An in-vitro study: The effect of surface properties on bioactivity of the oxide layer fabricated on Zr substrate by PEO

Abstract

Surface properties can affect cell attachment and growth ability on the biomaterials. In this work, the effect of the morphology and chemical composition of the surface coatings synthesized on zirconium (Zr) substrates on bioactivity was studied. The surface of pure Zr was modified via a plasma electrolytic oxidation process to obtain oxide-based coatings with different surface chemical compositions and pore sizes by changing the process parameters in the same electrolyte. The characterization of the coatings was conducted using X-ray diffraction (XRD), scanning electron microscopy (SEM), contact angle goniometry (CAG) and energy dispersive spectroscopy (EDS), and Fourier transform infrared spectroscopy (FTIR). The results showed that the coatings comprised t-ZrO₂ and Ca_0.134Zr_0.86O_1.86 phases. During the oxidation of Zr substrates, applying an anode current with different densities and frequencies altered the surface roughness Ra of the samples. The relationship between the surface morphology, chemical composition, and bioproperties was investigated by simulated body fluid (SBF) and cell culture tests. The coated samples were immersed into SBF solution and the formation of hydroxyapatite (HA) structure on the surface was detected after 7 days of immersion. A close relationship between the development of HA, the size of the porosity and chemical composition of the coating surface was observed. Additionally, the cell attachment and cell growth on the surface were found to be dependent on the chemistry and topography of coated surfaces. The cell culture test revealed that the growth of the cells was improved with increasing pore sizes on the coating surface.

Introduction

Due to the growing demand for implant materials with enhanced corrosion resistance, mechanical properties, and bioactivity, the number of research reports focused on the development of novel biomaterials for orthopedic and dental applications has soared in the last couple of decades [1], [2], [3], [4], [5], [6], [7]. Zirconium based materials are of great interest for biomedical applications owing to their high chemical stability, low thermal conductivity, high mechanical strength and fracture toughness, low elastic modulus as well as good biocompatibility [1,2,[4], [5], [6], [7], [8], [9], [10]]. Although the formation of a very thin (2-5 nm), native oxide layer (ZrO₂) on the surface of Zr substrates renders these materials corrosion resistant, it deteriorates the bioactivity of Zr due to the bio-inert nature of ZrO₂ by hindering the formation of chemical bonds between bone tissues and Zr implant [11], [12], [13]. Another important challenge related to the thin native ZrO₂ film is that when these materials are employed as load-bearing implants such as total hip and knee arthroplasties, the oxide layer may be destroyed due to being subjected to wear forces. In addition, it was reported that Zr is vulnerable to pitting corrosion in simulated body fluid (SBF) even at low applied potentials [7]. Thus, although Zr-based materials have high corrosion resistance under most service conditions, the corrosion resistance may be lost under severe service conditions, which in turn results in either decrease in the service life of the implant or practicality of the implant. In order to circumvent the aforementioned challenges, the surface properties of Zr require to be further understood and improved.

The modification of Zr surface using various methods including physical and chemical vapor deposition, sol-gel process, plasma spray coating, thermal oxidation, anodization and plasma electrolytic oxidation (PEO) has been reported [3,[14], [15], [16], [17]]. Among these surface modification techniques, environment friendly PEO is considered as one of the most promising methods owing to being capable of fabrication of adherent, crystalline, porous, relatively rough, and thick oxide films on Zr substrates [1,2,5,[17], [18], [19], [20], [21], [22]]. Furthermore, the chemical and morphological properties of the fabricated oxide layers can be tailored easily and the desired properties can be achieved by simply changing the process parameters such as temperature, concentration and pH of electrolyte, substrate composition as well as duty cycle, applied current density, voltage, frequency and duration of the process [1,2,5,7,8,10,12,13,[17], [18], [19], [20], [21], [22], [23], [24]]. One of the important tools used to control the electrical parameter is the type of power supply. AC, DC, and DC-pulsed power supply in PEO process plays a significant role in the preparation of the high-quality coatings for orthopedic, dental, biomedical and catalysis applications. The discharge occurring during the PEO process changes the coating microstructure, which can be controlled by electrical parameters and/or power supply. DC-pulsed power supply is preferred because it allows better control of energy discharge input resulting in surface coatings with more controlled microstructures and surface properties [2,5,7,19,[25], [26], [27], [28]].

It has been reported that the surface properties of the oxide films including morphology (roughness, pore characteristics etc.) and chemical composition play key roles on the cell interaction with the surface [28,29]. The effect of pore structure, pore size and total pore volume on the bioactivity of implants for bone formation has also been reported [30,31]. It was reported that mesoporous glasses with larger surface area and higher pore volume had enhanced in-vitro bioactivity compared to regular dense bioactive glasses [32,33].

The number of studies on the modification of Zr surface via HA formation either through single-step or multi-step methods to tailor the bioactivity of these materials have started to increase recently [2,4,5,11,13]. However, the publications dealing with the effect of surface morphology of the implant materials on the in-vitro bioactivity and cell attachment behavior are very limited. In the literature, the effect of different pore sizes on the coating have hardly been reported systematically. In addition, the effect of pore size morphology and biological responses of the coatings are not very clear [2,5,10,12,13]. Therefore, in the present study we aimed to investigate the effect of pore size of the oxide layer synthesized on Zr substrate on the bioactivity and cell interaction properties. Oxide-based films with different morphologies (pore size and surface roughness) were fabricated on the surface of Zr by PEO process. To achieve oxide layers with different pore sizes and different surface roughness, Ra, the process parameters including duty cycle, frequency, and applied voltage were modulated. The coatings were characterized using X-ray Diffractometer (XRD), Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS), Contact Angle Goniometer (CAG) and Fourier Transform Infrared Spectroscopy (FTIR). In addition, in-vitro bioactivity and cell interaction properties of the coating surfaces were evaluated by SBF and cell culture tests.