Skin-on-a-chip models: General overview and future perspectives

Abstract

Over the last few years, several advances have been made toward the development and production of in vitro human skin models for the analysis and testing of cosmetic and pharmaceutical products. However, these skin models are cultured under static conditions that make them unable to accurately represent normal human physiology. Recent interest has focused on the generation of in vitro 3D vascularized skin models with dynamic perfusion and microfluidic devices known as skin-on-a-chip. These platforms have been widely described in the literature as good candidates for tissue modeling, as they enable a more physiological transport of nutrients and permit a high-throughput and less expensive evaluation of drug candidates in terms of toxicity, efficacy, and delivery. In this Perspective, recent advances in these novel platforms for the generation of human skin models under dynamic conditions for in vitro testing are reported. Advances in vascularized human skin equivalents (HSEs), transferred skin-on-a-chip (introduction of a skin biopsy or a HSE in the chip), and in situ skin-on-a-chip (generation of the skin model directly in the chip) are critically reviewed, and currently used methods for the introduction of skin cells in the microfluidic chips are discussed. An outlook on current applications and future directions in this field of research are also presented.

FIG. 1.

Skin structure. The outermost stratum is the epidermis, a stratified layer of keratinocytes; the dermis is found underneath, which consists of dense irregular connective tissues and cushions the body from stress and strain. Finally, the hypodermis is the innermost layer of the skin, mainly functioning as fat storage. (a) Schematic view. Modified image. Republished with permission from Sutterby et al., Small 16, 39 (2020). Copyright 2021 John Wiley and Sons, permission conveyed through Copyright Clearance Center, Inc. (b) Histological section. Modified image. Reprinted with permission from Mine et al., PLoS One 3, 12 (2018). Copyright 2021 Authors, licensed under a Creative Commons Attribution (CCBY) License 4.0.

FIG. 2.

Engineered in vitro human skin equivalents. (a) Schematic and pictures of a dermo-epidermal human skin equivalent (HSE) cultured in a transwell insert. Images are kindly provided by Cristina Quílez from our group. (b) Epidermal skin equivalent at 7 and 28 days of culture at the air-liquid interface, scale bars: 100 μm. (c) Dermo-epidermal skin equivalent at 7 and 28 days of culture at the air-liquid interface, scale bars: 30 μm. (b) and (c) are modified images. Reprinted with permission from Roger et al., J. Anat. 234, 4 (2019). Copyright 2019 Authors, licensed under a Creative Commons Attribution (CCBY) License 4.0.

FIG. 3.

Vascularized skin equivalents. (a) Platform developed by Abaci et al., (i) showing the process for the vascularized skin generation and (ii) showing different vasculature patterns created with the sacrificial alginate networks. Reprinted with permission from Abaci et al., Adv Health Mater. 5, 14 (2016). Copyright 2021 John Wiley and Sons. (b) Platform developed by Mori et al., (i) the device structure and layout, (ii) schematic of the obtained skin equivalent, and (iii) lateral layout and top image of the device. Reprinted with permission from Mori et al., Biomaterials 116, 48–56 (2017). Copyright 2021 Elsevier.

FIG. 4.

Transferred skin-on-a-chip platforms. (a) Device developed for studying a neutrophil response to skin infection, where the circuit for blood circulation and the channel for the skin biopsy can be appreciated. The magnification shows the presence of bacteria in the skin fragment without and with antibiotic treatment. Republished with permission from Kim et al., Lab Chip 19, 3094–3103 (2019). Copyright 2021 Royal Society of Chemistry, permission conveyed through Copyright Clearance Center, Inc. (b) Pumpless chip with transferred skin designed for testing HSEs viability and maintenance. Skin fragment is placed inside the hole in the chip. Republished with permission from Abaci et al., Lab Chip 15, 3 (2015). Copyright 2021 Royal Society of Chemistry, permission conveyed through Copyright Clearance Center, Inc. (c) BioChip modified to construct a skin-on-a-chip using EpiDerm™ commercial equivalent. Reprinted with permission from Alexander et al., Genes (Basel) 9, 2 (2018). Copyright 2018 Authors, licensed under a Creative Commons Attribution (CCBY) License 4.0. (d) Multiorgan platform including intestine (1), liver (2), skin (3), and kidney (4). Republished with permission from Maschmeyer et al., Lab Chip 15, 12 (2015). Copyright 2021 Royal Society of Chemistry, permission conveyed through Copyright Clearance Center, Inc.

FIG. 5.

In situ skin-on-a-chip platforms. (a) Device composed of two channels and a well-like structure in the middle for casting the dermal compartment. Keratinocytes were inoculated through the upper channel. Reprinted with permission from Sriram et al., Mater Today. 21, 4 (2018). Copyright 2018 Authors, licensed under a Creative Commons Attribution (CCBY) License 4.0. (b) Chip designed with two channels and adapted for TEER measurements. Republished with permission from Ramadan et al., Lab Chip 16, 10 (2016). Copyright 2021 Royal Society of Chemistry, permission conveyed through Copyright Clearance Center, Inc. (c) Three-layered chip containing keratinocytes, fibroblasts, and HUVECs recreating the three layers of the skin. Reprinted with permission from Wufuer et al., Sci Rep. 6, 1–12 (2016). Copyright 2016 Authors, licensed under a Creative Commons Attribution (CCBY) License 4.0. (d) Pumpless microfluidic chip in which skin was directly cast inside the well located in a hole of the middle of the device. Reprinted with permission from Kim et al., Int. J. Mol. Sci. 21, 11 (2020). Copyright 2020 Authors, licensed under a Creative Commons Attribution (CCBY) License 4.0.