Autonomous Catalytic Motors with Energy Accumulation in Hydrogel Dome Structure for High-Power Intermittent Firing

Autonomous motors that move freely in liquid space enable local stimulation and sensing in natural and in vivo environments, and are expected to have applications such as improving water quality and treating disease. Catalytic motors derive their propulsion from the catalytic reaction of molecules in solution, such as hydrogen peroxide (H₂O₂); thus, they continue to move in solution as long as the fuel chemicals are present [1]. However, the catalytic propulsion power is limited in the real environment due to the low concentration of the fuels. In this study, we propose a high-power motor system which accumulates catalytic driving energy in the body. Our motor accumulated a catalytically produced bubble, followed by instantaneous release of the bubble with high-speed travel.<br/>To fabricate the motor, a hydrogel film was anchored to a disk glass substrate with silane coupling. The silane pattern was removed from a part of the hydrogel/glass interface to create a dome structure by hydrogel buckling [2]. The buckling structure was composed of two parts, a large hemisphere to accumulate the bubble and a narrow tunnel to release the bubble to the outside. A platinum (Pt) layer was deposited on the inside of the hydrogel dome to catalyze the reaction of environmental H₂O₂ to oxygen bubble. The hydrogel/Pt/glass device was placed in H₂O₂ solution, and its behavior was observed under a microscope.<br/>After the motor was placed in the H₂O₂solution, small bubbles formed around the Pt layer and gradually coalesced into a large bubble in the hydrogel dome because the H₂O₂fuel continued to access the internal Pt catalyst through the polymer network of the hydrogels. However, the hydrogel and glass walls prevented the bubble products from exiting. The internal bubble was expanded to a critical amount of accumulation and then released all at once to the outside. At the release of the accumulated bubble, the mm-sized motors showed significant horizontal movement, in contrast to the immobility of the motors with normal bubble release without accumulation. The direction of the horizontal movement was controlled by the buckling tunnel of the hydrogel. The cycle of bubble accumulation and release was observed repeatedly because sufficient H₂O₂ was present. In addition, the motor started to float during bubble accumulation due to the increased buoyancy and showed instantaneous diving to the bottom upon bubble release; thus, our motor showed three-dimensional motion in solution. The system enables mm-scale motors to move in the inner liquid space, which requires more driving force compared to the movement at the liquid surface. Furthermore, fluorescence observation of the hydrogel showed a stable deformation cycle during bubble accumulation and release, indicating that the device is sufficiently robust to tolerate the high-power intermittent firing.<br/>In conclusion, we have developed a high-power autonomous motor that accumulates catalytic driving energy in the hydrogel body. The system will pave the way for motors with low energy production efficiency. We will explore applications that take advantage of the instantaneous high-power firing and the accumulation of small environmental energy.<br/><br/>[References]<br/>[1] Y. Mei <i>et al.</i>, <i>Chem Soc Rev</i>, 40, 5, 2109–2119, (2011).<br/>[2] R. Takahashi <i>et al.,</i> <i>Adv Funct Mater</i>, 33, 24, 2300184, (2023).