Micromixers based on molecular diffusion - Micromixers Fundamentals Design and Fabrication - page 191

and sinks. Wu et al. solve this problem by using a fixed concentration field established in a hydrogel
membrane. A concentration gradient with an arbitrary shape can be formed by running microchannels
across the static concentration field in a given way. The porous hydrogel membrane is sandwiched
between two PDMS parts ( Fig. 5.23 (a)). The upper PDMS part contains the source and the sink, the
lower PDMS contains the microchannels for the concentration gradient. The gradient is established in
the hydrogel by molecular diffusion. The concentration in the microchannels of the lower PDMS parts
came in equilibrium with the concentration of the hydrogel. Thus, the shape of the concentration
distribution is determined by the shape of the microchannels ( Fig. 5.23 (b,c)). The amount of molecules
released at the source can be controlled by an applied pressure [39] . Controlling individual injection
nozzles with integrated microvalves allows dynamic control of complex concentration distributions [40] .
FIGURE 5.23
Free-diffusion gradient generator with arbitrary shapes of concentration distribution: (a) design concept;
(b) the shape of the microchannel; and (c) corresponding shape of the concentration gradient.
(Reprinted with permission from [38] .)