NANOSECOND FIBER LASER MICRO-TEXTURING OF TITANIUM SURFACE FOR BIOMEDICAL APPLICATIONS

M502

Habib Abou Saleh#1, Ehsan Toyserkani*2, Fathy Ismail#3

#1*2#3Department of Mechanical and Mechatronics Engineering, University of Waterloo200 University Avenue West, Waterloo, N2L 3G1, Ontario, Canada

#1habousal@uwaterloo.ca*2etoyserk@uwaterloo.ca

#3fmismail@mecheng1.uwaterloo.ca

Abstract

This paper investigates the feasibility of generating micro-self-assembled structures on pure titanium using a nanosecond Ytterbium fiber laser. The effect of process parameters, including laser frequency, power, processing speed and spot size, on the induction of the micro-self-assembled structures isinvestigated. Scanning Electron Microscope (SEM) and Profilometry analyses are carried out to demonstrate the size, shape, and roughness of the generated micro-structures. Analysis of the experimental results suggests that the generation of self-assembled structures with a desired roughness is viable. It is also observed that the laser spot size can potentially control the local surface roughness when the other process parameters are fixed.

Keywords - Laser micro-texturing, Ytterbium fiber laser, Micro-self-assembled structures.

Introduction

The understanding of surface modification of materials with laser beams along with the investigation of the underlying principles of laser radiation interaction with the ablated surfaces are essential to realize industrial and medical applications of laser beams [1, 2]. Laser surface modification of titanium and its alloys are of great interest [1, 2, 3]. Titanium has been widely used in biomedical applications due to its bio-stability, biocompatibility, light weight, high mechanical strength and long term durability [1, 2, 3]. Surface topography has a great influence on the implant performance during in vivo and in vitro studies [1, 3]. The micro structures generated at the surface of titanium influence the biological processes at the implant interface after the interaction with the biological environment [1, 3]. To generate desired microstructures on the surface of titanium, several

methods have been employed and studied such as grit-blasting, chemical etching, titanium plasma spray, electrochemical treatment as well ascombinations of these methods [1, 3]. These methods generate microstructures with different irregular patterns, with varying forms and diameters of structural elements [4]. Osteoinductive qualities and mechanical stability is determined by the degree of contamination of the titanium surface. Considerable amounts of contamination with numerous foreign elements were found on the processed surfaces using the above methods [4]. Such surfaces can lead to scar tissue formation due to the development of random bone cell orientations and an increase in cytotoxic concentration elements at the implant surface which will lead eventually to biological rejection of the implant, and thus implant failure [1]. Recent studies have shown that laser processing of implanted surfaces are of a great importance since they provide uniformly distributed surface topography with uniform patterns and less surface contamination when compared with other methods [3, 4]. Microstructures have been produced using long-pulse lasers, including nanosecond Nd:YAG laser, copper vapor laser, nanosecond excimer lasers, picosecond Nd:YAG laser, and sub-picosecond excimer laser [2, 3, 5, 6, 7, 8, 9]. The laser patterned surfaces were found suitable for cell adhesion, proliferation and spreading, but none of the studies have dealt with generating self-assembled micro-patterns using nanosecond Ytterbium fiber laser, and its interaction with Titanium surfaces.

In this paper, the feasibility of generating self-assembled structures on pure titanium samples using a nanosecond Ytterbium fiber laser is investigated. Titanium samples were laser irradiated at different laser process parameters which include laser frequency, laser power, interaction time and spot size. The process parameters that result in the local surface roughness ranging from 0.9 to 2 µm (which is

suitable for osteoblasts tissue integration) are revealed and the deviation of surface roughness vs. laser spot size is explained.

Materials and Fabrication Procedures

Titanium Samples Preparation

Titanium specimens, 12mm x 10mm x 10mm werecut from titanium plates. The specimens were castinto a baked sample casting using Phenolic powder. After casting, the samples were ground using silicon carbide (SiC) papers. SiC papers of number 320, 400, 600, and 1200 were used with an average grain size of 32 μm, 22 μm, 15 μm, 5 μm, respectively for fine grinding. Specimens were cleaned with water and alcohol and dried out after the use of each SiC paper to eliminate any residuals on the surface of the specimens.

Laser Workstation

The titanium specimens were irradiated by nanosecond Ytterbium fiber laser (YLR-1/100/20/20, IPG Photonics, MA, USA), generating a laser beam wavelength of 1062 nm with pulse duration of 100 ns, pulse energy of 1 mJ, and maximum power of 24watts. A NIR 10X laser head lens (Edmund Optics, USA) with a focal length of 20 mm was used to focus the laser beam. The focal spot size was estimated at4.5 microns. The specimens were mounted on the controlled X-Y translational stages, Heavy Load, High Torque Stages # 55-788 by (Edmund Optics, USA) under the scan head (Fig.1). The depth of focus was 4.1 μm. The laser system in Fig.1 was controlled through a code developed in MATLAB, meanwhile the X-Y stages are controlled through a controller software. The X-Y stages have a travel distance of 50 mm with an accuracy of ±1.0 μm per 25 mm of travel. The operational frequency range of the laser was between 20 KHz - 80 KHz. The laser system was operated at a room temperature of 25 0 C.

Figure 1. Laser Workstation.

Experimental Procedures

Experiments were carried out with laser power level varying from 10% to 100% of the maximum power of the laser system with frequency range of 20 KHz to 80 KHz at 20 KHz increments. The processing speed of the specimens varied from 1 to 2 mm/s. The spot size diameter was varied from 4.5 to 1262 µmwith an approximate incremental step of 10µm(Fig.2). No surface modification was noticed for spot size diameter greater than 462 µm. A series of laser irradiated titanium specimens along the horizontal direction of the scan field were produced.

Figure 2. Graphic model of the Laser System

Surface Characterization

Scanning Electron Microscope (SEM)

The inspection of the irradiated surfaces was done with the scanning electron microscope (SEM). A LEO 1530 field emission SEM (Zeiss, Cambridge, England) was used to characterize the surface morphology of the laser induced features. The scanned specimens were coated with gold to make sure that the specimens were electrically conductive at least at the surface before SEM can take place. The inspected specimens were cleaned to avoid any contamination of the working environment.

Profilometry Tests

Profilometry was done using WYKO NT1100 optical profiling system (Veeco, AZ, USA) to determine any alteration in the surface elevation through the computer processing of the digital image.

(a) (b)

(c) (d)

(e)Figure 3. SEM Results with X1000 magnification, P = 17 Watts, F= 20 KHz, V = 2 mm/s and spot size diameter of : (a) Dz = 284 µm, (b) Dz = 295 µm, (c) Dz = 389 µm, (d) Dz = 400 µm, (e) Dz = 410 µm.

Figure 4. Profilometry Result of one specimen (# 4), at P = 17 Watts, F = 20 KHz, V = 2 mm/s, Dz = 400 µm.

Results and Discussion

The goal of this study was to investigate the feasibility of producing microstructures with surfaceroughness ranging from 1 µm to 2 µm which enhances osteoblasts tissue integration. Laser irradiated zones produced at a processing speed of 2 mm/s with laser power of 17 Watts, frequency of 20 KHz, and spot size diameter of 284 µm, 295 µm, 389 µm, 400 µm, and 410 µm were selected for detailed visual and surface metrological characterization using SEM and profilometry tests. It should be noted that numerous experiments were conducted; however, the abovementioned parameters have resulted in the generation of microstructures with the desired surface roughness for the specific application. Table 1 lists surface roughness parameters including the average roughness, Rl and root mean square Rq, which are calculated over the measured array, Furthermore, the value of the peak-to-valley was measured and found to be between 10 to 16 µm. The surface roughness parameters were obtained by selecting a subregion of dimension of 75 µm x 75 µm of the entire measured array. Furthermore, the corresponding values for the effective energy are listed in the table. The effective energy (Ep) is calculated as:

Where, E is the laser pulse energy (J); F is laser frequency (Hz); V is the laser processing speed (m/s); and D is the spot size diameter (m).

Fig. 3 shows SEM images at different process parameters. As seen, the size of the self-assembled structures (grains) reduces when the spot size increases. The profilometry result is shown in Fig. 4. The figure also shows the extreme peak-to-valley value in the measured array.

Table1. Specimens Surface Roughness Parameters

Specimens Spot Size Ep Rl Rq

1 284 µm 3.52x107 1.68 µm 2.25µm

2 295 µm 3.38x107 1.32 µm 1.72µm

3 389 µm 2.57x107 1.33 µm 1.73µm

4 400 µm 2.50x107 0.926µm 1.23µm

5 410 µm 2.44x107 0.957µm 1.27µm

The generated surface parameters Rl and Rt can be used to study the effect of surface roughness on the cell integration and adhesion. It has been reported that the osteoblasts tissue integration is favoured with surface roughness ≤ 2 µm, and features depth between 8 µm to 12 µm [1, 7, 10, 11, 12, 13, 14, 15, 16]. According to Ball et al. 2005 [16], osteoblasts tissue integration is favoured with microstructures of micron scale topography. In his study,

osteointegration was noticed on titanium surfaces with surface roughness ranging from 0.05 µm to 1.75 µm. According to our results, osteoblasts tissue integration would occur with our generated microstructures which have a local surface roughness range approximately between 1 µm to 2 µm. As a result, nanosecond Ytterbium fiber laser for titanium micro-texturing is well suited for micron scale topographical surface modifications to influence and increase osteointegration of bone contacting devices.

Surface roughness was found to be inversely proportional to the spot size diameter of the laser beam. A decrease in the surface roughness and surface grain size was noticed at the same time with the increase in the spot size of the laser beam which also coincided with decrease in the laser effective energy. As a future work, we will analyse the role of wide range of the process parameters on the top viewed features size of micro self-assembled structures.

Conclusion

Nanosecond Ytterbium fiber laser micro-texturing of titanium specimens was investigated in this paper In conclusion:

1- Nanosecond Ytterbium fiber laser has the potential to be used for generating self-assembled micro-structures.

2- The roughness ranging from 1 µm to 2 µm was produced at a processing speed of 2 mm/s with 17 Watts laser power, frequency of 20 KHz, and spot size diameter of approximately 284 µm, 295 µm, 389 µm, 400 µm, and 410 µm.

3- A decrease in the surface roughness was identified with an increase in the spot diameter of the laser beam.

Acknowledgments

The authors would like to acknowledge technical help of Negar Rasti and Hamidreza Alemohammad. The authors also acknowledge the financial support provided by NSERC.

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