3D-printed Soft Optical sensor with a Lens (SOLen) for light guidance in mechanosensing

Additive manufacturing is enabling soft robots with increasingly complex geometries, creating a demand for sensing solutions compatible with single-material, one-step fabrication. Optical soft sensors are attractive for monolithic printing, but their performance is often degraded by uncontrolled light propagation (ambient coupling, leakage, scattering), while common mitigation strategies require multimaterial interfaces. Here, we present a 3D-printed soft optical sensor with a lens (SOLen), in which a printed lens is placed in front of an emitter within a Y-shaped waveguide. The sensing mechanism relies on deformation-induced lens rotation and focal-spot translation, redistributing optical power between two branches to generate a differential output that encodes motion direction and amplitude. An acrylate polyurethane resin was modified to improve compliance, and single-layer characterization was used to derive wavelength-dependent optical properties while minimizing printed layer-related artifacts. The refractive index was used in simulations to design a lens for a target focal distance, which was then printed with sub-millimeter fidelity. Rotational tests demonstrated reproducible branch-selective signal switching over multiple cycles, increasing sensitivity by over 65-fold compared with a configuration without a lens. These results establish a transferable material-to-optics workflow applicable to any printable transparent material, opening new perspectives for monolithic soft robots with direction-sensitive optical sensing.