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The development of highly efficient actuation systems for miniature robots remains a significant challenge in the industry. However, researchers at The Chinese University of Hong Kong (CUHK)’s Department of Mechanical and Automation Engineering have developed a new technology inspired by the seed dispersal mechanism of the squirting cucumber.
This innovation termed the accumulated strain energy-fracture power amplification mechanism (ASEF), has been realised in a light-driven hydrogel launcher, which outperforms conventional micro-engineered systems in power output and motion performance. The study results were published in the journal Nature Materials.
Miniature robots, roughly the size of insects, are capable of performing tasks in confined spaces that are challenging for larger instruments, such as clinical sampling inside human bodies. However, their small size limits their capacity to carry energy resources and components, necessitating new technologies to accumulate and release energy instantaneously, thereby amplifying their power.
The ASEF mechanism is inspired by the squirting cucumber (Ecballium elaterium), which accumulates elastic energy in its fruit wall until it reaches a critical pressure, causing the wall to fracture and eject seeds at high velocity and acceleration. Drawing from this biological phenomenon, the CUHK research team developed the ASEF mechanism and implemented it in a hydrogel launcher.
The launcher is made from a highly tough and stretchable hydrogel material doped with graphene, which imparts excellent photothermal conversion ability. When irradiated with near-infrared light, the graphene heats up rapidly, causing water vaporisation and volume expansion. This deforms the hydrogel network, storing substantial strain energy. Once the stored energy reaches its limit, the bottom of the launcher fractures, releasing the accumulated elastic energy within 0.3 milliseconds as the strain energy converts to kinetic energy.
A hydrogel launcher based on the ASEF mechanism, with a diameter of 7 mm and thickness of 3 mm, achieves a vertical launch height of over 193 cm—equivalent to 643 times the launcher’s body length. It also reaches a take-off velocity of approximately 7.5 m/s (27 km/h) and an acceleration of 25,000 m/s², or 2,500 times the acceleration of gravity. This performance surpasses that of current conventional micro-engineered systems in terms of power output and motion performance.
Professor Zhang Li, who led the research team, likened the ASEF mechanism to archery, where pulling back the bowstring gradually accumulates energy, which is then instantaneously released upon release, amplifying the power. The ASEF-based hydrogel launcher, according to Zhang, does not involve complex structures or fabrication processes and can be made from various materials at a low cost. Currently, each hydrogel launcher is single-use due to the bottom fracture after launching, but the team is exploring self-healing hydrogel materials for repeated use.
This new mechanism has the potential to replace existing actuating components in miniature robots, significantly enhancing their power output. This could enable tasks such as stent delivery, tissue sampling, and excision within deep tissues, benefiting patients. In agriculture, a light-driven hydrogel launcher could provide robots with sufficient power to carry and eject seeds and RFID tags over long distances. This could facilitate timed water supply in farmland, with the hydrogel robot releasing seeds upon absorbing water and swelling.
Moreover, the exceptional mobility performance based on this new actuation mechanism holds promise for exploration robots in the gravity-free or microgravity environments of the Moon or outer space. The ASEF-based technology represents a significant advancement in the field of miniature robotics, with broad potential applications in medicine, agriculture, and space exploration.
