Mechanically Tunable Electronic Ink for Additive Manufacturing of Body-Temperature Softening Bioelectronics

Bioelectronics, encompassing wearable and implantable devices, hold immense potential for health monitoring and therapeutic intervention by interfacing directly with biological organs. However, conventional rigid bioelectronics often induce discomfort, inflammation, or tissue injuries due to mechanical mismatches with biological tissues, despite their convenience and robustness. While soft electronics mimic tissue mechanics, challenges persist in handling, maintaining shape, and precise positioning. Transformative bioelectronics, which integrate advantages of rigid and soft electronics into a single device, have emerged as a promising solution. As its core material, gallium stood out for its exceptional stiffness tuning ratio (~10⁴) and a melting point (~29.8°C) close to body temperature. Yet, its poor rheological traits (i.e. ultrahigh surface tension and low viscosity) have challenged high-resolution patterning, limiting manufacturing techniques to mold casting or microfluidics. This, despite its high conductivity (3.4 × 10⁶ S m⁻¹), has resulted in poor patterning resolution, with gallium primarily used as mechanical frameworks for building transformative electronics.<br/>Here, we propose a one-step preparable, body-temperature softening electronic ink for high-resolution (~50 µm) additive manufacturing of mechanically transformative bioelectronics. Through direct-ink-write (DIW) printing, we deposit highly viscous inks layer-by-layer to form intricate 3-dimensional structures with high precision. To optimize the ink rheology for high printing stability, we sonicate 5.0 wt% of copper microparticles and gallium to form a gallium-copper composite. Exceptional thermal conductivity of copper (320.72 W m⁻¹K⁻¹) lowers the ink’s melting temperature to 27°C, which accelerates its solid-to-liquid phase transition time by 31% (from 64.18 to 16.96 seconds) at body temperature (~37°C). The ink showcases enhanced electrical conductivity (3.69 × 10⁶ S m⁻¹) and an exceptional mechanical tuning ratio (a negligible elastic modulus at the soft mode and a high modulus comparable to pure gallium (~10 GPa) at the rigid mode), highlighting its dual functionality: electrical functionality and mechanical adaptability.<br/>Our electronic ink showcases excellent rheological, thermal, electrical, and mechanical properties, as demonstrated through DIW of two transformative bioelectronic devices: an epidermal PPG (Photoplethysmogram) sensor for pulse monitoring and a wireless optoelectronic device for optogenetics and phototherapy. The transformative epidermal PPG sensor maintains rigid at room temperature for easy handling and transitions to a soft, skin-like texture at body temperature for comfortable wear, even during motion. Meanwhile, the stiffness-tunable wireless optoelectronic device is intricately patterned with interconnects of varying widths and thicknesses (100 - 400 µm) and built with 15 vertical interconnect accesses and 17 surface-mounted devices, designed to optimize wireless power transfer efficiency by accommodating skin effects in RF antenna design. Additionally, it transitions bidirectionally between stiff and stretchable states (&gt;206% strain) with temperature changes, adapting to tissue deformation. These demonstrations underscore the ink’s transformative nature, substrate versatility, and compatibility with high-resolution additive manufacturing techniques, thus amplifying its potential in transformative electronics for consumer electronics, robotics, sensors, and more.