It uses small balls containing a chemical compound that, when heated and then kept insulated, will stay at 37 °C (approx.
This allows cultures to be tested without the need for a laboratory or an expensive portable incubator.
The phase-change incubator was developed in the late 1990s by Amy Smith, when she was a graduate student at MIT.
Smith has also started a non-profit organization called A Drop in the Bucket to distribute the incubators and to train people on how to use them to test water quality.
[1] Embrace, an organization that from Stanford University, is applying a similar concept to design low-cost incubators for premature and low birth weight babies in developing countries.