Fusion demonstration (DEMO) concepts are facing challenges in safety and licensing in part due to uncertainties with in-vessel components validation, which relate directly to the materials and design criteria used for assessments. These challenges come from the designed operation under unprecedented environmental conditions and reliance on the performance of complex in-vessel components over time spans of years. These critical components, including the divertor and breeding blanket, will rely on complex structures and multi-material interface that also utilize novel (in regards to nuclear environments) materials. Along with the high operational temperature and fusion spectrum irradiation effects the use of complex components provides key differences in the structural integrity and required design criteria for existing nuclear systems and the DEMO concepts. Overall addressing the challenges in the structural integrity case for fusion requires a new approach to the materials engineering and design interface and design criteria. The current pathways towards development of new DEMO specific design criteria are reviewed, with a focus on current issues and new approaches that are being adopted. Finally thoughts on potential internationalization of new fusion specific design criteria are postulated.