Biomed. Eng.-Biomed. Tech. 2015; 60(2): 105–114
Karolina Schickle, Jose L. Gerardo-Nava*, Sabrina Puidokas, Sharareh Samadian Anavar, Christian Bergmann, Philipp Gingter, Benjamin Schickle, Kirsten Bobzin and Horst Fischer
Preparation of spherical calcium phosphate granulates suitable for the biofunctionalization of active brazed titanium alloy coatings Abstract: Titanium-based alloys can be actively brazed onto bio-inert ceramics and potentially be used as biocompatible coatings. To further improve their bioactivity in vivo, introduction of calcium phosphate (CaP)-based granulates onto their surface layer is possible. For this, mechanically stable CaP-based granulates need to be able to withstand the demand of the brazing process. In this study, spherical granulates, made of a calcium phosphate composite composed primarily of β-tricalcium phosphate and hydroxyapatite, a bioactive glass, and a mixture of the previous two, were manufactured by spray drying. The influence of organic additives (Dolapix CE64, trisodium citrate) and solids content (30–80 wt%) in the slurry on the physical characteristics of granulates was investigated. X-ray diffraction, Brunauer, Emmett, Teller specific surface area standard method, scanning electron microscopy, granulate size analysis, and single granule strength were performed. Our results showed that trisodium citrate permitted the production of granulates with regular morphology, high density, and increased failure stress values. The strong granules also withstood the brazing process. These results show that CaP bioactive agents can be generated and be integrated during the demanding metallurgical processes, allowing for one-step bioactivation of metal brazes.
*Corresponding author: Jose L. Gerardo Nava, MSc, Department of Dental Materials and Biomaterials Research, RWTH Aachen University Hospital, Pauwelsstrasse 30, 52074 Aachen, Germany, Phone: +49 241 8088265, Fax: +49 241 8082027, E-mail: [email protected] Karolina Schickle, Christian Bergmann and Horst Fischer: Department of Dental Materials and Biomaterials Research, RWTH Aachen University Hospital, Pauwelsstrasse 30, 52074 Aachen, Germany Sabrina Puidokas, Sharareh Samadian Anavar and Kirsten Bobzin: Surface Engineering Institute, RWTH Aachen, Kackertstrasse 15, 52072 Aachen, Germany Philipp Gingter and Benjamin Schickle: Institute of Mineral Engineering (GHI), RWTH Aachen, Mauerstrasse 5, 52064 Aachen, Germany
Keywords: bioglass; Dolapix CE64; hydroxyapatite; TCP; trisodium citrate. DOI 10.1515/bmt-2014-0017 Received February 17, 2014; accepted October 14, 2014; online first November 8, 2014
Abbreviations BG, bioglass; BG40Tr., BG 40 wt% trisodium citrate; CaP, calcium phosphate; HAp, hydroxyapatite; TH, β-tricalcium phosphate/hydroxyapatite; TH30, TH 30 wt%; TH40, TH 40 wt%; TH/BG40Tr, TH/BG 40 wt% trisodium citrate; TH/BG60Tr, TH/BG 60 wt% trisodium citrate; TH40D, TH 40 wt% Dolapix CE64; TH50TD, TH 50 wt% Dolapix CE64; TH60TD, TH 60 wt% Dolapix CE64; TH40Tr, TH 40 wt% trisodium citrate; TH70Tr, TH 70 wt% trisodium citrate; TH80Tr, TH 80 wt% trisodium citrate.
Introduction Ceramic-on-ceramic bearings in hip replacement implants are the preferred choice in young active adults due to their low wear rates. High-performance oxide ceramics such as alumina and zirconia are commonly used due to their high strength and a well-proven biocompatibility derived from their inertness. This inertness, however, leads to deficient bone-ceramic interaction, rendering them unsuitable as implant components on the acetabular side of the implant as this translates into poor boneimplant interaction [8]. Therefore, to improve implant fixation, acetabular pieces are modularly designed with the ceramic piece placed into a metallic shell, made of microstructured titanium or chrome cobalt alloys. The addition of the metallic shell increases the size of the implant, leading to an increased defect size on the receiving bone which in the end translates into an increased
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106 K. Schickle et al.: Calcium phosphate granules for active brazes burden to the patient. This opens the door for the development of new methods aiming to achieve metal-based coatings for the improvement of high-performance oxide ceramics. Very limited information regarding the coating of high-performance ceramics with metals is available. The chemical inertness of the ceramic limits the interaction with metallic substrates, preventing direct covalent bonding between both materials. Direct ceramic-metal bonding has thereby been limited to metallurgical processes. Diffusion bonding, for example, has been proposed as a way to join titanium to alumina for biomedical applications [17, 26]. However, close-fitting of titaniumbased linings onto ceramic prosthesis remains the standard used in the biomedical field. A technique currently being explored in our group for the coating of high-performance ceramics in the biomedical field is active brazing. In this technique, the brazing filler material incorporates an activating element such as titanium, hafnium, or zirconium that reacts with the ceramic surface, generating complex phases that can subsequently be wet by the filler [12]. Active brazing allows for thin (micrometer to millimeter range), defined, homogeneous metal-based layers to be deposited on inert ceramic surfaces. The possibility to disperse additional nonmetallic phases in the metal matrix during brazing has been already reported [21]. By combining metal fillers with bioactive agents, it could be possible to achieve, in a one-step process, bioactive metal coatings on surfaces of ceramics to improve bone integration of implants. Calcium phosphates (CaP) are commonly used as coatings to bioactivate metals [5–7] and ceramics [10, 25] as a way to improve bone-implant interaction. In order to disperse CaPs into filler materials, it is necessary to tailor granules with defined characteristics such as shape, size and strength. High strength is specially required to secure the structural integrity of the CaP granules during the brazing process and to avoid flaws in the final bioactive braze. These requirements can be addressed by using the spray drying technique [13], where the generated granulates can be modified by varying the spray drying para meters, such as inlet and outlet air temperatures, nozzle diameter, and pressure, and by altering slurry properties such as solids content, particle size of the initial powder, organic additives, viscosity, and the pH value [14, 27]. Spray drying of CaP materials has been previously reported in applications related to drug delivery systems, 3D printing, and bone cement powders. However, their characterization has been limited to studying the morphology, microstructure, and degradability [1, 2, 22–24], with little emphasis on single granule characterization. In the present study, we evaluated the spray drying
parameters needed to tailor CaP-based granulates, suitable for their use in titanium-based brazes. By primarily modifying the parameters of the slurries, we established the optimal conditions for manufacturing spherical, mechanical resistant, and dense CaP-based granulates capable to withstand the highly demanding conditions presented during the active brazing process.
Materials and methods Calcium phosphate materials Three different materials were used to synthesize CaP-based granulates: A CaP composite (TH) composed primarily of β-tricalcium phosphate (β-TCP) and hydroxyapatite (HAp), a bioactive glass (BG) and a mixture of the previous two (TH/BG). TH was generated by calcining commercially purchased calcium deficient HAp powder (Merck, Darmstadt, Germany) at 1000°C for 1 h to achieve a composite containing two phases, β-TCP and HAp. BG in the system SiO2– Na2O–CaO–P2O5 (similar to type 45S5) was used. Its manufacturing process has already been described in detail in two previous studies [2, 20]. The obtained casted glass was ball-milled and subsequently sieved (125 μm) to obtain a defined small grain fraction. TH/BG was synthesized by mixing the powders in the proportion of 40 wt% TH to 60 wt% BG.
Preparation of calcium phosphate-based granulates Suspensions from the three base materials were prepared by ball milling for 8 h the respective powder mixed with water and dissolved organic deflocculant. Slurries were obtained using different solid contents and organic deflocculants as follows: TH slurries at 30 wt% (TH30) and 40 wt% (TH40) solids content were carried out without any organic additive. Dolapix CE64, a commercially available deflocculant (Zschimmer & Schwarz, Lahnstein, Germany), was used at 2 wt% to prepare TH slurries containing 40 wt% (TH40D), 50 wt% (TH50D), and 60 wt% (TH60D) solids content. It was not possible to produce slurries above 70 wt% TH, even with the addition of higher concentrations of Dolapix CE64; therefore, trisodium citrate (sodium cistrate tribasic dihydrate, Sigma-Aldrich, Taufkirchen, Germany) was applied at 2 wt% to obtain TH slurries with solids content of 40 wt% (TH40Tr), 70 wt% (TH70Tr), and 80 wt% (TH80Tr). Slurries made of BG 40 wt% (BG40Tr), TH/BG 40 wt% (TH/BG40Tr), and 60 wt% (TH/BG60Tr) solids content were prepared using only trisodium citrate. To study the influence of different organic deflocculants on granule formation, slurries with the same solids content of 40 wt% but different deflocculants (TH40, TH40D, and TH40Tr) were characterized. Table 1 gives an overview of all variations of parameters used for the studied slurries. Suspensions were characterized by measuring their pH value (GPHR 1400, Greisinger Electronic, Regenstauf, Germany) and viscosity (Rheolab QC, Anton Paar, Graz, Austria). The CaP-based granulates were then prepared by spray drying. Production parameters were kept the same with all the slurries to avoid influence on the granulate properties. The parameters of the
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K. Schickle et al.: Calcium phosphate granules for active brazes 107 Table 1 Parameters of the synthesized slurries with respective organic additive and solids content. Suspension TH30 TH40 TH40D TH40Tr TH50D TH60D TH70Tr TH80Tr TH/BG40Tr TH/BG60Tr BG40Tr
Base material Solids content (Wt%) TCP-Hap TCP-Hap TCP-Hap TCP-Hap TCP-Hap TCP-Hap TCP-Hap TCP-Hap TCP-HAp/bioglass TCP-HAp/bioglass Bioglass
30 40 40 40 50 60 70 80 40 60 40
Organic additive (2%) None None Dolapix CE64 Trisodium citrate Dolapix CE64 Dolapix CE64 Trisodium citrate Trisodium citrate Trisodium citrate Trisodium citrate Trisodium citrate
spray drying machine (Mobile minor 2000, Niro, Soeborg, Denmark) used for granulate production were as follows: inlet air temperature, 220°C; outlet air temperature, 102°C; air flow rate, 40%; air pressure, 1.5 bar; and pump capacity, 20–30 r/min (dependent on outlet air temperature).
Characterization of the CaP-based granulates The phases of the materials before (powder) and after spray drying (granulates) were analyzed by X-ray diffraction using copper radiation Kα (XRD, PW 3710, Philips, Eindhoven, the Netherlands) to determine the phase composition of the materials and exclude any changes of the crystalline phases during the spray drying process. Particle size analysis of the granulates after spray drying was carried out using a laser diffraction particle size analyzer (Mastersizer 2000 version 5.12G, Malvern Instruments, Malvern, UK). Besides granule size, the shape and morphology of the obtained spheres were evaluated by scanning electron microscopy (SEM, Leo1530, Carl Zeiss, Oberkochen, Germany). Images from granules cross-sections were generated to assess possible internal defects and evaluate the internal density of the granules. Samples TH40, TH40D, TH40Tr, and TH70Tr were further characterized to determine the influence of both the solids content and dispersing agent on the physical properties of the granulate. Specific surface area measurements were obtained using the single-point differential method according Haul and Dümbgen (AREA meter, Ströhlein, Kaarst, Germany) in agreement with DIN 66132. The mechanical properties of the granulates were measured using a particle strength automatic test system (etewe GmbH, Karlsruhe, Germany). Single granulates (fraction size 63–100 μm) were placed between two parallel plates exerting a compressive force measured by an automated system with a resolution of 0.1 mN. Fracture load values were measured and used to calculate failure stress values as described by the following formula:
σ B = 4FB / ( πdG2 )
(1)
where σB is the failure stress value (MPa), FB is the measured breaking load (N), and dG2 is the diameter of the granule (μm).
Preparation of titanium-based brazes containing CaP-granulates Granulates made of TH, BG, and TH/BG were used for the application in Ti-based coatings to prove their suitability for the manufacturing of bioactive coatings. A titanium-based braze paste composed of Ti72-Co26-Si2 (wt%, TiCoSi) containing 40 wt% P41 binder (Brazetec, Hanau, Germany) was mixed with the different CaP-based granulates (10 vol%), applied on an alumina surface and subsequently brazed at 1090°C for 40 min. Cross-section images were made to determine the effect of the brazing process on the morphology of granules obtained from the three different materials (TH70Tr, BG40Tr, and TH/BG60Tr). TH granules produced by different deflocculants (TH40, TH60D, and TH70Tr) were also embedded into the braze, and the surfaces of these brazes were characterized using SEM/energy-dispersive X-ray (EDX) mapping (Leo1530, Carl Zeiss, Oberkochen, Germany) to visualize the morphology and distribution of the granulates on the surface of the metal matrix.
Results Characterization of the base materials and suspensions Characteristic data of the prepared slurries are summarized in Table 2. The pH values of the slurries were strongly dependent on both concentration of solids content in the suspension and the addition of organic deflocculants. Higher loading of solid material resulted in an increase of the pH values of the prepared suspensions without deflocculants (suspension TH30 and TH40). The addition of both Dolapix CE64 and trisodium citrate further increased the pH value in suspensions with the same solids content (TH40, TH40D, and TH40Tr). However, no influence of solids content on pH values in samples prepared with the same dispersing agent was observed (e.g., suspensions TH40D, TH50D, and TH60D). The pH of the samples containing BG was higher than those containing only TH. Solids content increase in the suspensions caused a raise in the viscosity values. The addition of deflocculants significantly lowered the viscosity of the slurries (e.g., suspensions TH40, TH40D, and TH40Tr). Increase in viscosity of the slurry with increase of solid matter was also observed after addition of deflocculants. An abrupt rise in viscosity was observed in samples TH70Tr and TH80Tr. The characterized suspensions were subsequently spray dried to obtain CaP-based granulates. High solids content prevented suspension TH80Tr from being spray dried. Characterization by XRD showed no changes of the based materials before and after spray drying. Two crystalline phases, mainly β-TCP and approximately 5% of HAp,
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108 K. Schickle et al.: Calcium phosphate granules for active brazes Table 2 Suspension characteristics (pH-value and viscosity) and the size of the granulates, d(0.1), d(0.5), and d(0.9) values. Suspension TH30 TH40 TH40D TH40Tr TH50D TH60D TH70Tr TH80Tr TH/BG40Tr TH/BG60Tr BG40Tr
pH value
Viscosity at 100 s-(mPas)
Viscosity at 1000 s-(mPas)
d(0.1) (μm)
d(0.5) (μm)
d(0.9) (μm)
7.6 8.0 8.9 9.0 8.5 8.9 9.6 9.5 9.9 11.0 13.1
26.8 56.2 4.5 5.8 4.6 6.8 142.5 1059.3 6.5 130.1 7.7
6.9 12.2 3.9 4.4 4.1 5.3 105.5 – 5.4 46.6 5.7
16.1 10.0 5.3 18.2 14.8 16.9 41.1 – 17.8 26.3 23.4
37.6 44.0 31.0 32.0 38.5 60.8 82.4 – 30.3 56.7 54.4
68.9 76.3 58.1 57.1 81.8 185.7 148.5 – 50.3 110.4 125.7
were detected by XRD for TH (Figure 1, green pattern). An exclusive amorphous behavior of the synthesized BG was observed as no crystalline peaks were detected (Figure 1,
blue pattern). TH/BG mixture spectra show weaker peaks of the TH crystalline phases, without additional phases being formed (Figure 1, red pattern).
Morphological and mechanical characterization of the spray-dried granulates
Figure 1 Phase composition of the materials after the spray drying process determined by XRD measurements. Spectra obtained from granulate produced with TH (green), BG (blue), and TH/BG (red). Similar spectra were recorded from the materials before spray drying.
Curves representing particle size distribution of the granules produced with Dolapix show a shift toward bigger granule sizes and a wider distribution with increasing solids content (Figure 2 and Table 2). The biggest average granule size 82.4 μm at d(0.5) was obtained with the slurry TH70Tr presenting the highest solids content. Granulates that were manufactured out of slurries without any organic additives exhibited spherical shapes and showed no defects in the cross section (Figures 3A and B and 4A and B, respectively). However, their surface morphology showed low packing density, and especially in sample TH40, appendages could be observed (Figures 3B and 4B). The addition of Dolapix CE64 significantly improved the surface morphology of the spheres, but granule defects such as doughnut shape
Figure 2 Influence of solids content on particle size distribution of the CaP-composite granulates. Blue pattern corresponds to the granulates made of slurry TH40D, red pattern to granulates made of slurry TH50D, and green pattern to granulates made of slurry TH60D.
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K. Schickle et al.: Calcium phosphate granules for active brazes 109
spherical morphology; however, the large glass grains produced a loosely packed surface (Figure 3I). The internal density of the granules was primarily affected by the solids content and to a lesser degree by the organic deflocculant. The internal structures of TH samples prepared without additives and with trisodium citrate were less densely packed compared to Dolapix CE64 samples (Figure 4B, C, and E). Equally, high solids content increased the internal density of the granules (Figure 4D and G). The specific surface area of granulates decreased with increasing size of the spheres, from 5 m2/g for TH40 to 2.5 m2/g for TH70Tr (Table 3). Comparing the specific surface area values of the samples prepared with slurries containing the same solids content but different organic additives (samples TH40, TH40D, and TH40Tr), it could be observed that the highest value was achieved by the sample prepared with Dolapix CE64 as dispersing agent (TH40D). Granulate strength characterization was performed on 50 single granules per condition, except for sample TH40, where only 15 could be studied due to agglomeration/appendage problems. Results from the strength characterization show the strong influence organic additives have on failure stress values, where Dolapix CE64 significantly increased the rigidity of the granules compared. Equally, granulates generated with TH slurries with higher solids content and similar organic additive showed higher failure stress values than BG-containing granules (Table 4).
Evaluation granulates after the brazing process
Figure 3 SEM images of the surface structure of granulates manufactured of slurries with various parameters. (A) TH30, (B) TH40, (C) TH40D, (D) TH60D, (E) TH40Tr, (F)TH70Tr, (G) TH/BG40Tr, (H) TH/ BG60Tr, and (I) BG40Tr. Inserts show the surface of granules from the respective material. Scale bars: all = 100 μm, all inserts = 10 μm.
and inner holes occurred (Figures 3C and D and 4C and D). The introduction of trisodium citrate as organic additive improved surface morphology, compared with samples with no organic additive, and a decrease in defects was observed, especially in the sample with the highest solids content TH70Tr (Figures 3E and F and 4E and F). Granules produced with TH/BG showed similar spherical morphology but low surface packing density, and large BG particles could be easily identified on the surface of the TH/BG granules (Figure 3G and H). BG granules also presented a
SEM cross-section images of TiCoSi-based coatings with granulates prepared from the three different materials showed that those granules consisting solely of TH (TH70Tr) maintained their integrity after the brazing process (Figure 5A). Contrary to this, BG and TH/BG (BG40Tr and TH/BG60Tr) granules were altered by the brazing process, losing completely their spherical form (Figure 5B and C). The effect of brazing on granules prepared using different deflocculants was also analyzed by SEM/EDX. Granulates generated without additives completely lost their shape as indicated by the irregular distribution of the Ca+P spectra (TH40, Figure 6A and B). Those generated with Dolapix CE64 showed significant deformations, and no spheric granule was observed in the braze (TH50D, Figure 6C and D). In contrast, TH samples prepared using trisodium citrate preserved their shape during the brazing process (TH70Tr, Figure 6E and F) as previously described.
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110 K. Schickle et al.: Calcium phosphate granules for active brazes
Figure 4 SEM images from cross sections of the generated granulates showing their inner structure. (A) TH30, (B) TH40, (C) TH40D, (D) TH60D, (E) TH40Tr, and (F) TH70Tr. Inserts show the inner structure of granules from the respective material. Scale bars: all = 100 μm, all inserts = 20 μm.
Discussion Biofunctionalization of inert surfaces is of great importance in the biomedical field. Titanium-based brazed coatings containing CaP-based granulates represent a new
approach to functionalize ceramic implant surfaces. To achieve this, the CaP-based granules have to fulfill several requirements including high internal density, regular shape, high strength, and a defined size distribution. These parameters are essential to achieve good dispersion
Table 3 Specific surface area of selected CaP-based granulates prepared from the respective suspension.
Table 4 Fracture load and failure stress values of selected CaP-based granules prepared from the respective suspension.
Granulate made from suspension
Granulate made from suspension
TH40 TH40D TH40Tr TH70Tr
Specific surface area (m2/g) 5.0 9.3 5.1 2.5
TH40 TH40D TH40Tr TH70Tr
Diameter (μm)
Fracture Load (mN)
Failure Stress (MPa)
96 ± 3 77 ± 10 72 ± 8 79 ± 10
1.0 ± 0.24 7.2 ± 3.1 1.8 ± 0.6 8.1 ± 2.2
0.14 ± 0.03 1.63 ± 0.86 0.42 ± 0.10 1.65 ± 0.30
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K. Schickle et al.: Calcium phosphate granules for active brazes 111
Figure 5 SEM images of the cross sections of granules embedded in the TiCoSi matrix. (A) TH70Tr granules maintain their spherical shape after the brazing process. (B) BG40Tr and (C) TH/BG60Tr on the other hand show loss of their spherical morphology. * = granules; scale bars: all = 40 μm.
in the metal matrix coating and enough mechanical stability to withstand the demanding conditions of the brazing process. TH, BG, and TH/BG were used to synthesize slurries for granulate fabrication by the spray drying process. Achieving slurries with high solids content in order to generate granulates with high densities was the primary goal. Slurry stability is crucial for the spray drying process and is strongly dependent on the pH value, which directly affects the surface charge on the particles [15]. The results confirmed that the pH values of the synthesized slurries increased with the addition of solids content [9]. This effect can be explained by the hydration process of charged particles which occurs immediately after contact of the particles with water molecules [15]. The pH values of the suspensions were slightly changed by the addition of deflocculants as well (suspensions TH40D and TH40Tr). These changes were dependent on the pH values of the applied deflocculants itself and the process of interaction and adsorption of the dispersant molecules on the charged particles. Another parameter, which is fundamental for the spray drying process, is the viscosity of the suspension. High solid content of the suspension is required to obtain compact, dense granules. Increase in solids content translates to a significant increase in the viscosity (e.g., suspension TH30 and TH40) [4, 16]. It is necessary to obtain a good fluidity of the slurry with high solids content in order to make it suitable for spray drying. Our results confirmed that all the CaP-based suspensions exhibited a shear thinning behavior (Table 2) [4]. Addition of both deflocculants (Dolapix CE64 and trisodium citrate) significantly decreased the viscosity of the suspensions enabling the preparation of slurries with higher solids content (Table 2). The dispersion of the suspensions using Dolapix CE64 is driven by an electrosteric mechanism [16]. The carbonic acid-based polyelectrolyte chains
interact with the charged surface of the particles, causing the formation of a thin organic layer on the particles which prevent the attractive van-der-Waals forces between particles, and thus flocculation is avoided [3, 16, 19]. However, our results have shown that this mechanism is limited, since it was not possible to synthesize slurries containing more than 60 wt% of solids content (TH). Synthesizing slurries containing 70 and 80 wt% was possible by adding trisodium citrate, commonly used as a pH control agent. The addition of this deflocculant resulted in significant changes in the particle surface charge (electrostatic mechanism) and in consequence an increase in repulsive interparticle forces that overcame the attractive Van der Waals forces [11, 18]. All prepared slurries were spray dried to obtain spherical granulates. Granule size was strongly dependent on slurry concentration (Table 2 and Figure 2), as indicated by greater granule sizes obtained with increased solids content of the slurry. Size distribution results confirmed the dependency of the granule size on solids content in the slurry and can be described by the following formula [14].
Dmean = CP -n ηγ ρ-t
(2)
where Dmean is the average granule size (μm); C is a constant; P is the atomization pressure; η is the viscosity; ρ is the solid content of slurry; and n, γ, and t are constants depending on the process conditions. The size of the granules is strongly dependent on the shrinkage during the drying process, which is related to the water loss from the droplets. As expected, the higher the water content in the slurry, the greater the shrinkage. Moreover, the slurry concentration appeared to have an influence on the shape of the size distribution curves as well. We could observe that the solids content has an effect on size distribution [27]. The distribution curves of granulates made of slurries
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112 K. Schickle et al.: Calcium phosphate granules for active brazes
Figure 6 SEM and EDX mapping images (Ca in magenta+P in cyan = purple) of TH granulate on the surface of a TiCoSi braze matrix. TH granulates produced (A, B) without additive, (C, D) with Dolapix CE64, and (E, F) with trisodium citrate. Scale bars: all = 50 μm.
with a lower solids content (TH40D) showed a narrow distribution in contrast to the curves of granulates prepared of slurries with a higher solids content (TH50D and TH60D). Granule morphology does not depend on solid content. However, the significant changes in morphology, density, and shape of the obtained spheres were dependent on the organic additives. The granulates, prepared from slurries without any organic additives, exhibited
poor surface density but spherical morphology in comparison to those made of slurries containing Dolapix CE64 as deflocculant. This occurred primarily due to the adsorption of organic molecules from the deflocculant on the surface of the particles. During spray drying, slurry droplets shrink until the outer shell is created. The presence of organic molecules on the particles caused tightening of the outer shell preventing the remaining water
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K. Schickle et al.: Calcium phosphate granules for active brazes 113
from evaporating and increasing the pressure inside the granules. Since the temperature during the spray drying process (between Tin = 230°C and Tout = 102°C) was not high enough to cause pyrolysis of the additive Dolapix CE64, which is reported to occur at temperatures between 230 and 490°C [19], granulate defects such as doughnut structure, hollow structure, or fracture of the spheres occurred [14]. The other applied organic deflocculant, trisodium citrate, did not show this effect since its pyrolysis temperature of approximately 150°C (manufacturer data) was surpassed. Even if particles were partially covered with trisodium citrate molecules, the temperature achieved during the drying process was high enough to decompose this organic substance, allowing water evaporation from inside the granules to take place. This allowed granulates synthesized using trisodium citrate as deflocculant to exhibit both spherical shape and a regular surface morphology. This could be observed by direct comparison of granulates manufactured from slurries containing the same solids content but different additives (TH40, TH40D, and TH40Tr). The achieved results showed a beneficial influence of trisodium citrate as dispersion agent on the spray drying process as it also allowed the formation of regular, spherically shaped granulates of both BG and TH/BG. Four granulates (TH40, TH40D, TH40Tr, and TH70Tr) were chosen to examine the influence of different organic additives and slurry concentrations on the specific surface area and mechanical properties of the granulates. As expected, the specific surface area decreased (from 5 to 2.5 m2/g) with increased solids content (from 40 to 70 wt%, Table 3). This can be explained as the result of differences in the granulate size. A decrease of the granulate size led to an increase of the specific surface area. The values of the specific surface area of samples made of slurries containing 40 wt% of solids prepared either without organic additives or trisodium citrate were comparable (5 m2/g). On the other hand, the values of granulates made of the same solids content but with Dolapix CE64 were much higher (9.3 m2/g). This is mainly the result of granulate defects that occurred only in suspensions containing Dolapix CE64. In this case, the specific surface area increased nearly two times due to the doughnut/hollow structure of the granulates. Dolapix CE64 had also a great influence on the rigidity of the samples. This, like in the surface area results, could be explained by residual Dolapix CE64 molecules stabilizing the surface of the granules as reflected by their increased failure stress value compared with samples with no additives and trisodium citrate. Increased solids content also
increased failure stress values and could be a reflection of the high internal density of the granules. A selection of the CaP-based granulates was used to determine their suitability for the production of bioactive brazed coatings. The results of initial tests confirmed the feasibility of the incorporation of TH granulates into metal coating. Granules that presented low packing density on their surface, such as those prepared without deflocculants or those containing bioglass, collapsed under the hard conditions posed by the brazing process. Granules generated with slurries using Dolapix, which also presented stronger mechanical properties, also remained unaffected by the brazing process. However, their irregular morphologies made them unsuitable for our application. The mechanically and thermally most resistant granulates appeared to be those generated using 70% TH and trisodium citrate as no damage after brazing was observed. The obtained results are very promising for possible novel coatings to increase the bioactivity of an implant surface. Therefore, further studies addressing the phase composition, the mechanical behavior, and the biological response are ongoing.
Conclusion The use of trisodium citrate as deflocculant allowed for the generation of CaP-based granules through spray drying. The granules showed regular spherical shapes and high internal densities that translated into mechanically strong spheres capable to withstand the mechanical and thermal conditions imposed by the active brazing process. The generation of such granules opens the door for a one-step biofunctionalization of materials during metallurgical processes, such as that of the titanium alloy-CaP granulate brazing system presented in this study, with the potential to improve the development of orthopedic implants. Acknowledgments: The authors would like to acknowledge the financial support for this project by the German Research Foundation (DFG, grant FI 975/16-1). The authors would like to also thank Mrs. Petra Schott and Mr. Peter König, Institute of Mineral Engineering, RWTH Aachen University, for their help with the particle size distribution measurements and the XRD analyses. Special thanks go to Mr. Malte Schmachtenberg, Welding and Joining Institute, RWTH Aachen University, for operating the SEM/EDX equipment.
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114 K. Schickle et al.: Calcium phosphate granules for active brazes
References [1] Andrianjatovo H, Jose F, Lemaitre J. Effect of β-TCP granularity on setting time and strength of calcium phosphate hydraulic cements. J Mater Sci Mater Med 1996; 7: 34–39. [2] Bergmann C, Lindner M, Zhang W, et al. 3D printing of bone substitute implants using calcium phosphate and bioactive glasses. J Eur Ceram Soc 2010; 30: 2563–2567. [3] Cesarano J, Aksay IA, Bleier A. Stability of aqueous alphaAl2O3 suspensions with poly(methacrylic acid) polyelectrolyte. J Am Ceram Soc 1988; 71: 250–255. [4] Chen B, Zhang Z, Zhang J, Lin Q, Jiang D. Fabrication and mechanical properties of β-TCP pieces by gel-casting method. Mater Sci Eng C 2008; 28: 1052–1056. [5] Fontana F, Rocchietta I, Addis A, Schupbach P, Zanotti G, Simion M. Effects of a calcium phosphate coating on the osseo integration of endosseous implants in a rabbit model. Clin Oral Implants Res 2011; 22: 760–766. [6] Gao Y, Hu J, Guan TH, Wu J, Zhang CB, Gao B. Physical properties and cellular responses to calcium phosphate coating produced by laser rapid forming on titanium. Lasers Med Sci 2012; 29: 9–17. [7] Goyenvalle E, Aguado E, Nguyen J-M, et al. Osteointegration of femoral stem prostheses with a bilayered calcium phosphate coating. Biomaterials 2006; 27: 1119–1128. [8] Ignatius A, Peraus M, Schorlemmer S, et al. Osseointegration of alumina with a bioactive coating under load-bearing and unloaded conditions. Biomaterials 2005; 26: 2325–2332. [9] Kim DJ, Lee JK. Dependence of the rheological behaviour of electrostatically stabilized alumina slurries on pH and solid loading. J Mater Sci 1998; 33: 2931–2935. [10] Kim H-W, Georgiou G, Knowles JC, Koh Y-H, Kim H-E. Calcium phosphates and glass composite coatings on zirconia for enhanced biocompatibility. Biomaterials 2004; 25: 4203–4213. [11] Leeuwenburgh SCG, Ana ID, Jansen JA. Sodium citrate as an effective dispersant for the synthesis of inorganic-organic composites with a nanodispersed mineral phase. Acta Biomater 2010; 6: 836–844. [12] Lin C-C, Chen R-B, Shiue R-K. A wettability study of Cu/Sn/Ti active braze alloys on alumina. J Mater Sci 2001; 36: 2145–2150. [13] Lukasiewicz SJ. Spray-drying ceramic powders. J Am Ceram Soc 1989; 72: 617–624. [14] Luo P, Nieh TG. Preparing hydroxyapatite powders with controlled morphology. Biomaterials 1996; 17: 1959–1964.
[15] Parks GA, Bruyn PL de. The zero point of charge of oxides. J Phys Chem 1962; 66: 967–973. [16] Rao SP, Tripathy SS, Raichur AM. Dispersion studies of submicron zirconia using Dolapix CE 64. Colloids Surfaces A Physicochem Eng Asp 2007; 302: 553–558. [17] Rocha LA, Ariza E, Costa AM, Oliveira FJ, Silva RF. Electrochemical behavior of Ti/Al2O3 interfaces produced by diffusion bonding. Mater Res 2003; 6: 439–444. [18] Sacks MD, Tseng T-Y. Role of sodium citrate in aqueous milling of aluminum oxide. J Am Ceram Soc 1983; 66: 242–247. [19] Sarraf H, Sabet A, Herbig R, Havrda J, Hulinsky V, Maryska M. Advanced colloidal techniques for characterization of the effect of electrosteric dispersant on the colloidal stability of nanocrystalline ZrO2 suspension. J Ceram Soc Japan 2009; 117: 302–307. [20] Schickle K, Zurlinden K, Bergmann C, et al. Synthesis of novel tricalcium phosphate-bioactive glass composite and functionalization with rhBMP-2. J Mater Sci Mater Med 2011; 22: 763–771. [21] Schlegel A, Bobzin K, Kopp N, Bagcivan N. Die Entwicklung des Breitspaltaktivlôtens als neue Fügetechnologie für KeramikMetall-Mischverbunde mit Einsatztemperaturen oberhalb 500°C. Keramische Zeitschrift 2011; 63: 329–333. [22] Silva NA, Salgado AJ, Sousa RA, et al. Development and characterization of a novel hybrid tissue engineering-based scaffold for spinal cord injury repair. Tissue Eng Part A 2010; 16: 45–54. [23] Sun L, Chow LC, Frukhtbeyn SA, Bonevich JE. Preparation and properties of nanoparticles of calcium phosphates with various Ca/P ratios. J Res Natl Inst Stand Technol 2010; 115: 243–255. [24] Sun R, Lu Y, Chen K. Preparation and characterization of hollow hydroxyapatite microspheres by spray drying method. Mater Sci Eng C 2009; 29: 1088–1092. [25] Takaoka T, Okumura M, Ohgushi H, Inoue K, Takakura Y, Tamai S. Histological and biochemical evaluation of osteogenic response in porous hydroxyapatite coated alumina ceramics. Biomaterials 1996; 17: 1499–1505. [26] Travessa D, Ferrante M. The Al2O3-titanium adhesion in the view of the diffusion bonding process. J Mater Sci 2002; 37: 4380–4390. [27] Wang A, Lu Y, Zhu R, Li S, Ma X. Effect of process parameters on the performance of spray dried hydroxyapatite microspheres. Powder Technol 2009; 191: 1–6.
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Karolina Schickle, Jose L. Gerardo-Nava*, Sabrina Puidokas, Sharareh Samadian Anavar, Christian Bergmann, Philipp Gingter, Benjamin Schickle, Kirsten Bobzin and Horst Fischer
Preparation of spherical calcium phosphate granulates suitable for the biofunctionalization of active brazed titanium alloy coatings Abstract: Titanium-based alloys can be actively brazed onto bio-inert ceramics and potentially be used as biocompatible coatings. To further improve their bioactivity in vivo, introduction of calcium phosphate (CaP)-based granulates onto their surface layer is possible. For this, mechanically stable CaP-based granulates need to be able to withstand the demand of the brazing process. In this study, spherical granulates, made of a calcium phosphate composite composed primarily of β-tricalcium phosphate and hydroxyapatite, a bioactive glass, and a mixture of the previous two, were manufactured by spray drying. The influence of organic additives (Dolapix CE64, trisodium citrate) and solids content (30–80 wt%) in the slurry on the physical characteristics of granulates was investigated. X-ray diffraction, Brunauer, Emmett, Teller specific surface area standard method, scanning electron microscopy, granulate size analysis, and single granule strength were performed. Our results showed that trisodium citrate permitted the production of granulates with regular morphology, high density, and increased failure stress values. The strong granules also withstood the brazing process. These results show that CaP bioactive agents can be generated and be integrated during the demanding metallurgical processes, allowing for one-step bioactivation of metal brazes.
*Corresponding author: Jose L. Gerardo Nava, MSc, Department of Dental Materials and Biomaterials Research, RWTH Aachen University Hospital, Pauwelsstrasse 30, 52074 Aachen, Germany, Phone: +49 241 8088265, Fax: +49 241 8082027, E-mail: [email protected] Karolina Schickle, Christian Bergmann and Horst Fischer: Department of Dental Materials and Biomaterials Research, RWTH Aachen University Hospital, Pauwelsstrasse 30, 52074 Aachen, Germany Sabrina Puidokas, Sharareh Samadian Anavar and Kirsten Bobzin: Surface Engineering Institute, RWTH Aachen, Kackertstrasse 15, 52072 Aachen, Germany Philipp Gingter and Benjamin Schickle: Institute of Mineral Engineering (GHI), RWTH Aachen, Mauerstrasse 5, 52064 Aachen, Germany
Keywords: bioglass; Dolapix CE64; hydroxyapatite; TCP; trisodium citrate. DOI 10.1515/bmt-2014-0017 Received February 17, 2014; accepted October 14, 2014; online first November 8, 2014
Abbreviations BG, bioglass; BG40Tr., BG 40 wt% trisodium citrate; CaP, calcium phosphate; HAp, hydroxyapatite; TH, β-tricalcium phosphate/hydroxyapatite; TH30, TH 30 wt%; TH40, TH 40 wt%; TH/BG40Tr, TH/BG 40 wt% trisodium citrate; TH/BG60Tr, TH/BG 60 wt% trisodium citrate; TH40D, TH 40 wt% Dolapix CE64; TH50TD, TH 50 wt% Dolapix CE64; TH60TD, TH 60 wt% Dolapix CE64; TH40Tr, TH 40 wt% trisodium citrate; TH70Tr, TH 70 wt% trisodium citrate; TH80Tr, TH 80 wt% trisodium citrate.
Introduction Ceramic-on-ceramic bearings in hip replacement implants are the preferred choice in young active adults due to their low wear rates. High-performance oxide ceramics such as alumina and zirconia are commonly used due to their high strength and a well-proven biocompatibility derived from their inertness. This inertness, however, leads to deficient bone-ceramic interaction, rendering them unsuitable as implant components on the acetabular side of the implant as this translates into poor boneimplant interaction [8]. Therefore, to improve implant fixation, acetabular pieces are modularly designed with the ceramic piece placed into a metallic shell, made of microstructured titanium or chrome cobalt alloys. The addition of the metallic shell increases the size of the implant, leading to an increased defect size on the receiving bone which in the end translates into an increased
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106 K. Schickle et al.: Calcium phosphate granules for active brazes burden to the patient. This opens the door for the development of new methods aiming to achieve metal-based coatings for the improvement of high-performance oxide ceramics. Very limited information regarding the coating of high-performance ceramics with metals is available. The chemical inertness of the ceramic limits the interaction with metallic substrates, preventing direct covalent bonding between both materials. Direct ceramic-metal bonding has thereby been limited to metallurgical processes. Diffusion bonding, for example, has been proposed as a way to join titanium to alumina for biomedical applications [17, 26]. However, close-fitting of titaniumbased linings onto ceramic prosthesis remains the standard used in the biomedical field. A technique currently being explored in our group for the coating of high-performance ceramics in the biomedical field is active brazing. In this technique, the brazing filler material incorporates an activating element such as titanium, hafnium, or zirconium that reacts with the ceramic surface, generating complex phases that can subsequently be wet by the filler [12]. Active brazing allows for thin (micrometer to millimeter range), defined, homogeneous metal-based layers to be deposited on inert ceramic surfaces. The possibility to disperse additional nonmetallic phases in the metal matrix during brazing has been already reported [21]. By combining metal fillers with bioactive agents, it could be possible to achieve, in a one-step process, bioactive metal coatings on surfaces of ceramics to improve bone integration of implants. Calcium phosphates (CaP) are commonly used as coatings to bioactivate metals [5–7] and ceramics [10, 25] as a way to improve bone-implant interaction. In order to disperse CaPs into filler materials, it is necessary to tailor granules with defined characteristics such as shape, size and strength. High strength is specially required to secure the structural integrity of the CaP granules during the brazing process and to avoid flaws in the final bioactive braze. These requirements can be addressed by using the spray drying technique [13], where the generated granulates can be modified by varying the spray drying para meters, such as inlet and outlet air temperatures, nozzle diameter, and pressure, and by altering slurry properties such as solids content, particle size of the initial powder, organic additives, viscosity, and the pH value [14, 27]. Spray drying of CaP materials has been previously reported in applications related to drug delivery systems, 3D printing, and bone cement powders. However, their characterization has been limited to studying the morphology, microstructure, and degradability [1, 2, 22–24], with little emphasis on single granule characterization. In the present study, we evaluated the spray drying
parameters needed to tailor CaP-based granulates, suitable for their use in titanium-based brazes. By primarily modifying the parameters of the slurries, we established the optimal conditions for manufacturing spherical, mechanical resistant, and dense CaP-based granulates capable to withstand the highly demanding conditions presented during the active brazing process.
Materials and methods Calcium phosphate materials Three different materials were used to synthesize CaP-based granulates: A CaP composite (TH) composed primarily of β-tricalcium phosphate (β-TCP) and hydroxyapatite (HAp), a bioactive glass (BG) and a mixture of the previous two (TH/BG). TH was generated by calcining commercially purchased calcium deficient HAp powder (Merck, Darmstadt, Germany) at 1000°C for 1 h to achieve a composite containing two phases, β-TCP and HAp. BG in the system SiO2– Na2O–CaO–P2O5 (similar to type 45S5) was used. Its manufacturing process has already been described in detail in two previous studies [2, 20]. The obtained casted glass was ball-milled and subsequently sieved (125 μm) to obtain a defined small grain fraction. TH/BG was synthesized by mixing the powders in the proportion of 40 wt% TH to 60 wt% BG.
Preparation of calcium phosphate-based granulates Suspensions from the three base materials were prepared by ball milling for 8 h the respective powder mixed with water and dissolved organic deflocculant. Slurries were obtained using different solid contents and organic deflocculants as follows: TH slurries at 30 wt% (TH30) and 40 wt% (TH40) solids content were carried out without any organic additive. Dolapix CE64, a commercially available deflocculant (Zschimmer & Schwarz, Lahnstein, Germany), was used at 2 wt% to prepare TH slurries containing 40 wt% (TH40D), 50 wt% (TH50D), and 60 wt% (TH60D) solids content. It was not possible to produce slurries above 70 wt% TH, even with the addition of higher concentrations of Dolapix CE64; therefore, trisodium citrate (sodium cistrate tribasic dihydrate, Sigma-Aldrich, Taufkirchen, Germany) was applied at 2 wt% to obtain TH slurries with solids content of 40 wt% (TH40Tr), 70 wt% (TH70Tr), and 80 wt% (TH80Tr). Slurries made of BG 40 wt% (BG40Tr), TH/BG 40 wt% (TH/BG40Tr), and 60 wt% (TH/BG60Tr) solids content were prepared using only trisodium citrate. To study the influence of different organic deflocculants on granule formation, slurries with the same solids content of 40 wt% but different deflocculants (TH40, TH40D, and TH40Tr) were characterized. Table 1 gives an overview of all variations of parameters used for the studied slurries. Suspensions were characterized by measuring their pH value (GPHR 1400, Greisinger Electronic, Regenstauf, Germany) and viscosity (Rheolab QC, Anton Paar, Graz, Austria). The CaP-based granulates were then prepared by spray drying. Production parameters were kept the same with all the slurries to avoid influence on the granulate properties. The parameters of the
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K. Schickle et al.: Calcium phosphate granules for active brazes 107 Table 1 Parameters of the synthesized slurries with respective organic additive and solids content. Suspension TH30 TH40 TH40D TH40Tr TH50D TH60D TH70Tr TH80Tr TH/BG40Tr TH/BG60Tr BG40Tr
Base material Solids content (Wt%) TCP-Hap TCP-Hap TCP-Hap TCP-Hap TCP-Hap TCP-Hap TCP-Hap TCP-Hap TCP-HAp/bioglass TCP-HAp/bioglass Bioglass
30 40 40 40 50 60 70 80 40 60 40
Organic additive (2%) None None Dolapix CE64 Trisodium citrate Dolapix CE64 Dolapix CE64 Trisodium citrate Trisodium citrate Trisodium citrate Trisodium citrate Trisodium citrate
spray drying machine (Mobile minor 2000, Niro, Soeborg, Denmark) used for granulate production were as follows: inlet air temperature, 220°C; outlet air temperature, 102°C; air flow rate, 40%; air pressure, 1.5 bar; and pump capacity, 20–30 r/min (dependent on outlet air temperature).
Characterization of the CaP-based granulates The phases of the materials before (powder) and after spray drying (granulates) were analyzed by X-ray diffraction using copper radiation Kα (XRD, PW 3710, Philips, Eindhoven, the Netherlands) to determine the phase composition of the materials and exclude any changes of the crystalline phases during the spray drying process. Particle size analysis of the granulates after spray drying was carried out using a laser diffraction particle size analyzer (Mastersizer 2000 version 5.12G, Malvern Instruments, Malvern, UK). Besides granule size, the shape and morphology of the obtained spheres were evaluated by scanning electron microscopy (SEM, Leo1530, Carl Zeiss, Oberkochen, Germany). Images from granules cross-sections were generated to assess possible internal defects and evaluate the internal density of the granules. Samples TH40, TH40D, TH40Tr, and TH70Tr were further characterized to determine the influence of both the solids content and dispersing agent on the physical properties of the granulate. Specific surface area measurements were obtained using the single-point differential method according Haul and Dümbgen (AREA meter, Ströhlein, Kaarst, Germany) in agreement with DIN 66132. The mechanical properties of the granulates were measured using a particle strength automatic test system (etewe GmbH, Karlsruhe, Germany). Single granulates (fraction size 63–100 μm) were placed between two parallel plates exerting a compressive force measured by an automated system with a resolution of 0.1 mN. Fracture load values were measured and used to calculate failure stress values as described by the following formula:
σ B = 4FB / ( πdG2 )
(1)
where σB is the failure stress value (MPa), FB is the measured breaking load (N), and dG2 is the diameter of the granule (μm).
Preparation of titanium-based brazes containing CaP-granulates Granulates made of TH, BG, and TH/BG were used for the application in Ti-based coatings to prove their suitability for the manufacturing of bioactive coatings. A titanium-based braze paste composed of Ti72-Co26-Si2 (wt%, TiCoSi) containing 40 wt% P41 binder (Brazetec, Hanau, Germany) was mixed with the different CaP-based granulates (10 vol%), applied on an alumina surface and subsequently brazed at 1090°C for 40 min. Cross-section images were made to determine the effect of the brazing process on the morphology of granules obtained from the three different materials (TH70Tr, BG40Tr, and TH/BG60Tr). TH granules produced by different deflocculants (TH40, TH60D, and TH70Tr) were also embedded into the braze, and the surfaces of these brazes were characterized using SEM/energy-dispersive X-ray (EDX) mapping (Leo1530, Carl Zeiss, Oberkochen, Germany) to visualize the morphology and distribution of the granulates on the surface of the metal matrix.
Results Characterization of the base materials and suspensions Characteristic data of the prepared slurries are summarized in Table 2. The pH values of the slurries were strongly dependent on both concentration of solids content in the suspension and the addition of organic deflocculants. Higher loading of solid material resulted in an increase of the pH values of the prepared suspensions without deflocculants (suspension TH30 and TH40). The addition of both Dolapix CE64 and trisodium citrate further increased the pH value in suspensions with the same solids content (TH40, TH40D, and TH40Tr). However, no influence of solids content on pH values in samples prepared with the same dispersing agent was observed (e.g., suspensions TH40D, TH50D, and TH60D). The pH of the samples containing BG was higher than those containing only TH. Solids content increase in the suspensions caused a raise in the viscosity values. The addition of deflocculants significantly lowered the viscosity of the slurries (e.g., suspensions TH40, TH40D, and TH40Tr). Increase in viscosity of the slurry with increase of solid matter was also observed after addition of deflocculants. An abrupt rise in viscosity was observed in samples TH70Tr and TH80Tr. The characterized suspensions were subsequently spray dried to obtain CaP-based granulates. High solids content prevented suspension TH80Tr from being spray dried. Characterization by XRD showed no changes of the based materials before and after spray drying. Two crystalline phases, mainly β-TCP and approximately 5% of HAp,
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108 K. Schickle et al.: Calcium phosphate granules for active brazes Table 2 Suspension characteristics (pH-value and viscosity) and the size of the granulates, d(0.1), d(0.5), and d(0.9) values. Suspension TH30 TH40 TH40D TH40Tr TH50D TH60D TH70Tr TH80Tr TH/BG40Tr TH/BG60Tr BG40Tr
pH value
Viscosity at 100 s-(mPas)
Viscosity at 1000 s-(mPas)
d(0.1) (μm)
d(0.5) (μm)
d(0.9) (μm)
7.6 8.0 8.9 9.0 8.5 8.9 9.6 9.5 9.9 11.0 13.1
26.8 56.2 4.5 5.8 4.6 6.8 142.5 1059.3 6.5 130.1 7.7
6.9 12.2 3.9 4.4 4.1 5.3 105.5 – 5.4 46.6 5.7
16.1 10.0 5.3 18.2 14.8 16.9 41.1 – 17.8 26.3 23.4
37.6 44.0 31.0 32.0 38.5 60.8 82.4 – 30.3 56.7 54.4
68.9 76.3 58.1 57.1 81.8 185.7 148.5 – 50.3 110.4 125.7
were detected by XRD for TH (Figure 1, green pattern). An exclusive amorphous behavior of the synthesized BG was observed as no crystalline peaks were detected (Figure 1,
blue pattern). TH/BG mixture spectra show weaker peaks of the TH crystalline phases, without additional phases being formed (Figure 1, red pattern).
Morphological and mechanical characterization of the spray-dried granulates
Figure 1 Phase composition of the materials after the spray drying process determined by XRD measurements. Spectra obtained from granulate produced with TH (green), BG (blue), and TH/BG (red). Similar spectra were recorded from the materials before spray drying.
Curves representing particle size distribution of the granules produced with Dolapix show a shift toward bigger granule sizes and a wider distribution with increasing solids content (Figure 2 and Table 2). The biggest average granule size 82.4 μm at d(0.5) was obtained with the slurry TH70Tr presenting the highest solids content. Granulates that were manufactured out of slurries without any organic additives exhibited spherical shapes and showed no defects in the cross section (Figures 3A and B and 4A and B, respectively). However, their surface morphology showed low packing density, and especially in sample TH40, appendages could be observed (Figures 3B and 4B). The addition of Dolapix CE64 significantly improved the surface morphology of the spheres, but granule defects such as doughnut shape
Figure 2 Influence of solids content on particle size distribution of the CaP-composite granulates. Blue pattern corresponds to the granulates made of slurry TH40D, red pattern to granulates made of slurry TH50D, and green pattern to granulates made of slurry TH60D.
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K. Schickle et al.: Calcium phosphate granules for active brazes 109
spherical morphology; however, the large glass grains produced a loosely packed surface (Figure 3I). The internal density of the granules was primarily affected by the solids content and to a lesser degree by the organic deflocculant. The internal structures of TH samples prepared without additives and with trisodium citrate were less densely packed compared to Dolapix CE64 samples (Figure 4B, C, and E). Equally, high solids content increased the internal density of the granules (Figure 4D and G). The specific surface area of granulates decreased with increasing size of the spheres, from 5 m2/g for TH40 to 2.5 m2/g for TH70Tr (Table 3). Comparing the specific surface area values of the samples prepared with slurries containing the same solids content but different organic additives (samples TH40, TH40D, and TH40Tr), it could be observed that the highest value was achieved by the sample prepared with Dolapix CE64 as dispersing agent (TH40D). Granulate strength characterization was performed on 50 single granules per condition, except for sample TH40, where only 15 could be studied due to agglomeration/appendage problems. Results from the strength characterization show the strong influence organic additives have on failure stress values, where Dolapix CE64 significantly increased the rigidity of the granules compared. Equally, granulates generated with TH slurries with higher solids content and similar organic additive showed higher failure stress values than BG-containing granules (Table 4).
Evaluation granulates after the brazing process
Figure 3 SEM images of the surface structure of granulates manufactured of slurries with various parameters. (A) TH30, (B) TH40, (C) TH40D, (D) TH60D, (E) TH40Tr, (F)TH70Tr, (G) TH/BG40Tr, (H) TH/ BG60Tr, and (I) BG40Tr. Inserts show the surface of granules from the respective material. Scale bars: all = 100 μm, all inserts = 10 μm.
and inner holes occurred (Figures 3C and D and 4C and D). The introduction of trisodium citrate as organic additive improved surface morphology, compared with samples with no organic additive, and a decrease in defects was observed, especially in the sample with the highest solids content TH70Tr (Figures 3E and F and 4E and F). Granules produced with TH/BG showed similar spherical morphology but low surface packing density, and large BG particles could be easily identified on the surface of the TH/BG granules (Figure 3G and H). BG granules also presented a
SEM cross-section images of TiCoSi-based coatings with granulates prepared from the three different materials showed that those granules consisting solely of TH (TH70Tr) maintained their integrity after the brazing process (Figure 5A). Contrary to this, BG and TH/BG (BG40Tr and TH/BG60Tr) granules were altered by the brazing process, losing completely their spherical form (Figure 5B and C). The effect of brazing on granules prepared using different deflocculants was also analyzed by SEM/EDX. Granulates generated without additives completely lost their shape as indicated by the irregular distribution of the Ca+P spectra (TH40, Figure 6A and B). Those generated with Dolapix CE64 showed significant deformations, and no spheric granule was observed in the braze (TH50D, Figure 6C and D). In contrast, TH samples prepared using trisodium citrate preserved their shape during the brazing process (TH70Tr, Figure 6E and F) as previously described.
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110 K. Schickle et al.: Calcium phosphate granules for active brazes
Figure 4 SEM images from cross sections of the generated granulates showing their inner structure. (A) TH30, (B) TH40, (C) TH40D, (D) TH60D, (E) TH40Tr, and (F) TH70Tr. Inserts show the inner structure of granules from the respective material. Scale bars: all = 100 μm, all inserts = 20 μm.
Discussion Biofunctionalization of inert surfaces is of great importance in the biomedical field. Titanium-based brazed coatings containing CaP-based granulates represent a new
approach to functionalize ceramic implant surfaces. To achieve this, the CaP-based granules have to fulfill several requirements including high internal density, regular shape, high strength, and a defined size distribution. These parameters are essential to achieve good dispersion
Table 3 Specific surface area of selected CaP-based granulates prepared from the respective suspension.
Table 4 Fracture load and failure stress values of selected CaP-based granules prepared from the respective suspension.
Granulate made from suspension
Granulate made from suspension
TH40 TH40D TH40Tr TH70Tr
Specific surface area (m2/g) 5.0 9.3 5.1 2.5
TH40 TH40D TH40Tr TH70Tr
Diameter (μm)
Fracture Load (mN)
Failure Stress (MPa)
96 ± 3 77 ± 10 72 ± 8 79 ± 10
1.0 ± 0.24 7.2 ± 3.1 1.8 ± 0.6 8.1 ± 2.2
0.14 ± 0.03 1.63 ± 0.86 0.42 ± 0.10 1.65 ± 0.30
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K. Schickle et al.: Calcium phosphate granules for active brazes 111
Figure 5 SEM images of the cross sections of granules embedded in the TiCoSi matrix. (A) TH70Tr granules maintain their spherical shape after the brazing process. (B) BG40Tr and (C) TH/BG60Tr on the other hand show loss of their spherical morphology. * = granules; scale bars: all = 40 μm.
in the metal matrix coating and enough mechanical stability to withstand the demanding conditions of the brazing process. TH, BG, and TH/BG were used to synthesize slurries for granulate fabrication by the spray drying process. Achieving slurries with high solids content in order to generate granulates with high densities was the primary goal. Slurry stability is crucial for the spray drying process and is strongly dependent on the pH value, which directly affects the surface charge on the particles [15]. The results confirmed that the pH values of the synthesized slurries increased with the addition of solids content [9]. This effect can be explained by the hydration process of charged particles which occurs immediately after contact of the particles with water molecules [15]. The pH values of the suspensions were slightly changed by the addition of deflocculants as well (suspensions TH40D and TH40Tr). These changes were dependent on the pH values of the applied deflocculants itself and the process of interaction and adsorption of the dispersant molecules on the charged particles. Another parameter, which is fundamental for the spray drying process, is the viscosity of the suspension. High solid content of the suspension is required to obtain compact, dense granules. Increase in solids content translates to a significant increase in the viscosity (e.g., suspension TH30 and TH40) [4, 16]. It is necessary to obtain a good fluidity of the slurry with high solids content in order to make it suitable for spray drying. Our results confirmed that all the CaP-based suspensions exhibited a shear thinning behavior (Table 2) [4]. Addition of both deflocculants (Dolapix CE64 and trisodium citrate) significantly decreased the viscosity of the suspensions enabling the preparation of slurries with higher solids content (Table 2). The dispersion of the suspensions using Dolapix CE64 is driven by an electrosteric mechanism [16]. The carbonic acid-based polyelectrolyte chains
interact with the charged surface of the particles, causing the formation of a thin organic layer on the particles which prevent the attractive van-der-Waals forces between particles, and thus flocculation is avoided [3, 16, 19]. However, our results have shown that this mechanism is limited, since it was not possible to synthesize slurries containing more than 60 wt% of solids content (TH). Synthesizing slurries containing 70 and 80 wt% was possible by adding trisodium citrate, commonly used as a pH control agent. The addition of this deflocculant resulted in significant changes in the particle surface charge (electrostatic mechanism) and in consequence an increase in repulsive interparticle forces that overcame the attractive Van der Waals forces [11, 18]. All prepared slurries were spray dried to obtain spherical granulates. Granule size was strongly dependent on slurry concentration (Table 2 and Figure 2), as indicated by greater granule sizes obtained with increased solids content of the slurry. Size distribution results confirmed the dependency of the granule size on solids content in the slurry and can be described by the following formula [14].
Dmean = CP -n ηγ ρ-t
(2)
where Dmean is the average granule size (μm); C is a constant; P is the atomization pressure; η is the viscosity; ρ is the solid content of slurry; and n, γ, and t are constants depending on the process conditions. The size of the granules is strongly dependent on the shrinkage during the drying process, which is related to the water loss from the droplets. As expected, the higher the water content in the slurry, the greater the shrinkage. Moreover, the slurry concentration appeared to have an influence on the shape of the size distribution curves as well. We could observe that the solids content has an effect on size distribution [27]. The distribution curves of granulates made of slurries
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112 K. Schickle et al.: Calcium phosphate granules for active brazes
Figure 6 SEM and EDX mapping images (Ca in magenta+P in cyan = purple) of TH granulate on the surface of a TiCoSi braze matrix. TH granulates produced (A, B) without additive, (C, D) with Dolapix CE64, and (E, F) with trisodium citrate. Scale bars: all = 50 μm.
with a lower solids content (TH40D) showed a narrow distribution in contrast to the curves of granulates prepared of slurries with a higher solids content (TH50D and TH60D). Granule morphology does not depend on solid content. However, the significant changes in morphology, density, and shape of the obtained spheres were dependent on the organic additives. The granulates, prepared from slurries without any organic additives, exhibited
poor surface density but spherical morphology in comparison to those made of slurries containing Dolapix CE64 as deflocculant. This occurred primarily due to the adsorption of organic molecules from the deflocculant on the surface of the particles. During spray drying, slurry droplets shrink until the outer shell is created. The presence of organic molecules on the particles caused tightening of the outer shell preventing the remaining water
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K. Schickle et al.: Calcium phosphate granules for active brazes 113
from evaporating and increasing the pressure inside the granules. Since the temperature during the spray drying process (between Tin = 230°C and Tout = 102°C) was not high enough to cause pyrolysis of the additive Dolapix CE64, which is reported to occur at temperatures between 230 and 490°C [19], granulate defects such as doughnut structure, hollow structure, or fracture of the spheres occurred [14]. The other applied organic deflocculant, trisodium citrate, did not show this effect since its pyrolysis temperature of approximately 150°C (manufacturer data) was surpassed. Even if particles were partially covered with trisodium citrate molecules, the temperature achieved during the drying process was high enough to decompose this organic substance, allowing water evaporation from inside the granules to take place. This allowed granulates synthesized using trisodium citrate as deflocculant to exhibit both spherical shape and a regular surface morphology. This could be observed by direct comparison of granulates manufactured from slurries containing the same solids content but different additives (TH40, TH40D, and TH40Tr). The achieved results showed a beneficial influence of trisodium citrate as dispersion agent on the spray drying process as it also allowed the formation of regular, spherically shaped granulates of both BG and TH/BG. Four granulates (TH40, TH40D, TH40Tr, and TH70Tr) were chosen to examine the influence of different organic additives and slurry concentrations on the specific surface area and mechanical properties of the granulates. As expected, the specific surface area decreased (from 5 to 2.5 m2/g) with increased solids content (from 40 to 70 wt%, Table 3). This can be explained as the result of differences in the granulate size. A decrease of the granulate size led to an increase of the specific surface area. The values of the specific surface area of samples made of slurries containing 40 wt% of solids prepared either without organic additives or trisodium citrate were comparable (5 m2/g). On the other hand, the values of granulates made of the same solids content but with Dolapix CE64 were much higher (9.3 m2/g). This is mainly the result of granulate defects that occurred only in suspensions containing Dolapix CE64. In this case, the specific surface area increased nearly two times due to the doughnut/hollow structure of the granulates. Dolapix CE64 had also a great influence on the rigidity of the samples. This, like in the surface area results, could be explained by residual Dolapix CE64 molecules stabilizing the surface of the granules as reflected by their increased failure stress value compared with samples with no additives and trisodium citrate. Increased solids content also
increased failure stress values and could be a reflection of the high internal density of the granules. A selection of the CaP-based granulates was used to determine their suitability for the production of bioactive brazed coatings. The results of initial tests confirmed the feasibility of the incorporation of TH granulates into metal coating. Granules that presented low packing density on their surface, such as those prepared without deflocculants or those containing bioglass, collapsed under the hard conditions posed by the brazing process. Granules generated with slurries using Dolapix, which also presented stronger mechanical properties, also remained unaffected by the brazing process. However, their irregular morphologies made them unsuitable for our application. The mechanically and thermally most resistant granulates appeared to be those generated using 70% TH and trisodium citrate as no damage after brazing was observed. The obtained results are very promising for possible novel coatings to increase the bioactivity of an implant surface. Therefore, further studies addressing the phase composition, the mechanical behavior, and the biological response are ongoing.
Conclusion The use of trisodium citrate as deflocculant allowed for the generation of CaP-based granules through spray drying. The granules showed regular spherical shapes and high internal densities that translated into mechanically strong spheres capable to withstand the mechanical and thermal conditions imposed by the active brazing process. The generation of such granules opens the door for a one-step biofunctionalization of materials during metallurgical processes, such as that of the titanium alloy-CaP granulate brazing system presented in this study, with the potential to improve the development of orthopedic implants. Acknowledgments: The authors would like to acknowledge the financial support for this project by the German Research Foundation (DFG, grant FI 975/16-1). The authors would like to also thank Mrs. Petra Schott and Mr. Peter König, Institute of Mineral Engineering, RWTH Aachen University, for their help with the particle size distribution measurements and the XRD analyses. Special thanks go to Mr. Malte Schmachtenberg, Welding and Joining Institute, RWTH Aachen University, for operating the SEM/EDX equipment.
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114 K. Schickle et al.: Calcium phosphate granules for active brazes
References [1] Andrianjatovo H, Jose F, Lemaitre J. Effect of β-TCP granularity on setting time and strength of calcium phosphate hydraulic cements. J Mater Sci Mater Med 1996; 7: 34–39. [2] Bergmann C, Lindner M, Zhang W, et al. 3D printing of bone substitute implants using calcium phosphate and bioactive glasses. J Eur Ceram Soc 2010; 30: 2563–2567. [3] Cesarano J, Aksay IA, Bleier A. Stability of aqueous alphaAl2O3 suspensions with poly(methacrylic acid) polyelectrolyte. J Am Ceram Soc 1988; 71: 250–255. [4] Chen B, Zhang Z, Zhang J, Lin Q, Jiang D. Fabrication and mechanical properties of β-TCP pieces by gel-casting method. Mater Sci Eng C 2008; 28: 1052–1056. [5] Fontana F, Rocchietta I, Addis A, Schupbach P, Zanotti G, Simion M. Effects of a calcium phosphate coating on the osseo integration of endosseous implants in a rabbit model. Clin Oral Implants Res 2011; 22: 760–766. [6] Gao Y, Hu J, Guan TH, Wu J, Zhang CB, Gao B. Physical properties and cellular responses to calcium phosphate coating produced by laser rapid forming on titanium. Lasers Med Sci 2012; 29: 9–17. [7] Goyenvalle E, Aguado E, Nguyen J-M, et al. Osteointegration of femoral stem prostheses with a bilayered calcium phosphate coating. Biomaterials 2006; 27: 1119–1128. [8] Ignatius A, Peraus M, Schorlemmer S, et al. Osseointegration of alumina with a bioactive coating under load-bearing and unloaded conditions. Biomaterials 2005; 26: 2325–2332. [9] Kim DJ, Lee JK. Dependence of the rheological behaviour of electrostatically stabilized alumina slurries on pH and solid loading. J Mater Sci 1998; 33: 2931–2935. [10] Kim H-W, Georgiou G, Knowles JC, Koh Y-H, Kim H-E. Calcium phosphates and glass composite coatings on zirconia for enhanced biocompatibility. Biomaterials 2004; 25: 4203–4213. [11] Leeuwenburgh SCG, Ana ID, Jansen JA. Sodium citrate as an effective dispersant for the synthesis of inorganic-organic composites with a nanodispersed mineral phase. Acta Biomater 2010; 6: 836–844. [12] Lin C-C, Chen R-B, Shiue R-K. A wettability study of Cu/Sn/Ti active braze alloys on alumina. J Mater Sci 2001; 36: 2145–2150. [13] Lukasiewicz SJ. Spray-drying ceramic powders. J Am Ceram Soc 1989; 72: 617–624. [14] Luo P, Nieh TG. Preparing hydroxyapatite powders with controlled morphology. Biomaterials 1996; 17: 1959–1964.
[15] Parks GA, Bruyn PL de. The zero point of charge of oxides. J Phys Chem 1962; 66: 967–973. [16] Rao SP, Tripathy SS, Raichur AM. Dispersion studies of submicron zirconia using Dolapix CE 64. Colloids Surfaces A Physicochem Eng Asp 2007; 302: 553–558. [17] Rocha LA, Ariza E, Costa AM, Oliveira FJ, Silva RF. Electrochemical behavior of Ti/Al2O3 interfaces produced by diffusion bonding. Mater Res 2003; 6: 439–444. [18] Sacks MD, Tseng T-Y. Role of sodium citrate in aqueous milling of aluminum oxide. J Am Ceram Soc 1983; 66: 242–247. [19] Sarraf H, Sabet A, Herbig R, Havrda J, Hulinsky V, Maryska M. Advanced colloidal techniques for characterization of the effect of electrosteric dispersant on the colloidal stability of nanocrystalline ZrO2 suspension. J Ceram Soc Japan 2009; 117: 302–307. [20] Schickle K, Zurlinden K, Bergmann C, et al. Synthesis of novel tricalcium phosphate-bioactive glass composite and functionalization with rhBMP-2. J Mater Sci Mater Med 2011; 22: 763–771. [21] Schlegel A, Bobzin K, Kopp N, Bagcivan N. Die Entwicklung des Breitspaltaktivlôtens als neue Fügetechnologie für KeramikMetall-Mischverbunde mit Einsatztemperaturen oberhalb 500°C. Keramische Zeitschrift 2011; 63: 329–333. [22] Silva NA, Salgado AJ, Sousa RA, et al. Development and characterization of a novel hybrid tissue engineering-based scaffold for spinal cord injury repair. Tissue Eng Part A 2010; 16: 45–54. [23] Sun L, Chow LC, Frukhtbeyn SA, Bonevich JE. Preparation and properties of nanoparticles of calcium phosphates with various Ca/P ratios. J Res Natl Inst Stand Technol 2010; 115: 243–255. [24] Sun R, Lu Y, Chen K. Preparation and characterization of hollow hydroxyapatite microspheres by spray drying method. Mater Sci Eng C 2009; 29: 1088–1092. [25] Takaoka T, Okumura M, Ohgushi H, Inoue K, Takakura Y, Tamai S. Histological and biochemical evaluation of osteogenic response in porous hydroxyapatite coated alumina ceramics. Biomaterials 1996; 17: 1499–1505. [26] Travessa D, Ferrante M. The Al2O3-titanium adhesion in the view of the diffusion bonding process. J Mater Sci 2002; 37: 4380–4390. [27] Wang A, Lu Y, Zhu R, Li S, Ma X. Effect of process parameters on the performance of spray dried hydroxyapatite microspheres. Powder Technol 2009; 191: 1–6.
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