Effects of sample delivery on analyte capture in porous bead sensors

Abstract

Sample delivery is a crucial aspect of point-of-care applications where sample volumes need to be low and assay times need to be short, while providing high analytical and clinical sensitivity. In this paper, we explore the influence of the factors surrounding sample delivery on analyte capture in an immunoassay-based sensor array manifold of porous beads resting in individual wells. We model using computational fluid dynamics and a flow-through device containing beads sensitized specifically to C-reactive protein (CRP) to explore the effects of volume of sample, rate of sample delivery, and use of recirculation vs. unilateral delivery on the effectiveness of the capture of CRP on and within the porous bead sensor. Rate of sample delivery lends to the development of a time-dependent, shrinking depletion region around the bead exterior. Our findings reveal that at significantly high rates of delivery, unique to porous bead substrates, capture at the rim of the bead is reaction-limited, while capture in the interior of the bead is transport-limited. While the fluorescence signal results from the aggregate of captured material throughout the bead, multiple kinetic regimes exist within the bead. Further, under constant pressure conditions dictated by the array architecture, we reveal the existence of an optimal flow rate that generates the highest signal, under point-of-care constraints of limited-volume and limited-time. When high sensitivity is needed, recirculation can be implemented to overcome the analyte capture limitations due to volume and time constraints. Computational simulations agree with experimental results performed under similar conditions.

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“…From the simulation result of the model IV with various inlet rates (15,30,60, and 120 µL/min) (Figure 2e), we could calculate the dimensionless constant on the order of 0.5-20. The Peckert number greater than 1 indicates that the mass flux got the contribution from convection much more than diffusion [33,34]. Thus, the optimized inlet flow rate in this model was from 30 to 60 µL/min in order to reach a balance between the high mass transfer and low fluid shear stress, which would eventually be applied to the microwell.…”

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“…The signal, however, was not distinguishable from the background at lower concentration, suggesting a minimum detection limit of 25 cells per mL. Several studies have shown that biosensor performances are often affected by the analyte transport in the vicinity of the sensing area 28 38 as well as dispensing of the cells in the microfluidic system 35 38 . Therefore in flow through systems like microcantilevers, it is possible to achieve a low detection limit by controlling the fluid delivery with a proper mixing regime.…”

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“…44 This design provides convective transport to the interior of the porous beads, efficient diffusion distances, and short depletion layers which improves sensitivity compared to flat microfluidic channels. 45 Forming immunocomplexes throughout a 3-D matrix allows the signal to be layered via a higher density of reagent capture within a 3-D lattice as opposed to 2-D capture in the case of lateral flow devices. This mini-sensor ensemble is capable of multiplexing fluorescence immunoassays for proteins, oligonucleotides, and small molecules, outperforming laboratory-based ELISA in terms of analysis time, limits of detection, and ease of use.…”

“…Our previous work explored the effect of various sample flow rates on analyte capture within the p-BNC system. 45 The porous bead substrates have a unique multiple kinetic regime in which the exterior of the bead is reaction-limited while the interior of the bead is transport-limited. Thus, the amount of captured material on the bead largely depends on the delicate interplay between these two effects.…”