Flow Control in Biomedical Microdevices using Thermally Responsive Fluids 125
Figure 6.5. A schematic illustration of the principle of an active valve using a thermally
responsive ﬂuid in a microﬂuidic network. (a) Fluid from one channel is diverted into two
channels at a channel bifurcation; (b) activating an integrated heater leads to localized gel
formation in the corresponding microchannel, which subsequently blocks this channel to
ﬂow. For color reference, see page 266.
achieved in an arbitrary location and at any time in the microﬂuidic system.
gel formation of Pluronic solutions is completely reversible, the hydrogel returns
to its liquid phase upon cooling, and the gel block disappears, opening the channel
again to ﬂow.
184.108.40.206 Microdevice Design and Fabrication
The device shown in Fig. 6.6 was fabricated using standard MEMS technology.
The fabrication process for this test device should be considered as one possible
example process. When designing a speciﬁc process for a given application,
process compatibility will other devices of a mictoTAS needs to be considered as
Figure 6.7 shows the schematic of the cross section of the device in Fig. 6.6. The
silicon substrate with the heaters and the glass substrate with the ﬂow channels are
fabricated separately, and bonded together using anodic bonding. A silicon wafer
is coated with a thin ﬁlm (0.2 μm) of silicon nitride using low pressure chemical
vapor deposition (LPCVD). This ﬁlm is patterned by photolithography and plasma
etching to open areas that will be used for routing of electrical leads and heaters.
A subsequent silicon wet etch forms 4.5 μm deep recesses in the silicon substrate,
following which the silicon nitride hard mask is removed by plasma etching.
Another silicon nitride layer for electric insulation is deposited onto the substrate
using LPCVD, followed by an LPCVD thin ﬁlm of doped polysilicon with a sheet
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