Original sentence: As a result a third layer was added - a coating of PU on the inside of the fabric to protect the membrane.Early Gore-Tex fabric replaced the inner layer of PU with a thin, porous fluoropolymer membrane (Teflon) with a polyurethane coating that is bonded to a fabric, usually nylon or polyester.The inner one is polyurethane, aka PU, and provides water resistance, at the cost of breathability.This membrane had about 9 billion pores per square inch (around 1.4 billion pores per square centimeter), each of which is approximately 1/20,000 the size of a water droplet, making it impenetrable to liquid water while still allowing the smaller sized water vapour molecules to pass through
Edited sentence: As a result a third layer was added - either a loose fabric shell layer, or a bonded coating (typically fabric, or occasionally a carbon layer as in Gore-Tex Paclite Shell).Each pore is approximately 1/20,000 the size of a water droplet, making it impenetrable to liquid water while still allowing the smaller sized water vapour molecules to pass through.Early Gore-Tex fabric replaced the inner layer of PU with a thin, porous fluoropolymer membrane (Teflon) with a polyurethane coating that is bonded to a fabric.The inner one is polyurethane (abbreviated: PU), and provides water resistance, at the cost of breathability.This membrane had about 9 billion pores per square inch (around 1.4 billion pores per square centimeter)