138 Medical Time Travel dering about, molecules just vibrate in one place. Without freely moving molecules, all chemistry stops. For living cells to survive this process, chemicals called cryo- protectants must be added. Cryoprotectants, such as glycerol, are small molecules that freely penetrate inside cells and limit the percentage of water that converts into ice during cooling. This allows cells to survive freezing by remaining in isolated pockets of unfrozen solution between ice crystals. [14] Below the glass transition temperature, molecules inside these pock- ets lock into place, and cells remain preserved inside the glassy water-cryoprotectant mixture between ice crystals. This approach for preserving individual cells by freezing was first demonstrated half a century ago. [15] It is now used rou- tinely for many different cell types, including human embryos. Preserving organized tissue by freezing has proven to be more difficult. While isolated cells can accommodate as much as 80% of the water around them turning into ice, organs are much less forgiving because there is no room between cells for ice to grow. [16] Suda’s cat brains survived freezing because the  relatively  warm  temperature  of  -20C  allowed  modest quantities  of  glycerol  to  keep  ice  formation  between  cells within tolerable limits. In 1984 cryobiologist Greg Fahy proposed a new approach to the problem of complex tissue preservation at low tempera- ture. [17] Instead of freezing, Fahy proposed loading tissue with so much cryoprotectant that ice formation would be com- pletely prevented at all temperatures. Below the glass transition temperature, entire organs would become a glassy solid (a solid with the molecular structure of a liquid), free of any damage from ice. This process was called “vitrification”. Preservation by vitrification, first demonstrated for embryos [18], has now been successfully applied to many different cell types and tis- sues of increasing complexity. In 2000, reversible vitrification of transplantable blood vessels was demonstrated. [19]