polymers Article
Mass-Production and Characterizations of Polyvinyl Alcohol/Sodium Alginate/Graphene Porous Nanofiber Membranes Using Needleless Dynamic Linear Electrospinning Ting-Ting Li 1,2 , Mengxue Yan 1 , Wenting Xu 1 , Bing-Chiuan Shiu 3 , Ching-Wen Lou 1,4,5,6, * and Jia-Horng Lin 1,3,5,6,7,8, * 1
2 3 4 5 6 7 8
*
Innovation Platform of Intelligent and Energy-Saving Textiles, School of Textiles, Tianjin Polytechnic University, Tianjin 300387, China;
[email protected] (T.-T.L.);
[email protected] (M.Y.);
[email protected] (W.X.) Tianjin and Education Ministry Key Laboratory of Advanced Textile Composite Materials, Tianjin Polytechnic University, Tianjin 300387, China Laboratory of Fiber Application and Manufacturing, Department of Fiber and Composite Materials, Feng Chia University, Taichung 40724, Taiwan;
[email protected] Department of Bioinformatics and Medical Engineering, Asia University, Taichung 41354, Taiwan Department of Chemical Engineering and Materials, Ocean College, Minjiang University, Fuzhou 350108, China College of Textile and Clothing, Qingdao University, Qingdao 266071, China School of Chinese Medicine, China Medical University, Taichung 40402, Taiwan Department of Fashion Design, Asia University, Taichung 41354, Taiwan Correspondence:
[email protected] (C.-W.L.);
[email protected] (J.-H.L.); Tel.: +886-4-2451-7250 (ext. 3405) (J.-H.L.); Fax: +886-4-24510871 (J.-H.L.)
Received: 14 August 2018; Accepted: 13 October 2018; Published: 19 October 2018
Abstract: The aim of this study was to investigate the feasibility of large-scale preparation of porous polyvinyl alcohol/sodium alginate/graphene (Gr) (Gr-AP) nanofiber membranes using a copper wire needleless dynamic linear electrode electrospinning machine. Furthermore, the effects of Gr concentrations (0, 0.0375, 0.075, 0.25, 0.5, and 0.75 wt.%) on the morphology, electrical, hydrophilicity and thermal properties were evaluated. Results indicate that the dynamic linear electrospun Gr-AP membranes have a high yield of 1.25 g/h and are composed of porous finer nanofibers with a diameter of 141 ± 31 nm. Gr improved the morphology, homogeneity, hydrophobicity and thermal stability of Gr-AP nanofiber membranes. The critical conductive threshold is 0.075 wt.% for Gr, which provides the nanofiber membranes with an even distribution of diameter, an optimal conductivity, good hydrophilicity, appropriate specific surface area and optimal thermal stability. Therefore, needleless dynamic linear electrospinning is beneficial to produce high quality Gr-AP porous nanofiber membranes, and the optimal parameters can be used in artificial nerve conduits and serve as a valuable reference for mass production of nanofiber membranes. Keywords: needleless electrospinning; linear electrode; graphene (Gr); nanofibers; conductivity
1. Introduction The incidence of nerve injuries has risen year by year as a result of the prosperity of public transportation and the increase in gross national income [1]. Considering long-term disability and poor surgery outcomes, peripheral nerve injury presents a substantial challenge to reconstructive surgeons, and autologous nerve grafts is thus a commonly employed measure [2]. Up until now, diverse natural Polymers 2018, 10, 1167; doi:10.3390/polym10101167
www.mdpi.com/journal/polymers
Polymers 2018, 10, 1167
2 of 15
and synthetic polymer biomaterials have been used for the development of scaffolds in the peripheral nerve tissue engineering field [3–8]. Electrospun nanofibers have characteristics of high porosity, high specific surface area, and good mechanical properties, which qualify for their use in environmental monitoring, healthcare, environmental protection, and electronics [9]. Electrospinning can transform suitable materials into nanofiber scaffolds, which can effectively substitute neural bridge connection [10–13]. Moreover, sodium alginate (SA) has good biocompatibility, biodegradability and has commonly been used as an artificial substitute of peripheral nerve tissues. However, due to its high viscosity, the SA solution cannot be electrospun into nanofibers individually, and is usually blended with polymers like PVA with good fiberizability to form nanofibers [14–16]. In addition, graphene (Gr) has good mechanical, electrical, thermal, optical properties, and a high specific surface area. Some scholars have used Gr to modify the properties of tissue engineering PVA/alginate scaffolds, thereby obtaining extraordinary electric conductivity and accelerating cell proliferation [17–22]. Despite of a competitive potential with advantages of efficiency, a lower production cost, manageable processes, and zero pollution, electrospinning has never been applied to industrialization. Traditional needle electrospinning fails to proceed to efficient production, and the issue of needle blocking also restricts its development. Likewise, needleless electrospinning also has drawbacks and falls short of the demands of the market [23–29]. For example, static bubble spinning produces fibers with a diameter dependent on the bubble size, and the fibers diameters are commonly uneven [30]. Similarly, using a dynamic rotating cylinder as the spinning nozzle produces a great range in the diameters of nanofibers [31,32]. In addition, when using PVA in the electrospinning materials, cone metallic coil electrospinning has a yield of 2.5 g/h, which is thirteen times higher than conventional single needle electrospinning (less than 0.3 g/h) [33–36] and the nanofibers have a lower diameter than needle electrospinning. For example, Golafshan et al. used needle electrospinning to form Gr-AP nanofiber stents that were suitable for nerve engineering and the nanofibers had a diameter above 300 nm. However, its low yield cannot satisfy market demand, and the discontinuous electrospinning process is not suitable for mass production [19]. In our previous study, using the cylindrical dynamic linear electrode as the spinning nozzle can achieve continuous electrospinning, with finer diameter nanofibers and a higher yield of nanofiber membranes [37]. In order to improve the electrospinning process to the greatest extent, increase product yields, and produce composite nanofibers with a high specific surface area and excellent properties, this study uses a custom-made copper wire needleless dynamic linear electrode electrospinning machine [38] (patent #2015208045157) to make Gr-AP porous nanofiber membranes out of PVA, SA, and Gr. The optimal Gr content, collection distance, and electrospinning voltage are adjusted based on the morphology and diameter distribution of the nanofibers. Moreover, the influence of the presence of Gr is studied in terms of electrical properties, hydrophilicity, and thermal stability of the nanofiber membranes. This study achieves its goal of the preparation of Gr-AP nanofiber membranes with a high production yield. This process can serve as a valuable stepping stone for the newly and highly efficient manufacture of nanofiber membranes for man-made neutral conduits. 2. Experimental Section 2.1. Materials Polyvinyl alcohol (PVA, Mw = 84,000–89,000) was purchased from Changchun Chemical, Changchun, China. Sodium alginate (SA, purity = 90%) and polyvinylpyrrolidone (PVP, K13-18, Mr = 10,000) were purchased from Shanghai Macklin Biochemical, Shanghai, China. P-ML20Multi-layered graphene (Gr) was provided from Enerage, Taiwan. The average thickness of Gr is 50–100 nm (Figure 1), specific surface area is 700 S/m. Deionized water was used in this study.
Polymers 2018, 10, 1167
3 of 15
Polymers 2018, 10, x FOR PEER REVIEW
3 of 15
Figure 1. SEM and TEM observations of graphene. Figure 1. SEM and TEM observations of graphene. 2.2. Formulation of and Gr and Gr-AP Suspensions 2.2. Formulation of Gr Gr-AP Suspensions Gr was firstlydispersed dispersed into 1%1% PVPPVP dispersing agent, and then underwent oscillation Gr was firstly into dispersing agent, and then ultrasonic underwent ultrasonic at 40 kHz for 2 h to obtain the stabilized Gr solutions. Moreover, 13.5 g of PVA was added to Gr oscillation at 40 kHz for 2 h to obtain the stabilized Gr solutions. Moreover, 13.5 g of PVA was added solutions and then stirred with a magnetic heating stirrer at 90 ◦ C for 2 h, formulating PVA/Gr to Gr solutions and then stirred with a magnetic heating stirrer at 90 °C for 2 h, formulating PVA/Gr suspensions with diverse concentrations. Finally, 2% of SA solution (20 mL) was added to the PVA/Gr suspensions with diverse concentrations. 2%forming of SA solution (20 mL) was added the PVA/Gr suspension and then stirred at 60 ◦ C for Finally, 2 h, evenly PVA/SA/Gr suspensions withtodifferent suspension and then stirred at 60 °C evenly forming PVA/SA/Gr Gr contents. The concentrations offor Gr 2inh, PVA/SA/Gr suspensions were 0,suspensions 0.0375, 0.075,with 0.25, different 0.5, Gr contents. Gr in PVA/SA/Gr suspensions were 0,suspensions 0.0375, 0.075, 0.25, 0.5, and 0.75 The wt.%concentrations respectively. Theofviscosity and conductivity of PVA/SA/Gr are shown in and Tablerespectively. 1. 0.75 wt% The viscosity and conductivity of PVA/SA/Gr suspensions are shown in Table 1. Table 1. Viscosity and conductivity of PVA/SA/Gr suspensions with different Gr content.
Table 1. Viscosity and conductivity of PVA/SA/Gr suspensions with different Gr content. Gr Content (wt.%)
0
Gr Content (wt%) 0 Viscosity (±0.1 mPa·s) 331 Viscosity (±0.1 mPa·s) −1 ) 865331 Conductivity (±0.01 µS·cm −1 Conductivity (±0.01 μS·cm ) 865
0.0375
0.0375 320 320 795 795
0.075
0.075 340 340 818 818
0.25
0.25 346.2 346.2 858 858
0.50
0.50 348.9 348.9 878 878
0.75
0.75 397 397900 900
2.3. Preparation of Linear Electrospinning Nanofiber Membranes
2.3. Preparation of Linear Electrospinning Nanofiber Membranes
PVA/SA/Gr suspensions with different Gr contents were electrospun into AP and Gr-AP nanofiber membranes for 5 hwith using different a custom-made copper wire needleless dynamic linear PVA/SA/Gr suspensions Gr contents were electrospun into APelectrode and Gr-AP electrospinning machine (Figure 2). The collection distance was set as 25, 27.5 and 30 cm, respectively. nanofiber membranes for 5 h using a custom-made copper wire needleless dynamic linear electrode Additionally,machine the voltage was set 70,collection 75 and 80 kV, respectively. membranes dried in an electrospinning (Figure 2).as The distance was setThe as 25, 27.5 andwere 30 cm, respectively. ◦ oven at 60 C for 2 h and evaluated in terms of the morphology and property evaluations. The whole Additionally, the voltage was set as 70, 75 and 80 kV, respectively. The membranes were dried in an process of the preparation of linear electrospinning nanofiber membranes are shown in Figure 3. oven at 60 °C for 2 h and evaluated in terms of the morphology and property evaluations. The whole The electrospinning principle was as follows: Four 0.8-mm-diameter copper wires are used as the process of the preparation of lineartoelectrospinning nanofiber membranes are shown Figure linear electrode that is connected a high voltage and immersed in a suspension. A motorinrotates the3. The electrospinning principle was as follows: Four 0.8-mm-diameter copper wires are used as the copper linear electrodes, ensuring a complete adhesion of the electrospinning solution. The electrostaticlinear electrode that is connected a linear high electrode voltage and immersed in a suspension. A motor rotates force is strengthened whentothe approaches the collector, and the spherical liquid over the copper electrodes, a complete of and the the electrospinning solution. the linear electrode eventually ensuring forms a Taylor cone. The adhesion triggered jets electrical jets become fine The nanofibers because of the drawing force, during which the solvent evaporates, and the nanofibers electrostatic force is strengthened when the linear electrode approaches the collector, can and the deposit onto the collector irregularly to form a nanofiber membrane. spherical liquid over the electrode eventually forms a Taylor cone. The triggered jets and the electrical
jets become fine nanofibers because of the drawing force, during which the solvent evaporates, and the nanofibers can deposit onto the collector irregularly to form a nanofiber membrane.
Polymers 2018, 10, 1167
4 of 15
Polymers 2018, 10, x FOR PEER REVIEW Polymers 2018, 10, x FOR PEER REVIEW
4 of 15 4 of 15
Figure 2. Schematic diagram of the needleless linear electrospinning.
Figure 2.Figure Schematic diagram needleless electrospinning. 2. Schematic diagramof of the the needleless linearlinear electrospinning.
Figure 3. Process of the preparation of linear electrospinning nanofiber membranes.
3. Process the preparationofoflinear linear electrospinning nanofiber membranes. Figure 3.Figure Process of theofpreparation electrospinning nanofiber membranes.
2.4. Measurements and Characterizations
2.4. Measurements and Characterizations 2.4. Measurements and Characterizations
The morphology of AP and Gr-AP nanofiber membranes was observed using scanning electron
The morphology of AP and Gr-AP nanofiber membranes was observed using scanning electron
microscopy (SEM, TM3030, HITACHI, Tokyo, Japan). The SEM images analyzedusing using ImageThe morphology of AP and HITACHI, Gr-AP nanofiber membranes waswere observed scanning electron microscopy (SEM, TM3030, Japan). The SEM analyzed using ImagePro Plus 6.0 software. A bundle of 100 Tokyo, nanofibers per image wereimages used towere measure the diameter of microscopyPro (SEM, TM3030, The SEM images analyzed usingofImage-Pro Plus 6.0 software. A bundle ofTokyo, 100 per image were used measure thestandard diameter the nanofibers, and HITACHI, Origin was used tonanofibers plotJapan). the diameter distribution and towere compute the thedeviations. nanofibers, and Origin to plot the diameter distribution and toato compute the standard functional groups of nanofiber membranes were analyzed using NICOLET iS10 FT- diameter of Plus 6.0 software. AThe bundle ofwas 100used nanofibers per image were used measure the IR spectrometer (Thermogroups FisherofScientific, Waltham, MA,were USA), and the surface resistivityiS10 of FTdeviations. The functional nanofiber membranes analyzed using a NICOLET the nanofibers, and Origin was used to plot the diameter distribution and to compute the standard membranes wasFisher measured using surface resistance instrument OHM-STAT, IR nanofiber spectrometer (Thermo Scientific, Waltham, MA, USA), and (RT-1000, the surface resistivity of deviations.nanofiber The groups of nanofiber membranes were analyzed using a NICOLET iS10 Staticfunctional Solutions Inc, Hudson, NY, USA), examining the influence of Gr content on the conductive membranes was measured using surface resistance instrument (RT-1000, OHM-STAT, threshold. The crystalline structure of the AP and Gr-AP nanofiber membranes was characterized by FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), and the surface resistivity of Static Solutions Inc, Hudson, NY, USA), examining the influence of Gr content on the conductive X-ray diffraction (D8 DISCOVER, BRUKER, Billerica, MA,nanofiber USA) withmembranes CuKa radiation (λ = 1.5406 Å). by threshold. The crystalline structure of the AP and Gr-AP was characterized nanofiber membranes was measured using surface resistance instrument (RT-1000, OHM-STAT, The wettability and hydrophilicity of nanofiber membranes were characterized by the water contact X-ray diffraction (D8 DISCOVER, BRUKER, Billerica, MA, USA) with CuKa radiation (λ =on 1.5406 Static Solutions Hudson, USA), the(JC2000DM, influence of Gr content theÅ). conductive angleInc, measured using aNY, surface contactexamining angle instrument Shanghai Zhongchen Digital The wettability and hydrophilicity of nanofiber membranes were characterized by the water contact Technic Apparatus, Shanghai, of China) and Image-Pro Plus 6.0 software. The contact anglewas between threshold. angle The crystalline structure the AP and Gr-AP nanofiber membranes characterized by measured using a surface contact angle instrument (JC2000DM, Shanghai Zhongchen Digital the droplet at first, second and surface of samples was measured five times to calculate the mean. The X-ray diffraction (D8 DISCOVER, BRUKER, Billerica, MA, USA) withThe CuKa radiation (λ = 1.5406 Å). Technic Apparatus, China) andwas Image-Pro Plus 6.0 software. contact angle between melting pattern of Shanghai, nanofiber membranes measured using a differential scanning calorimetry the(DSC200F3, droplet first, secondBavaria, and surface of samples was measured five times to calculate the mean. The The wettability andathydrophilicity of Germany) nanofiber membranes characterized by the contact NETZSCH, with nitrogen as thewere shielding gas. Samples of 5–10 mgwater melting patternfrom of nanofiber membranestowas measured using The a differential scanning calorimetry were heated the room temperature 300 °C at 10 °C/min. thermal stability of nanofiber angle measured using a surface contact angle instrument (JC2000DM, Shanghai Zhongchen Digital (DSC200F3, NETZSCH, Bavaria, Germany) with nitrogen as the shielding Samples Bavaria, of 5–10 mg membranes was measured using a thermogravimetric analyzer (TG 209F3,gas. NETZSCH, Technic Apparatus, China) and Image-Pro Plus 6.0 software. The contact angle between were heatedShanghai, from the room 300 Samples °C at 10 °C/min. Thewere thermal stability of nanofiber Germany) with nitrogen astemperature the shieldingtogas. of 5–10 mg heated from the room the dropletmembranes at first, second and surface of samples was measured five times to calculate was measured using a thermogravimetric analyzer (TG 209F3, NETZSCH, Bavaria, the mean. Germany) with nitrogen asmembranes the shielding was gas. Samples of 5–10 mgawere heated from the room The melting pattern of nanofiber measured using differential scanning calorimetry (DSC200F3, NETZSCH, Bavaria, Germany) with nitrogen as the shielding gas. Samples of 5–10 mg were heated from the room temperature to 300 ◦ C at 10 ◦ C/min. The thermal stability of nanofiber membranes was measured using a thermogravimetric analyzer (TG 209F3, NETZSCH, Bavaria, Germany) with nitrogen as the shielding gas. Samples of 5–10 mg were heated from the room temperature to 700 ◦ C at a rate of 10 ◦ C/min. The adsorption and surface area were characterized using a Quantachrome Autosorb instrument (IQ-C, KANGTA, Boynton Beach, FL, USA) with nitrogen as the adsorbate. The Barret-Joyner-Halenda (BJH) model was used to determine the distribution of mesopores.
using a Quantachrome Autosorb instrument (IQ-C, KANGTA, Boynton Beach, FL, USA) with nitrogen as the adsorbate. The Barret-Joyner-Halenda (BJH) model was used to determine the distribution of mesopores. 3. Results and Discussion Polymers 2018, 10, 1167
5 of 15
3.1. Effect of Gr Content, Collection Distance and Voltage on Morphology and Diameter of Gr-AP Nanofiber 3. Results and Discussion Membranes 3.1. Effect of Gr Content, Collection Distance and VoltageGr-AP on Morphology and membranes Diameter of Gr-AP Figure 4 shows the SEM images of electrospun nanofiber containing 0.0375 Nanofiber Membranes wt% (0.0375Gr-AP). A large number of droplets were adsorbed over the surface of nanofibers when Figure 4 shows the SEM of electrospun Gr-APexhibit nanofiber membranes containing the collection distance is small, andimages the nanofiber membrane many beads as seen in Figure 0.0375 wt.% (0.0375Gr-AP). A large number of droplets were adsorbed over the surface of nanofibers 4a–c. As the collection distance increases, the morphology of the nanofibers improved. This is because when the collection distance is small, and the nanofiber membrane exhibit many beads as seen in the attenuating attraction of static electricity allows a longer time for droplets to be drawn, which Figure 4a–c. As the collection distance increases, the morphology of the nanofibers improved. This is gives the solvent more time to evaporate. However, an excessive collection distance debilitates the because the attenuating attraction of static electricity allows a longer time for droplets to be drawn, electrical fields and produces unstable electrospinning jets, which render nanofibers with a larger which gives the solvent more time to evaporate. However, an excessive collection distance debilitates diameter and an uneven surface, as unstable shown in Figure 4g. The morphology is related to the the electrical fields and produces electrospinning jets,nanofibers which render nanofibers with a larger applied field (E∞ according Coulomb’s law,4g.and Gr-AP nanofiber membranes diameter and =anV/D) uneven surface, to as shown in Figure Thethe nanofibers morphology is related to present the optimal microstructure within an appropriate distance (D)the and voltage (V). As seen in Figure 4, the applied field (E∞ = V/D) according to Coulomb’s law, and Gr-AP nanofiber membranes present optimal microstructure withinoccurs an appropriate distance (D) and voltage (V). As seen in Figure 4, the of optimal membrane morphology at 27.5 cm (D) and 70 kV (V). The regulation of structure optimal membrane morphology occurs at 27.5 cm (D) and 70 kV (V). The regulation of structure of Gr-AP nanofiber membranes with other Gr contents was consistent with that of 0.0375 Gr-AP Gr-AP nanofiber membranes with otherand Gr contents wasdistribution consistent with that of 0.0375 Gr-APmembranes nanofiber at nanofiber membranes. The morphology diameter of Gr-AP nanofiber membranes. The morphology and diameter distribution of Gr-AP nanofiber membranes at optimal optimal distance and voltage are displayed in Figure 5. It can be seen in Figure 5 that at the optimal distance and voltage are displayed in Figure 5. It can be seen in Figure 5 that at the optimal distance distance and voltage, all the Gr-AP suspensions with different Gr content can be successfully and voltage, all the Gr-AP suspensions with different Gr content can be successfully electrospun electrospun into nanofibers and that the nanofibers exhibited a smooth surface with few beads and into nanofibers and that the nanofibers exhibited a smooth surface with few beads and uniform fiber uniform fiber distribution. diameter distribution. The massmade production made a prominent improvement with a diameter The mass production a prominent improvement with a yield of Gr-AP yield of Gr-AP membrane reaching 1.25 g/h, which is higher than needle electrospun Gr-AP nanofiber membrane reaching 1.25 g/h, which is higher than needle electrospun Gr-AP nanofiber membranes membranes indicated by Golafshan indicated by Golafshan et al. [19]. et al. [19].
FigureFigure 4. SEM images of Gr-AP nanofiber containing0.0375 0.0375 wt% collection 4. SEM images of Gr-AP nanofibermembranes membranes containing wt.% of of Gr.Gr. TheThe collection distance 25, 27.5, 30 cm, and electrospinning voltage and 80 80 kV.kV. distance is 25,is27.5, andand 30 cm, and thethe electrospinning voltageisis70, 70,75,75, and
Polymers 2018, 10, 1167
6 of 15
Polymers 2018, 10, x FOR PEER REVIEW
6 of 15
(a) AP Percentage/%
40 30 20 10 0 50
100
150
200
250
300
Fiber size/nm
350
400
60
(b)0.0375GrPercentage/%
50 40 30 20 10 0 50
(c)0.075 Gr-AP
100
150
Fiber size/nm
200
250
Percentage/%
60
40
20
0 50
100
150
Fiber size/nm
200
250
50
(d) 0.25Gr-AP Percentage/%
40 30 20 10 0 100
150
200
250
Fiber size/nm
300
350
40
(e) 0.5Gr-AP
35
Percentage/%
30 25 20 15 10 5 0 50
100
150
200
250
Fiber size/nm
300
350
400
35
(f) 0.75Gr-AP
30
Percentage/%
25 20 15 10 5 0 50
100
150
200
250
Fiber size/nm
300
350
400
Figure5.5.SEM SEMimages imagesand andfiber fiberdiameter diameter distributions AP-Gr membranes with different Figure distributions ofof AP-Gr membranes with different GrGr contents. contents.
Polymers 2018, 10, 1167
7 of 15
Figure 6 shows the average diameter and standard deviation (SD) of Gr-AP nanofiber membranes as related to different Gr concentrations. With the increase of Gr content, the diameter of nanofibers proportionally increases, and the nanofibers also have an increasingly uneven surface. The viscosity and conductivity of PVA/SA/Gr suspensions is related to the fiber diameter according to the electro hydrodynamic model. The viscosity of PVA/SA/Gr mixture increased with the addition of Gr as seen in Table 1, and higher voltage was required to obtain the stretched jet. While the applied electric field (V/D) is constant, the spinning process is more difficult and as a result, and the mean diameter of the fibers increases [39]. At less than 0.25 wt.% of Gr, the finest Gr-AP nanofibers of 141 ± 31 nm, is much smaller than that of Gr-AP membrane produced by the needle electrospining (296 ± 40 nm) proposed by Golafshan et al. [19], and the overall nanofibers have good morphology and even diameters. The smaller diameter of nanofibers is due to multiple factors, a small amount of Gr improves the conductivity of the suspension, rendering droplets over the linear electrodes with considerable electric charges. Simultaneously, the repulsion between the electric charges decreases the surface tension of the liquid, which makes the nanofibers subject to splitting during electrospinning. When surpassing 0.25 wt.%, an excessive amount of Gr aggregate and generate nodes over the nanofibers. Subsequently, the conductivity of the suspension increases, which is detrimental to the electrospinning due to whipping instability, and results in coarser and more uneven diameter of fibers [40].
Figure 6. Diameter of nanofibers of Gr-AP nanofiber membranes as related to the Gr content.
3.2. Far Infrared Ray Spectrum of Gr-AP Nanofiber Membranes Figure 7 shows that Gr-AP and AP nanofiber membranes have comparable spectra of absorbance. The presence of characteristic peak at 2850 cm−1 indicated the stretching vibration between the C–H and O–H from PVA, SA, and the residual water. The absorption peak at 2210.9 cm−1 corresponded to C=O for –COOH. The peak of the stretching vibration of C=C was presented at wave numbers of 1992.2 cm−1 , while the peak of the stretching vibration of C=O for C–O–C was presented at wave numbers of 1119.8 cm−1 [41]. The corresponding characteristic peaks of the functional groups of AP and Gr-AP nanofiber membranes were presented, suggesting that Gr and PVA/SA suspensions were successfully mixed.
8 of 15
Polymers Polymers 2018, 2018, 10, 10, xx FOR FOR PEER PEER REVIEW REVIEW
88 of of 15 15
Absorbance/% /% Absorbance
Polymers 2018, 10, 1167
a: a: AP AP b: b: 0.0375Gr-AP 0.0375Gr-AP c: c: 0.075Gr-AP 0.075Gr-AP d: d: 0.25Gr-AP 0.25Gr-AP e: e: 0.5Gr-AP 0.5Gr-AP f: f: 0.75Gr-AP 0.75Gr-AP
ff ee dd cc
bb aa
3500 3500
3000 3000
2500 2500
2000 2000
1500 1500
-1 -1
Wavenumber/cm Wavenumber/cm
1000 1000
Figure Figure 7. 7. FIR FIRspectrum spectrum of of Gr-AP Gr-AP nanofiber nanofiber membranes membranes as Figure 7. FIR spectrum of Gr-AP nanofiber membranes as related related to to the the Gr Gr content. content.
3.3. X-Ray Diffraction Diffraction of Gr-AP Gr-AP Nanofiber Membranes Membranes 3.3. 3.3. X-Ray X-Ray Diffraction of of Gr-AP Nanofiber Nanofiber Membranes In identify the existence of Gr in the electrospun nanofibers, XRD results are displayed in In XRD In order order to to identify the the existence existence of of Gr Gr in in the the electrospun electrospun nanofibers, nanofibers, XRD results results are are displayed displayed ◦ , indicating the interaction and Figure 8. AP membrane exhibited a broad diffraction peak at 2θ = 19.6 in Figure 8. AP membrane exhibited a broad diffraction peak at 2θ = 19.6°, indicating the interaction in Figure 8. AP membrane exhibited a broad diffraction peak at 2θ = 19.6°, indicating the interaction blending between SA and PVA. In addition, this broad peak is because is hydrogen bond interactions and and blending blending between between SA SA and and PVA. PVA. In In addition, addition, this this broad broad peak peak is because because hydrogen hydrogen bond bond between –OH and –COOH from–COOH SA and between –OH groups formed non-crystalline nanofibers [19]. interactions between –OH and from SA and between –OH groups formed non-crystalline interactions between –OH and –COOH from SA and between –OH groups formed non-crystalline ◦ for AP membrane. After Gr addition, these Furthermore, a sharp diffraction peak occurred at 2θ = 27 nanofibers at nanofibers [19]. [19]. Furthermore, Furthermore, aa sharp sharp diffraction diffraction peak peak occurred occurred at 2θ 2θ == 27° 27° for for AP AP membrane. membrane. After After ◦ twoaddition, diffraction peaks shifted towards to the left (2θ = 19.1 toand 2θ =(2θ 26.5=◦ 19.1° ), andand the diffraction intensity Gr these two diffraction peaks shifted towards the left 2θ Gr addition, these two diffraction peaks shifted towards to the left (2θ = 19.1° and 2θ == 26.5°), 26.5°), and and the the also changed. This is because Gr addition caused Gr a larger lattice constant [19] lattice which constant confirms[19] the diffraction intensity also changed. This is because addition caused a larger diffraction intensity also changed. This is because Gr addition caused a larger lattice constant [19] existence of Gr. the existence of Gr. which which confirms confirms the existence of Gr.
Figure Figure 8. 8. XRD XRDpattern patternof of Gr-AP Gr-AP nanofiber nanofiber membranes membranes as Figure 8. XRD pattern of Gr-AP nanofiber membranes as related related to to the the Gr Gr content. content.
3.4. Electrical Properties Properties of Gr-AP Gr-AP Nanofiber Membranes 3.4. 3.4. Electrical Electrical Properties of of Gr-AP Nanofiber Nanofiber Membranes The surface resistivity resistivity of Gr-AP Gr-AP nanofiber membranes membranes as related related to the the Gr content content is shown shown in The The surface surface resistivity of of Gr-AP nanofiber nanofiber membranes as as related to to the Gr Gr content is is shown in in Figure 9. Compared to AP membrane, Gr significantly decreased the surface resistance and improved Figure Figure 9. 9. Compared Compared to to AP AP membrane, membrane, Gr Gr significantly significantly decreased decreased the the surface surface resistance resistance and and improved improved the conductivity of of membranes. With With the addition addition ofGr, Gr, the surface surface resistivity first first decreases sharply sharply the the conductivity conductivity of membranes. membranes. With the the addition of of Gr, the the surface resistivity resistivity first decreases decreases sharply wt.% Gr)and and then increases increases (0.075–0.25 wt% wt.% Gr)and and slightly decreases decreases (0.25–0.75 wt% wt.% Gr). (0–0.075 (0–0.075 wt% wt% Gr) Gr) and then then increases (0.075–0.25 (0.075–0.25 wt% Gr) Gr) and slightly slightly decreases (0.25–0.75 (0.25–0.75 wt% Gr). Gr). 9 When Gr content is 0.075 wt%, the surface resistivity remarkably decreased to 3.13 × 10 . This 9 When Gr content is 0.075 wt%, the surface resistivity remarkably decreased to 3.13 × 10 . This is is because because aa low low Gr Gr content content allows allows Gr Gr to to form form broad broad conductive conductive paths paths in in aa network network that that accelerates accelerates
Polymers 2018, 10, x FOR PEER REVIEW Polymers 2018, 10, 1167
9 of 15 9 of 15
electric conduction. As Gr increases continuously (>0.075 wt%), the surface resistance increases first When Grslightly, content is while 0.075 wt.%, surface resistivity remarkably to 3.13 × 109 . This is and then decreases the the conductivity decreases firstdecreased and then increases correspondingly. because a low Gr content allows Gr to form broad conductive paths in a network that accelerates This result iselectric due to the aggregation of Gr in excessive which in turnincreases produces conduction. As Gr increases continuously (>0.075amounts, wt.%), the surface resistance first impedance and then nanofiber decreases slightly, while the conductivity decreases first and then increases correspondingly. paths. In addition, composites demonstrate a percolation phenomenon, which involves a This result is due to the aggregation of Gr in excessive amounts, which in turn produces impedance balanced critical concentration of the interacting materials. At a percolation threshold, the network paths. In addition, nanofiber composites demonstrate a percolation phenomenon, which involves a that transmits electrons isconcentration dependentofon coagulation [42]. It can bethe surmised balanced critical thethe interacting materials.ofAtfillers a percolation threshold, network that Gr has a critical concentration 0.075 iswt% as anon electrical percolation threshold, the adjacent Gr forms that transmits of electrons dependent the coagulation of fillers [42]. It can beand surmised that Gr has a critical concentration of 0.075 wt.% as an electrical percolation threshold, and the adjacent Gr a tunneling effect [43]. As a result, the conductivity of Gr-AP nanofiber membranes containing 0.075 forms a tunneling effect [43]. As a result, the conductivity of Gr-AP nanofiber membranes containing wt% Gr or more is ten APthat nanofiber membranes. 0.075 wt.% Grtimes or morethat is tenof times of AP nanofiber membranes.
Figure 9.resistivity Surface resistivity of Gr-AP nanofiber membranes as related to the Grto content. Figure 9. Surface of Gr-AP nanofiber membranes as related the Gr content.
3.5. Water Contact Angle of Gr-AP Nanofiber Membranes
3.5. Water Contact Angle of Gr-AP Nanofiber Water contact angle was applied toMembranes investigate the hydrophilic properties of Gr-AP nanofiber membranes (Figure 10). Due to hydrophilic nature, AP membranes revealed low hydrophilic properties
Water contact angle was applied to investigate the hydrophilic properties of Gr-AP nanofiber at a water contact angle of 45◦ ± 1.6◦ . Moreover, water contact angle enhanced significantly in membranes relation (Figure 10). Duedemonstrating to hydrophilic AP membranes revealed low hydrophilic to Gr content, superior nature, hydrophobicity. For the Gr-AP nanofiber membrane ◦ ± 1.3◦ , a factor of 20◦ higher containing 0.75 wt.% Gr, the water contact angle increased to 81.14 properties at a water contact angle of 45° ± 1.6°. Moreover, water contact angle enhanced significantly than needle electrospun AP-Gr membrane at the same Gr content [19]. The presence of Gr and more in relation compact to Gr nanofiber content,structure demonstrating superior hydrophobicity. For the Gr-AP nanofiber could explain the difference between them. As described in Figure 6, membrane containing 0.75 of wt% Gr,membranes the water contact angle increased to 81.14° ± 1.3°, a factor of 20° the fiber diameters AP-Gr formed compact structures. The increased hydrophobicity is advantageous for nerve tissuemembrane engineering applications. one hand, hydrophilic higher than needle electrospun AP-Gr at the sameOn Grthe content [19].theThe presence of Gr and scaffold has the important characteristic of a high degradation rate, which greatly decreased the more compact nanofiber structure could explain the difference between them. As described in Figure mechanical property in the tissue regeneration process, and thus limits the application of fiber scaffold 6, the fiber diameters of AP-Gr membranes formed compact structures. The increased in nerve tissue engineering [44]. On the other hand, the hydrophobic nanofiber membranehydrophobicity can effectively inhibit the cell proliferation, and the hydrophobic inner shell can guarantee the resistance to is advantageous for nerve tissue engineering applications. On the one hand, the hydrophilic scaffold fibroblasts [45,46]. Therefore, the contact angle result confirms the Gr addition can effectively enhance has the important characteristic of a high degradation rate, which greatly decreased the mechanical the hydrophobicity of fibrous membranes, which is beneficial to the nanofiber membrane applied to property in the tissue process, and thus limits the application of fiber scaffold in nerve neural tissue regeneration engineering. tissue engineering [44]. On the other hand, the hydrophobic nanofiber membrane can effectively inhibit the cell proliferation, and the hydrophobic inner shell can guarantee the resistance to fibroblasts [45,46]. Therefore, the contact angle result confirms the Gr addition can effectively enhance the hydrophobicity of fibrous membranes, which is beneficial to the nanofiber membrane applied to neural tissue engineering.
Polymers 2018, 10, 1167
10 of 15
Polymers 2018, 10,x xFOR FORPEER PEERREVIEW REVIEW Polymers 2018, 10,
1010ofof 1515
Figure 10. Water contact angle ofofGr-AP Gr-AP nanofiber membranes asasrelated related totoGr Gr content. Figure10. 10.Water Watercontact contactangle angleof Gr-APnanofiber nanofibermembranes membranesas relatedto Grcontent. content. Figure
Exothermic Exothermic
-1 DSC/(mW·mg DSC/(mW·mg-1))
3.6. Thermal Properties of Gr-AP Nanofiber Membranes 3.6.Thermal ThermalProperties PropertiesofofGr-AP Gr-APNanofiber NanofiberMembranes Membranes 3.6. Figure 11 shows that DSC curves of AP and Gr-AP nanofiber membranes display similar, and both Figure1111shows showsthat thatDSC DSCcurves curvesofofAP APand andGr-AP Gr-APnanofiber nanofibermembranes membranesdisplay displaysimilar, similar,and and Figure have a melting peak within 220–230 ◦ C. The melting peak of AP nanofiber membranes was 227.1 ◦ C, both have a melting peak within 220–230 °C. The melting peak of AP nanofiber membranes was 227.1 both have a melting peak within 220–230 °C. The melting peak of AP nanofiber membranes was 227.1 and the addition of Gr shifted the melting peak toward higher temperatures. Adversely, a Gr content °C,and andthe theaddition additionofofGr Grshifted shiftedthe themelting meltingpeak peaktoward towardhigher highertemperatures. temperatures.Adversely, Adversely,a aGr Gr °C, that exceeds 0.075 wt.% shifted the melting peak toward lower temperatures. The results may be contentthat thatexceeds exceeds0.075 0.075wt% wt%shifted shiftedthe themelting meltingpeak peaktoward towardlower lowertemperatures. temperatures.The Theresults resultsmay may content ascribed to the dispersion of Gr. As –COOH and –OH are generated over the surface of Gr, –OH ascribedtotothe thedispersion dispersionofofGr. Gr.As As–COOH –COOHand and–OH –OHare aregenerated generatedover overthe thesurface surfaceofofGr, Gr,–OH –OH bebeascribed and its oxgenous functional groups of the PVA molecular chains are hydrogen bonded, the resulting anditsitsoxgenous oxgenousfunctional functionalgroups groupsofofthe thePVA PVAmolecular molecularchains chainsare arehydrogen hydrogenbonded, bonded,the theresulting resulting and interaction adversely affects the mobility of PVA and SA molecular chains [47]. By contrast, an excessive interactionadversely adverselyaffects affectsthe themobility mobilityofofPVA PVAand andSA SAmolecular molecularchains chains[47]. [47].By Bycontrast, contrast,anan interaction Gr content causes agglomeration and then decreases the melting point. excessiveGr Grcontent contentcauses causesagglomeration agglomerationand andthen thendecreases decreasesthe themelting meltingpoint. point. excessive
160 160
AP AP 0.0375Gr-AP 0.0375Gr-AP 0.075Gr-AP 0.075Gr-AP 0.25Gr-AP 0.25Gr-AP 0.5Gr-AP 0.5Gr-AP 0.75Gr-AP 0.75Gr-AP
180 180
200 200
220 220
240 240
Temperature/°C Temperature/°C Figure11. 11.DSC DSCcurves curvesof ofGr-AP Gr-APnanofiber nanofibermembranes membranesas asrelated relatedto theGr Grcontent. content. Figure Figure 11. DSC curves of Gr-AP nanofiber membranes as related totothe the Gr content.
Polymers 2018, 10, 1167
11 of 15
Polymers 2018, 10, x FOR PEER REVIEW
11 of 15
Figure 12 12 shows shows TG TG curves curves that that demonstrate demonstrate three three weight weight loss loss phases. phases. The The first first phase phase occurred occurred Figure ◦ C. It was caused by the evaporation of the residual when the temperature was between 50 and 100 when the temperature was between 50 and 100 °C. It was caused by the evaporation of the residual water or thethe weight lossloss was was relatively smaller. The second weight loss phase occurred water or the thesolvent, solvent,and and weight relatively smaller. The second weight loss phase ◦ C. A considerable thermal decomposition of due to the degradation peaks between 100 and 350 occurred due to the degradation peaks between 100 and 350 °C. A considerable thermal nanofiber membranes occurredmembranes in this phase,occurred due to theinbreakage of macromolecular of PVA decomposition of nanofiber this phase, due to the chains breakage of and SA. Comparing the slopes, the maximum decomposition rate was also seen in this phase with macromolecular chains of PVA and SA. Comparing the slopes, the maximum decomposition rate ◦ when the breakage of a weight loss of The third weight loss loss of phase occurred at 350–600 was also seen in 55–75%. this phase with a weight 55–75%. The third weightCloss phase occurred at molecular andbreakage cyclization rendered to PVA and SA. More molecular of and residual 350–600 °Cchains when the of were molecular chains and cyclization were renderedchains to PVA SA. polymer polyenes further broke and were converted into micromolecule polyenes, with a weight More molecular chains of residual polymer polyenes further broke and were converted into loss of 10–20% polyenes, [48]. micromolecule with a weight loss of 10–20% [48]. 100
100 98
80
96
Mass /%
94
60
92 90
40 20 0
88 210 220 230 240 250 260 270
AP 0.0375Gr-AP 0.075Gr-AP 0.25Gr-AP 0.5Gr-AP 0.75Gr-AP
100
200
300
400
500
600
700
Temperature/°C Figure12. 12.TG TGcurves curvesof ofGr-AP Gr-APnanofiber nanofibermembranes membranesas asrelated related to to the the Gr Gr content. content. Figure
can be be found found from from Figure Figure 12 12 that that the the TG TG curves curves at at the the first first and and second second stages stages are are similar similar ItIt can regardless of the Gr content. As described in Table 2, at 90% mass of nanofiber membranes, the water regardless of the Gr content. As described in Table 2, at 90% mass of nanofiber membranes, the water in the completely evaporated, and PVA started Gr-AP nanofiber in the membranes membraneswas was completely evaporated, and and PVASAand SA decomposing. started decomposing. Gr-AP membranes had a higher decomposing temperature of 90% mass compared to AP membranes. nanofiber membranes had a higher decomposing temperature of 90% mass compared to AP The TG curve of TG Gr-AP nanofiber exhibits a shallower thanslope that of APthat nanofiber membranes. The curve of Gr-APmembranes nanofiber membranes exhibits a slope shallower than of AP membranes, indicatingindicating a slow degradation status. It was surmised that the presence of Gr increases nanofiber membranes, a slow degradation status. It was surmised that the presence of Gr the decomposition temperature of nanofiber membranes, thereby retarding the decomposition increases the decomposition temperature of nanofiber membranes, thereby retarding rate the and improvingrate the thermal stability.the Specifically, composed of 0.075 wt.% of Gr, the Gr-AP nanofiber decomposition and improving thermal stability. Specifically, composed of 0.075 wt% of Gr, membranes have the membranes highest decomposition temperature. The higher the Gr content, the more the Gr-AP nanofiber have the highest decomposition temperature. The higher the the Gr ◦ C. residual mass of Gr-AP nanofiber membranes, suggesting that Gr is not easily decomposed at 700 content, the more the residual mass of Gr-AP nanofiber membranes, suggesting that Gr is not easily
decomposed at 700 °C.
Table 2. DSC and TG results of Gr-AP nanofiber membranes as related to the Gr content.
Table 2. DSC and TG results of Gr-AP nanofiber membranes as related to the Gr content. Mass Change Mass Change Melting TG Residual Samples Melting ◦ T90%T90%Mass (◦ C) Change at the First MassatChange the Second at at the Mass TG Residual Point ( C) (%) Stage (%) Stage (%) Samples
Point (°C) AP 227.1 AP 227.1 0.0375Gr-AP 227.9 0.0375Gr-AP 0.075Gr-AP 227.9227.7 0.075Gr-AP 227.7226.4 0.25Gr-AP 0.5Gr-AP 0.25Gr-AP 226.4225.8 0.75Gr-AP 0.5Gr-AP 225.8225.7 0.75Gr-AP 225.7
(°C) 253.0 254.9 255.3 254.1 253.3 253.2
the First Stage (%) 72.55 72.55 63.26 63.26 63.89 63.89 62.89 62.89 59.88 59.88 58.02 58.02
253.0 254.9 255.3 254.1 253.3 253.2
Second Stage (%) 13.28 13.28 20.38 20.38 21.16 21.16 20.76 19.30 20.76 18.24 19.30 18.24
Mass (%) 12.5 12.5 14.07 14.07 14.42 14.42 14.84 19.70 14.84 22.89 19.70 22.89
Polymers 2018, 10, 1167
12 of 15
Polymers 2018, 10, x FOR PEER REVIEW
12 of 15
3.7. BET Specific Surface Area of Gr-AP Nanofiber Membranes 3.7. BET Specific Surface Area of Gr-AP Nanofiber Membranes Porous structure of Gr-AP membrane is analyzed by BET measurement. Figure 13 shows the N2 Porousisothermal structure ofcurves Gr-APofmembrane is analyzed measurement. Figure 13 shows the N2 adsorption Gr-AP membrane. ThebyNBET 2 adsorption/desorption isothermal curve adsorption isothermal curves of Gr-AP membrane. The N 2 adsorption/desorption isothermal curve presents a rapid increase when relative vapor pressure of P/P0 is lower than 0.15, then an increase to presents a rapid increase when arelative vapor pressure of P/P0 vapor is lower than 0.15, then an increase to an inflection point, and finally linear increase when relative pressure of P/P 0 is higher than an inflection point, and finally a linear increase when relative vapor pressure of P/P 0 is higher than 0.22. This adsorption isotherm curve represents an S-type isotherm and belongs to the macropore 0.22. Thisfree adsorption isothermreversible curve represents an process. S-type isotherm and belongs to the macropore (>50 nm) single multilayer adsorption The tendency is in conformity with type (>50 nm) free single multilayer reversible adsorption process. The tendency is in conformity II adsorption behavior [49,50], indicating the macropore structure of the Gr-AP membrane. Thewith BET type II adsorption behavior [49,50], indicating the macropore structure of the Gr-AP membrane. 2 /g, result further showed that the Gr-AP nanofiber membrane has high specific surface area of 5.347 mThe BET result further showed that the Gr-AP nanofiber membrane has high specific surface area of 5.347 and high specific surface area has relative high porosity, which is beneficial to cell attachment and 2/g, and high specific surface area has relative high porosity, which is beneficial to cell attachment m growth in the nerve tissue engineering. and growth in the nerve tissue engineering.
Volume Adsorbed(cc/g STP)
1.7
1.6
1.5
1.4
0.15
0.20
0.25
Relative Pressure(P/P0)
Figure 13. 13. N N22adsorption adsorptionisothermal isothermalcurves curvesof of0.075Gr-AP 0.075Gr-AP membrane membrane based based on on the the BJH BJH model. model. Figure
4. Conclusions Conclusions 4. In this this study, study, it it was that the the custom-made custom-made needleless needleless linear linear electrodes electrodes can can In was demonstrated demonstrated that successfully electrospin into porous well-formed Gr-AP nanofiber membranes. The test results successfully electrospin into porous well-formed Gr-AP nanofiber membranes. The test results show showthe thatpresence the presence Gr improved properties Gr-APnanofiber nanofibermembranes. membranes.The The average average that of Grofimproved the the properties of of Gr-AP diameter of Gr-AP nanofiber membranes is 141 ± 31 nm, which is 150 nm finer than of diameter of Gr-AP nanofiber membranes is 141 ± 31 nm, which is 150 nm finer than that of that needle needle electrospun nanofibers. Moreover, the achieved mass production makes over progress other electrospun nanofibers. Moreover, the achieved mass production makes progress otherover methods methods with a yield of 1.25 g/h. When made with different Gr contents, the morphology of with a yield of 1.25 g/h. When made with different Gr contents, the morphology of Gr-AP nanofiber Gr-AP nanofiber membranes can be manipulated using different voltages with a specified collection membranes can be manipulated using different voltages with a specified collection distance. With a distance. Withusing a critical value, using improvesofthe properties of Gr-AP nanofiber membranes. critical value, Gr improves theGr properties Gr-AP nanofiber membranes. Increasing Gr Increasing Gr content from 0 to 0.075 wt.% improves the morphology and homogeneity of Gr-AP content from 0 to 0.075 wt% improves the morphology and homogeneity of Gr-AP nanofiber nanofiber membranes, contributing hydrophobicity and thermal stability those pure AP AP membranes, contributing greater greater hydrophobicity and thermal stability thanthan those ofofpure nanofiber membranes. Gr Gr content is 0.075 conductivity of Gr-AP nanofiber nanofiber membranes.When When content is wt.%, 0.075 the wt%, the conductivity of Gr-APmembranes nanofiber remarkably changes and then stabilizes. Conversely, the Gr content that exceeds wt.% renders membranes remarkably changes and then stabilizes. Conversely, the Gr content 0.075 that exceeds 0.075 uneven diameter of nanofibers and weakened thermal stability of nanofiber membranes. Hence, a Gr wt% renders uneven diameter of nanofibers and weakened thermal stability of nanofiber membranes. content of 0.075 wt.% is presumed to be the threshold when producing Gr-AP nanofiber membranes. Hence, a Gr content of 0.075 wt% is presumed to be the threshold when producing Gr-AP nanofiber As a result, the achieved a achieved highly efficient production of Gr-AP nanofiber membranes. As alinear result,electrospinning the linear electrospinning a highly efficient production of Gr-AP membranes. This study serves as a valuable reference to prepare nanofibers for neural conduits. nanofiber membranes. This study serves as a valuable reference to prepare nanofibers for neural 2 /g, good Furthermore, Gr-AP nanofiber membranes have high specific surface area of 5.347 m conduits. Furthermore, Gr-AP nanofiber membranes have high specific surface area of 5.347 m2/g, hydrophobicity, good good biodegradability and appropriate degradation rate,rate, which willwill be used for good hydrophobicity, biodegradability and appropriate degradation which be used regenerated fibrous membrane in the nerve tissue engineering. for regenerated fibrous membrane in the nerve tissue engineering.
Polymers 2018, 10, 1167
13 of 15
Author Contributions: In this study, the concept and design for the needleless electrospinning experiment were supervised by J.-H.L. and C.-W.L. Experiment and data processing were conducted by W.X. Text composition and results analysis were performed by T.-T.L. and M.Y. The experimental results were examined by B.-C.S. Founding: This work is supported by the National Natural Science Foundation of China (grant numbers 51503145,11702187); the Natural Science Foundation of Tianjin City (grant number 18JCQNJC03400); the Natural Science Foundation of Fujian Province (grant numbers 2018J01504, 2018J01505); and the Program for Innovative Research Team in University of Tianjin (grant number TD13-5043). This work is also funded by the College Students Innovation Entrepreneurship Project of Tianjin (201710058076). Conflicts of Interest: The authors declare no conflicts of interest.
References 1. 2. 3.
4.
5. 6. 7.
8.
9.
10. 11. 12. 13.
14.
15. 16.
17.
Ficke, B.W.; Chaudhari, N.M. Nerve Injury. In Orthopedic Surgery Clerkship; Eltorai, A., Eberson, C., Daniels, A., Eds.; Springer: New York, NY, USA, 2017; pp. 302–306. Isaacs, J. Major peripheral nerve injuries. Hand Clin. 2013, 29, 371–382. [CrossRef] [PubMed] Müller, W.E.G.; Neufurth, M.; Ackermann, M.; Tolba, E.; Wang, S.; Feng, Q.; Schröder, H.C.; Wang, X. Fabrication of a new physiological macroporous hybrid biomaterial/bioscaffold material based on polyphosphate and collagen by freeze-extraction. J. Mater. Chem. B 2017, 5, 3823–3835. [CrossRef] Naahidi, S.; Jafari, M.; Logan, M.; Wang, Y.; Yuan, Y.; Bae, H.; Dixon, B.; Chen, P. Biocompatibility of hydrogel-based scaffolds for tissue engineering applications. Biotechnol. Adv. 2017, 35, 530–544. [CrossRef] [PubMed] Nectow, A.R.; Marra, K.G.; Kaplan, D.L. Biomaterials for the development of peripheral nerve guidance conduits. Tissue Eng. Part B Rev. 2012, 18, 40–50. [CrossRef] [PubMed] Santoro, M.; Shah, S.R.; Walker, J.L.; Mikos, A.G. Poly(lactic acid) nanofibrous scaffolds for tissue engineering. Adv. Drug Deliv. Rev. 2016, 107, 206–212. [CrossRef] [PubMed] Shera, S.S.; Sahu, S.; Banik, R.M. Preparation of drug eluting natural composite scaffold using response surface methodology and artificial neural network approach. Tissue Eng. Regen. Med. 2018, 15, 1–13. [CrossRef] Zhuravleva, M.; Gilazieva, Z.; Grigoriev, T.E.; Shepelev, A.D.; Tenchurin, T.K.; Kamyshinsky, R.; Krasheninnikov, S.V.; Orlov, S.; Caralogli, G.; Archipova, S.; et al. In vitro assessment of electrospun polyamide-6 scaffolds for esophageal tissue engineering. J. Biomed. Mater. Res. B 2018. [CrossRef] [PubMed] Choi, S.; Persano, L.; Camposeo, A.; Jang, J.; Koo, W.; Kim, S.J.; Cho, H.J.; Kim, I.D.; Pisignano, D. Electrospun nanostructures for high performance chemiresistive and optical sensors. Macromol. Mater. Eng. 2017, 302, 1–37. [CrossRef] Jia, L.; Zhang, H.; Wang, X. Research and development of aligned nanofibers prepared by electrospinning. J. Text. Res. 2016, 37, 159–166. Teimouri, A.; Ghorbanian, L.; Dabirian, I. Preparation and characterization of Silk/Diopside composite nanofibers via electrospinning for tissue engineering application. Tectonophysics 2016, 446, 77–96. Zhang, B.; Kang, F.; Tarascon, J.M.; Kim, J.K. Recent advances in electrospun carbon nanofibers and their application in electrochemical energy storage. Prog. Mater. Sci. 2016, 76, 319–380. [CrossRef] Zhang, H.; Xie, Z.; Wang, Y.; Shang, X.; Nie, P.; Liu, J. Electrospun polyacrylonitrile/β-cyclodextrin based porous carbon nanofiber self-supporting electrode for capacitive deionization. RSC Adv. 2017, 7, 55224–55231. [CrossRef] Kaassis, A.Y.; Young, N.; Sano, N.; Merchant, H.A.; Yu, D.G.; Chatterton, N.P.; Williams, G.R. Pulsatile drug release from electrospun poly(ethylene oxide)-sodium alginate blend nanofibers. J. Mater. Chem. B 2014, 2, 1400–1407. [CrossRef] Min, S.K.; Kim, G.H. Three-dimensional electrospun polycaprolactone (PCL)/alginate hybrid composite scaffolds. Carbohydr. Polym. 2014, 114, 213–221. Yang, J.M.; Yang, J.H.; Tsou, S.C.; Ding, C.H.; Hsu, C.C.; Yang, K.C.; Yang, C.C.; Chen, K.S.; Chen, S.W.; Wang, J.S. Cell proliferation on PVA/sodium alginate and PVA/poly(γ-glutamic acid) electrospun fiber. Mater. Sci. Eng. C 2016, 66, 170–177. [CrossRef] [PubMed] Cao, Y.M.; Wu, Q.K. Application progress of graphene, graphene derivatives and graphene composites in tissue engineering. J. Shanghai Jiaotong Univ. 2017, 37, 110–113.
Polymers 2018, 10, 1167
18.
19. 20. 21.
22. 23. 24. 25.
26. 27.
28. 29. 30. 31. 32. 33. 34. 35.
36.
37. 38. 39. 40.
14 of 15
Díezpascual, A.M.; Díezvicente, A.L. Poly(propylene fumarate)/polyethylene glycol-modified graphene oxide nanocomposites for tissue engineering. ACS Appl. Mater. Interfaces 2016, 8, 17902–17914. [CrossRef] [PubMed] Golafshan, N.; Kharaziha, M.; Fathi, M. Tough and conductive hybrid graphene-PVA: Alginate fibrous scaffolds for engineering neural construct. Carbon 2017, 111, 752–763. [CrossRef] Li, C.; Hou, T.; She, X.; Wei, X.; She, F.; Gao, W.; Kong, L. Decomposition properties of PVA/graphene composites during melting-crystallization. Polym. Degrad. Stab. 2015, 119, 178–189. [CrossRef] Pourjavadi, A.; Pourbadiei, B.; Doroudian, M.; Azari, S. Preparation of PVA nanocomposites using salep-reduced graphene oxide with enhanced mechanical and biological properties. RSC Adv. 2015, 5, 92428–92437. [CrossRef] Shadjou, N.; Hasanzadeh, M. Graphene and its nanostructure derivatives for use in bone tissue engineering: Recent advances. J. Biomed. Mater. Res. Part A 2016, 104, 1250–1275. [CrossRef] [PubMed] ˇ Cengiz-Çallıoglu, F.; Jirsak, O.; Dayik, M. Investigation into the relationships between independent and dependent parameters in roller electrospinning of polyurethane. Text. Res. J. 2013, 83, 718–729. [CrossRef] Jiang, G.; Zhang, S.; Qin, X. High throughput of quality nanofibers via one stepped pyramid-shaped spinneret. Mater. Lett. 2013, 106, 56–58. [CrossRef] Ramalingam, K.J.; Dhineshbabu, N.R.; Srither, S.R.; Saravanakumar, B.; Yuvakkumar, R.; Rajendran, V. Electrical measurement of PVA/graphene nanofibers for transparent electrode applications. Synth. Met. 2014, 191, 113–119. [CrossRef] Wang, X.; Niu, H.; Wang, X.; Lin, T. Needleless electrospinning of uniform nanofibers using spiral coil spinnerets. J. Nanomater. 2012, 2012, 785920. [CrossRef] Wang, X.; Hu, X.; Qiu, X.; Huang, X.; Wu, D.; Sun, D. An improved tip-less electrospinning with strip-distributed solution delivery for massive production of uniform polymer nanofibers. Mater. Lett. 2013, 99, 21–23. [CrossRef] Yan, G.; Niu, H.; Zhou, H.; Wang, H.; Shao, H.; Zhao, X.; Lin, T. Electro-aerodynamic field aided needleless electrospinning. Nanotechnology 2018, 29, 235302. [CrossRef] [PubMed] Yian, L.; Li, J.; Pan, Z. Research status of multi-jet electrospinning technology. J. Text. Res. 2013, 34, 150–156. Liu, Y.; He, J.H.; Yu, J.Y. Bubble-electrospinning: A novel method for making nanofibers. J. Phys. Conf. Ser. 2008, 96, 012001. [CrossRef] He, J.H.; Liu, Y.; Xu, L.; Yu, J.Y.; Sun, G. BioMimic fabrication of electrospun nanofibers with high-throughput. Chaos Solitons Fractals 2008, 37, 643–651. [CrossRef] Yarin, A.L.; Zussman, E. Upward needleless electrospinning of multiple nanofibers. Polymer 2004, 45, 2977–2980. [CrossRef] Wang, X.; Niu, H.; Lin, T.; Wang, X. Needleless electrospinning of nanofibers with a conical wire coil. Polym. Eng. Sci. 2010, 49, 1582–1586. [CrossRef] Senturk-Ozer, S.; Aktas, S.; He, J.; Fisher, F.T.; Kalyon, D.M. Nanoporous nanocomposite membranes via hybrid twin-screw extrusion-multijet electrospinning. Nanotechnology 2017, 28, 025301. [CrossRef] [PubMed] Li, Y.; Zhang, P.; Ouyang, Z.; Zhang, M.; Lin, Z.; Li, J. Nanoscale graphene doped with highly dispersed silver nanoparticles: Quick synthesis, facile fabrication of 3D membrane-modified electrode, and super performance for electrochemical sensing. Adv. Funct. Mater. 2016, 26, 2122–2134. [CrossRef] Su, Z.; Li, J.; Li, Q.; Ni, T.; Wei, G. Chain conformation, crystallization behavior, electrical and mechanical properties of electrospun polymer-carbon nanotube hybrid nanofibers with different orientations. Carbon 2012, 50, 5605–5617. [CrossRef] Li, T.T.; Yan, M.; Wu, Z.H. The spinnability study of the dynamic linear electrode electrospun PVA nanofibers. Mater. Rev. 2018, 24. in press (In Chinese) Lin, J.H.; Lou, C.W.; Jiang, K.J. Electrospinning Group. Patent CN205152410U, 13 April 2016. Rutledge, G.C.; Fridrikh, S.V. Formation of fibers by electrospinning. Adv. Drug Deliv. Rev. 2007, 59, 1384–1391. [CrossRef] [PubMed] Yang, S.; Lei, P.; Shan, Y.; Zhang, D. Preparation and characterization of antibacterial electrospun chitosan/poly(vinyl alcohol)/graphene oxide composite nanofibrous membrane. Appl. Surf. Sci. 2018, 435, 832–840. [CrossRef]
Polymers 2018, 10, 1167
41.
42.
43. 44. 45.
46.
47. 48.
49. 50.
15 of 15
Noori, M.; Ravari, F.; Ehsani, M. Preparation of PVA nanofibers reinforced with magnetic graphene by electrospinning method and investigation of their degradation kinetics using master plot analyses on solid state. J. Therm. Anal. Calorim. 2017, 6, 1–10. [CrossRef] Zhou, T.N. The preparation of the poly(vinyl alcohol)/graphene nanocomposites with low percolation threshold and high electrical conductivity by using the large-area reduced graphene oxide sheets. Express Polym. Lett. 2013, 7, 747–755. [CrossRef] Bao, W.S.; Meguid, S.A.; Zhu, Z.H.; Weng, G.J. Tunneling resistance and its effect on the electrical conductivity of carbon nanotube nanocomposites. J. Appl. Phys. 2012, 111, 093726. [CrossRef] Wang, J.; Wang, X.; Xu, C.; Zhang, M.; Shang, X. Preparation of graphene/poly(vinyl alcohol) nanocomposites with enhanced mechanical properties and water resistance. Polym. Int. 2011, 60, 816–822. [CrossRef] Tang, Y.; Chen, L.; Zhao, K.; Wu, Z.; Wang, Y.; Tan, Q. Fabrication of PLGA/HA (core)-collagen/amoxicillin (shell) nanofiber membranes through coaxial electrospinning for guided tissue regeneration. Compos. Sci. Technol. 2016, 125, 100–107. [CrossRef] He, M.; Xue, J.; Geng, H.; Gu, H.; Chen, D.; Shi, R.; Zhang, L. Fibrous guided tissue regeneration membrane loaded with anti-inflammatory agent prepared by coaxial electrospinning for the purpose of controlled release. Appl. Surf. Sci. 2015, 335, 121–129. [CrossRef] Vickery, J.L.; Patil, A.J.; Mann, S. Fabrication of graphene-polymer nanocomposites with higher-order three-dimensional architectures. Adv. Mater. 2010, 21, 2180–2184. [CrossRef] Covelo, A.; Gómez, K.K.; Corona-Lira, P.; Ramírez-Reivich, A.C.; Hernández, M. Electrochemical characterization of PVA/SA nanofibers obtained by electrospinning processing. Surf. Interface Anal. 2018. [CrossRef] Zhao, X.; Wang, Y.L.; Guo, J.L. Modification of the micro-interface adsorption model on particles with fractal theory-langmuir, freundlich and surface complexation adsorption model. Acta Sci. Circumst. 2005, 25, 52–57. Kawamura, Y.; Yoshida, H.; Asai, S.; Tanibe, H. Breakthrough curve for adsorption of mercury (II) on polyaminated highly porous chitosan beads. Water Sci. Technol. 1997, 35, 97–105. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
