text: Heat retention can be a disadvantage when used in mattresses and pillows, so in second-generation memory foam, companies began using open cell structure to improve breathability.
  In 2006, the third generation of memory foam was introduced. Gel visco or gel memory foam consists of gel particles fused with visco foam to reduce trapped body heat, speed up spring back time and help the mattress feel softer. This technology was originally developed and patented by Peterson Chemical Technology, and gel mattresses became popular with the release of Sertas iComfort line and Simmons Beautyrest line in 2011. Gel-infused memory foam was next developed with what were described as "beads" containing the gel which, as a phase-change material, achieved the desired temperature stabilization or cooling effect by changing from a solid to a liquid "state" within the capsule. Changing physical states can significantly alter an element's heat absorption properties.
  Since the development of gel memory foam, other materials have been added. Aloe vera, green tea extract and activated charcoal have been combined with it to reduce odors and even provide aromatherapy while sleeping. Rayon has been used in woven mattress covers over memory foam beds to wick moisture away from the body to increase comfort. Phase-change materials (PCMs) have also been used in covers on memory foam pillows, beds, and mattress pads. Materials other than polyurethane also have the properties necessary to make memory foam. Polyethylene terephthalate, one such polymeric material, provides certain benefits over polyurethane, such as recyclability, lightness, and thermal insulation.