Scientists have long understood that human cells and tissues can be preserved by cryogenically freezing them. All of the body's biological processes, including the decay of tissues and the death of cells, can be halted at extremely low temperatures, such as minus 196 degrees Celsius.

This knowledge has led to the theory that terminally ill patients could be cryogenically frozen in "suspended animation" until a cure can be found.

There's been just one major problem with cryogenically freezing people: The process tends to destroy vital cells when the body's temperature is lowered, or when it is defrosted.

For example, when a body's tissues are frozen, the water in the cells leaves the cell membrane. This water then surrounds the cell and freezes, which can crush the cell membrane. Another risk is that once the water leaves the cell, the cell will become dehydrated and unable to function. Finally, water can even freeze before it leaves the cell. Just as a balloon filled with water will often break when frozen, the expansion caused by freezing can rupture the cell membrane.

However, a great deal of progress has been made toward reducing these risks. Over the past three decades, scientists have improved their mastery of the techniques of slow freezing, which gradually reduce the tissue's temperature at a highly controlled pace. This has allowed hospitals to freeze blood, stem cells, sperm, human eggs, and embryos.

According to one estimate, 20 percent of the live births from in vitro fertilizations around the world involve frozen embryos. This is the equivalent of 300,000 to 400,000 people on our planet who started life at a temperature that is colder than on the planet Saturn.

Now, however, advances in cryopreservation could make the practice of freezing tissues and organs more reliable and more widespread. One new approach, called vitrification, prevents ice crystals from forming in one of two ways.

The first technique, called natural vitrification, can be achieved by quickly lowering the temperature.

The second technique, artificial vitrification, relies on adding substances called "cryoprotectants" before the freezing process begins. One commonly used cyroprotectant is dimethyl sulfoxide.