Preparation and testing of hydrogel–elastomer microfluidic assembly

Ecoflex microfluidic channel was prepared by curing Ecoflex resin onto silicon wafer mould with predetermined positive SU-8 patterns following conventional soft-lithography techniques²⁶. Physically crosslinked PAAm-alginate hydrogel was prepared by the previously described procedure. Physically crosslinked PAAm-algiante hydrogel was gently assembled on top of the Ecoflex microfluidic channel. The assembly was then exposed to ultraviolet irradiation for an hour. To verify microfluidic function and diffusion through hydrogel matrix, 2% aqueous solution of red, blue and green food dyes (McCormick) were supplied through three inlets of microfluidic channels.

The hydrogel–elastomer microfluidic chip for the diffusion-reaction test was prepared following the abovementioned procedures except that 0.1 wt.% of pH indicator solution was added into the PAAm-alginate pre-gel solution. To test the diffusion-reaction of waterborne chemicals, acid solution (0.1 M aqueous hydrogen chloride solution) and base solution (0.1 M aqueous sodium hydroxide solution) were supplied in two microfluidic channels. Note that all hydrogel–elastomer microfluidic chips were kept in humid chamber during tests to avoid dehydration.