Freezing is one of the simplest methods of preserving food, biological tissue and other perishables, but the formation of ice crystals can damage cells. Now, researchers from Massachusetts General Hospital (MGH) have developed a new way to “supercool” water and similar solutions to keep them in a liquid form well below the usual freezing point.
Everybody knows that water freezes at 0° C (32° F), but not everybody knows that the process isn’t quite that simple. That’s the point where the liquid begins to freeze, as water molecules at the surface begin to crystallize into ice, which spreads to neighboring molecules until the whole body of water is frozen. In a sense, lowering the temperature just increases the probability that any given molecule of a liquid will freeze.
There are pros and cons to freezing things. On the one hand, the colder something is, the slower certain metabolic processes and chemical reactions occur. That means bacteria won’t grow and tissue won’t degrade as quickly. Unfortunately, the freezing process causes ice crystals to form between cells, damaging them. Finding ways to keep things very cold without them properly freezing could help us store and transport food, tissue, organs and medicine for much longer.
Knowing that the freezing process begins at the surface, where water meets cold air, the MGH technique is surprisingly simple – keep the water and air separate. To do so, the team sealed the surface with a hydrocarbon-based oil, and found that they could stop ice formation in a 1-ml (0.03-oz) sample stored at -13° C (9° F) for up to a week.
“Our approach, which we dubbed ‘deep supercooling,’ is simply to cover the surface of such a liquid with a solution that does not mix with water, like mineral oil, to block the interface between water and air, which is the major site of crystallization,” says O. Berk Usta, co-corresponding author of the study. “This surprisingly simple, practical and low-cost approach to supercooling solutions for extended periods can enable many medical and food preservation methods, as well as fundamental experiments that were not previously possible.”
Building on those first tests, the researchers experimented with more complex oils and pure simple hydrocarbons like alcohols and alkanes. Using these, they were able to supercool 1-ml samples of water and cell suspensions to -20° C (-4° F) for 100 days. Scaling it up to more usable quantities of liquid, they managed to keep 100-ml (3.2-oz) samples chilled for a week.
The team then moved on to test how well the technique could preserve things such as red blood cells. Their preliminary experiments managed to keep 100 ml of red blood cell suspensions supercooled at -13° C for up to 100 days – twice as long as the 42-day maximum possible with existing practices.
“We currently are conducting experiments to increase the volume of red blood cell storage samples up to the more clinically relevant 300 to 500 ml (10 to 17 oz) range,” says Usta. “We also are working on applying this method to other cells and on translating it to large tissues and whole organs like the liver. Along with potential applications in medicine and food preservation, we also believe this invention could be used to study chemical reactions in the liquid state at low temperatures without the usual costly and complicated high-pressure equipment.”