Triboelectric and Piezoelectric Nanogenerators for Self-Powered Healthcare Monitoring Devices: Operating Principles, Challenges, and Perspectives

The internet of medical things (IoMT) is used for the acquisition, processing, transmission, and storage of medical data of patients. The medical information of each patient can be monitored by hospitals, family members, or medical centers, providing real-time data on the health condition of patients. However, the IoMT requires monitoring healthcare devices with features such as being lightweight, having a long lifetime, wearability, flexibility, safe behavior, and a stable electrical performance. For the continuous monitoring of the medical signals of patients, these devices need energy sources with a long lifetime and stable response. For this challenge, conventional batteries have disadvantages due to their limited-service time, considerable weight, and toxic materials. A replacement alternative to conventional batteries can be achieved for piezoelectric and triboelectric nanogenerators. These nanogenerators can convert green energy from various environmental sources (e.g., biomechanical energy, wind, and mechanical vibrations) into electrical energy. Generally, these nanogenerators have simple transduction mechanisms, uncomplicated manufacturing processes, are lightweight, have a long lifetime, and provide high output electrical performance. Thus, the piezoelectric and triboelectric nanogenerators could power future medical devices that monitor and process vital signs of patients. Herein, we review the working principle, materials, fabrication processes, and signal processing components of piezoelectric and triboelectric nanogenerators with potential medical applications. In addition, we discuss the main components and output electrical performance of various nanogenerators applied to the medical sector. Finally, the challenges and perspectives of the design, materials and fabrication process, signal processing, and reliability of nanogenerators are included.


Introduction
Due to the health problems caused by COVID-19, the World Health Organization (OMS) has implemented strategies for monitoring, controlling, and treating chronically ill patients [1,2]. An interesting strategy is digital home hospitalization, which could be used in future health contingencies [3][4][5]. This strategy is justified by the lack of hospital beds and the increase in the world population of older adults [6,7]. The world population could Nanomaterials 2022, 12, 4403 2 of 35 reach 8.5 billion and 9.7 billion in 2030 and 2050, respectively [8]. In addition, digital home hospitalization is suitable for people with chronic diseases who live in isolated areas [9,10]. Thus, patients could be cared for from their homes, preserving their integrity with the help of the implementation of a wireless patient monitoring tool.
In recent years, the internet of medical things (IoMT) and the internet of things (IoT) have registered great interest from the scientific and medical communities. The IoMT is a subarea developed and applied within the IoT to focus on different current medical applications. This process uses the acquisition, processing, transmission, and storage of medical information through specific devices and the security of patient data [11][12][13][14][15]. The IoMT processes and contains patients' confidential data. Thus, a critical challenge of the IoMT is achieving data security [16,17]. The IoMT requires devices with soft, elastic, and biocompatible materials, safe performance, and stable electrical response [18][19][20]. In addition, these devices require a great capacity for monitoring different patient signals using electrical energy sources of small size and long lifetime. These devices must provide real-time data on the health status of patients, informing the patients, family members, medical centers, or doctors. In addition, the IoMT devices must alert possible health risks of patients.
Wireless and portable electronics have increased with the gradual technological development of 5th generation mobile networks around the world [21][22][23][24][25]. Using portable electronic devices and wireless connections, the IoMT technology may allow the possibility to control and assist patients. Thus, this technology can help patients' health by providing and controlling medical treatments [26][27][28][29]. However, the future IoMT portable devices will require energy sources that replace conventional batteries, which have limitations such as limited lifetimes, large volume and weight, and toxic materials that can contaminate the environment. To improve the performance and lifetime of energy storage devices, recent investigations [30][31][32][33][34][35] have developed rechargeable metal-air batteries and metalion batteries, as well as supercapacitors. Another alternative to substitute conventional batteries is the fabrication of nanogenerators, which can harvest green energy from the environment or the human body. The nanogenerators offer advantages such as a long lifetime, small volume, light weight, a non-expensive fabrication process, and high output power density [36][37][38][39][40]. The nanogenerators can use the triboelectric and piezoelectric effects for harvesting biomechanical energy into electricity [41][42][43][44][45][46][47][48][49]. The piezoelectric nanogenerators can convert the mechanical deformations of the piezoelectric materials used in their structures into electrical voltages. On the other hand, triboelectric nanogenerators use both contact electrification and electrostatic induction between two moving triboelectric layers to generate an electrical potential difference. Table 1 depicts several advantages and disadvantages of piezoelectric and triboelectric nanogenerators, metal-air batteries, and supercapacitors.

Device Type
Advantages Disadvantages Ref.

Metal-air batteries
High energy density, low cost, flat discharge voltage, and high safety Performance is affected by environmental conditions, dendrite formation on the anode, carbonation of alkaline electrolyte, and limited range of operating temperature [30,32] Supercapacitor High energy density, high specific surface area, long cycle life, and good conductivity and stability Capacitance and charge storage depend on the employed electrode materials and non-simple fabrication process [33][34][35] Piezoelectric nanogenerator Simple structure and easy fabrication process, good electromechanical stability, and non-complex signal processing system Performance depends on the properties and structural configuration of the piezoelectric material [36] Triboelectric nanogenerator High electrical performance, compact structure, simple working principle, low-cost materials, and good electrical stability Wear of triboelectric material by friction, and performance depends on the properties and working mode of the triboelectric film [36] Metal-air batteries High energy density, low cost, flat discharge voltage, and high safety formation on the anode, carb tion of alkaline electrolyte, limited range of operating tem ature Supercapacitor High energy density, high specific surface area, long cycle life, and good conductivity and stability Capacitance and charge stor depend on the employed elec materials and non-simple fab tion process

Piezoelectric nanogenerator
Simple structure and easy fabrication process, good electromechanical stability, and non-complex signal processing system Performance depends on the erties and structural configur of the piezoelectric materi Triboelectric nanogenerator High electrical performance, compact structure, simple working principle, low-cost materials, and good electrical stability Wear of triboelectric materia friction, and performance dep on the properties and work mode of the triboelectric fi We reviewed piezoelectric and triboelectric nanogenerators that can energy from environmental sources such as human body motion, mechani and wind. The harvested energy can be converted into electrical energy to p medical devices that monitor and process signals of different health para tients. These signals could be transmitted to hospitals or medical centers us The operating principle, materials, and manufacturing processes of nano discussed. Future medical applications of piezoelectric and triboelectric n are reported, including materials, signal processing components, and pack we proposed the challenges and perspectives on the output performance o tors, taking into account the topics of design, materials and fabrication pr processing, and reliability. We reviewed piezoelectric and triboelectric nanogenerators that can harvest green energy from environmental sources such as human body motion, mechanical vibrations, and wind. The harvested energy can be converted into electrical energy to power potential medical devices that monitor and process signals of different health parameters of patients. These signals could be transmitted to hospitals or medical centers using the IoMT. The operating principle, materials, and manufacturing processes of nanogenerators are discussed. Future medical applications of piezoelectric and triboelectric nanogenerators are reported, including materials, signal processing components, and packaging. Finally, we proposed the challenges and perspectives on the output performance of nanogenerators, taking into account the topics of design, materials and fabrication processes, signal processing, and reliability.

Operating Principle, Materials, and Fabrication
Triboelectric and piezoelectric nanogenerators have potential applications in the medical sector due to their high electrical output performance, simple structure and operation principle, and cost-effective fabrication process.
In 2017, Chen et al. [103] reported a flexible hybrid nanogenerator that can be attached on soft surfaces, such as human skin, for body motion harvesting and monitoring physiological signals. A single-electrode TENG and a PENG are integrated into the nanogenerator to improve its electromechanical performance ( Figure 2). The PENG module used poly(vinylidenefluoride-co-trifluoroethylene) P(VDF-TrFE) nanofiber mats as the piezoelectric structure, and polydimethylsiloxane (PDMS) as the protective layer. Thermoplastic polyurethane (PU) nanofibers coated with carbon nanotubes (CNT) and silver nanowires (AgNWs) were employed in the electrodes. Thus, the PU electrodes sandwiched the piezoelectric layer. The PENG part was separated from the TENG part by a PDMS isolation film to produce piezoelectric and triboelectric outputs. Another PDMS layer operated as triboelectric material, and the PU film as an electrode for the single-electrode TENG. Based on the triboelectric and piezoelectric mechanisms, this hybrid nanogenerator registered a maximum peak power up to 84 mW/m 2 and 0.11 mW/m 2 for the TENG and PENG modules under compressive stress ( Figure 3). This nanogenerator can be attached to any soft hybrid to convert tapping and pressing energy into electricity. The hybrid nanogenerator can be placed on the skin for real-time monitoring of human physiological signals, such as respiratory information and radial artery pulse. Figure 4 shows the hybrid nanogenerator attached to the back of the hand to collect the body motion energy of closing the fist and punching. This nanogenerator can be mounted on the abdomen for monitoring the human respiratory rate and depth. Thus, the nanogenerator could be employed for monitoring the respiratory condition of patients. Also, this nanogenerator has potential applications for self-powered healthcare monitoring systems and e-skins. eration principle, and cost-effective fabrication process.
In 2017, Chen et al. [103] reported a flexible hybrid nanogenerator that ca tached on soft surfaces, such as human skin, for body motion harvesting and mo physiological signals. A single-electrode TENG and a PENG are integrated into th generator to improve its electromechanical performance ( Figure 2). The PENG used poly(vinylidenefluoride-co-trifluoroethylene) P(VDF-TrFE) nanofiber mats a ezoelectric structure, and polydimethylsiloxane (PDMS) as the protective layer. T plastic polyurethane (PU) nanofibers coated with carbon nanotubes (CNT) and sil owires (AgNWs) were employed in the electrodes. Thus, the PU electrodes sand the piezoelectric layer. The PENG part was separated from the TENG part by a isolation film to produce piezoelectric and triboelectric outputs. Another PDMS l erated as triboelectric material, and the PU film as an electrode for the single-e TENG. Based on the triboelectric and piezoelectric mechanisms, this hybrid nano tor registered a maximum peak power up to 84 mW/m 2 and 0.11 mW/m 2 for th and PENG modules under compressive stress ( Figure 3). This nanogenerator ca tached to any soft hybrid to convert tapping and pressing energy into electricity. brid nanogenerator can be placed on the skin for real-time monitoring of human logical signals, such as respiratory information and radial artery pulse. Figure 4 sh hybrid nanogenerator attached to the back of the hand to collect the body motion of closing the fist and punching. This nanogenerator can be mounted on the abdo monitoring the human respiratory rate and depth. Thus, the nanogenerator could ployed for monitoring the respiratory condition of patients. Also, this nanogener potential applications for self-powered healthcare monitoring systems and e-skin     In 2018, Sun et al. [104] designed a biocompatible triboelectric-piezoelectric-pyroelectric hybrid nanogenerator with high flexibility. This nanogenerator has a transparent structure formed by polyvinylidene fluoride (PVDF), PDMS, and Ag nanowires. The PVDF material acts as piezoelectric and pyroelectric film and the PDMS acts as a triboelectric film. The Ag nanowires are used as high-performance transparent electrodes (TEs) and can be distributed into a network like a leaf venation (LV), inspired by optimized LV ( Figure 5). Thus, the working mechanism of the hybrid nanogenerator is integrated by a triboelectric nanogenerator, a piezoelectric nanogenerator (TENG-PiENG), and a pyroelectric nanogenerator (pyENG). This nanogenerator can be mounted on different body parts to collect mechanical and thermal energy from the human body. This nanogenerator has an environmentally friendly design, which the structure of LV can reuse many times. The nanogenerator structure registered a minimum sheet resistance of 1.4 Ωsq −1 with 82% transmission and sheet resistance of 68.2 Ωsq −1 with an ultra-high transmission of up to 99%. The hybrid nanogenerator can harvest a maximum open-circuit output voltage of 55 V and 86 V from mechanical and thermal energy with the three coupled transduction mechanisms ( Figure 6). The nanogenerator has a potential application for assessing the patient's health status with a cold, using frequency, coughing, and breathing through the amplitude of the voltage (Figure 7). The hybrid nanogenerator can monitor various human physiological signals, such as heartbeat, swallowing, and neck tilting. This nanogenerator could be used for costeffective medical diagnostics and prognosis of cardiovascular, Parkinson, and esophagus diseases. Also, the nanogenerator integrated with a thermochromic liquid crystal display (LCD) could operate as a thermometer for medical diagnostics.   In 2019, Zhu et al. [105] investigated a self-powered and self-functional cotton sock (S 2 -sock) based on a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)coated fabric TENG and lead zirconate titanate (PZT) piezoelectric device ( Figure 8). The PEDOT:PSS is a polymer with high conductivity and mechanical stability. A Cu substrate was bonded to the piezoelectric device to improve the ductility and optimize the mechanical behavior. This sock can produce the featured waveforms for walking pattern recognition and motion tracking of patients under home care. The experimental tests show that in-shoe situation, the TENG part of the sock can generate the output power of 66 µW and 137 µW at 1 Hz walking and 2 Hz jumping with a load resistance of 10 MΩ, respectively ( Figure 9). For indoor use, the TENG part of the sock stepping on a polytetrafluoroethylene (PTFE) film, which is mounted on the ground, registers maximum powers of 1.17 and 1.71 mW under a load resistance of 59.7 MΩ. This allows a power density of 11 µW/cm 2 considering a contact area close to 150 cm 2 . For the piezoelectric device (size of 5 cm × 5 cm), the maximum output power was 32 µW under load resistance of 0.4 MΩ and 12 N at 1 Hz. The maximum power density of the PZT device is 128 µW/cm 2 , which offers a good performance for smart textiles. For the indoor case using bare shocks at home, the pattern recognition and motion tracking can be measured for home care applications of the IoMT. The S 2 -shock was used to measure the mimetic motions of a Parkinson's disease patient regarding normal motion, loss of stride, and freezing of gait (FOG). Moreover, this sock can collect energy from body motion and sense diversified physiological signals for healthcare and home sports. In addition, this cotton sock can be enhanced by incorporating wireless transmission modules and integrated circuits.
transmission and sheet resistance of 68.2 Ωsq −1 with an ultra-high transmi 99%. The hybrid nanogenerator can harvest a maximum open-circuit outpu V and 86 V from mechanical and thermal energy with the three coupled mechanisms ( Figure 6). The nanogenerator has a potential application for patient's health status with a cold, using frequency, coughing, and breathin amplitude of the voltage (Figure 7). The hybrid nanogenerator can monito man physiological signals, such as heartbeat, swallowing, and neck tilting. erator could be used for cost-effective medical diagnostics and prognosis o lar, Parkinson, and esophagus diseases. Also, the nanogenerator integrate mochromic liquid crystal display (LCD) could operate as a thermometer f agnostics. Figure 5. Network of LV-like Ag nanowires of the hybrid nanogenerator fabricate [104]. Images of (a) the structure of a ramified leaf used as a mold to fabricate the (b) The skeleton of the leaf venation and (c) LV network were prepared in a square s as a mold. (d) SEM image of Ag nanowires. These nanowires were synthesized in a Schematic of vacuum filtration of the Ag nanowires. (f) Image of Ag nanowires film a cellulose acetate membrane. (g) Schematic of a modified dry transfer printing tec cate the LV-like Ag nanowires network. (h) Image of the LV-like Ag network on a P (size of 3.5 cm × 3 cm). Reprinted with permission from [105]. Copyright 2018, Els In 2019, Li et al. [106] reported a hybrid nanogenerator based on cellulose nanofibril (CNF) as a triboelectric layer and bacterial cellulose (BC) with nanoparticles of BaTiO 3 and a multi-walled carbon nanotube (MWCNT) as piezoelectric layer. The CNF fibers were treated by employing a mixed solution of HNO 3 and H 2 SO 4 . In addition, a -NO 3 group is applied to enhance the electronegativity of the CNF fibers. This hybrid nanogenerator is formed by two composite layers. The top arched layer acts as PENG, using a functionalized BC paper with two Ni electrode films. The PENG has an arched shape to improve the strength of the effective strain applied to the bacterial cellulose film. The bottom layer operates as TENG integrated by a Nitro-CNF paper and a bottom Cu electrode film. The working mechanism of the PENG consists of the strain of the piezoelectric layer caused by external forces on its top surface, which generates a voltage variation between the two Ni electrodes. On the other hand, TENG operates with contact electrification and electrostatic induction. The triboelectric and piezoelectric parts have high output electrical performance, achieving short-circuit current density and open-circuit voltage of 1.23 µA/cm 2 , 37 V, 220 nA/cm 2 , and 22 V, respectively. With a rectifier circuit to integrate the outputs of both triboelectric and piezoelectric parts, the hybrid nanogenerator has a short-circuit current density and open-circuit voltage of 1.6 µA/cm 2 and 18 V, respectively. This cellulose-composed nanogenerator has an environmentally friendly simple structure, light weight, and cost-effective fabrication. Thus, this nanogenerator could apply to wearable or implantable devices and self-powered electronic sensors.  Schematic view of the components of the hybrid nanogenerator. (b) Image of erator placed on the wrist. Working mechanism of the (c) TENG-PiENG and brid nanogenerator. Open-circuit voltage of the (e) TENG-PiENG and (f) Py permission from [104]. Copyright 2018, Elsevier Ltd.  In 2020, Syu et al. [107] reported a biomimetic and flexible hybrid self-powered sensor (BHSS) that was formed by triboelectric and piezoelectric elements. The triboelectric structure is based on the shell of Mytilidae and is fabricated with PDMS film. The piezoelectric component contains porous PVDF fibers deposited on a printed circuit board (PCB) substrate. To develop the biomimetic Mytilidae surface, Mytilidae nano-structured patterns on a PDMS layer were applied using a soft transfer molding process. This hybrid sensor registered an open-circuit voltage and short-circuit current of 15 V and 115 nA, respectively. For a load resistance of 10 MΩ, the hybrid sensor achieved a maximum average power density of 675 µW/m 2 . The hybrid sensor was tested. This self-powered sensor was attached on the thumb, index, and middle fingers of a latex glove for monitoring bottleholding actions. For this experiment, its electrical signals under three plastic bottles with different water volumes (loading weight of 0.65 kg, 1.3 kg, and 2.06) were examined. The electrical signals of the sensors were studied for various contact positions along the bottles. This self-powered sensor could be used for wearable electronic devices and monitoring biomechanical motion, and human gesture recognition, using a machine learning algorithm of long short-term memory. home, the pattern recognition and motion tracking can be measured for home care applications of the IoMT. The S 2 -shock was used to measure the mimetic motions of a Parkinson's disease patient regarding normal motion, loss of stride, and freezing of gait (FOG). Moreover, this sock can collect energy from body motion and sense diversified physiological signals for healthcare and home sports. In addition, this cotton sock can be enhanced by incorporating wireless transmission modules and integrated circuits.  Huang et al. [108] fabricated a flexible biocompatible triboelectric-piezoelectric nanogenerator by employing recombinant spider silk and poly(ethylene terephthalate) (PET)/ PVDF films. Graphene was incorporated into PVDF to improve the piezo-PVDF performance. Thus, the PET/PVDF-graphene composite film was added with genetically engineered spider silk. The electron-cloud-potential-well model was used to describe the charge transfer between spider silk and PET, with or without the PVDF film. The electrons in molecular orbits of spider silk protein can be transferred to the empty orbits of PET film. This transference of electrons depends on the difference in potential-well depths (difference of the surface electron potential) between both materials. PVDF, under mechanical strain, can alter the surface electron potential of the coupling material. Due to this, PVDF was added to PET film to enhance its surface potential. Thus, the improved surface-potential difference between PET/PVDF and spider silk can increase their electrons transfer and amount of harvested energy. Figure 10 depicts the output electrical signals and stability results of the hybrid nanogenerator. This hybrid nanogenerator (area of 6.25 cm 2 ) registered a maximum output power density of 4016 mW/m 2 under a load resistance of 8 MΩ. For this resistance, the nanogenerator reached the output voltage, output short-circuit current, and output power close to 200 V, 12 µA, and 2.51 mW, respectively. This nanogenerator could be used as wearable hand-gesture sensors, body motion sensors, bioenergy harvesters, and implantable organ monitors ( Figure 11). For instance, this nanogenerator could be included in implantable sensors for monitoring signals of the heart, stomach, chest, and bladder. In 2019, Li et al. [106] reported a hybrid nanogenerator based on cellul (CNF) as a triboelectric layer and bacterial cellulose (BC) with nanoparticles a multi-walled carbon nanotube (MWCNT) as piezoelectric layer. The CN treated by employing a mixed solution of HNO3 and H2SO4. In addition, a applied to enhance the electronegativity of the CNF fibers. This hybrid nan formed by two composite layers. The top arched layer acts as PENG, using ized BC paper with two Ni electrode films. The PENG has an arched shape t strength of the effective strain applied to the bacterial cellulose film. The operates as TENG integrated by a Nitro-CNF paper and a bottom Cu electr In 2022, Du et al. [109] designed a shoe insole hybrid nanogenerator (IHN) composed of a multilayered TENG and arched PENG. This nanogenerator can convert the mechanical energy of footsteps into electrical energy. Moreover, this nanogenerator can identify three types of motion states: walking, stepping, and jumping. This nanogenerator has high performance of electrical stability and durability, which is suitable to collect biomechanical energy of feet to power wearable electronic sensors. In addition, this nanogenerator has potential application for self-powered biosensor systems in the fields of sports and medicine. Figure 12 illustrates an application of the IHN integrated with a dorsalis pedis artery sensor and a processor module. The sensor is placed inside the tongue of the shoe and the processor module is mounted on the outside of the shoe. The sensor contains a PVDF film that is packaged with PTFE and Kapton films. The PVDF film harvests the vibration signal of the pulse and converts it into an electrical signal. The hindfoot and forefoot of the IHN are formed by the arched PENG and TENG, respectively. The TENG part has three triboelectric films (PTFE-Al-PTFE), in which two sponge layers with through-holes are added to separate the Al sheet from both PTFE films. Both PTFE films and the Al layer are contacted and separated due to the motion (e.g., walking, stepping, and jumping) of the foot. On the back of PTFE is attached the Cu electrode that operates as a source for the charge transference caused by electrostatic induction. In addition, the Al sheet can act as both an electrode and a triboelectric layer. On the other hand, a surface area (2 cm × 7 cm) of the sponge is extracted to incorporate the arched PENG. This PENG is composed of a PVDF film that is packaged by an arched PE and Kapton film. Furthermore, the IHN was encapsulated using a cotton cloth and PE film, which avoids the effect of sweat on the electrical performance of the hybrid nanogenerator. and amount of harvested energy. Figure 10 depicts the output electrica bility results of the hybrid nanogenerator. This hybrid nanogenerator registered a maximum output power density of 4016 mW/m 2 under a lo MΩ. For this resistance, the nanogenerator reached the output voltage, cuit current, and output power close to 200 V, 12 μA, and 2.51 mW, nanogenerator could be used as wearable hand-gesture sensors, body m oenergy harvesters, and implantable organ monitors ( Figure 11). For in generator could be included in implantable sensors for monitoring sig stomach, chest, and bladder. (e) Photographs of the spider silk and PVDF/graphene films (both fi to A5 paper), and white LED light (3 W). This LED was lightened using a hy with an area of 48 cm 2 . (f) The output current density of the hybrid nanogenerat of hand tapping. The inset illustrates the input pressure generated by hand-tappi range. Reprinted with permission from [108]. Copyright 2020, WILEY-VCH V KGaA, Weinheim. (e) Photographs of the spider silk and PVDF/graphene films (both films with area equal to A5 paper), and white LED light (3 W). This LED was lightened using a hybrid nanogenerator with an area of 48 cm 2 . (f) The output current density of the hybrid nanogenerator during one cycle of hand tapping. The inset illustrates the input pressure generated by hand-tapping under a distance range. Reprinted with permission from [108]. Copyright ©2020, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Figure 13 shows the working mechanism of the IHM reported by Du et al. [109] during a walking cycle. Due to the pressure of the heel of the shoe in a walking cycle, the PENG shape is altered from arch to plane. This deformation generates a potential difference between both electrodes of the PENG. When the forefoot falls on the ground, the PTFE and Al layers are contacted in the through-hole of the sponge. The increment of the pressure of the forefoot increases the contact area between both PTFE and A layers. After the contact of both layers, a surface charge transfer induces between them a potential difference in the back Cu electrode (Figure 13b). This potential difference generates the electrons flow from the Cu electrode to the Al electrode using the external circuit. The PENG returns to arch shape after the heel lifts, generating a current opposite to that when the hindfoot falls ( Figure 13c). When the forefoot is lifted, the PTFE and Al layers of the TENG are separated. This causes the electrons flow from the Al electrode to the Cu electrode in the external circuit, which has an opposite direction to that when the forefoot falls (Figure 13d). This sequence of four states occurs during one cycle of walking. Due to the parallel design of TENG and PENG, both nanogenerators have the same electron flow direction. It allows the composite signal output of the IHN to be higher than those of the TENG and PENG. From human body motions, the IHN generated a maximum open circuit voltage of 150 V and a short-circuit current of 4.5 µA, respectively. Figure 14g shows the pulse signal of a dorsal pedis monitoring system that is supplied by the IHN. In 2022, Du et al. [109] designed a shoe insole hybrid nanogenerator (IHN) composed of a multilayered TENG and arched PENG. This nanogenerator can convert the mechanical energy of footsteps into electrical energy. Moreover, this nanogenerator can identify three types of motion states: walking, stepping, and jumping. This nanogenerator has high performance of electrical stability and durability, which is suitable to collect biomechanical energy of feet to power wearable electronic sensors. In addition, this nanogenerator has potential application for self-powered biosensor systems in the fields of sports and medicine. Figure 12 illustrates an application of the IHN integrated with a dorsalis pedis  Figure 13 shows the working mechanism of the IHM reported by Du et al. [109] during a walking cycle. Due to the pressure of the heel of the shoe in a walking cycle, the PENG shape is altered from arch to plane. This deformation generates a potential difference between both electrodes of the PENG. When the forefoot falls on the ground, the PTFE and Al layers are contacted in the through-hole of the sponge. The increment of the pressure of the forefoot increases the contact area between both PTFE and A layers. After the contact of both layers, a surface charge transfer induces between them a potential difference in the back Cu electrode (Figure 13b). This potential difference generates the electrons flow from the Cu electrode to the Al electrode using the external circuit. The PENG returns to arch shape after the heel lifts, generating a current opposite to that when the hindfoot falls (Figure 13c). When the forefoot is lifted, the PTFE and Al layers of the TENG are separated. This causes the electrons flow from the Al electrode to the Cu electrode in the external circuit, which has an opposite direction to that when the forefoot falls ( Figure  13d). This sequence of four states occurs during one cycle of walking. Due to the parallel design of TENG and PENG, both nanogenerators have the same electron flow direction. It allows the composite signal output of the IHN to be higher than those of the TENG and PENG. From human body motions, the IHN generated a maximum open circuit voltage of 150 V and a short-circuit current of 4.5 μA, respectively. Figure 14g shows the pulse signal of a dorsal pedis monitoring system that is supplied by the IHN.   Reprinted with permission from [109]. Copyright ©2022, John Wiley and Sons. Figure 13. Working mechanisms of the PENG and TENG parts of the IHN during one walking cycle [109]. Schematic of the charge transfer process of the PENG and TENG parts when (a) the hindfoot falls, (b) the forefoot falls, (c) the hindfoot lifts, and (d) the forefoot lifts, respectively. Reprinted with permission from [109]. Copyright 2022, John Wiley and Sons.

Potential Applications
This section describes the performance of potential biomedical applications of triboelectric or piezoelectric nanogenerators.
The PENGs and TENGs can be used in the modulation of neuronal activity, diagnostics for cardiovascular treatment, heart implants, drug delivery, and abdominal implants, among others. Ouyang et al. [110] reported an implanted symbiotic pacemaker integrated by a triboelectric nanogenerator, a power management unit (PMU), and pacemaker unit. The implantable triboelectric nanogenerator (iTENG) can harvest and store energy from cardiac motion. The energy collected by the iTENG is stored in the capacitor of the PMU. Next, this stored energy can be used to drive the pacemaker unit, generating pacing electrical pulses, and controlling the rate of cardiac motion ( Figure 15). Figure 16 shows the operating principle of the PTFE-based iTENG. Figure 17 depicts the electrical performance of the iTENG implanted in the chest of a Yorkshire porcine. For this case, the iTENG was located between the heart and pericardium of the pig. In addition, the PTFE component was placed on the left ventricular surface. Figure 18 illustrates the symbiotic cardiac pacemaker system in vivo using a wireless passive trigger. The pacemaker unit produces electrical pulses that can induce myocardial contraction and adjust the heart rate using pacing electrodes. The electrical pulses registered the output voltage and duration of 3 V and 0.5 ms, respectively. The symbiotic pacemaker could correct sinus arrhythmia and prevent damage. The iTENG achieved an open circuit voltage of 65.2 V and harvested energy from each cardiac motion cycle up to 0.495 mJ. This energy harvested is higher than that required for endocardial pacing (0.377 mJ). This iTENG could be employed for implantable medical electronic devices due to its high output power density and suitable stability. Thus, conventional batteries could be substituted. ms, respectively. The symbiotic pacemaker could correct sinus arrhythmia an damage. The iTENG achieved an open circuit voltage of 65.2 V and harvested en each cardiac motion cycle up to 0.495 mJ. This energy harvested is higher tha quired for endocardial pacing (0.377 mJ). This iTENG could be employed for im medical electronic devices due to its high output power density and suitable Thus, conventional batteries could be substituted.  Reprinted with permission from [110]. Copyright ©2019, Springer Nature.
Ouyang et al. [111] fabricated a flexible, self-powered, ultrasensitive pulse sensor (SUPS) formed by a triboelectric active sensor, which has an output voltage of 1.52 V, a peak signal-noise ratio of 45 dB, and good stability close to 107 cycles, as well as no expensive components. The SUPS structure ( Figure 19) is composed of a nanostructured Kapton (n-Kapton) triboelectric film (100 mm thick) with an electrode Cu film (50 nm thick) deposited on its back side, a second Cu film (50 nm thick) deposited on other Kapton film to form a nanostructured Cu (n-Cu) film, and a spacer. These components are encapsulated using an elastomer. The working mechanism of the SUPS depends on the coupling of contact electrification and electrostatic induction (Figure 19d). The concave structures of the device allow the contact electrification and electrostatic induction to occur simultaneously. This characteristic of the SUPS structure achieves a good relationship between applied external force and the electrical response of the device. Thus, the electrical signal of the SUPS can be modulated by the applied external force. Based on four different structures of triboelectric films, the electrical characterization of the SUPS was measured by applying a vertically compressive force (close to 50 N) with a linear motor on the triboelectric films (Figure 20a). The triboelectric layers integrated by n-Kapton and n-Cu films registered the best electrical output performance of the SUPS. For this case, the maximum output voltage, current, and transferred charge of the device were approximately 109 V, 2.73 mA, and 7.6 nC, respectively. Furthermore, the electrical performance of the device was measured on the radial arteria of a man (24 years old), as shown in Figure 20b. For this second case, the maximum output voltage, current, and transferred charge of the structures of n-Kapton and n-Cu were 1.52 V, 5.4 nA, and 1.08 nC, respectively. Also, the SUPS performance was tested to detect the mechanical oscillations of honeybee wings, which were converted to electrical signals (Figure 20c). Figure 16. Working mechanism of the PTFE-based iTENG designed by Ouyang et al. [110]. matic view of a corona discharge system. (b) Working mechanism of the iTENG. (c-e) R electrical performance (output voltage, transferred charge, and current) of the polarized polarized PTFE film considering a linear motor. Reprinted with permission from [110]. C 2019, Springer Nature. Two SUPS devices placed at distinct locations on the radial artery of the wrist of a man (24 years old) were used to measure the pulse wave velocity (PWV), as shown in Figure 21a,b. The PWV variations of this male patient before and after exercise consisting of 500 m jogs were recorded as shown in Figure 21c. Figure 21d depicts a schematic view of the main elements of the wireless pulse sensor system. This system includes the SUPS, analog-digital conversion, storage unit (8M-bit data), and Bluetooth interface. The SUPS output signals can be digitized using the ADC and wirelessly transmitted to a smart phone or laptop with the Bluetooth interface ( Figure 21h). Moreover, the SUPS can be adjusted to different human body parts for monitoring the pulse signals of the ankle artery, the finger, the radial artery, the brachial artery, and carotid artery zones, as shown in Figure 21e. Also, the SUPS could detect variations of the heart rates of patients due to different diary movement activities. In the future, this SUPS could be used in self-powered and wearable mobile diagnosis devices for cardiovascular diseases. Figure 16. Working mechanism of the PTFE-based iTENG designed by Ouyang et al. [110]. matic view of a corona discharge system. (b) Working mechanism of the iTENG. (c-e) R electrical performance (output voltage, transferred charge, and current) of the polarized a polarized PTFE film considering a linear motor. Reprinted with permission from [110]. Co 2019, Springer Nature.   Ouyang et al. [111] fabricated a flexible, self-powered, ultrasensitive pulse sensor (SUPS) formed by a triboelectric active sensor, which has an output voltage of 1.52 V, a peak signal-noise ratio of 45 dB, and good stability close to 107 cycles, as well as no expensive components. The SUPS structure ( Figure 19) is composed of a nanostructured Kapton (n-Kapton) triboelectric film (100 mm thick) with an electrode Cu film (50 nm thick) deposited on its back side, a second Cu film (50 nm thick) deposited on other Kapton film to form a nanostructured Cu (n-Cu) film, and a spacer. These components are encapsulated using an elastomer. The working mechanism of the SUPS depends on the , systolic blood pressure (sBP), heart rate (HR), femoral artery pressure (FAP), and ECG during the stimulation stage of the symbiotic pacemaker unit. (g) ECG signal of the heart rate considering a normal systolic blood pressure. (h) ECG signal under a pacing stimulus in the refractory period, considering a normal systolic blood pressure. (i) ECG signal of the heart successfully placed by the symbiotic pacemaker unit, with decreased systolic blood pressure. (j) ECG signal under failed pacing stimuli, regarding a restored systolic blood pressure. Reprinted with permission from [110]. Copyright ©2019, Springer Nature. adjusted to different human body parts for monitoring the pulse signals of the ankle artery, the finger, the radial artery, the brachial artery, and carotid artery zones, as shown in Figure 21e. Also, the SUPS could detect variations of the heart rates of patients due to different diary movement activities. In the future, this SUPS could be used in self-powered and wearable mobile diagnosis devices for cardiovascular diseases.    Chu et al. [112] proposed an active pulse sensing system for monitoring the vibration signals of the human radial artery. This system includes a sandwich-structure piezoelectret with high equivalent piezoelectricity. This structure is composed of fluorinated ethylene propylene (FEP)/Ecoflex/FEP layers. For the initial stage of this structure, the two electrodes (Al and Cu layers) have the same electrical potential. When the structure is deformed and compressed by external pressure, its dipole moment suffers a variation that generates a positive current in the external circuit. The reported system with wireless transmission and big data analyses could operate as a wearable m-Health system (Figure 22a). Figure 23 depicts the results of pulse wave intervals and corresponding Poincare plots measured from ten volunteers. In addition, Chu et al. [112] developed a three-channel pulse sensor array to measure the pulse waves at the Cun, Guan, and Chi pulse regions on the radial artery of the wrist considering the traditional Chinese medicine (TCM) technique ( Figure 24). Thus, a pulse-sensing system could detect and collect a large amount of the pulse signals in these three regions for potential applications of big data analyses and diagnoses. The high precision and stability of the proposed system are suitable for potential application as medical assessments, considering the identification of common heart diseases and measurement of blood pressure. the wings of a honeybee. (d) Output voltage of SUPS due to mechanical vibrations signals (10 kH generated by a loudspeaker. (e) Stability tests of the open-circuit voltage of SUPS during one milli operating cycles. For this test, external forces of 30 N were applied on SUPS using a linear mot Reprinted with permission from [111]. Copyright 2017, WILEY-VCH Verlag GMbH & Co. KGa Weinheim.  Liu et al. [113] designed a self-powered photodynamic therapy (s-PDT) system for potential application in cancer treatment. This s-PDT system is composed of a wearable twinning structure piezoelectric nanogenerator (ts-PENG), a power management unit (PMU), a miniaturized LED (m-LED), and a photosensitizer (PS) (Figure 25). This nanogenerator converts biomechanical energy into electrical energy to power a m-LED. This m-LED is used to stimulate light over tumor tissues, which inhibits tumor growth. The performance of this LED is controlled by the PMU with dimensions of 1.7 cm × 4.8 cm × 1.3 cm. This PMU can drive two irradiation modes: intermittent continuous light stimulation (ICLS) and pulsed light stimulation (PLS). For the ICLS mode, the tumor tissue can suffer strong radiation intermittently, which renders it dead. For the PLS case, small residual tumors can be destroyed by applying continuous low-dose irradiation.

OR PEER REVIEW
The ts-PENG structure developed by Liu et al. [113] is formed by a Parylene-C/PET packaging layer, a PET substrate, and a double piezoelectric layer of PVDF and Ag electrodes. The ts-PENG achieves the open-circuit voltage (V oc ) and short-circuit transferred charge (Q sc ) close to 200 V and 0.46 µC (Figure 26b). Furthermore, the ts-PENG reaches 340 mW/m 2 under load resistances between 10 kΩ and 10 GΩ (Figure 26d). Moreover, the wearable ts-PENG was tested on human and rat models ( Figure 27). First, the ts-PENG was attached on the knee joint and the elbow joint of a human body. The maximum values of the V oc and Q sc of the ts-PENG placed on knee joint and the elbow joint achieve 200 V, 220 V, 0.5 µC, and 0.65 µC, respectively. Also, an LED can be lighted using the biomechanical energy harvested by the wearable ts-PENG. In future applications, the PMU of the ts-PENG could drive the LED to apply both PLS and ICLS irradiation modes to tumors of the human body. On the other hand, the ts-PENG was positioned on the leg of a rat to convert its motion into electrical energy. A linear motor was used to pull the leg and simulate its mechanical motion. For this rat model, the maximum results of the V oc and Q sc of the ts-PENG are close to 8 V and 10.5 nC (Figure 27d). On the other hand, the s-PDT system was used for inhibiting tumors in vivo with intermittent continuous light stimulation ( Figure 28). Finally, the ts-PENG offers advantages such as simple structural configuration, high flexibility, good stability, light weight, and long service life. These ts-PENG characteristics are suitable for self-powered wearable medical devices. deformed and compressed by external pressure, its dipole moment suffers a generates a positive current in the external circuit. The reported system transmission and big data analyses could operate as a wearable m-Health s 22a). Figure 23 depicts the results of pulse wave intervals and correspon plots measured from ten volunteers. In addition, Chu et al. [112] developed nel pulse sensor array to measure the pulse waves at the Cun, Guan, and Ch on the radial artery of the wrist considering the traditional Chinese medicin nique ( Figure 24). Thus, a pulse-sensing system could detect and collect a la the pulse signals in these three regions for potential applications of big data diagnoses. The high precision and stability of the proposed system are suita tial application as medical assessments, considering the identification of c diseases and measurement of blood pressure.   Figure 23. Results of the pulse-sensing system measured from ten volunteers data of the peak-to-peak pulse wave intervals for the ten volunteers. The ninth a (V9 and V10) have pulse intervals with larger dispersion compared to the other is due to the ninth and tenth volunteers having arrhythmia that was previous ECG tests in hospitals. (b) Amplitude of pulse waves and (c) corresponding Po first volunteer (V1), who was not diagnosed with arrhythmia. (d) Amplitude of corresponding Poincare plots for the tenth volunteer, who has arrhythmia. Poin first and tenth (g) volunteers. The volunteer V10 has larger dispersion of pulse w dispersion of the pulse wave intervals could be related to arrythmia disease. R mission from [112]. Copyright 2018, WILEY-VCH Verlag GMbH & Co. KGaA Figure 23. Results of the pulse-sensing system measured from ten volunteers [112]. (a) Statistical data of the peak-to-peak pulse wave intervals for the ten volunteers. The ninth and tenth volunteers (V9 and V10) have pulse intervals with larger dispersion compared to the other eight volunteers. It is due to the ninth and tenth volunteers having arrhythmia that was previously diagnosed using ECG tests in hospitals. (b) Amplitude of pulse waves and (c) corresponding Poincare plots for the first volunteer (V1), who was not diagnosed with arrhythmia. (d) Amplitude of pulse waves and (e) corresponding Poincare plots for the tenth volunteer, who has arrhythmia. Poincare plots for the (f) first and tenth (g) volunteers. The volunteer V10 has larger dispersion of pulse wave intervals. Large dispersion of the pulse wave intervals could be related to arrythmia disease. Reprinted with permission from [112]. Copyright ©2018, WILEY-VCH Verlag GMbH & Co. KGaA, Weinheim.

R PEER REVIEW
ECG tests in hospitals. (b) Amplitude of pulse waves and (c) corresponding Poincare plots for first volunteer (V1), who was not diagnosed with arrhythmia. (d) Amplitude of pulse waves and corresponding Poincare plots for the tenth volunteer, who has arrhythmia. Poincare plots for the first and tenth (g) volunteers. The volunteer V10 has larger dispersion of pulse wave intervals. La dispersion of the pulse wave intervals could be related to arrythmia disease. Reprinted with p mission from [112]. Copyright 2018, WILEY-VCH Verlag GMbH & Co. KGaA, Weinheim.  Liu et al. [113] designed a self-powered photodynamic therapy (s-PDT) system for potential application in cancer treatment. This s-PDT system is composed of a wearable twinning structure piezoelectric nanogenerator (ts-PENG), a power management unit (PMU), a miniaturized LED (m-LED), and a photosensitizer (PS) (Figure 25). This nanogenerator converts biomechanical energy into electrical energy to power a m-LED. This m-LED is used to stimulate light over tumor tissues, which inhibits tumor growth. The performance of this LED is controlled by the PMU with dimensions of 1.7 cm × 4.8 cm × 1.3 cm. This PMU can drive two irradiation modes: intermittent continuous light stimulation (ICLS) and pulsed light stimulation (PLS). For the ICLS mode, the tumor tissue can suffer strong radiation intermittently, which renders it dead. For the PLS case, small residual tumors can be destroyed by applying continuous low-dose irradiation.    [113]. Copyright 2020, American Chemical Society.
The ts-PENG structure developed by Liu et al. [113] is formed by a Parylene-C/PET packaging layer, a PET substrate, and a double piezoelectric layer of PVDF and Ag electrodes. The ts-PENG achieves the open-circuit voltage (Voc) and short-circuit transferred charge (Qsc) close to 200 V and 0.46 μC (Figure 26b). Furthermore, the ts-PENG reaches  [113]. Copyright ©2020, American Chemical Society. 340 mW/m 2 under load resistances between 10 kΩ and 10 GΩ (Figure 26d). Moreover, the wearable ts-PENG was tested on human and rat models ( Figure 27). First, the ts-PENG was attached on the knee joint and the elbow joint of a human body. The maximum values of the Voc and Qsc of the ts-PENG placed on knee joint and the elbow joint achieve 200 V, 220 V, 0.5 μC, and 0.65 μC, respectively. Also, an LED can be lighted using the biomechanical energy harvested by the wearable ts-PENG. In future applications, the PMU of the ts-PENG could drive the LED to apply both PLS and ICLS irradiation modes to tumors of the human body. On the other hand, the ts-PENG was positioned on the leg of a rat to convert its motion into electrical energy. A linear motor was used to pull the leg and simulate its mechanical motion. For this rat model, the maximum results of the Voc and Qsc of the ts-PENG are close to 8 V and 10.5 nC (Figure 27d). On the other hand, the s-PDT system was used for inhibiting tumors in vivo with intermittent continuous light stimulation ( Figure 28). Finally, the ts-PENG offers advantages such as simple structural configuration, high flexibility, good stability, light weight, and long service life. These ts-PENG characteristics are suitable for self-powered wearable medical devices.    These results were measured after 12 days. Repri sion from [113]. Copyright 2020, American Chemical Society. Reprinted with permission from [113]. Copyright ©2020, American Chemical Society.

Challenges and Perspectives
This section describes various challenges and perspectives of piezoelectric and triboelectric nanogenerators to power future healthcare monitoring devices, including the topics of design, materials and fabrication processes, signal processing, output performance, and reliability.

Design
The nanogenerator design process is key to developing better piezoelectric and triboelectric structural configurations that optimize energy harvesting to power healthcare monitoring devices. For each potential medical device, the design of a nanogenerator must examine the main performance requirements and limitations of this device, optimal materials and manufacturing process, surrounding conditions (e.g., temperature, relative humidity, pressure, dust, and so on), encapsulated type, signal processing components, and the characteristics of the original energy sources (e.g., biomechanical, wind, mechanical vibrations, and so on). For instance, nanogenerators can harvest biomechanical motion to power medical devices. However, optimal energy harvesting from human body motion has great challenges. In order to develop efficient and stable nanogenerators attached to the human body, the design of nanogenerators must consider scalable, wearable, durable, and stretchable materials. In the coming years, smart textile-based nanogenerators could be incorporated into clothing using piezoelectric or triboelectric materials with special characteristics such as being wearable, stretchable, durable, and washable. These nanogenerator types could power smart sensors that monitor real-time signals on the health status of patients. By using the IoMT, these signals could be transmitted to medical hospitals or doctors. In addition, future nanogenerators could be attached to different parts of the human body, which will require designs of electromechanical components composed of materials with high flexibility, high reproducibility, easy fabrication, and friendly performance. Future implantable self-powered medical devices will require nanogenerators with biocompatible packaging materials.
To achieve optimal designs for the output performance of nanogenerators, multioptimization techniques can be implemented, including the various objective functions and constraints. For instance, the output power density and cost of a nanogenerator can be the objective functions to be maximized and minimized. The constraints equations of a nanogenerator can consider its size, service time, reliability parameters, and so on. For these optimization models, the mechanical and physical properties of the materials of the nanogenerator must be known.

Materials and Fabrication Processes
The output performance and reliability of piezoelectric and triboelectric nanogenerators for monitoring healthcare devices depend on the suitable selection of their materials and manufacturing processes. These materials should have electromechanical properties that allow for the development of nanogenerators with the benefits of cost-effectiveness, wearability, flexibility, easy fabrication, stability, and robust performance. Furthermore, the optimal selection of materials can be useful to fabricate smart nanogenerators with both functions for energy scavenging and working as active/self-powered sensors, which reduce their size and weight. Moreover, smart nanogenerators can be eco-friendly to the environment, eliminating the use of harmful materials from conventional batteries.
Recent investigations of nanogenerators have taken into account organic or waste materials, including chitin, spider silk, tomato, rice paper, peanut, walnut, pistachio, almond, fish gelatin, fish bladder, eggshell, sunflower husks, garbage soda cans, silk fibroin, and so on [114][115][116][117][118][119][120][121]. For these materials, two research challenges are the analytical modeling and experimental measurement of their piezoelectric and triboelectric transduction mechanisms. In addition, future investigations must consider standard rules and tests to measure the performance and reliability of nanogenerators. Another challenge is the development of scalable manufacturing processes for nanogenerators, which decrease costs and satisfy the future commercial market.

Signal Processing
Generally, the output voltage and current of triboelectric nanogenerators are not direct current (DC) signals. Due to this limitation, triboelectric nanogenerators need rectifier circuits to convert their AC output signals into DC signals. This DC voltage can be stored using capacitors or batteries to power the monitoring healthcare devices. A research challenge in this topic is the development of cost-effective energy storage systems coupled with nanogenerators.
Recent investigations [122][123][124] on nanogenerators have included novel electrical interfaces of high efficiency and minimum power consumption. The electrical interfaces could be self-powered and include cold-start circuit architectures [125,126]. In addition, the electrical interfaces could incorporate a small footprint to decrease the size of the nanogenerators. For this, the Application-Specific Integrated Circuit (ASIC) could be implemented.

Reliability
The implementation of reliability standard tests and rules is required to measure the safe and stable performance of nanogenerators for medical applications. The reliability analyses of the electromechanical performance of nanogenerators under different environments and operating conditions are required to study their stability, durability, and working capability. These reliability tests can provide the main electromechanical failures of the nanogenerators due to various factors, such as high relative humidity, large temperature variations, dust, mechanical impact, wear, crack growth, fatigue, and so on. Furthermore, the different packaging types and signal-processing components of nanogenerators must be included in the reliability tests. In order to improve the reliability of nanogenerators, the designers could consider simple operating mechanisms, minimum electrical and structural components, robust and durable materials, safe packaging, and so on. Figure 29 depicts various challenges of the piezoelectric and triboelectric nanogenerators for their application in IoMT electronic devices.
working capability. These reliability tests can provide the main electromechanical failures of the nanogenerators due to various factors, such as high relative humidity, large temperature variations, dust, mechanical impact, wear, crack growth, fatigue, and so on. Furthermore, the different packaging types and signal-processing components of nanogenerators must be included in the reliability tests. In order to improve the reliability of nanogenerators, the designers could consider simple operating mechanisms, minimum electrical and structural components, robust and durable materials, safe packaging, and so on. Figure 29 depicts various challenges of the piezoelectric and triboelectric nanogenerators for their application in IoMT electronic devices.

Conclusions
Recent progress on piezoelectric and triboelectric nanogenerators for self-powered monitoring healthcare devices was reviewed. The descriptions of the working principle, materials, and fabrication processes of a variety of these nanogenerators were reported. Discussions on the output electrical performance of piezoelectric and triboelectric nanogenerators are included. Furthermore, we presented the signal processing components and packaging types of different nanogenerators. The design, materials, and electromechanical behavior of different piezoelectric and triboelectric nanogenerators for biomechanical energy harvesting to power potential medical applications were considered. Also, we proposed some challenges and perspectives of these nanogenerator types, including the design, materials and manufacturing processes, signal processing, and reliability.