Stimuli-responsive hydrogels are materials with great potential for development of active functionalities in fluidics and micro-fluidics. Based on the current state of research on pH sensors, hydrogel sensors are described qualitatively and quantitatively for the first time. The review introduces the physical background of the special properties of stimuli-responsive hydrogels. Following, transducers are described which are able to convert the non-electrical changes of the physical properties of stimuli-responsive hydrogels into an electrical signal. Finally, the specific sensor properties, design rules and general conditions for sensor applications are discussed.

Glaciers are indicators of ongoing anthropogenic climate change¹. Their melting leads to increased local geohazards², and impacts marine³ and terrestrial^4,5 ecosystems, regional freshwater resources⁶, and both global water and energy cycles^7,8. Together with the Greenland and Antarctic ice sheets, glaciers are essential drivers of present^9,10 and future^11–13 sea-level rise. Previous assessments of global glacier mass changes have been hampered by spatial and temporal limitations and the heterogeneity of existing data series^14–16. Here we show in an intercomparison exercise that glaciers worldwide lost 273 ± 16 gigatonnes in mass annually from 2000 to 2023, with an increase of 36 ± 10% from the first (2000–2011) to the second (2012–2023) half of the period. Since 2000, glaciers have lost between 2% and 39% of their ice regionally and about 5% globally. Glacier mass loss is about 18% larger than the loss from the Greenland Ice Sheet and more than twice that from the Antarctic Ice Sheet¹⁷. Our results arise from a scientific community effort to collect, homogenize, combine and analyse glacier mass changes from in situ and remote-sensing observations. Although our estimates are in agreement with findings from previous assessments^14–16 at a global scale, we found some large regional deviations owing to systematic differences among observation methods. Our results provide a refined baseline for better understanding observational differences and for calibrating model ensembles^12,16,18, which will help to narrow projection uncertainty for the twenty-first century^11,12,18.

This paper describes two types of polymeric micropumps based on the temperature-sensitive hydrogel poly(N-isopropylacrylamide). The gel actuators are realised as photopolymerised patterns and microgels. They are electrothermically controlled by resistive heating elements. The diffusion-based micropump contains a photopatterned monolithic actuator, which is placed within the pump chamber, and provides a valveless single layer set-up. The diffusion micropump is intended for low performance applications and can operate in two modes: peristaltic or pulsatile. The maximum operating parameters are a flow rate of 2.8 ± 0.35 µl min⁻¹ and a back pressure of 1.28 kPa. The second type, a displacement pump, provides a higher performance (maximal 4.5 µl min⁻¹ and 15 kPa). The pump comprises a microgel-based actuator, which is placed within a separate actuator layer, and active microvalves. The specific features of the design and performance of the pumps are discussed.