The medical device industry, driven by innovation and new technologies, has become one of the biggest markets in healthcare. The explosion in sophistication and application of devices makes it possible to help improve human health in ways that were unthinkable just a few years ago. Over the last decade, this unprecedented growth has resulted in the development of state-of-the-art health and medical instruments that range from simple tongue depressors and bedpans to complex programmable pacemakers with microchip technology and laser surgical devices.

Static water droplets on modified polymer treated with Aculon’s superhydrophobic coating.

These devices are all used in the treatment, mitigation, diagnosis and/or prevention of diseases and abnormal physical conditions, but all are manufactured from unnatural materials to which human bodies have a natural resistance. Humans have a propensity to create poor biological interactions to devices and the tools that deliver devices made from silicone or latex rubber, solid steel or nitinol core wires, and other nonbiological materials. The insertion of foreign materials can create pain and discomfort for the patient and raise the risk of damage, infections, and even more life-threatening ailments. Complications are a major concern among healthcare professionals, as well as patients, and many view the risk management process as the most critical for successful medical product design and development. Fueled by increasing awareness about healthcare acquired infections, the door has been opened for hydrophilic and hydrophobic coating formulations for the medical device industry to help minimize complications.

Hydrophilic and Hydrophobic Coatings

The medical coatings market is segmented into two types: hydrophilic and hydrophobic coatings, each of which has specific demands differentiated by application and efficiency. Where hydrophilic molecules are polar and ionic, which make them lubricious, abrasion resistant, nonthrombogenic, and biocompatible, hydrophobic coatings are nonpolar repellants.

Hydrophilic coating technologies make polymeric devices susceptible to fluids by grafting polymers into covalent bonds to create water-attracting surfaces. The lubricity and water retention characteristics reduce the force required to manipulate intravascular medical devices during interventional procedures. They can decrease the frictional force between devices 10- to 100-fold and help reduce risk of damage to blood vessel walls, prevent vasospasm, and allow navigation in tortuous vascular pathways and lesions inaccessible to uncoated devices. Hydrophilic coatings have expanded the range of treatment sites for procedures such as balloon catheter angioplasty, neurological interventions, lesion crossing, and site-delivered drug therapies while reducing thrombogenicity. Reduced friction between therapy and support catheters has improved outcomes and reduced procedure time and cost.

Nanocoated hydrophilic technologies average between 8 and 12 g of pull force, which reduces friction over an uncoated surface by as much as 98 percent. This type of low-friction performance increases the device’s ability to navigate through tortuous anatomical pathways, improves device control, reduces tissue damage, and adds to patient comfort. Additionally, the chemistry of new hydrophilic coatings can be matched with a substrate to develop a chemical bond without any separation or delamination from its surfaces. A second benefit of hydrophilic coatings on medical devices is that they create an interface that the human immune system does not recognize as artificial, significantly reducing the risk of problems. Nanoenabled hydrophilic coatings are taking some credit for expanding medical device functionality.

For devices, surgical tools, and instruments that become fouled with fluids or tissue debris, hydrophobic coatings keep surgical tools cleaner overall and for longer periods. With hydrophobic coating repellency to fluids, the blood, urine, or tissue sheets slide off easily. In some cases, these hydrophobic coatings incorporate fluorocarbon functionality in order to improve repellency of hydrocarbons (i.e., lipids); these coatings are typically called oleophobic.

When applied to a variety of substrates, hydrophobic coatings demonstrate water-repellant, self-cleaning, antifouling and/or anticorrosive effects. Medical devices treated with hydrophobic coatings greatly reduce risks of contamination and infections in patients.

Superhydrophobic coatings were biologically inspired by the lotus leaf, which has an extremely high water contact angle of >120° and low sliding angle of <10°. The micro- and nanoscopic architecture on the surface minimizes the droplet’s adhesion. Superhydrophobic refers to extreme water repellency and is a recent nomenclature used by many to describe any surface that repels liquids. As with many industry buzzwords, it inaccurately represents the technology. A superhydrophobic coating has a water contact angle greater than 120° and a sliding angle less than 10° and, interestingly enough, no evidence for lasting superhydrophobicity in nonbiological natural surfaces exists. Superhydrophobic coatings become destabilized under adverse conditions and performance is lost.

Coatings that functionalize surfaces most effectively are hydrophobic. A medical device manufacturer that requests a superhydrophobic coating is typically not versed in the differences between superhydrophobic and hydrophobic coatings. When the difference in durability is explained, most device companies are completely satisfied with stable, covalently bonded hydrophobic coatings that are much more durable.

Coating Applications

Water droplet wets out when applied to a surface treated with Aculon AcuWet hydrophilic coating, which forms a covalent bond to create water-attracting surfaces.

Equally as important as the coatings are the applications, and each method has inherent pros and cons. It is important to note that not all methods are applicable to all devices or materials.

Dip coating is a common process for coating medical devices. It encompasses five steps:

  1. Surface preparation/washing.

  2. Submersion of the device in a coating liquid (with a certain dwelling time in some instances.

  3. Withdrawal from the coating liquid, i.e., coating application/deposition.

  4. Drying and/or curing via heat or UV.

  5. Postprocessing. It is a batch process and is time intensive.

Spray coat systems use a nozzle and driver to nebulize the coating solution and apply it to the surface as a mist. Some use ultrasound transducers to control spray droplet size, which impacts the thickness and quality of the coating. These systems are too slow for mass production.

Reel-to-reel coatings are not applicable to small intricate devices. The process includes a reel of wire or film that is unraveled and travels through a reservoir of coating solution and then into an oven for drying or curing before being rolled up onto the second reel. Although reel-to-reel is a continuous process, it’s ineffective for most devices.

Robotic coating is applicable for complicated shapes and is amenable to a continuous system. Tiny nozzles are directed robotically to trace along struts and other structures.

The viscosity of the coating solution can be programmed as needed.

Spin coating is another common technique for applying thin films to substrates. Although it quickly and easily produces uniform films, ranging from a few nanometers to a few microns in thickness, it can only coat flat surfaces and is a rather slow process.

Submersion coating, although rare for most coatings manufacturers, is the easiest to apply and has the most efficacy. Many devices require only a 30-second submersion to be fully functional and require no cure. Submersion coating is the most effective method that medical coating manufacturers are capable of executing.

Medical Devices Driving Coatings Innovation

The growth in the application of coatings onto medical devices is driven by the medical device industry, which, according to Market Research Reports, is expected to reach $543.9 billion by 2020. Delivering value in such a dynamic environment requires a keen set of partnerships and relationships. With more than 6,500 medical device companies in the United States alone, how quickly and effectively each individual challenge is handled distinguishes qualified firms.

Due to the complex and sophisticated processes of developing hydrophilic and hydrophobic coatings for medical devices, the ability to identify and classify key activities performed in the lab well in advance can improve the quality and reliability of project decision-making and control. Prior experience in developing coatings for medical devices — the more the better — coupled with expertise in developing key chemical formulations is crucial to success. To best meet the needs of different medical devices, even the most advanced formulations typically require refinements during the optimization phase as well as continued testing and analysis of coated devices through verification and validation.

Medical device manufacturers are well advised to establish a close partnership with their coating provider as far upstream in the product design process as possible, ideally at the concept phase. With FDA more heavily focused on medical device coatings, using Critical to Quality Indicators can be a useful tool to develop the framework for collaboration between coating providers and medical device companies.

Conclusion

As coatings are often the only part of a medical device that comes into contact with patients, it would be foolhardy for a device manufacturer to make its choice lightly. There is no room for failure. Applying coatings to medical devices is a multifaceted process that requires skill and careful thought. Understanding the intricacies of the technology as well as how to navigate the complex vendor relationships are success factors for a taking a medical device to market.

A coatings manufacturer should have the ability to create technologies that are biocompatible and maintain stringent quality control methods while improving the performance of the medical devices. Early involvement, preceded by careful vendor selection, is the best way to ensure that the product development process meets internal goals for deadlines efficiently and at acceptable costs.

This article was written by Edward Hughes, CEO of Aculon, Inc. (San Diego, CA). For more information, Click Here .


Medical Design Briefs Magazine

This article first appeared in the October, 2017 issue of Medical Design Briefs Magazine.

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