Journal of Molecular Structure 924–926 (2009) 208–213

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Journal of Molecular Structure

journal homepage: www.elsevier .com/locate /molstruc

Nanocomposite fibres for medical applications

E. Stodolak a,*, C. Paluszkiewicz b, M. Bogun c, M. Blazewicz a

a AGH – University of Science and Technology, Faculty of Materials Science and Ceramics, Department of Biomaterials, Al. Mickiewicza 30, 30-059 Krakow, Polandb AGH – University of Science and Technology, Faculty of Materials Science and Ceramics, Department of Silicate Chemistry, Al. Mickiewicza 30, 30-059 Krakow, Polandc Technical University of Lodz, Faculty of Textile Engineering and Marketing, Department of Man-Made Fibres, 50-952 Lodz, Poland

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 October 2008Accepted 9 January 2009Available online 20 January 2009

Keywords:FT-IR spectroscopyRaman spectroscopyFibrous nanocompositesAlginate

0022-2860/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.molstruc.2009.01.018

* Corresponding author. Tel.: +48 1261734.E-mail address: stodolak@agh.edu.pl (E. Stodolak).

Fibrous nanocomposites based on natural polysaccharides originated from algas were produced. Nano-composite fibres were obtained by a wet method. Two kinds of ceramic nanofillers were applied i.e.,nanometric amorphous silica (nSiO2) and natural hydroxyapatite (HAp). The presence of the nanofillersin a biopolymer matrix was determined using point X-ray microanalysis (SEM/EDS). FT-IR studies andRaman spectroscopy revealed that the presence of the nanofillers effected structure of the biopolymerchain, and structural changes depended on the introduced modifier type. Stronger effects were observedin case of material containing HAp as the nanofiller.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

The main concept of nanocomposite materials is based on dis-persion of nanometric particles (1 nm = 10�9 m) within polymer,ceramic or metallic matrices. Nanoscale of the dispersed phaseconcerns at least one of its dimensions which should not exceed100 nm. This requirement is not a strict rule and depends on char-acteristics of nanoparticles and the area of interest. Often objects of200 nm size and larger falls into the ‘‘nano” category [1–3].

The nanoparticles used for nanocomposites production arethermodynamically instable systems. They are characterised byhigh surface area which means a high amount of atoms showingexcessive surface energy, which in turn favours particles agglomer-ation and makes their dispersion within the matrix difficult. Dur-ing production of a nanocomposite a new interface between thenanofiller and the matrix is created. That is why even a smallamount of the nanofiller may have a profund effect on the nano-composite structure and thus its properties.

Last few years brought significant development of polymernanocomposites. This is caused by the possibility of significantimprovement of physicochemical properties of the polymer matrixand relatively simple methods of the nanocomposites production(e.g., in situ polymerisation, casting, drawing or injection) [4–6].

The key factor responsible for improvement of the nanocom-posite properties is degree of dispersion of the nanofiller withinthe polymer matrix. Only the sufficient dispersion of the smallamount of the nanoparticles in the matrix (up to 10 wt.%) allows

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obtaining the nanocomposite material with better mechanicalproperties than the conventional one. This is directly related tochanges taking place on the interface between a nanoparticle anda polymer chain. In the interface area, a polymer chain may under-go conformation changes and new chemical bonds or electrostaticinteractions may be created. These phenomena give rise to novelproperties of the nanocomposites, which are inaccessible for theconventional micrometric composites.

In order to follow the structural changes in the nanocompositematerials a variety of investigative methods suitable for verificationof complex phenomena taking place in the materials is utilised. Themost popular ones are microscopic (AFM, SEM, TEM) methods, ther-mal (DSC) and spectroscopic (FT-IR, Raman, XPS) [7–10].

Presence of specific nanoparticles in the polymer matrix allowsto obtain new properties such as: fire resistance (MMT), bacterio-cidity (Ag particles) or bioactivity (HAp, TCP) [11,12].

Last from the above mentioned properties of the nanocompos-ites made them interesting for biomaterials engineering. Nanopar-ticles of hydroxyapatite (HAp) or tricalcium phosphate (TCP)connected with a fibrous phase create a bio-mimetic system simi-lar to the natural fibrous-ceramic system building a bone.

Technology of the fibrous nanocomposites production bases ona standard ‘‘wet” method of fibres formation. Prior to the fibres for-mation bioactive nanofillers are introduced into the matrix solu-tion. The main purpose of the introduction of a bioactivemodifier into the polymer matrix is formation of a chemical bondbetween a bone and an implant according to the bioactivity mech-anism proposed by Hench [13,14].

This work presents a method of production of fibrous nanocom-posites based on natural biopolymer i.e., alginate. An initial sodium

E. Stodolak et al. / Journal of Molecular Structure 924–926 (2009) 208–213 209

alginate (NaAlg) solution was solidified in CaCl2 water solutionwhich in consequence led to calcium alginate fibres Ca(Alg)2. Theion exchange leads to obtain fibres which are more durable in invitro/in vivo conditions. Sodium alginate is soluble in water,whereas calcium alginate is sparingly soluble and thus more resis-tant to degradation in in vivo conditions.

In order to modify the biopolymer matrix two kinds of nanofil-lers i.e., nanometric hydroxyapatite (nHAp) and nanometric silica(nSiO2) were introduced into the initial NaAlg solution. The pres-ence of the nanofiller and its influence on the chemical structureof biopolymer chain was verified using vibrational (Raman) andoscillation (FT-IR) spectroscopy as well as by means of X-raymicroanalysis (SEM/EDS).

2. Experimental

Two kinds of modifiers were used to produce fibrous nanocom-posite materials i.e., nanometric hydroxyapatite of the natural ori-gin (nHA) and nanometric amorphous silica (nSiO2). Thehydroxyapatite was produced in Department of Advanced CeramicAGH-UST, Krakow by extraction from pig femural bones [15]. Theparticle size distribution was determined by DTS method usingZetasizer Nano-ZS (Malvern Inst., UK). An average size of thehydroxyapatite filler was about 50–70 nm. Specific surface areaof n-HA was determined by BET method using Nova 1200e appara-tus (Quantachrome Inst.). The specific surface area of the hydroxy-apatite was 73.6 m2/g. The second nanofiller used as fibrousnanocomposite modifier was commercially available silica (AldrichCo.). The main particle fraction for the nano-silica was 5–10 nm,and its specific surface area was 57.9 m2/g. As a precursor of thepolymer matrix a natural biopolymer i.e., sodium alginate (FMGBioPolymers) was used. The biopolymer consisted in �65% ofmanuurane acid (M) mers.

Fibres were spun from sodium alginate solution by the wet pro-cess, using a spinneret with 500 orifices of 0.08 mm in diameter. Alaboratory spinning machine was used, which made it possible tostabilise the technological parameters at a predetermined leveland keep them under continuous control. The nano-additives (aftertheir dispersion by ultrasounds) in the form of a suspension in asolvent were introduced into the spinning solution over the prep-aration period. The solidification of fibres was performed in a bathcontaining 3% aqueous CaCl2 solution and 0.03% HCl at tempera-ture below 40 �C. The fibre drawing process was performed intwo stages: in a plasticising bath with the same concentration asthat of the solidification bath, and then in an atmosphere of over-heated steam at 140 �C. After the solidification the bath residueswere washed off, and the fibres were dried at 40–60 �C under iso-metric conditions.

Fourier transform infrared spectroscopy (FTIR) studies werecarried out using Digilab FTS 60 spectrometer in the range of400–4000 cm�1. The transmission technique was applied and thesamples were prepared as standard KBr pellets with resolution2 cm�1.

FT-Raman spectra were collected using FTS6000 spectrometerequipped with Ge detector. The samples have been excited with1064 nm line of diode pumped, Nd-YAG Spectra Physics laser. Eachspectrum is the averaging of two repeated measurements of 8000scans each and 2 cm�1 resolution.

In order to determine fine structural changes taking place in thebiopolymer precursor – NaAlg, and then in a wet-formed CaAlg fi-bres (treated as the reference samples) Raman vibration spectros-copy was applied. This method was also used for structuralinvestigations of the nanocomposite biopolymer fibres. An addi-tional method confirming the presence of nHAp and nSiO2 waspoint and linear X-ray analysis in microareas (EDS/SEM) (Nova,

NanoSEM 200). This method was also used to determine degreeof dispersion of the nanofillers within the biopolymer matrix.

3. Results and discussion

Alginate are the common name of a family of linear polysaccha-ride-like polymers containing three different functional groups i.e.,–COO� (carboxyl), –C–O–C– (ether) and –OH (alcohol). The pres-ence of a carboxyl group results in weak acidic properties of poly-saccharides. Alginate acid reacts with metallic ions giving salts as aresult.

The precursor utilised during this investigations i.e., sodiumalginate is a salt containing a metallic ion connected to the car-boxyl group –COONa. During the solidification of sodium alginate(NaAlg) fibres in CaCl2 solution Na+ cation is exchanged withCa2+. The substitution of the monovalent cation with the divalentone results in a change of the polymer chain structure. The divalentcalcium cation fits into the biopolymer block structure like eggs inan egg box (Fig. 1). This binds the alginate polymer chains togetherby forming junction zones, resulting in gelation of the solution. Thestronger gelation effect is observed in case of the chains richer inG-type units i.e., guluronic acid mers [16–18]. This process was ap-plied to formation of calcium alginate (Ca(Alg)2) fibres. Fig. 2 pre-sents FT-IR spectrum of the biopolymer precursor i.e., sodiumalginate and a spectrum of calcium alginate fibres. The ionic ex-change did not changed characteristic vibrations within the car-boxyl group. The observed shift of bands positions concernedsymmetric and asymmetric stretching vibrations i.e., m(COO)sym

and m(COO)asym well known from the literature [19,20]. The car-boxyl group exhibits two bands: strong antisymmetrical stretchingband (for NaAlg at 1621 cm�1, for Ca(Alg)2 at 1608 cm�1) and aweaker symmetrical stretching band (for NaAlg at 1415 cm�1, forCa(Alg)2 at 1424 cm�1). The most important vibrational modes ofsodium alginate and calcium alginate are presented in Table 1.

The more detailed information about changes in the biopolymerchains after the ionic exchange may be provided by Raman spec-troscopy. The normal Raman spectra of alginates can be dividedinto two parts: vibrations of the polymer backbone in the rangeof <1300 cm�1 and stretching vibrations of the carboxyl functionalgroups with Raman shifts at P1300 cm�1 [21–23]. Interactions ofalginate with calcium ions lead to changes of band positions withthe most pronounced band shift of the symmetric COO� stretchingband from 1415 (NaAlg) to 1423 cm�1 (Ca(Alg)2). As previously de-scribed in an IR spectroscopy study, band shifts in this range are anindicator for the calcium content in alginate ‘egg-box’ structure.The most important Raman bands of sodium alginate and calciumalginate are presented in Table 2, while the selected FT-Ramanspectra are drawn in Fig. 3. The detailed analysis of spectra of so-dium and calcium alginate revealed that bands related to skeletalvibrations C–C and stretching vibrations C–O were relativelyshifted one to another (Table 2).

In the second part of the experiment nanometric ceramic parti-cles i.e., natural hydroxyapatite or amorphous silica were intro-duced into the biopolymer matrix consisting of sodium alginatesolution.

The process of the nanocomposite fibres production was carriedout in a way similar to the one applied for the pure alginate fibresi.e., by solidification of NaAlg fibres in CaCl2 solution. In the effectof the wet treatment and the ionic exchange (Na+ ? Ca2+) nano-composite materials with calcium alginate matrix were produced.FT-IR studies revealed that the applied solidification method al-lowed introduction of nanofillers into the bio-polymer matrix.Fig. 4 presents spectra of a nanometric HAp and the alginate fibrecontaining 3 wt.% of HAp. In the nanocomposite spectrum bandscharacteristic for the natural HAp were present. They were partic-

Fig. 1. Chemical structure of sodium alginate, NaAlg (a); calcium alginate, CaAlg (b); and the egg-box-type structure (c).

1800 1600 1400 1200 1000 800 600

Wavenumber (cm-1)

Ab

sorb

ance

(a.

u.)

a

b

Fig. 2. FT-IR spectrum of sodium alginate (a) and calcium alginate (b).

Table 1The most prominent FT-IR bands assignment for sodium alginate and calciumalginate.

Assignment NaAlg (cm�1) Ca(Alg)2 (cm�1)

m(COO)sym 1621 1605m(COO)asym 1415 1424m(CO) + m(CC) 1090 1088m(CO) + d(CCO) + d(CC) 1033 1033d(CO) + d(CCH) 948 946

Table 2The most prominent Raman bands for sodium alginate and calcium alginate.

Assignment NaAlg(cm�1)

Ca(Alg)2

(cm�1)

Skeletal C–C, C–O stretching, and C–C–H,C–C–O bending modes

810 816890 890956 958

Glycoside ring breathing mode 1095 1095Carboxyl stretching vibration: symmetric

stretching or C–O single bond stretching vibration1310 1310

Symmetric carboxyl stretching vibration 1412 1421

210 E. Stodolak et al. / Journal of Molecular Structure 924–926 (2009) 208–213

ularly visible in the range of PO3�4 group vibrations i.e.,

m = 603 cm�1 and m = 566 cm�1 [24,25]. The other HAp bands over-lapped with the ones characteristic for calcium alginate e.g., 1465and 1418 cm�1 band of CO2�

3 group vibrations overlapped the anti-symmetrical vibration of COO� group occurring at 1424 cm�1. It isknown that the ‘‘egg-box” model, in which every calcium ion is lo-cated inside electronegative cavities formed by two pairs of gulur-onate residues belonging to two polymer chains, can be used to

describe interactions between phosphate anions and the guluro-nate residues [26,27]. In this arrangement, the cation has tetrahe-dral structure i.e., the same as the phosphate anion. It is believedthat the stereo-chemical matching between inorganic (i.e., Ca2+

1500 1400 1300 1200 1100 1000 900 800

Wavenumber (cm-1)

Inte

nsi

ty (

a.u

.)

a

b

Fig. 3. FT-Raman spectra of sodium alginate (–) and calcium alginate (�).

1800 1600 1400 1200 1000 800 600

Wavenumber (cm-1)

Ab

sorb

ance

(a.

u.)

a

b

Fig. 4. FT-IR spectra of the natural hydroxyapatite (a) and calcium alginate fibrescontaining 3 wt.% of the hydroxyapatite (b).

E. Stodolak et al. / Journal of Molecular Structure 924–926 (2009) 208–213 211

or PO3�4 ) and organic parts results in the observed changes of the

nanocomposite material structure (Fig. 8) [28].Fig. 5 presents spectra of nanometric amorphous silica and cal-

cium alginate fibres containing 3 wt.% of nSiO2. Analysis of thenanocomposite spectrum confirmed the presence of nSiO2 in thebiopolymer matrix, which was indicated by a band in

1800 1600 1400 1200 1000 800 600

Wavenumber (cm-1)

Ab

sorb

ance

(a.

u.)

a

b

Fig. 5. FT-IR spectra of the amorphous nanometric silica (a) and calcium alginatefibres containing 3 wt.% of the nanometric silica (b).

m = 470 cm�1 which was absent in Ca(Alg)2 spectrum (Fig. 2b).The additional change observed in the nanocomposite spectrumwas change of ratio of intensities of bands from the wave numberrange of 1090–1035 cm�1. The higher intensity of 1090 cm�1 bandwas a result of overlapping of Si–O–Si group vibrations with C–Cand C–O skeletal vibrations of the glycoside ring.

Detailed analysis of Raman spectra of the pure calcium alginatefibres, and the composite calcium alginate fibres containinghydroxyapatite or silica revealed subtle differences in bands inten-sities and their mutual relation (Fig. 6). The small differences in thebands positions were close to the resolution of the applied methodwhich is 2 cm�1.

In the spectrum of the Ca(Alg)2 nanocomposite fibres contain-ing nSiO2 a shift of the relation of a band attributed to pulsatoryvibrations of the glycoside ring (m = 1094 cm�1) was observed.The shift was probably caused by stronger interaction o the nano-metric SiO2 on the polysaccharide chain structure. In the watersolution of sodium alginate the nanometric silica was hydrated,and in such state it may compete with calcium ions during theion exchange taking place in the alginate fibres solidification step.Examples of formation of organometallic complexes in which gly-coside rings of polysaccharide chains acted as ligands are knownfrom the literature [29,30]. The presence of a chelate ion doesnot influenced positions of the other bands characteristic for therings, which is similar to the calcium alginate nanocomposites.

The bands characteristic for carboxyl group stretching vibra-tions i.e., symmetric stretching (m = 1421 cm�1) or C–O single bondstretching vibration (m = 1310 cm�1) in the Raman spectra ofCa(Alg)2 with HAp had different relation that analogous bands inthe pure Ca(Alg)2 and Ca(Alg)2 with SiO2.

Similar change in bands relations were observed in differentrange of spectrum of Ca(Alg)2 with HAp. In the range attributedto C–C skeletal vibrations and C–O stretching vibrations and C–C–H and C–C–O vibrations (m1 = 816 cm�1, m2 = 890 cm�1,m3 = 958 cm�1) an increase of intensity of m2 = 890 cm�1 andm3 = 958 cm�1 bands was observed. In the spectra of pure Ca(Alg)2

and Ca(Alg)2 + SiO2 the relation of intensity of m1: m2:m3 bands was1:1:1, whereas in Ca(Alg)2 with 3 wt.% HAp the intensity was 2:2:1(Fig. 6). The observed change was probably caused by the presenceof relatively big particle between the polysaccharide chains, whichmight lead to deformation of glycoside ring or change of the bondslength.

Additional confirmation of the presence of nanometrichydroxyapatite in the alginate matrix was brought by FT-Ramanshift in which the spectra of HAp, Ca(Alg)2 and Ca(Alg)2 + HAp

1500 1400 1300 1200 1100 1000 900 800

Wavenumber (cm-1)

Inte

nsity

(a.u

.)

Fig. 6. FT-Raman spectra of calcium alginate (�), calcium alginate with 3 wt.% ofthe nanometric silica (–), and calcium alginate with 3 wt.% of the hydroxyapatite(���).

500 450 400 350 300 Wavenumber (cm-1)

Inte

nsity

(a.u

.)

Fig. 7. FT-Raman spectra of hydroxyapatite (�), calcium alginate (���), and calciumalginate with 3 wt.% hydroxyapatite (–).

Fig. 8. A schema of structure Ca(Alg)2 and anion PO3�4 showing the complementa

Fig. 9. Microphotographs of nanocomposite fibres modified by 3 wt.% HAp and 3 wt.% SiO2

212 E. Stodolak et al. / Journal of Molecular Structure 924–926 (2009) 208–213

were put together (Fig. 7). The analytical band was an intenseband at m = 480 cm�1 formed by overlapping of bands of HApand Ca(Alg)2 present in this spectrum range. The change of rela-tion between the bands (m1 = 480 cm�1, m2 = 440 cm�1,m3 = 350 cm�1) in Ca(Alg)2 with HAp spectrum in the relationto the bands from the same range present in calcium alginateand hydroxyapatite was a proof of formation of a nanocompositesystem.

The point X-ray analysis in micro areas (EDS/SEM) confirmedthe presence of the nanofillers in the calcium alginate matrix. OnEDS spectra (Fig. 9) energetic lines characteristic for a given ele-ment were visible i.e., Si line in Ca(Alg)2 + 3 wt.% SiO2 nanocom-posite (Fig. 9), and P line in Ca(Alg)2 + 3 wt.% HApnanocomposite (Fig. 8b). Moreover, in case of the nanocompositefibres with HA, the amount of calcium was higher because of itspresence in apatite structure Ca10(PO4)6(OH)2. This result of SEM/EDS study proves well dispersed both of nanoparticles into algi-nate matrix.

rity between the alginate – Ca complex and corresponding phosphate anion.

. EDS spectrum with chemical analysis confirmation nanofilliers into alginate matrix.

E. Stodolak et al. / Journal of Molecular Structure 924–926 (2009) 208–213 213

4. Conclusions

Nanocomposite materials: calcium alginate fibres containing3 wt.% of nHAp or 3 wt.% of SiO2 particles were characterised withhigh dispersion of the nanofillers in the polymer matrix. FT-IR andFT-Raman study showed that the nanoparticles influence thechemical structure of the biopolymer chain. Additionally, this workproved that spectroscopic tools such as FT-IR and FT-Raman can beused as complementary methods to characterise polymeric nano-composite materials.

The successful modification of the alginate fibres with the nano-filiers creates an opportunity to use the fibres as bioactive materi-als for tissue regeneration.

Acknowledgment

This work was supported by The Ministry of Science and Educa-tion, project No. R0804303.

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