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.

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 microl min(-1) and a back pressure of 1.28 kPa. The second type, a displacement pump, provides a higher performance (maximal 4.5 microl 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.

A large‐scale integration technology for MEMS based on the optoelectrothermic control of a temperature‐sensitive hydrogel is described and exemplified using an imaging array system, a so‐called artificial skin. The hydrogel itself acts as active functional unit, i.e., as actuator. The artificial skin comprises more than 4 000 individual actuators and provides both, visual and palpable artificial impressions of a surface.

A chemofluidic oscillator circuit that employs a hydrogel‐based chemofluidic transistor for chemical‐fluidic coupling is presented. It shows a period between 200 and 1000 s and alcohol concentrations oscillating between 2 wt% and 10 wt%. Because of the direct interaction with chemistry, chemofluidic transistors have the potential to facilitate labs‐on‐chips with enhanced functionality and scalability.

a b s t r a c tPermeation through polymeric membranes can be controlled by surface coating of a polyethylene terephthalate (PET) membrane with poly(N-isopropylacrylamide) (PNIPAAm) and inserting pores of defined geometry. When the temperature of the system rises above the volume phase transition temperature, the pores open, which allows permeation of formerly blocked particles. The exact control of the temperature allows defined change of the pore size and therefore enables separation abilities. Free swelling experiments are conducted to obtain the swelling behaviour of PNIPAAm. Then, a temperature expansion model is derived in order to simulate this behaviour with the finite element tool ABAQUS. The gained results are in excellent agreement with the observed shape change. Membranes with permeation control of particles can be used for biomedical application in microfluidics to analyse the size distribution of cells or in chemical information processing as a transistor-like component for an information-bearing chemical species. The possibility to simulate the behaviour of such permeation systems allows computer aided design and prediction of permeation abilities in these areas.