Mesoporous Hydrogels from Microemulsions and Similar Structures for Hydrogel-based Sensors

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Hydrogels are crosslinked polymers with the ability of uptaking huge amounts of water. This characteristic behavior leads to an increase of the hydrogel volume. Thus, hydrogels are the proper materials for applications in sensors and actuators, especially since hydrogels can be employed for utilization in micro systems.The swelling of huge hydrogel volumes results in detectable forces which can either be measured by means of a sensor or work as actuator components. However, the diffusion properties of water and chemical species in the crosslinked polymer structure of the hydrogel affect the response characteristics. Hence, high sensitivity often goes along with long response times.A solution may be the employment of porous hydrogels which can accelerate the cooperative diffusion process. Porous hydrogels in hydrogel-based piezoresistive pH sensors showed remarkable improvements in our previous works. Here, the response time of a pH sensor based on an acrylic acid/acrylamide based hydrogel was reduced considerably. Also, the swelling degree of the hydrogel increased noticeably. However, the sensitivity of the sensor decreased due to a decreased stiffness of the hydrogel. For this previous work, porous layers were synthesized by using porogens which cause a phase separation during polymerization. The use of porogens yields materials with macroporosity. Due to the synthesis method, the adjustment of pore sizes and pore size distribution is insufficient though.A possible route for synthesizing hydrogel layers with defined pore geometry is a template method described as follows. Mixtures of organic phase, water and surfactant are prepared. Stable, transparent solutions with defined aggregate structures (micelles, cylindrical micelles, bicontinuous, lamellae, hexagonal and cubic phases) occur at certain compositions. By dissolving monomers or polymers in the aqueous or organic phase of the template solution, the aggregate structures can be used as templates for the synthesis of mesoporous hydrogels with different morphologies.By using this method, firstly, the template is defined before polymerization. Secondly, replacement of macropores with mesopores leads to a higher hydrogel volume in the total volume of the hydrogel layer. For these reasons, this template method can evolve to be the preferred route to produce fast-responsive and sensitive hydrogels for microsystems technology.