Ag Nanowire Reinforced Highly Stretchable Conductive Fibers for Wearable Electronics

Background

Stretchable conductive fibres can be woven into fabrics for wearable electronics, but high conductivity and high stretchability are difficult to obtain together. Fibres with a liquid metal core can collapse or be damaged under concentrated pressure, large strain or cutting, and a fibre of Ag-decorated multiwalled carbon nanotubes in a fluoropolymer reached an initial conductivity of 17 460 S cm−1 but ruptured at about 60% strain. In fibres filled only with AgNPs, the particle network breaks apart as the fibre is stretched.

Design and approach

SBS containing dispersed AgNWs was wet-spun from a N,N-dimethylformamide/tetrahydrofuran (DMF/THF) solution into a polyvinyl alcohol (PVA) coagulation bath, with the solvent and nonsolvent chosen by their solubility parameters. The fibre was then soaked in silver trifluoroacetate (AgCF3COO) in ethanol and treated with hydrazine, and repeating this cycle formed AgNPs both inside and on the outer surface of the fibre. When the fibre is stretched and the AgNP network cracks, AgNWs aligned along the strain direction are intended to bridge the separated particle clusters and keep a conducting path.

How it was evaluated

The authors examined the fibre by field emission scanning electron microscopy with energy dispersive spectrometry, measured stress–strain curves of SBS and composite fibres, and measured the initial conductivity as a function of absorption–reduction cycles and the conductivity under uniaxial strain as a function of AgNW concentration (0, 0.34, 0.56, 1.12 and 1.68 wt%) and AgNW length (10 and 20 µm). AgNW alignment was quantified from backscattered electron images of fibres without AgNPs at 0% and 50% strain. Resistance was recorded during stretch–release at 20% to 100% strain, over 1000 cycles between 0% and 10% strain and during folding, the illumination of light-emitting diodes (LEDs) connected by the fibres was observed during stretching, and five fibres were attached to a glove read by a five-channel analog-to-digital converter.

Key results

The wet-spun fibre had a bean-shaped cross-section 150 to 200 µm wide, and a 100 cm length was collected at 1.14 cm s−1. AgNPs formed throughout the cross-section, and the conductivity saturated after three absorption–reduction cycles. The 0.56 wt% composite fibre broke at 900% elongation compared with 1182% for as-spun SBS, while its yield strength rose from 2.4 to 2.8 MPa. Removing surface AgNPs with tape lowered the conductivity to 603 S cm−1, but the fibre still conducted up to 190% strain, which the authors take to indicate that the internal AgNPs maintain conduction under strain.

Initial conductivity increased with AgNW content, reaching 2450 S cm−1 at 1.68 wt%. The maximum strain at which conduction was preserved was 80%, 200%, 220%, 160% and 140% for 0, 0.34, 0.56, 1.12 and 1.68 wt% AgNWs, so the 0.56 wt% fibre gave the best combination, with σ/σ0 of 4.4% at 100% strain; the authors attribute the decline at higher loading to AgNW aggregation. Longer AgNWs (20 versus 10 µm) raised both conductivity and maximum strain. At 50% strain, AgNWs were seen spanning cracks in the AgNP network, and the fraction of AgNWs within |θ| ≤ 10° of the fibre axis rose from 56% to 91%.

Three LEDs connected by the fibres stayed lit up to 150% strain at 0.1 mA. The resistance change was proportional to strain between 20% and 100%, with a gauge factor of about 15 at 100% strain. Over 1000 cycles between 0% and 10% strain, the peak and baseline resistance rose during the first 600 cycles and then remained unchanged, and folding changed the resistance from 7.2 Ω to 7.8 Ω. A glove with five fibres detected the bending of each finger and recognized the sign language letters Y, S and U.

Open questions

At 50% cyclic strain, the peak and baseline resistance increased markedly and did not stabilize, which the authors attribute to displacement of the fillers by friction with the SBS matrix and to fracture of AgNWs, observed in a fibre with 6.75 wt% AgNWs at 100% strain. The authors also report voids between AgNWs and the polymer, which they relate to weak adhesion, and resistance hysteresis between 0% and 50% strain that they attribute to the SBS elastomer.

How it connects to other research

The precursor absorption and hydrazine reduction follow the method cited from Park et al. (Nat. Nanotechnol. 2012), which formed AgNP networks in SBS fibre mats; this study wet-spins single SBS fibres and adds AgNWs that align with strain and bridge the AgNP network once it cracks.