When a person suffers a stroke, the interruption of blood flow to the brain can cause lasting loss of function in the limbs. Persistent physical therapy can improve motor control by strengthening connections between the limb and brain. Now, a group at Northeastern University has developed several portable robotic devices that may aid in the rehabilitation process; unlike other rehabilitation devices, these may also let patients continue therapy at home.

Stroke is the leading cause of disability in the United States; over two-thirds of stroke survivors are left with a disability, according to the National Stroke Association. Repetitive physical therapy that applies force to an affected limb can encourage motor signals to reach the brain and build new pathways of control. These exercises can help not just people recovering from a stroke, but also those suffering from other conditions, such as cerebral palsy or degenerative muscle diseases.

“It’s well understood that the more you do it, the better you get,” says Tariq Rahman, director of the Center for Orthopedic Research and Development at the Nemours Foundation and an associate professor at Drexel University.

Traditionally, physical therapists apply force to a limb manually: A group of therapists, for example, will help a patient walk on a treadmill by moving the legs and steadying the patient. In the last few decades, many researchers have looked to robotics for devices that provide forces to a patient’s legs, arms, hands, or pelvis. Researchers hope that such devices will create smoother motion, react more precisely to patient improvements, measure progress more exactly, and make for a more comfortable, effective recovery. Several rehabilitation devices currently in use, such as Rocomo’s Lokomat machine or the University of Twente’s Lopes, were designed to help people walk better–but these systems tend to be bulky and expensive.

The Northeastern researchers have developed devices for the knee, wrist, pelvis, and ankle that they say are portable and cheap enough to be rented by small rehabilitation or medical centers, and potentially even individual patients. The team kept the devices small by using a substance called electro-rheological fluid, which becomes stickier when an electric current is applied, thus creating a stronger resistive force in the device. The fluid contains particles that form chains when electricity is applied, turning the liquid into more of a gel in a few milliseconds.

“With this fluid, we are able to reduce the size of the mechanical components, like the brake,” says Constantinos Mavroidis, professor and director of the Biomedical Mechatronics Laboratory at Northeastern. The group also says it’s reduced the motor size by at least half compared to typical motors. In addition to the smaller size and reduced weight, the fluid-based motors also give a smoother motion, says Mavroidis. “You have the feeling that it’s not really a mechanical device but a soft spring.”

Rahman says that the Northeastern work looks promising. “We’re always looking to make [devices] cheaper, lighter, smaller, and invisible. This is all in the right direction,” says Rahman, who develops robotic rehabilitation devices for children with muscular disorders at the Alfred DuPont Children’s Hospital. Most of the rehab devices Rahman sees are large and unwieldy and thus impractical for patients to use in the home.

Northeastern’s second version of an active knee rehabilitation orthotic device, dubbed AKROD, uses electro-rheological fluid to create a brake on the device. AKROD consists of two lightweight circular braces above and two below the knee, with the power-generating fluid brake–containing gears and sensors–resting alongside the knee.

An upcoming issue of the IEEE Transactions on Mechatronics reports on the Northeastern team’s tests of the device on nine healthy patients. The subjects underwent standard stroke exercises and used AKROD as well. The researchers found that AKROD helped the subjects achieve comparable results to a bigger, commercial rehabilitation system called the Biodex System 3, which consists of a specially made chair, foot brace, and computer system.

A newer version of Northeastern’s AKROD uses a NASA-inspired gear-based system instead of the special fluid. The gear-bearing drive lets the system lift a patient’s leg to correct walking, rather than just apply resistive force. The device is still relatively small and light, due to a compact gearbox design. The device acts as if it has a virtual spring, say the researchers, using careful force to push the patient into the correct position.

The Northeastern team has also tested a rehabilitation device for the hand that’s made of a gripper handle connected to sensors and gears. The device is driven by two actuators with the electro-rheological fluid, which increases or decreases its resistance as the patient uses the handle to navigate through a video game maze. The device exercises not only hand muscles but also forearm muscles, and records the force and position of the patient’s hand. The researchers also created a version that can be used in an MRI to image the brain while a patient is undergoing the hand exercises. This could allow a doctor to see the effect of the exercise on a patient’s brain, according to Mavroidis.

“We’re so interested in this technology because it allows patients to perform repetitions of certain types of movements,” says Paolo Bonato, Harvard Medical School assistant professor and director of the Motion Analysis Laboratory at the Spaulding Rehabilitation Hospital. Bonato collaborates with Mavroidis to test the devices with patients at the hospital. They are currently testing a small number of patients with the AKROD, pelvic, and hand devices, Mavroidis says.

“We can envision a home-care type of application where these devices are used by the patient in the home or the community,” says Bonato.

The devices still need to go through clinical trials before they can be made available to the public.