Go with the Flow

Modeling flexible microscopic sheets that sculpt themselves

The Swiss Army Knife is a necessity for survivalists and the outdoorsy, providing tools for any situation you might encounter. Researchers at the University of Pittsburgh’s De­partment of Chemical Engineering have created a compu­tational model for just that—on a microscopic scale. Re­searchers Abhrajit Laskar, Oleg Shklyaev, and Anna Balazs have developed a model for flexible, chemically-active sheets capable of interacting with a fluid environment to create fluid flow, which in turn moves and alters the shape of the sheet. Each sheet is composed of a network of flexi­ble chemical bonds connecting nodes coated with catalysts, chemical compounds that speed up chemical reactions. This potential for autonomous deformation and locomo­tion opens up new opportunities for mechanical and bio­medical engineering on the microscopic scale.

A key of the model is a chemical reaction that has reactants and products of different volume. “Think of the reactants as being little balloons and the products as being big balloons,” Balazs said. “The big balloon and little balloon displace dif­ferent amounts of fluid in the container, and so they make these local density gradients in the fluid.” This phenomenon, known as solutal buoyancy, is the crux of the model.

The reaction occurs through catalysts which break down reactants introduced into the fluid. An example reaction is the decomposition of hydrogen peroxide (H2O2) into water (H2O) and oxygen (O2). This reaction is sped up with an en­zyme, called catalase, coated onto a sheet in the shape of a pet­aled flower. The less-dense products rise upward in the fluid, changing the direction of fluid flow and causing the petals to rise. As the amount of H2O2 remaining in solution decreas­es, the sheet flattens again due to the drop in products made. By intermittently introducing H2O2 into the system, the cat­alyst-coated petals can be made to open and close cyclically.

Coating the petals with different catalysts and setting up a chain of catalytic reactions allow for further control over the sheet’s shape. However, differing reactions’ rates sometimes pose a challenge if the reaction rate of one is significantly slow­er than another’s. This can be compensated for by coating the walls of the container with an appropriate catalyst to increase the surface area catalyzing the reaction, or by changing the ar­eal density of catalyst on the surface of the sheet.

The sheets can be made to move through the chamber by two primary methods. One is modeled by small bumps representing unevenness along the bottom surface of the container. A catalase-coated sheet would tumble over each bump because of its flexibility and chemical activity gen­erating upward local fluid flow. In this way, an active sheet can be made to move through the channel over obstacles at the bottom until there is no more reactant to be consumed.

The second method is to have a catalyst-coated sheet with edges heavier than the interior of the sheet, allowing the sheet to move like an inchworm—the products flow upward and cre­ate a bulge in the lighter center. After the reactant is consumed, the sheet flattens out, displaced to a new position. Addition of more H2O2 can repeat the process and propel the sheet further. Another method to induce inchworm locomotion would be to coat the sheet with an uneven concentration of catalyst. “You’d have more of the catalyst in one region than the other, and you could potentially achieve the same thing,” Balazs said.

The researchers think the sheets could be used to iden­tify diseased cells and, acting as claws, remove them from the bloodstream for examination. “The gripper itself has to be on the same size scale as the cell and also has to be very flexible and gentle,” Balazs said. Another use of the sheets would be for construction. “You can have these lit­tle hands that can pick up things in the microfluidic de­vice and then do construction.” This could pave the way for what could be thought of as microscopic foundries.

“It’s still challenging to make the sheet sufficiently flex­ible and compliant that it will fold and bend nicely with­out crumpling,” Balazs said. Coating flexible sheets with catalyst is not a technology most are familiar with, so bringing the model to life in a wet lab may be difficult. Balazs and her collaborators have certainly put us on the right track, and, regardless of the challenges, this tech­nology is sure to do more than open cans and bottles.