Synergistic Mechano-Fluidic Self-Fusion of Poly(Vinyl Alcohol)-Modified Liquid-Metal Particles for Post-Treatment-Free Soft Electronics

Background

Gallium-based liquid metals combine metallic conductivity with liquid-like deformability, but their high surface tension hinders patterning on elastomers. Processing them into liquid metal particles (LMPs) improves printability, yet each particle is enclosed by an insulating oxide shell of about 3 nm, so printed LMP films usually need thermal, mechanical, laser or chemical treatment before they conduct. Earlier PVA–liquid-metal systems used PVA for wettability, hydrogels or laminated layers, and earlier spray approaches atomized bulk metal that was already conductive.

Design and approach

Eutectic gallium–indium (EGaIn) is sonicated in 5 wt% aqueous PVA, and centrifugation removes unbound PVA, leaving an interfacial layer estimated at 7–10 nm over a 3–5 nm native oxide. The particles are redispersed in a deionized (DI) water/ethanol mixture (3:1 v/v) and sprayed with an airbrush through a mask. Conductive networks are proposed to form in two stages: spray impact ruptures the oxide/PVA shells and starts metallic contact, and evaporation of the binary solvent then drives Marangoni convection and capillary forces that redistribute particles and promote neck growth. The thin PVA layer lowers surface charge and surface energy and bridges particles to each other and to the substrate.

How it was evaluated

The authors compared PmLMP with a PVA-free control (DI_LMP) and an over-coated control (PVA_LMP) using TEM, STEM-EDS, FT-IR, zeta potential, rheometer adhesion and contact angles on SEBS, Ecoflex, PDMS and TPU. Shell rupture was examined by cross-sectional SEM and XPS of spray- versus screen-printed films and by AFM nanoindentation of single particles. Spray parameters (sonication amplitude, carrier-gas pressure, working distance, solvent ratio and substrate temperature) were varied against four-point-probe conductivity, and drying flows were measured by micro-particle image velocimetry. Electromechanical tests covered monotonic stretching (n = 5) and cyclic stretching at 100% strain (n = 3). Devices included a pick-and-place microcontroller circuit, an LED wound-healing patch tested in a murine full-thickness excisional wound model (n = 4 per group), and textile ECG/EMG electrodes worn by human participants under IRB approval.

Key results

Particle interface: centrifugal removal of excess PVA was required for conduction. The over-coated PVA_LMP film reached 0.19 × 10^6 S m−1, about eightfold lower than PmLMP (1.5 × 10^6 S m−1). PVA modification shifted the zeta potential from −70.1 mV to about −38 mV, and the contact-angle-derived effective surface tension of PmLMP was about 54.6 mN m−1, compared with about 72.8 mN m−1 for DI_LMP. Under stretching, DI_LMP beaded and PVA_LMP cracked, whereas PmLMP kept a continuous network with PVA fibrils bridging particles and substrate.

Impact activation: spray-printed films showed ruptured particles near the substrate and XPS signals of metallic Ga0 and In that were negligible in screen-printed films. AFM indentation gave a yield force of 266.08 nN, comparable to the 200–250 nN reported for bare oxide-shelled particles, and the corresponding critical impact velocity was estimated at about 4 m s−1. Conductivity was highest at 20% sonication amplitude, saturated above 30 psi and peaked at a 3 cm working distance.

Flow-assisted consolidation: in drying droplets, the DI water/ethanol mixture produced flow velocities up to 50 times faster (average 22.4-fold) than DI water, mitigated size segregation and filled voids between larger particles. TEM showed necks at particle contacts, and the binary-solvent formulation gave a threefold higher conductivity than the DI-only control. A capillary model estimated about 79 nN per interparticle meniscus, which the authors treat as a secondary consolidation force rather than the primary rupture mechanism.

Patterning and devices: traces conducted down to a nominal 50 µm linewidth, and a 4 × 4 array averaged 1.55 × 10^6 S m−1. PmLMP traces kept continuity up to 800% strain with R/R0 of about 2.8, while PVA_LMP fractured near 100%. Components were pick-and-placed directly onto as-printed traces, and in mice the LED patch (640 nm, 10 Hz) accelerated macroscopic wound closure relative to untreated controls over 7 days. Textile and Tegaderm electrodes with a LiCl hydrogel interface recorded ECG and EMG, and EMG signals controlled a remote-controlled car over Bluetooth Low Energy.

Open questions

The authors note that mask-assisted printing produced variations in trace height and cross-section, so conduction at 50 µm does not imply equal conductivity across linewidths. The capillary-stress estimate relies on an assumed contact radius, the solvent–temperature matrix was not a full response-surface optimization, and adhesion on plasma-treated PDMS and Ecoflex was compared only qualitatively. They suggest high-speed in situ studies to resolve oxide cracking during impact.

How it connects to other research

The solution-processed liquid-metal features reported in npj Flexible Electronics (DOI 10.1038/s41528-021-00123-x) pattern liquid metal through a fully solution-based process under ambient conditions, whereas PmLMP forms conductive paths within a single spray deposition by combining impact rupture of oxide/PVA-shelled particles with solvent-driven consolidation.