Three-dimensional Mesh-assisted Plasma Immersion Technology for Enhancement of Corrosion Resistance and Antimicrobial Properties of NiTi-based Orthopedic Devices

Plasma surface engineering has many applications in microelectronics, metallurgy, and biomedical engineering. In plasma immersion ion implantation and deposition (PIII&D), the energetic plasma treatment produces synergistic effects such as ion mixing as well as those inherent to conventional coating technologies. The technique is thus very useful and commercially viable for a variety of biomedical products including orthopedic implants. However, in spite of recent technological advances, conventional PIII&D suffers from some intrinsic limitations such as ion fluence non-uniformity for specimens with a complex geometry because the expanding plasma sheath cannot accurately mimic the sample surface topography. As a result, the ion incident angles vary across the surface leading to laterally non-uniform ion penetration depths and retained doses. In addition, owing to plasma extinction at high sample bias, the modified layer is typically quite thin and may not be adequate for biomedical implants that encounter extensive fretting and abrasive wear. With regard to biomedical implants with complex surfaces such as bone clamps and fixation devices used in the surgical treatment of multiple and complex bone fractures, there are likely shadowed areas that receive inadequate plasma treatment if conventional plasma immersion techniques are adopted. These drawbacks have thus hampered more widespread applications of PIII&D to many biomedical components. In this project, we aim at developing advanced plasma immersion technologies suitable for orthopedic implants with a complex shape. We will focus on biomedical nickel titanium shape memory alloys which are superior to traditional metals such as stainless steels and titanium alloys in orthopedic applications, and the proper surface modification can drastically enhance their mechanical and biological properties. Our strategy is to modify the shape of the plasma sheath by enshrouding the specimens with a 3-dimensional cage and metallic mesh designed with a geometry similar to the sample. In this way, expansion of the plasma sheath is stopped by the grounded cage when negative high voltage pulses are applied to the sample and more uniform and conformal surface treatment can be conducted through the more conformal mesh. We will first theoretically investigate the plasma sheath dynamics using particle-in-cell and fluids models under different instrumental conditions such as plasma density, gas pressure, and sample voltage. After gaining a better fundamental understanding of the plasma physics and plasma-materials interactions, we will conduct experiments using the optimized parameters on bone fixation devices to verify the process efficacy and produce biomedical implants with better mechanical and antimicrobial properties. Here, in addition to mitigating the leaching of toxic nickel from the materials, additional plasma treatment will be performed to improve the surface antibacterial properties in order to minimize post-surgical infection and repeated surgeries. The various surface characteristics including surface composition, hardness, corrosion resistance, barrier properties, as well as pertinent biological properties such as bacteria adhesion and proliferation will be determined, and the results will be used iteratively to optimize the experimental protocols. In corporation with our industrial partner which will provide us with some of the commercial bone fixation implants used in this project, we expect to patent and commercialize the process after project completion. It should also be noted that the fundamental understanding and experimental protocols developed from this project can be easily extended to other types of biomedical implants. The objective of this proposal is to develop new plasma immersion techniques to cater to the more stringent requirements demanded by the orthopedic community.