Abstract: A network layer is provided that contains at least one thermoplastic biodegradable substance. The network layer contains at least one colored first material in a first direction, the first colored material exhibiting a first absorption coefficient, and at least one colored second material in a second direction, the second colored material exhibiting a second absorption coefficient. The colored first material and the colored second material are in contact with each other, a coloration of the first material is different from a coloration of the second material, and the second absorption coefficient is different from the first absorption coefficient.