Antibacterial self-assembled monolayers for hydroxyapatite implants




Torres, Nelson Sigfrido

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Open fractures are common in battlefields, motor vehicle accidents, gunshot wounds, sports injuries, and high-energy falls. Such fractures are often treated using hydroxyapatite (HA)-based bone graft substitutes. However, open fracture wounds are highly susceptible to bacterial infections. Hence, this study was focused on incorporating anti-bacterial properties to HA using silver (Ag) carrying self-assembled monolayers (SAMs). Self-assembled monolayers (SAMs) are molecular coatings composed of a single layer (1-3 nm thick, 10-18 carbon atoms in length) of hydrocarbon chains with reactive head groups that have high affinity toward the substrate material. Furthermore, a monolayer can have reactive terminal groups such as carboxylic acid or amine groups that may serve as a starting point for further modification. Initially, the --COOH terminated phosphonic acid SAMs of two different chain lengths (11 carbon atoms -- short chain and 16 carbon atoms -- long chain) were deposited on HA. Anti-bacterial SAMs (ASAMs) were prepared by chemically attaching Ag to short and long chain SAMs coated HA. X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle goniometry collectively confirmed the attachment of Ag onto SAMs coated HA. The bacterial adhesion study showed that the adherence of Staphylococcus aureus was significantly reduced on ASAMs coated HA when compared to control HA. The stability of Ag carrying SAMs on HA was investigated under different sterilization conditions, including autoclave, dry heat, ethylene oxide gas, oxygen gas plasma, and ultraviolet irradiation, and under physiological conditions by immersion in TBS at 37°C and pH 7.4. The stability studies showed that gas plasma, dry heat and autoclave based sterilization techniques degraded most of the ASAMs on HA. UV irradiation did not damage the short chain ASAMs as vigorously as other treatments, while it degraded the long chain ASAMs completely. Ethylene oxide treatment did not degrade the long chain ASAMs unlike all other treatments but it severely damaged the short chain ASAMs. Both short and long chain ASAMs significantly desorbed from the HA surfaces under simulated physiological conditions although long chain ASAM exhibited better stability than short chain ASAM. Thus, this thesis has demonstrated the potential for using ASAMs to provide anti-bacterial properties to HA, their relative stability after sterilization and under physiological conditions, and the need for developing techniques to further improve stability of SAMs.


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Antibacterial, Hydroxyapatite, Orthopedic Implants, Self-Assembled Monolayers, Silver Nitrate



Biomedical Engineering