Scratch the surface26 October 2018
Biocompatibility, durability and infection resistance are important factors for all medical devices, but especially for implants. With these devices, it’s crucial to pick the right coating or the device could be rejected by the body. In this round-up, Emily Gibbs explores some of the latest innovations in implant coatings, from diamonds to synthetic silk.
The medical implants market is evolving fast. Comprising devices such as orthopaedic implants, cardiac pacing devices, neurostimulators and drug implants, it is growing at a rate of over 7% a year. According to a recent report by Market Research Future, its global value will hit $171.5 billion by 2023, up from $94.5 billion in 2015.
This in turn is fuelling demand for protective coatings. As new types of implants are commercialised, and new industry standards are developed, the coatings market will need to keep pace. We are witnessing not just market growth, but a wide range of technical advancements.
After all, coatings perform a critical role in medical implants. Depending on the device, the coating might deliver antimicrobial drugs, serve as a bloodimpermeable barrier, or aid incorporation between the device and the tissue. It needs to be robust, resilient, durable and, above all, biocompatible.
While this might sound like a lot to ask, it’s important to get it right. To take just one variable, the wrong kind of coating could increase the risk of infection. Estimates vary, but orthopaedic implants are thought to carry an infection risk of 1–16%. One study found that the financial burden from orthopaedic-implant-associated infections in the US would reach $1.6 billion by 2020. This issue is likely to become more serious as antimicrobial resistance increases.
It could also increase the risk of rejection. If the implant is not biocompatible with the patient’s tissues, they may suffer serious adverse reactions, including inflammation and allergic responses. While the coating is not the only variable that has an impact on biocompatibility, it is nonetheless a crucial one.
Then there is the risk of corrosion, which affects orthopaedic and orthodontic implants in particular. If the implant corrodes, it will lose mechanical strength, meaning it may need to be replaced. It can also lead to the release of harmful metallic ions, causing allergic reactions.
In short, the coating selection can make or break the implant’s efficacy, which is why there have been many recent innovations in implant coatings, each with its own potential benefits.
Atomic layer deposition technology
A Finnish company recently rolled out its atomic layer deposition (ALD) technology to medical devices. A thin-film coating technology, it had previously been used in the manufacturing of computer chips and LEDs, and is therefore suitable for the microelectronic devices now common in the healthcare sector.
In this coating method, the film is built up little by little – one atomic layer at a time – through a series of gas-phase chemical reactions. The resulting film is dense and impermeable, and its structural characteristics can be precisely controlled on an atomic level. This protects the device against corrosion.
“As several ALD materials are intrinsically biocompatible, and as the ALD method creates the highest quality coatings that form always pinhole-free, conformal, uniform and hermetic encapsulation around the coated object, ALD finds uses in all sectors of medical industries,” said the company in a press release. “It offers atomic-level precise solutions to various challenges that medical equipment manufacturers are now facing.”
Focus on antifouling
Dr Jie Zheng, a chemical and bioengineering professor at the University of Akron, US, recently received a grant for research into ‘antifouling’ coatings. The $340,866 grant, from the National Science Foundation, will be used to develop a better data-mining method for testing and designing these coatings.
Essentially, fouling refers to the accumulation of unwanted organisms on a wet surface. When that surface is an implanted medical device, it can cause serious problems – not least rejection.
“Antifouling materials and coatings are critically important for biomedical implants because they will prevent any unwanted interactions and infections with biomolecules, and thus reduce the risk of foreign-body reaction in patients,” says Zheng.
A diamond idea
In March 2018, researchers at RMIT University found that diamonds could improve the biocompatibility of 3D-printed titanium implants. Titanium, considered the gold standard for medical implants, has many benefits, but biocompatibility can pose a challenge. Diamonds do not have this problem; since they are made from carbon, they interact well with living tissue.
In this study, the implant was coated in detonation nanodiamonds (a cheap synthetic material), via a chemical vapour deposition method. It was seeded with mammalian cells and preserved in conditions similar to the inside of the body. Compared with an uncoated implant, the diamond-crusted one showed less bacterial activity and increased cell growth.
“This coating not only promotes better cellular attachment to the underlying diamond-titanium layer, but encourages the proliferation of mammalian cells,” explained lead author Dr Kate Fox. “The diamond enhances the integration between the living bone and the artificial implant, and reduces bacterial attachment over an extended period of time.”
The researchers think their technique might be particularly useful for ‘just in time’ implants – orthopaedic implants produced on-demand using techniques like 3D printing, and they expect diamond coatings to become common in orthopaedics in the near future.
Silk coatings for breast implants
Silicone implants, such as those used in breast reconstruction surgery after cancer, are particularly susceptible to rejection. Among reconstruction patients, a staggering 46% will undergo reoperation within three years, often because their body has rejected the implant.
German biotech company AMSilk has come up with a potential solution, in the form of synthetic silk biopolymers. Together with implant manufacturer Polytech Health & Aesthetics, it has initiated an international clinical trial to test out silk-coated breast implants in humans. This marks the first time that bioengineered silk will be inside the human body.
Although the substance mimics the properties of silk – it can be moulded into fibres, for example – it is actually a protein and is therefore more likely to be accepted by the body. And unlike most proteins, it is strong enough to withstand the sterilisation process.
If the clinical trial proves successful, the company intends to test the coating on other types of implant.
“Through our multiyear partnership with the experts at Polytech, we’ve created a new and groundbreaking product for the medical device market,” said Jens Klein, CEO of AMSilk. “The SILKline implants are the first product providing our silk coating in the medical device sector – but other products using the extraordinary biocompatibility of our silk coatings will soon follow.”
Bioactive glass (a type of glass made from high-purity chemicals) is emerging as a potential choice of coating for orthopaedic implants. The material is already used widely across other biomedical applications, such as tissue engineering and dental reconstruction, and has garnered a reputation for its antimicrobial properties.
Because the glass increases the pH of the surrounding body fluids, it creates an environment hostile to bacteria. If bacteria do approach, they are incapable of adhering to its surface and propagating there.
Over the past few years, the industry has witnessed the development of the first bioactive-glass-coated implants across orthopaedic and dental applications. In some cases, ions (such as silver, boron or copper) are added to boost the antimicrobial effects.
This ties in to a broader trend: coatings that combine several materials.
“Medical device manufacturers are looking towards the adoption of disruptive multimetallic coatings that contain more than one metallic component and polyelectrolyte coatings in a wide variety of applications,” Raghu Tantry, principal analyst at Frost and Sullivan Visionary Science, commented in a recent release. “These developments are likely to increase the unit price of materials and coatings, propelling market growth.”
Nanosurfacing for spinal implants
Many implants are made of, or coated in, bioceramic materials, which help ensure the device is not rejected by the body. However, these materials are often susceptible to microcracks and other flaws.
In September 2018, nanotechnology company Nanovis announced a licensing agreement with the University of Nevada for one of their technology patents, which could create stronger bioceramic coatings. Essentially, the coating contains tiny nanopores, improving its functional properties.
Depending on the purpose of the implant, it can also improve osseointegration (the fusion between the implant and the surrounding bone) and bactericidal properties (through the release of an antimicrobial drug).
“We can control the length, the height, the pore openings and the pore volumes within the ceramic nanosurface,”said Mano Misra, professor of material sciences and engineering at the University of Nevada. “The sizes and shapes of the nanosurface pores can be changed so the ceramic coating releases drugs over a longer period of time, providing superior anti-infection properties.”
Antibacterial silvercoating Technology
Silver has long been known for its antibacterial properties, and in recent years, a number of manufacturers have added silver ions to their coatings. These ions are capable of penetrating bacterial cells, damaging their DNA. However, there are a number of technical challenges to surmount – not least ensuring that no healthy cells are damaged in the process.
One Berlin-based company recently received a funding boost for its antibacterial silver-coating technology. The grant, totaling around €700,000, will be used to finance the next stage of research, as the company builds towards a human clinical study.
“The internationally IP-protected silver-coating technology developed by aap is intended to protect the surface of implants from colonisation by bacteria,” the company announced upon its release. “Thereby aap is addressing one of the biggest challenges in trauma: the reduction of surgical site infection risks.”
With research teams around the world working hard to redress the challenges, this is an exciting time for the coatings industry. Over the next few years, we can expect to see an array of new solutions hit the market, improving the safety and efficacy of implants.