Drop-on-demand high-speed 3D printing of flexible milled carbon fiber/silicone composite sensors for wearable biomonitoring devices

Three-dimensionally (3D) printed flexible piezoresistive composite sensors have provided valuable solutions for the personalized therapeutic development due to their promising capability in biomonitoring applications. Silicone rubber (SR) matrix is an important candidate to enable flexibility to the 3D printed devices. However, 3D printing of silicone inks blended with conducive fillers is limited due to the high viscosity, long curing time, and high percolation threshold. In the present study, a novel high-speed material jetting (MJ) 3D printing of high-viscosity conducive inks based on the mixture of a UV crosslinkable silicone rubber and milled carbon fibers (MCF) is demonstrated. The MCF content was optimized for printability, UV curability, and electrical conductivity. The sensors (with 30 wt. % MCF content) show high flexibility and foldability as well as a high resistance sensitivity to sever bending tests. The stretchability of 3D printed sensors was further improved by sandwiching the MCF/SR sensing layer between the SR layers. The electromechanical evaluation of the sandwiched MCF/SR sensors (S-MCF/SR) confirmed the high piezoresistive sensitivity of sensors (gauge factor in order of similar to 400). Finally, the 3D printed sensors were employed for monitoring human joint motions to demonstrate the potential application in monitoring biosignals.