MIT researchers outfitted a baby diaper with an RFID tag that emits a wireless signal when the surrounding material gets wet. The wetness "sensor" is actually a type of hydrogel that's commonly found in diapers to absorb liquid. As the hydrogel gets wet, it swells and its conductivity increases, triggering the RFID tag. The RFID tags are printed as stickers for around 2 cents each compared to other Internet-connected diapers in development with reusable sensors that cost as much as $40/each. From MIT News:

Over time, smart diapers may help record and identify certain health problems, such as signs of constipation or incontinence. The new sensor may be especially useful for nurses working in neonatal units and caring for multiple babies at a time...

(MIT AutoID Lab researcher Pankhuri Sen) envisions that an RFID reader connected to the internet could be placed in a baby’s room to detect wet diapers, at which point it could send a notification to a caregiver’s phone or computer that a change is needed. For geriatric patients who might also benefit from smart diapers, she says small RFID readers may even be attached to assistive devices, such as canes and wheelchairs to pick up a tag’s signals.

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https://boingboing.net/2020/02/16/mits-smart-diaper-alerts.html