Quantitative analysis of molecular absorption into PDMS microfluidic channels

Microfluidic devices fabricated using poly(dimethylsiloxane) (PDMS) polymer are routinely used for in vitro cell culture for a wide range of cellular assays. These assays typically involve the incubation of cultured cells with a drug molecule or a fluorescent marker while monitoring a cellular response. The accuracy of these assays depends on achieving a consistent and reproducible concentration of solute molecules in solution. However, hydrophobic therapeutic and fluorescent molecules tend to diffuse into the PDMS walls of the microfluidic devices, which reduce their concentration in solution and consequently affect the accuracy and reliability of these assays. In this paper, we quantitatively investigate the relationship between the partition coefficient (log P) of a series of markers routinely used in in vitro cellular assays including [3H]-dexamethasone, [3H]-diazepam, [14C]-mannitol, [3H]-phenytoin, and rhodamine 6G and their absorption into PDMS microfluidic channels. Our results show that the absorption of a given solute into PDMS depends on the hydrophilic/hydrophobic balance defined by its log P value. Specifically, results demonstrate that molecules with log P less than 2.47 exhibit minimal absorption (<10%) into PDMS channels whereas molecules with log P larger than 2.62 exhibit extensive absorption (>90%) into PDMS channels. Further investigations showed that TiO(2) and glass coatings of PDMS channels reduced the absorption of hydrophobic molecules (log P > 2.62) by 2- and 4.5-folds, respectively.