Synthetic hydrogels with hydrophobic interactions, which show excellent mechanical performance and good anti-swelling ability in saltwater, have great potential in various industries, such as soft robots, 3D printing, and wearable sensors. Normally, hydrophobic molecules inside a hydrophobic hydrogel tend to aggregate to form a large hydrophobic domain, leading to a phase separation phenomenon because water is a poor solvent of the hydrophobic domain. This aggregation, however, inhibits the adhesion of the hydrophobic hydrogel to various dry materials and thus limits its application in device and sensor industries. In this study, we report the synthesis of hybrid hydrogels with ionically and hydrophobically cross-linked networks. This novel hybrid hydrogel can strongly adhere to various substrates, such as glass, polypropylene, silicone, wood, and polytetrafluoroethylene, with a maximum adhesion strength measured to be 100 kPa. Meanwhile, this hybrid hydrogel can be stretched beyond 8–10 times of its initial length. We attribute this observed strong adhesion and high toughness properties to the synergy of electrostatic interactions and hydrophobic associations. With the strong adhesion and excellent tensile performance, these hydrogels may serve as a model system to explore the strong adhesion mechanism of hydrophobic hydrogels and expand the scope of hydrogel applications.

Materials with stimuli-switchable properties are widespread in nature. Through biomimicry, stimuli-responsive hydrogels have received increasing research in recent years. In particular, hydrogels with stimuli-tunable mechanical properties provide an important platform for designing new materials with specific applications such as 3D printing, soft robot, and tunable adhesion. Herein, the state-of-the-art of hydrogels with stimuli-responsive toughness is reviewed in detail. Characteristic reinforcement mechanisms are fully discussed first, followed by systematical demonstrations of various smart hydrogels responding to various stimuli. In particular, special attention is paid to thermal, light, pH, and salt triggers because of their broad applications and easy implementation. Furthermore, the applications of these smart hydrogels are included. In addition to the overview of recent advances, we finally summarize the critical challenges of current discoveries and make an outlook for the following work, which we anticipate to lead to substantial progress of these responsive materials in the future.

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The Korea-United States Air Quality Study (KORUS-AQ) took place in spring 2016 to better understand air pollution in Korea. In support of KORUS-AQ, 2554 whole air samples (WAS) were collected aboard the NASA DC-8 research aircraft and analyzed for 82 C 1-C 10 volatile organic compounds (VOCs) using multi-column gas chromatography. Together with fast-response measurements from other groups, the air samples were used to characterize the VOC composition in Seoul and surrounding regions, determine which VOCs are major ozone precursors in Seoul, and identify the sources of these reactive VOCs. (1) The WAS VOCs showed distinct signatures depending on their source origins. Air collected over Seoul had abundant ethane, propane, toluene and n-butane while plumes from the Daesan petrochemical complex were rich in ethene, C 2-C 6 alkanes and benzene. Carbonyl sulfide (COS), CFC-113, CFC-114, carbon tetrachloride (CCl 4) and 1,2-dichloroethane were good tracers of air originating from China. CFC-11 was also elevated in air from China but was surprisingly more elevated in air over Seoul. (2) Methanol, isoprene, toluene, xylenes and ethene were strong individual contributors to OH reactivity in Seoul. However methanol contributed less to ozone formation based on photochemical box modeling, which better accounts for radical chemistry. (3) Positive Matrix Factorization (PMF) and other techniques indicated a mix of VOC source influences in Seoul, including solvents, traffic, biogenic, and long-range transport. The solvent and traffic sources were roughly equal using PMF, and the solvents source was stronger in the KORUS-AQ emission inventory. Based on PMF, ethene and propene were primarily associated with traffic, and toluene, ethylbenzene and xylenes with solvents, especially non-paint solvents for toluene and paint solvents for ethylbenzene and xylenes. This suggests that VOC control strategies in Seoul could continue to target vehicle exhaust and paint solvents, with additional regulations to limit the VOC content in a variety of non-paint solvents.

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