Original source: Materials Today

A dense forest of mushroom-like gold nanowires grown on soft, flexible substrates could enable a new generation of wearable or implantable stretchable electronic devices, believe researchers [Wang et al., ACS Nano (2018), DOI: 10.1021/acsnano.8b05019]. Elastronics – electronic devices that can bend and flex repeatedly without impacting on performance – are ideally suited to on-the-skin monitoring or diagnostic applications.

Producing devices that are both conductive and flexible is challenging, however. Typically, elastronic devices are either extrinsically or intrinsically stretchable and conductive. Extrinsic elastronic devices rely on stretchable structures, while intrinsic devices are made from conformable, conductive materials. The design of elastic materials that retain their conductivity has focused on embedding conductive nanomaterials, one-or two-dimensional materials such as nanoparticles and nanowires, into elastomers. Now, however, researchers at Monash University, the Melbourne Centre for Nanofabrication, and the Royal College of Art have come up with a new intrinsic elastronic material in the form of gold nanowires grown vertically on an elastomeric substrate such as PET (polyethylene terephthalate), PDMS (polydimethylsiloxane), or silicone rubber (Ecoflex). A simple, nanoparticle-seeded solution growth process yields nanowires securely attached to the flexible substrate. The vertical nanowires resemble Japanese enokitake mushrooms with long parallel stems topped by nanoparticle ‘caps’ (Fig. 1).

“This is the first time that ‘standing’ gold nanowires [have been] grown on elastomeric substrates [and shown to] exhibit unconventional Janus materials properties and extremely high stretchability,” says Wenlong Cheng, who led the research, along with colleagues George Simon and Stephen Wang. “Conventional conductive films exhibit sharp, ‘cliff-like’ cracks upon stretching [because of] the mechanical mismatch between rigid conductive segment and soft substrates,” he explains.

Instead, the nanowire/elastomer film can be stretched to up to eight times its original length without buckling or failing (Fig. 2). Unlike typical metal films, the enokitake-like nanowire/elastomer films exhibit V-shaped cracks, for strains of up to 300%, which recover once the strain is removed. Moreover, conductivity is retained – over 90% of the original conductance is preserved after 2000 cycles of stretching to 800% strain and releasing. At lower strain levels, the researchers found no structural changes to the enokitake-like nanowire/elastomer film after 60 000 cycles of stretching/releasing at 185% strain.

“Our results show our vertical nanowire-bonded elastomers can be stretched much more before losing conductivity than traditional metallic films or horizontal nanowire percolation systems,” says Cheng. “The tiny cracks effectively prevent mechanical delamination and electrical failure, enabling excellent stretchability, recovery upon release of strain, and durability.”

The enokitake-like nanowire/elastomer system also appears to be robust, maintaining its combination of stretchability and conductivity even after storage in air for 40 weeks.

“Based on the superior elasticity, adjustable sensitivity, durability, and excellent skin conformability of our vertical enokitake-like nanowire film, we believe our approach has great potential for next-generation wearable and implantable applications. Solution-based, electroless gold coating on elastomers is advantageous in comparison to other reported approaches,” Cheng says.

The team also demonstrated a proof-of-concept wearable smart facial recognition sensor system based on the enokitake-like nanowire/elastomer film and has developed other devices including supercapacitors, transistors, conductors, and chemical biosensors.

“This concept – using standing Au nanowires to minimize the mechanical footprint of an elastomeric substrate to effectively reduce the interfacial strain between the hard and soft materials – is very new and should open up lots of exciting opportunities in hybridized approaches for fabricating soft electronic devices,” comments Sheng Xu of the University of California, San Diego.

Cunjiang Yu of the University of Houston agrees that the enokitake-like nanowire film is an ideal material with which to build soft wearable electronics. “[It is a] novel and unique material with unprecedented stretchability,” he comments. “The nanowire-based film can be stretched to nine times [its original length] while retaining electrical conductance, which is truly remarkable.”

This article was originally published in Nano Today 23 (2018) 3-4.