Stability of virtual air walls on micropallet arrays.

Arrays of micropallets have been used to pattern adherent cells as well to sort mixtures of cells. These artificial surfaces are composed of micrometer-sized, SU-8 structures formed on a hydrophobic glass surface. Successful application of these arrays requires stable Cassie-Baxter wetting by aqueous biological solutions. This paper systematically studies the factors governing the stability of Cassie-Baxter wetting on the arrays, including the surface properties of the array components as well as the physical and chemical properties of the wetting solutions. To establish stable Cassie-Baxter wetting with water, the surface of the array must be coated with a perfluoroalkylsilane of sufficient hydrophobicity, and the roughness of array must be greater than 1.6. Additionally, long-term stability of the Cassie-Baxter wetting depends on the properties of the wetting solutions, including the surface tension (>40 mM/m), salt concentration (>10 mM), and protein concentration (<5 mg/mL) of the wetting liquid. Under optimal conditions, Cassie-Baxter wetting of the micropallet array is stable for up to 1 month in the presence of tissue culture medium containing fetal calf serum. These experimental results provide critical information regarding the stability and endurance of the virtual air walls when the micropallet arrays are used as a bioanalytical tool.