Fast-response, stiffness-tunable (FRST) soft actuators — or movable machine elements — were developed that could be used in soft robots.

A robotic gripper with three FRST actuators can grasp and lift objects with arbitrary shapes and various weights spanning from less than 10 g to up to 1.5 kg.

Traditional robotic systems with rigid parts often pose a threat to human operators. In contrast, soft actuators and robots offer excellent adaptivity to their surroundings and are safer around humans; however, the inherent softness of materials like silicone rubber limits their ability to bear loads.

To help increase the load capacity of soft robotic systems without sacrificing their compliance during robot-object interaction, Singapore University of Technology and Design researchers have explored the use of thermally activated shape memory polymers (SMPs). Not only are SMPs capable of reversibly changing stiffness by two to three orders of magnitude, they are also compatible with 3D printing. Thus far, though, it has been reported that SMP-based soft actuators generally suffer from limitations such as slow responses and small deformations.

In this work, finite-element simulations and hybrid multimaterial 3D printing were used to design and manufacture FRST soft actuators that can complete a softening-stiffening cycle within 32 seconds. A commercial inkjet multimaterial 3D printing technology was combined with the direct-ink writing approach to fabricate fully printed FRST actuators. The stiffness tunability is provided by an embedded SMP layer and the fast response is enabled by embedded heating and cooling elements.

The integration of the SMP layer into the actuator body enhanced its stiffness by up to 120 times without sacrificing flexibility and adaptivity. The researchers relied on a deformable conductive circuit printed with a silver nanoparticle ink to activate the rubbery state of the SMP by localized heating. Upon deforming the actuator with pressurized air, the SMP is chilled with coolant driven through a fluidic channel, locking the actuator in a specific shape.

The researchers have also built computational models to simulate the mechanical and thermal-electrical behaviors of the FRST actuator. These models can be used to guide the design of FRST actuators and provide insights into enhancing load capacity. To showcase the high load capacity and shape adaptivity of the prototype, a robotic gripper with three FRST actuators was developed that can grasp and lift objects with arbitrary shapes and various weights spanning from less than 10 g to up to 1.5 kg.

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