Influence of surface modification on the adhesion between Nitinol wire and fluoropolymer films

One of the current challenges for the medical device industry is how to manufacture and assemble biomedical implants consisting of a metallic wire component and a fluorocarbon film without the use of an adhesive. In an attempt to answer this question, samples of Nitinol wire and fluorinated ethylene-propylene (FEP) film were surface modified by various treatments before being thermally bonded into a composite pull-out strength test specimen and mechanically tested to determine their adhesion strength.

The two surface treatments for Nitinol wire included mechanical roughening with sandpaper, and adding a fluorocarbon coating followed by polymerization and curing by a helium plasma treatment. The two surface treatments for FEP film included helium plasma and helium-oxygen plasma under atmospheric conditions. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and contact angle measurements were also taken to characterize the appearance and chemistry of the surfaces before and after modification. A unique pull-out strength test method was developed to assess the level of adhesion between these various candidates.

The pull-out force for untreated Nitinol bonded to untreated FEP film was 30.5 +/- 2.4 N. Significant improvements of up to 14% in this level of adhesion were obtained with the mechanically roughened Nitinol wire bonded to the helium plasma treated films. However, coating the wire with a liquid fluorocarbon monomer mixture (TG-10) was not successful and after thermal bonding to FEP film the level of adhesion decreased by over 80%.

Thus, the bonding strength between the wire and the film can be significantly improved by mechanically roughening the Nitinol wire and treating the FEP film with helium plasma prior to thermal bonding.