FIGURE 5 (A) A biocompatible supercapacitor based on body fluids as electrolyte combined with solar cells for energy harvesting and storage. Reproduced with permission.[98] Copyright 2017, Elsevier. (B) An miniature implantable supercapacitor based on a biocompatible aligned carbon nanotube fibre electrode in PBS. Reproduced with permission.[99] Copyright 2017, Elsevier. (C) A carbon-based flexible micro-supercapacitor-like system for various types of bioelectronic interfaces (scale bar on the top right, 5 mm). Reproduced with permission.[100] Copyright 2020, Springer Nature. (D) Solid-state supercapacitors powering the flexible smart contact lenses wirelessly. Reproduced with permission.[101] Copyright 2019, Science. (E) A biomolecule-based fibre-type implantable supercapacitor with micro diameter for in vivo energy storage. Reproduced with permission.[27] Copyright 2018, Elsevier.
3.2. Human Body Energy Harvester
Many forms of energy contained in the living organism, including chemical energy from the reaction of organic molecules such as glucose oxidation and mechanical energy from such as heart beating and respiration, have been widely studied for energy harvesting in vivo from the human body. The commonly studied internal energy-harvesting devices include the nanogenerator (NG) converting mechanical energy (cardiovascular, respiratory and gastrointestinal) into electrical energy by piezoelectric and triboelectric effects, and another kind of harvester is biofuel cells generating energy from glucose oxidation. With these permanent energy sources in the human body, the implanted energy harvesters can be implanted once for all and work for a long period of time enough to support the electronics to finish the missions during the lifetime of the hosts, which is a promising method to reduce the pain and incision caused by a repeated second surgery.
3.2.1. Nanogenerators (NGs)
Recently, various NGs have been implanted in organisms for energy harvesting, sensing, and stimulating nerves and muscles.[102-103] Two kinds of energy-harvesting devices are included: PENG and TENG. Due to the piezoelectric effect, abundant mechanical energy can be harvested by PENG from human bodies including essential motions such as heartbeat, gastrointestinal moving, daily walking and breathing. Since 1880, the piezoelectric effect was discovered by the Curie brothers, many piezoelectric materials were studied. There are several common piezoelectric materials such as zinc oxide (ZnO), lead zirconate titanate (PZT), barium titanate (BaTiO3), polyvinyl chloride (PVC), poly (lactic acid) and polyvinylidene fluoride (PVDF). However, inorganic piezoelectric materials such as ZnO and PZT have good performance but are usually rigid and brittle, which causes difficulty to match well with soft tissues. The flexible ZnO nanowire fabricated by Li et al. improved the flexibility of PENG for real-time rat heart rate monitor, the two-ends-bonded piezoelectric nanowire generator can drive the electron motion by mechanical deformation.[104]The output voltage and current are usually less than 50 mV and 500 pA, the robustness of the nanowire need to be improved with flexible polymer to isolate it from the biological surrounding. The average output voltage and current from the heartbeat of a live rat were about 3 mV and 30 pA (Figure 6A). While ZnO/rGO can reinforce the mechanical stress with increased local deformation, the enhanced piezoelectric output can be generated. According to Azimi et al., the optimized composite of PVDF nanofibres with ZnO and rGO as hybrid filler generated an output voltage which is nearly 10-fold increase than that of the pristine PVDF nanofibres.[29] The optimized PENG was sutured on the heart of an adult dog and every heartbeat can generate electrical energy as high as 0.487 μJ which is enough to power commercial pacemakers as demonstrated in Figure 6B. To further improve the output current, Kim et al. fabricated a soft PENG with Mn-doped PMN-PZT ((1−x)Pb(Mg1/3Nb2/3)O3−(x)Pb(Zr, Ti)O3) thin film on a PET substrate (Figure 6C). The PMN-PZT-based PENG can provide an open-circuit voltage of 17.8 V and a short-circuit current of 1.74 µA from porcine rhythmical cardiac contraction and relaxation which is higher than the previous research.[105-106] The PENG system successfully enabled ECG signal recording, controlled a light bulb wirelessly and transferred signals. Hypertension (high-blood pressure) is another health risk factor for heart disease that need long-term intensive real-time monitor but traditional air-filled cuffs for blood pressure test can only provide interrupted results.[107-108] To realize real-time and accurate monitoring of blood pressure, a flexible continuous battery-free electronic system was developed for in vivo blood pressure monitoring with a miniaturised thin-film PENG wrapped on the aorta as illustrated in Figure 6D. The devices were encapsulated by the biocompatible flexible polyimide film sandwiched in a 200 μm piezoelectric polarized PVDF thin film and thus can produce current continuously with the periodical expansion and retraction of the aorta. The long-term stability and possibility of long-time in vivo use enable the PENG to be a real-time continuous power source for the visualized blood pressure monitoring system. However, even though PENGs have advantages including high output voltage and good scalability as energy supply, the most widely used material PZT of PENG is toxic and risky to be implanted inside the human body even though it can be sealed by other nontoxic materials such as PDMS and polyimide encapsulation layer, while other biocompatible materials such as ZnO and PVDF can only provide millivolts (mV) and nano-amperes (nA) output lower than PZT-based PENG devices.[16]
Similarly, the TENG relying on the triboelectric effect at the interface between two different dielectrics also provided a promising way to harvest energy from body movements. Generally, the triboelectric effect of TENG occurs between inorganic and organic films with different frictional electric sequences. Various materials in nature such as silk, metal, wood and polymer have such effects. At present, TENG has been successfully applied in biomechanical energy collection for biomedical signal sensing such as respiratory,[109] cardiovascular and digestive systems.[110-111] For example, Zheng et al. fabricated a self-powered cardiac pacemaker with an energy supply of TENG harvesting energy from respiration (Figure 6E).[30] The size of the TENG device inserted into the left chest skin of a rat is about 1.2 cm wide and 0.2 cm thick. The output voltage and current generated by the TENG device are 3.73 V and 0.14 μA, which was directly applied to power a cardiac pacemaker for heartbeats regulation. Also, the TENG can harvest mechanical energy from gastric peristalsis and can be converted to biphasic electric pulses through contact and separation of the triboelectric layers.[112] The generated electric signals were delivered to the fibres of the vagus nerve stimulation (VNS) device and decreased the food intake, achieving the final purpose of weight control as illustrated in Figure 6F. Comparatively, TENGs have the advantages such as low-cost, selective materials and high energy conversion efficiency.[113-114] However, both TENGs and PENGs as biomechanical energy harvesters face unique challenges such as the stability of continuous energy supply, adaptability to the biological surroundings with different humidity and temperature, long-term safety inside the body and the limitation for long-term application.[16]