Membrane separated flow

The next hypothesis of this chapter is that the flow sensing 3!RT will be capable of measuring fluid flow through membranes and solid dividers. In this case the solid divider symbolizes,

Hypha Cell Wall

3!RT

Figure 6.6: An isometric illustration of a hypha growing across a 3! RT. Mass transport inside of hypha is thought to be laminar [173]. This is represented by the red arrow annota- tions inside the hypha. (Inset) A simplified two-dimensional representation, of the hypha-RT system. A similar scale model of this can be fabricated in PDMS. Providing a controlled testing methodology.

for example, the cell wall of a microorganism. This is desired as it will allow the system to be used as a mass flow sensor for filamentous biological systems such as hyphae. The extension (growth) of hyphae is driven by pressure inside the cell wall (turgor pressure) [92, 173]. This internal pressure creates fluid transport inside the hypha [92, 172, 173, 177]. Figure 6.6 shows an illustration of a filamentous species, in this case a fungal hypha, growing across a 3! RT. Internal fluid velocity is indicated by the red arrow annotations. Inset in this illustration is a simplified two-dimensional representation of the same idea; a 3! RT on the base of the device while a hypha grows above it. In each case, there is fluid flow separated from the RT by a solid membrane.

To test the 3!methods ability to function as a mass transport sensor for hypha, a microfluidic chip, which mimics the simplified two-dimensional representation shown in Figure 6.6 Inset), was designed and fabricated. A schematic diagram of this chip is shown in Figure 6.7. This chip allowed the RT response to beyond membrane fluid flow to be characterised in isolation from other factors. While this was the specific goal of the experiment, other interesting aspects which could be tested include, the accuracy of using the thermal wavelength for

Base Layer Top Layer

Microscope Slide with RT

Figure 6.7: A schematic cross-section diagram of the designed PDMS microfluidic chip. Two PDMS layers are stacked vertically to form two microfluidic channels, separated by a thin PDMS membrane. The base of the channel will be formed by a microscope slide.

measurement, the systems response to bi-directional fluid flow, AC waves in varying media, and how the RT responds to di↵erent fluids.

The chip design consisted of two 100 µm ⇥ 100 µm channels, stacked vertically with a 20 µm PDMS membrane isolating the two channels. The base of the channel was formed by a microscope slide with an integrated 3! RT. To fabricate this design, a combination of dry-film lithography, negative replica moulding, exclusion moulding, and plasma bonding was used, as described in the previous sections. In brief, the two microfluidic channels were formed by two di↵erent PDMS castings. The bottom layer consisted of a 100µm_⇥100µmbase channel, and a 20µm membrane. This structure was formed using the exclusion moulding process. The top channel consisted of the second 100µm_⇥100µmmicrofluidic channel, and a thick PDMS roof greater than 1mmthick in order to support external fluid connections. The top and bottom channels were manually aligned with each other and the RT, and bonded together with oxygen plasma (Tergeo, PIE Scientific). The sealing of the plasma bonds, tubing interfaces, and membrane were confirmed by infusing two colours of food dye into each of the channels, and visually confirming that no mixing or leaks occurred.

After the correct formation of the chip had been confirmed, the coloured food dye was ex- changed for de-ionised (DI) water. In the same manner as Section 6.2.6, a control measure- ment was recorded with the channels full of stationary fluid. The measurement frequency range was calculated assuming homogenous media above the RT, i.e. a constant thermal

di↵usivity. This is a simplification which warrants further discussion; thermal waves in this system will di↵use di↵erent lengths through the varying media i.e. thermal waves will travel through water–PDMS–water–PDMS rather than one medium alone. As such, the thermal wavelengths displayed will not accurately represent PDMS features.

A dual syringe pump (Harvard Apparatus, PHD 2000) was used to actuate fluid flow in the channels. The exit of the channel was connected through an external flow sensor (Elveflow, 1.5 - 80 µl/min) to calibrate the fluid flow rate. A range of flow rates were induced in the microfluidic channels in both simultaneous and exclusive combinations. The 3!response was then recorded for each of these configurations. Specifically, for each volumetric flow rate a 3! response was recorded for, both channels no-flow (the control), top channel flow only, bottom channel flow only, and both channels simultaneously.

6.3

Results and Discussion

The results to this experiment are divided into two sections: the single channel fluid velocity response, and the dual channel (through membrane). In the single channel velocity response a 3!RT is integrated into a microfluidic channel, and its response to fluid velocity is examined. In the dual channel results, a 3!RT is integrated into a vertically stacked microfluidic device, and the through membrane fluid velocity characteristics are assessed.