Corrosion mechanisms of magnetic microrobotic platforms in protein media

The field of biomedical small-scale swimmers has made major progress during the last two decades. While their locomotion aspects and functionalities have been demonstrated, there are key aspects that have been often overlooked such as their service live durability, which difficult their translation to the clinics. Several swimmers consist of combinations of metals and alloys that, while they excel in their functionalities, they fail in their stability due to corrosion in highly aggressive complex body fluids. Here, for the first time the corrosion mechanism of a widely employed design in magnetic microrobots, a gold-coated magnetic NiCo alloy, is assessed. A systematic approach by combining electrochemical and surface analysis techniques is reported, which shed light on the degradation mechanisms of these systems in simulated body fluids. While results demonstrate that Au coatings remarkably enhance the surface nobility and resistance to corrosion/biodegradation of NiCo in an aggressive environment containing albumin protein, Au coatings' intrinsic defects lead to a galvanic coupling with the NiCo substrate. The coordination of protein with NiCo further accelerates corrosion causing morphological changes to the swimmers' surface. Yet, the formation of a phosphate-based layer acts as a barrier to the metal release after long immersion periods.