New technology detects eye blinks with ultrasound

People blink 15-20 times per minute – approximately 10,000 times per day! In fact, some researchers are interested in observing eye blinks. For example, people blink more when they are tired. If we could monitor blinking while a person is driving, they could be warned when they are too tired to drive safely, potentially reducing traffic accidents. Or a person who cannot speak due to injury or disability could potentially communicate through blinking, which can be translated by a computer.

Current eyeblink monitoring methods have many limitations. one uses a camera to capture images of the eyes, but requires high computing power. Poor lighting can also reduce image quality. Another relies on a small sensor attached to the skin near the eye. This can get uncomfortable and is not very portable. More another method uses infrared to monitor eye blinks, but it carries many safety risks.

Researchers in China recently developed a small device integrated into a pair of glasses that can reliably track eye blinks with ultrasound. Ultrasound is a common medical imaging tool that measures high-energy sound waves that are reflected by objects of interest. Ultrasound is also not affected by poor lighting and is known to be safe for the eyes.

To keep their device small and light, the scientists used a technology called microelectromechanical systems, or MEMS for short. MEMS technology combines electrical and mechanical parts into a small device, typically less than a millimeter. The scientists designed a MEMS device with a top and bottom electrode and a piezoelectric layer sandwiched between them. Piezoelectric is a unique material that builds up an electrical charge in response to physical force, including sound waves, making it an excellent material for ultrasonic detection. The scientists chose aluminum nitride as the piezoelectric layer because it is non-toxic and highly sensitive to vibrations.

A single MEMS layered structure like this called a transducer, is so small that it does not produce a strong signal by itself. To amplify it, the scientists connected several transducers in a 15×15 array, resulting in a chip that is only 2.5mm by 2.5mm. They then attached this tiny chip to a pair of glasses.

The transducer is electrically excited in air and vibrates due to the piezoelectric effect. These vibrations emit an ultrasonic wave. When a person wears glasses, the ultrasound wave is reflected by their eye or eyelid. This reflected wave generates a signal in the transducer. Using the laws of physics, scientists calculate the distance the reflected wave has traveled. Because of the eyelid, it will be slightly longer when the eye is open and shorter when it is closed.

To evaluate their new device, the scientists conducted a series of eye blink tests. First, they collected data for only one eye open or closed. For an open eye, the reflected wave was measured at 72 microseconds. When the eye was closed, the reflected wave was measured at 51 microseconds. The scientists calculated the change in distance as 3.6 mm, which corresponds to the thickness of a human eyelid. They then ran tests to confirm that they could reliably measure similar changes with both eyes open or closed, as well as with one eye open and one closed.

The scientists then ran these tests on six people to see if the device worked for different eye shapes and eyelid thicknesses. They were able to successfully detect whether each person had their eyes open or closed. However, the time taken for the reflected wave to return to the device is different for each volunteer due to differences in eyelid and eye shapes. This result shows that the time difference in the measurements, not the exact time, is what matters when determining whether a person’s eyes are open or closed.

Finally, the scientists wanted to monitor their device in real time. They designed a computer algorithm that would record the ultrasound waves measured by the transducer, calculate the time difference of each reflected wave, and infer whether the person’s eyes were open or closed. They found that they could successfully monitor a person’s eyes for one minute and recorded the blinking state in real time.

The scientists in this study designed and built a small device attached to a pair of glasses that reliably detects and records whether a person’s eyes are open or closed. Not only was the device reliable, but it is portable and safe. The authors suggest longer-term human testing and further research to optimize the design, such as circuit size and other features.

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