Flexible Humidity Sensitive Fiber with Swellable Metal–Organic Frameworks

Humidity sensors have become an essential need for improving daily life quality. Here, a novel humidity sensitive fiber was fabricated using the soft polydimethylsiloxane (PDMS) as the core layer and the non-swellable polyvinylidenedifluoride (PVDF) embedded with a swellable MIL-88A as the outer layer. Noticeably, the PDMS fiber was chosen as a carrier due to its smoothness, softness and good tensile properties. The isoreticular FeIII dicarboxylate MIL-88 family with swellable metal–organic frameworks (MOFs) can undergo reversible dynamic structural transformations with a response to external humidity change. The resulted fiber with swellable MOFs showed three kinds of deformation for a single solvent and also had good deformation performance for two-component miscible solution with different volume ratios. As a proof of concept, a shape-memory effect at relative humidity from 10 to 90% and the simulated salt solutions instead of relative humidity changes were used to evaluate humidity change. In addition, a deep insight into the self-shape-change mechanism among those phenomena was investigated, wherein expansion deformation of the PDMS fiber as well as the structural transformation of the MIL-88A worked in different conditions.