Biosensors and Bioelectronics 24 (2008) 435–441 Contents lists available at ScienceDirect Biosensors and Bioelectronics journal homepage: www.elsevier.com/locate/bios Biocompatible ion selective electrode for monitoring metabolic activity during the growth and cultivation of human cells Anna Radomska a,∗ , Suket Singhal a , Hua Ye b , Mayasari Lim b , Athanasios Mantalaris b , Xicai Yue c , Emmanuel M. Drakakis c , Christofer Toumazou a , Anthony E.G. Cass a a Institute of Biomedical Engineering, Imperial College, London, United Kingdom b Department of Chemical Engineering, Imperial College, London, United Kingdom c Department of Bioengineering, Imperial College, London, United Kingdom article info Article history: Received 13 February 2008 Received in revised form 9 April 2008 Accepted 29 April 2008 Available online 8 May 2008 Keywords: Ion-selective electrode Biofouling Stem cell culture Real-time monitoring Bioreactor abstract Ammonia is the main nitrogenous waste product of cellular metabolism and if accumulated in culture media may limit cell growth and affect the quality of cultured cell lines. Therefore, it is crucial to control levels of this metabolite during the in vitro expansion of human cells. This paper describes the success- ful application of ion selective electrodes (ISE) to continuously monitor ammonium concentrations in a perfused cell bioreactor. The polymeric membranes of the ISE were cast from carboxylated poly(vinyl chloride) (PVC-COOH) and doped with highly hydrophilic poly(ethylene glycol) (PEG). The PEG was incor- porated into the surface of the sensors in order to reduce the effect of biofouling without impairing their analytical characteristics. The electrodes developed enabled fast and selective measurements of ammonia in the range 0.5–5mM, corresponding well with the concentration determined off-line. Additionally, the UV sterilised sensors were small and flexible enough to be readily inserted into the limited space of the bioreactor. Long-term analytical performance of PEG-modified ISE during continuous measurements in mammalian cell cultures was investigated. The sensors remained stable for the duration of the bioprocess, 7 days. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Haematopoietic stem cells (HSC) have a great potential in the treatment of a broad range of haematological diseases, as well as immune and metabolic disorders. This is due to their unique abil- ity to self-renew and to differentiate into all types of blood cells, including myeloid and lymphoid lineages. In spite of being used in a wide variety of therapeutic applications, the success of HSCs transplantation largely depends on their availability. Getting suffi- cient numbers of acceptable quality HSCs from established sources like bone marrow and peripheral blood is an elaborate procedure. Cord blood stem cells provide an attractive alternative but is still limited in the number of stem cells present in a single cord unit (Nielsen, 1999). Ex vivo expansion could alleviate several of the problems experienced with current stem cell availability. However, this promising concept is limited by a lack of knowledge about how to expand these cells in vitro without inhibition of the very long- term multilineage growth and differentiation properties required for their clinical utility. The identification and control of operating ∗ Corresponding author. Tel.: +44 2075945325; fax: +44 2075945196. E-mail address: a.radomska@imperial.ac.uk (A. Radomska). parameters during the ex vivo cultivation is challenging because of the complex dynamics of the culture, involving a delicate inter- play of molecular and cellular factors. An understanding of these is necessary to provide the basis for more rational approaches to the ex vivo expansion of HSCs (Lim et al., 2007). A general fea- ture of living cells is the uptake of nutrients (glucose, glutamine) and oxygen, the production of energy, excretion of acid waste products (lactic acid, carbonic acid) and ammonia (Hafner, 2000). Oxygen consumption, glucose depletion and metabolite genera- tion are important parameters suited for the sensitive detection of cellular responses. Mass transport of these and other chemi- cal species is essential for the efficient ex vivo cultivation of stem cells. There are two applications that represent different situa- tions with respect to mass transport phenomena (Collins et al., 1996). Conventional static culture systems (well-plates, T-flasks) remain the most popular choice for expanding HSCs. Despite their wide-spread usage, however, these systems have several inherent limitations. The inhomogeneous nature of a static system results in concentration gradients in culture media, with local depletion of nutrients and accumulation of waste materials. Furthermore, static surroundings are characterised by low process reproducibil- ity and productivity, as well as difficult bioprocess monitoring and control. Alternatively, perfused bioreactors can be used in place of 0956-5663/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.bios.2008.04.026