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The influence of polymer concentrations on the structure and mechanical properties of porous polycaprolactone-coated hydroxyapatite scaffolds
Applied Surface Science 256 (2010) 4586–4590
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The influence of polymer concentrations on the structure and mechanical properties of porous polycaprolactone-coated hydroxyapatite scaffolds J. Zhao, K. Duan, J.W. Zhang, X. Lu, J. Weng ∗ Key Laboratory of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu 610031, PR China
a r t i c l e
i n f o
Article history: Received 21 July 2009 Received in revised form 16 February 2010 Accepted 17 February 2010 Available online 25 February 2010 Keywords: Porous scaffold Hydroxyapatite Composite Polymer coating Mechanical properties
a b s t r a c t Polycaprolactone (PCL)-coated porous hydroxyapatite (HA) composite scaffolds were prepared by combining polymer impregnating method with dip-coating method. Three different PCL solution concentrations were used in dip-coating process to improve the mechanical properties of porous HA scaffolds. The results indicated that as the concentration of PCL solution increases the compressive strength significantly increased from 0.09 MPa to 0.51 MPa while the porosity decreased from 90% to 75% for the composite scaffolds. An interlaced structure was found inside the pore wall for all composite scaffolds due to the penetration of PCL. The porous HA/PCL composite scaffolds dip-coated with 10% PCL exhibited optimal combination of mechanical properties and pore interconnectivity, and may be a potential bone candidate for the tissue engineering applications. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Porous HA bioceramics scaffolds have been widely studied in bone tissue engineering field due to the importance of threedimensional (3D) porosity for bone regeneration [1–4]. Numerous methods have been developed for fabricating porous HA scaffolds, such as pore forming, foaming, gel-casting, freeze-drying and polymer impregnating method (PIM) [5–9]. Of these, only PIM leads to high porosity (more than 90%) and great structure similarity to the natural bone, which supports faster rate of osseointegration and therefore leads to enhanced mechanical properties in vivo [10,11]. However, a high porosity is inevitably associated with a decrease in strength of the porous HA scaffolds. Therefore, it is significant to improve the strength of porous HA scaffolds with highly interconnected porosity to get better clinic results. The introduction of polymers phase seems to be a promising choice to overcome low mechanical strength of porous ceramics because of its excellent toughness and plasticity by forming ceramics–polymer composite systems [12,13]. Among the most often utilized biodegradable synthetic polymers, PCL has been regarded as a preferable candidate in polymer–ceramics composite system due to its low cost and sufficient mechanical properties to serve as resorbable suture, drug delivery system, and bone graft substitutes [14,15]. As one of compound methods, polymer-coating method has attracted more attention owing to its quick and facile
∗ Corresponding author. Tel.: +86 28 8760 1371; fax: +86 28 8760 1371. E-mail addresses: [email protected] (J. Zhao), [email protected] (J. Weng).
process and polymer diversity according to the clinical demands [16]. Kim et al. have reported the HA/PCL composite scaffold with antibiotic for drug delivery system while the compressive strength has been improved from 0.16 MPa to 0.45 MPa by different concentration of polymer solution [17]. Peroglio et al. adopted the PCL solution and PCL nanodispersion infiltrating into the porous alumina scaffolds with increased apparent fracture energy and fracture resistance [18]. However, our knowledge about the effect of polymer concentration on the microstructure and mechanical properties of composite scaffolds is still very scarce. The present study concentrated on preparing a series of HA/PCL composite scaffolds and characterizing the influences of polymer concentration on the composite scaffold structure and mechanical properties. The concentration of PCL solution was optimized so as to obtain the composite scaffold with improved strength for potential scaffolds in bone tissue engineering. 2. Materials and methods 2.1. Materials HA powder was synthesized by the wet chemical method in our laboratory. The size of HA particles was in the range of 0.1–8.0 m with mean size of 2.1 m by particle size analyzer (HOBRIBA La920, Japan). Polyvinyl alcohol (PVA) with an average molecular weight of 75,000 was used as surfactant and dispersant additives. Polyurethane foams with pore size of 1 mm were used as duplicating template and cut into Ø 8 mm × 12 mm cylinder. PCL (Daicel Chemical Industries, Ltd., Japan) used in this study had an aver-
0169-4332/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.02.053
J. Zhao et al. / Applied Surface Science 256 (2010) 4586–4590
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age molecular weight of 50,000. Chloroform was purchased from chemical engineering factory (China, Chengdu). 2.2. Scaffold fabrication The porous HA scaffold was obtained by impregnating porous polyurethane foams with the slurry containing HA powder, distilled water and additives as our previous study [9]. PCL was used to enhance the strength and toughness of porous HA scaffolds. Three concentrations of PCL solutions were selected to investigate the influence of polymer solution concentration on the structure and mechanical properties of composite scaffolds. First PCL was dissolved in chloroform at different concentrations of 5%, 10% and 20% (w/v), respectively. Then the porous HA scaffolds were fully immersed in polymer solutions and treated by ultrasonic oscillation for 1 min each. PCL-coated HA composite scaffolds were centrifuged and then dried in vacuum drying chamber 48 h at room temperature. The names of composite scaffolds with 5%, 10% and 20% PCL were denoted as HA-5P, HA-10P and HA-20P, respectively.
Fig. 1. XRD patterns for three PCL-coated HA samples.
3. Results and discussion 2.3. Characterization 2.3.1. Phase composition X-ray diffraction analysis (XRD) was used to characterize the phase composition of the PCL coatings on scaffolds. In order to observe the phase composition of PCL coating with different concentrations, pressed HA sheets were dipped into varied PCL solutions to form a PCL coating and then dried in air for 24 h. XRD instrument is a Phillips X’Pert, using CuK␣ radiation at 35 mA and 45 kV. Scans were performed with a 2Â range of 10–60◦ with 0.033◦ steps. 2.3.2. Porosity The mass variations before and after dip-coating process were recorded to study the effect of coating concentration on mass. The mass variation of porous HA scaffolds with varied coatings was measured with a precise electronic balance (BS224S, Sartorius) with a sensitivity of 5 × 10−4 . The total porosity for HA scaffold and PCL-coated HA scaffolds were calculated by Archimedes method and Gravimetry method, respectively [19,20]. Six scaffolds of each group were determined and averaged.
3.1. Phase composition of composite scaffolds The XRD patterns of PCL coatings containing different concentration are shown in Fig. 1. The XRD peaks for three composite samples show typical PCL and HA peaks without additional peaks for other phases or peak shifts, which indicated that no chemical reactions occurred during the sintering process. After dip-coating process, the increase in the concentration of PCL solutions increased the peak intensities of PCL. 3.2. The effect of PCL concentrations on porosity The sintered porous HA scaffold showed excellent interconnectivity and porosity (Fig. 2). Porosity is one of the most important factors affecting the morphological properties of biomaterials scaffold in bone regeneration process. Higher porosity favors tissue ingrowth, bone formation and forming biological fixation with surrounding tissue [20]. Therefore, the prepared scaffold is competent for nutrient diffusion, metabolite elimination, cell migration and bone ingrowth into the interior scaffold in vivo [21].
2.3.3. Microstructure Before and after the dip-coating process, the microstructures of varied scaffolds were observed by scanning electronic microscopy (SEM, Quanta200). Clearly, the boundary of PCL and HA would be influenced by the ductility of PCL. Therefore, it is necessary to keep the real interface between the scaffold and PCL coating and investigate the interaction. The composite scaffolds were firstly placed in liquid nitrogen, and then rapidly fractured by brittle failure in order to overcome the polymer effect on the boundary. The cross section of porous scaffolds were fixed on a copper stud and coated with gold for SEM observation to investigate the polymer morphologies and distributions. 2.3.4. Mechanical properties In compression tests, cylindrical samples (Ø 8 mm × 12 mm) were used. The tests were carried out in ambient atmospheric condition (20 ± 5 ◦ C and 50 ± 5% RH) using an INSTRON 5567 mechanical tester (USA) with 30 kN load cell. The crosshead speed was set at 0.5 mm/min and the load was applied until the scaffold was fractured. At least five samples of each scaffold were measured and the results were averaged.
Fig. 2. The appearance of sintered HA porous scaffold.
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porosity requirement about 70% for cell infiltration, proliferation and the ingrowth of new bone tissue [22–24]. The apparent densities for all scaffolds are as much as the trabecular bone of 0.30 ± 0.1 g/cm3 [25]. Accordingly, all the porous HA/PCL composite scaffolds fulfill the porosity for clinical demands as bone implant in vivo.
3.3. Microstructure of composite scaffolds
Fig. 3. The alteration of scaffold mass and porosity of HA/PCL composite scaffolds.
After dip-coating process, the scaffold mass and porosity were altered owing to the introduction of PCL coating. Actually, the mass gain was dependent on the polymer solution concentration, which will directly influence the porosity of HA/PCL composite scaffold. Therefore, it is important to understand the relationship between the scaffold mass gain and porosity decrease. Fig. 3 indicates the results of scaffold mass gain, porosity of composite scaffolds coated with different PCL solution concentrations. The mass gain increased linearly with the concentration, and the porosity decreased linearly with the concentration. Nevertheless, the porosity for all composite scaffolds matched the range for trabecular bone (75–95%), as well as satisfied the minimum
The struts of porous scaffolds before and after dip-coating process were observed to investigate the interaction of scaffold combing with varied polymer coating, as shown in Fig. 4. Many micropores (<5 m) were found on the cross section of sintered porous HA scaffold (Fig. 4a). It may have accelerated the crack propagation and thus may decrease the scaffold integrity of mechanical property in the initial stage for implantation [26]. There were no obvious polymer coatings on the outer surface for HA/PCL composite scaffolds. However, distinct microstructure on fracture surface was observed for varied HA/PCL composite scaffolds. Many polymers in the form of fibers presented in-between the HA grains. It was clear that the mean diameter of polymer fibers increased with PCL concentration. It can fill the structure defects on one hand and strengthen the conjugation between HA grains on the other, both of which may improve the mechanical property for porous scaffolds. Note that the excessively high concentration of PCL solution had negative effect on the 3D porous structure and interconnectivity of scaffold. In the dip-coating process, the micropores on scaffold surface played a vital role in affording a channel for polymer infiltration into struts interior.
Fig. 4. Morphologies of cross section for porous HA (a), HA-5P (b), HA-10P (c) and HA-20P (d).
J. Zhao et al. / Applied Surface Science 256 (2010) 4586–4590
Fig. 5. Compressive strengths of the porous scaffolds.
3.4. Mechanical properties Compressive strength has universally used to evaluate the mechanical properties of ceramics materials. Fig. 5 shows the compressive strength results of all porous scaffolds coated with varied concentrations of PCL solutions. Peak compressive strength of the uncoated scaffold was measured at 0.09 MPa rising to 0.51 MPa for the coated composite scaffolds, close to the low end value (1–10 MPa) of cancellous bone with 70% porosity. In addition, it might not be necessary to produce a scaffold with a mechanical strength equal to bone since cultured cells on the scaffold and new tissue formation in vitro will enhance the time-dependent strength of the scaffold [27]. With increasing PCL solution concentration, the compressive strength gradually increased. It indicated that PCL as reinforced phase conglutinates very well with HA ceramics scaffold for reinforcing the polymer–ceramic composite system. Although higher concentration of PCL solution can result in obtaining stronger composite scaffold, the excessive amount of PCL coating on scaffold will decrease the porosity and interconnection of composite scaffold with poor biological properties in vivo. Therefore, 10% PCL to composite scaffold is optimal to achieve excellent mechanical strength and hold high porosity of 85%. It is well known that porous HA scaffolds failed in an elastic brittle manner as other characteristic of open cell ceramic structure [28]. Fig. 6 shows three typical stress–strain curves of HA/PCL composite scaffolds. The trends of stress–strain curves for all composite scaffolds are in agreement with the behavior of this polymer-coated scaffold discussed [18,29]. PCL can participate load supporting for
4589
whole composite scaffolds; in addition, PCL in-between HA grains can obstruct or deflect the propagation of crack. Indeed the behavior of all composite scaffolds seemed to be dominated by the effect of scaffold densification after the introduction of PCL. The mechanical property of composite scaffold with high porosity is notably enhanced by adopting polymer-coating process. However, the bioactivity of composite scaffold may be decreased due to the introduction of PCL, a hydrophobic polymer. In clinical application, the interface of implant is expected to be endowed with excellent bioactivity for forming bone bonding with the surrounding tissue. Therefore, enhancing the bioactivity of composite scaffold is necessary while holding reinforced mechanical properties. Some bioactive phase can be mixed into polymer coating such as HA powders, can markedly improve the bioactivity of composite scaffold. Moreover, proteins can also be encapsulated in polymer solution as controlled delivery of cell-signaling molecules for tissue development [30]. In addition, an effective method is to deposit a bone-like apatite layer on whole composite scaffolds in simulated body fluid, which present excellent bone bonding ability to surrounding tissue that untreated scaffolds [31]. Thus, the highly porous HA scaffold combined with the polymer-coating method have great potential applications in the drug delivery systems and bone implant. 4. Conclusions The 3D interconnected porous HA/PCL composite scaffold were successfully fabricated by combining polymer impregnating method with polymer-coating method resulted in the porous bodies with 90∼75% porosity and a compressive strength from 0.09 MPa up to 0.51 MPa. Clearly, the PCL introduced in porous HA scaffold effectively enhanced the compressive strength for composite scaffold. The introduction of PCL altered the load distribution and transmittal mode, which can participate load supporting and obstruct or change the propagation of crack by in-between HA grains. The 10% PCL reinforced porous HA scaffold with interpenetrating network structure has the optimal mechanical properties and interconnected pore structure. Acknowledgements The present study was supported by National Natural Science Foundation of China (30870630), 2007 Cultivating Fund of Key Program for Scientific and Technological Innovation (MOE, 704039) and Doctoral Innovation Fund (2006) from Southwest Jiaotong University, China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Fig. 6. Typical stress–strain curves of composite scaffolds.
R.B. Martin, Mater. Sci. Forum. 293 (1999) 5. U. Heise, J.F. Osborn, F. Duwe, Int. Orthop. 14 (1990) 329. S. Yang, K.F. Leong, Z. Du, C.K. Chua, Tissue. Eng. 7 (2001) 679. P.A. Gunatillak, R. Adhikari, Eur. Cell. Mater. 5 (2003) 1. O. Lyckfeldt, J.M.F. Ferreira, J. Eur. Ceram. Soc. 18 (1998) 131. Z.H. Dong, Y.B. Li, Q. Zou, Appl. Surf. Sci. 255 (2009) 6087. P. Sepulveda, J.G.P. Binner, S.O. Rogero, O.Z. Higa, J.C. Bressiani, J. Biomed. Mater. Res. 50 (2000) 27. L. Jiang, Y. Li, X. Wang, L. Zhang, J. Wen, M. Gong, Carbohyd. Polym. 74 (2008) 680. J. Zhao, L.Y. Guo, X.B. Yang, J. Weng, Appl. Surf. Sci. 255 (2008) 2942. H.R. Ramay, M. Zhang, Biomaterials 24 (2003) 3293. K.A. Hing, S.M. Nest, K.E. Tanner, W. Bonfield, J. Mater. Sci: Mater. Med. 10 (1999) 663. M. Sivakumar, I. Manjubala, K. Panduranga Rao, Carbohyd. Polym. 49 (2002) 281. W.J.E.M. Habraken, J.G.C. Wolke, J.A. Jansen, Adv. Drug. Deliv. Rev. 59 (2007) 234. M.R. Williamson, A.G.A. Coombes, Biomaterials 25 (2004) 459. C.M. Agrawal, R.B. Ray, J. Biomed. Mater. Res. 55 (2001) 141. J.A. Roether, A.R. Boccaccini, L.L. Hench, V. Maquet, S. Gautier, R. Jerome, Biomaterials 23 (2002) 3871.
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[17] H.W. Kim, J.C. Knowlesa, H.E. Kim, Biomaterials 25 (2004) 1279. [18] M. Peroglio, L. Gremillard, J. Chevalier, L. Chazeau, C. Gauthier, T. Hamaide, J. Eur. Ceram. Soc. 27 (2007) 2679. [19] R. Nazarov, H.J. Jin, D.L. Kaplan, Biomacromolecules 5 (3) (2004) 718. [20] V. Karageorgiou, D. Kaplan, Biomaterials 26 (2005) 5474. [21] M. Gutierres, M.A. Lopes, N.S. Hussain, A.F. Lemos, J.M.F. Ferreira, A. Afonso, A.T. Cabral, L. Almeida, J.D. Santos, Acta. Biomater. 4 (2008) 370. [22] A.J. Salgada, O.P. Coutinho, R.L. Reis, Macromol. Biosci. 4 (2004) 743. [23] D.W. Hutmacher, Biomaterials 21 (2000) 2529. [24] P.X. Ma, Mater. Today. 7 (5) (2004) 30.
[25] F.S. Kaplan, W.C. Hayes, T.M. Keaveny, A. Boskey, T.A. Einhorn, J.P. Iannotti, Form and function of bone, Orthop. Basic. Sci. (1994) 128–184. [26] X.G. Miao, D.M.F. Tan, J. Li, Y. Xiao, R. Crawford, Acta. Biomater. 4 (2008) 638. [27] J.R. Jones, L.L. Hench, Curr. Opin. Solid. ST. M. 7 (2003) 301. [28] L.J. Gibson, M.F. Ashby, Cellular Solids: Structure and Properties, Pergamon, Oxford, 1999 (pp. 117–118 and 209–212). [29] T. Tian, D.L. Jiang, J.X. Zhang, Q.L. Lin, Mater. Sci. Eng. C. 28 (2008) 51. [30] J. Sohier, R.E. Haan, K. de Groot, J.M. Bezemer, J. Control. Rel. 87 (2003) 57. [31] D. Lickorish, L. Guan, J.E. Davies, Biomaterials 28 (2007) 1495.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The influence of polymer concentrations on the structure and mechanical properties of porous polycaprolactone-coated hydroxyapatite scaffolds J. Zhao, K. Duan, J.W. Zhang, X. Lu, J. Weng ∗ Key Laboratory of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu 610031, PR China
a r t i c l e
i n f o
Article history: Received 21 July 2009 Received in revised form 16 February 2010 Accepted 17 February 2010 Available online 25 February 2010 Keywords: Porous scaffold Hydroxyapatite Composite Polymer coating Mechanical properties
a b s t r a c t Polycaprolactone (PCL)-coated porous hydroxyapatite (HA) composite scaffolds were prepared by combining polymer impregnating method with dip-coating method. Three different PCL solution concentrations were used in dip-coating process to improve the mechanical properties of porous HA scaffolds. The results indicated that as the concentration of PCL solution increases the compressive strength significantly increased from 0.09 MPa to 0.51 MPa while the porosity decreased from 90% to 75% for the composite scaffolds. An interlaced structure was found inside the pore wall for all composite scaffolds due to the penetration of PCL. The porous HA/PCL composite scaffolds dip-coated with 10% PCL exhibited optimal combination of mechanical properties and pore interconnectivity, and may be a potential bone candidate for the tissue engineering applications. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Porous HA bioceramics scaffolds have been widely studied in bone tissue engineering field due to the importance of threedimensional (3D) porosity for bone regeneration [1–4]. Numerous methods have been developed for fabricating porous HA scaffolds, such as pore forming, foaming, gel-casting, freeze-drying and polymer impregnating method (PIM) [5–9]. Of these, only PIM leads to high porosity (more than 90%) and great structure similarity to the natural bone, which supports faster rate of osseointegration and therefore leads to enhanced mechanical properties in vivo [10,11]. However, a high porosity is inevitably associated with a decrease in strength of the porous HA scaffolds. Therefore, it is significant to improve the strength of porous HA scaffolds with highly interconnected porosity to get better clinic results. The introduction of polymers phase seems to be a promising choice to overcome low mechanical strength of porous ceramics because of its excellent toughness and plasticity by forming ceramics–polymer composite systems [12,13]. Among the most often utilized biodegradable synthetic polymers, PCL has been regarded as a preferable candidate in polymer–ceramics composite system due to its low cost and sufficient mechanical properties to serve as resorbable suture, drug delivery system, and bone graft substitutes [14,15]. As one of compound methods, polymer-coating method has attracted more attention owing to its quick and facile
∗ Corresponding author. Tel.: +86 28 8760 1371; fax: +86 28 8760 1371. E-mail addresses: [email protected] (J. Zhao), [email protected] (J. Weng).
process and polymer diversity according to the clinical demands [16]. Kim et al. have reported the HA/PCL composite scaffold with antibiotic for drug delivery system while the compressive strength has been improved from 0.16 MPa to 0.45 MPa by different concentration of polymer solution [17]. Peroglio et al. adopted the PCL solution and PCL nanodispersion infiltrating into the porous alumina scaffolds with increased apparent fracture energy and fracture resistance [18]. However, our knowledge about the effect of polymer concentration on the microstructure and mechanical properties of composite scaffolds is still very scarce. The present study concentrated on preparing a series of HA/PCL composite scaffolds and characterizing the influences of polymer concentration on the composite scaffold structure and mechanical properties. The concentration of PCL solution was optimized so as to obtain the composite scaffold with improved strength for potential scaffolds in bone tissue engineering. 2. Materials and methods 2.1. Materials HA powder was synthesized by the wet chemical method in our laboratory. The size of HA particles was in the range of 0.1–8.0 m with mean size of 2.1 m by particle size analyzer (HOBRIBA La920, Japan). Polyvinyl alcohol (PVA) with an average molecular weight of 75,000 was used as surfactant and dispersant additives. Polyurethane foams with pore size of 1 mm were used as duplicating template and cut into Ø 8 mm × 12 mm cylinder. PCL (Daicel Chemical Industries, Ltd., Japan) used in this study had an aver-
0169-4332/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.02.053
J. Zhao et al. / Applied Surface Science 256 (2010) 4586–4590
4587
age molecular weight of 50,000. Chloroform was purchased from chemical engineering factory (China, Chengdu). 2.2. Scaffold fabrication The porous HA scaffold was obtained by impregnating porous polyurethane foams with the slurry containing HA powder, distilled water and additives as our previous study [9]. PCL was used to enhance the strength and toughness of porous HA scaffolds. Three concentrations of PCL solutions were selected to investigate the influence of polymer solution concentration on the structure and mechanical properties of composite scaffolds. First PCL was dissolved in chloroform at different concentrations of 5%, 10% and 20% (w/v), respectively. Then the porous HA scaffolds were fully immersed in polymer solutions and treated by ultrasonic oscillation for 1 min each. PCL-coated HA composite scaffolds were centrifuged and then dried in vacuum drying chamber 48 h at room temperature. The names of composite scaffolds with 5%, 10% and 20% PCL were denoted as HA-5P, HA-10P and HA-20P, respectively.
Fig. 1. XRD patterns for three PCL-coated HA samples.
3. Results and discussion 2.3. Characterization 2.3.1. Phase composition X-ray diffraction analysis (XRD) was used to characterize the phase composition of the PCL coatings on scaffolds. In order to observe the phase composition of PCL coating with different concentrations, pressed HA sheets were dipped into varied PCL solutions to form a PCL coating and then dried in air for 24 h. XRD instrument is a Phillips X’Pert, using CuK␣ radiation at 35 mA and 45 kV. Scans were performed with a 2Â range of 10–60◦ with 0.033◦ steps. 2.3.2. Porosity The mass variations before and after dip-coating process were recorded to study the effect of coating concentration on mass. The mass variation of porous HA scaffolds with varied coatings was measured with a precise electronic balance (BS224S, Sartorius) with a sensitivity of 5 × 10−4 . The total porosity for HA scaffold and PCL-coated HA scaffolds were calculated by Archimedes method and Gravimetry method, respectively [19,20]. Six scaffolds of each group were determined and averaged.
3.1. Phase composition of composite scaffolds The XRD patterns of PCL coatings containing different concentration are shown in Fig. 1. The XRD peaks for three composite samples show typical PCL and HA peaks without additional peaks for other phases or peak shifts, which indicated that no chemical reactions occurred during the sintering process. After dip-coating process, the increase in the concentration of PCL solutions increased the peak intensities of PCL. 3.2. The effect of PCL concentrations on porosity The sintered porous HA scaffold showed excellent interconnectivity and porosity (Fig. 2). Porosity is one of the most important factors affecting the morphological properties of biomaterials scaffold in bone regeneration process. Higher porosity favors tissue ingrowth, bone formation and forming biological fixation with surrounding tissue [20]. Therefore, the prepared scaffold is competent for nutrient diffusion, metabolite elimination, cell migration and bone ingrowth into the interior scaffold in vivo [21].
2.3.3. Microstructure Before and after the dip-coating process, the microstructures of varied scaffolds were observed by scanning electronic microscopy (SEM, Quanta200). Clearly, the boundary of PCL and HA would be influenced by the ductility of PCL. Therefore, it is necessary to keep the real interface between the scaffold and PCL coating and investigate the interaction. The composite scaffolds were firstly placed in liquid nitrogen, and then rapidly fractured by brittle failure in order to overcome the polymer effect on the boundary. The cross section of porous scaffolds were fixed on a copper stud and coated with gold for SEM observation to investigate the polymer morphologies and distributions. 2.3.4. Mechanical properties In compression tests, cylindrical samples (Ø 8 mm × 12 mm) were used. The tests were carried out in ambient atmospheric condition (20 ± 5 ◦ C and 50 ± 5% RH) using an INSTRON 5567 mechanical tester (USA) with 30 kN load cell. The crosshead speed was set at 0.5 mm/min and the load was applied until the scaffold was fractured. At least five samples of each scaffold were measured and the results were averaged.
Fig. 2. The appearance of sintered HA porous scaffold.
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porosity requirement about 70% for cell infiltration, proliferation and the ingrowth of new bone tissue [22–24]. The apparent densities for all scaffolds are as much as the trabecular bone of 0.30 ± 0.1 g/cm3 [25]. Accordingly, all the porous HA/PCL composite scaffolds fulfill the porosity for clinical demands as bone implant in vivo.
3.3. Microstructure of composite scaffolds
Fig. 3. The alteration of scaffold mass and porosity of HA/PCL composite scaffolds.
After dip-coating process, the scaffold mass and porosity were altered owing to the introduction of PCL coating. Actually, the mass gain was dependent on the polymer solution concentration, which will directly influence the porosity of HA/PCL composite scaffold. Therefore, it is important to understand the relationship between the scaffold mass gain and porosity decrease. Fig. 3 indicates the results of scaffold mass gain, porosity of composite scaffolds coated with different PCL solution concentrations. The mass gain increased linearly with the concentration, and the porosity decreased linearly with the concentration. Nevertheless, the porosity for all composite scaffolds matched the range for trabecular bone (75–95%), as well as satisfied the minimum
The struts of porous scaffolds before and after dip-coating process were observed to investigate the interaction of scaffold combing with varied polymer coating, as shown in Fig. 4. Many micropores (<5 m) were found on the cross section of sintered porous HA scaffold (Fig. 4a). It may have accelerated the crack propagation and thus may decrease the scaffold integrity of mechanical property in the initial stage for implantation [26]. There were no obvious polymer coatings on the outer surface for HA/PCL composite scaffolds. However, distinct microstructure on fracture surface was observed for varied HA/PCL composite scaffolds. Many polymers in the form of fibers presented in-between the HA grains. It was clear that the mean diameter of polymer fibers increased with PCL concentration. It can fill the structure defects on one hand and strengthen the conjugation between HA grains on the other, both of which may improve the mechanical property for porous scaffolds. Note that the excessively high concentration of PCL solution had negative effect on the 3D porous structure and interconnectivity of scaffold. In the dip-coating process, the micropores on scaffold surface played a vital role in affording a channel for polymer infiltration into struts interior.
Fig. 4. Morphologies of cross section for porous HA (a), HA-5P (b), HA-10P (c) and HA-20P (d).
J. Zhao et al. / Applied Surface Science 256 (2010) 4586–4590
Fig. 5. Compressive strengths of the porous scaffolds.
3.4. Mechanical properties Compressive strength has universally used to evaluate the mechanical properties of ceramics materials. Fig. 5 shows the compressive strength results of all porous scaffolds coated with varied concentrations of PCL solutions. Peak compressive strength of the uncoated scaffold was measured at 0.09 MPa rising to 0.51 MPa for the coated composite scaffolds, close to the low end value (1–10 MPa) of cancellous bone with 70% porosity. In addition, it might not be necessary to produce a scaffold with a mechanical strength equal to bone since cultured cells on the scaffold and new tissue formation in vitro will enhance the time-dependent strength of the scaffold [27]. With increasing PCL solution concentration, the compressive strength gradually increased. It indicated that PCL as reinforced phase conglutinates very well with HA ceramics scaffold for reinforcing the polymer–ceramic composite system. Although higher concentration of PCL solution can result in obtaining stronger composite scaffold, the excessive amount of PCL coating on scaffold will decrease the porosity and interconnection of composite scaffold with poor biological properties in vivo. Therefore, 10% PCL to composite scaffold is optimal to achieve excellent mechanical strength and hold high porosity of 85%. It is well known that porous HA scaffolds failed in an elastic brittle manner as other characteristic of open cell ceramic structure [28]. Fig. 6 shows three typical stress–strain curves of HA/PCL composite scaffolds. The trends of stress–strain curves for all composite scaffolds are in agreement with the behavior of this polymer-coated scaffold discussed [18,29]. PCL can participate load supporting for
4589
whole composite scaffolds; in addition, PCL in-between HA grains can obstruct or deflect the propagation of crack. Indeed the behavior of all composite scaffolds seemed to be dominated by the effect of scaffold densification after the introduction of PCL. The mechanical property of composite scaffold with high porosity is notably enhanced by adopting polymer-coating process. However, the bioactivity of composite scaffold may be decreased due to the introduction of PCL, a hydrophobic polymer. In clinical application, the interface of implant is expected to be endowed with excellent bioactivity for forming bone bonding with the surrounding tissue. Therefore, enhancing the bioactivity of composite scaffold is necessary while holding reinforced mechanical properties. Some bioactive phase can be mixed into polymer coating such as HA powders, can markedly improve the bioactivity of composite scaffold. Moreover, proteins can also be encapsulated in polymer solution as controlled delivery of cell-signaling molecules for tissue development [30]. In addition, an effective method is to deposit a bone-like apatite layer on whole composite scaffolds in simulated body fluid, which present excellent bone bonding ability to surrounding tissue that untreated scaffolds [31]. Thus, the highly porous HA scaffold combined with the polymer-coating method have great potential applications in the drug delivery systems and bone implant. 4. Conclusions The 3D interconnected porous HA/PCL composite scaffold were successfully fabricated by combining polymer impregnating method with polymer-coating method resulted in the porous bodies with 90∼75% porosity and a compressive strength from 0.09 MPa up to 0.51 MPa. Clearly, the PCL introduced in porous HA scaffold effectively enhanced the compressive strength for composite scaffold. The introduction of PCL altered the load distribution and transmittal mode, which can participate load supporting and obstruct or change the propagation of crack by in-between HA grains. The 10% PCL reinforced porous HA scaffold with interpenetrating network structure has the optimal mechanical properties and interconnected pore structure. Acknowledgements The present study was supported by National Natural Science Foundation of China (30870630), 2007 Cultivating Fund of Key Program for Scientific and Technological Innovation (MOE, 704039) and Doctoral Innovation Fund (2006) from Southwest Jiaotong University, China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Fig. 6. Typical stress–strain curves of composite scaffolds.
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