Effect of polyvinylidene fluoride electrospun fiber orientation on neural stem cell differentiation

Scaffold-Based Poly(Vinylidene Fluoride) and Its Copolymers: Materials, Fabrication Methods, Applications, and Perspectives

Tissue engineering involves implanting grafts into damaged tissue sites to guide and stimulate the formation of new tissue, which is an important strategy in the field of tissue defect treatment. Scaffolds prepared in vitro meet this requirement and are able to provide a biochemical microenvironment for cell growth, adhesion, and tissue formation. Scaffolds made of piezoelectric materials can apply electrical stimulation to the tissue without an external power source, speeding up the tissue repair process. Among piezoelectric polymers, poly(vinylidene fluoride) (PVDF) and its copolymers have the largest piezoelectric coefficients and are widely used in biomedical fields, including implanted sensors, drug delivery, and tissue repair. This paper provides a comprehensive overview of PVDF and its copolymers and fillers for manufacturing scaffolds as well as the roles in improving piezoelectric output, bioactivity, and mechanical properties. Then, common fabrication methods are outlined such as 3D printing, electrospinning, solvent casting, and phase separation. In addition, the applications and mechanisms of scaffold-based PVDF in tissue engineering are introduced, such as bone, nerve, muscle, skin, and blood vessel. Finally, challenges, perspectives, and strategies of scaffold-based PVDF and its copolymers in the future are discussed.

Revealing an important role of piezoelectric polymers in nervous-tissue regeneration: A review

Nerve injuries pose a drastic threat to nerve mobility and sensitivity and lead to permanent dysfunction due to low regenerative capacity of mature neurons. The electrical stimuli that can be provided by electroactive materials are some of the most effective tools for the formation of soft tissues, including nerves. Electric output can provide a distinctly favorable bioelectrical microenvironment, which is especially relevant for the nervous system. Piezoelectric biomaterials have attracted attention in the field of neural tissue engineering owing to their biocompatibility and ability to generate piezoelectric surface charges. In this review, an outlook of the most recent achievements in the field of piezoelectric biomaterials is described with an emphasis on piezoelectric polymers for neural tissue engineering. First, general recommendations for the design of an optimal nerve scaffold are discussed. Then, specific mechanisms determining nerve regeneration via piezoelectric stimulation are considered. Activation of piezoelectric responses via natural body movements, ultrasound, and magnetic fillers is also examined. The use of magnetoelectric materials in combination with alternating magnetic fields is thought to be the most promising due to controllable reproducible cyclic deformations and deep tissue permeation by magnetic fields without tissue heating. In vitro and in vivo applications of nerve guidance scaffolds and conduits made of various piezopolymers are reviewed too. Finally, challenges and prospective research directions regarding piezoelectric biomaterials promoting nerve regeneration are discussed. Thus, the most relevant scientific findings and strategies in neural tissue engineering are described here, and this review may serve as a guideline both for researchers and clinicians.

Review on electrically conductive smart nerve guide conduit for peripheral nerve regeneration

At present, peripheral nerve injuries (PNIs) are one of the leading causes of substantial impairment around the globe. Complete recovery of nerve function after an injury is challenging. Currently, autologous nerve grafts are being used as a treatment; however, this has several downsides, for example, donor site morbidity, shortage of donor sites, loss of sensation, inflammation, and neuroma development. The most promising alternative is the development of a nerve guide conduit (NGC) to direct the restoration and renewal of neuronal axons from the proximal to the distal end to facilitate nerve regeneration and maximize sensory and functional recovery. Alternatively, the response of nerve cells to electrical stimulation (ES) has a substantial regenerative effect. The incorporation of electrically conductive biomaterials in the fabrication of smart NGCs facilitates the function of ES throughout the active proliferation state. This article overviews the potency of the various categories of electroactive smart biomaterials, including conductive and piezoelectric nanomaterials, piezoelectric polymers, and organic conductive polymers that researchers have employed latterly to fabricate smart NGCs and their potentiality in future clinical application. It also summarizes a comprehensive analysis of the recent research and advancements in the application of ES in the field of NGC.

Recent progress of electroactive interface in neural engineering

Neural tissue is an electrical responsible organ. The electricity plays a vital role in the growth and development of nerve tissue, as well as the repairing after diseases. The interface between the nervous system and external device for information transmission is called neural electroactive interface. With the development of new materials and fabrication technologies, more and more new types of neural interfaces are developed and the interfaces can play crucial roles in treating many debilitating diseases such as paralysis, blindness, deafness, epilepsy, and Parkinson's disease. Neural interfaces are developing toward flexibility, miniaturization, biocompatibility, and multifunctionality. This review presents the development of neural electrodes in terms of different materials for constructing electroactive neural interfaces, especially focus on the piezoelectric materials-based indirect neuromodulation due to their features of wireless control, excellent effect, and good biocompatibility. We discussed the challenges we need to consider before the application of these new interfaces in clinical practice. The perspectives about future directions for developing more practical electroactive interface in neural engineering are also discussed in this review. This article is categorized under: Implantable Materials and Surgical Technologies > Nanomaterials and Implants Implantable Materials and Surgical Technologies > Nanotechnology in Tissue Repair and Replacement.

Porous biomaterials for tissue engineering: a review

The field of biomaterials has grown rapidly over the past decades. Within this field, porous biomaterials have played a remarkable role in: (i) enabling the manufacture of complex three-dimensional structures; (ii) recreating mechanical properties close to those of the host tissues; (iii) facilitating interconnected structures for the transport of macromolecules and cells; and (iv) behaving as biocompatible inserts, tailored to either interact or not with the host body. This review outlines a brief history of the development of biomaterials, before discussing current materials proposed for use as porous biomaterials and exploring the state-of-the-art in their manufacture. The wide clinical applications of these materials are extensively discussed, drawing on specific examples of how the porous features of such biomaterials impact their behaviours, as well as the advantages and challenges faced, for each class of the materials.

Electrical Stimulation Enabled via Electrospun Piezoelectric Polymeric Nanofibers for Tissue Regeneration

Electrical stimulation has demonstrated great effectiveness in the modulation of cell fate in vitro and regeneration therapy in vivo. Conventionally, the employment of electrical signal comes with the electrodes, battery, and connectors in an invasive fashion. This tedious procedure and possible infection hinder the translation of electrical stimulation technologies in regenerative therapy. Given electromechanical coupling and flexibility, piezoelectric polymers can overcome these limitations as they can serve as a self-powered stimulator via scavenging mechanical force from the organism and external stimuli wirelessly. Wireless electrical cue mediated by electrospun piezoelectric polymeric nanofibers constitutes a promising paradigm allowing the generation of localized electrical stimulation both in a noninvasive manner and at cell level. Recently, numerous studies based on electrospun piezoelectric nanofibers have been carried out in electrically regenerative therapy. In this review, brief introduction of piezoelectric polymer and electrospinning technology is elucidated first. Afterward, we highlight the activating strategies (e.g., cell traction, physiological activity, and ultrasound) of piezoelectric stimulation and the interaction of piezoelectric cue with nonelectrically/electrically excitable cells in regeneration medicine. Then, quantitative comparison of the electrical stimulation effects using various activating strategies on specific cell behavior and various cell types is outlined. Followingly, this review explores the present challenges in electrospun nanofiber-based piezoelectric stimulation for regeneration therapy and summarizes the methodologies which may be contributed to future efforts in this field for the reality of this technology in the clinical scene. In the end, a summary of this review and future perspectives toward electrospun nanofiber-based piezoelectric stimulation in tissue regeneration are elucidated.

Preparation of Polyvinylidene Fluoride–Gold Nanoparticles Electrospinning Nanofiber Membranes

In this work, gold nanoparticles (AuNPs) and curcumin drug were incorporated in polyvinylidene fluoride (PVDF) nanofibers by electrospinning as a novel tissue engineering scaffold in nerve regeneration. The influence of AuNPs on the morphology, crystallinity, and drug release behavior of nanofiber membranes was characterized. A successful composite nanofiber membrane sample was observed by scanning electron microscopy (SEM). The addition of AuNPs showed the improved as well as prolonged cumulative release of the drug. The results indicated that PVDF–AuNPs nanofiber membrane could potentially be applied for nerve regeneration.

Methods to Characterize Electrospun Scaffold Morphology: A Critical Review

Electrospun scaffolds can imitate the hierarchical structures present in the extracellular matrix, representing one of the main concerns of modern tissue engineering. They are characterized in order to evaluate their capability to support cells or to provide guidelines for reproducibility. The issues with widely used methods for morphological characterization are discussed in order to provide insight into a desirable methodology for electrospun scaffold characterization. Reported methods include imaging and physical measurements. Characterization methods harbor inherent limitations and benefits, and these are discussed and presented in a comprehensive selection matrix to provide researchers with the adequate tools and insights required to characterize their electrospun scaffolds. It is shown that imaging methods present the most benefits, with drawbacks being limited to required costs and expertise. By making use of more appropriate characterization, researchers will avoid measurements that do not represent their scaffolds and perhaps might discover that they can extract more characteristics from their scaffold at no further cost.

Understanding multiscale structure-property correlations in PVDF-HFP electrospun fiber membranes by SAXS and WAXS

Electrospinning is a versatile technique to produce nanofibrous membranes with applications in filtration, biosensing, biomedical and tissue engineering. The structural and therefore physical properties of electrospun fibers can be finely tuned by changing the electrospinning parameters. The large parameter window makes it challenging to optimize the properties of fibers for a specific application. Therefore, a fundamental understanding of the multiscale structure of fibers and its correlation with their macroscopic behaviors is required for the design and production of systems with dedicated applications. In this study, we demonstrate that the properties of poly(vinylidene fluoride-co-hexafluoro propylene) (PVDF-HFP) electrospun fibers can be tuned by changing the rotating drum speed used as a collector during electrospinning. Indeed, with the help of multiscale characterization techniques such as scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS), and wide-angle X-ray scattering (WAXS), we observe that increasing the rotating drum speed not only aligns the fibers but also induces polymeric chain rearrangements at the molecular scale. Such changes result in enhanced mechanical properties and an increase of the piezoelectric β-phase of the PVDF-HFP fiber membranes. We detect nanostructural deformation behaviors when the aligned fibrous membrane is uniaxially stretched along the fiber alignment direction, while an increase in the alignment of the fibers is observed for randomly aligned samples. This was analyzed by performing in situ SAXS measurements coupled with uniaxial tensile loading of the fibrous membranes along the fiber alignment direction. The present study shows that fibrous membranes can be produced with varying degrees of fiber orientation, piezoelectric β-phase content, and mechanical properties by controlling the speed of the rotating drum collector during the fiber production. Such aligned fiber membranes have potential applications for neural or musculoskeletal tissue engineering.

Pyrrole Plasma Polymer-Coated Electrospun Scaffolds for Neural Tissue Engineering

Promising strategies for neural tissue engineering are based on the use of three-dimensional substrates for cell anchorage and tissue development. In this work, fibrillar scaffolds composed of electrospun randomly- and aligned-oriented fibers coated with plasma synthesized pyrrole polymer, doped and undoped with iodine, were fabricated and characterized. Infrared spectroscopy, thermogravimetric analysis, and X-ray diffraction analysis revealed the functional groups and molecular integration of each scaffold, as well as the effect of plasma polymer synthesis on crystallinity. Scanning microscopy imaging demonstrated the porous fibrillar micrometric structure of the scaffolds, which afforded adhesion, infiltration, and survival for the neural cells. Orientation analysis of electron microscope images confirmed the elongation of neurite-like cell structures elicited by undoped plasma pyrrole polymer-coated aligned scaffolds, without any biochemical stimuli. The MTT colorimetric assay validated the biocompatibility of the fabricated composite materials, and further evidenced plasma pyrrole polymer-coated aligned scaffolds as permissive substrates for the support of neural cells. These results suggest plasma synthesized pyrrole polymer-coated aligned scaffolds are promising materials for tissue engineering applications.

PVDF and P(VDF-TrFE) Electrospun Scaffolds for Nerve Graft Engineering: A Comparative Study on Piezoelectric and Structural Properties, and In Vitro Biocompatibility

Polyvinylidene fluoride (PVDF) and its copolymer with trifluoroethylene (P(VDF-TrFE)) are considered as promising biomaterials for supporting nerve regeneration because of their proven biocompatibility and piezoelectric properties that could stimulate cell ingrowth due to their electrical activity upon mechanical deformation. For the first time, this study reports on the comparative analysis of PVDF and P(VDF-TrFE) electrospun scaffolds in terms of structural and piezoelectric properties as well as their in vitro performance. A dynamic impact test machine was developed, validated, and utilised, to evaluate the generation of an electrical voltage upon the application of an impact load (varying load magnitude and frequency) onto the electrospun PVDF (15-20 wt%) and P(VDF-TrFE) (10-20 wt%) scaffolds. The cytotoxicity and in vitro performance of the scaffolds was evaluated with neonatal rat (nrSCs) and adult human Schwann cells (ahSCs). The neurite outgrowth behaviour from sensory rat dorsal root ganglion neurons cultured on the scaffolds was analysed qualitatively. The results showed (i) a significant increase of the β-phase content in the PVDF after electrospinning as well as a zeta potential similar to P(VDF-TrFE), (ii) a non-constant behaviour of the longitudinal piezoelectric strain constant d33, depending on the load and the load frequency, and (iii) biocompatibility with cultured Schwann cells and guiding properties for sensory neurite outgrowth. In summary, the electrospun PVDF-based scaffolds, representing piezoelectric activity, can be considered as promising materials for the development of artificial nerve conduits for the peripheral nerve injury repair.

Physical Cues of Matrices Reeducate Nerve Cells

The behavior of nerve cells plays a crucial role in nerve regeneration. The mechanical, topographical, and electrical microenvironment surrounding nerve cells can activate cellular signaling pathways of mechanical transduction to affect the behavior of nerve cells. Recently, biological scaffolds with various physical properties have been developed as extracellular matrix to regulate the behavior conversion of nerve cell, such as neuronal neurite growth and directional differentiation of neural stem cells, providing a robust driving force for nerve regeneration. This review mainly focused on the biological basis of nerve cells in mechanical transduction. In addition, we also highlighted the effect of the physical cues, including stiffness, mechanical tension, two-dimensional terrain, and electrical conductivity, on neurite outgrowth and differentiation of neural stem cells and predicted their potential application in clinical nerve tissue engineering.

Electroactive Biomaterials and Systems for Cell Fate Determination and Tissue Regeneration: Design and Applications

During natural tissue regeneration, tissue microenvironment and stem cell niche including cell-cell interaction, soluble factors, and extracellular matrix (ECM) provide a train of biochemical and biophysical cues for modulation of cell behaviors and tissue functions. Design of functional biomaterials to mimic the tissue/cell microenvironment have great potentials for tissue regeneration applications. Recently, electroactive biomaterials have drawn increasing attentions not only as scaffolds for cell adhesion and structural support, but also as modulators to regulate cell/tissue behaviors and function, especially for electrically excitable cells and tissues. More importantly, electrostimulation can further modulate a myriad of biological processes, from cell cycle, migration, proliferation and differentiation to neural conduction, muscle contraction, embryogenesis, and tissue regeneration. In this review, endogenous bioelectricity and piezoelectricity are introduced. Then, design rationale of electroactive biomaterials is discussed for imitating dynamic cell microenvironment, as well as their mediated electrostimulation and the applying pathways. Recent advances in electroactive biomaterials are systematically overviewed for modulation of stem cell fate and tissue regeneration, mainly including nerve regeneration, bone tissue engineering, and cardiac tissue engineering. Finally, the significance for simulating the native tissue microenvironment is emphasized and the open challenges and future perspectives of electroactive biomaterials are concluded.

Recent advances of polymer-based piezoelectric composites for biomedical applications

Over the past decades, electronics have become central to many aspects of biomedicine and wearable device technologies as a promising personalized healthcare platform. Lead-free piezoelectric materials for converting mechanical into electrical energy through piezoelectric transduction are of significant value in a diverse range of technological applications. Organic piezoelectric biomaterials have attracted widespread attention as the functional materials in the biomedical devices due to their advantages of excellent biocompatibility. They include synthetic and biological polymers. Many biopolymers have been discovered to possess piezoelectricity in an appreciable amount, however their investigation is still preliminary. Due to their piezoelectric properties, better known synthetic fluorinated polymers have been intensively investigated and applied in biomedical applications including controlled drug delivery systems, tissue engineering, microfluidic and artificial muscle actuators, among others. Piezoelectric polymers, especially poly (vinylidene fluoride) (PVDF) and its copolymers are increasingly receiving interest as smart biomaterials due to their ability to convert physiological movements to electrical signals when in a controllable and reproducible manner. Despite possessing the greatest piezoelectric coefficients among all piezoelectric polymers, it is often desirable to increase the electrical outputs. The most promising routes toward significant improvements in the piezoelectric response and energy-harvesting performance of such materials is loading them with various inorganic nanofillers and/or applying some modification during the fabrication process. This paper offers a comprehensive review of the principles, properties, and applications of organic piezoelectric biomaterials (polymers and polymer/ceramic composites) with special attention on PVDF-based polymers and their composites in sensors, drug delivery and tissue engineering. Subsequently focuses on the most common fabrication routes to produce piezoelectric scaffolds, tissue and sensors which is electrospinning process. Promising upcoming strategies and new piezoelectric materials and fabrication techniques for these applications are presented to enable a future integration among these applications.

Design Challenges in Polymeric Scaffolds for Tissue Engineering

Numerous surgical procedures are daily performed worldwide to replace and repair damaged tissue. Tissue engineering is the field devoted to the regeneration of damaged tissue through the incorporation of cells in biocompatible and biodegradable porous constructs, known as scaffolds. The scaffolds act as host biomaterials of the incubating cells, guiding their attachment, growth, differentiation, proliferation, phenotype, and migration for the development of new tissue. Furthermore, cellular behavior and fate are bound to the biodegradation of the scaffold during tissue generation. This article provides a critical appraisal of how key biomaterial scaffold parameters, such as structure architecture, biochemistry, mechanical behavior, and biodegradability, impart the needed morphological, structural, and biochemical cues for eliciting cell behavior in various tissue engineering applications. Particular emphasis is given on specific scaffold attributes pertaining to skin and brain tissue generation, where further progress is needed (skin) or the research is at a relatively primitive stage (brain), and the enumeration of some of the most important challenges regarding scaffold constructs for tissue engineering.

Conductive Composite Fiber with Optimized Alignment Guides Neural Regeneration under Electrical Stimulation

Conductivity and alignment of scaffolds are two primary factors influencing the efficacy of nerve repair. Herein, conductive composite fibers composed of poly(ɛ-caprolactone) (PCL) and carbon nanotubes (CNTs) with different orientation degrees are prepared by electrospinning at various rotational speeds (0, 500, 1000, and 2000 rpm), and meanwhile the synergistic promotion mechanism of aligned topography and electrical stimulation on neural regeneration is fully demonstrated. Under an optimized rotational speed of 1000 rpm, the electrospun PCL fiber exhibits orientated structure at macroscopic (mean deviation angle = 2.78°) or microscopic crystal scale (orientation degree = 0.73), decreased contact angle of 99.2° ± 4.9°, and sufficient tensile strength in both perpendicular and parallel directions to fiber axis (1.13 ± 0.15 and 5.06 ± 0.98 MPa). CNTs are introduced into the aligned fiber for further improving conductivity (15.69-178.63 S m^-1 ), which is beneficial to the oriented growth of neural cells in vitro as well as the regeneration of injured sciatic nerves in vivo. On the basis of robust cell induction behavior, optimum sciatic nerve function index, and enhanced remyelination/axonal regeneration, such conductive PCL/CNTs composite fiber with optimized fiber alignment may serve as instructive candidates for promoting the scaffold- and cell-based strategies for neural repair.

Tunable Substrate Functionalities Direct Stem Cell Fate toward Electrophysiologically Distinguishable Neuron-like and Glial-like Cells

Engineering cellular microenvironment on a functional platform using various biophysical cues to modulate stem cell fate has been the central theme in regenerative engineering. Among the various biophysical cues to direct stem cell differentiation, the critical role of physiologically relevant electric field (EF) stimulation was established in the recent past. The present study is the first to report the strategy to switch EF-mediated differentiation of human mesenchymal stem cells (hMSCs) between neuronal and glial pathways, using tailored functional properties of the biomaterial substrate. We have examined the combinatorial effect of substrate functionalities (conductivity, electroactivity, and topography) on the EF-mediated stem cell differentiation on polyvinylidene-difluoride (PVDF) nanocomposites in vitro, without any biochemical inducers. The functionalities of PVDF have been tailored using conducting nanofiller (multiwall-carbon nanotube, MWNT) and piezoceramic (BaTiO₃, BT) by an optimized processing approach (melt mixing-compression molding-rolling). The DC conductivity of PVDF nanocomposites was tuned from ∼10^-11 to ∼10^-4 S/cm and the dielectric constant from ∼10 to ∼300. The phenotypical changes and genotypical expression of hMSCs revealed the signatures of early differentiation toward neuronal pathway on rolled-PVDF/MWNT and late differentiation toward glial lineage on rolled-PVDF/BT/MWNT. Moreover, we were able to distinguish the physiological properties of differentiated neuron-like and glial-like cells using membrane depolarization and mechanical stimulation. The excitability of the EF-stimulated hMSCs was also determined using whole-cell patch-clamp recordings. Mechanistically, the roles of intracellular reactive oxygen species (ROS), Ca²⁺ oscillations, and synaptic and gap junction proteins in directing the cellular fate have been established. Therefore, the present work critically unveils complex yet synergistic interaction of substrate functional properties to direct EF-mediated differentiation toward neuron-like and glial-like cells, with distinguishable electrophysiological responses.

Electrospun PVDF Nanofibers for Piezoelectric Applications: A Review of the Influence of Electrospinning Parameters on the β Phase and Crystallinity Enhancement

Polyvinylidene fluoride (PVDF) is among the most attractive piezo-polymers due to its excellent piezoelectricity, lightweight, flexibility, high thermal stability, and chemical resistance. PVDF can exist under different forms of films, membranes, and (nano)fibers, and its piezoelectric property related to its β phase content makes it interesting for energy harvesters and wearable applications. Research investigation shows that PVDF in the form of nanofibers prepared by electrospinning has more flexibility and better air permeability, which make them more suitable for these types of applications. Electrospinning is an efficient technique that produces PVDF nanofibers with a high β phase fraction and crystallinity by aligning molecular dipoles (-CH2 and -CF2) along an applied voltage direction. Different nanofibers production techniques and more precisely the electrospinning method for producing PVDF nanofibers with optimal electrospinning parameters are the key focuses of this paper. This review article highlights recent studies to summarize the influence of electrospinning parameters such as process (voltage, distance, flow rate, and collector), solution (Mw, concentration, and solvent), and ambient (humidity and temperature) parameters to enhance the piezoelectric properties of PVDF nanofibers. In addition, recent development regarding the effect of adding nanoparticles in the structure of nanofibers on the improvement of the β phase is reviewed. Finally, different methods of measuring piezoelectric properties of PVDF nanofibrous membrane are discussed.

Composites, Fabrication and Application of Polyvinylidene Fluoride for Flexible Electromechanical Devices: A Review

The technological development of piezoelectric materials is crucial for developing wearable and flexible electromechanical devices. There are many inorganic materials with piezoelectric effects, such as piezoelectric ceramics, aluminum nitride and zinc oxide. They all have very high piezoelectric coefficients and large piezoelectric response ranges. The characteristics of high hardness and low tenacity make inorganic piezoelectric materials unsuitable for flexible devices that require frequent bending. Polyvinylidene fluoride (PVDF) and its derivatives are the most popular materials used in flexible electromechanical devices in recent years and have high flexibility, high sensitivity, high ductility and a certain piezoelectric coefficient. Owing to increasing the piezoelectric coefficient of PVDF, researchers are committed to optimizing PVDF materials and enhancing their polarity by a series of means to further improve their mechanical-electrical conversion efficiency. This paper reviews the latest PVDF-related optimization-based materials, related processing and polarization methods and the applications of these materials in, e.g., wearable functional devices, chemical sensors, biosensors and flexible actuator devices for flexible micro-electromechanical devices. We also discuss the challenges of wearable devices based on flexible piezoelectric polymer, considering where further practical applications could be.

Cellular and Subcellular Contact Guidance on Microfabricated Substrates

Topography of the extracellular environment is now recognized as a major biophysical regulator of cell behavior and function. The study of the influence of patterned substrates on cells, named contact guidance, has greatly benefited from the development of micro and nano-fabrication techniques, allowing the emergence of increasingly diverse and elaborate engineered platforms. The purpose of this review is to provide a comprehensive view of the process of contact guidance from cellular to subcellular scales. We first classify and illustrate the large diversity of topographies reported in the literature by focusing on generic cellular responses to diverse topographical cues. Subsequently, and in a complementary fashion, we adopt the opposite approach and highlight cell type-specific responses to classically used topographies (arrays of pillars or grooves). Finally, we discuss recent advances on the key subcellular and molecular players involved in topographical sensing. Throughout the review, we focus particularly on neuronal cells, whose unique morphology and behavior have inspired a large body of studies in the field of topographical sensing and revealed fascinating cellular mechanisms. We conclude by using the current understanding of the cell-topography interactions at different scales as a springboard for identifying future challenges in the field of contact guidance.

Maintenance and differentiation of human ES cells on polyvinylidene fluoride scaffolds immobilized with a vitronectin-derived peptide

Polyvinylidene fluoride (PVDF) is biocompatible, easy to fabricate, and has piezoelectric properties; it has been used for many biomedical applications including stem cell engineering. However, long-term cultivation of human embryonic stem cells (hESCs) and their differentiation toward cardiac lineages on PVDF have not been investigated. Herein, PVDF nanoscaled membrane scaffolds were fabricated by electrospinning; a vitronectin-derived peptide-mussel adhesive protein fusion (VNm) was immobilized on the scaffolds. hESCs cultured on the VNm-coated PVDF scaffold (VNm-PVDF scaffold) were stably expanded for more than 10 passages while maintaining the expression of pluripotency markers and genomic integrity. Under cardiac differentiation conditions, hESCs on the VNm-PVDF scaffold generated more spontaneously beating colonies and showed the upregulation of cardiac-related genes, compared with those cultured on Matrigel and VNm alone. Thus, VNm-PVDF scaffolds may be suitable for the long-term culture of hESCs and their differentiation into cardiac cells, thus expanding their application in regenerative medicine.

Polymeric Guide Conduits for Peripheral Nerve Tissue Engineering

Peripheral nerve injuries (PNIs) are usually caused by trauma, immune diseases, and genetic factors. Peripheral nerve injury (PNI) may lead to limb numbness, muscle atrophy, and loss of neurological function. Although an abundance of theories have been proposed, very few treatments can effectively lead to complete recovery of neurological function. Autologous nerve transplantation is currently the gold standard. Nevertheless, only 50% of all patients were successfully cured using this method. In addition, it causes inevitable damage to the donor site, and available donor sites in humans are very limited. Tissue engineering has become a research hotspot aimed at achieving a better therapeutic effect from peripheral nerve regeneration. Nerve guide conduits (NGCs) show great potential in the treatment of PNI. An increasing number of scaffold materials, including natural and synthetic polymers, have been applied to fabricate NGCs for peripheral nerve regeneration. This review focuses on recent nerve guide conduit (NGC) composite scaffold materials that are applied for nerve tissue engineering. Furthermore, the development tendency of NGCs and future areas of interest are comprehensively discussed.

Adhesive Sponge Based on Supramolecular Dimer Interactions as Scaffolds for Neural Stem Cells

Improving cell-material interactions of nonadhesive scaffolds is crucial for the success of biomaterials in tissue engineering. Due to their high surface area and open pore structure, sponges are widely reported as absorbent materials for biomedical engineering. The biocompatibility and biodegradability of polysaccharide sponges, coupled with the chemical functionalities of supramolecular dimers, make them promising combinations for the development of adhesive scaffolds. Here, a supramolecular tactic based on (UPy)-modified polysaccharide associated with three-dimensional structure of sponges was developed to reach enhanced cellular adhesion. For this purpose, three approaches were examined individually in order to accomplish this goal. In the first approach, the backbone polysaccharides with noncell adhesive properties were modified via a modular tactic using UPy-dimers. Hereupon, the physical-chemical characterizations of the supramolecular sponges were performed, showing that the presence of supramolecular dimers improved their mechanical properties and induced different architectures. In addition, small-angle neutron scattering (SANS) measurements and rheology experiments revealed that the UPy-dimers into agarose backbone are able to reorganize in thinning aggregates. It is also demonstrated that the resulted UPy-agarose (AGA-UPy) motifs in surfaces can promote cell adhesion. Finally, the last approach showed the great potential for use of this novel material in bioadhesive scaffolds indicating that neural stem cells show a spreading bias in soft materials and that cell adhesion was enhanced for all UPy-modified sponges compared to the reference, i.e. unmodified sponges. Therefore, by functionalizing sponge surfaces with UPy-dimers, an adhesive supramolecular scaffold is built which opens the opportunity its use neural tissues regeneration.

GPU-accelerated ray-casting for 3D fiber orientation analysis

Orientation analysis of fibers is widely applied in the fields of medical, material and life sciences. The orientation information allows predicting properties and behavior of materials to validate and guide a fabrication process of materials with controlled fiber orientation. Meanwhile, development of detector systems for high-resolution non-invasive 3D imaging techniques led to a significant increase in the amount of generated data per a sample up to dozens of gigabytes. Though plenty of 3D orientation estimation algorithms were developed in recent years, neither of them can process large datasets in a reasonable amount of time. This fact complicates the further analysis and makes impossible fast feedback to adjust fabrication parameters. In this work, we present a new method for quantifying the 3D orientation of fibers. The GPU implementation of the proposed method surpasses another popular method for 3D orientation analysis regarding accuracy and speed. The validation of both methods was performed on a synthetic dataset with varying parameters of fibers. Moreover, the proposed method was applied to perform orientation analysis of scaffolds with different fibrous micro-architecture studied with the synchrotron μCT imaging setup. Each acquired dataset of size 600x600x450 voxels was analyzed in less 2 minutes using standard PC equipped with a single GPU.

High-resolution electrohydrodynamic bioprinting: a new biofabrication strategy for biomimetic micro/nanoscale architectures and living tissue constructs

Electrohydrodynamic (EHD) printing is a newly emerging additive manufacturing strategy for the controlled fabrication of three-dimensional (3D) micro/nanoscale architectures. This unique superiority makes it particularly suitable for the biofabrication of artificial tissue analogs with biomimetic structural organizations similar to the scales of native extracellular matrix (ECM) or living cells, which shows great potentials to precisely regulate cellular behaviors and tissue regeneration. Here the state-of-the-art advancements of high-resolution EHD bioprinting were reviewed mainly including melt-based and solution-based processes for the fabrication of micro/nanoscale fibrous scaffolds and living tissues constructs. The related printing materials, innovations on structure design and printing processes, functionalization of the resultant architectures as well as their effects on the mechanical and biological properties of the EHD-printed structures were introduced and analyzed. The recent explorations on the EHD cell printing for high-resolution cell-laden microgel patterning and 3D construct fabrication were highlighted. The major challenges as well as possible solutions to translate EHD bioprinting into a mature and prevalent biofabrication strategy were finally discussed.

Development of a Piezoelectric PVDF-TrFE Fibrous Scaffold to Guide Cell Adhesion, Proliferation, and Alignment

Severe peripheral nervous system injuries currently hold limited therapeutic solutions. Existing clinical techniques such as autografts, allografts, and newer nerve guidance conduits have shown variable outcomes in functional recovery, adverse immune responses, and in some cases low or minimal availability. This can be attributed in part to the lack of chemical, physical, and electrical cues directing both nerve guidance and regeneration. To address this pressing clinical issue, electrospun nanofibers and microfibers composed of piezoelectric polyvinylidene flouride-triflouroethylene (PVDF-TrFE) have been introduced as an alternative template for tissue engineered biomaterials, specifically as it pertains to their relevance in soft tissue and nerve repair. Here, biocompatible scaffolds of PVDF-TrFE are fabricated and their ability to generate an electrical response to mechanical deformations and produce a suitable regenerative microenvironment is examined. It is determined that 20% (w/v) PVDF-TrFE in (6:4) dimethyl formamide (DMF):acetone solvent maintains a desirable piezoelectric coefficient and the proper physical and electrical characteristics for tissue regeneration. Further, it is concluded that scaffolds of varying thickness promoted the adhesion and alignment of Schwann cells and fibroblasts. This work offers a prelude to further advancements in nanofibrous technology and a promising outlook for alternative, autologous remedies to peripheral nerve damage.

Aligned graphene/silk fibroin conductive fibrous scaffolds for guiding neurite outgrowth in rat spinal cord neurons

Graphene, as a highly conducting material, incorporated into silk fibroin (SF) substrates is promising to fabricate an electroactive flexible scaffold toward neural tissue engineering. It is well known that aligned morphology could promote cell adhesion and directional growth. The purpose of this study was to develop aligned conductive scaffolds made of graphene and SF (G/SF) by electrospinning technique for neural tissue engineering applications. The physicochemical characterization of scaffolds revealed that the mechanical and electrochemical property of aligned G/SF scaffolds continually raised with the increasing contents of graphene (A0% G/SF, A1% G/SF, A2% G/SF, and A3% G/SF), but the mechanical property descended when the graphene concentration reached to 4% (the A4% G/SF group). The results of the cell experiment in vitro indicated that all the aligned G/SF scaffolds were no neurotoxic to primary cultured spinal cord neurons. In addition, the neurite elongation in all aligned groups was significantly enhanced by the upregulation of Netrin-1 expression compared to them in the control group. Thus, A3% G/SF scaffolds not only possessed the optimal property based on the mechanical and electrochemical performances but also displayed the beneficial capability to neurite outgrowth, which might perform a suitable candidate to successfully scaffold electrically active tissues during neural regeneration or engineering.

Electrospinning Piezoelectric Fibers for Biocompatible Devices

The field of nanotechnology has been gaining great success due to its potential in developing new generations of nanoscale materials with unprecedented properties and enhanced biological responses. This is particularly exciting using nanofibers, as their mechanical and topographic characteristics can approach those found in naturally occurring biological materials. Electrospinning is a key technique to manufacture ultrafine fibers and fiber meshes with multifunctional features, such as piezoelectricity, to be available on a smaller length scale, thus comparable to subcellular scale, which makes their use increasingly appealing for biomedical applications. These include biocompatible fiber-based devices as smart scaffolds, biosensors, energy harvesters, and nanogenerators for the human body. This paper provides a comprehensive review of current studies focused on the fabrication of ultrafine polymeric and ceramic piezoelectric fibers specifically designed for, or with the potential to be translated toward, biomedical applications. It provides an applicative and technical overview of the biocompatible piezoelectric fibers, with actual and potential applications, an understanding of the electrospinning process, and the properties of nanostructured fibrous materials, including the available modeling approaches. Ultimately, this review aims at enabling a future vision on the impact of these nanomaterials as stimuli-responsive devices in the human body.

Extracellular matrix and biomimetic engineering microenvironment for neuronal differentiation

Extracellular matrix (ECM) influences cell differentiation through its structural and biochemical properties. In nervous system, neuronal behavior is influenced by these ECMs structures which are present in a meshwork, fibrous, or tubular forms encompassing specific molecular compositions. In addition to contact guidance, ECM composition and structures also exert its effect on neuronal differentiation. This short report reviewed the native ECM structure and composition in central nervous system and peripheral nervous system, and their impact on neural regeneration and neuronal differentiation. Using topographies, stem cells have been differentiated to neurons. Further, focussing on engineered biomimicking topographies, we highlighted the role of anisotropic topographies in stem cell differentiation to neurons and its recent temporal application for efficient neuronal differentiation.

Electrospun Polyvinylidene Fluoride-Based Fibrous Scaffolds with Piezoelectric Characteristics for Bone and Neural Tissue Engineering

Polyvinylidene fluoride (PVDF) and polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE) with excellent piezoelectricity and good biocompatibility are attractive materials for making functional scaffolds for bone and neural tissue engineering applications. Electrospun PVDF and P(VDF-TrFE) scaffolds can produce electrical charges during mechanical deformation, which can provide necessary stimulation for repairing bone defects and damaged nerve cells. As such, these fibrous mats promote the adhesion, proliferation and differentiation of bone and neural cells on their surfaces. Furthermore, aligned PVDF and P(VDF-TrFE) fibrous mats can enhance neurite growth along the fiber orientation direction. These beneficial effects derive from the formation of electroactive, polar β-phase having piezoelectric properties. Polar β-phase can be induced in the PVDF fibers as a result of the polymer jet stretching and electrical poling during electrospinning. Moreover, the incorporation of TrFE monomer into PVDF can stabilize the β-phase without mechanical stretching or electrical poling. The main drawbacks of electrospinning process for making piezoelectric PVDF-based scaffolds are their small pore sizes and the use of highly toxic organic solvents. The small pore sizes prevent the infiltration of bone and neuronal cells into the scaffolds, leading to the formation of a single cell layer on the scaffold surfaces. Accordingly, modified electrospinning methods such as melt-electrospinning and near-field electrospinning have been explored by the researchers to tackle this issue. This article reviews recent development strategies, achievements and major challenges of electrospun PVDF and P(VDF-TrFE) scaffolds for tissue engineering applications.

Synthetic scaffolds for musculoskeletal tissue engineering: cellular responses to fiber parameters

Tissue engineering often uses synthetic scaffolds to direct cell responses during engineered tissue development. Since cells reside within specific niches of the extracellular matrix, it is important to understand how the matrix guides cell response and then incorporate this knowledge into scaffold design. The goal of this review is to review elements of cell-matrix interactions that are critical to informing and evaluating cellular response on synthetic scaffolds. Therefore, this review examines fibrous proteins of the extracellular matrix and their effects on cell behavior, followed by a discussion of the cellular responses elicited by fiber diameter, alignment, and scaffold porosity of two dimensional (2D) and three dimensional (3D) synthetic scaffolds. Variations in fiber diameter, alignment, and scaffold porosity guide stem cells toward different lineages. Cells generally exhibit rounded morphology on nanofibers, randomly oriented fibers, and low-porosity scaffolds. Conversely, cells exhibit elongated, spindle-shaped morphology on microfibers, aligned fibers, and high-porosity scaffolds. Cells migrate with higher velocities on nanofibers, aligned fibers, and high-porosity scaffolds but migrate greater distances on microfibers, aligned fibers, and highly porous scaffolds. Incorporating relevant biomimetic factors into synthetic scaffolds destined for specific tissue application could take advantage of and further enhance these responses.

Quanfima: An open source Python package for automated fiber analysis of biomaterials

Hybrid 3D scaffolds composed of different biomaterials with fibrous structure or enriched with different inclusions (i.e., nano- and microparticles) have already demonstrated their positive effect on cell integration and regeneration. The analysis of fibers in hybrid biomaterials, especially in a 3D space is often difficult due to their various diameters (from micro to nanoscale) and compositions. Though biomaterials processing workflows are implemented, there are no software tools for fiber analysis that can be easily integrated into such workflows. Due to the demand for reproducible science with Jupyter notebooks and the broad use of the Python programming language, we have developed the new Python package quanfima offering a complete analysis of hybrid biomaterials, that include the determination of fiber orientation, fiber and/or particle diameter and porosity. Here, we evaluate the provided tensor-based approach on a range of generated datasets under various noise conditions. Also, we show its application to the X-ray tomography datasets of polycaprolactone fibrous scaffolds pure and containing silicate-substituted hydroxyapatite microparticles, hydrogels enriched with bioglass contained strontium and alpha-tricalcium phosphate microparticles for bone tissue engineering and porous cryogel 3D scaffold for pancreatic cell culturing. The results obtained with the help of the developed package demonstrated high accuracy and performance of orientation, fibers and microparticles diameter and porosity analysis.

Structural insights into semicrystalline states of electrospun nanofibers: a multiscale analytical approach

A dedicated nanofiber design for applications in the biomedical domain is based on the understanding of nanofiber structures. The structure of electrospun nanofibers strongly influences their properties and functionalities. In polymeric nanofibers X-ray scattering and diffraction methods, i.e. SAXS and WAXD, are capable of decoding their structural insights from about 100 nm down to the Angström scale. Here, we present a comprehensive X-ray scattering and diffraction based study and introduce new data analysis approaches to unveil detailed structural features in electrospun poly(vinylidene fluoride-co-hexafluoropropylene) (PVDFhfp) nanofiber membranes. Particular emphasis was placed on anisotropic morphologies being developed during the nanofiber fabrication process. Global analysis was performed on SAXS data to derive the nanofibrillar structure of repeating lamella crystalline domains with average dimensions of 12.5 nm thickness and 7.8 nm spacing along with associated tie-molecules. The varying surface roughness of the nanofiber was evaluated by extracting the Porod exponent in parallel and perpendicular direction to the nanofiber axis, which was further validated by Atomic Force Microscopy. Additionally, the presence of a mixture of the monoclinic alpha and the orthorhombic beta PVDFhfp phases both exhibiting about 6% larger unit cells compared to the corresponding pure PVDF phases was derived from WAXD. The current study shows a generic approach in detailed understanding of internal structures and surface morphology for nanofibers. This forms the basis for targeted structure and morphology steering and the respective controlling during the fabrication process with the aim to engineer nanofibers for different biomedical applications with specific requirements.

Janus N,N-dimethylformamide as a solvent for a gradient porous wound dressing of poly(vinylidene fluoride) and as a reducer for in situ nano-silver production: anti-permeation, antibacterial and antifouling activities against multi-drug-resistant bacteria both in vitro and in vivo

The requirements for anti-permeation, anti-infection and antifouling when treating a malicious wound bed raise new challenges for wound dressing. The present study used N,N-dimethylformamide to treat poly(vinylidene fluoride) (PVDF) in order to obtain a dressing impregnated with in situ generated nano-silver particles (NS) via an immersion phase inversion method. Scanning electron microscopy (SEM) images showed that the film was characterized by a two-layer asymmetric structure with different pore sizes (top layer: ∼0.4 μm; bottom layer: ∼1.8 μm). The moisture permeability test indicated that the film had an optimal water vapor transmission rate (WVTR: ∼2500 g m-2 per day). TEM images revealed the successful formation of spherical NS, and Fourier-transform infrared spectroscopy (FTIR) demonstrated the integration of PVDF and NS (i.e., PVDF/NS). Correspondingly, the water contact angle measurements confirmed increased membrane surface hydrophobicity after NS integration. The inductively coupled plasma (ICP) spectrometry showed that the PVDF/NS displayed a continuous and safe release of silver ions. Moreover, in vitro experiments indicated that PVDF/NS films possessed satisfactory anti-permeation, antibacterial and antifouling activities against A. baumannii and E. coli bacteria, while they exhibited no obvious cytotoxicity toward mammalian HaCaT cells. Finally, the in vivo results showed that the nanoporous top layer of film could serve as a physical barrier to prevent bacterial penetration, whereas the microporous bottom layer could efficiently prevent bacterial infection caused by biofouling, leading to fast re-epithelialization via the enhancement of keratinocyte proliferation. Collectively, the results show that the PVDF/NS25 film has a promising application in wound treatment, especially for wounds infected by multi-drug-resistant bacteria such as A. baumannii.

Transplantation in the nonhuman primate MPTP model of Parkinson's disease: update and perspectives

In order to calibrate stem cell exploitation for cellular therapy in neurodegenerative diseases, fundamental and preclinical research in NHP (nonhuman primate) models is crucial. Indeed, it is consensually recognized that it is not possible to directly extrapolate results obtained in rodent models to human patients. A large diversity of neurological pathologies should benefit from cellular therapy based on neural differentiation of stem cells. In the context of this special issue of Primate Biology on NHP stem cells, we describe past and recent advances on cell replacement in the NHP model of Parkinson's disease (PD). From the different grafting procedures to the various cell types transplanted, we review here diverse approaches for cell-replacement therapy and their related therapeutic potential on behavior and function in the NHP model of PD.

Effect of electrospinning parameters on morphological properties of PVDF nanofibrous scaffolds

Smart materials like piezoelectric polymers represent a new class of promising scaffold in neural tissue engineering. In the current study, the fabrication processing parameters of polyvinylidine fluoride (PVDF) nanofibrous scaffold are found as a potential scaffold with nanoscale morphology and microscale alignment. Electrospinning technique with the ability to mimic the structure and function of an extracellular matrix is a preferable method to customize the scaffold features. PVDF nanofibrous scaffolds were successfully fabricated by the electrospinning technique. The influence of PVDF solution concentration and other processing parameters like applied voltage, tip-to-collector distance, feeding rate, collector speed and the solvent were studied. The optimal parameters were 30 w/v% PVDF concentration, 15 kV applied voltage, 18 cm tip-to-collector distance, 0.5 ml/h feeding rate, 2500 rpm collector speed and N,N'-dimethylacetamide/acetone as a solvent. The mean fiber diameter of the obtained scaffold was 352.9 ± 24 nm with uniform and aligned morphology. Finally, the cell viability and morphology of PC-12 cells on the optimum scaffold indicated the potential of PVDF nanofibrous scaffold for neural tissue engineering.