Depth, microchannel

The geometry of the device is important the terrace length and microchannel depth are size-determining factors. 

Unlike capillary electrophoresis, wherein absorbance detection is probably the most commonly utilized technique, absorbance detection on lab-on-a-chip devices has seen only a handful of applications. This can be attributed to the extremely small microchannel depths evident on microchip devices, which are typically on the order of 10 pm. These extremely small channel depths result in absorbance pathlengths that seriously limit the sensitivity of absorbance-based techniques. The Collins group has shown, however, that by capitalizing on low conductivity non-aqueous buffer systems, microchannel depths can be increased to as much as 100 pm without seeing detrimental Joule heating effects that would otherwise compromise separation efficiencies in such a large cross-sectional microchannel [38],... 

Although most photoresists are generally considered to be sacrificial materials, liquid-type negative photoresists, such as SU-8, can be used to create microchannels within microfluidic chips [20]. The photoresist then becomes a structural material, in such a way that its thickness determines the depth of the microchannel. A negative dry photoresist... 

In the ESy, a miniature FS membrane is supported by two small, identical pieces of PP plastic, constituting a miniaturized membrane unit called an ESy extraction card (see the inset in Figure 4.7), which is housed under mechanical pressure in a card holder. The two PP pieces have dimensions of 2 mm x 20 mm x 40 mm. The inner surface of each piece contains a machined groove defining a microchannel of 1.65 pL volume (0.125 mm depth x 0.6 mm width x 22 mm length). The very small piece of FS membrane (2 mm width x 22 mm length x 25 pm thickness) is fastened in... 

Infrared detection of toluene can be achieved on a microchip. The chip substrate, which should be IR-transparent, has been fabricated from CaF2. The microchannels were etched on CaF2 using a saturated Fe(NH4)(S04)2 solution at room temperature for 24 h. The etched depths were -18 im or - 8 im, when the etchant was stirred or unstirred, respectively. Bonding was achieved using photoresist as an adhesive layer and with heating (135°C, 30 min) [743,744],... 

When APFiow exceeds the maximum APLapiace, the organic phase flows toward the aqueous phase (Figure 13b). When APFiow is lower than the minimum APLaPiace/ the aqueous phase flows toward the organic phase (Figure 13c). When the flow rate ratio is changed, the pressure balance is maintained by changing the position of the liquid-liquid interface. This model indicates that the important parameters for microfluid control are the interfacial tension, the dynamic contact angle, and the depth of the microchannel. This model can also be applied to gas-liquid microflows. 

In their pioneering work, Jensen et al. demonstrated that photochemical transformation can be carried out in a microfabricated reactor [37]. The photomicroreactor had a single serpentine-shaped microchannel (having a width of 500 pm and a depth of 250 or 500 pm, and etched on a silicon chip) covered by a transparent window (Pyrex or quartz) (Scheme 4.25). A miniature UV light source and an online UV analysis probe were integrated to the device. Jensen et al. studied the radical photopinacolization of benzophenone in isopropanol. Substantial conversion of benzophenone was observed for a 0.5 M benzophenone solution in this microflow system. Such a high concentration of benzophenone would present a challenge in macroscale reactors. This microreaction device provided an opportunity for fast process optimization by online analysis of the reaction mixture. 

In a conventional microtiter plate assay, a 1.5-mm movement would be necessary for the most distandy located antibody molecule to react with the antigen fixed on the surface of the well, since the liquid depth was 1.5 mm. On the other hand, the liquid phase of the microchannel filled with polystyrene beads was much smaller. The longest distance from an antibody molecule to the reaction-solid surface may be less than 20 pm. Diffusion time is proportional to the squares of the diffusion distance, so the diffusion time of the antibody molecule to the antigen in the microchip would be more than 5600 times shorter than the conventional method. 

A microchannel valve having smooth surface and the stacked PDMS stractures were fabricated (Fig. 5.14). Underneath the microchannel, a membrane and a pressure applying chamber are formed. The depth of microchannel of 50 pm and the membrane thickness of 60 pm were considered. For a 300 pm x 500 pm square membrane, it was numerically identified that more than a 150 kPa pressure was needed to obtain the deflection of 50 pm, which can close the microchannel perfectly. 

On the other hand, using high machining voltages (more than 32 V) at low tool travel speeds results in a non-smooth channel surface with significant depth variation along the channel. As a general rule, the quality of the microchannels deteriorates as the tool travel speed is decreased. This can be attributed to the poor material removal rate and the accumulation of melted material inside the microchannel immediately behind the tool. 

Figure 6.16 MicroChannel depth as a function of the machining time. Machining was done with a stainless steel cathode at 28 V and 5 lm/s in 30 wt% NaOH. Reprinted from [23] with the permission of the Journal of Micromechanics and Microengineering.

Two parameters are useful in the design of a machined microchannel. The first parameter is z0, the channel depth at time zero. This value is obtained by extrapolating the channel profile curves, such as the one in Fig. 6.16, to time t = 0. The second parameter is md, the average channel depth increase rate over time. The depth z(t) of the channel over time is then given by ... 

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