Biomimetic materials design towards tough nanocomposites and strain-stiffening hydrogels

Biological organisms use only a few chemical elements to construct materials, such as proteins, polysaccharides, and minerals, which are efficiently processed into hierarchical structures across length scales. These clever multi-component designs produce materials and structure with remarkably improved mechanical properties and functionalities relative to the original components. The field of biomimetics aims to mimic these well-adapted design strategies, structures and functions to solve material engineering problems. This thesis focuses on two nature-inspired systems: (i) tough and strong nacre-mimetic nanocomposites, and (ii) strain-stiffening biopolymer hydrogels and their application for cell culturing. Both systems consist of nanoscale components and their mechanical properties and structure are studied. The design strategy for tough and strong composites is inspired by the nacre, a biomaterial with outstanding mechanical properties. Here the structural organization of nacre is mimicked via self-assembling core-shell structured colloidal platelets, i.e. nanoclay coated with a polymer, via vacuum filtration. In publications I and II, we alter the interactions between the colloidal platelets with DNA-based monophosphates and demonstrate a simple way to modify intermolecular interactions resulting in increased stiffness, strength, and toughness. Like natural nacre, these nanocomposites are sensitive to humidity. In publication III, we study the effect of water in polymer-clay nanocomposites and find that the glass transition temperature of the nanoconfined polymer is lowered due to residual water. The second part is inspired by typical extracellular matrix-based protein gels, which show strain-stiffening. In publication IV, we show that agarose hydrogels are strain-stiffening, consisting of helically twisted semiflexible fibrillar networks. In publication V, we analyze this strain-stiffening response more closely and simultaneously show, for the first time, that agarose gels also contract when sheared, which is seen as negative normal force and normal stress difference. Our main findings indicate that the mechanical response of agarose networks is enthalpic and that connectivity dictates their strain-stiffening response similarly as in collagen gels. Finally, in publication VI, we present an application for agarose hydrogels as a luminal cell identity and estrogen receptor α+-preserving scaffold for breast cancer tissue explant culture. Base on a biomimetic materials design approach, the first part of this thesis illustrates a simple method to control the mechanical properties of layered clay-polymer nanocomposites. The second part presents insights into the fibril network mechanics of agarose hydrogels. The final publication introduces a reliable agarose-based preclinical model, which can be used as a platform for breast cancer drug development and personalized cancer therapy.