Fabricated microvascular networks could create compact fluidic factories

Using direct-write assembly of organic ink, researchers at the University of Illinois at Urbana-Champaign have developed a technique for fabricating three-dimensional microvascular networks. These tiny networks could function as compact fluidic factories in miniature sensors, chemical reactors, or computers used in applications from biomedicine to information technology.

“The fabrication technique produces a pervasive network of interconnected cylindrical channels, which can range from 10 to 300 microns in diameter,” said Jennifer Lewis, a professor of materials science and engineering and of chemical engineering at Illinois. “Our approach opens up new avenues for device design that are currently inaccessible by conventional lithographic methods.”

The microvascular networks also could be combined with self-healing functionality, “providing an analog to the human circulatory system for the next generation of autonomous healing materials,” said Scott White, a professor of aeronautical and astronautical engineering and a researcher at the Beckman Institute for Advanced Science and Technology. “The embedded network would serve as a circulatory system for the continuous transport of repair chemicals to sites of damage within the material.”

The scientists report their findings in a paper that has been accepted for publication in the journal Nature Materials, and posted on its Web site www.nature.com/materials.

To create a microvascular network, Lewis, White and graduate student Daniel Therriault begin by fabricating a scaffold using a robotic deposition apparatus and a fugitive organic ink. A computer-controlled robot squeezes the ink out of a syringe, almost like a cake decorator, building the scaffold layer by layer.

“The ink exits the nozzle as a continuous, rod-like filament that is deposited onto a moving platform, yielding a two-dimensional pattern,” Lewis said. “After a layer is generated, the stage is raised and rotated, and another layer is deposited. This process is repeated until the desired structure is produced.”

Once the scaffold has been created, it is surrounded with an epoxy resin. After curing, the resin is heated and the ink — which liquefies — is extracted, leaving behind a network of interlocking tubes and channels.

In the final step, the open network is filled with a photocurable resin. “The structure is then selectively masked and polymerized with ultraviolet light to plug selected channels,” Lewis said. “Lastly, the uncured resin is drained, leaving the desired pathways in the completed network.”

To demonstrate the effectiveness of their fabrication technique, the researchers built square spiral mixing towers within their microvascular networks. Each of the integrated tower arrays was made from a 16-layer scaffold. The mixing efficiency of these stair-cased towers was characterized by monitoring the mixing of two dyed fluid streams using fluorescent microscopy.

“Due to their complex architecture, these three-dimensional towers dramatically improve fluid mixing compared to simple one- and two-dimensional channels,” White said. “By forcing the fluids to make right-angle turns as they wind their way up the tower, the fluid interface is made to fold on top of itself repeatedly. This chaotic advection, in addition to normal diffusion, causes the fluids to become well-mixed in a short linear distance.”

In addition to serving as highly efficient and space-saving mixers in microfluidic devices, the microvascular networks offer improved functionality in the design of self-healing materials.

“With our current approach, we distribute microcapsules of healing agent throughout the material,” White said. “Where damage occurs locally, the capsules break open and repair the material. With repeated damage in the same location, however, the supply of healing agent may become exhausted.”

Using capillaries instead of capsules to carry the healing agent could improve the performance of self-healing materials, White said. “By incorporating a microvascular network within the material, we could continuously transport an unlimited supply of healing agent, significantly extending the lifetime of the material.”