Researchers at Seoul National University of Science and Technology (SeoulTech), have developed carbon nanotube (CNT) nanocomposites specifically formulated for vat photopolymerization (VPP), using them to 3D print flexible piezoresistive sensors integrated into a smart insole. The work, led by Prof. Keun Park and Assoc. Prof. Soonjae Pyo, aims to transition VPP from producing structural polymers to functional, wearable electronics by balancing conductivity, stretchability, and print fidelity.
The study was posted online on August 25, 2025, and is set to appear in Composite Structures (Vol. 372) on November 15, 2025.
To create the material, the team dispersed multi-walled CNTs (0.1–0.9 wt%) into an aliphatic urethane diacrylate resin. They carefully tuned exposure and print parameters to mitigate agglomeration and light scattering, two issues that typically impair cure depth and feature definition in CNT-filled photopolymers. The formulation with a 0.9 wt% CNT loading achieved an elongation at break of 223% while maintaining a bulk conductivity of ~1.64×10⁻³ S/m and a feature resolution of ~0.6 mm. This combination of soft mechanics, useful conductivity, and reliable curing enables VPP to produce sensorized parts without resorting to hydrogels or post-infiltration steps.
From VPP-friendly inks to TPMS insoles
The group printed triply periodic minimal surface (TPMS) lattices as the sensing core. These TPMS architectures are ideal for this application as they compress smoothly and reversibly, concentrating strain in a way that maximizes the responsiveness of the embedded CNT network. In testing, the 3D printed lattices showed stable piezoresistive behavior under load.
When embedded at key plantar zones inside an insole, the sensors captured real-time pressure maps and changes associated with gait and posture. A key advantage is that the sensing element is printed as an integral structure, not a laminated layer. This allows geometry and sensitivity to be co-designed for different foot regions or specific clinical and athletic use cases.
Beyond footwear, the authors frame the materials platform as a general path to 3D-architected, skin-compatible electronics, soft robotics skins, joint sleeves, or custom wearables, where stretch, fine features, and repeatable curing are all must-haves. Crucially, this work keeps to standard VPP workflows and light-curable chemistries, lowering the barrier to entry for labs or companies already running resin printers and looking to add functional sensing.
SeoulTech track record: auxetic metamaterial sensors and SCOBY wound-printing ink
Previous work from SeoulTech demonstrated a DLP-printed auxetic tactile sensor built from a cubic lattice with spherical voids; the device can operate in both capacitive and CNT-piezoresistive modes and was shown as a smart insole and a pressure-mapping array. In separate bioprinting research, SeoulTech introduced a Kombucha SCOBY-derived bacterial nanocellulose bioink, reinforced with chitosan and kaolin and mixed via a twin-screw “biopen,” enabling direct deposition onto irregular wound geometries for potential in-situ tissue repair
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Featured image shows photopolymerization additive manufacturing of highly stretchable CNT nanocomposites for 3D architectured sensor applications. Image via Kang et al. / Composite Strucutres.
