Controlled antibody release from gelatin for on-chip sample preparation

A practical way to realize on-chip sample preparation for point-of-care diagnostics is to store the required reagents on a microfluidic device and release them in a controlled manner upon contact with the sample. For the development of such diagnostic devices{,} a fundamental understanding of the release kinetics of reagents from suitable materials in microfluidic chips is therefore essential. Here{,} we study the release kinetics of fluorophore-conjugated antibodies from (sub-) [small mu ]m thick gelatin layers and several ways to control the release time. The observed antibody release is well-described by a diffusion model. Release times ranging from [similar]20 s to [similar]650 s were determined for layers with thicknesses (in the dry state) between 0.25 [small mu ]m and 1.5 [small mu ]m{,} corresponding to a diffusivity of 0.65 [small mu ]m2 s-1 (in the swollen state) for our standard layer preparation conditions. By modifying the preparation conditions{,} we can influence the properties of gelatin to realize faster or slower release. Faster drying at increased temperatures leads to shorter release times{,} whereas slower drying at increased humidity yields slower release. As expected in a diffusive process{,} the release time increases with the size of the antibody. Moreover{,} the ionic strength of the release medium has a significant impact on the release kinetics. Applying these findings to cell counting chambers with on-chip sample preparation{,} we can tune the release to control the antibody distribution after inflow of blood in order to achieve homogeneous cell staining.