Dreadlocks with Kung Fu Grip

Researchers have found that dunking synthetic, nanosize hairs into a liquid and allowing the liquid to evaporate can coax the nanobristles to twist together like dreadlocks, sticking to each other or to an object in their center with a tenacious grip. Besides potential uses such as energy and information storage, photonics and adhesion, the ability could be harnessed to develop catch-and-release systems, such as a device that can clutch a drug-filled capsule and carry it through the bloodstream.

Nanobristles hugging a polystyrene sphere. Images courtesy of the Aizenberg lab at Harvard School of Engineering and Applied Sciences.

“We demonstrated a fascinating phenomenon: how a nanobristle immersed in an evaporating liquid self-assembles into an ordered array of helical bundles. This is akin to the way [that] wet, curly hair clumps together and coils to form dreadlocks – but on a scale 1000 times smaller,“ said Joanna Aizenberg, Gordon McKay professor of materials science at Harvard School of Engineering and Applied Sciences (SEAS).

The scientists used a synthetic system of evenly spaced, flexible polymer nanobristles growing out of a flat surface to investigate the conditions and design principles that lead to spiral formation. According to conventional wisdom, making coiled or “chiral” structures requires either chiral starting materials or a chiral driving force, such as stirring in one direction. In contrast, the twisted bundles created by the Harvard team emerged from a symmetrical system.

The researchers began by submerging soft bristles in liquid. As the liquid evaporated, surface tension drew pairs of bristles together. (The same capillary forces are responsible for the curved appearance or meniscus at the surface of a liquid in a narrow tube.) With further evaporation, sets of bristles began twisting together, maximizing the contact between the hairs.

“Our development of a simple theory allowed us to further characterize the combination of geometry and material properties that favor the adhesive, coiled self-organization of bundles and enabled us to quantify the conditions for self-assembly into structures with uniform, periodic patterns,” said Lakshminarayanan Mahadevan, Lola England de Valpine professor of applied mathematics at SEAS. He and Aizenberg are the lead authors on a paper reporting the research in the Jan. 9 issue of Science.

False-color image of self-assembled mesoscale helical structures. The scanning electron microscopy images show the morphogenesis of helical patterns, from the first-order unclustered nanobristle to the fourth-order coiled bundle (scale bars 4 mm). Note the hierarchical nature of the assembly reflected in the presence of the lower-order braids in the large clusters braided in a unique structure reminiscent of modern dreadlocks or the mythical Medusa.

By changing the direction in which the liquid evaporated and the stiffness of the bristle, the researchers produced tightly coiled nanonests and woven nanobraids. The bristles also could wind around a sphere, holding it fast even when subjected to sonication (sound energy). Relatively simple changes in the bristles' environment, such as a change in pH, could cause them to release their grip, suggesting a way to control a catch-and-release system at the micron scale.

Potential applications of the technique include the ability to store elastic energy and information embodied in adhesive patterns that can be created at will, or as part of a device to help direct the flow of a minute amount of chemicals through a so-called lab on a chip. This has implications for photonics in a manner similar to the way that the chirally ordered and circularly polarizing elytral filaments in a beetle define its unique optical properties.

Illustration of the adhesive and particle trapping potential of the helically assembling bristle. This low-magnification SEM shows the capture of the 2.5-mm polystyrene spheres (scale bar 10 mm).

“We have teased apart and replicated a ubiquitous form in nature by introducing greater control over a technique increasingly used in manufacturing while also creating a microphysical manifestation of the terrifying braids of the mythical Medusa,” Mahadevan said.

“Indeed, our helical patterns are so amazingly aesthetic that often we would stop the scientific discussion and argue about mythology, modern dreadlocks, alien creatures or sculptures,” Aizenberg added.

Aizenberg and Mahadevan's co-authors included Boaz Pokroy and Sung H. Kang, both in the Aizenberg Biomimetics Lab at SEAS. The research was supported by the Wyss Institute for Biologically Inspired Engineering at Harvard; the Harvard Materials Research Science and Engineering Center; and the Center for Nanoscale Systems, a member of the National Nanotechnology Infrastructure Network initiative.

For more information, visit: http://www.seas.harvard.edu/