junction proteins.33,34 A common AD drug was evaluated together with JAK inhibitors by using 3D bioprinted tissue from a recent study.35 They demonstrated that AD-related drugs have great efficacy to restore barrier function such as increased TEER and increased expression of filaggrin and Claudin-1. The proinflammatory cytokines and chemokines that present in AD patients are reduced significantly in the tissue culture system within the JAK inhibitor treated tissue. This again emphasized that skin barrier function can be a secondary event in chronic skin inflammation.17
Ex Vivo Models
Ex vivo human skin models are used to not only alleviate ethical constraints of in vivo studies, but to provide a more comprehensive and accurate representation of the skin’s natural response to a variety of stimuli, including inflammation, wound closure, anti-aging effects, or skin barrier function. Tissue for such models is collected typically during abdominoplasty or other reductive surgeries with donor consent and can be cultured for 7–10 days. Compared to in situ and reconstructed human epidermis models, these tissue explants have an intact physical and biochemical barrier. Culture conditions have been demonstrated to not only affect regenerated barrier integrity, but also proliferative activity.36,37 This not only allows for components of experimentally damaged barriers to be evaluated but full formulations as well. Skin explant barriers can be disrupted by both physical, via stripping, or chemical, such as SLS, methods. SLS treatment has been shown to increase trans-epidermal water loss.37
Skin-stripping is a minimal invasive method, used both in vitro and in vivo, which removes the SC cell layers using glue or adhesive films. The condition and area of the skin being stripped can influence the thickness of SC, the number and nature of corneocytes, as well as the composition and levels of SC lipids being removed.38–40 These factors are important in order for the successful homogeneity and uniform removal of the SC. In an ex vivo model, regenerated SC reached an identical thickness as native skin in addition to expressing terminal differentiation proteins following 8 days in culture.36 Regenerated SC in this model also demonstrated a shift towards hexagonal lateral lipid organization and an increase in ceramide quantity.36 In efforts to elucidate the relationship between regenerated stratum corneum post-tape stripping in both an in vivo and ex vivo model, one study analyzed shifts in the ceramide profile. Ceramides were quantified using liquid chromatography combined with mass spectrometry. It was found that both ceramide composition and lipid organization was identical in both models.41
Barrier repairing formulations can also be evaluated through the use of an impaired ex vivo skin model. One study tape stripped skin explants and applied a ceramide-containing ceramide to determine if the lipid barrier was restored.42 Lipid organization was studied using FTIR and small angle x-ray diffraction. The application of the formulation containing a single ceramide resulted in a shift to a more dense orthorhombic packing and no change to the lamellar organization.42 Application of a fatty acid-containing formula on stripped skin also increase the fraction of lipids forming a dense orthorhombic packing.43
Although this model is not high-throughput, learnings are more clinically relevant. Mechanical means of inducing barrier damage are well established, but information on other extrinsic factors, such as the relationship pollution or UV damage with barrier function, is limited. The relation between extrinsic factors and cosmetic formulas have yet to be elucidated as well.
Clinical Models
Mechanical Models
Skin Stripping
Under clinical settings, skin-stripping is often used to assess skin lipid and protein compositions and levels. For instance, super glue skin-strip was used to evaluate the differences in SC lipid compositions in healthy individuals from three different ethnic groups, revealing that African subjects had the lowest CER and CHOL ratio compared to Asian, who were similar to Danish subjects.44 Another study with subjects of skin phototype IV–V demonstrated to have a faster barrier recovery, enhanced SC integrity, and increase epidermal lipid content and lamellar body density compared to phototype I–II.45
Skin-stripping is also widely used to determine the distribution of barrier creams applied topically, along with sunscreen filter's protective efficacy, by measuring and optimizing sun absorption spectra and distribution homogeneity onto the skin.
Skin-stripping is an effective and basic method to study skin barrier integrity, skin lipid composition, in addition to penetration depth of various formulations after topical application. Moreover, the removed SC layers can further be examined by other methods, including histochemical, genetic, lipodomic, and proteomic means.
Suction Blister
Blister-induction model is another invasive technique used to clinically study skin barrier integrity. The most popular method is suction-blister, which involves the use of pumps to induce blisters that apply a constant negative pressure onto the skin, leading to the separation of the epidermis from the dermis and causing the formation of a blister.47 Once fully shaped and filled with interstitial or tissue fluid, the blister is excised, revealing an epidermal wound.47
In clinical settings, suction blister is used to examine wound healing pathways in relation to skin barrier. It was shown that
Ex Vivo Models
Ex vivo human skin models are used to not only alleviate ethical constraints of in vivo studies, but to provide a more comprehensive and accurate representation of the skin’s natural response to a variety of stimuli, including inflammation, wound closure, anti-aging effects, or skin barrier function. Tissue for such models is collected typically during abdominoplasty or other reductive surgeries with donor consent and can be cultured for 7–10 days. Compared to in situ and reconstructed human epidermis models, these tissue explants have an intact physical and biochemical barrier. Culture conditions have been demonstrated to not only affect regenerated barrier integrity, but also proliferative activity.36,37 This not only allows for components of experimentally damaged barriers to be evaluated but full formulations as well. Skin explant barriers can be disrupted by both physical, via stripping, or chemical, such as SLS, methods. SLS treatment has been shown to increase trans-epidermal water loss.37
Skin-stripping is a minimal invasive method, used both in vitro and in vivo, which removes the SC cell layers using glue or adhesive films. The condition and area of the skin being stripped can influence the thickness of SC, the number and nature of corneocytes, as well as the composition and levels of SC lipids being removed.38–40 These factors are important in order for the successful homogeneity and uniform removal of the SC. In an ex vivo model, regenerated SC reached an identical thickness as native skin in addition to expressing terminal differentiation proteins following 8 days in culture.36 Regenerated SC in this model also demonstrated a shift towards hexagonal lateral lipid organization and an increase in ceramide quantity.36 In efforts to elucidate the relationship between regenerated stratum corneum post-tape stripping in both an in vivo and ex vivo model, one study analyzed shifts in the ceramide profile. Ceramides were quantified using liquid chromatography combined with mass spectrometry. It was found that both ceramide composition and lipid organization was identical in both models.41
Barrier repairing formulations can also be evaluated through the use of an impaired ex vivo skin model. One study tape stripped skin explants and applied a ceramide-containing ceramide to determine if the lipid barrier was restored.42 Lipid organization was studied using FTIR and small angle x-ray diffraction. The application of the formulation containing a single ceramide resulted in a shift to a more dense orthorhombic packing and no change to the lamellar organization.42 Application of a fatty acid-containing formula on stripped skin also increase the fraction of lipids forming a dense orthorhombic packing.43
Although this model is not high-throughput, learnings are more clinically relevant. Mechanical means of inducing barrier damage are well established, but information on other extrinsic factors, such as the relationship pollution or UV damage with barrier function, is limited. The relation between extrinsic factors and cosmetic formulas have yet to be elucidated as well.
Clinical Models
Mechanical Models
Skin Stripping
Under clinical settings, skin-stripping is often used to assess skin lipid and protein compositions and levels. For instance, super glue skin-strip was used to evaluate the differences in SC lipid compositions in healthy individuals from three different ethnic groups, revealing that African subjects had the lowest CER and CHOL ratio compared to Asian, who were similar to Danish subjects.44 Another study with subjects of skin phototype IV–V demonstrated to have a faster barrier recovery, enhanced SC integrity, and increase epidermal lipid content and lamellar body density compared to phototype I–II.45
Skin-stripping is also widely used to determine the distribution of barrier creams applied topically, along with sunscreen filter's protective efficacy, by measuring and optimizing sun absorption spectra and distribution homogeneity onto the skin.
Skin-stripping is an effective and basic method to study skin barrier integrity, skin lipid composition, in addition to penetration depth of various formulations after topical application. Moreover, the removed SC layers can further be examined by other methods, including histochemical, genetic, lipodomic, and proteomic means.
Suction Blister
Blister-induction model is another invasive technique used to clinically study skin barrier integrity. The most popular method is suction-blister, which involves the use of pumps to induce blisters that apply a constant negative pressure onto the skin, leading to the separation of the epidermis from the dermis and causing the formation of a blister.47 Once fully shaped and filled with interstitial or tissue fluid, the blister is excised, revealing an epidermal wound.47
In clinical settings, suction blister is used to examine wound healing pathways in relation to skin barrier. It was shown that
