Fluid flows give helping hand

Automating the construction of three-dimensional structures that are tens of millimeters in size would revolutionize the manufacture of devices for optical, electrical and biomedical applications. One way to produce such 3D microstructures would be to ‘program’ the constituent parts to spontaneously come together so they build the structures themselves. However, getting micron to mesoscale components (from 0.1mm to 100mm in size) to line up and dynamically assemble into a desired structure remains an elusive goal.

Building on previous research, researchers at the University of Pittsburgh Swanson School of Engineering have managed to overcome the challenge of designing such properly self-aligning structures, using fluid mechanics, chemo-mechanical processes – and a little stickiness. They report this work in a paper in PNAS Nexus; lead author of the paper is Oleg Shklyaev, a post-doctoral associate in the research groups of Anna Balazs, professor of chemical and petroleum engineering.

“One of the fundamental challenges in building anything with micron-sized building blocks is to get the blocks to robustly organize on their own, with little intervention from external tools, which could interfere with the dynamic self-assembly,” explained Balazs. “What’s wonderfully brilliant about the system that Oleg designed is that the naturally occurring interplay between the fluid and chemistry performs the work to spontaneously construct a robust system.”

Using computer modeling, Shklyaev designed various two-dimensional polymeric sheets: a heavier sheet forms the foundation or base of the structure, while lighter sheets form the construction panels. Sticky bonds are added to specific points on the sheets to act as hinges – similar to the molecular bonds that join together the bases in a strand of DNA.

The researchers drop the panels into a solution, causing them to sink to the bottom in random areas of the tank, and then add a reactant to instigate a catalytic reaction. This generates fluid flows both vertical and horizontal to the confining walls. The horizontal fluid flow moves the sheets together along the floor of the tank, causing the sticky bonds to connect the appropriate panels to the base. Next, the vertical flow lifts the panels into the upright position, where they are connected to each other via more sticky bonds to complete the structure.

“This conversion of chemical energy (released from the catalytic reaction) into mechanical action (fluid flow) is an inherent property of the system,” said Shklyaev. “Namely, as the catalytic reaction converts reactants into products, it intrinsically produces density or concentration gradients in the solution. The gradients, in turn, generate a force that acts on the fluid and triggers the flow. The flow acts like ‘helping hands’ to assemble the structure.

“Through chemistry, you can engineer the spatially and temporally varying patterns that emerge in the flow, and thereby tailor the work done by these hands, which also initiate the cascade of events that leads to building a regular tetrahedron, cube or similar structure. In principle, the ‘sticky’ bonds on the panels can involve strands of DNA; the complementarity of DNA strands enables the bonds to be highly selective and recognize the regions to which it should stick.”

By engineering the fluid flows, Shklyaev found he could drive the self-organization of a cube and the closing of the cube’s lid, so that the entire structure resembled a takeout box. The chemically generated fluid flow acting on the panels (through mechanisms knows as solutal buoyancy and diffusioosmosis) eventually reaches a dynamic steady state as it completes the assembly of the structure, which could later be removed from of the fluid while still maintaining its integrity.

To further illustrate the potential of the fluidic machinery, the tops of each panel were decorated with long whiskers. As the panels fold upward and the whiskers extend into the fluid flow, the resulting forces drive the whiskers to rotate, much like moving propellers. Sticky bonds could be added to the whiskers to attract bacteria or other materials that need to be removed from the system.

“The use of chemical reactions to tailor the flow to act as a mechanical tool has not been broadly applied in man-made systems but is particularly valuable since the fluid flow performs the necessary work and replaces complicated machinery,” Balazs said. "The process is scalable: multiple structures with different shapes can be formed at one time.

“By providing these guidelines to experimentalists, we can automate manufacturing processes since the structure formation is driven by the dynamic self-assembly of the components. The resulting structures can be used for medical applications since the processes typically involve water, which provides a biologically friendly environment.”

This story is adapted from material from the University of Pittsburgh Swanson School of Engineering, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.