HEAD FXP 17 Multifilament Synthetic. Thanks to HEAD's revolutionary string construction, FXP creates the maximum ball pocket for great power and ultimate control. Special "Tri-Cluster" FiberGEL filaments are integrated in a Polyamide 6 core. Polyamide 6 Monofilament Wrap with three Polyester fibers inside each Monofilament. DuPont Synthetic 66 Coating. FiberGEL filament for maximum ball pocket, great power and superior control Polyester filaments for added durability, control and crisp feel FXP strings offer Power. In Control.