Self-healing liquid metal composite for reconfigurable and recyclable soft electronics

The study addresses the need for resilient soft electronic materials that can withstand damage and enable end-of-life regeneration. Unlike rigid electronics protected by stiff housings, soft systems are vulnerable to wear, tear, and catastrophic damage, necessitating materials that are not only soft, stretchable, and conductive but also self-healing, damage tolerant, reconfigurable, and recyclable. The authors propose a liquid metal–elastomer composite platform that enables programmable formation, maintenance, and erasure of conductive networks to achieve strain-invariant conductors, autonomous healing, circuit reconfiguration, and full recyclability, thereby extending device lifetime and reducing premature disposal.

Prior approaches to regenerative and stretchable electronics include: transient electronics that dissolve at end-of-life and can be made stretchable via geometric patterning; conductive inks deposited on stretchable textiles; dynamic covalent networks with rigid conductive fillers that are rehealable and recyclable but limited to low strains and require manual healing; and solid–liquid composites with liquid metals for softness and conductivity. Earlier liquid metal systems demonstrated manual repair or reclaimability with modest stretchability, autonomous electrical healing via microcapsule-inspired mechanisms, and sintering of droplets by scribing or lasers to form conductors. However, these methods often show resistance increasing with strain, limited control over trace resistance, and typically create permanent or only partially reconfigurable pathways that do not fully revert to an insulating state. This work targets these gaps by enabling tunable, highly stretchable conductors that can be erased and regenerated to the pristine insulating droplet state.

Materials and composite formulation: The matrix is a physically cross-linked styrene–isoprene–styrene (SIS) block copolymer plasticized with polybutadiene (PBD) to tune softness and processability. Eutectic gallium–indium (EGaIn, 3:1) serves as the liquid metal phase. SIS pellets are dissolved in toluene via dual-asymmetric centrifugal mixing; PBD is added at specified volume ratios to SIS. Liquid metal is then added at target volume fractions (e.g., φ = 50–60%), and high-shear mixing disperses EGaIn into discrete droplets (mean diameter ~27.8 ± 7.7 µm). Sheets (~1.5 mm thick) are solvent cast and dried (24 h RT + 24 h at 80 °C). Embossing to form conductors: Conductive traces are created by compressive embossing using 3D-printed molds with defined line features. During embossing, discrete insulating droplets are forced into percolated networks within stamped regions. A feedback-controlled setup synchronizes an Instron 5944 mechanical tester (applying load) with a Keithley 2460 SMU (measuring resistance under constant current). Load is increased until a target resistance is reached, then the process is automatically stopped, enabling precise tuning of trace resistance across 10 Ω to 1 kΩ. Mechanical and electromechanical testing: ASTM D412 C-type dogbones (50% scale) are laser cut. Traces are embossed along the gauge length. Uniaxial tensile tests measure modulus (linear fit to 5% strain) and strain at break. Cyclic tests include: (i) 100% strain for 1000 cycles; (ii) stepped cycling from 0 to 1000% in 100% increments (three cycles per step), with resistance monitored continuously to report R/R0. Damage tolerance and self-healing: A hole-punch protocol under load introduces four sequential holes into an actively stretched specimen. The sequence includes stretching to 100% strain, resting, hole punching (four damage events total), then stretching to failure, while continuously measuring R/R0 and visually documenting damage evolution. Reconfiguration (local erase): A solvent-erase approach locally reprocesses the SIS matrix to break previously formed liquid metal networks, returning regions to the discrete droplet, insulating state. New traces are embossed to alter circuit topology in situ (e.g., toggling which LED is powered). Recycling (global regeneration): End-of-life composites are cut (~1 cm squares) and dissolved in toluene using planetary shear mixing (6 min at 2000 rpm) to re-form a SIS/PBD/toluene solution with dispersed droplets. Films are recast, yielding insulating composites that can be re-embossed. Mechanical properties and electrical function are characterized for pristine and two recycling generations. Integration with components: Liquid metal contact pads (EGaIn + 7 vol% Cu microparticles to form a semi-solid paste) are stencil-printed at component interfaces. LEDs are bonded, and assemblies are encapsulated with a thin SIS layer (200 µm) at 150 °C and 25 mbar vacuum. Optical microscopy (Leica DMi8, dark-field) assesses droplet/network microstructure and embossed topography.

- Embossing creates percolated liquid metal networks with tunable initial resistance spanning 10 Ω–1 kΩ and initial conductivities up to ~150 S/cm (0% strain). Feedback control enables precise stop-at-resistance embossing. - Exceptional stretchability with strain-invariant or decreasing resistance: R/R0 decreases from 1.0 to ~0.56 at 1200% strain for conductors; predicted metallic wire behavior would increase to R/R0 ≈ 169 at the same strain. - Conductivity increases with strain due to geometric and network effects, reaching as high as ~45,400 S/cm at 1200% strain. - Resistive elements (10 Ω to 1 kΩ) maintain nearly constant resistance up to ~800% strain and function as stretchable circuit elements (e.g., brightness control in parallel resistor LED circuits). - Cyclic durability: over 1000 cycles to 100% strain, R/R0 remains ~1 at 0% strain and ~0.6 at peak strain with negligible drift (peak-to-valley ~0.001 near the 1000th cycle). Under stepped cycling up to 1000% strain, R/R0 remains ~0.6 at maximum strain with modest variation (~0.20 peak-to-valley). - Self-healing under severe damage: Four sequential hole-punch events during active loading maintain electrical conductivity throughout and allow stretching to ~950% strain without loss of circuit function. - Mechanical properties: Adding PBD and 60 vol% liquid metal softens the SIS matrix (tensile modulus ~500 kPa) while preserving extreme extensibility (>1000% strain). Embossing has minimal effect on modulus; strain-at-break remains >1000%. - Component integration: Embossed traces reliably power LEDs during stretching (shown to ~240% strain before failure at the component–trace interface) and during bending, folding, twisting, and stretching. - Reconfiguration: Local solvent erase breaks prior networks, enabling on-demand rewiring (e.g., switching which LED is powered) and restoration of insulating regions prior to re-embossing. - Recycling: Global dissolution and recasting fully erase networks, returning to discrete droplet microstructures. Recycled composites (two generations shown) retain softness, reach ~1000% strain, and can be re-embossed to form functional circuits with comparable performance, though pristine samples show somewhat higher strain-stiffening at large strains.

The findings demonstrate a unified soft electronic material platform that addresses durability and sustainability challenges by combining a reprocessable elastomer matrix with reconfigurable liquid metal microstructures. Embossing provides on-demand formation of conductive networks with resistance tunability over orders of magnitude and near strain-invariant performance at extreme deformations, overcoming the typical resistance increase seen in solid metallic conductors and many prior stretchable composites. Under mechanical cycling and severe damage (hole punching), autonomous reconfiguration of the liquid metal network maintains conductivity, illustrating robust, self-healing behavior critical for soft systems in unforgiving environments. Local solvent reprocessing erases and re-forms traces for in-field reconfiguration, while global recycling regenerates pristine composites, fully breaking networks into discrete droplets and enabling remanufacture of new devices. Collectively, these capabilities advance resilient soft electronics and suggest pathways to reduce electronic waste by extending lifetime and enabling material reuse.

This work introduces a multifunctional liquid metal–elastomer–plasticizer composite that achieves tunable, highly stretchable conductors with strain-invariant resistance, autonomous electrical self-healing under damage, in-field circuit reconfiguration via local solvent erase, and complete end-of-life recycling to regenerate new devices. The approach leverages embossing to form and control liquid metal networks and the reprocessability of a physically cross-linked SIS matrix to erase and rebuild microstructures locally or globally. Future work could explore systematic control of liquid metal droplet size (e.g., sonication to produce submicron droplets) to further tune trace resistance, embossing pressure–resistance profiles, dielectric breakdown behavior, and undercooling, as well as long-term durability and fatigue over multiple recycling generations and improved component–trace interfaces for higher strain operation of integrated devices.

- Component–trace interface durability currently limits integrated LED operation to lower strains (~240%) compared to the intrinsic trace capability (>1000%); improved interconnect strategies are needed. - Recycled samples show slightly reduced strain stiffening at high strains compared to pristine composites, indicating some mechanical property evolution with recycling. - The study primarily uses ~30 µm droplets; effects of droplet size on electrical/mechanical behavior, breakdown voltage, and embossing response remain to be systematically quantified. - Long-term aging, environmental stability, and multi-generation recycling fatigue were not fully characterized. - Processing relies on organic solvent (toluene) for fabrication and recycling; greener solvent systems or solvent recovery strategies were not explored.