Abstract: Disclosed are implantable devices that include biocompatible polyurethane materials. In particular, the disclosed polyurethane materials can maintain desired elastomeric characteristics while exhibiting thermoset-like behavior and can exhibit improved characteristics so as to be suitable in load-bearing applications. For example, the disclosed polyurethane materials can be suitable for use in artificial joints, including total joint replacement applications. The disclosed polyurethane materials include biocompatible cross-linking agents as chain extenders, more particularly chain extenders comprising a terminal group capable of side reactions and further comprising an electron withdrawing group immediately adjacent the terminal group. In addition, the reaction materials and conditions can be selected to encourage intermediate levels of cross-linking without the use of traditional cross-linking trifunctional reagents.