[kragen]

Because titanium is opaque, solid titanium looks the same as a thin shell of titanium on top of another material. Because water is liquid at body temperature, it isn't a suitable material to replace titanium for the outer shell; it would just splurt out of your eye socket.

[hamburglar]

> Because titanium is opaque, solid titanium looks the same as a thin shell of titanium on top of another material.

I obviously haven’t done the math and don’t have access to the data that would allow me to, but I suspect that for this application, you’d have to have an extremely compelling reason to complicate design and manufacturing with such an approach.

[kragen]

Most people would consider not burning their eye socket and possibly brain an extremely compelling reason. Heating an entire 7-cc titanium ball from 37° to the 44° necessary to start causing burn damage would take about 30 joules: 500 mW for a minute, for example. If the heat source is close to the surface of the eyeball (on the inside) you would need less energy because only a part of the eyeball is reaching burning temperatures, maybe as little as a joule. If the ball is almost entirely full of water and properly insulated, you could probably handle hundreds of joules before it started to burn you through the teflon or whatever. A hundred joules is a 3.7 volt Li-ion battery with 7.5 milliamp hours of capacity, so this is a practical amount of energy to put inside your eye socket.