Origin of transparency in scattering biomimetic collagen materials

Significance Disordered materials with heterogeneities at scales ranging from nano- to micrometers usually scatter visible light and become opaque beyond a certain thickness. However, transparent materials are found in nature. The physical origin of transparency in natural and artificial biopolymer materials is still not fully understood. Here, a gap of transparency is found in synthetic fibrillar collagen matrices within a very narrow range of concentration. The transparency results from structural partial order inhibiting light scattering, while preserving mechanical stability, stiffness, and nonlinearity. The potential to produce biomimetic hydrogels with such striking optical properties by self-assembly opens an interesting route in the study of photonic materials and provides new elements in the understanding of the appearance of transparency in natural materials.