Laboratory for Surface Science and Technology, Department of Materials, ETH Zürich, Switzerland.
Soft Matter. 2021 Jul 7;17(26):6394-6403. doi: 10.1039/d1sm00395j.
Hydrogel surfaces are of great importance in numerous applications ranging from cell-growth studies and hydrogel-patch adhesion to catheter coatings and contact lenses. A common method to control the structure and mechanical/tribological properties of hydrogel surfaces is by synthesizing them in various mold materials, whose influence has been widely ascribed to their hydrophobicity. In this work, we examine possible mechanisms for this "mold effect" on the surface of hydrogels during polymerization. Our results for polyacrylamide gels clearly rule out the effect of mold hydrophobicity as well as any thermal-gradient effects during synthesis. We show unequivocally that oxygen diffuses out of certain molding materials and into the reaction mixture, thereby inhibiting free-radical polymerization in the vicinity of the molding interface. Removal of oxygen from the system results in homogeneously cross-linked hydrogel surfaces, irrespective of the substrate material used. Moreover, by varying the amount of oxygen at the surface of the polymerizing solutions using a permeable membrane we are able to tailor the surface structures and mechanical properties of PAAm, PEGDA and HEMA hydrogels in a controlled manner.
水凝胶表面在许多应用中非常重要，从细胞生长研究和水凝胶贴剂粘附到导管涂层和隐形眼镜。控制水凝胶表面结构和机械/摩擦学性能的一种常见方法是通过在各种模具材料中合成它们，其影响已被广泛归因于它们的疏水性。在这项工作中，我们研究了聚合过程中“模具效应”在水凝胶表面可能的机制。我们对聚丙烯酰胺凝胶的结果清楚地排除了模具疏水性以及合成过程中任何温度梯度的影响。我们明确表明，氧气从某些成型材料扩散到反应混合物中，从而抑制了成型界面附近的自由基聚合。从系统中除去氧气会导致均相交联的水凝胶表面，而与所使用的基底材料无关。此外，通过使用透气膜在聚合溶液的表面上改变氧气的量，我们能够以可控的方式调整 PAAm、PEGDA 和 HEMA 水凝胶的表面结构和机械性能。
