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Fibre reinforcement itself isn't new—ever since the first masons mixed horsehair into mud, builders have been taking advantage of it to strengthen their materials.
The team at Drexel have created 'BioFiber,' a polymer fiber encased in a bacteria-laden hydrogel and a protective damage-responsive shell. A grid of BioFibers embedded in a concrete structure improves its durability, prevent cracks from growing and enable self-healing.
“This is an exciting development for the ongoing efforts to improve building materials using inspiration from nature,” said Amir Farnam, PhD, an associate professor in the College of Engineering who was a leader of the research team. “We are seeing every day that our ageing concrete structures are experiencing damage which lowers their functional life and requires critical repairs that are costly. Imagine, they can heal themselves? In our skin, our tissue does it naturally through multilayer fibrous structure infused with our self-healing fluid — blood. These biofibers mimic this concept and use stone-making bacteria to create damage-responsive living self-healing concrete.”
Increasing the lifespan of concrete could be vital when it comes to reducing greenhouse gas. The process of making concrete—burning limestone, clay or shale at more than 2000 degrees Fahrenheit, accounts for 8% of greenhouse gas emission, but concrete structures degrade in as little as 50 years.
“For several years, the concept of bio-self-healing cementitious composites has been nurtured within the Advanced Infrastructure Materials Lab,” said Mohammad Houshmand, a doctoral candidate in Farnam’s lab who was the lead author of the research. “The BioFiber project represents a collaborative, multidisciplinary endeavor, integrating expertise from the fields of civil engineering, biology, chemistry, and materials science. The primary objective is to pioneer the development of a multifunctional self-healing BioFiber technology, setting new standards at the intersection of these diverse disciplines.”
The team’s approach in creating BioFibers was inspired by skin tissue’s self-healing capability and vasculature system’s role in helping organisms heal their own wounds. And it uses a biological technique they developed to enable self-repairing in concrete infrastructure with the help of biomineralisng bacteria.
“One of the amazing things about this research is how everyone comes at the problem from their different expertise and the solutions to creating novel BioFibers are so much stronger because of that,” said Caroline Schauer, who led a team that collaborated on the research. “Selecting the right combination of bacteria, hydrogel and polymer coating was central to this research and to the functionality of BioFiber. Drawing inspiration from nature is one thing, but translating that into an application comprised of biological ingredients that can all coexist in a functional structure is quite an undertaking — one that required a multifaced team of experts to successfully achieve.”
To make BioFibre, the team used a strong polymer core that supports concrete. They covered this core with a special hydrogel full of tiny spores and enclosed it all in a tough polymer shell, like skin. The whole thing is just over half a millimetre thick.
When poured into concrete in a grid pattern, BioFibre helps strengthen it. But it really works when a crack breaks the outer shell.
Water gets into the crack and reaches BioFibre, causing the hydrogel to expand and fill the crack. At the same time, the spores become active and, with help from the concrete's calcium, produce a material that fills the crack.
How long it takes to fix depends on the crack size and how active the spores are – things the team is studying. But it looks like the spores could do the job in just one to two days.
“While there is much work to be done in examining the kinetics of self-repair, our findings suggest that this is a viable method for arresting formation, stabilizing and repairing cracks without external intervention,” Farnam said. “This means that BioFiber could one day be used to make a ‘living’ concrete infrastructure and extend its life, preventing the need for costly repairs or replacements.”
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.