Flexible and stretchable metal oxide gas sensors for healthcare

Capable of measuring volatile biomarker produced by the metabolism from several secretion pathways, flexible and stretchable metal oxide gas sensors have received increasing attention and their development for healthcare starts to gain momentum. Integration of semiconducting metal oxide on a soft, thin, flexible substrate is the key to enable the flexible property to the gas sensor and such integration typically involves either a direct growth or post transfer of the metal oxide on or to the flexible substrate. In addition to the planar plastic substrate, textile represents another important class of flexible substrates due to its ease of integration with clothing. Moreover, the integration of metal oxide on a single fiber provides a great versatility for different applications. Though flexible sensors can easily conform to the developable surface (e.g., cylinder or cone) from a bending deformation, the conformal contact between the sensor and the tissue surface that is often non-developable requires the sensor to be capable of stretching. Due to the intrinsically brittle nature of the semiconducting metal oxide, several stretchable structures have been explored. Despite the great strides made to the burgeoning area of flexible and stretchable metal oxide gas sensors, grand challenges still need to be overcome before the technology can be applied for the practical application. The selected challenges discussed in this mini-review also represent a fraction of possibilities and opportunities for the research community in the future.