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In a collaborative project that brings together a variety of sub-disciplines within the field of composites, researchers from the University of Delaware’s Center for Composite Materials (CCM) are attempting to tap into knowledge from nature to develop self-healing material systems.
Shridhar Yarlagadda, CCM Assistant Director of Research and Research Professor of Electrical & Computer Engineering, brings to the project a broad understanding of fibre-reinforced polymer composites. Kristi Kiick, Assistant Professor of Materials Science & Engineering, has expertise in the synthesis, characterization, and application of biologically inspired and biologically produced materials. Together, the two are carrying out very basic research that has the potential for tremendous long-term payoffs.
“The concept of self-healing materials is not novel,” Yarlagadda says, “but most previous efforts have focused on healing of the polymer and not the fibre. Polymer healing is important in some applications where matrix cracking is an issue—for example, in cryogenic environments.”
“However, for load-carrying applications, both the fibre and the matrix are important,” he continues, “so we’re interested in self-healing systems where not only the polymer but also the fibre can repair itself.”
While the damaged area of a composite with broken fibres may be very small, it compromises the integrity of the entire composite. Repairs to fix broken fibres are generally made by cutting out a wedge that is much larger than the damaged area to ensure load transfer across the repair.
Led by Center Director Jack Gillespie, CCM researchers conceived an idea for a repair based on the creation of a fibrous network across the break to heal the damage. They approached Kiick to determine whether the concept, based on biomineralization, was even feasible.
Kiick felt that the idea had potential, and the researchers began experimenting with silica-based systems. “Glass fibres are very common in composites,” says Yarlagadda, “and glass is silica-based.”
“Various organisms in nature can produce ornate silica-based structures,” Kiick says. “It’s amazing because the synthesis is conducted at ambient temperature, and the resulting materials have fabulous mechanical properties, which originate from the elaborate assembly of silica with proteinaceous materials.”
Examples include sponges and diatoms, which can create multi-layer fibres with a protein core. “It’s the multilayer aspect of these formations that provides the superior mechanical properties,” Kiick says. “We’re trying to mimic the process so that we can solve the problem of composite repair in the long term.”
“Research in the biomineralization field,” she continues, “has demonstrated that there are classes of proteins which serve not only as templates but also as catalysts in the biomineralization process. Although it is known which amino acids catalyze the silica formation, our ability to recreate the higher order structures in the lab is not even close to what nature can create.”
While the team’s long-term goal is to propagate silica formation across a break, they are taking “baby steps” along the way to that goal. The first is to control silica propagation so healing occurs at the break instead of a mass of silica forming randomly on the material. The results suggest the possibility for such control. The team is also addressing the issues of load transfer across the fibre-silica interface and optimization of silica formation.
“We need to produce something more robust,” says Kiick. “The silica products in nature are amorphous but strong, which is what we’d like to achieve. We want to duplicate the mechanically strong system that is formed from these layers of silica and protein ‘adhesive’.”
“We’ve achieved initial success at propagating load across the broken ends of a fibre,” says Yarlagadda. “We have a long way to go, but the long-term possibilities are very exciting. It’s a very multi-faceted problem, but the potential for this technology if we’re successful is tremendous.”
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