Skin-on-a-Chip Technology: Microengineering Physiologically Relevant In Vitro Skin Models

All-Metal Flexible Fiber by Continuously Assembling Nanowires for High Electrical Conductivity

Fiber electronics booms as a new important field but is currently limited by the challenge of finding both highly flexible and conductive fiber electrodes. Here, all-metal fibers based on nanowires are discovered. Silver nanowires are continuously assembled into robust fibers by salt-induced aggregation and then firmly stabilized by plasmonic welding. The nanowire network structures provide them both high flexibility with moduli at the level of MPa and conductivities up to 106 S m-1. They also show excellent electrochemical properties such as low impedance and high electrochemically active surface area. Their stable chronic single-neuron recording is further demonstrated with good biocompatibility in vivo. These new fiber materials may provide more opportunities for the future development of fiber electronics.

Advances in Skin-on-a-Chip Technologies for Dermatological Disease Modeling

Skin-on-a-chip (SoC) technologies are emerging as a paradigm shift in dermatology research by replicating human physiology in a dynamic manner not achievable by current animal models. Although animal models have contributed to successful clinical trials, their ability to predict human outcomes remains questionable, owing to inherent differences in skin anatomy and immune response. Covering areas including infectious diseases, autoimmune skin conditions, wound healing, drug toxicity, aging, and antiaging, SoC aims to circumvent the inherent disparities created by traditional models. In this paper, we review current SoC technologies, highlighting their potential as an alternative to animal models for a deeper understanding of complex skin conditions.

Skin-on-a-chip technologies towards clinical translation and commercialization

Skin is the largest organ of the human body which plays a critical role in thermoregulation, metabolism (e.g. synthesis of vitamin D), and protection of other organs from environmental threats, such as infections, microorganisms, ultraviolet radiation, and physical damage. Even though skin diseases are considered to be less fatal, the ubiquity of skin diseases and irritation caused by them highlights the importance of skin studies. Furthermore, skin is a promising means for transdermal drug delivery, which requires a thorough understanding of human skin structure. Current animal andin vitrotwo/three-dimensional skin models provide a platform for disease studies and drug testing, whereas they face challenges in the complete recapitulation of the dynamic and complex structure of actual skin tissue. One of the most effective methods for testing pharmaceuticals and modeling skin diseases are skin-on-a-chip (SoC) platforms. SoC technologies provide a non-invasive approach for examining 3D skin layers and artificially creating disease models in order to develop diagnostic or therapeutic methods. In addition, SoC models enable dynamic perfusion of culture medium with nutrients and facilitate the continuous removal of cellular waste to further mimic thein vivocondition. Here, the article reviews the most recent advances in the design and applications of SoC platforms for disease modeling as well as the analysis of drugs and cosmetics. By examining the contributions of different patents to the physiological relevance of skin models, the review underscores the significant shift towards more ethical and efficient alternatives to animal testing. Furthermore, it explores the market dynamics ofin vitroskin models and organ-on-a-chip platforms, discussing the impact of legislative changes and market demand on the development and adoption of these advanced research tools. This article also identifies the existing obstacles that hinder the advancement of SoC platforms, proposing directions for future improvements, particularly focusing on the journey towards clinical adoption.

Progress in Topical and Transdermal Drug Delivery Research-Focus on Nanoformulations

Skin is the largest organ and a multifunctional interface between the body and its environment. It acts as a barrier against cold, heat, injuries, infections, chemicals, radiations or other exogeneous factors, and it is also known as the mirror of the soul. The skin is involved in body temperature regulation by the storage of fat and water. It is an interesting tissue in regard to the local and transdermal application of active ingredients for prevention or treatment of pathological conditions. Topical and transdermal delivery is an emerging route of drug and cosmetic administration. It is beneficial for avoiding side effects and rapid metabolism. Many pharmaceutical, technological and cosmetic innovations have been described and patented recently in the field. In this review, the main features of skin morphology and physiology are presented and are being followed by the description of classical and novel nanoparticulate dermal and transdermal drug formulations. The biophysical aspects of the penetration of drugs and cosmetics into or across the dermal barrier and their investigation in diffusion chambers, skin-on-a-chip devices, high-throughput measuring systems or with advanced analytical techniques are also shown. The current knowledge about mathematical modeling of skin penetration and the future perspectives are briefly discussed in the end, all also involving nanoparticulated systems.

Recombinant Human Keratinocyte Growth Factor Ameliorates Cancer Treatment-Induced Oral Mucositis on a Chip

Oral mucositis (OM) is a severe complication of cancer therapies caused by off-target cytotoxicity. Palifermin, which is recombinant human keratinocyte growth factor (KGF), is currently the only mitigating treatment available to a subset of OM patients. This study used a previously established model of oral mucositis on a chip (OM-OC) comprised of a confluent human gingival keratinocytes (GIE) layer attached to a basement membrane-lined subepithelial layer consisting of human gingival fibroblasts (HGF) and human dermal microvascular endothelial cells (HMEC) on a stable collagen I gel. Cisplatin, radiation, and combined treatments are followed by a recovery period in the OM-OC to determine possible cellular and molecular mechanisms of OM under effects of KGF. Cancer treatments affected the keratinocyte layer, causing death and epithelial barrier loss. Both keratinocytes and subepithelial cells died rapidly, as evidenced by propidium iodide staining. In response to radiation exposure, cell death occurred in the apical epithelial layer, predominantly, within 24h. Cisplatin exposure predominantly promoted death of basal epithelial cells within 32-36h. Presence of KGF in OM-OC protected tissues from damage caused by cancer treatments in a dose-dependent manner, being more effective at 10 ng/mL. As verified by F-actin staining and the Alamar Blue assay, KGF contributed to epithelial survival and induced proliferation of GIE and HGF as well as HMEC within 120h. When the expression of eighty inflammatory cytokines is evaluated at OM induction (Day 12) and resolution (Day 18) stages in OM-OC, some cytokines are identified as potential novel therapeutic targets. In comparison with chemoradiation exposure, KGF treatment showed a trend to decrease IL-8 and TNF-a expression at Day 12 and 18, and TGF-β1 at Day 18 in OM-OC. Taken together, these findings support the utility of OM-OC as a platform to model epithelial damage and evaluate molecular mechanisms following OM treatment.

Fibrosis-on-Chip: A Guide to Recapitulate the Essential Features of Fibrotic Disease

Fibrosis, which is primarily marked by excessive extracellular matrix (ECM) deposition, is a pathophysiological process associated with many disorders, which ultimately leads to organ dysfunction and poor patient outcomes. Despite the high prevalence of fibrosis, currently there exist few therapeutic options, and importantly, there is a paucity of in vitro models to accurately study fibrosis. This review discusses the multifaceted nature of fibrosis from the viewpoint of developing organ-on-chip (OoC) disease models, focusing on five key features: the ECM component, inflammation, mechanical cues, hypoxia, and vascularization. The potential of OoC technology is explored for better modeling these features in the context of studying fibrotic diseases and the interplay between various key features is emphasized. This paper reviews how organ-specific fibrotic diseases are modeled in OoC platforms, which elements are included in these existing models, and the avenues for novel research directions are highlighted. Finally, this review concludes with a perspective on how to address the current gap with respect to the inclusion of multiple features to yield more sophisticated and relevant models of fibrotic diseases in an OoC format.

Hair and Nail-On-Chip for Bioinspired Microfluidic Device Fabrication and Biomarker Detection

The advent of biosensors has tremendously increased our potential of identifying and solving important problems in various domains, ranging from food safety and environmental analysis, to healthcare and medicine. However, one of the most prominent drawbacks of these technologies, especially in the biomedical field, is to employ conventional samples, such as blood, urine, tissue extracts and other body fluids for analysis, which suffer from the drawbacks of invasiveness, discomfort, and high costs encountered in transportation and storage, thereby hindering these products to be applied for point-of-care testing that has garnered substantial attention in recent years. Therefore, through this review, we emphasize for the first time, the applications of switching over to noninvasive sampling techniques involving hair and nails that not only circumvent most of the aforementioned limitations, but also serve as interesting alternatives in understanding the human physiology involving minimal costs, equipment and human interference when combined with rapidly advancing technologies, such as microfluidics and organ-on-a-chip to achieve miniaturization on an unprecedented scale. The coalescence between these two fields has not only led to the fabrication of novel microdevices involving hair and nails, but also function as robust biosensors for the detection of biomarkers, chemicals, metabolites and nucleic acids through noninvasive sampling. Finally, we have also elucidated a plethora of futuristic innovations that could be incorporated in such devices, such as expanding their applications in nail and hair-based drug delivery, their potential in serving as next-generation wearable sensors and integrating these devices with machine-learning for enhanced automation and decentralization.

The frontline of alternatives to animal testing: novel in vitro skin model application in drug development and evaluation

The FDA Modernization Act 2.0 has brought nonclinical drug evaluation into a new era. In vitro models are widely used and play an important role in modern drug development and evaluation, including early candidate drug screening and preclinical drug efficacy and toxicity assessment. Driven by regulatory steering and facilitated by well-defined physiology, novel in vitro skin models are emerging rapidly, becoming the most advanced area in alternative testing research. The revolutionary technologies bring us many in vitro skin models, either laboratory-developed or commercially available, which were all built to emulate the structure of the natural skin to recapitulate the skin's physiological function and particular skin pathology. During the model development, how to achieve balance among complexity, accessibility, capability, and cost-effectiveness remains the core challenge for researchers. This review attempts to introduce the existing in vitro skin models, align them on different dimensions, such as structural complexity, functional maturity, and screening throughput, and provide an update on their current application in various scenarios within the scope of chemical testing and drug development, including testing in genotoxicity, phototoxicity, skin sensitization, corrosion/irritation. Overall, the review will summarize a general strategy for in vitro skin model to enhance future model invention, application, and translation in drug development and evaluation.

Constructing epidermal rete ridges using a composite hydrogel to enhance multiple signaling pathways for the maintenance of epidermal stem cell niche

The undulating microstructure rete ridge (RR) located at the junction between the dermis and epidermis plays a crucial role in improving skin mechanical properties and maintaining skin homeostasis. However, the investigation of RR microstructures is usually neglected in current tissue engineering for skin regeneration. Here, to create an epidermal model with RR microstructures, keratinocytes were cultured on a patterned GelMA-PEGDA hydrogel constructed using molding technology. Furthermore, grafting acryloylated Arg-Gly-Asp (RGD) peptides on the hydrogel surface significantly improved cell adhesion, fusion, and development. RT-PCR, Western blot, and immunofluorescence staining confirmed that cells on RR microstructures exhibited higher gene and protein expression associated with epidermal stem cells. RNA sequencing analysis of cells on RR microstructure showed higher gene expression profiles related to stem cell maintenance, basement membrane formation, and epidermal development. Furthermore, RT-PCR analysis of epidermal models of various dimensions demonstrated that smaller microstructures were more conducive to epidermal stem cell marker gene expression, which is analogous to human skin. Overall, we have successfully developed a method for integrating RR microstructures into an epidermal model that mimics natural skin to maintain epidermal stem cell niche, providing a valuable reference for researching skin regeneration within the fields of tissue engineering and regenerative medicine. STATEMENT OF SIGNIFICANCE: This study presents a method for precisely fabricating microstructures of skin rete ridges using composite hydrogels, thereby creating a skin model that mimics natural human skin. The findings reveal that this microstructure provides a stem cell niche that regulates the pathways and promotes the expression of proteins related to epidermal stem cells. This work advances the functional properties of tissue engineered skin and holds promise for improving the therapeutic efficacy of artificial skin grafts for the skin wounds.

Integrating spheroid-on-a-chip with tubeless rocker platform: A high-throughput biological screening platform

Spheroid-on-a-chip platforms are emerging as promising in vitro models that enable screening of the efficacy of biologically active ingredients. Generally, the supply of liquids to spheroids occurs in the steady flow mode with the use of syringe pumps; however, the utilization of tubing and connections, especially for multiplexing and high-throughput screening applications, makes spheroid-on-a-chip platforms labor- and cost-intensive. Gravity-induced flow using rocker platforms overcomes these challenges. Here, a robust gravity-driven technique was developed to culture arrays of cancer cell spheroids and dermal fibroblast spheroids in a high-throughput manner using a rocker platform. The efficiency of the developed rocker-based platform was benchmarked to syringe pumps for generating multicellular spheroids and their use for screening biologically active ingredients. Cell viability, internal spheroid structure as well as the effect of vitamin C on spheroids' protein synthesis was studied. The rocker-based platform not only offers comparable or enhanced performance in terms of cell viability, spheroids formation, and protein production by dermal fibroblast spheroids but also, from a practical perspective, offers a smaller footprint, requires a lower cost, and offers an easier method for handling. These results support the application of rocker-based microfluidic spheroid-on-a-chip platforms for in vitro screening in a high-throughput manner with industrial scaling-up opportunities.

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.

Organoid-on-a-chip: Current challenges, trends, and future scope toward medicine

In vitro organoid models, typically defined as 3D multicellular aggregates, have been extensively used as a promising tool in drug screening, disease progression research, and precision medicine. Combined with advanced microfluidics technique, organoid-on-a-chip can flexibly replicate in vivo organs within the biomimetic physiological microenvironment by accurately regulating different parameters, such as fluid conditions and concentration gradients of biochemical factors. Since engineered organ reconstruction has opened a new paradigm in biomedicine, innovative approaches are increasingly required in micro-nano fabrication, tissue construction, and development of pharmaceutical products. In this Perspective review, the advantages and characteristics of organoid-on-a-chip are first introduced. Challenges in current organoid culture, extracellular matrix building, and device manufacturing techniques are subsequently demonstrated, followed by potential alternative approaches, respectively. The future directions and emerging application scenarios of organoid-on-a-chip are finally prospected to further satisfy the clinical demands.

Organ-on-a-chip meets artificial intelligence in drug evaluation

Drug evaluation has always been an important area of research in the pharmaceutical industry. However, animal welfare protection and other shortcomings of traditional drug development models pose obstacles and challenges to drug evaluation. Organ-on-a-chip (OoC) technology, which simulates human organs on a chip of the physiological environment and functionality, and with high fidelity reproduction organ-level of physiology or pathophysiology, exhibits great promise for innovating the drug development pipeline. Meanwhile, the advancement in artificial intelligence (AI) provides more improvements for the design and data processing of OoCs. Here, we review the current progress that has been made to generate OoC platforms, and how human single and multi-OoCs have been used in applications, including drug testing, disease modeling, and personalized medicine. Moreover, we discuss issues facing the field, such as large data processing and reproducibility, and point to the integration of OoCs and AI in data analysis and automation, which is of great benefit in future drug evaluation. Finally, we look forward to the opportunities and challenges faced by the coupling of OoCs and AI. In summary, advancements in OoCs development, and future combinations with AI, will eventually break the current state of drug evaluation.

Recent advances in in vitro skin-on-a-chip models for drug testing

INTRODUCTION

The skin is an organ that has the largest surface area and provides a barrier against external environment. While providing protection, it also interacts with other organs in the body and has implications in various diseases. Development of physiologically realistic in vitro models of the skin in the context of the whole body is important for studying these diseases, and will be a valuable tool for pharmaceutical, cosmetics, and food industry.

AREA COVERED

This article covers the basic background in skin structure, physiology, as well as drug metabolism in the skin, and dermatological diseases. We summarize various in vitro skin models currently available, and novel in vitro models based on organ-on-a-chip technology. We also explain the concept of multi-organ-on-a-chip and describe recent developments in this field aimed at recapitulating the interaction of the skin with other organs in the body.

EXPERT OPINION

Recent development in the organ-on-a-chip field has enabled the development of in vitro model systems that resemble human skin more closely than conventional models. In near future, we will be seeing various model systems that allow researchers to study complex diseases in a more mechanistic manner, which will help the development of new pharmaceuticals for such diseases.

Preclinical study models of psoriasis: State-of-the-art techniques for testing pharmaceutical products in animal and nonanimal models

Local and systemic treatments exist for psoriasis, but none can do more than control its symptoms because of its numerous unknown mechanisms. The lack of validated testing models or a defined psoriatic phenotypic profile hinders antipsoriatic drug development. Despite their intricacy, immune-mediated diseases have no improved and precise treatment. The treatment actions may now be predicted for psoriasis and other chronic hyperproliferative skin illnesses using animal models. Their findings confirmed that a psoriasis animal model could mimic a few disease conditions. However, their ethical approval concerns and inability to resemble human psoriasis rightly offer to look for more alternatives. Hence, in this article, we have reported various cutting-edge techniques for the preclinical testing of pharmaceutical products for the treatment of psoriasis.

Organs-on-chips technologies – A guide from disease models to opportunities for drug development

Current in-vitro 2D cultures and animal models present severe limitations in recapitulating human physiopathology with striking discrepancies in estimating drug efficacy and side effects when compared to human trials. For these reasons, microphysiological systems, organ-on-chip and multiorgans microdevices attracted considerable attention as novel tools for high-throughput and high-content research to achieve an improved understanding of diseases and to accelerate the drug development process towards more precise and eventually personalized standards. This review takes the form of a guide on this fast-growing field, providing useful introduction to major themes and indications for further readings. We start analyzing Organs-on-chips (OOC) technologies for testing the major drug administration routes: (1) oral/rectal route by intestine-on-a-chip, (2) inhalation by lung-on-a-chip, (3) transdermal by skin-on-a-chip and (4) intravenous through vascularization models, considering how drugs penetrate in the bloodstream and are conveyed to their targets. Then, we focus on OOC models for (other) specific organs and diseases: (1) neurodegenerative diseases with brain models and blood brain barriers, (2) tumor models including their vascularization, organoids/spheroids, engineering and screening of antitumor drugs, (3) liver/kidney on chips and multiorgan models for gastrointestinal diseases and metabolic assessment of drugs and (4) biomechanical systems recapitulating heart, muscles and bones structures and related diseases. Successively, we discuss technologies and materials for organ on chips, analyzing (1) microfluidic tools for organs-on-chips, (2) sensor integration for real-time monitoring, (3) materials and (4) cell lines for organs on chips. (Nano)delivery approaches for therapeutics and their on chip assessment are also described. Finally, we conclude with a critical discussion on current significance/relevance, trends, limitations, challenges and future prospects in terms of revolutionary impact on biomedical research, preclinical models and drug development.

Organ-on-a-chip technologies for biomedical research and drug development: A focus on the vasculature

Current biomedical models fail to replicate the complexity of human biology. Consequently, almost 90% of drug candidates fail during clinical trials after decades of research and billions of investments in drug development. Despite their physiological similarities, animal models often misrepresent human responses, and instead, trigger ethical and societal debates regarding their use. The overall aim across regulatory entities worldwide is to replace, reduce, and refine the use of animal experimentation, a concept known as the Three Rs principle. In response, researchers develop experimental alternatives to improve the biological relevance of in vitro models through interdisciplinary approaches. This article highlights the emerging organ-on-a-chip technologies, also known as microphysiological systems, with a focus on models of the vasculature. The cardiovascular system transports all necessary substances, including drugs, throughout the body while in charge of thermal regulation and communication between other organ systems. In addition, we discuss the benefits, limitations, and challenges in the widespread use of new biomedical models. Coupled with patient-derived induced pluripotent stem cells, organ-on-a-chip technologies are the future of drug discovery, development, and personalized medicine.

Advances in 3D skin bioprinting for wound healing and disease modeling

Even with many advances in design strategies over the past three decades, an enormous gap remains between existing tissue engineering skin and natural skin. Currently available in vitro skin models still cannot replicate the three-dimensionality and heterogeneity of the dermal microenvironment sufficiently to recapitulate many of the known characteristics of skin disorder or disease in vivo. Three-dimensional (3D) bioprinting enables precise control over multiple compositions, spatial distributions and architectural complexity, therefore offering hope for filling the gap of structure and function between natural and artificial skin. Our understanding of wound healing process and skin disease would thus be boosted by the development of in vitro models that could more completely capture the heterogeneous features of skin biology. Here, we provide an overview of recent advances in 3D skin bioprinting, as well as design concepts of cells and bioinks suitable for the bioprinting process. We focus on the applications of this technology for engineering physiological or pathological skin model, focusing more specifically on the function of skin appendages and vasculature. We conclude with current challenges and the technical perspective for further development of 3D skin bioprinting.