Tissue engineered human skin equivalents

In vivo characterization of minipig skin as a model for dermatological research using multiphoton microscopy

Minipig skin is one of the most widely used non-rodent animal skin models for dermatological research. A thorough characterization of minipig skin is essential for gaining deeper understanding of its structural and functional similarities with human skin. In this study, three-dimensional (3-D) in vivo images of minipig skin was obtained non-invasively using a multimodal optical imaging system capable of acquiring two-photon excited fluorescence (TPEF) and fluorescence lifetime imaging microscopy (FLIM) images simultaneously. The images of the structural features of different layers of the minipig skin were qualitatively and quantitatively compared with those of human skin. Label-free imaging of skin was possible due to the endogenous fluorescence and optical properties of various components in the skin such as keratin, nicotinamide adenine dinucleotide phosphate (NAD(P)H), melanin, elastin, and collagen. This study demonstrates the capability of optical biopsy techniques, such as TPEF and FLIM, for in vivo non-invasive characterization of cellular and functional features of minipig skin, and the optical image-based similarities of this commonly utilized model of human skin. These optical imaging techniques have the potential to become promising tools in dermatological research for developing a better understanding of animal skin models, and for aiding in translational pre-clinical to clinical studies.

A Concise Review on Tissue Engineered Artificial Skin Grafts for Chronic Wound Treatment: Can We Reconstruct Functional Skin Tissue In Vitro?

Chronic wounds occur as a consequence of a prolonged inflammatory phase during the healing process, which precludes skin regeneration. Typical treatment for chronic wounds includes application of autografts, allografts collected from cadaver, and topical delivery of antioxidant, anti-inflammatory, and antibacterial agents. Nevertheless, the mentioned therapies are not sufficient for extensive or deep wounds. Moreover, application of allogeneic skin grafts carries high risk of rejection and treatment failure. Advanced therapies for chronic wounds involve application of bioengineered artificial skin substitutes to overcome graft rejection as well as topical delivery of mesenchymal stem cells to reduce inflammation and accelerate the healing process. This review focuses on the concept of skin tissue engineering, which is a modern approach to chronic wound treatment. The aim of the article is to summarize common therapies for chronic wounds and recent achievements in the development of bioengineered artificial skin constructs, including analysis of biomaterials and cells widely used for skin graft production. This review also presents attempts to reconstruct nerves, pigmentation, and skin appendages (hair follicles, sweat glands) using artificial skin grafts as well as recent trends in the engineering of biomaterials, aiming to produce nanocomposite skin substitutes (nanofilled polymer composites) with controlled antibacterial activity. Finally, the article describes the composition, advantages, and limitations of both newly developed and commercially available bioengineered skin substitutes.

De novo production of human extracellular matrix supports increased throughput and cellular complexity in 3D skin equivalent model

Three-dimensional (3D) tissue models of human skin are being developed to better understand disease phenotypes and to screen new drugs for potential therapies. Several factors will increase the value of these in vitro 3D skin tissues for these purposes. These include the need for human-derived extracellular matrix (ECM), higher throughput tissue formats, and greater cellular complexity. Here, we present an approach for the fabrication of 3D skin-like tissues as a platform that addresses these three considerations. We demonstrate that human adult and neonatal fibroblasts deposit an endogenous ECM de novo that serves as an effective stroma for full epithelial tissue development and differentiation. We have miniaturized these tissues to a 24-well format to adapt them for eventual higher throughput drug screening. We have shown that monocytes from the peripheral blood can be incorporated into this model as macrophages to increase tissue complexity. This humanized skin-like tissue decreases dependency on animal-derived ECM while increasing cellular complexity that can enable screening inflammatory responses in tissue models of human skin.

Development of a polyvinyl alcohol/sodium alginate hydrogel-based scaffold incorporating bFGF-encapsulated microspheres for accelerated wound healing

In the present study, a hybrid microsphere/hydrogel system, consisting of polyvinyl alcohol (PVA)/sodium alginate (SA) hydrogel incorporating PCL microspheres is introduced as a skin scaffold to accelerate wound healing. The hydrogel substrate was developed using the freeze-thawing method, and the proportion of the involved polymers in its structure was optimized based on the in-vitro assessments. The bFGF-encapsulated PCL microspheres were also fabricated utilizing the double-emulsion solvent evaporation technique. The achieved freeze-dried hybrid system was then characterized by in-vitro and in-vivo experiments. The results obtained from the optimization of the hydrogel showed that increasing the concentration of SA resulted in a more porous structure, and higher swelling ability, elasticity and degradation rate, but decreased the maximum strength and elongation at break. The embedding of PCL microspheres into the optimized hydrogel structure provided sustained and burst-free release kinetics of bFGF. Besides, the addition of drug-loaded microspheres led to no significant change in the degradation mechanism of the hydrogel substrate; however, it reduced its mechanical strength. Furthermore, the MTT assay represented no cytotoxic effect for the hybrid system. The in-vivo studies on a burn-wound rat model, including the evaluation of the wound closure mechanism, and histological analyses indicated that the fabricated scaffold efficiently contributed to promoting cell-induced tissue regeneration and burn-wound healing.

Microphysiological Systems: Next Generation Systems for Assessing Toxicity and Therapeutic Effects of Nanomaterials

Microphysiological systems, also known as organ-on-a-chip platforms, show promise for the development of new testing methods that can be more accurate than both conventional two-dimensional cultures and costly animal studies. The development of more intricate microphysiological systems can help to better mimic the human physiology and highlight the systemic effects of different drugs and materials. Nanomaterials are among a technologically important class of materials used for diagnostic, therapeutic, and monitoring purposes; all of which and can be tested using new organ-on-a-chip systems. In addition, the toxicity of nanomaterials which have entered the body from ambient air or diet can have deleterious effects on various body systems. This in turn can be studied in newly developed microphysiological systems. While organ-on-a-chip models can be useful, they cannot pick up secondary and systemic toxicity. Thus, the utilization of multi-organ-on-a-chip systems for advancing nanotechnology will largely be reflected in the future of drug development, toxicology studies and precision medicine. Various aspects of related studies, current challenges, and future perspectives are discussed in this paper.

Optical investigation of three-dimensional human skin equivalents: A pilot study

Human skin equivalents (HSEs) are three-dimensional living models of human skin that are prepared in vitro by seeding cells onto an appropriate scaffold. They recreate the structure and biological behaviour of real skin, allowing the investigation of processes such as keratinocyte differentiation and interactions between the dermal and epidermal layers. However, for wider applications, their optical and mechanical properties should also replicate those of real skin. We therefore conducted a pilot study to investigate the optical properties of HSEs. We compared Monte Carlo simulations of (a) real human skin and (b) two-layer optical models of HSEs with (c) experimental measurements of transmittance through HSE samples. The skin layers were described using a hybrid collection of optical attenuation coefficients. A linear relationship was observed between the simulations and experiments. For samples thinner than 0.5 mm, an exponential increase in detected power was observed due to fewer instances of absorption and scattering.

Bioengineered Skin Substitutes: the Role of Extracellular Matrix and Vascularization in the Healing of Deep Wounds

The formation of severe scars still represents the result of the closure process of extended and deep skin wounds. To address this issue, different bioengineered skin substitutes have been developed but a general consensus regarding their effectiveness has not been achieved yet. It will be shown that bioengineered skin substitutes, although representing a valid alternative to autografting, induce skin cells in repairing the wound rather than guiding a regeneration process. Repaired skin differs from regenerated skin, showing high contracture, loss of sensitivity, impaired pigmentation and absence of cutaneous adnexa (i.e., hair follicles and sweat glands). This leads to significant mobility and aesthetic concerns, making the development of more effective bioengineered skin models a current need. The objective of this review is to determine the limitations of either commercially available or investigational bioengineered skin substitutes and how advanced skin tissue engineering strategies can be improved in order to completely restore skin functions after severe wounds.

Current Strategies and Future Perspectives of Skin-on-a-Chip Platforms: Innovations, Technical Challenges and Commercial Outlook

The skin is the largest and most exposed organ in the human body. Not only it is involved in numerous biological processes essential for life but also it represents a significant endpoint for the application of pharmaceuticals. The area of in vitro skin tissue engineering has been progressing extensively in recent years. Advanced in vitro human skin models strongly impact the discovery of new drugs thanks to the enhanced screening efficiency and reliability. Nowadays, animal models are largely employed at the preclinical stage of new pharmaceutical compounds development for both risk assessment evaluation and pharmacokinetic studies. On the other hand, animal models often insufficiently foresee the human reaction due to the variations in skin immunity and physiology. Skin-on-chips devices offer innovative and state-of-the-art platforms essential to overcome these limitations. In the present review, we focus on the contribution of skin-on-chip platforms in fundamental research and applied medical research. In addition, we also highlighted the technical and practical difficulties that must be overcome to enhance skin-on-chip platforms, e.g. embedding electrical measurements, for improved modeling of human diseases as well as of new drug discovery and development.

Dermo-epidermal organotypic cultures for in vitro evaluation of skin irritation and corrosion

In recent years, in-vitro skin models for chemical hazard identification have been developed. Most of them consist only of human keratinocytes, neglecting the contribution of other skin constituents. Cultures containing the dermal and epidermal component provide an attractive system to investigate, in a more realistic model, toxicological responses, which represents a distinct advantage over keratinocytes-based models that do not mimic faithfully the in vivo environment. This study aimed to validate dermo-epidermal organotypic cultures (ORGs) as a platform to perform irritation and corrosion tests. Skin models were constructed by seeding keratinocytes on fibroblast-containing fibrin gels. After 21 days, the ORGs were evaluated histologically, and the irritant and corrosion potential was determined by means of viability measurements (MTT assay) and cytokine release, according to 431 and 439 OECD tests guidelines. Skin models showed similar histological characteristics to native skin and were able to classify different substances with high accuracy, showing their applicability to skin irritation and corrosion tests. Although cytokines release seems to be chemical-dependent, a tendency was observed, leading to the improvement of the prediction capacity. Nevertheless, further studies should be done to reduce variability in order to increase prediction capacity.

Dual-chambered membrane bioreactor for coculture of stratified cell populations

We have developed a dual-chambered bioreactor (DCB) that incorporates a membrane to study stratified 3D cell populations for skin tissue engineering. The DCB provides adjacent flow lines within a common chamber; the inclusion of the membrane regulates flow layering or mixing, which can be exploited to produce layers or gradients of cell populations in the scaffolds. Computational modeling and experimental assays were used to study the transport phenomena within the bioreactor. Molecular transport across the membrane was defined by a balance of convection and diffusion; the symmetry of the system was proven by its bulk convection stability, while the movement of molecules from one flow line to the other is governed by coupled convection-diffusion. This balance allowed the perfusion of two different fluids, with the membrane defining the mixing degree between the two. The bioreactor sustained two adjacent cell populations for 28 days, and was used to induce indirect adipogenic differentiation of mesenchymal stem cells due to molecular cross-talk between the populations. We successfully developed a platform that can study the dermis-hypodermis complex to address limitations in skin tissue engineering. Furthermore, the DCB can be used for other multilayered tissues or the study of communication pathways between cell populations.

Coarse-grained molecular dynamics simulations of the effect of edge activators on the skin permeation behavior of transfersomes

Transfersomes (TRS) can provide sustained drug delivery and themselves are biocompatible, biodegradable and nontoxic. Edge activators (EAs) are key factors for increasing the deformability of TRS, and this active deformation mechanism is of commercial interest, especially at the molecular level. Accordingly, in this paper, the deformability of pure dipalmitoyl phosphatidylcholine (DPPC) vesicles, TRS with sodium cholate as an EA, and DPPC vesicles containing pogostone (POG) were compared via umbrella sampling technology. The DPPC conformation and membrane fluidity of these three types of bilayer systems were evaluated, and the changes in the membrane properties of vesicles caused by EAs were studied. EAs could increase the deformability of TRS by decreasing the deformation energy barrier due to their amphiphilic structures, which was similar to those of DPPC molecules. The membrane properties also changed via treatment with EAs including altering the tail chain angle, disturbing the ordered tail chain arrangement and prompting lateral diffusion of DPPC molecules. In addition, the impact of EAs on DPPC bilayers was further demonstrated to be concentration dependent. An ideal concentration was identified for the lowest amount of EA that offered a gel-liquid-crystalline phase transition of DPPC bilayers. Importantly, POG, a lipophobic transdermal drug, can also affect the skin permeation behavior of vesicles but had weaker effects than EA.

Bio-Functional Textiles: Combining Pharmaceutical Nanocarriers with Fibrous Materials for Innovative Dermatological Therapies

In the field of pharmaceutical technology, significant attention has been paid on exploiting skin as a drug administration route. Considering the structural and chemical complexity of the skin barrier, many research works focused on developing an innovative way to enhance skin drug permeation. In this context, a new class of materials called bio-functional textiles has been developed. Such materials consist of the combination of advanced pharmaceutical carriers with textile materials. Therefore, they own the possibility of providing a wearable platform for continuous and controlled drug release. Notwithstanding the great potential of these materials, their large-scale application still faces some challenges. The present review provides a state-of-the-art perspective on the bio-functional textile technology analyzing the several issues involved. Firstly, the skin physiology, together with the dermatological delivery strategy, is keenly described in order to provide an overview of the problems tackled by bio-functional textiles technology. Secondly, an overview of the main dermatological nanocarriers is provided; thereafter the application of these nanomaterial to textiles is presented. Finally, the bio-functional textile technology is framed in the context of the different dermatological administration strategies; a comparative analysis that also considers how pharmaceutical regulation is conducted.

Microfluidics-based skin irritation test using in vitro 3D angiogenesis platform

A global ban on animal experiments has been proposed. Hence, it is imperative to develop alternative models. Artificial skin models should reflect the responses of subcutaneous blood vessels and the immune system to elucidate disease and identify cosmetics' base materials. Notably, in vivo skin-irritation cascades involve disruption of the epidermal barrier and the release of proinflammatory mediators in response to chemical stimuli. Such proinflammatory factors promote angiogenesis and blood vessel permeability, as observed in irritant contact dermatitis. As an alternative to animal models, we propose a novel skin-irritation model based on a three-dimensional in vitro angiogenesis platform, in which irritated keratinocytes biochemically stimulate vascular endothelial growth factors. Our microfluidic platform hosts interactions between keratinocytes and dermal fibroblasts, which promote angiogenic sprouting. We use sodium lauryl sulfate (SLS) and steartrimonium chloride (SC) as chemical irritants. The irritative effects of SLS and SC are of particular interest due to the ubiquity of both SLS and SC in cosmetics. SLS was observed to significantly affect angiogenic performance, with increasing sprout length. Further promotion of vessel sprouting and lumen formation was observed with 10, 20, and 60 μM of SC, despite its classification as nonirritating and use in supposedly safe formulations. This platform provides an alternative to animal testing as a basis for testing cosmetics and pharmaceutical substances, in addition to serving as a disease model for irritant contact dermatitis.

Engineered Skin Tissue Equivalents for Product Evaluation and Therapeutic Applications

The current status of skin tissue equivalents that have emerged as relevant tools in commercial and therapeutic product development applications is reviewed. Due to the rise of animal welfare concerns, numerous companies have designed skin model alternatives to assess the efficacy of pharmaceutical, skincare, and cosmetic products in an in vitro setting, decreasing the dependency on such methods. Skin models have also made an impact in determining the root causes of skin diseases. When designing a skin model, there are various chemical and physical considerations that need to be considered to produce a biomimetic design. This includes designing a structure that mimics the structural characteristics and mechanical strength needed for tribological property measurement and toxicological testing. Recently, various commercial products have made significant progress towards achieving a native skin alternative. Further research involve the development of a functional bilayered model that mimics the constituent properties of the native epidermis and dermis. In this article, the skin models are divided into three categories: in vitro epidermal skin equivalents, in vitro full-thickness skin equivalents, and clinical skin equivalents. A description of skin model characteristics, testing methods, applications, and potential improvements is presented.

Microstructured hydrogel scaffolds containing differential density interfaces promote rapid cellular invasion and vascularization

INTRODUCTION

Insufficient vascularization of currently available clinical biomaterials has limited their application to optimal wound beds. We designed a hydrogel scaffold with a unique internal microstructure of differential collagen densities to induce cellular invasion and neovascularization.

METHODS

Microsphere scaffolds (MSS) were fabricated by encasing 1% (w/v) type 1 collagen microspheres 50-150 μm in diameter in 0.3% collagen bulk. 1% and 0.3% monophase collagen scaffolds and Integra® disks served as controls. Mechanical characterization as well as in vitro and in vivo invasion assays were performed. Cell number and depth of invasion were analyzed using Imaris™. Cell identity was assessed immunohistochemically.

RESULTS

In vitro, MSS exhibited significantly greater average depth of cellular invasion than Integra® and monophase collagen controls. MSS also demonstrated significantly higher cell counts than controls. In vivo, MSS revealed significantly more cellular invasion spanning the entire scaffold depth at 14 days than Integra®. CD31+ expressing luminal structures suggestive of neovasculature were seen within MSS at 7 days and were more prevalent after 14 days. Multiphoton microscopy of MSS demonstrated erythrocytes within luminal structures after 14 days.

CONCLUSION

By harnessing simple architectural cues to induce cellular migration, MSS holds great potential for clinical translation as the next generation dermal replacement product.

STATEMENT OF SIGNIFICANCE

Large skin wounds require tissue engineered dermal substitutes in order to promote healing. Currently available dermal replacement products do not always adequately incorporate into the body, especially in complex wounds, due to poor neovascularization. In this paper, we present a hydrogel with an innovative microarchitecture that is composed of dense type I collagen microspheres suspended in a less-dense collagen bulk. We show that cell invasion into the scaffold is driven solely by mechanical cues inherent within this differential density interface, and that this induces robust vascular cell invasion both in vitro and in a rodent model. Our hydrogel performs favorably compared to the current clinical gold standard, Integra®. We believe this hydrogel scaffold may be the first of the next generation of dermal replacement products.

Acid-Treated Water-Soluble Chitosan Suitable for Microneedle-Assisted Intracutaneous Drug Delivery

Chitosan has been widely used as a nature-derived polymeric biomaterial due to its high biocompatibility and abundance. However, poor solubility in aqueous solutions of neutral pH and multiple fabrication steps for the molding process limit its application to microneedle technology as a drug delivery carrier. Here, we present a facile method to prepare water-soluble chitosan and its application for sustained transdermal drug delivery. The water-soluble chitosan was prepared by acid hydrolysis using trifluoroacetic acid followed by dialysis in 0.1 M NaCl solutions. We successfully fabricated bullet-shaped microneedle (MN) arrays by the single molding process with neutral aqueous chitosan solutions (pH 6.0). The chitosan MN showed sufficient mechanical properties for skin insertion and, interestingly, exhibited slow dissolving behavior in wet conditions, possibly resulting from a physical crosslinking of chitosan chains. Chitosan MN patches loading rhodamine B, a model hydrophilic drug, showed prolonged release kinetics in the course of the dissolving process for more than 72 h and they were found to be biocompatible to use. Since the water-soluble chitosan can be used for MN fabrication in the mild conditions (neutral pH and 25 °C) required for the loading of bioactive agents such as proteins and achieve a prolonged release, this biocompatible chitosan MN would be suitable for sustained transdermal drug delivery of a diverse range of drugs.

Bioengineering the microanatomy of human skin

Recreating the structure of human tissues in the laboratory is valuable for fundamental research, testing interventions, and reducing the use of animals. Critical to the use of such technology is the ability to produce tissue models that accurately reproduce the microanatomy of the native tissue. Current artificial cell-based skin systems lack thorough characterisation, are not representative of human skin, and can show variation. In this study, we have developed a novel full thickness model of human skin comprised of epidermal and dermal compartments. Using an inert porous scaffold, we created a dermal construct using human fibroblasts that secrete their own extracellular matrix proteins, which avoids the use of animal-derived materials. The dermal construct acts as a foundation upon which epidermal keratinocytes were seeded and differentiated into a stratified keratinised epithelium. In-depth morphological analyses of the model demonstrated very close similarities with native human skin. Extensive immunostaining and electron microscopy analysis revealed ultrastructural details such as keratohyalin granules and lamellar bodies within the stratum granulosum, specialised junctional complexes, and the presence of a basal lamina. These features reflect the functional characteristics and barrier properties of the skin equivalent. Robustness and reproducibility of in vitro models are important attributes in experimental practice, and we demonstrate the consistency of the skin construct between different users. In summary, a new model of full thickness human skin has been developed that possesses microanatomical features reminiscent of native tissue. This skin model platform will be of significant interest to scientists researching the structure and function of human skin.

3D biomaterial matrix to support long term, full thickness, immuno-competent human skin equivalents with nervous system components

Current commercially available human skin equivalents (HSEs) are used for relatively short term studies (∼1 week) due in part to the time-dependent contraction of the collagen gel-based matrix and the limited cell types and skin tissue components utilized. In contrast, here we describe a new matrix consisting of a silk-collagen composite system that provides long term, stable cultivation with reduced contraction and degradation over time. This matrix supports full thickness skin equivalents which include nerves. The unique silk-collagen composite system preserves cell-binding domains of collagen while maintaining the stability and mechanics of the skin system for long-term culture with silk. The utility of this new composite protein-based biomaterial was demonstrated by bioengineering full thickness human skin systems using primary cells, including nerves and immune cells to establish an HSE with a neuro-immuno-cutaneous system. The HSEs with neurons and hypodermis, compared to in vitro skin-only HSEs controls, demonstrated higher secretion of pro-inflammatory cytokines. Proteomics analysis confirmed the presence of several proteins associated with inflammation across all sample groups, but HSEs with neurons had the highest amount of detected protein due to the complexity of the model. This improved, in vitro full thickness HSE model system utilizes cross-linked silk-collagen as the biomaterial and allows reduced reliance on animal models and provides a new in vitro tissue system for the assessment of chronic responses related to skin diseases and drug discovery.

Current and Future Perspectives on Skin Tissue Engineering: Key Features of Biomedical Research, Translational Assessment, and Clinical Application

The skin is responsible for several important physiological functions and has enormous clinical significance in wound healing. Tissue engineered substitutes may be used in patients suffering from skin injuries to support regeneration of the epidermis, dermis, or both. Skin substitutes are also gaining traction in the cosmetics and pharmaceutical industries as alternatives to animal models for product testing. Recent biomedical advances, ranging from cellular-level therapies such as mesenchymal stem cell or growth factor delivery, to large-scale biofabrication techniques including 3D printing, have enabled the implementation of unique strategies and novel biomaterials to recapitulate the biological, architectural, and functional complexity of native skin. This progress report highlights some of the latest approaches to skin regeneration and biofabrication using tissue engineering techniques. Current challenges in fabricating multilayered skin are addressed, and perspectives on efforts and strategies to meet those limitations are provided. Commercially available skin substitute technologies are also examined, and strategies to recapitulate native physiology, the role of regulatory agencies in supporting translation, as well as current clinical needs, are reviewed. By considering each of these perspectives while moving from bench to bedside, tissue engineering may be leveraged to create improved skin substitutes for both in vitro testing and clinical applications.

Human Skin Equivalents Demonstrate Need for Neuro-Immuno-Cutaneous System

A variety of human skin equivalents (HSEs) has been designed for clinical use or for exploratory skin research. In vitro HSE models have been used to target relationships between the skin and nervous or immune systems but have not yet considered the neuro-immuno-cutaneous (NIC) system. In this study, HSEs are described, with and without neural and immune components, to discern these types of effects. These systems are composed of only primary human cells and contain an epidermis, dermis, hypodermis (with immune cells), and human induced neural stem cells for the neuronal component. RNA sequencing is utilized to confirm differences between sample groups and to identify unique or important genes with respect to sample type. Only samples with both neural and immune components result in the upregulation of genes in all the key biological pathways explored. The analysis of protein secretion confirms that this group has measurable functions related to all key cell types. Overall, this novel skin tissue system confirms that designing HSEs that include the NIC system results in a tissue model that reflects key functions. These systems could be used to identify selected targets of interest in skin research related to healthy or diseased states.

Skin-on-a-Chip: Transepithelial Electrical Resistance and Extracellular Acidification Measurements through an Automated Air-Liquid Interface

Skin is a critical organ that plays a crucial role in defending the internal organs of the body. For this reason, extensive work has gone into creating artificial models of the epidermis for in vitro skin toxicity tests. These tissue models, called reconstructed human epidermis (RhE), are used by researchers in the pharmaceutical, cosmetic, and environmental arenas to evaluate skin toxicity upon exposure to xenobiotics. Here, we present a label-free solution that leverages the use of the intelligent mobile lab for in vitro diagnostics (IMOLA-IVD), a noninvasive, sensor-based platform, to monitor the transepithelial electrical resistance (TEER) of RhE models and adherent cells cultured on porous membrane inserts. Murine fibroblasts cultured on polycarbonate membranes were first used as a test model to optimize procedures using a custom BioChip encapsulation design, as well as dual fluidic configurations, for continuous and automated perfusion of membrane-bound cultures. Extracellular acidification rate (EAR) and TEER of membrane-bound L929 cells were monitored. The developed protocol was then used to monitor the TEER of MatTek EpiDerm^TM RhE models over a period of 48 h. TEER and EAR measurements demonstrated that the designed system is capable of maintaining stable cultures on the chip, monitoring metabolic parameters, and revealing tissue breakdown over time.

Bryostatin and its synthetic analog, picolog rescue dermal fibroblasts from prolonged stress and contribute to survival and rejuvenation of human skin equivalents

Skin health is associated with the day-to-day activity of fibroblasts. The primary function of fibroblasts is to synthesize structural proteins, such as collagen, extracellular matrix proteins, and other proteins that support the structural integrity of the skin and are associated with younger, firmer, and more elastic skin that is better able to resist and recover from injury. At sub-nanomolar concentrations (0.03-0.3 nM), bryostatin-1 and its synthetic analog, picolog (0.1-10 nM) sustained the survival and activation of human dermal fibroblasts cultured under the stressful condition of prolonged serum deprivation. Bryostatin-1 treatment stabilized human skin equivalents (HSEs), a bioengineered combination of primary human skin cells (keratinocytes and dermal fibroblasts) on an extracellular matrix composed of mainly collagen. Fibroblasts activated by bryostatin-1 protected the structural integrity of HSEs. Bryostatin-1 and picolog prolonged activation of Erk in fibroblasts to promote cell survival. Chronic stress promotes the progression of apoptosis. Dermal fibroblasts constitutively express all components of Fas associated apoptosis, including caspase-8, an initiator enzyme of apoptosis. Prolong bryostatin-1 treatment reduced apoptosis by decreasing caspase-8 and protected dermal fibroblasts. Our data suggest that bryostatin-1 and picolog could be useful in anti-aging skincare, and could have applications in tissue engineering and regenerative medicine.

Immunological challenges associated with artificial skin grafts: available solutions and stem cells in future design of synthetic skin

The repair or replacement of damaged skins is still an important, challenging public health problem. Immune acceptance and long-term survival of skin grafts represent the major problem to overcome in grafting given that in most situations autografts cannot be used. The emergence of artificial skin substitutes provides alternative treatment with the capacity to reduce the dependency on the increasing demand of cadaver skin grafts. Over the years, considerable research efforts have focused on strategies for skin repair or permanent skin graft transplantations. Available skin substitutes include pre- or post-transplantation treatments of donor cells, stem cell-based therapies, and skin equivalents composed of bio-engineered acellular or cellular skin substitutes. However, skin substitutes are still prone to immunological rejection, and as such, there is currently no skin substitute available to overcome this phenomenon. This review focuses on the mechanisms of skin rejection and tolerance induction and outlines in detail current available strategies and alternatives that may allow achieving full-thickness skin replacement and repair.

Three-dimensional hierarchical cultivation of human skin cells on bio-adaptive hybrid fibers

The human skin comprises a complex multi-scale layered structure with hierarchical organization of different cells within the extracellular matrix (ECM). This supportive fiber-reinforced structure provides a dynamically changing microenvironment with specific topographical, mechanical and biochemical cell recognition sites to facilitate cell attachment and proliferation. Current advances in developing artificial matrices for cultivation of human cells concentrate on surface functionalizing of biocompatible materials with different biomolecules like growth factors to enhance cell attachment. However, an often neglected aspect for efficient modulation of cell-matrix interactions is posed by the mechanical characteristics of such artificial matrices. To address this issue, we fabricated biocompatible hybrid fibers simulating the complex biomechanical characteristics of native ECM in human skin. Subsequently, we analyzed interactions of such fibers with human skin cells focusing on the identification of key fiber characteristics for optimized cell-matrix interactions. We successfully identified the mediating effect of bio-adaptive elasto-plastic stiffness paired with hydrophilic surface properties as key factors for cell attachment and proliferation, thus elucidating the synergistic role of these parameters to induce cellular responses. Co-cultivation of fibroblasts and keratinocytes on such fiber mats representing the specific cells in dermis and epidermis resulted in a hierarchical organization of dermal and epidermal tissue layers. In addition, terminal differentiation of keratinocytes at the air interface was observed. These findings provide valuable new insights into cell behaviour in three-dimensional structures and cell-material interactions which can be used for rational development of bio-inspired functional materials for advanced biomedical applications.

In vitro psoriasis models with focus on reconstructed skin models as promising tools in psoriasis research

Psoriasis is a complex chronic immune-mediated inflammatory cutaneous disease associated with the development of inflammatory plaques on the skin. Studies proved that the disease results from a deregulated interplay between skin keratinocytes, immune cells and the environment leading to a persisting inflammatory process modulated by pro-inflammatory cytokines and activation of T cells. However, a major hindrance to study the pathogenesis of psoriasis more in depth and subsequent development of novel therapies is the lack of suitable pre-clinical models mimicking the complex phenotype of this skin disorder. Recent advances in and optimization of three-dimensional skin equivalent models have made them attractive and promising alternatives to the simplistic monolayer cultures, immunological different in vivo models and scarce ex vivo skin explants. Moreover, human skin equivalents are increasing in complexity level to match human biology as closely as possible. Here, we critically review the different types of three-dimensional skin models of psoriasis with relevance to their application potential and advantages over other models. This will guide researchers in choosing the most suitable psoriasis skin model for therapeutic drug testing (including gene therapy via siRNA molecules), or to examine biological features contributing to the pathology of psoriasis. However, the addition of T cells (as recently applied to a de-epidermized dermis-based psoriatic skin model) or other immune cells would make them even more attractive models and broaden their application potential. Eventually, the ultimate goal would be to substitute animal models by three-dimensional psoriatic skin models in the pre-clinical phases of anti-psoriasis candidate drugs. Impact statement The continuous development of novel in vitro models mimicking the psoriasis phenotype is important in the field of psoriasis research, as currently no model exists that completely matches the in vivo psoriasis skin or the disease pathology. This work provides a complete overview of the different available in vitro psoriasis models and suggests improvements for future models. Moreover, a focus was given to psoriatic skin equivalent models, as they offer several advantages over the other models, including commercial availability and validity. The potential and reported applicability of these models in psoriasis pre-clinical research is extensively discussed. As such, this work offers a guide to researchers in their choice of pre-clinical psoriasis model depending on their type of research question.

Study of epithelial differentiation and protein expression of keratinocyte-mesenchyme stem cell co-cultivation on electrospun nylon/B. vulgaris extract composite scaffold

Employing of the composite electrospun scaffold containing herbal extract in conjugation with co-culturing of cells can open up new window to the design of efficient biomaterials for skin tissue regeneration. Here, we introduce the synergistic effect of composite electrospun nanofibrous scaffold of nylon66 loaded with Beta vulgaris (B. vulgaris) (extract of beet roots, a plants whose widely used in Iranian folk medicine as wound healing medicine) and co-culture of mesenchymal stem-cells (MSCs)-human keratinocyte (H-keratino) differentiation towards epithelial lineage. In vitro biocompatibility was examined through MTT assay and epithelial differentiation checked by real-time PCR and immunocytochemistry (ICC) assay after co-culturing of MSCs and H-keratino on proposed scaffold. Significant enhancement in cell proliferation was detected after cell culturing on the composite type of electrospun scaffold containing B. vulgaris. Moreover, after 14days of co-culturing process, gene expression results revealed that both composite and non-composite nylon66 electrospun scaffold promote epithelial differentiation compared to mono-cell culturing of H-keratino in terms of several markers as Cytokeratin 10, Cytokeratin 14 and Involucrin and ICC of some dermal proteins like Cytokeratin 14 and Loricrin. To the best of our knowledge, findings of this study will introduce new way for the generation of novel biomaterials for the development of current skin tissue engineering.

Preparation of a Three-Dimensional Full Thickness Skin Equivalent

In vitro test systems are a promising alternative to animal models. Due to the use of human cells in a three-dimensional arrangement that allows cell-cell or cell-matrix interactions these models may be more predictive for the human situation compared to animal models or two-dimensional cell culture systems. Especially for dermatological research, skin models such as epidermal or full-thickness skin equivalents (FTSE) are used for different applications. Although epidermal models provide highly standardized conditions for risk assessment, FTSE facilitate a cellular crosstalk between the dermal and epidermal layer and thus can be used as more complex models for the investigation of processes such as wound healing, skin development, or infectious diseases. In this chapter, we describe the generation and culture of an FTSE, based on a collagen type I matrix and provide troubleshooting tips for commonly encountered technical problems.

Human Skin Constructs with Spatially Controlled Vasculature Using Primary and iPSC-Derived Endothelial Cells

Vascularization of engineered human skin constructs is crucial for recapitulation of systemic drug delivery and for their long-term survival, functionality, and viable engraftment. In this study, the latest microfabrication techniques are used and a novel bioengineering approach is established to micropattern spatially controlled and perfusable vascular networks in 3D human skin equivalents using both primary and induced pluripotent stem cell (iPSC)-derived endothelial cells. Using 3D printing technology makes it possible to control the geometry of the micropatterned vascular networks. It is verified that vascularized human skin equivalents (vHSEs) can form a robust epidermis and establish an endothelial barrier function, which allows for the recapitulation of both topical and systemic delivery of drugs. In addition, the therapeutic potential of vHSEs for cutaneous wounds on immunodeficient mice is examined and it is demonstrated that vHSEs can both promote and guide neovascularization during wound healing. Overall, this innovative bioengineering approach can enable in vitro evaluation of topical and systemic drug delivery as well as improve the potential of engineered skin constructs to be used as a potential therapeutic option for the treatment of cutaneous wounds.

Growth of Self-Assembled Bio-Organic Nanomatrices for Skin Tissue Engineering — An in vitro Study

In this paper, we have developed self-assembled nanoscale assemblies that were prepared by conjugating furan-2-carboxylic acid-3-aminopropyl amide with the short peptide sequence Gly-His (abbreviated Gly-His-FCAP). To mimic the extracellular matrix of mammalian fibroblasts and keratinocytes, the assemblies were then conjugated with Type I collagen. We then integrated the collagen bound Gly-His-FCAP assemblies with a short peptide sequence derived from salamander skin into the nanoscale assemblies for the first time to impart regenerative and wound healing properties to the composites. The antioxidant, antimicrobial and biodegradable properties were examined and results indicate that the nanocomposites displayed antioxidant properties as displayed by 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay. The biodegradability was found to be gradual. The nanocomposites were also found to inhibit the growth of the fungus Rhizopus sporangia over an 18[Formula: see text]h growth period. As proof of concept, to demonstrate the development of three-dimensional (3D) engineered skin in vitro, 3D printed PLA scaffolds of 2.5[Formula: see text]mm thickness were submerged in media containing nanocomposites and co-cultures of dermal fibroblasts with epidermal keratinocytes mimicking three dimensional skin substitute was examined. Our results indicated that the nanocomposites adhered to and supported cell proliferation and mimicked the components of skin and may have potential applications in skin tissue regeneration.

Potential applications of keratinocytes derived from human embryonic stem cells

Although skin grafting is one of the most advanced cell therapy technique, wide application of skin substitutes is hampered by the difficulty in securing sufficient amount of epidermal substitute. Additionally, in understanding the progression of skin aging and disease, and in screening the cosmetic and pharmaceutical products, there is lack of a satisfactory human skin-specific in vitro model. Recently, human embryonic stem cells (hESCs) have been proposed as an unlimited and reliable cell source to obtain almost all cell types present in the human body. This review focuses on the potential off-the-shelf use of hESC-derived keratinocytes for future clinical applications as well as a powerful in vitro skin model to study skin function and integrity, host-pathogen interactions and disease pathogenesis. Furthermore, we discuss the industrial applications of hESC-derived keratinized multi-layer epithelium which provides a human-like test platform for understanding disease pathogenesis, evaluation of new therapeutic modalities and assessment of the safety and efficacy of skin cosmetics and therapeutics. Overall, we conclude that the hESC-derived keratinocytes have great potential for clinical, research and industrial applications.

Multimaterial and Multiscale Three-Dimensional Bioprinter

We have developed a three-dimensional (3D) bioprinting system capable of multimaterial and multiscale deposition to enable the next generation of “bottom-up” tissue engineering. This area of research resides at the interface of engineering and life sciences. As such, it entails the design and implementation of diverse elements: a novel hydrogel-based bioink, a 3D bioprinter, automation software, and mammalian cell culture. Our bioprinter has three components uniquely combined into a comprehensive tool: syringe pumps connected to a selector valve that allow precise application of up to five different materials with varying viscosities and chemistries, a high velocity/high-precision x–y–z stage to accommodate the most rapid speeds allowable by the printed materials, and temperature control of the bioink reservoirs, lines, and printing environment. Our custom-designed bioprinter is able to print multiple materials (or multiple cell types in the same material) concurrently with various feature sizes (100 μm–1 mm wide; 100 μm–1 cm high). One of these materials is a biocompatible, printable bioink that has been used to test for cell survival within the hydrogel following printing. Hand-printed (HP) controls show that our bioprinter does not adversely affect the viability of the printed cells. Here, we report the design and build of the 3D bioprinter, the optimization of the bioink, and the stability and viability of our printed constructs.

A review of herbal medicines in wound healing

Herbs have been integral to both traditional and non-traditional forms of medicine dating back at least 5000 years. The enduring popularity of herbal medicines may be explained by the perception that herbs cause minimal unwanted side effects. More recently, scientists increasingly rely on modern scientific methods and evidence-based medicine to prove efficacy of herbal medicines and focus on better understanding of mechanisms of their action. However, information concerning quantitative human health benefits of herbal medicines is still rare or dispersed, limiting their proper valuation. Preparations from traditional medicinal plants are often used for wound healing purposes covering a broad area of different skin-related diseases. Herbal medicines in wound management involve disinfection, debridement, and provision of a suitable environment for aiding the natural course of healing. Here we report on 22 plants used as wound healing agents in traditional medicine around the world. The aim of this review is therefore to review herbal medicines, which pose great potential for effective treatment of minor wounds.

Emerging drugs for the treatment of wound healing

INTRODUCTION

Wound healing can be characterized as underhealing, as in the setting of chronic wounds, or overhealing, occurring with hypertrophic scar formation after burn injury. Topical therapies targeting specific biochemical and molecular pathways represent a promising avenue for improving and, in some cases normalizing, the healing process.

AREAS COVERED

A brief overview of both normal and pathological wound healing has been provided, along with a review of the current clinical guidelines and treatment modalities for chronic wounds, burn wounds and scar formation. Next, the major avenues for wound healing drugs, along with drugs currently in development, are discussed. Finally, potential challenges to further drug development, and future research directions are discussed.

EXPERT OPINION

The large body of research concerning wound healing pathophysiology has provided multiple targets for topical therapies. Growth factor therapies with the ability to be targeted for localized release in the wound microenvironment are most promising, particularly when they modulate processes in the proliferative phase of wound healing.

Pumpless microfluidic platform for drug testing on human skin equivalents

Advances in bio-mimetic in vitro human skin models increase the efficiency of drug screening studies. In this study, we designed and developed a microfluidic platform that allows for long-term maintenance of full thickness human skin equivalents (HSE) which are comprised of both the epidermal and dermal compartments. The design is based on the physiologically relevant blood residence times in human skin tissue and allows for the establishment of an air-epidermal interface which is crucial for maturation and terminal differentiation of HSEs. The small scale of the design reduces the amount of culture medium and the number of cells required by 36 fold compared to conventional transwell cultures. Our HSE-on-a-chip platform has the capability to recirculate the medium at desired flow rates without the need for pump or external tube connections. We demonstrate that the platform can be used to maintain HSEs for three weeks with proliferating keratinocytes similar to conventional HSE cultures. Immunohistochemistry analyses show that the differentiation and localization of keratinocytes was successfully achieved, establishing all sub-layers of the epidermis after one week. Basal keratinocytes located at the epidermal-dermal interface remain in a proliferative state for three weeks. We use a transdermal transport model to show that the skin barrier function is maintained for three weeks. We also validate the capability of the HSE-on-a-chip platform to be used for drug testing purposes by examining the toxic effects of doxorubucin on skin cells and structure. Overall, the HSE-on-a-chip is a user-friendly and cost-effective in vitro platform for drug testing of candidate molecules for skin disorders.

Evaluation of in vitro vs. in vivo methods for assessment of dermal absorption of organic flame retardants: a review

There is a growing interest to study human dermal exposure to a large number of chemicals, whether in the indoor or outdoor environment. Such studies are essential to predict the systemic exposure to xenobiotic chemicals for risk assessment purposes and to comply with various regulatory guidelines. However, very little is currently known about human dermal exposure to persistent organic pollutants. While recent pharmacokinetic studies have highlighted the importance of dermal contact as a pathway of human exposure to brominated flame retardants, risk assessment studies had to apply assumed values for percutaneous penetration of various flame retardants (FRs) due to complete absence of specific experimental data on their human dermal bioavailability. Therefore, this article discusses the current state-of-knowledge on the significance of dermal contact as a pathway of human exposure to FRs. The available literature on in vivo and in vitro methods for assessment of dermal absorption of FRs in human and laboratory animals is critically reviewed. Finally, a novel approach for studying human dermal absorption of FRs using in vitro three-dimensional (3D) human skin equivalent models is presented and the challenges facing future dermal absorption studies on FRs are highlighted.

Biomimetic 3D Tissue Models for Advanced High-Throughput Drug Screening

Most current drug screening assays used to identify new drug candidates are 2D cell-based systems, even though such in vitro assays do not adequately re-create the in vivo complexity of 3D tissues. Inadequate representation of the human tissue environment during a preclinical test can result in inaccurate predictions of compound effects on overall tissue functionality. Screening for compound efficacy by focusing on a single pathway or protein target, coupled with difficulties in maintaining long-term 2D monolayers, can serve to exacerbate these issues when using such simplistic model systems for physiological drug screening applications. Numerous studies have shown that cell responses to drugs in 3D culture are improved from those in 2D, with respect to modeling in vivo tissue functionality, which highlights the advantages of using 3D-based models for preclinical drug screens. In this review, we discuss the development of microengineered 3D tissue models that accurately mimic the physiological properties of native tissue samples and highlight the advantages of using such 3D microtissue models over conventional cell-based assays for future drug screening applications. We also discuss biomimetic 3D environments, based on engineered tissues as potential preclinical models for the development of more predictive drug screening assays for specific disease models.

Using organotypic (raft) epithelial tissue cultures for the biosynthesis and isolation of infectious human papillomaviruses

Papillomaviruses have a strict tropism for epithelial cells, and they are fully reliant on cellular differentiation for completion of their life cycles, resulting in the production of progeny virions. Thus, a permissive environment for full viral replication in vitro-wherein virion morphogenesis occurs under cooperative viral and cellular cues-requires the cultivation of epithelium. Presented in the first section of this unit is a protocol to grow differentiating epithelial tissues that mimic many important morphological and biochemical aspects of normal skin. The technique involves growing epidermal cells atop a dermal equivalent consisting of live fibroblasts and a collagen lattice. Epithelial stratification and differentiation ensues when the keratinocyte-dermal equivalent is placed at the air-liquid interface. The apparent floating nature of the cell-matrix in this method led to the nickname "raft" cultures. The general technique can be applied to normal low passage keratinocytes, to cells stably transfected with papillomavirus genes or genomes, or keratinocytes established from neoplastic lesions. However, infectious papillomavirus particles have only been isolated from organotypic epithelial cultures initiated with cells that maintain oncogenic human papillomavirus genomes in an extrachomosomal replicative form. The second section of this unit is dedicated to a virion isolation method that minimizes aerosol and skin exposure to these human carcinogens. Although the focus of the protocols is on the growth of tissues that yields infectious papillomavirus progeny, this culture system facilitates the investigation of these fastidious viruses during their complex replicative cycles, and raft tissues can be manipulated and harvested at any point during the process. Importantly, a single-step virus growth cycle is achieved in this process, as it is unlikely that progeny virions are released to initiate subsequent rounds of infection.

Practical problems encountered during the cultivation of an open-source reconstructed human epidermis model on a polycarbonate membrane and protein quantification

During recent years, the importance of in vitro technology in skin research has increased significantly. A variety of skin culture models have been developed and commercialized. In this respect, the availability of reconstructed human epidermis (RHE) equivalents represents a significant improvement compared to the use of monolayer cultures. However, when an in-house RHE model is being developed, researchers might encounter some difficulties during cultivation. The scope of this paper is to report our experiences and practical problems with the development of a three-dimensional RHE model cultured on a polycarbonate membrane. Some important issues including cell density, the use of lysing enzymes, culture media, cell storage and viability, cell confluency and protein extraction are reported and optional solutions are given.