A wireless full waveform respiration monitor worn entirely inside user's mouth (intra-oral) that streams data to a Web Bluetooth enabled web application. We built this by hacking the tiniest ARM based activity tracker we could find!
The device measures the air pressure, humidity and skin temperature inside the user's airway. In short, BME280 air pressure sensor + MLX90615 thermopile thermometer + hacked miniature nRF51822 based activity tracker mounted on an ultra-thin custom dental retainer.
We also spun this off into a separate Hackaday project with more detail, check it out.