Development of Pressure-Insensitive Flexible Strain Sensors based on Bioinspired Adhesive and Active CNT Layers

Abstract

Flexible tactile sensors are required to maintain conformal contact with target objects and to differentiate different tactile stimuli (e.g., strain and pressure) to achieve high sensing performance. However, many existing tactile sensors cannot distinguish strain from pressure. Moreover, because they lack intrinsic adhesion capability, they require additional adhesive tapes for surface attachment. Herein, we present a self-attachable, pressure-insensitive strain sensor that can firmly adhere to target objects and selectively perceive tensile strain with high sensitivity. The first part of the thesis deals with the design and fabrication of the self-attachable flexible strain sensor. There are two main components of the sensor: a selectively coated percolating multiwalled CNT(MWCNT) layer and a mushroom-shaped micropillar array. The MWCNTs are deposited on the bottom surface of the strain sensor, except for micropillar. When a tensile strain is applied to the MWCNT layer, microscale cracks occur within the MWCNT percolation network. As the strain increases so too does the distance between the networks, resulting in a large change in the electrical resistance. On the other hand, the application of normal pressure does not significantly change the MWCNT percolation network because the layer of MWCNT is very thin (aprox. 200 nm thick), thus the deformation of the layer under pressure is very limited. The micropillar with the tip protruding also protects the active MWCNT layer from applied pressure. Therefore, the proposed sensor can show a high sensitivity to strain while ignoring the response to pressure. Part II concerns the adhesion behavior of the self-attachable flexible strain sensor. We evaluated its self-adhesion performance by measuring the pull-off strength of the sensor over a flat glass substrate. We measured the adhesion strength of four different devices: planar PDMS (P), MWCNT-coated planar PDMS (CP), PDMS micropillars coated with MWCNTs over the entire surface (ECM), and PDMS micropillars selectively coated with MWCNTs on the bottom surface (SCM). SCM, whose tip surface is not coated with CNT layer, showed significantly enhanced adhesion of up to 250 kPa. The SCM maintained a high adhesion strength even when the coating dose at the MWCNT layer was increased and the sheet resistance was significantly reduced, from ~107 Ω sq-1 to ~104 Ω sq-1. In addition to glass substrates, SCM sensors showed strong self-adhesion on various substrates, which include Si, Au, Ag, Al, Cu, and ITO. SCM sensors also showed high adhesion strength (Root Mean Square: 0.05, 0.33, 1.89, and 5.18 µm) with different surface roughness. It also maintained a high self-adhesion capability for more than 1000 cycles of attachment and detachment tests without showing any signs of adhesion degradation. The last part of the thesis concerns the sensing behavior of the self-attachable flexible strain sensor. The sensor showed linear changes in relative resistance in the GF of 0.26 and the tensile strain range of 0 to 80 percent of the wide plane. The strain sensor showed an immediate response (< 90 ms) and relaxation (< 150 ms) for all strain ranges applied. Through the iterative cycle of strain load and unloading endurance test using 60% applied strain, the sensor showed a stable and uniform change of relative resistance over 1000 cycles. The results showed that the sensor not only makes conformal contact with the target substrate but also detects a mechanical strain with reliable sensitivity and durability. SCM strain sensors under different bending radii (R) of 15mm, 5mm, and 2.5mm can sensitively detect the various bending stresses applied to the PET substrate. The SCM sensor reacted sensitively to the applied strain between 0 and 80% but showed no apparent reactivity to normal pressure ranging from 0 to 100 kPa. Time-over measurements of relative resistance further demonstrated the low pressure sensitivity and high strain sensitivity of the SCM sensor. The initial 100 kPa of applied pressure to the sensor did not induce a significant change in resistance. However, when an 80% strain was applied to the sensor, a linear increase of resistance was observed demonstrating the decoupling ability of strain and pressure. Subsequently, the electrical resistance was no longer changed even if 100 kPa pressure was applied while maintaining 80% of the strain.