The Propper Midweight Base Layer Top features mid-weight fabric that is ideal for hot or cool climates. Made using spandex and polyester, it stretches in order to provide a snug fit. Equipped with the Polygiene® technology, this shirt protects from bacteria, while also battling odor. The ergonomic seams allow you to move and stretch effortlessly. Made Using Spandex and Polyester Polygiene® Technology Ergonomic Seams.