Body-temperature softening electronic ink for additive manufacturing of transformative bioelectronics via direct writing

Phase-change metal ink with pH-controlled chemical sintering for versatile and scalable fabrication of variable stiffness electronics

Variable stiffness electronics represent the forefront of adaptive technology, integrating rigid and soft electronics in a single system through dynamic mechanical modulation. While gallium's high modulus tuning ratio and rapid phase transitions make it ideal for transformative electronic systems (TES), its liquid-state instability, high surface tension, and unintended phase transitions during processing pose substantial challenges. Here, we introduce STiffness-Adjustable temperature-Responsive ink (STAR ink), a chemically sinterable gallium composite electronic ink designed to overcome these obstacles. STAR ink enables high-resolution (~50 micrometers) circuit patterning, large-scale batch fabrication, and three-dimensional structure coating at room temperature. Through pH-controlled chemical sintering, STAR ink-based TES exhibits exceptional mechanical tunability (tuning ratio: 1465) and electrical conductivity (2.27 × 106 siemens per meter). Demonstrated applications-from multilayered variable stiffness printed circuit boards (PCBs) matching standard PCBs' complexity to body-temperature responsive neural probe-underscore STAR ink's potential for reconfigurable electronics across consumer electronics and biomedical devices.

Programmable and Spatial Stiffness Gradient Substrates for Highly Robust Artificial Skins

Stretchable artificial skins have garnered great interest for their potential applications in real-time human-machine interaction and equipment operation status monitoring. The local stiffer structure areas on the substrates for the functional elements have been verified to improve the robustness of the artificial skins, but it remains challenging to achieve robust sensing performance under mechanical deformation due to large mechanical mismatch and the intricate fabrication process. Herein, we propose an easy strategy for fabricating a substrate with spatial and programmable stiffness gradients to greatly decrease strain interference and increase the robustness under stretching and bending. The substrate was fabricated by direct writing PDMS with laser gelation, where the sensing elements lay on the place with higher stiffness. The modulus of the substrates varied up to 10-fold, and they also show excellent adhesive properties and durability. This configuration of the spatial stiffness gradient effectively inhibits the deformation strain effect of stretching and bending on the sensing elements. Prototype flexible sensors and light-emitting diodes can be integrated into stretchable artificial skins to exhibit highly robust performance during dynamic deformations, demonstrating an efficient pathway for fabricating robust stretchable electronics, especially for real-time health surveillance.

Reconfigurable double-sided smart textile circuit with liquid metal

Smart textiles have emerged as a promising alternative to printed circuit boards (PCBs) for electronics that are flexible, lightweight, and stretchable. However, many existing solutions fall short of providing sufficient electrical properties or are limited to single-sided circuit designs, significantly reducing their utility. In this study, we present a smart textile based on liquid metal and silver flakes that allows for double-sided circuit configurations without the need for via holes, offering advantages beyond conventional PCB technologies. This approach allows users to insulate or connect top and bottom circuits as needed, even when the circuits overlap or intersect. The inherent properties of liquid metal facilitate pressure-induced sintering, working in synergy with textiles to provide users with the ability to dynamically alter circuits. This unique feature enables real-time customization, allowing for the addition, removal, or replacement of circuits through straightforward cutting and stitching processes. Demonstrating these characteristics, we showcase diverse applications, including a wristband with a replaceable LED indicator circuit, a reversible teddy bear cloth with two distinct functions, and a customizable DIY heating glove. This double-sided textile circuit that is patterned with pressure-controlled drawing offers new possibilities for multifunctional wearable electronics, bridging the gap between traditional PCBs and flexible smart textiles.

Flexible self-rectifying synapse array for energy-efficient edge multiplication in electrocardiogram diagnosis

Edge computing devices, which generate, collect, process, and analyze data near the source, enhance the data processing efficiency and improve the responsiveness in real-time applications or unstable network environments. To be utilized in wearable and skin-attached electronics, these edge devices must be compact, energy efficient for use in low-power environments, and fabricable on soft substrates. Here, we propose a flexible memristive dot product engine (f-MDPE) designed for edge use and demonstrate its feasibility in a real-time electrocardiogram (ECG) monitoring system. The f-MDPE comprises a 32 × 32 crossbar array embodying a low-temperature processed self-rectifying charge trap memristor on a flexible polyimide substrate and exhibits high uniformity and robust electrical and mechanical stability even under 5-mm bending conditions. Then, we design a neural network training algorithm through hardware-aware approaches and conduct real-time edge ECG diagnosis. This approach achieved an ECG classification accuracy of 93.5%, while consuming only 0.3% of the energy compared to digital approaches, highlighting the strong potential of this approach for emerging edge neuromorphic hardware.

Erasable and Multifunctional On-Skin Bioelectronics Prepared by Direct Writing

The field of bioelectronics has witnessed significant advancements, offering practical solutions for personalized healthcare through the acquisition and analysis of skin-based physical, chemical, and electrophysiological signals. Despite these advancements, current bioelectronics face several challenges, including complex preparation procedures, poor skin adherence, susceptibility to motion artifacts, and limited personalization and reconfigurability capabilities. In this study, we introduce an innovative method for fabricating erasable bioelectronics on a flexible substrate coating adhered to the skin using a ballpoint pen without any postprocessing. Our approach yields devices that are thin, erasable, reconfigurable, dry-friction resistant, self-healing, and highly customizable. We demonstrate the multifunctionality of these on-skin bioelectronics through their application as strain sensors for motion monitoring, temperature and humidity sensors for breath monitoring, and heating elements for target point hyperthermia. The potential of our bioelectronics in personalized medicine is substantial, particularly in health monitoring. We provide a novel solution for achieving efficient and convenient personalized medical services, addressing the limitations of existing technologies and paving the way for next-generation wearable health devices.

Connecting the Dots: Sintering of Liquid Metal Particles for Soft and Stretchable Conductors

This review focuses on the sintering of liquid metal particles (LMPs). Here, sintering means the partial merging or connecting of particles (or droplets) to form a network of percolated and, thus, conductive electrical pathways. LMPs are attractive materials because they can be suspended in a carrier fluid to create printable inks or distributed in an elastomer to create soft, stretchable composites. However, films and traces of LMPs are not typically conductive as fabricated due to the native oxide that forms on the surface of the particles. In the case of composites, polymers can also get between particles, making sintering more challenging. Sintering can be done via a variety of ways, such as mechanical, thermal, and chemical processing. This review discusses the mechanisms to sinter these particles, patterning techniques that use sintering, unique properties of sintered LMPs, and their practical applications in fields such as stretchable electronics, soft robotics, and active materials.

A bio-inspired microwave wireless system for constituting passive and maintenance-free IoT networks

With the rapid expansion of wireless networks, the deployment and long-term maintenance of distributed microwave terminals have become increasingly challenging. To address these issues, we present a bio-inspired microwave system to constitute passive and maintenance-free wireless networks. Drawing inspiration from vertebrate skeletons and skins, we employ stimuli-responsive polymer with tunable stiffness to support and protect sensitive electromagnetic structures, and synthesize self-healable skin-like polymer for system encapsulation. Owing to the biomimetic strategy, our system combines outstanding flexibility, electromagnetic stability, structural robustness, and self-healable performance. On the other hand, to address power supply issues, our system modulates ambient electromagnetic waves to achieve long-range wireless communication, and the hybrid energy harvesting strategy allows the system to capture energy from ambient light and microwaves, thereby eliminating the need for batteries or power cables. Multidisciplinary innovation enables our system to be deployed almost anywhere and supports stable, battery-less, and maintenance-free wireless communication.

Sign-Switchable Poisson's Ratio Design for Bimodal Strain-to-Electrical Signal Transducing Device

Stretchable electronic devices that conduct strain-related electronic performances have drawn extensive attention, functioning as mechanical sensors, actuators, and stretchable conductors. Although strain-insensitive or strain-responsive nature is well-achieved separately, it remains challenging to combine these two characteristics in one single device, which will offer versatile adaptability in various working situations. Herein, a hybrid material with sign-switchable Poisson's ratio (SSPR) is developed by combining a phase-change gel based reentrantreentrant honeycomb pattern and a polydimethylsiloxane film. The phase-change gel featuring thermally-regulated Young's modulus enables the hybrid material to switch between negative and positive Poisson's ratios. After integrating with a pre-stretched silver nanowires film, the obtained stretchable device performs bimodal strain-to-electrical signal transducing (Bi-SET) functions, in which the SSPR-dominated strain-resistance response switches between strain-dependent and strain-insensitive behaviors. As a proof of concept, a mode-switchable grasping system is constructed using a Bi-SET device-based controller, enabling the adaptation of grasping behaviors to various target objects.

Printed Nanomaterials for All-in-One Integrated Flexible Wearables and Bioelectronics

Recent advancements in printing technologies allow for fabricating various wearable sensors, circuits, and integrated electronics. However, most printing tools have limited ranges of handling ink viscosity, a short working distance, and a limited feature size for developing sophisticated electronics. Here, this paper introduces an all-in-one integrated wearable electronic system via multilayer, multinanomaterial printing. Versatile, high-resolution aerosol-jet printing could successfully print Cu nanoparticles, Ag nanoparticles, PEDOT:PSS, and polyimide (PI) to manufacture nanomembrane composite structures, including skin-contact electrodes and wireless circuits. The printed polymer, PEDOT:PSS deposited on the Cu, ensures biocompatibility when making direct skin contact while enhancing electrical conductivity for electrodes. A self-assembled monolayer facilitates better adhesion of Cu nanoparticles on the PI. Also, using intensive pulsed light, a photonic sintering method minimizes Cu-oxidation while avoiding thermal damage. The combined experimental and computational study shows the mechanical flexibility and reliability of the printed integrated device. With human subjects, the flexible wireless bioelectronic system demonstrates superior performance in detecting high-fidelity physiological signals on the skin, including electromyograms, electrooculograms, electrocardiograms, and motions, proving its potential applications in portable human healthcare and persistent human-machine interfaces.

Customized surface adhesive and wettability properties of conformal electronic devices

Conformal and body-adaptive electronics have revolutionized the way we interact with technology, ushering in a new era of wearable devices that can seamlessly integrate with our daily lives. However, the inherent mismatch between artificially synthesized materials and biological tissues (caused by irregular skin fold, skin hair, sweat, and skin grease) needs to be addressed, which can be realized using body-adaptive electronics by rational design of their surface adhesive and wettability properties. Over the past few decades, various approaches have been developed to enhance the conformability and adaptability of bioelectronics by (i) increasing flexibility and reducing device thickness, (ii) improving the adhesion and wettability between bioelectronics and biological interfaces, and (iii) refining the integration process with biological systems. Successful development of a conformal and body-adaptive electronic device requires comprehensive consideration of all three aspects. This review starts with the design strategies of conformal electronics with different surface adhesive and wettability properties. A series of conformal and body-adaptive electronics used in the human body under both dry and wet conditions are systematically discussed. Finally, the current challenges and critical perspectives are summarized, focusing on promising directions such as telemedicine, mobile health, point-of-care diagnostics, and human-machine interface applications.

Softening implantable bioelectronics: Material designs, applications, and future directions

Implantable bioelectronics, integrated directly within the body, represent a potent biomedical solution for monitoring and treating a range of medical conditions, including chronic diseases, neural disorders, and cardiac conditions, through personalized medical interventions. Nevertheless, contemporary implantable bioelectronics rely heavily on rigid materials (e.g., inorganic materials and metals), leading to inflammatory responses and tissue damage due to a mechanical mismatch with biological tissues. Recently, soft electronics with mechanical properties comparable to those of biological tissues have been introduced to alleviate fatal immune responses and improve tissue conformity. Despite their myriad advantages, substantial challenges persist in surgical handling and precise positioning due to their high compliance. To surmount these obstacles, softening implantable bioelectronics has garnered significant attention as it embraces the benefits of both rigid and soft bioelectronics. These devices are rigid for easy standalone implantation, transitioning to a soft state in vivo in response to environmental stimuli, which effectively overcomes functional/biological problems inherent in the static mechanical properties of conventional implants. This article reviews recent research and development in softening materials and designs for implantable bioelectronics. Examples featuring tissue-penetrating and conformal softening devices highlight the promising potential of these approaches in biomedical applications. A concluding section delves into current challenges and outlines future directions for softening implantable device technologies, underscoring their pivotal role in propelling the evolution of next-generation bioelectronics.