A new knee brace that uses "smart fluids" to provide resistance could change post-injury rehabilitation for millions of people, making repetitive exercises simpler and the needed equipment lighter. Northeastern University mechanical engineer Constantinos Mavroidis and his collaborators have used electro-rheological fluids -- materials whose viscosity changes in response to an electric field -- to develop actuators that can provide controllable resistance with the flip of a switch. Grafting the actuators onto a standard knee brace converts it into a piece of exercise equipment, which could potentially replace bulky weight machines. And by using a computer to regulate the voltage applied to the actuators, the researchers can vary the brace's resistance over time, making it smarter than traditional gym machines. Mavroidis would eventually like to license the technology for use in exercise equipment, but for now he's concentrating on orthotics. An elbow brace should be finished by fall's end, and Mavroidis has talked with Spaulding Rehabilitation Hospital in Boston, MA, about tests of the knee brace that could begin as early as this fall.