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Characterization of Ti and Ti6Al4V Surfaces After Mechanical and Chemical Treatments: A Rational Approach to the Design of Biomedical Devices

¹Department of Industrial Engineering, University of Padova, via Marzolo 9, 35131 Padova, Italy

²Department of Biomorphology and Biotechnologies, University of Messina, viale Gazzi, 98125 Messina, Italy

Corresponding Author:

Andrea Bagno
Department of Industrial Engineering, University of Padova
via Marzolo 9, 35131 Padova, Italy
Tel: 39-049-8275004
Fax: 39-049-8275555
E-mail: andrea.bagno@unipd.it

Received date: September 10, 2012; Accepted date: October 18, 2012; Published date: October 21, 2012

Citation: Bagno A, Dettin M, Santoro G (2012) Characterization of Ti and Ti6Al4V Surfaces After Mechanical and Chemical Treatments: A Rational Approach to the Design of Biomedical Devices. J Biotechnol Biomater 2:151. doi:10.4172/2155-952X.1000151

Copyright: © 2012 Bagno A, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

At present, titanium-based biomaterials for the production of prosthetic devices have achieved a satisfactory quality level; current research aims at improving their surgical dependability and biomechanical performances. In this contest, a crucial aspect is represented by the osseointegration process, which implies the secure association of endosseous devices with the surrounding biological tissue. Osseointegration is largely controlled by surface characteristics with regard to both chemical composition and morphological properties. Therefore, the design of such devices might be guided by the characterization of surface morphology produced by mechanical and chemical treatments. This paper illustrates the results obtained by the application of a set of treatments on titanium (Ti2) and Ti6Al4V alloy (Ti5) samples. Mechanical treatments mainly affect the dimension of larger defects, acting on a macrometric scale and inducing specific patterns; chemical treatments (i.e., acid attack at room or higher temperature) can dissolve surface material altering defect dimensions on a micrometric scale. This study may represent a useful tool for the rational design of implant surface characteristics.

Keywords

Surface analysis; Surface modification; Surface morphology; Titanium; Endosseous devices

Introduction

Endosseous implants are widely used for dental applications since they provide good results in oral implantology [1]; at present, the restoration of one or more teeth is commonly performed by means of titanium screws, which allow successful rehabilitation of edentulous patients [2]. On the other hand, orthopedic devices have to support rapid, stable and durable fixation to bone tissue [3].

In order to improve both surgical dependability and biomechanical performances of dental implants and endosseous prosthetic devices, many efforts have been aimed at promoting secure and congruent association of the metallic surface with the surrounding biological tissue (osseointegration) [1,4]. The formation of newly-grown bone tissue on implant surface highly depends on the characteristics of the surface itself, in terms of both chemical composition and morphological properties; these features are critical in controlling interactions at metal/tissue interface [5-7].

From a general point of view, surface treatments are known to induce modifications in both chemical composition and morphology [8]; they are expressly produced to achieve greater bone-to-implant contact [9,10]. A higher level of biomechanical and functional performances of prosthetic devices can be obtained taking into consideration that surface morphology has to be shaped in order to improve cell adhesion [11,12].

As to the purpose of the present study, a large number of scientific contributions [13] investigates the effects of treatments applied on different surfaces (i.e., titanium and titanium alloy), focusing the influence of abrasive particles size, pollution phenomena, thermal/ mechanical effects, and roughness [9,14-18]. Many other papers concern blasting techniques, addressing different abrasive particle (i.e., rutile, hydroxyapatite, corundum, and alumina), particles size, surface composition and native topography [19-27]. All these studies analyze roughness characteristics measured by profilometry and most of them try to correlate the optimized roughness distribution [28] and the amount of cell adhesion in vitro: indeed, successful osteoblast adhesion allows foreseeing excellent clinical outcomes when increasing the surface area available for bone-to-implant contact, thus improving bone-to-implant interactions [12,29-32].

In the wake of previous investigations [33], the present paper illustrates how and how much mechanical and chemical treatments applied on titanium-based samples can modify surface morphology. Different treatments have been applied on the surface of Ti (Ti2) and Ti6Al4V (Ti5) samples; in particular, acid attack conditions have been adjusted varying both composition and temperature of acidic mixture.

Resulting surface morphology and defect dimensions have been measured by means of optical microscopy and profilometry. In particular, surface defects width (μm) and depth (μm) were estimated. A third adimensional parameter was calculated: the so called “shape ratio”, representing how much the length of the real surface profile increases compared with the geometrically projected one; this parameter allows a preliminary estimation of surface roughness.

The results obtained show that mechanical treatments mainly affect the dimension of larger defects, acting on a macrometric scale and inducing specific patterns, which are likely to be identified on sample surface; on the other hand, chemical treatments (i.e. acid attack at room or higher temperature), can dissolve surface material deeply altering defect dimensions on a micrometric scale.

Taken together, experimental results indicate that: i) defect depth is an amplitude parameter able to properly describe surface characteristics; ii) linear relationships between the amount of defect alteration due to chemical treatment and the duration of the process do exist; iii) the velocity of the attack is temperature dependent.

Materials and Methods

Samples treatments

Commercially pure titanium (Ti2, grade 2) and Ti6Al4V (Ti5, grade 5) disks (Ø 15 mm; height 2.5 mm) were obtained from cylindrical bars by turning. First, they were smoothed (80, 180 or 220 Grit Paper), then sand-blasted (corundum 350 μm for 10 s), passivated with HNO₃ (30% (w/w) solution) or attacked with acidic solution (7% HCl/H₂O (w/w) and 27% H₂SO₄/H₂O (w/w)) for 1 up to 60 min at room temperature and mixture boiling point. Few samples were also treated by adding few drops of HF solution. Afterwards, the samples were sand-blasted and thoroughly degreased, washed and sonicated in a sequence of solvents: ethanol (Carlo Erba, Milan, Italy), acetone (Janssen, Geel, Belgium), chloroform (LabScan, Dublin, Ireland).

In the following part of the text, sand-blasted surfaces are indicated as SL; sand-blasted and acid-attacked surfaces as SLA. Ti2 identifies commercially pure titanium, while Ti5 indicates titanium alloy (Ti6Al4V). Tables 1 and 2 summarize the sequence of mechanical and chemical treatments performed on Ti2 and Ti5 samples, respectively.

Table 1: Sequence of mechanical and chemical treatments applied on Ti2 samples.

Table 2: Sequence of mechanical and chemical treatments applied on Ti5 samples.

Morphological characterization

In order to characterize Ti2-SL, Ti5-SL, Ti2-SLA and Ti5-SLA surfaces, pictures were acquired with an optical microscope (Leica DMRM) at different magnification (from 50X up to 1000X) to focus both single defects and their local organization. Surface defects width (μm) and depth (μm) were measured by image analysis (Casti Micro Image 3.4 software). Values are averaged on 10 to 20 measurements on each surface. A third dimensional parameter was calculated: the “shape ratio” represents how much the area of real surface profile increases compared with the geometrically projected one. Shape ratio allowed a preliminary description of the amount and relevance of surface defects. A fourth parameter (“bending diameter”) was also calculated assuming surface defects as circular grooves.

Profilometer analysis

600x600 μm² surface areas on samples were examined under KLA-Tencor P10 Surface Profiler operated at 100 Hz sampling frequency and 50 m/s scanning rate. Seven roughness parameters were determined: S_a, average roughness (μm); S_q, root mean square roughness (Å); S_z, ten-points mean roughness (Å); S_ds, peak density (Δ/Ų); S_sk, profile asymmetry; S_ku, profile kurtosis; S_q, root mean square profile slope (°).

Results and Discussion

Mechanical and chemical treatments

Mechanical treatment (sand-blasting with 350 μm corundum particles) results in very rough surfaces: highly irregular profiles are revealed by optical microscope on Ti2 and Ti5 samples (Figures 1a and 1b); in particular, defects produced by blasting are bigger on Ti2 disks than on Ti5.

Figure 1: Microscope images (100X) of samples after sand-blasting with 350 μm corundum particles: both surfaces reveal very irregular profiles. (a) Optical Ti6Al4V and (b) Ti After acid attack at room temperature, surface defects are flattened on both (c) Ti6Al4V and (d) Ti samples.

The effects of acid attack at room temperature on Ti2 and Ti5 surfaces are depicted in Figures 1c and 1d. Generally speaking, experimental evidences suggest that nitric passivation preserves the surface configuration previously induced by blasting on Ti2 and Ti5 samples (data not shown), while acid attacks makes surface defects flattening. During chemical treatments with acidic mixture, it has been observed that higher amounts of hydrogen bubbles are usually developed on blasted surfaces than on the others and that Ti5 reacts more readily than Ti2. Furthermore, it is worthwhile mentioning that mild attack conditions reduce defects on blasted surfaces; severe conditions (boiling acid mixture or HF addition) result in immediate and strong reaction, thus dissolving great defects and deeply modifying surface aspect.

Amplitude parameters

Defect depth and width can be measured on sample surface by image analysis software (Figure 2): they are “amplitude parameters” since they describe morphological characteristics not dependent on the extension of the investigated area. Furthermore, another dimensional parameter termed “shape ratio” has been calculated as the ratio between the length of profile roughness and the length of profile waviness: it does not depend on the extension of the sampled area. Greater the value of this parameter, greater is the area of the real surface.

Figure 2: Optical microscope images are analyzed by image analysis software to estimate (a) defect depth and (b) width; (c) defect profile and (d) length.

A forth parameter can be calculated assuming that surface defects are circular grooves i.e. the “bending diameter”, resembling the mean size of the defects given by:

Tables 3 and 4 summarize values of the above mentioned parameters measured on Ti2 and Ti5 samples, respectively.

Table 3: Mean values of defect dimensions on Ti2 samples after mechanical and chemical treatments.

Table 4: Mean values of defect dimensions on Ti5 samples after mechanical and chemical treatments.

Data analysis results

Characteristic dimensions of defects on sample surfaces were evaluated by explorative analysis (boxplot). This method allows summarizing information belonging to a group of homogeneous experimental data in terms of distribution, mean value and variability. A graphical representation is obtained by associating each sample with a box; the box is divided by a horizontal line, representing median value, while its higher and lower edges correspond to the first and the third quartile, identifying the inter-quartile range. Other two lines extend the box below and above, and indicate 1.5 inter-quartile ranges from lower and higher quartile. If experimental points lie outside these lines, they are simply considered outliers.

By means of boxplot representation, depth and width values distributions are considered for Ti2 and Ti5 samples, respectively (Figures 3 and 4). By comparing these distributions, it is worthy to note that depth values are significant, but width vales are not. Therefore, only depth values of surface defects can be used for discriminating different surface configurations.

Figure 3: Boxplot representations of (a) depth and (b) width values distributions on Ti2 samples (white boxes) after treatments and on commercial dental implant surfaces taken as references (dark grey boxes). A3.3 and A4.1 are TiO₂ blasted dental screws of different diameter (3.3 and 4.1 mm); B3.5 and B4.0 are corundum blasted dental screws of different diameter (3.5 and 4.0 mm).

Figure 4: Boxplot representations of (a) depth and (b) width values distributions on Ti5 samples (white boxes) after treatments and on commercial dental implant surfaces taken as references (dark grey boxes). A3.3 and A4.1 are TiO₂ blasted dental screws of different diameter (3.3 and 4.1 mm); B3.5 and B4.0 are corundum blasted dental screws of different diameter (3.5 and 4.0 mm).

Ti2 and Ti5 samples have been projected as points into planes whose coordinates are represented by shape-ratio vs. depth values: these graphs allow clustering together samples that share similar surface characteristics. With respect to Ti2 samples, it is possible to identify the combination of treatments (T7) responsible for the production of morphological characteristics very similar to those measured on reference surfaces (commercial implants): the corresponding points on the plane are clustered together (Figure 5a). On the other hand, clustering is not unambiguous for T5 samples (Figure 5b): this hampers the identification of a single treated surface similar to references.

Figure 5: Projections of (a) Ti2 and (b) Ti5 samples on shape ratio vs. depth plane: commercial dental implants (references) are indicated with dark grey dots.

It is worthwhile mentioning a further result concerning the relation between bending diameter and conditions of acid attack; a linear behavior correlates bending diameter values and the duration of the attack but with opposite trends on Ti2 and Ti5 surfaces (Figures 6 and 7). In particular, on Ti2 surface at acidic mixture boiling point, longer the duration of the attack, higher is the bending diameter values (R²=0.98). The amount of the attack is quantified in 17 μm/min. With regard to the Ti5 samples treated at room temperature, bending diameter values decrease while increasing the duration of the attack (R²=0.97); using stronger acidic conditions, the rate of the attack increases from 1.6 up to 2.2 μm/min.

Figure 6: Linear relation (R2=0.9892) between bending diameter values and the duration of acid attack for Ti2 samples.

Figure 7: Linear relations between bending diameter values and the duration of acid attack for Ti5 samples under mild and strong conditions (R2=0.9767 and 0.9782, respectively).

Profilometry data

Roughness properties have been estimated by contact profilometry. Figure 8 shows Ti2 and Ti5 surfaces before (SL) and after acid-attack (SLA); Table 5 summarizes roughness parameters values averaged over three measurements on each sample. An overview of roughness parameters suggests that their mean values increase on Ti2 surfaces due to acid-attack, and decrease on Ti5 surfaces. Consequently, acid-attack produces different effects due to the different conditions of the attack and the different composition of the substrate.

Figure 8: Images of square surface areas (600 μm side) obtained by contact profilometry: (a) SL (b) SLA Ti2 surfaces; (c) SL (d) SLA Ti5 surfaces.

Table 5: Roughness parameters values measured on SL and SLA Ti2 and Ti5 sample surfaces.

Conclusions

At present, endosseous implants and prosthetic devices are commonly applied in the field of implantology, providing stable and durable fixation with the surrounding bone tissue.

This study illustrates combinations of mechanical and chemical treatments used to modify the morphology of titanium-based samples; resulting defect dimensions were analysed and characterized by means of optical microscopy and profilometry.

The results obtained can be summarized as: i) defect depth is an amplitude parameter suitable for significantly describing and discriminating different surface characteristics; ii) the amount of defect alteration due to chemical treatment is linearly dependent on the duration of the process; iii) the rate of the chemical attack is temperature dependent.

In conclusion, the present study allows identifying a methodology to address efforts for the rational design of implant surfaces in order to promote implant reliability.

Acknowledgements

Authors would like to acknowledge Prof. Franco Bonollo (Department of Management and Engineering, University of Padova) for optical microscope images; Dr. Stefania Turato (Italian National Council of Research, Padova) for contact profilometry analysis. This work has been in part supported by grant of the University of Padova (University Research Project No. 60A10-1194).

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