Direct Wiring of Liquid Metal on an Ultrasoft Substrate Using a Polyvinyl Alcohol Lift-off Method

Abstract

In recent years, wiring and system construction on ultrasoft materials such as biological tissues and hydrogels have been proposed for advanced wearable devices, implantable devices, and soft robotics. Among the soft conductive materials, Ga-based liquid metals (LMs) are both biocompatible and ultrasoft, making them a good match for electrodes on the ultrasoft substrates. However, gels and tissues are softer and less wettable to the LMs than conventional soft substrates such as Ecoflex and polydimethylsiloxane. In this study, we demonstrated the transfer of LM paste composed of Ga-based LM and Ni nanoparticles onto ultrasoft substrates such as biological tissue and gels using sacrificial polyvinyl alcohol (PVA) films. The LM paste pattern fabricated on the PVA film adhered to the ultrasoft substrate along surface irregularities and was transferred without being destroyed by the PVA film before the PVA's dissolution in water. The minimum line width that could be wired was approximately 165 μm. Three-dimensional wiring, such as the helical structure on the gel fiber surface, is also possible. Application of this transfer method to tissues using LM paste wiring allowed the successful stimulation of the vagus nerve in rats. In addition, we succeeded in transferring a temperature measurement system fabricated on a PVA film onto the gel. The connection between the solid-state electrical element and the LM paste was stable and maintained the functionality of the temperature-sensing system. This fundamental study of wiring fabrication and system integration can contribute to the development of advanced electric devices based on ultrasoft substrates.

Self-assembling bilayer wiring with highly conductive liquid metal and insulative ion gel layers

“…2 e). In similar methods that enable wiring on arbitrary substrates, the transfer of patterns of Au thin film 53 , Ag nanoparticles 54 , and LM 19 fabricated on polyvinyl alcohol (PVA) film have been reported. These methods enable fine line drawing and are effective for integration and systemization.…”

“…For instance, the Ga-based alloy Galinstan (68 wt% Ga, 22 wt% In, and 10 wt% Sn) is a biocompatible material with a high electrical conductivity of S/m 11 , melting point of 11 °C 12 , and viscosity of 13 . This liquid material has been used for soft and stretchable device wiring 14 , sensor reactors 15 , 16 , connections with different conductive materials 17 , bioelectrodes 18 , 19 , actuators 20 , 21 , and dielectric elastomers 22 . Although Galinstan is more difficult to process than other conductive materials due to its high surface tension (~ 535 mN/m) 23 , it has a much lower Young's modulus and smaller resistance change due to deformation.…”

“…Conventional technologies for wiring LMs, such as direct drawing [24][25][26] , screen printing 27,28 , activation by sintering [29][30][31] and transfer 19,32 have been reported. These technologies leave the LM bare on the substrate and require an encapsulation process to prevent its destruction and leakage after wiring.…”

Liquid Metal Patterning and Unique Properties for Next‐Generation Soft Electronics

“…Adapted with permission. [ 108 ] Copyright 2022, American Chemical Society. Image for “permeability”.…”

“…[ 114 ] Such limitations should be surmounted by surface treatment, encapsulation, elastomer selection, alloying or forming composite, or adhesive glue as well as the development of patterning methods for LM. [ 16,97,108,116,122 ]…”