Mechanical micro-texturing and characterization on Ti6Al4V for the improvement of surface properties
Introduction
Titanium and its alloys are the most commonly used biometals for orthopedic and dental implants due to their excellent strength, low weight, high corrosion resistance, superior mechanical properties, and biocompatibility. However, titanium implants lack in surrounding bone tissue integration because of poor adhesion and bio-inertness [1,2]. Therefore, surface modification is needed to enhance the cellular responses, protein absorbance, biocompatibility, and osseointegration rate. In the literature, several studies had been carried out to improve the biocompatibility and performance of Ti64 implants by chemical (i.e. acid etching, sol-gel treatment, electrochemical anodization, biochemical), physical (i.e. thermal spray, ion plating, sputtering, ion implantation, and deposition) and mechanical (i.e. sandblasting, grinding, polishing, laser machining and treatments) [[3], [4], [5], [6]]. Zhang et al. [7] used mask electrochemical machining technique to fabricate sandwich like micro dimples, in which they highlighted the difficulties of dimensional control and shape of the textures. Further, in another study, They utilized a porous metal cathode to control the size accuracy and shape of the dimple [8]. The commonly used chemical and physical methods have certain limitations such as acid etching damages the implant surface, affect the mechanical properties and can increase the chances of bacterial reproduction around the implants, the major problem with SLA process is a residue of sand particles and these are highly reactive to oxygen, which may result in bacterial reproduction about the implant [9]. Furthermore, the surface coating techniques have limitations such as breakage of the oxide layer may result in poor implant life, ectopic bone formation possibility and difficulties in the coating of complex shapes and porous materials [10,11]. Micro milling is the promising technique to fabricate directional textures on bio implant surfaces with desired dimensional accuracy, surface finish, shape, size, and density. However, the use of mechanical micro-milling is very limited in the open literature. A few numbers of studies have been found on the use of micro-milling for the surface texturing of titanium implants. Pratap and Patra [12] highlighted that mechanical micro-milling is an effective technique to fabricate controlled surface textures on titanium. In addition, Wang et al. [13] fabricated orderly micro/nano textures on titanium by micro-milling and anodic oxidation and found that orderly formed micro textures improved cellular responses. The wettability and SFE of implant surfaces are closely related to each other. Therefore, the implant surfaces with higher SFE are desirable because they increase the degree of osseointegration, protein adsorption and cellular responses [14,15]. Yan et al. [16] investigated the SFE of different polished and smoothed Ti64 samples using different approaches. The study concluded that the SFE has no clear relation with the surface roughness. Furthermore, Miyajima et al. [17] studied the effect of chemical, physical and biological surface modification of Ti (grade 2) on critical and theoretical SFE. They concluded that the chemical and physical surface treatments increased the SFE. However, the effect of biological treatment on SFE values was negligible. In reported literature, the effect of different surface textures of titanium implant on surface characteristics and cellular responses has been studied. Furthermore, it has been found that the depth, size, shape, and density of microtextures play a crucial role and to control these geometrical parameters is a very challenging task. Since, due to recent developments in micromachining techniques, these geometrical parameters can be controlled by micro-milling.
From the critical literature review, it has been found that micro-milling based micro dimples with an extruded center, their geometrical parameters, surface characteristics and its effect on wettability and SFE has not been investigated. Therefore, in the present work, we fabricated micro dimples with an extruded center of different geometrical aspects on Ti6Al4V surface by micro-milling. The effect of these textured surfaces on surface properties in relation to geometrical parameters has been studied in detail.
Section snippets
Fabrication of surface textures using end micro-milling
An experimental setup is illustrated in Fig. 1 (a). The microtextures were machined on micromachining center developed in microfabrication laboratory at IIT (ISM) Dhanbad. The machining center is self-designed and fabricated with high positional accuracy and stability at high spindle speed. The spindle can rotate up to 60,000 rpm with an adequate torque of 24 Ncm. The asynchronous motor was used to drive the spindle with a variable frequency drive. The machining center has linear X, Y and Z
Fabrication of microtextures on the surface of Ti6Al4V
Two sizes of micro dimple (400 μm and 200 μm) denoted as MDS 400 and MDS 200 were successfully fabricated by micro-milling. The variation of texture depth (h) and space width (Sw) of MDS 400 and MDS 200 is shown in Fig. 2 (a) to (d) and Fig. 3 (a) to (d) respectively. The form measurements clearly illustrate that the micro dimple textures were in good quality and shape with straight and sharp walls. An in vitro study carried out by Renu et al. [28] exhibited that the surface textures with sharp
Conclusions
The study presents texturing of micro dimple with the extruded center of different geometrical parameters on Ti6Al4V by micro-milling. The effect of geometrical parameters on surface properties has been investigated. The following conclusions have been drawn:
- a)
The micro dimple textures of two different size (400 μm and 200 μm diameter), space width (Sw) and depth (h) have been successfully fabricated. Defect-free with good dimensional accuracy and sharp corners of textures have been achieved.
Declaration of competing interest
No conflict of interest exists.
Acknowledgment
The presented work has been funded by ECR scheme of DST, Govt. of India, File number: ECR/2016/001956 and project number: MECH.ENGG./DST(SERB)/(178)/2017–2018/533.
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