## 4 Discussion  
Numerous acute injuries and chronic diseases often result in complex soft tissue defects. Wound healing in these cases is an advanced biochemical process in which numerous processes separated by place and time are involved. Particularly extensive burns or chronic wounds, such as diabetic pressure ulcers, often require special care and adequate wound closure to reduce long-term complications such as infection or amputation. However, current treatments do not yet offer optimal treatment for these cases [17]. Therefore, for the reasons mentioned above, there is an urgent need to support recovery treatments [18,19]. A promising approach to develop supporting treatments is the use of recombinant growth factors. Growth factors enhance processes important for tissue regeneration, including those involved in wound healing. In this study, rhGDF-5, a highly effective human peptide hormone that can promote wound healing [8,10,11,12,13,14,20], was investigated. Interestingly, all clinically approved therapies involving growth factors utilize a carrier made from a certain biomaterial for the successful delivery and targeted application of growth factors [21]. Stability and structural design are important determinants for skin tissue engineering, when constructing new dermal substitutes. In previous in vitro and in vivo studies, we already examined the innovative collagen-gelatin scaffold as a scaffold, and the results demonstrated an accelerated and improved dermal wound repair when applied as a single treatment, but even better when applied as multiple treatments [16,22,23]. Moreover, the manufacturing process of the scaffold allows the integration of bioactive substances  
Figure 4: Histological analysis of the epidermal cell density. The epidermal cell density was histologically evaluated within a section of 100 μm width from the former sore center of the epidermis. All images per sample were taken from three different sections. such as antibiotics and growth factors. Therefore, we developed a composite biomaterial with the scaffold and embedded rhGDF-5 in the carrier matrix. Previous data already showed that the open structure of the scaffold as a three-dimensional matrix enables the fast migration of different cell types (e.g., fibroblasts and keratinocytes) into the scaffold [24]. Deep dermal wounds were treated with the scaffold and rhGDF-5 at different concentrations applied multiple times during the study, which significantly accelerated wound closure, increased the epidermal cell density, and produced a thicker epidermis compared to the treatment with the scaffold itself or untreated controls [25]. When positioned in the wound, the nonwoven fibers come into contact with the wound fluid. Because of fluid absorption, the gelatin begins to swell. Additionally, when enough volume is absorbed, the gelatin dissolves. During this process, rhGDF-5 also becomes hydrated and starts to move in the matrix by diffusion. Through these diffusion processes, it travels to the cells located in the damaged tissue area of the wound. The continuous water uptake from the wound fluid progressively reduces the viscosity of the gelatin and thus also increases the mobility of rhGDF-5, resulting in a faster diffusion. The final phase is reached when the gelatin is completely dissolved and absorbed in the wound. To further investigate improvements in wound treatment with a biomaterial composite in detail, we investigated various wound treatment conditions (applied once and multiple times) with various concentrations of rhGDF-5 and compared it to wounds treated with the collagen-elastin matrix and untreated wounds. All examined skin substitute dressings accelerated dermal wound healing, and removal of all skin substitutes was completed on day 21. The most optimal wound healing was seen for the rhGDF-5 (100, 500, 1000, and 5000 ng rhGDF-5)-treated dermal wounds when applied multiple times.