Feeling the Ground With Your Arms

Oftentimes, we don’t realize how important something is until we have lost it. Consider the sensory input we receive from the bottoms of our feet, for instance. Unless you have just stepped on a pile of LEGO bricks, this is probably one of the last things on your mind. But for those who have lost feeling on the bottoms of their feet — due to stroke, spinal cord injury, or another condition — it is a constant concern.

Without that normal sensory feedback, even walking becomes very challenging. Fortunately, that may change in the near future. A group led by researchers at the Georgia Institute of Technology has developed a novel system that restores an individual’s awareness of how their feet are contacting the ground. While it cannot restore normal sensations, the system captures the information and presents it in a different way.

Special pressure-sensing insoles placed inside a user's shoes continuously monitor how weight is distributed across the feet. That information is then transmitted wirelessly over Bluetooth to a set of flexible haptic devices worn on the forearms. Instead of feeling pressure through the feet, users feel patterns of vibration and heat on their arms that correspond to what is happening underfoot.

The researchers describe the approach as a form of sensory substitution. Rather than attempting to restore damaged nerves, the system reroutes the missing information to another area of the body that still has intact sensation. For many people with spinal cord injuries, the forearms remain sensitive even when feeling in the feet has been lost, making them a practical location for receiving this feedback.

The forearm-mounted arrays contain 64 individually addressable nodes capable of delivering both thermal and vibrotactile stimulation. Together, they provide 128 degrees of freedom, allowing complex patterns of information to be displayed across the skin. A newly developed hybrid actuator enables the system to independently control both heat and vibration at each location.

In a series of trials, participants were reportedly able to learn the meaning of the haptic signals within about two hours of training. In exploratory testing involving individuals with stroke and spinal cord injury, the additional feedback led to measurable improvements in standing balance and walking performance.

The researchers also see uses beyond mobility. While vibration provides immediate information needed for walking and balance, thermal feedback could serve as a longer-term warning system. Persistent pressure hot spots, which can lead to foot ulcers in people with diabetes or pressure injuries in wheelchair users, could potentially be identified before serious damage occurs.

Because the system is lightweight, wireless, and fully untethered, it can be used outside of laboratory environments. The team is now working to further miniaturize the hardware and make it easier to adapt to different patients. If successful, this technology could eventually provide a practical way for people with sensory impairments to regain awareness of one of the body's most important sources of information — the ground beneath their feet.