**4. Outlooks**

HAR relates to a wide range of tasks that deal with daily life with digitalization, aiming to assist people to have a better quality of life. As the keystone, sensing skills for HAR tasks are still under intensive development. Based on the surveyed most prominent sensing techniques in this manuscript, we further conclude some outlooks on the development of sensing skills for HAR tasks.


sensing technique cannot supply. An example is a microelectrode-based biosensor, which has been proposed for long-term monitoring of sweat glucose levels [181]. The multi-function microelectrode-based biosensor is fabricated on a flexible substrate, which offers greater wearing comfort than rigid sensors, thus providing long-term on-skin healthy monitoring.

Besides that, sensors are becoming more compact and power-efficient to provide always-on monitoring, or the sensors will be only in an active state triggered by a specific event. The energy harvesting techniques also provide ambient energy for sensors to extend the power life. With the growing number of wearable devices, the health monitoring sensors [182] are being deployed more near the body for continuous and real-time analysis of sweat, blood, etc., such as EEG monitoring by smart watches, or stress sensing by the Fitbit smart band, which uses an electrodermal activity skin response sensor to obtain a reading when the palm of user's hand is pressing the metal outer rim, and then the corresponding app will analyze the overall stress.
