Biomimicry is a design discipline that seeks sustainable solutions by emulating nature's patterns and strategies. The goal is to create products, processes, and systems that are well-adapted to life on Earth over the long term, such as a solar cell inspired by a leaf. While biomimicry has produced many case studies, the challenges faced today increasingly demand a systems-level approach as they are too complex to solve in isolation. This research aims to perform a systematic analysis of biomimicry at the systems level, formulate an underlying theory, and apply circularity and biomimicry systematically in practice to contribute to the theoretical development of biomimicry for complex systems challenges.