A metal foam is a cellular structure consisting of a solid metal, frequently aluminium, as well as a large volume fraction of gas-filled s. The pores can be sealed (closed-cell foam), or they can form an interconnected network (open-cell foam). The defining characteristic of metal foams is a very high porosity: typically 75–95% of the volume consists of void spaces making these ultralight materials. The strength of foamed metal possesses a power law relationship to its density; i.e., a 20 ense material is more than twice as strong as a 10 ense material.