Layer-by-Layer Analysis of In Vitro Skin Models

In vitro human skin models are evolving into versatile platforms for the study of skin biology and disorders. These models have many potential applications in the fields of drug testing and safety assessment, as well as cosmetic and new treatment development. The development of in vitro skin models that accurately mimic native human skin can reduce reliance on animal models and also allow for more precise, clinically relevant testing. Recent advances in biofabrication techniques and biomaterials have led to the creation of increasingly complex, multilayered skin models that incorporate important functional components of skin, such as the skin barrier, mechanical properties, pigmentation, vasculature, hair follicles, glands, and subcutaneous layer. This improved ability to recapitulate the functional aspects of native skin enhances the ability to model the behavior and response of native human skin, as the complex interplay of cell-to-cell and cell-to-material interactions are incorporated. In this review, we summarize the recent developments in in vitro skin models, with a focus on their applications, limitations, and future directions.

Engineered-Skin of Single Dermal Layer Containing Printed Hybrid Gelatin-Polyvinyl Alcohol Bioink via 3D-Bioprinting: In Vitro Assessment under Submerged vs. Air-Lifting Models

Three-dimensional (3D) in vitro skin models are frequently employed in cosmetic and pharmaceutical research to minimize the demand for animal testing. Hence, three-dimensional (3D) bioprinting was introduced to fabricate layer-by-layer bioink made up of cells and improve the ability to develop a rapid manufacturing process, while maintaining bio-mechanical scaffolds and microstructural properties. Briefly, gelatin-polyvinyl alcohol (GPVA) was mixed with 1.5 × 106 and 3.0 × 106 human dermal fibroblast (HDF) cell density, together with 0.1% genipin (GNP), as a crosslinking agent, using 3D-bioprinting. Then, it was cultured under submerged and air-lifting conditions. The gross appearance of the hydrogel's surface and cross-section were captured and evaluated. The biocompatibility testing of HDFs and cell-bioink interaction towards the GPVA was analyzed by using live/dead assay, cell migration activity, cell proliferation assay, cell morphology (SEM) and protein expression via immunocytochemistry. The crosslinked hydrogels significantly demonstrated optimum average pore size (100-199 μm). The GPVA crosslinked with GNP (GPVA_GNP) hydrogels with 3.0 × 106 HDFs was proven to be outstanding, compared to the other hydrogels, in biocompatibility testing to promote cellular interaction. Moreover, GPVA-GNP hydrogels, encapsulated with 3.0 × 106 HDFs under submerged cultivation, had a better outcome than air-lifting with an excellent surface cell viability rate of 96 ± 0.02%, demonstrated by 91.3 ± 4.1% positively expressed Ki67 marker at day 14 that represented active proliferative cells, an average of 503.3 ± 15.2 μm for migration distance, and maintained the HDFs' phenotypic profiles with the presence of collagen type I expression. It also presented with an absence of alpha-smooth muscle actin positive staining. In conclusion, 3.0 × 106 of hybrid GPVA hydrogel crosslinked with GNP, produced by submerged cultivation, was proven to have the excellent biocompatibility properties required to be a potential bioinks for the rapid manufacturing of 3D in vitro of a single dermal layer for future use in cosmetic, pharmaceutic and toxicologic applications.

Toxicity Profiling of Biosurfactants Produced by Novel Marine Bacterial Strains

Surface active agents (SAAs), currently used in modern industry, are synthetic chemicals produced from non-renewable sources, with potential toxic impacts on humans and the environment. Thus, there is an increased interest for the identification and utilization of natural derived SAAs. As such, the marine environment is considered a promising source of biosurfactants with low toxicity, environmental compatibility, and biodegradation compared to their synthetic counterparts. MARISURF is a Horizon 2020 EU-funded project aiming to identify and functionally characterize SAAs, derived from a unique marine bacterial collection, towards commercial exploitation. Specifically, rhamnolipids produced by Marinobacter MCTG107b and Pseudomonas MCTG214(3b1) strains were previously identified and characterized while currently their toxicity profile was assessed by utilizing well-established methodologies. Our results showed a lack of cytotoxicity in in vitro models of human skin and liver as indicated by alamar blue and propidium iodide assays. Additionally, the use of the single gel electrophoresis assay, under oxidative stress conditions, revealed absence of any significant mutagenic/anti-mutagenic potential. Finally, both 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulphonicacid) (ABTS) and 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) cell-free assays, revealed no significant anti-oxidant capacity for neither of the tested compounds. Consequently, the absence of significant cytotoxicity and/or mutagenicity justifies their commercial exploitation and potential development into industrial end-user applications as natural and environmentally friendly biosurfactants.

Candida auris Phenotypic Heterogeneity Determines Pathogenicity In Vitro

Candida auris is an enigmatic yeast that provides substantial global risk in health care facilities and intensive care units. A unique phenotype exhibited by certain isolates of C. auris is their ability to form small clusters of cells known as aggregates, which have been to a limited extent described in the context of pathogenic traits. In this study, we screened several nonaggregative and aggregative C. auris isolates for biofilm formation, where we observed a level of heterogeneity among the different phenotypes. Next, we utilized an RNA sequencing approach to investigate the transcriptional responses during biofilm formation of a nonaggregative and aggregative isolate of the initial pool. Observations from these analyses indicate unique transcriptional profiles in the two isolates, with several genes identified relating to proteins involved in adhesion and invasion of the host in other fungal species. From these findings, we investigated for the first time the fungal recognition and inflammatory responses of a three-dimensional skin epithelial model to these isolates. In these models, a wound was induced to mimic a portal of entry for C. auris We show that both phenotypes elicited minimal response in the model minus induction of the wound, yet in the wounded tissue, both phenotypes induced a greater response, with the aggregative isolate more proinflammatory. This capacity of aggregative C. auris biofilms to generate such responses in the wounded skin highlights how this opportunistic yeast is a high risk within the intensive care environment where susceptible patients have multiple indwelling lines.IMPORTANCE Candida auris has recently emerged as an important cause of concern within health care environments due to its ability to persist and tolerate commonly used antiseptics and disinfectants, particularly when attached to a surface (biofilms). This yeast is able to colonize and subsequently infect patients, particularly those that are critically ill or immunosuppressed, which may result in death. We have undertaken analysis on two different phenotypic types of this yeast, using molecular and immunological tools to determine whether either of these has a greater ability to cause serious infections. We describe that both isolates exhibit largely different transcriptional profiles during biofilm development. Finally, we show that the inability to form small aggregates (or clusters) of cells has an adverse effect on the organism's immunostimulatory properties, suggesting that the nonaggregative phenotype may exhibit a certain level of immune evasion.

Polysaccharide-Based Bioink Formulation for 3D Bioprinting of an In Vitro Model of the Human Dermis

Limitations in wound management have prompted scientists to introduce bioprinting techniques for creating constructs that can address clinical problems. The bioprinting approach is renowned for its ability to spatially control the three-dimensional (3D) placement of cells, molecules, and biomaterials. These features provide new possibilities to enhance homology to native skin and improve functional outcomes. However, for the clinical value, the development of hydrogel bioink with refined printability and bioactive properties is needed. In this study, we combined the outstanding viscoelastic behavior of nanofibrillated cellulose (NFC) with the fast cross-linking ability of alginate (ALG), carboxymethyl cellulose (CMC), and encapsulated human-derived skin fibroblasts (hSF) to create a bioink for the 3D bioprinting of a dermis layer. The shear thinning behavior of hSF-laden bioink enables construction of 3D scaffolds with high cell density and homogeneous cell distribution. The obtained results demonstrated that hSF-laden bioink supports cellular activity of hSF (up to 29 days) while offering proper printability in a biologically relevant 3D environment, making it a promising tool for skin tissue engineering and drug testing applications.

Microfluidic skin chip with vasculature for recapitulating the immune response of the skin tissue

There is a considerable need for cell-based in vitro skin models for studying dermatological diseases and testing cosmetic products, but current in vitro skin models lack physiological relevance compared to human skin tissue. For example, many dermatological disorders involve complex immune responses, but current skin models are not capable of recapitulating the phenomena. Previously, we reported development of a microfluidic skin chip with a vessel structure and vascular endothelial cells. In this study, we cocultured dermal fibroblasts and keratinocytes with vascular endothelial cells, human umbilical vascular endothelial cells. We verified the formation of a vascular endothelium in the presence of the dermis and epidermis layers by examining the expression of tissue-specific markers. As the vascular endothelium plays a critical role in the migration of leukocytes to inflammation sites, we incorporated leukocytes in the circulating media and attempted to mimic the migration of neutrophils in response to external stimuli. Increased secretion of cytokines and migration of neutrophils was observed when the skin chip was exposed to ultraviolet irradiation, showing that the microfluidic skin chip may be useful for studying the immune response of the human tissue.

Fabrication of Bioengineered Skin by Injection Molding: A Feasibility Study on Automation

The use of bioengineered skin has facilitated fundamental and applied research because it enables the investigation of complex interactions between various cell types as well as the extracellular matrix. The predominantly manual fabrication of these living tissues means, however, that their quality, standardization, and production volume are extremely dependent on the technician's experience. Simple laboratory automation could facilitate the use of living tissues by a greater number of research groups. We developed and present here an injection molding technique for the fabrication of bilayered skin equivalents. The tissue was formed automatically by two separate injections into a customized mold to produce the dermal and epidermal skin layers. We demonstrated the biocompatibility of this fabrication process and confirmed the resulting bilayered morphology of the bioengineered skin using histology and immunohistochemistry. Our findings highlight the possibility of fabricating multilayered living tissue by injection molding, suggesting that further investigation into this automation method could result in the rapid and low-cost fabrication of standardized bioengineered skin.

An in vitro skin model to study the effect of mesenchymal stem cells in wound healing and epidermal regeneration

The development of new wound therapies, such as bioengineered skin equivalents, is an ongoing process. Multi-potent mesenchymal stem cells (MSCs) give rise to many tissue lineages and have been implicated in wound healing making them a potential candidate for cell-based bioengineered products for injured tissue. In this study, we investigated the mesenchymal/epithelial interactions of cultured MSCs in comparison to cultured fibroblasts on epidermal proliferation, differentiation, and extracellular matrix (ECM) protein expression using a de-epidermalized dermis (DED) skin model. We also studied whether MSCs can transdifferentiate to keratinocytes using the same model. Keratinocytes were cultured on unseeded DED or DED populated with fibroblasts or MSCs at an air-liquid interface to induce epidermal differentiation. Fibroblasts or MSCs were also seeded on the papillary surface of the DED alone or on the reticular surface. General histology and immunostaining was performed on the skin equivalents to examine the expression of pan keratin (K) (K1, K5, K6, and K18) and protein markers for epidermal differentiation (K10), hyperproliferation (K6), proliferation (PCNA), ECM component (collagen type IV), and mesenchymal marker (vimentin). Keratinocyte-fibroblast skin model and keratinocyte-MSC skin model both displayed an epidermal phenotype similar to epidermis in vivo. Positive expression of proliferation, differentiation and ECM protein markers was observed. MSCs failed to adopt an epithelial phenotype in the DED skin model. Our findings highlight the potential use of MSCs in bioengineered tissue for the treatment of wounds.