Materials Letters 65 (2011) 1503–1505

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Materials Letters

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Layered chitosan conduits with controllable inner diameters

Kai Shen, Qiaoling Hu ⁎Institute of Biomedical Macromolecules, Zhejiang University, Hangzhou 310027, PR China

⁎ Corresponding author. Tel./fax: +86 571 87953726E-mail address: huql@zju.edu.cn (Q. Hu).

0167-577X/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.matlet.2011.02.054

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 January 2011Accepted 17 February 2011Available online 23 February 2011

Keywords:Colloidal processingMultilayer structureChitosan conduit

The aim of this study was to propose a new method to prepare chitosan (CS) conduits with controllablediameters, which obviated the need to change the mold frequently. The prepared CS conduit had a layeredstructure due to the unique in-situ precipitation mechanism. The external diameter of the CS conduit wasapproximately equal to the inner diameter of the cylindrical glass mold, while the inner diameter of the CSconduit can be controlled by the precipitation time of CS gel in NaOH aqueous solution. When the externaldiameter of the CS conduit was fixed, the inner diameter of it decreased with the increase of precipitation time.Because of the enhancement effect of drying stresses from both the external surface and the inner surface duringthe dryingprocess, the prepared CS conduit had a relatively highbending strength,whichwasmore than80 MPa.

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1. Introduction

Chitosan (CS), the N-deacetylated derivative of chitin, is thecopolymer of glucosamine and N-acetyl glucosamine unit linked byβ-(1–4) glycosidic bonds. As it has many favorable properties such asbiocompatibility, biodegradability, bioactivity and non-toxicity, CShas various applications, especially in biomedical field [1]. CS has beenprocessed into many forms, including microsphere [2], membrane [3],fiber [4], gel [5], rod [6], conduit [7,8] and scaffold [9]. With respect toCS conduit, it is mainly used in blood vessel tissue engineering [10],esophagus defect repair [11], spinal cord injury repair [12] and nerveregeneration [13]. So far as reported, the CS conduits are fabricatedeither by casting CS solution or by electrospinning CS nanofibers onspecially designed molds, both of which are followed by thedemolding process. Therefore, the inner diameter of the CS conduitis determined by the external diameter of the mold. If CS conduitswith different internal diameters are needed, the mold has to bechanged frequently, whichmay cost a lot of extramoney and energies.Thus here we initiated a new method to prepare layered CS conduitswith controllable diameters, which utilized a unique hydrogelforming mechanism and was both economical and time-saving.

2. Experimental

2.1. Preparation

The CS conduits were prepared by an original in-situ precipitationmethod [6]. Briefly, 5% (w/v) CS solutionwas obtainedby adding25 gCS(Mw=60 kDa, DD=90%) powder in 500 mL 2% (v/v) acetic acid and

agitating them vigorously for 4 h at room temperature. After held instatic condition for another 6 h to remove the air bubbles trapped in,some amount of the CS solution was cast on the internal surface of acylindrical glass mold and the mold was soaked in 5% (w/v) NaOHaqueous solution for 2 h to precipitate a CS membrane. Subsequently,the mold with CS membrane template was filled with the CS solution,followed by the precipitation in 5% (w/v) NaOHaqueous solution for 10,20, 30, 40, 50 and 60 min to form CS gel rod. At each time point, one CSgel rod was taken out from the precipitant, that is, 5% (w/v) NaOHaqueous solution and two ends of the CS gel rodwere cut off to pour outthe CS solution that had not been precipitated. After beingwashedwithdistilled water until the pH of washing water turned neutral, the CS gelconduit was obtained. Finally, when the gel rod was dried in an oven at60 °C for 24 h, a transparent, yellow CS conduit could be obtained. Theexternal diameter of the CS conduit was approximately equal to theinner diameter of the cylindrical glass mold, which was 16 mm in ourstudy, while the inner diameter of the CS conduit could be controlled byabove-mentioned the precipitation time.

2.2. Characterization

The digital photographs of CS gel conduits and CS dried conduitswere recorded by Sony DSC-W50 digital camera. The fracture surfaceof the CS conduit was observed by SEM (JSM-5510 LV, JEOL, Japan).The bending strengths of the CS conduits were measured by three-point bending test on Shenzhen Reger Company's universal materialstestingmachinewith span length as 40 mm and loading rate as 2 mm/min. The bending strength of the CS conduit was calculated fromEq. (1):

σb =8FmaxL

πd3 1−α4� � ð1Þ

Fig. 2. Forming mechanism of layered CS conduit: (A) The mold with CS template wasfilled with CS/acetic acid solution; (B) The first layer of the gel was formed with thepenetration of OH- through the CS membrane; (C) The second layer of the gel wasformed with the penetration of OH- through the first layer; and (D) Step C wasceaselessly repeated until both ends of the CS gel rod were cut off and the CS solutionthat had not been precipitated was pour out, and then the CS conduit could be obtained.

1504 K. Shen, Q. Hu / Materials Letters 65 (2011) 1503–1505

where Fmax was the maximum load (N), L was support span (mm), dwas the external diameter of the CS conduit and α was the ratio ofinternal diameter to external diameter.

The swelling degrees of these CS conduits in PBS at 37 °C werecalculated from Eq. (2):

Swelling degree ¼ Ww �W0ð Þ=W0 × 100% ð2Þ

where W0 was the original weight of the CS conduit, Ww was the wetweight of the CS conduit.

3. Results and discussion

Fig. 1 shows the digital photographs of CS gel conduits precipitatedfor different time. It can be observed that with the increase ofprecipitation time, from 10 min to 60 min, the inner diameter of CS gelconduit keeps decreasing. It can be inferred from this result that theCS gel conduit was precipitated layer by layer and from the outside tothe inside.

Fig. 2 illustrates the forming mechanism of CS conduit. The CSmembrane template can be viewed as a semipermeable membranethat permits some small molecules like Na+, H+ and OH- to passthrough, but not the large molecules, like CS. Because the concentra-tions of ions are not the same in different sides of CS membrane, theconcentration gradient will force OH- to permeate from the outside ofCS membrane (5% (w/v) NaOH aqueous solution) to the inside of it.Then OH- will react with the protonated amino groups of CS solution.With the proceeding of this neutralization reaction, the solubilityproduct of CS in H2O will be reached and the first layer of CS will beprecipitated inside the CS membrane. Then OH- keeps permeatinginto the CS solution, and the second layer of CS will be formed whenthe solubility product of CS in H2O is reached again. In this way, moreandmore layers of CS will be formed and the CS gel rod with a layeredstructure will be obtained. Therefore, when the CS solution is notentirely precipitated, a CS gel conduit can be obtained by cutting offtwo ends of the CS gel rod and pouring out unreacted CS solution.

Fig. 3(A) shows the digital photograph of dried CS conduit. It canbe observed that after drying process, the volume of CS conduitshrinks a lot, due to the evaporation of water. Fig. 3(B) shows the SEMimage of the fracture surface of CS conduit. It is not hard to find thatthe CS conduit has a layered structure, which directly demonstratesthe forming mechanism of CS conduit proposed by us.

The inner diameters of both CS gel conduits and CS dried conduitsprecipitated for different time in 5% (w/v) NaOH aqueous solution arelisted in Table 1, and the bending strengths of CS dried conduits arealso included. When the precipitation time increases from 10 min to

Fig. 1. Photographs of CS gel conduits precipitated for different time in 5% (w/v) NaOHaqueous solution.

Fig. 3. (A) The photograph of dried CS conduit; and (B) SEM image of the fracturesurface of CS conduit.

Table 1Inner diameters of both CS gel conduits and CS dried conduits, and bending strengths ofCS dried conduits with different precipitation time in 5% (w/v) NaOH aqueous solution.

Precipitationtime (min)

Inner diameter ofCS gel conduit(mm)

Inner diameter of CSdried conduit(mm)

Bending strength ofCS dried conduit(MPa)

10 11.38±0.22 3.78±0.06 104.0±3.120 10.64±0.18 3.31±0.05 97.2±2.530 8.73±0.16 2.83±0.05 94.3±1.740 7.73±0.15 2.56±0.04 90.3±2.050 7.39±0.15 2.37±0.05 89.8±1.660 5.52±0.12 2.02±0.04 80.6±2.3

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60 min, the inner diameters of both CS gel conduit and CS driedconduit keep decreasing. The CS dried conduit has a high bendingstrength. It is because that during the drying process, the intensedrying stresses from both the external surface and the internal surfacewill squeeze the CS gel conduit and make the intermolecular distanceof CS decrease, contributing to more and stronger hydrogen bonding.During the mechanical testing, the layered structure was stable. Noseparation of layers could be observed. When subjected to stresses,the CS conduit was broken layer by layer. Instead of expanding fromone layer to another layer, the cracks would preferentially expand inone layer.

These layered CS conduits can swell in PBS at 37 °C. Their swellingdegrees reach maxima in 48 hours and range from 58.3% to 65.2%.With the increase of inner diameter, the maximum swelling degreeincreases too. This is because the CS conduit with bigger innerdiameter has larger specific surface area.

4. Conclusions

In summary, CS conduits with controllable diameters weresuccessfully obtained via the above-mentioned method. The externaldiameter of the CS conduit was approximately equal to the innerdiameter of the cylindrical glass mold, while the inner diameter of theCS conduit can be controlled by the precipitation time of CS gel in

NaOH aqueous solution.When the external diameter of the CS conduitwas fixed, the inner diameter of it decreased with the increase ofprecipitation time. The prepared CS conduit had a layered structure,and the bending strength of it was relatively high due to theenhancement effect of drying stresses from both the external surfaceand the inner surface during the drying process. This CS conduit hasthe potential to be used in the biomedical field as a candidate for bonefracture fixation, blood vessel tissue engineering.

In fact, according to the proposed forming mechanism of CSconduit, except for the precipitation time, the inner diameter ofchitosan conduit can be controlled by the concentration of NaOHaqueous solution too. Moreover, the CS gel conduit can be lyophilized.Therefore, more systematic work has to be done next.

Acknowledgements

This work has been supported by National Natural ScienceFoundation of China (Grant Nos. 50773070 and 50333020), the KeyBasic Research Development Plan (Project 973) of China (Grant No.2009CB930104), and Grand Science and Technology Special Project ofZhejiang Province (Grant No. 2008C11087).

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