Liquid metal film conductors enable highly stretchable electrodes on 3D surfaces

Stretchable and flexible electronics have attracted wide attention in many emerging fields such as wearable devices, biomedical devices, flexible optical devices, and soft robotics. In these applications, stretchable conductive materials are an important and fundamental component and are currently a hot topic. Research on stretchable conductors has been widely reported, such as conductive elastomers, metal nanowires, and patterned metal film conductors.

Although these stretchable conductive schemes are very promising in many applications, in some cases, these approaches may have limitations that affect their application. For example, a combination of one or more properties such as low volume conductivity, high electromechanical coupling, or poor elasticity may limit the functionality of conductive elastomers and metal nanowire conductors. Similarly, deterministic architectures often require advanced manufacturing techniques to pattern wavy/serpentine traces of thin film conductors. At the same time, due to the non-straight patterning, they have limited stretchability and low surface density.

To address the above issues, room temperature stretchable conductors composed of liquid metal (LM) have been reported. Liquid metal has properties suitable for stretchable conductors, such as high conductivity, strong stretchability, patternability, and conformality derived from inherent liquid properties. Liquid metal-based solutions solve the necessary functions of stretchable conductors.

Despite the extensive research efforts to date, the advantages of liquid metal conductors are still largely limited to substrates or enclosed microchannels. Circuits with substrates inevitably face an unsolvable dilemma, that is, the substrate and circuit are inseparable, and the substrate underneath the circuit will inevitably hinder the movement, attachment, repair, and adjustment of the patterned circuit, thus limiting its applicability.

To this end, researchers are working hard to develop thin and flexible substrates to minimize the negative effects caused by the substrate. However, the presence of the substrate makes some problems inevitable, and it is impossible to completely eliminate these problems without completely removing the substrate. In order to overcome the limitations caused by the substrate and expand the applicability of stretchable conductors, it is necessary to develop independent conductors or substrate-free conductors with excellent electromechanical properties.

According to MEMS Consulting, a collaborative research team from Seoul National University in South Korea and Carnegie Mellon University in the United States has developed an independent patterned liquid metal thin film conductor (FS-GaIn) that can be independently and directly applied to irregular surfaces without a substrate. FS-GaIn is made by introducing metal nanowires into liquid metal and then performing sequential selective laser processing and selective etching under maskless and room temperature conditions. Laser-assisted manufacturing facilitates ultrafast patterning (laser scanning speed of 100 mm/s) and rapid prototyping.

When integrated into circuits, FS-GaIn can withstand extreme strain with little change in resistance without encapsulation, form stable electrical connections with rigid components without post-processing, and maintain conformal contact with non-flat 3D surfaces. Circuits improved by FS-GaIn also show high stability.

FS-GaIn is a special metal-based super-stretchable thin film conductor that can be applied to uneven 3D surfaces or used for in-situ circuit trimming. With the characteristics of substrate-free thin film liquid metal conductor, it expands the application in the field of flexible and stretchable electronics.

Schematic diagram of the FS-GaIn fabrication process, which is prepared by direct laser patterning of vacuum-filtered liquid metal-silver nanowire (LM-AgNW) films with a continuous visible light laser and subsequent selective etching. (i) SEM micrograph of FS-GaIn; (ii, iii) SEM micrographs of liquid metal-silver nanowire films before and after laser exposure.

Left: Conformal connection of FS-GaIn to the human ear model and rigid components. Right: Dendritic patterned FS-GaIn.

FS-GaIn preparation principle and process

In summary, the researchers developed FS-GaIn in this study, which has high conductivity (5.79 × 10⁵ S/m), small resistance change within 1350% strain range, remains stable within 10,000 cycles of 100% strain, and can maintain stable contact with rigid parts when stretched. The excellent electromechanical properties of FS-GaIn are due to three main reasons, namely surface conformality, highly tortuous serpentine structure, and wettability between liquid metal and silver nanowires.

The circuits repaired by applying FS-GaIn patches also demonstrated stable connections and were able to withstand strains exceeding 1000% without electrical disconnection. The FS-GaIn fabrication method is simple, using a combination of direct laser writing and etching to rapidly fabricate patterned FS-GaIn without a mask and at room temperature. The fabrication of FS-GaIn was achieved by taking advantage of differential etching between multiphase sintered and non-sintered areas.

The properties of FS-GaIn also allow for in-situ circuit modification, attachment to 3D surfaces with sharp contours, and installation of circuits in confined environments. FS-GaIn overcomes the inevitable limitations of traditional thin film conductors, and its application in wearable electronics, flexible optical devices, and soft robots is expected to open up the next generation of stretchable electronic devices.