High‐Resolution Patterning of Liquid Metal on Hydrogel for Flexible, Stretchable, and Self‐Healing Electronics

Abstract

concentrations are incompatible with the physiological conditions, as the diffused ions may irritate biological tissues. [1c] It is also challenging to pattern ionic conductors with a high resolution because of the inevitable diffusion. [15] Conducting polymers such as poly(3,4-ethylene dioxythiophene) (PEDOT) possess relatively low conductivity, [16] while solid metallic conductors demonstrate remarkable mechanical mismatch with soft hydrogels. Although conductive nanocomposites like metal nanowires offer good conductivity and stretchability, the required synthesis is complicated and costly. Metal nanowires also have poor biocompatibility and can cause adverse health effects. [17] Last but not least, the adhesion of conductive components on hydrogel surfaces is still a problem to be addressed. [18] Hence, for practical applications, most conductors must be dispersed into the hydrogel matrix, making it difficult to form functional structures on the hydrogels. Ga-based alloys as room temperature liquid metals (LMs) are a unique type of material possessing high conductivity, fluidity, deformability, nontoxicity, and self-healing capability. [19] These properties make them ideal conductors for flexible and stretchable hydrogel electronics. [20] However, the lack of efficient patterning techniques hinders the realization of functional LMbased hydrogel electronics. Although numerous techniques have been developed for the patterning of LM, they are generally designed for dry and rigid substrates. [21] These patterning techniques have always involved the pretreatment of substrates to increase adhesion to LM, or use of mechanical force for postsintering. Therefore, these techniques cannot be readily applied to wet and delicate hydrogels. Although previous work has shown that it is possible to form helical LM structures in the hydrogel by microfluidic injection, discrete and complex LM patterns are still difficult to obtain due to the need for inlets and outlets of the microfluidic system. [22] Therefore, it is highly desirable to develop a new method for the patterning of LM on hydrogels for practical applications. We have previously developed a versatile LM patterning technique using a magnetic field. The LM droplets dispersed with magnetic microparticles can be patterned on various substrates simply by moving a magnet. [23] Despite its simplicity, it is still challenging to obtain high-resolution LM patterns using this technique. In this work, high-resolution patterning of LM on hydrogels is accomplished by in situ fabrication of a shadow Soft, wet, and biocompatible hydrogels have emerged as promising materials for flexible and stretchable electronics owing to their similar properties with biological tissues. However, most existing conductive materials used for hydrogel-based electronics have drawbacks such as poor biocompatibility, low conductivity, and/or high mechanical mismatch with soft hydrogels. In this work, direct patterning of nontoxic and highly conductive liquid metal (LM) on hydrogels is reported for soft...

Soft Electronics Based on Liquid Conductors

“…Reproduced with permission. [ 54 ] Copyright 2019, Wiley‐VCH. d) Direct painting an EGaIn tree on copper tape.…”

“…Such a patterned PET served as a shadow mask to allow the deposition of EGaIn in a patterned manner onto PVA hydrogel surface (Figure 10f). [ 54 ] EGaIn did not penetrate or spread on the hydrogel surface after the removal of PET mask owing to the formation of an oxide layer around it. Such a sensor could be stretched with tensile strain up to 300% without evidence of any fracture, simultaneously corresponding to a steady and repeatable increase of resistance (Figure 10g,h).…”

Advances in Materials for Soft Stretchable Conductors and Their Behavior under Mechanical Deformation

“…Wang et al anchored conductive fillers with EGaIn particles to produce printable and superelastic conductors [ 110 ]. Xu et al cleverly disperse Nickel (Ni) micoparticles into EGaIn and deploy a permanent magnet to flow the liquid metal droplet through a shadow mask [ 111 ]. These Ni particles also aided adhesion between the liquid metal and the underlying hydrogel substrate.…”

“…Other groups created hydrogel-based wireless antennas. Lim et al fabricate a wireless antenna with a nanocomposite AgNW/alginate hydrogel supported by a PAAm-hydrogel substrate [ 261 ] whereas Xu et al display high-resolution patterning of liquid metal on a PVA hydrogel for near-field communication (NFC) ( Figure 24 a,b) [ 111 ].…”

“… ( a ) Liquid metal-based hydrogel NFC tag with demonstrated use ( b ). Reproduced with permission from [ 111 ]. Copyright 2020 John Wiley and Sons.…”