The medical devices are mostly made of metals, which suffer from corrosion, wear, and rejection during their service in the human body. However, the rapidly growing biomedical industry benefits from the progress in material research, and the mentioned drawbacks can be addressed by a suitable coating. Physical vapour deposition (PVD) is a powerful technique to modify surface properties, provide additional features such as antibacterial activity, and extend the lifetime of medical devices. According to the form and duration of contact with the human body, medical devices can be classified into:
- surgical devices
- implantable medical devices
- interventional medical devices
Surgical devices are made from various type of austenitic and martensitic stainless steel and the material should meet the following requirements: high wear and corrosion resistance, low friction coefficient, resistance to various cleaning process, antibacterial properties, and last but not least low light reflection. Implanted medical devices are commonly made from titanium alloy, cobalt-chromium alloy, nickel-titanium alloy, stainless steel, or magnesium alloy. Long-term retention in the human body requires high resistance to body fluid corrosion, good biocompatibility, and low ion release rate. Coatings for interventional medical devices are required to have low friction, anticoagulant, and sterilization properties.
PVD technologies use various physical methods to vaporise the target material which is then deposited on the surface of the coated substrate. Currently the most used PVD technologies are vacuum evaporation, ion plating, magnetron sputtering, and arc evaporation. Significant benefit of PVD coating is the surface modification of the substrate material. Deposition of thin film also has significant economic benefits, since only a small amount of material with the demanding properties is consumed. Even a thin films deposited on the surface can change its chemical or physical properties, like hardness or wettability. Furthermore, PVD provides dense and uniform coatings with excellent adhesion, high corrosion resistance, super high nano-hardness, good biocompatibility, and strong bonding force to the substrate. Therefore, it has become a trend to use this technology in the biomedical field to enhance the performance of various medical devices.
Currently the PVD coatings used in medical devices includes TiN, TiCN, CrN, TiAlN, DLC (diamond-like carbon) and coatings doped by silver atoms. In the future, more advanced coating can be developed by alloying with other elements. For example, magnesium can improve the activity of osteoblasts, strontium ions can also enhance osteoblast activity and in addition inhibit osteoclast activation and silver provides the antimicrobial activity.
The preparation process can be further enhanced by using multiple deposition technologies, which combine advantages of various coatings, for example the hardness, wear and corrosion resistance of the PVD coating and super-hydrophilic and biocompatible properties of organic material prepared for example by electrodeposition.
Surgical instruments
Surgical instruments include knives, scissors, pliers, tweezers, needles, hooks, drills or any other specialized instruments used during clinical operations. In addition to biocompatibility, there are special requirements for strength, hardness, stiffness, toughness, wear and corrosion resistance. Therefore, the surgical instruments are made from austenitic, martensitic, and precipitation-hardened stainless steel. The corrosion resistance is provided by chromium. In combination with oxygen the chromium atoms form a thin self-passivation oxide film which prevents further corrosion of the steel matrix. However, more chloride or sulfur ions can damage the passivation film which eventually leads to the corrosion of the instrument. For medical devices, it is common to be in contact with salt containing saline or blood, various acid and alkaline cleaning agents, rust removers, or disinfectant which will accelerate the corrosion processes. A suitable coating will enhance the corrosion resistance and prolong the lifetime of surgical instruments. In addition to this, the operating room must have sufficient light, and the reflection of light from the surface of surgical instrument may disturb the surgeon during the operation. Therefore, the surface of the surgical instrument is required to have a low light reflectance.
Binary coatings
The commonly used stainless steels are not hard enough and wear quickly when used to cut dense tissues such as bones. Binary TiN coating is well known for its aesthetic gold colour, and has good wear resistance and a hardness of around 29 GPa. The performance test on 420 stainless steel scissors shows that the coated scissors worked on average 100,000 times before the blade needed to be reground, compared to 12,000 time for uncoated instruments, which represents a lifetime improvement by 800% [1]. It is not surprising that TiN is the most commonly used protective coating, however, also CrN is often used for its good toughness, wear and corrosion resistance.
Ternary coatings
Although TiN and CrN coatings provide high hardness and wear resistance, they have a large coefficient of friction and less resistance to chemical corrosion during cyclic cleaning and disinfection. Alloying with aluminum or carbon improves the corrosion resistance or lubrication properties and further increases the hardness and wear resistance. The well-known AlTiN coating is widely used for surgical instruments due to its high abrasion resistance, superior chemical stability, and low light reflectivity [2]. Clinical experience shows that instruments coated with AlTiN are not damaged after repeated cleaning and disinfection in various acid or alkali agents, and virtually any type of sterilization technique can be used.
Other coatings
Quaternary TiAlCN coatings can provide both high wear resistance and low friction coefficient for no-lubrication or low-lubrication applications. Therefore, TiAlCN coating is widely used for minimally invasive surgical instruments and orthopaedics instruments. Diamond-like carbon (DLC) coatings are also suitable due to their low friction coefficient and high wear resistance. Furthermore, stainless steel lacks the antimicrobial properties, which can be achieved using the silver-doped coating. Magnetron sputtering is a suitable technology for adding a small amount of Ag. Due to the potential development of silver-resistant bacteria, it is important to consider the environmental impact of the used technology. In comparison with the often-used wet process of immersing the substrate in colloidal silver, the PVD is a dry vacuum process that does not produce silver-containing liquid waste.
Implantable medical devices
Implantable medical devices are inserted into the human body and remain there for at least 30 days. It includes devices such as stents, bone nails and plates, artificial joints and femurs, dental implants, implantable hearing aid, and other auxiliary devices.
Due to the long period of contact with the human body, the coating on implantable medical devices should have high biocompatibility, corrosion and wear resistance, or low friction coefficient. The antimicrobial properties are also important. Commonly used materials for implantable devices are titanium alloys, cobalt chromium alloys, nickel titanium alloys, or alternatively, stainless steel and magnesium alloys.
Titanium alloys implants
Titanium and its alloys have good biocompatibility and are often used for knee, hip, ankle, and shoulder orthopaedic implants. The long-term use of the traditional Ti-6Al-4V alloy as implant material reveals issues such as poor wear resistance, low biological activity and unwanted ion release due to limited corrosion resistance [3].
Ti and Cr based coatings
Pure Ti or Ti-O coatings on the titanium alloys were researched with the aim to improve the surface properties [4]. The Ti-6Al-4V artificial femur coated with binary TiN film shows excellent wear resistance after 10 million cycles. PVD deposited TiN films are also used in dental implants due to their protective and aesthetic functions. Generally, TiN and CrN coatings are also very suitable for coating-to-coating type of spinal joint support parts. Functionalized Ti-Sr-O coating prepared by magnetron sputtering with sustainable release of strontium ions increases the peri-implant bone volume and could potentially contribute to enhancement of bone anchorage [5]. PVD was also utilized for decorating the implantable devices with Ag and Ag2O nanoparticles (NPs) which kill 100% of E. coli bacteria within 2 hours [6].
Hydroxyapatite-based coatings
Ca10(PO4)6(OH)2 hydroxyapatite (HA) is a well-known biocompatible and non-toxic material. Depositing HA coating on medical titanium alloy devices can combine the good mechanical properties of the alloy and the desired biological activity of HA. As a non-conductive material the HA films can be prepared by RF magnetron sputtering. However, the bonding strength to substrate is rather low, which results in the coating peeling after some time of implantation in the human body. Adding oxides into the HA can improve the bond strength. For example, the HA(+ZrO2+Y2O3) biocomposite coating on Ti-6Al-4V shows no delamination after soaking in the simulated body fluid [7]. The adhesion of HA to the titanium alloy can also be improved by adding a diamond-like carbon (DLC) buffer layer. Moreover, the DLC/HA coating effectively prevent the body fluid from entering the substrate alloy [8].
Tantalum-based coatings
Transition metal Ta is also considered as a biocompatible material since it does not exhibit any adverse effects when implanted in the human body. Pure tantalum and its oxides, nitrides, and carbides can promote cell adhesion, proliferation, and differentiation [9]. PVD prepared nanocrystalline Ta2O5 coating on the surface of Ti-6Al-4V alloy exhibits improved cell and blood compatibility, in addition to good adhesion, corrosion resistance, and antibacterial property. Among the TiN, TaCN, and TiC the first two are more effective in promoting cell proliferation.
Silver-based coating
The hard and wear resistant coatings can be functionalized with a low amount of silver, which provides the antibacterial properties. The TiAgN coating on pure titanium with strong antibacterial effect was prepared by PVD. After 24 hours of cell culturing, all samples showed increased vitality compared to Ti reference, and no cytotoxic effects were found [10]. Due to the cytotoxicity of Ag, it is important to control the release rate of Ag ions into the human body to maintain the biocompatibility of such coatings. The Ag-TiB2 coating prepared by magnetron co-sputtering exhibits a moderate Ag release rate and 97% growth inhibition of E. coli [11].
Cobalt-chromium alloys implants
Medical devices are often made of cobalt-chromium-molybdenum (Co-Cr-Mo) alloy thanks to its good wear resistance. However, the ion release is a severe drawback of this material and needs to be addressed. The solution can be a suitable passivation coating which will inhibit the ion release.
Nitride coatings
It was shown that Co-Cr-Mo implants coated with binary TiN, CrN, and ZrN films have ion release reduced by 60–90% compared to uncoated specimens. These coatings also reduce the friction coefficient and are often used in pedicle screws, or spinal guides and discs.
Tantalum coatings
The Co-Cr-Mo alloy lacks the effect of osseointegration which is required for the implants survival in the human body. Tantalum can provide such feature for biomedical coatings due to its well-known osteoconductive properties. The Co-Cr-Mo alloys coated by a 20–600 nm thin film of Ta prepared by DC magnetron sputtering show improved biocompatibility when tested for 4 weeks in the simulated body fluid [12].
Nickel-titanium alloys implants
Nickel-titanium alloys are known for their shape memory effect, phase transition phenomenon, excellent mechanical properties, and high damping characteristics. Moreover, their elastic modulus is similar to that of human bone. They are widely used in dental, orthopaedic, cardiology, otolaryngology, and other fields of medicine [13]. The drawback of Ti-Ni alloy is the possibility of inducing thrombosis, which needs to be addressed before implanting into the human body. Moreover, its corrosion and wear resistance are not ideal. The release of Ni ions around the tissue can also cause allergic reactions and toxic side effects.
Ti-based coatings
The nano-structured Ti/TiN multilayer deposited on the surface of Ni-Ti alloy significantly improves the biocompatibility of the alloy and supress the cytotoxicity [14]. The TiN coating prepared by filtered arc can increase also the wettability compared to uncoated Ni-Ti alloy and the adhesion, diffusion, and proliferation of cells on the surface of Ni-Ti can be enhanced.
Ta-based coatings
The anti-corrosion properties of Ni-Ti alloy can be improved by the tantalum coating. The barrier effect of Ta prohibits the Ni ion release and improves the radiopacity of the Ni-Ti alloy. The uniform and dense Ta coating prepared by PVD also improves the corrosion resistance of Ni-Ti.
DLC coatings
Diamond-like carbon films contain strong sp3, sp2 bonds, which provide excellent wear resistance and low friction coefficient. It was shown that the DLC coatings prepared by arc-enhanced magnetron sputtering are suitable for improving the mechanical properties of medical implants made from Ni-Ti alloy [15]. In addition to the enhanced wear resistance, the DLC coating can improve the corrosion resistance and effectively inhibit the Ni ion release, which could cause significant health problems.
Stainless steel implants
Medical-grade stainless steel is a cost-effective and easily machinable material that can offer comprehensive performance when used for implantable medical devices such as artificial joints. However, it has limited corrosion and wear resistance in the body fluid environment, which results in accelerated deterioration of its performance. Nevertheless, this drawback can be mitigated by a suitable coating.
Transition-metal nitride-based coatings
The anti-corrosion and mechanical properties of stainless steel can be improved when coated with thin films of TiN prepared by magnetron sputtering [16]. The application of TiN significantly enhances the wear resistance and reduces the friction coefficient. Similar effects can be achieved using CrN or ZrN thin films. Substrates coated by Cr, CrN, ZrN multilayer have lower Ni, Co, and Mo metal ions release after seven days testing as revealed by ICP spectrometry.
Ta-based coatings
Compared to hard but brittle TiN or DLC films, the Ta coating can withstand higher deformation of the substrate without cracking or delamination. PVD prepared tantalum/tantalum oxide coating provides both the passivation of the stainless steel substrate and biocompatibility of its surface [17]. The biocompatibility is ensured by the oxide layer, while the metal tantalum layer provides ductile properties and good adhesion.
Antibacterial Ag doped coatings
Similar to previous cases, the coatings can be functionalized by doping with silver atoms, which provide the antimicrobial properties. The CrN coatings prepared by arc evaporation and subsequently doped by Ag using ion implantation show clear antibacterial effect [18].
Magnesium alloy implants
Magnesium and its alloys are biocompatible and biodegradable materials with low density and similar elastic modulus to human bones. Moreover, compared to the biodegradable polymers, magnesium alloys have higher mechanical strength. The fast corrosion of magnesium in body fluids affects its mechanical properties and also results in the inability of tissue to heal [19]. Modification of the Mg surface by PVD prepared thin films allows to control the corrosion rate and improve the mechanical properties.
Composite oxide-organic coating
The physical vapour deposition enables to combine more materials and deposit multilayer composite coating. The combined PVD and electrodeposition were used to coat a biodegradable Mg‑1.2Ca-4.5Zn alloy with a bilayer thin film composed of silica (SiO2) and silver-doped fluorohydroxyapatite (Ag-FHAp) [19]. The resulting double layer has excellent corrosion resistance and higher wettability, which is beneficial for the cell attachment.
Metal-oxide coating
The Ag-ZnO coating on the surface of Mg-2C-0.5Mn-6Zn alloy provides higher corrosion potential, and in addition to this, the Ag provides an antibacterial effect against Escherichia coli and Staphylococcus aureus [20].
Coatings for interventional medical devices
Interventional medical devices are inserted into the human body for short-time treatment and then they are removed. These devices include endovascular catheters, stents, guide wires and sheaths, embolization device, etc. The most commonly used materials for the interventional medical devices are polymers and metals. Metal materials are used for stents and guide wires. To ensure an easy cruise through the blood vessels and to avoid possible puncture or friction damage, the suitable coating is required to have super-hydrophilic and super-lubricating properties.
For this purpose, the coatings are mostly prepared by electroplating, spraying, or sol-gel technology. Nevertheless, the PVD method can also be used. For example, tantalum coating deposited on the surface of stainless steel improves the corrosion resistance and is mainly used in the cardiovascular interventional therapy. Also, the DLC coating can be suitable for this application due to its very low friction coefficient, which provides good lubrication effect for a catheter.
References
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