Chitosan-based Silver Composite Materials For Antibacterial Applications

As a natural polymer material,chitosan(CS)is considered as a potential candidate for antibacterial materials due to its excellent biocompatible,biodegradable,antibacterial and hemostatic properties.However,the low solubility of CS in aqueous solutions at neutral pH has significantly hindered its widespread implementation as a functional biomaterial.In addition,many studies have reported that CS was not effective enough in killing bacteria.It is very desirable to incorporate antimicrobial agents into CS,such as silver nanoparticles(AgNPs),which is considered as potential antibacterial agents owing to the broad-spectrum antimicrobial activity.But the potential toxic effects of AgNPs towards the human body still restricts the development in biological applications.In this study,we try to develop several kind of composite materials to combine the unique properties of CS and AgNPs:effective antibacterial activities and low cytotoxicity.The study results are as follows:(1)A highly soluble chitosan derivative,catechol-conjugated chitosan(CSS)was synthesized by the conjugation of bio-inspired catechol to chitosan.As a stabilizing and reducing agent,CSS was used to prepare AgNPs due to the stabilizing effect of chitosan and the reductive activity of aromatic phenols in catechol.A narrow diameter distribution and excellent stability were observed in the target CSS-AgNPs.The entire preparation process complied with the principles of green chemistry.The resulting CSS-AgNPs combined the unique properties of chitosan and silver nanoparticles,showing significant potential in biological and antibacterial fields.(2)As the antibacterial mechanism of AgNPs and CS is still unclear,we tried to use electron microscopy to observe the antibacterial behaviors of CSS-AgNPs.The changes in the cellular ultrastructure of the bacteria provide useful insight into the action mechanism of CSS-AgNPs.Interestingly,we uncovered that CSS-AgNPs exhibited a higher toxicity against gram-negative bacteria than against gram-positive bacteria.The differences are caused by structural differences in the cell walls of gram bacteria.Gram-positive bacteria are protected from CSS-AgNPs by a thicker cell wall,while gram-negatives are more easily killed due to an interaction between a special outer membrane and the nanoparticles.Therefore,CSS-AgNPs killed gram-positive bacteria through a disorganization of the cell wall,while the primary mechanism of action on gram-negative bacteria was a change in membrane permeability.Our study offers an in-depth understanding of the antibacterial behaviors of CSS-AgNPs and provides insights into ultimately optimizing the design of AgNPs for treatment of bacterial infections.(3)A new kind of sustained-release silver-loaded chitosan-based sponge was developed by a simple and green method as a long-lasting antimicrobial dressing.CCS-AgNPs were incorporated into CS matrix through interactions between the catechol and amino groups of CS.As a bridge to bind AgNPs into the CS matrix,catechol extended the silver release time from 1 day to at least 4 days.The prolonged silver release endows CCS-AgNPs/CS sponges with long-lasting bacteriostatic effects.Bacterial growth was completely inhibited for up to 3 days.Meanwhile,the embedded CCS-AgNPs significantly improved the bactericidal effect.More than 99.99%bacteria could be killed by the composite sponges,which completely satisfies antibacterial requirements for wound healing.In general,the CCS-AgNPs/CS sponge with appropriate silver content is considered as a potential candidate for wound healing dressings due to its long-lasting bacteriostatic effect,powerful bactericidal activity,and excellent biocompatibility.(4)A novel pH-responsive multilayer film was built-up by self-assemble method based on electrostatic interactions between the negatively charged polyoxazoline derivative and the positively charged CSS-AgNPs.We synthesized a new kind of oxazoline monomer with terminal methoxycarbonyl group(M1).Three polyoxazoline derivatives with carboxyl group were synthesized through ring opening polymerizations of M1 and another kind oxazoline monomer with ethyl end groups.The charge density of the three oxazoline derivatives depend on the ratio of carboxyl group.We investigated the assembling process and found a critical charge density effect.When the charge density of the oxazoline derivatives is lower than a critical value,no significant layer growth could be observed.Above the critical charge density,the rate of film growth decreases with the charge density of the oxazoline derivatives.and one of oxazoline derivative(PT2)with proper charge density was chose for the follow experiments.The PT2/CSS-AgNPs multilayer film shows an exponential growth.As a high-hydrophilic polymer,PT2 improves the hydrophilic property of the multilayer film,which could prevent bacteria adhesion.The in vitro silver release experiment found a pH-sensitive release on the PT2/CSS-AgNPs multilayer film.The reduced local pH would trigger the release of AgNPs by reduce the electrostatic attraction between PT2 and CSS-AgNPs.These anti-adhesive property and pH-sensitive release make PT2/CSS-AgNPs multilayer film promising candidate for protecting biomaterial implants and devices against bacterial colonization.