Designing the ‘fill and flow’ (bio)sensor to give stable measurements from a dynamic system

A laminar ‘fill and flow’ channel containing band electrodes is presented with a redesigned sample reservoir. The channel allows well-defined electroanalytical currents to be measured at the electrodes, so that it is suitable for inclusion as part of a total analysis system. The goal in the use of the channel is an electroanalytical system which operates by ‘gravity-feed’ without pumping, but the measured electrochemical current is a function of sample flow rate within the channel, which varies according to the height of sample in the reservoir and may be influenced by device to device variation or the nature of the sample. In this paper, a method is explored whereby the sample flow rate can be extracted automatically from features in the recorded electrode current. Two designs for the sample reservoir are presented involving step constrictions in their internal profile. Equations are derived to correct the current readings for varying flow rate, using the markers generated by the sample meniscus passing these steps. The design with two step constrictions in the reservoir is shown to give results where correction for flow rate can be made independent of the current or the sample. The technique is also shown to provide an opportunity to determine and hence to correct for the viscosity and, by taking a two-point reading within current markers provided by discontinuities created by the step constrictions of the reservoir design, the data may also be corrected to eliminate the effects of tilt of the channel and device-to-device variation.