fluid in a controlled manner and falls in the research category of "transient electronics." [1] Such electronic systems can be integrated into the human body for personalized health monitoring and precision therapeutic functions. Compared to conventional electronics, they are entirely made of biocompatible and biodegradable materials, which can eliminate any adverse long-term effects through bioabsorption. Potential risks and financial burdens to patients are thus substantially reduced. Although significant progress has been made in biodegradable electronics in the materials and device communities, [2,3] the availability of power supply remains a key challenge to be solved.To date, the most common method of powering implantable bioelectronics is by using a battery, as it is dependable and convenient. However, some issues cannot be ignored. [4][5][6] First, the batteries are often rigid, bulky, and occupy a large volume of implantable devices. Second, a second surgical removal is needed due to their non-biodegradability. Finally, the components, such as electrodes and electrolytes, contain toxic substances, posing serious safety hazards. Recently, much effort has been paid to capturing ambient energy (e.g., body motions [7] and glucose oxidation) [8] and wirelessly transmitting the energy (e.g., electromagnetic/ acoustic waves [9,10] and solar radiation) [11] as a solution to power implantable devices. However, the methods either suffer from relatively low output power or limited implantation position/penetration depth. Recent reports presented biodegradable energy storage devices (batteries and supercapacitors) as an alternative promising energy solution, due to their independent deployment capability and achievable high energy/ power density. Typically, by introducing water-soluble metals and oxides, various biodegradable batteries and supercapacitors have been constructed, for example, magnesium (Mg) primary batteries, [12][13][14] Zn-ion batteries, [15] molybdenum (Mo) interdigitated micro-supercapacitors, [16] supercapacitors based on molybdenum oxide (MoO 3 ), [17,18] and zinc oxide (ZnO). [19] All the components have good biocompatibility because they can be decomposed by hydrolysis into non-toxic compounds or elements that are existing already in the human body. Alternative strategies using nonmetallic biocompatible electrode materials have also been proposed such as activated carbon, [20] Biodegradable implantable devices are of growing interest in biosensors and bioelectronics. One of the key unresolved challenges is the availability of power supply. To enable biodegradable energy-storage devices, herein, 2D heterostructured MoO 3 -MoS 2 nanosheet arrays are synthesized on water-soluble Mo foil, showing a high areal capacitance of 164.38 mF cm −2 (at 0.5 mA cm −2 ). Employing the MoO 3 -MoS 2 composite as electrodes of a symmetric supercapacitor, an asymmetric Zn-ion hybrid supercapacitor, and an Mg primary battery are demonstrated. Benefiting from the advantages of MoO 3 -MoS 2 heterostructure, the Zn-i...
