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Alignment of electrospun fibers using the whipping instability
Materials Letters 193 (2017) 248–250
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Alignment of electrospun fibers using the whipping instability Tingping Lei a,c,⇑,1, Qianqian Peng a,1, Qingqing Chen a,1, Jinyu Xiong a, Feng zhang a, Daoheng Sun b a
College of Mechanical Engineering and Automation, Huaqiao University, Xiamen 361021, China School of Aerospace Engineering, Xiamen University, Xiamen 361005, China c Fujian Key Laboratory of Special Energy Manufacturing, Huaqiao University, Xiamen 361021, China b
a r t i c l e
i n f o
Article history: Received 14 October 2016 Received in revised form 20 December 2016 Accepted 29 January 2017 Available online 4 February 2017 Keywords: Electrospinning Whipping instability Fiber alignment PVDF Piezoelectric materials Polymers
a b s t r a c t A novel and effective collecting system that consists of insulating hollow cylinder and grating-like electrodes has been developed to fabricate highly aligned electrospun fibers. Unlike previous studies, the whipping instability of the electrospinning jet was utilized to produce aligned fibers. Results from many experiments demonstrate that the proposed system is of effectiveness and ease to confine the charged fibers inside the cylinder and stretch them to span across the gaps and thus to become uniaxially aligned arrays over large areas. This work is believed to provide a new insight into the use of the chaotic nature of electrospinning for the preparation of high-performance aligned fibers for a myriad of applications. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Electrospinning is a powerful technique for the production of nano/micro scale fibers that are utilized in many fields, such as tissue engineering, sensors, and environmental engineering [1–4]. Under normal conditions, the whipping (bending) instability of the highly charged jet often causes randomly oriented fibers [4]. For use in device fabrication (e.g. in microelectronics) and skeletal muscle repair, however, aligned fibers are more desirable [5,6]. Several methods have been developed for suppression or elimination of the instability that yield aligned fibers with various degrees of order, which are well summarized in a topical review [2]. Such alignment is normally achieved through control of the electric field between the spinneret tip and the collector such as using a pair of parallel conducting electrodes [7], use of a dynamic collector like a rotating wire drum [8], or a combination of both [9]. In fact, the whipping jet can be utilized to produce aligned fibers by designing a special fiber collecting system [10]. In this letter, insulating hollow cylinder with grating-like electrodes inside (Fig. 1(A)) was used as collector. The collector was motionless and the electrodes were negatively connected with a DC power supply. Due to opposite charges between the needle
⇑ Corresponding author at: College of Mechanical Engineering and Automation, Huaqiao University, Xiamen 361021, China. E-mail address: [email protected] (T. Lei). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.matlet.2017.01.138 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
and the electrodes, fibers will attract towards the electrodes and align on the gaps over large areas. By adjusting some key parameters, such as the gap width, cylinder diameter and the applied voltage, highly aligned fibers can be readily obtained.
2. Experimental PVDF (Mw 625,000) powder was dissolved in dimethylformamide (DMF) and acetone to prepare the polymer solution comprising of 12 wt% PVDF with DMF/acetone volume ratio (VDMF/ Vacetone) at 5/5. All chemicals and solvents were used directly without further purification. The layout of the electrospinning setup is presented in Fig. 1(A). An insulating hollow cylinder with grating-like electrodes inside was used to guide the electrospinning jet and collect the fiber. The needle and the electrodes were respectively connected to positive and negative terminals of the same power supply, so that the whipping jet can be effectively confined inside the cylindrical collector, and span to align across the electrode gaps. Since there are residual positive charges on fibers, the degree of fiber alignment would be further promoted, as these charges would continue to pull fibers to the nearest neighbor electrodes until they are fully stretched out, or the repulsive forces of fibers may be greater than the pulling forces to straighten fibers across the electrode gaps [11,12]. The other parts of the electrospinning device are the same as reported previously [13], except that the syringe with a metallic
249
T. Lei et al. / Materials Letters 193 (2017) 248–250
Fig. 1. (A) Schematic illustration of electrospinning setup, (B) photo of aligned fibers on the collector (cylinder made of PET plastic sheet with diameter 35 cm, electrode gap width (W2) 4 cm, electrode width (W1) 1 cm which is also for the other size cylinders listed in Table 1), (C) SEM image and FFT image (inset) of aligned fibers, (D) distribution of the fiber alignment from (C).
needle was placed along the axis of the cylinder and kept outside the cylinder. Unless stated otherwise, the flow rate was kept at 180 lL/h, and the needle and the grating-like electrodes were connected to voltages of 5.5 kV and 4.5 kV, respectively. Parameters, including the cylinder diameter, gap width of the electrodes, and the needle tip-to-collector distance (working distance, WD), were investigated and their values are listed in Table 1. All experiments were performed under an air atmosphere with relative humidity between 55 and 60%. The surface morphology of electrospun fibers was examined by a scanning electron microscope (SEM, LEO 1530). The fiber diameter and fiber alignment were measured from SEM images using ImageJ software. Specifically, twenty images with c. a. 60 fibers in each were chosen for the analyses. The fiber alignment was obtained by using the two-dimensional Fast Fourier Transform (FFT) function to transform SEM images into FFT images [14], which were rotated by 90° to match the alignment of the original images. From the FFT images, the distribution of the fiber orientation was calculated and normalized by taking the oriented direction with the largest number of fibers as angle zero.
3. Results and discussion The voltage applied to the grating-like electrodes had a great effect on the fiber deposition. When the electrodes were grounded or positively connected, it was very difficult to obtain aligned fibers between the gaps. However, when the electrodes were negatively connected, fibers were preferentially spanned across within a very short period of electrospinning (less than 5 s) and to become aligned arrays over large areas. A macroscopic fiber alignment inside the collector (cylinder made of PET plastic sheet with diameter 35 cm, electrode gap width 4 cm, electrode width 1 cm) is
Table 1 Electrospinning conditions: cylinder diameter, gap width of the electrodes, and working distance (WD). Cylinder diameter (cm)
Gap widtha (cm)
WD (cm)
4 10 20 35 50
2 2–4 2–6 2–8 2–10
5 10 12 20 20
a The gap width increases from 2 to ‘‘X” at a rate of 2 cm for cylinder diameters between 10 and 50 cm.
shown in Fig. 1(B). Fig. 1(C) shows the typical SEM image and the corresponding FFT image (inset) of the fibers aligned across the gaps. These images clearly indicate that the polymer fibers were in a good alignment. The distribution of fiber alignment (orientation) was calculated and plotted in Fig. 1(D), which reveals that approximately 90% of the fibers are aligned in the range between 7.5° and 7.5°. Our initial experiments demonstrated that by changing the cylinder size and gap width of the electrodes, fibers were still able to deposit and align across the gaps. Fig. 2(A) shows the distribution of fiber alignment using different collectors (cylinder diameter 4, 10, 20, 35 and 50 cm with a smaller gap width of 2 cm) and specific WDs (Table 1). As can be seen from this figure, the cylinder of diameter 10 cm shows the best alignment, with nearly 50% of the fibers aligned parallel to each other and over 90% of the fibers aligned in the range between 5° and 5°. This result is comparable with the good alignment of recent reports by Kiselev and RosellLlompart [15] and Grasl et al. [16], where a strict control of the electrospinning jet is needed for the alignment. With increasing cylinder diameter from 20 to 50 cm, the fiber alignment gradually
T. Lei et al. / Materials Letters 193 (2017) 248–250
(A) 50
4 cm 10 cm 20 cm 35 cm 50 cm
30
12 9
Max 6
Min
3 0
20 WD (cm)
Percentage (%)
40
(B) Voltage (kV)
250
10 0 -15
-10
-5
0
5
10
15
30 25 20 15 10 5 0
Max Min 0
10
Angle (degree)
20
30
40
50
Cylinder diameter (cm)
Fig. 2. (A) Calculated alignment for electrospun fibers collected from different collectors (cylinder diameter 4, 10, 20, 35 and 50 cm with the same gap width of 2 cm), using the WD listed in Table 1, (B) workable WD (bottom) and applied voltage on the needle (top) for different size collectors mentioned in Fig. 2(A). The applied voltage was obtained at the same negative voltage of 4.5 kV and the minimum WD shown in red half-circle at the bottom (Fig. 2(B), bottom).
of alignment decreases with increasing gap width. For the same gap width, the degree of alignment increases with reducing the cylinder size (except the one with diameter of 4 cm). Specifically, cylinder of diameter 10 cm with 2 cm gap shows the highest degree of alignment, with an angular deviation of 4.5°, whereas cylinder of diameter 4 cm with the same gap shows the lowest degree of alignment, with an angular deviation of 15.5°. 4. Conclusion In summary, uniaxially aligned fiber arrays over large areas were prepared using a novel and effective collecting system that consists of insulating hollow cylinder and grating-like electrodes. The effectiveness of the proposed system was evaluated by experimenting on several collectors of different size cylinders and different gap widths, and the ease of this system was demonstrated by a wide range of the workable working distance and applied voltage for different size collectors. Fig. 3. The degree of fiber alignment with varying gap width for different size collectors using electrospinning conditions listed in Table 1. The degree of fiber alignment is plotted in terms of angular standard deviation of a Gaussian distribution (rg), larger rg indicating lower degree of alignment.
became poor; however, a comparatively good alignment was still available. In contrast, when the cylinder diameter was reduced to 4 cm, the fiber alignment was dramatically reduced, which may be attributed to restriction in whipping motion due to the too short WD and narrow space. As shown in Fig. 2(B), the workable WD and applied voltage for different size collectors are allowed in a wide range, indicating that the proposed collecting system is of ease to align fibers. Definitely, there are always some disorder fibers deposited on the electrodes during all experiments; however, fibers spanned across the electrode gaps are normally aligned. By varying gap width of the electrodes and using the specific WD (Table 1), the fibers collected using different size cylinders were analyzed. We found that to have fibers align within a few seconds, gap width for cylinders of diameter 4, 10, 20, 35 and 50 cm should be not bigger than 2, 4, 6, 8 and 10 cm, respectively. Fig. 3 shows the degree of fiber alignment for these collectors with gap width from 2 cm to the maximum one. It is emphasized that the degree of alignment was plotted in terms of angular standard deviation (rg), with greater deviation indicating a lesser degree of alignment. As shown in Fig. 3, the degree
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51405169 and U1505243) and Fujian Natural Science Foundation (No. 2015J01205). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
X. Lu, C. Wang, Y. Wei, Small 25 (2009) 2349–2370. W.E. Teo, S. Ramakrishna, Nanotechnology 17 (2006) R89–R106. A. Greiner, J. Wendorff. Angew. Chem. Int. Ed. 46 (2007) 5670–5703. D.H. Reneker, A.L. Yarin, Polymer 49 (2008) 2387–2425. N.A. Melosh, A. Boukai, F. Diana, B. Gerardot, A. Badolato, P.M. Petroff, J.R. Heath, Science 300 (2003) 112–115. K. Aviss, J. Gough, S. Downes, Eur. Cell. Mater. 19 (2010) 193–204. D. Li, Y. Wang, Y. Xia, Nano lett. 3 (2003) 1167–1171. P. Katta, M. Alessandro, R. Ramsier, G. Chase, Nano lett. 4 (2004) 2215–2218. W. Yee, A. Nguyen, P. Lee, M. Kotaki, Y. Liu, B. Tan, S. Mhaisalkar, X. Lu, Polymer 49 (2008) 4196–4203. M. Khamforoush, M. Mahjob, Mater. Lett. 65 (2011) 453–455. T. Lei, P. Zhu, X. Cai, L. Yang, F. Yang, Appl. Phys. A 120 (2015) 5–10. W.E. Teo, S. Ramakrishna, Nanotechnology 16 (2005) 1878–1884. T. Lei, X. Cai, X. Wang, L. Yu, X. Hu, G. Zheng, W. Lv, L. Wang, D. Wu, D. Sun, L. Lin, RSC Adv. 3 (2013) 24952–24958. C. Ayres, G.L. Bowlin, S.C. Henderson, L. Taylor, J. Shultz, J. Alexander, T.A. Telemeco, D.G. Simpson, Biomaterial 27 (2006) 5524–5534. P. Kiselev, J. Rosell-Llompart, J. Appl. Polym. Sci. 125 (2012) 2433–2441. C. Grasl, M.M.L. Arras, M. Stoiber, H. Bergmeister, H. Schima, Appl. Phys. Lett. 102 (2013) 053111–53114.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Alignment of electrospun fibers using the whipping instability Tingping Lei a,c,⇑,1, Qianqian Peng a,1, Qingqing Chen a,1, Jinyu Xiong a, Feng zhang a, Daoheng Sun b a
College of Mechanical Engineering and Automation, Huaqiao University, Xiamen 361021, China School of Aerospace Engineering, Xiamen University, Xiamen 361005, China c Fujian Key Laboratory of Special Energy Manufacturing, Huaqiao University, Xiamen 361021, China b
a r t i c l e
i n f o
Article history: Received 14 October 2016 Received in revised form 20 December 2016 Accepted 29 January 2017 Available online 4 February 2017 Keywords: Electrospinning Whipping instability Fiber alignment PVDF Piezoelectric materials Polymers
a b s t r a c t A novel and effective collecting system that consists of insulating hollow cylinder and grating-like electrodes has been developed to fabricate highly aligned electrospun fibers. Unlike previous studies, the whipping instability of the electrospinning jet was utilized to produce aligned fibers. Results from many experiments demonstrate that the proposed system is of effectiveness and ease to confine the charged fibers inside the cylinder and stretch them to span across the gaps and thus to become uniaxially aligned arrays over large areas. This work is believed to provide a new insight into the use of the chaotic nature of electrospinning for the preparation of high-performance aligned fibers for a myriad of applications. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Electrospinning is a powerful technique for the production of nano/micro scale fibers that are utilized in many fields, such as tissue engineering, sensors, and environmental engineering [1–4]. Under normal conditions, the whipping (bending) instability of the highly charged jet often causes randomly oriented fibers [4]. For use in device fabrication (e.g. in microelectronics) and skeletal muscle repair, however, aligned fibers are more desirable [5,6]. Several methods have been developed for suppression or elimination of the instability that yield aligned fibers with various degrees of order, which are well summarized in a topical review [2]. Such alignment is normally achieved through control of the electric field between the spinneret tip and the collector such as using a pair of parallel conducting electrodes [7], use of a dynamic collector like a rotating wire drum [8], or a combination of both [9]. In fact, the whipping jet can be utilized to produce aligned fibers by designing a special fiber collecting system [10]. In this letter, insulating hollow cylinder with grating-like electrodes inside (Fig. 1(A)) was used as collector. The collector was motionless and the electrodes were negatively connected with a DC power supply. Due to opposite charges between the needle
⇑ Corresponding author at: College of Mechanical Engineering and Automation, Huaqiao University, Xiamen 361021, China. E-mail address: [email protected] (T. Lei). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.matlet.2017.01.138 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
and the electrodes, fibers will attract towards the electrodes and align on the gaps over large areas. By adjusting some key parameters, such as the gap width, cylinder diameter and the applied voltage, highly aligned fibers can be readily obtained.
2. Experimental PVDF (Mw 625,000) powder was dissolved in dimethylformamide (DMF) and acetone to prepare the polymer solution comprising of 12 wt% PVDF with DMF/acetone volume ratio (VDMF/ Vacetone) at 5/5. All chemicals and solvents were used directly without further purification. The layout of the electrospinning setup is presented in Fig. 1(A). An insulating hollow cylinder with grating-like electrodes inside was used to guide the electrospinning jet and collect the fiber. The needle and the electrodes were respectively connected to positive and negative terminals of the same power supply, so that the whipping jet can be effectively confined inside the cylindrical collector, and span to align across the electrode gaps. Since there are residual positive charges on fibers, the degree of fiber alignment would be further promoted, as these charges would continue to pull fibers to the nearest neighbor electrodes until they are fully stretched out, or the repulsive forces of fibers may be greater than the pulling forces to straighten fibers across the electrode gaps [11,12]. The other parts of the electrospinning device are the same as reported previously [13], except that the syringe with a metallic
249
T. Lei et al. / Materials Letters 193 (2017) 248–250
Fig. 1. (A) Schematic illustration of electrospinning setup, (B) photo of aligned fibers on the collector (cylinder made of PET plastic sheet with diameter 35 cm, electrode gap width (W2) 4 cm, electrode width (W1) 1 cm which is also for the other size cylinders listed in Table 1), (C) SEM image and FFT image (inset) of aligned fibers, (D) distribution of the fiber alignment from (C).
needle was placed along the axis of the cylinder and kept outside the cylinder. Unless stated otherwise, the flow rate was kept at 180 lL/h, and the needle and the grating-like electrodes were connected to voltages of 5.5 kV and 4.5 kV, respectively. Parameters, including the cylinder diameter, gap width of the electrodes, and the needle tip-to-collector distance (working distance, WD), were investigated and their values are listed in Table 1. All experiments were performed under an air atmosphere with relative humidity between 55 and 60%. The surface morphology of electrospun fibers was examined by a scanning electron microscope (SEM, LEO 1530). The fiber diameter and fiber alignment were measured from SEM images using ImageJ software. Specifically, twenty images with c. a. 60 fibers in each were chosen for the analyses. The fiber alignment was obtained by using the two-dimensional Fast Fourier Transform (FFT) function to transform SEM images into FFT images [14], which were rotated by 90° to match the alignment of the original images. From the FFT images, the distribution of the fiber orientation was calculated and normalized by taking the oriented direction with the largest number of fibers as angle zero.
3. Results and discussion The voltage applied to the grating-like electrodes had a great effect on the fiber deposition. When the electrodes were grounded or positively connected, it was very difficult to obtain aligned fibers between the gaps. However, when the electrodes were negatively connected, fibers were preferentially spanned across within a very short period of electrospinning (less than 5 s) and to become aligned arrays over large areas. A macroscopic fiber alignment inside the collector (cylinder made of PET plastic sheet with diameter 35 cm, electrode gap width 4 cm, electrode width 1 cm) is
Table 1 Electrospinning conditions: cylinder diameter, gap width of the electrodes, and working distance (WD). Cylinder diameter (cm)
Gap widtha (cm)
WD (cm)
4 10 20 35 50
2 2–4 2–6 2–8 2–10
5 10 12 20 20
a The gap width increases from 2 to ‘‘X” at a rate of 2 cm for cylinder diameters between 10 and 50 cm.
shown in Fig. 1(B). Fig. 1(C) shows the typical SEM image and the corresponding FFT image (inset) of the fibers aligned across the gaps. These images clearly indicate that the polymer fibers were in a good alignment. The distribution of fiber alignment (orientation) was calculated and plotted in Fig. 1(D), which reveals that approximately 90% of the fibers are aligned in the range between 7.5° and 7.5°. Our initial experiments demonstrated that by changing the cylinder size and gap width of the electrodes, fibers were still able to deposit and align across the gaps. Fig. 2(A) shows the distribution of fiber alignment using different collectors (cylinder diameter 4, 10, 20, 35 and 50 cm with a smaller gap width of 2 cm) and specific WDs (Table 1). As can be seen from this figure, the cylinder of diameter 10 cm shows the best alignment, with nearly 50% of the fibers aligned parallel to each other and over 90% of the fibers aligned in the range between 5° and 5°. This result is comparable with the good alignment of recent reports by Kiselev and RosellLlompart [15] and Grasl et al. [16], where a strict control of the electrospinning jet is needed for the alignment. With increasing cylinder diameter from 20 to 50 cm, the fiber alignment gradually
T. Lei et al. / Materials Letters 193 (2017) 248–250
(A) 50
4 cm 10 cm 20 cm 35 cm 50 cm
30
12 9
Max 6
Min
3 0
20 WD (cm)
Percentage (%)
40
(B) Voltage (kV)
250
10 0 -15
-10
-5
0
5
10
15
30 25 20 15 10 5 0
Max Min 0
10
Angle (degree)
20
30
40
50
Cylinder diameter (cm)
Fig. 2. (A) Calculated alignment for electrospun fibers collected from different collectors (cylinder diameter 4, 10, 20, 35 and 50 cm with the same gap width of 2 cm), using the WD listed in Table 1, (B) workable WD (bottom) and applied voltage on the needle (top) for different size collectors mentioned in Fig. 2(A). The applied voltage was obtained at the same negative voltage of 4.5 kV and the minimum WD shown in red half-circle at the bottom (Fig. 2(B), bottom).
of alignment decreases with increasing gap width. For the same gap width, the degree of alignment increases with reducing the cylinder size (except the one with diameter of 4 cm). Specifically, cylinder of diameter 10 cm with 2 cm gap shows the highest degree of alignment, with an angular deviation of 4.5°, whereas cylinder of diameter 4 cm with the same gap shows the lowest degree of alignment, with an angular deviation of 15.5°. 4. Conclusion In summary, uniaxially aligned fiber arrays over large areas were prepared using a novel and effective collecting system that consists of insulating hollow cylinder and grating-like electrodes. The effectiveness of the proposed system was evaluated by experimenting on several collectors of different size cylinders and different gap widths, and the ease of this system was demonstrated by a wide range of the workable working distance and applied voltage for different size collectors. Fig. 3. The degree of fiber alignment with varying gap width for different size collectors using electrospinning conditions listed in Table 1. The degree of fiber alignment is plotted in terms of angular standard deviation of a Gaussian distribution (rg), larger rg indicating lower degree of alignment.
became poor; however, a comparatively good alignment was still available. In contrast, when the cylinder diameter was reduced to 4 cm, the fiber alignment was dramatically reduced, which may be attributed to restriction in whipping motion due to the too short WD and narrow space. As shown in Fig. 2(B), the workable WD and applied voltage for different size collectors are allowed in a wide range, indicating that the proposed collecting system is of ease to align fibers. Definitely, there are always some disorder fibers deposited on the electrodes during all experiments; however, fibers spanned across the electrode gaps are normally aligned. By varying gap width of the electrodes and using the specific WD (Table 1), the fibers collected using different size cylinders were analyzed. We found that to have fibers align within a few seconds, gap width for cylinders of diameter 4, 10, 20, 35 and 50 cm should be not bigger than 2, 4, 6, 8 and 10 cm, respectively. Fig. 3 shows the degree of fiber alignment for these collectors with gap width from 2 cm to the maximum one. It is emphasized that the degree of alignment was plotted in terms of angular standard deviation (rg), with greater deviation indicating a lesser degree of alignment. As shown in Fig. 3, the degree
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51405169 and U1505243) and Fujian Natural Science Foundation (No. 2015J01205). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
X. Lu, C. Wang, Y. Wei, Small 25 (2009) 2349–2370. W.E. Teo, S. Ramakrishna, Nanotechnology 17 (2006) R89–R106. A. Greiner, J. Wendorff. Angew. Chem. Int. Ed. 46 (2007) 5670–5703. D.H. Reneker, A.L. Yarin, Polymer 49 (2008) 2387–2425. N.A. Melosh, A. Boukai, F. Diana, B. Gerardot, A. Badolato, P.M. Petroff, J.R. Heath, Science 300 (2003) 112–115. K. Aviss, J. Gough, S. Downes, Eur. Cell. Mater. 19 (2010) 193–204. D. Li, Y. Wang, Y. Xia, Nano lett. 3 (2003) 1167–1171. P. Katta, M. Alessandro, R. Ramsier, G. Chase, Nano lett. 4 (2004) 2215–2218. W. Yee, A. Nguyen, P. Lee, M. Kotaki, Y. Liu, B. Tan, S. Mhaisalkar, X. Lu, Polymer 49 (2008) 4196–4203. M. Khamforoush, M. Mahjob, Mater. Lett. 65 (2011) 453–455. T. Lei, P. Zhu, X. Cai, L. Yang, F. Yang, Appl. Phys. A 120 (2015) 5–10. W.E. Teo, S. Ramakrishna, Nanotechnology 16 (2005) 1878–1884. T. Lei, X. Cai, X. Wang, L. Yu, X. Hu, G. Zheng, W. Lv, L. Wang, D. Wu, D. Sun, L. Lin, RSC Adv. 3 (2013) 24952–24958. C. Ayres, G.L. Bowlin, S.C. Henderson, L. Taylor, J. Shultz, J. Alexander, T.A. Telemeco, D.G. Simpson, Biomaterial 27 (2006) 5524–5534. P. Kiselev, J. Rosell-Llompart, J. Appl. Polym. Sci. 125 (2012) 2433–2441. C. Grasl, M.M.L. Arras, M. Stoiber, H. Bergmeister, H. Schima, Appl. Phys. Lett. 102 (2013) 053111–53114.