Foam Additive Manufacturing of Piezoresistive Thermoplastic Polyurethane

The advancement of multifunctional soft devices in robotics, biomedical engineering, and wearable technologies increasingly demands material systems that integrate structural compliance with embedded sensing capabilities. This study investigates the potential of Foam Additive Manufacturing (FAM) using thermoplastic polyurethane (TPU) filled with carbon-based conductive additives, aiming to produce lightweight, deformable, and piezoresistive structures through in situ microcellular foaming during Fused Deposition Modelling. A systematic process optimization was performed by varying solubilization and extrusion parameters—including blowing agent absorption and desorption times, extrusion temperature and speed, and nozzle diameter—to control foam morphology at both the micro- and macro-scale. Morphological analysis via scanning electron microscopy (SEM) enabled the qualitative evaluation of bubble distribution and process-dependent structural transformations. Results demonstrate that TPU’s inherently elastic behavior presents unique challenges in bubble expansion compared to PLA, but also facilitates the formation of homogeneously foamed filaments with reduced density—achieving up to a 42.2% decrease. Functional testing on printed phalanx and finger-like specimens highlights the ability of conductive TPU foams to respond predictably to mechanical stimuli, with sharp, repeatable reductions in electrical resistance under compressive loads. When assembled into tendon-actuated finger prototypes, these materials exhibit trajectory-controlled flexion with real-time resistance-based feedback during grasping and release phases, validating their suitability for soft, sensor-integrated actuation. This work establishes key processing-structure-function relationships in the FAM of conductive TPU composites, offering critical insights for tailoring foam behavior in high-performance, application-specific designs. The findings serve as a platform for future developments in embedded sensing, programmable compliance, and functionally graded soft architectures.