Passive nanorheological tool to characterise hydrogels

Hydrogels are highly versatile, multi-phase materials with a wide variety of applications, due to their complex structures and tuneable features at the micron and sub-micron scale. Physical and chemical properties within the local environments contribute to the overall properties of a hydrogel. Current quantitative techniques used to characterise the properties of a hydrogel usually focus on bulk properties and are limited to identifying macroscopic properties, providing little information about local variations and heterogeneity, or fail to provide insight into real-time dynamic responses to external stimuli. These issues are especially challenging when characterising soft hydrogels due to their high-water content, which induces weak signals and noisy data. Here, we present a passive nanorheological tool that indirectly enables the characterisation of soft hydrogels at the micro/nanoscale by tracking nanoprobes with a label-free optical microscopy technique, making this an inexpensive, time-efficient, and user-friendly tool. This tool allows effective mapping of the properties in local micro/nano environments in heterogeneous soft materials thus permitting the identification of real-time sol-gel phase transition in thermosensitive hydrogels. Hence, this novel nanorheological characterisation tool has great potential for use in soft material design, manufacturing and quality control processes.