A biochip for building up self assembled ferromagnetic structures was designed. The magnetic structures are self assembling onto magnetic seeding points which are arranged in a periodic hexagonal structure on the bottom of the chip. The individual shapes of the ferromagnetic structures can be controlled by an applied external magnetic field. This periodic arrangement of adaptable ferromagnetic structures is well suited for selective and dynamic cell trapping. A biochip designed for lab experiments and a simulation model for optimizing the magnetic structures is presented.