The reason why magnesium alloy, as an orthopedic implant material, limits its further application in medical clinical practice is the problem of excessive degradation rate. The corrosion process of magnesium alloys in the body releases a large amount of hydrogen gas and causes local microenvironment alkalinity, which can lead to emphysema and inflammation in severe cases. The preparation of bioactive coatings on magnesium alloy surfaces is considered one of the most effective surface modification methods. However, with the deepening of clinical trials, researchers have found that the reliability of the connection between the coating and the substrate is the key to whether the structure can serve for a long time in vivo.

Among various surface treatment methods, the most commonly used chemical conversion method, micro arc oxidation method, and intermediate layer method provide a platform for magnesium alloys to firmly connect with the outermost coating. However, while these methods are used for surface modification of magnesium alloys, there are still many issues, such as the biological safety of the conversion layer or intermediate layer, the brittleness of the micro arc oxidation layer, etc. They are sandwiched between the coating and the substrate, which also increases the risk of interface cracking. The plasma treatment method is often used in medical disinfection and sterilization processes, and the process is simple, low-cost, reliable, and highly reproducible. It can modify magnesium substrates without using a lot of chemical reagents. This provides the possibility of preparing coatings on the surface of magnesium alloys.

Plasma surface activation process

Polish the magnesium alloy sample with sandpaper, clean and dry it, place it on the sample stage of the plasma activation equipment, open the gas cylinder, and turn on the suction pump. Control the gas flow rate to 1.0L/min, discharge power to 200W, and wait for the vacuum degree inside the chamber to reach 40Pa before starting the surface activation process. For different plasma activation processes, a single plasma is used to activate the surface of magnesium alloy samples using oxygen, nitrogen, and argon, respectively; The activation time is set to 30-180 seconds, and the sample is immediately taken out after activation. Different process gas plasma activations are shown in Figure 1.

Figure 1 Plasma activation on the surface of magnesium alloy

The Effect of Single Plasma Activation Treatment on the Wettability of Magnesium Alloy Surface

In the exploration of single plasma surface activation process, the test results of the contact angle size of magnesium alloy surface at different activation times are shown in Figure 2 (a). Perform a plasma activation process on the magnesium alloy substrate for up to 180 seconds, and measure the contact angle of its surface every 30 seconds. As the activation time of single O2 plasma, single N2 plasma, and single Ar plasma increases, the contact angle on the surface of magnesium alloy shows a gradually decreasing trend, indicating that the wettability of magnesium alloy surface activated by single plasma has been improved. From the measurement results, it can be found that after 120 seconds of activation by different single plasmas, the contact angle of the magnesium alloy surface decreases to below 2.0 °, and the contact angle image shows that the droplet morphology on the magnesium alloy surface remains basically horizontal, showing a spreading state. Among these three single plasma activation processes, O2 plasma can rapidly decrease the contact angle of the substrate surface after 30 seconds of activation of magnesium alloy, indicating high activity of O2 plasma. Meanwhile, the optimal activation time of 120 seconds can be determined based on the influence of a single plasma on the surface wettability of magnesium alloys.

Figure 2 Contact angles of magnesium alloy surface under different single plasma activation times

The influence of a single plasma on the surface roughness of magnesium alloys

Figure 3 (a) shows the surface roughness measurement results of magnesium alloy after 120 seconds of single plasma activation. After being activated by single O2, N2, and Ar plasma, the surface roughness of magnesium alloy was improved, and the effect of O2 plasma was the most significant. By extracting the surface characteristic contour lines of the magnesium alloy before and after activation in Figure 3 (b), it was observed in the partially enlarged area of Figure 3 (c) that the untreated magnesium alloy surface contour lines were sharp needle like protrusions. After plasma activation, the raised structure of the scratch disappears due to plasma bombardment etching, and the surface structure uniformity is improved. The sharp part is weakened and becomes more rounded and smooth. At the same time, by using atomic force microscopy to measure the microstructure of the 1.0 μ m × 1.0 μ m area on the surface of the magnesium alloy before and after activation, it can be observed in Figure 3 (c) that the "ridge" morphology of the untreated magnesium alloy surface has changed to a "hill" morphology after single plasma activation. In order to further confirm this microstructural change, the plasma activation treatment time on the surface of magnesium alloy was extended to 10 minutes, and it was found that this phenomenon on the surface was more uniform and obvious. These nano convex spheres were caused by ion collisions. Ionic collisions accelerate the diffusion of surface metal ions and facilitate the formation of nanoscale surface structures. As shown in Figure 3 (d). This indicates that plasma has varying degrees of influence on the surface roughness and microstructure of magnesium alloys. According to the Wenzel model, when the surface of a solid sample is hydrophilic, the rough structure of the surface will make the originally hydrophilic surface even more hydrophilic. Therefore, the increase in surface roughness of magnesium alloys after plasma activation treatment is also beneficial for the subsequent infiltration of coating solutions on the substrate surface.

Figure 3: Surface roughness and microstructure of magnesium alloy before and after single plasma activation (a) roughness; (b) Contour line; (c) Microstructure; (d) Microstructure activated for 10 minutes