Laser Sintering of Liquid Metal Nanoparticles for Scalable Manufacturing of Soft and Flexible Electronics

Abstract

Soft, flexible, and stretchable electronics are needed to transmit power and information, and track dynamic poses in next-generation wearables, soft robots, and biocompatible devices. Liquid metal has emerged as a promising material for these applications due to its high conductivity and liquid phase state at room temperature; however, surface oxidation of liquid metal gives it unique behaviors that are often incompatible with scalable manufacturing techniques. This paper reports a rapid and scalable approach to fabricate soft and flexible electronics composed of liquid metal. Compared to other liquid metal patterning approaches, this approach has the advantages of compatibility with a variety of substrates, ease of scalability, and efficiency through automated processes. Nonconductive liquid metal nanoparticle films are sintered into electrically conductive patterns by use of a focused laser beam to rupture and ablate particle oxide shells, and allow their liquid metal cores to escape and coalesce. The laser sintering phenomenon is investigated through comparison with focused ion beam sintering and by studying the effects of thermal propagation in sintered films. The effects of laser fluence, nanoparticle size, film thickness, and substrate material on resistance of the sintered films are evaluated. Several devices are fabricated to demonstrate the electrical stability of laser-patterned liquid metal traces under flexing, multilayer circuits, and intricately patterned circuits. This work merges the precision, consistency, and speed of laser manufacturing with the material benefits of liquid conductors on elastic substrates to demonstrate decisive progress toward commercial-scale manufacturing of soft electronics.

Liquid Metal–Based Soft Microfluidics

“…LM offers a brilliant solution toward the settlement of these drawbacks, yet LM‐based electronics suffer from corrosion at the contact between liquid metal and metal. In recent years, various strategies have been explored to develop corrosion resistant coatings for flexible electronics and to improve the LM patterning techniques on soft substrates . Dickey and Franklin et al reported the use of printed carbon nanotubes as flexible electrode and interconnect material, which was used in conjunction with EGaIn to form reliable thin‐film transistors (TFTs).…”

“…The tangled CNTs on the surface can suppress the crack generation during the stretching process, facilitating the development of stretchable LM–solid metal contacts and circuits. Furthermore, LM nanoparticles (LMNPs) can be patterned to form complicate planar geometries using a modulated pulse laser irradiation process, followed by transplantation of the pattern to the desired substrates . Pulsed laser irradiation plays a pivotal role in triggering the rupture and evaporation of the LMNPs to form uniform solid–liquid biphasic conductive layer.…”

“…In recent years, various strategies have been explored to develop corrosion resistant coatings for flexible electronics and to improve the LM patterning techniques on soft substrates. [90,[107][108][109][110][111] Dickey and Franklin et al [83] reported the use of printed carbon nanotubes as flexible electrode and interconnect material, which was used in conjunc-tion with EGaIn to form reliable thin-film transistors (TFTs). The EGaIn LM can be deposited by both the vacuum-filling PDMS microchannel structures and direct-writing on CNTs.…”

Light‐Induced Shape Morphing of Liquid Metal Nanodroplets Enabled by Polydopamine Coating

“…LM nanorod was also prepared and employed as a swimming nanomachine to drill into cancer cells and then transfer into a droplet for photothermal therapy . Moreover, deformable LM nanodroplets were applied as mechanically sintered or laser‐sintered conductive inks for flexible and stretchable electronics . By applying low forces or exposing to a focused laser beam, the LM nanodroplets were ruptured and the low‐viscosity LM was released.…”

“…For instance, Hohman et al generated LM nanodroplets by sonication of bulk LMs in ethanol and stabilized by different surfactants including 1‐dodecanethiol, 3‐mercapto‐N‐propionamide, and polyvinylpyrrolidone (PVP) . This approach has been well‐studied and the resulting colloidal solution was employed as conductive ink for soft and flexible electronics . Stable aqueous suspensions of LM nanodroplets are not only for electronic applications, but also exhibit great potential in medicine and therapy, such as drug delivery, photothermal therapy, and bioimaging .…”