Development of a dynamic delivery method for in vitro bioassays.

Measuring the biological activity of hydrophobic chemicals using in vitro assays is challenging because their aqueous solubility is low and the high density of bio-suspensions strongly decreases the bioavailability of hydrophobic pollutants. Dynamic dosing by partitioning from a stable polymer has a potential to overcome these limitations. Poly(dimethylsiloxane) (PDMS) was chosen due to its documented bio-compatibility and excellent partitioning properties. PDMS sheets were loaded with five polycyclic aromatic hydrocarbons (PAHs) and then immersed in model bio-suspensions composed of membrane vesicles ("chromatophores", composed of 30% lipids and 70% proteins) isolated from the photosynthetic bacterium Rhodobacter sphaeroides or phospholipid bilayer vesicles (liposomes) composed of palmitoyl-oleoyl phosphatidylcholine (POPC). Method development included the determination of partition coefficients between chromatophores or liposomes and water, desorption rate constants from PDMS to bio-suspensions, and diffusion resistances in both PDMS and bio-suspensions. The release of the PAHs from the PDMS into the bio-suspensions was measured and modeled as a combination of diffusion in pure water and diffusion in a completely mixed solvent composed of water and bio-suspensions. The mass transfer resistance for the release was lower in the PDMS than in the tested solutions, which demonstrates that PDMS can efficiently deliver PAHs even to dense biosuspensions. The contribution of aqueous diffusion to the mass transfer decreased with increasing hydrophobicity of the PAHs indicating that hydrophobic chemicals are efficiently transported with suspended biomaterial. The passive dosing system is versatile and offers a number of applications. Promising are tests with instantaneous response, where the time-dependent effect can be translated to concentration-effect curves but the system is also applicable for assuring constant dosing for longer-term testing.