Elsevier

Ceramics International

Volume 45, Issue 1, January 2019, Pages 793-804
Ceramics International

Multiscale connections between morphology and chemistry in crystalline, zinc-substituted hydroxyapatite nanofilms designed for biomedical applications

https://doi.org/10.1016/j.ceramint.2018.09.246Get rights and content

Abstract

Hydroxyapatite (HA), Ca10(PO4)6(OH)2, is a key material used to manufacture thin, bioactive coatings on metal implants for hard tissue regeneration. Technical challenges in their fabrication include control of the coating structure and chemical composition, as well as managing surface activation via ionic substitution. In this study, 15 – 216 nm-thick zinc-doped HA (ZnHA) thin films were grown via magnetron sputtering at room temperature. The effect of substituting Zn2+ for Ca2+ on the ultrastructures of the HA films was analyzed during several stages of film growth. Infrared vibrational scattering scanning near-field optical microscopy (s-SNOM) using the ultra-broad-band IR beam from synchrotron radiation was compared with Fourier transform infrared spectroscopy in attenuated total reflectance (FTIR/ATR) to reveal the close relationships between local topography and the chemical compositions of the film surface and the bulk of the film. The energy delivered via ionic bombardment induced the formation of complex structures: island-shaped nanoparticles of amorphous zinc-doped calcium phosphate, coalescent islands, disordered non-stoichiometric ZnHA nanoparticles, and long-range structures of nearly stoichiometric and crystalline ZnHA. Grazing incidence X-ray diffraction (GIXRD) from synchrotron radiation, X-ray photoelectron spectroscopy (XPS), focusing ion beam (FIB), and high-resolution transmission electron microscopy (HRTEM) confirmed Zn substitution into the HA lattice. Zinc was distributed homogeneously within the film structure (1.6 at%) but its concentration increased slightly on the film surface. The bulk of the film consisted of crystalline, columnar ZnHA domains oriented perpendicular to the substrate surface while the regions near the film/substrate interface were preferentially disordered and non-stoichiometric.

Introduction

In recent decades, intense efforts have been devoted to improve the biological performance of dental and orthopedic metal implants. One critical challenge is the development of bioactive implant coatings that combine long-term stability, corrosion resistance, antibacterial properties, and improved bone regeneration efficiency. Hydroxyapatite (HA, Ca10(PO4)6(OH)2) has emerged as a candidate material due to its compatibility with components of biological media and plasma, osteoconductivity, ceramic properties, and potential for association with antibiotics and antimicrobials [1], [2]. Several chemical and physical techniques for coating implants with this material have been proposed and tested. They include electrodeposition, sol gel processes, biomimetic processes, and plasma spraying, as well as ion, electron beam, pulsed laser, and radio-frequency magnetron sputtering [3], [4], [5].

Since the pioneering works of Cooley et al. and Dijk et al. [6], [7], radio-frequency magnetron sputtering (RFMS) systems have been developed to produce new nanostructured HA coatings designs for metal implants intended for biomedical applications [5]. RFMS enables the deposition of thin films and provides control of the deposition rate, thickness, and nanometric roughness, thus maintaining the complex topography of the implant [4], [5], [8]. In recent years, significant advances have been made regarding the influences of deposition conditions and plasma parameters on physio-chemical and mechanical film properties [3], [5], [9]. Furthermore, in-vitro and in-vivo studies have revealed that HA coatings deposited via RFMS offer promising potential for future medical applications [5]. Chemical treatment is one method of improving HA adhesion to the substrate, while crystallinity increases the long-term in-vivo film stability. Cationic and anionic substitution of HA structures such as substituting Sr2+ for Ca2+, (SiO4)4- for (PO4)3-, and F- for OH- has been tested and shown to improve biological responses to the coatings [10], [11], [12], [13], [14]. In this field, the challenge is to understand the effects of ionic incorporation on film growth and physio-chemical properties.

Most metal replacements used in synthetic HA grafts for bone regeneration use zinc due to its importance to the bone biomineralization processes [15], [16], [17], [18]. In-vitro and in-vivo studies have revealed that Zn-substituted HA that contains a low zinc concentration (less than 1.0 at%) improves osteoconduction and osseointegration [19], [20]. On the other hand, a high HA zinc concentration imparts antimicrobial and antibacterial ability [21]. Despite promising results with Zn-substituted HA grafts, only a few studies have been performed regarding the development of Zn-substituted HA coatings deposited on metal surfaces via chemical and physical techniques [19], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. Recently, Robinson et al. [31] performed RF magnetron sputtering of Sr (SrHA) and Zn (ZnHA) doped HA targets using a cluster of two high-vacuum torus 3 M sputtering sources. The deposition process produced amorphous calcium phosphate coatings, which transformed into crystalline HA after heating at 400 °C. The authors showed that the incorporation of Zn and Sr into the film surfaces led to (Ca + M)/P ratios of 1.45 and 1.20, respectively. XRD peaks of SrHA coatings exhibited slight shifts to lower 2θ values relative to HA, which evidenced the substitution of Sr for Ca in the HA lattice. The XRD peak positions and intensities for the ZnHA coating were similar to those of the HA coating [31]. However, the apparent lack of hydroxylation indicated by the FTIR spectrum reinforced the hypothesis that Zn was substituted for Ca in the HA structure [31].

In this paper, we apply RFMS with right-angle geometry (RAMS) to produce crystalline, nearly stoichiometric Zn-substituted HA thin films at room temperature for the first time. Multiscale characterization techniques such as nano-FTIR were combined with XPS, GIXRD, FIB, and HRTEM to characterize the film ultrastructure at various stages of growth, from the formation of the early amorphous phases through film crystallization. The results reported here can be further applied to the development of new metal-substituted HA thin coating designs for biomedical applications.

Section snippets

ZnHA powder and target preparation

Zn-substituted HA (ZnHA) was synthesized via the dropwise addition of Ca(NO3)2·4H2O (0.699 g/mol) and Zn(NO3)2·4H2O (0.0789 g/mol) solutions into an (NH4)2HPO4 (0.466 g/mol) aqueous solution. The pH and temperature were maintained at 9.0 and 90 °C, respectively. The solid was washed, filtered several times with deionized water until it reached a pH of 7.0, and dried via lyophilization. Grains between 74 µm and 150 µm in size were separated via sieving. The ZnHA targets were prepared using a

ZnHA target compositions

XRD, FTIR, and chemical analyses show that the ZnHA powder is composed of HA with a Zn content of 7.8 at%, where (Ca + Zn)/P = 1.56 (Fig. S2). The XRD pattern of the targets sintered at 1150 °C for 2 h exhibits peaks from a HA phase [49], α and β tricalcium phosphate (α,β TCP) [50], [51], ZnO [52], and CaO [53], (Fig. 1a). The XPS spectrum of the sintered target surface includes the binding energies of Zn, Ca, P, O, and C with Ca/P and (Ca + Zn)/P atomic ratios of 1.45 and 1.70, respectively (

Conclusion

In this work, we produced films of Zn-substituted HA with thicknesses of 15–198 nm using a radio-frequency magnetron sputtering technique with right-angle geometry. The incorporation of Zn into HA and formation of ZnHA did not depend on the target composition. The critical parameters used to adjust the ZnHA film stoichiometry and to improve crystallinity were the plasma energy, RF power, and deposition time. Incorporation of Zn into the HA structure did not modify the nucleation of the apatite

Acknowledgment

The authors wish to thank the Synchrotron Light Brazilian National Laboratory (LNLS) for providing beam time at the XRD2 and IR1 stations; the X-ray Diffraction Multiuser Lab (RX) and Nanoscience and Nanotechnology Multiuser Lab (LABNANO) at CBPF for the X-ray reflectivity measurements (XRR) and the use of the transmission electron microscopy equipment (TEM) and the Brazilian government agencies CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior), CNPq (Conselho Nacional de

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