Polysaccharide base electrospun nanofibrous scaffolds for cartilage tissue engineering: Challenges and opportunities

Polysaccharides, known as naturally abundant macromolecular materials which can be easily modified chemically, have always attracted scientists' interest due to their outstanding properties in tissue engineering. Moreover, their intrinsic similarity to cartilage ECM components, biocompatibility, and non-harsh processing conditions make polysaccharides an excellent option for cartilage tissue engineering. Imitating the natural ECM structure to form a fibrous scaffold at the nanometer scale in order to recreate the optimal environment for cartilage regeneration has always been attractive for researchers in the past few years. However, there are some challenges for polysaccharides electrospun nanofibers preparation, such as poor solubility (Alginate, cellulose, chitin), high viscosity (alginate, chitosan, and Hyaluronic acid), high surface tension, etc. Several methods are reported in the literature for facing polysaccharide electrospinning issues, such as using carrier polymers, modification of polysaccharides, and using different solvent systems. In this review, considering the importance of polysaccharide-based electrospun nanofibers in cartilage tissue engineering applications, the main achievements in the past few years, and challenges for their electrospinning process are discussed. After careful investigation of reported studies in the last few years, alginate, chitosan, hyaluronic acid, chondroitin sulfate, and cellulose were chosen as the main polysaccharide base electrospun nanofibers used for cartilage regeneration.

Harnessing Natural Polymers for Nano-Scaffolds in Bone Tissue Engineering: A Comprehensive Overview of Bone Disease Treatment

Numerous surgeries are carried out to replace tissues that have been harmed by an illness or an accident. Due to various surgical interventions and the requirement of bone substitutes, the emerging field of bone tissue engineering attempts to repair damaged tissues with the help of scaffolds. These scaffolds act as template for bone regeneration by controlling the development of new cells. For the creation of functional tissues and organs, there are three elements of bone tissue engineering that play very crucial role: cells, signals and scaffolds. For the achievement of these aims, various types of natural polymers, like chitosan, chitin, cellulose, albumin and silk fibroin, have been used for the preparation of scaffolds. Scaffolds produced from natural polymers have many advantages: they are less immunogenic as well as being biodegradable, biocompatible, non-toxic and cost effective. The hierarchal structure of bone, from microscale to nanoscale, is mostly made up of organic and inorganic components like nanohydroxyapatite and collagen components. This review paper summarizes the knowledge and updates the information about the use of natural polymers for the preparation of scaffolds, with their application in recent research trends and development in the area of bone tissue engineering (BTE). The article extensively explores the related research to analyze the advancement of nanotechnology for the treatment of bone-related diseases and bone repair.

Biomimetic biphasic curdlan-based scaffold for osteochondral tissue engineering applications - Characterization and preliminary evaluation of mesenchymal stem cell response in vitro

Osteochondral defects remain a huge problem in medicine today. Biomimetic bi- or multi-phasic scaffolds constitute a very promising alternative to osteochondral autografts and allografts. In this study, a new curdlan-based scaffold was designed for osteochondral tissue engineering applications. To achieve biomimetic properties, it was enriched with a protein component - whey protein isolate as well as a ceramic ingredient - hydroxyapatite granules. The scaffold was fabricated via a simple and cost-efficient method, which represents a significant advantage. Importantly, this technique allowed generation of a scaffold with two distinct, but integrated phases. Scanning electron microcopy and optical profilometry observations demonstrated that phases of biomaterial possessed different structural properties. The top layer of the biomaterial (mimicking the cartilage) was smoother than the bottom one (mimicking the subchondral bone), which is beneficial from a biological point of view because unlike bone, cartilage is a smooth tissue. Moreover, mechanical testing showed that the top layer of the biomaterial had mechanical properties close to those of natural cartilage. Although the mechanical properties of the bottom layer of scaffold were lower than those of the subchondral bone, it was still higher than in many analogous systems. Most importantly, cell culture experiments indicated that the biomaterial possessed high cytocompatibility towards adipose tissue-derived mesenchymal stem cells and bone marrow-derived mesenchymal stem cells in vitro. Both phases of the scaffold enhanced cell adhesion, proliferation, and chondrogenic differentiation of stem cells (revealing its chondroinductive properties in vitro) as well as osteogenic differentiation of these cells (revealing its osteoinductive properties in vitro). Given all features of the novel curdlan-based scaffold, it is worth noting that it may be considered as promising candidate for osteochondral tissue engineering applications.

Gradient scaffolds for osteochondral tissue engineering and regeneration

The tissue engineering approach for repairing osteochondral (OC) defects involves the fabrication of a biological tissue scaffold that mimics the physiological properties of natural OC tissue (e.g., the gradient transition between the cartilage surface and the subchondral bone). The OC tissue scaffolds described in many research studies exhibit a discrete gradient (e.g., a biphasic or tri/multiphasic structure) or a continuous gradient to mimic OC tissue attributes such as biochemical composition, structure, and mechanical properties. One advantage of a continuous gradient scaffold over biphasic or tri/multiphasic tissue scaffolds is that it more closely mimics natural OC tissue since there is no distinct interface between each layer. Although research studies to this point have yielded good results related to OC regeneration with tissue scaffolds, differences between engineered scaffolds and natural OC tissue remain; due to these differences, current clinical therapies to repair OC defects with engineered scaffolds have not been successful. This paper provides an overview of both discrete and continuous gradient OC tissue scaffolds in terms of cell type, scaffold material, microscale structure, mechanical properties, fabrication methods, and scaffold stimuli. Fabrication of gradient scaffolds with three-dimensional (3D) printing is given special emphasis due to its ability to accurately control scaffold pore geometry. Moreover, the application of computational modeling in OC tissue engineering is considered; for example, efforts to optimize the scaffold structure, mechanical properties, and physical stimuli generated within the scaffold-bioreactor system to predict tissue regeneration are considered. Finally, challenges associated with the repair of OC defects and recommendations for future directions in OC tissue regeneration are proposed.

Functional polymeric nerve guidance conduits and drug delivery strategies for peripheral nerve repair and regeneration

Peripheral nerve injuries can be extremely debilitating, resulting in sensory and motor loss-of-function. Endogenous repair is limited to non-severe injuries in which transection of nerves necessitates surgical intervention. Traditional treatment approaches include the use of biological grafts and alternative engineering approaches have made progress. The current article serves as a comprehensive, in-depth perspective on peripheral nerve regeneration, particularly nerve guidance conduits and drug delivery strategies. A detailed background of peripheral nerve injury and repair pathology, and an in-depth look into augmented nerve regeneration, nerve guidance conduits, and drug delivery strategies provide a state-of-the-art perspective on the field.

Polymeric ionically conductive composite matrices and electrical stimulation strategies for nerve regeneration: In vitro characterization

Stem cell strategies and the use of electrical stimulation (ES) represent promising new frontiers for peripheral nerve regeneration. Composite matrices were fabricated by coating electrospun polycaprolactone/cellulose acetate micro-nanofibers with chitosan and ionically conductive (IC) polymers including, sulfonated polyaniline, and lignin sulfonate. These composite matrices were characterized for surface morphology, coating uniformity, ionic conductivity, and mechanical strength to explore as scaffold materials for nerve regeneration in conjunction with ES. Composite matrices measured conductivity in the range of 0.0049-0.0068 mS/m due to the uniform coating of sulfonated polymers on the micro-nanofibers. Thin films (2D) and composite fiber matrices (3D) of IC polymers seeded with human mesenchymal stem cells (hMSCs) were electrically stimulated at 0.5 V, 20 Hz for 1 h daily for 14 days to study the changes in cell viability, morphology, and expression of the neuronal-like phenotype. In vitro ES lead to changes in hMSCs' fibroblast morphology into elongated neurite-like structures with cell bodies for ES-treated and positive control growth factor-treated groups. Immunofluorescent staining revealed the presence of neuronal markers including β3-tubulin, microtubule-associated protein 2, and nestin in response to ES. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1792-1805, 2019.

Aligned microchannel polymer-nanotube composites for peripheral nerve regeneration: Small molecule drug delivery

Peripheral nerve injury accounts for roughly 2.8% of all trauma patients with an annual cost of 7 billion USD in the U.S. alone. Current treatment options rely on surgical intervention with the use of an autograft, despite associated shortcomings. Engineered nerve guidance conduits, stem cell therapies, and transient electrical stimulation have reported to increase speeds of functional recovery. As an alternative to the conduction effects of electrical stimulation, we have designed and optimized a nerve guidance conduit with aligned microchannels for the sustained release of a small molecule drug that promotes nerve impulse conduction. A biodegradable chitosan structure reinforced with drug-loaded halloysite nanotubes (HNT) was formed into a foam-like conduit with interconnected, longitudinally-aligned pores with an average pore size of 59.3 ± 14.2 μm. The aligned composite with HNTs produced anisotropic mechanical behavior with a Young's modulus of 0.33 ± 0.1 MPa, very similar to that of native peripheral nerve. This manuscript reports on the sustained delivery of 4-Aminopyridine (4AP, molecular weight 94.1146 g/mol), a potassium-channel blocker as a growth factor alternative to enhance the rate of nerve regeneration. The conduit formulation released a total of 30 ± 2% of the encapsulated 4AP in the first 7 days. Human Schwann cells showed elevated expression of key proteins such as nerve growth factor, myelin protein zero, and brain derived neurotrophic factor in a 4AP dose dependent manner. Preliminary in vivo studies in a critical-sized sciatic nerve defect in Wistar rats confirmed conduit suturability and strength to withstand ambulatory forces over 4 weeks of their implantation. Histological evaluations suggest conduit biocompatibility and Schwann cell infiltration and organization within the conduit and lumen. These nerve guidance conduits and 4AP sustained delivery may serve as an attractive strategy for nerve repair and regeneration.

Nanofibers and Microfibers for Osteochondral Tissue Engineering

The use of fibers into scaffolds is a way to mimic natural tissues, in which fibrils are embedded in a matrix. The use of fibers can improve the mechanical properties of the scaffolds and may act as structural support for cell growth. Also, as the morphology of fibrous scaffolds is similar to the natural extracellular matrix, cells cultured on these scaffolds tend to maintain their phenotypic shape. Different materials and techniques can be used to produce micrfibers- and nanofibers for scaffolds manufacturing; cells, in general, adhere and proliferate very well on PCL, chitosan, silk fibroin, and other nanofibers. One of the most important techniques to produce microfibers/nanofibers is electrospinning. Nanofibrous scaffolds are receiving increasing attention in bone tissue engineering, because they are able to offer a favorable microenvironment for cell attachment and growth. Different polymers can be electrospun, i.e., polyester, polyurethane, PLA, PCL, collagen, and silk. Other materials such as bioglass fibers, nanocellulose, and even carbon fiber and fabrics have been used to help increase bioactivity, mechanical properties of the scaffold, and cell proliferation. A compilation of mechanical properties and most common biological tests performed on fibrous scaffolds is included in this chapter.

HIGHLIGHTS

The use of microfibers and nanofibers allows for tailoring the scaffold properties. Electrospinning is one of the most important techniques nowadays to produce fibrous scaffolds. Microfibers and nanofibers use in scaffolds is a promising field to improve the behavior of scaffolds in osteochondral applications.

Tissue-engineered spiral nerve guidance conduit for peripheral nerve regeneration

Recently in peripheral nerve regeneration, preclinical studies have shown that the use of nerve guidance conduits (NGCs) with multiple longitudinally channels and intra-luminal topography enhance the functional outcomes when bridging a nerve gap caused by traumatic injury. These features not only provide guidance cues for regenerating nerve, but also become the essential approaches for developing a novel NGC. In this study, a novel spiral NGC with aligned nanofibers and wrapped with an outer nanofibrous tube was first developed and investigated. Using the common rat sciatic 10-mm nerve defect model, the in vivo study showed that a novel spiral NGC (with and without inner nanofibers) increased the successful rate of nerve regeneration after 6 weeks recovery. Substantial improvements in nerve regeneration were achieved by combining the spiral NGC with inner nanofibers and outer nanofibrous tube, based on the results of walking track analysis, electrophysiology, nerve histological assessment, and gastrocnemius muscle measurement. This demonstrated that the novel spiral NGC with inner aligned nanofibers and wrapped with an outer nanofibrous tube provided a better environment for peripheral nerve regeneration than standard tubular NGCs. Results from this study will benefit for future NGC design to optimize tissue-engineering strategies for peripheral nerve regeneration.

STATEMENT OF SIGNIFICANCE

We developed a novel spiral nerve guidance conduit (NGC) with coated aligned nanofibers. The spiral structure increases surface area by 4.5 fold relative to a tubular NGC. Furthermore, the aligned nanofibers was coated on the spiral walls, providing cues for guiding neurite extension. Finally, the outside of spiral NGC was wrapped with randomly nanofibers to enhance mechanical strength that can stabilize the spiral NGC. Our nerve histological data have shown that the spiral NGC had 50% more myelinated axons than a tubular structure for nerve regeneration across a 10 mm gap in a rat sciatic nerve. Results from this study can help further optimize tissue engineering strategies for peripheral nerve repair.

Spiral Layer-by-Layer Micro-Nanostructured Scaffolds for Bone Tissue Engineering

This Article reports the fabrication and characterization of composite micro-nanostructured spiral scaffolds functionalized with nanofibers and hydroxyapatite (HA) for bone regeneration. The spiral poly(lactic acid-co-glycolic acid) (PLGA) porous microstructure was coated with sparsely spaced PLGA nanofibers and HA to enhance surface area and bioactivity. Polyelectrolyte-based HA coating in a layer-by-layer (LBL) fashion allowed 10-70 μM Ca²⁺/mm² incorporation. These scaffolds provided a controlled release of Ca²⁺ ions up to 60 days with varied release kinetics accounting up to 10-50 μg. Spiral scaffolds supported superior adhesion, proliferation, and osteogenic differentiation of rat bone marrow stromal cells (MSCs) as compared to controls microstructures. Spiral micro-nanostructures supported homogeneous tissue ingrowth and resulted in bone-island formation in the center of the scaffold as early as 3 weeks in a rabbit ulnar bone defect model. In contrast, control cylindrical scaffolds showed tissue ingrowth only at the surface because of limitations in scaffold transport features.

Electrospun Nanofiber Scaffolds and Their Hydrogel Composites for the Engineering and Regeneration of Soft Tissues

Electrospinning has emerged as a simple, elegant, and scalable technique that can be used to fabricate polymeric nanofibers. Pure polymers as well as blends and composites of both natural and synthetic ones have been successfully electrospun into nanofiber matrices for many biomedical applications. Tissue-engineered medical implants, such as polymeric nanofiber scaffolds, are potential alternatives to autografts and allografts, which are short in supply and carry risks of disease transmission. These scaffolds have been used to engineer various soft tissues, including connective tissues, such as skin, ligament, and tendon, as well as nonconnective ones, such as vascular, muscle, and neural tissue. Electrospun nanofiber matrices show morphological similarities to the natural extracellular matrix (ECM), characterized by ultrafine continuous fibers, high surface-to-volume ratios, high porosities, and variable pore-size distributions. The physiochemical properties of nanofiber matrices can be controlled by manipulating electrospinning parameters so that they meet the requirements of a specific application.Nanostructured implants show improved biological performance over bulk materials in aspects of cellular infiltration and in vivo integration, taking advantage of unique quantum, physical, and atomic properties. Furthermore, the topographies of such scaffolds has been shown to dictate cellular attachment, migration, proliferation, and differentiation, which are critical in engineering complex functional tissues with improved biocompatibility and functional performance. This chapter discusses the use of the electrospinning technique in the fabrication of polymer nanofiber scaffolds utilized for the regeneration of soft tissues. Selected scaffolds will be seeded with human mesenchymal stem cells (hMSCs), imaged using scanning electron and confocal microscopy, and then evaluated for their mechanical properties as well as their abilities to promote cell adhesion, proliferation , migration, and differentiation.