<qa>  What is a new therapy option for chronic wounds with yet unproven therapeutic efficPlatelet rich fibrin PRF is a new therapy option for chronic wounds with yet unproven therapeutic efficacy. This randomised controlled trial aims to provide evidence of the efficacy of PRF as monotherapy as well as a growth promoting carrier matrix for antimicrobial compounds. The investigators therefore designed a four armed trial with three PRF arms which are compared to each other as well as to an active comparator. The treatment arms are as follows: 