A compliant, self-adhesive and self-healing epidermal strain sensor that shows excellent sensing performance at ultra-low and high strain.

Low-melting liquid metal is a hugely promising material for flexible conductive patterns due to its excellent conductivity and supercompliance, especially low-cost and environmental liquid processing technology. However, the ever-present fluidity characteristic greatly limits the stable shape and reliability of prepared liquid metal conductive electronics. Herein, a novel solidification strategy of liquid GaIn alloys by Ni doping and heat treatment is first reported, which can efficiently create a solid phase in the liquid metal and provide an effective solution for practical applications. Particularly, the liquid characteristic is preserved for conveniently fabricating different flexible electronic circuits, and then the solidification is carried out on prepared conductive patterns by heat treatment. The solidification mechanism is revealed by the interface chemical reaction between Ni and GaIn, creating the solid phase of intermetallic compound (Ga 4 Ni 3 and InNi 3 ) during heat treatment. Moreover, a biphasic GaInNi can be obtained by regulating the atomic ratio of gallium, indium, and nickel. As a result, the obtained GaInNi possesses extremely low sheet resistance (15 ± 4.5 to 135 ± 2.5 mΩ sq −1 ) and the variation of ΔR/R 0 exhibits low level (0-2) when strained up to 100%, which offers a promising strategy to prepare stretchable and reliable liquid metal electronics.

As a critical part of flexible electronics, flexible circuits inevitably work in a dynamic state, which causes electrical deterioration of brittle conductive materials (i.e., Cu, Ag, ITO). Recently, gallium-based liquid metal particles (LMPs) with electrical stability and self-repairing have been studied to replace brittle materials owing to their low modulus and excellent conductivity. However, LMP-coated Ga 2 O 3 needs to activate by external sintering, which makes it more complicated to fabricate and gives it a larger short-circuit risk. Coreshell structural particles (Ag@LMPs) that exhibit excellent initial conductivity (8.0 Ω sq −1 ) without extra sintering are successfully prepared by coating nanosilver on the surface of LMPs through in situ chemical reduction. The critical stress at which rigid Ag shells rupture can be controlled by adjusting the Ag shell thickness so that LM cores with low moduli can release, achieving real-time self-repairing (within 200 ms) under external destruction. Furthermore, a flexible circuit utilizing Ag@LMPs is fabricated by screen printing, and exhibits outstanding stability and durability (R/R 0 < 1.65 after 10 000 bending cycles in a radius of 0.5 mm) because of the functional core-shell structure. The self-repairable Ag@LMPs prepared in this study are a candidate filler for flexible circuit design through multiple processing methods.

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Recently, stimuli-responsive sensors and actuators have drawn significant attention as they are promising in a wide scope of applications, including wearable devices and soft robotics. However, they both face serious challenges in terms of interface management, that is, the weak sensor–skin interfacial interaction and the single binding function in actuators. Herein, we design an adhesive and modulus tailorable interface by in situ polymerization of dopamine (DA) along the poly­(vinyl alcohol) (PVA) macromolecular chains and the incorporation of ionic coordination between polymer molecular chains (PVA and polydopamine (PDA) chains) and metal ions (Fe3+). Note that the introduction of hygroscopic Fe3+ cations not only facilitates the coordination to increase the water tolerance of the interface but also improves the water uptake into the system, which is essential for tailoring both the adhesion and modulus of the interface. By adopting this interface, we successfully fabricate a soft (skin-like modulus 0.1–10.0 MPa), ultrathin (∼50 μm), self-adhesive, and skin-compliant sensor. Furthermore, the interface is also applied to bind two active layers to fabricate a multiresponsive (heat, near-infrared (NIR) light, voltage, and humidity) actuator. In addition to the adhesion function, the interface can also help lock the actuator states thanks to the tunable modulus of the interface, which is highly valuable for decreasing the power consumption of the actuator. This work provides a new avenue to address both sensing and actuating bottlenecks by focusing on the interface.