“Open-cell memory foam” is just a fancy way of saying memory foam. Decades ago, when the first memory foam mattresses hit the market, beds contained a mix of closed-cell and open-cell foams (depending on the manufacturer). However, closed-cell foams weren’t a hit amongst sleepers—they retained heat and moisture, caused uncomfortable sinkage, and led to sleep disturbances. Once mattress brands caught on to this, they modified their beds to include only open-cell foams.