Wound Models Based on ex vivo Human Skin for the Screening of Wound Healing Biomaterials

IntroductionWound healing is a complex process and the development of appropriate dressings for a given wound type requires careful consideration. The majority of dressings available on the market are designed to fulfil two fundamental functions: physical protection and exudate absorption. Nevertheless, there is still a demand for new and affordable materials with inherent or added properties directed at stimulating, improving, or speeding up the wound-healing process, especially in the case of complex injuries, which remains a challenge for clinicians. Insufficient blood perfusion, resulting in low nutrient and oxygen levels leads to delays in cell migration and proliferation, infections, chronic inflammation, and an oxidative and proteolytic environment. Significant advancements in biomaterial science have facilitated the development of biomaterials that exhibit antibacterial, anti-inflammatory, and regenerative properties. Nevertheless, to test these innovative new dressing materials, models are necessary that reproduce the key characteristics of complex wounds and incorporate the specific target of the biomaterial under investigation.Materials and MethodsWe used ex vivo human models in which wounds are generated on skin explants and infected [1] or cultured over a 12-day period at the air-liquid interface to observe the wound healing process [2]. To evaluate the functionality of these models, a range of biomaterials in the form of hydrogels, nanoparticles, and dressings have been examined [3,4]. In particular, we have tested highly engineered biodegradable and biocompatible nanoparticles (NPs) fabricated using the layer-by-layer technique. In the context of wound infection models, bacteria are extracted from treated tissue samples and quantified by the colony forming unit assay. In the models used to study wound healing, cell viability is determined by means of a lactate dehydrogenase (LDH) assay. The levels of inflammatory cytokines are measured by (ELISA) while re-epithelialisation and re-vascularisation are monitored by immunohistochemistry.ResultsEx vivo human wound models has proven to be a useful tool for assessing the efficacy of antimicrobial-loaded dressings, as well as their local toxicity. The wound healing model enabled the screening of different types of biocompatible layer-by-layer NPs, their effects on re-epithelialization, revascularization and inflammatory processes. The moderate increase of the inflammatory cytokines IL-6 and IL-8 confirmed the biocompatibility of these materials. In particular, NPs coated with hyaluronic acid exhibited substantial enhancement in re-epithelialization and angiogenesis. DiscussionHuman full-thickness skin is a three-dimensional scaffold with a complex environment that includes extracellular enzymatic activity, inflammatory mediators, and several cell populations such as keratinocytes, melanocytes, and Langerhans cells in the epidermis as well as fibroblasts, mast cells, dendritic cells, endothelial cells, and lymphocytes in the dermis. The absence of a vascular system represents a significant limitation when compared with in vivo tests. Nonetheless, ex vivo models partially mimic the local environment of chronic wounds, which are also characterised by poor blood perfusion. ConclusionsEx vivo wound models represent a valuable tool in addition to animal testing for the preclinical screening of active biomaterials, especially when studying solutions designed to improve the local wound environment.