Photocrosslinked hyaluronic acid hydrogels: natural,biodegradable tissue engineering scaffolds

Ideally, rationally designed tissue engineering scaffolds promote natural wound healing and regeneration. Therefore, we sought to synthesize a biomimetic hydrogel specifically designed to promote tissue repair and chose hyaluronic acid (HA; also called hyaluronan) as our initial material. Hyaluronic acid is a naturally occurring polymer associated with various cellular processes involved in wound healing, such as angiogenesis. Hyaluronic acid also presents unique advantages: it is easy to produce and modify, hydrophilic and nonadhesive, and naturally biodegradable. We prepared a range of glycidyl methacrylate-HA (GMHA) conjugates, which were subsequently photopolymerized to form crosslinked GMHA hydrogels. A range of hydrogel degradation rates was achieved as well as a corresponding, modest range of material properties (e.g., swelling, mesh size). Increased amounts of conjugated methacrylate groups corresponded with increased crosslink densities and decreased degradation rates and yet had an insignificant effect on human aortic endothelial cell cytocompatibility and proliferation. Rat subcutaneous implants of the GMHA hydrogels showed good biocompatibility, little inflammatory response, and similar levels of vascularization at the implant edge compared with those of fibrin positive controls. Therefore, these novel GMHA hydrogels are suitable for modification with adhesive peptide sequences (e.g., RGD) and use in a variety of wound-healing applications.     

Mesenchymal stromal cells in wound healing applications: role of the secretome,targeted delivery and impact on recessive dystrophic epidermolysis bullosa treatment

Mesenchymal stromal cells (MSCs) are multi-potent stromal-derived cells capable of self-renewal that possess several advantageous properties for wound healing, making them of interest to the field of dermatology. Research has focused on characterizing the unique properties of MSCs, which broadly revolve around their regenerative and more recently discovered immunomodulatory capacities. Because of ease of harvesting and expansion, differentiation potential and low immunogenicity, MSCs have been leading candidates for tissue engineering and regenerative medicine applications for wound healing, yet results from clinical studies have been variable, and promising pre-clinical work has been difficult to reproduce. Therefore, the specific mechanisms of how MSCs influence the local microenvironment in distinct wound etiologies warrant further research. Of specific interest in MSC-mediated healing is harnessing the secretome, which is composed of components known to positively influence wound healing. Molecules released by the MSC secretome can promote re-epithelialization and angiogenesis while inhibiting fibrosis and microbial invasion. This review focuses on the therapeutic interest in MSCs with regard to wound healing applications, including burns and diabetic ulcers, with specific attention to the genetic skin disease recessive dystrophic epidermolysis bullosa. This review also compares various delivery methods to support skin regeneration in the hopes of combating the poor engraftment of MSCs after delivery, which is one of the major pitfalls in clinical studies utilizing MSCs.     

The role of nuclear hormone receptors in cutaneous wound repair

The cutaneous wound repair process involves balancing a dynamic series of events ranging from inflammation, oxidative stress, cell migration, proliferation, survival and differentiation. A complex series of secreted trophic factors, cytokines, surface and intracellular proteins are expressed in a temporospatial manner to restore skin integrity after wounding. Impaired initiation, maintenance or termination of the tissue repair processes can lead to perturbed healing, necrosis, fibrosis or even cancer. Nuclear hormone receptors (NHRs) in the cutaneous environment regulate tissue repair processes such as fibroplasia and angiogenesis. Defects in functional NHRs and their ligands are associated with the clinical phenotypes of chronic non‐healing wounds and skin endocrine disorders. The functional relationship between NHRs and skin niche cells such as epidermal keratinocytes and dermal fibroblasts is pivotal for successful wound closure and permanent repair. The aim of this review is to delineate the cutaneous effects and cross‐talk of various nuclear receptors upon injury towards functional tissue restoration. Copyright © 2014 John Wiley & Sons, Ltd.     

Polymer‐based microparticles in tissue engineering and regenerative medicine

Different types of biomaterials, processed into different shapes, have been proposed as temporary support for cells in tissue engineering (TE) strategies. The manufacturing methods used in the production of particles in drug delivery strategies have been adapted for the development of microparticles in the fields of TE and regenerative medicine (RM). Microparticles have been applied as building blocks and matrices for the delivery of soluble factors, aiming for the construction of TE scaffolds, either by fusion giving rise to porous scaffolds or as injectable systems for in situ scaffold formation, avoiding complicated surgery procedures. More recently, organ printing strategies have been developed by the fusion of hydrogel particles with encapsulated cells, aiming the production of organs in in vitro conditions. Mesoscale self‐assembly of hydrogel microblocks and the use of leachable particles in three‐dimensional (3D) layer‐by‐layer (LbL) techniques have been suggested as well in recent works. Along with innovative applications, new perspectives are open for the use of these versatile structures, and different directions can still be followed to use all the potential that such systems can bring. This review focuses on polymeric microparticle processing techniques and overviews several examples and general concepts related to the use of these systems in TE and RE applications. The use of materials in the development of microparticles from research to clinical applications is also discussed. © 2011 American Institute of Chemical Engineers Biotechnol. Prog., 2011     

Design considerations and challenges for mechanical stretch bioreactors in tissue engineering

With the increase in average life expectancy and growing aging population, lack of functional grafts for replacement surgeries has become a severe problem. Engineered tissues are a promising alternative to this problem because they can mimic the physiological function of the native tissues and be cultured on demand. Cyclic stretch is important for developing many engineered tissues such as hearts, heart valves, muscles, and bones. Thus a variety of stretch bioreactors and corresponding scaffolds have been designed and tested to study the underlying mechanism of tissue formation and to optimize the mechanical conditions applied to the engineered tissues. In this review, we look at various designs of stretch bioreactors and common scaffolds and offer insights for future improvements in tissue engineering applications. First, we summarize the requirements and common configuration of stretch bioreactors. Next, we present the features of different actuating and motion transforming systems and their applications. Since most bioreactors must measure detailed distributions of loads and deformations on engineered tissues, techniques with high accuracy, precision, and frequency have been developed. We also cover the key points in designing culture chambers, nutrition exchanging systems, and regimens used for specific tissues. Since scaffolds are essential for providing biophysical microenvironments for residing cells, we discuss materials and technologies used in fabricating scaffolds to mimic anisotropic native tissues, including decellularized tissues, hydrogels, biocompatible polymers, electrospinning, and 3D bioprinting techniques. Finally, we present the potential future directions for improving stretch bioreactors and scaffolds. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:543–553, 2016     

Mesenchymal stem cells: Potential role in corneal wound repair and transplantation

Corneal diseases are a major cause of blindness in the world. Although great progress has been achieved in the treatment of corneal diseases, wound healing after severe corneal damage and immunosuppressive therapy after corneal transplantation remain prob-lematic. Mesenchymal stem cells(MSCs) derived from bone marrow or other adult tissues can differentiate into various types of mesenchymal lineages, such as osteocytes, adipocytes, and chondrocytes, both in vivo and in vitro. These cells can further differentiate into specific cell types under specific conditions. MSCs migrate to injury sites and promote wound healing by secreting anti-inflammatory and growth factors. In ad-dition, MSCs interact with innate and acquired immune cells and modulate the immune response through their powerful paracrine function. Over the last decade, MSCs have drawn considerable attention because of their beneficial properties and promising therapeutic prospective. Furthermore, MSCs have been applied to various studies related to wound healing, autoim-mune diseases, and organ transplantation. This review discusses the potential functions of MSCs in protecting corneal tissue and their possible mechanisms in corneal wound healing and corneal transplantation.     

A novel dermal tissue construct: Development and in vitro characterization

A dermal tissue construct composed of human dermal fibroblasts and a chitosan sponge has been developed, targeted towards the treatment of diabetic nonhealing ulcers. The construct has been designed in a way that the dermal fibroblasts are arranged as a three‐dimensional sheet adhered entirely on one side of the chitosan sponge. This design would allow maximal diffusion of growth factors from the cells to the wound bed when the construct is applied on the wound with the cellular sheet side making contact with the wound bed. The diffusion of secreted growth factors would take place directly from cells to the wound bed without being impeded by a matrix. The cells are present at a high density in the dermal construct, which would aid in accelerated wound healing. The construct has a porous chitosan sponge base, which would allow gas exchange, and renders the dermal construct very flexible so that it would take the shape of the wound contours well, while having mechanical integrity. The viability of cells in the construct is greater than 90%. The dermal construct produces a high amount of vascular endothelial growth factor, from 42 ng to 31 ng in 24 h. The construct also produces high amounts of Interleukin‐8 (IL‐8), from 375 ng to 1065 ng in the first 24 h. Both VEGF and IL‐8 have important roles in the healing of chronic diabetic ulcers. © 2010 American Institute of Chemical Engineers Biotechnol. Prog., 2010     

Helical spring template fabrication of cell‐laden microfluidic hydrogels for tissue engineering

Cell‐laden microfluidic hydrogels find great potential applications in microfluidics, tissue engineering, and drug delivery, due to their ability to control mass transport and cell microenvironment. A variety of methods have been developed to fabricate hydrogels with microfluidic channels, such as molding, bioprinting, and photopatterning. However, the relatively simple structure available and the specific equipment required limit their broad applications in tissue engineering. Here, we developed a simple method to fabricate microfluidic hydrogels with helical microchannels based on a helical spring template. Results from both experimental investigation and numerical modeling revealed a significant enhancement on the perfusion ability and cell viability of helical microfluidic hydrogels compared to those with straight microchannels. The feasibility of such a helical spring template method was also demonstrated for microfluidic hydrogels with complex three‐dimensional channel networks such as branched helical microchannels. The method presented here could potentially facilitate the development of vascular tissue engineering and cell microenvironment engineering. Biotechnol. Bioeng. 2013; 110: 980–989. © 2012 Wiley Periodicals, Inc.     

Therapeutic potential of electromagnetic fields for tissue engineering and wound healing

Ability of electromagnetic fields (EMF) to stimulate cell proliferation and differentiation has attracted the attention of many laboratories specialized in regenerative medicine over the past number of decades. Recent studies have shed light on bio‐effects induced by the EMF and how they might be harnessed to help control tissue regeneration and wound healing. Number of recent reports suggests that EMF has a positive impact at different stages of healing. Processes impacted by EMF include, but are not limited to, cell migration and proliferation, expression of growth factors, nitric oxide signalling, cytokine modulation, and more. These effects have been detected even during application of low frequencies (range: 30–300 kHz) and extremely low frequencies (range: 3–30 Hz). In this regard, special emphasis of this review is the applications of extremely low‐frequency EMFs due to their bio‐safety and therapeutic efficacy. The article also discusses combinatorial effect of EMF and mesenchymal stem cells for treatment of neurodegenerative diseases and bone tissue engineering. In addition, we discuss future perspectives of application of EMF for tissue engineering and use of metal nanoparticles activated by EMF for drug delivery and wound dressing.     

Effects of weak static magnetic fields on endothelial cells

Pulsed electromagnetic fields (PEMFs) have been used extensively in bone fracture repairs and wound healing. It is accepted that the induced electric field is the dose metric. The mechanisms of interaction between weak magnetic fields and biological systems present more ambiguity than that of PEMFs since weak electric currents induced by PEMFs are believed to mediate the healing process, which are absent in magnetic fields. The present study examines the response of human umbilical vein endothelial cells to weak static magnetic fields. We investigated proliferation, viability, and the expression of functional parameters such as eNOS, NO, and also gene expression of VEGF under the influence of different doses of weak magnetic fields. Applications of weak magnetic fields in tissue engineering are also discussed. Static magnetic fields may open new venues of research in the field of vascular therapies by promoting endothelial cell growth and by enhancing the healing response of the endothelium. Bioelectromagnetics 31:296–301, 2010. © 2010 Wiley‐Liss, Inc.     

Biomimetic approaches to protein and gene delivery for tissue regeneration

Novel therapeutic strategies that promote wound healing seek to mimic the response of the body to wounding, to regenerate rather than repair injured tissues. Many synthetic or natural biomaterials have been developed for this purpose and are used to deliver wound therapeutics in a controlled manner that prevents unwanted and potentially harmful side-effects. Here, we review the natural and synthetic biomaterials that have been developed for protein and gene delivery to enhance tissue regeneration. Particular emphasis is placed on novel biomimetic materials that respond to environmental stimuli or release their cargo according to cellular demand. Engineering biomaterials to release therapeutic agents in response to physiologic signals mimics the natural healing process and can promote faster tissue regeneration and reduce scarring in severe acute or chronic wounds.     

Revolutionizing orthopaedic biomaterials: The potential of biodegradable and bioresorbable magnesium-based materials for functional tissue engineering

In recent years, there has been a surge of interest in magnesium (Mg) and its alloys as biomaterials for orthopaedic applications, as they possess desirable mechanical properties, good biocompatibility, and biodegradability. Also shown to be osteoinductive, Mg-based materials could be particularly advantageous in functional tissue engineering to improve healing and serve as scaffolds for delivery of drugs, cells, and cytokines. In this paper, we will present two examples of Mg-based orthopaedic devices: an interference screw to accelerate ACL graft healing and a ring to aid in the healing of an injured ACL.     

Activity of mesenchymal stem cells in therapies for chronic skin wound healing

Chronic or non-healing skin wounds present an ongoing challenge in advanced wound care, particularly as the number of patients increases while technology aimed at stimulating wound healing in these cases remains inefficient. Mesenchymal stem cells (MSCs) have proved to be an attractive cell type for various cell therapies due to their ability to differentiate into various cell lineages, multiple donor tissue types, and relative resilience in ex-vivo expansion, as well as immunomodulatory effects during transplants. More recently, these cells have been targeted for use in strategies to improve chronic wound healing in patients with diabetic ulcers or other stasis wounds. Here, we outline several mechanisms by which MSCs can improve healing outcomes in these cases, including reducing tissue inflammation, inducing angiogenesis in the wound bed, and reducing scarring following the repair process. Approaches to extend MSC life span in implant sites are also examined.     

Outer root sheath cells of human hair follicle are able to regenerate a fully differentiated epidermis in vitro

During wound healing, interfollicular epidermis can be regenerated from the outer root sheath of hair follicles, showing that the cells of this structure can shift toward an interfollicular epidermal phenotype. Similarly, it has been shown that a multilayered epithelium originating from outer sheath cells can be obtained in vitro by culturing hair follicles. However, in the culture systems developed so far, the phenotypical shift was incomplete since the cells retained some of their original characteristics and did not acquire several key markers of terminally differentiated epidermis. In this paper, we describe a new tissue culture method for obtaining a multilayered epithelium from outer sheath cells. This is performed by implanting human hair follicles vertically into dermal equivalents and then raising the culture at the air-liquid interface. The morphological, immunological, and biochemical features of the in vitro reconstructed tissue are very similar to those observed in normal interfollicular epidermis, including those specific for terminally differentiated keratinocytes. Thus, under appropriate in vitro conditions, outer root sheath cells are able to express an interfollicular epidermal phenotype as occurs in vivo during wound healing.     

Tissue transglutaminase in normal and abnormal wound healing: Review article

Summary. A complex series of events involving inflammation, cell migration and proliferation, ECM stabilisation and remodelling, neovascularisation and apoptosis are crucial to the tissue response to injury. Wound healing involves the dynamic interactions of multiple cells types with components of the extracellular matrix (ECM) and growth factors. Impaired wound healing as a consequence of aging, injury or disease may lead to serious disabilities and poor quality of life. Abnormal wound healing may also lead to inflammatory and fibrotic conditions (such as renal and pulmonary fibrosis). Therefore identification of the molecular events underlying wound repair is essential to develop new effective treatments in support to patients and the wound care sector.Recent advances in the understating of the physiological functions of tissue transglutaminase a multi functional protein cross-linking enzyme which stabilises tissues have demonstrated that its biological activities interrelate with wound healing phases at multiple levels. This review describes our view of the function of tissue transglutaminase in wound repair under normal and pathological situations and highlights its potential as a strategic therapeutic target in the development of new treatments to improve wound healing and prevent scarring.     

Distinct phenotype and therapeutic potential of gingival fibroblasts

Gingiva of the oral mucosa provides a practical source to isolate fibroblasts for therapeutic purposes because the tissue is easily accessible, tissue discards are common during routine clinical procedures and wound healing after biopsy is fast and results in complete wound regeneration with very little morbidity or scarring. In addition, gingival fibroblasts have unique traits, including neural crest origin, distinct gene expression and synthetic properties and potent immunomodulatory functions. These characteristics may provide advantages for certain therapeutic approaches over other more commonly used cells, including skin fibroblasts, both in intraoral and extra-oral sites. However, identity and phenotype of gingival fibroblasts, like other fibroblasts, are still not completely understood. Gingival fibroblasts are phenotypically heterogeneous, and these…fibroblast subpopulations may play different roles in tissue maintenance, regeneration and pathologies. The purpose of this review is to summarize what is currently known about gingival fibroblasts, their distinct potential for tissue regeneration and their potential therapeutic uses in the future.     

Limb regeneration in axolotl: is it superhealing?

The ability of axolotls to regenerate their limbs is almost legendary. In fact, urodeles such as the axolotl are the only vertebrates that can regenerate multiple structures like their limbs, jaws, tail, spinal cord, and skin (the list goes on) throughout their lives. It is therefore surprising to realize, although we have known of their regenerative potential for over 200 years, how little we understand the mechanisms behind this achievement of adult tissue morphogenesis. Many observations can be drawn between regeneration and other disciplines such as development and wound healing. In this review, we present new developments in functional analysis that will help to address the role of specific genes during the process of regeneration. We also present an analysis of the resemblance between wound healing and regeneration, and discuss whether axolotls are superhealers. A better understanding of these animals' regenerative capacity could lead to major benefits by providing regenerative medicine with directions on how to develop therapeutic approaches leading to regeneration in humans.     

Polyhydroxyalkanoate (PHA)/inorganic phase composites for tissue engineering applications

Polyhydroxyalkanoates are emerging as a class of biodegradable polymers for applications in tissue engineering. Members of the polyhydroxyalkanoates family encompass a wide variety of materials, from hard and brittle materials to soft and elastomeric. Over the years, efforts have been made to extend the group of polyhydroxyalkanoates and to investigate their use in numerous biomedical applications, such as sutures, cardiovascular patches, wound dressings, guided tissue repair/regeneration devices, and tissue engineering scaffolds. Along with the development of polyhydroxyalkanoates, researchers have looked into the possibility of designing composites in combination with inorganic phases to further improve the mechanical properties, rate of degradation, and also impart bioactivity. Poly(3-hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate), and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) are some of the polymers which have been studied extensively to fabricate composites in combination with hydroxyapatite, bioactive glass, and glass-ceramic fillers or coatings. This paper reviews international research carried out toward development of polyhydroxyalkanoates/inorganic phase composites in terms of systems investigated, microstructures, properties achieved, and applications, with special focus on tissue engineering scaffolds. A comparison between different composite systems developed in the past few years is presented. The paper also addresses the prospect of potential further development of polyhydroxyalkanoates/inorganic phase composites with optimized microstructure and properties for improved tissue engineering scaffolds.     

Nanofibers and their applications in tissue engineering

Developing scaffolds that mimic the architecture of tissue at the nanoscale is one of the major challenges in the field of tissue engineering. The development of nanofibers has greatly enhanced the scope for fabricating scaffolds that can potentially meet this challenge. Currently, there are three techniques available for the synthesis of nanofibers: electrospinning, self-assembly, and phase separation. Of these techniques, electrospinning is the most widely studied technique and has also demonstrated the most promising results in terms of tissue engineering applications. The availability of a wide range of natural and synthetic biomaterials has broadened the scope for development of nanofibrous scaffolds, especially using the electrospinning technique. The three dimensional synthetic biodegradable scaffolds designed using nanofibers serve as an excellent framework for cell adhesion, proliferation, and differentiation. Therefore, nanofibers, irrespective of their method of synthesis, have been used as scaffolds for musculoskeletal tissue engineering (including bone, cartilage, ligament, and skeletal muscle), skin tissue engineering, vascular tissue engineering, neural tissue engineering, and as carriers for the controlled delivery of drugs, proteins, and DNA. This review summarizes the currently available techniques for nanofiber synthesis and discusses the use of nanofibers in tissue engineering and drug delivery applications.     

Novel chitin/nanosilica composite scaffolds for bone tissue engineering applications

Biopolymers like chitin are widely investigated as scaffolds in bone tissue engineering. Its properties like biocompatibility, biodegradability, non-toxicity, wound healing ability, antibacterial activity, hemostatic property, etc., are widely known. However, these materials are not much bioactive. Addition of material like silica can improve the bioactivity and biocompatibility of chitin. In this work, chitin composite scaffolds containing nanosilica were prepared using chitin hydrogel and their bioactivity, swelling ability and cytotoxicity was analyzed in vitro. These scaffolds were found to be bioactive in simulated body fluid (SBF) and biocompatible when tested with MG 63 cell line. These results suggest that chitin/nanosilica composite scaffolds can be useful for bone tissue engineering applications.