Freezing-tolerant and conductive hydrogels have attracted tremendous interest as promising materials for flexible sensors under the gelid condition. In this work, a freezing-tolerant, dual-responsive hydrogel sensor was developed by using an ionic–glycerol hybrid hydrogel. The fabricated hydrogel sensor was demonstrated to detect efficiently different temperatures ranging from −30 to 100 °C or to detect both small strains (e.g., 1%, 2%, 4%) and large strains (e.g., 10%, 20% and 30%) as a freezing-tolerant strain sensor. A systematic investigation was conducted to explore the thermal- and strain-sensitive mechanisms of the obtained hydrogel. It was found that the resistance of the hydrogel rapidly increased as the temperature (T) decreased. In addition, the strain sensitivity increased as T decreased from 0 °C to −30 °C. Surprisingly, an abrupt resistance increase was observed when the tensile strain of the gel reached a critical value (e.g., 2620% at 0 °C) or upon moving the hydrogel from 25 to −10 to −30 °C. Such a sharp increase in resistance was found to be mainly caused by the abrupt fracture of a vast amount of the inner hydrogel network at large strains or the appearance of microcrystals in the gel network at low temperatures. This work provides fundamental and practical insights into fabricating functional freezing-tolerant hydrogel sensors for engineering applications.

Hydrogel coatings have aroused tremendous research interest due to their excellent biocompatibility, lubricity, and flexibility. However, most hydrogel coatings are limited by their low mechanical strength and large-scale applications on diverse substrates. Herein, a mechanically tunable gelatin methacryloyl-based hydrogel coating is developed via a scalable ultraviolet-curing strategy combined with post-immersion treatment in a sodium citrate–water–glycerol solution. Notably, the hydrogel coating shows mechanically tunable properties by changing the soaking time or the concentration of sodium citrate. The compression modulus of the hydrogel is enhanced by 15 times after 120 min of soaking in sodium citrate–water–glycerol. Besides, the anti-freezing property endows the hydrogel coating with low-temperature (e.g., −40 °C) flexibility. The hydrogel coating is transparent with a transmittance of over 80% within the visible light region and exhibits long-term stability (over 60 days at 25 °C and 40% relative humidity). Moreover, the obtained hydrogel coating shows good lubrication properties with a friction coefficient of less than 0.01. The fabricated hydrogel coating can be potentially used in engineering applications where mechanically tunable properties, anti-freezing, lubrication, and excellent light transmittance are required.

Hydrogels containing hydrophobic materials have attracted great attention for their potential applications in drug delivery and biosensors. This work presents a kneading-dough-inspired method for dispersing hydrophobic particles (HPs) into water. The kneading process can quickly mix HPs with polyethyleneimine (PEI) polymer solution to form “dough”, which facilitates the formation of stable suspensions in aqueous solutions. Combining with photo or thermal curing processes, one type of HPs incorporated PEI-polyacrylamide (PEI/PAM) composite hydrogel exhibiting good self-healing ability, tunable mechanical property is synthesized. The incorporating of HPs into the gel network results in the decrease in the swelling ratio, as well as the enhancement of the compressive modulus by more than five times. Moreover, the stable mechanism of polyethyleneimine-modified particles has been investigated using surface force apparatus, where the pure repulsion during approaching contributes to the good stability of the suspension. The stabilization time of the suspension is dependent on the molecular weight of PEI: the higher the molecular weight is, the better the stability of the suspension will be. Overall, this work demonstrates a useful strategy to introduce HPs into functional hydrogel networks. Future research can be focused on understanding the strengthening mechanism of HPs in the gel networks.

Understanding the friction behavior of hydrogels is critical for the long-term stability of hydrogel-related bioengineering applications. Instead of maintaining a constant sliding velocity, the actual motion of bio-components (e.g., articular cartilage and cornea) often changes abruptly. Therefore, it is important to study the frictional properties of hydrogels serving under various sliding velocities. In this work, an unexpected low friction regime (friction coefficient μ < 10−4 at 1.05×10−3 rad/s) was observed when the polyacrylamide hydrogel was rotated against a glass substrate under alternative sliding velocity cycles. Interestingly, compared with the friction coefficients under constant sliding velocities, the measured μ decreased significantly when the sliding velocity changed abruptly from high speeds (e.g., 105 rad/s) to low speeds (e.g., 1.05×10−3 rad/s). In addition, μ exhibited a downswing trend at low speeds after experiencing more alternative sliding velocity cycles: the measured μ at 1.05 rad/s decreased from 2×10−2 to 3×10−3 after 10 friction cycles. It is found that the combined effect of hydration film and polymer network deformation determines the lubrication and drag reduction of hydrogels when the sliding velocity changes abruptly. The observed extremely low friction during alternative sliding velocity cycles can be applied to reduce friction at contacted interfaces. This work provides new insights into the fundamental understanding of the lubrication behaviors and mechanisms of hydrogels, with useful implications for the hydration lubrication related engineering applications such as artificial cartilage.