Sense of tactile glove, maps tactile stimuli

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When you pick up a balloon, the pressure to hold it is different from how you would when picking up a jar. And now the engineers of MIT and elsewhere have a way to accurately measure and map these subtleties of tactile dexterity.

The team designed a new tactile glove that can “feel” pressure and other tactile stimuli. The inside of the glove has a sensor system that detects, measures and maps small changes in pressure through the glove. The individual sensors are highly matched and can pick up very low vibrations through the skin, such as a person’s pulse.

When subjects wore the glove while picking up a balloon versus a beaker, the sensors generated task-specific pressure maps. Holding a balloon produced a relatively even pressure signal across the entire palm, while grabbing a beaker created more pressure at the fingertips.

Researchers say the tactile glove could help retrain motor function and coordination in people who have had a stroke or other fine motor condition. The glove could also be adapted to increase virtual reality and gaming experiences. The team plans to integrate the pressure sensors not only into touchscreen gloves, but also into flexible adhesives to track pulse, blood pressure and other vital signs more accurately than smartwatches and other portable monitors.

“The simplicity and reliability of our sensing structure holds great promise for a variety of healthcare applications, such as pulse detection and sensory recovery in patients with tactile dysfunction,” says Nicholas Fang , professor of mechanical engineering at MIT.

Fang and colleagues detail their findings in a study published today in Nature Communications. Study co-authors include Huifeng Du and Liu Wang at MIT, as well as Professor Chuanfei Guo’s group at Southern University of Science and Technology (SUSTech) in China.

Sensation with sweat

Glove pressure sensors are similar in principle to sensors that measure humidity. These sensors, found in HVAC systems, refrigerators, and weather stations, are designed as small capacitors, with two electrodes or metal plates, sandwiching a rubbery “dielectric” material that carries electrical charges between. both electrodes.

In humid conditions, the dielectric layer acts like a sponge to absorb charged ions from the surrounding moisture. This addition of ions changes the capacitance, or the amount of charge between the electrodes, in a way that can be quantified and converted to a measurement of humidity.

In recent years, researchers have adapted this capacitive sandwich structure for the design of thin and flexible pressure sensors. The idea is similar: when a sensor is squeezed, the charge balance in its dielectric layer shifts, in a way that can be measured and converted to pressure. But the dielectric layer of most pressure sensors is relatively bulky, limiting their sensitivity.

For their new touch sensors, the MIT and SUSTech team have removed the conventional dielectric layer in favor of a surprising ingredient: human sweat. Since sweat naturally contains ions such as sodium and chloride, they felt that these ions could serve as dielectric substitutes. Rather than a sandwich structure, they envisioned two thin, flat electrodes, placed on the skin to form a circuit with a certain capacity. If pressure were applied to a “sense” electrode, ions from the skin’s natural moisture would build up on the underside and change the capacitance between the two electrodes, by an amount they could measure.

They found that they could increase the sensitivity of the sensing electrode by covering its underside with a forest of tiny, curved, conductive hairs. Each hair would serve as a microscopic extension of the main electrode, so that if pressure was applied, for example, to a corner of the electrode, the hair in that specific region would bend in response and accumulate ions from the skin, degree and whose location could be accurately measured and mapped.

Pressure pillars
In their new study, the team fabricated thin, nucleus-sized sensing electrodes lined with thousands of microscopic gold filaments, or “micropillars.” They have demonstrated that they can accurately measure the degree of curvature of groups of micropillars in response to various forces and pressures. When they placed a sensing electrode and a monitoring electrode on the fingertip of a volunteer, they discovered that the structure was very sensitive. The sensors were able to pick up subtle phases of the person’s pulse, such as different peaks during the same cycle. They could also maintain accurate pulse readings, even when the person wearing the sensors waved their hands across a room.

“The pulse is a mechanical vibration that can also cause deformation of the skin, which we cannot feel, but the pillars can lift,” says Fang.

The researchers then applied the concepts of their new micro-pillar pressure sensor to the design of a highly sensitive tactile glove. They started with a silk glove, which the team bought off the shelf. To make pressure sensors, they cut small squares out of carbon fabric, a textile made up of many fine filaments similar to micropillars.

They turned each square of fabric into a sensing electrode by spraying it with gold, a naturally conductive metal. They then glued the cloth electrodes to various parts of the glove’s inner liner, including the fingertips and palms, and threaded conductive fibers throughout the glove to connect each electrode to the cuff of the glove, where the researchers have glued a control electrode.

Several volunteers took turns wearing the touchscreen glove and performing various tasks, including holding a balloon and grabbing a glass beaker. The team collected readings from each sensor to create a pressure map on the glove during each task. The maps revealed distinct and detailed pressure models generated during each task.

The team plans to use the glove to identify pressure patterns for other tasks, such as writing with a pen and handling other household items. Ultimately, they envision that such tactile aids could help patients with motor dysfunction calibrate and strengthen their hand’s dexterity and grip.

“Some fine motor skills require not only knowing how to handle objects, but also knowing what force needs to be exerted,” Fang explains. “This glove could provide us with more accurate measurements of grip strength for control groups versus patients recovering from stroke or other neurological disorders. It could increase our understanding and allow control.

This research was funded, in part, by the Joint Center for Mechanical Engineering Research and Education at MIT and SUSTech.


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