Porosity-Engineered Soft Robots Achieve Jointless Multidimensional Motion
Soft robotics continues to push beyond the constraints of rigid mechanisms, offering adaptable solutions for grasping, locomotion, and manipulation. A recent advance draws direct inspiration from the elephant trunk—a natural continuum manipulator capable of precise, powerful, and jointless motion. The research addresses a central challenge: creating highly deformable, reliable actuators with smoothly varying stiffness, enabling multiple programmed movements without discrete joints.
The team’s approach combines material-level microporosity with design-level macroporosity, realized through volumetric tessellation. Using a water-in-oil emulsion as the printable medium, the continuous phase cures into a stretchable polyurethane after 3D printing. As water droplets evaporate, they leave interconnected micropores within the solid walls. This microporosity, paired with a lattice of gradually dimensioned unit cells, allows fine-tuning of stiffness and deformation behavior. The structures are printed in a single step on a commercial digital light processing (DLP) system, then enveloped in a soft, convoluted skin to maintain biaxial deformability and prevent instability.
Mechanical testing of the emulsion-derived material revealed extensibility up to 523% and tensile strength of 1.57 MPa. Pore size, controlled by mixing method, directly influenced density and mechanical performance. Lattice topology further modulated extensibility and compressibility. Smaller unit cells increased stiffness and hysteresis, while larger cells enhanced deformation range. Cyclic tests confirmed that tensile strains up to 150% and compressive strains up to 40% could be repeated over thousands of cycles without failure.
Unlike bulk hyperelastic materials, the tessellated structures can undergo significant volumetric changes. Compression tests showed up to 80% contraction, with a characteristic three-stage response—linear elastic, plateau, and densification—similar to elastomeric foams. Larger unit cells achieved greater compressibility but lower strength, providing a tunable design space for specific actuation needs.
Two skin convolution profiles were modeled: V-shape and U-shape. The V-shape offered greater extensibility at equal pressure and was selected for fabrication. Skin thickness proved critical; below 1 mm, structures failed under operating pressures, while 1.5 mm provided reliable sealing without dominating stiffness.
The researchers demonstrated two principal pneumatic elastic lattice actuators (PELAs). The biaxial PELA, with symmetric stiffness distribution, achieved 32% contraction at ?25 kPa and 45% extension at 15 kPa. The bending PELA, with asymmetric stiffness across its width, delivered bidirectional bending up to ±25° depending on pressure polarity. In bending mode, the graded unit cells buckled at different loads, producing smooth curvature.
Building on this, a three-fingered gripper was assembled from modified bending PELAs with compliant external lattice layers. This configuration increased bending force and allowed adjustable opening angles. The gripper adapted to spherical objects from 24.5 to 40 mm in diameter, interlocking with varied shapes through enlarged contact surfaces, and supported payloads of 300 g.
A more complex continuum actuator combined axial and bending motions in a single monolithic structure. Gradient stiffness in the distal sections induced bending, while a symmetric central section enabled axial extension and contraction. Both ends moved synchronously, varying their separation and enabling grasping of objects such as cubes and hexagons without separate joints or linkages.
The elastic lattice design offers high deformation ratios at low pressures, yielding favorable force-to-weight performance. For example, the bending PELA with compliant layer achieved 1.13 N blocking force at 25 kPa while weighing only 15 g. Unlike multimaterial actuators, the monolithic lattice avoids delamination and weight penalties, and compared to origami- or kirigami-based designs, it maintains stroke length without interference from folding patterns.
By encoding stiffness gradients directly into the printed lattice, the method enables jointless actuators that move with the mechanical continuity of biological structures. This capability points toward soft robotic manipulators that integrate multiple motion modes in a single body, potentially scaling to meter-sized systems with larger printers. The porosity-based strategy also opens avenues for mimicking anisotropic muscle arrangements, advancing bioinspired mechanics toward robust, continuum architectures for adaptive tasks in complex environments.
