Much like the human circulatory system pumps blood and oxygen through the body, a new vascularized composite material developed by University of Illinois researchers can transport liquids or gases through the composite. The material relies on sacrificial fibers that degrade at high temperatures, without affecting the structural composite material, creating tiny channels for liquid or gas transport. The channels can wind through the material in one long line or branch out to form a network of capillaries.
“We can make a material now that’s truly multifunctional by simply circulating fluids that do different things within the same material system,” says Scott White, a professor of aerospace engineering who leads the research team. “We have a vascularized structural material that can do almost anything.”
White and peers in the Autonomous Materials Systems Laboratory in the Beckham Institute for Advanced Science and Technology have created a sacrificial polymer fiber that is woven into the reinforcing fiber fabric. The polymer fiber had to meet several requirements. First, it needed to be robust enough to withstand the weaving process. Next, the fiber had to disintegrate at a temperature slightly higher than normal composite curing conditions, but not so high that matrix degradation would occur. “This gives us a very narrow temperature window to work with – approximately 180 C to 220 C,” says White.
Finally, the fiber removal process must occur under gaseous conditions. “We do not melt the fiber and extract by vacuum pressure. At extremely high aspect ratio channels (length/diameter) the melted fiber would simply remain in the channel,” says White. “Instead, we depolymerize the fiber to a monomer, which is then highly volatile at the temperature we do the extraction. So the fiber becomes a gas and escapes the composite quite efficiently.”
The sacrificial fiber is a polylactide (PLA) fiber. The researchers treat it with a catalyst – typically tin oxalate – to lower its natural degradation temperature. They have used PLA fibers to create vascular composites in high-temperature cured epoxy with glass or graphite fibers.
White’s team, supported by the Air Force Office of Scientific Research, has demonstrated various ways the material is useful. It can create self-healing composites that mend multiple times when subject to damage, such as an interlaminar fracture. The vascularized material can circulate coolants to lower the temperature in a composite subjected to thermal loads or increase the service temperature of a composite by active cooling. In addition, the magnetic structure of the composite can be changed by circulating a ferrofluid within the channels, which might prove useful for electromagnetic modulation. Lastly, the researchers have circulated a conductive fluid, showing that you can create conductive pathways within a composite on demand.
The research team has reached several milestones, including extraction from a 1 meter long, .5 millimeter diameter channel. But its work is not done. “We are perfecting a new way of making sacrificial fibers via melt spinning,” says White. “We are also beginning to explore how to weave fibers within fabric preforms to yield interconnected vascular networks.”