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 .