KEEPING drugs, and particularly vaccines, potent in tropical climes is a challenge. Heat tends to damage them. Such medicines have therefore to be moved from one refrigerator to another, along what is referred to as a cold chain, until they arrive at the clinics where they are to be administered. 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 estimated 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 when treated in this way, 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 into tiny silk pouches, in a manner that makes them almost indifferent to heat.

Dr Kaplan and his team describe their technique in a recent issue of 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 resulting films 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 in this way does, indeed, help preserve them. It immobilises the molecules, preventing them from unfolding and thus losing their potency. It also minimises residual moisture even better than normal freeze-drying. Dr Kaplan and his team demonstrated the effectiveness of their new technique by trying it out on the MMR (measles, mumps and rubella) vaccine.

Prolonged storage at just 25°C will cause even the freeze-dried version of MMR to break down. After six months it retains only 60-75% of its potency. If kept for that length of time at 45°C (extreme, but not unknown in the tropics) it is practically worthless. When stored using Dr Kaplan’s silk sheets, however, it was still about 85% potent after six months, regardless of temperature.

When the researchers tried the same trick with tetracycline, a commonly prescribed antibiotic, they got similar results. Tetracycline stored at body temperature lost 80% of its potency after four weeks in ordinary solution, but lost none when stored in silk films. The new technique also worked with penicillin. Moreover, fibroin is harmless to people-silk is, after all, often used in sutures-so there is little risk of adverse side-effects. It probably does not matter if some silk gets into a vaccine when it is dissolved in water prior to inoculation.

That said, the vaccines and antibiotics stored in this new way have yet to be tested on people. That is the next step. But if they work, this new trick will help save lives being needlessly lost in some of the poorest parts of the world.