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Posted: Jun 29, 2009
How to make nanosilver non-cytotoxic with sugar
(Nanowerk Spotlight) You can find them in all kinds of products, from socks to food containers to coatings for medical devices – we are talking about silver nanoparticles. Valued for its infection-fighting, antimicrobial properties, silver in its modern incarnation as silver nanoparticles, has become the promising antimicrobial material in a variety of applications because the nanoparticles can damage bacterial cells by destroying the enzymes that transport cell nutrient and weakening the cell membrane or cell wall and cytoplasm.
Despite their wide use, the issue of possible adverse effects and toxicity of nanoparticles for the human body is progressively, albeit slowly, recognized as central by an increasing number of studies (see for instance our recent Spotlight "Nanosilver used in food storage materials found to interfere with DNA replication"). A widely accepted consensus on the detailed molecular mechanism of silver nanoparticles toxicity is still missing and very often the drive toward new formulations overwhelms the interest for a better assessment of the cytotoxicity of the nanoparticles.
"There is an increasing interest toward the exploitation of silver nanoparticles technology in the development of bioactive biomaterials, aiming at combining the relevant antibacterial properties of the metal with the peculiar performance of the biomaterial," Andrea Travan explains to Nanowerk. "So far, to the best of our knowledge, water-based biomaterials able to successfully combine antibacterial properties of silver nanoparticles with demonstrated absence of cytotoxicity have not yet been reported in the literature."
That this hydrogel appears to be non-toxic toward three different eukaryotic cell lines is due to the fact that the nanoparticles, immobilized in the gel matrix, can exert their antimicrobial activity by simple contact with the bacterial membrane, while they can not be uptaken and internalized by eukaryotic cells.
Another crucial issue about silver nanoparticles is their tendency to aggregate, thus losing the peculiar properties associated with the nanoscale. So far, the preparation and stabilization of metal nanoparticles has represented open challenges. The novel approach by the Italian team now also provides for an efficient stabilization of the silver nanoparticles against aggregation.
Travan says that the role of the branched polysaccharide Chitlac (lactose-modified chitosan) is fundamental in the formation and stabilization of well-dispersed silver nanoparticles having a mean diameter of about 35 nm.
This work has been carried out within the scope of an European Project of the 6th Framework Program called Newbone and was motivated by the fact that biomaterials able to prevent bacterial infections without undermining its biomimetic properties are of great interest for the field.
The nanocomposite hydrogels developed by Travan and his colleagues are able to display antibacterial activity without being harmful to mammalian cells. The simultaneous presence, in the final system, of a sugar-based bioactive polymer for cell stimulation and of silver nanoparticles for antibacterial activity represents a major achievement.
Top: TEM image of silver nanoparticles formed on the polymeric chains of Chitlac. Chitlac chains have been stained with a mixed solution of lead citrate (5 g/L) and uranyl acetate (5 g/L); middle and bottom: schematic representation of the polymeric chains of Chitlac providing the nitrogen atoms for the coordination and stabilization of silver nanoparticles. (Reprinted with permission from American Chemical Society)
Being at the crossover of nanotechnology and glycobiology, this approach might pave the way to facilitate the use of silver nanoparticle-biopolymer composites in the preparation of bioactive biomaterials. It also could lead to new tools to design engineered materials exploiting, for different purposes, the bioactivity provided by the carbohydrate component and the properties of silver at the nanoscale level.
Says Travan: "These nanocomposite materials will exploit, for different purposes, the bioactivity provided by the specific carbohydrate component and the properties of silver at the nanoscale level. As an example for the former property, the galactose terminal of the side chain of our polymer Chitlac specifically interacts with sugar-binding proteins, called galectins, on and around the cell surface, triggering the proliferation and the cellular signaling of chondrocytes of cartilage."
Moreover the non-demanding chemical approach used in this technique allows to obtain constructs with a variety of sizes and shapes like slabs, films, microspheres, etc.
Particularly appealing for biomedical applications – where an ideal candidate biomaterial must associate antibacterial properties with a demonstrated lack of cytotoxicity – with this novel material is the possibility of obtaining both three-dimensional highly hydrated structures (e.g. for tissue engineering applications) and more conventional products (like films, fibers, or coatings).
"In our cytotoxicity studies we show how the lack of physical barriers to nanoparticle diffusion into cells determines their generalized bio-availability, with the risk of a massive uptake by eukaryotic cells, which eventually leads to their death," Travan explains. "Conversely, the nanocomposite materials we have developed are able to solve the cytotoxicity problem by creating a gel structure that efficiently immobilizes the metal nanoparticles and ions within the material."
Future challenges in this research area are represented by a more detailed understanding of the antimicrobial mechanism of silver-based materials and by a generalization of the concept of "physical barrier" to diffusion of silver nanoparticles as fundamental to prevent their cytotoxicity.
Travan points out that a particularly interesting future possibility concerns the exploitation, for diagnostic purposes, of the SERS effect, which was shown by the polysaccharide-nanoparticle systems able to specifically reveal galectins in the pericellular areas. "Since galectins involved in many important biological processes, like tumor growth and development of rheumatoid arthritis, the possibilities of applications look very promising."