Determining the absolute surface hydrophobicity of microparticulates using thin layer wicking

Surface hydrophobicity is an important factor in the transport of microparticulates (MPs) across biological barriers. We have previously shown in our laboratory that the surface properties of polystyrene MPs influence the diffusion and transport through gastrointestinal (GI) mucus and mucin. Unfortunately, most currently used methods for evaluating the surface hydrophobicity of MPs involve a relative measurement resulting in a rank order rather than an absolute hydrophobicity value. Obtaining an absolute assessment of hydrophobicity is necessary in order to obtain meaningful comparisons and correlations across laboratories, polymers, methods of fabrication, and so on. A modified thin layer wicking (TLW) technique was developed and validated to allow for the determination of absolute surface hydrophobicity of intact MPs. The TLW method was validated by constructing a standard curve and comparing the rate of solvent rise through MPs dried on microscope slides to the known contact angle of PLGA polymers. MPs with surface contact angles ranging between 67.04 degrees and 90.18 degrees were evaluated using the TLW technique. The modified TLW technique was also successfully validated using surface-modified polystyrene and OVA MPs. Based on the results of the current study, the modified TLW technique appears to be a reliable and quantitative method for assessing the surface hydrophobicity of intact MPs.