Mark R. Towler

A preliminary comparison of the mechanical properties of chemically cured and ultrasonically cured glass ionomer cements, using nano-indentation techniques.

There is a requirement for a dental cement with properties comparable or superior to conventional glass ionomer cements (GICs) but with the command set properties of the resin-modified GICs. The objective of this work was to show that the application of ultrasound to conventional Fuji IX commercial glass ionomer cement imparts a command set, whilst improving the short-term surface mechanical properties. Nano-indentation techniques were employed to highlight the improvements in hardness and creep resistance imparted to the cement through the application of ultrasound. The instant set imparted by the application of ultrasound provides improved surface hardness and creep, particularly within the first 24 h after setting. The surface hardness of the chemically cured Fuji IX (176 M Pa) increased by an order of magnitude when set ultrasonically (2620 M Pa), whilst creep reduced to a negligible amount. Rapid setting allows for shorter chair time and an improved clinical technique, making restorations more convenient for both the patient and clinician.

Leaching of potential hazardous elements of coal cleaning rejects

The geochemical characteristics of coal cleaning rejects (CCR) in Santa Catarina State, Brazil, were investigated. Around 3.5 million ton/ year of coal waste are dumped in Santa Catarina State. Coal beneficiation by froth flotation results in large amounts of CCR composed of coaly and mineral matter, the latter characterised by the occurrence of sulphide minerals and a broad array of leachable elements. The total and leachable contents of more than 60 elements were analysed. Atmospheric exposure promotes sulphide oxidation that releases substantial sulphate loads as well as Ca2 + , K + , Mg2 + , Cl −  and Al3 + . The metals with the most severe discharges were Zn, Cu, Mn, Co, Ni and Cd. Most trace pollutants in the CCR displayed a marked pH-dependent solubility, being immobile in near-neutral samples. The results highlight the complex interactions among mineral matter solubility, pH and the leaching of potentially hazardous elements.

Preliminary work on the antibacterial effect of strontium in glass ionomer cements

Major problems associated with glass ionomer cement (GIC) dental materials are pulpal damage and the development of recurrent caries [1], which are caused by bacterial infection and not by the restorative material itself [2]. Bacteria can enter through the tooth/restoration interface by micro-leakage or it can multiply from single microbes remaining within the smear layer. Secondary caries is a result of these bacteria. Any antibacterial activity imparted by the restorative material will reduce pulpal damage and increase the restoration’s longevity. Although the population of micro-organisms is known to reduce around GIC restorations [3], the available information about the antibacterial effects of GICs is limited in both the number of materials evaluated and the bacteria tested [4]. GICs are being developed for use as in situ medical cements and in these applications the biocompatibility of the cement is important. Fluoride release is known to stimulate apatite deposition in bone [5], but excessive release has been associated with a cytotoxic response in cell culture [6, 7]. While the antibacterial activity of GICs has generally been attributed to fluoride, obtaining a correlation between fluoride release and antibacterial activity is complicated because cements may release other caries inhibitory species such as strontium, zinc and silver ions. The cements studied in this work have been shown to represent model materials of defined chemical composition, which are free of such complications [8]. Fluoride release from these cements is directly proportional to the fluorine content of the glass; fluoride being released principally by an ion exchange process [8, 9]. The purpose of this study was to undertake preliminary analysis of the antibacterial properties of these GICs and then to reevaluate these properties when strontium ions were added. The glasses employed were based on the generic composition:

Preliminary investigation of novel bone graft substitutes based on strontium–calcium–zinc–silicate glasses

Bone graft procedures typically require surgeons to harvest bone from a second site on a given patient (Autograft) before repairing a bone defect. However, this results in increased surgical time, excessive blood loss and a significant increase in pain. In this context a synthetic bone graft with excellent histocompatibility, built in antibacterial efficacy and the ability to regenerate healthy tissue in place of diseased tissue would be a significant step forward relative to current state of the art philosophies. We developed a range of calcium–strontium–zinc–silicate glass based bone grafts and characterised their structure and physical properties, then evaluated their in vitro cytotoxicity and in vivo biocompatibility using standardised models from the literature. A graft (designated BT109) of composition 0.28SrO/0.32ZnO/0.40 SiO2 (mol fraction) was the best performing formulation in vitro shown to induce extremely mild cytopathic effects (cell viability up to 95%) in comparison with the commercially available bone graft Novabone® (cell viability of up to 72%). Supplementary to this, the grafts were examined using the standard rat femur healing model on healthy Wister rats. All grafts were shown to be equally well tolerated in bone tissue and new bone was seen in close apposition to implanted particles with no evidence of an inflammatory response within bone. Complimentary to this BT109 was implanted into the femurs of ovariectomized rats to monitor the response of osteoporotic tissue to the bone grafts. The results from this experiment indicate that the novel grafts perform equally well in osteoporotic tissue as in healthy tissue, which is encouraging given that bone response to implants is usually diminished in ovariectomized rats. In conclusion these materials exhibit significant potential as synthetic bone grafts to warrant further investigation and optimisation.

The role of open and closed curing conditions on the leaching properties of fly ash-slag-based geopolymers.

This study deals with the synthesis of geopolymers from co-fired fly ash and blast furnace slags. Geopolymer bodies were simultaneously synthesized in open and closed curing conditions in order to elucidate the role of this parameter on their resultant properties. Open curing conditions produce solid bodies characterized by high porosity, low compressive strength and exacerbated leaching of certain oxyanionic metalloids. By contrast, protected curing promotes the binder development, giving rise to higher strength and less porous systems. This imposes physical restrictions to leaching which decreases and/or retards releases of oxyanionic metalloids in comparison to open curing conditions. Fly ash-slag-based geopolymers may immobilize a number of trace pollutants such as Be, Bi, Cd, Co, Cr, Cu, Nb, Ni, Pb, REE, Sn, Th, U, Y and Zr, regardless of the curing conditions. Due to geopolymers displaying weak assimilation capacity for oxyanionic species, their successful regarding oxyanionic retention is strongly dependent on porosity and therefore on curing conditions applied.

Bioactive glass reinforced elastomer composites for skeletal regeneration: A review.

Biodegradable elastomers have clinical applicability due to their biocompatibility, tunable degradation and elasticity. The addition of bioactive glasses to these elastomers can impart mechanical properties sufficient for hard tissue replacement. Hence, a composite with a biodegradable polymer matrix and a bioglass filler can offer a method of augmenting existing tissue. This article reviews the applications of such composites for skeletal augmentation.

Comparison of in vitro and in vivo bioactivity of SrO-CaO-ZnO-SiO2 glass grafts.

A range of calcium—strontium—zinc—silicate glass grafts are developed. Following characterization, their ability to form an apatite layer in simulated body fluid (SBF) is evaluated. Concurrently, their in vivo biocompatibility is determined. These glasses are incapable of forming an apatite layer in SBF. However, in vivo, each glass is well tolerated with new bone formation apparent in close apposition to implanted particles and no evidence of an inflammatory response. Such results are contrary to much of the literature and indicate that forecasting a materials ability to bond to bone based on SBF experiments may provide a false negative result.

The crystallisation of glasses from the ternary CaF2-CaAl2Si2O8-P2O5 system

A study of glasses from the ternary system CaF2-CaAl2Si2O8-P2O5 has been carried out. It has been shown that glasses with low phosphorus contents and high fluorite contents crystallise to fluorite. Fluorine reduces the glass transition temperature and is also required for the formation of fluorapatite (FAP). In the absence of fluorine in the glass no apatite phase is formed. Bulk nucleation of FAP is favoured for glasses with Ca:P ratios close to the apatite stoichiometry of 1.67 and with low crosslink densities. Thermal gravimetric analysis showed significant weight losses attributable to the formation of volatile silicon tetrafluoride to occur on crystallisation of the aluminium containing phases, anorthite and mullite, which supports the view that silicon tetrafluoride formation is hindered by fluorine bonding to the aluminium atoms of the glass network. Anorthite crystallisation always occurred by a surface nucleation mechanism and appeared to be favoured by the higher silicon to aluminium ratio in these glasses compared to previously studied glass compositions.

Inorganic hemostats: The state-of-the-art and recent advances.

Hemorrhage is the most common cause of death both in hospitals and on the battlefield. The need for an effective hemostatic agent remains, since all injuries are not amenable to tourniquet use. There are many topical hemostatic agents and dressings available to control severe bleeding. This article reviews the most commonly used inorganic hemostats, subcategorized as zeolite and clay-based hemostats. Their hemostatic functions as well as their structural properties that are believed to induce hemostasis are discussed. The most important findings from in vitro and in vivo experiments are also covered.

Investigation into the ultrasonic setting of glass ionomer cements Part II Setting times and compressive strengths

Glass ionomer cements (GICs) are formed by the reaction of an ion leachable alumino-silicate glass with an aqueous solution of poly (alkenoic acid), PAA. Water is used as the reaction medium [1]. A two-stage setting reaction occurs, resulting in a cement consisting of residual glass particles embedded in a hydrogel polysalt matrix [2]. During the first stage the material is susceptible to water uptake and during the second it is susceptible to dehydration. For example, when GICs are stored in water after an initial set of 15 min, a surface softening occurs, which may be caused by an inhibition of the setting reaction in a superficial layer of the cement [3]. This short-term relationship with water restricts the potential of GICs for healthcare applications. It is for this reason that resin modified GICs (RMGICs) were developed. These materials, conventional GICs into which an organic, photo-polymerisable monomer has been incorporated [4], can be command set by the application of an intense light source. However, RMGICs have drawbacks related to the presence of both non-polymerised monomer and the resin itself [5, 6]. Rapid setting is important both for providing resistance to attack from moisture in the mouth and improving wear resistance, but an alternative method to light-curing which does not require the incorporation of additional chemicals would be beneficial. The application of ultrasonic waves has long been used for setting cement in the building industry, and so studies were performed to utilise the same method for setting dental cements [7]. Most dental clinics have ultrasonic capability as it is used for de-scaling teeth. The effects of applying ultrasonic waves to the surface of the cement include a reduction in both porosity and mean glass particle size (due to breaking up of agglomerates), an improvement in the packing of the glass particles and a decrease in the mean molecular weight of the PAA [8]. Whilst it is not possible to further explain the curing mechanism at this stage, this work will determine the extent to which ultrasound affects the setting times and mechanical properties of a series of hand mixed and mechanically triturated GICs.