Polymeric hydrogels are a most interesting class of ''soft matter'' with several established and many more possible applications as functional materials. In this review we will focus on the combination of polymeric hydrogels and porous membranes which leads to composites with promising functionality for, e.g., mass separations, sensing and analytics, (bio)catalysis, biomedical engineering and microsystem technologies. The combination of a rigid porous membrane with a soft functional hydrogel by a suited preparation technique enables that the functionality of the hydrogel can be applied in a unique way. The most important preparation strategies for hydrogel composite membranes, i.e., pore-filling, various surface-grafting methods and combinations thereof, will be discussed. The structural diversity of the hydrogels is based on the use of a wide range of synthetic monomers, but biopolymers or their derivatives can also be applied. The interplay of the membrane pore structure, the structure of the hydrogel and the distribution of the hydrogel in the pore space can lead to different types of composite membranes with completely different potential applications. The focus will be on promising examples for the various types of functional composite membranes, i.e., macroporous membrane adsorbers, antifouling filtration membranes, hydrogel-based ultrafiltration membranes, other separation membranes with pore-filling hydrogel as selective material, stimuli-responsive membranes and porous membrane valves and gates, as well as biocompatible or bioactive membranes.

Hydrogel pore-filled composite membranes (HPFCM) based on polyethylene terephthalate (PET) track-etched membranes with pore diameters between 200 and 5000 nm and temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm) hydrogels were successfully prepared. A premodification of the pore walls by grafted linear PNIPAAm led to stable anchoring of crosslinked PNIPAAm prepared in a subsequent step. Proper tuning of photopolymerization conditions resulted in a desired microstructure of the hydrogels and thus tailored barrier properties of the composite membranes. The very interesting separation performance of HPFCM was due to diversification of the hydrogel network that caused adjustable sieving properties via synthesis conditions and also largely switchable barrier properties in response to the temperature. The interplay between the immobilized hydrogel and various pore sizes of the membrane support was also investigated. The base membrane provides mechanical support and confines the hydrogel within its pores, and it thus allows using the hydrogel mesh size for size-selective solute transport. Completely stable and selective HPFCM were only obtained with base pore sizes of about 2 mm or smaller. The size-selectivity (molecular weight cut-off) of the same HPFCM was higher under diffusive than under convective flow conditions; this is presumably mainly caused by elasticity deformation of the hydrogel network. The apparent cut-off from diffusion experiments was well correlated to the mesh-size of the hydrogel determined from the Darcy model applied to permeability data obtained under convective flow conditions. Upon temperature increase beyond 32 C, flux increased and rejection decreased very strongly; this remarkable change between macromolecule-size selective ultrafiltration and microfiltration/filtration behavior was fully reversible.

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