Preparation and characterization of chitosan-based ...

Materials Letters 132 (2014) 23–26

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Materials Letters journal homepage: www.elsevier.com/locate/matlet

Preparation and characterization of chitosan-based nanofibers by ecofriendly electrospinning Yanan Liu a, Mira Park b, Hye Kyoung Shin c, Bishweshwar Pant a, Soo-Jin Park c,n, Hak-Yong Kim a,nn a

Department of BIN Fusion Technology, Chonbuk National University, Jeonju 561-756, South Korea Department of Organic Materials and Fiber Engineering, Chonbuk National University, Jeonju 561-756, South Korea c Department of Chemistry, Inha University, 100 Inharo, Incheon 402-751, South Korea b

art ic l e i nf o

a b s t r a c t

Article history: Received 27 December 2013 Accepted 7 June 2014 Available online 16 June 2014

Cross-linked chitosan/poly(vinyl alcohol) (CS/PVA with weight ratios of 2:1, 1:1, 1:2 and 1:3) nanofibers have been successfully electrospun using 1% aqueous acetic acid. Viscosity average molecular weight of CS was reduced from 78.7 104 to 1.4 104 by electron beam irradiation (EBI) in order to improve its solubility. The effects of composition on morphologies and swelling property of electrospun nanofibers were investigated. Fourier transform infrared (FTIR) spectroscopy studies demonstrated main chemical structure of CS persisted after EBI treatment. Swelling behavior test after cross-linking confirmed that the non-toxic CS-based nanofibers have a potential application in the biomedical field. & 2014 Elsevier B.V. All rights reserved.

Keywords: Electrospinning Chitosan Poly(vinyl alcohol) EBI Nanofibers

1. Introduction Electrospinning is regarded as a well-known and versatile technique to fabricate micro and nanofibers with high porosity and surface area-to-volume ratio [1], and more importantly, morphological similarity to natural extracellular matrix. These architectural structural nanofibers are appropriate for biomaterials such as wound dressing, drug release, tissue engineering and so forth [2]. CS-based nanofibers have been identified as an excellent biomaterial, due to biodegradability, biocompatibility and antibacterial properties of CS [3]. Low solubility and stability of CS inhibit the electrospinnability of pure CS. Many methods such as alkalization, ultraviolet, gamma ray irradiation, and enzyme degradation have been utilized to improve the solubility [4,5]. Homayoni et al. [6] fabricated CS nanofibers from 90% CH3COOH solution after the hydrolysis of CS for 48 h. Recently, electrospun CS/PVA nanofibers have been successfully fabricated [7–9]. However, electrospinning conditions are relatively limited in terms of concentration, molecular weight, and degree of deacetylation of CS. Some solvents such as trifluoroacetic acid, dichloromethane or acrylic acid are employed in the process, residual toxic solvent

n

Corresponding author. Tel.: þ 82 32 876 7234; fax: þ82 32 867 5604. Corresponding author. Tel.: þ 82 63 270 2351; fax: þ82 63 270 4249. E-mail addresses: [email protected] (S.-J. Park), [email protected] (H.-Y. Kim).

nn

http://dx.doi.org/10.1016/j.matlet.2014.06.041 0167-577X/& 2014 Elsevier B.V. All rights reserved.

in electrospun products limits the applications in the biomedical field. Considering these aspects, it is an alternative approach to diminish molecular weight of polysaccharides by electron beam irradiation (EBI). The molecular weight of CS can be reduced without changing main structure under optimized conditions [10]. In this paper, CS was modified by EBI in order to dissolve completely in 1% aqueous CH3COOH. It provides a good way to get non-toxic and environmentally friendly system for electrospinning. Thus-obtained non-toxic CS-based nanofibers may become outstanding candidates for biomedical applications.

2. Experimental 8 wt% gelatinous CS (200,000 cps, the degree of deacetylation: 75–85%, Aldrich Co.) was made from 1% aqueous CH3COOH, and irradiated by EBI at a dose of 50 kGy. The irradiation was performed using an electron beam accelerator (beam energy of 2.5 MeV, beam current of 8.5 mA, conveyor velocity of 10 m/min, dose rate of 6.67 kGy/s, EBTECH Co., Ltd., Korea) at room temperature in an air atmosphere. 10 wt% PVA (Mw ¼ 85,000–124,000, Aldrich Co.) and CS solutions were mixed with different weight ratios (3:1, 2:1, 1:1, and 1:2). Glyoxal solution (40 wt% in H2O, Aldrich Co.) was added as a cross-linker (6 wt% with weight of PVA). The solution was electrospun at 18 kV by maintaining a tip-to-collector distance of 16 cm. A schematic diagram of the electrospinning process is shown in Fig. 1(A).

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Y. Liu et al. / Materials Letters 132 (2014) 23–26

Fig. 1. (A) The preparation process of CS/PVA nanofibrous mats; (B) SEM photographs of the CS/PVA nanofibrous mats 2:1, 1:1, 1:2, 1:3, respectively.

a b

Transmittance(a.u)

Intensity(a.u)

a b

c d

c d e f

1731

2933

g

1250

844

e f g

10

15

20

25

30

35

40

2Theta(deg.) Fig. 2. XRD patterns of (a) CS powder, (b) CS film after EBI, (c–f) CS/PVA nanofibrous mats 2:1, 1:1, 1:2, 1:3, respectively, and (g) PVA nanofibrous mats.

The chemical structures were confirmed by Fourier transform infrared spectroscopy (FT-IR, Varian 1000 Scimitar series). The surface morphologies and the diameters were determined by scanning electron microscopy (SEM, JSM-5900JEOL Co.). Information about the crystallinity was obtained by X-ray diffractometry (XRD, Rigaku Co.) with Cu Kα (λ ¼1.540 Å) radiation. Viscosity average molecular weight (M v ) of pristine and modified CS samples were measured with 1% CH3COOH solution at 30 1C by an Ubbelohde capillary viscometer and six dilutions (C: concentration) were tested for each CS sample. The flow times (tsolution and tsolvent) were used to calculate the relative viscosity (ηr ¼ t solution =t solvent ); M v were calculated based on the Mark Houwink equation as follows [11]: log ½η ¼ log K þ a log M v

4000

3500

3000

2500

2000

1500

1000

Wave number(cm-1) Fig. 3. FT-IR spectra of (a) CS powder, (b) CS film after EBI, (c–f) CS/PVA nanofibrous mats 2:1, 1:1, 1:2, 1:3, respectively, and (g) PVA nanofibrous mats.

(K, a: the constant values, 0.0474 and 0.723, respectively, and ½η: the intrinsic viscosity). The degrees of swelling were determined by incubating in DI water for 48 h and calculated as follows: Esw ¼ ðW e W 0 Þ=W 0 100 (Esw: the percentage water absorption at equilibrium, We: the weight at equilibrium, and W0: the original weight.)

3. Results and discussions The CS gelatinous paste became solution without any precipitation after EBI treatment. [η] values of pristine and modified CS samples were obtained by the limited value of ðηr 1Þ=C at C¼ 0,


25

PVA:CS (g:g) 1:2

1:1

2:1

3:1

360

Esw/%

300

240

180

120

PVA:CS(1:1) 60 3

6

9

12

glyoxal % Fig. 4. (A) SEM photographs of the CS/PVA (1:1) mats after incubating with different times 1, 5, 12, and 24 h, respectively; (B) The degrees of swelling for CS/PVA nanofibrous mats with different ratios and CS/PVA (1:1) nanofibrous mats with different concentration of glyoxal solution.

868.77 and 47.78 ml/g, respectively. After calculation, it was found that M v of CS was decreased from 78.7 104 to 1.4 104 after EBI treatment. Pure CS nanofibers were not made successfully from the irradiated CS solutions, while CS nanofibers were obtained by blending with PVA. Fig. 1(B) shows SEM photographs of nanofibrous mats. It is clear that the morphology depends on the content of PVA. As in Fig. 1B (a), many beads appeared in CS/PVA (2:1) nanofibers and diameter of all fibers was less than 100 nm. However, homogeneous and bead-free nanofibers were obtained with increasing PVA content. The average diameter of the fibers was approximately 160720 nm and distribution ranges were narrow, only from 1 nm to 300 nm, which was determined by averaging diameter of 100 random fibers. The results indicated the outstanding fiber-forming property of PVA. The higher content of CS in composite is better for biomaterial applications due to its antibacterial property [12]. It was implied that CS/PVA (1:1) nanofibrous mats were the best one which can be used as excellent biomaterials. Fig. 2 shows XRD patterns of CS and CS/PVA nanofibrous mats. CS powder had a relative spike peak at 2θ¼20.21 [13]. The other broader peaks (b) indicated that the crystallinity of CS was decreased by EBI–1%CH3COOH treatment, which led to dissolve CS completely. For PVA nanofibers, there is one typical peak around 2θ¼19.51 [14]. After blending PVA, the peak of composite shifted towards lower degree. With increasing CS content in the blend, further shifting of the peak took place (Fig. 2(c)–(f)), probably due to the ionic bonds interaction such as hydrogen bonding interaction occurred between CS and PVA molecules. XRD patterns could be expressed as a complex mixed pattern of chemical blending, not a simple mechanical mixing. The structures of CS (before and after EBI) and composite nanofibers were examined by FT-IR. In Fig. 3(a), CS exhibited – CH– stretching vibration at approximately 2900 cm 1. The broad peak at 3600–3050 cm 1 was due to the characteristic peak of – OH and –NH2, and the peak at 1585 cm 1 corresponds to the bending frequency of the amide N–H group [15]. After EBI treatment, amide N–H peak shifted to 1546 cm 1 and became bigger, meanwhile, –NH2 peak at 3300 cm 1 got broader. These changes were caused by the nitroxyl radical produced, but low-concentration level of nitroxyl radical implied that the backbone structure and main groups remained after EBI treatment [16]. The electrospun PVA nanofibrous mats showed absorption bands at 3325, 2933, 1731, 1250, 1096, and 844 cm 1, characteristic of ν (OH), ν (CH2), ν (CQO), ω (C–H), ν (C–O), and ν (C–C) resonances,

respectively [17]. Fig. 3(c)–(f) shows the characteristic broad band at 2933, 1731, 1250, and 844 cm 1, which became stronger with increased content of PVA. Fig. 4(A) depicts the effect of water on the CS/PVA nanofibers (1:1). The diameter of the nanofibers was found to be increasing with longer incubation time. However, the morphology of nanofibers showed fiber structures even after dipping for 1 day, which indicated that nanofibers possessed a good water resistant property after cross-linking. A dotted line in Fig. 4(B) exhibits the relationship between the degree of swelling and the ratios of CS and PVA. It showed that the degree of swelling decreased with increasing PVA content. CS/PVA (2:1) nanofibers exhibited the best swelling behavior in aqueous medium with more than 300% weight gain which pronounced less in the case of CS/PVA (1:3) due to higher stability by cross-linking. Furthermore, as-spun CS/PVA (1:1) nanofibrous mats containing 3, 6, 9, and 12 wt% of glyoxal solution are shown in solid line. The sample with 3 wt% glyoxal partly dissolved after immersing due to lower crosslinking degree, so the degree of swelling was only 166%. As in Fig. 4(B), the swelling property of cross-linked nanofibers was found to be dependent on the content of glyoxal solution. The nanofiber with 6 wt% glyoxal showed optimum swelling property.

4. Conclusions A series of cross-linked electrospun nanofibrous mats of CS/PVA were successfully prepared by 1% aqueous CH3COOH as an ideal solvent. This is a good approach to fabricate chitosanbased nanofibers by combined electron-beam irradiation and electrospinning processes. The advantage of this strategy lies on the fabrication of CS-based nanofibers with non-toxic and environmentally friendly aqueous solution. The composites showed good water uptake ability. This work may provide a new direction for biomedical applications.

Acknowledgments This research was financially supported by the Ministry of Education, Science Technology (MEST) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (No. 201210A0404613010100) and also by KRF grant funded by MEST (2012R1A2A2A01046086).

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