From Materials to Devices: An Engineering Perspective on Designing Gallium-Based Liquid Metal Bioelectronics

Abstract

Gallium-based liquid metals (LMs) are rapidly revolutionizing the field of soft and stretchable electronics, especially within the rapidly evolving biomedical sector. It has excellent biocompatibility, near-zero vapor pressure, and viscosity similar to that of water. It also has one of the highest surface tensions of any liquid at room temperature. However, it can adopt an adaptive shape form because a thin solid oxide skin forms spontaneously when exposed to oxygen. With the increasing need for flexible electronic devices capable of seamlessly interfacing with biological systems, LM offers unparalleled advantages due to their unique combination of metallic conductivity, fluidic flexibility, and inherent low mechanical modulus. The potential of these materials to dramatically enhance device functionality and comfort in wearable and implantable technologies has positioned them at the forefront of emerging bioelectronic solutions. Despite these attractive properties, integrating gallium-based LMs into practical electronic devices remains a considerable challenge, primarily due to intrinsic material behaviors such as high surface tension, instantaneous oxide layer formation, and difficulties in adhering effectively to target substrates. Successfully overcoming these limitations is critical not only for achieving reliable functionality but also for fully realizing the transformative potential of LM microfabrication technologies.In this Account, we systematically address the engineering challenges associated with integrating gallium-based LMs. We begin by thoroughly examining their fundamental material properties, such as electrical and thermal conductivity, surface tension, and oxide skin behavior, and explicitly link these attributes to practical implications in device manufacturing and performance. We provide a detailed analysis of oxide skin formation and explain how this influences LM fabrication handling and long-term device stability. Next, we outline and introduce various LM microfabrication techniques, including direct printing, stencil patterning, and selective metal wetting methods, and introduce our newly developed patterning method using an external magnetic field. Each LM technique is discussed in depth, highlighting its strengths and limitations, and emphasizing the critical material considerations required for each method, based on our experiences of research and findings. Current strategies for integrating LM into bioelectronics will be explained in two primary categories: (1) mechanically robust 3D interconnects used in flexible devices such as smart contact lenses and antennas, and (2) active electrodes designed for interfacing with soft biological tissues. Special attention is given to sensing and stimulation applications with a focus on optimizing performance, stability, and biocompatibility. We also highlight approaches to mitigate issues related to oxide layers and strategies to improve the electrode reliability through surface modifications. Finally, this Account culminates with practical design guidelines, presented through design maps, to assist engineers and researchers in making informed, application-specific decisions for soft targets such as organoids, brain, retina, and even dynamic surfaces like skin. Our comprehensive approach emphasizes the engineering tradeoffs required to fully harness the potential of gallium-based LM technologies.