MELEGROS: Monolithic Elephant-inspired Gripper with Optical Sensors

The elephant trunk exemplifies a natural gripper where structure, actuation, and sensing are seamlessly integrated. Inspired by the distal morphology of the African elephant trunk, we present MELEGROS, a Monolithic ELEphant-inspired GRipper with Optical Sensors, emphasizing sensing as an intrinsic, co-fabricated capability. Unlike multi-material or tendon-based approaches, MELEGROS directly integrates six optical waveguide sensors and five pneumatic chambers into a pneumatically actuated lattice structure (12.5 mm cell size) using a single soft resin and one continuous 3D print. This eliminates mechanical mismatches between sensors, actuators, and body, reducing model uncertainty and enabling simulation-guided sensor design and placement. Only four iterations were required to achieve the final prototype, which features a continuous structure capable of elongation, compression, and bending while decoupling tactile and proprioceptive signals. MELEGROS (132 g) lifts more than twice its weight, performs bioinspired actions such as pinching, scooping, and reaching, and delicately grasps fragile items like grapes. The integrated optical sensors provide distinct responses to touch, bending, and chamber deformation, enabling multifunctional perception. MELEGROS demonstrates a new paradigm for soft robotics where fully embedded sensing and continuous structures inherently support versatile, bioinspired manipulation.

💡 Research Summary

The paper introduces MELEGROS, a monolithic soft robotic gripper inspired by the distal morphology of the African elephant trunk. Unlike conventional soft robots that assemble separate sensing, actuation, and structural components—often using multiple materials and post‑assembly steps—MELEGROS integrates six optical waveguide sensors and five pneumatic chambers directly into a single lattice body fabricated in one continuous 3‑D printing process using a soft, transparent resin. The lattice is based on an I‑type triply periodic minimal surface (IWP‑TPMS) with a 12.5 mm cell size and 1.5 mm strut thickness, providing low bending stiffness while preserving structural continuity. This geometry enables the gripper to elongate, compress, and bend as a single piece, mimicking the seamless integration seen in a natural trunk.

Actuation is achieved with bladder‑like pneumatic chambers arranged in two opposing “fingers” (dorsal and ventral) at the tip—six chambers on the dorsal side and four on the ventral side—plus three six‑chamber actuators at the base for reaching, contraction, and overall elongation. Pressurizing the chambers causes the lattice to deform, producing reaching motions, independent dorsal/ventral bending, and pinching actions. The prototype (132 g) can lift a 264 g cylinder (twice its own weight) and manipulate delicate objects such as grapes without damage, demonstrating both strength and compliance.

Sensing relies on 3‑D‑printed polymer waveguides patterned with a regular array of wells (1 mm width, 0.65 mm depth). Light from an LED travels through the waveguide to a photodetector; bending increases scattering at the wells, reducing transmitted intensity and yielding a voltage drop proportional to curvature. Three sensor families are embedded: (1) proprioceptive bending sensors on each finger to capture joint angles, (2) pressure sensors within the pneumatic chambers to monitor internal actuation state, and (3) tactile sensors on the outer surface to detect external contact. All sensors share the same material as the body, eliminating mechanical mismatches and preserving the gripper’s uniform compliance.

Design and validation were driven by a simulation‑in‑the‑loop workflow using the SOFA framework. An effective Young’s modulus of ~12 kPa (derived from compression tests on bulk lattice samples) was applied to a homogenized model of the lattice. Simulations informed sensor placement, chamber geometry, and required actuation pressures. After only four design‑fabrication iterations, the final device met the target performance. Experimental characterization showed linear voltage responses to bending angles up to ±50 kPa and distinct voltage spikes for tactile events, confirming the ability to decouple proprioceptive and exteroceptive signals.

MELEGROS demonstrates a new paradigm for soft robotics: fully embedded optical sensing within a monolithic, support‑free printed structure that simultaneously serves as body, actuator, and sensor. This approach reduces fabrication complexity, improves reliability by removing bi‑material interfaces, and provides rich multimodal perception without compromising mechanical softness. The authors suggest future work on increasing sensor resolution, faster pneumatic control, and scaling the architecture to multi‑fingered hands, paving the way for more capable, bio‑inspired manipulators.