Embedded 3D Printing of Highly Flexible Nanocomposites With Piezoresistive Sensing Capabilities

In recent years, highly flexible nanocomposite sensors have been developed for the detection of a variety of human body movements. Due to their biocompatibility, these nanocomposites can be employed for the development of wearable sensors and smart devices. Currently, wearable sensors have been developed for various applications including quantification of biochemical profiles of athletes, rehabilitation of elderly patients, electromechanical detection of small physiological body movements such as blood flow, and human motion monitoring. One of the key challenges in wearable sensors is the ability to combine the desired sensing functionality with a satisfactory degree of integration with the body. Human motion sensors are often attached to the skin, therefore often require flexible devices to have seamless contact with the body to avoid slippage and ensure functionality. To precisely detect the bending motions of human joints, the sensors must be able to conform well to the human skin and produce signals that can accurately describe the amount of deformation applied to the material during bending. Typically, elastomers such as polydimethylsiloxane (PDMS) and Ecoflex are used as either a polymer matrix or substrate material for these highly flexible devices. PDMS notably has a high Poisson ratio (~ 0.5) which has proven to be beneficial to increase the sensitivity of these sensors. However, its high Young’s modulus between 0.4 MPa – 3.5 MPa (Sylgard 184) prevents comfortable and reliable adhesion with skin. In contrast, Ecoflex has a low elastic modulus typically between 100 kPa – 125 kPa, which is more appropriate for skin-attachable strain sensing applications. Recently, multiple skin-attachable and ultrastretchable strain sensors have been developed by dispersing conductive nanoparticles in an elastic matrix, such as carbon black and carbon nanotubes (CNTs), as well as coating highly flexible substrates with conductive surfaces such as gold nanosheets or graphite thin films. These sensors utilized the highly stretchable nature of Ecoflex and their selected conductive material to produce nanocomposites capable of detecting strain applied to the material through either piezoresistive or capacitive effects.

In this paper, a carbon nanotube-based piezoresistive strain sensor is developed via the direct ink writing based embedded 3D printing method. The weight concentration range of carbon nanotubes suitable for embedded 3D printing of the nanocomposite inks is identified first. Samples with complex 2D and 3D geometries are printed to demonstrate the manufacturing capabilities of the embedded printing process. The sensitivity of the piezoresistive strain sensor is optimized by determining the ideal nanofiller concentration, curing temperature, and nozzle size to produce the highest gauge factor over a strain range of 1.4 to 4.8. The piezoresistive and mechanical properties of the optimized sensors are fully characterized to verify the suitability for skin-attachable strain sensing applications. The developed sensors have wide sensing range, high sensitivity, and minimal strain rate dependence. In addition, their low elasticity and high biocompatibility allow them to be comfortably bonded on the human skin.