Tough, graded interfaces are a broadly conserved feature of biological attachment of two dissimilar materials, but the mechanisms by which toughening occurs are not fully known. In biological systems, graded structures are commonly found in the attachments of materials with very different properties such as bone, which is very stiff, and tendon, which is compliant 1. Previous research has shown that the interface between these two materials is composed mostly of collagen, proteoglycans, and carbonated apatite crystals (mineral). Our working hypothesis is that gradients in mineral content and collagen orientation combine to provide a compliant energy absorbing region between the tendon and bone 1.

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