Clinging on Tighter: Scientists Reenvision Bioinspired Suction Cup

Scientists at the University of California San Diego have taken another step forward in their effort to create a stronger suction cup inspired by the unassuming clingfish. The findings were published September 10 in the journal Applied Materials & Interfaces and follow a previous prototype developed in 2019. 

Clingfish are small fish widespread in tropical and temperate regions. They are common in intertidal areas, where they use a powerful suction disc on their undersides to adhere to rocks, seagrass, and other structures, whether smooth, rough, or porous. They can remain stuck to these surfaces even in powerful currents and when battered by waves.

Last year, Jessica Sandoval, a UC San Diego Jacobs School of Engineering PhD student in assistant professor Michael Tolley’s lab, worked with Scripps Institution of Oceanography researcher Dimitri Deheyn to reverse engineer these discs into suction cups that could potentially be used for a variety of human needs, including surgical devices. They developed suction cups that cling well to wet and dry objects both in and out of water, holding up to hundreds of times their own weight. But they encountered a snag; the suction cups did not perform well under wet shear motion – that is, when moved side to side along a surface. 

Bringing in researchers from San Diego State University and the University of San Diego, Sandoval returned to the clingfish, whose suction ability is perfectly adapted to withstand this exact problem when strong waves sweep horizontally over their rocky habitats. In her previous study she found that rows of hexagonal structures, called papillae, are covered in tiny fibers and seem to play a crucial role in the suction disc’s performance. 

“To answer the question of how to best stick when pulled parallel to a wet surface, we turned to look at the surface structures along the suction disc of the clingfish,” said Sandoval. “We wanted to understand how these very geometric shapes stabilize the grip of the clingfish to slippery underwater rocks.”

Sandoval analyzed clingfish specimens from the Scripps Marine Vertebrate Collection, ranging in size from less than an inch to a few inches, and reasoned that their clinging ability must rely on the precise positioning and structure of the papillae. The team took a closer look, developing an image processing tool that renders and automatically characterizes a 2D image of their pattern, size, shape, and distribution across the suction disc. 

The team emphasized that automated image processing is a key tool for understanding biological features, which was previously done mostly through observation by the naked eye or under a microscope in conjunction with written descriptions. 

“This was very time consuming and only performed on a small amount of replicates considering the amount of time and human effort involved,” said Deheyn, senior author of the study. “With this image processing technology we can analyze the anatomy of several specimens encompassing thousands of features, which to some extent represents a revolutionary tool that will greatly facilitate advancements in the field of bioinspiration and biomimicry.” 

The scans showed that the surface area of the papillae varied with size, but the channels between them did not, and those papillae that were elongated function to seal the suction disc to surfaces. They also found that the suction ability was greater the faster the water speed, meaning that the suction discs stick tighter in high energy environments, such as in the intertidal zone. 

“We were able to tease out the intricacies of the adhesive structures across five different sizes of clingfish,” said Sandoval. “We concluded that many characteristics, such as channel spacing, were consistent across body sizes. This was an exciting find. We even found similarities when comparing the adhesive surface textures of clingfish with those of the toe pads of tree frogs, suggesting similar attachment processes despite the completely different environments.”  

Next steps include developing a prototype based on these findings and testing its applicability for medical, food service, and manufacturing uses. 

This study was funded by the Office of Naval Research and the Air Force Office of Scientific Research. Jessica Sandoval is supported by the Gates Millennium Scholars Program.