Bounds (lines) and Monte Carlo estimates (circles) for the elastic modulus (E) of collagen fibrils containing mineral deposits up to the level of mineralization found in bone. The stiffening of collagen fibrils by mineral increases dramatically above a critical mineral concentration called the “percolation threshold” (arrows). This concentration is a function of the shape and distribution of mineral (red: aspect ratio of 1:1; blue: aspect ratio of 2:1). [Adapted with permission from Genin GM, Kent A, Birman V, Wopenka B, Pasteris JD, JP M, Thomopoulos S. Functional grading of mineral and collagen in the attachment of tendon to bone. Biophysical Journal. 2009;97(4).]