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 that remain 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 miti- gation strategies typically require multimaterial interfaces. Here, we present an approach for 3D printed soft optical sensing (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 the two branches to generate a differential output that encodes both motion direction and amplitude. An acrylate polyurethane resin was modified with lauryl acrylate to improve compliance and optical transmittance, and single-layer optical characterization was used to derive wavelength-dependent refractive index and transmittance while minimizing DLP layer-related artifacts. The measured refractive index was used in simulations to design a lens profile for a target focal distance, which was then printed with sub-millimeter fidelity. Rotational tests demonstrated reproducible branch-selective signal switching over multiple cycles. These results establish a transferable material-to-optics workflow for soft optical sensors with lens with new functionalities for next-generation soft robots

💡 Research Summary

Additive manufacturing, especially digital light processing (DLP) 3D printing, enables the fabrication of highly compliant, geometrically complex soft robotic structures in a single step. However, optical soft sensors that are printed monolithically suffer from uncontrolled light propagation: ambient illumination, leakage, and scattering within a uniformly transparent elastomer degrade signal‑to‑noise ratio and make calibration difficult. Existing mitigation strategies rely on opaque or reflective coatings, which introduce additional materials, multi‑step processes, and incompatibility with truly monolithic manufacturing.

In this work the authors propose a novel “Soft Optical sensor with a Lens” (SOLen) that integrates a printed micro‑lens directly in front of an LED emitter inside a Y‑shaped waveguide. The lens is fabricated from the same acrylate polyurethane (a‑PU) resin that forms the waveguide, eliminating the need for separate optical components. By adding 25 wt % lauryl acrylate (LA) to the base resin, the cross‑link density is reduced, yielding a material that is both softer (higher compliance) and more transparent in the visible–near‑infrared range.

Optical characterization is performed on single‑layer specimens to avoid artifacts from inter‑layer interfaces. UV‑Vis measurements show >85 % transmittance and low absorbance between 450 nm and 900 nm. The refractive index is extracted from reflectance and transmittance data using a slab model, giving an average n ≈ 1.50 with the expected wavelength dependence (higher n at shorter wavelengths). These values are fed into COMSOL ray‑optics simulations.

The lens profile is defined by a Cartesian oval equation that enforces a focal spot 20 mm inside the polymer, with the LED positioned 1 mm from the lens. Simulations predict that a ±3° rotation of the lens (caused by bending of the soft body) translates the focal spot laterally by several millimetres, preferentially coupling light into one of the two receiving branches. This produces a clear differential intensity signal that encodes both the magnitude and direction of deformation.

The authors print three lenses corresponding to refractive indices of 1.44, 1.49, and 1.54 (representing 860 nm, 500 nm, and 450 nm illumination, respectively). Microscopy confirms that the printed surfaces match the designed Cartesian ovals within the printer’s resolution limits (≈610 µm in‑plane, 25 µm layer height).

Experimental validation uses an 860 nm LED and a rotational stage to tilt the emitter/lens assembly by ±3°. In the undeformed state the focal spot lies midway between the two branches, yielding balanced photodiode voltages. When rotated right, the focal spot shifts into the right branch, increasing its intensity (voltage drop) while the left branch’s voltage rises, and the opposite occurs for left rotation. This switching is repeatable over at least five cycles with low variance. A control sensor without a lens shows negligible voltage change, confirming that the lens is responsible for the observed signal separation.

The SOLen concept therefore provides a coating‑free, monolithic method to steer light within soft optical waveguides, enabling direction‑sensitive strain sensing with a simple differential readout. The workflow—material formulation, single‑layer optical characterization, lens design via refractive‑index‑based simulations, and high‑resolution DLP printing—is transferable to other photopolymer systems. Future work could extend the approach to multi‑axis deformation sensing, faster dynamic response, and integration into closed‑loop soft‑robotic control architectures.