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Page 2 of 20 Li et al. Soft Sci 2023;3:37 https://dx.doi.org/10.20517/ss.2023.30

LM-enabled wearable and flexible biosensors are also discussed.

Keywords: Liquid metal, flexible electronics, biosensors, wearable electronics

INTRODUCTION
Wearable biosensors are directly worn inside or outside the body in the form of watches, bracelets, glasses,
[4,5]
[1-3]
and clothing to acquire real-time and useful information related to heart rate , blood oxygen ,
exercise , and other physiological signals [9-11] . The key consideration for wearable biosensors is the comfort
[6-8]
of individuals who wear the devices during routine activities without restricting sensing precision .
[12]
Conventionally, wearable biosensors have been fabricated from rigid materials, limiting their comfort
during use and compromising safety for implantation. Thus, the development of biosensors with flexible
and stretchable properties is crucial to allow for conformal integration onto human skin and tissue,
facilitating optimal signal acquisition and transmission.

In recent years, gallium (Ga)-based liquid metals (LMs) have transcended their role in many fields, finding
diverse applications across multiple disciplines. From soft robotics and advanced electronics to additive
manufacturing and biomedicine, their exceptional properties have fostered innovation and paved the way
for transformative breakthroughs in various fields [13-15] . Especially in the field of flexible and wearable
electronics, attributed to their unique mechanical/chemical properties, Ga-based LMs have attracted much
attention. These properties include (1) LMs are in a liquid state with good fluidity at room temperature,
resulting in negligible Young’s modulus that can avoid the mechanical interface mismatch between the
device and human tissue [15-17] ; (2) they are metallic materials with high electrical/thermal conductivity, which
are essential for conducting electricity and heat transfer of wearable biosensors [18,19] ; and (3) the low toxicity
of LMs satisfies the bio-friendly requirements of wearable electronics [20,21] . In addition, pattering
technologies depending on the native physical properties of LMs, such as channel injection [22,23] , direct
[27]
writing [24-26] , and spray-printing methods, have greatly enabled the fabrication of LM-based wearable and
flexible sensors. Various wearable biosensors that are mountable on human tissue have been developed
using LMs. These biosensors have expanded the scope of wearable electronics and can be utilized in
numerous areas, such as human-machine interface, motion detection, and health monitoring.

This review aims to summarize the basic physical properties of LM-based materials and provides an in-
depth introduction to the designs/applications of flexible/stretchable interconnects, pressure sensors, strain
sensors, and implantable electrodes based on LMs [Figure 1]. This review begins with providing a detailed
description of the physical properties and electrically conductive mechanisms of LMs-based materials,
including raw LMs, LM nanoparticles (LMNPs) inks, and LM-polymer composites. The second section
summarizes recent advancements in LM-based biosensing devices, including interconnections, pressure
sensors, strain sensors, and implantable bioelectrodes. These advancements include nano-scale LM circuits
for improved bioelectronic integration, enhanced sensitivity of LM pressure sensors for monitoring
gastrointestinal pressure, fiber-featured strain sensors for breathing monitoring, and implantable
bioelectrodes capable of sensing electroencephalogram (EEG)/electrocardio (ECG) signals. These recently
developed LM-based biosensors offer exciting opportunities for in vivo/vitro monitoring of heart rate,
breath rate, body movement, and EEG/ECG signals. At last, the final section discusses the challenges and
future research perspectives of LM-based biosensors.

GA-BASED LMS, LMNP INKS, AND LM-ELASTOMER COMPOSITES
Raw LMs
Ga and Ga-based alloys are the most popular used LMs for flexible electronics when compared to liquid-