Laser manufacturing of multi-functional and antibacterial surfaces for orthopaedic applications
With an increasingly aging population across the globe, there is an increased demand for implantable medical devices to treat long-term health conditions and injuries. Patients with implantable devices such as joint prosthesis, trauma fixation dental implants have a risk of experiencing device associated infection (DAI) which is caused by bacteria attaching to the associated device and forming antibiotic tolerant biofilms1. Attributable mortality rate of DAI across a range of devices can be as high as 25% and treatment is often costly with a much higher chance of reoccurring infections after the first one1. Laser surface treatment of commercially pure titanium (cp Ti or medical grade 2), which is a commonly used material in medical devices has been shown to improve response to bacteria attempting to attach to the surface2. The aim of this work was to investigate how laser treatment in an open-air environment made changes to the surface of cp Ti, the bacterial response to these changes and to propose a mechanism of this improved response to bacterial attachment. Surface characteristics to cp Ti were investigated before and after laser treatment in open air assisted via argon gas. Characterisation was conducted in terms of topography and roughness (SEM, WLI and AFM), surface chemistry (XPS and ToF-SIMS), phase composition (XRD), microstructure (SEM) and wettability (contact angle). The biofilm coverage of multiple species, S. aureus, S. epidermidis, E. coli, P. aeruginosa and P. mirabilis was measured via Live/Dead staining. SEM and FEG-SEM techniques were used to assess the position of bacteria and biofilms on the surface and FIB techniques were used to investigate the bacteria-material interface.Open-air laser surface treatment via a continuous wave laser assisted by argon gas was conducted on as received (AR) cp Ti at laser energy densities of 13.9 J/mm2 (LP), 19.8 J/mm2 (MP) and 28.2 J/mm2 (HP). Laser surface treatment changed both the micro and nano-topography of cp Ti in the form for laser tracks (micro-scale) and ripples (nano-scale). SEM images also revealed presence of micro sized re-solidified “droplets” of titanium on all surfaces as a result of laser-material interaction. Micro-roughness decreased initially from AR (Ra = 0.34 μm) to LP (Ra = 0.19 μm) and increased for MP (Ra = 0.41 μm) and HP (Ra = 0.63 μm). Laser treated surfaces were considered spiky (Rku > 3) with more peaks than troughs (Rsk > 0). Nano-roughness increased as a result of laser treatment from AR (Ra = 46.1 nm) to LP (Ra = 95.9 nm), MP (Ra = 100.4 nm) and HP (Ra = 169.9 nm). All surfaces remained smooth (Rku < 3), with LP & HP surfaces possessing more peaks across the surface (Rsk > 0) and MP surface possessing a surface with more troughs (Rsk < 0).vOnly α-phase titanium existed at the surface level after laser treatment, with changes to the microstructure involving a shift from HCP α-phase titanium to a mixture of acicular and platelet α-phase. XPS and ToF-SIMS results indicated a higher concentration of TiO2 at the surface with an increasing oxide thickness with increasing laser energy density. Surfaces remained hydrophilic after laser surface treatment, with contact angles not exceeding 59.6 ± 2.1°.Live/Dead staining and measurement of biofilm coverage indicated a statistically significant (p < 0.05) reduction in biofilm coverage for all tested species across all laser treated surfaces relative to the AR surface. There was no statistically significant difference in biofilm coverage on the laser treated surfaces themselves. SEM and FEG-SEM images depicted biofilms forming readily on the AR surfaces and only small micro-colonies and clusters forming around micro features (i.e. re-solidified droplets) on the laser treated surfaces. Bacteria were much more scattered across the nano-ripples on the laser treated surfaces with only a few small colonies forming. FIB sectioning of S. aureus on the LP surface did not possess sufficient magnification to identify any specific bacteria-material interactions in the nano-scale. In conclusion a combination of hierarchical micro and nano topographies on laser treated surfaces with an increased thickness in the oxide layer reduced the biofilm coverage of multiple species of bacteria associated with DAI on cp Ti.
Laser processing, titanium, material characterisation, biofilms, bacteria, surface treatment
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