High-Resolution, Multifunctional Variable Stiffness Electronics by Solution-Processible, Chemically-Sintered Liquid Metal Ink

Modern electronics are engineered with specific mechanical attributes, ranging from robust, rigid devices to soft, flexible systems, each tailored to their intended applications. While rigid electronics provide stable interfaces for stationary or handheld use, soft electronics excel in wearable and implantable applications, conforming to bodily movements. However, as electronic devices evolve towards multifunctional systems, fixed mechanical properties limit their ability to optimize stiffness for each distinct function, restricting broader applicability. To address this challenge, variable stiffness electronics have emerged, capable of adapting their shape, flexibility, and stretchability as needed. Gallium has emerged as a promising material for variable stiffness electronics due to its high electrical conductivity (3.4 × 10⁶ S m^-1), low toxicity, and substantial mechanical tuning ratio (552.3 kPa to 106.7 MPa), and phase transition temperature (29.76°C) close to room temperature. However, challenges such as fluidic leakage, high surface tension, low viscosity, and frequent phase transitions during processing have made high-resolution circuit patterning difficult, limiting gallium’s application to mechanical frameworks.
To overcome these challenges, we introduce a Thermally-actuated, Stiffness-adjustable Metal (TSM) ink that resolves the limitations of pristine gallium while enabling high-resolution patterning for conductive, variable stiffness electronics. This ink is formulated by dispersing gallium microparticles within a hydrophilic polyurethane template via sonication, resulting in a solution state with reduced surface tension and controllable phase transition properties. The TSM ink is compatible with conventional solution-based fabrication methods such as screen printing, dip coating, and nozzle printing, offering improved printing reliability and pattern resolution (~50 μm) compared to pure gallium. The ink is chemically-sintered using in-situ acidified dimethyl sulfoxide, creating conductive inter-gallium bridges within the polymer network to enhance both conductivity and solid-liquid phase transition capabilities. This unique attribute of TSM ink allows for the creation of high-resolution patterns that can act as both electrical circuit layers and mechanical frameworks.
We demonstrate the versatility of TSM ink through multiple variable stiffness electronics applications, including a multilayer, high-resolution transformative printed circuit board that converts between a rigid computing machine and a flexible, wearable health monitoring device, as well as an implantable neural probe that transitions from rigid to soft upon brain insertion. These advances open opportunities for a new class of adaptable electronics capable of transforming their configuration and function to suit diverse, dynamic environments.