I. Introduction
Research on the development of flexible, stretchable, conformal and wearable sensors which can be mounted on the human skin has been gaining increasing interests [1]–[4]. These sensors are capable of providing real time information about the vital signs of an individual's health condition. There have been reports on the use of these sensors in health tracking devices [5], temporary tattoos for hydration level measurement [6] and electronic nose [7]. The various applications of wearable sensors range from monitoring human health situations with target parameters such as body movement, body temperature, electrocardiogram (ECG), electroencephalogram (EEG) and blood pressure to different applications in the biomedical, epidermal and military industries [8].