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

水凝胶是一种能够吸收大量水分的交联型聚合物。这一特性导致了水凝胶体积的增加。因此，水凝胶是用于传感器和执行器的合适材料，特别是因为水凝胶可以用于微系统。巨大水凝胶体积的膨胀导致可检测的力，这些力既可以通过传感器测量，也可以作为执行器组件使用。然而，水和化学物种在水凝胶的交联聚合物结构中的扩散性质影响着响应特性。因此，高灵敏度往往伴随着较长的响应时间。解决方案可能是使用多孔水凝胶来加速合作扩散过程。基于水凝胶的压阻式pH传感器中的多孔水凝胶在我们以前的工作中表现出了显着的改进。在这里，基于丙烯酸/丙烯酰胺水凝胶的pH传感器的响应时间大大缩短。此外，水凝胶的溶胀度也显著增加。然而，由于水凝胶的硬度降低，传感器的灵敏度降低。在以前的工作中，多孔层是通过使用在聚合过程中导致相分离的致孔剂来合成的。使用造孔剂可生产出具有大孔隙度的材料。由于合成方法的原因，孔尺寸和孔尺寸分布的调整是不够的。合成具有定义的孔几何形状的水凝胶层的可能途径是下面描述的模板方法。制备了有机相、水和表面活性剂的混合物。具有特定聚集体结构(胶束、柱状胶束、双连续相、片状相、六方相和立方相)的稳定透明溶液在某些组成下出现。通过将单体或聚合物溶解在模板溶液的水相或有机相中，聚集体结构可用作合成不同形貌的介孔水凝胶的模板。其次，用中孔取代大孔导致水凝胶层总体积中的水凝胶体积增大。由于这些原因，这种模板方法可以发展成为为微系统技术生产快速响应和敏感的水凝胶的首选路线。

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.