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.  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.  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.  Sudas cat brains survived freezing because
the relatively warm temperature of -20°C 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.  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 , 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.