Reliable treatment of internal boundary conditions remains a central challenge in structural optimization, particularly for strongly coupled multiphysics systems. This talk presents recent methodological developments in density‑based topology optimization and cut‑element formulations, with emphasis on applications involving vibro‑acoustics with viscothermal effects, thermofluidics, fluid–structure interaction and surgical procedures. Special attention is given to computational techniques that ensure robust and consistent representations of complex, embedded interfaces within the design domain. These capabilities are shown to be crucial not only in classical multiphysics settings but also in emerging applications such as optimization of surgical procedures, where large interface deformations must be handled accurately. Through selected examples, it is demonstrated that such interface‑aware strategies are essential for solving demanding multiphysics design problems and that they open new conceptual and computational perspectives for advanced engineering and bio‑inspired applications.