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Nanocomposites of cross-linking polyanhydrides and hydroxyapatite needles: mechanical and degradable properties
Materials Letters 58 (2004) 2819 – 2823 www.elsevier.com/locate/matlet
Nanocomposites of cross-linking polyanhydrides and hydroxyapatite needles: mechanical and degradable properties Hao-Ying Li *, Yun-Fa Chen, Yu-Sheng Xie Institute of Chemical Metallurgy, Chinese Academy of Sciences, Beijing 100080, China Received 23 September 2003; received in revised form 19 April 2004; accepted 3 May 2004 Available online 20 June 2004
Abstract Three methacrylated anhydride monomers of citric acid (MCA), sebacic acid (MSA) and 1,4-bis(carboxyphenoxy) butane (MCPB) were utilized to prepare biodegradable nanocomposites with homogenously distributed hydroxyapatite (HAp) nanoneedles by in situ photopolymerization, where the needle-like HAp particles were homogenously dispersed in cross-linked polyanhydride network at nanoscale. The incorporation of HAp cannot only dramatically increase the compressive properties of the nanocomposites but also play an important role in decreasing the cumulative mass loss and in increasing the mechanical integrity of nanocomposites during the degradation. The results showed that it was possible to prepare such HAp/polyanhydride nanocomposite, with proper mechanical strength and controlled degradable rate, by adjusting the content of HAp in the polyanhydrides and by changing the compositions in the crosslinking network to meet the rehabilitation need of different fracture bones in human body, both in mechanical properties and in the biodegradable rate. D 2004 Elsevier B.V. All rights reserved. Keywords: Nanocomposites; Polymers; Hydroxyapatite; Nanoneedles; Biodegradable; Mechanical; Photopolymerization
1. Introduction In contrast with metallic implants, biodegradable polymers used as internal fixation devices eliminate the second surgery of removal, avoid some problems caused by stress shielding, and can simultaneously deliver antiinfectious drugs and/or bone growth-enhancing factors [1]. In order to overcome the problems associated with bulk degradation of the widely used polyester orthopedic devices, crosslinked polyanhydrides undergoing surface erosion during degradation have been developed. Besides, cross-linked polyanhydrides have improved mechanical strength, controlled biodegradable rate, and can be easily fabricated into the desirable shapes, which demonstrates a great potential in orthopedic applications and might be a better alternative than polyesters as the internal fixation devices [2 –4].
* Corresponding author. Welsh School of Pharmacy, Cardiff University, Redwood Building, King Edward VII Avenue, Cardiff CF1 3XF, UK. Tel.: +44-29-2087-7226; fax: +44-29-2087-4149. E-mail address: [email protected] (H.-Y. Li). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.05.003
However, the compressive strength of these reported cross-linked polyanhydrides is still far lower than cortical bones and needs to be further increased to meet the need of load-bearing bones during rehabilitation [5]. Hydroxyapatite (HAp) nanoneedles have been utilized to reinforce cross-linked polyanhydride by a novel in situ photopolymerization method that has been reported by our group [6], where nanosized HAp needles are homogenously distributed in the cross-linking polyanhydride network to form a nanocomposite with dramatically increased mechanical properties and improved biodegradable characteristics. Unfortunately, the compressive strength of HAp/poly(methacrylated pyromellitylimidoalanine) [PM(PMA-ala)] nanocomposite alone cannot completely meet the requirements of cortical bones, and its fast degradable rate does not fit well with the healing rate of load-bearing bones which usually need 3– 6 months for full rehabilitation. In this study, the mechanical strength and biodegradable properties of three kinds of nanocomposites, fabricated with different cross-linking polyanhydrides and HAp nanoneedles via in situ photopolymerization, are investigated in detail. This study might provide some clues for further research of
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manufacturing such nanocomposites with sufficient mechanical properties and controlled degradable rate for the specific requirements of the broken bone during healing. Three anhydride monomers of methacrylated citric acid (MCA), sebacic acid (MSA), and 1,4-bis(carboxyphenoxy) butane (MCPB) were synthesized. The compressive strength and the degradation properties of the nanocomposites containing polyanhydrides and HAp with different content are investigated in detail. The results demonstrate that it is possible to fabricate such HAp/polyanhydride nanocomposites with proper compressive strength and biodegradable properties to meet the rehabilitation need of different human bones.
2. Experimental The preparation of needle-like HAp was prepared following the method as mentioned before [6]. The HAp length was manipulated to under 100 nm by controlling the concentration of dispersant polymethylmethacrylate amid (Mw = 10,000; A-6114, Toagosei, Japan). The particles appeared a needle-like shape within nanosized scale under the investigation of transmission electronic microscopy (TEM; JEM200CX, JEOL, Japan). X-ray diffraction (XRD) analysis demonstrated that the chemical component of these nanoneedles was Ca10(PO4)6(OH)2. Methacrylated anhydride monomers of MCA, MSA, and MCPB were synthesized via reacting citric acid (CA), sebacic acid (SA), and 1,4bis(carboxyphenoxy) butane (synthesized as Conix method Ref. [7]; CPB) with excess methacrylic anhydride, respectively, then purified in hexane and dried in vacuum desiccator [5]. The absorbance peak of CMC in the methacrylated anhydride compounds appeared at about 1636 cm 1 on Fourier transform infrared spectroscopy (FTIR; Nicolet Magna-IR 750; Madison, WI) spectrum. The chemical shift (y) of MCH2 protons in the methacrylate end-group was clearly evident in 1H-nuclear magnetic resonance (1H-NMR; DMX300, Bruker) image at approximately y5.8 and y6.2 parts per million (ppm). These characteristics demonstrated that the acids of SA, CA, and CPB had been methacrylated and end-capped with methacrylate functionalities. Furthermore, the ratio between the area of methylene hydrogens (y1.3) and the area of vinyl hydrogens (y5.8 and y6.2) on 1HNMR spectrum indicated that MSA was an oligomer of seven monomers and that MCPB was basically a monomer. Elemental analysis of MSA and MCPB was C: 64.37, H: 8.77, O: 26.65 (calculated theoretical for n = 7, C: 64.91, H: 8.46, O: 25.82) and C: 65.7, H: 5.32, O: 28.16 (calculated theoretical for n = 1, C: 66.95, H: 5.57, O: 27.47), respectively. The possible formation of linear and circular oligomers of MCA made it difficult to determine the number of repeated units in MCA from its 1H-NMR spectrum. However, both the FTIR and 1H-NMR spectra of MCA showed that no contaminants of reactants were left in the liquid MCA after MCA was purified in excess hexane and was dried in vacuum desiccator.
Nanocomposites of HAp/polyanhydride were prepared as the reported method [6]. In short, photopolymerization was initiated by 2-dimethoxy-2-phenylacetophenone (DMPA; Aldrich), after HAp nanoneedles were homogenously dispersed in the liquid anhydride monomers by vigorous treatment of ultrasonic waves. The distribution of HAp needles in the nanocomposites was investigated by the TEM. The compressive strength and modulus of nanocomposite samples, with cylinder shape of the height 15 mm and the diameter 7.5 mm, were investigated by universal testing machine (SPL-25TA, Shimadzu, Japan). The studies of degradation behavior were carried out in the simulated body fluid (SBF) at pH 7.25 and 37 jC. SBF solution was changed every 24 h. The cumulative mass loss and the retention of elastic modulus were measured at different degradation time: 0, 50, 100, 150, 200 and 250 h for poly(MSA) (PMSA) and HAp/PMSA; 0, 100, 200, 300, 400, 500 and 600 h for poly(MCA) (PMCA) and HAp/ PMCA; and 0, 500, 1000, 1500, 2000 and 2500 h for poly(MCPB) (PMCPB) and HAp/PMCPB.
3. Results and discussion Fig. 1 demonstrated the preparation procedure of HAp/ polyanhydride nanocomposites. In short, the prepared needle-like HAp particles were homogenously distributed into methacrylated anhydride monomers via vigorous ultrasonic waves, followed by rapid photo-cure initiated via UV resulting in the in situ polymerization and leading to a nanocomposite where nanosized HAp needles were homogenously dispersed in the cross-linking network at nanoscale. Fig. 2 clearly demonstrated that HAp nanoneedles have a uniform distribution in the polyanhydride network at nanosized level via in situ photopolymerization after ultrasonic treatment. 3.1. Compressive properties of nanocomposites The compressive strength and modulus of cross-linking HAp/polyanhydride nanocomposites were related to the content of HAp in the composites (Fig. 3). As the increase of HAp content in the composite, the compressive strength and tensile modulus would be improved gradually. The maximum compressive strength and compressive elastic modulus of nanocomposites HAp/PMSA, HAp/PMCPB, and HAp/PMCA were about three times those of the corresponding pure cross-linked polyanhydrides of PMSA, PMCPB, and PMCA, respectively. The reason why the maximum ratio of HAp to MSA was only 20% was that, MSA was an anhydride oligomer of seven monomers resulting in a higher viscosity, in which to disperse HAp particles by ultrasonic treatment became more difficult. In addition, the compressive properties of the PMCA were superior to those of the PMSA and PMCPB, which might be that MCA had an increased functionality with a
H.-Y. Li et al. / Materials Letters 58 (2004) 2819–2823
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Fig. 1. The preparation scheme of nanocomposites composed of cross-linked polyanhydride and needle-like HAp particles.
carefully adjusting the content of HAp in the composite and the chemical composite in the cross-linking polyanhydride network.
concomitant enhancement in cross-linking efficiency, leading to an increase in strength and modulus. Furthermore, the compressive strength and modulus of these polymers and the nanocomposites covered a wide range from 18.2 MPa (PMSA) to 278 MPa [HAp/MCA (30 wt.%/wt.%)]. Therefore, it was possible to prepare such nanocomposite with proper compressive properties to meet the need of cortical bone (compressive strength 130 –220 Mpa) and trabecular bone (compressive strength 5 –20 MPa), through
The cumulative mass loss of the cross-linked polyanhydrides and the nanocomposite at different HAp content were plotted in Fig. 4. It was clearly demonstrated that the
Fig. 2. Transmission electron micrograph (TEM) of the distribution of HAp nanoneedles in PMCPB.
Fig. 3. Compressive strength and compressive elastic modulus of nanocomposites with different mass ratio between HAp and polyanhydride.
3.2. Biodegradation behavior of nanocomposites
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addition of HAp needles, whether in the PMCA, PMSA, or in PMCPB system, could decrease the mass loss of nanocomposites during the degradation procedure, and this tendency would be intensified as the content of HAp was increased. When the degradation time was 600 h, the cumulative mass loss of PMCA was 57%, while that of nanocomposite HAp/PMCA with HAp content of 10%, 20%, and 30% was decreased to 44%, 38%, and 32%, respectively. The same tendency can also be found in the nanocomposites of HAp/PMSA and HAp/PMCPB during their degradation procedure. The reason why HAp can inhibit the degradation rate had been explained elsewhere [6]. Therefore, the addition of HAp was also helpful for decreasing the mass loss during degradation, besides that HAp can play a crucial role in increasing the compressive strength and elastic modulus that was very important to meet the need of load-bearing bones. In addition, when the HAp content was the same, the mass loss of the HAp/ PMCPB nanocomposite was much lower than those of HAp/PMSA and HAp/PMCA which might be due to the strong hydrophobic properties of PMCPB polymer because of the aromatic group in its molecular structure, which indicated that it was possible to prepare such nanocomposite with controlled biodegradable rate by changing the composition in the cross-linked polyanhydride network. Fig. 5 showed the effect of HAp on the tensile modulus of cross-linked polyanhydrides PMCA, PMSA, PMCPB, and their nanocomposites with HAp. In these systems, as the HAp content increased, the tensile modulus normalized by the initial tensile modulus (E/E0) would be enhanced. Values of E/E0 of PMCA, HAp/PMCA (10%), HAp/PMCA (20%), HAp/PMCA (30%) were 61%, 71%, 73%, and 76% after the degradation for 600 h; and the nanocomposites of HAp/ PMSA and HAp/PCPB also showed an increased tendency in modulus retention as there is an increase of HAp content
Fig. 5. Tensile modulus normalized by the initial tensile modulus during the degradation time of HAp/polyanhydrides nanocomposites.
in the nanocomposite during the degradation procedure in SBF. These results obviously demonstrated that the addition of HAp was helpful for enhancing the mechanical integrity of the nanocomposite during the degradation process. In addition, it was also showed that the retention of modulus of the HAp/polyanhydrides (20%) was over 70% when the mass loss reached 50%, which was very beneficial for the rehabilitation of the bone fracture. Furthermore, the modulus retention of HAp/PMCPB was much higher than those of the other two nanocomposites during the same degradation time which can maintain over 92% elastic modulus even being degraded for 2500 h. However, the hydrophilic PMSA would lose 35% modulus in just 200 h during degradation. Therefore, it would be possible to fabricate such cross-linking nanocomposites, with expected biodegradation rate, by changing the composites in the polyanhydride cross-linking network.
4. Conclusion
Fig. 4. Cumulative mass loss as a function of degradation time for HAp/ polyanhydrides nanocomposites.
In situ photopolymerization was an effective way to prepare nanocomposites with homogenously distributed HAp needles in cross-linking polyanhydride network. The incorporation of HAp into the polyanhydrides can dramatically increase the mechanical properties and the maximum of compressive strength and modulus of nanocomposite were about three times the corresponding polyanhydride alone. Besides, HAp played an important role in decreasing the cumulative mass loss and in enhancing the mechanical integrity of the nanocomposite during the degradation process. Furthermore, by carefully designing the crosslinking polyanhydride network, the biodegradation rate of the nanocomposite can be manipulated from several days to years in order to meet the rehabilitation rate of different bones.
H.-Y. Li et al. / Materials Letters 58 (2004) 2819–2823
Acknowledgements The authors gratefully acknowledge the support of the President Special Foundation of Chinese Academy of Sciences.
References [1] R.R.M. Bos, F.R. Rozema, G. Boering, H.W.B. Jansen, Degradation of and tissue reaction to biodegradable poly(L-lactide) for use as internal fixation of fractures: a study in rats, Biomaterials 12 (1991) 32. [2] K.S. Anseth, C.N. Bowman, Reaction diffusion enhanced termination in polymerisations of multifunctional monomers, Polym. React. Eng. 1 (1993) 499.
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[3] K.S. Anseth, K.J. Anderson, C.N. Bowman, Radical concentrations, environments, and reactivities during crosslinking polymerizations, Macromol. Chem. Phys. 197 (1996) 833. [4] H.Y. Li, Y.F. Chen, C.X. Ruan, Y.S. Xie, Preparation of biodegradable and biocompatible organic – inorganic hybrid material for orthopedic application, Proceeding of 1st China – Japan Chemical Engineering, China Chemical Engineering Press, Beijing, 2000, pp. 598. [5] D.S. Muggli, A.K. Burkoth, K.S. Anseth, Crosslinked polyanhydrides for use in orthopedic application: degradation behaviour and mechanics, J. Biomed. Mater. Res. 46 (1999) 271. [6] H.Y. Li, Y.F. Chen, Y.S. Xie, Photo-crosslinking polymerization to prepare polyanhydride/needle-like hydroxyapatite biodegradable nanocomposite for orthopedic application, Mater. Lett. 57 (2003) 2848. [7] A. Conix, Poly[1,3-bis(carboxyphenoxy) propane anyhydride], Macromol. Chem. 24 (1957) 95.
Nanocomposites of cross-linking polyanhydrides and hydroxyapatite needles: mechanical and degradable properties Hao-Ying Li *, Yun-Fa Chen, Yu-Sheng Xie Institute of Chemical Metallurgy, Chinese Academy of Sciences, Beijing 100080, China Received 23 September 2003; received in revised form 19 April 2004; accepted 3 May 2004 Available online 20 June 2004
Abstract Three methacrylated anhydride monomers of citric acid (MCA), sebacic acid (MSA) and 1,4-bis(carboxyphenoxy) butane (MCPB) were utilized to prepare biodegradable nanocomposites with homogenously distributed hydroxyapatite (HAp) nanoneedles by in situ photopolymerization, where the needle-like HAp particles were homogenously dispersed in cross-linked polyanhydride network at nanoscale. The incorporation of HAp cannot only dramatically increase the compressive properties of the nanocomposites but also play an important role in decreasing the cumulative mass loss and in increasing the mechanical integrity of nanocomposites during the degradation. The results showed that it was possible to prepare such HAp/polyanhydride nanocomposite, with proper mechanical strength and controlled degradable rate, by adjusting the content of HAp in the polyanhydrides and by changing the compositions in the crosslinking network to meet the rehabilitation need of different fracture bones in human body, both in mechanical properties and in the biodegradable rate. D 2004 Elsevier B.V. All rights reserved. Keywords: Nanocomposites; Polymers; Hydroxyapatite; Nanoneedles; Biodegradable; Mechanical; Photopolymerization
1. Introduction In contrast with metallic implants, biodegradable polymers used as internal fixation devices eliminate the second surgery of removal, avoid some problems caused by stress shielding, and can simultaneously deliver antiinfectious drugs and/or bone growth-enhancing factors [1]. In order to overcome the problems associated with bulk degradation of the widely used polyester orthopedic devices, crosslinked polyanhydrides undergoing surface erosion during degradation have been developed. Besides, cross-linked polyanhydrides have improved mechanical strength, controlled biodegradable rate, and can be easily fabricated into the desirable shapes, which demonstrates a great potential in orthopedic applications and might be a better alternative than polyesters as the internal fixation devices [2 –4].
* Corresponding author. Welsh School of Pharmacy, Cardiff University, Redwood Building, King Edward VII Avenue, Cardiff CF1 3XF, UK. Tel.: +44-29-2087-7226; fax: +44-29-2087-4149. E-mail address: [email protected] (H.-Y. Li). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.05.003
However, the compressive strength of these reported cross-linked polyanhydrides is still far lower than cortical bones and needs to be further increased to meet the need of load-bearing bones during rehabilitation [5]. Hydroxyapatite (HAp) nanoneedles have been utilized to reinforce cross-linked polyanhydride by a novel in situ photopolymerization method that has been reported by our group [6], where nanosized HAp needles are homogenously distributed in the cross-linking polyanhydride network to form a nanocomposite with dramatically increased mechanical properties and improved biodegradable characteristics. Unfortunately, the compressive strength of HAp/poly(methacrylated pyromellitylimidoalanine) [PM(PMA-ala)] nanocomposite alone cannot completely meet the requirements of cortical bones, and its fast degradable rate does not fit well with the healing rate of load-bearing bones which usually need 3– 6 months for full rehabilitation. In this study, the mechanical strength and biodegradable properties of three kinds of nanocomposites, fabricated with different cross-linking polyanhydrides and HAp nanoneedles via in situ photopolymerization, are investigated in detail. This study might provide some clues for further research of
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manufacturing such nanocomposites with sufficient mechanical properties and controlled degradable rate for the specific requirements of the broken bone during healing. Three anhydride monomers of methacrylated citric acid (MCA), sebacic acid (MSA), and 1,4-bis(carboxyphenoxy) butane (MCPB) were synthesized. The compressive strength and the degradation properties of the nanocomposites containing polyanhydrides and HAp with different content are investigated in detail. The results demonstrate that it is possible to fabricate such HAp/polyanhydride nanocomposites with proper compressive strength and biodegradable properties to meet the rehabilitation need of different human bones.
2. Experimental The preparation of needle-like HAp was prepared following the method as mentioned before [6]. The HAp length was manipulated to under 100 nm by controlling the concentration of dispersant polymethylmethacrylate amid (Mw = 10,000; A-6114, Toagosei, Japan). The particles appeared a needle-like shape within nanosized scale under the investigation of transmission electronic microscopy (TEM; JEM200CX, JEOL, Japan). X-ray diffraction (XRD) analysis demonstrated that the chemical component of these nanoneedles was Ca10(PO4)6(OH)2. Methacrylated anhydride monomers of MCA, MSA, and MCPB were synthesized via reacting citric acid (CA), sebacic acid (SA), and 1,4bis(carboxyphenoxy) butane (synthesized as Conix method Ref. [7]; CPB) with excess methacrylic anhydride, respectively, then purified in hexane and dried in vacuum desiccator [5]. The absorbance peak of CMC in the methacrylated anhydride compounds appeared at about 1636 cm 1 on Fourier transform infrared spectroscopy (FTIR; Nicolet Magna-IR 750; Madison, WI) spectrum. The chemical shift (y) of MCH2 protons in the methacrylate end-group was clearly evident in 1H-nuclear magnetic resonance (1H-NMR; DMX300, Bruker) image at approximately y5.8 and y6.2 parts per million (ppm). These characteristics demonstrated that the acids of SA, CA, and CPB had been methacrylated and end-capped with methacrylate functionalities. Furthermore, the ratio between the area of methylene hydrogens (y1.3) and the area of vinyl hydrogens (y5.8 and y6.2) on 1HNMR spectrum indicated that MSA was an oligomer of seven monomers and that MCPB was basically a monomer. Elemental analysis of MSA and MCPB was C: 64.37, H: 8.77, O: 26.65 (calculated theoretical for n = 7, C: 64.91, H: 8.46, O: 25.82) and C: 65.7, H: 5.32, O: 28.16 (calculated theoretical for n = 1, C: 66.95, H: 5.57, O: 27.47), respectively. The possible formation of linear and circular oligomers of MCA made it difficult to determine the number of repeated units in MCA from its 1H-NMR spectrum. However, both the FTIR and 1H-NMR spectra of MCA showed that no contaminants of reactants were left in the liquid MCA after MCA was purified in excess hexane and was dried in vacuum desiccator.
Nanocomposites of HAp/polyanhydride were prepared as the reported method [6]. In short, photopolymerization was initiated by 2-dimethoxy-2-phenylacetophenone (DMPA; Aldrich), after HAp nanoneedles were homogenously dispersed in the liquid anhydride monomers by vigorous treatment of ultrasonic waves. The distribution of HAp needles in the nanocomposites was investigated by the TEM. The compressive strength and modulus of nanocomposite samples, with cylinder shape of the height 15 mm and the diameter 7.5 mm, were investigated by universal testing machine (SPL-25TA, Shimadzu, Japan). The studies of degradation behavior were carried out in the simulated body fluid (SBF) at pH 7.25 and 37 jC. SBF solution was changed every 24 h. The cumulative mass loss and the retention of elastic modulus were measured at different degradation time: 0, 50, 100, 150, 200 and 250 h for poly(MSA) (PMSA) and HAp/PMSA; 0, 100, 200, 300, 400, 500 and 600 h for poly(MCA) (PMCA) and HAp/ PMCA; and 0, 500, 1000, 1500, 2000 and 2500 h for poly(MCPB) (PMCPB) and HAp/PMCPB.
3. Results and discussion Fig. 1 demonstrated the preparation procedure of HAp/ polyanhydride nanocomposites. In short, the prepared needle-like HAp particles were homogenously distributed into methacrylated anhydride monomers via vigorous ultrasonic waves, followed by rapid photo-cure initiated via UV resulting in the in situ polymerization and leading to a nanocomposite where nanosized HAp needles were homogenously dispersed in the cross-linking network at nanoscale. Fig. 2 clearly demonstrated that HAp nanoneedles have a uniform distribution in the polyanhydride network at nanosized level via in situ photopolymerization after ultrasonic treatment. 3.1. Compressive properties of nanocomposites The compressive strength and modulus of cross-linking HAp/polyanhydride nanocomposites were related to the content of HAp in the composites (Fig. 3). As the increase of HAp content in the composite, the compressive strength and tensile modulus would be improved gradually. The maximum compressive strength and compressive elastic modulus of nanocomposites HAp/PMSA, HAp/PMCPB, and HAp/PMCA were about three times those of the corresponding pure cross-linked polyanhydrides of PMSA, PMCPB, and PMCA, respectively. The reason why the maximum ratio of HAp to MSA was only 20% was that, MSA was an anhydride oligomer of seven monomers resulting in a higher viscosity, in which to disperse HAp particles by ultrasonic treatment became more difficult. In addition, the compressive properties of the PMCA were superior to those of the PMSA and PMCPB, which might be that MCA had an increased functionality with a
H.-Y. Li et al. / Materials Letters 58 (2004) 2819–2823
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Fig. 1. The preparation scheme of nanocomposites composed of cross-linked polyanhydride and needle-like HAp particles.
carefully adjusting the content of HAp in the composite and the chemical composite in the cross-linking polyanhydride network.
concomitant enhancement in cross-linking efficiency, leading to an increase in strength and modulus. Furthermore, the compressive strength and modulus of these polymers and the nanocomposites covered a wide range from 18.2 MPa (PMSA) to 278 MPa [HAp/MCA (30 wt.%/wt.%)]. Therefore, it was possible to prepare such nanocomposite with proper compressive properties to meet the need of cortical bone (compressive strength 130 –220 Mpa) and trabecular bone (compressive strength 5 –20 MPa), through
The cumulative mass loss of the cross-linked polyanhydrides and the nanocomposite at different HAp content were plotted in Fig. 4. It was clearly demonstrated that the
Fig. 2. Transmission electron micrograph (TEM) of the distribution of HAp nanoneedles in PMCPB.
Fig. 3. Compressive strength and compressive elastic modulus of nanocomposites with different mass ratio between HAp and polyanhydride.
3.2. Biodegradation behavior of nanocomposites
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addition of HAp needles, whether in the PMCA, PMSA, or in PMCPB system, could decrease the mass loss of nanocomposites during the degradation procedure, and this tendency would be intensified as the content of HAp was increased. When the degradation time was 600 h, the cumulative mass loss of PMCA was 57%, while that of nanocomposite HAp/PMCA with HAp content of 10%, 20%, and 30% was decreased to 44%, 38%, and 32%, respectively. The same tendency can also be found in the nanocomposites of HAp/PMSA and HAp/PMCPB during their degradation procedure. The reason why HAp can inhibit the degradation rate had been explained elsewhere [6]. Therefore, the addition of HAp was also helpful for decreasing the mass loss during degradation, besides that HAp can play a crucial role in increasing the compressive strength and elastic modulus that was very important to meet the need of load-bearing bones. In addition, when the HAp content was the same, the mass loss of the HAp/ PMCPB nanocomposite was much lower than those of HAp/PMSA and HAp/PMCA which might be due to the strong hydrophobic properties of PMCPB polymer because of the aromatic group in its molecular structure, which indicated that it was possible to prepare such nanocomposite with controlled biodegradable rate by changing the composition in the cross-linked polyanhydride network. Fig. 5 showed the effect of HAp on the tensile modulus of cross-linked polyanhydrides PMCA, PMSA, PMCPB, and their nanocomposites with HAp. In these systems, as the HAp content increased, the tensile modulus normalized by the initial tensile modulus (E/E0) would be enhanced. Values of E/E0 of PMCA, HAp/PMCA (10%), HAp/PMCA (20%), HAp/PMCA (30%) were 61%, 71%, 73%, and 76% after the degradation for 600 h; and the nanocomposites of HAp/ PMSA and HAp/PCPB also showed an increased tendency in modulus retention as there is an increase of HAp content
Fig. 5. Tensile modulus normalized by the initial tensile modulus during the degradation time of HAp/polyanhydrides nanocomposites.
in the nanocomposite during the degradation procedure in SBF. These results obviously demonstrated that the addition of HAp was helpful for enhancing the mechanical integrity of the nanocomposite during the degradation process. In addition, it was also showed that the retention of modulus of the HAp/polyanhydrides (20%) was over 70% when the mass loss reached 50%, which was very beneficial for the rehabilitation of the bone fracture. Furthermore, the modulus retention of HAp/PMCPB was much higher than those of the other two nanocomposites during the same degradation time which can maintain over 92% elastic modulus even being degraded for 2500 h. However, the hydrophilic PMSA would lose 35% modulus in just 200 h during degradation. Therefore, it would be possible to fabricate such cross-linking nanocomposites, with expected biodegradation rate, by changing the composites in the polyanhydride cross-linking network.
4. Conclusion
Fig. 4. Cumulative mass loss as a function of degradation time for HAp/ polyanhydrides nanocomposites.
In situ photopolymerization was an effective way to prepare nanocomposites with homogenously distributed HAp needles in cross-linking polyanhydride network. The incorporation of HAp into the polyanhydrides can dramatically increase the mechanical properties and the maximum of compressive strength and modulus of nanocomposite were about three times the corresponding polyanhydride alone. Besides, HAp played an important role in decreasing the cumulative mass loss and in enhancing the mechanical integrity of the nanocomposite during the degradation process. Furthermore, by carefully designing the crosslinking polyanhydride network, the biodegradation rate of the nanocomposite can be manipulated from several days to years in order to meet the rehabilitation rate of different bones.
H.-Y. Li et al. / Materials Letters 58 (2004) 2819–2823
Acknowledgements The authors gratefully acknowledge the support of the President Special Foundation of Chinese Academy of Sciences.
References [1] R.R.M. Bos, F.R. Rozema, G. Boering, H.W.B. Jansen, Degradation of and tissue reaction to biodegradable poly(L-lactide) for use as internal fixation of fractures: a study in rats, Biomaterials 12 (1991) 32. [2] K.S. Anseth, C.N. Bowman, Reaction diffusion enhanced termination in polymerisations of multifunctional monomers, Polym. React. Eng. 1 (1993) 499.
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[3] K.S. Anseth, K.J. Anderson, C.N. Bowman, Radical concentrations, environments, and reactivities during crosslinking polymerizations, Macromol. Chem. Phys. 197 (1996) 833. [4] H.Y. Li, Y.F. Chen, C.X. Ruan, Y.S. Xie, Preparation of biodegradable and biocompatible organic – inorganic hybrid material for orthopedic application, Proceeding of 1st China – Japan Chemical Engineering, China Chemical Engineering Press, Beijing, 2000, pp. 598. [5] D.S. Muggli, A.K. Burkoth, K.S. Anseth, Crosslinked polyanhydrides for use in orthopedic application: degradation behaviour and mechanics, J. Biomed. Mater. Res. 46 (1999) 271. [6] H.Y. Li, Y.F. Chen, Y.S. Xie, Photo-crosslinking polymerization to prepare polyanhydride/needle-like hydroxyapatite biodegradable nanocomposite for orthopedic application, Mater. Lett. 57 (2003) 2848. [7] A. Conix, Poly[1,3-bis(carboxyphenoxy) propane anyhydride], Macromol. Chem. 24 (1957) 95.