Coupling of Phosphates on Alumina Surfaces for Bioactivation

Nadine Kaltenborn

Coupling of Phosphates on Alumina Surfaces for Bioactivation

Journal of the American Ceramic Society, 2007

Coupling of Phosphates on Alumina Surfaces for Bioactivation Nadine Kaltenborn, w,z Michael Sax, y Frank A. Mu¨ller, z Lenka Mu¨ller, z Henning Dieker, J Arno Kaiser, w Rainer Telle, w and Horst Fischer w,zz w Department of Ceramics and Refractory Materials, RWTH Aachen University, 52064 Aachen, Germany z Hermsdorfer Institut fu¨ r Technische Keramik e. V., 07629 Hermsdorf, Germany y IBS, 49457 Drebber, Germany z Department of Materials Science (III) Biomaterials, University of Erlangen-Nuernberg, 91052 Erlangen, Germany J Department of Physics, I. Physikalisches Institut, RWTH Aachen University, 52064 Aachen, Germany This paper reports on a new surface treatment, coupling phos- phates on alumina surfaces for bioactivation. Alumina samples were treated in monoaluminumphosphate solution at 14001C. Strongly coupled aluminum phosphates due to the treatment were proved by X-ray diffraction and infrared spectroscopy. Contact angle measurements proved the hydrophilic nature of the treated surface. In vitro tests in simulated body ﬂuid indi- cated a bioactive behavior by means of apatite-forming ability. Such functionalized alumina ceramics imply the potential for a new class of bioactive high-strength ceramic implant materials, due to a pronounced afﬁnity of bioactive phosphate groups to particular amino acid sequences of proteins. I. Introduction H IGHLY pure alumina is used as a biomedical implant such as dental prosthetic components and total joint replacements due to its excellent biocompatibility and a minimum of wear debris during dynamic in vivo loading. 1–3 The bioinert charac- teristic of this oxidic ceramic material is advantageous for ar- ticulating joint components, but is disadvantageous for implant components with direct bone tissue contact like monolithic ac- etabular sockets or all-ceramic dental implants where osseoin- tegration is required. A limited number of monolithic alumina acetabular sockets, implanted 20 years ago in patients, showed an acceptable mean duration in vivo until revision. 4 However, high failure rates due to loosening of monolithic alumina sockets clearly point out that the adhesion in terms of a bioactive os- seointegration between an oxide ceramic surface and the bone tissue needs to be improved. 5,6 One possibility for a bioactivation of inert implant surfaces is to coat them with a bioactive material like calcium phosphate ceramics. These ceramics are described to activate cell attach- ment and osseointegration, leading to a tight interconnection between bone tissue and implant surface. Using this strategy, a combination of bioactivity at the surface and good mechanical properties of the bulk material can be achieved. Nevertheless, uncontrollable resorption dynamics of the coating are a prob- lematic aspect. Furthermore, different thermal expansion coef- ﬁcients (coating vs. bulk material) promote mechanical stresses in the interface and could ﬁnally cause spalling of the coated material. 7–10 In order to avoid the disadvantages of bioactive coatings, one promising strategy is a bioactive modiﬁcation of the implant surface. A close interconnection between implant and bone should be achieved by coupling functional groups like PO 4 to the ceramic surface. Here, we report on the phosphatization of alumina surfaces as a suitable functionalization strategy. II. Experimental Procedure Cylindrical samples with a diameter of 20 mm and a thickness of 3 mm were prepared by compacting spray-dried alumina gran- ulate (CT3000 SG, purity: 99.8%, Alcoa, Pittsburgh, PA) using a uniaxial pressure of 100 MPa. The green alumina bodies were sintered at a temperature of 16501C and subsequently ground on a diamond-charged rotary grinding machine (ATM, Alden- kirchen, Germany). The density of the ﬁred and ground speci- mens was determined by the Archimedes principle. The alumina samples were exposed to a monoaluminumphosphate solution (FFB 705, Chemische Fabrik Budenheim, Budenheim, Ger- many) at pH 1, heated to 14001C, and slowly cooled down again to room temperature. After heat treatment, the excess of phosphate was removed and the alumina samples were ultra- sonically cleaned in acetone. After drying at 901C, the pH values of the samples in 100 mL aqua dest were measured (pH-meter ‘‘pH 530’’, WTW, Weilheim, Germany). Thus, pretreated alu- mina samples were analyzed by X-ray diffraction analysis (XRD) (PW 3710, Philips, Eindhoven, the Netherlands), infra- red spectroscopy (IFS 66-V, Bruker, Ettlingen), and scanning electron microscopy (SEM) (Leo 440i, Carl Zeiss, Jena, Germany). Additionally, tests in simulated body ﬂuid (SBF) and measurements of the contact angle were performed. Four alumina samples were stored in SBF at 371C for 3 weeks. The weight of the samples was measured before and after the treat- ment in simulated body ﬂuid, respectively. SBF10 solutions were prepared according to a procedure described elsewhere. 11 III. Results and Discussion Figure 1 shows the cross-sectional SEM-micrographs of a phos- phatized alumina sample with a penetration depth of approxi- mately 700 mm of the acidic monoaluminumphosphate solution D. Greenspan—contributing editor zz Author to whom correspondence should be addressed. e-mail: h.ﬁscher@rwth- aachen.de Manuscript No. 22477. Received November 10, 2006; approved January 11, 2007. J ournal J. Am. Ceram. Soc., 90 [5] 1644–1646 (2007) DOI: 10.1111/j.1551-2916.2007.01601.x r 2007 The American Ceramic Society 1644

(Fig. 1(a)) and a top view of the same sample (Fig. 1(c)), where the corrosive attack of the acid—the wash-out of grains (mean grain size approximately 2–5 mm, Fig. 1(b)—is clearly visible. Surface damages in terms of a porous microstructure are always associated with a decrease in mechanical strength. 12,13 This im- plies that phosphatization of the alumina surface may lower the mechanical reliablility. However, a complete porous scaffold was not generated by the treatment, but a graded porous surface layer supported by a core of dense alumina. The core material (determined density 3.89 g/cm 3 ) was not affected by the treat- ment. The dense core material (that meets the requirements in ISO 6476 14 for alumina implants regarding purity >99.5%), will thereby strengthen the ceramic component with the func- tionalized porous surface layer. A subsequent study will analyze the mechanical properties of the graded ceramic material in de- tail. From the biological point of view, the newly formed can- cellous bone-like porosity at the surface should promote the ingrowth of bone tissue into the surface of an implant that is functionalized using the presented treatment. Moreover, forma- tion of aluminum phosphate (AlPO 4 ) was proved on the treated materials by XRD analysis (Fig. 2). The curve in the infrared plot (Fig. 3) represents the relative reﬂectivity of a treated sam- ple compared with an untreated one. Any discrepancy in the plotted curve from a horizontal line indicates differences be- tween a treated and an untreated surface. The infrared spectrum of the phosphatized alumina surface reveals a significant differ- ence compared with the untreated alumina sample in the range between 1000 and 1500 cm 1 (Fig. 3). This vibration could be attributed to the PO 4 groups. The generated AlPO 4 bindings are neither soluble in water nor in alcohol. This is an evidence for a strong chemical bonding, which is an important requirement for the stability in vivo and during sterilization. The contact angle of phosphatized alumina (351) was significantly lower compared with untreated samples (911). This is more evidence for the bioactivation of the oxidic ceramic surfaces as there is a direct relationship between hydrophilic behavior and bioactivity. 15,16 The pH value of the phosphatized alumina samples in 100 mL aqua dest was 5.84, which is in a well-tolerable range for the Fig. 1. Scanning electron microscopy micrograph of an alumina surface treated with a 50% monoaluminumphosphate solution. (a) Lateral cut, (b) detailed view of lateral cut at the surface layer, and (c) view on top of the specimen. Fig. 2. X-ray diffraction plot of an alumina surface treated with mono- aluminumphosphate solution at 14001C. Fig. 3. Infrared spectrum of untreated versus treated alumina surface. The curve is a function of the difference between the curves of a treated and an untreated sample. This means any discrepancy in the plotted curve from a horizontal line indicates differences between a treated and an untreated surface. May 2007 Communications of the American Ceramic Society 1645

J. Am. Ceram. Soc., 90 [5] 1644–1646 (2007) DOI: 10.1111/j.1551-2916.2007.01601.x r 2007 The American Ceramic Society Journal Coupling of Phosphates on Alumina Surfaces for Bioactivation Nadine Kaltenborn,w,z Michael Sax,y Frank A. Müller,z Lenka Müller,z Henning Dieker,J Arno Kaiser,w Rainer Telle,w and Horst Fischerw,zz w Department of Ceramics and Refractory Materials, RWTH Aachen University, 52064 Aachen, Germany z Hermsdorfer Institut für Technische Keramik e. V., 07629 Hermsdorf, Germany y IBS, 49457 Drebber, Germany z Department of Materials Science (III) Biomaterials, University of Erlangen-Nuernberg, 91052 Erlangen, Germany J Department of Physics, I. Physikalisches Institut, RWTH Aachen University, 52064 Aachen, Germany lematic aspect. Furthermore, different thermal expansion coefﬁcients (coating vs. bulk material) promote mechanical stresses in the interface and could ﬁnally cause spalling of the coated material.7–10 In order to avoid the disadvantages of bioactive coatings, one promising strategy is a bioactive modiﬁcation of the implant surface. A close interconnection between implant and bone should be achieved by coupling functional groups like PO4 to the ceramic surface. Here, we report on the phosphatization of alumina surfaces as a suitable functionalization strategy. This paper reports on a new surface treatment, coupling phosphates on alumina surfaces for bioactivation. Alumina samples were treated in monoaluminumphosphate solution at 14001C. Strongly coupled aluminum phosphates due to the treatment were proved by X-ray diffraction and infrared spectroscopy. Contact angle measurements proved the hydrophilic nature of the treated surface. In vitro tests in simulated body ﬂuid indicated a bioactive behavior by means of apatite-forming ability. Such functionalized alumina ceramics imply the potential for a new class of bioactive high-strength ceramic implant materials, due to a pronounced afﬁnity of bioactive phosphate groups to particular amino acid sequences of proteins. II. Experimental Procedure Cylindrical samples with a diameter of 20 mm and a thickness of 3 mm were prepared by compacting spray-dried alumina granulate (CT3000 SG, purity: 99.8%, Alcoa, Pittsburgh, PA) using a uniaxial pressure of 100 MPa. The green alumina bodies were sintered at a temperature of 16501C and subsequently ground on a diamond-charged rotary grinding machine (ATM, Aldenkirchen, Germany). The density of the ﬁred and ground specimens was determined by the Archimedes principle. The alumina samples were exposed to a monoaluminumphosphate solution (FFB 705, Chemische Fabrik Budenheim, Budenheim, Germany) at pH 1, heated to 14001C, and slowly cooled down again to room temperature. After heat treatment, the excess of phosphate was removed and the alumina samples were ultrasonically cleaned in acetone. After drying at 901C, the pH values of the samples in 100 mL aqua dest were measured (pH-meter ‘‘pH 530’’, WTW, Weilheim, Germany). Thus, pretreated alumina samples were analyzed by X-ray diffraction analysis (XRD) (PW 3710, Philips, Eindhoven, the Netherlands), infrared spectroscopy (IFS 66-V, Bruker, Ettlingen), and scanning electron microscopy (SEM) (Leo 440i, Carl Zeiss, Jena, Germany). Additionally, tests in simulated body ﬂuid (SBF) and measurements of the contact angle were performed. Four alumina samples were stored in SBF at 371C for 3 weeks. The weight of the samples was measured before and after the treatment in simulated body ﬂuid, respectively. SBF10 solutions were prepared according to a procedure described elsewhere.11 I. Introduction H IGHLY pure alumina is used as a biomedical implant such as dental prosthetic components and total joint replacements due to its excellent biocompatibility and a minimum of wear debris during dynamic in vivo loading.1–3 The bioinert characteristic of this oxidic ceramic material is advantageous for articulating joint components, but is disadvantageous for implant components with direct bone tissue contact like monolithic acetabular sockets or all-ceramic dental implants where osseointegration is required. A limited number of monolithic alumina acetabular sockets, implanted 20 years ago in patients, showed an acceptable mean duration in vivo until revision.4 However, high failure rates due to loosening of monolithic alumina sockets clearly point out that the adhesion in terms of a bioactive osseointegration between an oxide ceramic surface and the bone tissue needs to be improved.5,6 One possibility for a bioactivation of inert implant surfaces is to coat them with a bioactive material like calcium phosphate ceramics. These ceramics are described to activate cell attachment and osseointegration, leading to a tight interconnection between bone tissue and implant surface. Using this strategy, a combination of bioactivity at the surface and good mechanical properties of the bulk material can be achieved. Nevertheless, uncontrollable resorption dynamics of the coating are a prob- D. Greenspan—contributing editor III. Results and Discussion Figure 1 shows the cross-sectional SEM-micrographs of a phosphatized alumina sample with a penetration depth of approximately 700 mm of the acidic monoaluminumphosphate solution Manuscript No. 22477. Received November 10, 2006; approved January 11, 2007. zz Author to whom correspondence should be addressed. e-mail: h.ﬁscher@rwthaachen.de 1644 May 2007 Communications of the American Ceramic Society 1645 Fig. 1. Scanning electron microscopy micrograph of an alumina surface treated with a 50% monoaluminumphosphate solution. (a) Lateral cut, (b) detailed view of lateral cut at the surface layer, and (c) view on top of the specimen. (Fig. 1(a)) and a top view of the same sample (Fig. 1(c)), where the corrosive attack of the acid—the wash-out of grains (mean grain size approximately 2–5 mm, Fig. 1(b)—is clearly visible. Surface damages in terms of a porous microstructure are always associated with a decrease in mechanical strength.12,13 This implies that phosphatization of the alumina surface may lower the mechanical reliablility. However, a complete porous scaffold was not generated by the treatment, but a graded porous surface layer supported by a core of dense alumina. The core material (determined density 3.89 g/cm3) was not affected by the treatment. The dense core material (that meets the requirements in ISO 647614 for alumina implants regarding purity >99.5%), will thereby strengthen the ceramic component with the functionalized porous surface layer. A subsequent study will analyze the mechanical properties of the graded ceramic material in detail. From the biological point of view, the newly formed cancellous bone-like porosity at the surface should promote the ingrowth of bone tissue into the surface of an implant that is functionalized using the presented treatment. Moreover, formation of aluminum phosphate (AlPO4) was proved on the treated materials by XRD analysis (Fig. 2). The curve in the infrared plot (Fig. 3) represents the relative reﬂectivity of a treated sample compared with an untreated one. Any discrepancy in the Fig. 2. X-ray diffraction plot of an alumina surface treated with monoaluminumphosphate solution at 14001C. plotted curve from a horizontal line indicates differences between a treated and an untreated surface. The infrared spectrum of the phosphatized alumina surface reveals a significant difference compared with the untreated alumina sample in the range between 1000 and 1500 cm1 (Fig. 3). This vibration could be attributed to the PO4 groups. The generated AlPO4 bindings are neither soluble in water nor in alcohol. This is an evidence for a strong chemical bonding, which is an important requirement for the stability in vivo and during sterilization. The contact angle of phosphatized alumina (351) was significantly lower compared with untreated samples (911). This is more evidence for the bioactivation of the oxidic ceramic surfaces as there is a direct relationship between hydrophilic behavior and bioactivity.15,16 The pH value of the phosphatized alumina samples in 100 mL aqua dest was 5.84, which is in a well-tolerable range for the Fig. 3. Infrared spectrum of untreated versus treated alumina surface. The curve is a function of the difference between the curves of a treated and an untreated sample. This means any discrepancy in the plotted curve from a horizontal line indicates differences between a treated and an untreated surface. 1646 Vol. 90, No. 5 Communications of the American Ceramic Society ramic acetabular sockets. In a next step of development, the mechanical properties of such implant components that are modiﬁed with the described porous and functionalized surface layer need to be analyzed in detail. References 1 Fig. 4. Scanning electron microscopy micrograph of a phosphatized alumina surface after 3 weeks in simulated body ﬂuid. human body.17 After 3 weeks of soaking in SBF, the phosphatized alumina samples increased their weight by 2 mg, indicating the precipitation of new phases. SEM and EDX analyses revealed the formation of a layer consisting of Ca, P, O, and Al from the substrate, and traces of Na. Thus, it can be concluded that the surface was covered by a calcium phospate layer (Fig. 4), indicating the in vitro bioactivity of the surface of the material. Cracks and spallings within the calcium phosphate layer indicate a weak adhesion between the substrate and the coating. However, soaking in SBF was performed in order to conﬁrm the in vitro apatite-forming ability and not to coat the material. Thus, the adhesion is of subordinate importance. IV. Conclusions Chemically stable AlPO4 was proved on alumina surfaces after treatment in monoaluminumphosphate at 14001C. Phosphate groups exhibit a pronounced afﬁnity to human proteins, i.e. they show a bioactive behavior in vivo. In vitro analyses conﬁrmed that the functionalized oxide ceramic surfaces exhibited bioactive characteristics after treatment. Such a functionalization implies potential for a new class of nonmetallic inorganic materials. Using the described surface treatment, the high strength and fracture toughness of structural ceramic materials could be combined with a bioactive surface. This is of interest for loadbearing implants with bone tissue contact like monolithic ce- S. F. Hulbert, F. A. Young, R. S. Mathews, J. J. Klawitter, C. D. Talbert, and F. H. Stelling, ‘‘Potential of Ceramic Materials as Permanently Implantable Skeletal Prostheses,’’ J. Biomed. Mater. Res., 4, 433–6 (1970). 2 P. Griss and E. Werner, ‘‘Alumina Ceramic, Bioglass and Silicon Nitride. A Comparative Biocompatibility Study’’; pp. 217–25 in Mechanical Properties of Biomaterials, Edited by G. W. Hastings and D. F. Williams. John Wiley & Sons Ltd., London, 1980. 3 G. Willmann, ‘‘Survival Rate and Reliability of Ceramic Femoral Heads for Total Hip Arthroplasty,’’ Mat.-wiss. u. Werkstofftech., 29, 595–604 (1998). 4 M. Hamadouche, P. Boutin, J. Daussange, M. E. Bolander, and L. Sedel, ‘‘Alumina-on-Alumina Total Hip Arthroplasty: A Minimum 18.5-Year FollowUp Study,’’ J. Bone Joint Surg. Am., 84, 69–77 (2002). 5 M. Böhler, K. Knahr, H. Plenk, A. Walter, and M. Salzer, ‘‘Long-Term Results of Uncemented Alumina Acetabular Implants,’’ J. Bone Joint Surg. Br., 76-B, 53–9 (1994). 6 M. Hamadouche, R. S. Nizard, A. Meunier, and L. Sedel, ‘‘Cementless Fixation With Alumina Sockets. Apropros of 62 Total Hybrid Prostheses With PressFit Alumina Sockets at 6-Year Mean Follow-Up,’’ Rev. Chir. Orthop. Reparatrice Appar. Mot., 86, 474–81 (2000). 7 H. Takagi, T. Yamamuro, K. Hyakuna, T. Nakamura, Y. Kotoura, and M. Oka, ‘‘Bone Bonding Behaviour of Bead-Coated Alumina Ceramic Under LoadBearing Conditions,’’ J. Biomed. Mater. Res. B, 23, 161–81 (1989). 8 T. Takaoka, M. Okumura, H. Ohgushi, K. Inoue, Y. Takaura, and S. Tamai, ‘‘Historical and Biomechanical Evaluation of Osteogenetic Response in Porous Hydroxyapatite Coated Alumina Ceramics,’’ Biomaterials, 17, 1499–505 (1996). 9 G. Jiang and D. Shi, ‘‘Coating of Hydroxyapatite on Highly Porous Al2O3 Substrate for Bone Substitutes,’’ J. Biomed. Mater. Res. B, 43, 77–81 (1998). 10 K. Yamashita, E. Yonehara, X. Ding, M. Nagai, T. Umegaki, and M. Matsuda, ‘‘Electrophoretic Coating of Multilayered Apatite Composite on Alumina Ceramics,’’ J. Biomed. Mater. Res. B., 43, 46–53 (1998). 11 L. Müller and F. A. Müller, ‘‘Preparation of SBF With Different HCO 3 Content and Its Inﬂuence on the Composition of Biomimetic Apatites,’’ Acta Biomater., 2, 181–9 (2006). 12 Z. Y. Deng, T. Fukasawa, M. Ando, G. J. Zhang, and T. Ohji, ‘‘Microstructure and Mechanical Properties of Porous Alumina Ceramics Fabricated by the Decomposition of Aluminum Hydroxide,’’ J. Am. Ceram. Soc., 84, 2638–44 (2001). 13 S. Bose, J. Darsell, H. L. Hosick, L. Yang, D. K. Sarkar, and A. Bandyopadhyay, ‘‘Processing and Characterization of Porous Alumina Scaffolds,’’ J. Mater. Sci. Mater. Med., 13, 23–8 (2002). 14 ISO 6474. Implants for Surgery—Ceramic Materials Based on High Purity Alumina. International Organization for Standardization, Geneve, Switzerland, 1994. 15 K. Webb, V. Hlady, and P. A. Tresco, ‘‘Relative Importance of Surface Wettability and Charged Funcional groups on NIH 3T3 Fibroblast Attachment, Spreading, and Cytoskeletal Organization,’’ J. Biomed. Mater. Res., 41, 422–30 (1998). 16 H. P. Jennissen, ‘‘Ultra-Hydrophilic Transition Metals as Histophilic Biomaterials,’’ Macromol. Symp., 225, 43–69 (2005). 17 D. M. Roy, ‘‘Recent Advances in Phosphate Chemically Bonded Ceramics,’’ Adv. Mater., 13, 213–27 (1989). &

With the exponential growth of the Internet of Things (IoT), indoor photovoltaics (IPVs) have emerged as a pivotal technology for powering low-power devices, drawing heightened interest due to their adaptability to indoor environments. The Photovoltaic Conversion Efficiency (PCE) of IPV cells is critically dependent on their ability to match the indoor spectrum with the device's response characteristics. In this realm, Antimony Trisulfide (Sb 2 S 3), characterized by its wide bandgap and high absorption coefficient, emerges as a promising candidate for low-light applications. Our study focuses on the modeling and numerical analysis of Sb 2 S 3 thin-film IPV cells by wxAMPS software, aiming to refine both the device structure and its photoelectric performance for effective indoor light harvesting. In a strategic shift from conventional CdS materials, we utilized SnO 2-known for its high transmissivity, non-toxicity, and wide bandgap-as the electron transport layer (ETL) in Sb 2 S 3 IPV cells. This substitution notably enhanced the shortwave response, elevating the spectral response from 45 % to 80 % at 400 nm. Additionally, we introduced a bandgap-tunable ZnOS buffer layer. This innovation proved instrumental in rectifying the band alignment mismatch between SnO 2 and Sb 2 S 3 layer, thereby optimizing interfacial electron transport properties. The integration of the ZnOS buffer layer effectively improved the fill factor from 40.0 % to 64.7 % of the Sb 2 S 3 IPV cell by solving the band mismatch problem. The resulting optimized Sb 2 S 3 IPV cell demonstrated exceptional response characteristics across the full visible spectrum (400-750 nm) and showed notable photoelectric performance under both fluorescent lamps (FLs) and lightemitting diodes (LEDs). Moreover, a detailed analysis was conducted on the performance differences of the device under indoor light sources compared to solar spectrum conditions, along with the underlying mechanisms. Finally, the Sb 2 S 3 IPV cell achieved a peak theoretical efficiency of 46.25 % under cold white FL lighting, a testament to the optimal match between the device structure and this specific emission power spectrum. This modeling research not only underscores the feasibility of employing antimony-based photovoltaic technologies in indoor settings but also offers theoretical guidance for further advancements in this domain.

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Coupling of Phosphates on Alumina Surfaces for Bioactivation