Approaches to deformable physical sensors: Electronic versus iontronic

For the past two decades, the researches on stretchable physical sensors have made great technological advances, and have been applied to various applications such as electronic skin for robots, haptic devices, bionics, and wearable/implantable healthcare sensors, etc. The deformable physical sensors have been investigated in two approaches: the electronic sensors with the well-developed fabrication technologies and the iontronic sensors as a new technological alternative. There has been a wide spectrum of researches branching from the two approaches. They have evolved from simple-structured sensors with a single function into high-resolution array sensors with multiple functions. A variety of technological methods and principles have been explored depending on the target applications and the materials in use. The deformable sensors can be differentiated according to the specific methodology and principle; i) type of electrical signal (resistance or impedance, capacitance, induction, voltage or current, frequency), ii) power consumption (use of external power (passive) or self-signaling (active)), iii) data acquisition method (time-division multiple access or event-driven parallel access), and iv) multiple functionality (combination of different sensing units or multimodality). It is not clear at this stage which approach and method are suitable for which applications and materials. This review begins with a comparison of the technological methodologies and summarizes the evolution of deformable physical sensors. Focusing on key conceptual achievements according to the technological methodologies, this review introduces the advances of the electronic sensors in section 2 and the iontronic sensors in section 3. Section 4 discusses the challenges and directions for future researches, along with some possible technological solutions.