Inclusion Criteria:  Age 18-65 years, systemically healthy, healthy periodontal conditions at the experimental region or conditions not interfering with proper biopsy harvesting, no known sensitivity to HY, no uncontrolled diabetes mellitus, no radiotherapy or chemotherapy, no current pregnancy or breastfeeding, no hormonal diseases, no infectious diseases, no autoimmune diseases or under immunotherapy, no systemic bone disease, no wound healing disorders or acute inflammatory/infectious processes, no systemic diseases with implications on mucous membranes (e.g., Morbus Crohn), no increased risk of bleeding (i.e., patient with coagulation disorders or under anticoagulant therapy, and no heavy smokers (i.e., no regular tobacco consumption > 10 cigarettes/day).  Exclusion Criteria:  not meeting the inclusion criteria.