## 4 Environment-Friendly Electronic Devices
presented the biodegradable RRAM consisting of three thin-films, that is, magnesium difluoride (MgF\({}_{2}\)) used as the switching layer, Mg and Fe acted as electrodes [204]. The switching layer (MgF\({}_{2}\)) is biodegradable. Such biodegradable RRAM devices were fabricated on a polyethylene terephthalate (PET) substrate, which also can be dissolved when they were placed in DI water. [PERSON] et al. developed a physical transient memristor with the MgO thin film as a resistive dielectric layer and the Ni metal layers as electrodes, which was fabricated on a SiO\({}_{2}\)/Si substrate [8]. The Ni/MgO/Ni structure can be exfoliated from the SiO\({}_{2}\)/Si substrate, and then submerged into the DI water. Due to the lack of a supporting substrate, the Ni and MgO thin-films dissolve in water. [PERSON] et al. for the first time proposed the all-inorganic transient perovskite CsPbBr\({}_{3}\) nonvolatile resistive switching memory device with Ag/CsPbBr\({}_{3}\)/PEDOT: PSS/ITO/PET structure [80]. The perovskite CsPbBr\({}_{3}\) thin-film was used as the active layer. The PEDOT: PSS layer was deposited on the ITO electrode to enhance the adhesion of CsPbBr\({}_{3}\) thin films, and the ITO layer was used as the bottom electrode. They verified that the resistive switching mechanism is ascribed to the formation and dissipation of Ag conducting filament at the CsPbBr\({}_{3}\) layer. Organic thin-film materials such as egg albumen (or egg white) and silk are natural polymer materials that can be biodegradable, bioabsorbable, and environmental-friendly. So, the use of the organic thin-film as the active resistive switching layer is promising in transient memristor fabrication. [PERSON] et al. employed natural chitosan thin-film as the solid polymer electrolyte layer and the dissolvable Mg as electrodes to fabricate fully biodegradable resistive switching memory on the plastic/rice paper substrate [205]. They investigated the dissolution evolution of the Mg/Au-doped chitosan/Mg resistive-switching devices in the water in detail, as well as resistive switching performances and retention capability. The obtained results implied that the fabricated chitosan-based organic memristor with Mg electrodes is highly promising for biodegradable and environmentally benign memory applications. Metal nanoparticles/organic thin-film hybrid materials acted as organic resistive-switching functional materials are another hot topic of organic-inorganic resistive-switching devices. In this case, metal thin-films/nanoparticles or metal-doping nanocrystals are embedded in organic RS functional materials. [PERSON] et al. reported a degradable resistive-switching nonvolatile memory based on Al/Au NPs: lignin/Al sandwiched structure, where gold nanoparticles embedded alkali lignin (Au NPs: lignin) was used as the biodegradable active layer fabricated by solution-process method, and the PLA thin-film was also used as a degradable skin-inspired substrate [206]. Au NPs in the active layer are responsible for the resistive-switching mechanism. Such memory devices can be degraded in a proteinase K aqueous solution. Other transient/degradable resistive-switching devices have also been developed using degradable, resorbable, and/or biocompatible thin-film materials, which are shown in Table 1.  
The above-investigated reviews suggest that the degradable, resorbable, dissolvable, and/or compatible thin films including metal oxide thin-film materials (e.g., ZnO, MgO, SiO\({}_{2}\)), and organic materials (e.g., protein, silk, cellulose) can be used effectively as memristive dielectric functional materials for eco-friendly resistive-switching devices. For electrode materials of the resistive-switching devices, metal thin-film such as Pt, Au, Mg, Mo, Al, W, Zn, and/or Fe. were used to prepare dissolvable top/down electrodes. However, as observed, the degradable and/or compatible resistive-switching devices based on the 2D thin-films are hardly reported.