Technical issues of in vitro microdialysis sampling in bioprocess monitoring N. Torto 1 , L. Gorton Department of Analytical Chemistry, Lund University, P.O. Box 124, S-221 00 Lund, Sweden T. Laurell Department of Electrical Measurements, Lund University, Box 118, S-221 00 Lund, Sweden G. Marko-Varga* Astra Draco AB, Pre-Clinical Research and Development, Bioanalytical Chemistry, P.O. Box 34, S-221 00 Lund, Sweden This review discusses the principles of in vitro microdialysis and presents an overview of the microdialysis sampling experiment and the role of the microdialysis membrane in the monitoring of bioprocesses. It also seeks to encourage efforts towards developing meth- odology that can tailor membrane units for bioprocess applications. The performance and characteristics of microdialysis mem- branes of different polymeric materials and with cut-offs ranging between 5 and 100 kDa are evaluated using a range of analytes including carbohydrates, peptide and non- peptide leukotrienes as well as pro- teins. z1999 Elsevier Science B.V. All rights reserved. Keywords: Microdialysis; Probe; Bioprocess monitoring; Membranes 1. Introduction Microdialysis is a sampling technique that is histor- ically linked to neuroscience. However, it has recently been employed as an in situ sampling and sample clean-up tool as well as for quenching biological reac- tions by preventing the diffusion of enzymes into the perfusion liquid during analytical bioprocess monitor- ing [ 1]. The scope of application is broadening, and there is a potential for a signi¢cant impact in biotech- nological processes involving cell biology and phar- macology. This new surge is facilitated by the progress in the design and con¢guration of microdialysis probes and membrane choice, as these de¢ne the appli- cation area. For applications in neuroscience, the microdialysis probe type is obviously limited by the size of the animal and the degree of perturbation prac- tically feasible in the region to be studied. Such design requirements are not critical in bioprocess work as the membrane's effective dialysis length ( and surface area ) can be increased to improve the extraction frac- tion (EF). In fact, larger and more robust probe designs are desired for pilot plant applications. The present contribution is not intended to review the application of microdialysis in bioprocesses. Rather, the reader is referred to recent reviews and the references therein, where a comparison of micro- dialysis with established sampling techniques for bio- process monitoring has been clearly presented [ 1^3 ]. However, a critical and focused analysis of microdial- ysis, and the future trends, based on our recent efforts at pioneering its use in the sampling of various bio- processes will be given. The potential to utilise microdialysis to monitor kinetics and inhibitory kinetics in cell cultures during process optimisation will also be demonstrated. Examples will be taken from cell biology, where cell lines and cells originating from animal models are screened for drug development. The use of microdial- ysis in these systems allows continuous sampling, in situ sample clean-up and monitoring. Microdialysis is much more preferred than the conventional add^incu- bate^separate methodology as it is only limited by the temporal resolution of the analytical measurement of the sample analyte [ 4 ]. 0165-9936/99/$ ^ see front matter ß 1999 Elsevier Science B.V. All rights reserved. PII:S0165-9936(98)00115-0 *Corresponding author. E-mail: gyorgy.marko-varga@draco.se.astra.com 1 On leave from the Department of Chemistry, University of Botswana, Gabaronne, P/Bag 0022, Botswana. 252 trends in analytical chemistry, vol. 18, no. 4, 1999