Effect of layer-by-layer confinement of polypeptides and polysaccharides onto thermoresponsive microgels: A comparative study

The confinement of polyelectrolyte multilayers of poly-L-lysine (PLL)/poly-L-glutamic acid (PGA) and chitosan (CHIT)/dextran sulfate (DEX) onto soft and porous thermoresponsive poly(N-isopropylacrylamide-co-methacrylic acid) (P(NiPAM-co-MAA)) microgel was studied by dynamic light scattering (DLS) and electrophoretic measurements. DLS demonstrates an "odd-even" effect in the hydrodynamic radius depending on the type of polyelectrolyte in the outermost layer and that coated microgels retain their native thermoresponsive property. Strong hysteresis is found between the swelling and deswelling processes of microgels coated with polypeptides, whilst for those coated with polysaccharides are nearly reversible. Electrophoretic mobility results indicate charge reversal after each layer deposition, which magnitude decreases with increasing number of layers. Microgels coated with polysaccharides exhibit higher mobility values. Stability studies reveal spatial and temporal reorganization of the polymer chains over several weeks. Upon increasing time, the hydrodynamic radius of polypeptide-terminated microgels decreases, whereas that of polysaccharide-terminated increases to almost the original size of the uncoated microgel. Both systems exhibit an exponential growth of the bilayer thickness with the number of deposition steps, more pronounced for microgels coated with polypeptides. Our results demonstrate the feasibility of the layer-by-layer assembly of these biopolymers onto microgels, which could have potential application for storage and release of biomolecules. (C) 2010 Elsevier Inc. All rights reserved.