Anti-friction gold-based stretchable electronics enabled by interfacial diffusion-induced cohesion

Stretchable electronics that prevalently adopt chemically inert metals as sensing layers and interconnect wires have enabled high-fidelity signal acquisition for on-skin applications. However, the weak interfacial interaction between inert metals and elastomers limit the tolerance of the device to external friction interferences. Here, we report an interfacial diffusion-induced cohesion strategy that utilizes hydrophilic polyurethane to wet gold (Au) grains and render them wrapped by strong hydrogen bonding, resulting in a high interfacial binding strength of 1017.6 N/m. By further constructing a nanoscale rough configuration of the polyurethane (RPU), the binding strength of Au-RPU device increases to 1243.4 N/m, which is 100 and 4 times higher than that of conventional polydimethylsiloxane and styrene-ethylene-butylene-styrene-based devices, respectively. The stretchable Au-RPU device can remain good electrical conductivity after 1022 frictions at 130 kPa pressure, and reliably record high-fidelity electrophysiological signals. Furthermore, an anti-friction pressure sensor array is constructed based on Au-RPU interconnect wires, demonstrating a superior mechanical durability for concentrated large pressure acquisition. This chemical modification-free approach of interfacial strengthening for chemically inert metal-based stretchable electronics is promising for three-dimensional integration and on-chip interconnection.

In the FTIR spectrum of WPU, the band at 3500 ~ 3200 cm -1 is attributed to -OH and -NHstretching vibration, and the band at 1800 ~ 1600 cm -1 is attributed to carboxyl stretching vibration 1,2 .This result proves that WPU molecular chain has hydrophilic functional groups.In contrast, FTIR spectra reveal that there are no hydrophilic polar groups in the molecular chains of PDMS and SEBS.
Supplementary Figure 4 Microscope images of WPU films as a function of urea content at (a) 5%, (b) 10%, (c) 15% and (d) 20%.Scale bar: 300 μm.Each experiment was repeated three times independently with similar results.
The WPU film with 5 wt% urea has sparse micropores.In addition, both WPU films with 10 wt% and 15 wt% urea show uniform permeable micropores.The pore size of the former is around 100 μm, while most of micropores in the latter are larger than 200 μm.The WPU film with 20 wt% urea obtains dense closed micropores that are not breathable.As for water vapor evaporation test, it was conducted by storing the samples in an incubator at 35 o C. The Au-RPU, Au-SEBS and Au-PDMS on-skin electronics were attached to the opening of a bottle containing 1 g deionized water, and the gas permeability was tested by measuring the weight loss of water.The water in the bottle with the Au-RPU device evaporated completely after one week, while the weight of the bottle barely decreased (87% of the water remain) over the same time.This result shows a high degree of gas permeability for our Au-RPU device.Au layer after the ninth etching is around 6.5 nm.We detected O signal in Au layer at a depth of 6.5 nm.This depth is higher than the XPS electron escape depth of O element (2.0 nm).These results therefore confirm the diffusion of oxygen molecules or oxygen-containing groups into the Au layer.

Supplementary
Based on the FTIR spectrum of WPU, each O 1s XPS spectrum for surface uncleaned WPU or cleaned WPU is the convolution of three components: a -OH group signal at 533.3 ± 0.2 eV, a -COOgroup signal at 532.1 ± 0.2 eV, and a C=O group signal at 531.0 ± 0.2 eV.
Figure 6 (a) SEM image of Au-RPU device with rough microporous structure.Scale bar: 200 μm.Each experiment was repeated three times independently with similar results.(b) Pore diameter statistics of Au-RPU from SEM images.The pvalue is 0.05.It indicates that the average pore diameter of Au-RPU device is 101.6 ± 20.7 μm (mean ± SD).(c) Water vapor permeability tests of Au-RPU, Au-SEBS and Au-PDMS on-skin electronics.
Figure 11 SEM images of Au-SEBS device (a) during and (b) after 200% strain.Each experiment was repeated three times independently with similar results.Microcracks are observed at the released state, indicating the slippage of Au layer after the stretching.As a result, the electrical conductivity of the device decreases.Supplementary

Table 2
Comparison of the interfacial binding strength of our device with other reported works.

Table 3
Comparison of electrical property and anti-friction performance of stretchable devices with different conductive materials.Comparison of electrical properties and anti-friction ability of gold-based stretchable electronics.Supplementary Table 6 Atomic relative concentration of C 1s, O 1s and Au 4f from XPS depth profiling of the 20 nm-thick Au layer on WPU substrate.Comparison of our pressure sensor array with the metalelectrode-based pressure sensors.XPS depth profile analyses are performed on 20-nm-thick Au/flat WPU (20-Au/WPU) sample.The contents change of Au 4f, O 1s and C 1s elements at the interface are analyzed by etching the Au layer with Ar ion gradually.Notably, the XPS electron escape depth at 1486.7 eV for Au, O and C elements is around 1.7, 2.0 and 2.8 nm respectively, which means if O element is detected in Au film thicker than the oxygen escape depth (2.0 nm), then it can prove the diffusion of O element in Au layer.Since the total thickness of the Au layer is 20 nm and approximately 1.5 nm-thick Au layer can be removed per etching, the thickness of the