Building blocks spontaneously constructed 3D objects in solution

Pitt engineers show self-organization of sticky micron-to-mesoscale 3D structures in confined fluids.

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Manufacturing devices for optical, electrical, and biological purposes would be revolutionized by automating the production of three-dimensional structures.

Constructing 3D microstructures requires programming the constituent parts to come together and spontaneously build the structures. However, getting micron-to-mesoscale components to line up and dynamically assemble into the necessary structures is still difficult.

Based on the previous study, chemical engineering researchers- at the University of Pittsburgh Swanson School of Engineering- have overcome the challenge of designing properly self-aligning structures using fluid mechanics, chemo-mechanical processes – and a little stickiness. 

The inherent coupling of chemical and mechanical behavior in fluid-filled microchambers enables the fluid to perform work autonomously, which in turn can direct the self-organization of objects immersed in the solution.

Anna Balazs, Distinguished Professor, John A. Swanson Chair of Engineering, said, “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.”

“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 construct a robust system spontaneously.”

Scientists created two-dimensional polymeric sheets using computer modeling. One sheet is the foundation or base, and the other is the building panels. Similar to the bonds found in DNA molecules (A, C, G, and T), which are made to fit together perfectly, sticky bonds are added to specified locations on the sheets to serve as hinges.

After being dropped into a solution, the panels sink to the bottom of the tank in various spots. A catalytic reaction starts when a reactant is added to the solution; this reaction creates fluid flows that are vertically and horizontally accelerated toward the restricting walls.

The sheets are brought together along the chamber floor by the horizontal flow, and the sticky bonds then join the proper panel to the foundation. The construction is then completed after the vertical flow lifts the sides of the structure into an upright position. This time, sticky bonds are used to join the panels.

Shklyaev explained, “This conversion of chemical energy (released from the catalytic reaction) into mechanical action (fluid flow) is an inherent property of the system. 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.”

Shklyaev controlled the fluid flows to cause a cube to self-organize and its lid to close, giving the structure the appearance of a takeaway box. The chemically produced fluid flow operating on the panels finally reaches a dynamic steady state as it completes the assembly of the object, which could then be removed from the fluid and maintain its integrity. This fluid flow is controlled by solutal buoyancy and diffusion osmosis.

Each panel had lengthy whiskers on the top to better highlight the potential of the fluidic machinery. The forces generated as the panels fold upward and the whiskers extend into the fluid flow cause the whiskers to rotate like rotating propellers. To draw bacteria or other substances that need to be eliminated from the system, sticky bonds could be introduced to the whiskers.

Balazs said“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. The process is scalable; multiple structures with different shapes can be formed simultaneously.”

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

Journal Reference:

  1. Oleg E Shklyaev, Abhrajit Laskar, Anna C Balazs. Engineering confined fluids to autonomously assemble hierarchical 3D structures. PNAS Nexus. DOI: 10.1093/pnasnexus/pgad232
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