Arc'teryx Mens Acrux FL Synthetic Keeping the science of the foot in mind these were constructed with durability, breathability, and efficiency for safe traveling in even the harshest of environments The interior liner is separate, which provides an adaptive fit that allows for incredible breathability and weather protection The liner is made from a stretch nylon mono-mesh that will not conform or absorb moisture Not to keep bragging about the liner, but the liner does not have a tongue, which reduces the build up of heat and pressure on the top of your foot to keep you comfortable The upper features a non-woven Durable Water Repellent Finish (DWR) microfiber, PU coated nylon yarn, and high abrasion TPU film for the utmost in maintaining air permeability, abrasion resistance, and moisture dispersion. So great.