Implementing Coating Techniques on Implantables

COATINGS

Besides enhancing the surfaces of medical products, coatings can be used to make crucial components that go inside devices.

Metrigraphics Precision Components (Wilmington, MA) uses coating techniques to make tiny multilayer flexible circuits for medical devices that are inserted or implanted in the body. Standard flex circuits usually have a minimum trace width of about 75 µm, but Metrigraphics can make circuits with trace widths of 5 µm or less, according to Luke Volpe, the company's product development director.

To make its flex circuits, Metrigraphics starts by depositing a very thin coating of polyimide on a glass base. Then a sputtering system is used to deposit a thin layer of titanium, which bonds to the polyimide. Traces are created using a photoresist layer applied on top of the metal, and the conductive blanket of titanium allows them to be electroplated. In most cases, gold is deposited into the openings in the photoresist to produce the trace pattern.

Once the traces have been created, the photoresist is stripped off and the titanium around the traces is removed using an ion milling process, which in-volves bombarding the metal with high-energy ions. As a final step, a second coating of polyimide is often applied to the circuit to keep contaminants out of the spaces between the traces. It also prevents shorts caused by traces contacting each other.

To ensure adequate adhesion between the polyimide and titanium layers, the company must make sure that the polyimide surface is clean before the metal coating is applied. According to Volpe, nitrogen is used to blow off any contaminants. In addition, ion milling can be used to remove a very thin layer of the top surface of the polyimide and any contaminants on that surface.

When the protective polyimide coating is being applied, Volpe says, it is easy to trap air between the traces, which reduces the dielectric constant in those areas. To remove this troublesome air, Metrigraphics can pull a vacuum around the circuit after the polyimide has been deposited but before it is cured. This vacuum forces the air to the surface in the form of bubbles, he says.

Some medical OEMs make flex circuits for their devices. But OEMs often need help from a supplier to make very small ones, notes Randy Sablich, vice president and general manager of Metrigraphics.

“There are companies that can make the flex circuits for the desktop components of a medical system,” Sablich says. “But the circuits for the implantable parts of the system are more complex by an order of magnitude. And when you're talking about [creating] line widths in the 5-µm size range, that's not typically something that companies have the ability to do.”

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