Femtosecond laser-fabricated multifunctional liquid metal flexible sensors and their applications

Significance  Liquid metals, owing to their high electrical conductivity, room-temperature fluidity, large deformability, and self-healing capability, have become important functional materials for flexible sensors, electronic skins, wearable health monitoring systems, soft robotics, and human–machine interfaces. Gallium-based liquid metals, especially eutectic gallium–indium, are particularly attractive because they combine metallic conductivity with fluid-like mechanical compliance. They can maintain conductive pathways under bending, stretching, twisting, and complex surface deformation, which makes them suitable for soft electronic systems that must operate under repeated mechanical loading. In addition, the fluid nature of liquid metals enables electrical reconnection after mechanical damage, giving them potential for self-healing circuits and long-term wearable devices.

However, the material features that make liquid metals attractive also bring major processing challenges. Gallium-based liquid metals possess high surface tension and rapidly form an oxide layer in air. High surface tension causes liquid metals to contract into droplets on many polymer substrates rather than spreading into continuous conductive lines. The oxide layer can help maintain non-spherical shapes, but it also strongly affects wetting, adhesion, interfacial contact, and electrical stability. These characteristics make it difficult to fabricate high-resolution, stable, and complex liquid metal patterns on flexible substrates. Conventional fabrication technologies, including microfluidic injection, three-dimensional printing, and transfer printing, have been used to construct liquid metal flexible sensors. Microfluidic methods provide good encapsulation but are limited by filling resistance, pressure-driven operation, and restricted structural freedom. Three-dimensional printing improves geometric design flexibility, but printing stability is affected by liquid metal rheology, surface oxidation, high surface tension, and line-edge control. Transfer printing is simple and compatible with various substrates, but it strongly depends on interfacial adhesion contrast and may face difficulties in fine pattern transfer, complex surfaces, and multilayer alignment. Therefore, developing a precise, low-damage, interface-regulatable fabrication method is essential for improving the performance and practical applicability of liquid metal flexible sensors.

Progress  Femtosecond laser processing provides an effective route for addressing these challenges. It features ultrashort pulse duration, high peak power density, small heat-affected zone, high spatial resolution, and non-contact processing. Laser energy can be deposited into a localized region within an extremely short time, which reduces thermal diffusion and allows precise ablation or surface modification. For flexible polymer substrates and transparent elastomers, femtosecond laser processing can induce controllable micro/nanostructures on surfaces or inside materials. These micro/nanostructures provide a direct way to regulate the wettability and adhesion of liquid metals, thereby enabling selective adhesion, high-resolution patterning, embedded microchannels, and stable flexible sensing devices.

The material characteristics of liquid metals provide the basis for this fabrication strategy. Liquid metals possess high conductivity, fluidity, surface oxidation behavior, self-healing capability, and adjustable interfacial wetting/adhesion properties. Their conductivity ensures stable electrical signal transmission, while their fluidity allows them to adapt to large deformation and curved surfaces. The oxide layer plays a dual role. It can help maintain patterned structures and assist conductive recovery after damage, but it may also cause unstable adhesion, variations in contact resistance, and long-term electrical drift. Therefore, liquid metal flexible sensor fabrication requires not only conductive pattern formation, but also control over wetting, adhesion, encapsulation, and interface stability.

Femtosecond laser processing can regulate these key interfacial factors through micro/nanostructure induction. Surfaces treated by femtosecond laser processing are covered with rough micro/nanostructures, which can reduce the effective contact area between liquid metals and the treated regions, forming low-adhesion areas. In contrast, untreated regions may retain stronger adhesion to liquid metals. This high/low adhesion contrast confines liquid metals to target regions and suppresses residue or spreading outside designed patterns. When combined with liquid metal oxidation control and sacrificial-layer-assisted processing, femtosecond laser structuring can produce liquid metal patterns with clear boundaries and high resolution. Laser processing can also form internal microchannels and interconnects inside transparent elastomers, which improves encapsulation and reduces the risk of leakage, oxidation, and surface wear.

Representative applications have been demonstrated in pressure sensors, strain sensors, tactile sensors, stretchable circuits, and multifunctional integrated devices. In pressure sensors, femtosecond-laser-fabricated microstructured liquid metal electrodes, microcone arrays, and surface protrusions enhance local deformation and amplify capacitance or resistance changes under weak pressure. Such structures improve sensitivity, lower detection limits, and reduce mechanical crosstalk in sensor arrays. For high-density electronic skins, this crosstalk suppression is important for accurate pressure mapping and tactile pattern recognition. In strain sensors, femtosecond laser regulation of substrate wettability and adhesion enables stable liquid metal patterning. Liquid metal microcrack structures and doped liquid metal electrodes can produce more regular conductive path changes during stretching, leading to improved linearity, repeatability, and cyclic stability. In multifunctional devices, femtosecond laser processing can be combined with interfacial alloying, liquid metal/copper composite electrodes, and embedded microchannel structures. These strategies improve pattern resolution, stretchability, curved-surface adaptability, and interfacial robustness, supporting the development of stretchable circuits, multilayer interconnects, and complex soft electronic systems.

Conclusions and Prospects  Femtosecond laser processing plays an important role in liquid metal flexible sensor fabrication by linking micro/nanostructure design with local interface regulation. It improves pattern accuracy, interfacial stability, and device performance, and provides a useful strategy for flexible pressure sensors, strain sensors, tactile sensors, stretchable circuits, and embedded soft electronic systems. Future research should focus on high-throughput processing, large-area consistency, complex substrate adaptability, heterogeneous material integration, reliable encapsulation, and long-term stability. With further improvement in parallel laser processing, dynamic focusing, multi-axis motion, and closed-loop monitoring, femtosecond laser-fabricated liquid metal flexible sensors are expected to show broader application potential in electronic skins, health monitoring, and soft robotics.