Influence of Ag/Ca ratio on the osteoblast growth and antibacterial activity of TiN coatings on Ti-6Al-4V by Ag and Ca ion implantation

Highlights

• Ag + Ca ions were implanted on TiN coated Ti6-Al4-V alloy via PIII&D.

• Ag + Ca ion implantation improve the osteoblast growth and antibacterial activity of TiN.

• Different Ag/Ca ratios in Ag/Ca-TiN are controlled by magnetic bias.

• Ag/Ca(2:1)-TiN shows the best corrosion, cell adhesion and antibacterial activity.

• Ag/Ca ratio is a key parameter for the biological properties of Ag/Ca-TiN.

Abstract

Titanium nitride (TiN) coating has many prominent properties such as chemical stability, high hardness, excellent wear properties, biocompatibility and so on, which make it as a potential candidate for inhibiting the release of toxic ions and improving mechanical properties of dental and orthopedic implants made by titanium‑aluminum‑vanadium (Ti-6Al-4V). Of course, the biologically inert and poor antibacterial property of TiN limits its application. In this work, TiN coating was firstly deposited on the surface of Ti-6Al-4V substrate to inhibit its toxic ions release. Then, silver (Ag) and calcium (Ca) with different ratios were implanted into TiN coatings by plasma immersion ion implantation and deposition (PIII&D) system to enhance both osteoblast growth and antibacterial activity on the surfaces of TiN coatings. The results show that TiN coating inhibits the release of toxic ions such as Al and V. X-ray photoelectron spectrometer (XPS) and X-ray diffraction (XRD) analyses proved the presence of Ag, Ca on the surface of Ag and Ca implanted TiN (Ag/Ca-TiN). Scanning electron microscopy (SEM) was used to observe the surfaces' morphologies of samples, which takes advantage of distinct detectors to visualize secondary and back-scattering electrons. The protein adsorption experiment showed that the protein adsorption capacities increased with the increase of Ca ratio. When the results of antibacterial and cell adhesion tests were combined, Ag/Ca(2:1)-TiN indicated the best osteoblast adhesion and antibacterial activity in vitro due to the synergistic effect of Ag and Ca. The improved inhibiting ability in releasing toxic ions, osteoblast adhesion and antibacterial activity of the Ag/Ca(2:1)-TiN suggest that it might be a potential choice for surface medication of orthopedic implants clinically.

Introduction

Titanium nitride (TiN) is a biocompatible [1], nontoxic, high wear and corrosion resistant [2], high chemically and physically stable [3,4] ceramic coating, which has been demonstrated for use in biomedical applications to enhance hardness and polarization resistance, and reduce wear damage of surgical alloy, orthodontics, orthopedic implants, hip prostheses, and cardiovascular biomaterials [[5], [6], [7]]. Titanium‑aluminum‑vanadium alloy (Ti-6Al-4V) as a widely used dental implant [8,9] as well as hard tissue replacement and fracture fixation device [10] has many good mechanical properties such as the high specific strength, good fatigue strength [11] and corrosion performance. However, the long-term release of toxic metallic ions (Al and V ions) in Ti-6Al-4V may enter the bloodstream, causing inflammation and pain [12,13]. Also, its bio-inert and insufficient osseointegration make the implant passively integrate with the peri-implant bone, which needs a long period for osseointegration [[14], [15], [16], [17]]. It is generally known that a rapid coalescence at the implant-bone interface may not only enhance stabilization of implant, but also shorten the hospitalization [18,19]. More importantly, although full disinfection work and antibiotic drug therapy have been done during and after the surgery, prosthesis implant failures may still occur owing to implant-associated bacterial infections [[20], [21], [22]] because of the poor antibacterial activity of Ti-6Al-4V [23,24]. Therefore, TiN is a good candidate for coating titanium alloy surface to overcome the above problems.

However, the application of the biological and antibacterial abilities of TiN still needs to be improved, because these defects may cause ultimate failure after long-term use. To antibacterial effect, many studies have shown that silver can effectively improve the antibacterial activity of the material [20,25,26]. Silver, as a broad-spectrum antibacterial agent, is able to act on a broad spectrum of bacterial and fungal species, including antibiotic-resistant strains, without local or systemic side effects in many in vivo experiments [27].

Thin film technology is a clean and uniform technology, which can improve the surface properties of samples [3,28,29]. In this work, a plasma immersion ion implantation and deposition (PIII&D) system is used to modify the surface of the sample. This system has four sources, which can deposit a dense coating on the surface of the sample by DC source and implant two kinds of high-speed ions into the sample surface at the same time by pulsed cathodic arc plasma sources. Fig. 1 showed PIII&D system. The advantages of PIII&D include the deposition of the coating with dense and uniform, good surface modification when the implanted ion reaches an appreciate concentration and depths, and the treatment of complex three-dimensional surface [30,31].

In order to improve the antibacterial activity and promote the rapid growth of bone cells, some research groups doped antibacterial silver with other element to promote the growth of bone cells, the purpose was to achieve the synergistic role of the two elements. Zhao et al. proved that co-doping of Mg and Ag exhibited a balance between the antibacterial performance and the osteogenic properties of the titanium [32]. In this work, we chose Ca to replace Mg to combine with antibacterial Ag to achieve synergistic effect of the osteoblast growth and antibacterial performance.

The researches evidenced that the introduction of appropriate amount of Ca, Mg and other elements can promote cell differentiation [18,32,33]. As one of the main components of human bones, calcium is a necessary ion for bone formation, which can regulate mRNA transcription and protein synthesis, and stimulate the growth and osteogenesis of osteoblasts [[34], [35], [36]]. At present, based on our knowledge, there are few reports of combining these advantages for irritating bone cell growth and antibacterial activity. So, in this work, TiN coating was deposited on Ti-6Al-4V firstly, which could not only increase the hardness of Ti-6Al-4V, but also prevent the release of toxic ions. Then silver and calcium ions were implanted into TiN coating via plasma immersion ion implantation and deposition system to form Ag/Ca-TiN, which is expected to enhance both its cell adhesion and its antibacterial ability. In our previous work, it has been proved that TiN coating can improve the mechanical properties of Ti-6Al-4V, and prevent the release of toxic substances such as Al and V in [20,37]. Therefore, in this paper, we focused on the release of the implanted Ag and Ca ions, in vitro osteoblast adhesion and antibacterial activity of Ag/Ca-TiN groups with different Ag/Ca ratios. This is also the difference and innovation in this work compared with our previous work.