Targeting the Complexity of In Vitro Skin Models: A Review of Cutting-Edge Developments

Skin in vitro models offer much promise for research, testing drugs, cosmetics, and medical devices, reducing animal testing and extensive clinical trials. There are several in vitro approaches to mimicking human skin behavior, ranging from simple cell monolayer to complex organotypic and bioengineered 3-dimensional models. Some have been approved for preclinical studies in cosmetics, pharmaceuticals, and chemicals. However, development of physiologically reliable in vitro human skin models remains in its infancy. This review reports on advances in in vitro complex skin models to study skin homeostasis, aging, and skin disease. Skin in vitro models offer much promise for research, testing drugs, cosmetics, and medical devices, reducing animal testing and extensive clinical trials. There are several in vitro approaches to mimicking human skin behavior, ranging from simple cell monolayer to complex organotypic and bioengineered 3-dimensional models. Some have been approved for preclinical studies in cosmetics, pharmaceuticals, and chemicals. However, development of physiologically reliable in vitro human skin models remains in its infancy. This review reports on advances in in vitro complex skin models to study skin homeostasis, aging, and skin disease. The generation of in vitro skin models that accurately reproduce all skin layers (epidermis, dermis, and subcutis), including its appendages such as sweat glands (SGs), hair follicles, or arrector pili muscle, represents a significant hurdle in the field of skin tissue engineering. However, we are still far from mimicking the full cellular and structural complexity of living human skin (Antoni et al., 2015Antoni D. Burckel H. Josset E. Noel G. Three-dimensional cell culture: a breakthrough in vivo.Int J Mol Sci. 2015; 16: 5517-5527Crossref PubMed Scopus (699) Google Scholar; Sanabria-de la Torre et al., 2020Sanabria-de la Torre R. Fernández-González A. Quiñones-Vico M.I. Montero-Vilchez T. Arias-Santiago S. Bioengineered skin intended as in vitro model for Pharmacosmetics, skin disease study and environmental skin impact analysis.Biomedicines. 2020; 8: 464Crossref PubMed Scopus (17) Google Scholar; Sutterby et al., 2022Sutterby E. Thurgood P. Baratchi S. Khoshmanesh K. Pirogova E. Evaluation of in vitro human skin models for studying effects of external stressors and stimuli and developing treatment modalities.View. 2022; 320210012Crossref Scopus (18) Google Scholar). The demand for innovative and effective 3-dimensional (3D) engineered skin tissue equivalents in vitro has increased not only for improving clinical purposes such as skin grafts but also for essential (patho)physiological investigations, such as to unravel the underlying mechanisms of skin disorders as well as assessing the effectiveness and potential toxicity of active substances in cosmetic and pharmaceutical research (Olejnik et al., 2022Olejnik A. Semba J.A. Kulpa A. Dańczak-Pazdrowska A. Rybka J.D. Gornowicz-Porowska J. 3D bioprinting in skin related research: recent achievements and application perspectives.ACS Synth Biol. 2022; 11: 26-38Crossref PubMed Scopus (40) Google Scholar; Salameh et al., 2021Salameh S. Tissot N. Cache K. Lima J. Suzuki I. Marinho P.A. et al.A perfusable vascularized full-thickness skin model for potential topical and systemic applications.Biofabrication. 2021; 13035042Crossref PubMed Scopus (30) Google Scholar; Sanabria-de la Torre et al., 2020Sanabria-de la Torre R. Fernández-González A. Quiñones-Vico M.I. Montero-Vilchez T. Arias-Santiago S. Bioengineered skin intended as in vitro model for Pharmacosmetics, skin disease study and environmental skin impact analysis.Biomedicines. 2020; 8: 464Crossref PubMed Scopus (17) Google Scholar). Current in vitro skin models aim to replicate critical skin characteristics, such as barrier function, elasticity, immune response, and blood flow, but technical and biological issues limit a true and functional model (Klicks et al., 2017Klicks J. von Molitor E. Ertongur-Fauth T. Rudolf R. Hafner M. In vitro skin three-dimensional models and their applications.J Cell Biotechnol. 2017; 3: 21-39Crossref Google Scholar). The evolution of skin substitutes paralleled technological advances and resulted in skin models with varying degrees of structural and functional complexity. Although their evolution is not linear, the main drive of innovation lies in improving physiologic relevance (Figure 1). The simplest approach to skin models is 2-dimensional (2D) culture of primary human keratinocytes and fibroblasts. Although such models do not accurately replicate cell–cell or cell–matrix communication and structural organization of the skin, they are very useful for drug screening; for cytotoxicity assays; and to study molecular mechanisms in homeostasis, aging, or diseases such as cancer (Duval et al., 2017Duval K. Grover H. Han L.H. Mou Y. Pegoraro A.F. Fredberg J. et al.Modeling physiological events in 2D vs. 3D cell culture.Physiology (Bethesda). 2017; 32: 266-277Crossref PubMed Scopus (1158) Google Scholar; Hofmann et al., 2023Hofmann E. Fink J. Pignet A.L. Schwarz A. Schellnegger M. Nischwitz S.P. et al.Human in vitro skin models for wound healing and wound healing disorders.Biomedicines. 2023; 11: 1056Crossref PubMed Scopus (17) Google Scholar; Moon et al., 2021Moon S. Kim D.H. Shin J.U. In vitro models mimicking immune response in the skin.Yonsei Med J. 2021; 62: 969-980Crossref PubMed Scopus (0) Google Scholar; Stanton et al., 2022Stanton D.N. Ganguli-Indra G. Indra A.K. Karande P. Bioengineered efficacy models of skin disease: advances in the last 10 years.Pharmaceutics. 2022; 14: 319Crossref PubMed Scopus (5) Google Scholar; Sutterby et al., 2022Sutterby E. Thurgood P. Baratchi S. Khoshmanesh K. Pirogova E. Evaluation of in vitro human skin models for studying effects of external stressors and stimuli and developing treatment modalities.View. 2022; 320210012Crossref Scopus (18) Google Scholar). Incorporation of extracellular matrix (ECM) components into 2D cell models provided cells with important cues of native 3D environment, promoting more representative cell growth, proliferation, and function and resembling some conditions in vivo (Antoni et al., 2015Antoni D. Burckel H. Josset E. Noel G. Three-dimensional cell culture: a breakthrough in vivo.Int J Mol Sci. 2015; 16: 5517-5527Crossref PubMed Scopus (699) Google Scholar; Langhans, 2018Langhans S.A. Three-dimensional in vitro cell culture models in drug discovery and drug repositioning.Front Pharmacol. 2018; 9: 6Crossref PubMed Scopus (1000) Google Scholar; Sarkiri et al., 2019Sarkiri M. Fox S.C. Fratila-Apachitei L.E. Zadpoor A.A. Bioengineered skin intended for skin disease modeling.Int J Mol Sci. 2019; 20: 1407Crossref PubMed Scopus (24) Google Scholar; Sutterby et al., 2022Sutterby E. Thurgood P. Baratchi S. Khoshmanesh K. Pirogova E. Evaluation of in vitro human skin models for studying effects of external stressors and stimuli and developing treatment modalities.View. 2022; 320210012Crossref Scopus (18) Google Scholar; Valdoz et al., 2021Valdoz J.C. Johnson B.C. Jacobs D.J. Franks N.A. Dodson E.L. Sanders C. et al.The ECM: to scaffold, or not to scaffold, that is the question.Int J Mol Sci. 2021; 2212690Crossref PubMed Scopus (60) Google Scholar). The evolution of more complex in vitro skin systems also includes organoids (Figure 1), a simplified 3D culture system in which the cells grow in a 3D-well chemically defined microenvironment—composed of ECM and media—to form self-organized clusters of cells that differentiate into distinct cell types that mimic the structure and function of the organ (Corrò et al., 2020Corrò C. Novellasdemunt L. Li V.S.W. A brief history of organoids.Am J Physiol Cell Physiol. 2020; 319: C151-C165Crossref PubMed Scopus (0) Google Scholar; Hofer and Lutolf, 2021Hofer M. Lutolf M.P. Engineering organoids.Nat Rev Mater. 2021; 6: 402-420Crossref PubMed Scopus (524) Google Scholar; Kim et al., 2020bKim J. Koo B.K. Knoblich J.A. Human organoids: model systems for human biology and medicine.Nat Rev Mol Cell Biol. 2020; 21: 571-584Crossref PubMed Scopus (0) Google Scholar). Whereas organoids recapitulate more complex 3D in vitro systems for different internal organs such as gut (Taelman et al., 2022Taelman J. Diaz M. Guiu J. Human intestinal organoids: promise and challenge.Front Cell Dev Biol. 2022; 10854740Crossref PubMed Scopus (31) Google Scholar), lung (Matkovic Leko et al., 2023Matkovic Leko I. Schneider R.T. Thimraj T.A. Schrode N. Beitler D. Liu H.Y. et al.A distal lung organoid model to study interstitial lung disease, viral infection and human lung development.Nat Protoc. 2023; 18: 2283-2312Crossref PubMed Scopus (0) Google Scholar), brain (Eichmüller and Knoblich, 2022Eichmüller O.L. Knoblich J.A. Human cerebral organoids - a new tool for clinical neurology research.Nat Rev Neurol. 2022; 18: 661-680Crossref PubMed Scopus (21) Google Scholar), bladder (Minoli et al., 2023Minoli M. Cantore T. Hanhart D. Kiener M. Fedrizzi T. La Manna F. et al.Bladder cancer organoids as a functional system to model different disease stages and therapy response.Nat Commun. 2023; 14: 2214Crossref PubMed Scopus (17) Google Scholar), or tumors (Xu et al., 2022Xu H. Jiao D. Liu A. Wu K. Tumor organoids: applications in cancer modeling and potentials in precision medicine.J Hematol Oncol. 2022; 15: 58Crossref PubMed Scopus (69) Google Scholar), the traditional and simplest in vitro 3D skin models consist of 2 dermo–epidermal layers formed by fibroblasts embedded in an ECM gel with keratinocytes cultured on the top of the gel, generated on an insert's porous membrane to allow epidermal keratinization (Gangatirkar et al., 2007Gangatirkar P. Paquet-Fifield S. Li A. Rossi R. Kaur P. Establishment of 3D organotypic cultures using human neonatal epidermal cells.Nat Protoc. 2007; 2: 178-186Crossref PubMed Scopus (124) Google Scholar). These 3D human skin equivalents (HSEs) are well-established and are available on the market for testing purposes (Hayden et al., 2003Hayden P.J. Ayehunie S. Jackson G.R. Kupfer-Lamore S. Last T.J. et al.In vitro skin equivalent models for toxicity testing.in: Alternative toxicological methods. CRC Press, Boca Raton2003: 229-248Google Scholar; Suhail et al., 2019Suhail S. Sardashti N. Jaiswal D. Rudraiah S. Misra M. Kumbar S.G. Engineered skin tissue equivalents for product evaluation and therapeutic applications.Biotechnol J. 2019; 14e1900022Crossref PubMed Scopus (46) Google Scholar). Presently, there are a range of commercially available HSEs offering a source of full-thickness models for drug screening, toxicity tests, or skin sensitivity tests (Table 1). Several patients, animal-based, and ex vivo assays have been used to identify potentially irritant chemicals but are not compatible with high-throughput drug screening testing required by industry. This, along with the legislative move toward nonanimal testing (European Regulation 1223/2009 and United States Federal Food, Drug, and Cosmetic Act, 2022), has prompted the development of commercial in vitro assays for regulatory toxicology, with several validated epithelial-only in vitro methods for skin corrosion and irritation adopted by the Organisation for Economic Co-operation and Development (Test Guidelines 431 and 439, respectively) (Kandárová et al., 2006Kandárová H. Liebsch M. Schmidt E. Genschow E. Traue D. Spielmann H. et al.Assessment of the skin irritation potential of chemicals by using the SkinEthic reconstructed human epidermal model and the common skin irritation protocol evaluated in the ECVAM skin irritation validation study.Altern Lab Anim. 2006; 34: 393-406Crossref PubMed Scopus (52) Google Scholar; Spielmann et al., 2007Spielmann H. Hoffmann S. Liebsch M. Botham P. Fentem J.H. Eskes C. et al.The ECVAM international validation study on in vitro tests for acute skin irritation: report on the validity of the Episkin and EpiDerm assays and on the Skin Integrity Function Test.Altern Lab Anim. 2007; 35: 559-601Crossref PubMed Scopus (184) Google Scholar). These tests, which are based on metabolic activity, provide an indirect measure of tissue viability, with a 50% threshold set for acceptable chemical safety levels. More sophisticated 2D approaches (KeratinoSens [Andreas et al., 2011Andreas N. Caroline B. Leslie F. Frank G. Kimberly N. Allison H. et al.The intra- and inter-laboratory reproducibility and predictivity of the KeratinoSens assay to predict skin sensitizers in vitro: results of a ring-study in five laboratories.Toxicol In Vitro. 2011; 25: 733-744Crossref PubMed Scopus (98) Google Scholar; Emter et al., 2010Emter R. Ellis G. Natsch A. Performance of a novel keratinocyte-based reporter cell line to screen skin sensitizers in vitro.Toxicol Appl Pharmacol. 2010; 245: 281-290Crossref PubMed Scopus (269) Google Scholar] and LuSens assay [Ramirez et al., 2014Ramirez T. Mehling A. Kolle S.N. Wruck C.J. Teubner W. Eltze T. et al.LuSens: a keratinocyte based ARE reporter gene assay for use in integrated testing strategies for skin sensitization hazard identification.Toxicol In Vitro. 2014; 28: 1482-1497Crossref PubMed Scopus (104) Google Scholar]) utilize a luciferase reporter assay on the basis of the chemically induced activation of the transcription factor, nuclear factor erythroid 2. However, these tests do not replicate the skin architecture and permeability barrier and have yet to be translated into 3D systems. A number of studies have used HSEs to identify altered gene expression profiles to chemicals and known irritants, with the SENS-IS assay being currently the most developed and validated (Cottrez et al., 2015Cottrez F. Boitel E. Auriault C. Aeby P. Groux H. Genes specifically modulated in sensitized skins allow the detection of sensitizers in a reconstructed human skin model. Development of the SENS-IS assay.Toxicol In Vitro. 2015; 29: 787-802Crossref PubMed Scopus (80) Google Scholar; Harding et al., 2021Harding A.L. Murdoch C. Danby S. Hasan M.Z. Nakanishi H. Furuno T. et al.Determination of chemical irritation potential using a defined gene signature set on tissue-engineered human skin equivalents.JID Innov. 2021; 1100011Abstract Full Text Full Text PDF PubMed Scopus (3) Google Scholar; Hasan et al., 2019Hasan M.Z. Kitamura M. Kawai M. Ohira M. Mori K. Shoju S. et al.Transcriptional profiling of lactic acid treated reconstructed human epidermis reveals pathways underlying stinging and itch.Toxicol In Vitro. 2019; 57: 164-173Crossref PubMed Scopus (13) Google Scholar; Saito et al., 2017Saito K. Takenouchi O. Nukada Y. Miyazawa M. Sakaguchi H. An in vitro skin sensitization assay termed EpiSensA for broad sets of chemicals including lipophilic chemicals and pre/pro-haptens.Toxicol In Vitro. 2017; 40: 11-25Crossref PubMed Scopus (33) Google Scholar). More recently, Harding et al., 2023Harding A.L. Colley H.E. Vazquez I.B. Danby S. Hasan M.Z. Nakanishi H. et al.c-Src activation as a potential marker of chemical-induced skin irritation using tissue-engineered skin equivalents.Exp Dermatol. 2023; 32: 220-225Crossref PubMed Scopus (1) Google Scholar analyzed the immediate up-stream signaling cascades upon treatment with chemical irritants to identify activation of specific kinases that could be tested in more rapid assay formats. Despite the evident progress in the complexity of HSEs, a critical constraint is the lack of a neural compartment, isolating the response of specific cell types to the different stimulus rather than providing an integrated and orchestrated feedback of the whole system (Basso et al., 2019Basso L. Serhan N. Tauber M. Gaudenzio N. Peripheral neurons: master regulators of skin and mucosal immune response.Eur J Immunol. 2019; 49: 1984-1997Crossref PubMed Scopus (0) Google Scholar; Cohen et al., 2020Cohen J.A. Wu J. Kaplan D.H. Neuronal regulation of cutaneous immunity.J Immunol. 2020; 204: 264-270Crossref PubMed Scopus (27) Google Scholar; Trier et al., 2019Trier A.M. Mack M.R. Kim B.S. The neuroimmune axis in skin sensation, inflammation, and immunity.J Immunol. 2019; 202: 2829-2835Crossref PubMed Scopus (36) Google Scholar).Table 1Commercially Available Full-Thickness HSECompanySystemRegional AvailabilityReferenceEpiSkin, Lyon, FranceT-SkinGlobalBataillon et al., 2019Bataillon M. Lelièvre D. Chapuis A. Thillou F. Autourde J.B. Durand S. et al.Characterization of a new reconstructed full thickness skin model, T-SkinTM, and its application for investigations of anti-aging compounds.Int J Mol Sci. 2019; 20: 2240Crossref PubMed Scopus (33) Google Scholar; Luu-The et al., 2009Luu-The V. Duche D. Ferraris C. Meunier J.R. Leclaire J. Labrie F. Expression profiles of phases 1 and 2 metabolizing enzymes in human skin and the reconstructed skin models Episkin and full thickness model from Episkin.J Steroid Biochem Mol Biol. 2009; 116: 178-186Crossref PubMed Scopus (75) Google ScholarMatTek, Ashland, MAEpiDermFTGlobalMallampati et al., 2010Mallampati R. Patlolla R.R. Agarwal S. Babu R.J. Hayden P. Klausner M. et al.Evaluation of EpiDerm full thickness-300 (EFT-300) as an in vitro model for skin irritation: studies on aliphatic hydrocarbons.Toxicol In Vitro. 2010; 24: 669-676Crossref PubMed Scopus (0) Google ScholarPhenion, Henkel AG KGaA, Düsseldorf, GermanyPhenion FTEurope (and wider1More specifically, supply to places reachable by courier service within 72 hours.)Pinto et al., 2022Pinto D. Trink A. Giuliani G. Rinaldi F. Protective effects of sunscreen (50+) and octatrienoic acid 0.1% in actinic keratosis and UV damages.J Investig Med. 2022; 70: 92-98Crossref PubMed Scopus (5) Google ScholarSterlab, Vallauris, FranceFull-thickness skin——Abbreviation: HSE, human skin equivalent.1 More specifically, supply to places reachable by courier service within 72 hours. Open table in a new tab Abbreviation: HSE, human skin equivalent. In academia, persisting efforts are made to sophisticate in vitro HSEs to increase experimental throughput and tissue complexity, while improving biological and methodological aspects to faithfully mimic human skin and skin diseases. Here, large collaborative networks are formed, similar to the European Network for Skin Engineering and Modeling (NETSKINMODELS) EU COST Action (CA21108, 2022-2026) in which over 330 participants from 40 countries from both academia and industry are united. With this review, members of NETSKINMODELS now aim to set the stage on current developments within the field to identify knowledge and technology gaps and to provide guidelines and resources in later stages to fuel the development and implementation of in vitro skin models in both academic and industrial research. Although useful for some cosmetic tests, the earlier-mentioned in vitro 2D and simple 3D skin models do not properly resemble skin physiopathology and lack the presence of a circulatory flow imitating blood vessels that distributes nutrients and other molecules. In addition, natural human skin growth occurs simultaneously as the human body is exposed to multiple stresses and environmental conditions. Therefore, recent diverse dynamic and microfluidic bioengineered devices are being used to stimulate and facilitate key physiologic events for the growth of skin tissues in vitro, for example, skin bioreactors and skin on chip (SoC) (Figure 2a). Skin bioreactors are complex bioengineered devices that are projected to induce in vivo–like biophysiological stimuli at the bench scale to stimulate, mature, monitor, and prolong healthy skin culture duration. More specifically, in vitro skin models have been developed using stretch-based bioreactors—either using unidirectional, bidirectional, or radial force—to provide in skin models with a more mature basement membrane (Tokuyama et al., 2015Tokuyama E. Nagai Y. Takahashi K. Kimata Y. Naruse K. Mechanical stretch on human skin equivalents increases the epidermal thickness and develops the basement membrane.PLoS One. 2015; 10e0141989Crossref Scopus (44) Google Scholar), stratum corneum (Jung et al., 2016Jung M.H. Jung S.M. Shin H.S. Co-stimulation of HaCaT keratinization with mechanical stress and air-exposure using a novel 3D culture device.Sci Rep. 2016; 633889Crossref Scopus (23) Google Scholar), stiffer dermis (Wahlsten et al., 2021Wahlsten A. Rütsche D. Nanni M. Giampietro C. Biedermann T. Reichmann E. et al.Mechanical stimulation induces rapid fibroblast proliferation and accelerates the early maturation of human skin substitutes.Biomaterials. 2021; 273120779Crossref PubMed Scopus (40) Google Scholar), or wrinkled skin (Lim et al., 2018Lim H.Y. Kim J. Song H.J. Kim K. Choi K.C. Park S. et al.Development of wrinkled skin-on-a-chip (WSOC) by cyclic uniaxial stretching.J Ind Eng Chem. 2018; 68: 238-245Crossref Scopus (37) Google Scholar). Because skin growth and maintenance in the body are a complex process involving multiple biophysiological, chemical, and mechanical stimuli, bioreactors tend to simplify and focus on introduction of few of these stimuli in their design development. Nowadays, we can find 2 main bioreactor techniques for dynamic cultivation of in vitro skin models: perfusion and mechanical stimulation (Figure 2b–h). Perfusion bioreactors supply fresh media to the cells while at the same time providing removal of waste metabolites. In addition, they also enable the application of shear stress on the cells, mimicking that produced by the blood flow in the endothelium, responsible for activation of various activation pathways within cells for protein synthesis and division. The simplest perfusion bioreactor is based on a simple channel placed just below (Lee et al., 2017Lee S. Jin S.P. Kim Y.K. Sung G.Y. Chung J.H. Sung J.H. Construction of 3D multicellular microfluidic chip for an in vitro skin model.Biomed Microdevices. 2017; 19: 22Crossref PubMed Scopus (97) Google Scholar; Rimal et al., 2021Rimal R. Marquardt Y. Nevolianis T. Djeljadini S. Marquez A.B. Huth S. et al.Dynamic flow enables long-term maintenance of 3-D vascularized human skin models.Appl Mater Today. 2021; 25101213Google Scholar; Song et al., 2018Song H.J. Lim H.Y. Chun W. Choi K.C. Lee T. Sung J.H. et al.Development of 3D skin-equivalent in a pump-less microfluidic chip.J Ind Eng Chem. 2018; 60: 355-359Crossref Scopus (38) Google Scholar) or around (Sriram et al., 2018Sriram G. Alberti M. Dancik Y. Wu B. Wu R. Feng Z. et al.Full-thickness human skin-on-chip with enhanced epidermal morphogenesis and barrier function.Mater Today. 2018; 21: 326-340Crossref Scopus (185) Google Scholar) the epidermis in which culture media passively flow in a gravity-based platform (Lee et al., 2017Lee S. Jin S.P. Kim Y.K. Sung G.Y. Chung J.H. Sung J.H. Construction of 3D multicellular microfluidic chip for an in vitro skin model.Biomed Microdevices. 2017; 19: 22Crossref PubMed Scopus (97) Google Scholar; Song et al., 2018Song H.J. Lim H.Y. Chun W. Choi K.C. Lee T. Sung J.H. et al.Development of 3D skin-equivalent in a pump-less microfluidic chip.J Ind Eng Chem. 2018; 60: 355-359Crossref Scopus (38) Google Scholar) or by an active peristaltic pump (Rimal et al., 2021Rimal R. Marquardt Y. Nevolianis T. Djeljadini S. Marquez A.B. Huth S. et al.Dynamic flow enables long-term maintenance of 3-D vascularized human skin models.Appl Mater Today. 2021; 25101213Google Scholar; Sriram et al., 2018Sriram G. Alberti M. Dancik Y. Wu B. Wu R. Feng Z. et al.Full-thickness human skin-on-chip with enhanced epidermal morphogenesis and barrier function.Mater Today. 2018; 21: 326-340Crossref Scopus (185) Google Scholar). In addition, a more complex perfusable channel across the dermal compartment has been developed to mimic vascular network as in in vivo tissues (Mori et al., 2017Mori N. Morimoto Y. Takeuchi S. Skin integrated with perfusable vascular channels on a chip.Biomaterials. 2017; 116: 48-56Crossref PubMed Scopus (38) Google Scholar; Salameh et al., 2021Salameh S. Tissot N. Cache K. Lima J. Suzuki I. Marinho P.A. et al.A perfusable vascularized full-thickness skin model for potential topical and systemic applications.Biofabrication. 2021; 13035042Crossref PubMed Scopus (30) Google Scholar). Owing to variable demands of stimulation needed for desired in vitro skin growth, there has not yet been a commercially available in vitro bioreactor specifically dedicated to HSEs. Nevertheless, with ample evidence of benefits of dynamic cultures, we soon expect the in vitro skin model industry to shift to dynamic cultures. This will lead to cheaper, more mature, and physiologically active skin models. For this, the upcoming bioreactor development design have to focus on simplification, ease of use, less manual input and modular integration of desired sensors, and stimulation apparatus within 1 single dynamic bioreactor platform. Current trends on in vitro testing and drug screening require not only a more realistic biophysiological environment but also high-throughput analysis. Although skin bioreactors succeed in the first, they are macroscale systems with limited scalability. In this regard, SoCs are miniaturized devices that allow the application of different stimuli such as microflows, mechanical forces, or chemical gradients to obtain more realistic models with a more accurate response to treatments and drugs (Abaci et al., 2016Abaci H.E. Guo Z. Coffman A. Gillette B. Lee W.-H. Sia S.K. et al.Human skin constructs with spatially controlled vasculature using primary and iPSC-derived endothelial cells.Adv Healthc Mater. 2016; 5: 1800-1807Crossref PubMed Google Scholar; Mori et al., 2018Mori N. Morimoto Y. Takeuchi S. Perfusable and stretchable 3D culture system for skin-equivalent.Biofabrication. 2018; 11011001Crossref PubMed Scopus (51) Google Scholar, Mori et al., 2017Mori N. Morimoto Y. Takeuchi S. Skin integrated with perfusable vascular channels on a chip.Biomaterials. 2017; 116: 48-56Crossref PubMed Scopus (38) Google Scholar). These SoC platforms can be divided into 2 groups, depending on the source of the tissue that is inside the chip (Table 2). First, microfluidics devices in which skin biopsies or mature HSEs are placed can be defined as transferred SoCs. Although far from the original definition of organ-on-a-chip given by Ingber (Bhatia and Ingber, 2014Bhatia S.N. Ingber D.E. Microfluidic organs-on-chips.Nat Biotechnol. 2014; 32: 760-772Crossref PubMed Scopus (2406) Google Scholar), they have shown good results when studying transdermal transport or bacterial infections (Abaci et al., 2015Abaci H.E. Gledhill K. Guo Z. Christiano A.M. Shuler M.L. Pumpless microfluidic platform for drug testing on human skin equivalents.Lab Chip. 2015; 15: 882-888Crossref PubMed Google Scholar; Kim et al., 2019aKim J.J. Ellett F. Thomas C.N. Jalali F. Anderson R.R. Irimia D. et al.A microscale, full-thickness, human skin on a chip assay simulating neutrophil responses to skin infection and antibiotic treatments.Lab Chip. 2019; 19: 3094-3103Crossref PubMed Google Scholar). This approach is commonly used for multiorgan-on-a-chip models (Maschmeyer et al., 2015Maschmeyer I. Lorenz A.K. Schimek K. Hasenberg T. Ramme A.P. Hübner J. et al.A four-organ-chip for interconnected long-term co-culture of human intestine, liver, skin and kidney equivalents.Lab Chip. 2015; 15: 2688-2699Crossref PubMed Google Scholar; Wagner et al., 2013Wagner I. Materne E.M. Brincker S. Süssbier U. Frädrich C. Busek M. et al.A dynamic multi-organ-chip for long-term cultivation and substance testing proven by 3D human liver and skin tissue co-culture.Lab Chip. 2013; 13: 3538-3547Crossref PubMed Scopus (0) Google Scholar). The second group refers to those in which the skin is directly generated inside the channels of the device. The most developed models are those involving only cell monolayers, either with 1 single monolayer of keratinocytes (Ramadan and Ting, 2016Ramadan Q. Ting F.C. In vitro micro-physiological immune-competent model of the human skin.Lab Chip. 2016; 16: 1899-1908Crossref PubMed Google Scholar) or with several monolayers simulating all the layers of the skin (Wufuer et al., 2016Wufuer M. Lee G. Hur W. Jeon B. Kim B.J. Choi T.H. et al.Skin-on-a-chip model simulating inflammation, edema and drug-based treatment.Sci Rep. 2016; 637471Crossref PubMed Scopus (234) Google Scholar). Some research groups have gone 1 step further, generating 3D structures inside the channels of the device, both as a single-differentiated epidermis (Zhang et al., 2021Zhang J. Chen Z. Zhang Y. Wang X. Ouyang J. Zhu J. et al.Construction of a high fidelity epidermis-on-a-chip for scalable in vitro irritation evaluation.Lab Chip. 2021; 21: 3804-3818Crossref PubMed Google Scholar) or full-thickness skin (Sriram et al., 2018Sriram G. Alberti M. Dancik Y. Wu B. Wu R. Feng Z. et al.Full-thickness human skin-on-chip with enhanced epidermal morphogenesis and barrier function.Mater Today. 2018; 21: 326-340Crossref Scopus (185) Google Scholar).Table 2Summary of Dynamic Skin Models Found in the LiteratureType of PlatformDermal MatrixCellsApplicationReferenceMicrofluidic platformNoneNormal human keratinocytesNHKs maintenance in dynamic cultureO'Neill et al., 2008O'Neill A.T. Monteiro-Riviere N.A. Walker G.M. Characterization of microfluidic human epidermal keratinocyte culture.Cytotechnology. 2008; 56: 197-207Crossref PubMed Scopus (0) Google ScholarVascularized HSECollagenNormal human fibroblasts and keratinocytes + HUVECsPercutaneous absorption, skin and vascular permeabilityMori et al., 2018Mori N. Morimoto Y. Takeuchi S. Perfusable and stretchable 3D culture system for skin-equivalent.Biofabrication. 2018; 11011001Crossref PubMed Scopus (51) Google Scholar, Mori et al., 2017Mori N. Morimoto Y. Takeuchi S. Skin integrated with perfusable vascular channels on a chip.Biomaterials. 2017; 116: 48-56Crossref PubMed Scopus (38) Google ScholarVascularized HSECollagenPrimary