KEEPING drugs, and particularly vaccines, potent in tropical climes is a challenge. Heat tends to damage them. Such medicines have therefore to be passed from one refrigerator to another, along what is referred to as a cold chain, until they arrive at the clinics whence they are to be deployed. Fridges, however, are expensive. They also require electricity, which is not always available—or is available only unreliably—in the poorer parts of the world. As a consequence, breaks in cold chains are reckoned by the World Health Organisation to destroy almost half of the vaccines produced around the world.

Some vaccines can be freeze-dried, which helps. But even treated thus, their lifetime out of the fridge is limited. Ways of keeping drugs and vaccines stable at tropical temperatures would therefore be welcome. And David Kaplan of Tufts University, in Massachusetts, thinks he has found one. Put simply, he and his colleagues have worked out how to pack medicines in tiny silk pouches, in a manner that makes them almost indifferent to heat.

Dr Kaplan and his team describe their technique in this week’s Proceedings of the National Academy of Sciences. They start with silkworm cocoons—the raw material for almost all silk production. They boil the cocoons in a solution of sodium carbonate to separate a protein called fibroin, which is the one they want, from another, called sericin, which they do not. They treat the fibroin with salt, then mix it with the substance to be preserved and spread the result out as films, before freeze-drying it. The films in question consist of a fibroin matrix filled with tiny pockets a few hundred nanometres (billionths of a metre) across. These pockets contain the medicine.

Packaging delicate medicines this way does, indeed, help preserve them. It immobilises the molecules and minimises residual moisture even better than normal freeze-drying, as Dr Kaplan and his team discovered when they tried the technique out on the MMR (measles, mumps and rubella) vaccine.