A class of “metacaps” whose ultrafast yet tunable snapping response can be leveraged to overcome the power and speed limitations of current soft robots

From minimally invasive surgical devices and smart wearables to grippers and rescue equipment in hazardous environments and deep seas, adaptive intrinsic compliance has empowered soft robots for safe and sustainable interactions with the surroundings are the priority. However, intrinsic compliance from conventional soft materials also infers slow response and weak force of the soft robots and the high degrees of freedom also means the use complex control systems is a necessity.

To address these challenges, the Yang lab has developed a new type of structure called ‘metacaps’, whose ultrafast yet tunable snapping response can be leveraged to overcome the power and speed limitations of current soft robots and simplify their control requirement. Conventional spherical caps, known for their ‘snap-through’ behavior, a quick ‘inside-out’ transformation mechanism upon mechanical loading, making it favored for fast robotic rendering. Yet they are sensitive to defects resulted in manufacturing and offer limited energy release profiles during the snap-through process.

By integrating architected structures into the spherical caps and adding an array of ribs, our metacaps surpass conventional designs in terms of snapping response time, bistability, and robustness against to defects for sustainable actuations. Endorsed by a combination of theoretical analysis, numerical simulations, and experimental validations, the research investigates the effect of geometric designs and material properties and illustrates how the ribs influence the structural bistability and energy release during the snapping. 

The capabilities of the metacaps have been demonstrated in several prototype robots, including an ultra-fast, sensorless gripper that can grasp objects in 3.75 milliseconds upon physical contact. This remarkable speed sets a record for soft grippers, especially considering the large size (centimeter scale) of the device. These grippers can be incorporated into robotic platforms and sports equipment for real-world applications. For example, the metacap gripper can be integrated inside a baseball glove to assist in catching flying baseballs without the use of fingers, making the sport more accessible to those with hand disabilities. 

The actuation speed of the gripper can be adapted to specific tasks by coupling it with a pneumatic chamber of adjustable volumes. By adjusting the volume of the chamber, we can precisely control the gripper’s opening and closing speeds. In addition, the metacaps have been employed to propel a swimming robot with amplified and tunable speeds as well as enable untethered, electronics-free swimming. Thereby, the swimming robots can be deployed in extreme environments where electronics would be vulnerable to damage or dangerous for spark ignition.

Nevertheless, limitations remain in the metacap-enabled soft robots. The sensing ability of the gripper allows it to grasp objects once the contact force surpasses a critical value, but it cannot release objects automatically. Another input of stimulus is needed. Also, the force must be applied at the center of the metacap to trigger the grasping process. The pneumatically controlled actuator with adjustable actuation speed requires a long inflation time to store sufficient elastic energy for fast actuation.

Overall, the innovative design and flexibility of metacaps open new doors in the field of soft robotics, offering a blend of rapid response and controllability that has not been possible previously.

About the Lab

The Yang Lab is interested in developing novel materials synthesis, assembly and eco-manufacturing of complex, multi-functional, nano- to macrostructured soft, sustainable materials and composites. By coupling chemistry, fabrication and external stimuli, the Yang lab addresses the fundamental questions at surface-interface in precisely controlled and sometimes extreme environments, and study environmental responsiveness and the related structure-property relationship. Special interests involve novel design, synthesis and engineering of well-defined polymers, gels, colloidal particles, liquid crystals, and organic-inorganic hybrids with controlled size, shape, and morphology over multiple length scales, and investigate dynamic responses, mechanical instabilities, and structural evolution in soft and geometric substrates. By directed patterning and assembly of nano- and micro-objects in solutions and on patterned surfaces, they explore unique surface, optical, and mechanical properties and their dynamic tuning for relevant and societally impactful applications, including coatings (e.g. superhydrophobic, superamphiphobic, underwater superoleophobic, structurally colored), dry and wet and reversible adhesives, smart windows, displays, (bio)sensors, soft robotics, biomedical devices, wearables, dehumidifiers, and carbon absorbing concrete for a better and more sustainable future.

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Snapping Metacaps Propel Soft Robot Design, Advanced Science News