The TPU membrane has a super density,cellular shaped molecular structure,which contains as many as 9 trillion microspores in a mere 1 sq.inch.The diameter of such hole is only 0.1-0.3μ,while that of water is 100-300μ,that si 1000 times larger than the microspore,so the water molecular can't come through it,but the diameter of water vapor is only 0.00004μ,therefore,the water vapor(or sweat vapor)can come out easily.