Biomaterials for MEMS

132 V. Bazargan and B. Stoeber

Figure 6.13. Concept of wall formation in a microfluidic channel at the diffusive interface

between two fluids.

6.5.3 Cross-Channel Transport

6.5.3.1 Concept of Cross-Channel Transport using a Gel Wall

Figure 6.13 shows the concept of formation of a wall between two fluids in a

microfluidic channel. Diffusion of compounds occurs across the interface between

both fluids. Presence of one compound in the other stream leads to formation of

a wall in the center of the channel. If further the material system and the flow

condition are chosen appropriately, this wall can be removed on one side and

regenerated on the other side at a constant rate leading to a steady displacement of

the wall perpendicular to the flow velocity of the fluid. During wall generation, if

objects such as compounds, particles or cells are incorporated into the wall, these

objects are slowly moved from one fluid stream into the other stream.

6.5.3.2 Generation of a Gel W all in a Microchannel

Figure 6.14 shows a microfluidic device with two inlet channels, one for a Pluronic

F127 solution and one for a saline solution. The channels were fabricated in

Figure 6.14. Top view of a microfluidic device. Pluronic solution and saline solution are

introduced in a 10 μm high microchannel at T

a

=24

◦

C. Gel formation occurs in the center of

the channel. The Pluronic stream is seeded with fluorescent particles. For color reference,

see page 268.

SO13997_text.indd 140SO13997_text.indd 140 26/01/2011 3:50 PM26/01/2011 3:50 PM

Flow Control in Biomedical Microdevices using Thermally Responsive Fluids 133

polydimethylsiloxane (PDMS) using standard soft lithography techniques; then

the PDMS was bonded to a glass substrate.

The 15 wt% Pluronic solution has a gel temperature of 26.5

◦

C, which is above

the ambient temperature T

a

=24

◦

C. A gel wall forms in the center of the channel

where sodium phosphate from the saline stream diffuses into the Pluronic stream.

This leads to a locally reduced gel formation temperature for the Pluronic solution

according to the phase diagram in Fig. 6.4. Fig. 6.14 also shows the distribution of

sodium phosphate ions across the channel. The ions diffuse from the saline stream

across the gel wall into the Pluronic solution. The ion concentration in the wall

near the Pluronic stream is the minimum concentration at which gel formation can

occur at ambient temperature. The concentration inside the wall near the saline

stream corresponds to the critical sodium phosphate concentration above which

gel formation cannot occur. At steady state, sodium phosphate diffuses from the

saline stream into the Pluronic stream at a constant flux. This increases the sodium

phosphate concentration in the wall near the saline stream above the critical

concentration and therefore the saline side of the wall liquefies. At the same time,

the steady diffusion of ions into the Pluronic stream increases the ion concentration

in this stream near the wall above the necessary concentration for gel formation

at ambient temperature T

a

= 24

◦

C. As a consequence, the wall is permanently

removed near the saline solution, while it is simultaneously rebuilt in the Pluronic

stream. This leads to a steady motion of the wall across the microchannel, as

documented through the PIV measurement in Fig. 6.15. The cross-channel velocity

of the wall is nearly constant at v =5μm/s, and it is insensitive to the magnitude

of the pressure driving the fluids and the concentration of the saline solution. This

suggests that the motion of the wall is defined by the gel removal rate near the

saline solution.

Figure 6.16 shows the wall thickness along the flow channel for different

experimental conditions. In all cases, the wall thickness increases almost linearly

ϭϬ

ϱ

ŵͬƐ

ϭϬϬђŵ

WůƵƌŽŶŝĐ^ŽůƵƚŝŽŶ

^ĂůŝŶĞ^ŽůƵƚŝŽŶ

Figure 6.15. Velocity field of a section of the wall from PIV.

SO13997_text.indd 141SO13997_text.indd 141 26/01/2011 3:50 PM26/01/2011 3:50 PM