To our forefathers, distinguishing the living from the dead centered on calidum innatum - vital heat. Aristotle showed that when the heart turns cold, compared to other organs, a person dies and postulated that the vital heat produced in the heart caused blood vessels to react like water bubbles in boiling water - heat which our lungs cooled with air, to keep the cycle going. Modern microbiology knows body heat is not what the ancients envisioned and is more complex than an organ; biological processes should produce thermal signatures no matter how small, even within single cells, it was just that nobody knew how to measure them.

Mechanical engineers may have solved this mystery. They have discovered a way to measure the "thermal conductivity" of three types of cells taken from human and rat tissues and placed in individual micro-wells. They showed that they could detect uniform heat signatures from the various cells and measured significant difference between dead and living ones, suggesting a new way to probe cells for biological activity. 

A lone cell is fantastically small, often only about 10 microns (10 millionths of a meter) across, and this size has thwarted thermodynamic measurements of single cells. Writing in Applied Physics Letters, a team describes how their novel nanoscale biosensing technique can measure thermal conductivity, even within a single cell. 

"In the short-term, this biosensing technique can be used to measure cell viability," said
Dongsik Kim
 from Pohang University of Science and Technology in Korea, who led the team with  Jaesung Park. "In the long-term, we hope to refine it to develop a non-invasive, rapid means for early diagnosis of diseases such as cancer based on differences in the thermal properties of cells."

While the fundamental heat signatures the researchers detected are not exactly what the ancient philosophers imagined, measuring them may answer more mysteries than they could have dreamed.