Most grad programs in physical science/engineering love interdisciplinary work. Any serious research experience is seen as a big plus, and your being open minded to working in a subfield different from your undergrad work is an advantage, not a detriment. And once you get started on your graduate research, you’ll find that breadth of experience only adds to your creative potential in the lab. Just make sure to highlight in your personal statement that you’re passionate about the biomaterials field, and why.