Evaluation of in vitro human skin models for studying effects of external stressors and stimuli and developing treatment modalities

Artificial keloid skin models: understanding the pathophysiological mechanisms and application in therapeutic studies

Keloid is a type of scar formed by the overexpression of extracellular matrix substances from fibroblasts following inflammation after trauma. The existing keloid treatment methods include drug injection, surgical intervention, light exposure, cryotherapy, etc. However, these methods have limitations such as recurrence, low treatment efficacy, and side effects. Consequently, studies are being conducted on the treatment of keloids from the perspective of inflammatory mechanisms. In this study, keloid models are created to understand inflammatory mechanisms and explore treatment methods to address them. While previous studies have used animal models with gene mutations, chemical treatments, and keloid tissue transplantation, there are limitations in fully reproducing the characteristics of keloids unique to humans, and ethical issues related to animal welfare pose additional challenges. Consequently, studies are underway to create in vitro artificial skin models to simulate keloid disease and apply them to the development of treatments for skin diseases. In particular, herein, scaffold technologies that implement three-dimensional (3D) full-thickness keloid models are introduced to enhance mechanical properties as well as biological properties of tissues, such as cell proliferation, differentiation, and cellular interactions. It is anticipated that applying these technologies to the production of artificial skin for keloid simulation could contribute to the development of inflammatory keloid treatment techniques in the future.

Bioinspired and Photo-Clickable Thiol-Ene Bioinks for the Extrusion Bioprinting of Mechanically Tunable 3D Skin Models

Bioinks play a fundamental role in skin bioprinting, dictating the printing fidelity, cell response, and function of bioprinted 3D constructs. However, the range of bioinks that support skin cells' function and aid in the bioprinting of 3D skin equivalents with tailorable properties and customized shapes is still limited. In this study, we describe a bioinspired design strategy for bioengineering double crosslinked pectin-based bioinks that recapitulate the mechanical properties and the presentation of cell-adhesive ligands and protease-sensitive domains of the dermal extracellular matrix, supporting the bioprinting of bilayer 3D skin models. Methacrylate-modified pectin was used as a base biomaterial enabling hydrogel formation via either chain-growth or step-growth photopolymerization and providing independent control over bioink rheology, as well as the mechanical and biochemical cues of cell environment. By tuning the concentrations of crosslinker and polymer in bioink formulation, dermal constructs were bioprinted with a physiologically relevant range of stiffnesses that resulted in strikingly site-specific differences in the morphology and spreading of dermal fibroblasts. We also demonstrated that the developed thiol-ene photo-clickable bioinks allow for the bioprinting of skin models of varying shapes that support dermis and epidermis reconstruction. Overall, the engineered bioinks expand the range of printable biomaterials for the extrusion bioprinting of 3D cell-laden hydrogels and provide a versatile platform to study the impact of material cues on cell fate, offering potential for in vitro skin modeling.

Biomimetic microfluidic chips for toxicity assessment of environmental pollutants

Various types of pollutants widely present in environmental media, including synthetic and natural chemicals, physical pollutants such as radioactive substances, ultraviolet rays, and noise, as well as biological organisms, pose a huge threat to public health. Therefore, it is crucial to accurately and effectively explore the human physiological responses and toxicity mechanisms of pollutants to prevent diseases caused by pollutants. The emerging toxicological testing method biomimetic microfluidic chips (BMCs) exhibit great potential in environmental pollutant toxicity assessment due to their superior biomimetic properties. The BMCs are divided into cell-on-chips and organ-on-chips based on the distinctions in bionic simulation levels. Herein, we first summarize the characteristics, emergence and development history, composition and structure, and application fields of BMCs. Then, with a focus on the toxicity mechanisms of pollutants, we review the applications and advances of the BMCs in the toxicity assessment of physical, chemical, and biological pollutants, respectively, highlighting its potential and development prospects in environmental toxicology testing. Finally, the opportunities and challenges for further use of BMCs are discussed.

Development of a Mass Spectrometry Imaging Method to Evaluate the Penetration of Moisturizing Components Coated on Surgical Gloves into Artificial Membranes

Skin dryness and irritant contact dermatitis induced by the prolonged use of surgical gloves are issues faced by physicians. To address these concerns, manufacturers have introduced surgical gloves that incorporate a moisturizing component on their inner surface, resulting in documented results showing a reduction in hand dermatitis. However, the spatial distribution of moisturizers applied to surgical gloves within the integument remains unclear. Using matrix-assisted laser desorption/ionization (MALDI)-mass spectrometry imaging (MSI), we investigated the spatial distribution of moisturizers in surgical gloves within artificial membranes. Recently, dermal permeation assessments using three-dimensional models, silicone membranes, and Strat-M have gained attention as alternative approaches to animal testing. Therefore, in this study, we established an in vitro dermal permeation assessment of commercially available moisturizers in surgical gloves using artificial membranes. In this study, we offer a methodology to visualize the infiltration of moisturizers applied to surgical gloves into an artificial membrane using MALDI-MSI, while evaluating commercially available moisturizer-coated surgical gloves. Using our penetration evaluation method, we confirmed the infiltration of the moisturizers into the polyethersulfone 2 and polyolefin layers, which correspond to the epidermis and dermis of the skin, after the use of surgical gloves. The MSI-based method presented herein demonstrated the efficacy of evaluating the permeation of samples containing active ingredients.

Modeling of solar UV-induced photodamage on the hair follicles in human skin organoids

Solar ultraviolet (sUV) exposure is known to cause skin damage. However, the pathological mechanisms of sUV on hair follicles have not been extensively explored. Here, we established a model of sUV-exposed skin and its appendages using human induced pluripotent stem cell-derived skin organoids with planar morphology containing hair follicles. Our model closely recapitulated several symptoms of photodamage, including skin barrier disruption, extracellular matrix degradation, and inflammatory response. Specifically, sUV induced structural damage and catagenic transition in hair follicles. As a potential therapeutic agent for hair follicles, we applied exosomes isolated from human umbilical cord blood-derived mesenchymal stem cells to sUV-exposed organoids. As a result, exosomes effectively alleviated inflammatory responses by inhibiting NF-κB activation, thereby suppressing structural damage and promoting hair follicle regeneration. Ultimately, our model provided a valuable platform to mimic skin diseases, particularly those involving hair follicles, and to evaluate the efficacy and underlying mechanisms of potential therapeutics.

Microfluidic-based skin-on-chip systems for safety assessment of nanomaterials

The skin is the body's largest organ, continuously exposed to and affected by natural and anthropogenic nanomaterials (materials with external and internal dimensions in the nanoscale range). This broad spectrum of insults gives rise to irreversible health effects (from skin corrosion to cancer). Organ-on-chip systems can recapitulate skin physiology with high fidelity and potentially revolutionize the safety assessment of nanomaterials. Here, we review current advances in skin-on-chip models and their potential to elucidate biological mechanisms. Further, strategies are discussed to recapitulate skin physiology on-chip, improving control over nanomaterials exposure and transport across cells. Finally, we highlight future opportunities and challenges from design and fabrication to acceptance by regulatory bodies and industry.

New insights into the role of Cutibacterium acnes-derived extracellular vesicles in inflammatory skin disorders

Cutibacterium acnes (C. acnes) is one of the most prevalent bacteria that forms the human skin microbiota. Specific phylotypes of C. acnes have been associated with the development of acne vulgaris, while other phylotypes have been linked to healthy skin. In this scenario, bacterial extracellular vesicles (EVs) play a role in the interkingdom communication role with the human host. The purpose of this study was to examine the impact of EVs generated by various phylotypes of C. acnes on inflammation and sebum production using different in vitro skin cell types. The main findings of this study reveal that the proteomic profile of the cargo embodied in the EVs reflects distinct characteristics of the different C. acnes phylotypes in terms of life cycle, survival, and virulence. The in vitro skin cell types showed an extended pro-inflammatory modulation of SLST A1 EVs consistently triggering the activation of the inflammation-related factors IL-8, IL-6, TNFα and GM-CSF, in comparison to SLST H1 and SLST H2. Additionally, an acne-prone skin model utilizing PCi-SEB and arachidonic acid as a sebum inducer, was employed to investigate the impact of C. acnes EVs on sebum regulation. Our findings indicated that all three types of EVs significantly inhibited sebum production after a 24-h treatment period, with SLST H1 EVs exhibiting the most pronounced inhibitory effect when compared to the positive control. The results of this study highlight the protective nature of C. acnes SLST H1 EVs and their potential use as a natural treatment option for alleviating symptoms associated with inflammation and oily skin.

Optical Methods for Non-Invasive Determination of Skin Penetration: Current Trends, Advances, Possibilities, Prospects, and Translation into In Vivo Human Studies

Information on the penetration depth, pathways, metabolization, storage of vehicles, active pharmaceutical ingredients (APIs), and functional cosmetic ingredients (FCIs) of topically applied formulations or contaminants (substances) in skin is of great importance for understanding their interaction with skin targets, treatment efficacy, and risk assessment-a challenging task in dermatology, cosmetology, and pharmacy. Non-invasive methods for the qualitative and quantitative visualization of substances in skin in vivo are favored and limited to optical imaging and spectroscopic methods such as fluorescence/reflectance confocal laser scanning microscopy (CLSM); two-photon tomography (2PT) combined with autofluorescence (2PT-AF), fluorescence lifetime imaging (2PT-FLIM), second-harmonic generation (SHG), coherent anti-Stokes Raman scattering (CARS), and reflectance confocal microscopy (2PT-RCM); three-photon tomography (3PT); confocal Raman micro-spectroscopy (CRM); surface-enhanced Raman scattering (SERS) micro-spectroscopy; stimulated Raman scattering (SRS) microscopy; and optical coherence tomography (OCT). This review summarizes the state of the art in the use of the CLSM, 2PT, 3PT, CRM, SERS, SRS, and OCT optical methods to study skin penetration in vivo non-invasively (302 references). The advantages, limitations, possibilities, and prospects of the reviewed optical methods are comprehensively discussed. The ex vivo studies discussed are potentially translatable into in vivo measurements. The requirements for the optical properties of substances to determine their penetration into skin by certain methods are highlighted.

Effect of Betulin Colloidal Particles on Proliferation and Cytokine Secretion of Human Skin Fibroblasts

The aim of the study was to obtain untreated and treated betulin colloidal particles and assess their effect on the viability, morphology, proliferation and cytokine secretion of human dermal fibroblasts. To improve bioavailability, betulin treatment was performed by an antisolvent precipitation technique. The average particle size after treatment in the aqueous dispersion decreased from 552.9 ± 11.3 to 278.2 ± 1.6 nm. Treated betulin colloidal particles showed no cytotoxicity up to a concentration of 400 µg·mL-1 in the colorimetric tetrazolium salt viability test (CCK-8). Moreover, the cell morphology was not changed in the presence of betulin colloidal particles at a concentration range from 0.78 to 400 µg·mL-1. The obtained results also show that betulin particles induce the secretion of the proinflammatory and angiogenesis-stimulating cytokine IL-8. However, further studies would be required to clarify the mechanism of IL-8 secretion induction.

Blank Spots in the Map of Human Skin: The Challenge for Xenotransplantation

Most of the knowledge about human skin homeostasis, development, wound healing, and diseases has been accumulated from human skin biopsy analysis by transferring from animal models and using different culture systems. Human-to-mouse xenografting is one of the fundamental approaches that allows the skin to be studied in vivo and evaluate the ongoing physiological processes in real time. Humanized animals permit the actual techniques for tracing cell fate, clonal analysis, genetic modifications, and drug discovery that could never be employed in humans. This review recapitulates the novel facts about mouse skin self-renewing, regeneration, and pathology, raises issues regarding the gaps in our understanding of the same options in human skin, and postulates the challenges for human skin xenografting.

Antimicrobial efficacy of composite irrigation solution against dominant pathogens in seawater immersion wound and in vivo wound healing assessment

Seawater immersion wound is inevitably accompanied by bacterial infection. Effective irrigation is critical for bacterial infection prevention and wound healing. In this study, the antimicrobial efficacy of a designed composite irrigation solution against several dominant pathogens in seawater immersion wounds was evaluated, and in vivo wound healing assessment was conducted in a rat model. According to the time-kill result, the composite irrigation solution exhibits excellent and rapid bactericidal effect against Vibrio alginolyticus and Vibrio parahaemolyticus within 30 s of treatment while eliminating Candida albicans, Pseudomonas aeruginosa, Escherichia coli, and the mixed microbes after 1 h, 2 h, 6 h, and 12 h of treatment, respectively. Significant bacterial count reduction of Staphylococcus aureus was observed after 5 h treatment. In addition to its skin non-irritating attribute, the in vivo wound healing results further demonstrated that the irrigation solution showed high repair efficiency in the skin defect model inoculated with the mixed microbes. The wound healing rate was significantly higher than that of the control and normal saline groups. It could also effectively reduce the number of viable bacteria on the wound surface. The histological staining indicated that the irrigation solution could reduce inflammatory cells and promote collagen fibers and angiogenesis, thereby promoting wound healing. We believed that the designed composite irrigation solution has great potential for application in the treatment of seawater immersion wounds.

An Innovative Fluid Dynamic System to Model Inflammation in Human Skin Explants

Skin is a major administration route for drugs, and all transdermal formulations must be tested for their capability to overcome the cutaneous barrier. Therefore, developing highly reliable skin models is crucial for preclinical studies. The current in vitro models are unable to replicate the living skin in all its complexity; thus, to date, excised human skin is considered the gold standard for in vitro permeation studies. However, skin explants have a limited life span. In an attempt to overcome this problem, we used an innovative bioreactor that allowed us to achieve good structural and functional preservation in vitro of explanted human skin for up to 72 h. This device was then used to set up an in vitro inflammatory model by applying two distinct agents mimicking either exogenous or endogenous stimuli: i.e., dithranol, inducing the contact dermatitis phenotype, and the substance P, mimicking neurogenic inflammation. Our in vitro system proved to reproduce inflammatory events observed in vivo, such as vasodilation, increased number of macrophages and mast cells, and increased cytokine secretion. This bioreactor-based system may therefore be suitably and reliably used to simulate in vitro human skin inflammation and may be foreseen as a promising tool to test the efficacy of drugs and cosmetics.

Investigating the effects of low intensity visible light on human keratinocytes using a customized LED exposure system

Photobiomodulation (PBM) refers to the use of light to modulate cellular processes, and has demonstrated utility in improving wound healing outcomes, and reducing pain and inflammation. Despite the potential benefits of PBM, the precise molecular mechanisms through which it influences cell behavior are not yet well understood. Inconsistent reporting of key light parameters has created uncertainty around optimal exposure profiles. In addition, very low intensities of light, < 0.1 J/cm², have not been thoroughly examined for their use in PBM. Here, we present a custom-made compact, and modular LED-based exposure system for studying the effects of very low-intensity visible light (cell proliferation, migration, ROS production, and mitochondrial membrane potential) of three different wavelengths in a parallel manner. The device allows for six repeats of three different exposure conditions plus a non-irradiated control on a single 24-well plate. The immortalised human keratinocyte cell line, HaCaT, was selected as a major cellular component of the skin epidermal barrier. Furthermore, an in vitro wound model was developed by allowing the HaCaT to form a confluent monolayer, then scratching the cells with a pipette tip to form a wound. Cells were exposed to yellow (585 nm, 0.09 mW, ~ 3.7 mJ/cm²), orange (610 nm, 0.8 mW, ~ 31 mJ/cm²), and red (660 nm, 0.8 mW, ~ 31 mJ/cm²) light for 10 min. 48 h post-irradiation, immunohistochemistry was performed to evaluate cell viability, proliferation, ROS production, and mitochondrial membrane potential. The results demonstrate increased proliferation and decreased scratch area for all exposure conditions, however only red light increased the mitochondrial activity. Oxidative stress levels did not increase for any of the exposures. The present exposure system provides opportunities to better understand the complex cellular mechanisms driven by the irradiation of skin cells with visible light.

5G Electromagnetic Radiation Attenuates Skin Melanogenesis In Vitro by Suppressing ROS Generation

Recently, the impacts of 5G electromagnetic radiation (EMR) with 28 GHz on human health have been attracting public attention with the advent of 5G wireless communication. Here, we report that 5G (28 GHz) EMR can attenuate the skin pigmentation in murine melanoma cells (B16F10) and a 3D pigmented human epidermis model (Melanoderm™). B16 cells were exposed to 5G (28 GHz) with or without α-MSH for 4 h per day. Interestingly, 5G attenuated α-MSH-induced melanin synthesis. Fontana-Masson staining confirmed that the dendritic formation of α-MSH stimulated B16 cells was diminished by 5G exposure. To confirm the anti-melanogenic effect of 5G EMR, MelanoDerm™ was irradiated with 5G at a power intensity of 10 W/m² for 4 h a day for 16 days and melanin distribution was detected with Fontana-Masson staining, which supported the anti-melanogenic effect of 5G EMR. Consistently, 5G EMR suppressed α-MSH induced upregulation of melanogenic enzymes; tyrosinase, TRP-1, and TRP-2. Of note, 5G EMR attenuated ROS production stimulated by α-MSH and H₂O₂, suggesting that 5G EMR may dissipate ROS generation, which is pivotal for the melanin synthesis. Collectively, we demonstrated that 5G EMR can attenuate skin pigmentation by attenuating ROS generation.