Figure 8.

A pH-sensitive hydrogel modified with a fatty acid surface layer. A cylindrical hydrogel with a diameter of 400 μm is polymerized in a microchannel. The hydrogel material is the same as described in Fig. 4e, which contains hydroxyl termini. Fatty acid segments then are covalently linked to the hydroxyl groups on the outer surface of the hydrogel via in-channel treatment (a). When a pH = 12 buffer is flowed through the channel, the modified hydrogel (♦) remains stable for hours without obvious change (b), whereas an unmodified hydrogel of the same composition and size (■) starts to swell instantly. After piercing the hydrogel with a micropipette tip to physically disrupt the surface layer (c), the modified hydrogel starts to swell at the pierced location (d and e) and eventually becomes fully swollen accompanied by a color change (f). (Bars = 100 μm.)