Flexible Electrodes and Sensors Insensitive to Lateral Strains for Electronic Skin
Authors: Zhang Runzhi , Yang Eui-Hyeok 
Published: February 2020

Applying stretching-insensitive flexible and stretchy devices into flexible electronics would benefit a wide range of applications in wearable and multifunctional electronics, including flexible displays, curved smart phones, electronic skins, and implantable medical devices. If flexible devices are used for skin-attachable electronic devices, the devices can be subjected to mechanical deformation. Therefore, devices insensitive to lateral strains induced by mechanical deformation (such as stretching, bending, twisting and wrinkling) needs to be developed.

In this work, we present a flexible and stretchy pressure sensor utilizing vertically aligned carbon nanotube (VACNT) carpets on Polydimethylsiloxane (PDMS)substrate, which reliably operates under dynamic deformations of the substrate such as stretching, bending, and twisting. We also present a flexible and stretchysupercapacitor composed of VACNTs partially embedded into PDMS. The devices are facilely fabricated by the partial-embedding of VACNTs in PDMS, permitting a strong hold of VACNT hybrid into PDMS, which facilitates a stable performance under varied strains.

We synthesized VACNTs using atmospheric-pressure chemical vapor deposition (APCVD) into carpet-like structures. The catalyst layer consisting of 5 nm Al and 3 nm Fe was deposited on the Si/SiO₂ substrate prepared using physical vapor deposition (PVD). Then the substrate was placed in the atmosphere pressure chemical vapor deposition (APCVD) chamber. The furnace temperature was increased to 750˚C with a constant 500 sccm Ar flow. VACNTs were grown at 750˚C for 15 minutes with 60 sccm H₂ and 100 sccm C₂H₄. Then the chamber was cooled down to the room temperature while maintaining the same Ar flow rate. The structure of the grown CNTs is vertically aligned in general, while the individual carbon nanotubes are entangled with each other. The morphology of the VACNTs were characterized using scanning electron microscopy (SEM). To fabricate flexible substrates with embedded VACNTs, we transferred the grown VACNTs onto partially cured PDMS. First, we used a liquid mixture of PDMS base and curing agent (Sylgard 184 Silicone Elastomer, Dow Corning) which were mixed with a ratio of 10:1 to form a PDMS substrate. After degasing under reduced pressure in a vacuum pump, the bubbles were all removed while PDMS was still liquid. Then the liquid PDMS was placed on a hot plate at 65˚C for about 30 minutes before it was fully cured. We optimized the curing condition of PDMS, where the partially cured PDMS was tacky but not fully wet. The grown VACNTs were then placed onto partially cured PDMS. Then the tips of CNTs were partially immersed into PDMS slowly. During the curing process, the embedded CNTs were eventually wetted by PDMS. Then after PDMS was fully cured, the VACNT-PDMS structure was successfully peeled off from the Si/SiO₂ substrate owing to the strong adhesion between PDMS and VACNTs. The entire fabrication process is rapid and facile, which permits the integration of VACNT-PDMS substrate.

For the pressure sensing, we applied three different pressure values to the sensor, 1.03 kPa (10g), 1.37 kPa (20g), and 1.82 kPa (50g), respectively. Various stretching and bending strains were applied to the VACNT-PDMS substrate. The substrate was laterally stretched to 200% and bent up to 180 degree under a constant pressure applied vertically against the substrate, demonstrating the consistent resistance up to 50% stretching and 180˚ bending. For the supercapacitor, the cyclic voltammetry (CV) showed good electrochemical stability and capacitive behaviors at scanning rate of 1000 mV/s. The capacitance can be calculated to be 2 mF/cm² at 1000 mV/s.