Any type of researc is really good. Penn is doing a lot of tissue engineering and dental research. As is (I believe, but don't quote me) Drexel.
But genetics is really good too. There's a lot of dental research focusing on genetics now and I think that might be the hot new area.
You could also try to volunteer at these schools (depending on where you go/GPA, etc). Try contacting some professors and seeing what they think. Expect to dig up some LORs from your current research profs.
THough genetics might be the way to go. Most schools jsut want you to have research. It doesn't matter what in.