Characterization of microvascular networks for self-healing using fluorescent digital image correlation

While the current generation of microencapsulated self-healing polymers and composites has proven remarkably successful, some limitations are inherent to this conceptual approach. Eventually the healing functionality will be compromised as the supply of healing agent is consumed. Natural systems overcome this limitation by incorporating a pervasive vascular network of pathways to supply the chemical components for healing on demand. The next generation of self-healing materials draws its inspiration from these natural circulatory systems by incorporating a microvascular network for the supply of healing agent to damaged regions. The microvascular networks are created through direct-write assembly of a fugitive ink in a layer-by-layer sequential build process to produce a three-dimensional structure, which is then infiltrated with a polymer matrix material. The ink is removed after curing the matrix, leaving behind a complex interconnected network of microchannels. Characterizing the mechanical integrity of these networks is critical to optimizing their strength and toughness. The fluorescent digital image correlation technique (FDIC), which couples conventional digital image correlation with fluorescent nanoparticle markers, was used to begin probing the strains and displacements of a simple microvascular system with 200 micron diameter channels. The FDIC technique was verified on a simple two-dimensional model of a microvascular system by comparison with an analytical solution. The more complicated effects of a fully three-dimensional microvascular network with channel spacing on the order of one millimeter were also investigated. The results give insight into the mechanical behavior of microvascular networks and demonstrate the utility of FDIC as a characterization tool at these length scales.