Q. Huang, K.N. Al-Milaji, Z. Li, H. Zhao
Virginia Commonwealth University,
Keywords: inkjet printing, silver nanowire, printed electronics, stretchable conductors
Summary:Stretchable conductors have attracted great attention for their potential applications as sensors for wearable electronic devices. Conventional printing methods for stretchable conductors include depositing conductive materials on an elastomer substrate or printing a composite mixture of conductive fillers and elastomer. In the former approach, buckling and fracture (slips and delamination) of conductive materials often occur due to the mechanical instability and poor adhesion between the printed structures and the underlying substrates, which irreversibly increases the electrical resistance of the conductors during stretching/releasing cycles. In the latter approach, the conductive fillers usually need to be as high as 60-80 wt% to render the required conductivity, which significantly sabotages the achievement of high stretchability and also leads to an increase of material and printing costs. Directly printing and embedding conductive micro- and nano-materials into the elastomer substrate provides a new strategy to overcome these problems. The percolation network of silver nanowires (AgNWs) has shown great potential as stretchable conductors due to their excellent electrical conductivity and good chemical stability. In this work, stretchable micro-conductors have been obtained by directly inkjet printing silver nanowires into a liquid polydimethylsiloxane (PDMS) film supported by a substrate. The momentum of the inkjet droplets pushes them into the liquid elastomer forming printed patterns inside the polymer. The surface wetting property of the supporting substrate affects the final morphology of the printed patterns. In addition to the advantage of directly printing and embedding AgNWs in an elastomer layer, this printing process also enables alignment of these AgNWs along the printing direction. When coalescing into other droplets forming lines, the AgNWs inside the droplet align themselves along the printing direction under the internal flow during droplet coalescence. More alignment has been introduced during solvent evaporation when the AgNW lines are squeezed by the surrounding liquid PDMS. As a result, high-resolution (~tens of microns), AgNW micro-conductors with aligned structures are embedded in the stretchable elastomer. An initial electrical conductivity of ~ 3000 S.cm-1 has been demonstrated. Benefited from the embedded structure and alignment of AgNWs, the electrical performance of the conductor at various strains keeps stable after hundreds of stretching/releasing cycles. The process of inkjet printing and embedding AgNWs directly into stretchable elastomers provides a viable approach for fabricating wearable electronic devices. This work is supported by Jeffress Trust Award in Interdisciplinary Research (The Thomas F. and Kate Miller Jeffress Memorial Trust, Bank of America, Trustee) and startup fund at the Virginia Commonwealth University.