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

Amphoteric surface hydrogels derived from hydrogen-bonded multilayers: Reversible loading of dyes and macromolecules

We used hydrogen-bonded multilayers of poly(N-vinylpyrrolidone) (PVPON) and poly(methacrylic acid) (PMAA) as precursors for producing surface-bound hydrogels and studied their pH-dependent swelling and protein uptake behavior using in situ attenuated total reflection Fourier transform infrared spectroscopy and in situ ellipsometry. The hydrogels were produced by selective chemical cross-linking between PMAA units using carbodiimide chemistry and ethylenediamine (EDA) as a cross-linking reagent, followed by complete removal of PVPON from the film obtained by exposing the film to pH 7.5. As shown by in situ ellipsometry, hydrogels exhibit distinctive polyampholytic swelling as a function of pH, with minimum swelling at pH 4.2-5.7, and increased film thickness at both lower and higher pH values. Film swelling at lower pH values occurs as a result of the presence of amino groups within the hydrogels, which originate from the one-end attachment of the EDA cross-linker to PMAA chains. The pH-switching of hydrogel swelling was fast and reversible. The degree of hydrogel swelling could be also controlled by varying the time allowed for cross-linking. The produced hydrogels were able to absorb large amounts of dyes and proteins of opposite charge reversibly, in response to pH variations. Finally, we demonstrate that proteins included within the hydrogel can easily be replaced with linear polycations. These surface hydrogels hold promise for bioseparation and controlled delivery applications

Multilayer-derived, ultrathin, stimuli-responsive hydrogels

In addition to a well-known capacity of the layer-by-layer (LbL) technique to create multilayers with strongly bound polymer chains, the technique also provides a unique opportunity to fabricate highly swollen, hydrogel-like films and capsules. Layered, ultrathin hydrogel-like membranes can be fabricated using electrostatically assembled or hydrogen-bonded multilayers as template matrices, or using a click chemistry approach. This review describes recent progress in the area, comparing various approaches used for the fabrication of these surface-mediated, LbL-templated structures, and discusses future applications of ultrathin hydrogel materials

Controlling Mechanical Properties of Poly(methacrylic acid) Multilayer Hydrogels via Hydrogel Internal Architecture

Hydrogel materials are crucial in many applications due to their versatility and ability to mimic biological tissues. While manipulating bulk hydrogel cross-link density, polymer content, chemical composition, and microporosity has been a main approach to controlling hydrogel rigidity, altering the internal organization of hydrogel materials through chain intermixing and stratification can bring finer control over hydrogel properties, including mechanical responses. We report on altering the mechanical properties of ultrathin poly(methacrylic acid) (PMAA) multilayer hydrogels by controlling the internal organization of the PMAA network. PMAA multilayer hydrogels were synthesized by cross-linking PMAA layers in poly(N-vinylpyrrolidone) (PVPON)/PMAA hydrogen-bonded multilayer templates prepared by dipped or spin-assisted (SA) layer-by-layer assembly using sacrificial PVPON interlayers with molecular weights of 40,000 or 280,000 g mol–1. The effect of PVPON molecular weight on PMAA hydrogel stratification and network swelling and hydration was assessed by in situ spectroscopic ellipsometry and neutron reflectometry (NR). In a new NR modeling of polymer intermixing, we have inferred nanoscopic structure and water distribution within the ultrathin-layered films from measured continuum neutron scattering length density (SLD) and related those to the mechanical properties of the hydrogel films. We have found that hydrogel swelling, the number of water molecules associated with the swollen hydrogel, and water density within the SA PMAA hydrogels can be controlled by choosing low- or high-Mw PVPON. While cross-link densities determined by ATR-FTIR were similar, greater swelling and hydration at pH > 5 were observed for SA PMAA hydrogels synthesized using higher-Mw PVPON. The enhanced swelling of these SA hydrogels resulted in softening with a lower Young’s modulus at pH > 5 as measured by colloidal probe atomic force microscopy (AFM). The effect of PMAA layer intermixing on hydrogel mechanical properties was also compared for dipped and SA (PMAA) multilayer hydrogels of similar thickness and cross-linking degree. Despite similar values of gigapascal-range Young’s modulus for dry PMAA multilayer hydrogel films, an almost twice greater softening of the SA (PMAA) hydrogel compared to that prepared by dipping was observed, with Young’s modulus values decreasing to tens of megapascals in solution at pH > 5. Our study demonstrates that, unlike simply changing bulk hydrogel cross-link density, programming polymer network architecture via controlling the nanostructured organization of SA PMAA hydrogels enables selective modulation of the cross-link density within hydrogel strata. Control of polymer chain intermixing through hydrogel stratification offers a framework for synthesizing materials with finely tuned hydrogel internal structures, enabling precise control of such physical properties as the internal architecture, hydrogel swelling, surface morphology, and mechanical response, which are critical for the application of these materials in sensing, drug delivery, and tissue engineering