Glycine: A potential coupling agent to bond to helium plasma treated PEEK?

To test the tensile bond strength (TBS) between two self-adhesive resin composite cements and PEEK after helium plasma treatment and used glycine as a potential coupling agent incorporated in different adhesives.

In summary, 896 air-abraded PEEK specimens were fabricated. Half of the specimens were treated with cold active inert helium plasma and the other half were left non-treated. Both groups were then split in two groups: In group 1 (n=256), 64 specimens were pre-treated with: (a) soft-liner liquid, (b) visio.link, (c) Ambarino P60 and (d) no pre-treatment (control), respectively. In group 2 (n=192), specimens were conditioned accordingly, but the adhesive materials were modified by including a commercially available glycine (Air-Flow PERIO). PEEK specimens were then luted using either RelyX Unicem or Clearfil SA Cement and TBS was measured initially and after 14 days water storage combined with 10'000 thermal cycles (16 specimens/subgroup). Fracture type analysis was performed. For statistical analyses Kolmogorov-Smirnov, Shapiro-Wilk tests, 1-, 4-way ANOVA (post hoc: Scheffé), and t-test were used (p<0.001).

Helium plasma pre-treatment without glycine showed no impact on initial TBS (p>0.348). In contrast, a combination between glycine application and Softline/Ambarino P60 allowed for significantly higher initial TBS was measured after helium plasma treatment (p=0.001). However, this effect was no evident after thermo-cycling. All groups conditioned with visio.link showed the highest TBS values.

The introduction of amine groups by simple provision of amino acids in the form of glycine can improve the bond strength after helium plasma treatment using different adhesive materials. However, using this simple approach, the method cannot withstand thermal challenge yet.