Flow Control in Biomedical Microdevices using Thermally Responsive Fluids 135
transition, the cells experience an increasing concentration of sodium phosphate,
until they arrive in an environment with the maximum concentration. Controlling
the transit time for these cells therefore defines the temporal rate of increase in
ion concentration to which the cells are being exposed. This concept therefore
represents a tool to control the rate of change of ion concentration around living
cells for investigating cell response to temporal ion gradients. In addition, this
concept can be used to limit the rate of change in ion concentration that a cell is
submitted to in order to prevent cell damage.
A similar influence of ions on the gel formation temperature of Pluronic
solutions has been observed for other compounds than sodium phosphate. Corre-
spondingly, other compounds can be employed to achieve a similar cross-channel
transport as described here for sodium phosphate, so that this concept is not
limited to the system Pluronic/sodium phosphate.
6.6 CONCLUSIONS
Three different flow control mechanisms based on thermally responsive polymer
solutions have been discussed, all of which are based on the presence of the
thermally responsive polymer Pluronic in the working fluid of the microfluidic
system. All of these concepts use very simple fabrication techniques that allow
easy integration into lab-on-a-chip devices. All concepts include no moving parts
making them very robust to failure.
First, selective heating of specific regions in microfluidic devices was used
to reversibly transform the polymer solution into a gel, which then blocked the
microfluidic channel. This concept was demonstrated with a heating element
integrated into the wall of a 10 μm deep channel. The fast valve response (1 ms
possible) was hereby limited by thermal diffusion. Future extension of this concept
to deeper channels and faster valve response could possibly be achieved with Joule
heating within the fluid.
Passive flow control was achieved for ambient temperatures slightly below
the gelation temperature of the Pluronic solution. Viscous heating generated
within the flow led to successive gel formation inside a microfluidic channel until
the channel was blocked to flow. Channel blockage occurred faster for ambient
temperatures closer to the gel temperature. This effect could serve as a mechanism
for automatic flow dosage in response to the ambient temperature.
Furthermore, a cross-channel transport concept has been developed in which
a gel wall is formed in the center of a microfluidic channel at the diffusive
interface between a Pluronic solution and a sodium phosphate solution. The wall
is generated on one of its sides and it is dissolved on its second side, leading to a
steady motion across the channel. During wall generation, particles or living cells
can be included in the wall, which then move at the same velocity as the wall and
are transported from one fluid stream into the second fluid stream. It is possible to
maintain a concentration gradient of diffusive compounds across the wall and the
transition time can be adjusted through the flow rates of the fluid streams and the
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