Biomaterials for MEMS

128 V. Bazargan and B. Stoeber

Figure 6.8. An image of the 0.7 μm large fluorescent seed particles in the microchannels

showninFig.6.6.

time between image frames. Valve closing and opening has been demonstrated

for 0.25 - 4 Hz cycles of 100 ms to 1 s long 400 mW pulses.

The time scale for thermal diffusion across the height of the channel limits

the dynamic response of this valve. Assuming the thermal diffusivity of water

α =1.41· 10

−7

m

2

/s as a good approximation for the thermal diffusivity of the

Pluronic solution, the thermal time scale t

d

∼ H

2

/α for heat diffusion across

a channel of height H =10μm is less than 1 ms as discussed in Section 6.2.

Optimizing the pulse shape of the heating voltage applied to the heaters should

yiels a valve responses below 1 ms.

6.5.2 Passive Valving

6.5.2.1 C oncept for Passive Flow Control

In addition to active flow control in microfluidic systems using Pluronic solutions,

their usefulness for passive flow control has been demonstrated as well.

26,53,72

The concept for active flow control discussed in the previous section relies on

a significant temperature change of the Pluronic solution that leads to a rapid

transition across the phase transition temperature, where rapid heating results in

rapid gel formation, and cooling leads to liquefaction.

A concept for passive flow control using Pluronic solutions uses a small

amount of heat generated in the fluid through viscous heating. Viscous heating

describes heat generated through internal friction in a fluid that is in motion,

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Flow Control in Biomedical Microdevices using Thermally Responsive Fluids 129

D

E

Figure 6.9. The flow field at the channel bifurcation shown in Fig. 6.6. The velocities

were evaluated from images of the seed particles as in Fig. 6.8 using PIV; (a) before valve

actuation, (b) 33 ms later; blue: below 40 μm/s, green: 40–80 μm/s, yellow: 80–120 μm/s,

orange: 120–160 μm/s. For color reference, see page 267.

where neighboring fluid layers in shear flow move at different speeds. The rate

of viscous heating Φ can be approximated by μ (

˙

γ)

2

with the shear rate

˙

γ=∂U/∂y

indicating the change in velocity perpendicular to the flow direction. This means

that at high shear rates, heat will be produced the fluid temperature will increase.

If the Pluronic solution is already near the phase boundary, only a small increase in

temperature is required for a transition from the liquid phase to the soft gel phase.

6.5.2.2 Demonstration of Passive Valving

A straight microchannel with a rectangular cross section was fabricated through

deep reactive ion etching (DRIE) of a trench into silicon followed by anodic

bonding of a Pyrex lid to the silicon substrate. The device was connected to a

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130 V. Bazargan and B. Stoeber

Figure 6.10. Experimental setup for the demonstration of passive flow control using

thermally responsive Pluronic solutions at constant flow rate in microchannels. For color

reference, see page 268.

syringe pump with flexible polymer tubing, and a gauge pressure sensor was

attached between the syringe pump and the microchannel as shown in Fig. 6.10.

The fluid drained from the channel through a piece of tubing to atmosphere.

The gauge pressure recorded therefore corresponded to the pressure along the

microchannel, neglecting the pressure drop along the wide flexible tubing on either

end of the microchannel. The microchannel was submerged in a temperature-

controlled water bath to achieve well-defined temperature conditions.

A 13 wt% Pluronic F127 solution was pumped through the microchannel at

aflowrateQ =0.5μL/min using a syringe pump. The device temperature was

maintained at 27

◦

C with the temperature-controlled water bath, which is well

below the gel formation temperature of 31

◦

C of the Pluronic solution. After

initiating the flow, the pressure along the channel reached a constant value as

shown in Fig. 6.11. The pressure then increased, first slowly, and then more rapidly

at a nearly constant rate. After holding ice against the device, the pressure rapidly

decreased.

Figure 6.11. Pressure recorded along a 10 mm long microchannel with rectangular cross

section (150 μm × 100 μm) for a 13 wt% Pluronic F127 solution at 27

◦

Cdriventhroughthe

channel at a flow rate 0.5 μL/min. At around 34 minutes, ice was held against the device to

lower its temperature.

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