  Due to the fact that roughly sixty percent of the human body is essentially
composed of water, the human body is inherently a conductive object, being able
to, firstly, form an inherent electric field from the body to the surroundings
and secondly, deform the distribution of an existing electric field near the
body. Body-area capacitive sensing, also called body-area electric field
sensing, is becoming a promising alternative for wearable devices to accomplish
certain tasks in human activity recognition and human-computer interaction.
Over the last decade, researchers have explored plentiful novel sensing systems
backed by the body-area electric field. On the other hand, despite the
pervasive exploration of the body-area electric field, a comprehensive survey
does not exist for an enlightening guideline. Moreover, the various hardware
implementations, applied algorithms, and targeted applications result in a
challenging task to achieve a systematic overview of the subject. This paper
aims to fill in the gap by comprehensively summarizing the existing works on
body-area capacitive sensing so that researchers can have a better view of the
current exploration status. To this end, we first sorted the explorations into
three domains according to the involved body forms: body-part electric field,
whole-body electric field, and body-to-body electric field, and enumerated the
state-of-art works in the domains with a detailed survey of the backed sensing
tricks and targeted applications. We then summarized the three types of sensing
frontends in circuit design, which is the most critical part in body-area
capacitive sensing, and analyzed the data processing pipeline categorized into
three kinds of approaches. Finally, we described the challenges and outlooks of
body-area electric sensing.
