The parylene coating process is carried out in a closed system under a controlled vacuum. The deposition chamber and items to be coated remain at room temperature throughout the process. Solvents, catalysts, or plasticizers are not used in the coating process, and additional curing isn’t required.

Many of today’s medical devices that approach micro- and nanolevel dimensions provide benefits that weren’t available 10 years ago. These devices have internal electronics (implanted or external) that need critical protection. Their small size rules out conventional protective conformal coatings, because they do not have sufficient weight to be dipped, sprayed, or brushed. Small devices can also be adversely affected by air gaps, lack of uniform thickness, and other issues that are inherent to many coatings. For implantable microelectronics, any protective coating must also be biocompatible. This article discusses how parylene conformal coatings offer a biocompatible, ultrathin alternative for implantable applications, especially those that demand protection for electronic components.

Applying Parylene

Parylene is the generic name for a unique series of polymeric organic coating materials that are polycrystalline and linear in nature. They possess useful dielectric and barrier properties per unit thickness and are chemically inert. Several types of parylene exist, all of which are free of fillers, stabilizers, solvents, catalysts, and plasticizers. As a result, the material presents no leaching, outgassing, or extraction issues. The overriding factor that makes parylene coatings so attractive to protect implantable medical devices is biocompatibility. Parylene has been used as a protective coating for medical implants for nearly 40 years.

Because parylene is vapor deposited, it can coat into very minute spaces and crevices. Parylene coatings are ultrathin, pinhole-free, and conform to components due to their molecular level polymerization—they literally grow onto the deposition surface one molecule at a time. Parylene also addresses the challenges posed by devices with small dimensions and low weights. It is important that anything used to protect microdevices does not exacerbate either of these aspects. Parylene does not add significant additional mass—a characteristic that can be critical to microimplantable devices.

Parylene coatings are typically applied in thickness ranging from 500 Å to 75 µm. A 25-µm coating, for example, will have a dielectric capability in excess of 5000 V. Other coating materials can’t be applied as thinly as parylene while also providing the same level of protection. Parylene can also strengthen delicate wire bonds by an estimated factor of 10.
Devices to be coated with parylene are placed in a room-temperature deposition chamber. A powdered raw material, called 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. It is then heated to a very high temperature, cracking the dimer vapor into a monomeric vapor. This vapor is then transferred into the ambient temperature deposition chamber, where it spontaneously polymerizes onto all surfaces, forming the ultrathin, uniform, and conformal parylene film. The coating process is carried out in a closed system under a controlled vacuum. The deposition chamber and items to be coated remain at room temperature throughout the process. No solvents, catalysts, or plasticizers are used in the coating process, and no curing process or added steps are required.

Because there is no liquid phase in the deposition process, there are no subsequent meniscus, pooling, or bridging effects, as seen in the application of many liquid coatings. As a result, dielectric properties are never compromised. The molecular growth of parylene coatings also ensures a uniform, conformal coating at the thickness specified by the manufacturer. Because parylene is formed from a gas, it penetrates into every crevice to ensure complete encapsulation of the substrate without blocking or bridging even the smallest openings.

Parylene in Implants

The movement from analog to digital electronic technology has enhanced the use of implantable medical microdevices and offers an ideal platform for using parylene protective conformal coatings. Some analog devices require external manual settings or replaceable batteries. These devices are typically nonimplantables such as those found in analog hearing-assist devices. In those instances, parylene coatings cannot be used where a case must open or a through-the-enclosure dial turned, but they can be used on the stationary internal electronics. However, many electronic implantable and external hearing-assist devices use digital technology. These digital devices often have no external controls and are wireless. As a result, they represent prime opportunities for seamless parylene protective coatings.
Applications that may benefit from parylene coatings continue to grow as devices become smaller. Examples of applications include micromedical electronics for vision and hearing-assist implantable devices, both of which tend to be extremely small by design.

Hearing-Assist Devices. The middle ear contains a group of bones—the malleus, incus, and stapes (often referred to as the hammer, anvil and stirrup)—called the ossicles, which connect to the inner ear. The motion of these bones is ultimately converted into the electrical signals in the cochlea that stimulate the auditory nerve. When the tympanic membrane doesn’t vibrate these bones due to genetic defect, disease, or injury, doctors can attach a tiny electromagnet to the incus bone. This electromagnet is energized by a sound processing system that is implanted behind the ear. As the electromagnet vibrates in response to sound, it enables the patient to hear again. The entire electromagnet coil assembly is about the size of a grain of rice. At this size, the device couldn’t properly be coated with any material that added significant weight or thickness or was not biocompatible. However, as a result of its micron-thick film, vapor-deposited parylene can protect this type of device.

Another component of this system is implanted behind the ear. It is electromagnetically coupled through the skin to an external electronics package that picks up the sound around the patient and processes it to the appropriate signals that ultimately reach the micromagnetic assembly. Electronic devices that are implanted subcutaneously rely heavily on several parylene properties—biocompatibility, moisture-barrier protection, and dielectric properties.

This type of hearing-assist device should not be confused with a cochlear implant. A cochlear implant employs a similar electronics package (externally and implanted behind the ear), but it uses a lead containing a series of conductors and electrodes along its length that is inserted into the cochlea. These electrodes stimulate the nerve endings within the cochlea to produce auditory nerve signals that are perceived as sound by the patient.

Parylene can be applied in much thinner coats than other coating materials while providing the same level of protection. It can also strengthen delicate wire bonds by an estimated factor of 10. Source: Specialty Coating Systems.

Although silicone can be used as a protective coating in these applications, it is highly porous and must be applied in thicker coatings than parylene. Other polymers have been used for their insulative properties, but they must also be applied at a greater thickness than parylene.

Moving from implantable to external hearing-assist devices, those that operate in the ear canal and behind the ear, many feature a dome that fits into the ear canal and holds the receiver (speaker) in place. The dome is typically made of molded silicone, which is inexpensive but highly porous. This porosity permits the dome to absorb body oils from the cerumen (ear wax) and soiling that quickly stain the silicone, which poses a hygienic and aesthetic problem. When coated with parylene, the dome’s silicone pores are sealed to prevent the staining, while the silicone retains its flexibility. Coating with parylene prevents unsightly staining, facilitates dome cleaning, and extends the dome’s use.

Parylene coatings are especially useful in the electronics portion of external hearing-assist devices, as these devices migrate from analog to digital format and are generally programmed by computers. When the audiology doctor or technician adjusts the device, the battery is removed, and a cable is inserted into a connector exposed in the battery compartment, which connects to a computer. All of the device adjustments are controlled by the computer and programs. There are typically no external controls or contacts on these devices, except those for the battery. The absence of external, manual adjustment dials or controls makes the electronics package a candidate for parylene protective coatings.

Vision-Assist Devices. Parylene coatings have an advantage in ocular implant devices, such as artificial lenses that replace a defective natural lens. They also have potential in a device that is being developed to aid patients with retinitis pigementosa.

Retinitis pigmentosa is an irreversible disease that destroys peripheral sight and leaves a patient with focal vision. There are two types of photoreceptor cells—rod cells (concentrated along the retina’s outer perimeter) and cone cells (concentrated in the macula, the center of the retina). Rod cells help us see images that come into the peripheral or side vision. They also aid vision in dark and dimly lit environments. Cone cells allow us to perceive color and see fine detail in the center of vision.

The rods and cones convert light into electrical impulses that are transmitted to the brain where vision occurs. The most common feature of retinitis pigmentosa is a gradual degeneration of the rods and cones. As the disease progresses and more rod cells degenerate, sight is gradually lost. Device developers and universities are working to find a way to stimulate the nerve endings within the retina to recreate vision.

One device under development has a camera that takes an image of the external world, converts it into electronic signals, and then sends it to the eye (where the implant is located). The patient wears a signal processor and a special pair of glasses that contains the camera.

The eyeglass assembly contains inductive coils for signal transmission into the eye. The electronics within this package convert the signals coupled through the coils into signals that are transmitted to an electrode matrix attached to the retina. The electrode matrix stimulates the retinal nerves to reproduce a basic image, similar to the manner in which a video display’s dot matrix is excited to create a picture. The stimulated nerve endings send the rudimentary image to the brain via the optic nerve. Using parylene coatings in this application could help device designers in several ways.
The ocular implant is the size of a matchstick, with the electronics and coil at one end and the electrode matrix at the other. The electronics and coil end is positioned behind the iris and the pupil, while the electrode matrix is against the retina. The device operates within the eyeball and thus must be resistant to the fluids within the eye. A parylene coating would protect the device both from the physiology of the eye as well as protect the eye from the device. The coating must be biocompatible and provide dielectric and moisture barrier protection without adding to the dimension or rigidity of the implant.

The success of this technology is in the creation and transmission of an electrical signal and the recreation of the image. A parylene coating would be thin enough to give the device flexibility and allow it to adapt to the curvature of the retina.
The device would allow enough light and dark contrast to enable the patient to navigate through surroundings and recognize people. Although far from perfect, this capability surpasses total blindness and returns a measure of quality of life and freedom for the patient. As this type of device is refined, designers will discover ways to increase the density of these electrode matrices and the images will improve.

Conclusion

Using parylene conformal coatings allows medical device developers to design, test, and improve devices as they continue to get smaller. Parylene coatings provide biocompatible barriers that protect the smallest devices and components, and their chemical inertness offers added benefits to implanted devices and electronics. Using this material coating will help designers ensure that future devices live up to their potential, even under the harsh conditions within the body.

Lonny Wolgemuth is senior medical market specialist at Specialty Coating Systems (Indianapolis).