Skin tissue engineering

Nanomaterials Regulate Bacterial Quorum Sensing: Applications, Mechanisms, and Optimization Strategies

Anti-virulence therapy that interferes with bacterial communication, known as "quorum sensing (QS)", is a promising strategy for circumventing bacterial resistance. Using nanomaterials to regulate bacterial QS in anti-virulence therapy has attracted much attention, which is mainly attributed to unique physicochemical properties and excellent designability of nanomaterials. However, bacterial QS is a dynamic and multistep process, and there are significant differences in the specific regulatory mechanisms and related influencing factors of nanomaterials in different steps of the QS process. An in-depth understanding of the specific regulatory mechanisms and related influencing factors of nanomaterials in each step can significantly optimize QS regulatory activity and enhance the development of novel nanomaterials with better comprehensive performance. Therefore, this review focuses on the mechanisms by which nanomaterials regulate bacterial QS in the signal supply (including signal synthesis, secretion, and accumulation) and signal transduction cascade (including signal perception and response) processes. Moreover, based on the two key influencing factors (i.e., the nanomaterial itself and the environment), optimization strategies to enhance the QS regulatory activity are comprehensively summarized. Collectively, applying nanomaterials to regulate bacterial QS is a promising strategy for anti-virulence therapy. This review provides reference and inspiration for further research on the anti-virulence application of nanomaterials.

Specifics of Cryopreservation of Hydrogel Biopolymer Scaffolds with Encapsulated Mesenchymal Stem Cells

The demand for regenerative medicine products is growing rapidly in clinical practice. Unfortunately, their use has certain limitations. One of these, which significantly constrains the widespread distribution and commercialization of such materials, is their short life span. For products containing suspensions of cells, this issue can be solved by using cryopreservation. However, this approach is rarely used for multicomponent tissue-engineered products due to the complexity of selecting appropriate cryopreservation protocols and the lack of established criteria for assessing the quality of such products once defrosted. Our research is aimed at developing a cryopreservation protocol for an original hydrogel scaffold with encapsulated MSCs and developing a set of criteria for assessing the quality of their functional activity in vitro. The scaffolds were frozen using two alternative types of cryocontainers and stored at either -40 °C or -80 °C. After cryopreservation, the external state of the scaffolds was evaluated in addition to recording the cell viability, visible changes during subsequent cultivation, and any alterations in proliferative and secretory activity. These observations were compared to those of scaffolds cultivated without cryopreservation. It was shown that cryopreservation at -80 °C in an appropriate type of cryocontainer was optimal for the hydrogels/adipose-derived stem cells (ASCs) tested if it provided a smooth temperature decrease during freezing over a period of at least three hours until the target values of the cryopreservation temperature regimen were reached. It was shown that evaluating a set of indicators, including the viability, the morphology, and the proliferative and secretory activity of the cells, enables the characterization of the quality of a tissue-engineered construct after its withdrawal from cryopreservation, as well as indicating the effectiveness of the cryopreservation protocol.

A novel multifunctional chitosan-gelatin/carboxymethyl cellulose-alginate bilayer hydrogel containing human placenta extract for accelerating full-thickness wound healing

The replication of skin's dermal and epidermal morphology within a full-thickness wound using a bi-layer hydrogel to cater to their distinct needs is a compelling pursuit. Moreover, human placenta extract (HPE), containing a diverse array of bioactive agents, has proven to be effective in promoting the wound healing process and enhancing epidermal keratinocytes. This study presents a multifunctional bi-layer hydrogel incorporating HPE for accelerating full-thickness wound healing through sustained HPE release, inhibition of bacteria invasion, and promotion of cell proliferation. The upper layer of the scaffold, known as the dressing layer, is composed of carboxymethyl cellulose and sodium alginate, serving as a supportive layer for cell proliferation. The under layer, referred to as the regenerative layer, is composed of chitosan and gelatin, providing an extracellular matrix-like, porous, moist, and antibacterial environment for cell growth. The scaffold was optimized to replicate the morphology of the dermal and epidermal layers, with suitable fibroblast infiltration and a pore size of approximately 283μm. Furthermore, the degradation rate of the samples matched the wound healing rate and persisted throughout this period. The sustained HPE release rate, facilitated by the degradation rate, was optimized to reach ~98% after 28 days, covering the entire healing period. The samples demonstrated robust antibacterial capabilities, with bacterial inhibition zone diameters of and 2.63±0.12cm for S. aureus and E. coli, respectively. The biocompatibility of the samples remained at approximately 68.33±4.5% after 21 days of fibroblast cell culture. The in vivo experiment indicated that the HPE@Bilayer hydrogel promotes the formation of new blood vessels and fibroblasts during the early stages of healing, leading to the appropriate formation of granulation tissue and a wound contraction rate of (79.31±3.1)%. Additionally, it resulted in the formation of a thick epidermal layer (keratinization) that effectively covered all the impaired areas, achieving a wound contraction rate of 95.83±6.3% at the late stage of wound healing. Furthermore, immunohistochemistry staining for CD31 and TGF-β revealed that the HPE@Bilayer group had 22 blood vessels/field and 34%-66% immunoactive cells, respectively, after 14 days of healing. However, by day 21, angiogenesis and TGF-β expression had declined, demonstrating that the wounds had been successfully treated with minimal scarring.

3D biofabrication of diseased human skin models in vitro

Human skin is an organ located in the outermost part of the body; thus, it frequently exhibits visible signs of physiological health. Ethical concerns and genetic differences in conventional animal studies have increased the need for alternative in vitro platforms that mimic the structural and functional hallmarks of natural skin. Despite significant advances in in vitro skin modeling over the past few decades, different reproducible biofabrication strategies are required to reproduce the pathological features of diseased human skin compared to those used for healthy-skin models. To explain human skin modeling with pathological hallmarks, we first summarize the structural and functional characteristics of healthy human skin. We then provide an extensive overview of how to recreate diseased human skin models in vitro, including models for wounded, diabetic, skin-cancer, atopic, and other pathological skin types. We conclude with an outlook on diseased-skin modeling and its technical perspective for the further development of skin engineering.

One-Pot, Open-Air Synthesis of Flexible and Degradable Multifunctional Polymer Composites with Adhesion, Water Resistance, Self-Healing, Facile Drug Loading, and Sustained Release Properties

Developing proper wound management via wound dressings represents a global challenge. Ideal wound dressings shall encompass multiple integrated functionalities for variable, complex scenarios; however, this is challenging due to the complex molecular design and synthesis process. Here, polymer composites, cross-linked poly(styrene oxide-co-hexaphenylcyclotrisiloxane)/crosslinked poly(hexaphenylcyclotrisiloxane) (cP(SO-co-HPCTS)/cPHPCTS) with multiple functionalities are prepared by a one-step, open-air method using catalytic ring-opening polymerization. The introduction of a mobile polymer cP(SO-co-HPCTS) endows the composite with good flexibility and self-healing properties at human body temperature. The hydrophobic groups in the main chain provide hydrophobicity and good water resistance, while the hydroxyl groups contained in the end groups enable good adhesion properties. Drugs can be efficiently loaded by blending and then sustainably release from the polymer composite. The material can rapidly degrade in a tetrahydrofuran solution of tetrabutylammonium fluoride due to its SiOSi bonds. The facile, one-step, open-air synthesis procedure and multiple functional properties integrated into the composites provide good prospects for their extensive application and batch production as wound dressing materials.

LPS-pretreatment adipose-derived mesenchymal stromal cells promote wound healing in diabetic rats by improving angiogenesis

Mesenchymal stem cells (MSCs) play a key role in wound healing, and the advantages of pretreated MSCs in wound healing have previously been reported. In the present study, we investigated the impact of LPS pretreated human adipose-derived MSCs on skin wound healing in diabetic rats. We found that some improvements occurred through improving angiogenesis. Then, we scrutinized the impact of lipopolysaccharide (LPS) treatment on human adipose-derived MSCs in a high-glucose (HG) medium, as an in vitro diabetic model. In vivo findings revealed significant improvements in epithelialization and angiogenesis of diabetic wounds which received LPS pre-MSCs. Particularly, LPS pre-MSCs-treated diabetic wounds reached considerably higher percentages of wound closure. Also, the granulation tissue of these wounds had higher pronounced epithelialization and more vascularization compared with PBS-treated and MSCs-treated diabetic ones by CD31, VEGF, CD90, collagen 1, and collagen 3 immunostaining. Western-blots analyses indicated that LPS pre-MSCs led to the upregulation of vascular endothelial growth factor (VEGF) and DNMT1. In addition, significantly higher cell viability (proliferation/colonie), and elevated VEGF and DNMT1 protein expression were observed when MSCs were treated with LPS (10 ng/ml, 6 h) in HG culture media. Based on these findings, it is suggested that LPS pre-MSCs could promote wound repair and skin regeneration, in some major processes, via the improvement of cellular behaviors of MSCs in the diabetic microenvironment. The beneficial advantages of LPS treated with mesenchymal stem cells on wound healing may lead to establishing a novel approach as an alternative therapeutic procedure to cure chronic wounds in diabetic conditions.

Antibacterial biomaterials for skin wound dressing

Bacterial infection and the ever-increasing bacterial resistance have imposed severe threat to human health. And bacterial contamination could significantly menace the wound healing process. Considering the sophisticated wound healing process, novel strategies for skin tissue engineering are focused on the integration of bioactive ingredients, antibacterial agents included, into biomaterials with different morphologies to improve cell behaviors and promote wound healing. However, a comprehensive review on anti-bacterial wound dressing to enhance wound healing has not been reported. In this review, various antibacterial biomaterials as wound dressings will be discussed. Different kinds of antibacterial agents, including antibiotics, nanoparticles (metal and metallic oxides, light-induced antibacterial agents), cationic organic agents, and others, and their recent advances are summarized. Biomaterial selection and fabrication of biomaterials with different structures and forms, including films, hydrogel, electrospun nanofibers, sponge, foam and three-dimension (3D) printed scaffold for skin regeneration, are elaborated discussed. Current challenges and the future perspectives are presented in this multidisciplinary field. We envision that this review will provide a general insight to the elegant design and further refinement of wound dressing.

Recent Applications of Electrospun Nanofibrous Scaffold in Tissue Engineering

Tissue engineering is a relatively new area of research that combines medical, biological, and engineering fundamentals to create tissue-engineered constructs that regenerate, preserve, or slightly increase the functions of tissues. To create mature tissue, the extracellular matrix should be imitated by engineered structures, allow for oxygen and nutrient transmission, and release toxins during tissue repair. Numerous recent studies have been devoted to developing three-dimensional nanostructures for tissue engineering. One of the most effective of these methods is electrospinning. Numerous nanofibrous scaffolds have been constructed over the last few decades for tissue repair and restoration. The current review gives an overview of attempts to construct nanofibrous meshes as tissue-engineered scaffolds for various tissues such as bone, cartilage, cardiovascular, and skin tissues. Also, the current article addresses the recent improvements and difficulties in tissue regeneration using electrospinning.

Construction of tissue-customized hydrogels from cross-linkable materials for effective tissue regeneration

Hydrogels are prevalent scaffolds for tissue regeneration because of their hierarchical architectures along with outstanding biocompatibility and unique rheological and mechanical properties. For decades, researchers have found that many materials (natural, synthetic, or hybrid) can form hydrogels using different cross-linking strategies. Traditional strategies for fabricating hydrogels include physical, chemical, and enzymatical cross-linking methods. However, due to the diverse characteristics of different tissues/organs to be regenerated, tissue-customized hydrogels need to be developed through precisely controlled processes, making the manufacture of hydrogels reliant on novel cross-linking strategies. Thus, hybrid cross-linkable materials are proposed to tackle this challenge through hybrid cross-linking strategies. Here, different cross-linkable materials and their associated cross-linking strategies are summarized. From the perspective of the major characteristics of the target tissues/organs, we critically analyze how different cross-linking strategies are tailored to fit the regeneration of such tissues and organs. To further advance this field, more appropriate cross-linkable materials and cross-linking strategies should be investigated. In addition, some innovative technologies, such as 3D bioprinting, the internet of medical things (IoMT), and artificial intelligence (AI), are also proposed to improve the development of hydrogels for more efficient tissue regeneration.

Preparation and characterization of sodium alginate-PVA polymeric scaffolds by electrospinning method for skin tissue engineering applications

Sodium alginate (SA) has proven its high potential in tissue engineering and regenerative medicine. One of the main weaknesses of this polysaccharide is its low spinnability. Nanofiber-based scaffolds are of interest to scientists for biomedical engineering. The main aim of this study was to improve the spinnability of SA in combination with polyvinyl alcohol (PVA). The main parameters in the electrospinning of the optimized SA:PVA ratio, including voltage, flow rate, and working space were also optimized. To achieve this, response surface methodology under central composite design was employed to design the experiments scientifically. The final nanofiber scaffolds were studied using scanning electron microscopy, Fourier transform infrared spectroscopy for degradability, swelling, tensile strength, porosity, nanofiber diameter, contact angle, and cytotoxicity. Based on the results, the best ratio for SA : PVA was 1 : 6.5 that was spinnable in various values for the process parameters. The fabricated scaffolds under these conditions revealed good physical, chemical, mechanical, and biological features. L929 cell lines revealed high viability during 48 h culture. The results revealed that uniform and homogeneous nanofibers with regular size distribution (166 nm) were obtained at 30 kV, 0.55 μL h^-1, and 12.50 cm. To sum up, the fabricated scaffolds with the optimized ratio under the reported conditions indicate at good biologically compatible candidates for skin tissue engineering.

Sequential Scalp Assessment in Hair Regeneration Therapy Using an Adipose-Derived Stem Cell-Conditioned Medium

BACKGROUND

An adipose-derived stem cell-conditioned medium (ADSC-CM) reportedly exerts skin-rejuvenating and hair growth-promoting effects. In the therapeutic application of ADSC-CM for alopecia, changes to the interfollicular scalp remain unclear although some evidence has indicated hair growth-promoting effects.

OBJECTIVE

To evaluate the effects of ADSC-CM not only on hair follicles, but also on the interfollicular scalp.

METHODS

Forty patients (21 men, 19 women; age range, 23-74 years) with alopecia were treated by intradermal injection of ADSC-CM every month for 6 months. Eighty fixed sites on patients were investigated by trichograms, physiological examinations, and ultrasonographic examinations at 4 time points (before treatment and 2, 4, and 6 months after the initial treatment).

RESULTS

Hair density and anagen hair rate increased significantly. As physiological parameters, transepidermal water loss value gradually increased, with significant differences at 4 and 6 months after the initial treatment, but hydration state of the stratum corneum and skin surface lipid level showed no obvious changes. As ultrasonographic parameters, dermal thickness and dermal echogenicity were increased significantly.

CONCLUSION

Intradermal administration of ADSC-CM on the scalp has strong potential to provide regenerative effects for hair follicles and the interfollicular scalp. An adipose-derived stem cell-conditioned medium offers a promising prospect as an alternative treatment for alopecia.

Pharmaceutical evaluation of atorvastatin-loaded nanostructured lipid carriers incorporated into the gelatin/hyaluronic acid/polycaprolactone scaffold for the skin tissue engineering

In this study, gelatin/hyaluronic acid (HA) scaffolds containing different amounts of atorvastatin-loaded nanostructured lipid carriers (NLCs) coated entirely with polycaprolactone (PCL) film were fabricated for skin regeneration. 12 atorvastatin-loaded NLCs formulations were synthesized, and particle size, zeta potential, drug entrapment efficiency (EE), and drug release of the formulations were determined. The optimum freeze-dried atorvastatin-loaded NLCs were added in 3 different weight percentages to the gelatin and HA membranous scaffolds. Thereafter, the membranes were coated entirely by a thin layer of the PCL. They were characterized, and then mechanical properties, in vitro degradation and in vitro drug release were assessed. Moreover, human dermal fibroblasts (HDF) were cultured on the prepared nanocomposite scaffolds in order to investigate the cytotoxicity by the MTT assay after the first day, third day, and fifth day. Results revealed that the most favorable atorvastatin-loaded NLCs had 99.54 nm average particle size, -24.30 mV zeta potential, 97.98% EE, and 75.24% drug release within 237 hrs. Mechanical tests indicated that all the three scaffolds had approximately a 90 MPa elastic modulus which was more than two-fold of tensile modulus of normal human skin. The in vitro degradation test demonstrated that the membranes were degraded up to 98% after 5 days, and the scaffolds drug release efficiency (DRE) was in a range of 75-79% during those 5 days. The MTT assay results confirmed the cytocompatibility of the scaffolds. The scaffold containing 54.1 wt% NCLs was the optimum sample (S₃). Scanning Electron Microscopy (SEM) images of the latter one showed the uniform distribution of the NLCs with an average size of 150 nm, and the images of cultured HDF illustrated the good cell attachment. In conclusion, suitable physicochemical and biological properties of the novel gelatin/HA/PCL nanocomposite scaffold containing 54.1 wt% atorvastatin-loaded NLCs (S₃) can be a good candidate for skin regeneration.

Poly(D,L-lactide)/PEG blend films for keratinocyte cultivation and skin reconstruction

The objective of this study was to develop a novel porous thin poly(D,L-lactide) (PLA) film as a tissue-engineering scaffold for keratinocytes used for the replacement of damaged skin. Poly(D,l-lactic acid)/poly(ethylene glycol) (PEG: Mw 6000 or 15 000) blend films were formed by a spin coating technique. The properties and structures of these blend films were investigated. PDLA/PEG (6000) blend films were modified by microfibrillar collagen after water incubation to increase hydrophilicity and improve keratinocyte adhesion. Primary keratinocytes were seeded on PLA films, cultivated for 9 d and transplanted to rats with a model skin defect wound. The wound's healing after keratinocyte transplantation was assayed with histological and immunochemical methods. It was found that skin damage recovery was the most effective after transplantation of keratinocytes on porous PLA film modified with collagen.

Cryogenic free-form extrusion bioprinting of decellularized small intestinal submucosa for potential applications in skin tissue engineering

A novel strategy of cryogenic 3D bioprinting assisted by free-from extrusion printing has been developed and applied to printing of a decellularized small intestinal submucosa (dSIS) slurry. The rheological properties, including kinetic viscosity, storage modulus (G'), and loss modulus (G″), were appropriate for free-from extrusion printing of dSIS slurry. Three different groups of scaffolds, including P₅₀₀, P₆₀₀, and P₇₀₀, with filament distances of 500, 600, and 700 μm, respectively were fabricated at a 5 mm s^-1 working velocity of the platform (V _xy) and 25 kPa air pressure of the dispensing system (P) at -20 °C. The fabricated scaffolds were crosslinked via 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) which resulted in a polyporous microstructure. The variations in the filament diameter and pore size were evaluated in the initial frozen state after printing, the lyophilized state, and after immersion in a PBS solution. The Young's modulus of the P₅₀₀, P₆₀₀, and P₇₀₀ scaffolds was measured in wet and dry states for EDC-crosslinked scaffolds. The cell experiment results showed improved cell adhesion, viability, and proliferation both on the surface and within the scaffold, indicating the biocompatibility and suitability of the scaffold for 3D cell models. Further, gene and protein expression of normal skin fibroblasts on dSIS scaffolds demonstrated their ability to promote the production of some extracellular matrix proteins (i.e. collagen I, collagen III, and fibronectin) in vitro. Overall, this study presents a new potential strategy, by combining cryogenic 3D bioprinting with decellularized extracellular matrix materials, to manufacture ideal scaffolds for skin tissue engineering applications.

3D Cell Printing of Perfusable Vascularized Human Skin Equivalent Composed of Epidermis, Dermis, and Hypodermis for Better Structural Recapitulation of Native Skin

Although skin cell-printing has exhibited promises for fabrication of functional skin equivalents, existing skin models through 3D cell printing are still composed of dermal and epidermal layers. However, a key hope for printing skin is to improve structural complexity of human skin over conventional construction, enabling the precise localization of multiple cell types and biomaterials. Here, the complexity of skin anatomy is increased using 3D cell printing. A novel printing platform is suggested for engineering a matured perfusable vascularized 3D human skin equivalent composed of epidermis, dermis, and hypodermis. The skin model is evaluated using functional markers representing each region of epidermis, dermis, and hypodermis to confirm tissue maturation. It is hypothesized that the vascularized dermal and hypodermal compartments that provide a more realistic microenvironment can promote cross-talks with the epidermal compartment, producing better recapitulation of epidermal morphogenesis. Skin stemness in epithelial tissue is investigated. These findings reveal that the full-thickness skin has more similarities to the native human skin compared with the dermal and epidermal skin model, indicating that it better reflects the actual complexity of native human skin. It is envisioned that it offers better predictive and reliable in vitro platform for investigation of mechanisms of pathological research and skin disease modeling.

Morphology-induced physico-mechanical and biological characteristics of TPU-PDMS blend scaffolds for skin tissue engineering applications

Composition and architecture of scaffolds are the most important factors determining the performance of skin substitutes. In this work, morphology induced unique physical and biological characteristics of compatibilized TPU-PDMS blend scaffolds at 90:10, 80:20, and 70:30 blend ratios of TPU and PDMS was studied. The fiber morphology, porosity, surface wettability, and mechanical properties of electrospun scaffolds were distinctly influenced by the presence of PDMS. Interestingly, the scaffold architecture varied from electrospun fibers to porous fibers and finally occurrence of unique porous beads noticed at 30% PDMS in the microstructure which was confirmed using FESEM. Micro-CT analysis revealed that the porosity of electrospun scaffolds was enhanced from 61% to 79% with 30 parts of PDMS addition. Moreover, MTT assay and cell proliferation were studied using human skin fibroblast cells and found to be significantly enhanced with the PDMS percentage. TPU-PDMS blends offer better overall performance at 70:30 blend ratio of TPU and PDMS (T70P30). Only 4% of hemolysis was observed for T70P30 blends, which establishes the hemocompatibility of the material. In comparison, the results reveal the potential of the cytocompatible T70P30 scaffold for the fabrication of skin substitutes for tissue engineering applications. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1634-1644, 2019.

Promotion of excisional wound repair by a menstrual blood-derived stem cell-seeded decellularized human amniotic membrane

This is the first study demonstrating the efficacy of menstrual blood-derived stem cell (MenSC) transplantation via decellularized human amniotic membrane (DAM), for the promotion of skin excisional wound repair. The DAM was seeded with MenSCs at the density of 3 × 10⁴ cells/cm² and implanted onto a rat's 1.50 × 1.50 cm² full-thickness excisional wound defect. The results of wound closure and histopathological examinations demonstrated that the MenSC-seeded DAM could significantly improve the wound healing compared with DAM-treatment. All in all, our data indicated that the MenSCs can be a potential source for cell-based therapies to regenerate skin injuries.

3D cell printing of in vitro stabilized skin model and in vivo pre-vascularized skin patch using tissue-specific extracellular matrix bioink: A step towards advanced skin tissue engineering

3D cell-printing technique has been under spotlight as an appealing biofabrication platform due to its ability to precisely pattern living cells in pre-defined spatial locations. In skin tissue engineering, a major remaining challenge is to seek for a suitable source of bioink capable of supporting and stimulating printed cells for tissue development. However, current bioinks for skin printing rely on homogeneous biomaterials, which has several shortcomings such as insufficient mechanical properties and recapitulation of microenvironment. In this study, we investigated the capability of skin-derived extracellular matrix (S-dECM) bioink for 3D cell printing-based skin tissue engineering. S-dECM was for the first time formulated as a printable material and retained the major ECM compositions of skin as well as favorable growth factors and cytokines. This bioink was used to print a full thickness 3D human skin model. The matured 3D cell-printed skin tissue using S-dECM bioink was stabilized with minimal shrinkage, whereas the collagen-based skin tissue was significantly contracted during in vitro tissue culture. This physical stabilization and the tissue-specific microenvironment from our bioink improved epidermal organization, dermal ECM secretion, and barrier function. We further used this bioink to print 3D pre-vascularized skin patch able to promote in vivo wound healing. In vivo results revealed that endothelial progenitor cells (EPCs)-laden 3D-printed skin patch together with adipose-derived stem cells (ASCs) accelerates wound closure, re-epithelization, and neovascularization as well as blood flow. We envision that the results of this paper can provide an insightful step towards the next generation source for bioink manufacturing.

Biomaterials for Skin Substitutes

Patients with extensive burns rely on the use of tissue engineered skin due to a lack of sufficient donor tissue, but it is a challenge to identify reliable and economical scaffold materials and donor cell sources for the generation of a functional skin substitute. The current review attempts to evaluate the performance of the wide range of biomaterials available for generating skin substitutes, including both natural biopolymers and synthetic polymers, in terms of tissue response and potential for use in the operating room. Natural biopolymers display an improved cell response, while synthetic polymers provide better control over chemical composition and mechanical properties. It is suggested that not one material meets all the requirements for a skin substitute. Rather, a composite scaffold fabricated from both natural and synthetic biomaterials may allow for the generation of skin substitutes that meet all clinical requirements including a tailored wound size and type, the degree of burn, the patient age, and the available preparation technique. This review aims to be a valuable directory for researchers in the field to find the optimal material or combination of materials based on their specific application.

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.

Coculture of mesenchymal stem cells and endothelial cells enhances host tissue integration and epidermis maturation through AKT activation in gelatin methacryloyl hydrogel-based skin model

A major challenge for clinical use of skin substitutes is insufficient host tissue integration leading to loosening and partial necrosis of the implant. In this present study, a three-dimensional (3D) coculture system constructed using human umbilical cord mesenchymal stem cells (uc-MSCs) and umbilical vein endothelial cells (HUVECs) encapsulated in gelatin methacryloyl (GelMA) hydrogels was evaluated to determine the outcomes of cell-cell interactions in vitro and in vivo. The results revealed that GelMA hydrogels displayed minor cytotoxicity on both cell types. An uc-MSC:HUVEC ratio of 50:50 demonstrated the highest cell proliferation and expression of angiogenic markers. The supplement of basic fibroblast growth factors (bFGF) in coculture system further induced cell proliferation and gene expression in vitro. In vivo transplantation of this cocultured constructs efficiently enhanced the implant and host tissue integration. Additionally, the proliferation of keratinocytes was well maintained on GelMA hydrogels and the gene expression related to cell proliferation and differentiation was significantly increased in coculture system comparing to monoculture. Mechanistically, AKT signaling pathways were activated in cocultures. Our findings suggest that coculturing MSC and EC in GelMA hydrogels could be a promising approach to substantially improve the integration of exogenous skin substitutes and host tissues.

STATEMENT OF SIGNIFICANCE

In this study, the co-culture of uc-MSCs and HUVECs in photocrosslinkable GelMA hydrogels significantly enhanced host tissue integration. Cell proliferation, ECM deposition and angiogenic genes expression were all substantially improved in vitro and the excellent host tissue integration into the implanted tissue was observed in vivo. When served as a dermal layer, the scaffold with co-cultured cells enhanced the proliferation and differentiation of keratinocytes. AKT signaling was proved to be involved in the regulation of cell survival and fate determination. This work demonstrated the importance of 3D cell co-culture to facilitate host tissue integration that can be a promising approach for long-term survival of skin substitutes.

Nanofibrous polycaprolactone scaffolds with adhered platelets stimulate proliferation of skin cells

OBJECTIVES

Faulty wound healing is a global healthcare problem. Chronic wounds are generally characterized by a reduction in availability of growth factors. New strategies are being developed to deliver growth factors more effectively.

METHODS

In this study, we introduced electrospun scaffolds composed of polycaprolactone (PCL) nanofibers functionalized with adhered platelets, as a source of numerous growth factors. Three concentrations of platelets were immobilized to nanofibrous scaffolds by simple adhesion, and their influence on adhesion, proliferation and metabolic activity of seeded cells (murine fibroblasts, keratinocytes and melanocytes) was investigated.

RESULTS

The data obtained indicated that presence of platelets significantly promoted cell spreading, proliferation and metabolic activity in all the skin-associated cell types. There were no significant differences among tested concentrations of platelets, thus even the lowest concentration sufficiently promoted proliferation of the seeded cells.

CONCLUSIONS

Such complex stimulation is needed for improved healing of chronic wounds. However, the nanofibrous system can be used not only as a skin cover, but also in broader applications in regenerative medicine.

Review of recent research on biomedical applications of electrospun polymer nanofibers for improved wound healing

Wound dressings play an important role in a patient's recovery from health problems, as unattended wounds could lead to serious complications such as infections or, ultimately, even death. Therefore, wound dressings since ancient times have been continuously developed, starting from simple dressings from natural materials for covering wounds to modern dressings with functionalized materials to aid in the wound healing process and enhance tissue repair. However, understanding the nature of a wound and the subsequent healing process is vital information upon which dressings can be tailored to ensure a patient's recovery. To date, much progress has been made through the use of nanomedicine in wound healing due to the ability of such materials to mimic the natural dimensions of tissue. This review provides an overview of recent studies on the physiology of wound healing and various wound dressing materials made of nanofibers fabricated using the electrospinning technique.

Silk Biomaterials with Vascularization Capacity

Functional vascularization is critical for the clinical regeneration of complex tissues such as kidney, liver or bone. The immobilization or delivery of growth factors has been explored to improve vascularization capacity of tissue engineered constructs, however, the use of growth factors has inherent problems such as the loss of signaling capability and the risk of complications such as immunological responses and cancer. Here, a new method of preparing water-insoluble silk protein scaffolds with vascularization capacity using an all aqueous process is reported. Acid was added temporally to tune the self-assembly of silk in lyophilization process, resulting in water insoluble scaffold formation directly. These biomaterials are mainly noncrystalline, offering improved cell proliferation than previously reported silk materials. These systems also have appropriate softer mechanical property that could provide physical cues to promote cell differentiation into endothelial cells, and enhance neovascularization and tissue ingrowth in vivo without the addition of growth factors. Therefore, silk-based degradable scaffolds represent an exciting biomaterial option, with vascularization capacity for soft tissue engineering and regenerative medicine.

Vascularized composite allotransplantation: a new concept in musculoskeletal regeneration

Over the past 20 years, the fields of biomaterial sciences and tissue engineering have evolved into new clinically relevant applications including regenerative medicine and cell based therapies. Tissue engineering therapies are based on different types of materials and scaffolds combined with cells and submitted to engineering processes in order to create bio-scaffolds which will improve or replace biological functions. Despite the effort, only a few therapies, such as bone, cartilage and nerve, succeeded in clinical applications. Furthermore, the major drawback in standard application of these therapies was the critical size defects which could be covered with engineered materials, as well as inability to provide sustainable vascular supply to the created bio-scaffolds. In 1998, the first successful hand transplantation was performed in France, and the field of vascularized composite allotransplantation (VCA) was introduced into the armamentarium of reconstructive surgery (Dubernard in Am J Transplant 5(6):1580-1, 2005; Petruzzo et al. in Am J Transplant 6(7):1718-24, 2006; Lanzetta et al. in Transplantation 79(9):1210-4, 2005). As a result, a new generation of transplants including hand, face, larynx abdominal wall, lower extremities and penile transplantation became available to patients who had lost these unique organs and were previously unable to achieve restored function using standard reconstructive procedures. Ethical debate on the need for life-long immunosuppressive therapy to prevent rejection of the VCA overshadowed the success of face and hand transplants. Thus, a new, challenging opportunity developed to combine approaches of tissue engineering and regenerative medicine and ultimately restore the framework, function, aesthetics and survival of the VSA transplants. This overview presents the unique opportunities of merging established and new technologies into the burgeoning field of reconstructive transplantation.

Burn wound healing and treatment: review and advancements

Burns are a prevalent and burdensome critical care problem. The priorities of specialized facilities focus on stabilizing the patient, preventing infection, and optimizing functional recovery. Research on burns has generated sustained interest over the past few decades, and several important advancements have resulted in more effective patient stabilization and decreased mortality, especially among young patients and those with burns of intermediate extent. However, for the intensivist, challenges often exist that complicate patient support and stabilization. Furthermore, burn wounds are complex and can present unique difficulties that require late intervention or life-long rehabilitation. In addition to improvements in patient stabilization and care, research in burn wound care has yielded advancements that will continue to improve functional recovery. This article reviews recent advancements in the care of burn patients with a focus on the pathophysiology and treatment of burn wounds.

Suspended, Shrinkage-Free, Electrospun PLGA Nanofibrous Scaffold for Skin Tissue Engineering

Electrospinning is a technique for creating continuous nanofibrous networks that can architecturally be similar to the structure of extracellular matrix (ECM). However, the shrinkage of electrospun mats is unfavorable for the triggering of cell adhesion and further growth. In this work, electrospun PLGA nanofiber assemblies are utilized to create a scaffold. Aided by a polypropylene auxiliary supporter, the scaffold is able to maintain long-term integrity without dimensional shrinkage. This scaffold is also able to suspend in cell culture medium; hence, keratinocyte cells seeded on the scaffold are exposed to air as required in skin tissue engineering. Experiments also show that human skin keratinocytes can proliferate on the scaffold and infiltrate into the scaffold.

Treatment of Hypertrophic Scar in Human with Autologous Transplantation of Cultured Keratinocytes and Fibroblasts along with Fibrin Glue

Objective

Hypertrophic scar involves excessive amounts of collagen in dermal layer and may be painful. Nowadays, we can’t be sure about effectiveness of procedure for hypertrophic scar management. The application of stem cells with natural scaffold has been the best option for treatment of burn wounds and skin defect, in recent decades. Fibrin glue (FG) was among the first of the natural biomaterials applied to enhance skin deformity in burn patients. This study aimed to identify an efficient, minimally invasive and economical transplantation procedure using novel FG from human cord blood for treatment of hypertrophic scar and regulation collagen synthesis.

Materials and Methods

In this case series study, eight patients were selected with hypertrophic scar due to full-thickness burns. Human keratinocytes and fibroblasts derived from adult skin donors were isolated and cultured. They were tested for the expression of cytokeratin 14 and vimentin using immunocytochemistry. FG was prepared from pooled cord blood. Hypertrophic scars were extensively excised then grafted by simply placing the sheet of FG containing autologous fibroblast and keratinocytes. Histological analyses were performed using Hematoxylin and eosin (H&E) and Masson’s Trichrome (MT) staining of the biopsies after 8 weeks.

Results

Cultured keratinocytes showed a high level of cytokeratin 14 expression and also fibroblasts showed a high level of vimentin. Histological analyses of skin biopsies after 8 weeks of transplantation revealed re-epithelialization with reduction of hypertrophic scars in 2 patients.

Conclusion

These results suggest may be the use of FG from cord blood, which is not more efficient than previous biological transporters and increasing hypertrophic scar relapse, but could lead to decrease pain rate.

Capillary Network-Like Organization of Endothelial Cells in PEGDA Scaffolds Encoded with Angiogenic Signals via Triple Helical Hybridization

Survival of tissue engineered constructs after implantation depends on proper vascularization. The differentiation of endothelial cells into mature microvasculature requires dynamic interactions between cells, scaffold, and growth factors, which are difficult to recapitulate in artificial systems. Previously, photocrosslinked poly(ethylene glycol) diacrylate (PEGDA) hydrogels displaying collagen mimetic peptides (CMPs), dubbed PEGDA‐CMP, that can be further conjugated with bioactive molecules via CMP‐CMP triple helix hybridization were reported. Here, it is shown that a bifunctional peptide featuring pro‐angiogenic domain mimicking vascular endothelial growth factor (VEGF) and a collagen mimetic domain that can fold into a triple helix conformation can hybridize with CMP side chains of the PEGDA‐CMP hydrogel, which results in presentation of insoluble VEGF‐like signals to endothelial cells. Presentation of VEGF‐like signals on the surface of micropatterned scaffolds in this way transforms cells from a quiescent state to elongated and aligned phenotype suggesting that this system could be used to engineer organized microvasculature. It is also shown that the pro‐angiogenic peptide, when applied topically in combination with modified dextran/PEGDA hydrogels, can enhance neovascularization of burn wounds in mice demonstrating the potential clinical use of CMP‐mediated matrix‐bound bioactive molecules for dermal injuries.

Autologous skin substitute for hard-to-heal ulcers: retrospective analysis on safety, applicability, and efficacy in an outpatient and hospitalized setting

Chronic ulcers ((arterio)venous, decubitus, or postoperative) have no tendency to heal within a period of at least 3 months despite optimal therapy according to internationally accepted guidelines. This retrospective study evaluates the safety and efficacy of an autologous, dermal-epidermal skin substitute (SS) for treating ulcers of various origins. Ulcers were treated within 7 Dutch centers over 5 years. Sixty-six ulcers (size: 0.75-150 cm²; duration: 0.25-32 years) with a follow-up time of 24 weeks after a single-skin substitute application were assessed. Wound-bed preparation consisted of vacuum-assisted-closure-therapy (5 days, hospitalized) or application of acellular dermis (5-7 days, outpatient). Time to heal, adverse events, and recurrence 1 year after complete healing were recorded. Complete ulcer healing occurred in 36 of 66 ulcers (55%) at 24 weeks. At that time point, a further 29% of ulcers showed decrease in ulcer size between 50 and 99%. No difference was observed between the hospitalized vs. outpatient treatment with complete healing. There were 32 of 36 healed ulcers that were available for follow-up 1 year after complete closure, of which 27 (84%) were still closed. Only two minor/moderate possibly related adverse events were recorded. This retrospective analysis shows that SS provides a safe and successful treatment for particularly chronic ulcers of various origins.

Adipose-derived mesenchymal stem cells and regenerative medicine

Adipose tissue-derived mesenchymal stem cells (ADSCs) are multipotent and can differentiate into various cell types, including osteocytes, adipocytes, neural cells, vascular endothelial cells, cardiomyocytes, pancreatic β-cells, and hepatocytes. Compared with the extraction of other stem cells such as bone marrow-derived mesenchymal stem cells (BMSCs), that of ADSCs requires minimally invasive techniques. In the field of regenerative medicine, the use of autologous cells is preferable to embryonic stem cells or induced pluripotent stem cells. Therefore, ADSCs are a useful resource for drug screening and regenerative medicine. Here we present the methods and mechanisms underlying the induction of multilineage cells from ADSCs.

The effect of secretory factors of adipose-derived stem cells on human keratinocytes

The beneficial effects of adipose-derived stem cell conditioned medium (ADSC-CM) on skin regeneration have been reported. Although the mechanism of how ADSC-CM promotes skin regeneration is unclear, ADSC-CM contained various growth factors and it is an excellent raw material for skin treatment. ADSC-CM produced in a hypoxia condition of ADSC—in other words, Advanced Adipose-Derived Stem cell Protein Extract (AAPE)—has great merits for skin regeneration. In this study, human primary keratinocytes (HKs), which play fundamental roles in skin tissue, was used to examine how AAPE affects HK. HK proliferation was significantly higher in the experimental group (1.22 μg/mL) than in the control group. DNA gene chip demonstrated that AAPE in keratinocytes (p < 0.05) notably affected expression of 290 identified transcripts, which were associated with cell proliferation, cycle and migration. More keratinocyte wound healing and migration was shown in the experimental group (1.22 μg/mL). AAPE treatment significantly stimulated stress fiber formation, which was linked to the RhoA-ROCK pathway. We identified 48 protein spots in 2-D gel analysis and selected proteins were divided into 64% collagen components and 30% non-collagen components as shown by the MALDI-TOF analysis. Antibody array results contained growth factor/cytokine such as HGF, FGF-1, G-CSF, GM-CSF, IL-6, VEGF, and TGF-β3 differing from that shown by 2-D analysis. Conclusion: AAPE activates HK proliferation and migration. These results highlight the potential of the topical application of AAPE in the treatment of skin regeneration.

Reconstruction of a large scalp defect by the sequential use of dermal substitute, self-filling osmotic tissue expander and rotational flap

Large scalp defects pose a challenge for the surgeon. Here, we present a 31-year-old male patient with a soft tissue defect on the temple with exposed bone. To allow reconstruction, we placed a self-filling osmotic expander in the subgaleal pocket for 12 weeks. The final volume of the tissue expander was 300 mL. In the last step, a rotational flap was created after removal of the tissue expander from its pocket. Thereby, a tension-free suturing was possible. The post-surgical healing was uncomplicated. Osmotic tissue expanders are a valuable tool for the closure of large tissue defects without the necessity of repeated filling procedures.

A bilayer construct controls adipose-derived stem cell differentiation into endothelial cells and pericytes without growth factor stimulation

This work describes the differentiation of adipose-derived mesenchymal stem cells (ASC) in a composite hydrogel for use as a vascularized dermal matrix. Our intent is that such a construct could be utilized following large-surface-area burn wounds that require extensive skin grafting and that are limited by the availability of uninjured sites. To develop engineered skin replacement constructs, we are pursuing the use of ASC. We have established that a PEGylated fibrin gel can provide a suitable environment for the proliferation of ASC over a 7 day time course. Further, we have demonstrated that PEGylated fibrin can be used to control ASC differentiation toward vascular cell types, including cells characteristic of both endothelial cells and pericytes. Gene expression analysis revealed strong upregulation of endothelial markers, CD31, and von Willebrand factor, up to day 11 in culture with corresponding evidence of protein expression demonstrated by immunocytochemical staining. ASC were not only shown to express endothelial cell phenotype, but a subset of the ASC expressed pericyte markers. The NG2 gene was upregulated over 11 days with corresponding evidence for the cell surface marker. Platelet-derived growth factor receptor beta gene expression decreased as the multipotent ASC differentiated up to day 7. Increased receptor expression at day 11 was likely due to the enhanced pericyte gene expression profile, including increased NG2 expression. We have also demonstrated that when cells are loaded onto chitosan microspheres and sandwiched between the PEGylated fibrin gel and a type I collagen gel, the cells can migrate and proliferate within the two different gel types. The matrix composition dictates the lineage specification and is not driven by soluble factors. Utilizing an insoluble bilayer matrix to direct ASC differentiation will allow for the development of both vasculature as well as dermal connective tissue from a single population of ASC. This work underscores the importance of the extracellular matrix in controlling stem cell phenotype. It is our goal to develop layered composites as wound dressings or vascularized dermal equivalents that are not limited by nutrient diffusion.

Xenogeneic extracellular matrix as an inductive scaffold for regeneration of a functioning musculotendinous junction

The prevailing dogma in tissue engineering is cell-centric. One shortcoming of this approach is the failure to provide the implanted cells with a suitable in vivo microenvironment that promotes tissue reconstruction. Extracellular matrix (ECM)-based scaffolds provide a three-dimensional microenvironment that can promote constructive and functional tissue remodeling rather than inflammation and scarring even in the absence of any implanted cells. The objective of this study was to determine the ability of an ECM-based scaffold to facilitate functional restoration of the distal gastrocnemius musculotendinous junction in a canine model after complete resection of the tissue. Within 6 months, vascularized, innervated skeletal muscle that was similar to normal muscle tissue had formed at the ECM-scaffold implantation site. This neo-tissue generated 48% of the contractile force of contralateral musculotendinous junction and represents the first report of de novo formation of contractile, vascularized, and innervated skeletal muscle in situ after significant tissue loss.

Evaluation of the bacterial diversity among and within individual venous leg ulcers using bacterial tag-encoded FLX and titanium amplicon pyrosequencing and metagenomic approaches

Background

Approximately 1 out of every 100 individuals has some form of venous insufficiency, which can lead to chronic venous disease and Venous Leg Ulcer (VLU). There are known underlying pathologies which contribute to the chronic nature of VLU including biofilm phenotype infections.

Results

Using pyrosequencing based approaches we evaluated VLU to characterize their microbial ecology. Results show that VLU infections are polymicrobial with no single bacterium colonizing the wounds. The most ubiquitous and predominant organisms include a previously uncharacterized bacteroidales, various anaerobes, Staphylococcus, Corynebacterium, and Serratia. Topological analysis of VLU show some notable differences in bacterial populations across the surface of the wounds highlighting the importance of sampling techniques during diagnostics. Metagenomics provide a preliminary indication that there may be protozoa, fungi and possibly an undescribed virus associated with these wounds.

Conclusion

The polymicrobial nature of VLU and previous research on diabetic foot ulcers and surgical site infections suggest that the future of therapy for such wounds lies in the core of the logical and proven multiple concurrent strategy approach, which has been termed "biofilm-based wound care" and the use of individualized therapeutics rather than in a single treatment modality.