Additive Manufacturing of Foam Pressure Sensors with Controlled Conductivity, Stiffness, and Shape

Foam-based pressure sensors are attractive for human motion tracking and health monitoring, as they integrate flexibility, compressibility, and sensitivity. However, their stiffness, geometry, and cellular architecture can hardly be independently controlled. Moreover, degradation upon cyclic use and multistep fabrication methods hamper their broad application. Here, we demonstrate single-step additive manufacturing of elastic polymer foams with programmable shapes and tunable stiffness. Using direct bubble writing (DBW), conductive foams are printed cell by cell. Silver nanoparticles are formed and embedded within the polymer matrix during the printing process, thereby eliminating the need for extensive postprocessing and degradation caused by the delamination of conductive layers. The conductivity (resistance) and sensitivity of the foams are measured as a function of the silver loading, revealing that the pressure sensitivity is weakly dependent on the silver content. The density of printed foams is controlled by adjusting the gas pressure applied during the printing process, resulting in elastic modulus from 11.4 ± 1.2 kPa to 22.0 ± 2.0 kPa. The sensitivity of the sensors ranges from 0.16 to 0.0014 kPa^-1 for pressures 1 kPa < P < 125 kPa, which is comparable to state-of-the-art foam-based pressure sensors. The sensors exhibit low hysteresis (below 10%) and stable cyclic performance up to 60% strain. We showcase their application potential in real-time detection of human motion (finger, hand, and foot). Altogether, direct bubble writing enables one-step, high-throughput manufacturing of pressure sensors with custom shape, stiffness, and conductivity.