Amphoteric Surface Hydrogels from Hydrogen-Bonded Multilayers: Reversible Protein Uptake

INTRODUCTION. The aim of producing bioactive materials motivated the use of hydrogels as materials which control binding, absorption and release of proteins1,2. Advantages of using multilayering to produce surface hydrogels include the applicability of the technique to virtually any substrate type, and the possibility to control the film thickness by a simple variation of the number of layers. In our study, we report on one-component hydrogels derived from hydrogen-bonded multilayers which show pH-dependent swelling/deswelling and are able to reversibly absorb dyes and/or proteins. EXPERIMENTAL METHODS. The hydrogels were produced by chemical crosslinking (using carbodiimide chemistry and ethylenediamine (EDA) as a crosslinker) between polyacid units within a 5-bilayer poly(N-vinylpyrrolidone) (PVPON) and poly(methacrylic acid) (PMAA) film followed by complete PVPON removal3. Hydrogel swelling and protein uptake and release were studied by in situ ATR-FTIR (attenuated total reflection Fourier transform infrared spectroscopy) and ellipsometry.RESULTS AND DISCUSSION. As shown by in situellipsometry in Fig.1, hydrogels exhibit distinctive polyampholytic swelling as a function of pH, with minimum swelling at neutral pH, and increased swelling at both lower and higher pH values. Film swelling occurs due to the presence of amino groups (which originate from one-end-attachment of EDA crosslinker to PMAA chains) and carboxylic groups, which are ionized at low and high pH values, respectively. At pH 7.5, the produced (PMAA)5 hydrogels demonstrated cell-resistant properties towards adhesion of mouse fibroblasts. The pH-switching of hydrogel swelling was fast and reversible. The amphoteric nature of PMAA hydrogels was used for controlled loading and release of negatively and positively charged dyes and proteins. Fig. 2 contrasts the inclusion of heparin and lysozyme (Lys) within the surface hydrogel as a function of pH. Specifically, positively charged Lys was included within the hydrogel matrix at pH > 5, when carboxylic groups within the (PMAA)5 matrix were ionized. The amount of adsorbed Lys correlated with PMAA ionization as directly determined by in situ ATR-FTIR. Negatively charged heparin did not interact with the hydrogel at pH 7, but had high affinity to the film at lower pH values (pH < 5) where the hydrogel carried positive charge due to unreacted protonated amino groups of EDA. Finally, we demonstrate that proteins included within the hydrogel can easily be replaced with linear polycations. CONCLUSIONS. The (PMAA) hydrogels were able to reversibly absorb large amounts of dyes and proteins of opposite charge reversibly in response to pH variations. These surface hydrogels hold promise for bioseparation and controlled delivery applications