Figure 4 - Diagram of the different applications of skin engineered substitutes. Adapted from (Sarkiri et al., 2019).
3D skin equivalents have been described in the literature such as a model composed by fibroblasts and keratinocytes grafted in a viscose rayon support, which was created to test potential skin irritants (Canton et al., 2010). Other model comprises the use of sucrose co-polymers and fibroblasts, thus leading to the formation of a macromolecular assembling which potentiate collagen deposition (Au - Benny et al., 2016). Uchino and co-workers developed a 3D human skin model containing vitrified collagen that supported the culture of dendritic cells, keratinocytes and fibroblasts in a layered construct (Uchino et al., 2009). Other three-layered constructs featuring a hypodermis-like layer have been reported as full thickness in vitro models of human skin (Trottier et al., 2008, Monfort et al., 2013).
Some reconstructed skin models are produced in the laboratory, for particular research purposes, however other models are already commercially available. Amongst these, there are different classes of systems, namely RHEs (eg, EpiSkin®, SkinEthic®, and EpiDerm®) and LSEs (eg, GraftSkin®, EpiDermFT®, and Pheninon®) models (reviewed in (Abd et al., 2016, Yun et al., 2018). Some of these models have been validated according to European Union (EU) guidelines and implemented into the EU and Organisation for Economic Cooperation and Development (OECD) guidelines for testing dangerous ingredients for the skin (Kandarova et al., 2004, OECD, 2015, Fentem, 1999, Fentem and Botham, 2004, Worth et al., 1998).
SkinEthic®, EpiDerm® and EpiSkin® are probably the most used models and their use was approved by the European Union Reference Laboratory for alternatives to animal testing (EURL – ECVAM) (OECD, 2011). The SkinEthic® and EpiDerm® consist of epidermal keratinocytes cultured on polycarbonate membrane whereas EpiSkin® is composed by stratified human keratinocytes cultured on collagen-based matrix (Yun et al., 2018).
Schäfer-Korting and co-workers have extensively reported a comparison of the permeation of several hydrophilic and lipophilic compounds in human epidermal membranes, porcine skin and three RHE models (SkinEthic®, EpiDerm® and EpiSkin®). The results pointed out that the RHE models, mostly SkinEthic®, were significantly more permeable than human epidermis and pig skin, however the permeation of the compounds through pig skin and the RHEs is similar to those obtained in human epidermis. Interestingly, they did not observe the expected improvement in reproducibility with the RHEs compared to the ex vivo skin (Schafer-Korting et al., 2008).
Other 3D models were designed and commercialized to mimic the SC , like Strat-M®, however, this model is not a cell-based system since it is absent of cells. Strat-M® is a synthetic membrane comprising multiple layers of different types of materials, as porous polyether sulfone and polyolefin, enclosed by a combination of lipids (ceramides, cholesterol, free fatty acids) and other components. Strat-M® was used to evaluate the permeation efficiency of hydrophilic molecules and to study the mechanism of passive transport (Uchida et al., 2015, Haq et al., 2018). Even though this model lacks the capacity to mimic the complex architecture of full human skin, it represents a valuable alternative since its composition closely resemble that of the SC layer and the results obtained using Strat-M® are highly reproducible due to the simple nature and standardized construction of this model (Yun et al., 2018).
These commercially available skin models have been used for several purposes, namely for the evaluation of the permeability of drug as well for irritation and toxicological studies (Alépée et al., 2015, Alépée et al., 2014, Kandarova et al., 2004, Kandarova et al., 2005, Kandarova et al., 2006). In the literature, several studies have compared LSE and HRE systems with animal and human skin models and the results pointed out their applicability as skin mimetic systems for the evaluation of skin absorption, testing of cosmetic formulations and for toxicological studies (van Ravenzwaay and Leibold, 2004, Schafer-Korting et al., 2008, Schreiber et al., 2005). Despite the fact that cell-based models can simulate better the human skin, due to the presence of human cell in their composition, these models present some disadvantages, namely due to the lack of skin appendages, their high cost of production and the extremely short shelf time (Flaten et al., 2015, Netzlaff et al., 2005).
A summary of the main advantages and disadvantages of the in vitro models reviewed in the last subsection is reported in Table 3.