Fig. 1. (Color online) The fabrication methods and energy sources for flexible energy storage devices and their applications in wearable biosensing^[8–15].

Fig. 2. (Color online) Chemical methods for flexible energy storage devices fabrication. (a) Two-step hydrothermal synthesis of MnO₂ nanosheet-assembled hollow polyhedrons on carbon cloth^[20]. (b) Metal-like conductive paper electrodes based on Au nanoparticle assembly followed by nickel electroplating^[10]. (c) A microwave-assisted rapid sysnthesis of nickel-iron-based catalysts for rechargeable zinc-air battery^[32]. (d) Synthesis of 3D nanofiber electrode via CVD^[36].

Fig. 3. (Color online) Physical methods for flexible energy storage devices fabrication. (a) The coating process to achieve flexible CNT-based cathodes^[42]. (b) Infiltration of electrospun porous polyimide nanowires for sulfide solid elelctrolyte membranes^[44]. (c) Fabrication of graphite/Si hybrid electrode via sputtering^[46]. (d) Schematic of the 3D printed interdigital electrodes for micro-supercapacitors^[11].

Fig. 4. (Color online) Self-powered systems consists of flexible energy storage devices and energy harvesting components. (a) Schematic of a printable self-powered system consists of solar cells, supercapacitors and gas sensor^[60]. (b) Design of a thermocell for harvesting body heat and charging supercapacitors^[63]. (c) Self-powered cloth consists of TENG, supercapacitor and wearable sensor^[69]. (d) An all-solid-state self-powered system with high performance PENG using a particular mesoporous film^[73].

Fig. 5. (Color online) Physiological sensing systems integrated with flexible energy storage devices. (a) An all-in-one, and flexible self-powered sodium sensing patch with wireless data transmission^[78]. (b) A self-powered smartwatch for non-invasive sweat glucose monitoring^[92]. (c) Schematic of a self-powered system with pH sensor and its performance under dynamic bending conditions^[79]. (d) A self-powered wristband that can power up LED as an indicator of gas detection^[60].

Fig. 6. (Color online) Physical sensing systems integrated with flexible energy storage devices. (a) A screen-printed flexible solid-state supercapacitor for self-powered pulse sensing^[106]. (b) Schematic illustration of an integrated self-powered tactile sensor^[114]. (c) The structure design of the FBG sensor for in-situ temperature measurement^[117]. (d) Integration of the dual-mode strain sensor and supercapacitor on a deformable substrate^[126].

Table 1. Summary of recent flexible energy storage devices integrated with sensing systems.