The OnZurf® Probe is based on the microdialysis technique and contain a 15 mm dialyze membrane with a pore size of 10kDa that is designed to achieve specific diffusion characteristics. The OnZurf® Probe is introduced in the body during open surgery. After insertion, the probe is fixated on the organ and can be used for sampling tissue fluid for up to 7 days.

Read more about microdialysis

Microdialysis is a technique suitable for measuring local metabolic changes in a specific tissue in vivo. Microdialysis sampling aims to identify local metabolic events in a specific tissue through continuous sampling. The results are useful for understanding clinical problems, and help to identify optimal patient conditions. In physiological terms, the technique is analogous to repeated local venous blood sampling techniques from a specific tissue without drawing any interstitial fluid or blod.

Microdialysis is based on a tubular or flat porous plastic membrane with a selected pore size that will define its range of activity in terms of molecular size (weight). The range of membrane pore sizes available is wide, and range cut-offs can typically be chosen from 10 to 100 kD (kiloDaltons). In general, the smaller pore size provides a better selectivity for small biomarker molecules. During sampling, the inner acceptor side of the membrane is supplied with a physiological and nearly isotonic solution and small molecules from the donor side are transported across the membrane by diffusion. The passive diffusion process is driven by the concentration gradient between the acceptor and the donor side, and the rate of diffusion across the membrane is also dependent on factors such as molecular size, membrane surface area and temperature.

The “recovery” of solute reflects the concentration in the dialysate of the molecules of interest in relation to the true concentration surrounding the microdialysis probe. The recovery is dependent on factors such as the perfusion (acceptor) solution flow rate, and a high recovery is promoted by a slow flow rate. However, a slow flow rate delays the response, and often a higher flow rate is used when only the relative change in concentration is monitored. The recovery is also influenced by the diffusion rate in the medium and in the membrane, as well as the design of the probe with respect to cut-off, diameter and length of the membrane, etc. The diffusion rate is influenced by temperature and the recovery typically increases by 1-2% per degree (˚C) of temperature increase. Typically, the concentration achieved in the dialysate (sample) will be lower than the corresponding concentration of a specific molecule in the tissue/interstitium. A high flow rate increases the hydrostatic pressure, such that the perfusion fluid is forced into the interstitial fluid, and resulting in lower recovery. On the other hand, a lower perfusate flow rate results in lower hydrostatic pressures in the probe channels; most of the perfusion volume returns for collection, and high recovery rate is achieved. In conclusion, to establish a high recovery rate, a long membrane with optimal pore size and low perfusion rate are optimal.