In recent years, with the increasing aging of the social population and the rapid increase in the number of orthopedic patients, researchers have developed many orthopedic medical implants. Zirconium-based amorphous alloy is regarded as a new potential material in medical orthopedic implant surgery for its non-degradability and high corrosion resistance and wear resistance. Since the zirconium-based amorphous alloy is a non-biological material, it requires surface-active treatment before surgery when it is used as a medical implant. We can prepare hydroxyapatite (HA) coating on the surface of the implant to improve its biological activity. Studies have shown that the surface of human bone is covered by microgrooves with a width of 10-100 μm and finer nanostructures (such as grooves with a width and depth of several hundred nanometers, pits, and nanoparticles, etc.). Therefore, the researchers use the surface modification technique to prepare similar micro-nano structures on the implant surface to provide a suitable external environment for HA deposition. Traditional surface processing techniques such as machining, sandblasting, and acid etching have been widely used in common metal implant materials. However, when the techniques are used to process the material surface, they have great randomness and low accuracy, making it impossible to accurately prepare the expected micro-nano structure. Because of its non-contact processing and energy-controllable merits, laser surface processing technology can overcome these problems, and it has been widely used to prepare micro-nano structures such as grooves and pores on the surface of metal materials.
In this paper, a nanosecond laser and femtosecond laser composite processing method for efficiently processing micro-nano structures on the surface of metal implants is proposed. The micro-nano structures of the natural bone surface are prepared on the Zr55Cu30Ni5Al10 surface through laser composite processing technology. A short pulse nanosecond laser is used to fabricate microgroove structures with a width of tens of micrometers on the material surface, which can provide more room for the deposition of HA on the surface of the implant. The finer nanostructures are then prepared using acid etching and ultra-short pulse femtosecond laser processing, respectively. The finer nanostructures can accelerate the agglomeration of HA crystal nuclei and enhance the deposition speed of HA. The experiments are divided into four groups: unprocessed samples, nanosecond laser processed samples, nanosecond laser and acid etching treated samples, and nanosecond and femtosecond laser processed samples. To compare wetting properties, the surface contact angles of samples treated with different surface modification methods are measured. Different samples are immersed in simulated body fluid to deposit HA on the surface, and their deposition speed and mass of HA are compared.
The morphology and water contact angle of the sample surface under different processing methods are shown in Fig.3. The water contact angle of an unprocessed sample is 62°. When the scanning time, scanning speed, laser pulse energy, pulse width, and repetition frequency of a nanosecond laser are 1, 100 mm/s, 0.12 mJ, 10 ns, and 150 kHz, respectively, the width and depth of the fabricated microgroove structure meet the requirements, and the contact angle is reduced to 53°. The water contact angle between the nanosecond laser and the acid-etched sample is 47°. When the scanning time, scanning speed, laser power, and repetition frequency of a femtosecond laser are 5, 20 mm/s, 100 mW, and 1 kHZ, respectively, the water contact angle is 26°. We can find that the hydrophilicity of the sample processed by the nanosecond laser and femtosecond laser is greatly improved. Four different samples are immersed in simulated body fluids, dried, and weighed every 3 days. Measurement of the mass growth of the different samples is shown in Fig. 4. We can find that samples processed by the nanosecond and femtosecond laser have the highest mass growth of HA throughout the soaking period. The distribution of the Ca elements can characterize the distribution of HA on the sample surface. Figure 5 shows the surface morphology of different samples and the distribution of Ca elements on the sample surface. We can find that the Ca element on the surface of the sample processed by the nanosecond laser and femtosecond laser is distributed uniformly.
A method of nanosecond laser and femtosecond laser composite processing of zirconium-based amorphous alloy is proposed in this paper. The wettability and HA deposition characteristics of Zr55Cu30Ni5Al10 samples treated by different surface modification methods are studied. Compared with the results of these four groups of experiments, the processing effect of the nanosecond and femtosecond laser composite processing methods is the most excellent. Since the Zr55Cu30Ni5Al10 is intrinsically hydrophilic, in the conditions of nanosecond and femtosecond laser composite processing, the roughness of the sample surface increases, and the hydrophilicity increases accordingly. The composite processing method of the nanosecond laser and femtosecond laser not only improves the hydrophilicity of the Zr55Cu30Ni5Al10 surface but also greatly promotes the deposition speed and mass of HA on the surface.
