Mechanobiocidal and Anticorrosion Effect of Nanopatterned Silk Film


Student thesis: Doctoral Thesis

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Awarding Institution
Award date30 Jul 2020


Bone implants are normally made from metallic alloys and elements such as, Pt-based, Fe-based, Mg-based and Ti-based materials. However, as most of them are not biodegradable, in some cases, a post-surgical operation is often required to remove implants after tissue healing, which can significantly increase the chances of infection. Therefore, scientists have started their research to find and develop biodegradable materials, metallic or polymeric based, to resolve this issue. Although magnesium alloys are categorized as biocompatible and bioresorbable materials, their poor corrosion properties limit their biomedical applications. Among the polymers, some of them, such as polycaprolactone (PCL), polyether ether ketone (PEEK), poly (methyl methacrylate) (PMMA), poly (lactic-co-glycolic acid) (PLGA), etc. have good biocompatibility and biodegradability, which increases their chances for further bioapplications. However, their poor mechanical properties still remain a big issue, and therefore, their usage in applications such as bone implants is far away from reality. In this respect, natural silk fibers extracted from Bombyx mori exhibit excellent mechanical properties and tunable biodegradability at the same time. This natural polymer, which has been approved by the Food and Drug Administration (FDA), has diverse applications ranging from biocarriers to bone implants.

Bacterial infection before and after surgery has become a serious problem around the world because of the increase in antimicrobial resistance (AMR) caused by the overuse of antibiotics. Although the use of biocidal elements, such as silver, has been proven to be effective for killing bacteria, these elements can also be dangerous for human cells. Therefore, researchers have searched for a better and more biocompatible way to combat bacterial colonization. In this regard, they have focused on nature and found some interesting results. From a lotus leaf to gecko skin and from dragonflies’ wings to shark skin, researchers have found unique and effective nanopatterns that help these creatures prevent bacterial colonization effectively. These nanopatterns kill or repel bacteria without secreting any chemical substances and make the surface mechanobiocidal. By decreasing the contact area of bacteria with the surface, the cell wall of the bacteria experiences high pressure which can lead to cell wall rupturing or bacterial detachment. Thus far, scientists have mostly focused on the fabrication of superhydrophobic surfaces to reduce the chances of bacteria colonization; however, hydrophobic surfaces are not suitable for cell growth. Hence, the fabrication of an antibacterial surface with a good ability to support cell growth is highly favorable.

A brief introduction about biomaterials including silk-based materials, bacterial infections and antibacterial surfaces is given in the first chapter. In this chapter, the main approaches to the prevention of bacterial attachment and proliferation such as antibiotic conjugation to a surface, using metallic ions and introducing nanofeatures to the surface are also explained.

In this research, being inspired by the nanopatterns discovered on the surface of dragonflies’ wings, a nanopatterned silk film was fabricated and investigated. The fabricated silk film was coated with hexagonally packed polystyrene nanoparticles, which acted as a sacrificial mask in the plasma etching process, via the colloidal lithography technique. Almost uniform nanocones, with the same height and center-to-center distance, were obtained after oxygen plasma etching for approximately 15 min. Pure silk has hydrophobic properties that can be changed to hydrophilic ones by oxygen plasma etching, thereby increasing the surface energy by approximately 176%. This nanostructure not only decreased the bacterial attachment, of both Gram-negative (Escherichia coli (E. coli)) and Gram-positive (Staphylococcus aureus (S. aureus)), by up to 90% but also enhanced the proliferation of osteoblast cells by more than 30%. For a better investigation of the effectiveness of these nanocones, nanopatterned silk samples were infected with three different concentrations of bacteria (103, 105 and 107 CFU/mL) for 6 h, and then, osteoblast cells were cultured on the surface. The results showed that the prevention of bacterial colonization was better on the surface of the nanopatterned samples compared to control ones; therefore, cell proliferation was improved, even when the samples were infected with a bacterial concentration of 105 CFU/mL.

Magnesium alloys, which are biodegradable biomaterials, have been investigated considerably as a bone implant; however, their poor corrosion properties have remained a concerning issue. These alloys corrode very fast in a physiological medium and lose their integrity, releasing huge amounts of Mg ions, which can lead to inflammation and increase the pH value of the medium. In this project, AZ31 magnesium alloy was coated with a silk film to act as a corrosion barrier, and the previously discussed approach regarding the introduction of nanofeatures on the surface of a silk film was applied to enhance the antibacterial behavior. Upon the application of the silk film on the surface of AZ31, the corrosion current density of silk coated AZ31 (SCAZ31) reduced significantly by approximately 104 times. The leaching of Mg ions also decreased by half after 1-day immersion in a simulated body fluid (SBF) at 37 °C consequently resulting in a lower pH. Meanwhile, magnesium alloys have an intrinsic antibacterial effect because of the Mg ions and the application of a silk film can decrease, the antibacterial effect of SCAZ31. However, the antibacterial results showed that fewer bacteria were attached and proliferated on the surface of SCAZ31 compared to AZ31. An improvement in the biocompatibility of SCAZ31 as compared to AZ31 was also observed.

At the end of thesis, the effect of nanofeatures on the antibacterial properties of the silk film and the effect of applying a nanopatterned silk film on the AZ31 surface on the corrosion and antibacterial properties of AZ31 are summarized. Some suggestion for future works are also presented.