Immobilisation of biomolecules on plasma-treated ultra-high-molecular-weight polyethylene

Bruggeman, Marc ORCID: 0000-0001-8523-2545 (2023). Immobilisation of biomolecules on plasma-treated ultra-high-molecular-weight polyethylene. University of Birmingham. Ph.D.

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Abstract

Total joint replacements play a vital role in enhancing the quality of life for individuals affected by joint diseases and injuries, a total of 190,223 total joint procedures were performed in the United Kingdom in 2021 alone. In the context of total joint replacements, ultra-high-molecular-weight polyethylene is the primary polymer of choice as the bearing material (i.e., the cup/liners that act as the interface between the implanted artificial joints and corresponding joint sockets) due to its low frictional coefficient, high strength, and bio-inert nature, it was used in at least 93.3% of all hip replacements in 2021. However, surface oxidation and subsequent accelerated debris generation at the polymeric cup insert is a frequent cause of implant rejection as it results in chronic inflammation and joint loosening. To address this, antioxidants like α-tocopherol are embedded into the polymer matrix, as was done in this work.

Next to implant failure due to wear and/or debris, bacterial adhesion and colonization resulting in biofilm formation are responsible for about 20% of biomaterial-associated infections and malfunctions. Fortunately, many of the currently used biomaterials are amenable to modification, allowing the incorporation of antimicrobial agents in/on the surface to combat inflammation and reduce the risk of infection. As polyethylene is chemically-inert, surface modification was required to allow further modification. Due to its non-toxic nature, plasma-nitriding has received increased industrial attention for metals since its initial introduction. To treat non-conductive materials such as polyethylene, the use of an active screen was required to serve as the cathode and allow the formation of plasma near the material surface.

Prior to this work, no research had been conducted on the active screen plasma-nitriding of α-tocopherol-doped ultra-high-molecular-weight polyethylene. The effect of α-tocopherol doping in combination with active screen plasma-nitriding on the ultra-high-molecular-weight polyethylene was investigated using a wide variety of characterisation techniques (e.g., FTIR, Raman, XPS, WXRD, SEM, etc.), to reveal the changes in both chemical and physical material properties. Amongst others, it was discovered that active screen plasma-nitriding introduced a mixture of polyene, amorphous carbon, and aromatics on the polyethylene-based material surfaces, which drastically increased the surface microhardness. The incorporation of α-tocopherol also resulted in an increased nitrogen presence after active screen plasma-nitriding, reduced surface oxidation, and prevented surface contamination and delamination.

In general, implant-associated infections have been treated by developing materials containing antibiotics and/or biocides. However, the use of antibiotics may result in developing antibiotic-resistant bacteria and the use of biocides comes with the risk of cytotoxicity. A recent alternative are antimicrobial peptides, as used in this work. Antimicrobial peptides are native to the human body and utilise modes-of-action that make it difficult for microbes to become resistant. However, due to the rapid metabolization of natural antimicrobial peptides by proteases, high antimicrobial peptide concentrations may be required to fully prevent infection which can induce cytotoxicity. By immobilizing antimicrobial peptides to the plasma-treated polyethylene-based surface, the cytotoxicity and degradative mechanisms can be reduced. Here, the undoped polyethylene-based surfaces allowed for the immobilisation of antimicrobial peptides and/or amino acids using various coupling chemistries.

The biological response of the polyethylene-based surfaces was evaluated following bacterial attachment and CellTiter-Glo luminescent cell viability assays. The biological evaluation of the polyethylene-based samples with immobilised AMPs were unable to be completed. Nonetheless, the polyethylene-based samples displayed increased bacterial growth of Staphylococcus aureus and Staphylococcus epidermidis on the plasma-treated samples when compared to their untreated counterparts. On the other hand, the human osteoblast-like Saos-2 cell proliferation demonstrated increased cell growth on untreated samples when compared to plasma-treated samples. Overall, this research has unveiled the potential of the developed ultra-high-molecular-weight polyethylene-based materials for use in total joint replacements.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):