FIGURE 4 (A) Injectable fibre sodium-ion battery for all-region power supply in vivo. Reproduced with permission.[26] Copyright 2021, Royal Society of Chemistry. (B) Biodegradable and rechargeable sodium-ion batteries with fibre configuration. Reproduced with permission.[25] Copyright 2021, Royal Society of Chemistry. (C) A tissue-like soft all-hydrogel battery powering a hydrogel strain sensor for the heartbeat monitor. Reproduced with permission.[90] Copyright 2022, Wiley-VCH GmbH. (D) Biodegradable rechargeable zinc ion battery with controlled degradation and stable electrochemical performance. Reproduced with permission.[91] Copyright 2022, Wiley-VCH GmbH.
Combined with bioresorbable features, the syringe-assisted injectable bioresorbable fibre battery can provide a better solution for the non-invasive implantation of electronics with low health risks. For example, Mei et al. demonstrated a biocompatible aqueous sodium-ion battery that can be injected into the body without immune responses and degraded after completing the mission.[25] They utilised biodegradable materials to construct the fibre battery, the polydopamine/polypyrrole composite as anode and MnO2 as the cathode was coated on the conducting fibre and the biodegradable chitosan was applied as the separator. After twisting the electrodes fibre together and injecting it into the abdominal subcutis of an experimental mouse, with the body fluid act as the electrolyte, the fibre battery showed a specific capacity of 25.6 mAh g-1, and a good cycling stability with retention of 69.1% after 200 cycles. As demonstrated as a power source for the biosensor, the fibre battery succeeds in detecting the pressure changes in the implanted area and is biodegraded as designed through hydrolysis and enzymolysis. The flexible battery also showed excellent stability during the pressing, bending and stretching as can be observed in Figure 4B, the biodegradable fibre sodium-ion battery was finally fully biodegraded after several weeks avoiding the need for surgery for removal. The minimised injectable batteries are very promising choices for IMEs due to their better biocompatibility, longer service time and better stability inside the human body. With a coplanar structure, the ultrasoft rechargeable batteries with low Young’s moduli based on lithium ion and zinc ion were exploited. Ye et al. proposed tissue-like ultra-soft batteries based on an all-hydrogel design based on the lithium-ion battery and zinc-ion battery system.[90] With the interfacial dry crosslinking strategy, superb electrical conductivity and high interface charge transfer efficiency were achieved. Benefiting from this strategy, the lithium-ion battery-based all-hydrogel battery achieved high specific capacities of 82 mAh g-1 and the all-hydrogel zinc-ion batteries represented 370 mAh g-1 at a current density of 0.5 A g-1. While integrated with a hydrogel strain sensor, the all-hydrogel battery can supply a stable power output for the heartbeat monitor while precisely detecting the strain change (Figure 4C). In addition, a biodegradable rechargeable zinc ion battery with controlled degradation and stable electrochemical performance was reported.[91] In this battery, a biodegradable cellulose aerogel-gelatin solid electrolyte was designed, a biodegradable highly flexible silk protein film was used as passivation, and the zinc thin-film battery achieved a stable output voltage of up to 1.6 V. After implantation, the degradation in subcutaneous area of rats can be observed in Figure 4D. The complete degradation of this thin-film battery in vivo demonstrated a non-toxic and harmless implantation procedure to the hosts (rats) and an advanced power source for clinical electronics.
3.1.3. Supercapacitors
With fast charging and discharging rates, high power density (>10 kW kg-1), and splendid cycling lifespan (>100,000 times), supercapacitor bridged the gap between electrolytic capacitors and rechargeable batteries. Such kind of energy storage devices can act well as an energy reservoir for electronics when unexpected interruptions in the power source occur, which showed exceptional rate capability and good stability[92-93]. Depending on different mechanisms, supercapacitors can be divided into two types. One is electrical double-layer capacitance (EDLC) relying on ionic electrochemical adsorption/desorption at the electrode/electrolyte interfaces, another is pseudo-capacitors depending on pseudo-capacitance from rapid redox reactions occurring on (or near) electrode surfaces. Due to the uncertainty of the pathological characteristics of most biocompatible materials, and the discontinuity of electrode elements under the complex and variable surface strain, new degradable energy devices often show unpredictable performance and huge safety risks in the diagnosis process. Therefore, with minimised dimension,[94] good flexibility and strong selectivity of non-toxic electrode materials, microscale supercapacitors have a bright application prospect in the area of medical auxiliary energy sources.[95] At present, wearable devices usually use micro-batteries and micro-supercapacitors as energy supply and have the potential to be integrated with electronic devices in a small size. For microelectronics requiring long-term service and high-power density, micro supercapacitors would be a good choice and therefore have drawn wide attention. In the field of biomedical electronics and sensors, micro-batteries as energy supply inevitably suffer from frequent replacement due to the short cycling life of about several thousand times. Comparatively, micro supercapacitors represent excellent cycling stability with negligible capacitance decay, which effectively avoids the frequent replacement issue. Also, unlike micro batteries, micro supercapacitors with higher power density with small volumes are more suitable for flexible IMEs requiring high power density than batteries requiring additional integration to achieve high power density.[96-97] However, conventional supercapacitors also suffer from rigid and heavy structures, unstable electrolytes and rigid sealing before implantation, which is unfit for IMEs. Some researchers reported strategies that body fluid was used as electrolytes without encapsulation. According to Chae et al., a conceptual system adopts solar cells as energy supply and supercapacitor as energy reservoir, MnO2 and carbon were used as electrodes inserted into the subcutaneous area of a rat, and biofluids were used as electrolytes (Figure5A ).[98] The working voltage window of the implanted supercapacitor is from 0.2 to 1.0 V and good cycling stability was achieved with over 1000 cycles. Similarly, as shown in Figure 5B , with biocompatible carbon nanotube fibres as electrode material and biofluid as the electrolyte, He et al. fabricated supercapacitors without additional encapsulation, a specific capacitance of 10.4 F cm-3 and long cycling stability (98.3 % retention after 10000 cycles in PBS) were achieved.[99] Furthermore, for the electrical modulation of tissues and organs, bio-interfaces at the macroscopic level are in demand. Fang et al. fabricated a flexible micro supercapacitor-like system with micelle-enabled self-assembly approach for various types of bioelectronic interfaces, as shown in Figure 5C . With interdigitated electrode design, the micro supercapacitors show a smaller size and are more suitable for subcellular interfaces. With this flexible micro supercapacitor-like electronic system, multifunction was achieved including modulation of cardiomyocytes in vitro, excitation of isolated heart and retinal tissues ex vivo, stimulation of sciatic nerves in vivo, and bioelectronic cardiac recording.[100] More than that, micro supercapacitors have also been applied to assist wireless power transfer for the continuous operation of electronic devices such as smart contact lenses due to the long cycling lifespan and high-power density. With a wireless charging antenna continuously receiving external inductive power, the integrated micro supercapacitors can store energy and enable independent operation of the smart electronic device inside the human body. Park J et al. reported a soft smart contact lens integrated with an antenna for wireless recharging and a solid-state supercapacitor for energy storage.[101] The monolithically integrated solid-state supercapacitors showed excellent cycling stability up to 10,000 cycles and a stable temperature was maintained during wireless operation ensured the safety of the wearer’s eyes as illustrated in Figure 5D . Furthermore, fibre supercapacitor with high aspect ratio structure and micro diameter was constructed and applied for surgical suture for wound stitching and implantation in the blood vessel.[27] The highly flexible and conductive electrode fibres was fabricated with biocompatible poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS)/ ferritin nanoclusters trapped multiwalled carbon nanotube (MWNT) sheets, which enabled the fibre supercapacitor operate well in a mouse with excellent biocompatibility. An areal capacitance of 32.9 mF cm-2 in PBS solution was achieved and above 90% of capacitance was maintained in the mouse after 8 days implantation as illustrated in Figure5E .
Though the supercapacitor suffers from low energy density, good cycling stability and high power density are highly desirable as an assistant energy storage device. It is promising that the miniaturised micro supercapacitors in collaboration with the wireless charging technologies will support the sustainable long-term service of the IMEs system.