Electrospun Fibers for Recruitment and Differentiation of Stem Cells in Regenerative Medicine

Interfacial blending in co-axially electrospun polymer core-shell fibers and their interaction with cells via focal adhesion point analysis

Electrospun polymer scaffolds have gained prominence in biomedical applications, including tissue engineering, drug delivery, and wound dressings, due to their customizable properties. As the interplay between cells and materials assumes fundamental significance in biomaterials research, understanding the relationship between fiber properties and cell behaviour is imperative. Nevertheless, altering fiber properties introduces complexity by intertwining mechanical and surface chemistry effects, challenging the differentiation of their individual impacts on cell behaviour. Core-shell fibers present an appealing solution, enabling the control of mechanical properties of scaffolds, flexibility in material and drug selection, efficient encapsulation, strong protection of bioactive drugs against harsh environments, and controlled, prolonged drug release. This study addresses a key challenge in core-shell fiber design related to the blending effect between core and shell polymers. Two types of fibers, PMMA and core-shell PC-PMMA, were electrospun, and thorough analyses confirmed the desired core-shell structure in PC-PMMA fibers. Surface chemistry analysis revealed PC diffusion to the PMMA shell of the core-shell fiber during electrospinning, subsequently prompting an investigation of the fiber's surface potential. Conducting cellular studies on osteoblasts by super-resolution confocal microscopy provided insights into the direct influence of interfacial polymer blending and, consequently, altered fiber surface and mechanical properties on cell focal adhesion points, bridging the gap between material attributes and cell responses in core-shell fibers.

Bioinspired scaffolds based on aligned polyurethane nanofibers mimic tendon and ligament fascicles

Topographical factors of scaffolds play an important role in regulating cell functions. Although the effects of alignment topography and three-dimensional (3D) configuration of nanofibers as well as surface stiffness on cell behavior have been investigated, there are relatively few reports that attempt to understand the relationship between synergistic effects of these parameters and cell responses. Herein, the influence of biophysical and biomechanical cues of electrospun polyurethane (PU) scaffolds on mesenchymal stem cells (MSCs) activities was evaluated. To this aim, multiscale bundles were developed by rolling up the aligned electrospun mats mimicking the fascicles of tendons/ligaments and other similar tissues. Compared to mats, the 3D bundles not only maintained the desirable topographical features (i.e., fiber diameter, fiber orientation, and pore size), but also boosted tensile strength (∼40 MPa), tensile strain (∼260%), and surface stiffness (∼1.75 MPa). Alignment topography of nanofibers noticeably dictated cell elongation and a uniaxial orientation, resulting in tenogenic commitment of MSCs. MSCs seeded on the bundles expressed higher levels of tenogenic markers compared to mats. Moreover, the biomimetic bundle scaffolds improved synthesis of extracellular matrix components compared to mats. These results suggest that biophysical and biomechanical cues modulate cell-scaffold interactions, providing new insights into hierarchical scaffold design for further studies.

Multifunctional Scaffolds Based on Emulsion and Coaxial Electrospinning Incorporation of Hydroxyapatite for Bone Tissue Regeneration

Tissue engineering is nowadays a powerful tool to restore damaged tissues and recover their normal functionality. Advantages over other current methods are well established, although a continuous evolution is still necessary to improve the final performance and the range of applications. Trends are nowadays focused on the development of multifunctional scaffolds with hierarchical structures and the capability to render a sustained delivery of bioactive molecules under an appropriate stimulus. Nanocomposites incorporating hydroxyapatite nanoparticles (HAp NPs) have a predominant role in bone tissue regeneration due to their high capacity to enhance osteoinduction, osteoconduction, and osteointegration, as well as their encapsulation efficiency and protection capability of bioactive agents. Selection of appropriated polymeric matrices is fundamental and consequently great efforts have been invested to increase the range of properties of available materials through copolymerization, blending, or combining structures constituted by different materials. Scaffolds can be obtained from different processes that differ in characteristics, such as texture or porosity. Probably, electrospinning has the greater relevance, since the obtained nanofiber membranes have a great similarity with the extracellular matrix and, in addition, they can easily incorporate functional and bioactive compounds. Coaxial and emulsion electrospinning processes appear ideal to generate complex systems able to incorporate highly different agents. The present review is mainly focused on the recent works performed with Hap-loaded scaffolds having at least one structural layer composed of core/shell nanofibers.

Advanced Nanofiber-Based Scaffolds for Achilles Tendon Regenerative Engineering

The Achilles tendon (AT) is responsible for running, jumping, and standing. The AT injuries are very common in the population. In the adult population (21–60 years), the incidence of AT injuries is approximately 2.35 per 1,000 people. It negatively impacts people’s quality of life and increases the medical burden. Due to its low cellularity and vascular deficiency, AT has a poor healing ability. Therefore, AT injury healing has attracted a lot of attention from researchers. Current AT injury treatment options cannot effectively restore the mechanical structure and function of AT, which promotes the development of AT regenerative tissue engineering. Various nanofiber-based scaffolds are currently being explored due to their structural similarity to natural tendon and their ability to promote tissue regeneration. This review discusses current methods of AT regeneration, recent advances in the fabrication and enhancement of nanofiber-based scaffolds, and the development and use of multiscale nanofiber-based scaffolds for AT regeneration.

Zein-polycaprolactone core-shell nanofibers for wound healing

In a previous study, we developed electrospun antimicrobial microfiber scaffolds for wound healing composed of a core of zein protein and a shell containing polyethylene oxide. While providing a promising platform for composite nanofiber design, the scaffolds showed low tensile strengths, insufficient water stability, as well as burst release of the antimicrobial drug tetracycline hydrochloride, properties which are not ideal for the use of the scaffolds as wound dressings. Therefore, the aim of the present study was to develop fibers with enhanced mechanical strength and water stability, also displaying sustained release of tetracycline hydrochloride. Zein was chosen as core material, while the shell was formed by the hydrophobic polymer polycaprolactone, either alone or in combination with polyethylene oxide. As compared to control fibers of pristine polycaprolactone, the zein-polycaprolactone fibers exhibited a reduced diameter and hydrophobicity, which is beneficial for cell attachment and wound closure. Such fibers also demonstrated sustained release of tetracycline hydrochloride, as well as water stability, ductility, high mechanical strength and fibroblast attachment, hence representing a step towards the development of biodegradable wound dressings with prolonged drug release, which can be left on the wound for a longer time.

Novel Poly(ester urethane urea)/Polydioxanone Blends: Electrospun Fibrous Meshes and Films

In this work, we report the electrospinning and mechano-morphological characterizations of scaffolds based on blends of a novel poly(ester urethane urea) (PHH) and poly(dioxanone) (PDO). At the optimized electrospinning conditions, PHH, PDO and blend PHH/PDO in Hexafluroisopropanol (HFIP) solution yielded bead-free non-woven random nanofibers with high porosity and diameter in the range of hundreds of nanometers. The structural, morphological, and biomechanical properties were investigated using Differential Scanning Calorimetry, Scanning Electron Microscopy, Atomic Force Microscopy, and tensile tests. The blended scaffold showed an elastic modulus (~5 MPa) with a combination of the ultimate tensile strength (2 ± 0.5 MPa), and maximum elongation (150% ± 44%) in hydrated conditions, which are comparable to the materials currently being used for soft tissue applications such as skin, native arteries, and cardiac muscles applications. This demonstrates the feasibility of an electrospun PHH/PDO blend for cardiac patches or vascular graft applications that mimic the nanoscale structure and mechanical properties of native tissue.

Antibacterial Porous Electrospun Fibers as Skin Scaffolds for Wound Healing Applications

Electrospun fiber scaffolds have a huge potential for the successful treatment of infected wounds based on their unique properties. Although several studies report novel drug-loaded electrospun fiber-based biomaterials, many of these do not provide information on their interactions with eukaryotic and bacterial cells. The main aim of this study was to develop antibacterial drug-loaded porous biocompatible polycaprolactone (PCL) fiber scaffolds mimicking the native extracellular matrix for wound healing purposes. Mechanical property evaluation and different biorelevant tests were conducted in order to understand the structure-activity relationships and reveal how the surface porosity of fibers and the fiber diameter affect the scaffold interactions with the living bacterial and eukaryotic fibroblast cells. Cell migration and proliferation assays and antibiofilm assays enabled us to enlighten the biocompatibility and safety of fiber scaffolds and their suitability to be used as scaffolds for the treatment of infected wounds. Here, we report that porous PCL microfiber scaffolds obtained using electrospinning at high relative humidity served as the best surfaces for fibroblast attachment and growth compared to the nonporous microfiber or nonporous nanofiber PCL scaffolds. Porous chloramphenicol-loaded microfiber scaffolds were more elastic compared to nonporous scaffolds and had the highest antibiofilm activity. The results indicate that in addition to the fiber diameter and fiber scaffold porosity, the single-fiber surface porosity and its effect on drug release, mechanical properties, cell viability, and antibiofilm activity need to be understood when developing antibacterial biocompatible scaffolds for wound healing applications. We show that pores on single fibers within an electrospun scaffold, in addition to nano- and microscale diameter of the fibers, change the living cell-fiber interactions affecting the antibiofilm efficacy and biocompatibility of the scaffolds for the local treatment of wounds.

Gelatin-Based Electrospun Fibers Insolubilized by Horseradish Peroxidase-Catalyzed Cross-Linking for Biomedical Applications

Gelatin-based water-insoluble nanofibers with a diameter of 160 nm were obtained from electrospinning aqueous solutions containing gelatin with phenolic hydroxyl (Ph) moieties (Gelatin-Ph) and horseradish peroxidase (HRP). The water insolubility of the nanofibers was accomplished through HRP-catalyzed cross-linking of the Ph moieties by exposing the electrospun nanofibers to air containing hydrogen peroxide. The HRP activity in the electrospun nanofibers was 65% that of native HRP. The cytocompatibility necessary for tissue engineering applications of the water-insoluble Gelatin-Ph nanofibers was confirmed by the adhesion and viability of human embryonic kidney-derived HEK293 cells.

Three-Dimensional Printing and Electrospinning Dual-Scale Polycaprolactone Scaffolds with Low-Density and Oriented Fibers to Promote Cell Alignment

Complex and hierarchically functionalized scaffolds composed of micro- and nanoscale structures are a key goal in tissue engineering. The combination of three-dimensional (3D) printing and electrospinning enables the fabrication of these multiscale structures. This study presents a polycaprolactone 3D-printed and electrospun scaffold with multiple mesh layers and fiber densities. The results show successful fabrication of a dual-scale scaffold with the 3D-printed scaffold acting as a gap collector with the printed microfibers as the electrodes and the pores a series of insulating gaps resulting in aligned nanofibers. The electrospun fibers are highly aligned perpendicular to the direction of the printed fiber and form aligned meshes within the pores of the scaffold. Mechanical testing showed no significant difference between the number of mesh layers whereas the hydrophobicity of the scaffold increased with increasing fiber density. Biological results indicate that increasing the number of mesh layers improves cell proliferation, migration, and adhesion. The aligned nanofibers within the microscale pores allowed enhanced cell bridging and cell alignment that was not observed in the 3D-printed only scaffold. These results demonstrate a facile method of incorporating low-density and aligned fibers within a 3D-printed scaffold that is a promising development in multiscale hierarchical scaffolds where alignment of cells can be desirable.

Differentiation of neonate mouse spermatogonial stem cells on three-dimensional agar/polyvinyl alcohol nanofiber scaffold

Electrospun nanofiber matrices sufficiently mimic the structural morphology of natural extracellular matrix. In this study, we aimed to examine the effects of agar/polyvinyl alcohol nanofiber (PVA) scaffold on the proliferation efficiency and differentiation potential of neonate mouse spermatogonial stem cells (SCCs). Testicular cells were isolated from testes of 40 mouse pups and were seeded in: 1) 2D cell culture plates in the absence (2D/-GF) or presence (2D/+GF) of growth factors and 2) onto agar/PVA scaffold in the absence (3D/-GF) or presence (3D/+GF) of growth factors. The cells were subsequently cultured for 4 weeks. First 2 weeks were dedicated to proliferative phase, whereas the next 2 weeks emphasized the differentiation phase. The identity of the SCCs was investigated at different time-points by flow cytometry and quantitative reverse transcription PCR (qRT-PCR) analyses against the germ cell markers, including PLZF, Id-4, Gfrα-1, Tekt-1, and Sycp-3. After 2 weeks of culture, the 3D/+GF group showed the highest percentage of PLZF-positive cells among culture systems (P < 0.05). The expression levels of pre-meiotic markers (Id-4 and Gfrα-1) decreased significantly in all groups, particularly in 3D/+GF group after 28 days of culture. Additionally, the cells in the 3D/+GF group displayed the highest expression of meiotic (Sycp-3) and post-meiotic markers (Tekt-1) 14 days after differentiation induction. Seemingly, the combination of the agar/PVA scaffold and growth factor-supplemented medium synergistically increased the differentiation rate of mouse SSCs into meiotic and post-meiotic cells. Thus, agar/PVA nanofiber scaffolds may have the potential for applications in the restoration of infertility, especially in azoospermic males.

ABBREVIATIONS

2D: two dimentional; 3D: three dimentional; bFGF: basic fibroblast growth factor; BMP-4: bone morphogenetic protein 4; DMEM: Dulbecco's modified Eagle's medium; ECM: extracellular matrix; FCS: fetal calf serum; FTIR: Fourier-transform infrared spectroscopy; GDNF: glial cell line-derived neurotrophic factor; GF: growth factors; Gfrα-1, GDNF family co-receptor α1; Id-4, Inhibitor of DNA Binding 4; MTT: methylthiazoltetrazolium; PLZF: promyelocytic leukemia zinc finger; PVA: polyvinyl alcohol; qRT-PCR: quantitative reverse transcription PCR; RA: retinoic acid; SACS: soft agar culture system; SD: standard deviation; SEM: scanning electron microscope; SSCs: spermatogonial stem cells; Sycp-3, Synaptonemal complex protein 3; Tekt-1, Tektin 1.

Experimental Protocol of MSC Differentiation into Neural Lineage for Nerve Tissue Regeneration Using Polymeric Scaffolds

The treatment of neurodegenerative diseases is still a challenging grindstone in reconstructive surgeries and regenerative medicine. The retention of mesenchymal stem cells (MSCs) to retain remarkable properties of differentiating into motor neuron-like cells and Schwann cells can prove to be effective in repairing disorders. Moreover, the ultrafine electrospun nanofibers provide a favorable and conducive platform for proliferation and differentiation of MSCs. The development of new 3D culture methods with electrospun scaffolds that closely mimic the physiological niche of cells will help us to understand the functional benefits of MSCs in regeneration process. This article highlights the protocols for isolation of MSCs from rat bone marrow and their subsequent culture on nanofiber scaffolds. Furthermore, this chapter summarizes the various procedures including isolation of the MSCs, their seeding on electrospun nanofibrous scaffolds, and their proliferation and differentiation into neural lineage upon appropriate induction. The materials and preparation of various reagents used at different steps of the protocol are also summarized in detail.

The poly (l-lactid-co-glycolide; PLGA) fiber component of brushite-forming calcium phosphate cement induces the osteogenic differentiation of human adipose tissue-derived stem cells

A brushite-forming calcium phosphate cement (CPC) was mechanically stabilized by addition of poly (l-lactid-co-glycolide; PLGA) fibers (≤10% w/w). It proved highly biocompatible and its fiber component enhanced bone formation in a sheep lumbar vertebroplasty model. However, possible effects on the osteogenic differentiation of resident mesenchymal stem cells (MSCs) remained unexplored. The present study used a novel approach, simultaneously analyzing the influence of a solid CPC scaffold and its relatively low PLGA proportion (a mimicry of natural bone) on osteogenic, chondrogenic, and adipogenic differentiation, as well as the pluripotency of human adipose tissue-derived mesenchymal stem cells (hASCs). hASCs were cultured on CPC discs with/without PLGA fibers (5% and 10%) in the absence of osteogenic medium for 3, 7, and 14 d. Gene expression of osteogenic markers (Runx2, osterix, alkaline phosphatase, collagen I, osteonectin, osteopontin, osteocalcin), chondrogenic markers (collagen II, Sox9, aggrecan), adipogenic markers (PPARG, Leptin, and FABP4), and pluripotency markers (Nanog, Tert, Rex) was analyzed by RT-PCR. The ability of hASCs to synthesize alkaline phosphatase was also evaluated. Cell number and viability were determined by fluorescein diacetate/propidium iodide staining. Compared to pure CPC, cultivation of hASCs on fiber-reinforced CPC transiently induced the gene expression of Runx2 and osterix (day 3), and long-lastingly augmented the expression of alkaline phosphatase (and its enzyme activity), collagen I, and osteonectin (until day 14). In contrast, augmented expression of all chondrogenic, adipogenic, and pluripotency markers was limited to day 3, followed by significant downregulation. Cultivation of hASCs on fiber-reinforced CPC reduced the cell number, but not the proportion of viable cells (viability > 95%). The PLGA component of fiber-reinforced, brushite-forming CPC supports long-lasting osteogenic differentiation of hASCs, whereas chondrogenesis, adipogenesis, and pluripotency are initially augmented, but subsequently suppressed. In view of parallel animal results, PLGA fibers may represent an interesting clinical target for future improvement of CPC- based bone regeneration.

Recent trends in peripheral nervous regeneration using 3D biomaterials

Mesenchymal stem cells (MSCs) owing their multipotency are known as progenitors for the regeneration of adult tissues including that of neuronal tissue. The repair and/or regeneration of traumatic nerves is still a challenging task for neurosurgeons. It is also a well-established fact that the microenvironment plays a primary role in determining the fate of stem cells to a specific lineage. In recent years, with the advent of nanotechnology and its positive influence on designing and fabrication of various 3D biomaterials have progressed to a greater extent. The production of 3D biomaterials such as nanofibers, conduits and hydrogels are providing a suitable environment for mimicking physiological niche of stem cells. These 3D biomaterials in combination with MSCs have been successfully analyzed for their potential in the regeneration of degenerative neurological disorders. This review primarily highlights the combinatorial effect of multipotent MSCs seeded on various 3D polymeric scaffolds in repair and regeneration of nervous tissue. The elaboration of MSCs from distinct sources reported so far in literature are summarized to understand their role in regeneration processes. Furthermore, we accentuate the application of 3D biomaterials especially the nanofibers, polymeric conduits, hydrogels infiltrated with MSCs harvested from distinct sources in the field of peripheral nerve regeneration studies.

Combining Stem Cells and Biomaterial Scaffolds for Constructing Tissues and Cell Delivery

Combining stem cells with biomaterial scaffolds serves as a promising strategy for engineering tissues for both in vitro and in vivo applications. This updated review details commonly used biomaterial scaffolds for engineering tissues from stem cells. We first define the different types of stem cells and their relevant properties and commonly used scaffold formulations. Next, we discuss natural and synthetic scaffold materials typically used when engineering tissues, along with their associated advantages and drawbacks and gives examples of target applications. New approaches to engineering tissues, such as 3D bioprinting, are described as they provide exciting opportunities for future work along with current challenges that must be addressed. Thus, this review provides an overview of the available biomaterials for directing stem cell differentiation as a means of producing replacements for diseased or damaged tissues.

Engineering the hard-soft tissue interface with random-to-aligned nanofiber scaffolds

Tendon injuries can be difficult to heal and have high rates of relapse due to stress concentrations caused by scar formation and the sutures used in surgical repair. Regeneration of the tendon/ligament-to-bone interface is critical to provide functional graft integration after injury. The objective of this study is to recreate the tendon-to-bone interface using a gradient scaffold which is fabricated by a one-station electrospinning process. Two cell phenotypes were grown on a poly-ε-caprolactone nanofiber scaffold which possesses a gradual transition from random to aligned nanofiber patterns. We assessed the effects of the polymer concentration, tip-to-collector distance, and electrospinning time on the microfiber diameter and density. Osteosarcoma and fibroblast cells were seeded on the random and aligned sections of scaffolds, respectively. A random-to-aligned cocultured tissue interface which mimicked the native transition in composition of enthesis was created after 96 h culturing. The results showed that the microstructure gradient influenced the cell morphology, tissue topology, and promoted enthesis formation. This study demonstrates a heterogeneous nanofiber scaffold strategy for interfacial tissue regeneration. It provides a potential solution for mimicking transitional interface between distinct tissues, and can be further developed as a heterogeneous cellular composition platform to facilitate the formation of multi-tissue complex systems.

Mechanical Considerations for Electrospun Nanofibers in Tendon and Ligament Repair

Electrospun nanofibers possess unique qualities such as nanodiameter, high surface area to volume ratio, biomimetic architecture, and tunable chemical and electrical properties. Numerous studies have demonstrated the potential of nanofibrous architecture to direct cell morphology, migration, and more complex biological processes such as differentiation and extracellular matrix (ECM) deposition through topographical guidance cues. These advantages have created great interest in electrospun fibers for biomedical applications, including tendon and ligament repair. Electrospun nanofibers, despite their nanoscale size, generally exhibit poor mechanical properties compared to larger conventionally manufactured polymer fiber materials. This invites the question of what role electrospun polymer nanofibers can play in tendon and ligament repair applications that have both biological and mechanical requirements. At first glance, the strength and stiffness of electrospun nanofiber grafts appear to be too low to fill the rigorous loading conditions of these tissues. However, there are a number of strategies to enhance and tune the mechanical properties of electrospun nanofiber grafts. As researchers design the next-generation electrospun tendon and ligament grafts, it is critical to consider numerous physiologically relevant mechanical criteria and to evaluate graft mechanical performance in conditions and loading environments that reflect in vivo conditions and surgical fixation methods.