Effect of Fabrication Processes on the Antimicrobial Properties of Silver Doped Nano-Sized HAp

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1 Key Engineering Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Effect of Fabrication Processes on the Antimicrobial Properties of Silver Doped Nano-Sized HAp K-S. Oh 1, K-J. Kim 1, Y-K. Jeong 1, and Y-H. Choa 2 1 Nano Ceramic Center, KICET, Seoul, Korea, ksoh@kicet.re.kr 2 Dept. of New Materials Technology, Hanyang University, Ansan, Kyunggi-do , Korea Keywords: Hydroxyapatite, Silver, Antimicrobial, Nano-particle, Freeze-Dry, Synthesis Abstract. In fabrication of antimicrobial hydroxyapatite (HAp) doped with silver, the influences of particle size and drying route were investigated and discussed in relation with silver contents. Stronger antibiotics require as much silver as possible to be doped in HAp. In the introduction of silver ion through ion-exchange process, there were two critical experimental steps for the increase of silver content. By eliminating the supernatant solution through sublimation route, the formation of agglomerates was minimized. And fine nano-particles were obtained by rapid stirring during precipitation. These two steps contributed to the great surface area of HAp and as a result, 4.87% of silver was introduced. The corresponding antimicrobial properties were also found to be the strongest in this work. Introduction Hydroxyapatite (HAp) is a prominent biomaterial capable of guiding bone formation and bonding with natural bone due to its biocompatibility and osteoconductivity. HAp can also serve as a carrier of antibiotics to reduce the bacterial infections in the bone-implant interface [1,2]. By taking advantage of the ion-exchange capabilities, HAp can be effective in controlling microorganisms with the introduction of transient metal ions such as silver. The amount of silver incorporable in HAp, however, is limited to less than 4% [3] in the ion exchange process due to the OH vacancies generated to meet the charge balance with ion exchange [4]. Enhanced antimicrobial effect was expected by using the nano-scale carrier taking advantage of the large surface area for the increase of silver content [5]. In this work, while fabricating HAp from popular precursors such as Ca(NO) 3 and (NH 4 ) 2 HPO 4, experimental parameters required for nano-sized HAp were investigated particularly in relation to the nucleation density which is basically influenced from stirring rates. The importance of drying route was also analyzed in relation to the formation of agglomerates. The silver uptake and consequent antimicrobial properties against E.Coli were observed with respect to the particle size and the route of eliminating the supernatant solution. Experimental Procedure HAp (Ca 10 (PO 4 ) 6 (OH) 2 ) powder was precipitated by mixing the solution of Ca(NO 3 ) 2 4H 2 O and (NH 4 ) 2 HPO 4 at 80 C. Both solutions had a concentration of 0.5 M and weighed so that the molar ratio between Ca and P is 1.667:1. The phs of both solution were adjusted to 10 by adding NH 4 OH. The solutions were mixed by pouring (NH 4 ) 2 HPO 4 solution under stirring rate of either 100 or 1000 rpm. The ph and temperature chosen were necessary for the successful fabrication of the HAp phase [6,7]. After aging for 2 h, the precipitated solution was filtered and washed repeatedly with distilled water until a ph lower than 9 was attained from the cleaning water. The filtered cream was dried through either evaporation or sublimation. The evaporation of solution was done in the oven at 90 o C and the sublimation was carried out at 80 o C and 80 mmhg in a freeze-drier. Particle size analyzer was used to observe the size distribution of particles with drying routes. Transmission Electron Microscopy (TEM) sample was prepared by dipping the Cu grid in the as precipitated solution. By immersing the dried HAp powder in the AgNO 3 solution, Ag ion was introduced. The All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-12/05/16,22:25:30)

2 584 Bioceramics 15 concentration of AgNO 3 solution was prepared so that Ag should occupy 5,8,12 and 15 mol% of total cations (Ca+Ag) assuming complete exchange. The amount of introduced silver was verified using Inductively Coupled Plasma (ICP). The antimicrobial effects were measured by determining the smallest amount of sample (minimum inhibitory concentration: MIC) needed to inhibit the growth of E.Coli. Results and Discussion Figure.1 shows the HAp particles precipitated as a result of mixing the Ca(NO 3 ) 2 4H 2 O and (NH 4 ) 2 HPO 4 solutions at 80 o C and ph=10. The mixed solutions were aged for 2 h under either 100 or 1000 rpm in a beaker with the diameter of 30 cm. Whisker shaped powders shown in Fig.1 were typical morphology of HAp and X-ray diffraction analysis carried out after heat treatment at 700 o C verified the formation of HAp phase. At 100 rpm, mean width and length of the particle were around 50 nm and 500 nm respectively as shown in Fig. 1(a). On the other hand, at the stirring rate of 1000 rpm, mean width and length of the particle decreased to below 20 nm and 100 nm respectively. 100 nm (a) 100 rpm (b) 1000 rpm Figure.1. HAp particles precipitated and aged for 2 h under stirring rates of (a) 100 rpm and (b) 1000 rpm. Crystallization of particles from a liquid can be expressed in terms of a supersaturation that is the concentration in excess of the equilibrium value in a given experimental condition [8]. The chemical potential of precipitate component in the solution is higher than that in the solid phase and the difference in chemical potential drives the reaction to form the solid particles. The creation of new surface is accompanied along the path to equilibrium and the ultimate size of a particle precipitated from solution is governed by kinetic factors such as the rate of nucleation of new particles [9]. An increase in nucleation rate leads to smaller particle sizes at constant temperature as proposed by Nyvlt et.al. [9]. The necessary conditions for the birth of nuclei are existence of driving force and homogeneity enough to build up the stoichiometric precipitate. Rapid stirring can effectively decrease the volume in which homogeneity is ensured and thus the nucleation density would increase. To fully materialize the advantages of fine particles, the agglomeration should be minimized. It is well known that the elimination route of solution heavily influences the formation of agglomerates [10]. Fine particles have stronger tendency of agglomeration, since the attraction force originates from Van der Waals attraction. The moisture among the particles forms water and binds the particles even stronger with capillary forces and thus results in agglomerates. During the evaporation of liquid phase, droplets of liquid are inevitable and the capillary pressure of droplets result in agglomerates. To minimize the formation of agglomerates, liquid phase should be eliminated without forming droplets and sublimation is the proven alternative route for elimination. Fig. 2 shows the particle size distribution of HAp cream dried through either sublimation or

3 Key Engineering Materials Vols evaporation. It is shown that the sublimated HAp particles have smaller (~20 nm) and narrower distribution than evaporated HAp particles that have the average size of around 60 nm. Due to the high anisotropy of HAp particles, the particle size measured by particle size analyzer does not show excellent agreement with the one observed from TEM. 14 Frequency of Particles (%) Sublimated Evaported Particle size (µm) Figure 2. Size distribution of HAp particles dried through either sublimation or evaporation route. Table 1 summarizes the amount of silver ion exchanged in the HAp synthesized under 1000 rpm and subsequently dried through either sublimation or evaporation. Ag replaces Ca sites to generate OH vacancies to meet the charge neutrality as represented by the chemical form of Ca 10-x Ag x (PO 4 ) 6 (OH) 2-x [4]. Since 2-X should be greater than a null, theoretical limit of silver exchange must be 20% of total Ca +Ag. Table 1 Amount of silver detected from the ion-exchanged HAp Silver (mol%) in the solution assuming complete exchange Evaporated HAp Sublimated HAp The exchange is limited to the vicinity of surface, and therefore the amount of Ag exchange is even smaller than the theoretical one. In the case of HAp dried through evaporation, even if the amount of silver in the solution is increased from 5 to 15 and thus the chemical potential gradient for ion exchange is increased, the amount of silver in the powder did not exceed 2.21% showing the tendency of strong saturation. With sublimated HAp, however, the sliver uptake increased to as high as 4.87%, probably due to minimizing agglomerates and consequent increase of surface area. Therefore, eliminating the solution through sublimation resulted in the increase of the upper limit of silver ion exchange. Table 2 is the amount of Ag and antimicrobial properties with respect to drying routes and stirring rates. The highest silver content as 4.87% was found for the HAp stirred at 1000 rpm and dried through sublimation. Silver content dropped to 2.21%, however, when the HAp powder was dried through evaporation due to the formation of agglomerates. And even if the solution was sublimated, the silver content was limited to 2.33% when the stirring rate was 100 rpm and therefore the particles were rather coarse. Thus only by drying the fine particles through sublimation, effective

4 586 Bioceramics 15 increase in silver content could be achieved. The increase in silver content resulted in reduced MIC. The MIC against E.Coli was found to be smallest for the HAp powder with highest silver content, suggesting a stronger antibiotic. Table 2 Amount of silver and MIC of ion-exchanged HAp with respect to the drying route and stirring rate. Drying Route Evaporation Sublimation Stirring Rate [rpm] Ag [% Ag/(Ag+Ca)] MIC against E.Coli [ppm] Conclusion In the fabrication of antimicrobial hydroxyapatite doped with silver, the influences of particle size and drying route were investigated and discussed in terms of the introduction of silver contents. To minimize the formation of agglomerates, the liquid must be removed through sublimation with rapid stirring being another necessary condition for obtaining fine HAp particles. The silver content was highest and antimicrobial properties were strongest when both conditions were fulfilled. Acknowledgements This work was supported by a Grant-in-Aid for Next-Generation New Technology Development Program from the Korea Ministry of Commerce, Industry and Energy (No.N11-A ). References 1. M.Shirkhanzadeh, M. Azadegan, G.Q.Liu, Mat.Letters, Vol.24, (1995), p.7 2. Q-L.Feng, T.-N.Kim, J.Wu, E-S.Park, J-O.Kim, D.-Y.Lim and F.Z.Cui, Thin Solid Films. Vol.335, (1998) p L.Badrour, A.Sadel, M.Zahir, L.Kimakh, A.El Hajbi, Ann.Chim.Sci.Mat, Vol.23, (1998), p L.Merker and H. Wondratscheck, Z.Kristallogr., Vol.109, (1957), p K.-S.Oh, K.-J.Kim, and Y.-K.Jeong, J.Kor.Pow.Met.Inst., Vol.8, (2001), p S.-J. Jun, S. Kim and J.-H. Han, J.Kor.Ceram.Soc., Vol.35, (1998), p A.Cuneyt Tas, F.Korkusuz, M.Timucin, N.Akkas, J.Mat.Sci.;Mat.in Med., Vol.8, (1997), p N.S. Bell, S-B. Cho, and J.H. Adair, J.Amer.Ceram.Soc., Vol. 81, (1998),p J.Nyvlt, O.Sohnel, M.Matuchova, and M.Broul: The Kinetics of Industrial Crystallization, Chemical Engineering Monographs 19. (Elsevier, New York, 1985). 10. J.S.Reed: Principles of Ceramic Processing (Wiley Interscience, New York, 1995) pp

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