Generation of spatio-temporal concentration proﬁles for cell culture systems: A case study in ammonia Xicai Yue a, * , Emmanuel M. Drakakis a , Athanasios Mantalaris b , Anthony Cass c a Department of Bioengineering, Imperial College London, London SW7 2AZ, UK b Biological Systems Engineering Laboratory, Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK c Institute of Biomedical Engineering, Imperial College London, London SW7 2AZ, UK article info Article history: Received 20 August 2009 Received in revised form 21 October 2009 Accepted 28 May 2010 Available online 10 June 2010 Keywords: Ammonia Sensor Bioreactor Data acquisition Cell culture abstract Cellular behavior is inﬂuenced by micro-concentration gradients existing in their immedi- ate spatio-environment. A practical real-time measurement system, composed of miniatur- ized long-lasting sensors placed within a two-dimensional bioreactor and a 32-channel data acquisition system for ammonia, is presented in this paper aiming to generate the spa- tio-temporal ammonia concentration proﬁles for the whole culture process. Our results demonstrate the feasibility of the system and its applicability in elucidating the cell growth micro-environment within a bioreactor, which can potentially facilitate the intelligent con- trol of ammonia levels in cell culture systems. The presented system is ﬂexible enough to interface other sensors for generating their spatio-temporal proﬁles of the sensed cell cul- ture parameters. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The micro-environment in which cells reside provides signals that direct them to proliferate, differentiate, or re- main dormant [1]. These factors include soluble molecules, the extracellular matrix, neighboring cells, and physical stimuli. Achieving intelligent control of the culture process necessitates an increased understanding of these phenom- ena, which are determined by the micro-concentration gradients in the local environment around the cells. The development of a monitoring system that generates spa- tio-temporal concentration proﬁles for important culture parameters will enable scientists and bioprocess engineers to more effectively direct cell fate, in particular stem cell cultures. Ammonia is an important metabolite produced in cell cultures used to grow mammalian cells in vitro. It has been reported to be toxic and inhibitory for mammalian cells leading to the reduction of growth rates and cell densities in batch cultures, changes in metabolic rates, as well as perturbation of protein processing and virus replication [2]. A variety of methods measuring ammonia concentra- tion in cell cultures have been reported. Traditional meth- ods include off-line gas sensor electrodes [3], on-line ﬂow injection analysis (FIA) measurements [4], high perfor- mance liquid chromatography (HPLC) [5], spectroscopy [6] and conductivity measurements [7]. However, many of these measurement methods do not record ammonia in real-time and on-line. Consequently they do not capture its dynamic changes during a cell culture process. Recently, a noninvasive measurement system using near-infrared spectroscopy has also been reported [8], which monitors certain important culture parameters in real-time. How- ever, a considerable standard measurement error is re- ported. A real-time multi-sensor ammonia measurement system, tailored to provide detailed spatio-temporal con- centration proﬁles within a two-dimensional cell culture system which would lead to culture process optimization by eliminating ammonia accumulation [9] or maintaining low ammonia level [10], is presented. The components of the measurement system including the miniaturized 0263-2241/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.measurement.2010.05.012 * Corresponding author. E-mail address: x.yue@imperial.ac.uk (X. Yue). Measurement 43 (2010) 1207–1216 Contents lists available at ScienceDirect Measurement journal homepage: www.elsevier.com/locate/measurement