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.