Healthcare management applications based on triboelectric nanogenerators

Since the beginning of humanity, taking good care of one’s health has been an ever-evolving requirement for a comfortable lifestyle. In the modern age of new technologies, research focus has been greatly on healthcare management. The healthcare-related spectrum covers real-time data acquisition; early diagnosis; and quick, efficient, and robust treatment systems. Major medical interests revolve around the diseases and conditions that are very common among the human population, such as cardiovascular diseases, respiration anomalies, obesity, diabetes, musculoskeletal disorders, and pain.^1–5 Significant primary parameters related to these conditions are the heart rate, pulse, body temperature, blood pressure, and respiration rate. Proper measurement and analysis of these para-meters might be helpful in sensing health irregularities. However, the monitoring and treatment systems are desired to be cost-effective, be minimally invasive, have a long working life, have small footprint, be wearable, and be degradable once their jobs are done after being implanted in the human body.^6–11 

The triboelectric nanogenerator (TENG) was invented in 2012.¹² Since then, it has gained an overwhelming admiration by the researchers working in various research areas. Basically, it is a device that converts mechanical energy into electrical energy using a combination of the triboelectric effect and electrostatic induction.^9,13,14 Human body motion from various organs and parts is a rich source of energy and has been utilized to develop self-powered TENG healthcare devices (see Fig. 1).¹⁵ TENG has been used in multiple ways to improve healthcare: (1) an energy harvesting device to power the external health monitoring sensors or devices,¹⁶ (2) self-powered sensors without using any external power source,¹⁷ and (3) self-powered electrical stimulators to treat a human condition.^18,19 TENGs are revolutionary devices having a number of remarkable qualities, such as low-cost, low-frequency operation, environment-friendly, easy to fabricate, lightweight, long-life, flexible, stretchable, washable, self-powered, self-healing, biocompatible and biodegradable, multiple operation modes, and easy integration.^20–33 All these brilliant properties of TENG make it an incredible technology to be utilized in healthcare management taking full advantage of its potential. However, there are some concerns about employing these TENG devices in such delicate healthcare applications. The very first challenge is about materials used to fabricate these devices. They can be harmful to humans if not selected carefully. This problem has been solved smartly by switching to synthetic and natural biocompatible and biodegradable materials.^34,35 A pressing issue with TENG is that it has insufficient electrical output to be beneficial. One of the few methods adopted to resolve this issue is electrode mapping for muscle stimulation,¹⁹ in which the electrode’s strategic design helps augment the effect of low current output. In another instance, smart device design using interdigital electrodes generating high electrical fields have been successfully used for the differentiation of osteoblasts.³⁶ 

In this Research Update, we have classified triboelectric-based healthcare management into three broad categories, as depicted in Fig. 2. In Sec. I, triboelectric techniques used to protect human health from external factors are briefed. Section II covers the sensing and monitoring applications based on the TENG devices. Section III is compiled of various healthcare treatments to improve multiple health conditions. Finally, we have concluded the overall discussion finding a way forward for TENG in healthcare management.

Due to progressive industrialization and advancements, our environment is filled with numerous contaminants that may be hazardous for living beings. Consequently, any medical condition or disease arising because of these pollutants, either curable or not, brings some sort of discomfort with it. As the old saying goes, prevention is better than cure, TENG has been used to develop a number of useful protective devices to minimize the harmful effects of such environmental toxins. Some of these applications are presented as follows.

Rapid industrialization and urbanization have been a major cause of air pollution in our cities. It is well-established that fine dust particles (PM10) in air have caused an increased risk of the death rate.³⁷ Ultrafine dust particles (PM2.5) cause respiratory disorders, such as asthma and lung inflammation.³⁸ Personal protection from such pollutants is the need of the hour. Triboelectric technology has provided solutions for this chronic problem in the form of personal protection equipment (PPE). Liu et al. have designed a self-powered electrostatic adsorption face mask (SEA-FM) using TENG to protect a person from the hazardous effects of fine and ultrafine dust particles.³⁹ The SEA-FM consists of a poly(vinylidene fluoride) electrospun nanofiber film (PVDF-ESNF) and a respiration-driven TENG (R-TENG). After installing this contact–separation mechanism into a multi-layer face mask, inhalation and exhalation during breathing drive the charge generation [Fig. 3(a1)]. Therefore, charged fine and ultrafine particulates get absorbed by the electrostatic charge saturated PVDF-ESNF layer. However, uncharged particulates are first charged by the PVDF-ESNF and then absorbed deep into the membrane. Even in such low-pressure drop conditions, R-TENG results with the removal efficiency of the coarse and fine particles more than 99%, while it remained at more than 86.9% for ultrafine particles since the mask was worn for 30 days, 4 h each day to prove its long-term service life. SEA-FM might lead toward providing a practical solution of a self-powered and wearable medical device for better health.

Viruses are another kind of particle that may travel through air. The recent pandemic of coronavirus (SARS-CoV-2) has caused millions of deaths across the world due to its respiratory consequences.⁴⁰ Wearing a mask is one of the prevention methods used to reduce the spread of this virus. Recently, a group of scientists from India and Russia has proposed a self-powered triboelectric face mask that can kill the SARS-CoV-2 virus by electrocution.⁴¹  Figures 3(b1) and 3(b2) show that 20 × 16 cm² sized body motion-activated face mask is composed of two main parts. First, a triboelectric (TE) filter is fabricated by sandwiching a tribo-positive layer between two electrode-separated tribo-negative layers for charge generation. The second part consists of two conductive meshes, separated by a 5 µm polypropylene layer to form an electrocution layer (EL). The TE filter and EL are electrically interfaced via a voltage tripler circuit unit (VTU) and a capacitor. TENG is being used to improve energy per cycle, total stored energy, and charging efficiency, instead of using full-wave rectifiers to charge the storage capacitor. The EL gets self-activated through the breathing cycle of the wearer due to the multilayered design of the TE filter. Human exhalation and inhalation, talking, and other facial movements cause to charge triboelectric materials due to the vertical contact–separation mode of TENG. It produces an electrical field in the mesh of the polypropylene electrocution layer (EL). When external virus-carrying droplets make contact with the electrically connected EL, they sense a slight shock, and the external protein of the charged viruses instantly deactivates due to current and heat. However, the generated heat is not high to disturb the respiratory system of the mask-wearing person. The latex rubber-PU pair is found to be the best among the four tested combinations to get the highest triboelectric charge density (TECD) and power induction [see Fig. 3(b3)]. In short, this face mask can be a design with great potential because of its cost-effective and self-powered nature.

An average adult human body is composed of more than 70% water.⁴² Among the other various nutrients, water is an essential requirement for humanity to survive. However, 2.2 billion people worldwide do not have access to clean drinking water, according to the 2017 WHO report.⁴³ Water-borne diseases, such as cholera, typhoid, hepatitis A and E, and many others, are responsible for 4500 deaths/day of children under 14 years of age globally.⁴⁴ An affordable and one-stop water cleaning solution is desired to provide clean water; therefore, Ding et al. devised a low-cost water disinfection system using the TENG.⁴⁵ The hand-powered TriboPump system is comprised of three parts [see Fig. 3(c1)]: a tubular coaxial-electrode copper (Cu) ionization cell (CECIC) used as the disinfection device, a disk TENG (D-TENG) used as the power source, and a coaxial mechanical structure that includes the water pump. Disk-shaped D-TENG utilized a rotational energy harvesting concept using the stator and the rotator [see Fig. 3(c2)]. The Cu layer on the stator and rotator is radially patterned in sectors to enhance the performance while operating. When the Cu rotator starts moving, it gets positively charged while the PTFE layer on the stator becomes negatively charged due to inherent different triboelectric polarities after numerous friction cycles. However, the positive charge density on the Cu rotator becomes double the negative charges on the stator PTFE layer to satisfy the law of charge conservation. Electrical current will start flowing once a load gets connected between the two output electrodes of the stator. D-TENG is connected to the water pump via a handle-operated gear transmission to start rotation and ultimately water disinfection. D-TENG is designed to keep the current constant against varying loads (i.e., water quality) without any additional power management circuits. Four strains of bacteria were removed from the river water when TriboPump was operated at 60 rpm. This hand-operated system is very affordable as low as $10 and can serve for two years. This water disinfection system is most suitable for rural neighborhoods with no access to clean drinking water and electrical power supply.

Electromagnetic radiations (EMRs) emitted from electromagnetic equipment around us may affect human health, especially affect body metabolism and hormone secretions, weaken the elderly, cause childhood leukemia, as well as may pose a risk to pregnancy.^46–49 Zhang et al. developed a TENG-based wearable hybrid NG electromagnetic shielding device (ES-HNG).⁵⁰ The portable and self-powered device can monitor health by detecting and harnessing abdominal motion while simultaneously protecting it from the EMR. The single-electrode mode TENG layer (700 µm) of the stretchable ES-HNG (12 × 3 cm²) is composed of Ag-coated fiber woven conductive anti-electromagnetic radiation fabric (AEMF) sandwiched between two layers of rubber. The TENG layer works on the principle of contact electrification and electrostatic induction. Rubber is triboelectrically more negative than the human skin. During the abdominal motion, once the skin starts moving away from the rubber layer of ES-HNG, both layers get positively and negatively charged, respectively, causing an electrical current flow from the ground toward the AEMF. Similarly, current starts flowing back from the AEMF to the ground when the skin moves away from the rubber layer. The fabrication of ES-HNG is illustrated in Fig. 3(d1). A stretchable and human abdomen motion powered larger area ES-HNG (55 × 35 cm²) with two smaller pyroelectric-piezoelectric NGs (PPENGs) (6 × 3 cm²) is employed to monitor breathing motion and provide protection from electromagnetic radiations [see Fig. 3(d2)]. More than 99.9978% of the electromagnetic waves of 0–1.5 GHz frequency range can be shielded with a stretching capability of ∼40% for the wearing comfort. Very high shielding efficiency and broad frequency range make ES-HNG very useful for a range of electromagnetic shielding applications, especially in health monitoring systems.

Early or timely diagnosis of a medical condition can be found useful and even vital in many medical cases. It may help manage, control, or treat the diseases before causing any irreversible damage. TENG has introduced multiple wearable and non-invasive methods to monitor and analyze medical parameters in real-time. Its applications extend to the heart, respiration, body movements, and many other circumstances, and a few of them are briefly described below.

According to the World Health Organization (WHO), cardiovascular anomalies are the leading cause of death worldwide.⁵¹ Meanwhile, air pollution has caused various respiratory issues causing chronic respiratory diseases, such as asthma, allergies, lung diseases, and pulmonary hypertension.^52,53 Therefore, it is enormously vital to keep track of cardiac and respiratory parameters in real-time, especially heart rate and rhythmic motion, to diagnose underlying diseases. It will be great to have a single solution to tackle both of these challenges. Ma et al. have proposed a self-powered, flexible, and implantable triboelectric active sensor (iTEAS) for continuous monitoring of heart and respiratory conditions.⁵⁴ The iTEAS consists of multiple purpose-specific biocompatible materials: (1) a nanostructured PTFE (n-PTFE) as a negative triboelectric layer, (2) an elastic Kapton substrate, (3) an ultrathin Au electrode, (4) an Al film used as a triboelectric layer and electrode, and (5) an elastic titanium strip over Kapton film to ensure contact and separation. Contraction and relaxation of the heart caused a contact separation between n-PTFE and Al triboelectric layers. This repetitive motion results in alternating current (AC) output signals via coupling contact electrification and electrostatic induction. The self-powered, core–shell packaged iTEAS is implanted into the pericardium [see Fig. 4(a1)] of a living swine while bidirectionally harvesting the motion of heart and respiration using the contact–separation mode of TENG. Multifunctional iTEAS is employed to continuously monitor the heart and respiratory rate and arrhythmia and to measure the blood pressure and the blood flow velocity while using an arterial pressure catheter. Because of a thin-film structure, the iTEAS becomes an ideal device for implantation into the body with minimally invasive surgery with up to 99% accurate heart rate monitoring. However, its application may be limited as the device has to be implanted in the human body to work. Recently, a noninvasive, self-powered ultrasensitive pulse sensor (SUPS) was introduced by Xu et al. to observe cardiovascular activities.⁵⁵ The multilayer pulse sensor is mainly composed of two elastic melamine sponge (MS) separated triboelectric layers, namely, fluorinated ethylene propylene (FEP) and polyamide (PA), along with other layers to ensure high measurement sensitivity and electrical output while keeping the device stable [see Fig. 4(a3)]. In addition to the thin and flexible multiplayer structure, the porous structure of MS enhances the sensitivity of SUPS to detect weak dynamic pressure of pulse. SUPS also works on the same principle of triboelectrification and electrostatic induction coupling as the other device in this section for heart and respiratory monitoring. Once attached to the desired artery, pulse movement drives the surface electron transfer from the PA layer to the FEP layer due to triboelectrification. As a result, an electrical potential between two triboelectric layers produces an alternating current (AC) output. Upon periodic low-pressure loading, the sensor results in an ultrasensitive electrical output with a fast response time of 30 ms. SUPS is found to be capable of generating 0.7 nA current against a very low weight of 5 mg, indicating its low detection limit. Sensor-obtained human heart rates (HRs), pulse transit time (PTT), and pulse wave velocity (PWV) are compared with ECG and other medical instruments to be found consistent. SUPS is the latest addition in noninvasive solutions to accurately monitor cardiovascular systems with potential in future intelligent medicine.

The most common cause of elderly injuries is falling, leading to disability and low quality of life. Each year, around every third person over the age of 65 years experiences fall.⁵⁶ Keeping it in view, triboelectric technology is applied to design a self-powered and cost-effective fall detection system by Jeon et al.⁵⁷  Figure 4(b1) depicts a 15-cell TENG electrically connected detection array (3 × 5) in which each cell is composed of a PTFE thin film sandwiched between top and bottom Al electrodes, packaged with Kapton tape to give every cell an arch-shape. Upon pressing any cell, positive charges are generated due to the triboelectric difference between PTFE, causing a charge flow from the top electrode to the bottom because of Al layers. Similarly, the electrical current changes its direction when both layers start restoring to their original positions due to the arch-shaped structure of the TENG. This specific shape of cells helps to effectively utilize contact–separation TENG pressure sensing with no external power source. Upon cell pressing, electric signals are generated and classified through a processor as falls or not falls. A 15-cell connected TENG array has been smartly used to cover the positioning of the body fall. The fall detection system is tested to discriminate among falls and daily activities that resulted in a very high accuracy of >95% keeping a time window of 0.5 s and training/test ratio at 56/40. A direct application of such a system can be in smart homes and smart hospitals to facilitate elderly management centers.

In the current era of fast growth, sleep quality has been undermined due to changing lifestyles. Poor sleeping behavior can lead to various diseases, including but not limited to depression, obesity, cardiovascular diseases, pulmonary diseases, and diabetes.⁵⁸ Lin et al. reported a self-powered, textile-based TENG array for sleep monitoring.⁵⁹ A 2 m by 1.5 m, washable three-layered bedsheet is designed by sandwiching a wavy-shaped PET thin film between two perpendicular Ag-coated conductive cotton fiber layers adhered to a fabric [see Fig. 4(c1)]. The PET layer has negative triboelectric properties, while the conductive fibers are positive. Therefore, when these two layers are contacted and separated, the conductive fibers become positively charged and the PET becomes negatively charged. Torso movement is the source of the external force, which presses the wavy PET layer to change its contact area with the two conductive layers to produce an electrical potential difference. It causes an electric current to flow from the conductive fibers to the ground. Once the force is released, a reverse current will flow from the ground to the conductive fibers. Pressing and releasing the smart bedsheet by external pressure produce electrical signals to observe sleeping behavior patterns with superb pressure sensitivity (0.77 V Pa⁻¹) and fast response time (<80 ms). A sleep quality report, including body position, posture, and pressure distribution, is also generated for an actual sleeper for a 9-h sleep indicating weak, light, and deep sleep patterns over time [see Fig. 4(c2)]. Moreover, the smart textile also showed that it can act as a self-powered warning system for emergency conditions, such as falling down from the bed [see Fig. 4(c3)]. This durable, washable, simple, and highly integrated smart textile-based sleep monitoring system is practically suitable for massive production. It is expected to impact various research areas, such as motion tracking, remote wireless healthcare service system, and tactile sensing.

Walking behaviors may reflect upon the health condition of a person. Observing and analyzing human gait can help in the reduction of injury risk and arthritis⁶⁰ and reducing additional knee surgeries⁶¹ among many other health hazards. Considering the high value of walking patterns, Lin et al. came up with an excellent idea of a TENG-based real-time smart insole to monitor various aspects of human gait [see Fig. 5(a1)].⁶² The sensing device is installed in the front and rear position of the insole of the shoes. The contact–separation mode TENG section of the two-part device consists of a convex Cu-integrated rubber environment shielding layer and a Cu bottom layer. The second layer of the sensor, named as “elastic air chamber (EAC),” is made of latex and acrylic films and serves as an elastic chamber for temporary air storage from the TENG part upon external pressure. The TENG section works in contact–separation mode employing one tribo-negative layer (rubber) and the other tribo-positive layer (copper). Following triboelectrification and electrostatic induction concepts, electrons flow from the copper film toward the ground due to a change in the electrical field as soon as the charged rubber film moves toward the copper film as external pressure on the insole is applied. Once the pressure is off, the electrical current starts moving back in the opposite direction toward the copper film. When a periodic external load ranging from 5 to 40 N is applied on the device, 7–35 V output is generated in a short time of 56 ms, indicating its sensitivity and quick response. A real-time gait monitoring system is devised that comprises a couple of TENG-based smart insoles and a data acquisition system coupled with analyzing software. Typical gait behaviors, such as stepping, walking, and running, are monitored and well-distinguished using the system. Additionally, the application of the device in sport training is also established by observing various motions of an athlete during a triple jump, making the use of expensive visual monitoring equipment redundant [see Fig. 5(a2)]. This wearable device may contribute to the fields of activity recognition and remote healthcare monitoring.

Sweat is a human bodily secretion dependent on various physiological conditions, and analyzing it may help diagnose life-threatening diseases, such as cystic fibrosis, diabetes, and cancer, using multiple biomarkers.^63–65 Recently, Song et al. developed a TENG-based wireless sweat sensor that does not use any external power source to work beside human motion.⁶⁶ A freestanding-mode TENG (FTENG)-powered wearable sweat sensor system (FWS³) is composed of various purpose-oriented segments, such as a sweat sensor patch and a flexible FTENG sensor connected with circuitry [see Fig. 5(b1)]. The flexible PCB-fabricated FTENG sensor is composed of a gold interdigital electrode sandwiched between polyimide and PTFE layer and a grating-patterned copper slider. FTENG works by coupling the contact electrification and in-plane sliding-induced charge transfer. Electrons gather on the PTFE surface during the interfacial sliding process due to its tribo-negative nature, as compared with copper. The one-directional sliding process produces an electrical current between the stator electrodes until the grating slider fully covers the second stator electrode. The FTENG stator is used for walking energy harvesting and faster storage in a capacitor while integrated with a power management integrated circuit (PMIC), a low-dropout voltage regulator, instrumentation amplifiers, and a Bluetooth low energy (BLE) programmed system on a chip (PSoC) module to acquire, regulate, amplify, and transmit the measurements to a smartphone via Bluetooth. While being a TENG-based, wearable, and wireless sensor system, it does not require a very long charging period as it gets a maximum power output of 0.94 mW or a power density of 416 W m⁻². Using the designed system, the Na⁺ ion concentration and pH value of sweat are measured under 1.5 Hz frequency, as evident from Fig. 5(b2). This system has an enormous potential in self-powered personalized health monitoring.

Parkinson’s disease (PD) is a common neurological disease, and about 1% of the population over 60 years of age in industrialized countries is suffering from it.⁶⁷ Effects of the PD are body shaking, pain, and sleep disturbance, and with the passage of time, it becomes difficult to walk, talk, or even perform simple tasks.^68,69 Early detection of PD is vital to start treatment because it gets severe as time passes. Recently, a team of scientists from Korea have published their work on a TENG-based, self-powered tremor sensor with the capability of sensing PD.⁷⁰ The sensor has been designed to measure the low-frequency hand vibration of PD patients while harvesting energy from human motion. A conductive and highly stretchable catechol-chitosan-diatom hydrogel (CCDHG) has been synthesized and fixed with a sixfold M-shaped Kapton-pasted PET film to develop a 3 × 3 cm² CCDHG-TENG device [see Fig. 5(c1)]. CCDHG is further wrapped with a tribo-negative PDMS film, and an aluminum electrode is connected to CCDHG. When a repetitive external force is applied to the device, the Al film and CCDHG act like positive and negative triboelectric layers to create an electron flow and finally produce an alternating current (AC) output. The performance of the device is proven by using it as a tapping, stretching, and bending-responsive electric skin. The tremor sensor is attached to the wrists of the real PD patients and the severity of the PD-generated tremors is detected using frequency domain analysis [see Fig. 5(c2)]. We understand that such a tremor sensor can be highly useful in real-time and wearable health monitoring applications and can be combined with artificial intelligence for data collection and training.

Since the invention of TENG, researchers have been using this technology for the improvement of human health. TENG is proven to improve various medical conditions using self-powered electrical stimulation devices while being minimally invasive. Some of the developments are skin wound recovery, neural tissue engineering, weight control, nerve stimulations, hearing loss repair, hair regeneration, and heart diseases that can be overcome by electrical stimulation, which is one of the therapeutic approaches to overcome health issues by harvesting the biomechanical energy of the organs into electrical energy.

Obesity is a problem that may lead to various serious diseases, including cardiac disorders, hypertension, diabetes, osteoarthritis, dyslipidemia, asthma, and psychological problems.^71–73 There is also an economic downside because overweight people are found incapable of doing certain jobs effectively and are prone to get more sick than non-obese persons.⁷⁴ Yao et al. presented a self-powered and spontaneously responsive vagal nerve stimulation (VNS) device using a TENG to manage weight control in Sprague–Dawley (SD) rats.⁷⁵ The biocompatible and flexible VNS device is implanted on the outer surface of the stomach to automatically generate biphasic electrical pulses taking advantage of very low-frequency (0.05 Hz) peristaltic stomach motion, as shown in Figs. 6(a2) and 6(a3). The multilayer VNS device works in the contact–separation mode under the repetitive motion of the stomach. It uses nanostructured PTFE and Cu electrodes as negative and positive triboelectric layers. When the stomach contracts, the bottom electrode layer (BEL) moves away from the PTFE layer, creating a potential difference and eventually a current flow between the BEL and the top electrode layer (TEL) [see Fig. 6(a2-ii)]. The current direction gets inverted once the stomach starts distending [see Fig. 6(a2-iv)]. The 1 mm thick VNS device adds PDMS packaging for the sake of structural flexibility and stability. Another layer of ecoflex is coated on top of PDMS so that the device can tightly fit on the curvy surface of the stomach while maintaining sensitivity in response to stomach motions. A bilateral VNS is executed using the electrical pulses by wrapping the electrodes around the vagal nerves. According to the impedance of the vagal nerve, the generated stimulation voltage due to low-frequency motion of stomach is around 200 mV that is proven sufficient to reduce the food intake, which caused a decrease in the bodyweight of the rats by 38% within 100 days [see Fig. 6(a4)]. This work provides a concept that coordinated body motions can be used to generate artificial nerve stimulation resulting in therapeutic technology.

Various pharmacological and surgical methods have been devised to treat hair loss, and they do deliver results but with many undesired effects, such as weight gain and mood changes, and can be less affordable.^76,77 Use of non-invasive electrical signals in the form of pulsed electrostatic fields (ETG) have proven effective in preventing or reducing hair loss in humans.⁷⁸ However, TENG takes it a step further by developing a self-powered device for hair growth. In 2019, Yao et al. developed a wearable, universal motion-activated electrical stimulation device (m-ESD) to promote hair regeneration.⁷⁹ The device is illustrated in Fig. 6(b1) showing two parts: an electrical pulse generating omnidirectional triboelectric generator (OTG) to harvest the body motion energy and transfer it to interdigitated electrodes via a charge transfer electrode (CTE). The interdigitated dressing electrodes provide a spatially distributed electric field. The designed device is attached to the backside of the Sprague–Dawley (SD) rats to generate similar amplitude electric pulses using random head movements as mechanical input [see Fig. 6(b2)]. The OTG is composed of a negative triboelectric layer of PTFE sandwiched between the positive triboelectric copper layer for triboelectric charge generation (CGE) and the gold CTE for charge transfer toward the working electrode. The working principle for the OTG is a sliding-mode TENG in which the CGE slides against the PTFE layer to produce positive and negative charges on the CGE and PTFE layers, respectively. When the m-ESD device stretches back and forth, it causes to slide the CGE against the PTFE, producing an alternating voltage output. The OTG-based device generates voltage outputs when m-ESD stretches in different directions, velocities, and distances. The significant hair regenerative effect is achieved with an electric field of 3 V/cm by getting the longest hair shaft length in this experimentation [see Fig. 6(b3)]. It has been shown that m-ESD can increase hair follicle (HF) proliferation and the secretion of vascular endothelial growth factor (VEGF) and keratinocyte growth factor (KGF) to reduce hair keratin disorder that eventually promotes hair regeneration. The length of the final hair shaft generated using the developed device is 1.8 times and 2.2 times longer than that treated by minoxidil (MNX) and vitamin D3 (VD3), respectively. Finally, m-ESD stimulation also improved hair follicle (HF) density to 141% of the MNX and VD3 regions. Encouraging treatment results with self-powered and non-invasive properties of the proposed device make it a most suitable candidate for a modern hair fall solution in humans.

Almost half a billion of the global population is living with diabetes.⁸⁰ Among many other diabetic complications, diabetic foot ulcers (DFUs) are known to be challenging medical issues causing physical, psychological, and financial damages to the patients. If DFU progresses, it often leads to neighboring tissue infection and amputation. In a 1999 US study, the estimated costs of treating a diabetic foot ulcer were US$ 28 000.⁸¹ In 2021, Jeong et al. have suggested a fully-stretchable and wearable ionic TENG (iTENG) patch made of a gel-based platform to accelerate the wound healing process in damaged tissue.³⁴ Silicon rubber microtubes are filled with LiCl-incorporated organogel and then woven into an elastic structure to fabricate a stretchable textile-like patch. iTENGs consisting of a TENG, wire, and patch are used to convert and deliver input biophysical energy into the symmetric electric potential to repair a damaged tissue evenly [see Fig. 6(c1)]. Using the contact–separation mode, once TENG touches the skin surface, electrons move from the skin to the iTENG surface due to the different electron affinities of the two materials. As soon as iTENG gets away from the dermal surface, electrons on iTENG repel anions in the organogel to accumulate them in the patch to produce an electric field. An alternating current (AC) generates upon repetitive contact–separation motion. The ionic fabric attached to the mouse’s skin produces up to 2 Vpp voltage spikes upon its active motion. In vitro testing of the iTENG device on fibroblasts indicates improved cell migration and proliferation [see Fig. 6(c2)]. Using a balb/c nude mouse model, electrical stimulation is applied every 2 days, resulting in ∼5% of the original wound size leaving no unwanted scar while control wound size remained at ∼20% after 14 days [see Fig. 6(c3)]. It is expected that such biocompatible and stretchable electrotherapy will help in treating other conditions, such as keloid scarring and alopecia.

As per WHO, about half a billion people are suffering from hearing loss and these numbers are estimated to increase to nearly a billion by 2050.⁸² Major type of hearing loss is known as sensorineural hearing loss, and a cochlear implant (CI) is used as a conventional technique to solve this problem that has its inherent problems such as high power consumption and financial cost and inconvenient device placement. In 2016, Jang et al. brought a new idea to introduce artificial basilar membranes (ABMs) that used the triboelectric phenomenon to mimic cochlear tonotopy, frequency distribution along the membrane.⁸³ A self-powered, low-cost triboelectric-based ABM (TEABM) is implemented to convert the incoming acoustic signal into an electrical signal with an eight-beam-channel device made of varying beam size [see Fig. 6(d1)]. Due to varying lengths and width, each beam resonates at a different frequency to implement acoustoelectric transduction under the vertical contact–separation TENG mode. Figure 6(d1) shows a layered structure of a single beam effective TENG with Kapton and aluminum (Al) triboelectric layers. The acoustic stimulus causes the Kapton film to contact the Al foil, which results in the transfer of electrons from the Al to the Kapton surface. When both of the films bend upward, they start parting away from each other due to different Young’s moduli. It produces a potential difference between both metallic layers, causing an electrical current flow between them. In the periodic motion, the current changes its direction when the Kapton film moves down again to contact the Al foil. In such a manner, the output voltage of TEABM varies with the sound pressure level for each channel in a distinct way due to its different width. [See Fig. 6(d2)]. In vivo testing of TEABM is performed on a group of deafened Henry guinea pigs by implanting a custom-made intra-cochlear array electrically connected to acoustically stimulated TEABM [see Fig. 6(d3)]. Meanwhile, a signal processor is used to convert the electrical output of the device into a stimulation pulse for the auditory neurons. Considering this device as a stepping stone in this research area, there is still a lot of room left to mimic a broad range of audible frequencies with ultrasensitive devices.

Around 150 billion USD are spent every year for nerve injuries only in the United States.⁸⁴ Neural tissue engineering (NTE) is known as a useful technique to treat neural injuries. NTE methods include scaffolds, tissue grafts, and cellular therapies with mesenchymal stem cells (MSCs) being the most important.^85,86 Guo et al. reported a self-powered, small-sized TENG-based electrical stimulation system for stem cell differentiation that can work for a long time.⁸⁷ Highly conductive rGO–PEDOT (reduced graphene oxide with poly(3,4-ethylenedioxythiophene)) hybrid microfibers (80 µm in diameter) are prepared by mixing graphene oxide (GO) suspension with poly(3,4-ethylenedioxythiophene) (PEDOT) solution and are used as neural scaffolds. The step-driven TENG has a multilayer structure, with four springs attached at the corners of the PMMA substrate to perform contact–separation energy harvesting [see Fig. 7(a1)]. Aluminum (Al) and Kapton films are tribo-positive and tribo-negative triboelectric layers in the TENG. The coupling between triboelectric and electrostatic effects produced pulsed electrical energy using the motor-triggered TENG. The electrical output of 250 V and 30 µA is generated when TENG was stepped-on. TENG-combined biodegradable rGO–PEDOT hybrid microfiber electrically stimulated the MSCs and increased cell proliferation, differentiation, and expression levels of neural-specific genes, proving its nerve regeneration capability. MSCs cultured on the rGO–PEDOT hybrid also showed great cytocompatibility. This type of integrated device can be presented as a futuristic application for self-power nerve regeneration.

One of the complications of surgical treatments is muscle weight reduction or also known as skeletal muscle atrophy (SMA), and it may increase rehabilitation duration as well.⁸⁸ Electrical muscle stimulation is a proven method to maintain the skeletal muscle function for an immobilized patient.⁸⁹ Wang et al. presented a self-powered TENG device for direct muscle stimulation using an intramuscular electrode [see Fig. 7(b1)].¹⁹ The stimulation system consists of two parts: a hand-tappable stacked TENG and a flexible multiple-channel intramuscular electrode. To assist the repetitive pressing operation in a contact–separation mode, a stacked TENG is fabricated. A PET sheet is folded to form a zigzag structure, while aluminum films are attached to each surface as the tribo-positive layer and the electrode. An additional PTFE film is attached to every second aluminum film as a tribo-negative layer. In this stacked TENG, every layer contact and separate each other to provide alternating current stimulating the muscle tissue. The stacked TENGs can harvest energy from human motions, such as feet or hand tapping, meanwhile offering control flexibility. The intramuscular electrode is sutured in the tibias anterior (TA) muscle belly of a rat using a suture wire, and only electrode sites are in contact with the muscle tissue. A flexible printed cable (FPC) connector is used to connect TENG and electrode sections of the device, and current was delivered to the electrode via hand tapping. The strategic design of electrodes with broad electrode-motoneuron position range and stimulation waveform polarity ensured motoneurons mapping and enabled muscle stimulation, in the form of forward leg-kicking movement, with 35 µA instead of typical mA current. The smart design of multiple-channel electrodes can be a way forward toward practical application as it helps in optimizing the stimulation efficiency, and integrating it with TENG will make it useful as a self-powered system to electrically stimulate the wasted muscles.

Controllable drug delivery is desired for various human diseases, such as diabetes and cancer.⁹⁰ It can help deliver the accurate drug dose at the right time.⁹¹ A number of drug delivery systems have been developed that include ultrasound, electric field, magnetic field, and thermal and laser-assisted drug delivery.^92–96 However, they do need some external source to initiate the process. In this context, Ouyang et al. proposed an on-demand transdermal drug delivery (TDD) system using TENG that does not need any external power source to function.⁹⁷ A miniaturized TDD system provides a tunable drug release rate using a non-invasive patch, making on-demand drug delivery possible without using any needle. The TDD system is composed of a human movement harvesting rotary TENG, a power management circuit to convert the AC output into transdermal compatible voltage, and two drug and iontophoresis transdermal patches for guided drug release [see Fig. 7(c1)]. Sliding-mode type TENG entails a stator (copper) layer and a rotator (PTFE-covered copper) layer. When the rotator moves from state 1 to state 2 using human hand motion, electrons move from the copper rotator to PTFE, making it negatively charged [see Fig. 7(c2)]. The continuous rotation of the rotator caused an alternating potential difference and a current flow between electrodes 1 and 2. The ex vivo tests on porcine skin showed that the TENG-based system can deliver more than five times more drug amount across the skin as compared with the traditional power supply [see Fig. 7(c3)]. With a TENG-based transdermal drug delivery system, now, it will be possible to make high output, self-powered, and wearable devices while benefiting from simple and low-cost fabrication. Highly effective and controllable drug release can give us a direction toward more possibilities in personalized healthcare.

Here, advancements of TENG prevention, monitoring, and treatment are systematically summarized for healthcare and biomedical applications. Although TENGs have shown great potential in the materials, device designs, and miniaturized power management circuits, however, for applications of health monitoring, there are still myriad challenges that need to be addressed. Further advancement toward the development of a self-powered healthcare system can be achieved by making efforts to resolve the following limitations. Some of these challenges are optimization of the device design, device size, and material fabrication while maintaining the flexibility, cost-effectiveness, and sustainability for the in vivo applications. Moreover, TENGs as implantable medical devices (IMDs) can also provide great opportunities to sense and treat various internal vital organs of the human body by serving as cardiac pacemakers, tissue stimulators, and physiological sensors. However, material compatibility, device performance, and power sourcing are a few of the many challenges that have to be overcome to get significant benefits out of it. Some possible solutions can be using biologically safe biodegradable materials, specially designed and treated film surfaces for high electrical performance, and wireless but non-invasive external power sources. These developments soon can lead to significant progress in the commercialization of TENG-based self-powered systems. In the coming decades, new research should focus not only on optimizing the TENG devices but also on their effective integration with other emerging technologies involving new bio-implantable and biodegradable materials. The advances of a long-term stable encapsulation layer to protect the device under harsh in vivo conditions will also play a definitive role. Highly efficient, small size, and cost-effective wireless signal transmission systems will also be required as a part of the overall solution. Before industrialization and commercialization of TENG-based healthcare and biomedical systems, in-depth research and extensive animal experiments are immensely required to make it a perfect technology for modern healthcare.

This work was supported by the Nano Material Technology Development Program (Grant No. 2020M3H4A1A03084600) and the Basic Science Research Program (Grant No. 2019R1A2B5B03069968) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology.