PLLA Fibers Research Articles

Influence of precursor solvent and confined environment on the polymorphic transition in electrospun Poly(l-lactide) fibers

The present study examined the thermal and crystallization properties of poly(l-lactide) (PLLA) chains confined in electrospun fibers. The heat of fusion and crystallite sizes decreased with a decrease in fiber diameter because of the confinement effect. The crystal structure of PLLA fibers exhibited α′ and α forms, which comprised loosely and closely packed chains, respectively. As-electrospun PLLA fibers exhibited an unstable α′ phase because of the rapid solidification of polymer chains during electrospinning. Subsequent thermal annealing at a temperature higher than 140 °C induced a polymorphic transition from the α′ phase to the stable α phase. This finding differed from that observed in bulk PLLA, which exhibited α crystals regardless of annealing temperature. Furthermore, the polymorphic transition of α′ crystals to α crystals in fibers fabricated with 95:5 chloroform/trifluoroethanol solvent occurred with lower annealing temperatures or shorter annealing times relative to those fabricated with 60:40 chloroform/trifluoroethanol solvent. This behavior was attributed to the low entanglement content in the PLLA chains in fibers fabricated with 95:5 chloroform/trifluoroethanol solvent; the low entanglement content increased chain mobility, thereby accelerating polymorphic transition.

Assessing the combination of magnetic field stimulation, iron oxide nanoparticles, and aligned electrospun fibers for promoting neurite outgrowth from dorsal root ganglia in vitro

Magnetic fiber composites combining superparamagnetic iron oxide nanoparticles (SPIONs) and electrospun fibers have shown promise in tissue engineering fields. Controlled grafting of SPIONs to the fibers post-electrospinning generates biocompatible magnetic composites without altering desired fiber morphology. Here, for the first time, we assess the potential of SPION-grafted scaffolds combined with magnetic fields to promote neurite outgrowth by providing contact guidance from the aligned fibers and mechanical stimulation from the SPIONs in the magnetic field. Neurite outgrowth from primary rat dorsal root ganglia (DRG) was assessed from explants cultured on aligned control and SPION-grafted electrospun fibers as well as on non-grafted fibers with SPIONs dispersed in the culture media. To determine the optimal magnetic field stimulation to promote neurite outgrowth, we generated a static, alternating, and linearly moving magnet and simulated the magnetic flux density at different areas of the scaffold over time. The alternating magnetic field increased neurite length by 40% on control fibers compared to a static magnetic field. Additionally, stimulation with an alternating magnetic field resulted in a 30% increase in neurite length and 62% increase in neurite area on SPION-grafted fibers compared to DRG cultured on PLLA fibers with untethered SPIONs added to the culture media. These findings demonstrate that SPION-grafted fiber composites in combination with magnetic fields are more beneficial for stimulating neurite outgrowth on electrospun fibers than dispersed SPIONs. Statement of significanceAligned electrospun fibers improve axonal regeneration by acting as a passive guidance cue but do not actively interact with cells, while magnetic nanoparticles can be remotely manipulated to interact with neurons and elicit neurite outgrowth. Here, for the first time, we examine the combination of magnetic fields, magnetic nanoparticles, and aligned electrospun fibers to enhance neurite outgrowth. We show an alternating magnetic field alone increases neurite outgrowth on aligned electrospun fibers. However, combining the alternating field with magnetic nanoparticle-grafted fibers does not affect neurite outgrowth compared to control fibers but improves outgrowth compared to freely dispersed magnetic nanoparticles. This study provides the groundwork for utilizing magnetic electrospun fibers and magnetic fields as a method for promoting axonal growth.

Aligned Electrospun PLLA/Graphene Microfibers with Nanotopographical Surface Modulate the Mitochondrial Responses of Vascular Smooth Muscle Cells

AbstractAlthough electrospun polymer fibers with unique (bio)physicochemical features have been demonstrated to regulate cellular phenotype, their specific effect on organelles has not been examined closely. Herein, this work investigates the impact of aligned electrospun poly(L‐lactic acid) (PLLA)/graphene (Gr) microfibers with the nanoporous surface on the mitochondrial responses of human aorta smooth muscle cells. Highly aligned PLLA/Gr microfibers with an average fiber diameter of ≈1.3 µm are prepared via a novel stable jet electrospinning strategy. Nanoporous topography is formed onto individual fiber surfaces using highly volatile solvent dichloromethane and suitable ambient humidity. Further, it is found that the addition of Gr in PLLA fibers significantly facilitates the formation of mitochondrial reactive oxygen species (mROS) and adenosine triphosphate (ATP) as well as mitochondrial autophagy and division by upregulating the protein expression of microtubule‐associated protein light chain 3, mitochondrial fission 1 protein, and dynamin‐related protein 1. In contrast, the introduction of the nanoporous structure greatly inhibits the generation of mROS and ATP as well as mitochondrial autophagy and division. The work thus indicates that the incorporation of nanotopography and Gr in/on the PLLA fibers can offer additional physicochemical stimuli to regulate the mitochondrial responses of cells.

Fabrication of hierarchical porous poly (l-lactide) (PLLA) fibrous membrane by electrospinning

Nanofibres with high surface area have various applications in many fields, such as filtration, tissue engineering and drug delivery. In the previous reported works, a lot of researchers have fabricated porous polymer nanofibres with porous surface structure, but few researchers could produce thoroughly hollow porous nanofibres by electrospinning. Here, we developed porous poly(l-lactide) (PLLA) fibres with good internal pore connection by two methods, i) removal of a pore-forming agent from PLLA composite fibres; ii) non-solvent induced re-crystallization of pure PLLA fibres. In the first method, electrospun PLLA/cellulose acetate butyrate (CAB) composite fibres were immersed into acetone to remove CAB. Results showed that the fibres with less CAB could produce porous structures. However, with the increasing of CAB content, the fibre surface had only a collapsed structure. In the second method, while pure PLLA fibres were immersed into acetone, the solvent-induced recrystallization of PLLA generated porous structures throughout the whole fibres. Owing to the higher crystallization, the mechanical properties of PLLA fibres were also highly improved.

Biodegradable polymeric occluder for closure of atrial septal defect with interventional treatment of cardiovascular disease

The next-generation closure device for interventional treatment of congenital heart disease is regarded to be biodegradable, yet the corresponding biomaterial technique is still challenging. Herein, we report the first fully biodegradable atrial septal defect (ASD) occluder finally coming into clinical use, which is made of biodegradable poly(l-lactic acid) (PLLA). We characterized the physico-chemical properties of PLLA fibers as well as the raw polymer and the operability of the as-fabricated occluders. Cell behaviors on material were observed, and in vivo fiber degradation and inflammatory responses were examined. ASD models in piglets were created, and 44 PLLA ASD occluders were implanted via catheter successfully. After 36 months, the PLLA ASD occluders almost degraded without any complications. The mechanical properties and thickness between newborn and normal atrial septum showed no significant difference. We further accomplished the first clinical implantation of the PLLA ASD occluder in a four-year boy, and the two-year follow-up up to date preliminarily indicated safety and feasibility of such new-generation fully biodegradable occluder made of synthetic polymers.

Quasistatic direct electromechanical responses from as-electrospun submicron/micron fiber mats of several polymers

Electromechanically active submicron/micron polymer fibers are promising components of wearable pressure sensors because they are mechanically flexible and lightweight. Recently, an as-electrospun microfiber mat comprising atactic polystyrene (aPS) demonstrated significantly high quasistatic direct electromechanical responses, although aPS films are not normally piezoelectric in nature. In this study, we investigated the quasistatic direct electromechanical properties of as-electrospun submicron/micron fiber mats composed of several polymers, namely poly(methyl methacrylate) (PMMA), aPS, poly(vinyl alcohol) (PVA), poly(D,l-lactic acid) (PDLLA), poly(l-lactic acid) (PLLA), and poly[(R)-3 hydroxybutyric acid] (PHB). As-electrospun PMMA, aPS, PDLLA, PLLA, and PHB fiber mats demonstrated significantly high direct electromechanical responses 22 h after fabrication, despite the nonpiezoelectric nature of PMMA, aPS, and PDLLA, whose films normally do not exhibit electromechanical responses. In contrast, the as-electrospun PVA fiber mat hardly showed any direct electromechanical response. We examined the surface potentials of these fiber mats, which revealed that the surface potential of the PVA fiber mat decayed rapidly and was essentially quenched within 2 h after fabrication, demonstrating that electrospinning forms polymer fibers with different abilities to retain charge and, as a consequence, different electromechanical responses. The direct electromechanical properties of the PMMA and aPS fiber mats were stable, even 30 d after fabrication, which is mainly attributable to the lower volume conductivities of these polymers. Selective discharging of real charges, including surface charges and space charges, through spraying with isopropanol mist revealed that the direct electromechanical responses of the fiber mats are due to real charges stored in the fiber mats through electrospinning. These fundamental findings provide novel material-choice opportunities for the development of large-area, flexible, lightweight, and inexpensive pressure sensors.

Shortening of electrospun PLLA fibers by ultrasonication

This research work is aimed at studying the effect of ultrasounds on the effectiveness of fiber fragmentation by taking into account the type of sonication medium, processing time, and various PLLA molecular weights. Fragmentation was followed by an appropriate filtration in order to decrease fibers length distribution. It was evidenced by fiber length determination using SEM that the fibers are shortened after ultrasonic treatment, and the effectiveness of shortening depends on the two out of three investigated parameters, mostly on the sonication medium, and processing time. The gel permeation chromatography (GPC) confirmed that such ultrasonic treatment does not change the polymers' molecular weight.Our results allowed to optimize the ultrasonic fragmentation procedure of electrospun fibers while preliminary viscosity measurements of fibers loaded into hydrogel confirmed their potential in further use as fillers for injectable hydrogels for regenerative medicine applications.

Collagen and chondroitin sulfate functionalized bioinspired fibers for tendon tissue engineering application

Functional tendon tissue engineering depends on harnessing the biochemical and biophysical cues of the native tendon extracellular matrix. In this study, we fabricated highly-aligned poly(L-lactic acid) (PLLA) fibers with surfaces decorated by two of the crucial tendon ECM components, type 1 collagen (COL1) and chondroitin sulfate (CS), through a coaxial stable jet electrospinning approach. Effects of the biomimetic COL1-CS (shell)/PLLA (core) fibers on the tenogenic differentiation of human mesenchymal stem cells (hMSCs) in vitro were investigated. Higher rates of cell spreading and proliferation are observed on the aligned COL1-CS/PLLA fibers compared to that on the plain PLLA fibers. Expression of the tendon-associated genes scleraxis (SCX) and COL1 as well as protein tenomodulin (TNMD) are significantly increased. Introduction of mechanical stimulation gives rise to synergistic effect on tenogenic differentiation of hMSCs. Higher expression of TGF-β2, TGFβR-II, and Smad3 by the cells on the COL1-CS/PLLA fiber substrates are observed, which indicates that COL1-CS/PLLA ultrafine fibers dictate the hMSC tenogenic differentiation through activating the TGF-β signaling pathway. Animal study in rat Achilles tendon repair model corroborated the promoting role of COL1-CS/PLLA in regenerating a tendon-like tissue. Thus, our highly aligned biomimicking fibers may serve as an efficient scaffolding system for functional tendon regeneration.

Hierarchical porous silk fibroin/poly(L-lactic acid) fibrous membranes towards vascular scaffolds

Fibrous membranes played an important role to prepare tubular scaffolds for muscular artery regeneration. In this study, a strategy has been developed to combine silk fibroin (SF) with highly porous electrospun poly(L-lactic acid) (PLLA) fibrous membrane towards vascular scaffolds. After PLLA fibres were electrospun and collected, they were immersed into acetone to generate a porous structure with ultra-high surface area. While the pores on PLLA fibres were fulfilled with SF solution and dried, SF was coated uniformly and tightly on PLLA fibres. A multi-layer tubular structure of the tunica media was simulated by winding and stacking a strip of electrospun fibrous membrane. In vitro viability and morphology studies of A7r5 smooth muscle cells were undertaken for up to 14 days. Because the hydrophilicity of SF/PLLA composite fibres were improved dramatically, it had a positive effect on cell adhesion rate (97%) and proliferation (64.4%). Moreover, good cell morphology was observed via a multiphoton laser confocal microscope on SF/PLLA bioactive materials. These results demonstrated that the hierarchical porous SF/PLLA fibrous membranes are promising off-the-shelf scaffolds for muscular artery regeneration.

Quantitative characterization of dielectric properties of polymer fibers and polymer composites using electrostatic force microscopy

We use a new method based on electrostatic force microscopy (EFM) to perform quantitative measurements of the dielectric constants of individual electrospun nanofibers of poly(L-lactic acid) (PLLA), as well as composite fibers of PLLA with embedded multiwall carbon nanotubes (MWCNT-PLLA). The EFM data record the oscillation phase of an atomic force microscope (AFM) cantilever as a function of the AFM tip position. In our experiments the relative dielectric constants ϵ of the sample are measured from the EFM phase shifts vs. the tip-surface separation, according to a simple analytical model describing the tip-surface interactions. We perform a comprehensive study of how the dielectric constant depends on the fiber diameter for both electrospun PLLA and MWCNT/PLLA fiber composites. Our measurements show that EFM can distinguish between dielectric properties of PLLA fibers and fiber composites with different diameters. Dielectric constants of both PLLA and MWCNT-PLLA composite fibers decrease with increasing fiber diameter. In the limit of large fiber diameters (D > 100 nm), we measure dielectric constants in the range: ϵ = 3.4–3.8, similar to the values obtained for unoriented PLLA films: ϵfilm = 2.4–3.8. Moreover, the dielectric constants of the small diameter MWCNT-PLLA composites are significantly larger than the corresponding values obtained for PLLA fibers. For MWCNT-PLLA nanofiber composites of small diameters (D < 50 nm), ϵ approaches the values measured for neat MWCNT: ϵCN = 12 ± 2. These results are consistent with a simple fiber structural model that shows higher polarizability of thinner fibers, and composites that contain MWCNTs. The experimental method has a high-resolution for measuring the dielectric constant of soft materials, and is simple to implement on standard atomic force microscopes. This non-invasive technique can be applied to measure the electrical properties of polymers, interphases, and polymer nanocomposites.

Parameter optimization of O2/He atmospheric pressure plasma for surface modification of poly (L-lactic) acid oriented fiber membranes: Improving cell adhesion and proliferation

Non-thermal atmospheric pressure plasma jet (APPJ) is an effective tool for modifying surface properties and improving the biocompatibility of polymers owing to its advantages of simple structure, low cost, and flexible capacity. Effects of APPJ treatment on surface properties are influenced by operating parameters such as applied voltage, gas ratio, and treatment time. In this study, we investigated how these parameters affect surface properties of poly (l-lactic acid) (PLLA) fiber membranes when the O2/He APPJ treatment was employed, and optimized parameters for improving cell adhesion and proliferation. Specifically, highly oriented PLLA fiber membranes were fabricated via electrospinning and then treated using the O2/He APPJ with different processing parameters. Gelatin grafting was also performed on a sub-set of APPJ-treated PLLA membranes. We investigated electrical properties and optical emission spectra (OES) of APPJ, and employed water contact angles (WCA), X-ray photoelectron spectroscopy (XPS) analysis to examine changes of surface properties. Processing parameters were optimized based on experimental results. Cell culture studies showed that attachment and spreading of cells on the APPJ-treated membranes were much better than on untreated samples, and a significant increase of cell proliferation rate was observed for the APPJ-treated membranes under the optimized processing parameters. The optimization method presented has potential to be applied in similar APPJ-based surface treatment, which in turn broaden applications of APPJ in modifying surface properties of biopolymers.

Hierarchical porous poly(l-lactic acid)/SiO2 nanoparticles fibrous membranes for oil/water separation

A two-step strategy has been developed to introduce silica nanoparticles into highly porous poly(l-lactic acid) (PLLA) nanofibers. Silica nanoparticles (SiNPs) were firstly synthesized and then modified to be hydrophobic. After PLLA/SiNPs composite fibrous membranes were electrospun and collected, they were re-crystallized by acetone at room temperature for a few minutes. With the re-arrangement of PLLA chains, the nano-/micro-electrospun fibres were transformed from non-porous ones to be porous ones with high surface area. Consequently, SiNPs that were completely covered by PLLA before acetone treatment showed up at the fibre surface. Higher PLLA crystallization also enhanced the Young’s modulus and tensile strength (420 and 8.47 MPa) of the composite membrane. However, incorporation of SiNPs into porous PLLA membranes reduced their modulus and tensile strength (280.66 and 5.92 MPa), but an increase in strain to fracture (80.82%) was observed. Scanning electron microscopy (SEM), focused ion beam SEM, transmission electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction were applied to confirm the presence of SiNP in PLLA fibres. The presence of SiNPs inside and outside fibres enhances the hydrophobicity of PLLA/SiNPs nano-fibrous membrane as the water contact angle is greater than 150°. The oil absorption of these porous composite membranes was also tested using four different oils, which can reach the highest absorption capacity when the weight ratio of PLLA and SiNPs is 1:1. The flux of prepared membranes was investigated, and results indicated that SiNPs-loaded membrane effectively enhanced the flux (5200 Lm−2 h−1).

Electrospun highly porous poly(L-lactic acid)-dopamine-SiO2 fibrous membrane for bone regeneration.

Electrospinning has been widely used to fabricate polymer fibrous scaffolds for bone tissue engineering because of their highly porous structures. In order to improve the biocompatibility of polymer scaffolds, some nano particles have been introduced into electrospun fibres. For example, silica nanoparticles (SiNPs), with high surface area and good biocompatibility, have been used for bone tissue engineering for better bone cell attachment. In this work, porous poly(L-lactic acid) (PLLA) fibrous membrane with high surface area was fabricated by electrospinning and post-treatment process. The membrane can serve as substrates of SiNPs for bone tissue engineering. Dopamine (DOP) was applied to modify the surface of PLLA fibres, which improved the coating strength of SiNPs on PLLA fibres. SiNP coating significantly improved the mechanical properties and hydrophilicity of PLLA/DOP/SiNP composite membranes. As a result of SiNPs coating, PLLA/DOP/SiNP membrane exhibited better cellular biocompatibility, more cell attachment and proliferation. These results demonstrate that porous PLLA/DOP/SiNP composite membrane with high surface area has high potential for periosteum in the field of bone regeneration applications.

Poly(Glycerol Succinate) as an Eco-Friendly Component of PLLA and PLCL Fibres towards Medical Applications.

This study was conducted as a first step in obtaining eco-friendly fibres for medical applications using a synthesised oligomer poly(glycerol succinate) (PGSu) as an additive for synthetic poly(L-lactic acid) (PLLA) and poly (L-lactide-co-caprolactone) (PLCL). The effects of the oligomer on the structure formation, morphology, crystallisation behaviour, and mechanical properties of electrospun bicomponent fibres were investigated. Nonwovens were investigated by means of scanning electron microscopy (SEM), wide angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), and mechanical testing. The molecular structure of PLLA fibres is influenced by the presence of PGSu mainly acting as an enhancer of molecular orientation. In the case of semicrystalline PLCL, chain mobility was enhanced by the presence of PGSu molecules, and the crystallinity of bicomponent fibres increased in relation to that of pure PLCL. The mechanical properties of bicomponent fibres were influenced by the level of PGSu present and the extent of crystal formation of the main component. An in vitro study conducted using L929 cells confirmed the biocompatible character of all bicomponent fibres.

Shape Memory and Osteogenesis Capabilities of the Electrospun Poly(3-Hydroxybutyrate-co-3-Hydroxyvalerate) Modified Poly(l-Lactide) Fibrous Mats.

Poly(l-lactide) (PLLA) as one of the most well-known biodegradable polyesters has been studied extensively for bone tissue engineering. If being properly programmed, scaffolds from PLLA can also be endowed with the capability of shape memory. However, several noted issues, for example, mechanical brittleness, high glass transition temperature Tg, and relatively poor shape retention and recovery properties, necessitate modification of the PLLA to improve its application efficacy in physiological conditions. This study is proposed to modify PLLA by having the biodegradable poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) incorporated to form ultrafine composite fibers (i.e., PLLA-PHBV) through electrospinning. Different pairs of PLLA-PHBV at the varying mass ratios of 10:0, 9:1, 8:2, 7:3, 6:4, and 0:10 can be successfully electrospun into fibrous form with the fineness of 2-3 μm. Incorporation of PHBV enables to give rise to desired Tg decreases and also, interestingly, increases in the Young's modulus of the PLLA-PHBV blends, while gradually increasing the PHBV mass ratios up to 30%. The PLLA-PHBV (7:3) formulation is identified to present excellent shape memory properties with high shape fixing ratio (>98%) and shape recovery ratio (>96%) compared to the unmodified PLLA fiber counterpart. Moreover, the PLLA-PHBV (7:3) fibers also show enhanced osteogenesis-inducing ability in the mouse bone mesenchymal stem cells, even under nonosteoinductive conditions. Collectively, for the first time this study demonstrates the enhanced shape memory and osteogenesis capabilities of the electrospun PLLA-PHBV composite fibers, and the researched PLLA-PHBV (7:3) fiber system could be potentially applied as a multifunctional scaffolding material for applications in bone tissue repair and regeneration. Impact statement By first converting the poly(l-lactide) (PLLA)-poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) hybrids into fibrous form at varied mass ratios followed by a thorough characterization, we reasonably demonstrated that incorporation of an appropriate amount of PHBV (i.e., 30%) into the PLLA fibers could give rise to significant improvement on the shape memory capability of the PLLA, along with the desired decreases in the transition temperature (Tg). Moreover, the fibrous PLLA-PHBV (7:3) scaffold was also found to significantly promote the osteogenic commitment in bone mesenchymal stem cells with osteoinductive factors in a synergistic manner. Our biomimicking and shape memory enabled fibrous scaffold of PLLA-PHBV could be used to construct multifunctional three-dimensional scaffold with shape memory effect for bone regeneration.

Ultrafast bone-like apatite formation on highly porous poly(l-lactic acid)-hydroxyapatite fibres.

In order to provide a favourable environment for living bone formation, it is an essential condition to grow bone-like apatite layer at the interface between the tissue-implant and its surrounding tissues. Inspired by the chemical composition and the nano porous structure of natural bones, we developed an ultrafast and accessible route to accelerate effectively the formation of bone-like apatite on the surface of porous poly(l-lactic acid)-hydroxyapatite (PLLA-HA) composite fibres in 5 times simulated body fluid (5SBF). The key of the method lays in successful exposure of HA nanoparticles on the surface of PLLA fibres by acetone treatment of electrospun PLLA-HA nano/micro fibres. The recrystallization of PLLA chains uncovers more HA nanoparticles on the surface of every fibre which provide nucleation sites for calcium and phosphate ions. After only 2h of immersing in 5SBF, a full layer of apatite completely covered on the surface of porous PLLA-HA fibres. The results indicate that HA nanoparticles on porous fibre surface can accelerate the kinetic deposition of apatite on fibre surface. Biological in vitro cell culture with human osteoblast-like cell for up to 7days demonstrates that the incorporation of HA nanoparticles on the surface of porous PLLA fibrous membranes leads to significant enhance osteoblast adhesion and proliferation. The route can open avenues for development of fibrous PLLA biomaterials for hard tissue repair and substitution.

An Integrative Dual-Layer Poly-L-Lactic Acid Fibrous Membrane Prevents Peritendinous Adhesions.

Anti-adhesion membranes are prospective scaffolds for preventing peritendinous adhesion after injury. However, currently available scaffolds have some limitations, such as low efficacy for anti-adhesion, low quality of tendon healing, and unknown drug interactions. Thus, in this study, we designed an innovative structure involving an integrated dual-layer poly(L-lactic acid) (PLLA) electrospun membrane for preventing peritendonous adhesion by promoting tendon gliding. We investigated the surface morphology and wettability of the fiber scaffold. The adhesion and proliferation of fibroblasts were low on the PLLA fibrous membrane. Compared with single-layer membranes, the dual-layer PLLA fiber scaffold reduced adhesion to the tissues. The gliding space persisted until recovery in chicken extensor flexor tendons in vivo. Thus, this innovative PLLA membrane scaffold could prevent adhesion and promote gliding to facilitate tendon healing.

Effect of introduction of tetrabutylammonium bromide on properties of poly (L‐lactic acid) tubular scaffold prepared by electrospinning

In this work, poly (L-lactic acid) (PLLA) tubular scaffold was prepared by adding tetrabutylammonium bromide (TBAB) to the PLLA spinning solution. The effects of the introduction of TBAB on the morphology, crystallisation and mechanical properties of PLLA scaffolds were systematically studied by means of scanning electron microscope, differential scanning calorimetry (DSC), X-ray diffraction (XRD) and attenuated total reflectance Fourier-transform infrared spectroscopy. The results show that the introduction of TBAB improves crystallinity and mechanical property of the PLLA scaffold. As the TBAB content in the spinning solution increases, the mechanical properties of the PLLA tubular scaffold increase first and then decrease. The circumferential tensile strength of the PLLA tubular scaffold prepared by the spinning solution with addition of TBAB were found to increase by up to 323%. The results of DSC and XRD show that the introduction of TBAB can improve the crystallinity of the PLLA fibres. The Fourier-transform infrared spectroscopy test results demonstrate that the chain orientation of the PLLA fibres increases with the TBAB content in the spinning solution and induces the formation of α /α ′ crystal phase structure in the PLLA fibres, which is the main reason for the substantial improvement of the mechanical properties of the tubular scaffold.

Influence of Mesenchymal Stem Cells Seeding on Morphology of Tissue-Engineered Vascular Graft Based on Poly (L-lactide) Scaffold

Introduction - The problem of small-diameter vascular grafts still exists in modern vascular surgery. Autologous or synthetic vascular grafts are used routinely for arterial bypass in patients with cardiovascular diseases. However, some patients either lack suitable autologous tissue or cannot receive synthetic grafts. Such patients could benefit from a vascular graft produced by tissue engineering. The aim of the study is to evaluate the effects of adipose-derived mesenchymal stem cells (AD-MSC) on the formation of tissue-engineered vascular graft (TEVG). Methods - Tubular scaffolds 1.1 mm inner diameter were obtained by the electrospinning method on the basis of nano- and microfibers of poly(L-lactide) (PLLA). AD-MSC were seeded on the scaffolds by the developed filtration method. The subsequent culturing for 14 days was carried out in the constructed flow bioreactor. Cell distribution in the scaffold wall was assessed by the fluorescence microscopy. Obtained grafts were implanted into rat aorta: group 1 without AD-MSC (n=36) with follow-up till 16 months, group 2 with seeded AD-MSC (n=36) with follow-up till 16 months. The material was subjected to histological examination, electron microscopy, immunohistochemistry CD 31+, aSMA and morphometric analysis (cell counting, neointima and neoadventia thickness). To track AD-MSC in vivo PKH-26 labeling and subsequent fluorescence microscopy were performed in 2, 7 and 14 days. Results - Filtration seeding and subsequent cultivation in the flow bioreactor led to a uniform distribution of AD-MSC in the scaffold wall. Graft patency in the first group was 86%, in the second 96%. All grafts formed neointima, with no signs of hyperplasia in the area of anastomoses. The total resorption of PLLA fibers in group 1 was observed by histological examination, new vascular wall consisted of endothelial lining and connective tissue without smooth muscle cells (SMC); in all cases aneurysm formation was detected. Total cells number of implant wall was more in group 2, including SMC that formed neomedia. Also the outer connective tissue layer was much thicker. Histology identified the uniform formation of new tissues in group 2 with no signs of aneurysm. Conclusion - AD-MSC seeding leads to the formation of TEVG, morphologically similar to the natural structure of the vessels. The study was supported by Russian Science Foundation (project 14-33-00003)

Biodegradable electrospun fibers enriched with struvite crystal seeds for the recovery of phosphorous and nitrogen

One of the greatest global challenges of the 21st century is facing the depletion of phosphorous reserves, widely used in fertilizers. One possibility is to recover the phosphorous discharged in the water by human activities, with the consequent additional benefit of improving water quality by limiting eutrophication. In this work, biodegradable poly(l-lactic acid) (PLLA) nonwovens produced by electrospinning are proposed as highly porous fibrous materials characterized by high surface-to-volume ratio for promoting the crystallization of struvite crystals as a means to recover both phosphorus and nitrogen from a nutrient rich solution. To this aim, PLLA fibers were intentionally loaded with a small amount of struvite crystals (3,7 wt%) by applying different preparation procedures, including sonication steps, single-nozzle and coaxial electrospinning. PLLA fibers containing struvite crystals were immersed in a synthetic solution of suitable ions to investigate the occurrence of struvite accumulation through the analysis of the soaked nonwovens with several techniques, i.e. Scanning and Transmission Electron Microscopy, Wide Angle X-Ray Diffraction, Thermogravimetric analysis and Atomic Emission Spectrometry. We demonstrated that struvite loaded in the PLLA fibers successfully acted as crystal seeds and enabled the precipitation of struvite crystals firmly attached to fiber surface. Results showed that the quality of crystal seeds dispersion in PLLA fibers as well as their location along the fiber section heavily affect struvite recovery. Prospectively, these materials can be directly used, after being in contact with wastewater and enriched with struvite, as ready-to-use fertilizers.