As a kid, physicist Seth Fraden loved the movie “Fantastic Voyage,” about a microscopic submarine traveling through a human bloodstream. Almost 10 years ago, Fraden began a quest to create a robotic eel he could send on a similar journey, though it wouldn’t be for entertainment. The eel would be designed to deliver a drug to cells or genes. And to capture the flexibility of the real sea creature, it would take the form of a gel that could glide through water.