Sputter deposition is a versatile low-pressure process where ions bombard a material and liberate atoms, depositing on a substrate to form a coating. The process offers excellent control over coating thickness and energy of arriving atoms and can use targets tailored to product shapes.
Picture courtesy: Procedia CIRP 89 (2020) 250–262
Sputtering has been used for many years in biomedicine to produce the charge transfer layers on pacemaker electrodes and hard coatings on surgical tools. More recently, sputtering has been investigated to deposit radiopaque layers on implantable devices because of its versatility and ability to deposit precisely controlled thin layers of numerous materials.
Picture: Micrograph of a sputtered TiN coating used for sensing and effecting electrodes. The high surface area of the coating increases the capacitance and enables efficient charge transfer with tissue.
Sputtered coatings are used in sensing and effecting electrodes for cardiac pacing, as well as neural stimuli-affecting electrodes for cardiac pacing and neural stimulation, to enhance the charge transfer between the electrodes and tissue. The flexibility of sputtering enables the microstructure of the deposited material to be controlled, thereby producing high surface areas in the electrodes, leading to large capacitances.
For this purpose, we use titanium nitride (TiN) and iridium oxide (Ir2O3). The surface of the sputtered TiN coating is porous, as shown in the picture. Ir2O3 has an additional property where changes between different oxidation states related to pulsing also play a role in charge transfer.
Picture: A nitinol stent
Picture: A Nitinol stent coated with a 5-micrometre thick layer of sputtered Ta significantly improves its visibility under X-rays.
Interventional devices are becoming smaller each generation, making it harder to see them during fluoroscopy.
However, using thin layers of high atomic number materials like Ta or Au can significantly enhance the visibility of these devices under X-rays. A comparison of pictures showing a Nitinol peripheral stent without and with a 10-micron thick sputtered Ta coating highlights the difference in visibility.
Applying a coating or surface modification to a medical device can profoundly affect the regulatory status of the device. It could result in a modified device or create a combination product.
Everyday purposes for developing and using a coating on a medical device include the following:
nanomechanical strength, and
Biomaterials and coating technologies can help address significant healthcare challenges.
The most critical challenges facing coatings and medical devices are miniaturisation of devices, cost reduction and prevention of infection.
Miniaturisation of devices
As medical technology advances, we see a trend towards smaller and smaller medical devices. This is particularly evident in clinical fields embracing minimally invasive percutaneous therapies or procedures. The reasons behind this shift are clear: smaller devices can help reduce trauma, speed up recovery times, and shorten hospital stays. Ultimately, this can help ease the burden on the healthcare system and patients while increasing these procedures' cost-effectiveness.
As medical devices become smaller, there is a higher demand for biomaterials that meet design requirements. For instance, if a catheter's wall thickness needs to decrease to make the device smaller, the tubing made from traditional materials must maintain the same mechanical performance and endurance as the original. Biomaterials can help decrease the device size while providing the necessary strength and durability for its components, making them an attractive option for device designers.
The pace of growth in healthcare spending is not sustainable. Medical device product reimbursement is increasingly under cost pressure, affecting biomaterials technology suppliers and solutions providers. This cost pressure poses a challenge for biomaterials solution providers and presents an opportunity for innovation. Therefore, biomaterials researchers and companies should focus on innovation while developing and marketing their novel medical coatings technologies.
To provide customers with a comprehensive solution, it is essential to focus on developing top-quality coating chemistry and a robust coating process for its application. Additionally, offering appropriate technical support for on-site validation and transfer of the coating processes to the customer is recommended. This approach can help maintain a competitive total cost of ownership for the customers.
Prevention of infection
Using alternative technologies can eliminate the need for coatings in medical devices. One way to achieve this is by modifying the surface of the biomaterial. By carefully designing the chemistry of medical polymers, they can be given desirable characteristics, like hydrophilicity or antimicrobial activity. This allows designers to do away with the coating step entirely. In other words, a surface-modified medical device can be created directly from the raw material substrate, making it more cost-effective.
Picture: For coating a batch of stents with Ta, the stents rotate about their own axis while the tables rotate, ensuring thickness uniformity around and along the stents. Thickness uniformity is achieved using two-spaced cylindrical targets
It has been reported that reimbursement agencies are now threatening to refuse coverage for treating hospital-acquired infections. Device-associated infections' financial and medical consequences can be severe and potentially drastic. It may require surgery to remove the infected device and intensive antibiotics to eradicate it. Depending on the original clinical indication of the medical device, further intervention may be necessary to replace the device once the infection has been treated. Retreatment from a device infection can be up to six times more expensive than the original placement. Therefore, it is no surprise that this change in reimbursement policy has created a high demand for antimicrobial materials technologies. Biomaterial technology developers are developing innovative materials such as non-biofouling coatings, “contact-killing” surfaces, and antibiotic-releasing materials to meet this demand.
TiAlCN-based coatings are well-suited for low-friction, light-load applications with little or no lubrication. Graphite carbon in the coating acts as a low-friction lubricant along the crystal bond edges, helping to prevent parts from wearing out. These coatings are widely used in minimally invasive surgical and orthopaedic instruments. DLC coatings are also famous for medical scissors and forceps because they can withstand multiple sterilisation cycles. However, stainless steel 316L - commonly used in surgical tools - lacks antibacterial properties, so PVD magnetron sputtering Ag/AgTa2O5 nanocomposite thin films are used to limit bacterial adhesion on surgical tool surfaces. Heat treatment can enhance the film’s mechanical properties and improve bonding between the amorphous structure and stainless steel 316L substrate. As a result, adhesion strength can increase to 140% (up to 3000 mN) after annealing at 400°C. The hydrophobicity properties of the film can also be improved through annealing, up to 100°C, by about 120%.
Using a PVD magnetron sputtering process, a Ti-Sr-O functionalised coating was applied to a titanium surface. This coating showed sustained release profiles of strontium, and when combined with a nanopatterned surface, it increased peri-implant bone volume. This could potentially enhance the bone anchorage of Osseo-integrated implants. Additionally, TiO2 was coated on Ti-6Al-4V substrates, followed by heat treatment at 500°C for 90 minutes, to improve the implants' biological behaviour and antibacterial activity.