CatchGel—A Molecular to Macroscale Investigation of Catch Bond-Crosslinked Polysaccharide Hydrogels

Nature’s exquisite designs are a constant source of inspiration for materials scientists, yet our ability to replicate biological interactions on the macroscale in engineered systems is often severely limited. Biological systems exist in perfectly balanced environmental conditions that regulate the functionality of the processes within the system. Thus, a major obstacle in exploiting biological interactions in macroscale materials is a lack of understanding of how the changing chemical environment - from the molecular to the macroscale - affects the intrinsic molecular functionality of the biological unit, ultimately impacting macroscale behaviour.<br/>From a material design standpoint bacterial adhesins are particularly interesting proteins. These proteins are expressed on bacterial cell surfaces to allow them to bind to, and colonise, a host. Many of these bacterial adhesins exhibit so-called ‘catch bond’ behaviour. Essentially, the adhesive force of these receptor-ligand complexes increases when subjected to high shear, in contrast to traditional ‘slip’ bonds, which decrease/rupture under high shear. On reaching a maximum applied force, the catch bond then reverts to slip bond behaviour resulting in a catch-and-roll type action that bacterial cells use to move in a targeted fashion along a surface. This behaviour also allows pathogens to continue to colonise host organisms despite aggressive actions from the host to remove colonies.<br/>Transplanting catch bond-forming protein receptors and ligands onto polysaccharide chains opens up the possibility of creating bio-based hydrogel networks with unique tensile properties. Specifically, networks that display both shear-thinning and shear-thickening properties depending on the applied force, which also have the potential to self-heal as complexes rupture and reform. However, the complexity of intermolecular interactions in both catch-bond forming adhesin complexes and bio-based polymers mean the road to macroscale materials is less than straightforward.<br/>Focusing on adhesins isolated from Staphylococcus epidermidis (serine-aspartate repeat protein G (sdrG)) and Staphylococcus aureus (serine-aspartate repeat protein E (sdrE), clumping factor A (clfA) and clumping factor B (clfB)), we use single molecule force spectroscopy to probe the fundamental interactions between these adhesins and their specific ligands to understand the impact of orientation, substrate, mutations and environmental composition on the stability of the catch bond behaviour. Expressing adhesins on yeast cells and probing substrate interactions with spinning disk microscopy, a deeper understanding of cooperative interactions as a function of environmental complexity is gained. Finally, functionalisation of polymer chains with adhesins and their corresponding ligands creates the building blocks for the catch bond cross-linked hydrogel networks, whose macroscale mechanical properties are probed using rheological measurements, completing the molecular to macroscale transition.<br/>This multi-length scale approach to the design of catch bond cross-linked materials provides fundamental understanding of the impact of protein orientation, mutations and environment as we transition from the cell to a synthetically designed macroscale matrix. Examining how protein behaviour and macroscale materials properties change as a function of protein composition and environmental complexity allows more accurate development and manipulation of adhesin-derived responsive biomimetic materials. Thus, leading to a greater understanding of the nano-macroscale relationships in complex multiphase systems and enhanced strategies for the translation of biological interactions into macroscale materials.