Sterilization effects on hard-soft combined polymers for medical application

Polymeric hard-soft combinations manufactured by multi-component-injection molding are a suitable way to combine strength properties of a hard thermoplastic component with an elastomer for damping, sealing or haptic functionalities. The use of multi-component injection molding has been transferred to medical applications, mostly applied for sterile products. Consequently, it has to be ascertained that necessary sterilization processes do not affect the adhesion between hard and soft component during the life cycle of the plastic part. So far, there is a lack of sufficient studies on sterilization effects within hard-soft combinations. For this reason, the adhesion strength of different polymeric hard-soft combinations after sterilization treatment was analyzed in this study. The tested material combinations are different thermoplastic elastomers (TPE) on thermoplastic substrates. Moreover, silicone rubber was combined with thermoplastic substrates after atmospheric pressure plasma treatment. Hot steam sterilization (20 min., 121°C) and gamma irradiation (50 kGy) were applied on the specimens with varied intensities. To quantify the influence of sterilization treatments, adhesion strength was analyzed by 90°-peel testing. SEM images of the peeled off surfaces were compared for qualitative analysis of the bond strength. Hot steam sterilization was particularly critical to polar materials as e.g. polycarbonate (PC), resulting in reduced peel force, as well as to heat sensitive material combinations. Gamma irradiation can be applied to nearly all tested polymers, for some thermoplastic - silicone rubber combinations the peel force can even be enhanced due to post-curing reactions. Beneath the influence of sterilization on adhesion, other effects as yellowing of e.g. PC and TPU or changes in mechanical characteristics have to be respected.