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A TINY sensor barely bigger than a heart cell may improve your chances of
surviving heart disease. In the long run, it could be built into an implant to
monitor hypertension or provide early warning of cardiac failure.

For now, the device will let scientists measure the beating of individual
heart cells in the lab. This information is crucial in the battle against heart
disease. At the moment, such measurements have to be carried out with bulky
pipettes and needles, using cultures of heart cells. The results are not always
accurate because the cells have to be removed from their nutrient solution when
tested, which affects their behaviour.

So a team led by Gisela Lin at the University of California in Los Angeles
decided to try a new tack by etching a tiny sensor into a silicon chip. To do
this, they carved two cantilever beams out of silicon dioxide, with clamps at
one end of the beams for attaching to cells. At the base of one of the beams
they made a strain gauge out of polycrystalline silicon. When the beams bend
under strain, they change the gauge鈥檚 electrical resistance in proportion to the
force acting on the beam.

To test the sensor, Lin鈥檚 team clamped a single heart cell from a rat to the
beams and submerged the entire device in a saline bath. Then they pumped
solutions with alternately high and low calcium concentrations over the cell to
make it contract and relax. The strain gauge measured the force of this pushing
and pulling. The scientists found that individual heart cells produced forces
between 100 nanonewtons and 50 micronewtons.

Don Devoe, a micromachines researcher at the University of Maryland in
College Park, says: 鈥淭he fact that a device of this complexity can be produced
without any real modifications to an existing commercial process for
manufacturing integrated circuits is fairly impressive.鈥�

One of Lin鈥檚 aims is to study cells from enlarged or 鈥渉ypertrophic鈥�
hearts鈥攁 condition sometimes caused by high blood pressure. The overworked
heart cells are bigger and stronger than normal ones, but they eventually die,
triggering heart failure. 鈥淚f we could detect these changes at the cellular
level, before it spreads to your whole heart, then it would be like an early
warning system,鈥� says Lin, who now works at micromachine maker Standard MEMS in
Rancho Palos Verdes, California.