Authors: Lawson, M.C., Hoth, K.B., DeForest, C.A., Bowman, C.N., Anseth, K.S., University of Colorado, Aurora, CO
Title: Inhibition of S. epidermidis Biofilms on Orthopedic PMMA Bone Cement and Ti-6Al-4V Alloy using Polymerizable Vancomycin Derivatives
Purpose: Biofilm formation on indwelling orthopaedic biomaterials including metallic hardware and poly(methyl methacrylate) bone cement remains problematic. Antibiofilm strategies with polymerizable antibiotics were examined.
Methods: Polymerizable derivatives of vancomycin were used to modify both Ti-6Al-4V alloy and poly(methyl methacrylate) (PMMA) bone cement with the intent of interfering with Staphylococcus epidermidis biofilm formation. In this study, a Ti-6Al-4V alloy was functionalized with a methacrylated silane reagent in preparation for subsequent polymer attachment. X-ray photoelectron spectroscopy (XPS) was used to access silane surface coverage. Surfaces were then coated with polymers formed from poly(ethylene glycol)(375)-acrylate [PEG(375)-acrylate] or a vancomycin-PEG(3400)-PEG(375)-acrylate copolymer. Following functionalization with vancomycin-based polymers and surface exposure to biofilm-forming Staph. epidermidis, scanning electron microscopy (SEM) was used to quantify adherent bacteria. Additionally, PMMA bone cement was loaded with various species including vancomycin and several polymerizable vancomycin derivatives (acrylamides). Mechanical properties of the loaded cements including compressive modulus, yield strength, resilience, and fracture strength were measured. A gravimetric biofilm assay was employed in subsequent experiments to evaluate the ability of the additive species to interfere with bacterial proliferation on PMMA surfaces.
Results: XPS showed the Si elemental surface composition to be ca. 18% after silanizaton, which suggests monolayer or greater surface coverage. Scanning electron microscopy results showed the vancomycin-PEG(3400)-acrylate-type titanium surface to reduce adherent bacteria numbers by approximately 4 fold when compared to PEG(375)-acrylate alone (P = 0.007). Consistent with well-established literature, vancomycin-loading of PMMA bone cement at 10 wt% significantly reduced all mechanical properties tested, including a ca. 20% reduction in compressive modulus. In contrast, loading a vancomycin-acrylamide derivative (polymerizable) restored these mechanical deficits. However, no antibiofilm properties were demonstrated. Conversely, a polymerizable, PEGylated vancomycin derivative reduced biofilm attachment but resulted in arguably poor cement mechanical properties.
Discuassion and Conclusion: Several polymerizable vancomycin derivatives were examined. Data with coated Ti alloy surfaces suggest that copolymerizing a PEGylated vancomycin species, VPA(3400), with PEG(375)-acrylate is more effective than PEG alone at blocking S. epidermidis biofilms. The PEG spacer itself likely contributes to the antibiofilm effect, but SEM data indicate that the pendant vancomycin molecule improves the antimicrobial effect under some growth conditions. In addition, loading PMMA bone cement with certain polymerizable vancomycin derivatives may eventually be useful for retarding biofilm adherence without compromising mechanical properties, though the current formulations were found to do one or the other—not both. A vancomycin-acrylamide additive resulted in compressive mechanical properties almost identical to PMMA controls but did not inhibit biofilm growth. VPA(3400) inhibited biofilm growth but compromised mechanical properties. From a design perspective, it would be convenient to have a set of antibacterial materials that could be used for the modification of both metallic and bone cement surfaces. Polymerizable antibiotic monomers have potential for this purpose, and experiments described here should facilitate that end.