In this paper we present the first results on the effect of specimen size and branching on the fatigue properties of polyisobutylene-based thermoplastic elastomers measured by the hysteresis method. It was verified that smaller specimens were inherently stronger as expected; at the same loading rate microdumbbells induced higher strain rates so they can be considered as the "worst case" scenario. Microdumbbells, which can be implanted into small animals for in vivo studies, were used for dynamic fatigue testing of linear poly(styrene-b-isobutylene-b-styrene) triblock copolymers (L_SIBS) in comparison with long-chain branched (tree-like or dendritic) versions (D_SIBS). In dynamic stress relaxation studies, D_SIBS performed better than L_SIBS. Simulated physiological conditions had negligible effect on the dynamic properties. (C) 2008 Elsevier Ltd. All rights reserved.