The use of microfiber--high-tech synthetic material composed of extremely fine filaments--in lingerie has pretty much revolutionized bra design. Softer and smoother than silk, microfiber bras can be are great seamless bras and extremely lightweight, while still looking luxurious. The fabric is also breathable. The material is usually composed of of a blend of polyester and nylon fibers with spandex added for fit and shape.
Many microfiber bras are made with seamless cups. Molded from one piece of fabric, a seamless bra will be perfectly smooth under knits and clingy fabrics. Seamless microfiber bras are the standard for t-shirt bras. There are also soft-cup styles, minimizers, demi-cups, sports bras, even nursing bras available.