Cellular human tissue-engineered skin substitutes investigated for deep and difficult to heal injuries

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.

Approach to the Atypical Wound

The heterogeneity of atypical wounds can present diagnostic and therapeutic challenges; however, as the prevalence of atypical wounds grows worldwide, prompt and accurate management is increasingly an essential skill for dermatologists. Addressing the underlying cause of an atypical wound is critical for successful outcomes. An integrated approach with a focus on pain management and patient engagement is recommended to facilitate enduring wound closure. Advances in treatment, in addition to further research and clinical training, are necessary to address the expanding burden of atypical wounds.

Recent advances of electrospun strategies in topical products encompassing skincare and dermatological treatments

As the potential applications of electrospinning in healthcare continue to be explored, along with advancements in industrial-scale solutions and the emergence of portable electrospinning devices, some researchers have explored electrospinning technology in topical products, including its application in skincare, such as facial masks, beauty patches, sunscreen, and dermatological treatments for conditions like atopic dermatitis, psoriasis, acne, skin cancer, etc. In this review, we first outline the fundamental principles of electrospinning and provide an overview of existing solutions for large-scale production and the components and functionalities of portable spinning devices. Based on the essential functionalities required for skincare products and the mechanisms and treatment methods for the aforementioned dermatological diseases, we summarize the potential advantages of electrospinning technology in these areas, including encapsulation, sustained release, large surface area, and biocompatibility, among others. Furthermore, considering the further commercialization and clinical development of electrospinning technology, we offer our insights on current challenges and future perspectives in these areas, including issues such as ingredients, functionality, residue concerns, environmental impact, and efficiency issues.

Phototoxicity in vitro and safety in vivo of the emulsion photosensitizer based on furanocoumarins of Heracleum sosnowskyi

The use of plants such as giant hogweed as raw materials for the manufacture of dosage forms has been little explored. In this study, we utilized furanocoumarins from the Heracleum sosnowskyi plant to create an experimental emulsion dosage form (EmFHS). The EmFHS was finely dispersed (481.8 nm ± 71.1 nm), shelf-stable, and contained predominantly 8-methoxypsoralen at a concentration of 1 mg/ml. Phototoxicity analysis of EmFHS for THP-1 cells under UV (365 nm) irradiation showed an IC50 of 19.1 µg/ml (24 h) and 6.3 µg/ml (48 h). In relation to spheroids (L929), EmFHS exhibited a phototoxic effect in the concentration range of 31.25-125 µg/ml8-MOP. A full phototoxic effect was observed 48 h after UV irradiation. The phototoxic effect of EmFHS in vitro was dose-dependent and comparable to the effect of emulsion synthetic 8-methoxypsoralen and chlorin e6 solution. EmFHS cytotoxicity was caused solely by UV radiation, and toxicity in the dark was minimal. EmFHS, administered at a dose of 3 mg/kg8-MOP, was found to be safe after a single intravenous administration to rats. It had a photosensitizing effect in the form of local photodermatitis when exposed to UV irradiation at a dose of 44 J/cm2. The biokinetics of emulsion furanocoumarins showed that the phototoxic effect of EmFHS is due to the high penetration ability of the emulsion into cells of spheroids. At the same time, it has a low degree of cumulation when administered intravenously. The obtained data suggest that EmFHS may be a promising treatment for PUVA therapy of various dermatological diseases. Additionally, the plant Heracleum sosnowskyi shows potential as a basis for creating new dosage forms with phototherapeutic effects.

Skin graft with dermis and appendages generated in vivo by cell competition

Autologous skin grafting is a standard treatment for skin defects such as burns. No artificial skin substitutes are functionally equivalent to autologous skin grafts. The cultured epidermis lacks the dermis and does not engraft deep wounds. Although reconstituted skin, which consists of cultured epidermal cells on a synthetic dermal substitute, can engraft deep wounds, it requires the wound bed to be well-vascularized and lacks skin appendages. In this study, we successfully generate complete skin grafts with pluripotent stem cell-derived epidermis with appendages on p63 knockout embryos' dermis. Donor pluripotent stem cell-derived keratinocytes encroach the embryos' dermis by eliminating p63 knockout keratinocytes based on cell-extracellular matrix adhesion mediated cell competition. Although the chimeric skin contains allogenic dermis, it is engraftable as long as autologous grafts. Furthermore, we could generate semi-humanized skin segments by human keratinocytes injection into the amnionic cavity of p63 knockout mice embryos. Niche encroachment opens the possibility of human skin graft production in livestock animals.

Cellular therapeutics and immunotherapies in wound healing - on the pulse of time?

Chronic, non-healing wounds represent a significant challenge for healthcare systems worldwide, often requiring significant human and financial resources. Chronic wounds arise from the complex interplay of underlying comorbidities, such as diabetes or vascular diseases, lifestyle factors, and genetic risk profiles which may predispose extremities to local ischemia. Injuries are further exacerbated by bacterial colonization and the formation of biofilms. Infection, consequently, perpetuates a chronic inflammatory microenvironment, preventing the progression and completion of normal wound healing. The current standard of care (SOC) for chronic wounds involves surgical debridement along with localized wound irrigation, which requires inpatient care under general anesthesia. This could be followed by, if necessary, defect coverage via a reconstructive ladder utilizing wound debridement along with skin graft, local, or free flap techniques once the wound conditions are stabilized and adequate blood supply is restored. To promote physiological wound healing, a variety of approaches have been subjected to translational research. Beyond conventional wound healing drugs and devices that currently supplement treatments, cellular and immunotherapies have emerged as promising therapeutics that can behave as tailored therapies with cell- or molecule-specific wound healing properties. However, in contrast to the clinical omnipresence of chronic wound healing disorders, there remains a shortage of studies condensing the current body of evidence on cellular therapies and immunotherapies for chronic wounds. This review provides a comprehensive exploration of current therapies, experimental approaches, and translational studies, offering insights into their efficacy and limitations. Ultimately, we hope this line of research may serve as an evidence-based foundation to guide further experimental and translational approaches and optimize patient care long-term.

Bacterial fluorescence imaging as a predictor of skin graft integration in burn wounds

BACKGROUND

Split-thickness skin graft (STSG)1 integration rates are susceptible to improvement. Infection and/or biofilm should be appropriately addressed prior to grafting to improve the likelihood of graft-take. Incorporating technological aids such as fluorescence (FL)2 imaging (MolecuLight®), which accurately locates areas of bacterial loads above 104 CFU/gr, for graft site assessment and preparation could yield better outcomes.

METHODS

This single-center, prospective observational study included adult burn patients with previously infected wounds that had been deemed clinically and microbiologically clean and were therefore candidates for grafting. Prior to grafting, a FL imaging assessment (blinded to the surgical team) localized areas positive for moderate-high bacterial loads (>104 CFU/gr). Intra-operatively, a standard swab sample from the recipient site was collected by the surgical team. Postoperatively, areas positive/negative for FL and areas of graft take and failure were overlapped and measured (cm2) over a 2D schematic. The performance and accuracy of FL imaging and swab sampling in relation to graft outcomes were assessed.

RESULTS

38 patients were enrolled in the study. The mean total body surface area (TBSA)3 involvement was 14.5 ± 12.4 % [range 0.8 - 40.2 %]. 25/38 of the subjects enrolled had complete graft take while 13 had partial graft losses. There were no total losses. FL-imaging was positive in 100 % of losses versus 31 % (4/13) of the swab microbiology. FL-imaging was found to have a sensitivity of 86 %, specificity of 98 %, PPV of 72 %, NPV of 99 %, and an accuracy of 94 % for predicting any type or range of graft loss in the entire cohort. Meanwhile, the sensitivity of microbiology from swab samples was 30 %, with a specificity of 76 %.

CONCLUSIONS

FL imaging is an accurate method for assessing recipient sites and predicting the outcome of a skin graft among burn patients. These findings suggest that FL imaging can inform better decision-making surrounding grafts that may lead to better outcomes.

LEVEL OF EVIDENCE

Level IIA, Therapeutic study.

Rapid innervation and physiological epidermal regeneration by bioengineered dermis implanted in mouse

Tissue-engineered skin substitutes are promising tools to cover large and deep skin defects. However, the lack of a synergic and fast regeneration of the vascular network, nerves, and skin appendages limits complete skin healing and impairs functional recovery. It has been highlighted that an ideal skin substitute should mimic the structure of the native tissue to enhance clinical effectiveness. Here, we produced a pre-vascularized dermis (PVD) comprised of fibroblasts embedded in their own extracellular matrix (ECM) and a capillary-like network. Upon implantation in a mouse full-thickness skin defect model, we observed a very early innervation of the graft in 2 weeks. In addition, mouse capillaries and complete epithelialization were detectable as early as 1 week after implantation and, skin appendages developed in 2 weeks. These anatomical features underlie the interaction with the skin nerves, thus providing a further cue for reinnervation guidance. Further, the graft displays mechanical properties, collagen density, and assembly features very similar to the host tissue. Taken together our data show that the pre-existing ECM components of the PVD, physiologically organized and assembled similarly to the native tissue, support a rapid regeneration of dermal tissue. Therefore, our results suggest a promising potential for PVD in skin regeneration.

Compound Microalgae-Type Biofunctional Hydrogel for Wound Repair during Full-Thickness Skin Injuries

A dual biofunctional hydrogel (HQCS-SP) wound dressing, offering antibacterial properties and a biological response, was innovatively designed and developed to repair full-layer skin defects. The HQCS-SP hydrogel creates an artificial matrix that facilitates cell recruitment, extracellular matrix deposition, exhibiting exceptional tissue affinity, robust self-healing, effective hemostatic capabilities and accelerates wound healing. It is synthesized by crosslinking modified chitosan (HQCS) with spirulina protein (SP) and Fe3+. The HQCS provides antibacterial, antioxidant, good tissue affinity and excellent hemostasis performance. The incorporation of SP not only reinforces the antioxidant, antibacterial, anti-inflammatory, and pro-angiogenesis effects but also participates in the regulation of signal pathways and promotes wound healing. Therefore, this study offers a new visual angle for the design of advanced functional trauma dressings with great application potential in the bio-medical field.

Dual delivery gene-activated scaffold directs fibroblast activity and keratinocyte epithelization

Fibroblasts are the most abundant cell type in dermal skin and keratinocytes are the most abundant cell type in the epidermis; both play a crucial role in wound remodeling and maturation. We aim to assess the functionality of a novel dual gene activated scaffold (GAS) on human adult dermal fibroblasts (hDFs) and see how the secretome produced could affect human dermal microvascular endothelial cells (HDMVECs) and human epidermal keratinocyte (hEKs) growth and epithelization. Our GAS is a collagen chondroitin sulfate scaffold loaded with pro-angiogenic stromal derived factor (SDF-1α) and/or an anti-aging β-Klotho plasmids. hDFs were grown on GAS for two weeks and compared to gene-free scaffolds. GAS produced a significantly better healing outcome in the fibroblasts than in the gene-free scaffold group. Among the GAS groups, the dual GAS induced the most potent pro-regenerative maturation in fibroblasts with a downregulation in proliferation (twofold, p < 0.05), fibrotic remodeling regulators TGF-β1 (1.43-fold, p < 0.01) and CTGF (1.4-fold, p < 0.05), fibrotic cellular protein α-SMA (twofold, p < 0.05), and fibronectin matrix deposition (twofold, p < 0.05). The dual GAS secretome also showed enhancements of paracrine keratinocyte pro-epithelializing ability (1.3-fold, p < 0.05); basement membrane regeneration through laminin (6.4-fold, p < 0.005) and collagen IV (8.7-fold, p < 0.005) deposition. Our findings demonstrate enhanced responses in dual GAS containing hDFs by proangiogenic SDF-1α and β-Klotho anti-fibrotic rejuvenating activities. This was demonstrated by activating hDFs on dual GAS to become anti-fibrotic in nature while eliciting wound repair basement membrane proteins; enhancing a proangiogenic HDMVECs paracrine signaling and greater epithelisation of hEKs.

Analysis of bioprinting strategies for skin diseases and injuries through structural and temporal dynamics: historical perspectives, research hotspots, and emerging trends

This study endeavors to investigate the progression, research focal points, and budding trends in the realm of skin bioprinting over the past decade from a structural and temporal dynamics standpoint. Scholarly articles on skin bioprinting were obtained from WoSCC. A series of bibliometric tools comprising R software, CiteSpace, HistCite, and an alluvial generator were employed to discern historical characteristics, evolution of active topics, and upcoming tendencies in the area of skin bioprinting. Over the past decade, there has been a consistent rise in research interest in skin bioprinting, accompanied by an extensive array of meaningful scientific collaborations. Concurrently, diverse dynamic topics have emerged during various periods, as substantiated by an aggregate of 22 disciplines, 74 keywords, and 187 references demonstrating citation bursts. Four burgeoning research subfields were discerned through keyword clustering-namely, #3 'in situbioprinting', #6 'vascular', #7 'xanthan gum', and #8 'collagen hydrogels'. The keyword alluvial map reveals that Module 1, including 'transplantation' etc, has primarily dominated the research module over the previous decade, maintaining enduring relevance despite annual shifts in keyword focus. Additionally, we mapped out the top six key modules from 2023 being 'silk fibroin nanofiber', 'system', 'ionic liquid', 'mechanism', and 'foot ulcer'. Three recent research subdivisions were identified via timeline visualization of references, particularly Clusters #0 'wound healing', #4 'situ mineralization', and #5 '3D bioprinter'. Insights derived from bibliometric analyses illustrate present conditions and trends in skin bioprinting research, potentially aiding researchers in pinpointing central themes and pioneering novel investigative approaches in this field.

Advanced function, design and application of skin substitutes for skin regeneration

The development of skin substitutes aims to replace, mimic, or improve the functions of human skin, regenerate damaged skin tissue, and replace or enhance skin function. This includes artificial skin, scaffolds or devices designed for treatment, imitation, or improvement of skin function in wounds and injuries. Therefore, tremendous efforts have been made to develop functional skin substitutes. However, there is still few reports systematically discuss the relationship between the advanced function and design requirements. In this paper, we review the classification, functions, and design requirements of artificial skin or skin substitutes. Different manufacturing strategies for skin substitutes such as hydrogels, 3D/4D printing, electrospinning, microfluidics are summarized. This review also introduces currently available skin substitutes in clinical trials and on the market and the related regulatory requirements. Finally, the prospects and challenges of skin substitutes in the field of tissue engineering are discussed.

Current Biomedical Applications of 3D-Printed Hydrogels

Three-dimensional (3D) printing, also known as additive manufacturing, has revolutionized the production of physical 3D objects by transforming computer-aided design models into layered structures, eliminating the need for traditional molding or machining techniques. In recent years, hydrogels have emerged as an ideal 3D printing feedstock material for the fabrication of hydrated constructs that replicate the extracellular matrix found in endogenous tissues. Hydrogels have seen significant advancements since their first use as contact lenses in the biomedical field. These advancements have led to the development of complex 3D-printed structures that include a wide variety of organic and inorganic materials, cells, and bioactive substances. The most commonly used 3D printing techniques to fabricate hydrogel scaffolds are material extrusion, material jetting, and vat photopolymerization, but novel methods that can enhance the resolution and structural complexity of printed constructs have also emerged. The biomedical applications of hydrogels can be broadly classified into four categories-tissue engineering and regenerative medicine, 3D cell culture and disease modeling, drug screening and toxicity testing, and novel devices and drug delivery systems. Despite the recent advancements in their biomedical applications, a number of challenges still need to be addressed to maximize the use of hydrogels for 3D printing. These challenges include improving resolution and structural complexity, optimizing cell viability and function, improving cost efficiency and accessibility, and addressing ethical and regulatory concerns for clinical translation.

Current Biomaterials for Wound Healing

Wound healing is the body's process of injury recovery. Skin healing is divided into four distinct overlapping phases: hemostasis, inflammation, proliferation, and remodeling. Cell-to-cell interactions mediated by both cytokines and chemokines are imperative for the transition between these phases. Patients can face difficulties in the healing process due to the wound being too large, decreased vascularization, infection, or additional burdens of a systemic illness. The field of tissue engineering has been investigating biomaterials as an alternative for skin regeneration. Biomaterials used for wound healing may be natural, synthetic, or a combination of both. Once a specific biomaterial is selected, it acts as a scaffold for skin regeneration. When the scaffold is applied to a wound, it allows for the upregulation of distinct molecular signaling pathways important for skin repair. Although tissue engineering has made great progress, more research is needed in order to support the use of biomaterials for wound healing for clinical translation.

Metal-Phenolic Networks for Chronic Wounds Therapy

Chronic wounds are recalcitrant complications of a variety of diseases, with pathologic features including bacterial infection, persistent inflammation, and proliferation of reactive oxygen species (ROS) levels in the wound microenvironment. Currently, the use of antimicrobial drugs, debridement, hyperbaric oxygen therapy, and other methods in clinical for chronic wound treatment is prone to problems such as bacterial resistance, wound expansion, and even exacerbation. In recent years, researchers have proposed many novel materials for the treatment of chronic wounds targeting the disease characteristics, among which metal-phenolic networks (MPNs) are supramolecular network structures that utilize multivalent metal ions and natural polyphenols complexed through ligand bonds. They have a flexible and versatile combination of structural forms and a variety of formations (nanoparticles, coatings, hydrogels, etc.) that can be constructed. Functionally, MPNs combine the chemocatalytic and bactericidal properties of metal ions as well as the anti-inflammatory and antioxidant properties of polyphenol compounds. Together with the excellent properties of rapid synthesis and negligible cytotoxicity, MPNs have attracted researchers' great attention in biomedical fields such as anti-tumor, anti-bacterial, and anti-inflammatory. This paper will focus on the composition of MPNs, the mechanisms of MPNs for the treatment of chronic wounds, and the application of MPNs in novel chronic wound therapies.

Manufacturing the multiscale vascular hierarchy: progress toward solving the grand challenge of tissue engineering

In human vascular anatomy, blood flows from the heart to organs and tissues through a hierarchical vascular tree, comprising large arteries that branch into arterioles and further into capillaries, where gas and nutrient exchange occur. Engineering a complete, integrated vascular hierarchy with vessels large enough to suture, strong enough to withstand hemodynamic forces, and a branching structure to permit immediate perfusion of a fluidic circuit across scales would be transformative for regenerative medicine (RM), enabling the translation of engineered tissues of clinically relevant size, and perhaps whole organs. How close are we to solving this biological plumbing problem? In this review, we highlight advances in engineered vasculature at individual scales and focus on recent strategies to integrate across scales.

In vitro comparison of human plasma-based and self-assembled tissue-engineered skin substitutes: two different manufacturing processes for the treatment of deep and difficult to heal injuries

Background

The aim of this in vitro study was to compare side-by-side two models of human bilayered tissue-engineered skin substitutes (hbTESSs) designed for the treatment of severely burned patients. These are the scaffold-free self-assembled skin substitute (SASS) and the human plasma-based skin substitute (HPSS).

Methods

Fibroblasts and keratinocytes from three humans were extracted from skin biopsies (N = 3) and cells from the same donor were used to produce both hbTESS models. For SASS manufacture, keratinocytes were seeded over three self-assembled dermal sheets comprising fibroblasts and the extracellular matrix they produced (n = 12), while for HPSS production, keratinocytes were cultured over hydrogels composed of fibroblasts embedded in either plasma as unique biomaterial (Fibrin), plasma combined with hyaluronic acid (Fibrin-HA) or plasma combined with collagen (Fibrin-Col) (n/biomaterial = 9). The production time was 46-55 days for SASSs and 32-39 days for HPSSs. Substitutes were characterized by histology, mechanical testing, PrestoBlue™-assay, immunofluorescence (Ki67, Keratin (K) 10, K15, K19, Loricrin, type IV collagen) and Western blot (type I and IV collagens).

Results

The SASSs were more resistant to tensile forces (p-value < 0.01) but less elastic (p-value < 0.001) compared to HPSSs. A higher number of proliferative Ki67+ cells were found in SASSs although their metabolic activity was lower. After epidermal differentiation, no significant difference was observed in the expression of K10, K15, K19 and Loricrin. Overall, the production of type I and type IV collagens and the adhesive strength of the dermal-epidermal junction was higher in SASSs.

Conclusions

This study demonstrates, for the first time, that both hbTESS models present similar in vitro biological characteristics. However, mechanical properties differ and future in vivo experiments will aim to compare their wound healing potential.

Updates on Recent Clinical Assessment of Commercial Chronic Wound Care Products

Impaired wound healing after trauma, disorders, and surgeries impact millions of people globally every year. Dysregulation in orchestrated healing mechanisms and underlying medical complications make chronic wound management extremely challenging. Besides standard-of-care treatments including broad spectrum antibiotics and wound-debridement, novel adjuvant therapies are clinically tested and commercialized. These include topical agents, skin substitutes, growth factor delivery, and stem cell therapies. With a goal to overcome factors playing pivotal role in delayed wound healing, researchers are exploring novel approaches to elicit desirable healing outcomes in chronic wounds. Although recent innovations in wound care products, therapies, and devices are extensively reviewed in past, a comprehensive review summarizing their clinical outcomes is surprisingly lacking. Herein, this work reviews the commercially available wound care products and their performance in clinical trials to provide a statistically comprehensive understanding of their safety and efficacy. The performance and suitability of various commercial wound care platforms, including xenogeneic and allogenic products, wound care devices, and novel biomaterials, are discussed for chronic wounds. The current clinical evaluation will provide a comprehensive understanding of the benefits and drawbacks of the most-recent approaches and will enable researchers and healthcare providers to develop next-generation technologies for chronic wound management.

Adipose Mesenchymal Stem Cell Derived Exosomes Promote Keratinocytes and Fibroblasts Embedded in Collagen/Platelet-Rich Plasma Scaffold and Accelerate Wound Healing

Engineered skin substitutes derived from human skin significantly reduce inflammatory reactions mediated by foreign/artificial materials and are consequently easier to use for clinical application. Type I collagen is a main component of the extracellular matrix during wound healing and has excellent biocompatibility, and platelet-rich plasma can be used as the initiator of the healing cascade. Adipose mesenchymal stem cell derived exosomes are crucial for tissue repair and play key roles in enhancing cell regeneration, promoting angiogenesis, regulating inflammation, and remodeling extracellular matrix. Herein, Type I collagen and platelet-rich plasma, which provide natural supports for keratinocyte and fibroblast adhesion, migration, and proliferation, are mixed to form a stable 3D scaffold. Adipose mesenchymal stem cell derived exosomes are added to the scaffold to improve the performance of the engineered skin. The physicochemical properties of this cellular scaffold are analyzed, and the repair effect is evaluated in a full-thickness skin defect mouse model. The cellular scaffold reduces the level of inflammation and promotes cell proliferation and angiogenesis to accelerate wound healing. Proteomic analysis shows that exosomes exhibit excellent anti-inflammatory and proangiogenic effects in collagen/platelet-rich plasma scaffolds. The proposed method provides a new therapeutic strategy and theoretical basis for tissue regeneration and wound repair.

Engineered extracellular vesicles from human skin cells induce pro-β-cell conversions in pancreatic ductal cells

Direct nuclear reprogramming has the potential to enable the development of β cell replacement therapies for diabetes that do not require the use of progenitor/stem cell populations. However, despite their promise, current approaches to β cell-directed reprogramming rely heavily on the use of viral vectors. Here we explored the use of extracellular vesicles (EVs) derived from human dermal fibroblasts (HDFs) as novel non-viral carriers of endocrine cell-patterning transcription factors, to transfect and transdifferentiate pancreatic ductal epithelial cells (PDCs) into hormone-expressing cells. Electrotransfection of HDFs with expression plasmids for Pdx1, Ngn3, and MafA (PNM) led to the release of EVs loaded with PNM at the gene, mRNA, and protein level. Exposing PDC cultures to PNM-loaded EVs led to successful transfection and increased PNM expression in PDCs, which ultimately resulted in endocrine cell-directed conversions based on the expression of insulin/c-peptide, glucagon, and glucose transporter 2 (Glut2). These findings were further corroborated in vivo in a mouse model following intraductal injection of PNM- vs sham-loaded EVs. Collectively these findings suggest that dermal fibroblast-derived EVs could potentially serve as a powerful platform technology for the development and deployment of non-viral reprogramming-based cell therapies for insulin-dependent diabetes.

Comparison of Two Human Skin Cell Isolation Protocols and Their Influence on Keratinocyte and Fibroblast Culture

For the development of advanced therapies, the use of primary cells instead of cell lines is preferred. The manufacture of human tissue-engineered skin substitutes requires efficient isolation and culture protocols allowing a massive expansion of the cells in culture from an initial specimen of a minimal size. This study compared two skin cell isolation protocols, routinely applied in two clinical laboratories. Epithelial (keratinocytes) and dermal (fibroblasts) cells were isolated and cultured from three human skin biopsies (N = 3). The two-step digestion protocol (LOEX-Protocol) firstly used thermolysin to enzymatically disrupt the dermal-epidermal junction while, for the one-step digestion protocol (UPCIT-Protocol), mechanical detachment with scissors was applied. Then, the epidermal and dermal layers were digested, respectively, to achieve cell isolation. The cell size, viability, yield and growth were analyzed over five passages (P). The colony-forming efficiency (CFE) and Keratin 19 (K19) expression of epithelial cells were also assessed after P0 and P1. Regarding the dermal cells, no significant differences were observed in the tested parameters of isolation and culture. However, for the epithelial cells, viability was higher (93% vs. 85%) and the number of cells extracted per cm2 of skin was 3.4 times higher using the LOEX-Protocol compared to the UPCIT-Protocol. No significant difference was observed for any parameter once the keratinocytes were cultured from P1 to P4. The CFE and K19 expression decreased from P0 to P1 in both protocols, probably due to the culture process. This study shows that both protocols enable the efficient isolation of skin dermal and epithelial cells and subsequent culture to produce grafts destined for the treatment of patients.

Skin substitutes as treatment for chronic wounds: current and future directions

Chronic wounds such as diabetic foot ulcers and venous leg ulcers place a significant burden on the healthcare system and in some cases, have 5-year mortality rates comparable to cancer. They negatively impact patients' quality of life due to pain, odor, decreased mobility, and social isolation. Skin substitutes are an advanced therapy recommended for wounds that fail to show decrease in size with standard care. The choice of substitute used should be based on evidence, which often differs based on wound etiology. There are more than 75 skin substitutes currently available, and that number is rising. In this review, we discuss current management and future directions of chronic wounds while providing a review of available randomized control trial data for various skin substitutes.

Progress in Research on Stem Cells in Neonatal Refractory Diseases

With the development and progress of medical technology, the survival rate of premature and low-birth-weight infants has increased, as has the incidence of a variety of neonatal diseases, such as hypoxic-ischemic encephalopathy, intraventricular hemorrhage, bronchopulmonary dysplasia, necrotizing enterocolitis, and retinopathy of prematurity. These diseases cause severe health conditions with poor prognoses, and existing control methods are ineffective for such diseases. Stem cells are a special type of cells with self-renewal and differentiation potential, and their mechanisms mainly include anti-inflammatory and anti-apoptotic properties, reducing oxidative stress, and boosting regeneration. Their paracrine effects can affect the microenvironment in which they survive, thereby affecting the biological characteristics of other cells. Due to their unique abilities, stem cells have been used in treating various diseases. Therefore, stem cell therapy may open up the possibility of treating such neonatal diseases. This review summarizes the research progress on stem cells and exosomes derived from stem cells in neonatal refractory diseases to provide new insights for most researchers and clinicians regarding future treatments. In addition, the current challenges and perspectives in stem cell therapy are discussed.

Engineered Extracellular Vesicles Derived from Dermal Fibroblasts Attenuate Inflammation in a Murine Model of Acute Lung Injury

Acute respiratory distress syndrome (ARDS) represents a significant burden to the healthcare system, with ≈200 000 cases diagnosed annually in the USA. ARDS patients suffer from severe refractory hypoxemia, alveolar-capillary barrier dysfunction, impaired surfactant function, and abnormal upregulation of inflammatory pathways that lead to intensive care unit admission, prolonged hospitalization, and increased disability-adjusted life years. Currently, there is no cure or FDA-approved therapy for ARDS. This work describes the implementation of engineered extracellular vesicle (eEV)-based nanocarriers for targeted nonviral delivery of anti-inflammatory payloads to the inflamed/injured lung. The results show the ability of surfactant protein A (SPA)-functionalized IL-4- and IL-10-loaded eEVs to promote intrapulmonary retention and reduce inflammation, both in vitro and in vivo. Significant attenuation is observed in tissue damage, proinflammatory cytokine secretion, macrophage activation, influx of protein-rich fluid, and neutrophil infiltration into the alveolar space as early as 6 h post-eEVs treatment. Additionally, metabolomics analyses show that eEV treatment causes significant changes in the metabolic profile of inflamed lungs, driving the secretion of key anti-inflammatory metabolites. Altogether, these results establish the potential of eEVs derived from dermal fibroblasts to reduce inflammation, tissue damage, and the prevalence/progression of injury during ARDS via nonviral delivery of anti-inflammatory genes/transcripts.

Autologous Skin Grafts, versus Tissue-engineered Skin Constructs: A Systematic Review and Meta-analysis

For over 100 years, autologous skin grafts have remained the gold standard for the reconstruction of wounds but are limited in availability. Acellular tissue-engineered skin constructs (acellular TCs) and cellular tissue-engineered skin constructs (cellular TCs) may address these limitations. This systematic review and meta-analysis compare outcomes between them.

Methods

A systematic review was conducted using PRISMA guidelines, querying MEDLINE, Embase, Web of Science, and Cochrane to assess graft incorporation, failure, and wound healing. Case reports/series, reviews, in vitro/in vivo work, non-English articles or articles without full text were excluded.

Results

Sixty-six articles encompassing 4076 patients were included. No significant differences were found between graft failure rates (P = 0.07) and mean difference of percent reepithelialization (p = 0.92) when split-thickness skin grafts were applied alone versus co-grafted with acellular TCs. Similar mean Vancouver Scar Scale was found for these two groups (p = 0.09). Twenty-one studies used at least one cellular TC. Weighted averages from pooled results did not reveal statistically significant differences in mean reepithelialization or failure rates for epidermal cellular TCs compared with split-thickness skin grafts (p = 0.55).

Conclusions

This systematic review is the first to illustrate comparable functional and wound healing outcomes between split-thickness skin grafts alone and those co-grafted with acellular TCs. The use of cellular TCs seems promising from preliminary findings. However, these results are limited in clinical applicability due to the heterogeneity of study data, and further level 1 evidence is required to determine the safety and efficacy of these constructs.

3D-printed placental-derived bioinks for skin tissue regeneration with improved angiogenesis and wound healing properties

Extracellular matrix (ECM)-based bioinks has attracted much attention in recent years for 3D printing of native-like tissue constructs. Due to organ unavailability, human placental ECM can be an alternative source for the construction of 3D print composite scaffolds for the treatment of deep wounds. In this study, we use different concentrations (1.5%, 3% and 5%w/v) of ECM derived from the placenta, sodium-alginate and gelatin to prepare a printable bioink biomimicking natural skin. The printed hydrogels' morphology, physical structure, mechanical behavior, biocompatibility, and angiogenic property are investigated. The optimized ECM (5%w/v) 3D printed scaffold is applied on full-thickness wounds created in a mouse model. Due to their unique native-like structure, the ECM-based scaffolds provide a non-cytotoxic microenvironment for cell adhesion, infiltration, angiogenesis, and proliferation. In contrast, they do not show any sign of immune response to the host. Notably, the biodegradation, swelling rate, mechanical property, cell adhesion and angiogenesis properties increase with the increase of ECM concentrations in the construct. The ECM 3D printed scaffold implanted into deep wounds increases granulation tissue formation, angiogenesis, and re-epithelialization due to the presence of ECM components in the construct, when compared with printed scaffold with no ECM and no treatment wound. Overall, our findings demonstrate that the 5% ECM 3D scaffold supports the best deep wound regeneration in vivo, produces a skin replacement with a cellular structure comparable to native skin.

Natural Polymer-Based Thin Film Strategies for Skin Regeneration in Lieu of Regenerative Dentistry

Wound healing (WH) is a complex and dynamic process that comprises of a series of molecular and cellular events that occur after tissue injury. The injuries of the maxillofacial and oral region caused by trauma or surgery result in undesirable WH such as delayed wound closure and formation of scar tissue. Skin tissue engineering (TE)/regeneration is an emerging approach toward faster, superior, and more effective resolution of clinically significant wounds effectively. A multitude of TE principles approaches are being put to action for the fabrication of hydrogels, electrospun sheets, 3D scaffolds, and thin films that can be used as wound dressings materials, sutures, or skin substitutes. Thin films are advantageous over other materials owing to their flexibility, ability to provide a barrier against external contamination, easy gaseous exchange, and easy monitoring of wounds. This review focuses on wound-dressing films and their significance and discusses various fabrication techniques. In addition, we explore various natural biopolymers that can be used for fabrication of skin TE materials. Impact Statement In this review article, critical evaluations of natural polymers used in skin regeneration were discussed. Further, the fabrication technology of the 2D and 3D material in wound healing were discussed.

Accelular nanofibrous bilayer scaffold intrapenetrated with polydopamine network and implemented into a full-thickness wound of a white-pig model affects inflammation and healing process

Treatment of complete loss of skin thickness requires expensive cellular materials and limited skin grafts used as temporary coverage. This paper presents an acellular bilayer scaffold modified with polydopamine (PDA), which is designed to mimic a missing dermis and a basement membrane (BM). The alternate dermis is made from freeze-dried collagen and chitosan (Coll/Chit) or collagen and a calcium salt of oxidized cellulose (Coll/CaOC). Alternate BM is made from electrospun gelatin (Gel), polycaprolactone (PCL), and CaOC. Morphological and mechanical analyzes have shown that PDA significantly improved the elasticity and strength of collagen microfibrils, which favorably affected swelling capacity and porosity. PDA significantly supported and maintained metabolic activity, proliferation, and viability of the murine fibroblast cell lines. The in vivo experiment carried out in a domestic Large white pig model resulted in the expression of pro-inflammatory cytokines in the first 1-2 weeks, giving the idea that PDA and/or CaOC trigger the early stages of inflammation. Otherwise, in later stages, PDA caused a reduction in inflammation with the expression of the anti-inflammatory molecule IL10 and the transforming growth factor β (TGFβ1), which could support the formation of fibroblasts. Similarities in treatment with native porcine skin suggested that the bilayer can be used as an implant for full-thickness skin wounds and thus eliminate the use of skin grafts.

Local Drug Delivery Strategies towards Wound Healing

A particular biological process known as wound healing is connected to the overall phenomena of growth and tissue regeneration. Several cellular and matrix elements work together to restore the integrity of injured tissue. The goal of the present review paper focused on the physiology of wound healing, medications used to treat wound healing, and local drug delivery systems for possible skin wound therapy. The capacity of the skin to heal a wound is the result of a highly intricate process that involves several different processes, such as vascular response, blood coagulation, fibrin network creation, re-epithelialisation, collagen maturation, and connective tissue remodelling. Wound healing may be controlled with topical antiseptics, topical antibiotics, herbal remedies, and cellular initiators. In order to effectively eradicate infections and shorten the healing process, contemporary antimicrobial treatments that include antibiotics or antiseptics must be investigated. A variety of delivery systems were described, including innovative delivery systems, hydrogels, microspheres, gold and silver nanoparticles, vesicles, emulsifying systems, nanofibres, artificial dressings, three-dimensional printed skin replacements, dendrimers and carbon nanotubes. It may be inferred that enhanced local delivery methods might be used to provide wound healing agents for faster healing of skin wounds.

Emerging Therapies for Full-Thickness Skin Regeneration

The classical treatment of extensive full-thickness skin loss due to trauma or burns has been the split-thickness skin graft. While split-thickness skin grafts close the wound, they leave patients with visible scars, dry skin, pruritis, pain, pigmentation alterations, and changes in sensation. The optimal replacement for full-thickness skin loss is replacement with intact full-thickness skin. New technologies combined with advances in the understanding of the mechanisms behind wound healing have led to the development of techniques and products that may eventually recapitulate the functions, appearance, and physical properties of normal skin. Autologous homologous skin constructs, minimal functional skin units, and composite bioengineered skin with dermal substitutes all represent potential avenues for full-thickness composite skin development and application in extensive wounds. This article summarizes the progress, state, and future of full-thickness skin regeneration in burn and massive wound patients.

The Role of Cell-Based Therapies in Acute Burn Wound Skin Repair: A Review

Tissue engineering solutions for skin have been developed over the last few decades with a focus initially on a two-layered structure with epithelial and dermal repair. An essential element of skin restoration is a source of cells capable of differentiating into the appropriate phenotype. The need to repair areas of skin when traditional techniques were not adequate addressed led to cell based therapies being developed initially as a laboratory-based tissue expansion opportunity, both as sheets of cultured epithelial autograft and in composite laboratory-based skin substitutes. The time to availability of the cell-based therapies has been solved in a number of ways, from using allograft cell-based solutions to the use of point of care skin cell harvesting for immediate clinical use. More recently pluripotential cells have been explored providing a readily available source of cells and cells which can express the broad range of phenotypes seen in the mature skin construct. The lessons learnt from the use of cell based techniques has driven the exploration of the use of 3D printing technology, with controlled accurate placement of the cells within a specific printed construct to optimise the phenotypic expression and tissue generation.

Position Paper Progress in the development of biomimetic engineered human tissues

Tissue engineering (TE) is the multi-disciplinary approach to building 3D human tissue equivalents in the laboratory. The advancement of medical sciences and allied scientific disciplines have aspired to engineer human tissues for three decades. To date there is limited use of TE tissues/organs as replacement body parts in humans. This position paper outlines advances in engineering of specific tissues and organs with tissue-specific challenges. This paper outlines the technologies most successful for engineering tissues and key areas of advancement.

Novel Electrospun Polycaprolactone/Calcium Alginate Scaffolds for Skin Tissue Engineering

After decades of research, fully functional skin regeneration is still a challenge. Skin is a multilayered complex organ exhibiting a cascading healing process affected by various mechanisms. Specifically, nutrients, oxygen, and biochemical signals can lead to specific cell behavior, ultimately conducive to the formation of high-quality tissue. This biomolecular exchange can be tuned through scaffold engineering, one of the leading fields in skin substitutes and equivalents. The principal objective of this investigation was the design, fabrication, and evaluation of a new class of three-dimensional fibrous scaffolds consisting of poly(ε-caprolactone) (PCL)/calcium alginate (CA), with the goal to induce keratinocyte differentiation through the action of calcium leaching. Scaffolds fabricated by electrospinning using a PCL/sodium alginate solution were treated by immersion in a calcium chloride solution to replace alginate-linked sodium ions by calcium ions. This treatment not only provided ion replacement, but also induced fiber crosslinking. The scaffold morphology was examined by scanning electron microscopy and systematically assessed by measurements of the pore size and the diameter, alignment, and crosslinking of the fibers. The hydrophilicity of the scaffolds was quantified by contact angle measurements and was correlated to the augmentation of cell attachment in the presence of CA. The in vitro performance of the scaffolds was investigated by seeding and staining fibroblasts and keratinocytes and using differentiation markers to detect the evolution of basal, spinous, and granular keratinocytes. The results of this study illuminate the potential of the PCL/CA scaffolds for tissue engineering and suggest that calcium leaching out from the scaffolds might have contributed to the development of a desirable biological environment for the attachment, proliferation, and differentiation of the main skin cells (i.e., fibroblasts and keratinocytes).

Novel strategies for designing regenerative skin products for accelerated wound healing

Healthy skin protects from pathogens, water loss, ultraviolet rays, and also maintains homeostasis conditions along with sensory perceptions in normal circumstances. Skin wound healing mechanism is a multi-phased biodynamic process that ultimately triggers intercellular and intracellular mechanisms. Failure to implement the normal and effective healing process may result in chronic injuries and aberrant scarring. Chronic wounds lead to substantial rising healthcare expenditure, and innovative methods to diagnose and control severe consequences are urgently needed. Skin tissue engineering (STE) has achieved several therapeutic accomplishments during the last few decades, demonstrating tremendous development. The engineered skin substitutes provide instant coverage for extensive wounds and facilitate the prevention of microbial infections and fluid loss; furthermore, they help in fighting inflammation and allow rapid neo-tissue formation. The current review primarily focused on the wound recovery and restoration process and the current conditions of STE with various advancements and complexities associated with different strategies such as cell sources, biopolymers, innovative fabrication techniques, and growth factors delivery systems.

Recent Advances in Nano-Formulations for Skin Wound Repair Applications

Skin injuries caused by accidents and acute or chronic diseases place a heavy burden on patients and health care systems. Current treatments mainly depend on preventing infection, debridement, and hemostasis and on supplementing growth factors, but patients will still have scar tissue proliferation or difficulty healing and other problems after treatment. Conventional treatment usually focuses on a single factor or process of wound repair and often ignores the influence of the wound pathological microenvironment on the final healing effect. Therefore, it is of substantial research value to develop multifunctional therapeutic methods that can actively regulate the wound microenvironment and reduce the oxidative stress level at the wound site to promote the repair of skin wounds. In recent years, various bioactive nanomaterials have shown great potential in tissue repair and regeneration due to their properties, including their unique surface interface effect, small size effect, enzyme activity and quantum effect. This review summarizes the mechanisms underlying skin wound repair and the defects in traditional treatment methods. We focus on analyzing the advantages of different types of nanomaterials and comment on their toxicity and side effects when used for skin wound repair.

Advancements in Extracellular Matrix-Based Biomaterials and Biofabrication of 3D Organotypic Skin Models

Over the last decades, three-dimensional (3D) organotypic skin models have received enormous attention as alternative models to in vivo animal models and in vitro two-dimensional assays. To date, most organotypic skin models have an epidermal layer of keratinocytes and a dermal layer of fibroblasts embedded in an extracellular matrix (ECM)-based biomaterial. The ECM provides mechanical support and biochemical signals to the cells. Without advancements in ECM-based biomaterials and biofabrication technologies, it would have been impossible to create organotypic skin models that mimic native human skin. In this review, the use of ECM-based biomaterials in the reconstruction of skin models, as well as the study of complete ECM-based biomaterials, such as fibroblasts-derived ECM and decellularized ECM as a better biomaterial, will be highlighted. We also discuss the benefits and drawbacks of several biofabrication processes used in the fabrication of ECM-based biomaterials, such as conventional static culture, electrospinning, 3D bioprinting, and skin-on-a-chip. Advancements and future possibilities in modifying ECM-based biomaterials to recreate disease-like skin models will also be highlighted, given the importance of organotypic skin models in disease modeling. Overall, this review provides an overview of the present variety of ECM-based biomaterials and biofabrication technologies available. An enhanced organotypic skin model is expected to be produced in the near future by combining knowledge from previous experiences and current research.

Constructing Tissue-Engineered Dressing Membranes with Adipose-Derived Stem Cells and Acellular Dermal Matrix for Diabetic Wound Healing: A Comparative Study of Hypoxia- or Normoxia-Culture Modes

Diabetes foot ulcer (DFU) is a serious complication of diabetes, characterized by impaired vascular function, limited angiogenesis, and chronic inflammation. Direct stem cell injection on treating DFU is far from satisfactory in clinical practice, as this therapy neither protects nor localizes the injected cell suspension at the chronic ulcer site. Meanwhile, most of injected cells gradually perished within several days due to senescence or apoptosis. Acellular dermal matrix (ADM) has the potential to act as excellent cell delivery vehicles, considering it is highly biomimetic to native dermal tissue, has low immunogenicity, and suitable for stem cell attachment and proliferation. Hypoxia culture has significantly enhanced effects on the survival ability of in vitro cultured stem cells, indicating this culture mode is a suitable way for inhibiting the senescence or apoptosis of transplanted cells. In the current study, we, respectively, culture adipose-derived stem cells (ADSCs) on an ADM membrane under a hypoxia or normoxia condition to construct two kinds of tissue-engineered dressing membranes (H-ADSCs/ADM and N-ADSCs/ADM) and then comparatively evaluated their efficacy on DFU healing using a diabetic rat model. In vitro results showed that hypoxia precondition could stimulate the ADSCs secreting VEGF-A, and the culture medium from hypoxia-preconditioned ADSCs could enhance the proliferation, migration, and angiogenesis of HUVECs. In vivo results indicated that compared to the N-ADSCs/ADM membrane, the transplanted cells in the H-ADSCs/ADM membrane can survive longer at the chronic ulcer site, consequently improve angiogenesis, inhibit inflammation, and increase extracellular matrix remodeling, eventually accelerating DFU closure. This study provides an innovative covering graft for the treatment of DFU in the clinic.

Skin regeneration, repair, and reconstruction: present and future

Background

Large skin defects caused by trauma (e.g., burns) or due to other reasons (e.g., tumor-related skin resections) require sufficient skin replacement. The constant improvement of innovative methods of skin replacement and skin expansion mean that even burn victims with more than 80% body surface burned have a realistic chance of survival. Due to these new developments, not only has survival rate increased, but also quality of life has increased tremendously over the past decades.

Methods

The aim of this review is to present an overview of current standards and future trends concerning the treatment of skin defects. The main focus is placed on the most important technologies and future trends.

Results

Autologous skin grafting was developed more than 3500 years ago. Several approaches and techniques have been discovered and established in burn care and plastic surgery since then. Great achievements were made during the 19th and 20th centuries. Many of these old and new techniques are still part of modern burn and plastic surgery. Today, autologous skin grafting is still considered to be the gold standard for many wounds, but new technologies have been developed, ranging from biological to synthetic skin replacement materials.

Conclusion

Today, old and new technologies are available which allow us new treatment concepts. All this has led to the reconstructive clockwork for reconstructive surgery of the 21st century.

Chitosan/Gelatin Composite Nonwoven Fabric Scaffold Seeding Minimal Function Unit of Skin for Functional Skin Regeneration

The construction of intact functional skin is a challenging field in tissue engineering. Traditional skin tissue engineering, using "seed cells" as a bioactive source for scaffolding materials maybe not efficient enough. Here a new strategy is shown for constructing functional tissue-engineered skin with Minimal Functional Unit of Skin (MFUS) as the source of bioactivity. Chitosan/gelatin non-woven fabric is used as the scaffold. MFUS is derived from autologous skin with full-thickness skin microstructure and complete functional skin unit harvesting. A mathematical model is used to calculate the MFUS Minimal Harvest Diameter and Angle (MHDA). Chitosan/gelatin non-woven fabric (CS+GEL) is porous and absorbable, with an elastic modulus meeting the requirement of skin engineering. It supports layered and 3D growth of MFUS. The degradation rate of chitosan, including filament diameter and density is evaluated in vivo. MFUS-engineered skin could reduce the density of local nerve fibers in the early stage, potentially reducing pain during wound healing, as well as could limit excessive fibroblast cell migration in the later stage, potentially reducing scar formation. This study proposes a new strategy for the clinical treatment of large full-thickness skin defects by constructing intact functional at minimal cost.

Epidermolysis Bullosa: A Review of the Tissue-Engineered Skin Substitutes Used to Treat Wounds

Skin wound healing is a crucial process for regenerating healthy skin and avoiding the undesired consequences associated with open skin wounds. For epidermolysis bullosa (EB), a debilitating group of fragile skin disorders currently without a cure, skin blistering can often be severe and heal poorly, increasing susceptibility to life-threatening complications. To prevent these, investigational therapies have been exploring the use of tissue-engineered skin substitutes (TESSs) aimed at replacing damaged skin and promoting long-term wound closure. These products have either been developed in house or commercially sourced and are composed of allogeneic or autologous human skin cells, often with some form of bioscaffolding. They can be broadly classified based on their cellular composition: keratinocytes (epidermal substitutes), fibroblasts (dermal substitutes) or a combination of both (composite substitutes). Encouraging long-term wound healing has been achieved with epidermal substitutes. However, these substitutes have not demonstrated the same efficacy for all patients, which may be due to the molecular heterogeneity observed between EB subtypes. Autologous composite TESSs, which more closely resemble native human skin, are therefore being investigated and may hold promise for treating an extended range of patients. Additionally, future TESSs for EB are focused on using gene-corrected patient skin cells, which have already demonstrated remarkable long-term wound healing capabilities. In this review, we provide an overview of the different TESSs that have been investigated in clinical studies to treat patients with EB, as well as their long-term wound healing results. Where available, we describe the methods used to develop these products to inform future efforts in this field.

Inflammatory Immune Responses Trigger Rejection of Allogeneic Fibroblasts Transplanted into Mouse Skin

Fibroblasts, or their homolog stromal cells, are present in most tissues and play an essential role in tissue homeostasis and regeneration. As a result, fibroblast-based strategies have been widely employed in tissue engineering. However, while considered to have immunosuppressive properties, the survival and functionality of allogeneic fibroblasts after transplantation remain controversial. Here, we evaluated innate and adaptive immune responses against allogeneic fibroblasts following intradermal injection into different immune-deficient mouse strains. While allogeneic fibroblasts were rejected 1 week after transplantation in immunocompetent mice, rejection did not occur in immunodeficient γ chain–deficient NOD-SCID (NSG) mice. T-cell- and B-cell-deficient RAG1 knockout mice showed greater loss of fibroblasts by day 5 after transplantation compared with NSG mice (P ≤ 0.05) but prolonged persistence compared with wild-type recipient (P ≤ 0.005). Loss of fibroblasts correlated with the expression of proinflammatory chemokine genes and infiltration of myeloid cells in the transplantation site. Depletion of macrophages and neutrophils delayed rejection, revealing the role of innate immune cells in an early elimination of fibroblasts that is followed by T-cell-mediated rejection in the second week. These findings indicate that the application of allogeneic fibroblasts in tissue engineering products requires further improvements to overcome cell rejection by innate and adaptive immune cells.

Engineering functional skin constructs: A quantitative comparison of three-dimensional bioprinting with traditional methods

Tissue engineering has been successful in reproducing human skin equivalents while incorporating new approaches such as three-dimensional (3D) bioprinting. The latter method offers a plethora of advantages including increased production scale, ability to incorporate multiple cell types and printing on demand. However, the quality of printed skin equivalents compared to those developed manually has never been assessed. To leverage the benefits of this method, it is imperative that 3D-printed skin should be structurally and functionally similar to real human skin. Here, we developed four bilayered human skin epidermal-dermal equivalents: non-printed dermis and epidermis (NN), printed dermis and epidermis (PP), printed epidermis and non-printed dermis (PN), and non-printed epidermis and printed dermis (NP). The effects of printing induced shear stress [0.025 kPa (epidermis); 0.049 kPa (dermis)] were characterized both at the cellular and at the tissue level. At cellular level, no statistically significant differences in keratinocyte colony-forming efficiency (CFE) (p = 0.1641) were observed. In the case of fibroblasts, no significant differences in the cell alignment index (p < 0.1717) and their ability to contract collagen gel (p = 0.851) were detected. At the tissue levels, all the four skin equivalents were characterized using histological and immunohistochemical analysis with no significant differences found in either epidermal basal cell count, thickness of viable epidermis, and relative intensity of filaggrin and claudin-1. Our results demonstrated that 3D printing can achieve the same high-quality skin constructs as have been developed traditionally, thus opening new avenues for numerous high-throughput industrial and clinical applications.

Biomaterials and Their Biomedical Applications: From Replacement to Regeneration

The history of biomaterials dates back to the mists of time: human beings had always used exogenous materials to facilitate wound healing and try to restore damaged tissues and organs. Nowadays, a wide variety of materials are commercially available and many others are under investigation to both maintain and restore bodily functions. Emerging clinical needs forced the development of new biomaterials, and lately discovered biomaterials allowed for the performing of new clinical applications. The definition of biomaterials as materials specifically conceived for biomedical uses was raised when it was acknowledged that they have to possess a fundamental feature: biocompatibility. At first, biocompatibility was mainly associated with biologically inert substances; around the 1970s, bioactivity was first discovered and the definition of biomaterials was consequently extended. At present, it also includes biologically derived materials and biological tissues. The present work aims at walking across the history of biomaterials, looking towards the scientific literature published on this matter. Finally, some current applications of biomaterials are briefly depicted and their future exploitation is hypothesized.

An overview on the manufacturing of functional and mature cellular skin substitutes

There are different types of skin diseases due to chronic injuries that impede the natural healing process of the skin. Tissue engineering (TE) has focused on the development of bioengineered skin or skin substitutes that cover the wound, providing the necessary care to restore the functionality of injured skin. There are two types of substitutes: acellular skin substitutes (ASSs), which offer a low response of the body, and cellular skin substitutes (CSSs), which incorporate living cells and appear as a great alternative in the treatment of skin injuries due to them presenting a greater interaction and integration with the rest of the body. For the development of a CSS, it is necessary to select the most suitable biomaterials, cell components, and methodology of biofabrication for the wound to be treated. Moreover, these CSSs are immature substitutes that must undergo a maturing process in specific bioreactors, guaranteeing their functionality. The bioreactor simulates the natural state of maturation of the skin by controlling parameters such as temperature, pressure, or humidity, allowing a homogeneous maturation of the CSSs in an aseptic environment. The use of bioreactors not only contributes to the maturation of the CSSs, but also offers a new way of obtaining large sections of skin substitutes or natural skin from small portions acquired from the patient, donor, or substitute. Based on the innovation of this technology and the need to develop efficient CSSs, this work offers an update on bioreactor technology in the field of skin regeneration.

Surviving an Extensive Burn Injury Using Advanced Skin Replacement Technologies

There have been significant improvements in the technology available for treating extensive burns in the past decade. This case presents two unique, skin replacement technologies that were used to treat an 86% surface area flame burn in a pediatric patient. A temporary dermal replacement, known as "Novosorb™ Biodegradable Temporizing Matrix" was first used to stabilize the burn injury and remained in place for approximately three months. Given the large burn size and lack of available donor skin for grafting, a permanent skin replacement product known as "Self-Assembled Skin Substitute (SASS)" was then utilized to cover the burns. SASS is a novel technology that was developed to replace skin as an autologous skin graft and is currently available in Canada through a clinical trial for major burns. Ultimately, the concurrent use of these two technologies allowed for the unprecedented survival of a child following an extensive and life-threatening burn injury.