FIGURE 3(A) Biodegradable Mg-MoO3 primary battery work for about 13 days. Reproduced with permission.[22] Copyright 2018, Wiley-VCH GmbH. (B) Fully biodegradable Mg-Mo primary batteries. Reproduced with permission.[19] Copyright 2014, Wiley-VCH GmbH. (C) Bioresorbable zinc primary battery with zinc microparticle network coated with chitosan and Al2O3 double shells as anode. Reproduced with permission.[20] Copyright 2021, American Chemical Society. (D) Biodegradable thin film Mg primary battery with silk fibroin-IL electrolyte. Reproduced with permission.[21] Copyright 2017, American Chemical Society.
3.1.2. Rechargeable batteries
Compared with the single-used primary battery, rechargeable batteries are still a compelling topic in the future research of IMEs energy storage devices since they can be recharged after the first implantation and provide long-term stable power in vivo without the need for replacement surgery. With one-dimensional structures and miniaturised coplanar configuration, rechargeable batteries are more beneficial for minimally invasive IMEs application and enable the implanted system to work with stable power sources. Recently, Peng et al. designed a one-dimensional fibre-like rechargeable aqueous sodium-ion battery that can be injected into different parts of the body for energy supply as can be seen in Figure 4A.[26] With aligned carbon nanotube (CNT)/Na0.44MnO2 (NMO) hybrid fibre as cathode and CNT/molybdenum trioxide/polypyrrole (CNT/MoO3/PPy) hybrid fibre as anode, good mechanical and electrochemical performance were achieved. With the remarkable one-dimension structure, the fibre sodium ion battery possessed advantages including good flexibility and minimised implantation incision size. Also, the fibre battery is exposed to biofluid which acts as electrolyte and can be injected into tissue directly. Due to the fibre structure with high flexibility, the battery can match the soft tissue well and achieve stable interface contact. The electrochemical performance of the fibre battery with a power density of 78.9 mW cm-3 can power most of the biomedical applications in vivo including the implanted sensor for respiration monitoring.