Untethered microrobots have great potential as they could play an important role in areas such as precision medicine, imaging, antibiotic alternatives, and environmental monitoring and clean up, among others. While promising current examples move only in liquids, which limits the environments that they can be applied in.

“Improving the structure and motion mechanism of microrobots is an important way to improve their working efficiency,” said Dong Sun, professor at the Department of Biomedical Engineering in City University of Hong Kong.

“Microrobots that can achieve surface motion can not only swim but also walk, which greatly enhances their environmental adaptability,” he continued. “Some microswimmers come and go freely in environments such as the blood and tissue fluids but are helpless in environments without fluid immersion. Our designed microwalker can still move as usual on wet tissue surfaces”

In a new study published in Advanced Intelligent Systems, Sun, along with lab members Yuanjun Jia and Pan Liao, developed a magnetically-powered microwalker constructed using two, equal-length rigid segments connected by a joint. “The rigid segments and joint increase structural stability to avoid collapse,” said Sun.

Parallel, gecko setae-like tentacles were then placed at the bottom of the segments as contact feet to generate friction with the contact surface. The feet have different amounts of friction in different directions, just like a saw, which can easily be pulled in one direction, but has resistance in another.

“Bionic technology provides a good solution to develop a microwalker with surface motion ability,” said Sun. “The application of directional friction in materials science and mechanical engineering has become a new research hot topic. The gecko uses the directional friction of its bristles to achieve adsorption on a wall, which inspired the structural design of our microwalker.”

The microwalker was fabricated from biocompatible materials using 3D laser lithography. Afterward, they are coated withnickel and titanium. “Nickel is for magnetic actuation and titanium is for biocompatibility,” explained Sun. “Deposition of nickel with a purity of 99.99% has been reported to impart good ferromagnetic performance.

“The microwalker is driven by an external oscillating magnetic field and its movement speed is affected by the amplitude and frequency of the magnetic field oscillation,” he continued. “Taking advantage of the frictional anisotropy of the bionic contact feet, the two segments of the microwalker alternately advance under the oscillating magnetic field. The motion direction of the microwalker can be controlled by controlling the balance direction of the oscillating magnetic field.”

Experiments were conducted to test the performance of the microwalker. Among them, movement was demonstrated on the surface of a cell fragment and climbing up a slope provided a proof-of-concept demonstration for the microwalker’s capabilities in non-liquid environments.

“Although there are still many hurdles to clear on the road to clinical application, such as drug delivery, microsurgery, blood clot removal, cell manipulation, and in vivo sensing, we will continue to research in materials, structures and control,” explained Sun, “We will continue to work hard and look forward to the day when microrobots will provide healthcare solutions.”

Reference: Yuanjun Jia, Pan Liao, Yong Wang, and Dong Sun, Magnet-driven Microwalker in Surface Motion Based on Frictional Anisotropy, Advanced Intelligent Systems (2022). DOI: 10.1002/aisy.202200118