A foil structure 1 for covering graft material 54 in a bone graft site in a bone augmentation process comprises a perforated thin metal sheet 2, such as anodised titanium, provided with a coating layer 5 of a biocompatible polymer, preferably parylene, forming a non-adherent cell surface. The perforation through-holes have a diameter of 1 micrometer or less, allowing nutrients to pass but hindering the passage of pathogens. The coating 5 may comprise an active coating for release of a substance over time.
