Micromembrane Probes for Tissue Engineering Monitoring

Currently, the assessment of a developing construct is almost always carried out destructively using biochemical or histological methods to determine cell number, viability, and tissue growth throughout the construct (Martin et al., 2004 Obradovic et al., 2000). 

Microdialysis is used to collect solutes present in the extracellular fluid via a microdialysis probe with a semipermeable membrane at its tip (see Fig. 14.8). The probe is perfused by a buffer (perfusate), and solutes from the environment surrounding the probe diffuse though the membrane into the perfused solution. Perfusate with the solutes (dialysate) is then collected for ex situ analysis. Microdialysis has been used in vivo to monitor the local concentrations of solutes in the extracellular fluids in a number of different tissues. The first microdialysis experiments were conducted on the brain and blood plasma (Bito et al., 1966). It has since been used to study metabolism in numerous tissues such as brain, muscles, tendons, subcutaneous adipose tissue, lungs, kidneys, and liver (Flock and Kloft, 2005 de la Pena et al., 2000 Jackson, 2005 Siddiqui and Shuaib, 2001). Microdialysis is widely used for pharmacokinetic research (Davies, 1999 de Lange et al., 2000 Verbeeck, 2000) and has also been used to monitor cell metabolites in cell culture medium (Wu et al., 2001). 

There are two main challenges related to the application of micromembrane probes (1) how to quantify the true value of the measured parameters based on the assay of their concentrations in the collected samples and (2) whether this determined quantitative link would change with time due to membrane fouling. 

To link the concentrations of the solute of interest in the dialysate (collected samples) and in the probe surrounding (to be measured), a parameter called relative recovery is often used. The concentration of solute in dialysate can be equal to the solute concentration in the probe surrounding if flow rate of perfusate equals 0 (in equilibrium). Otherwise the concentration of the solute in the dialysate will be lower than that in the surrounding probe. Relative recovery (RR) is expressed as a percentage of the concentration of the solute in the probe (the dialysate) relative to that in the surrounding external solution, that is. 

The recovery of solutes from the interstitial fluid by the probe ean be evaluated by introducing internal standards into the perfusate and measuring the relative loss (RL). Relative loss of a standard was estimated as a percentage of the standard solute that diffused from the probe perfusate into the surrounding interstitial fluid  

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