According to a 2009 estimate by the U.S. Society of Civil Engineers, more than one in four U.S. bridges are either structurally deficient or functionally obsolete. While newer "smart" bridges have embedded wired networks of sensors to monitor their structural integrity, the high cost of installing such systems on existing bridges is simply unaffordable for strained city, state and federal budgets. Now University of Maryland electrical engineering researcher Mehdi Kalantari has developed a tiny wireless sensor that monitors and transmits minute-by-minute data on a bridge's structural integrity that he estimates is one-hundredth the cost of a wired network approach.

The wireless sensors, which Kalantari is marketing under the name SenSpot, measure all the variables reflecting the structural integrity of bridges that conventional wired systems do, such as strain, vibration, tilt, acceleration, deformation and cracking. The sensors themselves are less than five millimeters thick and are comprised of four thin, flexible layers. The first senses and measures structural parameters; the second stores energy; the third transmits data to central computer for analysis; and the outer layer harvests energy from ambient light and radio waves.

While serious problems would quickly trigger an alarm, more subtle early-stage problems may take up to a few days until the system is confident enough to report a structural integrity issue.

Kalantari says the sensors are rugged and, because they are self-adhesive, require no potentially damaging drilling into the bridge structure. He says they should last at least a decade with practically no maintenance required. As they harvest energy from ambient light and radio waves, they don't require any wires, batteries or dedicated external power source. With each unit costing about US$20, the total cost for an average-sized highway bridge needing about 500 sensors would be about $10,000.

"If this kind of technology had been available in Minnesota four years ago, there's a good chance the fatal bridge collapse could have been avoided," Kalantari says, referring to the August 1, 2007 bridge collapse along Minneapolis I-35W that killed 13 and injured 145. "This new approach makes preventive maintenance affordable - even at a time when budgets are tight. Officials will be able to catch problems early and will have weeks or month to fix a problem."

In conjunction with the Maryland Department of Transportation, Kalantari has been testing the sensors by measuring the structural parameters of highway bridges in a real setting for almost a year. He says this has allowed him to optimize the device's performance and energy consumption with the updated model smaller and 10 times more energy efficient than its predecessor. The field testing has also allowed him to track the bridges' response to changes in weather conditions and traffic.

To commercialize his technology, Kalantari founded Resensys LLC through the University of Maryland's Technology Advancement Program incubator. He expects to scale up production of the sensors in September this year.