TPU/CNTs flexible strain sensor with auxetic structure via a novel hybrid manufacturing process of fused deposition modeling 3D printing and ultrasonic cavitation-enabled treatment

Abstract

The broad technological concept as so-called "hybrid additive manufacturing" is introduced into the fabrication of flexible strain sensing structure, employing the combination process of fused deposition modeling (FDM) 3D printing technique and ultrasonic cavitation-enabled treatment. We customize the FDM settings to print auxetic structures with a negative Poisson's ratio (NPR), using the flexible thermoplastic polyurethane (TPU) material. Then, we employ the ultrasonic cavitation-enabled treatment conducted towards the flexible auxetic structure immersing in carbon nanotubes (CNTs) liquid, aiming to embed the CNTs into the surface layer of the auxetic structures. It is equivalent to two additive manufacturing steps, proving effective in designing and fabricating flexible strain sensors. Instead of 3D printing after the hybridization of the TPU matrix material, this manufacturing procedure can ensure that the intrinsic excellent mechanical properties of TPU are not weakened due to the internally mixed conductive materials but able to exhibit usable sensing performances. The Poisson's ratio of the sensor structure is - 0.8. The sensor achieves over 300% strain. The sensitivity within the initial strain range is 2.661. The reliability of signal acquisition during the cyclic deformations is proved. Its response-recovery duration under 5% strain at 500 mm/min deformation rate is 130 ms(response)− 250 ms(recovery). It gives accurate feedback to external stimuli at different frequencies. It can maintain stable output signals during 3000 cycles of stretching. It also can sense 1% minor-strain stimuli. The strain sensing behavior of the fabricated flexible sensor with the auxetic structure is detailed. The CNTs are combined tightly with the TPU surface layer to form a conductive network with variable resistance during recoverable deformation of the auxetic structure. The macro and micro-mechanisms of sensing are dissected, considering the tunneling effect of discontinuous CNTs. The hybrid additive manufacturing process is verified to be feasible, credible, and low-cost for easy fabrication of flexible strain sensors with complex geometry configurations.