According to the American Diabetes Association, more than 29 million people have been diagnosed with diabetes in the United States alone, and an additional person is diagnosed every 23 seconds. As the disease has turned into an epidemic, medical manufacturers have developed numerous devices to manage both Type I and Type II diabetes. Over the years, these devices have become more sophisticated, using more advanced technologies with the goal of providing more effective, fail-safe, and painless diabetes management.

Diabetes care is most commonly accomplished by a finger-prick test multiple times a day and, when needed, insulin delivery is completed via a simple syringe. As new monitoring and delivery systems become available (e.g., small wearable devices with wireless receivers, implantable monitoring devices, cloud-based data storage, etc.), they must continue to meet the standards of safety, effectiveness, and reliability — lives often depend on it. To ensure this high reliability and safety for patients, the industry often relies upon conformal coatings to provide the protection that today's advanced devices and components need.

Why Device Protection Is Critical

Any medical device that comes into contact with the human body — via skin contact or implantation — must be biocompatible and biostable. These requirements, in and of themselves, can drive designers to use a conformal coating. However, device protection goes much deeper than simply ensuring surface biocompatibility.

An illustration of Parylene compared with liquid coatings.

Depending on the device and its use, there can be many reasons to protect components. Needles, for example, may require a coating to ensure biocompatibility, but they also may benefit from dry-film lubricity characteristics (low coefficient of friction) to enable their smooth movement against tissue during insertion and removal. With the growth in communications and monitoring devices, connected directly or remotely to the body, the use of high-tech electronics, sensors, transducers, and batteries has dramatically increased.

Meanwhile, the size of such electronics has decreased. The electronics used in advanced diabetes management systems must be protected from biofluids, moisture, and humidity, all while maintaining biocompatibility. For devices that communicate externally, such as those sending signals to monitor, instruct, or track the accuracy of drug delivery, the components required to transmit such information need to be protected without distorting the signal to avoid information delivery failure. Where other coatings may offer one or two of these attributes, Parylene conformal coatings offer all of them and many more.

Understanding the Parylene Difference

Parylene is the name for a group of vapor-deposited, organic poly(para-xylylene) polymers that are often used as moisture and dielectric barriers. What differentiates Parylene from many other types of conformal coatings is that it is applied by vapor deposition — not via spraying, dipping, brushing, or another mechanical form of liquid application (see Figure 1). It is deposited by a gas phase process that does not rely on line-of-sight physics, but literally grows on device surfaces at a molecular level, which means it provides uniform, conformal coverage of all component surfaces, even penetrating the smallest crevices to provide a complete, biocompatible barrier that is only microns in thickness.

Devices to be coated with Parylene are placed in a room-temperature deposition chamber (see Figure 2). A powdered raw material, known as dimer, is placed in the vaporizer at the opposite end of the coating system. The double molecule dimer is heated, sublimating it directly to a vapor. The vapor is then rapidly heated to a very high temperature that cracks (pyrolizes) it into a monomeric vapor. This vapor then travels into an ambient temperature deposition chamber where it spontaneously polymerizes onto all surfaces, forming the ultrathin, uniform, and extremely conformal Parylene film. The entire Parylene coating process is carried out in a closed system under a controlled vacuum. The deposition chamber and items to be coated remain at ambient temperature throughout the entire process and no additional cure process or steps are required.

Parylene vapor deposition. Ultrathin Parylene conformal coatings are applied as a vapor at room temperature. Parylene N is illustrated.

Parylene is typically applied in thickness ranging from 500 Å to 75 μm, maintaining excellent properties in ultrathin films. A 25-μm coating, for example, will have an electrical insulating capability of 7,000 V. No other coating material can be applied as thin as Parylene and still provide the same level of protection. The molecular “growth” of Parylene coatings ensures not only a uniform conformal coating at the thickness specified by the manufacturer, but because Parylene is formed from a gas, it ensures complete encapsulation of all surfaces without blocking or bridging small openings. All Parylene variants are free of fillers, stabilizers, solvents, catalysts, and plasticizers. As a result, the Parylenes present no leaching, outgassing, or extraction issues. Its naturally low coefficient of friction provides dry-film lubricity properties that benefit moving parts, including syringe and infusion device applications, eliminating the need for silicone oils that are often used as a lubricant in syringe-type devices (see Figure 3).

For implanted devices, Parylene provides components with both biocompatibility and biostability so the device is compatible within the human body and is also not itself compromised by biofluids.

A key feature of Parylene for fluid-delivery devices, and any implantable device or system, is that the ultrathin film delivers necessary protective benefits without compromising the mechanical or functional attributes of the device. As an extremely thin coating, Parylene does not impact the dimension of the device — regardless of how small it is — and ensures biocompatibility within the body. For devices requiring electrical communication, it has other sought after electrical properties such as low dielectric constant and dissipation factors.

Parylene Options

Table 1 — Parylene properties.

There are three commonly utilized variants of Parylene: N, C, and Parylene HT. Each Parylene has unique properties that make it advantageous for medical device applications (see Table 1). All three Parylene formulations are biocompatible and biostable, as confirmed by ISO 10993 and USP Class VI biological evaluations, and all are compatible with today's common sterilization methods used (e.g., e-beam, gamma, EtO, peroxide plasma, steam autoclave, etc.).

Parylene N is a nonchlorinated poly(para-xylylene) that has low values for dissipation factor and dielectric constant that do not vary with frequency. While all the Parylenes have excellent penetrating power, the N and Parylene HT variants surpass that of Parylene C due to their high molecular activity in the monomer state.

Parylene C is produced from the same dimer used to make Parylene N, but it is modified by a chlorine atom attached to the molecule's benzene ring. Its combination of electrical and physical properties, plus a low permeability to moisture, fluids and corrosive gases make it the most common Parylene variant used. Its ability to provide pinhole-free conformal barriers makes it the coating of choice for many critical medical electronic components and assemblies. Because of its excellent barrier properties, Parylene C is often the first choice for the protection of insulin syringes, pens, and other medical fluid/medication holding devices.

Parylene HT utilizes fluorine in the connecting bonds of the long chain polymer structure instead of hydrogen atoms. This results in some highly enhanced attributes. Parylene HT is the most stable Parylene in the presence of ultraviolet light and is thermally stable in the presence of oxygen at high temperatures (350 °C long term, 450 °C short term). It has the lowest dielectric constant and dissipation factor of all the Parylenes and has the best crevice penetration capability.

Advanced Parylene Adhesion Technologies

Strong adhesion is critical to enable Parylene coatings to provide maximum benefit to the application. Some of the more advanced surface materials coming into use for many medical devices can be difficult to bond to for all coatings, including Parylene. To negate these issues, adhesion promotion technologies have been developed.

AdPro Plus and AdPro Poly adhesion promotion technologies improve adhesion of Parylene coatings to materials such as titanium, stainless steel, gold, chromium, solder mask, and many polymeric materials, including polyimide (Kapton®) substrates. Both technologies have demonstrated improved stability at elevated temperatures and are biocompatible and biostable for medical device applications. When Parylene coatings are paired with AdPro Plus or AdPro Poly adhesion promotion technologies, the long-term reliability and protection of surfaces is greatly improved.

Application Examples

Coefficient of friction measurements for Parylene-coated rubber specimens.

In the diabetes management arena, two devices provide excellent examples for the use of Parylene.

Continuous Glucose Monitoring. These devices measure glucose levels 24/7. Devices vary in the special features they offer, but all basically work by using a tiny electrode/glucose sensor inserted under the skin to measure glucose levels in tissue fluid. A transmitter sends data wirelessly to a monitor that, in turn, alerts the patient if the glucose level is in a danger zone (too high or too low). This area is advancing rapidly, but most devices currently on the market still require minimal finger pricking to calibrate their sensor accuracy.

Automatic Insulin Pumps. These systems provide a precisely controlled rate of insulin delivery to patients who would normally need multiple daily injections to regulate blood glucose levels. Traditional pumps are very small, wearable devices with a user-replaceable insulin cartridge inside the pump. The insulin syringe is controlled by a piston in the system, and all are managed by miniaturized electronics. The pumps separate insulin doses into basal rates, bolus doses to cover carbohydrates in meals, and correction/supplemental doses. Patch pumps are another type of device and are worn directly on the body with integral reservoir, pump, and infusion. These are controlled wirelessly by a separate programming device.

Parylene Protection. Parylene offers protection of these diabetes management devices and others by providing:

  • Biocompatibility and biostability.
  • Excellent chemical, moisture, and biofluid barrier properties.
  • Complete encapsulation of all surfaces without compromising device dimensions.
  • Dielectric barrier protection without signal loss or transfer.

Protection of both operational components and signal transmission integrity is extremely critical in diabetes management devices. These needs will only continue to grow as devices become smaller and more powerful.

Choosing the Right Match

The advances in glucose monitoring and insulin infusion devices for diabetes management illustrate one discipline on the leading edge of medical technology, and Parylene is well suited to help protect both current and future device generations. Selecting the right coating can be as important as choosing the materials that go into the device itself and should be considered during the development phase of the device. There may be several options to consider, but Parylene protection is one solution that will meet many material and sterilization process requirements for numerous medical devices and systems.

The suitability of Parylene coatings must be determined through testing and process confirmation by each medical device manufacturer, but suppliers, when available, may provide reference access to their FDA Device and Drug Master files. In this case, test results of ISO 10993 and USP Class VI testing can be referenced in customer FDA submissions.

This article was written by Dick Molin, Sr. Medical Market Specialist for Specialty Coating Systems, Indianapolis, IN. For more information, Click Here .


Medical Design Briefs Magazine

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

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