A relative used to have a bike shop and sold stuff called 'slime'.
'Orrible
innit?

Yep that's the stuff, iirc I saw some in Asda at a reasonable price.

Filled the inner tube with goo so it couldn't puncture.
Also sold solid bike tyres (as seen on Tomorrows World), the inner core
was like a soft honeycomb. He said they were lousy on a bike as they were
so heavy.

Yeah I've seen them, I always wondered how bad they were to use on a bike,
would be the ideal solution on a wheelbarrow where weight and comfort
weren't such an issue, I would have thought.

Henry