Advances in Liquid Metal-Enabled Flexible and Wearable Sensors

Abstract

Sensors are core elements to directly obtain information from surrounding objects for further detecting, judging and controlling purposes. With the rapid development of soft electronics, flexible sensors have made considerable progress, and can better fit the objects to detect and, thus respond to changes more sensitively. Recently, as a newly emerging electronic ink, liquid metal is being increasingly investigated to realize various electronic elements, especially soft ones. Compared to conventional soft sensors, the introduction of liquid metal shows rather unique advantages. Due to excellent flexibility and conductivity, liquid-metal soft sensors present high enhancement in sensitivity and precision, thus producing many profound applications. So far, a series of flexible and wearable sensors based on liquid metal have been designed and tested. Their applications have also witnessed a growing exploration in biomedical areas, including health-monitoring, electronic skin, wearable devices and intelligent robots etc. This article presents a systematic review of the typical progress of liquid metal-enabled soft sensors, including material innovations, fabrication strategies, fundamental principles, representative application examples, and so on. The perspectives of liquid-metal soft sensors is finally interpreted to conclude the future challenges and opportunities.2 of 25 to monitor skin strain and muscle movement [32]. Introduction of soft sensors has greatly propelled the development of intelligent artificial skin, which provides not only aesthetic functions, but also perception of tactile sensation and temperature measurement [33,34]. They can also be applied to realize intelligent perception and control for robots [35][36][37]. Moreover, flexible sensors can tightly fit the object and maintain performance even while being stretched. In this way, they can respond to changes more sensitively and detect position and posture alteration in real time.Up to now, materials and techniques to achieve flexible sensors have been widely explored. To obtain better application output for the human body, both flexibility and biocompatibility of the substrates are required. Soft thermoplastic polymers and silicone elastomers are the most common substrates used to fabricate flexible sensors, such as polyethylene terephthalate (PET), poly urethane (PU), polydimethylsiloxane (PDMS) and Eco Flex [38]. Moreover, the sensing elements are other important parts, which are mainly made up of conductive materials, including organic and inorganic nanomaterials [39,40]. Carbon-based materials, such as carbon nanotube and graphene, have been investigated in various soft circuits [39,[41][42][43][44][45][46]. Although circuits fabricated by carbon-based materials are highly stretchable, they are not as conductive as those fabricated by metals. Nanoparticles of rigid metals are applied to design soft circuits with high conductivity, among which silver nanoparticles have been extensively studied [47][48][49][50][51][52][53]. Moreover, liquid me...

Nano‐Biomedicine based on Liquid Metal Particles and Allied Materials

“…LMs confined in microfluidic channels and elastomers are used as soft and stretchable conductive wires, interconnects, antennas, and sensors in wearables for movement monitoring and electrophysiological signal detection. [ 17,18 ] Through modifying the wetting behavior of LM interfacial property, the drawbacks of the high surface tension can be overcome, thus enabling LMs printed directly on skin as electronic tattoos for healthcare functionality. [ 19 ] When combined with other metals or nonmetals, such as Mg and polymers, these composites are endowed with certain rigidity and shaping capability and can be applied as conformable skin patches for tumor photothermal therapy or magnetic‐mediated hyperthermia.…”

Editorial on the Special Issue on Microelectrode Arrays and Application to Medical Devices

“…As an emerging technology, wearable and flexible sensing have been widely studied due to their important role in personalized medicine and their capabilities in real-time monitoring of an individual’s health. The application of liquid metals, which can provide a higher degree of flexibility and conductivity compared to conventional soft sensors, in different biomedical areas was reviewed by Ren et al [ 9 ]. The authors presented the recent progress of liquid metal-enabled soft sensors from different aspects, including innovations in material selection and fabrication techniques, fundamental principles, and typical applications.…”

“…In this editorial note, we briefly review the major findings of the 10 articles published in the Special Issue on microelectrode arrays and application to medical devices. The articles are categorized into three sections, i.e., fabrication techniques [ 1 , 2 , 3 , 4 ], application demonstration [ 5 , 6 , 7 , 8 ], and review of recent advances in this field [ 9 , 10 ].…”

Liquid Metal Based Flexible and Implantable Biosensors

“…The low Young’s modulus ensures that small forces are required to achieve large deformation. As a type of metal with a low melting point, liquid metal shows higher electrical conductivity than most soft polymer materials [ 21 ]. At the same time, bulk liquid metal can achieve stable electrical signal transmission under deformation of ~700%.…”