Covalent Hydrogels with Dual Temperature and Time Memory Function Based on Supramolecular Host–Guest Complexation

Temperature is among the most important measured physical parameters. Despite that numerous polymeric temperature sensors having been reported, none of these systems can record the time that the object was heated to a certain temperature. Here, we report a supramolecular approach for developing a smart polymeric hydrogel capable of measuring the temperature and the duration of the heat exposure. This double memory principle is based on dual kinetic control of the dissociation of cyclobis(paraquat-p-phenylene) tetrachloride (CBPQT⁴⁺,4Cl^–)-naphthalene host–guest complexes within a thermoresponsive naphthalene functionalized poly(N-isopropylacrylamide) hydrogel. Heating-induced collapse of this LCST polymer hydrogel induces host–guest complex dissociation, followed by the diffusion-controlled release of the free CBPQT⁴⁺,4Cl^– host into the medium surrounding the hydrogel. This partial host release is governed by the heat exposure time and the heating temperature that controls the extent of dehydration of the hydrogel and, thus, the diffusion rate of the CBPQT⁴⁺,4Cl^–. Hence, the amount of host released is an indicator for both the heating temperature and the heating time of the hydrogel and can be recorded by determining the reswelling capability of the hydrogel in water. As such, measuring the reswelling capability provides information on how long the hydrogel system was exposed to a certain temperature.