title: Atomic layer deposition of conformal silver as an ultra-thin anti-microbial coating for orthopaedic implants
Creators: name: Golrokhi, Z
Publisher: name: University of Liverpool Repository
Year: 2017

description: The controlled deposition of ultra-thin conformal silver nanoparticle films is of interest for applications including anti-microbial surfaces, plasmonics, catalysts and sensors. Although various techniques can produce silver films, only a limited number of techniques can offer highly conformal ultra-thin coatings on high aspect ratio surfaces and complex geometries, together with sub-nanometre control and scalability. Here we develop a self-limiting atomic layer deposition (ALD) process for the deposition of conformal metallic silver nanoparticle films. In this study, silver films have been deposited using direct liquid injection thermal ALD with ((hexafluoroacetylacetonato) silver (I) (1,5-cyclooctadiene)) as the metal source. The ALD process has been compared and contrasted by using propan-1-ol as a co-reactant with the ALD process using tertiary butyl hydrazine as a co-reactant. A narrow ALD temperature window between 123 and 128 °C is identified for the propan-1-ol process with a nominal mass deposition rate of ~17.5 ng/cm2/cycle. The ALD reaction mechanisms have been elucidated using in-situ quartz crystal microbalance (QCM) measurements, showing chemisorption of the silver precursor, followed by heterogeneous catalytic dehydrogenation of the alcohol to form metallic silver and an aldehyde. A significantly wider temperature window between 105 and 128 °C (23 °C) is identified for the hydrazine based process with a nominal mass deposition rate of ~20.2 ng/cm2/cycle (a nominal growth rate of 0.18 Å/cycle). The effects of temperature, co-reactant dose and cycle number on the deposition rate and on the physico- chemical and electrical properties of the films have been systematically investigated. Under self- limiting conditions, films grown using propan-1-ol are non-conductive metallic silver with a nano- textured surface topography. The size distribution of nanoparticles is narrow under ALD conditions and the number of ALD cycles can be used to accurately control nanoparticle sizes up until neighbouring particles begin to merge. The hydrazine based process produces less textured, more film like coatings. The films are found to be metallic silver and are electrically conductive. Also, better surface adhesion was achieved with scotch tape test in hydrazine based process compared with propan-1-ol. Silver is the most favourable metal for antimicrobial coatings due to its excellent antimicrobial activity against a wide range of microorganisms including inhibition of bacterial adhesion, broad anti-bacterial spectrum, and its tendency for being less prone to the increase of bacteria resistance compared to antibiotic. The need for artificial implants has raised due to aging populations and obesity and resulted in the number of implant-related infections. These infections result in the implant failure, revision surgeries, pain for the patients, more hospitalisation time and also hugely increase the financial burden on health services. One of the major bacteria associated with joint replacement complications is Staphylococcus epidermidis, having strong biofilm forming capabilities in deep wounds and on prostheses. In order to inhibit biofilm formation on surfaces of implants we developed 3D titanium structures using the selective laser melting technique and subsequently coated them with an ultra-thin conformal layer of metallic silver nanoparticles using (ALD). Silver coated implants showed high antimicrobial effect on S. epidermidis by reducing it up to 2-log fold. Ultrastructural examination of human fibroblasts (HS27), keratinocytes (HaCaT), endothelium (HMVEC and HUVEC) and bone (SAOS2) cells showed robust growth on both silver coated and control Titanium implant surfaces. The study shows that a nano-layer of silver coated SLM manufactured titanium implants have significant effect in reducing the pathogenic biofilm formation while retaining their biocompatible properties, making these surface-modified implants promising candidates for clinical orthopaedic applications.