The Bio ST seats and backs have been designed with the highest level of functionality while equally considering the need for durability. Every seat and back is constructed from a ½" sheet of High Density Polyethylene (HDPE), a medium density foam module (1 ½" of Sunmate and Confor for Custom Solid b... The Bio ST seats and backs have been designed with the highest level of functionality while equally considering the need for durability. Every seat and back is constructed from a ½" sheet of High Density Polyethylene (HDPE), a medium density foam module (1 ½" of Sunmate and Confor for Custom Solid backs and 2" of Sunmate and Confor for Custom Solid seats and cushions), and a choice of a variety of mounting options (all made from steel for added strength). The HDPE "skeleton" used provides significantly greater strength than plywood and will also retard warping or rotting when subjected to moisture. Screws and bolts will not come loose from HDPE as they can when plywood is used as the "skeleton" of a seat or back. All hardware used (nuts, bolts, washers and screws) are stainless steel that we treat with a black ox finish. This assures the greatest strength and rust prevention while blending into the seat and back for improved aesthetics