'Tea leaf paradox' leads to new generation diagnostics

30 October 2006

Monash scientists have used an 80-year-old scientific principle to separate red blood cells from blood plasma in a discovery that could lead to pocket diagnostics such as a 'smart card' that performs on-the-spot blood tests.

The principle is the 'tea leaf paradox', discovered in 1926 by Albert Einstein when he noticed that tea leaves accumulated at the centre of the bottom of a stirred cup of tea, instead of spinning outward. He reasoned that friction at the base of the cup retarded the usual centrifugal force in the liquid at the bottom of the cup, resulting in a downward swirling flow that pushed particles radially inward instead of outward.

Dr Leslie Yeo, Dr James Friend and Dr Dian Arifin of Monash's Micro/Nanophysics Research Laboratory in the Department of Mechanical Engineering have used the tea leaf principle to separate whole blood into cells and plasma by passing a tiny, electrically-charged probe over a drop of blood in a container measuring about eight millimetres in diameter.

Dr Yeo said this micro-centrifugation technology was extremely useful for the engineering of chip-based devices for miniaturised portable medical diagnostic kits that could be used to measure blood glucose, for example.

"Like lightning, the high voltage of the probe ionises the air around it and that generates a rapid air flow over the surface of the blood, which leads to the swirling flow described by Einstein," Dr Yeo said. "The flow then spins the red blood cells in the blood down to a stagnation point at the base of the micro-chamber, leaving the clear plasma fluid on top.

"We imagine that this technology could be incorporated into a card a bit thicker than a credit card. It could include a tiny, retractable needle that could draw a drop of blood into the card where the cells and plasma would be separated, and the concentrated blood cells or plasma analysed quickly."