Electrospinning of Chitosan

Electrospinning of Chitosan

Kousaku Ohkawa,*1Dongil Cha,1Hakyong Kim,2Ayako Nishida,1Hiroyuki Yamamoto1

1Institute of High Polymer Research,Faculty of Textile Science and Technology,Shinshu University,Tokida3-15-1, Ueda386-8567,Japan

Fax:(t81)268-21-5571;E-mail:kohkawa@giptc.shinshu-u.ac.jp

2Department of Textile Engineering,Chonbuk National University,Chonju644-765,Republic of Korea

Received:June16,2004;Revised:July29,2004;Accepted:July30,2004;DOI:10.1002/marc.200400253 Keywords:chitosan;electron microscopy;electrospinning;?bers;poly(vinyl alcohol)

Introduction

The nano?ber technology of present interest focuses on the electrospinning technique,which conveniently allows the preparation of?brous materials with very?ne diameters ranging from submicron to several nanometers.[1,2]The electrospinning phenomenon itself involves basic and signi?cant issues in polymer science for solution dynamics, in which viscoelastic parameters,surface free energy (surface tension)and electroconductivity are critical factors for the successful spinning of nano?bers.[3,4]

The systematic understanding of the spinnability with an applied electric?eld and the polymer solution parameters has been investigated.[5–8]A pioneering research group in this?eld comprehensively investigated the detailed mech-anism of electrospinning,including jet initiation,growth of bending instability,elongation of the jet and solidi?cation of the jet into the nano?ber.[9–12]The polymer solution to be electrospun is usually placed in a pipette connected to the positive electrode.First,as the electric?eld is applied,a charged jet is emitted from the droplet on a pipette tip. Second,the electri?ed liquid jet of the polymer solution evolves its electrically driven bending instability towards the collector,which neutralizes the charge of the jet.Third, the?uid jet solidi?es into a nano?ber through evaporation of the solvent,resulting in the deposition of the nonwoven ?brous membrane on the collector.Thus,the solvent per-forms two crucial roles in electrospinning.One is to solvate the polymer molecules,ready to form the electri?ed jet. The other is to carry the solvated polymer molecules towards the collector,then to leave the polymer?bers by rapid vaporization of the solvent molecules.The appro-priate selection of a solvent system is a prerequisite for successful electrospinning.

Summary:An electrospun nonwoven fabric of a cationic polysaccharide,chitosan,was successfully prepared.The present study focuses on the effect of the electrospinning solvent and the chitosan concentration on the morphology of the resulting nonwoven fabrics.The solvents tested were dilute hydrochloric acid,acetic acid,neat formic acid and tri?uoroacetic acid.As the chitosan concentration was in-creased,the morphology of the deposition on the collector changed from spherical beads to interconnected?brous networks.The addition of dichloromethane to the chitosan-TFA solution improved the homogeneity of the electrospun chitosan?ber.Under optimized conditions,homogenous (not interconnected)chitosan?bers with a mean diameter of 330nm were

Electrospinning of Chitosan

prepared.

Effects of the coexisting dichloromethane(MC)in the pre-

spun chitosan-TFA solution on the morphology of the elec-

trospun chitosan?bers.The volume ratio of TFA:MC was

70:30(?5000).

1600Communication

Nano?bers from various synthetic polymers have been reported,as well as those from natural polymers,including proteins,[13,14]nucleic acids[15]and polysaccharides.[16,17] Electrospun nonwoven?brous materials of several natural polymers have been characterized with respect to their app-lications as biocompatible or bioresorbable materials.[16] Because of its abundant production in nature and excellent biocompatibility,the cationic polysaccharide chitosan (Scheme1)is a very promising polymer for this purpose.[18,19]Recently,we reported the utilization of chito-san as a new type of?ber[20–23]or capsule material.[24,25] Chitosan is an interesting polymer because of its phy-sicochemical properties,including its solid-state struc-ture[26,27]and the dissolving state conformation.[28,29]In the solid state,relatively rigid crystallites form due to the regu-larly arranged hydroxyl and amino groups at the equatorial positions in the b(1,4)-linked D-glucosamine repeating units,[30]while in solution,hydrogen bonding drives the formation of micro?brils,depending on the chitosan con-centration.[31]Such characteristics of chitosan guide the methodology for the successful electrospinning of this material and will further form an interplay between basic polymer chemistry and advanced materials science.There have been several attempts to prepare a nonwoven fabric of chitosan by the electrospinning technique.However,these attempts were not for a pure chitosan system but used a blended system of chitosan with poly(ethylene glycol)[32] or employed chitosan derivatives.[33]Thus,an electrospun nonwoven fabric has not yet been prepared from a pure chitosan system.Therefore,the present study was under-taken to examine the electrospinning of pure chitosan. The present study involved two parts to prepare the electrospun nonwoven fabric of chitosan.In the?rst part, the blended system was examined.The chitosan was mixed with another polymer,which can interfere with the rigid association of the chitosan molecules.Poly(vinyl alcohol) (PV A)was chosen for two reasons:(i)PV A strongly inter-acts with chitosan through hydrogen bonding on a molec-ular level;[34,35](ii)PV A can be conveniently electrospun from an aqueous medium.[5,36]In the second part,the pure chitosan system was con?rmed.The appropriate solvent for the electrospinning of chitosan was experimentally determined from a range of several acidic media,including dilute hydrochloric acid,acetic acid,neat formic acid,di-chloroacetic acid and tri?uoroacetic acid.These solvents were selected because they are frequently used in solution dynamic studies of chitosan.[28,29,34,37,38] Experimental Part

Materials

The viscosity average molecular weights of the chitosan samples were determined according to Robert.[38]For the elec-trospinning experiments,two grades of commercial chitosan were purchased from Wako Pure Chemical Industries,Ltd., Japan.The?rst was chitosan10(viscosity average molecular weight,M v?2.1?105;[20]degree of deacetylation,0.78)and the second was chitosan100(M v?1.3?106;degree of deacetylation,0.77).Poly(vinyl alcohol)(PV A;degree of polymerization,approximately2000;M n?8.8?104)was purchased from Wako.Acetic acid(AcOH),formic acid (FA)and dichloromethane were also purchased from Wako. Tri?uoroacetic acid(TFA)was obtained from Tokyo Chemical Industry Co.,Ltd.All the solvents were used without further puri?cation.

Electrospinning Apparatus

The apparatus for the electrospinning experiments was assem-bled based on previous studies.[6,7]The electrospinning experiments were performed at room temperature.The poly-mer solution was placed into a3mL syringe with a capillary tip having an inner diameter of0.6mm.A copper wire connected to the positive electrode was inserted into the polymer solution.

A copper plate wrapped with aluminum foil was used as the collector and the collector was connected to the ground.A high voltage power supply(HAR-50P2,Matsusada Precision Inc., Japan)was employed to generate the electric?eld(0–30kV). The applied voltage and the tip-to-collector distance were?xed at15kV and150mm,respectively.

Electrospinning Procedures

First,PV A was dissolved in distilled water(DW)at a concen-tration of9wt.-%,and chitosan10was dissolved in neat FA at 7wt.-%.A PV A-DW solution(9wt.-%)was mixed with a chitiosan10-FA solution(7wt.-%)in the volume ratios90:10, 70:30,50:50and30:70.Separately,a chitosan100-neat FA (2wt.-%,or0.2M AcOH)solution was mixed with a PV A-DW solution(9wt.-%)in a volume ratio of50:50.The mixed solutions were then subjected to the electrospinning experi-ments with the apparatus parameters as described above. Second,the chitosan10was dissolved at concentrations ranging from3to9wt.-%in the following solvents:neat FA, DCA,TFA and aqueous acetic acid(0.2M AcOH)and hydro-chloric acid(0.1M HCl),and their mixtures with methanol, ethanol,1,4-dioxane,dichloromethane,N,N-dimethylform-amide or dimethylsulfoxide were used as the solvents.The electrospinning of pure chitosan?bers was examined with the apparatus parameters described

Electrospinning of Chitosan

Electrospinning of Chitosan

above.

The morphologies of the electrospun?bers were observed using a Hitachi S-2380N scanning electron microscope(SEM) or a Hitachi S-5000FE-SEM at an accelerating voltage of5or 10kV.All samples were sputtered with platinum for2min prior to their observation.

Results and Discussion

Nonwoven Fabric from Chitosan/PVA Blend System The solvents used for dissolving chitosan10and PV A were formic acid(FA)and distilled water(DW),respectively.At a chitosan10:PV A ratio of100:0(corresponding to a7wt.-% chitosan10-FA solution),no jet was seen upon applying the high voltage even above25kV.Therefore,under these conditions,an electrospun?ber of chitosan could not be obtained.

Figure1shows SEM photographs of the chitosan/PV A blended electrospun fabrics.When a small portion of the PV A was mixed with chitosan,as presented in Figure1a (chitosan:PVA?90:10),beads were deposited on the col-lector.As the ratio of the chitosan solution increased (chitosan:PVA?70:30),the size of the beads became smal-ler and thin?bers coexisted among the beads(Figure1b). When equal volumes of the chitosan and PV A(50:50) solutions were blended,homogenous?bers with an average diameter of120nm could be spun(Figure1c;diameter distribution,83–170nm).At a chitosan:PV A ratio of

Electrospinning of Chitosan

30:70, Figure1.SEM photographs of the chitosan and PV A blended electrospun?bers(magni?cation?

10000).The chitosan10was dissolved in formic acid at7wt.-%and PV A was dissolved in distilled

water at9wt.-%.The two solutions were mixed in the speci?ed volume ratios and then electrospun.

The volume ratios were chitosan10:PVA?90:10(panel a),70:30(panel b),50:50(panel c),

30:70(panel d)and0:100(panel e).Chitosan100was dissolved in formic acid(or0.2M acetic acid)at

2wt.-%and the solution was mixed with9wt.-%PV A in a volume ratio of50:50,then the mixed

solution was electrospun(panel f).

1602K.Ohkawa,D.Cha,H.Kim,A.Nishida,H.Yamamoto

the?bers were thicker(Figure1d,average diameter,170nm; diameter distribution,110–220nm)than those prepared at50:50.This could be due to the electrospun pure PV A ?ber having a relatively thick mean diameter of470nm (Figure1e;diameter distribution,370–620nm).Electro-spun?bers were also prepared from a blended solution of chitosan100(2wt.-%)-FA(or0.2M AcOH)and PV A (9wt.-%)-DW in the volume ratio50:50(Figure1f;average diameter,170nm;diameter distribution,120–220nm). Electrospinnig of Chitosan Fibers

Among the solvents tested for the electrospinning of chito-san(0.2M AcOH,0.1M HCl,neat FA,DCA and their mixtu-res with the volatile organic solvents such as methanol,ethanol and1,4-dioxane,dichloromethane,and also with aprotic solvents including N,N-dimethylformamide and dimethylsulfoxide),none of them produced a visible jet as the electric?eld was applied(data not shown).

Only when TFA was used as the solvent,chitosan?bers were deposited onto the collector.The SEM photographs of the deposited chitosan are represented in Figure2.The morphology of the deposited chitosan depended on its con-centration in the TFA solution.When the chitosan concen-tration was6wt.-%or less,the beads and?bers coexisted in the SEM images(Figure2a and2b).At a chitosan concen-tration of7wt.-%,?bers were predominantly deposited while the bead fraction remarkably decreased(Figure2c and2d;average diameter,490nm;diameter distribution, 330–610nm).An almost homogenous network of

Electrospinning of Chitosan

the Figure2.Morphological changes in the electrospun?bers of chitosan10.Chitosan10was dissolved

in tri?uoroacetic acid(TFA)at the speci?ed concentration,then the chitosan-TFA solutions were

electrospun.Chitosan10concentrations were5wt.-%(panel a;magni?cation,?1000),6wt.-%(panel

b,?1000),7wt.-%(panel c,?1000;panel d,?10000)and8wt.-%(panel e,?1000;panel f,

?10000).

Electrospinning of Chitosan1603

electrospun chitosan?bers was observed at8wt.-% (Figure2e;average diameter,490nm;diameter distribu-tion,390–610nm).There are two possible reasons why the electrospinning of chitosan is successful when using TFA: (i)TFA forms salts with the amino groups of chitosan[28] and this salt formation destroys the rigid interaction between the chitosan molecules,making them ready to be electrospun;(ii)the high volatility of TFA is advantageous for the rapid solidi?cation of the electri?ed jet of the chitosan-TFA solution.

In the case of8wt.-%chitosan-TFA solution,small beads (Figure2e)and interconnected?bers(Figure2f)were still found,suggesting that optimization of the electrospinning conditions would be necessary for the preparation of a homogenous(not interconnected)?ber network of chit-osan.One possible approach to this optimization was to mix a volatile organic solvent with TFA.Hence,a TFA and dichloromethane(MC)mixed solvent was examined (Figure3).When the network morphology prepared from the TFA:MC?80:20solvent(Figure3b;average diameter, 380nm;diameter distribution,200–660nm)was compared with that from90:10(Figure3a;average diameter,390nm; diameter distribution,230–650nm),the network of chito-san?bers became more homogenous.At a TFA:MC ratio of 70:30(Figure3c),small beads and interconnected?bers mostly cannot be seen,indicating that a homogenous?ber network could be prepared under these conditions.The mean diameter of the chitosan?ber thus obtained was 330nm and the?ber diameters were distributed from210to 650nm.By increasing the MC ratio above70:30,the chitosan partly separated from the solution. Conclusion

In this study,we succeeded for the?rst time in preparing pure chitosan?bers by an electrospinning technique.Both the chitosan/PVA blended system and the pure chitosan system produced homogenous?brous materials,having average diameters in the submicron range.Our ongoing studies will focus on the effectiveness of the solvent,TFA, in the electrospinning of chitosan in order to prepare?ner electrospun?bers in the nanometer range.In parallel,elec-trospinning experiments using other polysaccharides, especially cellulose,crosslinkable chitosan derivatives[22] and synthetic polypeptides are in progress and these results will be published

Electrospinning of Chitosan

elsewhere.

Figure3.Effects of the coexisting dichloromethane(MC)in the prespun chitosan-TFA solution on

the morphology of the electrospun chitosan?bers.Concentration of chitosan10was?xed at8wt.-%.

Chiotsan10was dissolved in a series of the TFA:MC mixed solvents and then the chitosan-TFA-MC

solutions were electrospun.The volume ratios of TFA:MC were90:10(panel a;magni?cation,

?5000),80:20(panel b,?5000),and70:30(panel c,?5000;panel d,?10000).

1604K.Ohkawa,D.Cha,H.Kim,A.Nishida,H.Yamamoto

Acknowledgements:This work was supported by the Grants-in-aid for21st Century COE Program for Scienti?c Research(No. 13555178,No.14750709,No.16651064)from the Ministry of Education,Culture,Sports,Science,and Technology of Japan.

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