Before revision:  Wrist-worn wearable devices such as smartwatches and smartbands became ubiquitous. They are equipped with photoplethysmogram (PPG) sensors which enable to extract heart rate variability (HRV) features.
After revision:  Ubiquitous wrist-worn wearable devices are equipped with photoplethysmogram (PPG) sensors which enable to extract heart rate variability (HRV) features.