Sustainable wearable energy storage devices self‐charged by human‐body bioenergy

Charging wearable energy storage devices with bioenergy from human‐body motions, biofluids, and body heat holds great potential to construct self‐powered body‐worn electronics, especially considering the ceaseless nature of human metabolic activities. To bridge the gap between human‐body bioenergy and storage of energy, wearable triboelectric/piezoelectric nanogenerators (TENGs/PENGs), biofuel cells (BFCs), thermoelectric generators (TEGs) have been designed to harvest energy from body‐motions, biofluids, and body heat, respectively. Researchers have explored various strategies using bioenergy harvesters to charge wearable supercapacitors and batteries to relieve or even fully eliminate the recharging process from external power stations, thus, making wearable electronics more sustainable, autonomous, and user friendly. In this article, we review the advances in the design of sustainable energy storage devices charged by human‐body energy harvesters. The progress in multifunctional wearable energy storage devices that cater to the easy integration with human‐body energy harvesters will be summarized. Then, the focus is laid on the integrating strategies (single‐cell strategy and separated‐cell strategy), device design, materials selection, and characteristics of different self‐charging human‐body energy harvesting‐storage systems. Finally, the challenges that impede the wide application of human‐body energy harvesters charged supercapacitors/batteries and prospects will be discussed both from materials and structural design aspects.


INTRODUCTION
The wide applications of wearable sensors and therapeutic devices await reliable power sources for continuous operation. [1][2][3][4] Electrochemical rechargeable energy storage devices, including supercapacitors (SCs) and batteries, energy storage devices. [9][10][11] However, most of the existing wearable energy storage devices exhibit low-energy density, on account of the limited thickness and area of the flexible devices. Meanwhile, the self-discharging nature of electrochemical energy storage, especially for SCs, inevitably drains the stored energy. A frequent recharging process with cumbersome wired power transmission from an immovable electrical grid is required to elongate the working periods, causing inconvenience to users. Integrating wearable energy harvesting devices with energy storage devices to form a self-sustainable power source has been an attractive route to replenish the consumed energy of the SCs/batteries, and thus, decrease the frequency of recharging or even enable a fully self-sustainable wearable electronics system. 12 Since most wearable electronics are adapted to the human body, harvesting energy from the human body, including biomechanical, biofuel (glucose/lactate), and body heat energy using PENG/TENG, biofuel cells (BFCs), and thermoelectric generators (TEGs), respectively, has received considerable interest. 13 Compared with the lightdependent wearable solar cells, human-body energy highly relies on human motions or continuous metabolic activities, thus, providing a reliable power source for wearable electronics. Meanwhile, wearable energy harvesters possess the advantages to integrate with various sensors for a self-powered sensing patch as they need to be mounted on the human body. This proof-of-concept design can be separated into two types. One is to use the output of the human-body energy harvesters to directly indicate the physiological intensity. The power, current, or voltage outputs of human body energy harvesters are proportional to the intensity of human physiological signals such as frequency of human motions, 14 concentrations of lactate/glucose in sweat/blood, 15 or temperature of the human body. 16 Another way is to utilize a form of human-body energy harvester to provide energy for other biological signals or external stimuli, such as using the triboelectric nanogenerators (TENGs) to power a temperature sensor and chemical sensors. 17,18 More specifically, the human-body energy harvesters can be possibly fabricated into a self-sustainable implantable device, to eliminate the requirement of the recharging process, and avoid further surgery for changing the energy source that would impart suffering, high cost, and risk to wearers. 19 For example, implantable TENG and BFC have been demonstrated with the capability to power the implantable pacemakers. [20][21][22][23] Even though the human-body energy harvesters implement on or in the human body are limited with lower energy generation, the miniaturized, flexible, and energy-saving developing trend of microelectronics technology is broadening the application of human-body energy. [24][25][26] F I G U R E 1 Wearable supercapacitors/batteries self-charged by TENG/PENG, BFC, and TEG harvesting energy from body motions, biofluids, and heat, respectively The merits of human-body energy harvesters stimulate the development of charging flexible SCs/batteries with energy converted from body motion, body heat, and biofuels to provide sustainable power sources for wearable or implantable devices. [27][28][29][30] This design enables thin wearable SCs and batteries to be free of frequent recharging from immobile power stations. On the other side, integration with SC/battery could accumulate irregular energy from the human body and then deliver stable outputs for the performance of TENG/PENGs, BFCs, and TEGs that are affected by movement status and surrounding environment, greatly expanding the practical application of human-body energy harvesters. 12,31 Most human-body energy harvesters are fabricated with conformability and mechanical compliance to have intimate contact with soft and curvilinear human tissues such as human skin, heart, and eyes. Thus, thin polymer films, tattoos, contact lens, and textiles have been used as platforms for wearable energy harvesters to not only function as traditional energy harvesters but also with specific properties such as comfort for wearing, biocompatibility, flexibility, stretchability, durability to washing, or ease of fabrication. [32][33][34][35] To have a high-level integration with existing energy harvesters, energy storage devices need to have the same functionalities and compatible fabrication techniques with energy harvesters.
In this review, we summarize the recent progress on charging wearable electrochemical energy storage devices with different human-body bioenergy harvesters, including TENG/PENGs, which generate energy from human-body motion, BFCs extracting energy from biofluids and TEGs harvesting energy from body heat ( Figure 1). First, the advances in multifunctional wearable energy storage devices that cater to the easy integration with human-body energy harvesters will be shortly summarized. Then, the focus is laid on the integrating strategies (single-cell strategy and separated-cell strategy) for the self-charging human-body energy harvesting-storage system for wearable electronic devices. Charging wearable electrochemical energy storage devices with energy from the human body holds the potential to relieve or even eliminate the frequent recharging from stationary power stations. Meanwhile, wearable energy storage devices could buffer irregular or unstable outputs of human-body energy harvesters and subsequently release them as the constant power source. Lastly, the challenges that impede the wide application of human-body energy harvesters charged SCs/batteries and prospects will be discussed with perspectives given from the materials and structural design aspects.

WEARABLE ELECTROCHEMICAL ENERGY STORAGE DEVICES
Wearable SCs and batteries consisting of an anode and a cathode with a separator containing dissociated salts to enable ions transfer are two main types of electrochemical energy storing devices for wearable electronics. Flexibility is essential for devices to endure bending mechanical deformation. The current status quo of flexible power sources mainly focuses on the replacement of the brittle materials with flexible functional materials to endure the mechanical deformation such as recycling, bending, or stretching. For SCs, two different mechanisms are applied to fabricate electrical double-layer capacitors (EDLC) or pseudocapacitors. ELDCs store charge at the electrode/electrolyte interlayers by electrostatic double layers, while a surface or near-surface Faradic reaction which is reversible is involved in a pseudocapacitor. 36 Pseudocapacitors show much higher energy density and capacitance than that of ELDCs, but their power density is lower, resulting from the slower surface redox or intercalation on electrodes. 36,37 Battery is a kind of transducer that converts electrical energy to chemical energy when charged from external power sources. In a charging and discharging cycle, the ions, such as lithium for lithium-ion batteries (LIB), transfer between bulk anode and cathode through an intercalation/deintercalation process to complete a reversible redox reaction. This process is controlled by the diffusion of ions in electrolyte and bulk electrodes, making the power density and charge-discharging rate of batteries lower than these of SCs. However, batteries exhibit much higher energy density, lower self-charging rates than SCs.
Among various battery systems, LIBs play a dominant role in the portable secondary battery market, owing to the superiorities in high voltage, high-energy density, low selfdischarge, and long cycle life. However, the widespread adoption of wearable LIBs is hindered by concerns about safety issues and high costs. Various battery systems have been invented to replace LIBs. Sodium/potassium-ion batteries (SIBs/KIBs) involving similar Na + /K + intercalation/deintercalation processes as Li + were considered as the replacement of LIBs, due to the abundances of Na/K. 38 The commercially used graphite anode in LIBs cannot be used for Na + /K + intercalation. Wang and coworkers designed robust and monolayer hydrogen boride (HB) as the anode of KIBs. Beyond super-high areal capacity improved by the H vacancies (1138 mAh/g), the HB exhibited ultralow bending and high in-plane stiffness, making it an ideal material for the fabrication of wearable KIBs. 39 Zinc ion batteries (ZIBs,) applying metallic zinc as the anode, have a high theoretical capacity (820 mAh/g), low electrochemical potential (−0.763 V versus SHE), resource abundance, low toxicity, and safe nature from aqueous nature. Various rechargeable ZIBs have been developed, including the reaction mechanisms, selection of cathode materials, performance improvement of Zn anode, and electrolytes innovations, dramatically boosting the application of ZIBs for wearable electronics. 40 Ag-Zn batteries with comparable specific capacity with LIBs have been widely used in portable biomedical electronics. Lee and coworkers designed the first example of a stretchable Ag-Zn battery by embedding the silver nanowires into the PDMS elastomer as the current collectors and electroactive cathodes simultaneously. 41 The Zn anodes were produced by electrodepositing Zn on top of embedded Ag nanowires electrodes. The fabricated battery could maintain the performance at both relaxed and stretched states during 1000 cycles of 80% stretching, suggesting its great durability to severe mechanical deformations. Alkaline electrolytes enable ZIBs with high operation voltage, high capacity, and tolerance to high-current discharging, compared with the neutral or acidic electrolytes. Yin et al. developed a printed rechargeable AgO-Zn battery with a high capacity of 54 mAh/cm 2 and robust resiliency to mechanical deformation. The high capacity comes from the utilization of higher oxidation state oxide-AgO, whereas only Ag 2 O is utilized in most of the Ag 2 O-Zn batteries. 42 This work presents a practical way for the fabrication of power sources for wearable electronics. The utilization of safer electrolyte may further widen the application of ZIBs, for example, a solid-state wearable and very safe ZIB utilizing gelatin and polyacrylamide (PAM)-based hierarchical polymer as the electrolyte, and α-MnO 2 nanorod/CNT as the cathode was fabricated by Li et al. in 2018. 43 The electrolyte is produced by involving the grafting of PAM onto gelatin and the filling of gelatin chains in PAN (polyacrylonitrile) electrospinning networks.
The use of hazardous-free materials is also one important aspect of wearable batteries. Biocompatible and even biodegradable materials or fluids have been considered to replace the traditional electrodes or electrolytes. Bandodkar et al. designed a biocompatible battery using Mg as the anode and human sweat as the electrolyte. 44 The fast sweat capture and storage was enabled by the utilization of a microfluidic channel fabricated using silicone and paper. The Mg battery was thin, flexible, and reliable for powering wireless epidermal sweat sensors and the heart rate sensor, suggesting the promising future of Mg batteries to serve as wearable power sources. Wang et al. imparted the stretchability to their "green" Mg battery (lemon juice as the electrolyte) by introducing the kirigami structure design. 45 The Mg battery can endure bending, twisting and stretching, paving the way for the fabrication of "green" wearable power sources.
Endowing the electrochemical energy storages with flexibility/bendability is an initial step in the innovation of wearable power sources. When being used in more mechanically challenging scenarios, such as biointegrated devices, they are required to be able to endure harsher stretching deformations. 46 Detailed review and discussion on wearable electrochemical energy storages are available in previous reviews. 11,47,48 Herein, we focus on the specific aspects of harvesting bioenergy from human body for charging wearable energy storage.

ENERGY HARVESTING FROM THE HUMAN BODY
Biomechanical motions, body fluids, and body heat are rich power pools for renewable energy from the human body. To effectively extract bioenergy from the human body, TENG/PENG, BFCs, and TEGs with different working mechanisms, have been established. One challenge of human-body energy harvesters is the stable generation of electricity for wearable or implantable electronic devices in harsh working scenarios such as during the stretching for skin-worn harvesters, or the complicated physiological environment for implantable harvesters. Recent progress on advanced materials synthesis, fabrication techniques, and working mechanisms has dramatically speed up the practical applications of human-body energy harvesters. 49-56

TENG/PENG for mechanical energy harvesting
Nanogenerator, based on displacement current, is an emerging energy harvesting technology that efficiently converts ambient random mechanical energy into elec-tricity for small electronics such as distributed sensors and wearable devices. 57 In 2006, Wang et al. firstly reported a piezoelectric nanogenerator (PENG), which utilized ZnO nanowires to generate potential differences upon a strain. 58 The working principle of PENGs is depicted in Figure 2A. The PENG is composed of a piezoelectric material that is sandwiched by the top and bottom electrodes. Under vertical strain, the piezoelectric material generates an electrostatic potential and induces the flow of electrons between the two electrodes. When the strain is released, a reversed current is formed by the backflow of the electrons. PENG is widely applied in self-charging units and self-powered sensors. To increase the current output, Gu et al. proposed a PENG with three-dimensional intercalation electrodes ( Figure 2B). 59 The multilayer structure created many boundary interfaces between the piezoelectric materials (Sm doped PMN-0.31PT NWs) and electrodes, which increased the total amount of polarization charges (1690 μC/m 2 ) as well as the current density (290 μA/cm 2 ). The PENG could charge a 1 μF capacitor very fast by harvesting walking energy.
In 2012, Fan et al. reported a flexible triboelectric nanogenerator (TENG), 60 which has higher output and a wider choice of materials than PENG. The working mechanism of a TENG is based on the conjunction of triboelectrification and electrostatic induction ( Figure 2C). A TENG has two dissimilar materials, which are positively and negatively charged respectively by triboelectrification. The relative movement of these two charged materials creates a potential difference and the flow of charge between two back electrodes. The TENG has four kinds of working modes: contact-separation, lateral sliding, single electrode, and free-standing mode, showing the versatility to scavenge energy from different kinds of mechanical movements like contacting, bending, pressing, sliding, and rotating. The flexible TENG is very suitable for biomechanical energy harvesting from low-frequency human motion. Xiong et al. proposed a textile-based wearable TENG involving the black phosphorus for the durable triboelectric electron-trapping coating to efficiently harvest body movement energy ( Figure 2D). 61 The black phosphorus protected by cellulose-derived hydrophobic nanoparticles makes the wearable TENG with excellent long-term water repellency and durability regardless of various extreme conditions, broadening the application of the ambient-sensitive materials in TENGs.

BFC for biochemical energy harvesting
BFCs are constructed to consume biomass for electricity generation through natural processes. Glucose and Working mechanism of wearable thermoelectric energy generators. Reproduced with permission. 73 Copyright 2017, Elsevier. (H) Schematics of thermoelectric fibers for the construction of stretchable fabric energy harvesters. Reproduced with permission. 75 Copyright 2020, Springer Nature lactate are two widely used fuels for implantable or skin-worn BFCs due to their abundance in blood and sweat, respectively. 13,[62][63][64] Figure 2E schematically shows the working mechanism of a typical glucose-oxygen BFC composing of a glucose oxidase (GOx) immobilized anode to generate protons and electrons through glucose oxidation reaction, and a bilirubin oxidase (BOx) enzyme immobilized cathode to catalyze the oxygen reduction reaction. 63 When the anode and cathode are connected by an external loading, electrons transfer from the GOx anode to the BOx cathode, and thus, current and power are generated. The current intensity is proportional to the biofuel concentration until the BFC is saturated. Major challenges of human-body BFCs are the stability of enzyme-based electrodes in dynamic surrounding environments (eg, pH, temperature, oxygen levels, or humidity changes) and functional materials to manifest specific capability in different working places such as mechanical resiliency for skin-worn BFCs. The power density of human-body BFCs can reach several mW/cm 2 and be able to power commercially available radio transmitters for the wireless communication between electronics and data receiver stations. For example, Bandodkar et al. reported a stretchable high-power-density epidermal BFC consuming lactate in human sweat through the combination of thick screen-printed CNT-based anode and cathode pellets and stretchable metal connection framework in an "island-bridge" configuration. 1 During exercise, these BFCs could utilize the high areal surface to generate a high-power density of approximately 1 mW/cm 2 and power a Bluetooth Low Energy (BLE) radio, as shown in Figure 2F. Recently, Yu et al. designed flexible epidermal BFCs with a high-power density of 3.5 mW/cm 2 by integrating zero-(Pt-Co nanoparticles) to three-dimensional (CNT network) nanomaterials in a highly conductive photolithographic Cu interconnection. 66 The fabricated BFCs manifested a high-power density, conformable contact with human skin, and capability for powering wireless epidermal sensors (metabolic analytes and temperature), suggesting the great potential of sweat-based BFCs to power wearable electronic devices.

TEGs for biothermal energy harvesting
Released heat from the body accounts for the biggest proportion of energy provided by food and the mechanical efficiency of the body is only around 15-30%. 67 Human body temperature is stabilized at 37°C through metabolism and, thus, enables the body heat to be a continuous energy source. Appropriately, 0.6-1.8 W power which is enough to power many wearable sensors could be generated if thermoelectric can harvest dissipated heat from one whole body (60-180 W) at a approximately 1% conversion efficiency. 68 Wearable thermoelectric generators (TEGs) conformally contacting with human skin are designed to convert human body heat into electricity through the Seebeck effect. 52,[69][70][71][72] Figure 2G shows the cross-section of wrist-based TEGs having several P-type and N-type legs, copper strips, PDMS encapsulation, and a flexible thermal interface layer (TIL) to enable the wearability of TEGs on curved skin. 73 The differences between human skin and ambient temperature cause a stable heat flux flowing through the TEGs, thus, could generate the voltage between two legs. The maximization of conversion efficiency, intimacy TEG-body attachment, wearing convenience, low weight, and reliability are key factors that need to be solved when wearable TEGs are designed at different levels (materials, devices, and systems). 74 Recently, Sun et al. reported the first example to wove selectively doped CNT fibers ( Figure 2H) with a wrapping of acrylic fiber into stretchable π-type 3D fabric TEGs. 75 The truly textile-based TEGs have an excellent stretchability of 80% from interlocked thermoelectric modules, a superior peak power density of 70 mW/m 2 at 44 K temperature difference, and compatibility with body movement. This work would greatly facilitate the development of textile-based TEGs to harvest energy from body heat. Wearable TEGs can also be used in augmented reality and virtual reality applications. A comprehensive review on the recent adoption of thermal technology for augmented reality and virtual reality can be found in a recent review. 76 The working mechanism and output characteristics define the specific applications of each bioenergy harvesting technology. TENG/PENGs are flexible, simple in fabrication, and very efficient in harvesting the motion of the human body with low-frequency nature, making them suitable both for implantable and wearable use. The energy is generated in pulse mode with high voltage but low current. BFCs work like traditional fuel cells and can generate high-power density at the mW/cm 2 range and current density at around mA/cm 2 level in the presence of biofluids. However, the involvement of enzymes limited the performance stability in the prolonged operation. As the human body is a continuous source of heat, and nonbioactive materials are involved in the fabrication of TEGs, wearable TEGs are logically fit for nonstop energy harvesting. The energy generated by TEGs also can be high to the microwatt level. However, the amount of energy generated by a certain TEG highly relies on the temperature difference between the human body and the surrounding environment. Thus, it is still challenging to guarantee a stable output considering the varied surrounding temperature over a day. Also, the application of TEGs for implantable energy harvesting has rarely been reported for the minor temperature difference inside the human body. The details about the fabrication, characteristics, and challenges of wearable PENG/PENGs, BFCs, and TEGs have been fully presented in previous reviews. 49,52,63

HUMAN-BODY MOTIONS CHARGED ENERGY STORAGE DEVICES
The movements of the human body are a fountain of mechanical energy sources that can be harvested for powering portable electronics such as the voluntary movements of skeletal muscle and involuntary activities of digestion, respiration, and heartbeat. Nanogenerator is a promising technology for human-motion energy harvesting due to the advantages of wearable, lightweight, and a high response to the low trigger. Many efforts have been exerted to combine the nanogenerators and capacitor/battery for the wearable self-charging power units (SCPUs), which could provide a continuously manageable current source.

PENG for mechanical energy harvesting
The direct conversion of mechanical energy into the chemical energy of a battery was first reported in 2014 by Zhang et al., known as a SCPU. 77 A novel PVDF-PZT nanocomposite film was developed, serving as both the piezoelectric material of PENG and the separator of the battery. This all-in-one hybrid device design was compact and concise. Under strain, the piezoelectric potential of PVDF-PZT film driven ions to migrate through the film and incorporate All-in-one core-shell fiber-based SCPU with TENG outside and SC inside. 89 Reproduced with permission. 86 Copyright 2018, American Chemical Society. (F) A super stretchable textile-based coplanar SCPU with TENG and SC. Reproduced with permission. 90 Copyright 2020, American Chemical Society into the anode electrode. Following this work, the authors demonstrated a new all-solid-state SCPU ( Figure 3A). 78 The mesoporous PVDF-LiPF 6 film was used as the piezoseparator. The fabricated SCPU showed high power generation and high energy conversion/storage efficiency. Under periodic compressive deformation, the SCPU was quickly charged to 0.118 μAh within approximately 240 s. The SCPU was demonstrated in wearable power sources for sports bracelets and smart watches. Parida et al. reported a fast-charging SCPU that significantly shorten the charging time for near one order of magnitude (Figure 3B). 79 The porous P(VDF-TrFE) film was impregnated with PMMA/PC/LiClO 4 gel electrolyte to act as the piezoelectric separator. Under strain, the P(VDF-TrFE) film generated a potential and induced an electric double layer at the carbon nanotube (CNT) electrodes, leading to fast adsorb and desorb ions. Although these kinds of all-in-one SCPU are compact, the power density is relatively low.

TENG for self-charging battery
The combination of high output TENG and high-energy density battery enables the fast charging and longtime operation of the SCPU. Zi et al. firstly reported a SCPU that combined a TENG and a LIB, as shown in Figure 3C. 80 The arch shape TENG harvested energy from surrounding mechanical vibrations and simultaneously charged the LIB. The SCPU independently provided a constant current of 2 μA at 1.55 V voltage and continuously powered a UV sensor, while the full-charged LIB alone could only power the sensor for a few hours. The SCPU was mounted under a shoe to harvest walking energy, serving as a self-sustainable and portable power source. However, these SCPUs have poor stretchability because of using common polymer film and aluminum electrodes. 81 Fibers/yarns shape TENGs and batteries are more flexible and can be integrated into wearable textiles. Pu et al. reported a wearable SCPU based on textile TENG and flexible LIB belt (as shown in Figure 3D). 82 A piece of common polyester fabric was coated with Nickle (Ni) to act as electrodes for the TENG, and the LiFePO 4 and Li 4 Ti 5 O 12 were coated on the Ni-cloth as the cathode and the anode of LIB. The SCPU had good flexibility and could be worn in different positions to harvest human-motion energy for powering a remote heartbeat meter. Besides, the Zn-ion battery (ZIB) was also combined with TENG to form a wearable all fabric-based SCPU. 83 The SCPU knitted into the pixel of a flexible fabric substrate could harvest the energy of human finger motion and charge the battery for powering an electronic watch.

TENG for self-charging supercapacitor
A SC, especially the solid-state SC, is preferable for TENG energy storage because of the superior flexibility, reliability, and safety. 84-For wearable devices, fiber and fabric provide a promising platform. Wang et al. reported a wearable SCPU consisting of all fiber-based SC and TENG for the first time, which was weaved on a coat to harvest the jogging energy. 88 Besides, a highly integrated core-shell fiberbased SCPU was demonstrated, which had SC inside and TENG outside. In such a coaxial fiber, carbon fiber bundles were utilized as the electrode material for the TENG and SC ( Figure 3E). 89 The coaxial SCPU was knitted into a fabric to scavenge patting energy and power an electric watch. A washing test was conducted to verify the good launderability of the SCPU. The SCPU devices could also be constructed directedly on textile. Cong et al. printed a super stretchable SCPU on the textile through a resist-dyeinganalogous method. The Ni and reduced graphene oxide were used as electrode of TENG and micro-SC, which could maintain excellent conductivity even at 600% tensile strain. (Figure 3F). 90 The SC reached 50.6 mF/cm, 2 while the output of TENG was 94.5 mW/m. 2 It was demonstrated in powering a watch.
However, in the system scale, the impedance match between nanogenerator and battery/capacitor is very important for efficiency and lifetime. The pulsed high voltage and low current characteristics of nanogenerators influence the charging efficiency and the stability of the battery/capacitor. A power management system is preferred to bridge the energy generation and storage, which usually involves a step-down transformer and an AC-DC converter. Song et al. developed a power management and biosensors system on a flexible printed circuit board, which could transmit data to a mobile phone in real time. 18 In the future, efforts are needed to improve the generator's output, enhance the efficiency of the power management circuit, and develop suitable application systems.

HUMAN-BODY BIOFLUIDS CHARGED ENERGY STORAGE DEVICES
Utilizing energy from human-body biofluids to charge energy storage devices can be derived from the BFCcharged SCs due to their high-power density, safety, long cycling life, and high speed of the charging-discharging process. The charged SC can deliver much higher power than BFCs harvesting energy from human blood, sweat, tears, and mitigate the unstable outputs of BFCs caused by the variable levels of biofluids, thus, broadening the capabilities of biofluid energy to power wearable electronic devices. Until now, there are two configurations in integrating SCs and BFCs including a single-cell strategy that functionalizes enzyme on capacitive electrodes or a separated-cell strategy in which two separated modules are connected by an external circuit.

Single-cell strategy
The advance of highly conductive, superior capacitive, and electrochemically inert materials fostered the development of immobilizing enzymes on those materials to construct hybrid devices, called biosupercapacitors, with the function of harvesting energy and storing the harvested energy in the presence of biofluids. 91 104 have been explored to endow traditional BFCs with the capability to store energy. The hybrid device could release electricity in a much higher power density than BFCs resulting from the fast charge-discharge rate of SCs. As shown in Figure 4A, a biosupercapacitor was constructed by Agnès et al. to wire GOx enzyme and laccase directly on two CNT matrixes as the anode and the cathode, respectively, for BFC setup. The high electrical conductivity, high specific surface of CNT matrixes enabled the storage of charge in the same device. Energy converted from biocatalytic reaction (glucose oxidation on anode and oxygen reduction on the cathode) can continuously charge the CNT matrix. Thanks to the fast discharging-charging nature of the CNT electrochemical double layer, the fabricated biosupercapacitor can release a high-power density of 15 mW/cm 2 at 0.5 V in a pulse mode for more than 40 000 cycles. The biosupercapacitor could recover itself after each discharge pulse through the energy generated from continued oxidation of glucose on the anode and oxygen reduction on the cathode. 105 Most of the reported biosupercapacitors involving the reduction of oxygen on the cathode. However, the concentration of dissolved oxygen in the biofluids is much lower than that of glucose, for example, only 0.14 mM oxygen in arterial blood and 0.08 mM in intestinal tissue, whereas 4.8 and 3.3 mM glucose in plasma and muscle, respectively. To mitigate the unstable deficiency of The schematics of a glucose BFC-SC integrated energy system and its charging-discharging performance. Reproduced with permission. 106 Copyright 201, Elsevier. (C) The structure and the photo image of a microelectronic temperature sensing system which includes an implantable glucose BFC and SC integrated energy system as the power source. Reproduced with permission. 107 Copyright 2020, Wiley-VCH. (D) Schematics of a hybrid stretchable sweat-based BFC-SC energy harvesting-storage system on two sides of a piece of textile and its in-vivo charging performance from a subject's exercise. Reproduced with permission. 30 Copyright 2018, The Royal Society of Chemistry oxygen for in vivo application, Xiao et al. utilized solid nanoporous gold/MnO 2 to replace enzymatic cathode based on oxygen reduction. During discharging, the fraction of Mn(IV) (∼1.3%) accepts one electron and converts to insoluble Mn(III) and then the redistribution of surface Mn(III) and Mn(IV) in the bulk through the charge transfer causes the recovery of cathode potential in the reset step. Combing with a supercapacitive glucose dehydrogenase/redox polymer biocathode, the designed biosupercapacitor delivered 294 times higher power density than the continuous mode in N 2 -bubbled 10 mM glucose solution, overcoming the limitation of oxygen supply in most biosupercapacitors. 104 The simple configuration of biosupercapacitor integrating two modules in one device can deliver high power output compared with classic BFC. However, the reported devices still utilize materials with limited flexibility, impeding the application of biosupercapacitors for wearable electronics. Another issue is about the shared electrolyte may constrain the performance of each module. The electrolyte with a high concentration of ions is preferred by SCs, but it is unrealistic in standard biofluids.

BFC charge energy storage devices in a lateral connection
When BFCs and SCs are fabricated separately and then integrated on the same substrate, several merits can be reached: (1) the electrode materials and device configurations of each part can be optimized for better performance; (2) SC can be free from sharing the electrolyte with BFC, thus, specific and high-performance electrolytes can be used; (3) flexibility of integrating BFCs and SCs based on specific power requirements; (4) power released from SCs can be in not only a pulsed mode but also a continuous mode with high-power density. The advantages of charging energy storage by human-body BFCs in a lateral connection have encouraged the innovations of integrating two devices in a platform and connecting them through an external circuit.
Hou et al. developed an integrated system combining enzymatic BFCs extracting energy from glucose and SC storing harvested energy on the same substrates, including a piece of glass or flexible PET, for biofluid energy harvesting and storage, as shown in Figure 4B. The bioanode and biocathode of the BFC module were fabricated by immobilizing flavin adenine dinucleotide-glucose dehydrogenase for the oxidation of glucose and laccase for the reduction of oxygen on two MCNTs buckypapers, respectively. For the SC part, PVA-H 3 PO 4 solid-state electrolyte was sandwiched by two MCNTs electrodes modified with pseudocapacitive PANI conductive polymer, inducing a much lower internal resistance than the two-in-one strategy. When two BFCs connected in-series were utilized to charge the SC, a 0.8 V charging voltage can be reached in SC and a maximum 608 μW/cm 2 can be delivered in the subsequent continuous discharging process with a current density of 0.8 mA/cm 2 . This strategy allows for the versatile connection among BFCs and SCs, and thus, casts more possibility to power implantable or wearable electronic devices. 106 A prototype of using the implantable glucose BFCs to charge SC and then powering the temperature sensor was developed by Bollella et al., as shown in Figure 4C. The energy accumulating devices, the SC was charged by a millimeter-scale implantable BFC with a size of 2 × 2 × 3 mm 3 . Beyond the energy harvestingstorage system, they also designed a power management circuit to tune the power output from BFC-SC energy harvesting-storage system depending on the requirement of the temperature sensor system (including data memory and wireless data delivery). The complete system could work independently in the hemolymph of the slug which contains glucose, including energy extraction, sensing process, and wireless data read-out, demonstrating the great potential of BFC-SC to serve as a reliable power source. 107 The necessity of fabricating two modules separately can be further highlighted by a sweat-based biofluid energy conversion-storage system. Sweat-based BFCs scavenge lactate to generate and deliver electricity only in the presence of sweat. However, irregular perspiration of users leads to unstable sweat levels and lactate concentration, and thus, a constant power output is hard to be delivered. Lv et al. screen printed a hybrid stretchable BFC-SC system on two sides of a single spandex textile to harvest energy from lactate in sweat and then store harvested energy for subsequent use, as shown in Figure 4D. By formulating the stretchable CNT ink and MnO 2 /CNT ink for the BFC and SC, respectively, and printing in a stretching durable serpentine structure, they realized the success of multiple-layer printing of integrated two stretchable modules on a single textile without deteriorating the softness of textiles. The integrated textile system was able to conform to human skin and endure repeated stretching deformation. When tested on the sweating subject's arm, the SC was charged to 0.4 V from sweat BFC and still delivered power even after sweat was evaporated. 30

HUMAN-BODY HEAT CHARGED ENERGY STORAGE DEVICES
As the human body is a constant pool of thermal energy and there always exists a temperature difference between the human body and the surrounding environment, charging wearable electrochemical energy storage devices with TEGs is a reliable measure for powering wearable electronics continuously. The temperature gap between the hot side and cold side of wearable TEGs is varied when humans stay in environments with different ambient temperatures and wind speeds. 108 The electrochemical energy storage devices could store the unstable electrical energy produced by wearable TEGs and then release them as stable power sources. For traditional solid-state thermoelectric devices with p-and n-type semiconductors, the integration with energy storage could only be enabled by fabricating two modules separately on the same platform and then connected by conductive circuits. Newly developed ionic TEGs rely on the different thermodiffusion of ionic charge carriers in the presence of heat gradients and, thus, hold the attribute to be fabricated as two-in-one thermal energy harvesting and storage devices. 109

Single-cell strategy
The SC could be simply charged by the temperature difference between the human body and ambient air by using two capacitive electrodes to sandwich a highly ionic conductive electrolyte, in which the ion movement can be driven by thermal flux. The Eastman entropies caused by ions solvation environment changes in variable temperatures and change of charge density at two electrode surfaces induced by the Soret effect result in simultaneous temperature-dependent potential difference and energy storage on two electrodes. 110 The Soret effect is a phenomenon that the gradient of ion would be induced by the mobility difference of ions with the presence of temperature gradient. Kim et al. developed a human body thermal charged solid-state SC by sandwiching polystyrene sulfonic acid (PSSH) film through two symmetric highly capacitive electrodes containing graphene/CNT and PANI. The PSSH served as both the voltage generator and solid electrolyte. When one electrode was contacted with the human body, and another electrode was contacted with ambient air (5.3 K temperature difference for an in-vitro test), the H + diffused from hot electrode to cold electrode and PSS-moved less, causing the H + concentration on the cold electrode is much higher than that of the hot side. The H + gradient creates a much higher potential (8 mV/K) on the cold electrode than regular TEGs or thermocells. With the external connection, potential difference-induced electron flow caused the PANI oxidation and reduction on the hot electrode and cold electrode, respectively, to finish the charging procedure. After the removal of the temperature gradient, the H + gradient gradually diminished and the previous hot electrode tuned to be approximately 0.038 V more positive than the cold electrode. The voltage change in the whole process is shown in Figure 5A, both the charging step and stored energy could power electronic devices. 29 The voltage output of the thermocharged SC is mainly determined by the gradient of ion concentrations on electrode surfaces at a certain temperature difference. An extremely high thermal gradient ratio, 24 mV/K, was achieved by using an aligned cellulosic membrane serving as an ion transfer chamber to enable the selective diffusion of sodium ions, as shown in Figure 5B. 111 The aligned ion conductor was simply produced by filling the NaOH into the perpendicularly cut natural wood after the delignification process by highly concentrated NaOH. This cellulosic membrane contains oxidized chains of cellulose molecules to attract sodium ions to form the cellulose-Na complex and repel OHions of NaOH electrolyte. Also, the density of negative charge on the cellulose nanofiber can be enhanced by the oxidation of 2,2,6,6-tetramethylpiperidine-1-oxyl. Both factors contribute to a high Soret coefficient. Additionally, the membrane is flexible and biocompatible, making it is suitable to harvest energy from the human body. This work provides a promising route for the fabrication of ionic thermocharged SC to harvest and store low-grade heat from the human body for wearable electronics.

TEGs charge energy storage devices in a lateral connection
The single-cell strategy for the human body heat harvesting-storage system suffers from the long charging time caused by slow ion diffusion of Soret effect, electrical leakage current, and self-discharging induced by external loading which is essential for the charging process. Charging SC by independent traditional wearable TEGs or thermocells is, thus, regarding as a promising tunnel to reach a fast speed charging and a high discharging and charging ratio. 112 Yang et al. integrated series-connected traditional TEGs, one in-planar micro-SC, and a metal pad to maximize the efficiency of heat transfer on the same substrate to harvest the thermal energy from a human finger, as shown in Figure 5C. The EDLC micro-SC contained CNT/graphene porous interdigitated electrodes and a H 3 PO 4 -PVA gel film as the electrolyte. When a human finger touched the ring shape metal pad, the heat transfer to the TEG component and the micro-SC can be charged by series-connected p-Ag 2 Te/n-Ag 2 Se TEG to the maximum potential within 10 s. The discharging and charging ratio of the integrated system was 97.6% at an 8.6 K temperature difference. 113 Textile-based TEG-charged SC was developed to maximize the contact between wearable TEGs and human skin, ease the integration with wearable electronic circuits and sensors. Deng et al. integrated four TEG modules with one flexible SC on a T-shirt to harvest thermal energy from users ( Figure 5D). The electricity generation capacity of TEGs was well studied by checking the power and voltage output in different body parts, motion states, environments, and wind speeds. At 5°C ambient temperature, four TEG modules containing around 800 thermocouples generated approximately 1 mW power. Once the TEG arrays connected with the SC at different ambient temperatures, fast charging was booted, and the voltage of SC could reach a similar level with that of TEGs after 3-4 min. This work demonstrated the workability of TEGs and integrated TEGs-SC in different practical environments, paving the way for the massive utilization of TEGs-SC to power wearable electronic devices. 108 A thermocell was also integrated on a T-shirt to charge a capacitor. The voltage of a single thermocell relying on temperature-dependent Fe(CN) 6 3-/Fe(CN) 6 4redox reaction is much higher than that of traditional TEG, and thus, the capacitor could be charged to 2 mV after 1-2 s. 114

COMPENSATION FROM SOLAR ENERGY FOR THE HUMAN BODY-CHARGED ENERGY STORAGE DEVICES
Human beings are living on sunlight-radiated earth, thus, harvesting energy from sunlight is a good compensation for human-body energy to charge wearable electrochemical storage devices, especially considering each human-body energy harvester requires specific conditions to deliver the best power output. For example, only when a wearer metabolizes enough sweat containing a high concentration of lactate then epidermal lactate BFCs can display superior performance; continuous human movement is needed for wearable TENG to harvest mechanical energy. The compensation from sunlight can be enabled by either incorporating independent solar cells with bioenergy harvesters or designing all-in-one devices.
Wang and coworkers have combined TENG and solar cells to charge SCs and LIBs battery simultaneously to design sustainable power packages for wearable electronics since 2016. [115][116][117][118] In one of their design ( Figure 6A), a stretchable SC, a flexible TENG, and three fiber-shaped dye-sensitized solar cells (DSSCs) are integrated on a bracelet. The rectified TENG could harvest energy from body motion, be compensated by fiber-shaped DSSCs extracting energy from ambient, and then charge the wearable SC. Compared with the single TENG module (∼100 s), the hybrid energy harvesters consumed only 43 s to charge the SC from 0 to 1.8 V in the presence of simulated sunlight, suggesting the compensative effect from fibershaped DSSCs. The practical application was shown by an electric watch that can work in both a bright and dark environment when it utilized a hybrid self-sustainable power bracelet as the power source. 117 Combining BFCs and solar cells can harvest energy from human biofluids and sunlight irradiation simultaneously. The development of wearable systems that combine BFCs, solar cells, and energy storage devices is still not being well explored. However, previous reports about photoelectrochemical BFCs can instruct utilizing both energies generated by BFCs and solar cells to charge wearable electrochemical energy-storage systems. This kind of BFCs fabricated by coupling biocatalysts with photoactive materials, thus, solar energy can be harvested by BFCs, [119][120][121][122][123][124][125][126][127][128] as shown in Figure 6B. The open-circuit potential of photoelectrochemical BFC can be around approximately 1 V which is higher than that of solely BFC and solar cells. The shortage lies in the low-power density and the optimizations in material design and enzyme/semiconductor conjunction will significantly enhance the efficiency of hybrid devices. 119 The energy output of TEGs is highly related to the temperature difference. Apart from integrating solar cells with TEGs to charge wearable energy storage devices, the sunlight also can be used to increase the temperature difference for TEGs through a layer of the light absorber to fabricate solar TEGs. The light absorber can produce heat by converting the solar irradiation and then transfer the concentrated heat to the legs of TEGs through a highly thermal-conductive substrate, as shown in Figure 6C. [129][130][131] Jung et al. simplified the system by printing the TEG legs onto the thin solar absorber made of fiveperiod Ti/MgF 2 superlattice film (∼ 500 nm). By doing so, the temperature difference was increased to 20°C under solar irradiation. 132 The energy storage devices could be fastly charged by solar TEGs, especially in outdoor places.

CONCLUSION AND PERSPECTIVE
Charging flexible electrochemical energy storage devices by human-body energy (body motion, heat, and biofluids) is becoming a promising method to relieve the need of frequent recharging, and, thus, enable the construction of a self-sustainable wearable or implantable system including sensing, therapy, and wireless data transmission. Electrochemical energy storage devices can accumulate the irregular or unstable harvested energy for use as stable power sources for wearable or implantable electronics. To be wellintegrated with human-body energy harvesters, wearable SCs and batteries need to be conformal to the soft human body or organs. Therefore, multifunctionalities, such as stretchability, self-healing capability, and transparency, are endowed to newly developed electrochemical energy storage devices. 46,[133][134][135] Based on the characteristics and mechanisms of energy harvesters (PENGs/TENGs, BFCs, and TEGs), two strategies are utilized either by singlecell strategy or by connecting through external circuits to design different human-body energy harvesting-storage systems. The single-cell strategy integrates the capability of energy harvesting and charges storage in one pair of electrodes, simplifying the device construction, while divided modules enable the performance maximizations of each module and flexibility of connection depending on the power consumption of electronic devices. Beyond the human-body energy, sunlight energy can be a versatile compensation in the presence of sunlight for human body energy to charge electrochemical energy storage devices. The human-body charged SCs/batteries have been demonstrated to be promising for powering wearable electronics. The practical application of the human-body energy harvesting-storage system is still encountering several challenges, even though the conceptual progress in this field has been achieved. The most obvious challenge is that the stored energy in electrochemical energy storage devices from the human body is still far below that of the traditional cable charging method, thus, only wearable electronic devices with low energy consumption can be powered. 136 Also, most of the energy storage modules in reported systems relied on capacitive SCs, because batteries need stronger power sources to charge. SCs are suffering from a faster self-discharging nature comparing with batteries, impeding the subsequent use of the stored energy. This challenge is mainly attributed to the low power efficiency of human-body energy harvesters and low integration efficiency.
The progress in materials science and device structural design would enable the elevated performance of human-body energy harvesters and electrochemical energy storage devices to minimize the impedance/voltage mismatch between two modules and increase energy output. Meanwhile, integrating different types of energy harvesters generating energy simultaneously from body motion, biofluids, and body heat together would increase the power generation by fully harvesting human-body energy, and weaken the influence of human activities and surroundings on the performance of single-energy harvesters. 137,138 For example, a textile-based microgrid system has been constructed by integrating TENGs and epidermal BFCs on one textile platform to simultaneously extract energy from human-body motions and sweat stimulated by body exercise, and then to deliver more energy to wearable energy storage devices. 139 Two energy harvesters alleviated the dependence of a single harvester on the subject's exercise status. Also, the system can compensate the limitation of BFC coming from the delay in perspiration and of the TENGs coming from the lack of motion, and thus, elongates the operation for powering a wearable chemical sensor from a 10-min exercise to over 30 min, exhibiting the great prospects in combining hybrid energy harvesters for the charging of energy storage devices. Other challenges, including the compatible and cheap fabrication for different components, tunable energy capacity depending on the specific energy consumption, and multifunctional performance, such as high stretchability and self-healing behavior of the whole integrated system, are also impeding the wide application of human-body energy harvesting storage. Materials selection, mechanism exploration, device structural design, and electronic circuit optimization are four aspects that can be considered to eliminate the shades in the development of human-body self-sustainable energy systems. The growing focus on wearable health care monitoring, therapies, and humanmachine interfaces would attract joint efforts from material scientists, chemists, and electronics experts to develop practical human body-system self-powered electronics.

A C K N O W L E D G M E N T S Jian Lv and Jian
Chen contributed equally to this work. This work was supported by the National Research Foundation, Prime Minister's Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program.

C O N F L I C T O F I N T E R E S T
The authors declare no conflict of interest.
[Correction added on 29 June 2021, after first online publication: Conflict of Interest section has been added.]