The pressure to hold a balloon is different from grasping a jar. Engineers at Massachusetts Institute of Technology (MIT) have a way to precisely measure and map tactile dexterity using a touch-sensing glove that feels pressure.
The glove’s pressure sensors are like humidity measurement sensors found in refrigerators and weather stations; small capacitors, with two electrodes sandwiching a rubbery dielectric material shuttling electric charges between the two electrodes.
In humid conditions, the dielectric layer soaks up charged ions from surrounding moisture, changing the amount of charge between the electrodes in a way that can be measured.
Sensing with sweat
The dielectric layer in most pressure sensors is relatively bulky, limiting its sensitivity, but the MIT team did away with this layer in favor of human sweat which contains sodium and chloride to serve as dielectric stand-ins. They envisioned two thin, flat electrodes, placed on the skin to form a circuit with a certain charge. If pressure was applied to one sensing electrode, ions from the skin’s moisture would accumulate on the underside, changing the charge between both electrodes by a measurable amount.
The team could boost sensitivity of the electrode by covering its underside with tiny, bendy, conductive hairs which serve as an extension of the electrode. If pressure were applied to a corner of the electrode, specific hairs would bend in response, accumulating ions from the skin to be measured and mapped.
“The simplicity and reliability of our sensing structure holds great promise for a diversity of healthcare applications, such as pulse detection and recovering the sensory capability in patients with tactile dysfunction,” Professor of Mechanical Engineering at MIT Nicholas Fang says.
Pressure pillars
The team fabricated thin, kernel-sized sensing electrodes lined with thousands of gold microscopic filaments, or micropillars, and then measured the degree groups of micropillars bent in response to forces and pressures. When they placed a sensing electrode and a control electrode onto a volunteer’s fingertip, the structure was highly sensitive and could pick up subtle phases in the person’s pulse.
The researchers then applied the sensor concept to the design of a highly sensitive tactile glove, cutting out small squares from carbon cloth, a textile composed of many thin filaments like micropillars. They turned each cloth square into a sensing electrode by spraying it with gold, a naturally conductive metal, and gluing the electrodes to the glove’s inner lining of the fingertips and palms. Then they threaded conductive fibers throughout the glove to connect each electrode to the glove’s wrist, where a control electrode was glued.
When subjects wore the glove while picking up a balloon versus a beaker, the sensors generated pressure maps specific to each. Holding a balloon produced a relatively even pressure signal across the entire palm, while grasping a beaker created stronger pressure at the fingertips.
The team plans to identify pressure patterns when writing with a pen and handling other household objects. This could help patients with motor dysfunction calibrate and strengthen their hand dexterity and grip. “Some fine motor skills require not only knowing how to handle objects, but also how much force should be exerted,” Fang says. “This glove could provide more accurate measurements of gripping force for control groups versus patients recovering from stroke or other neurological conditions. This could increase our understanding and enable control.”
