Chloroprene rubber

The most common cover is a chloroprene rubber with a nylon surface, similar to wet suit material.

Inside is open cell foam and a polyurethane solid GEL.

It is referred to as a ‘SOLID GEL’ and will not leak out.  If the cover is punctured, it may feel a little sticky.  To remove the stickiness, just coat the gel with a little baby powder.