Polyelectrolyte multilayers : nanofabricated architectures for bio-interface materials

Abstract

The layer-by-layer process, whereby aqueous solutions of oppositely charged polymers are alternately and repeatedly deposited onto a substrate, has emerged in recent years as a promising approach for creating thin films with nanoscale control of structure, composition, and surface properties. Applications ranging from surface modification to optical and electronic devices have arisen from the versatility of this nanocomposite fabrication technique. The additional ability to assemble into films a wide variety of biological entities, such as enzymes and DNA, has expanded the use of polyelectrolyte multilayers for biosensor and other biomaterials applications. This thesis further explores the rationale of using multilayers as biomaterials, with particularly emphasis on the importance of the underlying molecular architecture. Many of the results presented here concern films assembled from weak polyions, i.e., ones with pH-dependent charge densities, including poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH). Using weak polyions enables the creation of thin films with chemical and structural properties controlled with nanoscale precision by simply adjusting the pH of the polymer solutions. Under certain assembly conditions, initially nonporous PAA/PAH films become nano- and/or microporous through a simple pH-induced phase separation in acidic water (pH [approx.] 2.4), even with an exposure time of just a few seconds. By adjusting several processing parameters (e.g., the time, temperature, ionic strength, and a secondary rinse with neutral water), it is possible to generate either interconnected or discrete porous morphologies.

(cont.) The interaction of a highly adhesive mammalian NR6WT fibroblast cell line with various PAA/PAH films and other multilayer systems of differing compositions, structures, and charge densities has also been explored. This thesis demonstrates that by manipulating the multilayer pH assembly conditions, which in turn dictates the molecular architecture of the thin films, one may powerfully direct a single multilayer combination to be either cell adhesive or cell resistant. Highly ionically stitched multilayers attract cells, whereas weakly ionically crosslinked multilayers, which swell substantially in physiological conditions and thereby present richly hydrated surfaces, resist cell attachment. This unprecedented ability to fine-tune a multilayer to be either cell adhesive or bioinert, along with the unique feature of controllable porosity, allows polyelectrolyte multilayers to be envisioned for membranes, controlled release, and biocompatible implant coating applications.