Pressurized Vascular Systems for Self-healing Materials

Vascular epoxy specimen containing two pairs of microchannels (280 microns in diameter) positioned to intersect cracks and release liquid healing agents into regions of damage. Each microchannel contains either a liquid resin (dyed red) or a liquid hardener (dyed blue), which react to form a polymer adhesive upon mixing. Inlets inserted into each microchannel enable connection of the microchannels to external pumps. Specimen height: is 40 mm.

Researchers at the Beckman Institute have demonstrated that an active pumping capability for pressurized delivery of liquid healing agents in microvascular systems significantly improves the degree of healing compared with capillary force methods.

Artificial microvascular systems for self-repair of materials damage, such as cracks in a coating applied to a building or bridge, have relied on capillary force for transport of the healing agents. Now, researchers at the Beckman Institute have demonstrated that an active pumping capability for pressurized delivery of liquid healing agents in microvascular systems significantly improves the degree of healing compared with capillary force methods.

In a paper for the Royal Society journal Interface, Nancy Sottos, Scott White, and former graduate student Andrew Hamilton report on their investigation into using an active pumping method for microvascular systems in a paper titled Pressurized vascular systems for self-healing materials. Their inspiration, they write, comes from the fact that nature in its wisdom gives that ability to many biological systems: “Fluid flow in these natural vascular systems is typically driven by a pressure gradient induced by the pumping action of a heart, even in primitive invertebrates such as earthworms.”

Sottos and White, faculty in the College of Engineering at the University of Illinois, and their fellow collaborators from Beckman’s Autonomous Materials Systems (AMS) group, have developed different methods for self-healing, including microvascular systems for self-repair of polymers. The vascular system works when reactive fluids are released in response to stress, enabling polymerization that restores mechanical integrity. 

For this project, Sottos, White, and Hamilton sought to determine the effectiveness of an active pumping mechanism in a microvascular system because, they wrote, relying on capillary flow to disperse the healing agents “limits the size of healable damage” and because “unpressurized delivery of healing agents requires diffusional mixing – a relatively slow and highly localized process for typical resin-hardener systems – to occur for the healing reaction to initiate.”

To achieve active pumping the researchers experimented with an external “pump” composed of two computer-controlled pressure boxes that allowed for more precise control over flow. The healing agents in the pump were fed into two parallel microchannels.

The results of using active pumping to deliver healing agents from external reservoirs showed that “a damage volume larger than the total vascular volume was effectively filled and healed. Employing pressure-driven flow removes the reliance upon capillary forces and, in the cases of dynamic pumping, mixing of the two healing agents in the damaged region was enhanced.”

The improved mixing of the reactive agents results in “the formation of consistently tougher healed material over the course of numerous damage-heal cycles when compared with the alternative strategy of employing a dense spatial distribution of vascular features to achieve mixing via diffusion alone, resulting in inferior mechanical recovery.”

The method reported on in the paper relied on the most effective healing technique for microvascular systems (two separated liquid-phase components that react upon contact following release caused by damage), while the use of an external, actively-controlled pump to deliver the healing agents to the damaged region “removed the constraints of limited healing agent supply.”

They found that active pumping improves the degree of mechanical recovery, and that a continuous flow of healing agents from dynamic pumping extends the repeatability of the self-healing response.

“Significant improvements,” they write, “are achieved in the degree of healing and the number of healing events possible, compared with prior passive schemes that utilize only capillary forces for the delivery of healing agents.”

Sottos said the study was a first step toward integrating active pumping into microvascular systems. Earlier this year Sottos, White, and fellow AMS member Jeff Moore reported on a method for microvascular systems that uses structural composites reinforced with fibers that, when heated, vaporize, leaving tiny channels with multiple functionalities.

“This set-up could be used with any microvascular network, including the structural composites reported on recently,” Sottos said. “In future materials, it would be ideal to have the pumping integrated in the materials itself. 

“The advance of this paper is the study of active pumping/mixing for healing. We haven't applied this to healing with the structural composites yet; the present study was essential to understand what happens when we pump the healing agents.”