- Email: [email protected]
β-cyclodextrin composite nanofibers using electrospinning techniques
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 251–254
journal homepage: www.elsevier.com/locate/jmatprotec
Synthesis of poly(N-vinylpyrrolidone)/-cyclodextrin composite nanofibers using electrospinning techniques Jie Bai a , Qingbiao Yang a , Meiye Li b , Chaoqun Zhang a , Li Yiaoxian a,∗ a
Department of Chemistry, Jilin University, Changchun 130021, PR China National Analytical Research Center of Electrochemistry and Spectroscopy, Changchun Institute of Applied Chemistry, Chinese Academic of Science, Changchun 130022, PR China b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Poly(N-vinylpyrrolidone) (PVP) nanofibers (NFs) containing -cyclodextrin have been pre-
Received 24 August 2007
pared by electrospinning technology. The morphology of fibers has been characterized
Received in revised form
by scanning electron microscopy (SEM). The structure of composite was characterized by
17 October 2007
fourier transform infrared spectroscopy (FTIR).
Accepted 23 December 2007
© 2008 Elsevier B.V. All rights reserved.
Keywords: Electrospinning Nanofibers -Cyclodextrin Electron microscopy
1.
Introduction
With the development of nanotechnology, the study of onedimensional (1D) nanostructure materials has attracted great interests, such as nanorods, nanowires, nanofibers, nanotubes, and nanocables (Cobden, 2001; Reneker and Chun, 1996). Recently, synthesis of fibers structure materials was developed rapidly. Among the methods of producing polymer nanofibers, electrospinning technology was the simplest and the most effective to produce ultrafine polymer nanofibers. At first, this technology was reported at 1934 by Formhals, and it had attracted much attention in the past decade because of its potential to produce ultrafine fibers with diameters in the range of nanometer to micrometer. In the electrospinning process, a strong electrostatic force that was produced through a high static voltage was applied to the capillary containing a polymer solution, and the charged solution droplet
∗
Corresponding author. Tel.: +86 431 8499845; fax: +86 431 8499845. E-mail address: [email protected] (L. Yiaoxian). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.109
formed a compound “Taylor” cone at the edge of the nozzles. As the electric forces overcame the surface tension, a charged solution of polymer was ejected. After the solvents were evaporated during the course of jet flying, the nanofibers were collected on a grounded collector (Li and Xia, 2004; Bergshoef and Vancso, 1999; Yarin et al., 2000; Formhals, 1934). At the present time, about 50 kinds of polymers have been exploited to prepare single polymer nanofiber by electrospinning (Chronakis, 2005; Royen et al., 2006; Jin et al., 2005; Piperno et al., 2006), and these fibers have been applied to various fields, such as biomedical engineering, filtration, chemical sensors (Min et al., 2004a,b; Gopal et al., 2006; Liu et al., 2004) and so on. Today, the preparation of composite nanofibers has become study hotspot in many fields. Some functional materials were incorporated into polymer nanofibers, and the result composites had many excellent chemical and physical properties (Min et al., 2004c; Forster and Antonietti, 1998; Zhou
252
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 251–254
Scheme 1 – Chemical structure of poly(vinylpyrrolidone).
et al., 2005). Cyclodextrins (CDs), cyclic starch oligomers, may be represented as shallow truncated cones consisting of six, seven, or eight glucose units. They are named alpha (␣-), beta (-), or gamma (␥-) cyclodextrins (CDs), respectively. The CDs is considered as one of the supermolecules pioneers, it has been widely applied in all kinds of fields, such as host–guest interaction, molecular recognition, drug delivery and enzyme catalysis, foods, cosmetics and pesticides (Nepogodiev and Stoddart, 1998; Wenz, 1994). In this work, we use electrospinning technology to prepare sub-micron diameters composite fibers of PVP (chemical structure, Scheme 1) and -CD (chemical structure, Scheme 2). The morphology and structure of the mat were measured and discussed.
Scheme 3 – Schematic electrospinning apparatus. (1) Collector, (2) glass pipette (aperture: 0.5 mm), and (3) dc power supply.
were electrospun at 18 ◦ C and collection distance was fixed about 20 cm. The feed rate of the solution was 2 ml/h through a syringe pump. 2.4.
Characterization of composite nanofibers
All ultrafine fiber samples were observed with scanning electron microscopy (FEI XL30 ESEM). The fourier transform infrared (FTIR) spectra were collected on an IRPRESTIGE-21 fourier transform infrared spectrophotometer in the wave number range of 400–4000 cm−1 . The samples were prepared in the form of nanofibers film.
2.
Experimental
2.1.
Materials
3.
All the chemicals we used were of analytical grade. Polyvinylpyrrolidone (PVP, Mw = 1,300,000) was purchased from Aldrich. -Cyclodextrin and ethanol were supplied by Beijing Chemicals Co. (China). 2.2.
Preparation of spinning solutions
PVP was dissolved in solvent mixtures (PH 10) of water/ethanol with ratios of 40/60 (w/w) when its concentration was 8 wt.%, and stirred 24 h at room temperature. A controlled amount of 0.09 and 0.2 g -cyclodextrin was added into 10 g PVP solutions (8 wt.%), respectively. The reaction was rapidly stirred at room temperature about 12 h. 2.3.
Electrospinning process
The schematic setup of the electrospinning process we used in this study was shown in Scheme 3. Each of the electrospinning solutions was placed in a 2 ml glass syringe, the open end of the glass syringe was connected to a pinhead (the inner diameter = 0.6 mm) to be used as the nozzle. The utilized electrical potential which is attached to the pinhead is 12 kV. An aluminum sheet was connected to the grounding electrode and it was used as the collector plate. These solutions
Scheme 2 – Chemical structure of -cyclodextrin.
Results and discussion
During the electrospinning process, the electrified jet (solution of PVP and -CD) under a high electrostatic field was continuously stretched and splitted into multiple branches due to the complicated actions such as bending instability and the high frequency of whipping. With the solvent evaporation, the PVP and -CD composites were solidified and became filaments whose diameters decreased sharply to several hundreds nanometers. The morphology and the diameters of the polymers nanofibers were always measured by SEM. In this study, 8 wt.% of PVP in ethanol/water (6:4) solvents (PH 10) was chosen to study the effect of adding -CD molecules on fibrous PVP produced by elecrospinning. Respectively, 0.09 and 0.2 g -cyclodextrin was added into 10 g PVP solutions (8 wt.%), and the mass number of cyclodextrin was 10% and 20% in corresponding composite nanaofibers. Fig. 1 shows the SEM images of 0, 10 and 20 wt.% of -CD in PVP nanofibers. It is observed that pure PVP nanofibers have uniform diameters and are very straight. However, when the -CD was mixed to the PVP solution, the case is of obvious difference. The main changes of composite nanofibers were as followed: Firstly, when the -CD was added to the PVP nanofibers, the diameters of nanofibers changed remarkably (Fig. 1). When the content of the -CD were 0, 10.0 and 20.0 wt.% in the composite fibers, the average diameters of the nanofibers were 498, 529 and 621 nm, respectively. The results showed that the average diameters of the composite nanofibers increased with adding -CD. Secondly, when the content of -CD was increased, many curvatures and large knots appeared, and so the morphologies of PVP/-CD composite nanofibers changed (Fig. 2). Thirdly, nanofibers appeared not uniform with adding the -CD. It was clear that the -CD influenced
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 251–254
253
Fig. 1 – The SEM images of various PVP/-CD fibers with different -CD concentrations. (a) 0 wt.%, (b) 10.0 wt.%, and (c) 20.0 wt.%.
the morphologies of -CD/PVP composite nanofibers significantly. Fig. 3 shows the FTIR spectra of PVP fibers film, PVP/CD (9/1) and PVP/-CD (8/2) composite nanofibers film. The spectrum of pure PVP (Fig. 3a) shows the peak whose wave numbers are in the region 1670 cm−1 dues to amide carbonyl group of PVP. The peak at 3485 cm−1 are attributed to the band of water in PVP. The composite of -CD/PVP (Fig. 3b and c) also shows the peak due to amide carbonyl group of PVP at
1670 cm−1 . At the same time, when the contents of the -CD were increased, the band shifted from 3485 to 3440 cm−1 and the band at 3440 cm−1 was gradually stronger in matrix. It is because the band of OH in ethanol and the band of 3485 cm−1 encounter in this region. Another change is that with the content of -CD increased, the spectra showed that the intensities increased in the band at 1030 cm−1 . It indicated the presence of CH2 OH. Above data proved the present of -CD in composite nanofibers.
Fig. 2 – The SEM images of various PVP/-CD fibers with different -CD concentrations. (a) 0 wt.%, (b) 10.0 wt.%, and (c) 20.0 wt.%.
254
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 251–254
Fig. 3 – The FTIR of various PVP/-CD fibers with different -CD concentrations. (a) 0 wt.%, (b) 10.0 wt.%, and (c) 20.0 wt.%.
4.
Conclusion
The PVP/-CD composite nanofibers were fabricated by electrospun the composite solution which was mixed by PVP/-CD and the solvent of ethanol/water (6/4). How -CD had effect on PVP nanofibers was investigated by measuring SEM. The composite nanofibers were also characterized by the FTIR spectra, and the FTIR data confirmed the presence of -CD in the composites.
Acknowledgements The authors gratefully acknowledge the support of the National Natural Science Foundation of China (No. 20674023).
references
Bergshoef, M.M., Vancso, G.J., 1999. Transparent nanocomposites with ultrathin, electrospun nylon-4,6 fiber reinforcement. Adv. Mater. 11, 1362–1365. Chronakis, I.S., 2005. Novel nanocomposites and nanoceramics based on polymer nanofibers using electrospinning process—a review. J. Mater. Process Technol. 167, 283–293. Cobden, D.H., 2001. Molecular electronics nanowires begin to shine. Nature 409, 32–33. A. Formhals, US patent 1,975,504 (1934).
Forster, S., Antonietti, M., 1998. Amphiphilic block copolymers in structure-controlled nanomaterial hybrids. Adv. Mater. 10, 195–217. Gopal, R., Kaur, S., Ma, Z., Chan, C., Ramakrishna, S., Matsuura, T., 2006. Electrospun nanofibrous filtration membrane. J. Membr. Sci. 281, 581–586. Jin, W.J., Lee, H.K., Jeong, E.H., Park, W.H., Youk, J.H., 2005. Preparation of polymer nanofibers containing silver nanoparticles by using poly(N-vinylpyrrolidone). Macromol. Rapid Commun. 26, 1903–1907. Li, D., Xia, Y.N., 2004. Electrospinning of nanofibres: reinventing the wheel. Adv. Mater. 16, 1151–1170. Liu, H., Kameoka, J., Czaplewski, D.A., Craighead, H.G., 2004. Polymeric nanowire chemical sensor. Nano Lett. 4, 671–675. Min, B.M., Lee, G., Kim, S.H., Nam, Y.S., Lee, T.S., Park, W.H., 2004a. Electrospinning of silk fibroin nanofibers and its effect on the adhesion and spreading of normal human keratinocytes and fibroblasts in vitro. Biomaterials 25, 1289–1297. Min, B.M., Jeong, L., Nam, Y.S., Kim, J.M., Kim, J.Y., Park, W.H., 2004b. Formation of silk fibroin matrices with different texture and its cellular response to normal human keratinocytes. Int. J. Biol. Macromol. 34, 223–230. Min, B.M., You, Y., Kim, J., Lee, M., Park, W.H., 2004c. Formation of nanostructured poly(lactic-co-glycolic acid)/chitin matrix and its cellular response to normal human keratinocytes and fibroblasts. Carbohyd. Polym. 57, 285–292. Nepogodiev, S.A., Stoddart, J.F., 1998. Cyclodextrin-based catenanes and rotaxanes. Chem. Rev. 98, 1959–1976. Piperno, S., Lozzi, L., Rastelli, R., Passacantando, M., Santucci, S., 2006. PMMA nanofibers production by electrospinning. Appl. Surf. Sci. 252, 5583–5586. Reneker, D.H., Chun, I., 1996. nanometre diameter fibres of polymer, produced by electrospinning. Nanotechnology 7, 216–223. Royen, P.V., dos Santos, A.M., Schacht, E., Ruys, L., Vaeck, L.V., 2006. Characterisation of electrospun nanowebs with static secondary ion mass spectrometry (S-SIMS). Appl. Surf. Sci. 252, 6992–6995. Wenz, G., 1994. Cyclodextrins as building blocks for supramolecular structures and functional units. Angew. Chem. Int. Ed. Engl. 33, 803–822. Yarin, A.L., Koombhongse, S., Reneker, D.H., 2000. Bending instability of electrically charged liquid jets of poly-mersolutionsinelectrospinning. J. Appl. Phys. 89, 3018–3026. Zhou, W.P., Wu, Y.L., Wei, F., Luo, G.H., Qian, W.Z., 2005. Elastic deformation of multiwalled carbon nanotubes in electrospun MWCNTs–PEO and MWCNTs–PVA nanofibers. Polymer 46, 12689–12695.
journal homepage: www.elsevier.com/locate/jmatprotec
Synthesis of poly(N-vinylpyrrolidone)/-cyclodextrin composite nanofibers using electrospinning techniques Jie Bai a , Qingbiao Yang a , Meiye Li b , Chaoqun Zhang a , Li Yiaoxian a,∗ a
Department of Chemistry, Jilin University, Changchun 130021, PR China National Analytical Research Center of Electrochemistry and Spectroscopy, Changchun Institute of Applied Chemistry, Chinese Academic of Science, Changchun 130022, PR China b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Poly(N-vinylpyrrolidone) (PVP) nanofibers (NFs) containing -cyclodextrin have been pre-
Received 24 August 2007
pared by electrospinning technology. The morphology of fibers has been characterized
Received in revised form
by scanning electron microscopy (SEM). The structure of composite was characterized by
17 October 2007
fourier transform infrared spectroscopy (FTIR).
Accepted 23 December 2007
© 2008 Elsevier B.V. All rights reserved.
Keywords: Electrospinning Nanofibers -Cyclodextrin Electron microscopy
1.
Introduction
With the development of nanotechnology, the study of onedimensional (1D) nanostructure materials has attracted great interests, such as nanorods, nanowires, nanofibers, nanotubes, and nanocables (Cobden, 2001; Reneker and Chun, 1996). Recently, synthesis of fibers structure materials was developed rapidly. Among the methods of producing polymer nanofibers, electrospinning technology was the simplest and the most effective to produce ultrafine polymer nanofibers. At first, this technology was reported at 1934 by Formhals, and it had attracted much attention in the past decade because of its potential to produce ultrafine fibers with diameters in the range of nanometer to micrometer. In the electrospinning process, a strong electrostatic force that was produced through a high static voltage was applied to the capillary containing a polymer solution, and the charged solution droplet
∗
Corresponding author. Tel.: +86 431 8499845; fax: +86 431 8499845. E-mail address: [email protected] (L. Yiaoxian). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.109
formed a compound “Taylor” cone at the edge of the nozzles. As the electric forces overcame the surface tension, a charged solution of polymer was ejected. After the solvents were evaporated during the course of jet flying, the nanofibers were collected on a grounded collector (Li and Xia, 2004; Bergshoef and Vancso, 1999; Yarin et al., 2000; Formhals, 1934). At the present time, about 50 kinds of polymers have been exploited to prepare single polymer nanofiber by electrospinning (Chronakis, 2005; Royen et al., 2006; Jin et al., 2005; Piperno et al., 2006), and these fibers have been applied to various fields, such as biomedical engineering, filtration, chemical sensors (Min et al., 2004a,b; Gopal et al., 2006; Liu et al., 2004) and so on. Today, the preparation of composite nanofibers has become study hotspot in many fields. Some functional materials were incorporated into polymer nanofibers, and the result composites had many excellent chemical and physical properties (Min et al., 2004c; Forster and Antonietti, 1998; Zhou
252
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 251–254
Scheme 1 – Chemical structure of poly(vinylpyrrolidone).
et al., 2005). Cyclodextrins (CDs), cyclic starch oligomers, may be represented as shallow truncated cones consisting of six, seven, or eight glucose units. They are named alpha (␣-), beta (-), or gamma (␥-) cyclodextrins (CDs), respectively. The CDs is considered as one of the supermolecules pioneers, it has been widely applied in all kinds of fields, such as host–guest interaction, molecular recognition, drug delivery and enzyme catalysis, foods, cosmetics and pesticides (Nepogodiev and Stoddart, 1998; Wenz, 1994). In this work, we use electrospinning technology to prepare sub-micron diameters composite fibers of PVP (chemical structure, Scheme 1) and -CD (chemical structure, Scheme 2). The morphology and structure of the mat were measured and discussed.
Scheme 3 – Schematic electrospinning apparatus. (1) Collector, (2) glass pipette (aperture: 0.5 mm), and (3) dc power supply.
were electrospun at 18 ◦ C and collection distance was fixed about 20 cm. The feed rate of the solution was 2 ml/h through a syringe pump. 2.4.
Characterization of composite nanofibers
All ultrafine fiber samples were observed with scanning electron microscopy (FEI XL30 ESEM). The fourier transform infrared (FTIR) spectra were collected on an IRPRESTIGE-21 fourier transform infrared spectrophotometer in the wave number range of 400–4000 cm−1 . The samples were prepared in the form of nanofibers film.
2.
Experimental
2.1.
Materials
3.
All the chemicals we used were of analytical grade. Polyvinylpyrrolidone (PVP, Mw = 1,300,000) was purchased from Aldrich. -Cyclodextrin and ethanol were supplied by Beijing Chemicals Co. (China). 2.2.
Preparation of spinning solutions
PVP was dissolved in solvent mixtures (PH 10) of water/ethanol with ratios of 40/60 (w/w) when its concentration was 8 wt.%, and stirred 24 h at room temperature. A controlled amount of 0.09 and 0.2 g -cyclodextrin was added into 10 g PVP solutions (8 wt.%), respectively. The reaction was rapidly stirred at room temperature about 12 h. 2.3.
Electrospinning process
The schematic setup of the electrospinning process we used in this study was shown in Scheme 3. Each of the electrospinning solutions was placed in a 2 ml glass syringe, the open end of the glass syringe was connected to a pinhead (the inner diameter = 0.6 mm) to be used as the nozzle. The utilized electrical potential which is attached to the pinhead is 12 kV. An aluminum sheet was connected to the grounding electrode and it was used as the collector plate. These solutions
Scheme 2 – Chemical structure of -cyclodextrin.
Results and discussion
During the electrospinning process, the electrified jet (solution of PVP and -CD) under a high electrostatic field was continuously stretched and splitted into multiple branches due to the complicated actions such as bending instability and the high frequency of whipping. With the solvent evaporation, the PVP and -CD composites were solidified and became filaments whose diameters decreased sharply to several hundreds nanometers. The morphology and the diameters of the polymers nanofibers were always measured by SEM. In this study, 8 wt.% of PVP in ethanol/water (6:4) solvents (PH 10) was chosen to study the effect of adding -CD molecules on fibrous PVP produced by elecrospinning. Respectively, 0.09 and 0.2 g -cyclodextrin was added into 10 g PVP solutions (8 wt.%), and the mass number of cyclodextrin was 10% and 20% in corresponding composite nanaofibers. Fig. 1 shows the SEM images of 0, 10 and 20 wt.% of -CD in PVP nanofibers. It is observed that pure PVP nanofibers have uniform diameters and are very straight. However, when the -CD was mixed to the PVP solution, the case is of obvious difference. The main changes of composite nanofibers were as followed: Firstly, when the -CD was added to the PVP nanofibers, the diameters of nanofibers changed remarkably (Fig. 1). When the content of the -CD were 0, 10.0 and 20.0 wt.% in the composite fibers, the average diameters of the nanofibers were 498, 529 and 621 nm, respectively. The results showed that the average diameters of the composite nanofibers increased with adding -CD. Secondly, when the content of -CD was increased, many curvatures and large knots appeared, and so the morphologies of PVP/-CD composite nanofibers changed (Fig. 2). Thirdly, nanofibers appeared not uniform with adding the -CD. It was clear that the -CD influenced
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 251–254
253
Fig. 1 – The SEM images of various PVP/-CD fibers with different -CD concentrations. (a) 0 wt.%, (b) 10.0 wt.%, and (c) 20.0 wt.%.
the morphologies of -CD/PVP composite nanofibers significantly. Fig. 3 shows the FTIR spectra of PVP fibers film, PVP/CD (9/1) and PVP/-CD (8/2) composite nanofibers film. The spectrum of pure PVP (Fig. 3a) shows the peak whose wave numbers are in the region 1670 cm−1 dues to amide carbonyl group of PVP. The peak at 3485 cm−1 are attributed to the band of water in PVP. The composite of -CD/PVP (Fig. 3b and c) also shows the peak due to amide carbonyl group of PVP at
1670 cm−1 . At the same time, when the contents of the -CD were increased, the band shifted from 3485 to 3440 cm−1 and the band at 3440 cm−1 was gradually stronger in matrix. It is because the band of OH in ethanol and the band of 3485 cm−1 encounter in this region. Another change is that with the content of -CD increased, the spectra showed that the intensities increased in the band at 1030 cm−1 . It indicated the presence of CH2 OH. Above data proved the present of -CD in composite nanofibers.
Fig. 2 – The SEM images of various PVP/-CD fibers with different -CD concentrations. (a) 0 wt.%, (b) 10.0 wt.%, and (c) 20.0 wt.%.
254
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 251–254
Fig. 3 – The FTIR of various PVP/-CD fibers with different -CD concentrations. (a) 0 wt.%, (b) 10.0 wt.%, and (c) 20.0 wt.%.
4.
Conclusion
The PVP/-CD composite nanofibers were fabricated by electrospun the composite solution which was mixed by PVP/-CD and the solvent of ethanol/water (6/4). How -CD had effect on PVP nanofibers was investigated by measuring SEM. The composite nanofibers were also characterized by the FTIR spectra, and the FTIR data confirmed the presence of -CD in the composites.
Acknowledgements The authors gratefully acknowledge the support of the National Natural Science Foundation of China (No. 20674023).
references
Bergshoef, M.M., Vancso, G.J., 1999. Transparent nanocomposites with ultrathin, electrospun nylon-4,6 fiber reinforcement. Adv. Mater. 11, 1362–1365. Chronakis, I.S., 2005. Novel nanocomposites and nanoceramics based on polymer nanofibers using electrospinning process—a review. J. Mater. Process Technol. 167, 283–293. Cobden, D.H., 2001. Molecular electronics nanowires begin to shine. Nature 409, 32–33. A. Formhals, US patent 1,975,504 (1934).
Forster, S., Antonietti, M., 1998. Amphiphilic block copolymers in structure-controlled nanomaterial hybrids. Adv. Mater. 10, 195–217. Gopal, R., Kaur, S., Ma, Z., Chan, C., Ramakrishna, S., Matsuura, T., 2006. Electrospun nanofibrous filtration membrane. J. Membr. Sci. 281, 581–586. Jin, W.J., Lee, H.K., Jeong, E.H., Park, W.H., Youk, J.H., 2005. Preparation of polymer nanofibers containing silver nanoparticles by using poly(N-vinylpyrrolidone). Macromol. Rapid Commun. 26, 1903–1907. Li, D., Xia, Y.N., 2004. Electrospinning of nanofibres: reinventing the wheel. Adv. Mater. 16, 1151–1170. Liu, H., Kameoka, J., Czaplewski, D.A., Craighead, H.G., 2004. Polymeric nanowire chemical sensor. Nano Lett. 4, 671–675. Min, B.M., Lee, G., Kim, S.H., Nam, Y.S., Lee, T.S., Park, W.H., 2004a. Electrospinning of silk fibroin nanofibers and its effect on the adhesion and spreading of normal human keratinocytes and fibroblasts in vitro. Biomaterials 25, 1289–1297. Min, B.M., Jeong, L., Nam, Y.S., Kim, J.M., Kim, J.Y., Park, W.H., 2004b. Formation of silk fibroin matrices with different texture and its cellular response to normal human keratinocytes. Int. J. Biol. Macromol. 34, 223–230. Min, B.M., You, Y., Kim, J., Lee, M., Park, W.H., 2004c. Formation of nanostructured poly(lactic-co-glycolic acid)/chitin matrix and its cellular response to normal human keratinocytes and fibroblasts. Carbohyd. Polym. 57, 285–292. Nepogodiev, S.A., Stoddart, J.F., 1998. Cyclodextrin-based catenanes and rotaxanes. Chem. Rev. 98, 1959–1976. Piperno, S., Lozzi, L., Rastelli, R., Passacantando, M., Santucci, S., 2006. PMMA nanofibers production by electrospinning. Appl. Surf. Sci. 252, 5583–5586. Reneker, D.H., Chun, I., 1996. nanometre diameter fibres of polymer, produced by electrospinning. Nanotechnology 7, 216–223. Royen, P.V., dos Santos, A.M., Schacht, E., Ruys, L., Vaeck, L.V., 2006. Characterisation of electrospun nanowebs with static secondary ion mass spectrometry (S-SIMS). Appl. Surf. Sci. 252, 6992–6995. Wenz, G., 1994. Cyclodextrins as building blocks for supramolecular structures and functional units. Angew. Chem. Int. Ed. Engl. 33, 803–822. Yarin, A.L., Koombhongse, S., Reneker, D.H., 2000. Bending instability of electrically charged liquid jets of poly-mersolutionsinelectrospinning. J. Appl. Phys. 89, 3018–3026. Zhou, W.P., Wu, Y.L., Wei, F., Luo, G.H., Qian, W.Z., 2005. Elastic deformation of multiwalled carbon nanotubes in electrospun MWCNTs–PEO and MWCNTs–PVA nanofibers. Polymer 46, 12689–12695.