Large infection spreading rates combined with increasing microbial resistance to antibiotics remain one of the biggest healthcare challenges. In this context, biomaterials based on natural biopolymers with antibacterial properties have recently come to the fore as potential candidate vehicles as both preventative measures and treatments of severe infections. In this study, ε-polylysine (ε-PL), a naturally occurring antibacterial cationic homopolyamide, and a key component of the extracellular matrix – bioactive hyaluronic acid, were both used to synthesize functional composite hydrogels. The aim of this study was to synthesize and evaluate functional and antibacterial activity of the chemically cross-linked hydrogels based on ε-polylysine (ε-PL) and hyaluronic acid (HA) and investigate key interactions from polymer design leading to efficient bactericidal effects.
The chemically cross-linked hydrogels were synthesized with ε-PL to HA mass ratios of 40:60, 50:50, 60:40, 70:30 and 80:20 wt%. For covalent linkage between biopolymers 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC)/ N-hydroxysuccinimide (NHS) cross-linking strategy was used. Physicochemical properties including molecular structure, morphology and mechanical properties of the fabricated ε-PL/HA hydrogels were fully characterized using Fourier transform infrared spectroscopy (FT-IR), gel fraction, scanning electron microscopy (SEM) and oscillatory rheology. Viscoelastic properties were characterized by an amplitude (0.001-1000 % strain interval at 1 Hz frequency) and frequency sweeps (0.01-100 frequency interval at a constant 0.2 % strain). Antibacterial activity was studied against selection of bacterial strains including Gram– (E. coli (ATCC 25922) and P. aeruginosa 9); and Gram+ (S. aureus JAR (010631), S. epidermidis (ATCC 35984) and S. pyogenes (ATCC 19615)). The statistical significance of obtained data was calculated using IBM SPSS software.
FT-IR spectra of prepared ε-PL/HA hydrogels unraveled successful amide bond formation between ε-PL and HA. SEM micrographs revealed formation of three-dimensional networks with highly macro- and microporous homogeneous structure for all synthesized hydrogels. Gel fraction studies indicated that the mean value of the insoluble fraction in the fabricated hydrogels among all ε-PL to HA mass ratios is 55 %. All hydrogels showed larger shear storage moduli (G’) than loss moduli (G’’) in the linear viscoelastic regime (at strains ε=0.01-1%), indicative of a solid-like structure of material. Irrespective of the polymer ratio, on average the chem ԑ-PL/HA hydrogels exhibited 13.2 ± 3.5 kPa G′ value, indicative of the dominance of the chemical crosslinks over physical entanglements for this type of crosslinking method. During antibacterial studies a positive correlation between increase of ε-PL mass ratio and bacterial inhibition value was observed. Moreover, starting from ε-PL mass ratio of 50 wt% hydrogel samples showed statistically significant bacteria reduction after 24h contact time (p<0.05).
The covalently bonded hydrogels demonstrated highly porous homogenous structure and appropriate viscoelastic properties. Evaluation of antibacterial activity revealed great impact of ε-PL mass ratio on the bacterial inhibition. Antibacterial studies of hydrogel samples revealed both fast (up to 24h) and prolonged (up to 168h) antibacterial effect against Gram-positive and Gram-negative bacteria. Described features open up potentials for the material to be used in tissue engineering (e.g., musculoskeletal disorders or post-surgery site infection treatment).