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

水凝胶是具有吸收大量水的能力的交联聚合物。这种特征行为导致水凝胶体积的增加。因此，水凝胶是传感器和致动器中应用的合适材料，特别是因为水凝胶可以用于微系统中。巨大的水凝胶体积的溶胀导致可检测的力，该力可以通过传感器测量或作为致动器组件工作。然而，水和化学物质在水凝胶的交联聚合物结构中的扩散性质影响响应特性。因此，高灵敏度往往沿着着长的响应时间。解决方案可能是采用多孔水凝胶，可以加速合作扩散过程。多孔水凝胶在基于水凝胶的压阻式pH传感器中显示出显着的改进，在我们以前的工作。在此，基于丙烯酸/丙烯酰胺基水凝胶的pH传感器的响应时间显著缩短。此外，水凝胶的溶胀度显著增加。然而，由于水凝胶的刚度降低，传感器的灵敏度降低。对于这一先前的工作，多孔层是通过使用在聚合过程中引起相分离的致孔剂来合成的。致孔剂的使用产生具有大孔隙率的材料。然而，由于合成方法的原因，孔径和孔径分布的调节是不充分的。用于合成具有限定的孔几何形状的水凝胶层的可能途径是如下所述的模板方法。制备有机相、水和表面活性剂的混合物。具有确定的聚集体结构（胶束、圆柱形胶束、双连续相、层状相、六方相和立方相）的稳定、透明的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，形成的聚集体结构可以作为模板合成不同形貌的介孔水凝胶。其次，用中孔代替大孔导致水凝胶层的总体积中更高的水凝胶体积。由于这些原因，这种模板方法可以发展成为生产用于微系统技术的快速响应和敏感水凝胶的首选途径。

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.