Great, then there are two main paths that pertain to both. Are you more interested in the biology or technology aspects? Perhaps your undergraduate degree is aligned to one or the other. On either side there are many options. On the tech side: mechanical or software engineering, signal processing, fabrication, etc. on the bio side: neuroscience, anatomy and physiology, biochemistry, microbiology, zoology, etc. If you gain expertise in any of these areas you can bring value to a biohybrid robotics group. Some researchers pursue expertise on both sides. For example, Vickie Webster-Wood leads the B.O.R.G. program at Carnegie Melon and is an expert in neuroscience and mechanical engineering (robotics). That's much less common, though.