Dry-jet Wet Spinning Research Articles

Mesopores variation in polyacrylonitrile fibers during dry-jet wet spinning process

The mesopore structures in polyacrylonitrile (PAN) fibers during dry-jet wet spinning process were investigated by high-resolution transmission electron microscopy (HRTEM) and image analysis utilizing the ultrathin section technique. The morphologies and dimension distribution of the mesopores in the surface and core regions of the nascent fibers and PAN fibers are presented. All fibers exhibited lamellar-like structures perpendicular to the fiber axis and the mesopores were distributed between the lamellae. For nascent fibers, the size and volume of the mesopores increased with increasing air gap and decreased with increasing drawing ratio. In addition, the widths of the mesopores were larger than their lengths. Consequently, the size and content of the mesopores in nascent fibers could be adjusted by controlling coagulation conditions. During the post-spinning process, the size and volume of the mesopores in PAN fibers decreased efficiently by hot drawing in a hot water washing bath, in hot steam chambers or on hot rollers. Moreover, the lengths of the mesopores were larger than their widths. In all fiber samples, the number and size of the mesopores in the core region were larger than those in the surface region. In addition, the mechanical properties of fibers were correlated with dimension of the mesopores. Their tensile strength increased with decreasing mesopore widths and lengths.

Preparation and Properties of High-performance Polyimide Copolymer Fibers Derived from 5-Amino-2-(2-hydroxy-5-aminobenzene)-benzoxazole

A series of polyamic acid copolymers (co-PAAs) with para-hydroxyl groups was synthesized using two diamine monomers, namely p-phenylenediamine (p-PDA) and 5-amino-2-(2-hydroxy-5-aminobenzene)-benzoxazole (m-pHBOA), of different molar ratios through copolymerization with 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) in N,N-dimethyacetamine (DMAc). The co-PAA solutions were used to fabricate fibers by dry-jet wet spinning, and thermal imidization was conducted to obtain polyimide copolymer (co-PI) fibers. The effects of the m-pHBOA moiety on molecular packing and physical properties of the prepared fibers were investigated. Fourier transform infrared (FTIR) spectroscopic results confirmed that intra/intermolecular hydrogen bonds originated from the hydroxyl group and the nitrogen atom of the benzoxazole group and/or the hydroxyl group and the oxygen atom of the carbonyl group of cyclic imide. As-prepared PI fibers displayed homogenous and smooth surface and uniform diameter. The glass transition temperatures (Tgs) of PI fibers were within 311−337 °C. The polyimide fibers showed 5% weight loss temperature (T5%) at above 510 °C in air. Two-dimensional wide-angle X-ray diffraction (WXRD) patterns indicated that the homo-PI and co-PI fibers presented regularly arranged polymer chains along the fiber axial direction. The ordered molecular packing along the transversal direction was destroyed by introducing the m-pHBOA moiety. Moreover, the crystallinity and orientation factors increased with increasing draw ratio. Small-angle X-ray scattering (SAXS) results showed that it is beneficial to reduce defects in the fibers by increasing the draw ratio. The resultant PI fibers exhibited excellent mechanical properties with fracture strength and initial modulus of 2.48 and 89.73 GPa, respectively, when the molar ratio of p-PDA/m-pHBOA was 5/5 and the draw ratio was 3.0.

Preparation of High-Quality Polyacrylonitrile Precursors for Carbon Fibers Through a High Drawing Ratio in the Coagulation Bath During a Dry-Jet Wet Spinning Process

Excellent poly(acrylonitrile-co-itaconic acid) (99/1) (PAI) nascent fibers, which have an important role in preparing high-quality precursors for carbon fibers, were prepared by a dry-jet wet spinning process. Their structures were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM) and an ultrasound solvent etching method, as well their properties being determined by a strength and extension meter and a fineness meter, both designed specifically for fibers. When a high drawing ratio, over 300%, was applied to the fibers in the dry-jet wet spinning coagulation bath, the molecular chains were easy to orient and regularly arrange, resulting in the relative crystallinity, crystal size and amorphous orientation degree of the nascent fibers being improved. The fibrils with large diameter were formed, increasing the bulk density with the overall porosity and pore numbers decreasing. Therefore, the nascent fibers had smaller diameters, higher strength, higher rupture elongation and smaller coefficients of variation. The optimum high performance PAI precursor fibers, with 0.59dtex in titer, 7.51cN/dtex in tensile strength, 7.9% in rupture elongation and the final carbon fiber with 5.54GPa in tensile strength, were obtained through a post-spinning treatment in which they were subjected to a high coagulation bath draw ratio and carbonization.

Tensile and interfacial properties of dry-jet wet-spun and wet-spun polyacrylonitrile-based carbon fibers at cryogenic condition

For carbon fiber–reinforced polymer composites applied in aerospace industry, the mechanical performance of carbon fiber in extreme temperature environment is of great importance. In this study, carbon fiber produced by dry-jet wet spinning and wet spinning approaches were cryogenically conditioned at different cooling rates. After cryogenically conditioned at sharp cooling rate, both fibers have around 10% decreases in tensile strength due to the huge hoop stress induced by the quenching process. However, after cryogenically conditioned at slow cooling rate, the interfacial shear strength between the two kinds of carbon fibers and epoxy resin was significantly enhanced. Furthermore, scanning electron microscopy, atomic force microscopy, and the Raman spectroscopy were conducted to detect the micro-structures and surface morphologies of the cryogenically conditioned carbon fibers. This study provided fundamental data for the material design and application of the carbon fiber at extreme temperature environments such as aerospace or other industrial fields.

Effects of the aspect ratio of multi-walled carbon nanotubes on the structure and properties of regenerated collagen fibers

Collagen is a natural one-dimensional nanomaterial. Multi-walled carbon nanotubes (MWNTs) have been previously shown to interact with biomolecules and to have promising applications in reinforced biopolymers for tissue engineering and regenerative medicine. In this work, collagen/MWNT composite fibers are prepared using dry-jet wet-spinning technology. Three types of MWNTs with aspect ratios of 40, 150, and 4000 are used to investigate the effects of the MWNT aspect ratio on the properties of the composite fibers. There results show that there are strong molecular interactions between the MWNTs and collagen molecules. The mechanical properties and thermal stability of the composite fibers are significantly improved compared to those of the collagen fibers. The diameter and aspect ratio of the MWNTs are the main factors affecting the self-assembled structure of the collagen molecules, the alignment of the microfibrils, and the mechanical and thermal performance of the composite fibers.

Highly adsorptive polysulfone/hydrous iron-nickel-manganese (PSF/HINM) nanocomposite hollow fiber membrane for synergistic arsenic removal

The integration of nanotechnology and membrane technology has led to the development of nanocomposite membranes that endowed with outstanding performances for arsenic removal in water system. In this study, a novel adsorptive nanocomposite membrane was fabricated by incorporating novel hydrous iron-nickel-manganese trimetal oxides (HINM) nanoparticles into polysulfone (PSf) membrane. HINM with atomic ratio of Fe:Ni:Mn of 3:2:1 was first synthesized by oxidation and coprecipitation method. The PSf/HINM membranes were fabricated by dry-jet wet spinning phase inversion technique. The physicochemical properties of the membranes were observed by contact angle goniometer, scanning electron microscope (SEM) and atomic force microscope (AFM). H1.5 PSf/HINM membrane shows maximum adsorption capacity of 41.90 mg/g and water flux of 173.82 L/m2 h.bar. This membrane was fabricated by impregnating 1.5 wt% of HINM nanoparticles within 18 wt% of PSf, 5% PVP, and 78% NMP solution. The experimental data of arsenite adsorption were best fitted into pseudo-second order and Freundlich isotherm models. The mechanism of arsenite removal on PSf/HINM membrane involved both chemisorption and physisorption. Chemisorption is concerned to hydroxyl substitution by arsenite ions through the formation of inner-sphere complex, while physisorption associated to electrostatic attraction between arsenite ions and the surface charge of HINM nanoadsorbents. Leaching test demonstrated that HINM was stably incorporated within the membrane matrix. Continuous adsorption-filtration study showed that H1.5 membrane has successfully treated arsenic contaminated water to meet the maximum level (0.01 mg/L) set by World Health Organization (WHO). The membrane could be easily regenerated using 0.1 M NaOH solution. Hence, development of adsorptive nanocomposite PSf/HINM membrane is a promising solution for synergistic arsenic removal in water system due to both adsorption and filtration properties of the membrane.

Light driven PVDF fibers based on photochromic nanosilica@naphthopyran fabricated by wet spinning

Photoresponsive polyvinylidene fluoride (PVDF) fibers doped with naphthopyran-functionalized silica nanoparticles (SiO2@NPT) were successfully prepared by dry–jet wet spinning. The incorporated photochromic nanomaterials, SiO2@S1 and SiO2@S2, were previously prepared by covalent post-grafting of the silylated NPTs – 2H-naphtho[1,2-b]pyran derivatives, S1 and S2 – onto nano-sized SiO2 (15 ± 3 nm).The morphological and chemical characterization of the resulting doped fibers PVDF@SiO2@S1 and PVDF@SiO2@S2 was evaluated by scanning electron microscopy with energy dispersive spectroscopy and Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR). Both PVDF@SiO2@S1 and PVDF@SiO2@S2 fibers presented average diameters of 133 ± 8 μm and 98 ± 7 μm, respectively, and a porous outer surface. The presence of the SiO2@NPT within the fibers was confirmed through the observation of dense clusters embedded within the polymeric matrix. Furthermore, the FTIR-ATR spectra of the fibers revealed that the PVDF matrix was composed of α and β crystalline and amorphous phases.The PVDF@SiO2@S1 and PVDF@SiO2@S2 fibers showed photoresponsive properties under UV and sunlight irradiation, exhibiting fast coloration kinetics and good optical contrast (ΔE*ab = 7.2 and 15.1, respectively), and changing from a pale orange and an off-white color to a more intense yellow-orange and purple coloration, respectively, in less than 1 min. Moreover, they showed an initial fast bleaching, losing half of their color in less than 30 min (t1/2 = 28 and 20 min for PVDF@SiO2@S1 and PVDF@SiO2@S2 fibers, respectively), but displaying a residual coloration that took 1 and 2 h to return to the initial uncolored state (t3/4 = 95 and 49 min, respectively). The PVDF@SiO2@S2 fibers presented the best photochromic performance.

Preparation of Polybenzimidazole Hollow-Fiber Membranes for Reverse Osmosis and Nanofiltration by Changing the Spinning Air Gap.

High-performance polybenzimidazole (PBI) hollow-fiber membranes (HFMs) were fabricated through a continuous dry-jet wet spinning process at SRI International. By adjusting the spinning air gap from 4″ (10.2 cm) to 0.5″ (1.3 cm), the HFM pore sizes were enlarged dramatically without any significant change of the fiber dimensional size and barrier layer thickness. When fabricated with an air gap of 2.5″ (6.4 cm) and a surface modified by NaClO solution, the PBI HFM performance was comparable to that of a commercial reverse osmosis (RO) HFM product from Toyobo in terms of salt (NaCl) rejection and water permeability. The PBI RO HFM was positively surface charged in acidic conditions (pH < 7), which enhanced salt rejection via the Donnan effect. With an air gap of 1.5″ (3.8 cm), the PBI HFM rejected MgSO4 and Na2SO4 above 95%, a result that compares favorably with that achieved by nanofiltration. In addition, the PBI HFM has a defect-free structure with an ultra-thin barrier layer and porous sublayer. We believe PBI HFMs are ideal for water purification and can be readily commercialized.

Facilitated fibrillation of regenerated cellulose fibers by immiscible polymer blending using an ionic liquid

Cellulose (Cell) microfibrils were prepared from regenerated Cell fibers with facilitated fibrillation character by immiscible polymer blending with polyacrylonitrile (PAN) as a minor component. The blend fibers were spun by dry-jet wet spinning of the liquid–liquid phase-separated Cell/PAN solution using the well-known ionic liquid 1-buthyl-3-methyl imidazolium chloride as a cosolvent. Wide angle X-ray diffraction, small angle X-ray scattering, and electron microscopy analyses revealed that the PAN phase in the fiber existed as an elongated island owing to the large draft force used on the spinning solutions. The PAN phase in the fiber was removed with dimethyl sulfoxide to obtain neat Cell fibers with elongated pores and nanometer-sized cross-sections. The generated pores, acting as starting points for fibrillation, allowed the lateral cohesion of the fibers to be weakened and facilitated the fibrillation tendency. Cell fibers regenerated from a 100/20 weight ratio blend of Cell/PAN were easily fibrillated to microfibrils with an average diameter of 114 ± 67 nm by mechanical treatment.

Use of mechanically ground lignocellulosic native fines (LF) in the all-cellulosic composite filaments: fines properties and plasticizers

The influence of the physical and colloidal properties of W-stone ground lignocellulose native fines (LF) on the properties of lignocellulosic composite filaments was investigated. W-stone ground LF is a low-cost material exhibiting a microfibrillar structure with the chemical structure of native wood. The physical properties of manufactured LFs were investigated by utilising SEM imaging, turbidity measurements and image-based particle analysis using a Kajaani fibre analyser. The properties of LFs were varied by adjusting the process energy input that altered the produced material’s particle size and shape and subsequent fractionation with a wire. The reduction in particle size was observed to increase the colloidal stability of produced LFs, but no significant changes in the chemical profile of the LFs were observed. The effect of the properties of LF on the manufacture of composite filaments with carboxymethyl cellulose (CMC) was studied by using a dry-jet wet spinning approach. The smaller particle size had a positive effect on the mechanical properties of composite filaments (tenacity increased from 5.5 to up to 7.6 cN/tex). The compatibility of different plasticisers with LF–CMC composite filaments was also studied. It was observed that the number of free hydroxyls per a monomer unit of the plasticiser had a positive correlation with the plasticisation effect in the LF–CMC composite filaments. Regenerated cellulose filaments are often rather expensive to be used in many applications such as composites. The investigated filaments could thus be used in low-cost applications requiring a fully biodegradable material profile. Here, the presence of lignin may increase the structural compatibility of the produced matrix.Graphical abstract

Cellulose acetate fibers with improved mechanical strength prepared with aqueous NMMO as solvent

Cellulose diacetate (CA) was successfully dissolved in N-methylmorpholine N-oxide aqueous solution (NMMO·H2O). The CA degraded little during the dissolving process and the homogenous CA/NMMO·H2O solutions were obtained even when the concentration was up to 18%. From the rheological behavior of the CA/NMMO·H2O solution, it was found that the viscosity of the solution was lower than that of lyocell solution, which is due to the lower degree of polymerization and the substitute of acetyl group. Then CA fibers were prepared by dry-jet wet spinning process. The tenacity and the E-modulus of the regenerated CA fibers were up to 3.0 cN/dtex and 75 cN/dtex respectively, and the former was larger than that of industrial CA fibers prepared by dry spinning process. This is related to the structure of the CA fibers, because the crystallinity, crystal orientation of the fibers can be improved by high draw ratio during dry-jet wet spinning process. Otherwise, the CA fibers showed no fibrillation tendency, which showed improved wearing comfort compared to lyocell fibers. This work provided an effective way to produce new CA fibers with higher tensile strength.

Synthesis and AO Resistant Properties of Novel Polyimide Fibers Containing Phenylphosphine Oxide Groups in Main Chain

A series of co-polyimide (PI) fibers containing phenylphosphine oxide (PPO) group were synthesized by incorporating the bis(4-aminophenoxy) phenyl phosphine oxide (DAPOPPO) monomer into the PI molecular chain followed by dry-jet wet spinning. The effects of DAPOPPO molar content on the atomic oxygen (AO) resistance of the fibers were investigated systematically. When the AO fluence increased from 0 atoms·cm−2 to 3.2 × 1020 atoms·cm−2, the mass loss of the fibers showed the dependence on DAPOPPO molar content in co-PI fibers. The PI fiber containing 40% DAPOPPO showed lower mass loss compared to those containing 0% and 20% DAPOPPO. At higher AO fluence, the higher DAPOPPO content gave rise to dense carpet-like surface of fibers. XPS results indicated that the passivated phosphate layer was deposited on the fiber surface when exposed to AO, which effectively prevented fiber from AO erosion. With the DAPOPPO content increasing from 0% to 40%, the retentions of tensile strength and initial modulus for the fibers exhibited obvious growth from 44% to 68%, and 59% to 70%, after AO exposure with the fluence of 3.2 × 1020 atoms·cm−2. The excellent AO resistance benefits the fibers for application in low Earth orbit as flexible construction components.

Research on PAN Nascent Fiber Interior Microstructure through Ultrasonic Etching and Ultrathin Sectioning

The interior microstructures of polyacrylonitrile nascent fibers is studied by the scanning electronic microscopy and the high-resolution transmission electron microscopy through ultrasonic etching and ultrathin sectioning. Due to the orientation and fold of molecular chains, the lamellae of 50–80 nm in thickness are formed. A high number of pores, ranging from dozens to two hundred nanometers in diameters exist between the lamellae, which result from residual solvent. The fibril structure is formed in the nascent fiber during the coagulation process, which are oriented along the fiber axis. An uneven tensile stress distribution leads to the formation of skin-core structures in the nascent fiber during the dry-jet wet spinning process.

Improved yield of carbon fibres from cellulose and kraft lignin

Abstract To meet the demand for carbon-fibre-reinforced composites in lightweight applications, cost-efficient processing and new raw materials are sought for. Cellulose and kraft lignin are each interesting renewables for this purpose due to their high availability. The molecular order of cellulose is an excellent property, as is the high carbon content of lignin. By co-processing cellulose and lignin, the advantages of these macromolecules are synergistic for producing carbon fibre (CF) of commercial grade in high yields. CFs were prepared from precursor fibres (PFs) made from 70:30 blends of softwood kraft lignin (SW-KL) and cellulose by dry-jet wet spinning with the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate ([EMIm][OAc]) as a solvent. In focus was the impact of the molecular mass of lignin and the type of cellulose source on the CF yield and properties, while membrane-filtrated kraft lignin and cellulose from dissolving kraft pulp and fully bleached paper-grade SW-KP (kraft pulp) served as sources. Under the investigated conditions, the yield increased from around 22% for CF from neat cellulose to about 40% in the presence of lignin, irrespective of the type of SW-KL. The yield increment was also higher relative to the theoretical one for CF made from blends (69%) compared to those made from neat celluloses (48–51%). No difference in the mechanical properties of the produced CF was observed.

Conversion of wood-biopolymers into macrofibers with tunable surface energy via dry-jet wet-spinning

Surface chemistry of regenerated all-wood-biopolymer fibers that are fine-tuned by composition of cellulose, lignin and xylan is elucidated via revealing their surface energy and adhesion. Xylan additive resulted in thin fibers and decreased surface energy of the fiber outer surfaces compared to the cellulose fibers, or when lignin was used as an additive. Lignin increased the water contact angle on the fiber surface and decreased adhesion force between the fiber cross section and a hydrophilic probe, confirming that lignin reduced fiber surface affinity to water. Lignin and xylan enabled fiber decoration with charged groups that could tune the adhesion force between the fiber and an AFM probe. The fibers swelled in water: the neat cellulose fiber cross section area increased 9.2%, the fibers with lignin as the main additive 9.1%, with xylan 6.8%, and the 3-component fibers 5.5%. This indicates that dimensional stability in elevated humidity is improved in the case of 3-component fiber compared to 2-component fibers. Xylan or lignin as an additive neither improved strength nor elongation at break. However, improved deformability was achieved when all the three components were incorporated into the fibers.Graphical

Research on the multi-scale microstructure of polyacrylonitrile precursors prepared by a dry-jet wet spinning process

An understanding of the properties of polyacrylonitrile (PAN) precursors is an essential precondition for manufacturing high-performance carbon fibres, and the structure of the precursors has a direct and profound effect on the performance of carbon fibres. In this study, PAN precursors, formed in a multistage coagulation bath, were spun by a dry-jet wet spinning process, and the multi-scale microstructure and morphology of the precursors were investigated by separating the fibrils from the precursors. Scanning electron microscopy and high-resolution transmission electron microscopy were employed to examine the surface morphology, cross-sectional morphology and microstructure of the precursors. X-ray diffraction was used to characterize the crystal structure. The micropore sizes of the precursors were determined with nitrogen adsorption experiments; the adsorption increased after ultrasonic etching and decreased with an increase in the treated concentration. All the results demonstrated that the PAN precursors had a multi-scale microstructure, the precursors consisted of fibrils with diameters of 80–200 nm and the fibrils consisted of some microfibrils with diameters of 20–40 nm, including the periodic tissues with thicknesses of 16–30 nm perpendicular to the fibre axis.

Fibril microstructural changes of polyacrylonitrile fibers during the post-spinning process

Ultrathin sectioning and solvent etching were used to assess microstructural changes in polyacrylonitrile (PAN) fibrils following a dry-jet wet spinning process. Morphologic assessment revealed the coexistence of fibrils and lamellae within the PAN fibers, with a frill-like structure found on the fibrils. The size of the lamella-like layers was consistent with a crystal size of 13.2 nm as characterized by X-ray diffraction and microfibrils were composed of alternate dark lamella-like crystal layers and surrounded by bright amorphous chains. Further, fibril arrangement followed a weave-like pattern. During the post-spinning process, break-rearrangement of chain-folded lamellae had an important role in fibril structure evolution. The high stream stretching and heat treatment induced the breaking of lamellae and the formation of new lamellae through the molecular chains rotating and slipping, resulting in a decrease in voids and the coalescence of microfibrils. Consequently, the fibrils were arranged in a more orderly manner and interlinked more tightly, a process conducive for the preparation of high quality PAN fibers.

The influence of cross-sectional morphology on the compressive resistance of polymeric nerve conduits

Artificial nerve conduits are proposed as an alternative to repair peripheral nerves injuries; while mechanical properties are of great importance for successful clinical application of nerve conduits. In this work, the poly (d,l-lactide-co-glycolide) (PLGA) nerve conduits were fabricated by the dry-jet wet spinning, and the compressive resistance of nerve conduits was systematically measured. It was found that the compressive resistance of nerve conduits significantly increased with the PLGA concentration of dope fluids. A numerical model was developed to simulate the compression tests of nerve conduits; where the hyperelastic–plastic constitutive law and the morphological characters of the cross sections of nerve conduits were implemented. The numerical results indicated the layered morphology of the cross sections of nerve conduits played the most important role in determining the compressive resistance. In addition, the accuracy of the numerical model was well validated by good agreement of experimental and numerical results of nerve conduit compression tests. This study helps understanding how to characterize the compressive resistance of porous polymeric nerve conduits and how the morphology influences the compressive resistance, leading to better design of fabrication setup and material selection for nerve conduits.

Formation of Surface Morphology in Polyacrylonitrile (PAN) Fibers during Wet-Spinning

The effects of concentration of dimethyl sulfoxide (DMSO) in the coagulation bath, draw ratio and extrusion speed on surface roughness of polyacrylonitrile (PAN) fibers prepared by wet spinning and dry-jet wet spinning were investigated. The surface roughness was much higher for PAN fibers produced by wet spinning than dry-jet wet spinning. The surface roughness of the fiber increased linearly with increasing concentration of DMSO in the coagulation bath. Higher roughness was observed at higher draw ratios during spinning. The surface roughness of the PAN fibers decreased initially until at 90m/h, then increased with further increases in extrusion speed. A mechanism for formation of PAN fiber surface morphology based on deformation of the soft skins of single fibers during solidification of PAN/DMSO solution caused by stress perpendicular to the fiber axis is proposed. The stress results from recovery of the aligned PAN macromolecules due to shear in the spinneret during extrusion.

Polyacrylonitrile Nerve Conduits With Inner Longitudinal Grooved Textures to Enhance Neuron Directional Outgrowth

Topographical guidance cues are considered effective factors for enhancing the efficiency of nerve conduits in repairing nerve injuries. However, very few investigations so far have been reported to design and optimize the topographical patterns. In this paper, polyacrylonitrile nerve conduits with aligned longitudinal grooves on the inner surface were manufactured by the dry-jet wet spinning methods, where the groove width was precisely tunable. PC12 cells (mouse neuroblastic and eosinophilic cells) were cultured in the nerve conduits, and the results demonstrated that our designed inner grooved pattern effectively improved neurite alignment even when the groove width is significantly larger than the neuron size. Furthermore, the results revealed that both the groove and ridge width played important roles in controlling the neurites growth: parallel growth to the longitudinal direction was more obvious in the nerve conduits within narrower grooves. Here, we showed a simple yet effective technique to implement never conduits with a functional grooved inner texture. Controllable guidance of neurite growth, as well as the in-conduit morphological heterogeneity, showed promise for nerve regeneration and stem cell therapy in the future. [2017-0196]