Flow-through microdialysis

If a single analyte is to be detected, a simple flow-through microdialysis/microchip system may suffice. If multianalyte determination is needed, a separation-based microchip device can allow resolution and detection of several analytes in a single sample (Section 48.5.4). In flow-through devices, the perfusate is directed to an array of sensors, which may be also be modified to allow detection of different analytes and improve detection sensitivity. 

The drawback of this approach turned out to be samples of submicrolitre level, which necessitated the development of a dedicated small measuring device with a very low dead volume. A portable lightweight apparatus, consisting in a miniaturized flow-through biosensor connected to a microdialysis probe at one side, and to a semi-vacuum pulse-free pump at the other was then built. A portable potentiostat equipped for data collection and storage was used to handle data. 

Glucose and lactate were simultaneously monitored by Wei Min et al. [191] during the fermentation of Lactococcus lactis. They made use of a microdialysis probe connected to a dual flow-through cell in which two amperometric biosensors were located. The biosensors were based on the co-immobilization of the respective oxidase enzymes together with HRP in a carbon paste matrix. Both analytes were monitored on-hne for about 14 h in a very complex aqueous two-phase fermentation process, and the results were in good agreement with independent ofiF-line HPLC measurements. 

Determination of the effects of changes in blood flow through the various regions of the cutaneous microvasculature is obviously not possible using traditional Franz-type isolated membrane diffusion cell studies. The ultimate goal of experimental systems is usually to allow quantitative prediction of the absorption and distribution of topically applied solutes that wfll be applicable to the in vivo situation. Therefore, we can deduce that studies examining the effects of changes in cutaneous blood flow are limited to experimental models in which the microvasculature has been preserved and can be effectively perfused and manipulated. Models reported in the htraature to date include isolated perfused tissue models, anesthetized animal studies, and more recently human and animal cutaneous microdialysis studies. 

Microdialysis sampling has also been coupled to separation-based microchip devices. External probes have been used in microchip electrophoresis for the monitoring of enzyme reactions [32] and have been applied to the in vivo analysis of glutamate in the striatum of an anesthetized rat [33]. Considerations for coupling microdialysis sampling to flow-through and separation-based microchip devices are similar and will now be discussed in more detail, with reference to specific examples from the literature. 

Pij anowska, D. G, et al., A flow-through potentiometiic sensor for an integrated microdialysis system. Sensors and Actuators, B Chemical, B103, 350, 2004. 

Hsieh YC, Zahn JD (2007) On-chip microdialysis system with flow-through glucose sensing capabilities. J Diabetes Sci Technol l(3) 375-83... 

The most common type of microdialysis probe is constructed as a concentric tube as shown in Fig. 1. The probes usually consist of a semiper-meable membrane, such as polysulfone, polyethersulfone, polyamide, polycarbonate-poly ether copolymer, or cuprophan [4], glued between the tip of the inner cannula and the outer shaft, which are made of steel or plastic. The perfusion fluid (perfusate) enters the inlet flowing through the inner tube to its distal end and exits the inner tube to enter the space between the inner tube and the outer dialysis membrane where molecular exchange takes place. After the exchange, the fluid containing the molecules of interest (dialysate) is transferred towards the proximal end of the probe and is collected at the outlet for later... 

Figure 4.6 The tip of a microdialysis probe, expanded to show dialysis tubing around a steel cannula through the base of which fluid can flow out and then up and over the membrane. The length of membrane below the probe support can be altered (1-10 mm) to suit the size of the animal and the brain area being studied. Flow rates are normally below 2 pl/min...

In vivo microdialysis is based on the principle of dialysis, the process whereby concentration gradients drive the movement of small molecules and water through a semipermeable membrane. In vivo microdialysis involves the insertion of a small semipermeable membrane into a specific region of a living animal, such as the brain. The assembly that contains this semipermeable membrane is called a probe, which is composed of an inlet and an outlet compartment surrounded by a semipermeable membrane (see O Figure 9-1). Using a microinfusion pump set at a low flow rate (0.2-3 /rL/min), an aqueous solution known as the perfusate is pumped into the inlet compartment of the microdialysis probe. Ideally, the... 

Benveniste H, Drejer J, Schousboe A, Diemer NH. 1987. Regional cerebral glucose phosphorylation and blood flow after insertion of a microdialysis fiber through the dorsal hippocampus in the rat. J Neurochem 49(3) 729-734. 

Figure 6.1 Schematic illustrating in vivo membrane separation processes. Left microdialysis sampling probe with curved lines indicating analyte tortuous diffusion through the tissue into and out of the probe. Right ultrafiltration representing interstitial fluid flow into the membrane device. Tissue is represented with cells and hlood vessels (dark circles in light circles).

Compared to basic research microdialysis sampling devices, those used for clinical studies are much longer with typical membrane lengths between 10 and 30 mm. Flow rates of the perfusion fluid through these devices are also much lower (0.3 pL/min) than typically applied in basic research studies. This combination results in high EE% values for low molecular weight and hydrophilic analytes such as glucose. 

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