U-cups (usually made of rubber or polyurethane) are used in dynamic applications. Because they are pressure-actuated lip type seals (rather than squeeze seals), they operate with lower break-out and running friction. They can also maintain smoother, steadier movement. U-cups are unidirectional, however, so two seals will be required for bidirectional sealing. U-cups are more expensive than O-rings. Figure 141 illustrates the cross-section of a U-cup.