Weak Polyelectrolyte Multilayers And Their Anti-bacterial Properties

Infections arising from the use of biomaterials, such as vascular and urinary catheters, vascular implants, heart valves etc, have become a major concern in the biomedical industry. As only the biomaterials surface is in direct contact with the biological environment, the infections first of all depends on the biomaterials surface properties involved in reactions occurring at the biomaterials-biosystem interface. In the present work, the layer-by-layer (LbL) self-assembly technique was introduced to the surface modification of the biomaterials to improve their anti-bacterial properties. The effects of the pH of dipping solution on the composition, wettability and morphology of the weak polyelectrolyte multilayer films were systemtatically investigated, and two kinds of multilayer films with anti-fouling properties were designed, which will be introduced in the following section.Chitosan/heparin multilayer films and their anti-infection properties—Chitosan as an antibacterial agent and heparin as an anti-adhesive agent were alternatively deposited onto aminolyzed poly (ethylene terephthalate) (PET) to construct anti-infection multilayer films, and the pH of dipping solution was adjusted to change the composition of the multilayer films, and harmonize the anti-bacterial and anti-adhesive properties of the multilayer films to construct more powerful anti-infection coating. The contact angle and UV data verified the progressive buildup of the multilayer film by alternate deposition of the polyelectrolytes. The results of initial adhesion of Escherichia coli (E. coli) showed that the number of adhesive bacteria decreased with decreasing the assembly pH. The in vitro antibacterial test indicated that the multilayer of chitosan/heparin could kill the bacteria effectively and assembly pH has remarkable effect on the antibacterial property of the multilayer. Chitosan/heparin multilayer films were also used as a template to load nano-silver for more powerfull anti-infection coating. Chitosan-silver nitrate complex and heparin were alternately deposited onto an aminolyzed PET film surface, and subsequently, the silver ions within the multilayer films were reduced with ascorbic acid to form silver nanoparticles. UV-visible spectroscopy and transmission electron microscopyconfirmed the formation of well-dispersed nanosilver particles with sizes (10-40 nm) that depended on the initial concentration of silver ions in chitosan solution and the pH of ascorbic acid solution. The chitosan/heparin multilayer films were possessed of bactericidal effect on Escherichia coli, and this antibacterial effect could be significantly enhanced by the incorporation of silver nanoparticles into the multilayer films. The multilayer films containing nanosilver were not only effective as antibacterial but also as anticoagulant coating. And cell toxicity evaluation suggested that the multilayer films containing nanosilver did not show any cytotoxicity. The multilayer films containing nanosilver may have good potentials for surface modification of medical devices, especially for cardiovascular implants.pH amplified exponential growth multilayer films and their anti-fouling function—A facile method to amplify the growth of the exponential multilayers by the alternate deposition of PEI solution at high pH and PAA solution at low pH was demonstrated. A model was given to explain why pH can amplify the growth of multilayer films based on the mechanism of polyelectrolyte "diffusion in and out" of multilayers. Since both PEI and PAA are weak polyelectrolyte, the ionization degree of their functional groups change with the alternate deposition steps, which will facilitate the diffusion of the PEI molecules "in and out" of the whole film and amplify the amount of the polyelectrolytes deposited onto the surface during each adsorption step. The method of pH amplified exopential growth multilayers may provide a novel and convenient pathway to produce multilayer films more efficiently. A hierarchal micro and nano structures on the multilayers can be achieved simultaneously by deposition of PEI solution at high pH and PAA solution at low pH. When the multilayers were constructed with PAA solutions at low pH and PEI solution at high pH, such as PAA2.95/PEI9.00, PAA chains and PEI chains are adsorbed onto the surface with globular conformation due to its low ionization degree, at the same time, a pH-induced ionic-bond-breaking and morphological reorganization process happens in the pH alternative build-up steps. Larger aggregates from more loopy PAA chains and bigger pores by pH-induced morphological reorganization lead to a rougher surface with micro- and nano- structures on themultilayers. A superhydrophobic surface was developed from pH amplified exponential growth multilayers with micro- and nano- structures after being modified by a chemical vapor deposition of (tridecafluoroctyl) triethoxysilane. The pH amplified exponential multilayers with micro-nano structures can be constructed on the PEI-modified hydrophobic substrates such as Teflon, followed by depositing a thin layer of fluorinated silane onto the multilayers. Free-standing superhydrophobic multilayers could be prepared sucessfully after the multilayers were taken away from the hydrophobic subatrates. The amplified exponential growth multilayer films with characteristic size changing from several hundred nanometers to about five micrometers were constructed by adjusting the pH of dipping solution. The effects of the morphology and wettability of the surfaces of pH amplified exponential growth multilayer films on their anti-fouling properties were investigated. The biological tests suggested that the superhydrophobic surfaces having self-cleaning ability can resist the adhesion of the bacteria, and other surfaces increase their anti-fouling properties with increasing the hydrophilicity. The multilayers which structure size is less than or equal to the size of bacteria can more effectively resist the adhesion of bacteria, which is caused by the few contact area between bacteria and the surface.