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Engineering Insight

Parylene film is applied using a vapour deposition process, beginning with a solid raw material. Coated objects remain at room temperature during the entire coating process. (click image to enlarge)

A polyurethane gel used in prosthetic liners developed by Otto Bock HealthCare (Duderstadt, Germany) is designed to protect the user’s residual limbs from pressure points and friction. The material was chosen for its mechanical characteristics, but, in its natural state, it tends to have a tacky surface. To remedy this, Otto Bock initially tested a thermoplastic polyurethane that is sprayed on the liner. While the polymer eliminated the tackiness, the deposition process proved unwieldy. After further research in consultation with engineers at the manufacturer’s US division in Utah and the University of Utah, the company opted for a parylene surface-treatment process offered by Para Tech Coating Inc. (PTC; Aliso Viejo, CA, USA; www.parylene.com).

Discovering Parylene Surface Treatment

The polyurethane liners have a Shore OO durometer of 40 to 50. Providing damping properties along with flexibility and durability, the liners can be preshaped to flex easily and reduce pressure on sensitive areas. Left untreated, however, the gel’s surface tackiness could cause it to adhere to skin and adjacent surfaces. The spray-on thermoplastic polyurethane remedied this problem, but it raised another set of issues, says Oleg Pianykh, development engineer. The manufacturing deficiencies included “variability in coverage from operator to operator, postspray curing delays, and the material’s potential for peeling during use. So, the Otto Bock design team sought a more consistent, timely and durable alternative,” explains Pianykh. That quest led them to parylene.

A USP Class VI polymer, parylene is applied under vacuum in a gas phase process. Because the coating polymerises directly from a gas, the resulting film conforms closely to the underlying surface and builds with essentially equal thickness on all surfaces. By nature, vacuum-deposited parylene film is free of pinholes
and other imperfections.

Part of the material analysis process at Otto Bock involved coating polyurethane samples and testing them to characterise film strength, adherence and stiffness. Parylene was found to provide the desired surface modification without adding rigidity or otherwise changing the substrate’s physical properties, says Pianykh. ASTM tape tests showed good film adhesion and peel resistance without need for silane adhesion promotion. “It was important early on to determine the elongation properties of parylene, because liners may be stretched in use,” adds Pianykh. Handling and stretching coated samples showed that the thin film tolerates up to 1500% elongation without loss of adhesion, he notes.

Productivity Takes the Prize

When the time came to source suppliers, the company selected three parylene-coating centres for development studies. While all three delivered acceptable coating results, PTC took the advantage in productivity.

Extruded liners must be washed to remove silicone mould release agents, and residues left on liner surfaces tend to outgas in the parylene coating chamber as pressure is reduced. This extends cycle time and degrades production efficiency. Thanks to proprietary design refinements in deposition methods, PTC was able to coat the polyurethane liners without any increase in conventional deposition cycle times.

To minimise manufacturing delays and centralise functions, Otto Bock chose to develop its parylene coating capabilities in-house. Representatives from Otto Bock’s design, production and maintenance departments collaborated with PTC in system specification, equipment refinements, part fixturing and the setting of process parameters.

During the coating development effort, Otto Bock perfected a modified, stronger polyurethane for liners, and found parylene coating to be equally functional on this latest elastomer formulation.

Patient comfort is an important goal at Otto Bock HealthCare. The parylene surface-treatment process supports this corporate objective while shortening production time and improving product performance, notes Pianykh.