The Axiom SP Fluid uses a Hydrolite fluid bladder instead of Viscoelastic Foam. The bladder rests directly under the super stretch outer cover, allowing it to conform to the tissues supporting the Ischial Tuberosities.
Hydrolite is a non-Newtonian fluid that behaves as a solid under the force of gravity, but flows as a viscous fluid under load, which can help minimize shear. It is also incredibly light, eliminating the "weight penalty" of other fluid cushions. It is less temperature sensitive than other fluids, meaning it will provide more consistent performance regardless of climate.