This paper investigates the surface morphology of polyisobutylene-polystyrene (PIB-PS) block-type thermoplastic elastomers, emerging new biomaterials, by atomic force microscopy (AFM). A novel aborescent PIB-PS block polymer (AR15) was synthesized through inimer-type living carbocationic polymerization and characterized in comparison with a semi-commercial PIB-PS thermoplastic elastomer (TPE) (TS30). Tapping-mode AFM revealed that PS spheres with a domain size of 45-60 urn were very irregularly distributed in the continuous PIB phase of the aborescent AR15 polymer, whereas ordered and mixed cylindrical/lamellar morphology with 25-31 mn hard phase appeared in a linear TS30 triblock polymer. Moreover, AFM revealed rough surface features of a 1 mm thick compression molded TS30 polymer disk coated with a probiotie protein. AR15 disks were used for in vivo urinary tract encrustation study. Encrustation of the PIB-PS disks was comparable or better than that of medical-grade silicone rubber. The preliminary results imply the prospect of PIB-PS thermoplastic elastomers as emerging soft biomaterials. Copyright (C) 2003 John Wiley Sons, Ltd.