Russian scientists from the Lebedev Physical Institute (LPI) have proposed a new method for titanium surface modification, giving it the unique optical and mechanical properties, as well as improving the biocompatibility of the sample. Implants made of the nanocrystalline titanium with the new biocompatible coating are currently in preclinical testing.
The proposed modifications of the titanium coating production consist of irradiating the surface of the nanocrystalline titanium using femtosecond laser, causing the formation of periodic structures with dimension scale ranging from submicron to micron.
The accomplished studies showed that it is possible to control the sizes of occurring structures, and hence the properties of the material, by varying the laser emission parameters, while maintaining the unique mechanical properties of nanocrystalline material.
“We decided to explore this area together with our colleagues from the Center for Nanostructured Materials and Nanotechnology of the Belgorod State University, led by professor Yuri Kolobov. His staff has successfully studied the creation and practical applications of nanocrystalline titanium, which is attractive for its unique mechanical properties – high strength and super-plasticity. However, nano- and microtexture surfaces, which create an “incubator” for osteoblast cells and thus improve the biocompatibility, are required for biomedical applications – particularly for the surface modification of titanium implants”, said Dr. Andrey Ionin, head of the Gas Lasers Laboratory at the LPI.
Initially, it was very difficult to create the surface-based nano- and microtexture while maintaining the inherent structure of nanocrystalline material using the conventional treatment. For example, plasma etching or annealing techniques heat up a pretty thick surface layer – the sintering or fusion of the nanocrystals occurs and consequently microcrystals are formed, which greatly affect the mechanical properties of titanium. But when the sample is exposed to ultra-short laser pulses, only a thin surface layer is heated to high temperatures due to the effect of high beam power amplitude. After that, the energy begins to spread deeper into material and the temperature drops, therefore there are no undesirable sintering of nanocrystals.
The resulting nanostructured surface is a well-defined one-dimensional lattice with a characteristic step of 70-600 nm. Thus, when the energy density of the laser radiation is 17 mJ/cm2, the pulse series of 500 pulses form a sequence of narrow grooves (thickness about 100 nm) spaced apart by 400 nm in average on the surface of a titanium target. The scientists explain the emergence of such quasi-periodic structures by the impact of interference between electric field of the incident radiation and the field of surface electromagnetic waves that are generated by the femtosecond laser pulses.
The width of the grooves increases by increasing the energy density and the relief grid becomes well-defined and almost harmonic. However, the further increase of the laser energy density results in almost full disappearance of this lattice, and the visibly expressed origins of microcones gradually appear in the surface. The entire surface is covered with an array of such microcones during the prolonged or high-intensity exposure, giving the surface virtually 100% absorption capacity. Thus, it is possible to control the morphology of the surface of the nanocrystalline titanium by changing various parameters of laser radiation – the energy density, the radiation time, pulse duration and their number – and using the proposed method.
The surface of the nanocrystalline titanium with nano- and microscale relief retains increased strength and ductility – parameters that are typical to the volumetric material, and also acquires high hydrophilicity necessary for its biocompatibility. Therefore, this morphology of the titanium surface makes it an ideal candidate for the development of new implants, which are currently undergoing pre-clinical trials at the P.A. Hertsen Research Institute of Oncology in Moscow.
“Dozens of works on the applications of nanocrystalline titanium have been published over the last five years. It is possible to make a plate for the skull, dental implants, different rod-shaped constructions using this material. Another very interesting application is the production of microsurgical instruments with cutting edges made from the nanotitanium. The nanocrystalline titanium production facilities are now appearing all over the world. Russia has advantages in this area associated with a well-developed production of titanium. The main advantage of the new method – it is cheaper than the current production technology. Also, a high-tech processing of nanocrystalline titanium is possible”, commented Sergey Kudryashov, the co-author of the development.