of wastes. [4] Furthermore, self-destructive property induced by external stimuli such as moisture, light, and heat can expand its potential application to security-related electronics for protecting private and confidential information. [5-7] Combined uses of natural or synthesized biodegradable polymers with non-transient electronic materials were examples of early research direction of water-soluble electronic devices, which allowed to partially disintegrate into fragmentation due to collapse of the substrates. [8-10] Recognition of chemical reactivity of monocrystalline silicon nanomembranes (Si NMs) in aqueous solutions reached a turning point in transient system through integration with other inorganicbased conductors and insulators, enabling to fabricate almost any types of existing silicon-based electronic devices in a transient form with moderate modifications. [11] Demonstrated examples included device components, sensors, actuators, and energy sources with electrical behaviors comparable to those of state-ofthe-art devices. Successful follow-ups to initial works provided various interpretations of the dissolution kinetics and versatile organic-and inorganic-based materials and devices to accomplish a wide spectrum of this technology for potential applications in biological researches to clinical medicine and other possible areas. In the following, we discuss various types of biodegradable inorganic/organic materials along with their degradation mechanisms and biocompatible properties, diverse manufacturing processes for electronic devices and systems, examples of basic transient electronic components, and different strategies/concepts of transience. Representative applications in the fields of bioengineering, green-electronics, security systems, and others are reviewed, and concluding remarks address some challenges and perspectives for the future of transient electronics. 2. Bioresorbable Materials 2.1. Inorganic Materials Figure 1a shows a representative example of water-soluble electronic devices that contained electronic components, involving a transistor, diode, inductor, capacitor, resistor, and interconnects. [11] All components consisted of various inorganic biodegradable constituents such as Si NMs, magnesium oxide (MgO), and magnesium (Mg) for semiconductors, gate and interlayer dielectrics, and conductors, that can be degraded into benign and biocompatible end products. In addition, many other inorganic Transient electronics refers to an emerging class of advanced technology, defined by an ability to chemically or physically dissolve, disintegrate, and degrade in actively or passively controlled fashions to leave environmentally and physiologically harmless by-products in environments, particularly in bio-fluids or aqueous solutions. The unusual properties that are opposite to operational modes in conventional electronics for a nearly infinite time frame offer unprecedented opportunities in research areas of eco-friendly electronics, temporary biomedical implants, data-secure hardware system...

A novel transient electronics technology that is capable of completely dissolving or decomposing in certain conditions after a period of operation offers unprecedented opportunities for medical implants, environmental sensors, and other applications. Here, we describe a biodegradable, flexible silicon-based electronic system that detects NO species with a record-breaking sensitivity of 136 Rs (5 ppm, NO2) and 100-fold selectivity for NO species over other substances with a fast response (~30 s) and recovery (~60 s). The exceptional features primarily depend on not only materials, dimensions, and design layouts but also temperatures and electrical operations. Large-scale sensor arrays in a mechanically pliable configuration exhibit negligible deterioration in performance under various modes of applied loads, consistent with mechanics modeling. In vitro evaluations demonstrate the capability and stability of integrated NOx devices in severe wet environments for biomedical applications.

Recent advances in the engineering or strategy of materials and device design have established ultrathin, soft, lightweight, and skin-conformable characteristics in wearable/implantable electronic systems, allowing precise, long-term monitoring of biological signals from skin/internal organs while reducing signal artifacts upon daily body motions or other external effects. Such a soft, flexible platform offers an opportunity capable of recording and analyzing diverse physical, chemical, and electrophysiological parameters for clinically useful information in the effective prevention, treatment, and management of illness as well as the preservation of physical and mental well-being. Combination with other peculiar functions such as bioresorbable and self-healing properties can enhance the biosafety/reliability of devices and realize unprecedented applications in the fields of biology and medicine or other areas of interest. This Review summarizes the underlying mechanisms of materials science in terms of a mechanical balance between devices and biological structures, discusses the latest biomedical applications with a focus on technological advances and significance, and concludes with an overview of current challenging points and perspectives for future research directions in wearable/implantable electronics.

Biocompatible and biodegradable polymer composite systems equipped with electrical/mechanical functions have been researched as a tool that can be applicable for bio‐integrated electronics to obtain a variety of useful information, through formation of reliable interfaces with soft, non‐planar skin and organs in the human body. Although previous review articles have explored various types of organic materials (natural/synthetic polymers), including intrinsically conducting polymers (ICPs) as constituent materials for such composites, there has been a lack of consideration of inorganic materials, which may narrow the scope of material options for diverse electrical/functional properties. Indeed, biocompatible and/or biodegradable inorganic elements exhibit high electrical conductivity, various morphologies for effective percolation networks, cytocompatibility, soft mechanical characteristics compared to ICPs, and many functions such as magnetism, photoluminescence, and electrochemical reactivity. This review introduces recent advances in biologically safe and biodegradable conductive polymer composites, including materials selection, relevant fabrication strategies, and various potential applications, aimed at revealing the potential applicability of inorganic elements in conductive composites, which has not been discussed in other reviews. Finally, it concludes with remarks on key issues and challenges to overcome corrosion and oxidation issues of inorganic metals and to improve the electrical performance of composite systems.

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The lifetime of transient electronic components can be programmed via the use of encapsulation/passivation layers or of on-demand, stimuli-responsive polymers (heat, light, or chemicals), but yet most research is limited to slow dissolution rate, hazardous constituents, or byproducts, or complicated synthesis of reactants. Here we present a physicochemical destruction system with dissolvable, nontoxic materials as an efficient, multipurpose platform, where chemically produced bubbles rapidly collapse device structures and acidic molecules accelerate dissolution of functional traces. Extensive studies of composites based on biodegradable polymers (gelatin and poly-(lactic-co-glycolic acid)) and harmless blowing agents (organic acid and bicarbonate salt) validate the capability for the desired system. Integration with wearable/recyclable electronic components, fast-degradable device layouts, and wireless microfluidic devices highlights potential applicability toward versatile/multifunctional transient systems. In vivo toxicity tests demonstrate biological safety of the proposed system.

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The photo-catalytic activities of TiO2 thin films grown by remote plasma atomic layer deposition (RPALD) were investigated as a function of grain size and crystal structure. As the deposition temperature increased, it was shown that grain size of the anatase phase in the as-deposited TiO2 thin films progressively decreased. When the TiO2 films grown by RPALD were annealed at 900°C, the anatase TiO2 phase with relatively small grains provided large number of nucleation sites for formation of more stable rutile phase. On the other hand, the phase transition to rutile was retarded due to the reduction in grain boundaries in the case of the anatase TiO2 films with large grains. Thus, the anatase TiO2 phase with large grains has excellent thermal stability against the phase transition to rutile. Because the large grain boundary volume is a primary cause of degradation of photo-catalytic activity, the photo-catalytic activity of the TiO2 thin film was maximized when the anatase phase with large grains was formed in the TiO2 thin film. Therefore, RPALD can be effectively utilized to control the grain size of the as-deposited anatase TiO2 thin film with high thermal stability for potential application of photo-catalysts.