Sensing the future

Nothing tests hardware like space travel, hence a long history of `technology transfer’ from flight projects.

US-based Sensors 2000 (S2K), has been particularly effective at this in developing chemical and biological sensors to measure responses to space flight. These sensors are of importance to gravitational biology; as the accuracy of post-flight measurements is often compromised by re-entry.

Current chemical/biological sensor technologies tend not to offer the necessary resolution, accuracy or stability, but S2K addresses these problems with techniques like membrane technology, catheter and electrode approaches, and micromachining.

A good example of this is a system which measures the pH and temperature of circulating media in a rat tumour cell investigation carried out during space flight.

The BIONA-C Rail (Biological Ion Analysis in the Cell Culture Module) contains four fluid paths that circulate media to support cell growth. Each path contains a bioreactor where fluids cross a permeable membrane to the cell culture. The system can also measure CO2 and other ions in solution, and control the collection of samples and addition of nutrients for the cell cultures.

In two of the fluid paths the sensors are located directly in the circulating media paths. In the other two, media samples are periodically pumped from the circulating path to the sensors.

Originally designed to measure ion concentrations in blood plasma, these sensors are catheter-type ion selective electrodes, which have membranes selective to only one ionic species.

To measure concentration with ion-selective electrodes, a reference electrode is needed to provide a concentration independent potential. The potential of the ion selective electrode depends on the concentration of the ion it is selective to. The difference between indicator and reference potential can then be related to the ion concentration. This calibration process ensures that errors are minimised.

In the BIONA-C system, 3 pH indicator electrodes and one reference electrode are inserted into a piece of tubing to form an array. A thermistor is also inserted in the array to monitor the temperature at the measurement site. Fluid flows through the array across the tips of the sensors. The lead wires of each sensor are attached directly to the circuit boards.