Biodegradable inorganic materials for transient electronics

Inorganic transient electronics that exploit the excellent electrical properties and stability of inorganic materials have expanded rapidly after the initial demonstration of the dissolution of electrical-grade single-crystal Si nanomembranes. Hwang et al. showed that ∼100-nm-thick single-crystal Si dissolved in a phosphate buffer solution at 37°C at a rate of ∼3 nm/day, suggesting the potential use of Si as transient or biodegradable electrical functional materials.^{Reference Hwang, Tao, Kim, Cheng, Song, Rill, Brenckle, Panilaitis, Won, Kim, Yu, Ameen, Li, Su, Yang, Kaplan, Zakin, Slepian, Huang, Omenetto and Rogers1} The versatile electrical properties of Si provide various types of transient electronics, including electronic components such as transistors, diodes, and inductors, logic circuits, and optical, temperature, and strain sensors.^{Reference Hwang, Tao, Kim, Cheng, Song, Rill, Brenckle, Panilaitis, Won, Kim, Yu, Ameen, Li, Su, Yang, Kaplan, Zakin, Slepian, Huang, Omenetto and Rogers1} The study of dissolution chemistry and kinetics of potential transient electronic materials has naturally become important. Follow-up studies demonstrate that Ge, SiGe, and amorphous indium gallium zinc oxide (a-IGZO) are also dissoluble in a phosphate buffer solution and can be alternative semiconductor layers.^{Reference Kang, Park, Kim, Hwang, Cheng, Shin, Chung, Kim, Yin, Lee, Lee and Rogers20,Reference Jin, Kang, Cho, Han, Chung, Lee, Shin, Baek, Kim, Lee and Rogers28} Dissolution kinetics strongly depend on materials microstructure such as density of films and crystallinity, and on environmental conditions such as temperature, pH, and ion species.^{Reference Hwang, Tao, Kim, Cheng, Song, Rill, Brenckle, Panilaitis, Won, Kim, Yu, Ameen, Li, Su, Yang, Kaplan, Zakin, Slepian, Huang, Omenetto and Rogers1,Reference Kang, Hwang, Cheng, Yu, Kim, Kim, Huang and Rogers17–Reference Lee, Yu, Song, Farimani, Vitale, Xie, Yoon, Kim, Richardson, Luan, Wu, Xie, Lucas, Crawford, Mei, Feng, Huang, Litt, Aluru, Yin and Rogers19} The study of dissolution kinetics provides a tunable design factor for transient electronics by controlling the transient dissolution rate of individual components and overall systems.

Transient dielectrics play an additional important role as encapsulation materials that determine the life cycle of electronics. Transient electronics lose their electrical function when the electrically functional materials dissolve. Transient encapsulation slows down water diffusion toward the active circuit and extends the valid functional lifetime of the overall device. Representative metal or semiconductor oxides and nitrides such as MgO, SiO₂, and SiN_x have been introduced as transient dielectric materials.^{Reference Kang, Hwang, Cheng, Yu, Kim, Kim, Huang and Rogers17} The key challenge in inorganic encapsulation is to minimize the defects that can cause water leakage. Several possible methods have been introduced using multiple layers, atomic-layer-deposited layers, and single-crystal Si nanomembranes for encapsulation.^{Reference Kang, Hwang, Cheng, Yu, Kim, Kim, Huang and Rogers17,Reference Lee, Yu, Song, Farimani, Vitale, Xie, Yoon, Kim, Richardson, Luan, Wu, Xie, Lucas, Crawford, Mei, Feng, Huang, Litt, Aluru, Yin and Rogers19}

Silicon transient electronics are used in biodegradable medical sensing and treatment as minimally invasive implants based on the biocompatibility of Si where SiOH₄, a byproduct of Si dissolution, is naturally present in the human body and usually evacuated through urine.^{Reference Archibald and Fenner29} Eliminating the necessity of secondary surgery greatly reduces the risks of surgery as well as the difficulties caused by permanent electronic implants. A recent demonstration showed comparable performance of biodegradable sensors in monitoring brain pressure or EEG and ECoG for diagnosis of symptoms with incubation periods such as traumatic brain injury.^{Reference Kang, Murphy, Hwang, Lee, Harburg, Krueger, Shin, Gamble, Cheng, Yu, Liu, McCall, Stephen, Ying, Kim, Park, Webb, Lee, Chung, Wie, Gujar, Vemulapalli, Kim, Lee, Cheng, Huang, Lee, Braun, Ray and Rogers2,Reference Yu, Kuzum, Hwang, Kim, Juul, Kim, Won, Chiang, Trumpis, Richardson, Cheng, Fang, Thomson, Bink, Talos, Seo, Lee, Kang, Kim, Lee, Huang, Jensen, Dichter, Lucas, Viventi, Litt and Rogers3} Biodegradable devices are now available for biomedical actuation using wireless electrical stimulators to enhance or promote biological activity and regeneration.^{Reference Koo, MacEwan, Kang, Won, Stephen, Gamble, Xie, Yan, Chen, Shin, Birenbaum, Chung, Kim, Khalifeh, Harburg, Bean, Paskett, Kim, Zohny, Lee, Zhang, Luo, Ji, Banks, Lee, Huang, Ray and Rogers4}

The article by Choi et al. overviews the dissolution chemistry of inorganic transient materials in biofluids, focusing on their kinetic behavior and biomedical applications.^{Reference Choi, Koo and Rogers23}

Biodegradable and flexible organic electronic materials

Biodegradable organic materials are useful as electrical functional layers—a semiconductor, conductor, and dielectric, a substrate to support the electrical circuit, and an encapsulation to protect the device. Organic materials offer tunability in electrical, mechanical, and biodegradable properties through modification of functional groups and blending of functional chains; and large-scale, low-cost production is available employing solution processes. Organic materials are suitable for flexible and stretchable devices that require wide deformation and conformal contact because of their soft and flexible mechanical properties relative to inorganic materials. The development of biodegradable electronic organic materials offers a chance to build a wide range of biocompatible flexible and stretchable devices with transient capabilities.

One of the first demonstrations of a transient organic thin-film transistor (OTFT) used fully biodegradable 5,5′-bis(7-dodecy119H-fluoren-2-yl)-2,2′-bithiophene (DDFTTF) semiconductor, poly(vinyl alcohol) (PVA) dielectric and PLGA.^{Reference Bettinger and Bao13} The mobility reached 0.253 cm²/Vs and the ON/OFF ratio was 9.4 × 10³ with PVA as the dielectric. A 10 mm × 10 mm × 2 mm OTFT was fully degraded in 70 days in a citrate buffer (pH 4.0) at 37°C.^{Reference Bettinger and Bao13}

Many researchers have tried to adopt eco-friendly and biodegradable natural molecules to exploit their biocompatible features.^{Reference Irimia-Vladu, Troshin, Reisinger, Shmygleva, Kanbur, Schwabegger, Bodea, Schwödiauer, Mumyatov, Fergus, Razumov, Sitter, Sariciftci and Bauer14} A representative example is indigo, which has semiconducting properties due to strong hydrogen bonding and intramolecular π interaction. Indigo is degraded not only by water, but also by enzymes, bacteria, light, and oxidizing agents.^{Reference Irimia-Vladu, Troshin, Reisinger, Shmygleva, Kanbur, Schwabegger, Bodea, Schwödiauer, Mumyatov, Fergus, Razumov, Sitter, Sariciftci and Bauer14} Other naturally obtained molecules have been suggested for biodegradable electronics—semiconductors, including eumelanin, β-carotene, indanthrene yellow G, indanthrene brilliant orange RF, and perylene diimide; and dielectrics, including sugars (glucose, lactose, and sucrose), nucleobases (adenine and guanine), and caffeine.^{Reference Irimia-Vladu, Troshin, Reisinger, Shmygleva, Kanbur, Schwabegger, Bodea, Schwödiauer, Mumyatov, Fergus, Razumov, Sitter, Sariciftci and Bauer14}

Efforts have recently been made to improve mobility by synthesizing biodegradable conjugated polymers and implementing them in organic electronic devices. Although synthesizing bonds capable of both conjugation and decomposition is highly challenging, imine bonds provide both biodegradability and biocompatibility. Pioneering work in this field^{Reference Lei, Guan, Liu, Lin, Pfattner, Shaw, McGuire, Huang, Shao, Cheng, Tok and Bao15} was the synthesis of poly(diketopyrrolopyrrole-p-phenylenediamine) (PDPP-PD) for a demonstrated electronic device. PDPP-PD was synthesized by a condensation reaction of diketopyrrolopyrrole-aldehyde (DPP-CHO) and p-phenylenediamine with p-toluenesulfonic acid catalyst.^{Reference Lei, Guan, Liu, Lin, Pfattner, Shaw, McGuire, Huang, Shao, Cheng, Tok and Bao15} A TFT with mobility of 0.21 ± 0.03 cm²/Vs was demonstrated using PDPP-PD (molecular weight M_r = 39.6 kDa), Al₂O₃, cellulose (800 nm, 2 g/m²) and Au as the active layer, dielectric layer, substrate, and electrode, respectively.^{Reference Lei, Guan, Liu, Lin, Pfattner, Shaw, McGuire, Huang, Shao, Cheng, Tok and Bao15} The device provided conformal contact to human brain models and an avocado with a rough surface because of its flexibility and softness.

The article by Liu et al. surveys biodegradable organic electronic materials, from natural organics to synthetic polymers, and their application and performance in electronic devices.^{Reference Liu, Tran, Feig and Bao24}

Fabrication strategies for transient electronics

Traditional semiconductor fabrication processes require high temperatures and reactive solvents, and thus have limited applicability to typical biodegradable materials due to their low thermostability and high chemical reactivity. Moreover, strategies are needed to fabricate hybrid layers of organic–inorganic materials so as to utilize both the outstanding electrical performance of inorganic materials and the mechanical flexibility of organic ones. Representative methods for the fabrication of transient electronics include (1) transfer-printing processes that separate reactive-sensitive materials from the electronic fabrication and integrate them after printing, and (2) solution processes that print composites of inorganic functional particles in an organic matrix.

In one of the first demonstrations of inorganic transient electronics, transient devices were fabricated by physical vapor deposition of dielectric layers (MgO or SiO₂) and electrode layers (Mg) using stencil masks on a substrate (silk), combined with transfer printing of silicon nanomembranes from silicon-on-insulator (SOI) wafers.^{Reference Hwang, Tao, Kim, Cheng, Song, Rill, Brenckle, Panilaitis, Won, Kim, Yu, Ameen, Li, Su, Yang, Kaplan, Zakin, Slepian, Huang, Omenetto and Rogers1} This procedure involves no thermal and chemical processes to damage biodegradable organic substrates such as silk and PLGA because most of the inorganic layer is formed by physical deposition and transfer printing.

Hwang et al. demonstrated the mass transfer of MOSFETs using anisotropic etching of SOI wafers with Si(111) handles, which provide the full wafer process without limitations on thermal processing and feature size.^{Reference Hwang, Kim, Tao, Kim, Kim, Yu, Panilaitis, Jeong, Song, Omenetto and Rogers16} The MOSFETs consisted of Si nanomembranes (∼100 nm) for the active channel, SiO₂ (∼100 nm) for the gate dielectric, and Mg (∼200 nm) for contact metals formed by 950∼1050°C doping, 250°C plasma-enhanced chemical vapor deposition (PECVD), and electron-beam evaporation deposition, respectively. Selective etching using reactive ion etching (RIE) formed the trench to allow the anisotropic undercut of the Si(111) handle toward the (110) direction using tetramethylammonium hydroxide (TMAH) to release the preformed MOSFETs from the SOI wafer. Undercut MOSFETs could be individually transferred by PDMS stamps on biodegradable polymers such as PLGA.

This type of modified Si fabrication processing and transfer-printing technique offers potential mass production of biodegradable Si transient devices through foundry facilities.^{Reference Chang, Fang, Bower, Song, Yu and Rogers22} High- and uniform-performance components (e.g., diodes and MOSFETs) with well-defined processing conditions can be produced in bulk on 6-in. Si(111) wafers. SiO₂ (dielectric), SiN_x (passivation), W (metal interconnect), and Ti/TiN (adhesion layer) were chosen for adaptation to the foundry process. The device, separated by selective etching using TMAH or KOH using a SOI (111) transfer method, could be transferred onto a biodegradable polymer (PLGA).^{Reference Chang, Fang, Bower, Song, Yu and Rogers22} This suggests the possibility of using transient electronics as chip units capable of performing complex calculation or logic functions, including integrated circuit levels beyond laboratory-scale demonstration.

Solution process-based printing of inorganic–organic hybrid composites are also available for transient electronics and have the advantages of low temperature, low cost, and simple processing. Huang et al. developed conductive organic–inorganic composites with in-plane conductivity of ∼3500 S/m (W) and 0.024 S/m (Zn) by blending W or Zn microparticles (4 ∼ 12 μm) with poly(ethylene oxide) (PEO).^{Reference Huang, Liu, Hwang, Kang, Patnaik, Cortes and Rogers30} Conductive composites formed by screen printing can be used as inductive coils for wireless communication as well as circuit electrodes. Lee et al. have reported conductive composites that maintain conductivity in 37°C deionized water for nine days by blending Mo microparticles (1∼5 μm) and hydrophobic polybutanedithiol 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione pentenoic anhydride) (PBTPA).^{Reference Lee, Koo, Kang, Park, Lee, Chen, Lim, Lee and Rogers31} The electrical properties and dissolution behavior of inorganic–organic blend mixtures can be controlled by particle size, concentration, and the polymer matrix.

The article by Han et al. details fabrication strategies to integrate high-performance inorganic transient materials and soft organic substrates. It also describes recent developments in electronic printing using transient composites.^{Reference Han, Ko, Shin and Hwang25}

Transient energy devices

The development of transient energy sources is essential to supply power to transient devices. There are two main streams of energy supply: (1) energy storage using batteries and capacitors, and (2) energy harvesting from the environment using materials such as piezoelectrics, photovoltaics, and triboelectrics.^{Reference Hwang, Tao, Kim, Cheng, Song, Rill, Brenckle, Panilaitis, Won, Kim, Yu, Ameen, Li, Su, Yang, Kaplan, Zakin, Slepian, Huang, Omenetto and Rogers1,Reference Yin, Huang, Xu, Zhang, Lam, Cheng and Rogers32–Reference Zheng, Zou, Zhang, Liu, Shi, Wang, Jin, Ouyang, Li and Wang34} It is challenging to design transient energy sources due to the few materials available, although the lifespan required is shorter than those in conventional usage. The primary goal of current research in transient energy devices is to design combinations of materials and structures that can deliver sustainable power to achieve desirable operational characteristics.

A battery is a good energy source due to its high energy density and stable energy supply. Figure 2a shows a galvanic cell that uses biodegradable metal electrodes (Mg for the anode, Fe, W, Mo for the cathode), phosphate buffer saline (PBS) electrolyte, and polyanhydride packaging.^{Reference Yin, Huang, Xu, Zhang, Lam, Cheng and Rogers32} Single-cell batteries composed of Mg-X metal foil (X = Fe, W, or Mo) under a constant discharge current density (0.1 mA/cm²) can stably produce voltages of ∼0.75 V, ∼0.65 V, ∼0.45 V for Mg-Fe, Mg-W, and Mg-Mo, respectively. Series stacking of single cells can enhance the output voltage; for example, four stacks of Mg-Mo generate 1.5∼1.6 V under a constant-discharge current density condition (0.1 mA/cm²). Modulating the combinations of electrode materials, electrolyte and coating materials, and anode thickness offers tunability in current density, output voltage, and lifespan.

Figure 2.

Schematics of transient energy devices. (a) Battery with four stacking Mg-Mo cells in series. Adapted with permission from Reference 27. © 2014 Wiley. (b) Mechanical energy harvester using triboelectricity. Scale bar = 10 um. Adapted with permission from Reference 29. © 2016 AAAS. (c) Wireless energy transfer with near-field inductive coupling. Adapted with permission from Reference 2. © 2016 Springer Nature. Note: PLGA, poly(lactic-co-glycolic acid); NFC, near-field communication; BDP, biodegradable polymer.

Energy harvesters that convert physical energy into electrical energy can also be used as transient energy sources. Figure 2b shows several examples of transient energy harvesters. A representative energy harvester has been suggested using biodegradable and piezoelectric metal oxide, ZnO with biodegradable Mg electrode, and silk substrates.^{Reference Dagdeviren, Hwang, Su, Kim, Cheng, Gur, Haney, Omenetto, Huang and Rogers33} As a mechanical energy harvester, this device generates ∼0.55 nA current, ∼1.14 V output voltage, and ∼10 nW/cm² power density with 0.28% energy-conversion efficiency. A recent study explored another type of mechanical energy harvester, a triboelectric generator: the friction between two different biodegradable polymers such as PLGA, PVA, PCL, and PHB/V can generate a power of 32.6 mW/m².^{Reference Zheng, Zou, Zhang, Liu, Shi, Wang, Jin, Ouyang, Li and Wang34}

Device size, life span, and energy densities remain key issues in batteries and various energy harvesters for biodegradable applications where miniaturization, long-term operation, and sufficient power are critical. Wireless transmitting power using near-field inductive coupling has been proposed as an alternative to implantable transient devices. The effectiveness of this type of energy transfer has been verified by recent biomedical applications in wireless hyperthermia to minimize residual bacteria after surgery, electrical stimulation to accelerate peripheral nerve regeneration, and heat modulation to control drug delivery.^{Reference Kang, Murphy, Hwang, Lee, Harburg, Krueger, Shin, Gamble, Cheng, Yu, Liu, McCall, Stephen, Ying, Kim, Park, Webb, Lee, Chung, Wie, Gujar, Vemulapalli, Kim, Lee, Cheng, Huang, Lee, Braun, Ray and Rogers2,Reference Yu, Kuzum, Hwang, Kim, Juul, Kim, Won, Chiang, Trumpis, Richardson, Cheng, Fang, Thomson, Bink, Talos, Seo, Lee, Kang, Kim, Lee, Huang, Jensen, Dichter, Lucas, Viventi, Litt and Rogers3,Reference Lee, Kim, Harburg, Park, Ma, Pan, Kim, Lee, Kim, Jang, Kang, Huang, Kim, Lee, Leal and Rogers5} Figure 2c shows wireless power and data transmitting devices using inductive coupling for wireless monitoring of intracranial pressure.^{Reference Kang, Murphy, Hwang, Lee, Harburg, Krueger, Shin, Gamble, Cheng, Yu, Liu, McCall, Stephen, Ying, Kim, Park, Webb, Lee, Chung, Wie, Gujar, Vemulapalli, Kim, Lee, Cheng, Huang, Lee, Braun, Ray and Rogers2}

The article by Jia et al. summarizes transient energy sources, focusing especially on energy-storage devices from batteries to supercapacitors.^{Reference Jia, Wang, Lee, Yu and Wallace26}

Triggered transient electronics for active lifetime control

The lifetimes of most eco-/bioresorbable or water-soluble forms of transient electronics are, as previously described, passively determined by the selection of constituent materials and system geometries, where ambient conditions (e.g., pH, temperature, and ionic content of the surrounding aqueous environments) dictate the overall dissolution kinetics. Advanced approaches, sometimes referred to as triggered transience,^{Reference Hernandez, Kang, Lee, Hwang, Kaitz, Inci, Park, Chung, Sottos, Moore, Rogers and White7–Reference Lee, Jeong, Liu, Zhang, Shi, Kang, Kim, Kim, Lee, Kim, Jang, Yin, Kim, Banks, Paik, Huang and Rogers10} make possible deterministic, active control over the initiation of dissolution, disintegration, depolymerization, or other chemical or physical processes. Such capabilities can be exploited in biomedical devices to allow the release of drugs at specific intervals or in military systems to enable destruction of secured hardware upon recovery by an adversary. Several studies illustrate on-demand, fast, and accurate control over the behavior of transient electronics with diverse materials systems, device structures, and trigger stimuli.

Figure 3a shows an example of triggering in which disintegration of a supporting substrate of cyclic poly(phthalaldehyde) (cPPA) occurs upon exposure to light.^{Reference Hernandez, Kang, Lee, Hwang, Kaitz, Inci, Park, Chung, Sottos, Moore, Rogers and White7} Here, the cPPA contains 2.5–5% of 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine (MBTT) as a dopant that yields photoactivated reactive Cl· radical species and hydrochloric acid (HCl), to cleave the acetal backbone of the cPPA. Exposure to UV light (λ_max = 365 nm) initiates depolymerization of the cPPA, leading to the disintegration of the ultrathin electronic components on the substrate.

Figure 3.

Triggered transient electronics for active lifetime control. (a) Photoinitiated transiency of 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine (MBTT)/cyclic poly(phthalaldehyde) (cPPA) substrate with electronics. Adapted with permission from Reference 7. © 2014 Wiley. (b) Transiency of acid-sensitive substrates (cPPA) and circuit (Mg) by release of methanesulfonic acid dispersed in wax encapsulation initiated by heat melting. Adapted with permission from Reference 9. © 2015 Wiley. (c) Structure of mechanical destruction of electronics by using an expandable polymer. Adapted with permission from Reference 8. © 2017 Wiley. (d) Wireless microfluidic device with thermal expansion polymer, and series of images showing the selective and sequential injection of water with dye by heat triggering the expandable polymer. Adapted with permission from Reference 10. © 2014 Wiley.

Thermal triggering mechanisms provide a complementary set of opportunities. In one example, metastable substrates such as cyclododecane (C₁₂H₂₄) sublime under ambient conditions; this accelerates at elevated temperatures to induce disintegration of the supported electronics.^{Reference Yoon, Lee, Choi, Lee, Kim, Kim, Moon, Lim, Kim and Choi35}Figure 3b shows further examples of triggering by thermal means, where advanced chemistries allow additional flexibility.^{Reference Park, Kang, Hernandez, Kaitz, Wie, Shin, Lee, Sottos, Moore, Rogers and White9} Park et al. describe a wax with 3% methanesulfonic acid introduced by stirring at 70°C; upon melting, rapid degradation of acid-sensitive metal interconnect lines (i.e., Mg) and substrate (i.e., cPPA) occurs by chemical etching.^{Reference Park, Kang, Hernandez, Kaitz, Wie, Shin, Lee, Sottos, Moore, Rogers and White9} A radio-frequency approach allows wireless delivery of thermal power for remote triggering.

Other strategies rely on the thermal expansion of polymers to fracture the essential components of electrical circuits^{Reference Gumus, Alam, Hussain, Mishra, Wicaksono, Sevilla, Shaikh, Diaz, Velling, Ghoneim, Ahmed and Hussain8} or to cause the release of active liquid species from microfluidic reservoirs^{Reference Lee, Jeong, Liu, Zhang, Shi, Kang, Kim, Kim, Lee, Kim, Jang, Yin, Kim, Banks, Paik, Huang and Rogers10} for chemical reactions or pharmacological delivery. As an example of the former, Figure 3c shows the instant destruction of a device on a thermally expandable polymer (Expancel 031 DU 40, AkzoNobel) composed of polymeric microspheres that encapsulate small amounts of liquid hydrocarbon.^{Reference Gumus, Alam, Hussain, Mishra, Wicaksono, Sevilla, Shaikh, Diaz, Velling, Ghoneim, Ahmed and Hussain8} When an integrated thermal actuator reaches temperatures above 80°C, the polymer expands irreversibly by nearly 10 times due to a phase change of the hydrocarbon from liquid to gas. Figure 3d shows a microfluidic system that releases etchants upon heating of a polymer beyond a critical level.^{Reference Lee, Jeong, Liu, Zhang, Shi, Kang, Kim, Kim, Lee, Kim, Jang, Yin, Kim, Banks, Paik, Huang and Rogers10} Water and sodium hydroxide act to dissolve Mg (Mg + 2H₂O → Mg(OH)₂ + H₂) and Al (2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H) electrodes, respectively, thereby destroying an antenna by etching.

The article by You et al. surveys strategies in triggered transient electronics to initiate degradation or terminate device function on demand via various stimuli.^{Reference You, Zhao, Guo and Mei27}

Outlook

Transient electronics research has seen explosive recent expansion in the discovery and analysis of water-soluble or degradable materials, and demonstrations of versatile devices for applications. The potential of transient electronics and the possibility of translation to applications relevant and useful to society has been clearly demonstrated.

Moving forward, we believe that the following areas of materials research are necessary: (1) precise control of degradation and operational lifetime adjustment, (2) development of new classes of transient electronic materials to increase the choice of electronic and degradation properties and other functionality, and (3) evolution toward high-performance electronics to translate the techniques to industry.

Transient electronics require sophisticated efforts to reduce the gap between materials degradation and the functional lifetime of electronic devices. This gap arises from the quick diffusion of water molecules through the protective layer into the electronic circuit before the protective layer can dissolve out. Enhancing the water-barrier properties of encapsulation layers is critical for precise control of the functional lifetime without increased layer thickness or total degradation time. This requires understanding the dissolution kinetics of water-diffusion behavior and developing nanostructural or molecular approaches to control it. The surface modification of transient encapsulation, one potential solution, requires understanding the surface chemistry at the molecular level to control the interaction between water molecules and the surface. Work on forming extremely dense encapsulation structures is also essential to slow down vapor diffusion into the layer. All of these considerations must be explored in transient passive materials, and the fields under consideration will no doubt expand.

The choice of electronic materials in transient electronics is limited to some inorganics and polymers whose dissolution chemistry and kinetics are known, especially at this new thickness scale. There are still great opportunities to discover new water-soluble or biodegradable electronic components that are electrically conductive and have semiconducting and dielectric properties. This effort can be furthered by exploring new classes of metal alloys with biodegradable components, designing nanostructured materials with transient metal and metal oxides, and synthesizing new types of polymers. Composites of transient inorganics and polymers are another great option. This will enrich the design choices for electronic devices by consideration of various electrical, transient, mechanical, and other properties.

In another direction, current techniques must be translated to industry so that they can influence medical practice. The first step will be moving forward to chip-scale devices made by reliable, high-performance processing. Most current demonstrations of transient electronics remain in the realm of materials development with one or two simple functional operations. Designing and developing chip-scale devices will be the first step toward the sophisticated transient devices with extensive performance and applications. Continuous efforts must be made to find materials and fabrication methods that are compatible with industrial processes. Extensive research in this translation stage should also explore the materials issues and reliability problems that will arise in stacking and packaging such chips

The articles in this issue not only summarize and overview recent developments in transient electronics in representative fields, they also offer information, suggestions, and ideas for the next generation of transient electronics devices.