So the researchers started tinkering with the protocol in three important ways. First, they perfected a preservation cocktail to replace blood. The final recipe contained, crucially, two sugars called trehalose and glycerol. These sugars essentially act as antifreeze, helping the liver to stay unfrozen even below zero. The researchers also realized that ice tends to start forming at the interface of air and liquids, so they got rid of any air in the cocktail. Lastly, they used machines to perfuse, or gently circulate, the cocktail through the liver's blood vessels, which helped it seep into the tissues evenly. This also allowed the team to bring the temperature down gradually