Overcoming the brittleness of glass through bio-inspiration and micro-architecture.

Highly mineralized natural materials such as teeth or mollusk shells boast unusual combinations of stiffness, strength and toughness currently unmatched by engineering materials. While high mineral contents provide stiffness and hardness, these materials also contain weaker interfaces with intricate architectures, which can channel propagating cracks into toughening configurations. Here we report the implementation of these features into glass, using a laser engraving technique. Three-dimensional arrays of laser-generated microcracks can deflect and guide larger incoming cracks, following the concept of 'stamp holes'. Jigsaw-like interfaces, infiltrated with polyurethane, furthermore channel cracks into interlocking configurations and pullout mechanisms, significantly enhancing energy dissipation and toughness. Compared with standard glass, which has no microstructure and is brittle, our bio-inspired glass displays built-in mechanisms that make it more deformable and 200 times tougher. This bio-inspired approach, based on carefully architectured interfaces, provides a new pathway to toughening glasses, ceramics or other hard and brittle materials.

Cohesive behavior of soft biological adhesives: experiments and modeling.

Extracellular proteins play a key role in generating and maintaining cohesion and adhesion in biological tissues. These "natural glues" are involved in vital biological processes such as blood clotting, wound healing and maintaining the structural integrity of tissues. Macromolecular assemblies of proteins can be functionally stabilized in a variety of ways in situ that include ionic interactions as well as covalent crosslinking to form protein networks that can extend both within and between tissues. Within tissues, myriad cohesive forces are required to preserve tissue integrity and function, as are additional appropriate adhesive forces at interfaces both within and between tissues of differing composition. While the mechanics of some key structural adhesive proteins have been characterized in tensile experiments at both the macroscopic and single protein levels, the fracture toughness of thin proteinaceous interfaces has never been directly measured. Here, we describe a novel and simple approach to measure the cohesive behavior and toughness of thin layers of proteinaceous adhesives. The test is based on the standard double-cantilever beam test used for engineering adhesives, which was adapted to take into account the high compliance of the interface compared with the beams. This new "rigid double-cantilever beam" method enables stable crack propagation through an interfacial protein layer, and provides a direct way to measure its full traction-separation curve. The method does not require any assumption of the shape of the cohesive law, and the results provide abundant information contributing to understanding the structural, chemical and molecular mechanisms acting in biological adhesion. As an example, results are presented using this method for thin films of fibrin-a protein involved in blood clotting and used clinically as a tissue bio-adhesive after surgery-with the effects of calcium and crosslinking by Factor XIII being examined. Finally, a simple model is proposed, demonstrating how a bell-shaped cohesive law forms during the failure of the fibrin interface based on an eight-chain model whose structure degrades and changes configuration with stress.