An approach for assessing hydrogel hydrophobicity

Hydrogel hydrophobicity is an important modulator of mammalian cell behavior, drug payload release, and medical device fouling. Contact angle and protein adsorption measures are two common methods for evaluating hydrogel hydrophobicity. However, protein adsorption is a complex phenomenon which is challenging to interpret in terms of gel hydrophobicity alone. In addition, the permeability of hydrogels can be problematic for contact angle assessment, as this method can only be strictly applied to smooth, solid, and nonpermeable surfaces. Therefore, the development of a technique for measuring hydrogel hydrophobicity which is simple, sensitive, and independent of variations in gel permeability would significantly advance the ability to finely tune this variable. The present technical note develops a method for quantifying the hydrophobicity of hydrogels by exploiting their capacity to swell differentially in solvents of distinct polarities. To validate this technique, hydrogels of varying hydrophobicities were prepared by combining hydrophilic poly(ethylene glycol) diacrylate (PEGDA) with either hydrophobic 3‐(trimethoxysilyl) propyl methacrylate (TMSPM) or hydrophilic 2‐hydroxyethyl methacrylate (HEMA). The ratio of hydrogel swelling in 70% isopropanol to that in water was termed the hydrophobicity index (H‐index) and was determined for each gel type. The measured H‐indices reflected known differences in the hydrophobicities of HEMA, TMSPM, and PEGDA and, in contrast to contact angle assessments, appeared to be independent of variations in hydrogel permeability. In addition, the trend in H‐indices agreed well with the trend in protein adsorption across hydrogel formulations, although the H‐indices appeared to be able to resolve more subtle differences in gel hydrophobicity than protein adsorption measures. © 2012 Wiley Periodicals, Inc. J Biomed Mater Res Part A 100A:2855–2860, 2012.