At about nineteen-thousand meters, the air is so thin and the amount of oxygen so small that a person needs a lot of special equipment to survive.
VOA: special.2011.04.06
The first is the lack of oxygen in the extremely thin air.
VOA: special.2010.06.30
The other established approach is to take two wafers of silicon, coat each with a thin layer of glass on one side (which is done by baking them in oxygen), and then fuse them together to produce a silicon-glass-silicon sandwich.