rus
Issue #3/2014
S.Kudryashov, Y.Kolobov, A.Ligachev
Multi-Scale Femtosecond Laser Texturing And Chemical Modification Of The Surface Of Medical Titanium Implants
Multi-Scale Femtosecond Laser Texturing And Chemical Modification Of The Surface Of Medical Titanium Implants
Femtosecond laser texturing (FLT) of the surface of medical titanium alloys allows considerable modification of wettability, corrosive characteristics, biocompatibility and bioactivity of materials. The article reviews the results of original research demonstrated on the scales of 0.06µm to 30µm and higher.
Теги: ablation surface processing medical titanium materials multi-scale texturing ultra-short pulse lasers абляционная обработка поверхности лазеры ультракоротких импульсов мультимасштабная текстура титановые материалы для медицины
Besides volumetric mechanical and chemical characteristics, surface topographic characteristics (relief texture) have great significance for the titanium materials used for medical purposes. They considerably influence on the wettability, corrosive characteristics, biocompatibility and bioactivity of materials [1, 2]. Nano-scale and micro-scale textures, which describe the surface energy/wettability [3] and biocompatibility of titanium surface respectively, are of great interest for the researchers. As it is known, technologies for the formation of such multi-scale textures are based on the use of the several methods including plasma and ion-plasma spraying, impact of charged particle beams, different abrasive treatments with different scales of abrasive grain etc. [4] as well as laser methods of the surface nano- and micro-structuring [5-10]. At the same time, femtosecond laser texturing (FLT) of titanium and its alloys with the removal (ablation) of the material gives ample opportunities for the prototyping of prospective surface textures for laboratory research [11-16]. Unique characteristics of the femtosecond laser radiation are used for it: high peak power at the moderate energy of ultrashort laser pulses (USP) due to their ultrashort duration. It offers important experimental opportunities:
Electromagnetic-power adjustment of the material optical characteristics during exciting USP [13, 17-18];
Absorption of the laser radiation energy without the impact of ablation laser torch [19];
Ablation occurs at the times comparable with the time of electron-lattice thermalization [19] resulting in the low depth of target warming up at the moment of the material ablation removal [20] and it is very important for the elimination of annealing of nano- and sub-microcrystalline titanium materials.
Use of the listed opportunities implemented under different conditions of laser action demonstrated interesting results. In this paper representing the overview of the results of original research, results of FLT of the surface of biomedical titanium materials at nano-, submicro-, micro- and multi-scales, which were obtained by the authors and their colleagues, are given. Studies were carried out using the following instruments: 1) standard laboratory femtosecond laser system based on titanium-sapphire active medium (Start-248 М, Avesta-Project: laser generation wavelength – 744 nm, width at half-height – 12 nm, pulse duration at half-height – 110 fs, energy in ТЕМ00-mode – up to 8 mJ, repetition frequency – 10 Hz) (Fig. 1) suitable for the microscopic demonstration of FLT effect, and 2) unique femtosecond laser system with the active medium based on ytterbium-doped fiber (Satsuma, Amplitude Systemes: laser generation wavelength – 1030 nm, width at half-height – 5 nm, pulse duration at half-height – 300 fs, energy in ТЕМ00-mode – up to 10 µJ, repetition frequency – 0-1 MHz) (Fig. 2) having multiply higher mean radiation power and thus intended for the prototyping of prospective textures on the areas of several square centimeters and higher. FLT was performed by the weakly-focused USPs (lens of glass K-8 with the focal distance of 70 mm) under the conditions of scanning of titanium material targets located on motorized three-dimensional translation platform with the target exposure variation (number of USPs which are incident at the point) (Fig. 3).
Sub-100-nanometer (in general case – sub-wave) surface nano-gratings with the abnormal (Fig. 4a) or normal (Fig. 4c, 4d) orientation (marks are parallel or perpendicular to polarization vector) occur under the weak subthreshold (by the density of USP energy) conditions of multi-pulse FLT. And above the threshold of surface structuring (occurrence of gratings) normal gratings on the titanium surface have periods which are greater by an order and equal to 0.45-0.6 µm [13] (Fig. 4b). Abnormal nano-gratings represent marks of hollow nano-spikes or crests (Fig. 1a) [15] as in the case with aluminum [21] occurring as the result of unfinished slabbing ablation [19]. Sub-wave nano-gratings occur due to the efficient excitation of the surface plasmonic resonance [22] (see calculations for aluminum in the paper [23]). It favors the distribution and mutual interference of opposite short-wave and intense surface plasmons [24] with the formation of the stationary longitudinal electromagnetic wave. Effect causes the reduction of grating period in half. Developed nano-scale surface texture results in the increase of the material surface energy and for this reason its wettability is considerably improved; contact angles measured after the plasma cleaning are approximately 10° or lower.
By analogy, sub-wave surface nano-gratings with the normal orientation and period of about 100 nm occur upon multi-pulse FLT of the wet surface of titanium [11] (Fig. 5) and other materials under greatly exceeded superthreshold conditions (by the density of USP energy). It appears that the reason for this also lies in the efficient excitation of the surface plasmon resonance [22]. In this case, generation of nano-gratings of the surface relief proceeds more efficiently due to the nonlinear-optical generation of individual harmonics in the liquid layer and broad-band intense "white" radiation with femtosecond duration (supercontinuum) [23]. Radiation generated by supercontinuum maintains the excitation of the surface plasmonic resonance even upon the considerable variation of titanium optical constants during the exciting USP. High USP peak powers are required for the formation of many light microfilaments with high intensity of electromagnetic radiation in the liquid layer (before the linear focus of USP) and efficient proceeding of nonlinear-optical processes [23]. Super-hydrophilic nano-scale texture formed under these conditions is also developed. It is more evident upon high (Fig. 5a, 5b) than low (Fig. 5c, 5d) exposures. It should be noted that plasma cleaning before the measurements of wettability in situ is not required due to the absence of surface contamination with environmental hydrophobic agents.
On the contrary, quasi-regular arrays of high-contrast micro-cones with the typical distance of about 5 µm occur upon the multi-pulse FLT of dry titanium surface under greatly superthreshold conditions (by the density of USP energy) (Fig. 6). These conclusions are drawn according to the data of Fourier analysis.
Micro-cone vertexes are approximately at the level of initial surface (Fig. 6b) and micro-cones spontaneously grow at the expense of the intense fragmentation inhomogeneous near-field ablation along the cone periphery [16]. It is stimulated by the optical diffraction of USP on the cone when the aspect ratio of cone geometrics (height/diameter of basis) gets within the specified range. As a result, the maximum diameter of cones correlates with the variation of laser radiation wavelength (lower – for UV USP [16]). And ablation of their periphery can penetrate very deeply (Fig. 6d). Besides, the depth of surface layer warming up under the action of USP grows up to several micrometers even under the conditions of intense material ablation with its removal (Fig. 6c). But due to the short pulse duration the penetration depth turns out to be submicron, about 0.3 µm, which is very important for the retention of "volumetric" nano-crystalline or sub-microcrystalline structure of the surface layer of titanium material because the unique mechanical properties of titanium material are connected with this structure [1, 4].
In cases when it is necessary to ensure super-diffraction scales of surface textures, multi-pulse FLT is performed by hard-focused USPs under the conditions of laser one- or two-dimensional "milling" of grooves sequence (Fig. 7). Upon such technology the focusing parameters determine the size of groove and focus (or sample) transition determines the period of their repetition. Efficient removal of the material with the rate of about 0.1 µm/pulse during the surface scanning occurs within the framework of the mechanism of fragmentation ablation through the hydrodynamic dispersion of material supercritical fluid [19].
Finally, there are modes at which it is possible to realize such conditions when the surface will be only textured or together with the textured structure it will contain loosely coupled particles. These conditions can be created if optically-partially-transparent layer of nano-crystalline hydroxilapatite (HAP) is previously applied on the surface and then focusing the laser beam on the titanium surface the density of USP energy is varied. The surface can be only textured (Fig. 8a) or have partially or almost completely applied layer of HAP (Fig. 6b, 6c) on the texture, or only certain nanoparticles of HAP loosely coupled with the textured surface (Fig. 8d) [14]. The surface content can be determined on the basis of the results of energy dispersive X-ray spectroscopy. Taking into account the achievements of overlay laser welding technology and tangible printing, the technology of HAP application can be considerably improved.
Thus, current opportunities of the technology of ablation FLT allow the formation of multi-scale surface textures on the surfaces of medical titanium materials keeping the crystalline structure of the surface layer, which becomes deeper, even under the conditions of intense considerable ablation deepening of the material as well as overlay welding of nanocrystalline particles of hydroxilapatite for the purpose of the surface biocompatibility increase.
Authors are grateful to the workers of the Lebedev Physical Institute of the Russian Academy of Sciences and Belgorod State National Research University who provided the material for the illustrations given in this overview.
The work was executed with the financial support from the Ministry of Education and Science of the Russian Federation (project No. 02.G25.31.0103) and Russian Foundation for Fundamental Research (No. 13-02-01107). ■
Electromagnetic-power adjustment of the material optical characteristics during exciting USP [13, 17-18];
Absorption of the laser radiation energy without the impact of ablation laser torch [19];
Ablation occurs at the times comparable with the time of electron-lattice thermalization [19] resulting in the low depth of target warming up at the moment of the material ablation removal [20] and it is very important for the elimination of annealing of nano- and sub-microcrystalline titanium materials.
Use of the listed opportunities implemented under different conditions of laser action demonstrated interesting results. In this paper representing the overview of the results of original research, results of FLT of the surface of biomedical titanium materials at nano-, submicro-, micro- and multi-scales, which were obtained by the authors and their colleagues, are given. Studies were carried out using the following instruments: 1) standard laboratory femtosecond laser system based on titanium-sapphire active medium (Start-248 М, Avesta-Project: laser generation wavelength – 744 nm, width at half-height – 12 nm, pulse duration at half-height – 110 fs, energy in ТЕМ00-mode – up to 8 mJ, repetition frequency – 10 Hz) (Fig. 1) suitable for the microscopic demonstration of FLT effect, and 2) unique femtosecond laser system with the active medium based on ytterbium-doped fiber (Satsuma, Amplitude Systemes: laser generation wavelength – 1030 nm, width at half-height – 5 nm, pulse duration at half-height – 300 fs, energy in ТЕМ00-mode – up to 10 µJ, repetition frequency – 0-1 MHz) (Fig. 2) having multiply higher mean radiation power and thus intended for the prototyping of prospective textures on the areas of several square centimeters and higher. FLT was performed by the weakly-focused USPs (lens of glass K-8 with the focal distance of 70 mm) under the conditions of scanning of titanium material targets located on motorized three-dimensional translation platform with the target exposure variation (number of USPs which are incident at the point) (Fig. 3).
Sub-100-nanometer (in general case – sub-wave) surface nano-gratings with the abnormal (Fig. 4a) or normal (Fig. 4c, 4d) orientation (marks are parallel or perpendicular to polarization vector) occur under the weak subthreshold (by the density of USP energy) conditions of multi-pulse FLT. And above the threshold of surface structuring (occurrence of gratings) normal gratings on the titanium surface have periods which are greater by an order and equal to 0.45-0.6 µm [13] (Fig. 4b). Abnormal nano-gratings represent marks of hollow nano-spikes or crests (Fig. 1a) [15] as in the case with aluminum [21] occurring as the result of unfinished slabbing ablation [19]. Sub-wave nano-gratings occur due to the efficient excitation of the surface plasmonic resonance [22] (see calculations for aluminum in the paper [23]). It favors the distribution and mutual interference of opposite short-wave and intense surface plasmons [24] with the formation of the stationary longitudinal electromagnetic wave. Effect causes the reduction of grating period in half. Developed nano-scale surface texture results in the increase of the material surface energy and for this reason its wettability is considerably improved; contact angles measured after the plasma cleaning are approximately 10° or lower.
By analogy, sub-wave surface nano-gratings with the normal orientation and period of about 100 nm occur upon multi-pulse FLT of the wet surface of titanium [11] (Fig. 5) and other materials under greatly exceeded superthreshold conditions (by the density of USP energy). It appears that the reason for this also lies in the efficient excitation of the surface plasmon resonance [22]. In this case, generation of nano-gratings of the surface relief proceeds more efficiently due to the nonlinear-optical generation of individual harmonics in the liquid layer and broad-band intense "white" radiation with femtosecond duration (supercontinuum) [23]. Radiation generated by supercontinuum maintains the excitation of the surface plasmonic resonance even upon the considerable variation of titanium optical constants during the exciting USP. High USP peak powers are required for the formation of many light microfilaments with high intensity of electromagnetic radiation in the liquid layer (before the linear focus of USP) and efficient proceeding of nonlinear-optical processes [23]. Super-hydrophilic nano-scale texture formed under these conditions is also developed. It is more evident upon high (Fig. 5a, 5b) than low (Fig. 5c, 5d) exposures. It should be noted that plasma cleaning before the measurements of wettability in situ is not required due to the absence of surface contamination with environmental hydrophobic agents.
On the contrary, quasi-regular arrays of high-contrast micro-cones with the typical distance of about 5 µm occur upon the multi-pulse FLT of dry titanium surface under greatly superthreshold conditions (by the density of USP energy) (Fig. 6). These conclusions are drawn according to the data of Fourier analysis.
Micro-cone vertexes are approximately at the level of initial surface (Fig. 6b) and micro-cones spontaneously grow at the expense of the intense fragmentation inhomogeneous near-field ablation along the cone periphery [16]. It is stimulated by the optical diffraction of USP on the cone when the aspect ratio of cone geometrics (height/diameter of basis) gets within the specified range. As a result, the maximum diameter of cones correlates with the variation of laser radiation wavelength (lower – for UV USP [16]). And ablation of their periphery can penetrate very deeply (Fig. 6d). Besides, the depth of surface layer warming up under the action of USP grows up to several micrometers even under the conditions of intense material ablation with its removal (Fig. 6c). But due to the short pulse duration the penetration depth turns out to be submicron, about 0.3 µm, which is very important for the retention of "volumetric" nano-crystalline or sub-microcrystalline structure of the surface layer of titanium material because the unique mechanical properties of titanium material are connected with this structure [1, 4].
In cases when it is necessary to ensure super-diffraction scales of surface textures, multi-pulse FLT is performed by hard-focused USPs under the conditions of laser one- or two-dimensional "milling" of grooves sequence (Fig. 7). Upon such technology the focusing parameters determine the size of groove and focus (or sample) transition determines the period of their repetition. Efficient removal of the material with the rate of about 0.1 µm/pulse during the surface scanning occurs within the framework of the mechanism of fragmentation ablation through the hydrodynamic dispersion of material supercritical fluid [19].
Finally, there are modes at which it is possible to realize such conditions when the surface will be only textured or together with the textured structure it will contain loosely coupled particles. These conditions can be created if optically-partially-transparent layer of nano-crystalline hydroxilapatite (HAP) is previously applied on the surface and then focusing the laser beam on the titanium surface the density of USP energy is varied. The surface can be only textured (Fig. 8a) or have partially or almost completely applied layer of HAP (Fig. 6b, 6c) on the texture, or only certain nanoparticles of HAP loosely coupled with the textured surface (Fig. 8d) [14]. The surface content can be determined on the basis of the results of energy dispersive X-ray spectroscopy. Taking into account the achievements of overlay laser welding technology and tangible printing, the technology of HAP application can be considerably improved.
Thus, current opportunities of the technology of ablation FLT allow the formation of multi-scale surface textures on the surfaces of medical titanium materials keeping the crystalline structure of the surface layer, which becomes deeper, even under the conditions of intense considerable ablation deepening of the material as well as overlay welding of nanocrystalline particles of hydroxilapatite for the purpose of the surface biocompatibility increase.
Authors are grateful to the workers of the Lebedev Physical Institute of the Russian Academy of Sciences and Belgorod State National Research University who provided the material for the illustrations given in this overview.
The work was executed with the financial support from the Ministry of Education and Science of the Russian Federation (project No. 02.G25.31.0103) and Russian Foundation for Fundamental Research (No. 13-02-01107). ■
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