Abstract The use of cell based biosensors outside of the laboratory has been limited due to many issues including preparation of the sample, maintenance of the biological environment, incorporation of the appropriate sensors, and integration of the electronics for data collection and analysis. This paper describes these system issues and briefly presents current efforts to address packaging, fluidics, and data interpretation. These include the development of an integrated silicon-PDMS cell cartridge system that provides reliable electrical and fluidic interconnect and a regulated cell environment, the development of low power and low overhead sensors and electronics, and the necessary fluidic sample preparation. Keywords: Cell based sensors, Portable biological sensors, Sample preparation 1. Introduction Cell based biosensors, which utilize living cells as the primary transducer (the interface between cell and electronics is the secondary transducer), have evolved from techniques originally used for biological and neurological research. There has been increased interest in expanding their sensitivity and broad based detection capability to general sensing applications [1]. The potential uses of cell based biosensors range from high-throughput drug discovery to chronic environmental monitoring and toxin detection. Although there has been significant progress toward producing practically useful biosensors [1-5], most of this work has been confined to a laboratory environment. 2. Cellular Requirements The living component of a cell based biosensor requires a controlled environment to sustain viability. The major environmental properties that must be controlled for cell culture are the incubation temperature, the substrate material (for anchorage dependent cells), the media, and the gas phase mixture. The optimal incubation temperature for a cell culture varies according to cell type and origin, but most mammalian cell lines will grow satisfactorily at 37 °C. Although temperatures a few degrees below optimal do not affect viability, cells cannot tolerate higher temperatures and will die rapidly at temperatures above 40 °C. Since both the metabolism and growth rate of cells are dependent on temperature, precision is more important than accuracy, and regulation should be controlled to within ± 0.5 °C [6]. The most significant constituents for the gas phase are oxygen (necessary for the oxidative metabolism of eukaryotic cells) and carbon dioxide (necessary only indirectly as it is involved in the bicarbonate buffer system). Most cell cultures are incubated in an atmosphere of 5 to 10% CO 2 in air that equilibrates with the bicarbonate buffer in the media to provide the proper pH. However, for short durations, a non-bicarbonate buffer (such as HEPES) can be used and may be preferable since bicarbonate can form precipitates with heavy metals and other compounds. Unless the pH of the media is used as a sensor for cellular metabolism, the pH of the media should be regulated to within the range of 7.4 ± 0.2. SYSTEM REQUIREMENTS FOR A PORTABLE CELL BASED SENSOR B. Derek DeBusschere, David A. Borkholder, and Gregory T.A. Kovacs Electrical Engineering, Stanford University CIS 202X, Stanford, CA 94305-4075