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Applications

aser-induced novel patterns: As smart strain actuators for new-age dental implant surfaces

03/08/2016  |  Tags: dental implant surfaces, laser,
Surfacemorphologiesoftitaniumimplantsareofcrucialimportanceforlong-termmechanicaladaptation for following implantation. One major problem is the stress shielding effect which originates from the mismatch of the bone and the implant elasticity. It is time for a paradigm shift and for an exploration of novel smart surfaces to prevent this problem. Several surface treatment methods have traditionally been used to modify the surface morphology of titanium dental implants. The laser micro-machining can be considered as a unique and promising, non-contact, no media, contamination free, and flexible treatmentmethodformodifyingsurfacepropertiesofmaterialsinthebiomedicalindustry.Theaimofthe present study is two folds; to develop novel 3D smart surfaces which can be acted as strain actuators by nanosecondlaserpulseenergiesandirradiationstrategies.Andanalyzethesesmartsurfacemorphologies using finite element methods in order to estimate their internal stiffness values which play a great role on stress shielding effect. Novel 3D smart strain actuators were prepared using an ytterbium fiber laser with 200–250ns pulse durations on commercial pure titanium dental implant material specimen surfaces and optimum operation parameters were suggested.
1. Introduction
The main biomedical uses of titanium and its alloys are in bone-contact applications, i.e. orthopedics and in implant dentistry.Titanium is to be found to be well tolerated and nearly an inert material in the human body environment. Osseointegration is the vital process which occurs during bone healing and formation of new bone and it is defined as “a direct structural and functional connection between ordered, living bone and the surface of the load carrying implant”. So the osseointegration progress is a function of surface characteristics and cell contact on the implant material. There are two main factors which govern osseointegra-tion; they are physico-chemical properties and micro morphology of the implant. Micro morphology is a crucial characteristic of an implant. The term micro morphology refers to the irregularities of the surface that are smaller than 100. Surface micro morphology could be categorized under a couple of titles as “roughness” that is composed of irregular discontinuities and as “texture” that is composed of regular patterns. Ellingsen suggested that surface roughness could be classified in three levels as microstructural, macrostructural, and ultrastructural.Besides, factors that are related to the design of the implant have an influence on bone-implant anchorage. These factors that have an effect on bone healing are, overall surface area of the implant,implant length and configuration of the threads. Hence, increasing surface area is an important factor for rapid osseointegration. There are also some clinical requirements such as good oral hygiene, excellent blood supply (i.e. non-smokers), etc.In recent studies, it is also pointed out that surface topog-raphy has crucial importance for micro-mechanical interlocking between bone and implant. Mechanical interlocking that is based on micro-porous surface structures is an important criterion for osseointegration. The mechanical properties of bone-implant interface and stress distribution with mechanical interlocking of the interface have influence at the macroscopic level – on higher surface roughness of 10. It has been emphasized by Wen-nerberg et al. that a high surface roughness value is not the only criterion for micro-mechanical interlocking and the pattern of the roughness, the size and the distribution of regular patterns have also significant influences on micro-mechanical interlocking of the bone-implant interface There are several conventional manufacturing methods that have been utilized to modify the surface micro-morphology for an enhanced mechanical retention between the implant and bone.These conventional methods include photolithography, sandblast-ing, electron beam texturing, ion beam, plasma spraying, etching,anodizing and laser texturing, etc. Although sandblasting is a very popular method, it has unavoidable disadvantages like toxic Al and Si contamination in the peri-implant tissue; and depending on the typeoftheblastingmedia(e.g.powderblastingprocess)thesurfaceroughness could be random owing to the brittle nature of ceramics.
Although different types of blasting media such as biphasic calcium phosphate (BCP) and TiO2(rutile) have been developed, they cause distortions and unfavorable geometrical structures on the surface of implants. The remaining conventional methods also generate anisotropic surface structures. Considering the above mentioned limitations, laser surface micro-machining is a promising method for manufacturing reproducible, more isotropic, distinct and reg-ular surface structures and textures. Furthermore, laser surface structuring has several advantages such as the complete removal of surface contamination by laser treatment, precise control of com-plicated features in a fast and clean process, and the fact that no force is applied to the work piece. Recent investigations have been emphasized that laser energy can produce rough implant surfaces free from contamination.

2. Materials and methods
Bach et al. emphasized that the “crucial  requirements”on workpiece surfaces should be determined by the particular load conditions under the final product operates. An implant placed in bone encounters harsh load conditions. The surrounding bone acts by changing its local density. When the system load is car-ried with an implant, stress shielding of bone occurs. The concept of stress shielding was a complex phenomenon and it relates to bone resorption. The stiffness of the implant can be considered as the most important factor. The stress transition is deteri-orated because of stiffness mismatch between bone and implant.
At this point, immediate loading is accommodated and adequate prestressing must be provided through smart structures. So there is a great necessity to develop “working surfaces”. The feature of biomedical industry will belong to novel implant designs that use smart surfaces instead of anisotropic irregularities. Any smart sur-face system, however well designed, that attempts to manage mechanical responses of implants will always be in a state of devel-opment with investigations constantly being continued as a result of feedback from the various scientists. Thus unforeseen interfa-cial problems can be solved. In the light of this unique knowledge,we have to make that careful planning at all design stages and involvement of as many different surface patterns as possible is essential. Without this essential step, a complex, difficult produc-tion becomes unmanageable. So distinctive micro-patterns were carefully designed to prevent stress shielding effect. Then we pro-duced these novel micro-patterns on 2mm thick 10mm×10mm
square shaped ASTM B265 grade 2 commercial pure titanium spec-imen surfaces. Alpha alloys, especially CP titanium, are used for a variety of biomedical applications where corrosion resistance is important. All specimens were moulded in acrylic resin. Grinding was carried out under water irrigation using 320, 600, 1200 grit waterproof silicon carbide abrasive papers followed by polishingm diamond paste on a medium nap rayon cloth.
Total twenty specimens were used, they were mounted tightly and controlled in order to prevent misplacement. The laser micro-machining system consists of a 20W ytterbium fiber laser source manufactured by Telesis Zenith (Ohio, USA), beam focusing optics and x–ymotorizedtranslationstage.
demonstratestheexperi-mentalprocess.BeinganactivelyQ-switchedfiberlaserat1064nm,the pulse repetition rate ranges from single pulse to 125kHz. Com-plex 3D patterns can be fabricated using predetermined graphical image packages. At virtually first stage of the machining process,2D digital patterns were designed carefully as honeycomb, diag-onal, spherical, etc. using lines, splines and dots. They were used to adjust the beam transition pathways thus the original specimen was ready to shape on to the next stage. They help to ensure stan-dardization by establishing the same process conditions on all next specimens. Digital images were imported in a graphics package via Merlin software. Honeycomb pattern was manipulated in two dif-ferent shapes, which have inner and outer channels using rastering mode via a galvo system.
In this system, the laser is steered by mirrors and each mirror move along on an axis mounted onto galvo motors to produce these novel structures. These galvos move rapidly and therefore can form every complex structure at high speed. The laser scanning speed is noteworthy effect on the fabri-cation process because it changes vaporization time. Also working at rastering mode, one advantage is that the entire power of the laser beam is used for direct writing. Polished pure titanium spec-imens were micro-machined by single and multiple, 200–250ns pulses. The laser pulse energy used, ranged from 1.6mJ to 1.8mJ and the laser scan speed values were in the range of 50–150mm/s.
With anf=160mmf-theta lens, a beam waist diameter of 27m was obtained, yielding power densities up to 13.92×108W/cm2.The resulting surface patterns were imaged by optical and scanning electron microscopy.
3. Results
We produced novel regular 3D surface patterns which can be acted as smart strain actuators during the biomechanical loading on commercially pure titanium dental implant material spec-imens’ surfaces using ytterbium fiber laser irradiation in the range of 150–250ns pulse durations, with single and multiple passes. Morphological characterization was performed using opti-cal microscopy and scanning electron microscopy for implant surfaces. The samples were examined with a scanning electron microscope (JSM-6060 JEOL) working at secondary electron mode.
In order to characterize the regular geometries, microphotographs of each regular periodic pattern were taken at 250×, 400×and1000×magnifications. Three different geometries were carefully designed in order to increase the effective surface area for bone-implant contact. Each pattern has distinct features and named as inner honeycomb pattern, outer honeycomb pattern diagonal pat-tern, and spherical pattern. Also the SEM micrograph of our spherical and novel moon pattern can be seen in 4 Measured sizes and other dimensional details of our laser micro machined novel patterns can be seen in Fig. 5. The new morpholo-gies were characterized by periodical micro-features. Honeycomb pattern had 49–116m inner and outer edge length respectively and 200m distance between the centers of the each feature.Spherical pattern can be characterized with semi-spheres which had a diameter within the range of 68–70m. Dimensional details of diagonal and moon patterns were given at the same sketch.Average depth for honeycomb, diagonal and spherical patterns was measured about 20 and also average height for moon pattern was observed about 10um.