Antimicrobial activity of silver and copper nanoparticles incorporated with biodegradable polymeric scaffolds for wound healing application

Abstract

There has been a significant increase in the prevalence of chronic diseases such as diabetes and cancer which cause wounds like diabetic foot ulcers and pressure ulcers. This usually causes infection(s), leading to the amputation of limbs or even death. Chronic wounds do not follow the typical stages of wound healing and thus either take a very long time to heal or do not heal at all. Therefore, the need for developing advanced and novel wound treatment products that offer infection control and sharp debridement is vital. To combat this issue, this project focussed on fabricating bioactive nanomaterials to design an advanced wound dressing in the form of 3D scaffolds. The scaffolds were made of nanocomposites containing biocompatible and biodegradable polymers decorated with antimicrobial active Cu and Ag NPs. Prior to designing the scaffolds, pristine Ag and Cu NPs were fabricated and their antimicrobial activity and cytotoxicity were tested. The antimicrobial activity of these NPs was tested against E. coli and S. aureus bacterial strains, while their cytotoxicity was evaluated using baby hamster kidney fibroblasts (BHK-21) cells. Ag NPs with various sizes and spherical shapes were synthesized using dopamine (DA) as a reducing agent. Ag NPs with the smallest diameter exhibited the most antimicrobial activity compared to those with big diameters. The cytotoxicity of Ag NPs was very low, with cell viability of over 70% between 2 μg/mL and 32 μg/mL. At concentrations above 32 μg/mL, a decrease in cell viability was observed, indicating increasing toxicity, especially for small NPs. Cu-based NPs were also synthesized using hydrazine and DA as reducing and capping agents, respectively. Pure metallic Cu, Cu 2 O and the mixture of Cu and Cu 2 O (Cu/Cu2 O) NPs were separately prepared. Cu and Cu2 O NPs exhibited promising antimicrobial activity against E. coli and S. aureus. An improved antimicrobial activity was observed for Cu/Cu2 O NPs, compared to the Cu and Cu2O NPs, and this was attributed to the synergistic effect. The cytotoxicity of Cu2 O NPs was higher compared to that of Cu and Cu/Cu 2 O NPs. The antimicrobial activity and cytotoxicity of both Ag and Cu NPs incorporated with polydopamine (PDA) were also studied. Cu and Ag NPs were either embedded within the matrix of PDA (Cu-PDA or Ag-PDA) or incorporated on the surface of PDA (Cu@PDA or Ag@PDA). Cu@PDA and Ag@PDA showed stronger antimicrobial activity than Cu-PDA and Ag-PDA due to the NPs exposure to the bacteria. These nanocomposites showed very iii low cytotoxicity with cell viability of over 80 % at concentrations as high as 250 μg/mL. However, a drastic decrease in cell viability was observed when the concentrations of Cu@PDA and Ag@PDA were increased above 250 μg/mL. Taking advantage of the strong biological activity exhibited by Cu and Ag NPs, bimetallic nanoalloys of these two metals were prepared. Nanoalloys with a fibrous shape were obtained as bimetallic nanofibres (BNF). The mole ratio of Cu:Ag was found to have an effect the antimicrobial activity of the BNFs. The BNFs prepared with a mole ratio of 1:2 (BNF-2) exhibited a strong antimicrobial activity compared to 1:1 (BNF-1) and 2:1 (BNF-3). Furthermore, the BNFs showed minimal cytotoxicity towards the BHK-21 cells at low concentrations (7.8 μg/mL), with the cell viability ranging between 75-90 %. Compared to BNF-1 and BNF-2, BNF-3 was found to have a negative impact on cell viability when the concentrations were increasing. The novel and porous 3D scaffolds were prepared by coating chitosan/gelatine (CS/Gel) scaffolds with PDA using the freeze-drying method. This was followed by decorating the surface of the scaffolds with either Ag or Cu NPs, in situ. The presence of the catechol functional group inherent from PDA facilitated the reduction of Ag ions to form pure zerovalent Ag NPs on the surface of the scaffolds. However, to reduce Cu ions to form Cu NPs, an access reducing agent (hydrazine) was added. The scaffolds showed reasonably high fluid uptake (FU) with time-dependent biodegradability, a property that would be advantageous in controlling wound exudates. These scaffolds also showed excellent antimicrobial activity against E. coli and S. aureus with low cytotoxicity towards human fibroblast cells. Taken together, the designed scaffolds could act as a barrier from the external environment, help prevent bacterial infections and further accelerate skin cells regeneration. The most important feature of these scaffolds is their potential ability to penetrate deep wounds and provide a conducive environment for skin cells to regenerate without any difficulties from bacterial colonization.