Materials Science and Engineering C 33 (2013) 340–346
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Effects of rheological properties on ice-templated porous hydroxyapatite ceramics Yan Zhang, Kechao Zhou, Yinxiang Bao, Dou Zhang ⁎ State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, P.R. China
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Article history: Received 21 October 2011 Received in revised form 1 August 2012 Accepted 29 August 2012 Available online 5 September 2012 Keywords: Porous ceramics Suspensions Hydroxyapatite Ice-templated method Biomedical applications
a b s t r a c t Freeze casting of aqueous suspension was investigated as a method for fabricating hydroxyapatite (HA) porous ceramics with lamellar structures. The rheological properties of HA suspensions employed in the ice-templated process were investigated systematically. Well aligned lamellar pores and dense ceramic walls were obtained successfully in HA porous ceramics with the porosity of 68–81% and compressive strength of 0.9–2.4 MPa. The results exhibited a strong correlation between the rheological properties of the employed suspensions and the morphology and mechanical properties of ice-templated porous HA ceramics, in terms of lamellar pore characteristics, porosities and compressive strengths. The ability to produce aligned pores and achieve the manipulation of porous HA microstructures by controlling the rheological parameters were demonstrated, revealing the potential of the ice-templated method for the fabrication of HA scaffolds in biomedical applications. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Hydroxyapatite (HA) with the chemical formula Ca10 (PO4)6(OH) has been used extensively in biomedical applications owing to its high biocompatibility and the similarity of the composition to the mineral constituents of bones and teeth [1]. Porous HA exhibits strong bonding to the bone, as the pores provide the framework for the mechanical interlock leading to a firm fixation of the material [2]. The porous architecture is more resorbable and osteoconductive than its dense counterpart, and enhances the surface area greatly, which allows more cells to be carried in comparison with dense hydroxyapatite. It is essential to satisfy certain criteria in the development of pore structures for the application of bone tissue engineering, including the appropriate mechanical strength and the pore characteristics, i.e. porosity, pore size, pore shape, and pore orientation, in order to promote the migration of cells through the pores, with appropriate space for the nutrient transportation, tissue infiltration and ultimately, the vascularisation [3]. Several techniques have been developed in order to meet the required demands and produce porous scaffolds [4,5], including gel casting [6], gas foaming [7] and polymer foam replication [8]. However, these methods are only able to provide an isotropic microstructure and commonly employ the template or porogen that needs to be eliminated in the successive steps, such as calcinations and chemical removal, which increase the production costs and the risk of failure [9]. For example, gel casting of foams is a common technique for fabricating ceramic scaffolds with high mechanical strength, but it usually results in a structure of poorly interconnected 2
⁎ Corresponding author. E-mail address: [email protected] (D. Zhang). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2012.08.048
pores, and nonuniform pore size distribution. It would be beneficial to develop scaffolds with oriented microstructure and controlled architecture to fulfill a range of requirements needed for bone replacement application. Taking advantage of the principle of unidirectional solidification of a liquid vehicle such as water, ice-templating was developed as a simple and novel method for the preparation of porous structures [10]. As a physical process and a near net shape forming technique, it has many advantages such as rapid manufacturing, less drying cracks and simplified binder burnout process. It is versatile as a range of materials [11,12] and compositions [13,14] can be utilized, and therefore has attracted significant attention in recent years. Recent investigations have demonstrated the ability to prepare porous HA scaffolds with a lamellar oriented architecture by ice-templated process. Major efforts have been focused on the adjustment of freezing conditions, including the cooling rate [15], the manner of freezing [16], and the temperature gradient [17], in order to control the pore characteristics, microstructures and mechanical properties of porous HA ceramics. The aim of our work was to reveal the influence of key rheological properties of the suspensions employed in the ice-templated process on the pore characteristics and mechanical properties of the resulted porous HA ceramics. Understanding the relationships between the rheological properties and the microstructure, porosity and mechanical strength of porous HA ceramics are critical for controlling the microstructure and mechanical properties of HA scaffolds for bone repair and replacement applications. 2. Materials and methods HA suspensions were prepared by mixing HA powders (d50 =1.0 μm, Nanjing Emperor Nano Material Co., Ltd., P. R. China), PVA binder (420, Kuraray Co. Ltd., Japan), and the dispersant, ammonium polyacrylate
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(HydroDisper A160, Shenzhen Highrun Chemical Industry Co. Ltd., P. R. China) in deionized water. Suspensions were ball-milled for 24 h in zirconia media and de-aired by stirring in a vacuum desiccator, until complete removal of air bubbles. 1 M HCl and concentrated (35% solution) NH4OH were utilized to modify the pH value. Freeze casting the suspension was carried out by pouring the suspension into a transparent cylindrical polydimethylsiloxane (PDMS) mold (ϕ: 10 mm, H: 15 mm), which was then transported to a copper cold finger placed in a liquid nitrogen container. Frozen samples were then demoulded and then placed in the vacuum chamber (b 10 Pa) of a freeze-drier (FD-1A-50, Beijing Boyikang Medical Equipment Co. China) for 24 h to allow the ice sublimate. The dried samples were heated at 550 °C for 3 h in order to burn out the organic additives and sintered at 1250 °C for 3 h. The Zeta potential was measured by a Zeta potential analyzer (Zetasizer nano-zs, Malvern Instruments, UK) using 1.0 vol.% HA suspensions. The pH value of the suspension was measured using a pH meter (PHB-4, Shanghai Precision and Scientific Instrument Co. Ltd). The rheological properties of the suspensions were investigated by a rheometer (AR 2000, TA Instruments, New Castle, DE) at 20 °C using a stainless steel parallel-plate configuration with 40 mm diameter and 500 μm gap. The as-prepared suspensions were pre-sheared at a shear stress of 200 Pa for 10 s before recording. X-ray diffraction (XRD, Rigaku D/max-2550) was used to evaluate the phases of the green and sintered samples. The apparent porosity was derived from the density data obtained by the Archimedes's method [18]. The microstructures of the porous samples were analyzed by environmental scanning electron microscopy (ESEM, Quantan 200, JEOL, Tokyo, Japan). The compressive strength of the sintered sample (diameter ~ 9 mm × height ~ 13 mm) was tested with a crosshead speed of 0.2 mm/min using an Electronic Universal Testing Machine (KD11 − 2, Shenzhen KEJALI Technology Co. Ltd., China).
3. Results and discussion 3.1. Phase identification by XRD Fig. 1 shows the XRD pattern of porous ceramics before and after heat treatment at 1250 °C. All peaks were consistent with the characteristic XRD peaks of HA (JCPDS 9–432) and diffraction peaks became narrower and sharper after sintering at 1250 °C, identifying pure HA phase with no trace of any second phase after sintering at 1250 °C, which was similar to the result reported by Naruporn et al. [19].
Fig. 1. XRD patterns of (A) HA powder, and (B) HA porous ceramic after sintering at 1250 °C.
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3.2. Effect of the dispersant concentration on the rheological properties of the suspension Fig. 2 shows the effect of the dispersant concentration on the viscosity of HA suspension of 20 vol.% solid loading with 1 wt.% PVA binder at a constant shear rate of 100 s −1. All viscosities of HA suspensions were low with a range of dispersant concentrations. Initially, the viscosity decreased apparently with the addition of small amounts of dispersant, and then reached the minimum value of 6.91 mPa·s at the dispersant concentration of 1 wt.%, followed by the increase of the viscosity with further increase of the dispersant concentration. Suspensions with suitable homogeneity and enough dispersive ability can avoid segregation and agglomeration during the freezing process, enabling a homogeneous structure with desirable orientation in the sintered samples. The lowest viscosity of 6.91 mPa·s was obtained at the optimal dispersant concentration of 1 wt.%, indicating the best dispersive ability, lowest resistance to flow and the best fluidity of the suspension during the unidirectional solidification process, which were favorable to obtain homogeneous structures and improved mechanical properties in the final sintered porous samples. 3.3. Effect of the pH value on the rheological properties of the suspension and the pore morphology Fig. 3 shows the relationship between the pH value and the Zeta potential of 1 vol.% suspension with 1 wt.% HydroDisper A160 dispersant. The pH value of the initial prepared suspension was 7.6, with the corresponding Zeta potential of − 31.9 mV. No isoelectric point (IEP) existed in the pH range of 4–11. This was similar to the report by E. Cunningham et al. [20], but different to the result reported by Y. Bao et al. [21], which indicated an IEP at pH 9.5–9.7. The discrepancy may well result from the difference between the surface chemistry states of HA particles prepared by different methods. The absolute value of the Zeta potential rose continuously with the increase of pH value. For the absolute value of Zeta potential of the HA suspension, the change was smooth with only 2.8% to 5.3% increase comparing with the initial Zeta potential in the pH range of 8–11. However, in the pH range of 7.4–4.0, the change was abrupt with 1.3% to 34.2% decrease of the absolute value comparing with the initial Zeta potential. The HA suspension with the initial pH value was homogeneous enough and showed better dispersive state which can be processed easily in the following freeze casting steps. Consequently, there was no need to adjust the pH value to improve the dispersibility of the aqueous HA suspension in this experiment, as the initial suspension showed a well dispersed state. Fig. 4 shows the cross-sectional SEM micrographs of porous HA ceramics obtained from suspension of 20 vol.% solid loading with
Fig. 2. Viscosity vs. NH4PAA concentration of 20 vol.% HA suspension and 1 wt.% PVA binder at the shear rate of 100 s−1.
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Newtonian behavior, while the suspensions with 30 and 40 vol.% solid loading exhibited a shear-thinning behavior. For suspensions involving colloidal interactions, the extent of the coupling between the solid and liquid phases can be measured by the Stokes number (St) in Eq. (2) [26]: St ¼
ρp r 3 γ k μ0
ð2Þ
where ρp is the particle density, r is the characteristic length of the solid particles, γ is the shear rate, k is a coefficient and depends on
Fig. 3. Zeta potential vs. pH value of 1 vol.% HA suspensions obtained from 1 wt.% dispersant concentration.
different pH values. For the sintered HA sample prepared using the water-based HA suspension of pH 4, the poor parallelism and the collapse of the freeze-cast structures were observed, as shown in Fig. 3(A), indicating the poor dispersive ability and stability of the suspension with low absolute Zeta potential value. When utilizing the suspension of pH 7.5 and 11, the porous ceramics with uniform and clear oriented distribution were obtained. Suspensions with enough homogeneity and suitable dispersive ability can avoid segregation and agglomeration during the freezing process. The pore width of 20–25 μm and lamellar thickness of 4–6 μm were obtained from the suspension with pH 7.6, and the pore width of about 20 μm and lamellar thickness of 5–7 μm were obtained from the suspension with pH 11, as shown in Fig. 3(B) and (C) respectively, without remarkable difference between them. Thermodynamically, a particle can only be rejected if the free energy of the system Δσ0 is positive [22], as shown in Eq. (1): Δσ 0 ¼ σ ps − σ pl þ σ sl > 0
ð1Þ
where σps, σpl and σsl are the energies between the particle and the solid phase, the particle and the liquid phase, and the solid phase and the liquid phase respectively. In the acid condition with pH 4, the HA particles are susceptible to dissolution [23]. Therefore, the polymer chains of the dispersant absorbed on the surfaces of the particles can be easily peeled off into the liquid phase, resulting in lots of agglomerations and sedimentations, making the Δσ0 decrease. Consequently, the ceramic particles were easily engulfed in the solid phase solvent and the long-range ordered porous architectures cannot be formed completely during the unidirectional process. However, the homogenous lamellar architectures were obtained from the suspensions with pH 7.6 and 11, which were favorable to eliminate the dissolution and inhibit the hydrolysis of the HA particles, leading to a well dispersive state due to the effect of the steric and electrostatic repulsions. On the other hand, the pH of the body fluid [24] is ranging between 7.3 and 7.5, so the samples prepared by the suspension of the initial pH 7.6 can fulfill the need of the human body in future biomedical application. 3.3. Effects of the solid loading on the rheological properties of the suspension and the pore morphology The different characteristics in the viscosity of HA suspension with a range of solid loading from 10 to 40 vol.% at the shear rates from 0.1 to 500 s −1 have been illustrated in our previous work [25]. Both suspensions with 10 and 20 vol.% solid loading exhibited an approximate
Fig. 4. SEM micrographs of sintered porous ceramics prepared by different pH values from 20 vol.% solid loading suspension, 1 wt.% dispersant concentration and 1 wt.% binder concentration. The solidification direction was parallel to the page and the lamellar orientation. A. pH=4, B. pH=7.6, C. pH=11.
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the shape, size and orientation of the particle. For a sphere, k is equal to 3πr[26], μ0 is the viscosity of the suspending liquid which is the water in this work. The St numbers of all the suspensions with a range of solid loading were «1 under all the shear rates from 0.1 to 500 s −1, indicating the movement of the particles during the solidification process was subject to not only the solid particles themselves, but also the water solvent. It is well known that the viscosity is strongly dependent on the solid loading of a suspension. Determination of the viscosity curve with a wide range of solid loading is essential for predicting and adjusting the viscosity of the suspension. A large number of models have been proposed to define the relationship between viscosity and volume fraction of solids [27–29]. In this study, German equation Eq. (3) [30] and Krieger-Dougherty equation Eq. (4) [31] were utilized to fit the measurement results. By using the data at a shear rate of 100 s −1, the fitting curves of relative viscosity as a function of solid loading are shown in Fig. 4. The fitting parameters are summarized in Table 1. φ −n ηr ¼ A 1− φm
ð3Þ
φ −n ηr ¼ 1− φm
ð4Þ
where ηr is the relative viscosity defined as the apparent viscosity of the suspension η, divided by the viscosity of the solvent (water) ηL (1.005 mPa·s at 20 °C). A is a coefficient and n is power law exponent. φ is the volume fraction of the particles, φm is the volume fraction of particles at which the viscosity becomes practically infinite. The maximum HA particle concentration obtained in these models were 58 vol.%. As listed in Table 1, the fit parameters of A = 4.2, n = 2.23, φm = 0.63 for German model and n = 1.98, φm = 0.58 for Krieger-Dougherty model were represented. At the shear rate of 100 s −1, the viscosities of the suspensions fitted well to Krieger-Dougherty model with corresponding correlative factor of 99.0 %. By identifying the theoretical model fitted to the measurement data, it is convenient to make reasonable evaluations and modifications on the viscosity of the suspension before starting the ice-templated process (Fig. 5). Fig. 6(A)–(D) show the cross-sectional SEM micrographs of porous HA ceramics obtained from the suspensions with the solid loading of 10–40 vol.%. With the increase of the solid loading, the ceramic wall turned to be thicker and the lamellar pore width became smaller and even disappeared when the solid loading reached to 40 vol.%. During the solvent solidification process, when a balance between the van der Waals repulsive force and the viscous drag attractive force was achieved, the critical velocity of the freezing front can be calculated using Eq. (5) [32]: vcr ¼
Δσ 0 a0 12ημr
ð5Þ
where vcr is the critical velocity of the freezing front, σ0 is the free energy of the system, a0 is the mean distance between the molecules in the liquid layer, η is the dynamic viscosity of the liquid vehicle, μ is the thermal conductivity ratio of the particles and the liquid vehicle and r is radius of the spherical particle. Higher viscosity of the suspension Table 1 Comparison of rheological properties obtained from two flow models. Model
Parameters
φm
Shear rate
German
A= 4.2, n = 2.23 n = 1.98
0.63
100 s−1
Krieger-Dougherty
Correlative factor 57.1% 99.0%
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Fig. 5. Relative viscosity at 100 s−1 as a function of solid loading of the HA suspensions and flow models. German plot and Krieger-Dougherty plot are shown by using the critical volume fraction Φm of 0.63.
will result in lower vcr during the ice-templated process. As a result, the HA particles were easy to be engulfed in the solid phase solvent instead of concentrating between the adjacent solidified solvent crystals. The increase of the viscosity with the increase of the solid loading led to greater resistance the freezing front experienced and the difficulty to rearrange the particles densely during the solidification process. Fig. 6(E) and (F) show lamellar structures perpendicular to the page and lamellar orientation, which are similar to the porous structures with perpendicular orientation in Fig. 6(A). These three micrographs indicated good interconnectivity of the porous structure, which could provide useful pathways for biofluids when being employed as the scaffold. 3.4. Effects of the binder concentration on the rheological properties of the suspension and the pore morphology Fig. 7 shows the effects of the binder concentration on the viscosity of 20 vol.% solid loading suspensions at the shear rate from 0.1 to 500 s−1. The viscosity of the suspension increased with the increase of the binder concentration. The suspensions without and with 1 wt.% PVA binder showed little difference in viscosity at all shear rates, both exhibiting an approximate Newtonian behavior. The viscosities of the suspensions with 3 and 5 wt.% PVA binders were 3–11 times higher than those of the suspensions without and with 1 wt.% PVA binder. The viscosity of the suspension with 3 wt.% PVA binder was about 2.2 times lower than that of the suspension with 5 wt.% PVA binder, while both suspensions exhibited a shear-thinning behavior. The addition of higher concentration of PVA binder from 3 wt.% to 5 wt.% could cause the flocculation and inhomogeneous dispersion of the suspension, resulting in higher resistance the particle experienced at all the shear rates in the viscosity test. In the aqueous system, the HA particles encountered various forces, including the one from soluble polymers [33]. With the addition of the binder into the suspension, the polymers act as a part of the solid loading, resulting in the increase of the viscosity, as well as a larger force the freezing front could experience during the solidification of the liquid vehicle. Fig. 8 shows the cross-sectional SEM micrographs of porous HA ceramics obtained from 20 vol.% solid loading suspension containing different binder concentrations. With the increase of the binder concentration, the lamellar pore width decreased from 25–40 to 5–10 μm and the lamellar thickness increased from 2–3 to 20–25 μm. During the unidirectional solidification process, the freezing-front velocity determines whether a particle in the suspension will be rejected and pushed ahead, or engulfed and trapped by the approaching liquid– solid interface, which finally results in the particle rearrangement formed as a replica of the ice structures. When freezing the suspensions
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Fig. 6. SEM micrographs of sintered HA porous ceramics prepared by a range of initial HA solid loadings with the initial pH 7.6, 1 wt.% dispersant concentration and 1 wt.% binder concentration. The solidification direction was parallel to the page and the lamellar orientation. (A)10 vol.%, (B) 20 vol.%, (C) 30 vol.% , (D) 40 vol.%. (E) 10 vol.% solid loading with the solidification direction perpendicular to the page and the lamellar orientation, (F) High magnification of (E).
without and with 1 wt.% binder concentration, the freezing front experienced little difference in the resistance due to little change in the viscosity. However, the green strength of the freeze dried sample without the binder addition was too low to handle, resulting in big cracks and large distortions in the lamellar architecture. For the addition of 3 wt.% and 5 wt.% binder concentrations, the resistance the freezing front experienced became stronger due to higher viscosities, and which resulted in smaller lamellar pore width and larger lamellar wall thickness in the final sintered porous samples. Moreover, when the binder concentrations increased to 3 and 5 wt.%, a great number of
the pore defects including some with large width were observed in the ceramic walls, as shown in Fig. 8(C) and (D). The addition of higher concentration PVA binder could cause the flocculation and inhomogeneous dispersion of the suspension, which leads to the agglomeration in the suspension and the pore defects in the sintered body. 3.5. Porosity and compressive strength of the lamellar porous ceramic Fig. 9(A) shows the effects of suspensions' parameters on the porosities and compressive strengths of sintered HA porous ceramics.
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Fig. 7. Viscosity vs. binder concentration of 20 vol.% suspension at the shear rate from 0.1-500 s−1 with 1 wt.% dispersant concentration.
With the addition of PVA binder to 1 wt.% concentration, the porosity decreased from 81 vol.% to 68 vol.%, and the corresponding compressive strength increased from 0.9 MPa to 2.4 MPa. While the binder concentration increased continually from 1 wt.% to 5 wt.%, the porosity increased from 68 vol.% to 75 vol.%, and the corresponding compressive strength decreased from 2.4 MPa to 1.5 MPa. The lowest porosity of 68 vol.% and highest compressive strength of 2.4 MPa were obtained with the addition of 1 wt.% PVA binder. Without the addition of PVA binder, cracks and distortions existed in the lamellar
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structures, and the ceramic walls turned to be porous, resulting in the highest porosity and lowest compressive strength of the sintered samples. With the addition of 3 wt.% and 5 wt.% binders, the pore defects existed in the ceramic walls owing to the poor dispersion and high viscosity of the suspension, which were unfavorable to the densification of the ceramic wall and improvement of the compressive strength of the final sintered porous samples. With the addition of 1 wt.% binder, the suspension showed the best dispersive state and the green body with the highest green strength can be obtained, which were beneficial to the formation and maintenance of the lamellar pore structure. Fig. 9(B) shows a typical compression load– displacement curve of porous HA ceramic. The sample failed gradually, showing a jagged load–displacement curve, which is a typical feature of porous ceramics in compression [34]. In biomedical applications, as the pore geometry and interconnectivity need to satisfy the requirements for body fluid penetration and bone cell ingrowth, isotropic pore structures can hardly fulfill a wide range of needs with different functions and locations of the bone structures [35]. Designing an ideal scaffold is to balance the need between large interconnected pores for tissue ingrowth and nutrient transportation, and the required mechanical properties. The compressive strength and porosity of the cancellous bone [36,37] are 2–12 MPa and 90 vol.%–57 vol.%, respectively. Ice-templated process provides an easy route to fabricate porous ceramics with similar ranges of porosities and compressive strengths. The long-range ordered lamellar architectures obtained by ice-templated process mimics the structures of nacre, contributing to much higher mechanical strength than those of conventional porous HA [38]. As the minimum pore size of 100 μm with
Fig. 8. SEM micrographs of sintered HA porous ceramics prepared by a range of binder concentrations from 20 vol.% solid loading suspension with initial pH 7.6 and 1 wt.% dispersant concentration. The solidification direction was parallel to the page and the lamellar orientation. (A) No binder, (B) 1 wt.%, (C) 3 wt.%, (D) 5 wt.%.
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and the compressive strength increased monotonically by around 3 times. PVA binder with concentrations ranging from 0 to 5 wt.% resulted in large change in the viscosity, i.e. up to 11 times. As a result, the lowest porosity of 68 vol.% and highest compressive strength of 2.4 MPa were obtained with the addition of 1 wt.% PVA binder. The results demonstrated that it is a versatile and useful method by adjusting the suspension's parameters for the fabrication of the lamellar porous HA ceramics with a range of pore sizes, porosities and compressive strengths. Acknowledgments The authors are grateful to the New Century Excellent Talents program funded by the Ministry of Education of the People's Republic of China (No. NCET-10-0802), the National Natural Science Foundation of China (No. 51172288), the Ph.D. Programs Foundation of Ministry of Education of China (No. 20110162130003), Hunan Provincial Natural Science Foundation of China (No. 11JJ1008) and Hunan Provincial Innovation Foundation for Postgraduate (No. CX2011B113) for financial support. References
Fig. 9. (A) Measured porosities and compressive strengths of the samples obtained from the suspension with the addition of 1 wt.% PVA binder and different binder concentrations. (B) Typical compression load–displacement curves of porous HA ceramic obtained from the suspension with the addition of 1 wt.% PVA binder. The compressive tests were carried out parallel to the solidification direction.
three-dimensional interconnectivity is generally considered to be suitable for tissue in-growth into porous ceramic structures [39], the pore sizes of 5–40 μm in HA ceramics prepared by the ice-templating method in this study were too small to allow implant vascularisation and bone growth. On the other hand, the unidirectional lamellar porous structures along the freezing direction have better pore interconnectivity than those structures perpendicular to the freezing direction. Therefore, future studies will focus on enlarging the pore size, improving the pore interconnectivity, and finally analyzing the behaviors both in vitro and in vivo reacting with the cells and host tissue using these lamellar oriented porous HA scaffolds. 4. Conclusions Porous HA ceramics with lamellar oriented structures were fabricated by an ice-templated method. The rheological properties of the initial HA suspension employed in the ice-templated process have shown strong influence on the resultant porous ceramics, in aspects of pore characteristic, porosity, interconnectivity and compressive strength. The viscosities of the suspension showed good fit to Krieger-Dougherty model. With the increase of the solid loading from 10 to 40 vol.%, the porosity of sintered HA ceramic decreased monotonically around 40 vol.%,
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Effects of rheological properties on ice-templated porous hydroxyapatite ceramics Yan Zhang, Kechao Zhou, Yinxiang Bao, Dou Zhang ⁎ State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, P.R. China
a r t i c l e
i n f o
Article history: Received 21 October 2011 Received in revised form 1 August 2012 Accepted 29 August 2012 Available online 5 September 2012 Keywords: Porous ceramics Suspensions Hydroxyapatite Ice-templated method Biomedical applications
a b s t r a c t Freeze casting of aqueous suspension was investigated as a method for fabricating hydroxyapatite (HA) porous ceramics with lamellar structures. The rheological properties of HA suspensions employed in the ice-templated process were investigated systematically. Well aligned lamellar pores and dense ceramic walls were obtained successfully in HA porous ceramics with the porosity of 68–81% and compressive strength of 0.9–2.4 MPa. The results exhibited a strong correlation between the rheological properties of the employed suspensions and the morphology and mechanical properties of ice-templated porous HA ceramics, in terms of lamellar pore characteristics, porosities and compressive strengths. The ability to produce aligned pores and achieve the manipulation of porous HA microstructures by controlling the rheological parameters were demonstrated, revealing the potential of the ice-templated method for the fabrication of HA scaffolds in biomedical applications. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Hydroxyapatite (HA) with the chemical formula Ca10 (PO4)6(OH) has been used extensively in biomedical applications owing to its high biocompatibility and the similarity of the composition to the mineral constituents of bones and teeth [1]. Porous HA exhibits strong bonding to the bone, as the pores provide the framework for the mechanical interlock leading to a firm fixation of the material [2]. The porous architecture is more resorbable and osteoconductive than its dense counterpart, and enhances the surface area greatly, which allows more cells to be carried in comparison with dense hydroxyapatite. It is essential to satisfy certain criteria in the development of pore structures for the application of bone tissue engineering, including the appropriate mechanical strength and the pore characteristics, i.e. porosity, pore size, pore shape, and pore orientation, in order to promote the migration of cells through the pores, with appropriate space for the nutrient transportation, tissue infiltration and ultimately, the vascularisation [3]. Several techniques have been developed in order to meet the required demands and produce porous scaffolds [4,5], including gel casting [6], gas foaming [7] and polymer foam replication [8]. However, these methods are only able to provide an isotropic microstructure and commonly employ the template or porogen that needs to be eliminated in the successive steps, such as calcinations and chemical removal, which increase the production costs and the risk of failure [9]. For example, gel casting of foams is a common technique for fabricating ceramic scaffolds with high mechanical strength, but it usually results in a structure of poorly interconnected 2
⁎ Corresponding author. E-mail address: [email protected] (D. Zhang). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2012.08.048
pores, and nonuniform pore size distribution. It would be beneficial to develop scaffolds with oriented microstructure and controlled architecture to fulfill a range of requirements needed for bone replacement application. Taking advantage of the principle of unidirectional solidification of a liquid vehicle such as water, ice-templating was developed as a simple and novel method for the preparation of porous structures [10]. As a physical process and a near net shape forming technique, it has many advantages such as rapid manufacturing, less drying cracks and simplified binder burnout process. It is versatile as a range of materials [11,12] and compositions [13,14] can be utilized, and therefore has attracted significant attention in recent years. Recent investigations have demonstrated the ability to prepare porous HA scaffolds with a lamellar oriented architecture by ice-templated process. Major efforts have been focused on the adjustment of freezing conditions, including the cooling rate [15], the manner of freezing [16], and the temperature gradient [17], in order to control the pore characteristics, microstructures and mechanical properties of porous HA ceramics. The aim of our work was to reveal the influence of key rheological properties of the suspensions employed in the ice-templated process on the pore characteristics and mechanical properties of the resulted porous HA ceramics. Understanding the relationships between the rheological properties and the microstructure, porosity and mechanical strength of porous HA ceramics are critical for controlling the microstructure and mechanical properties of HA scaffolds for bone repair and replacement applications. 2. Materials and methods HA suspensions were prepared by mixing HA powders (d50 =1.0 μm, Nanjing Emperor Nano Material Co., Ltd., P. R. China), PVA binder (420, Kuraray Co. Ltd., Japan), and the dispersant, ammonium polyacrylate
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(HydroDisper A160, Shenzhen Highrun Chemical Industry Co. Ltd., P. R. China) in deionized water. Suspensions were ball-milled for 24 h in zirconia media and de-aired by stirring in a vacuum desiccator, until complete removal of air bubbles. 1 M HCl and concentrated (35% solution) NH4OH were utilized to modify the pH value. Freeze casting the suspension was carried out by pouring the suspension into a transparent cylindrical polydimethylsiloxane (PDMS) mold (ϕ: 10 mm, H: 15 mm), which was then transported to a copper cold finger placed in a liquid nitrogen container. Frozen samples were then demoulded and then placed in the vacuum chamber (b 10 Pa) of a freeze-drier (FD-1A-50, Beijing Boyikang Medical Equipment Co. China) for 24 h to allow the ice sublimate. The dried samples were heated at 550 °C for 3 h in order to burn out the organic additives and sintered at 1250 °C for 3 h. The Zeta potential was measured by a Zeta potential analyzer (Zetasizer nano-zs, Malvern Instruments, UK) using 1.0 vol.% HA suspensions. The pH value of the suspension was measured using a pH meter (PHB-4, Shanghai Precision and Scientific Instrument Co. Ltd). The rheological properties of the suspensions were investigated by a rheometer (AR 2000, TA Instruments, New Castle, DE) at 20 °C using a stainless steel parallel-plate configuration with 40 mm diameter and 500 μm gap. The as-prepared suspensions were pre-sheared at a shear stress of 200 Pa for 10 s before recording. X-ray diffraction (XRD, Rigaku D/max-2550) was used to evaluate the phases of the green and sintered samples. The apparent porosity was derived from the density data obtained by the Archimedes's method [18]. The microstructures of the porous samples were analyzed by environmental scanning electron microscopy (ESEM, Quantan 200, JEOL, Tokyo, Japan). The compressive strength of the sintered sample (diameter ~ 9 mm × height ~ 13 mm) was tested with a crosshead speed of 0.2 mm/min using an Electronic Universal Testing Machine (KD11 − 2, Shenzhen KEJALI Technology Co. Ltd., China).
3. Results and discussion 3.1. Phase identification by XRD Fig. 1 shows the XRD pattern of porous ceramics before and after heat treatment at 1250 °C. All peaks were consistent with the characteristic XRD peaks of HA (JCPDS 9–432) and diffraction peaks became narrower and sharper after sintering at 1250 °C, identifying pure HA phase with no trace of any second phase after sintering at 1250 °C, which was similar to the result reported by Naruporn et al. [19].
Fig. 1. XRD patterns of (A) HA powder, and (B) HA porous ceramic after sintering at 1250 °C.
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3.2. Effect of the dispersant concentration on the rheological properties of the suspension Fig. 2 shows the effect of the dispersant concentration on the viscosity of HA suspension of 20 vol.% solid loading with 1 wt.% PVA binder at a constant shear rate of 100 s −1. All viscosities of HA suspensions were low with a range of dispersant concentrations. Initially, the viscosity decreased apparently with the addition of small amounts of dispersant, and then reached the minimum value of 6.91 mPa·s at the dispersant concentration of 1 wt.%, followed by the increase of the viscosity with further increase of the dispersant concentration. Suspensions with suitable homogeneity and enough dispersive ability can avoid segregation and agglomeration during the freezing process, enabling a homogeneous structure with desirable orientation in the sintered samples. The lowest viscosity of 6.91 mPa·s was obtained at the optimal dispersant concentration of 1 wt.%, indicating the best dispersive ability, lowest resistance to flow and the best fluidity of the suspension during the unidirectional solidification process, which were favorable to obtain homogeneous structures and improved mechanical properties in the final sintered porous samples. 3.3. Effect of the pH value on the rheological properties of the suspension and the pore morphology Fig. 3 shows the relationship between the pH value and the Zeta potential of 1 vol.% suspension with 1 wt.% HydroDisper A160 dispersant. The pH value of the initial prepared suspension was 7.6, with the corresponding Zeta potential of − 31.9 mV. No isoelectric point (IEP) existed in the pH range of 4–11. This was similar to the report by E. Cunningham et al. [20], but different to the result reported by Y. Bao et al. [21], which indicated an IEP at pH 9.5–9.7. The discrepancy may well result from the difference between the surface chemistry states of HA particles prepared by different methods. The absolute value of the Zeta potential rose continuously with the increase of pH value. For the absolute value of Zeta potential of the HA suspension, the change was smooth with only 2.8% to 5.3% increase comparing with the initial Zeta potential in the pH range of 8–11. However, in the pH range of 7.4–4.0, the change was abrupt with 1.3% to 34.2% decrease of the absolute value comparing with the initial Zeta potential. The HA suspension with the initial pH value was homogeneous enough and showed better dispersive state which can be processed easily in the following freeze casting steps. Consequently, there was no need to adjust the pH value to improve the dispersibility of the aqueous HA suspension in this experiment, as the initial suspension showed a well dispersed state. Fig. 4 shows the cross-sectional SEM micrographs of porous HA ceramics obtained from suspension of 20 vol.% solid loading with
Fig. 2. Viscosity vs. NH4PAA concentration of 20 vol.% HA suspension and 1 wt.% PVA binder at the shear rate of 100 s−1.
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Newtonian behavior, while the suspensions with 30 and 40 vol.% solid loading exhibited a shear-thinning behavior. For suspensions involving colloidal interactions, the extent of the coupling between the solid and liquid phases can be measured by the Stokes number (St) in Eq. (2) [26]: St ¼
ρp r 3 γ k μ0
ð2Þ
where ρp is the particle density, r is the characteristic length of the solid particles, γ is the shear rate, k is a coefficient and depends on
Fig. 3. Zeta potential vs. pH value of 1 vol.% HA suspensions obtained from 1 wt.% dispersant concentration.
different pH values. For the sintered HA sample prepared using the water-based HA suspension of pH 4, the poor parallelism and the collapse of the freeze-cast structures were observed, as shown in Fig. 3(A), indicating the poor dispersive ability and stability of the suspension with low absolute Zeta potential value. When utilizing the suspension of pH 7.5 and 11, the porous ceramics with uniform and clear oriented distribution were obtained. Suspensions with enough homogeneity and suitable dispersive ability can avoid segregation and agglomeration during the freezing process. The pore width of 20–25 μm and lamellar thickness of 4–6 μm were obtained from the suspension with pH 7.6, and the pore width of about 20 μm and lamellar thickness of 5–7 μm were obtained from the suspension with pH 11, as shown in Fig. 3(B) and (C) respectively, without remarkable difference between them. Thermodynamically, a particle can only be rejected if the free energy of the system Δσ0 is positive [22], as shown in Eq. (1): Δσ 0 ¼ σ ps − σ pl þ σ sl > 0
ð1Þ
where σps, σpl and σsl are the energies between the particle and the solid phase, the particle and the liquid phase, and the solid phase and the liquid phase respectively. In the acid condition with pH 4, the HA particles are susceptible to dissolution [23]. Therefore, the polymer chains of the dispersant absorbed on the surfaces of the particles can be easily peeled off into the liquid phase, resulting in lots of agglomerations and sedimentations, making the Δσ0 decrease. Consequently, the ceramic particles were easily engulfed in the solid phase solvent and the long-range ordered porous architectures cannot be formed completely during the unidirectional process. However, the homogenous lamellar architectures were obtained from the suspensions with pH 7.6 and 11, which were favorable to eliminate the dissolution and inhibit the hydrolysis of the HA particles, leading to a well dispersive state due to the effect of the steric and electrostatic repulsions. On the other hand, the pH of the body fluid [24] is ranging between 7.3 and 7.5, so the samples prepared by the suspension of the initial pH 7.6 can fulfill the need of the human body in future biomedical application. 3.3. Effects of the solid loading on the rheological properties of the suspension and the pore morphology The different characteristics in the viscosity of HA suspension with a range of solid loading from 10 to 40 vol.% at the shear rates from 0.1 to 500 s −1 have been illustrated in our previous work [25]. Both suspensions with 10 and 20 vol.% solid loading exhibited an approximate
Fig. 4. SEM micrographs of sintered porous ceramics prepared by different pH values from 20 vol.% solid loading suspension, 1 wt.% dispersant concentration and 1 wt.% binder concentration. The solidification direction was parallel to the page and the lamellar orientation. A. pH=4, B. pH=7.6, C. pH=11.
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the shape, size and orientation of the particle. For a sphere, k is equal to 3πr[26], μ0 is the viscosity of the suspending liquid which is the water in this work. The St numbers of all the suspensions with a range of solid loading were «1 under all the shear rates from 0.1 to 500 s −1, indicating the movement of the particles during the solidification process was subject to not only the solid particles themselves, but also the water solvent. It is well known that the viscosity is strongly dependent on the solid loading of a suspension. Determination of the viscosity curve with a wide range of solid loading is essential for predicting and adjusting the viscosity of the suspension. A large number of models have been proposed to define the relationship between viscosity and volume fraction of solids [27–29]. In this study, German equation Eq. (3) [30] and Krieger-Dougherty equation Eq. (4) [31] were utilized to fit the measurement results. By using the data at a shear rate of 100 s −1, the fitting curves of relative viscosity as a function of solid loading are shown in Fig. 4. The fitting parameters are summarized in Table 1. φ −n ηr ¼ A 1− φm
ð3Þ
φ −n ηr ¼ 1− φm
ð4Þ
where ηr is the relative viscosity defined as the apparent viscosity of the suspension η, divided by the viscosity of the solvent (water) ηL (1.005 mPa·s at 20 °C). A is a coefficient and n is power law exponent. φ is the volume fraction of the particles, φm is the volume fraction of particles at which the viscosity becomes practically infinite. The maximum HA particle concentration obtained in these models were 58 vol.%. As listed in Table 1, the fit parameters of A = 4.2, n = 2.23, φm = 0.63 for German model and n = 1.98, φm = 0.58 for Krieger-Dougherty model were represented. At the shear rate of 100 s −1, the viscosities of the suspensions fitted well to Krieger-Dougherty model with corresponding correlative factor of 99.0 %. By identifying the theoretical model fitted to the measurement data, it is convenient to make reasonable evaluations and modifications on the viscosity of the suspension before starting the ice-templated process (Fig. 5). Fig. 6(A)–(D) show the cross-sectional SEM micrographs of porous HA ceramics obtained from the suspensions with the solid loading of 10–40 vol.%. With the increase of the solid loading, the ceramic wall turned to be thicker and the lamellar pore width became smaller and even disappeared when the solid loading reached to 40 vol.%. During the solvent solidification process, when a balance between the van der Waals repulsive force and the viscous drag attractive force was achieved, the critical velocity of the freezing front can be calculated using Eq. (5) [32]: vcr ¼
Δσ 0 a0 12ημr
ð5Þ
where vcr is the critical velocity of the freezing front, σ0 is the free energy of the system, a0 is the mean distance between the molecules in the liquid layer, η is the dynamic viscosity of the liquid vehicle, μ is the thermal conductivity ratio of the particles and the liquid vehicle and r is radius of the spherical particle. Higher viscosity of the suspension Table 1 Comparison of rheological properties obtained from two flow models. Model
Parameters
φm
Shear rate
German
A= 4.2, n = 2.23 n = 1.98
0.63
100 s−1
Krieger-Dougherty
Correlative factor 57.1% 99.0%
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Fig. 5. Relative viscosity at 100 s−1 as a function of solid loading of the HA suspensions and flow models. German plot and Krieger-Dougherty plot are shown by using the critical volume fraction Φm of 0.63.
will result in lower vcr during the ice-templated process. As a result, the HA particles were easy to be engulfed in the solid phase solvent instead of concentrating between the adjacent solidified solvent crystals. The increase of the viscosity with the increase of the solid loading led to greater resistance the freezing front experienced and the difficulty to rearrange the particles densely during the solidification process. Fig. 6(E) and (F) show lamellar structures perpendicular to the page and lamellar orientation, which are similar to the porous structures with perpendicular orientation in Fig. 6(A). These three micrographs indicated good interconnectivity of the porous structure, which could provide useful pathways for biofluids when being employed as the scaffold. 3.4. Effects of the binder concentration on the rheological properties of the suspension and the pore morphology Fig. 7 shows the effects of the binder concentration on the viscosity of 20 vol.% solid loading suspensions at the shear rate from 0.1 to 500 s−1. The viscosity of the suspension increased with the increase of the binder concentration. The suspensions without and with 1 wt.% PVA binder showed little difference in viscosity at all shear rates, both exhibiting an approximate Newtonian behavior. The viscosities of the suspensions with 3 and 5 wt.% PVA binders were 3–11 times higher than those of the suspensions without and with 1 wt.% PVA binder. The viscosity of the suspension with 3 wt.% PVA binder was about 2.2 times lower than that of the suspension with 5 wt.% PVA binder, while both suspensions exhibited a shear-thinning behavior. The addition of higher concentration of PVA binder from 3 wt.% to 5 wt.% could cause the flocculation and inhomogeneous dispersion of the suspension, resulting in higher resistance the particle experienced at all the shear rates in the viscosity test. In the aqueous system, the HA particles encountered various forces, including the one from soluble polymers [33]. With the addition of the binder into the suspension, the polymers act as a part of the solid loading, resulting in the increase of the viscosity, as well as a larger force the freezing front could experience during the solidification of the liquid vehicle. Fig. 8 shows the cross-sectional SEM micrographs of porous HA ceramics obtained from 20 vol.% solid loading suspension containing different binder concentrations. With the increase of the binder concentration, the lamellar pore width decreased from 25–40 to 5–10 μm and the lamellar thickness increased from 2–3 to 20–25 μm. During the unidirectional solidification process, the freezing-front velocity determines whether a particle in the suspension will be rejected and pushed ahead, or engulfed and trapped by the approaching liquid– solid interface, which finally results in the particle rearrangement formed as a replica of the ice structures. When freezing the suspensions
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Fig. 6. SEM micrographs of sintered HA porous ceramics prepared by a range of initial HA solid loadings with the initial pH 7.6, 1 wt.% dispersant concentration and 1 wt.% binder concentration. The solidification direction was parallel to the page and the lamellar orientation. (A)10 vol.%, (B) 20 vol.%, (C) 30 vol.% , (D) 40 vol.%. (E) 10 vol.% solid loading with the solidification direction perpendicular to the page and the lamellar orientation, (F) High magnification of (E).
without and with 1 wt.% binder concentration, the freezing front experienced little difference in the resistance due to little change in the viscosity. However, the green strength of the freeze dried sample without the binder addition was too low to handle, resulting in big cracks and large distortions in the lamellar architecture. For the addition of 3 wt.% and 5 wt.% binder concentrations, the resistance the freezing front experienced became stronger due to higher viscosities, and which resulted in smaller lamellar pore width and larger lamellar wall thickness in the final sintered porous samples. Moreover, when the binder concentrations increased to 3 and 5 wt.%, a great number of
the pore defects including some with large width were observed in the ceramic walls, as shown in Fig. 8(C) and (D). The addition of higher concentration PVA binder could cause the flocculation and inhomogeneous dispersion of the suspension, which leads to the agglomeration in the suspension and the pore defects in the sintered body. 3.5. Porosity and compressive strength of the lamellar porous ceramic Fig. 9(A) shows the effects of suspensions' parameters on the porosities and compressive strengths of sintered HA porous ceramics.
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Fig. 7. Viscosity vs. binder concentration of 20 vol.% suspension at the shear rate from 0.1-500 s−1 with 1 wt.% dispersant concentration.
With the addition of PVA binder to 1 wt.% concentration, the porosity decreased from 81 vol.% to 68 vol.%, and the corresponding compressive strength increased from 0.9 MPa to 2.4 MPa. While the binder concentration increased continually from 1 wt.% to 5 wt.%, the porosity increased from 68 vol.% to 75 vol.%, and the corresponding compressive strength decreased from 2.4 MPa to 1.5 MPa. The lowest porosity of 68 vol.% and highest compressive strength of 2.4 MPa were obtained with the addition of 1 wt.% PVA binder. Without the addition of PVA binder, cracks and distortions existed in the lamellar
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structures, and the ceramic walls turned to be porous, resulting in the highest porosity and lowest compressive strength of the sintered samples. With the addition of 3 wt.% and 5 wt.% binders, the pore defects existed in the ceramic walls owing to the poor dispersion and high viscosity of the suspension, which were unfavorable to the densification of the ceramic wall and improvement of the compressive strength of the final sintered porous samples. With the addition of 1 wt.% binder, the suspension showed the best dispersive state and the green body with the highest green strength can be obtained, which were beneficial to the formation and maintenance of the lamellar pore structure. Fig. 9(B) shows a typical compression load– displacement curve of porous HA ceramic. The sample failed gradually, showing a jagged load–displacement curve, which is a typical feature of porous ceramics in compression [34]. In biomedical applications, as the pore geometry and interconnectivity need to satisfy the requirements for body fluid penetration and bone cell ingrowth, isotropic pore structures can hardly fulfill a wide range of needs with different functions and locations of the bone structures [35]. Designing an ideal scaffold is to balance the need between large interconnected pores for tissue ingrowth and nutrient transportation, and the required mechanical properties. The compressive strength and porosity of the cancellous bone [36,37] are 2–12 MPa and 90 vol.%–57 vol.%, respectively. Ice-templated process provides an easy route to fabricate porous ceramics with similar ranges of porosities and compressive strengths. The long-range ordered lamellar architectures obtained by ice-templated process mimics the structures of nacre, contributing to much higher mechanical strength than those of conventional porous HA [38]. As the minimum pore size of 100 μm with
Fig. 8. SEM micrographs of sintered HA porous ceramics prepared by a range of binder concentrations from 20 vol.% solid loading suspension with initial pH 7.6 and 1 wt.% dispersant concentration. The solidification direction was parallel to the page and the lamellar orientation. (A) No binder, (B) 1 wt.%, (C) 3 wt.%, (D) 5 wt.%.
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and the compressive strength increased monotonically by around 3 times. PVA binder with concentrations ranging from 0 to 5 wt.% resulted in large change in the viscosity, i.e. up to 11 times. As a result, the lowest porosity of 68 vol.% and highest compressive strength of 2.4 MPa were obtained with the addition of 1 wt.% PVA binder. The results demonstrated that it is a versatile and useful method by adjusting the suspension's parameters for the fabrication of the lamellar porous HA ceramics with a range of pore sizes, porosities and compressive strengths. Acknowledgments The authors are grateful to the New Century Excellent Talents program funded by the Ministry of Education of the People's Republic of China (No. NCET-10-0802), the National Natural Science Foundation of China (No. 51172288), the Ph.D. Programs Foundation of Ministry of Education of China (No. 20110162130003), Hunan Provincial Natural Science Foundation of China (No. 11JJ1008) and Hunan Provincial Innovation Foundation for Postgraduate (No. CX2011B113) for financial support. References
Fig. 9. (A) Measured porosities and compressive strengths of the samples obtained from the suspension with the addition of 1 wt.% PVA binder and different binder concentrations. (B) Typical compression load–displacement curves of porous HA ceramic obtained from the suspension with the addition of 1 wt.% PVA binder. The compressive tests were carried out parallel to the solidification direction.
three-dimensional interconnectivity is generally considered to be suitable for tissue in-growth into porous ceramic structures [39], the pore sizes of 5–40 μm in HA ceramics prepared by the ice-templating method in this study were too small to allow implant vascularisation and bone growth. On the other hand, the unidirectional lamellar porous structures along the freezing direction have better pore interconnectivity than those structures perpendicular to the freezing direction. Therefore, future studies will focus on enlarging the pore size, improving the pore interconnectivity, and finally analyzing the behaviors both in vitro and in vivo reacting with the cells and host tissue using these lamellar oriented porous HA scaffolds. 4. Conclusions Porous HA ceramics with lamellar oriented structures were fabricated by an ice-templated method. The rheological properties of the initial HA suspension employed in the ice-templated process have shown strong influence on the resultant porous ceramics, in aspects of pore characteristic, porosity, interconnectivity and compressive strength. The viscosities of the suspension showed good fit to Krieger-Dougherty model. With the increase of the solid loading from 10 to 40 vol.%, the porosity of sintered HA ceramic decreased monotonically around 40 vol.%,
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