Wound closure with EDC cross-linked cultured skin substitutes grafted to athymic mice

Closure of Long Surgical Incisions with Hemostatic Tissue Adhesive in a Porcine Skin Model

OBJECTIVE

Skin adhesives offer many advantages over traditional wound-closure devices. Recently, the current research group reported on tissue adhesives composed of natural polymers (gelatin and alginate), which are biocompatible with mechanical properties suitable for tissue adhesion. The objective of the present study was to conduct clinical and histologic assessment of this hemostatic bioadhesive in the healing of long skin incisions (≥4 cm) in comparison with traditional and commercially available methods.

METHODS

Researchers created 24 long incisions on the ventral side of two domestic pigs to compare four different treatment modalities: two topical bioadhesives based on gelatin and alginate combined with the hemostatic agent kaolin, nylon sutures, and commercial tissue adhesive N-butyl-2-cyanoacrylate. The bioadhesive compounds were spread on the incision surface and then mixed either manually or with a double-headed syringe. After 14 days, clinical and histologic measurements were performed to evaluate the healing phase of the wounds.

RESULTS

The bioadhesive formulation that contained a relatively low crosslinker concentration demonstrated superior results to the formulation that contained a standard crosslinker concentration. However, no significant statistical differences were observed compared with the control incisions (sutures and commercial adhesive N-butyl-2-cyanoacrylate). This was verified by immunohistochemical analysis for epithelial integrity and scar formation as well as by clinical assessment.

CONCLUSIONS

This newly developed bioadhesive demonstrated suitable properties for the closure of long incisions in a porcine skin model.

Comparison of biopolymer scaffolds for the fabrication of skin substitutes in a porcine wound model

This study compared three acellular scaffolds as templates for the fabrication of skin substitutes. A collagen-glycosaminoglycan (C-GAG), a biodegradable polyurethane foam (PUR) and a hybrid combination (PUR/C-GAG) were investigated. Scaffolds were prepared for cell inoculation. Fibroblasts and keratinocytes were serially inoculated onto the scaffolds and co-cultured for 14 days before transplantation. Three pigs each received four full-thickness 8 cm × 8 cm surgical wounds, into which a biodegradable temporising matrix (BTM) was implanted. Surface seals were removed after integration (28 days), and three laboratory-generated skin analogues and a control split-thickness skin graft (STSG) were applied for 16 weeks. Punch biopsies confirmed engraftment and re-epithelialisation. Biophysical wound parameters were also measured and analysed. All wounds showed greater than 80% epithelialisation by day 14 post-transplantation. The control STSG displayed 44% contraction over the 16 weeks, and the test scaffolds, C-GAG 64%, Hybrid 66.7% and PUR 67.8%. Immunohistochemistry confirmed positive epidermal keratins and basement membrane components (Integrin alpha-6, collagens IV and VII). Collagen deposition and fibre organisation indicated the degree of fibrosis and scar produced for each graft. All scaffold substitutes re-epithelialised by 4 weeks. The percentage of original wound area for the Hybrid and PUR was significantly different than the STSG and C-GAG, indicating the importance of scaffold retainment within the first 3 months post-transplant. The PUR/C-GAG scaffolds reduced the polymer pore size, assisting cell retention and reducing the contraction of in vitro collagen. Further investigation is required to ensure reproducibility and scale-up feasibility.

Free-standing multilayer films as growth factor reservoirs for future wound dressing applications

Chronic skin wounds place a high burden on patients and health care systems. The use of angiogenic and mitogenic growth factors can facilitate the healing but growth factors are quickly inactivated by the wound environment if added exogenously. Here, free-standing multilayer films (FSF) are fabricated from chitosan and alginate as opposing polyelectrolytes in an alternating manner using layer-by-layer technique. One hundred bilayers form an about 450 μm thick, detachable free-standing film that is subsequently crosslinked by either ethyl (dimethylaminopropyl) carbodiimide combined with N-hydroxysuccinimide (E-FSF) or genipin (G-FSF). The characterization of swelling, oxygen permeability and crosslinking density shows reduced swelling and oxygen permeability for both crosslinked films compared to non-crosslinked films (N-FSF). Loading of fibroblast growth factor 2 (FGF2) into the films results in a sustained release from crosslinked FSF in comparison to non-crosslinked FSF. Biocompatibility studies in vitro with human dermal fibroblasts cultured underneath the films demonstrate increased cell growth and cell migration for all films with and without FGF2. Especially G-FSF loaded with FGF2 greatly increases cell proliferation and migration. In vivo biocompatibility studies by subcutaneous implantation in mice show that E-FSF causes an inflammatory tissue response that is absent in the case of G-FSF. N-FSF also represents a biocompatible film but shows early degradation. All FSF possess antibacterial properties against gram+ and gram- bacteria demonstrated by an agar diffusion disc assay. In summary, FSF made of alginate and chitosan crosslinked with genipin can act as a reservoir for the sustained release of FGF2, possessing high biocompatibility in vitro and in vivo. Moreover, G-FSF promotes growth and migration of human dermal fibroblasts and has antibacterial properties, which makes it an interesting candidate for bioactive wound.

3D Bioprinting of Functional Skin Substitutes: From Current Achievements to Future Goals

The aim of this review is to present 3D bioprinting of skin substitutes as an efficient approach of managing skin injuries. From a clinical point of view, classic treatments only provide physical protection from the environment, and existing engineered scaffolds, albeit acting as a physical support for cells, fail to overcome needs, such as neovascularisation. In the present work, the basic principles of bioprinting, together with the most popular approaches and choices of biomaterials for 3D-printed skin construct production, are explained, as well as the main advantages over other production methods. Moreover, the development of this technology is described in a chronological manner through examples of relevant experimental work in the last two decades: from the pioneers Lee et al. to the latest advances and different innovative strategies carried out lately to overcome the well-known challenges in tissue engineering of skin. In general, this technology has a huge potential to offer, although a multidisciplinary effort is required to optimise designs, biomaterials and production processes.

In Vivo Evaluation of Three-Dimensional Printed, Keratin-Based Hydrogels in a Porcine Thermal Burn Model

Keratin is a natural material that can be derived from the cortex of human hair. Our group had previously presented a method for the printed, sequential production of three-dimensional (3D) keratin scaffolds. Using a riboflavin-sodium persulfate-hydroquinone (initiator-catalyst-inhibitor) photosensitive solution, we produced 3D keratin-based constructs through ultraviolet crosslinking in a lithography-based 3D printer. In this study, we have used this bioink to produce a keratin-based construct that is capable of delivering small molecules, providing an environment conducive to healing of dermal burn wounds in vivo, and maintaining stability in customized packaging. We characterized the effects of manufacturing steps, such as lyophilization and gamma irradiation sterilization on the properties of 3D printed keratin scaffolds prepared for in vivo testing. Keratin hydrogels are viable for the uptake and release of contracture-inhibiting Halofuginone, a collagen synthesis inhibitor that has been shown to decrease collagen synthesis in fibrosis cases. This small-molecule delivery provides a mechanism to reduce scarring of severe burn wounds in vitro. In vivo data show that the Halofuginone-laden printed keratin is noninferior to other similar approaches reported in literature. This is indicative that the use of 3D printed keratin is not inhibiting the healing processes, and the inclusion of Halofuginone induces a more organized dermal healing after a burn; in other words, this treatment is slower but improves healing. These studies are indicative of the potential of Halofuginone-laden keratin dressings in dermal wound healing. We aim to keep increasing the complexity of the 3D printed constructs toward the production of complex scaffolds for the treatment and topographical reconstruction of severe burn wounds to the face.

Keratin scaffolds with human adipose stem cells: Physical and biological effects toward wound healing

Keratin, a natural biomaterial derived from wool or human hair, has the intrinsic ability to interact with different types of cells and the potential to serve as a controllable extracellular matrix that can be used a scaffold for tissue engineering. In this study, we demonstrated a simple and fast technique to construct 3D keratin scaffolds for accelerated wound healing using a lyophilization method based on extraction of keratin from human hair. The physical properties of the keratin scaffolds such as water uptake, pore size, and porosity can be adjusted by changing the protein concentrations during the fabrication process. The keratin scaffolds supported human adipose stem cells (hASCs) adhesion, proliferation, and differentiation. In vivo study performed on ICR mice showed that keratin scaffolds with hASCs shortened skin wound healing time, accelerated epithelialization, and promoted wound remodeling. Therefore, keratin scaffolds alone or together with hASCs may serve as therapeutic agents for repairing wounded tissue.

Inflammatory response and biomechanical properties of coaxial scaffolds for engineered skin in vitro and post-grafting

Engineered skin (ES) offers many advantages over split-thickness skin autografts for the treatment of burn wounds. However, ES, both in vitro and after grafting, is often significantly weaker, less elastic and more compliant than normal human skin. Biomechanical properties of ES can be tuned in vitro using electrospun co-axial (CoA) scaffolds. To explore the potential for coaxial scaffold-based ES use in vivo, two CoA scaffolds were fabricated with bioactive gelatin shells and biodegradable synthetic cores of polylactic acid (PLA) and polycaprolactone (PCL), and compared with gelatin monofilament scaffolds. Fibroblast and macrophage production of inflammatory cytokines interleukin 6 (IL-6) and transforming growth factor β-1 was significantly higher when cultured on PLA and PCL monofilament scaffolds compared to gelatin monofilament scaffolds. The core-shell fiber configuration significantly reduced production of pro-inflammatory cytokines to levels similar to those of gelatin monofilament scaffolds. In vitro, ES mechanical properties were significantly enhanced using CoA scaffolds; however, after grafting CoA- and gelatin-based ES to full-thickness excisional wounds on athymic mice, the in vitro mechanical advantage of CoA grafts was lost. A substantially increased inflammatory response to CoA-based ES was observed, with upregulation of IL-6 expression and a significant M2 macrophage presence. Additionally, expression of matrix metalloproteinase I was upregulated and collagen type I alpha 1 was downregulated in CoA ES two weeks after grafting. These results suggest that while coaxial scaffolds provide the ability to regulate biomechanics in vitro, further investigation of the inflammatory response to core materials is required to optimize this strategy for clinical use. STATEMENT OF SIGNIFICANCE: Engineered skin has been used to treat very large burn injuries. Despite its ability to heal these wounds, engineered skin exhibits reduced biomechanical properties making it challenging to manufacture and surgically apply. Coaxial fiber scaffolds have been utilized to tune the mechanical properties of engineered skin while maintaining optimal biological properties but it is not known how these perform on a patient especially with regards to their inflammatory response. The current study examines the biomechanical and inflammatory properties of coaxial scaffolds and uniaxial scaffolds in vitro and in vivo. The results show that the biological response to the scaffold materials is a critical determinant of tissue properties after grafting with reduced inflammation and rapid scaffold remodeling leading to stronger skin.

Dynamic multiphoton imaging of acellular dermal matrix scaffolds seeded with mesenchymal stem cells in diabetic wound healing

Significantly effective therapies need to be developed for chronic nonhealing diabetic wounds. In this work, the topical transplantation of mesenchymal stem cell (MSC) seeded on an acellular dermal matrix (ADM) scaffold is proposed as a novel therapeutic strategy for diabetic cutaneous wound healing. GFP-labeled MSCs were cocultured with an ADM scaffold that was decellularized from normal mouse skin. These cultures were subsequently transplanted as a whole into the full-thickness cutaneous wound site in streptozotocin-induced diabetic mice. Wounds treated with MSC-ADM demonstrated an increased percentage of wound closure. The treatment of MSC-ADM also greatly increased angiogenesis and rapidly completed the reepithelialization of newly formed skin on diabetic mice. More importantly, multiphoton microscopy was used for the intravital and dynamic monitoring of collagen type I (Col-I) fibers synthesis via second harmonic generation imaging. The synthesis of Col-I fibers during diabetic wound healing is of great significance for revealing wound repair mechanisms. In addition, the activity of GFP-labeled MSCs during wound healing was simultaneously traced via two-photon excitation fluorescence imaging. Our research offers a novel advanced nonlinear optical imaging method for monitoring the diabetic wound healing process while the ADM and MSCs interact in situ. Schematic of dynamic imaging of ADM scaffolds seeded with mesenchymal stem cells in diabetic wound healing using multiphoton microscopy. PMT, photo-multiplier tube.

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.

Gene-activated matrix/bone marrow-derived mesenchymal stem cells constructs regenerate sweat glands-like structure in vivo

It is a significant challenge to regenerate full-thickness skin defects with sweat glands. Various skin substitutes have been developed to resolve this issue with minimal success. In this study, to yield a novel construct for in situ regeneration of sweat glands, the collagen-chitosan porous scaffold was combined with Lipofectamine 2000/pDNA-EGF complexes to obtain the gene-activated scaffold (GAS), which was then seeded with bone marrow-derived mesenchymal stem cells (BM-MSCs). The porous scaffold functionalized as a reservoir for the incorporated gene complexes which were released in a sustained manner. The seeded BM-MSCs were transfected in situ by the released complexes and specially differentiated into sweat gland cells in vitro under the induction of the expressed epidermal growth factor (EGF). Application in vivo of the GAS/BM-MSCs constructs on the full-thickness skin defects of SD rats confirmed that GAS/BM-MSCs could accelerate the wound healing process and induce the in situ regeneration of the full-thickness skin with sweat gland-like structures. Analyzed by immunohistochemical staining, RT-qPCR and Western-blotting, the levels of the major sweat gland markers such as carcino-embryonic antigen (CEA), cytokeratin 8 (CK8) and cytokeratin 14 (CK14) were all up-regulated, indicating that GAS/BM-MSCs can facilitate the regeneration of sweat glands-like structure in vivo.

Burn Scar Biomechanics after Pressure Garment Therapy

BACKGROUND

The current standard of care for the prevention and treatment of scarring after burn injury is pressure garment therapy. Although this therapy has been used clinically for many years, controversy remains regarding its efficacy. The authors evaluated the efficacy of pressure garment therapy in a female red Duroc pig burn model in which wound depth could be tightly controlled.

METHODS

Full-thickness burn wounds were generated on female red Duroc pigs. At day 28 after burn, pressure garment therapy was applied to half the wounds (10 mmHg), with control wounds covered with garments that exerted no compression. Scar area, perfusion, hardness, and elasticity were quantified at days 0, 28, 42, 56, and 72 using computerized planimetry, laser Doppler, and torsional ballistometry. Scar morphology was assessed at days 28, 56, and 76 using histology, immunohistochemistry, and transmission electron microscopy.

RESULTS

Pressure garment therapy significantly hindered scar contraction, with control scars contracting to 64.6 percent + 13.9 percent original area at day 72, whereas pressure garment therapy scars contracted to 82.7 percent + 17.9 percent original area. Pressure garments significantly reduced skin hardness and increased skin strength by 1.3 times. No difference in perfusion or blood vessel density was observed. The average collagen fiber diameter was greater in control burns than in pressure garment therapy.

CONCLUSIONS

Pressure garment therapy was effective at reducing scar contraction and improving biomechanics compared with control scars. These results confirm the efficacy of pressure garments and highlight the need to further investigate the role of pressure magnitude and the time of therapy application to enhance efficacy for optimal biomechanics and patient mobility.

Facile fabrication of egg white macroporous sponges for tissue regeneration

The availability of 3D sponges combining proper biochemical, biophysical, and biomechanical properties with enhanced capacity of in vivo engraftment and vascularization is crucial in regenerative medicine. A simple process is developed to generate macroporous scaffolds with a well-defined architecture of interconnected pores from chicken egg white (EW), a material with protein- and growth factor-binding features which has not yet been employed in regenerative medicine. The physicomechanical properties and degradation rates of the scaffold are finely tuned by using varying concentrations of the cross-linker, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride, without alteration of the biochemical traits. In vitro, EW scaffolds supported active metabolism, proliferation, and migration of human dermal fibroblasts, thereby generating uniform cellular constructs. In vivo, subcutaneous implantation in mice reveals negligible immune reaction and efficient cell and tissue ingrowth. Angiogenesis into EW scaffolds is enhanced as compared to standard collagen type I sponges used as reference material, likely due to significantly higher adsorption of the proangiogenic factor vascular endothelial growth factor. In summary, a material is presented derived by facile processing of a highly abundant natural product. Due to the efficient subcutaneous engraftment capacity, the sponges can find utilization for soft tissue regeneration.

The effect of crosslinking agent on sustained release of bFGF–collagen microspheres

Initial burst release and loss of bioactivity of drugs are the shortcomings of drug delivery systems (DDSs) used for in vivo treatment.

Review collagen-based biomaterials for wound healing

With its wide distribution in soft and hard connective tissues, collagen is the most abundant of animal proteins. In vitro, natural collagen can be formed into highly organized, three-dimensional scaffolds that are intrinsically biocompatible, biodegradable, nontoxic upon exogenous application, and endowed with high tensile strength. These attributes make collagen the material of choice for wound healing and tissue engineering applications. In this article, we review the structure and molecular interactions of collagen in vivo; the recent use of natural collagen in sponges, injectables, films and membranes, dressings, and skin grafts; and the on-going development of synthetic collagen mimetic peptides as pylons to anchor cytoactive agents in wound beds.

Tunable engineered skin mechanics via coaxial electrospun fiber core diameter

Autologous engineered skin (ES) offers promise as a treatment for massive full thickness burns. Unfortunately, ES is orders of magnitude weaker than normal human skin causing it to be difficult to apply surgically and subject to damage by mechanical shear in the early phases of engraftment. In addition, no manufacturing strategy has been developed to tune ES biomechanics to approximate the native biomechanics at different anatomic locations. To enhance and tune ES biomechanics, a coaxial (CoA) electrospun scaffold platform was developed from polycaprolactone (PCL, core) and gelatin (shell). The ability of the coaxial fiber core diameter to control both scaffold and tissue mechanics was investigated along with the ability of the gelatin shell to facilitate cell adhesion and skin development compared to pure gelatin, pure PCL, and a gelatin-PCL blended fiber scaffold. CoA ES exhibited increased cellular adhesion and metabolism versus PCL alone or gelatin-PCL blend and promoted the development of well stratified skin with a dense dermal layer and a differentiated epidermal layer. Biomechanics of the scaffold and ES scaled linearly with core diameter suggesting that this scaffold platform could be utilized to tailor ES mechanics for their intended grafting site and reduce graft damage in vitro and in vivo.

Effect of pore size and cross-linking of a novel collagen-elastin dermal substitute on wound healing

Collagen-elastin (CE) scaffolds are frequently used for dermal replacement in the treatment of full-thickness skin defects such as burn wounds. But little is known about the optimal pore size and level of cross-linking. Different formulations of dermal substitutes with unidirectional pores were tested in porcine full-thickness wounds in combination with autologous split skin mesh grafts (SSG). Effect on wound healing was evaluated both macro- and microscopically. CE scaffolds with a pore size of 80 or 100 μm resulted in good wound healing after one-stage grafting. Application of scaffolds with a larger average pore size (120 μm) resulted in more myofibroblasts and more foreign body giant cells (FBGC). Moderate crosslinking impaired wound healing as it resulted in more wound contraction, more FBGC and increased epidermal thickness compared to no cross-linking. In addition, take rate and redness were negatively affected compared to SSG only. Vascularization and the number of myofibroblasts were not affected by cross-linking. Surprisingly, stability of cross-linked scaffolds was not increased in the wound environment, in contrast to in vitro results. Cross-linking reduced the proliferation of fibroblasts in vitro, which might explain the reduced clinical outcome. The non-cross-linked CE substitute with unidirectional pores allowed one-stage grafting of SSG, resulting in good wound healing. In addition, only a very mild foreign body reaction was observed. Cross-linking of CE scaffolds negatively affected wound healing on several important parameters. The optimal non-cross-linked CE substitute is a promising candidate for future clinical evaluation.

Nanofiber-expanded human umbilical cord blood-derived CD34+ cell therapy accelerates murine cutaneous wound closure by attenuating pro-inflammatory factors and secreting IL-10

Nanofiber-expanded human umbilical cord blood-derived CD34+ cell therapy is under consideration for treating peripheral and cardiac ischemia. However, the therapeutic efficacy of nanofiber-expanded human umbilical cord blood-derived (NEHUCB) CD34+ cell therapy for wound healing and its mechanisms are yet to be established. Using an excision wound model in NOD/SCID mice, we show herein that NEHUCB-CD34+ cells home to the wound site and significantly accelerate the wound-healing process compared to vehicle-treated control. Histological analysis reveals that accelerated wound closure is associated with the re-epithelialization and increased angiogenesis. Additionally, NEHUCB-CD34+ cell-therapy decreases expression of pro-inflammatory cytokines, such as TNF-α, IL-1β, IL-6 and NOS2A in the wound bed, and concomitantly increases expression of IL-10 compared to vehicle-treated control. These findings were recapitulated in vitro using primary dermal fibroblasts and NEHUCB-CD34+ cells. Moreover, NEHUCB-CD34+ cells attenuate NF-κB activation and nuclear translocation in dermal fibroblasts through enhanced secretion of IL-10, which is known to bind to NF-κB and suppress transcriptional activity. Collectively, these data provide novel mechanistic evidence of NEHUCB-CD34+ cell-mediated accelerated wound healing.

Plant-derived human collagen scaffolds for skin tissue engineering

Tissue engineering scaffolds are commonly formed using proteins extracted from animal tissues, such as bovine hide. Risks associated with the use of these materials include hypersensitivity and pathogenic contamination. Human-derived proteins lower the risk of hypersensitivity, but possess the risk of disease transmission. Methods engineering recombinant human proteins using plant material provide an alternate source of these materials without the risk of disease transmission or concerns regarding variability. To investigate the utility of plant-derived human collagen (PDHC) in the development of engineered skin (ES), PDHC and bovine hide collagen were formed into tissue engineering scaffolds using electrospinning or freeze-drying. Both raw materials were easily formed into two common scaffold types, electrospun nonwoven scaffolds and lyophilized sponges, with similar architectures. The processing time, however, was significantly lower with PDHC. PDHC scaffolds supported primary human cell attachment and proliferation at an equivalent or higher level than the bovine material. Interleukin-1 beta production was significantly lower when activated THP-1 macrophages where exposed to PDHC electrospun scaffolds compared to bovine collagen. Both materials promoted proper maturation and differentiation of ES. These data suggest that PDHC may provide a novel source of raw material for tissue engineering with low risk of allergic response or disease transmission.

Healing potential of mesenchymal stem cells cultured on a collagen-based scaffold for skin regeneration

BACKGROUND

Wound healing of burned skin remains a major goal in public health. Previous reports showed that the bone marrow stem cells were potent in keratinization and vascularization of full thickness skin wounds.

METHODS

In this study, mesenchymal stem cells were derived from rat adipose tissues and characterized by flowcytometry. Staining methods were used to evaluate their differentiation ability. A collagen-chitosan scaffold was prepared by freeze-drying method and crosslinked by carbodiimide-based crosslinker.

RESULTS

The results of immunecytochemistry and PCR experiments confirmed the adipose-derived stem cells (ASC) in differentiation to the keratinocytes under the treatment of keratinocyte growth factor. The isolated ASC were seeded on the scaffolds and implanted at the prepared wounds. The scaffolds without cells were considered as a control and implanted on the other side of the rat. Histopathological analyses confirmed the formation of new tissue on the scaffold-cell side after 14 days with the formation of dermis and epidermis.

CONCLUSION

These results indicated the capacity of ASC in differentiation to keratinocytes and also wound healing in vivo.

Evaluation of dermal substitute in a novel co-transplantation model with autologous epidermal sheet

The development of more and more new dermal substitutes requires a reliable and effective animal model to evaluate their safety and efficacy. In this study we constructed a novel animal model using co-transplantation of autologous epidermal sheets with dermal substitutes to repair full-thickness skin defects. Autologous epidermal sheets were obtained by digesting the basement membrane (BM) and dermal components from rat split-thickness skins in Dispase II solution (1.2 u/ml) at 4 °C for 8, 10 and 12 h. H&E, immunohistochemical and live/dead staining showed that the epidermal sheet preserved an intact epidermis without any BM or dermal components, and a high percentage of viable cells (92.10 ± 4.19%) and P63 positive cells (67.43 ± 4.21%) under an optimized condition. Porcine acellular dermal matrixes were co-transplanted with the autologous epidermal sheets to repair full-thickness skin defects in Sprague-Dawley rats. The epidermal sheets survived and completely re-covered the wounds within 3 weeks. Histological staining showed that the newly formed stratified epidermis attached directly onto the dermal matrix. Inflammatory cell infiltration and vascularization of the dermal matrix were not significantly different from those in the subcutaneous implantation model. Collagen IV and laminin distributed continuously at the epidermis and dermal matrix junction 4 weeks after transplantation. Transmission electron microscopy further confirmed the presence of continuous lamina densa and hemidesmosome structures. This novel animal model can be used not only to observe the biocompatibility of dermal substitutes, but also to evaluate their effects on new epidermis and BM formation. Therefore, it is a simple and reliable model for evaluating the safety and efficacy of dermal substitutes.

Full-thickness skin wound healing using human placenta-derived extracellular matrix containing bioactive molecules

The human placenta, a complex organ, which facilitates exchange between the fetus and the mother, contains abundant extracellular matrix (ECM) components and well-preserved endogenous growth factors. In this study, we designed a new dermal substitute from human placentas for full-thickness wound healing. Highly porous, decellularized ECM sheets were fabricated from human placentas via homogenization, centrifugation, chemical and enzymatic treatments, molding, and freeze-drying. The physical structure and biological composition of human placenta-derived ECM sheets dramatically supported the regeneration of full-thickness wound in vivo. At the early stage, the ECM sheet efficiently absorbed wound exudates and tightly attached to the wound surface. Four weeks after implantation, the wound was completely closed, epidermic cells were well arranged and the bilayer structure of the epidermis and dermis was restored. Moreover, hair follicles and microvessels were newly formed in the ECM sheet-implanted wounds. Overall, the ECM sheet produced a dermal substitute with similar cellular organization to that of normal skin. These results suggest that human placenta-derived ECM sheets provide a microenvironment favorable to the growth and differentiation of cells, and positive modulate the healing of full-thickness wounds.

Sterilization of tendon allografts: a method to improve strength and stability after exposure to 50 kGy gamma radiation

Terminal sterilization of tendon allografts with high dose gamma irradiation has deleterious effects on tendon mechanical properties and stability after implantation. Our goal is to minimize these effects with radio protective methods. We previously showed that radio protection via combined crosslinking and free radical scavenging maintained initial mechanical properties of tendon allografts after irradiation at 50 kGy. This study further evaluates the tissue response and simulated mechanical degradation of tendons processed with radio protective treatment, which involves crosslinking in 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide followed by soaking in an ascorbate/riboflavin-5-phosphate solution. Control untreated and treated tendons were irradiated at 50 kGy and implanted in New Zealand White rabbit knees within the joint capsule for four and 8 weeks. Tendons were also exposed to cyclic loading to 20 N at one cycle per 12 s in a collagenase solution for 150 cycles, followed by tension to failure. Control irradiated tendons displayed increased degradation in vivo, and failed prematurely during cyclic processing at an average of 25 cycles. In contrast, radio protected irradiated tendons displayed greater stability following implantation over 8 weeks, and possessed strength at 59 % of native tendons and modulus equivalent to that of native tendons after cyclic loading in collagenase. These results suggest that radio protective treatment improves the strength and the stability of tendon allografts.

Biomimetic mineralization of woven bone-like nanocomposites: role of collagen cross-links

Ideal biomaterials for bone grafts must be biocompatible, osteoconductive, osteoinductive and have appropriate mechanical properties. For this, the development of synthetic bone substitutes mimicking natural bone is desirable, but this requires controllable mineralization of the collagen matrix. In this study, densified collagen films (up to 100 μm thick) were fabricated by a plastic compression technique and cross-linked using carbodiimide. Then, collagen-hydroxyapatite composites were prepared by using a polymer-induced liquid-precursor (PILP) mineralization process. Compared to traditional methods that produce only extrafibrillar hydroxyapatite (HA) clusters on the surface of collagen scaffolds, by using the PILP mineralization process, homogeneous intra- and extrafibrillar minerals were achieved on densified collagen films, leading to a similar nanostructure as bone, and a woven microstructure analogous to woven bone. The role of collagen cross-links on mineralization was examined and it was found that the cross-linked collagen films stimulated the mineralization reaction, which in turn enhanced the mechanical properties (hardness and modulus). The highest value of hardness and elastic modulus was 0.7 ± 0.1 and 9.1 ± 1.4 GPa in the dry state, respectively, which is comparable to that of woven bone. In the wet state, the values were much lower (177 ± 31 and 8 ± 3 MPa) due to inherent microporosity in the films, but still comparable to those of woven bone in the same conditions. Mineralization of collagen films with controllable mineral content and good mechanical properties provide a biomimetic route toward the development of bone substitutes for the next generation of biomaterials. This work also provides insight into understanding the role of collagen fibrils on mineralization.

Comparative study of bovine, porcine and avian collagens for the production of a tissue engineered dermis

Combining bovine collagen with chitosan followed by freeze-drying has been shown to produce porous scaffolds suitable for skin and connective tissue engineering applications. In this study collagen extracted from porcine and avian skin was compared with bovine collagen for the production of tissue engineered scaffolds. A similar purity of the collagen extracts was shown by electrophoresis, confirming the reliability of the extraction process. Collagen was solubilized, cross-linked by adding chitosan to the solution and freeze-dried to generate a porous structure suitable for tissue engineering applications. Scaffold porosity and pore morphology were shown to be source dependant, with bovine collagen and avian collagen resulting into the smallest and largest pores, respectively. Scaffolds were seeded with dermal fibroblasts and cultured for 35 days to evaluate the suitability of the different collagen-chitosan scaffolds for long-term tissue engineered dermal substitute maturation in vitro. Cell proliferation and scaffold biocompatibility were found to be similar for all the collagen-chitosan scaffolds, demonstrating their capability to support long-term cell adhesion and growth. The scaffolds contents was assessed by immunohistochemistry and showed increased deposition of extracellular matrix by the cells as a function of time. These results correlate with measurements of the mechanical properties of the scaffolds, since both the ultimate tensile strength and tensile modulus of the cell seeded scaffolds had increased by the end of the culture period. This experiment demonstrates that porcine and avian collagen could be used as an alternative to bovine collagen in the production of collagen-chitosan scaffolding materials.

Carbodiimide conjugation of fibronectin on collagen basal lamina analogs enhances cellular binding domains and epithelialization

To improve the regenerative potential of biomaterials used as bioengineered scaffolds, it is necessary to strategically incorporate biologically active molecules that promote in vivo cellular processes that lead to a fully functional tissue. This work evaluates the effects of strategically binding fibronectin (FN) to collagen basal lamina analogs to enhance keratinocyte functions necessary for complete skin regeneration. We found that FN that was passively adsorbed to collagen-glycosaminoglycan basal lamina analogs enhanced epithelial thickness and keratinocyte proliferation compared with nontreated basal lamina analogs at 3 days of air/liquid (A/L) interface culture. Additionally, we evaluated the availability of FN cellular binding site domains when FN was either passively adsorbed or [1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride] conjugated to basal lamina analogs fabricated from collagen-glycosaminoglycan coprecipitate or self-assembled type I collagen. It was found that 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride conjugation of FN significantly enhanced FN binding site presentation as well as epithelial thickness. Overall, the results gained from this study will be used to improve the regenerative capacity of basal lamina analogs for bioengineered skin substitutes as well as the development of bioengineered scaffolds for other tissue engineering applications.

Collagen-chitosan polymer as a scaffold for the proliferation of human adipose tissue-derived stem cells

The architecture and biomaterial are vital for three-dimensional culture of cells in scaffolds, so collagen-chitosan scaffolds suitable for the proliferation of adipose tissue-derived stem cells (ADSCs) were fabricated in this study. Chitosan was fully mixed with collagen with different volume ratio and cross-linked. The microstructure, pore size, bibulous ability, water content, interval porosity, enzyme degradation and affinity were examined before and after cross-linking. During ADSCs cultured in scaffold, the viability and metabolic rates were measured. After 14 days, the surface markers, specific transcription factors and multi-differentiation potential were assayed to identify the stemness of expanded cells. According to the pore size, bibulous ability, interval porosity, degradation rate and affinity of the scaffold, we chose cross-linked scaffolds of 7:3 material ratio as a better scaffold for ADSCs proliferation, and ADSCs could be expanded by more than 20 times. All expanded cells still maintained stem cell characteristics and pluripotency. So our developed collagen-chitosan scaffolds can promote ADSCs adhesion, expansion, and maintain pluripotency.