreover, the
high linearity of the R-R and P–P intervals of pulse waveform captured by
the SUPS (Fig. 4b) can be directly used for feature analysis. Moreover, the
pulse waveform can be used for diagnosing specific cardiovascular diseases such as arrhythmia, coronary artery disease, and atrial fibrillation
(AF). Therefore, combined with Bluetooth technology, the SUPS can
achieve more accurate, convenient, and real-time wireless monitoring of
cardiovascular diseases based on pulse signals. At present, such wearable
TENG based SUPS has been successfully applied in the diagnosis of cardiovascular diseases.
Fu et al. developed a fibrous stretchable TENG-based sensor (FSTENG) with core-sheath structure [18]. Such ultra-sensitive sensor with a
pressure limit of detection of 0.02 N can be used to detect various human
physical activities such as knee bending, finger flexing, walking and
physiological activities like pulse, throat vibration, and facial expression.
The output voltage of this sensor can reach nearly 10 V, and with the
support of elastic support and stretchable electrodes, the output voltage
remains stable at 60% tensible strain (Fig. 4c). Such wearable device with
good elasticity can with stand bigger deformation of the body parts and
in combine with the TENG self-powered technology, it will have greater
potential in health-care monitoring application.
4.2.1. Implantable cardiac pacemakers
Since 2012, TENGs has received wide attention by virtue of various
energy collection harvesting methods applicable to the human body.
Subsequently, the multi-party team also made further improvements to
the TENG's capacity efficiency and output energy.
Zheng et al. realized for the first implantation of TENG in the cardiovascular system in a living organism. Such implantable triboelectric
nanogenerator (iTENG) has a tiny working area of only 0.8 cm 0.8 cm
[19]. The iTENG was prepared by compositing PDMS film with Au
deposited Kapton film as the friction layer and nano-structured
aluminum foil as electrode. The iTENG was implanted in the left chest
of a rat. The chest of the rat will produce slight but regular fluctuations
with breathing movement, making the Kapton thin layer and
nano-structured aluminum foil periodically contact and then separate
with each other. Through this mechanism, the iTENG can generate an
output voltage of 3.73 V and current of 0.14 μA (Fig. 5a). Such electrical
energy converting from mechanical movement can be stored in a
capacitor, and powering TENG for the cardiac pacemaker when needed.
Ouyang et al. successfully utilized an iTENG-based implantable
symbiotic pacemaker (SPM) in large animals (35 kg Yorkshire pigs) to
accomplish cardiac pacing function as well as correction of sinus arrhythmias [20]. The whole device is driven by the energy obtained by
biological cardiac pulsation, while the heart is electrically stimulated by
4.2. Implantable cardiovascular electronics devices
Since the first implantable pacemaker was developed in 1958, great
improvements of cardiovascular implantable electronic devices (CIEDs)
have been made. Modern CIEDs, including implantable pacemakers,
implantable cardioverter-defibrillators (ICDs), cardiac resynchronization
therapy devices, implantable loop recorders and implantable hemodynamic monitoring devices, have already saved millions of lives by
providing more accurate and continuous diagnostic (including heart
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Fig. 5. (a) TENG was first implanted into the cardiovascular system by Zheng in 2014, whose output current is synchronized with the heartbeat [19]. (b) Cardiac
pacing and sinus arrhythmia correction was achieved by using SPM in 2019 [20]. Implantable TENGs are getting smaller: (c) Implantable TENG(6 4 0.1 cm3)
manufactured by Zheng et al., in 2016 [21]. (d) One-stop implantable friction-electric active cardiac sensor(6 4 0.1 cm3) created by Ma et al. can monitor blood
pressure signals in 2016 [22]. (e) Smaller size SEPS(1 1.5 0.1 cm3) implanted in the heart chambers by Liu in 2019 [23]. (f) BTS successfully detected Abnormal
Cardiovascular Event Identification and Abnormal Respiratory Event Identification abnormal signals in large and small animals [24]. (g)NSTENG developed by Zhao in
2021 [25].
clinical therapeutic diagnoses like symbiotic bioelectronic drugs in vivo.
This “symbiotic pacemaker” still needs to improve the efficiency of energy harvesting and conversion to achieve energy storage.
SPM to ensure normal physiological functioning (Fig. 5b). The iTENG
implanted between the pig's heart and pericardium can derive an average
of 0.495 μJ energy from one pacing cycle, which is higher than the pacing
threshold energy required by the endocardium. Experimentally, ice was
placed near the sinus lymph node to induce significant arrhythmic
symptoms, and the open-circuit voltage was up to 65.2 V, which was
sufficient for the normal operation of the pacemaker. After electrical
stimulation, sinus arrhythmia heart rate turned to normal heart rate, and
blood pressure gradually recovered. The excellent output performance
exhibited by such systems makes people expect that they can be used in
4.2.2. Implantable heart sensor
The achievement of implanting a TENG as part of a heart sensor in an
animal requires a more sophisticated design of the device. This also
means that the miniaturization of device size, biocompatibility of packaging materials, and accuracy of signal collection should be considered.
Zheng et al. further fabricated an implantable TENG with a size of 6
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cm 4 cm 0.1 cm based on the previous design. The device could reach
an output voltage and output current of 14 V and 5 μA after implanted
into the inferior wall of the left ventricle of adult pig, which were 3.5 and
25 times higher than the pre-modified version in 2014, respectively [21].
In the experiment, by injecting adrenaline into the body, we can observe
a good correlation between signal fluctuations and the cardiac beat
(Fig. 5c), which proves the advantage of the implantable sensor in signal
acquisition with high accuracy. In addition to the basic triboelectric film,
electrode, and packaging parts, it also adds a keel structure that can
effectively ensure the contact-separation process between the triboelectric layer and the electrode layer. So that the mechanical properties of the
overall structure are significantly enhanced, and higher output is
generated. The converted energy was used to drive an implanted wireless
signal transmitter, thereby enabling real-time wireless signal transmission from a self-powered implantable cardiac sensor. In addition, the
device stably monitored dynamic data in adult Yorkshire pigs for over 72
h. This work demonstrates the great potential of nano power generation
technology for the development of medical devices for self-powered
cardiovascular systems.
Ma et al. adopted the same design principle to create a self-powered,
one-stop implantable friction-electric active cardiac sensor measuring 3
cm 2 cm 0.1 cm. The voltage and current output of the device can
reach to 75 V and 12 μA [22]. The output electrical signals are compared
with heartbeat rhythm and respiratory frequency, and these fluctuations
show up to 99% accuracy (Fig. 5d). Two weeks after the device was
implanted between the epicardium and pericardium of adult pigs, the
experimenters anesthetized the pigs to check the integrity and efficacy of
the device. Compared with the control group, myocardial tissue was not
infected. These tests have ensured that the TENG has good biological
adaptability in vivo and can be developed as a multifunctional medical
device. The current size needs to be further improved for in vivo
implantation.
Liu et al. published a TENG-based implantable endocardial pressure
monitoring approach. The device not only has a smaller device size (1.0
cm 1.5 cm 0.1 cm), but also achieves the advancement of selfpowered endocardial pressure sensors (SEPS) from surface monitoring
in the subcutaneous region of the heart to implantation in the heart
chambers (ventricles, atrium) (Fig. 5e). This taps into the implantable
TENG's potential of multifunctional and multi-scenario applications of
medical devices for cardiovascular diseases [23]. Subsequent experimental data from surgically implanted experimental pigs showed that the
device was biocompatible and responsive. And symptoms such as ventricular extrasystole and ventricular fibrillation were successfully detected in a pig heart signal with epilepsy.
Ouyang et al. reported a bioresorbable triboelectric sensor (BTS)
based on the against each other to convert the biomechanical signal into
an electrical signal [24]. The maximum voltage output of the BTS was up
to 4.2 V in a non-liquid environment and remained stable under mechanical stress stimulation, and its sensitivity was up to 11 mV/mmHg
with linearity close to 99.3%. After 12 weeks of implantation in the back
of rats, the BTS was completely degraded by biodegradation measurements. After implantation in small animals (rats), abnormalities in respiratory distress were successfully monitored; and arrhythmias were
successfully identified in large animals (adult hounds). This fully
biodegradable, real-time instrument that detects abnormal vital signs
will provide safer monitoring of patients after surgery (Fig. 5f).
Zhao et al. developed a non-spaced triboelectric nanogenerator
(NSTENG), which can avoid the obstacle of perception of fine motions by
the spacer layer and has more displacement and deformation capability
than the conventional TENG with the spacer layer [25]. NSTENG uses a
layer of copper as the electrode, and the outer layer is wrapped with
rubber (Fig. 5g). The surface of the copper foil at the junction of the two
layers is etched by plasma etching, thus forming a micro-nano structure
air gap inside. Due to the excellent biocompatibility of the NSTENG, the
researchers implanted it in rats and obtained an open-circuit voltage and
short-circuit current of 2.2 V and 44.5 nA, and measured heart rate with
99.73% accuracy, and also found that the NSTENG is able to provide
subtle information about heart motion beyond the ECG, providing a new
idea of implantable TENG.
5. TENGs for nerve system
5.1. Electrical nerve stimulation
The nervous system plays a key role in the reception and transmission
of physiological signals in various parts of the human body. Clinically,
signals generated by the nervous system are crucial for the diagnosis and
treatment of diseases. Pulse currents can stimulate and control motor
nerve fibers, afferent sensory nerve fibers, and promote the plasticity of
the human brain, thereby regulating various physiological functions
[64]. In addition, nerve electrical stimulation can also be used as a
clinical method for the treatment of muscle non-effective responses due
to central nervous system injury. Its principle of action is to replace
neurons that cannot transmit signals to activate muscles or manipulate
limbs, usually by stimulating motor neurons or directly acting on the
target muscle tissue. Due to its excellent electrical properties, plasticity,
and biocompatibility, TENG has received increased attention in the field
of nerve stimulation. It can be used as a sustainable power supply to drive
nerve stimulation devices, thus improving the portability, long-term
effectiveness, and intelligence of the entire system.
Lee et al. developed a TENG consisting of multiple units connected in
a serrated structure [26]. The individual units consist of a friction layer of
nanopatterned Cu and PDMS films, a gold interlayer and two layers of
flexible polyimide were selected for the stimulation electrode part, and
each unit was mechanically connected through a polyethylene terephthalate (PET) plate. Under slight hand movements, the sawtooth
structure is squeezed and restored, and mechanical energy is converted
into electrical energy by this cyclic process (Fig. 6a). In the experiment,
when five units are connected in parallel, the TENG generates an output
voltage of 68 V and an output current of 1.9 μA. The output will act on the
sciatic nerve of the rat by stimulating the electrodes to make the tibialis
anterior (TA) and gastrocnemius medialis (GM) muscles contract, and the
leg of the rat appears twisted, while the myoelectric frequencies of TA
and GM recorded in Electromyography (EMG) and the nerve signal of
electrode stimulation match.
The two teams above use electrical stimulation to act on reflex arc
structures to modulate muscle activity, but TENG can also be used to
control organ function(Fig. 6c) [27]. Yao et al. implanted an implantable
TENG on the gastric surface of rats to stimulate the vagus nerve (VNS)
using the distension and contraction of the stomach to convert mechanical energy into electrical energy. This causes a reduction in food
intake and thus weight loss in rats. After 100 days of the experiment, part
of the epididymis and kidney of the experimental group were removed,
which were 58% and 67% respectively compared with the control group,
and the average weight was 38% lighter than the control group. In
addition to controlling gastric function, this foldable TENG can also
stimulate the bladder nerves in the pelvic cavity to regulate and improve
bladder function.
In addition to the control of organ function, nanogenerator technology can be combined with low-level vagal nerve stimulation therapy to
inhibit autonomic nervous system remodeling to prevent the worsening
of AF. Sun et al. proposed a closed-loop self-powered electrical stimulation system powered by a triboelectric, piezoelectric hybrid nanogenerator (H-NG) [28]. The pulse signal is monitored in real-time by a
PENG sensor on the skin surface and transmitted to a mobile phone
through a Bluetooth module. Once the property is detected, the patient is
reminded to actuate the system by hand (Fig. 6d). Data from experiments
in rats showed that the system was safe and intelligent to achieve
excellent efficacy: the time to AF was significantly reduced by 90%, and
the cardiac pathological response caused by AF was significantly
reduced.
Flexible electronic sensors that mimic the sense of smell can also be
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Fig. 6. (a) TENG with serrated connection for sciatic nerve stimulation [26]. (b) The triboelectric-brain-behavior biomimetic closed loop of controllable behavior
[29]. (c) Stimulation of VNS with an implantable TENG for feeding control [27]. (d) Self-powered H-NG enables human intervention of atrial fibrillation automatically [28].
avoids the ethical issues associated with the use of human embryos but
also effectively reduces the risk of high tumor incidence caused using
multifunctional stem cells.
In order to enhance the differentiation of MSC into nerve cells, Guo
et al. combined small TENG with improved conductivity of Wiener fibers
[30]. The friction layer consists of polymethyl methacrylate (PMMA) and
Al, with Cu as the electrode covering the outside of the friction layer. The
two rectangular friction layers compressed and stretched with a spring at
the four corners. As the experimenter walks around, the TENG can output
nearly 300 V and 30 μA of current, and the output will be electrically
stimulated by reduced graphene oxide (rGO) nanofibers mixed with 15%
poly (3,4-ethylene dioxythiophene) (PEDOT) as a nerve scaffold. In the
experiment, Tuj1 (neuronal spectrum marker gene) and GFAP (glial
spectrum marker gene) were significantly expressed in MSCs subjected to
electrical stimulation for 21 days in the experimental group compared to
the control group that was not subjected to electrical stimulation
(Fig. 7a).
Jin et al. proposed that fibroblasts could be directly incorporated into
nerve cells by transcription factors (TFs) under a triboelectric stimulator
(TES) to regenerate and repair the nervous system [31]. AL and PDMS
combined with nano-power generation technology to form a
triboelectric-brain-behavior closed loop to control behavior. Zhong et al.
fabricated an olfactory detector consisting of eight sensing units of 0.6 0.6 cm2 in size. Polypyrrole (PPy)on the friction layer of each sensing unit
is mixed with different dopants or surfactants [29]. The chemical reaction between the gas and some polypyrrole derivatives in the environment is used to change the output current. Then, the left primary
somatosensory cortex of the mouse brain receives stimulation and causes
the mouse to turn right. In this way, the recognition of specific odors by
the mice was successfully used to simulate their behavior of avoiding
toxic gases (Fig. 6b).
5.2. Nerve regeneration and nerve injury repair
Neural tissue engineering (NTE), as an effective technology to repair
the nervous system, relies on the induction of stem cell differentiation to
promote the regeneration and recovery of the nervous system. It is
important to effectively induce therapeutic mesenchymal stem cells
(MSCs) to differentiate into nerve cells through biological/chemical/
physical factors [65]. This approach to neural regeneration not only
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Fig. 7. (a) TENG-based electrical stimulation increases the proportions of cells differentiating into neuronal cells [30]. (b) In vivo experiments, Encoding TFs into
neurons based on TES to achieve 14.7% differentiation rate [31]. (c) Control of nerve cell growth direction by BD-TENG generated electric field [32].
pressure sensor attached to the human chest corresponded to the periodic
respiratory motion signal (Fig. 8a). In addition, this sensor system based
on micro-sphere TENG can also be used for non-invasive medical diagnosis, such as wrist pulse signal detection of vital signs.
Compared with chest skin-applied respiratory motion sensors, noseor mouth-applied airflow respiratory sensors have less power output but
can be used for more abundant physiological information monitoring
such as respiratory temperature, humidity, and exhale molecules.
Wang et al. proposed an airflow-driven respiratory sensor based on
TENG principle. Using acrylic airflow tube embedded with flexible
nanostructured PTFE film that can vibrates periodically with beathing,
the output waveform of TENG has a corresponding correlation with
different breathing states [34]. Experimental data show that the amount
of charge transferred per unit time during respiration is highly correlated
with the volume of exchanged gas (Fig. 8b).
Zhang et al. developed a breathing-driven human-computer interaction system [37]. The team used airflow fluctuations from breathing to
drive a single-electrode mode TENG. In addition to sending control
breathing commands to allow human-computer interaction, the system
can also distinguish between normal and deliberate breathing states
(Fig. 8c).
were also selected as the triboelectric layer, but the combination method
adopted by the two groups of experimental teams was different. The
device used Cu electrodes and Kapton films as substrates, and PDMS
layers then bonded with the microcolumn structure Kapton (Fig. 7b).
Under periodic mechanical force, TES can output about 30 V and 280 nA
of current. The final rate of successful differentiation into neuronal cells
by metal wire output to the cell culture substrate was as high as 14.17%,
which is the highest efficiency of induced neuronal cells achieved so far
using the non-viral gene delivery method. This study demonstrates that it
is entirely possible to reprogram neurons by TF injection.
Zheng et al. proposed a biodegradable TENG (BD-TENG) that is
biodegradable in vitro and in vivo, and the degradability of the implantable TENG can be controlled by changing the encapsulation material
[32]. An output voltage of up to 40 V output was obtained in vitro by
modulating the proportion of constituent materials. BD-TENG was used
to apply an electric field of 10 Vm-1 to the culture medium of rat nerve
cells in the experimental group to regulate the growth direction of nerve
cells, which is of great value for neural repair. After 5 days of culture,
compared with the control group, it can be seen by observing the nucleus
and cytoskeleton in the experimental group that most of the neuronal
cells were in parallel with the electric field direction and grew well
(Fig. 7c).
6.2. Exhaled gas molecular detection
6. TENGs for respiratory system
Respiration is an important physiological process of exchange between the body and the environment, while exhaled breath molecules
can be used for various applications like testing of driving under the
influence (DUI), hazardous exposure, and disease diagnosis etc. Kim et al.
used a 3D printed triboelectric respiratory sensor (TRS) (Fig. 8d) not only
identified four respiratory motion patterns: strong, weak, long, and short,
but also successfully distinguished the state of inhalation and exhalation
by detection of CO2 concentrations [35]. This system realized the combination of human respiration monitoring and molecular detection and
showed the great potential for automatic respiration sensors based on
triboelectric effects.
6.1. Respiratory mechanics signal acquisition
It is a common method to detect the body's dynamic signals caused by
breathing through direct contact of the device with the skin of the chest,
abdomen, or throat. Liu et al. designed an ultra-sensitive system for
respiration monitoring based on the miniature sphere TENG pressure
sensor [33]. The triboelectric layer of the TENG pressure sensor was
made from thermally expandable microspheres in PDMS mixture, to
respond to pressure changes through changes of frictional contact areas
and surface charges. In the experiment, the output waveform of the TENG
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Fig. 8. (a) Respiratory detection system based on micro spherical TENG [33]. (b) Self-powered respiratory sensor driven by airflow [34]. (c) The human-computer
interaction breathing detection system based on TENG can distinguish between normal breathing and deliberate breathing [37]. (d) Analysis of respiratory data
collected by TRS [35]. (e) TSRS achieves distinguishing different respiration patterns using NH3 concentration [39]. (f) Quantitative detection of NO2 by AIMS based
on TENG [40].
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in water, in which TENG self-powered technology is driven by water
waves [41]. In the experiment, 50 arched TENGs elements with the size
of 15 cm 30 cm each were placed in parallel in a swimming pool with
periodic waves (Fig. 9a). The average output voltage of each TENG was
about 270 V and current was about 120 μA. This system achieved
removal rate of three model bacteria of 99.9999%. Moreover, the electrolysis process also has a high removal efficiency for mixed marine
algae.
Tian et al. designed a electroporation sterilization system using ZnO/
Ag nanoparticle brush electrodes TENG driven by water wave [42]. The
triboelectric layer made from rubber balls and aluminum foil can fluctuate with water waves and generate output voltage of 50 V and output
current of about 2 μA. The system showed efficient removal of E. coli,
Staphylococcus aureus and total colonies in natural river water (Fig. 9b).
DUI breath test is currently an effective means of detecting whether a
person is driving drunk. Wen et al. developed a self-powered alcohol
breath detector, whose power supply mechanism is a blow-driven TENG
(BD-TENG) [38]. The detector can achieve a fast response of 11 s, a fast
recovery period of 20 s, and a wide sensing range of 10–200 ppm. Xue
et al. developed an elastic olfactory electronic skin that can display
alcohol concentration in respiration driven by TENG [36]. It uses a PET
outer layer and Polyaniline (PANI)/PTFE/PANI nanostructure friction
layer, which combines nano-frictional electrical technology and
gas-sensitive properties to achieve a visual display of alcohol from
breath.
An increasing trend of ammonia (NH3) concentration in exhaled
breath is usually associated with the occurrence of diseases, such as
kidney diseases. Wang et al. developed a triboelectric self-powered respiratory sensor (TSRS) based on nanocomposite film for respiratory energy collection and detection of NH3 concentration [39]. ZnO with Ce
doping is used not only as a tribo-layer for TENG, but also as a sensing
material for detecting NH3. When the system is attached to the chest skin,
the mechanical energy of the chest's periodic motion can be harvested.
This system can also achieve real-time monitoring of breathing and differentiation of various breathing patterns such as normal, deep, shallow,
and fast breathing. In addition, the TSRS for identification of NH3 from
six other interfering substances in a simulated respiratory environment at
97% relative humidity (RH) was carried out, and the results showed that
TSRS has a good recognition ability for trace amounts of NH3 with good
anti-interference ability (Fig. 8e).
In addition to the testing of disease breath biomarkers and drunk
driving, breath testing is also used to detect toxic gases such as nitrogen
dioxide (NO2). Su et al. manufactured a TENG-based wearable alveolusinspired membrane sensor (AIMS). The AIMS in principle is a breathdriven single-electrode mode TENG that uses latex membrane as friction layer and NO2 sensitive WO3 decorated copper electrode as sensing
layer, the pressure change during respiration circle can cause the latex
membrane periodic contact with electrode, while the amount of NO2
bonding with WO3 can change the concentration of free electrons in the
sensing layer, thus altering the output waveform of TENG [40]. In
detection of 80 ppm of NO2, the response of AIMS is 340.24% and
linearity of 0.976. The AIMS also showed good anti-interference ability of
specific responses to NO2 in various interfering substances (Fig. 8f).
7.1.2. Antibiofouling activity
Shipping, coastal construction, oil pipelines and other industrial infrastructures suffer from biofouling, organisms such as bacteria, biofilms,
plants, or animals are easy to attach and grow on the wetted surface of
environmental objects, thereby corroding and accelerating the aging of
components. Superficial high-voltage EF can disturb the inherent distribution of organisms, thus achieving the purpose of inhibiting biological
adhesion.
Zhao et al. developed a triboelectric wave harvester (TEWH) by
combining high EF biofouling prevention and water wave-driven triboelectric harvesting techniques [43]. A test block was immersed in the
water tank, while part of the test block was connected to TEWH through
electrodes. The TEWH can generate up to 300 V output voltage from
waves and such high EF can significantly protect the test block from
microbial adhesion (Fig. 9c). The anti-adhesion efficiency of E. coli,
positive-gram bacteria S. aureus, and diatoms (Bacillariophyceae) was as
high as 99.3%, 99.1%, and 96.0%, respectively.
7.1.3. Postcharge disinfection
Tian et al. used the ball-ball TENG as the power supply of the ZnO/Ag
electrode brush to carry out the sterilization experiment [42]. It was
unexpectedly found that the system could also achieve bacterial disinfection within dozens of minutes after the TENG power supply was
turned off(Fig. 9b). On the one hand, after stopping charging, disinfection is found in the replaced new solution, so the substances produced
during the charging process of TENG can be excluded. On the other hand,
if the electrodes are not charged by TENG, they have only a basic
disinfection capacity. So how does the AC signal, which is not rectified to
direct current (DC) but output directly to the electrode brush, achieve the
post-charge disinfection function? This remains to be further cognitive
learning of AC signals and capacitive materials.
7. Others
7.1. TENGs for disinfection
With the acceleration of population growth and industrialization, the
scarcity of clean water resources has become a worldwide problem,
especially in developing countries. Numerous gastrointestinal infections
spread through microbial contamination of drinking water. On the other
hand, additional nutrients from industrial discharge and farm runoff
water supply promote algal blooms, cause a large consumption of dissolved oxygen in the water, suffocate other aquatic organisms, and pose a
huge ecological risk [65]. Disinfection has been widely used in production of drinking water, food, and dairy products, but the cost and convenience still need to be innovated and strengthened.
7.2. TENGs for tissue repair (bone repair and wound repair)
The bone injury requires a long time to recover, the proliferation and
differentiation of osteocytes is the main physiological factor of this process. How to make osteocytes proliferate and differentiate stably and
effectively is the main problem at present. It has been found that a
suitable dose of physical stimulation like light or electrical stimulation
can promote the bone repair process. Because the repair process is relatively long and the system needs to operate in vivo for a long time, the
TENG self-powered devices have great potential for applications in this
field.
Tang et al. developed an implantable self-powered low-level laser
cure (SPLC) system that can promote the proliferation and differentiation
of mouse embryonic osteoblasts (Fig. 10a) [44]. The SPLC consists of a
flexible TENG and an in vitro laser unit. The friction layer of the TENG
made from PDMS film and indium tin oxide (ITO) and is fabricated into
an arch to accommodate the flexural motion of the knee joint. The TENG
was implanted between the mouse diaphragm and the liver. When the
diaphragm moved, the TENG generated an output of short-circuit current
7.1.1. Microbe inactivation by electroporation
The principle of high electric field disinfection is that when the
electric field (EF) applied to living cells exceeds 106 Vm1, irreversible
electroporation occurs, and the cell membrane will be destroyed. Such
sterilization method is efficient and fast, does not produce by-products,
and is very suitable for food processing. Combined with nanotechnology, effective microbial disinfection can be achieved at a voltage
below 100 V, thereby reducing the production cost of high voltage
sources in EF disinfection applications.
Jiang et al. developed a self-powered electrochemical water treatment system combined with algae removal and sterilization technology
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Fig. 9. (a, b) Electroporation for sterilization powered by TENG [41,42]. (c) Inhibition of biological adhesion activity by TEWH [43].
deposition in MC3T3-E1 cells, and then promoting the proliferation and
differentiation of osteoblasts. The group also observed increased
expression of the intracellular Ca2þ concentration and ALP, two markers
of osteogenic differentiation, hint the effectiveness of bone repair promoted by electrical stimulation.
When the integrity of the skin is compromised, the wound generates
an endogenous electric field that initiates a series of physiological reactions to seal the wound. The presence of an endogenous electric field in
the wound can promote all stages of wound healing. For wounds that do
not heal on their own, exogenous electric fields can assist. In recent years,
TENG-based electrical stimulation therapy devices have been constructed for wound healing, and the feasibility has been verified at the
cellular and animal levels.
Long et al. developed a TENG-based electronic bandage for wound
healing (Fig. 10d) [48]. The mechanism is TENG generated small electrical pulses lead to cells around the wound migrating and proliferating
and differentiating. The experimental equipment locally converts the
kinetic energy generated by the mouse's breathing into a discrete alternating voltage, which overlaps the wound with the electric field and
promotes skin regeneration at the wound site. The study showed that the
healing time of the experimental group with the electronic bandage was
only 3 days compared with 12 days in the non-intervention group, and
the low level and safe current produced also partially reduced the pain
and discomfort of the patients.
Jeong et al. designed a fully extendable TENG patch based on
hydrogel assembly for skin wound healing (Fig. 10e) [47]. The principle
is that electrical stimulation can induce charged ions to move through
epidermal ion channels and destroy the transepithelial electrical potential to induce endogenous electric field formation. The dermal cells such
as keratinocytes, endothelial cells, and fibroblasts at the edge of the
wound are guided to migrate to the center of the wound, to repair the
wound skin. For normal human skin fibroblasts, the migration rate of the
of 0.06 nA and an open-circuit voltage of 0.2 V. The electricity then was
collected and used to drive the laser unit. The level of alkaline phosphatase (ALP) in the TENG laser therapy group increased by 16.9%
compared with the non-intervention group, which gave rise to bone
matrix synthesis and MC3T3-E1 extracellular matrix maturation. TENG
laser irradiation increased mineral deposition in MC3T3-E1 cells, indicating that TENG the laser therapy system can promote osteoblast proliferation, differentiation, and bone formation.
Yao et al. developed an implantable biodegradable self-powered
electrical stimulation fracture healing device consisting of a TENG and
a pair of dressing electrodes(Fig. 10b) [45]. Even on irregular tissue
surfaces, the device can still attach and treat, showing a good flexibility.
The device can generate steady biphasic electrical pulses that stimulate
MC3T3-E1 cells to proliferate and repair the bone. Fracture recovery was
achieved within 6 weeks with electrical stimulation. Compared with the
non-intervention group, the bone mineral density and flexural strength of
stimulation group increased by 27% and 83%, respectively. For the in
vitro experiment, the whole device degraded in 18 weeks after rapid
autocatalytic hydrolysis. Degradation observed after 14 weeks implanted
in vivo, but the degradation rate is highly dependent on the dynamic
internal environment of the animal.
Tian et al. designed a self-powered TENG-based electrical stimulation
device (Fig. 10c) to stimulate the proliferative differentiation of osteoblasts, thereby enhancing bone formation and healing [46]. The device
consists of a triboelectric nanogenerator, a rectifier, and forefinger
electrodes. Al and PTFE were used as the friction layer of TENG, Au and
Al were used as passive and negative electrodes respectively. TENG was
implanted on the surface of the rat femur, and the mechanical energy
generated by rat movement was converted into electrical energy, which
could be used for bone in situ therapy. The rectifying pulse direct current
stimulation delivered by the device significantly improved the adhesion
and proliferation of MC3T3-E1 cells, resulting in increased mineral
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Fig. 10. (a) TENG powered system to stimulate proliferation and differentiation of osteoblasts [44]. (b) A biodegradable device works by electrical stimulation to
healing fracture, bone mineral density and bone bending resistance of mice were improved and returned to normal level [45]. (c) TENG promote bone repair through
electrical stimulation [46]. (d) Electronic bandages produce small pulses of electricity to promote wound healing [48]. (e) A patch consisting of hydrogel and TENG
was used for wound healing [47].
Meanwhile, using mesh connection material that combines memory recovery and malleability to secure the internal components, and to reduce
the safety risk by preventing internal response hardware from misaligning [69].
For the safety issue, besides from existing fillet and welding process,
implantable devices can be coated with materials with high cushioning
and good biocompatibility to reduce collision damage. Wearable devices
need to use non-slip and flexible materials to adapt the scenario with
sweat or irregular regions.
For the controllability issue, in addition to developing intrinsically
flexible and scalable materials with low Young's modulus, providing
tissue-level soft mechanical properties, and eliminating surface barriers
between electrodes and organisms [70], stable fixation of the device is
also a major issue. Physical fixation within the body is hard to accomplish, substances for adsorption generated in situ through chemical reactions is another option, but its biosafety is another issue worth
considering.
TENG group was about 3.5 times higher than that of the control group,
and the wound healing rate of the TENG therapy group was about 3 times
faster than that of the control group, which could safely and effectively
heal the surface wounds.
8. Challenges and future perspectives
8.1. Body-device interface
Stability and controllability of medical devices are critical in clinical
practice. Stable and effective interface can maximize the efficiency of
medical devices. Both the skin and soft organs are flexible, the medical
devices must conformally integrated with those flexible interfaces to
minimize physical damage to the body and ensure the stability of bodydevice interfaces. In practical diagnosis and therapy, the incompatibility
interface between flexible human body and rigid medical devices
weakens the credibility of the acquired physiological data. A medical
device with higher conformal integration to the human body can increase
the success rate of the body-device interfaces, improve the efficiency of
data transmission and energy conversion, and reduce the risk of data
distortion due to mechanical mismatch [66]. Currently, various issues
such as integration, safety, and controllability are facing challenges. To
address these issues, there are several potential solutions:
For integration issue, we need to flexibly select different TENG
operating modes based on the application scenarios, and use corrosionresistant and strong sealing materials for packaging [67,68].
8.2. Miniaturization
Thanks to the continuous and high efficiency output of TENG, conventional batteries may eventually be replaced by self-powered devices.
The heart of the issue is the mass and volume of energy harvesters in selfpowered devices. The choice of material can determine the size of the
energy harvesters to some extent. The sizes of four TENG operating
modes are vary with usage scenarios, so the optimization of energy
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8.5. Standardization
harvesters should also consider the actual situation.
To meet the miniaturization, the output performance may be drastically reduced. To meet the high output, the device size may be too large
to meet the actual requirements. It may be necessary to use mathematical
modeling to analyze the balance between the size and output. Some
studies of TENG and PENG confirmed that the output performance of NG
is not always proportional to the size, and they also have the potential for
further miniaturization [71].
In addition to solving the size issue of energy harvesters, we can try to
design new self-powered devices or apply new structures while maintaining the current output power. Advances in every field, from circuits
to materials to manufacturing processes, could lead to better solutions for
smaller medical electronics.
The main principle of evaluation and standardization of self-powered
medical devices is to improve the quality and performance of existing
medical devices, prevent the abuse of unnecessary, low-quality applications. The major standards should include:
8.5.1. Output
The acquisition of clinical data relies on the good electrical performance of the device. The output performance, such as output voltage
(VOC), output current (ISC), energy density, output power, and energy
conversion rate etc. need to be standardized, this is not only for needs of
current devices, but also for future self-powered technology upgrades.
High-quality output performance is the fundamental of further development and iterations of devices.
8.3. Power management
8.5.2. Connection
The choice of connection material depends on the structural and
functional requirements for the connections of components within the
medical devices. Considering the service life of the medical devices, the
connecting parts need to have excellent properties like mechanical strain
properties [68], corrosion resistance [67], stretchability, strong rigidity,
etc. Connections for clinical use need to be conspicuous and easily
identifiable during surgery. Overly complicated connection may interfere
with surgery, therefore additional evaluation for this situation is
required. The regulation of electronic connections may limit the selection
of components, but also maximize overall performance of the system.
Reasonable and effective power management can prolong the working life of medical devices and preventing the deterioration of service life
caused by long-term high-power operation. Here we present some power
management strategies.
The first strategy is to harvest unstable periodic native energy. For
instance, the AC output generated by the TENG's periodic operation
cannot directly power medical device. Instead, it should be stored in a
capacitor or battery then fed into the device. It is worth noting that
impedance mismatches between energy storage and devices can result in
energy losses, decreased transmission efficiency, or even circuit failures
[72]. The impedance mismatch may also cause reflection, especially for
digital signal, which makes the signal to overshoot and hook, interfere
with its normal reception, and destroy the signal integrity. It must be
carefully considered when developing the power management system for
TENG.
The second strategy is to plan energy consumption according to
biorhythms. The energy production and consumption of human body
varies from day to night, so the energy gains of the entire system change
over time. After learning the energy consumption by the human body and
devices in different time periods, artificial intelligence algorithms can be
built and used to plan the release of energy stored in capacitors or batteries. Dual power supplies with different rhythms work collaboratively
in a planned way, switching on and off alternately to meet the actual
energy consumptions, thereby extend the working life of energy storage
and the devices [73]. But the layout must be further modified to meet the
miniaturization requirements.
8.5.3. Implantation correlation
Implantable devices typically remain in the body for ten years or more
[76]. The immune response to foreign implants usually causes local or
systemic inflammation, making the internal environment unfavorable for
device operation. Therefore, the biosafety and biocompatibility of medical devices need to be regulated and standardized. Implantation sites
and fixation method must be carefully evaluated before the surgery, to
reduce the immune response related side effects.
8.5.4. Data handling
Data handling is important for both wearable and implantable devices. It not only reflects a device's processing power but also the device's
dependability. Large amount of data is not only indispensable for training
accurate processing algorithms, but also crucial for building big data
handling schemes for diseases. Utilizing machine learning [77] and deep
learning [78] algorithms, the accuracy and processing speed of clinical
big data will be largely increased, and future applications of clinical big
data will benefit from the high throughput, high accuracy and real time
individual data gathered by wearable and implantable medical devices.
The clinical big data algorithms can identify the specific patterns of
suspected etiology and degree of diseases, and it is particularly valuable
in management of cardiovascular and neurological diseases. However,
standards must be established for the collection, storage, and use of
clinical big data to comply with the regulations of medical ethics and
privacy protection.
8.4. Wireless technology
In the past, it was nearly impossible for wearable and implantable
devices on different body parts to communicate with each other.
Currently, wireless technology solves the problems of wired connections
like discomfort and infection, also improves the accuracy, frequency, and
distance of communication. In the future, wearable and implantable
medical devices may work together under the body area network, this
puts forward higher requirements for wireless technology [74]. However, issues with current wireless technology limit its application in medical
devices, such as energy consumption, communication distance, interference and shielding etc. Integrated with wireless technology, medical
devices could take a giant step towards more accurate understanding of
the disease through imperceptibly real-time data acquisition and analysis. With continuous optimization of internal circuits, signal transmitters, antennas, and communication networks, the next generation of
self-powered medical devices utilizing TENG and PENG could have
more accurate and efficient signal transmission with lower distortion
rate. With development of high-speed cellular network such as 5G, it is
possible to use artificial intelligence algorithm in the cloud to review and
analyze patient data in real time using, and formulate data-driven
patient-friendly treatment plans [75].
8.6. Closed-loop system design
As the name suggests, a closed-loop system is a system that run
cyclically, continuously, and automatically. Closed-loop systems in
medical devices, such as the TENG application in cardiovascular diseases
consists of three major parts: energy harvesters or converters, output
circuits, and cardiovascular electronic devices (CEDs). The system is
designed to expand limited energy supply of conventional CEDs, making
them more effective in monitoring and identifying cardiovascular
symptoms.
Typical CEDs, such as temporary cardiac pacemaker, often face issues
like uncomfortable or infection. It has been demonstrated in a study that
15
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Medicine in Novel Technology and Devices 16 (2022) 100195
interests or personal relationships that could have appeared to influence
the work reported in this paper.
a transient wireless closed-loop system can resolve these issues with
excellent monitoring and data-feedback capabilities [79]. This study
demonstrates the feasibility of implantable closed-loop system and offers
a potential solution to problems that NG-based implantable devices may
be encountered in the future.
In the treatment of neurological diseases, the closed-loop system is
used more frequently. Nervous system precisely controls the work of
human body through a natural closed-loop system that consists of a
receiver, algorithm, and effector [80]. Medical devices can convert
multiple biophysical (temperature, flow, pressure, etc.) and biochemical
(metabolites, proteins, hormones, etc.) stimulations into electric signal
and utilize algorithms to perceive the environment and regulate the internal environment of human body through various drives (e.g., optical,
electrical, pharmacology), to intervene and correct patient homeostatic
imbalances. The bionic working principle provides framework for future
closed-loop systems of medical devices.
Acknowledgement
Y. Q., X. W. and S. Z. contributed equally to this work.
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9. Conclusion
Compared with conventional medical devices, TENG-based selfpowered medical devices are more intelligent, efficient, and accurate.
The application of TENGs in medical devices bring longer service life,
more effective intervention methods, and more humanized data presentation. For patients, TENG-based self-powered medical devices can
lower the financial costs and physical pain of implantable battery
replacement. In clinical practice, TENG-based self-powered medical devices can participate in the entire process of clinical diagnosis and
treatment. Utilizing TENG-based self-powered medical devices for preoperative data diagnosis, intraoperative data monitoring of internal
environment changes, and postoperative monitoring of recovery status,
healthcare practitioner can gain a clearer picture of physical condition of
patient and make clinical decisions more accurately. In the future, the
clinical application of TENGs will be more diversified. TENG-based selfpowered wearable and implantable medical devices can be applied to
more parts, muscles, bones, and other tissues for repair and treatment.
The possibilities are endless.
Author statement
Yichang Quan: Conceptualization, Methodology, Data curation,
Writing- Original draft preparation, Visualization. Xujie Wu: Conceptualization, Methodology, Data curation, Writing- Original draft preparation, Visualization. Simian Zhu: Conceptualization, Methodology,
Visualization, Writing- Reviewing and Editing, Funding acquisition,
Project administration. Xiangyu Zeng: Funding acquisition, Supervision,
Investigation, Project administration. Zeng Zhu: Supervision, Validation,
Project administration. Qiang Zheng: Conceptualization, Methodology,
Visualization, Writing- Reviewing and Editing, Funding acquisition,
Project administration.
Funding sources
This work was supported by the National Natural Science Foundation
of China (82001982 to Q. Z.), The Science and Technology Fund of
Guizhou Provincial Health Commission (gzwkj2022-444 to X. Z.), China
Postdoctoral Science Foundation (2021M700974 to S. Z.), Guizhou
Provincial Natural Science Foundation (ZK[2021]475 to S. Z.), Natural
Science Foundation of Education Department of Guizhou Province (KY
[2021]176 to S. Z.), Science Foundation of Guizhou Medical University
(J[2020]022 and 20NSP057 to S. Z.), College Students Innovation and
Entrepreneurship
Training
Program
of
Guizhou
Province
(S202110660052 and S202210660029 to S. Z.).
Declaration of competing interest
The authors declare that they have no known competing financial
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