due to the strong influence that silk polymorphism has on the permeability to gases like CO2 and O2. The increase in beta-sheet content, in fact, yields a less hydrated material (decrease in free water and freezing bound water) where the molecules are more packed and organized in nanoscopic beta-sheet structures26,37. O2 and CO2 are not soluble in silk proteins, while they are soluble in water. It is then possible that the dissolution of gas decreases for silk materials that have a decreased amount of water (i.e. higher beta-sheet content) in their structure, resulting in a decreased diffusion of water-soluble gases through silk membranes.