Reece N. Oosterbeek

Topologically ordered magnesium-biopolymer hybrid composite structures

Magnesium and its alloys are intriguing as possible biodegradable biomaterials due to their unique combination of biodegradability and high specific mechanical properties. However, uncontrolled biodegradation of magnesium during implantation remains a major challenge in spite of the use of alloying and protective coatings. In this study, a hybrid composite structure of magnesium metal and a biopolymer was fabricated as an alternative approach to control the corrosion rate of magnesium. A multistep process that combines metal foam production and injection molding was developed to create a hybrid composite structure that is topologically ordered in all three dimensions. Preliminary investigations of the mechanical properties and corrosion behavior exhibited by the hybrid Mg-polymer composite structures suggest a new potential approach to the development of Mg-based biomedical devices.

Gold sputtered Blu-Ray disks as novel and cost effective sensors for surface enhanced Raman spectroscopy

Surface Enhanced Raman spectroscopy (SERS) offers sensitive and non-invasive detection of a variety of compounds as well as unparalleled information for establishing the molecular identity of both inorganic and organic compounds, not only in biological fluids but in all other aqueous and non-aqueous media. The localized hotspots produced through SERS at the solution/nanostructure interface of clustered gold or silver nano-particles enables detection levels of parts per trillion. Recent developments in advanced fabrication methods have enabled the manufacture of SERS substrates with repeatable surface nanostructures which provide reproducible quantitative analysis, historically a weakness of the SERS technique. In this paper we describe the novel use of gold sputtered Blu-Ray surfaces as SERS substrates. Blu-Ray disks provide ideal surfaces of SERS substrates with their repeatable and regular nano-gratings. We show that the unique surface features and composition of the recording surface enables the formation of gold nano-islands with nanogaps, simply through gold sputtering, and relate this to a 600 fold signal increase of the melamine Raman signal in aqueous solutions and detection to 68 ppb. Melamine is a triazine compound and appears not only as environmental contaminant in environmental groundwater but also as an adulterant in foods due to its high nitrogen content. We have shown significant SERS signal enhancements for spectra of melamine using gold-sputtered Blu-Ray disk surfaces, with reproducibility of 12%. Blu-Ray disks have a unique combination of design, surface features and composition of the recording surface which makes them ideal for preparation of SERS substrates by gold sputter-coating.

Protein arrangement on modified diamond-like carbon surfaces – An ARXPS study

Abstract Understanding the nature of the interface between a biomaterial implant and the biological fluid is an essential step towards creating improved implant materials. This study examined a diamond-like carbon coating biomaterial, the surface energy of which was modified by Ar + ion sputtering and laser graphitisation. The arrangement of proteins was analysed by angle resolved X-ray photoelectron spectroscopy, and the effects of the polar component of surface energy on this arrangement were observed. It was seen that polar groups (such as CN, CO) are more attracted to the coating surface due to the stronger polar interactions. This results in a segregation of these groups to the DLC–protein interface; at increasing takeoff angle (further from to DLC–protein interface) fewer of these polar groups are seen. Correspondingly, groups that interact mainly by dispersive forces (CC, CH) were found to increase in intensity as takeoff angle increased, indicating they are segregated away from the DLC–protein interface. The magnitude of the segregation was seen to increase with increasing polar surface energy, this was attributed to an increased net attraction between the solid surface and polar groups at higher polar surface energy ( γ S p ).

Ultrashort pulse laser interactions with cortical bone tissue for applications in orthopaedic surgery

Using a femtosecond pulsed laser system (pulse width = 100fs, repetition rate = 1kHz, λ = 800nm), ablation threshold studies of freshly culled bovine and ovine cortical bone samples were identified using the diameter regression technique. Using the D2 technique, the ablation threshold was found to lie within a range of 0.83 - 0.96 Jcm−2 and 0.89 - 0.95 Jcm−2 for ovine and bovine cortical bone respectively indicating that laser ablation of bone is irrespective of target species. The relationship between cortical bone tissue removal and the number of applied pulses was explored. By altering the laser spot translation rate, we varied the number of pulses at each point along scribed linear cuts. Optical Coherence Tomography (OCT) and PDMS casting indicates that cut depth is linearly dependent on the number of pulses applied to the tissue, irrespective of donor species. For single pulse ablation of ovine and bovine cortical bone, we determined that the ablation rates were 0.41 - 0.75 μm per pulse and 0.28 - 0.90 μm per pulse when pulses of fluences in the range 0.52 - 2.63 Jcm−2 were applied to ovine and bovine cortical bone tissue, respectively. Structural analysis of the ablation features using environmental scanning electron microscopy and optical microscopy were utilized to assess the ablation features and identify signs of damage to surrounding tissue. We observed no structural indications of thermal shockwave cracking, molten debris deposition or charring of the tissue whilst leaving hydroxyapatite crystal structure intact.

Ultrafast pulsed Bessel beams for enhanced laser ablation of bone tissue for applications in LASSOS

Using a femtosecond pulsed laser system (pulse width = 100fs, repetition rate = 500 Hz, λ=800nm), a zero-order Bessel beam was generated using a LCOS-Spatial light modulator (LCOS-SLM) with an effective cone angle of 4.56°. Ablation threshold studies of fresh bovine and ovine load bearing cortical bone was identified using the method of least damage and found to be identical at φth = 0.15 ± 0.03 J cm-2, irrespective of the target species. The ablation threshold is significantly reduced compared to the ablation threshold determined for Gaussian beams in bovine and ovine cortical bone (Load Bearing: φth = 0.91 ± 0.03 J cm-2, Skull: φth = 1.19 ± 0.06 J cm-2). Incubation effects were investigated and the incubation coefficient was determined to be ζ = 0.93 ± 0.06, indicating no incubation effects are present. The relationship between tissue removal and the number of pulses applied was explored. By altering the translation rate of the sample under the Bessel region of the incident laser, the number of pulses applied at each point along the linear ablation features was varied. Cross sections of ablation features were measured using scanning electron microscopy (SEM) and maximum depths of the ablation features measured. The ablation rate of bovine and ovine cortical was found to be 2.69 – 13.21 ± 0.05 μm pulse-1 and 2.49 – 12.79 ± 0.03 μm pulse-1 respectively for fluence values ranging from 25.0 – 2.5 Jcm-2, significantly higher than those of Gaussian beams. Structural analysis of the ablation features using SEM and optical microscopy showed no signs of heat affected zone (HAZ) in the form of thermal shockwave cracking, molten debris deposition or charring of the tissue.

Ultrafast laser patterning and defect generation in titania nanotubes for the enhancement of optical and photocatalytic properties

This study demonstrates the first known instance of the templating of titanium dioxide nanotube arrays by laser induced periodic surface structures (LIPSS) and subsequent electrochemical anodization. Titanium dioxide is an established photocatalyst, however it suffers from poor visible light absorption, thus limiting its use under solar irradiation. Thermal annealing can enhance the visible light absorption, with the downside of introducing more defect traps that reduce the lifetime of the charge separated state. Hence, this study proposes an alternative to chemical methods, by modulating the surface profile of the nanotube array to trap visible light. The enhanced visible light absorption is predicted via computational modelling and the morphological evolution of the anodization process was investigated. This study provides the basis for further work into LIPSS templating of other anodized transition metal oxide materials.

Enhancement of laser machining resolution using the two photon absorption effect

One of the main strengths of ultrafast laser machining is its ability to produce very high resolution cuts. This is useful in a range of fields for manufacturing purposes, however being able to incorporate two photon absorption (TPA) to further enhance this would allow even smaller fabrication. In this study we look at the effect of doping amorphous polycarbonate with two different molecules (R1 and R2) which have been identified to have a high TPA cross-section. Using the diameter regression (D2) technique, we observe a notable decrease in the ablation thresholds as dopant concentration increases from 0 – 10%. In addition, these results provide a measurement of effective beam waist (ωeff). Beam waist values for R2 doped samples show a decrease to half the reference, which can be linked to an increase in the occurrence of multiphoton absorption. Linear ablation features produced at identical peak powers and pulse numbers show improved machining resolution as dopant concentration increases. This increase in resolution is more prolific for R2 than R1 despite their similar TPA cross sections. We believe that this is due to the relative bandgaps of the materials causing three photon absorption for R2 whereas multiphoton absorption is limited to the second order process for R1. This means by doping appropriate materials with these molecules it is possible to improve resolution whilst maintaining most properties of the main material.

Investigating ultrashort laser pulses as a LASer scalpel for orthopedic surgery (LASSOS)

We investigate ultrashort laser pulses for orthopedic surgery upon two species of fresh, unaltered bone. Ablation thresholds and rates are determined and we explore non-conventional beam types to optimize ablation efficiency and feature properties.

Fast femtosecond laser ablation for efficient cutting of sintered alumina and quartz substrates

Ultrafast laser machining of ceramic and crystalline substrates offers many benefits versus mechanical dicing. We optimized femtosecond laser parameters for cutting industry sintered alumina and quartz wafers, yielding drastic improvements in cutting speed and quality.