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Development and spectral characterization of poly(methyl methacrylate)/hydroxyapatite composite for biomedical applications.

Currently, composite materials have gained momentum in the field of orthopaedics. Among the composite materials (ceramic/metals, ceramic/polymers and ceramic/ceramics) available, ceramic/polymer possess significant advantages of high mechanical reliability (polymers) and excellent biocompatibility (ceramics) for applications in load bearing areas. The present study is an attempt to develop a ceramic/polymer composite made by grafting PMMA with HAP by using suitable grafting agent. The synthesized composite was characterized using spectral techniques.

Introduction

The past three decades have witnessed a tremendous increase in the use of biomaterials for bone related surgical applications. In certain applications involving artificial bones and teeth, the thrust for high biocompatibility, bioactivity, ability for biodegradation and mechanical properties equivalent to bone and teeth is ensured from the clinical experience. Though the present generation of biomaterials like bioceramics and metallic alloys ensures for biocompatible and mechanical properties respectively, the requirement for a particular defect mismatches with the original bone. Previous reports indicate that the performance of composite materials is better towards individual components. Much work has been explored on ceramictmetal and ceramic/ceramic composites and hence the present study is aimed to develop ceramictpolymer composites (1).

It is a well-known fact that hydroxyapatite granules have been used clinically as substitutes for autografts in filling bone defects. Unlike other implant materials, HAP is biocompatible, non-toxic, resorbable, excellent osteo-conductive ability, possesses structure similar to bone mineral and form direct bond with bone. However, the migration of individual particles from the implant site before tissue in growth cause inconvenience, difficult to fill the surface of irregular bone defect and reconstruction. In recent years, several kinds of bioactive bone cements have been developed to over come this problem. This shortcoming of HAP was overcame by employing biodegradable HAP particulate composites and composites based on PMMA, polyethylene polyacrylic acid, chitosan, gelatin etc are reported (2).

Such biodegradable composites have the ability to degrade gradually and induce new bone growth, thus enabling the load to transfer gradually from the material to newly grown bone. Applying calcium phosphate on copolymer has shown to enhance bone-bonding rates and therefore the use of HAP/polymer composites may improve both bone bonding rate and mechanical properties (3).

Various methods have been developed to improve the interface of HAP with a polymer matrix such as Silane coupling agents (4). Zirconyl salts (5), polyacids (6) and chemically coupling hydroxyethyl methacrylate to octacalcium phosphate through co-precipitation methods. Licu et al., have shown that it is feasible to chemically graft PMMA to the surface of nano-HAP using isocyanato ethylmethacrylate (ICEM) as a binding moiety on the mineral surface (7). Hence in the present investigation, an attempt has been made to modify HAP with hydroxy functionlised PMMA using coupling and grafting technique.

Materials and Methods

Preparation of Polymethylmethacrylate

Poly (methyl methacrylate) and chain-terminated poly (methylmethacrylate) were prepared using methylmethacrylate (MMA) as a monomer. 80 ml of monomer was taken in a 500 ml round bottom three-necked flask fitted with mechanical stirrer, nitrogen inlet and separating funnel. The aqueous redox initiators consisting of 1.6g ferroussulphate (FeS[O.sub.4]) and 3.2g of potassiumpersulphate ([K.sub.2][S.sub.2][O.sub.8]) were dissolved in 200ml distilled water and added to methylmethacrylate monomer.

The reaction was carried out at 80[degrees]C for three hours. After completion, the polymer was precipitated with isopropyl alcohol and dried at 35[degrees]C for 48 hours. Chain terminating agent such as thioglycolic acid (10% of the monomer) was added along with the methyl methacrylate monomer to prepare a low molecular weight functional group containing PMMA. The same experimental condition mentioned above was followed in the preparation of the functional group containing PMMA.

A mixture of 100 ml viscous solution containing poly (vinyl alcohol) (PVA) (1.25 g), poly(vinyl pyrolidine) (PVP) (0.25 g) and polyethylene glycol (PEG) 1000 (1.5 g) was first introduced into the reaction vessel and heated to ambient temperatures followed by stirring at 700 rpm/min for 15 min. A mixture containing 15ml of MMA (GMA 10% of MMA), 2-mercapto ethanol (chain transferring agent 10% of BPO initiator, 10% PMMA) and ethylene glycol dimethacrylate (EGDMA) cross-linking agent was added to the reaction mixture drop by drop. The reaction was allowed to proceed for three hrs. Triton x-100 was used to settle the microsphere. The settled microsphere were washed with distilled water for several times and finally dried at 60[degrees]C for 12 hrs.

Synthesis of Hydroxyapatite

Hydroxyapatite (HAP) was synthesized by the slow addition of 0.6M [H.sub.3]P[O.sub.4] (Phosphoric acid) to the aqueous suspension of 1.0M Ca[(OH).sub.2] (Calcium hydroxide) under constant stirring as per the procedure detailed elsewhere (8). The resultant was filtered and dried at 50[degrees]C for 3 hours and sintered in air atmosphere at 1100[degrees]C for 2 hours. To obtain principal HAP critical control of the pH of the reaction and concentration of the reactant is required. The final product was characterized for its crystallinity and phase behaviour through instrumentation techniques.

10 Ca[(OH).sub.2] + 6[H.sub.3]P[O.sub.4] [right arrow] [Ca.sub.10][(PO4).sub.6][(OH).sub.2] + 18[H.sub.2]O

Coupling of Hydroxy apatite (HAP) Isocyanatoethyl methacrylate (ICEM)

20 gm of sintered HAP powder was suspended in 200 ml of dry dimethyl formamide (DMF) under nitrogen atmosphere. 4 ml of Isocyanateoethyl methacrylate (ICEM) and 0.3 ml of catalyst (isobutyl tindilaurate) was added, then the reaction mixture was kept at 50[degrees]C for 24 hrs under stirring. Centrifuging separated the resulant product and washed with DMF as well as with CH[Cl.sub.3] and finally dried at room temperature.

Grafting of poly (methyl methacrylate) on to coupled Hydroxyapatite

To graft PMMA with ICEM coupled HAP 3 gm of the powder was mixed with 3.5 gm of MMA and 3.5gm of PMMA beads of cold acrylic resin system. Benzyl peroxide (BPO) 1 % and N,N-dimethyl-P-toludine (DMPT) 2% were used as initiator and accelerator. After curing, the resin blocks were dissolved in chloroform, separated by centrifugation, washed with large amount of solvents and finally dried at 60[degrees]C. Grafted polymer was isolated by dissolving in methanol-hydrochloric acid mixtures and the remaining insoluble were washed with distilled water. The product obtained is Hydroxyapatite-isocyanato ethyl methacrylate--Poly (methyl methacrylate) (9). The mixture can be schematically explained as follows,

O-OH + OCN-C[H.sub.2]-C[H.sub.2]OC(O)(CC[H.sub.3]) = C[H.sub.2](ICEM) PMMA (MMA)

[right arrow]O [approximately equal to] [approximately equal to] [approximately equal to] (C[H.sub.3])=C[H.sub.2]) [right arrow] O [approximately equal to] [approximately equal to] [approximately equal to] PMMA

Results and Discussion

The FT-IR spectrum of PMMA (Figure 1) indicates the details of functional groups present in the synthesized PMMA. A sharp intense peak at 1731 [cm.sup.-1] appeared due to the presence of ester carbonyl group stretching vibration. The broad peak ranging form 1260-[1000.sup.-1] can be explained owing to the C-O (ester bond) stretching vibration. The broad band from 950-650 [cm.sup.-1] is due to the bending of C-H. The broad peak ranging form 3100-2900 [cm.sup.-1] is due to the presence of stretching vibration. Figure 2 shows the NMR spectra of PMMA. The main features of the [sup.1]H-NMR spectra is the peak corresponding to the methoxy carbon (OC[H.sub.3]) at [delta]=3.57-3.64, which confirms the PMMA microsphere.

[FIGURES 1-2 OMITTED]

Characterization of HAP powder

Figure 3 shows the IR spectrum of raw HAP powder. The broad peak ranging from 3300-3600 [cm.sup.-1] can be explained owing to O-H group stretch vibration. The band at 1450 [cm.sup.-1] is assigned to the C[O.sub.3.sup.2-] stretching. An intense P[0.sub.4.sup.3-] peak appeared at 1048 [cm.sup.-1]. Additional phosphate group bands are found in the region 963, 875, 633 and 472 [cm.sup.-1]. Figure 4 shows the IR spectrum of HAP powder sintered at 1100[degrees]C. In this spectrum, a sharp peak of OH stretch vibration band appeared at 3570 [cm.sup.-1] and a less intense C[O.sup.3-] peak appeared at 1450 [cm.sup.-1].

[FIGURES 3-4 OMITTED]

An intense P[O.sub.4.sup.3-] peak appeared at 1048 [cm.sup.-1]. The additional phosphate peaks are found in the region 1110, 975, 600 and 470 [cm.sup.-1]. Also the XRD pattern shown in the figure 5 indicates the presence of pure HAP phase matching with JCPDS files.

[FIGURE 5 OMITTED]

Characterisation of isocyanato ethyl methacrylate coupled hydroxyapatite

Figure 6 illustrates the IR spectra of ICEM coupled Hydroxyapatite. The C[H.sub.2] stretch and the ester carbonyl peak can be seen clearly at 2960 and 1740 [cm.sup.-1] respectively. The amide absorption bands at 1660 [cm.sup.-1] and 1570 [cm.sup.-1] are all visible in the ICEM-HAP spectrum. An intense P[O.sub.4.sup.3-] peak is found at 1040 [cm.sup.-1] is found in the regions of 1100, 963, 875 [cm.sup.-1].

[FIGURE 6 OMITTED]

Characterization of HAP/PMMA grafted composite using isocyanato ethyl methacrylate (ICEM) as coupling agent

Figure 7 shows the IR spectrum of HAP/PMMA grafted product via ICEM. The band at 3339 [cm.sup.-1] is assigned to the hydrogen-bonded, N-H group. The band at 3000 [cm.sup.-1] is assigned to C[H.sub.3] stretch vibration. A very strong band appeared at 1735 [cm.sup.-1] belongs to the carbonyl group, as well as amide I and amide II bands at 1623 and 1580 [cm.sup.-1] respectively. Band at 2940 [cm.sup.-1] is from -C[H.sub.2] stretch vibration.

[FIGURE 7 OMITTED]

The C-O-C peak at 1254, 1200 [cm.sup.-1] and an intense phosphate peak is found is found at 1040 [cm.sup.-1]. The additional phosphate group bands at 810 [cm.sup.-1] and 757 [cm.sup.-1]. In this spectrum, there is no peak at 2270 [cm.sup.-1]. Indicating no residual isocyanate group. The FT-IR spectra provide strong proof that the functionalized PMMA were actually grafted to HAP through ICEM. In the spectra, therefore, a PMMA spectrum is clearly recognizable.

Conclusion

The experimental results indicate that the surface hydroxyl group of Hydroxyapatite has the ability to react with organic isocyanate groups [10]. Therefore the hydroxyl group on the surface of HAP provides reactive sites for chemically coupled organic polymers. The present investigation illustrates that PMMA can be chemically bonded to Hydroxyapatite surface using ICEM as coupling agent. The grafted PMMA on HAP showed the existence of polymer on the surface of HAP that was confirmed from the spectral data.

As a class, PMMA was successfully coupled and grafted onto HAP particles via covalent bonding of isocyanate groups. The possibility of realizing a chemical bonding between HAP and a polymer matrix provides a wide range of possibilities for integrating HAP in composites.

Acknowledgement

The authors are thankful to Indian Council of Medical Research (ICMR) for providing financial assistance.

References

(1.) S. Kannan, A. Balamurugan, S. Rajeswari and M. Subbaiyan, Corrosion reviews, 20 (4-5), 339-358 (2002).

(2.) K. Kargupta, P. Rao and A. Kumar, J. Appl. Polymer Sci., 49, 1309-1317 (1993).

(3.) P. Calvert and S. Mann, J. Mater. Sci., 23, 3001-3013 (1988).

(4.) A. M. P. Dupraz, J. R. de Wijn, S. A. T. Vander Meer and K. de Groot, J. Biomed. Mater. Res., 30, 231-238 (1996).

(5.) D. N. Mishra, J. Dent. Res., 12,1405-1408 (1985).

(6.) Q. Liu, J. R. de Wijn, D. Bakker and C. A. Van Blitters Wijk, J. Mat. Sci. Mater. Med., 7, 551-557 (1996).

(7.) J. R. de Wijn and C. A. Van Blitters Wijk, "Grafting PMMA on Hydroxyapatite powder particles using isocyanato ethyl methacrylate", [Abstract], in Transactions of 5th world Biomaterials Congress, Toronto, 663-678 (1996).

(8.) T. M. Sridhar, Ph.D thesis, Submitted to University of Madras, 2002.

(9.) V. Delpech and M. Lebugle, Clin. Mater., 5, 209-216 (1990).

(10.) J. R. de Wijn and C. A. Van Blitters Wijk,, "Surface modification of nano-apatite by grafting organic polymer", in Transactions of 5th world Biomaterials Congress, Toronto, 443-449 (1996).

A. Balamurugan, S. Kannan, V. Selvaraj and S. Rajeswari

Department of Analytical Chemistry

University of Madras, Guindy Campus, Channel 600 025

e-mail: anbala@indiatimes.com
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Author:Balamurugan, A.; Kannan, S.; Selvaraj, V.; Rajeswari, S.
Publication:Trends in Biomaterials and Artificial Organs
Geographic Code:1USA
Date:Jul 1, 2004
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