[rjmunro]

I want someone to engineer a fabric / fibre that grows with each wash rather than shrinks. I feel like with modern materials science it must be possible.

I'd use it for children's clothes. After you wash the clothes, you wait for the kid to grow a bit before they wear it again. No more growing out of your favourite things.

[mmooss]

Where would the extra mass come from? Water? CO2 in the air?

[rjmunro]

It doesn't need extra mass, it needs to stretch out a bit.