The effect of composition on ion release from Ca-Sr-Na-Zn-Si glass bone grafts.

Controlled delivery of active ions from biomaterials has become critical in bone regeneration. Some silica-based materials, in particular bioactive glasses, have received much attention due to the ability of their dissolution products to promote cell proliferation, cell differentiation and activate gene expression. However, many of these materials offer little therapeutic potential for diseased tissue. Incorporating trace elements, such as zinc and strontium, known to have beneficial and therapeutic effects on bone may provide a more viable bone graft option for those suffering from metabolic bone diseases such as osteoporosis. Rational compositional design may also allow for controlled release of these active ions at desirable dose levels in order to enhance therapeutic efficacy. In this study, six differing compositions of calcium-strontium-sodium-zinc-silicate (Ca-Sr-Na-Zn-Si) glass bone grafts were immersed in pH 7.4 and pH 3 solutions to study the effect of glass composition on zinc and strontium release in a normal and extreme physiological environment. The zinc release levels over 30 days for all zinc-containing glasses in the pH 7.4 solution were 3.0-7.65 ppm. In the more acidic pH 3 environment, the zinc levels were higher (89-750 ppm) than those reported to be beneficial and may produce cytotoxic or negative effects on bone tissue. Strontium levels released from all examined glasses in both pH environments similarly fell within apparent beneficial ranges--7.5-3500 ppm. Glass compositions with identical SrO content but lower ZnO:Na(2)O ratios, showed higher levels of Sr(2+) release. Whereas, zinc release from zinc-containing glasses appeared related to ZnO compositional content. Sustainable strontium and zinc release was seen in the pH 7.4 environment up to day 7. These results indicate that the examined Ca-Sr-Na-Zn-Si glass compositions show good potential as therapeutic bone grafts, and that the graft composition can be tailored to allow therapeutic levels of ions to be released.

Differential-pulse voltammetric determination of clenbuterol in bovine urine using a Nafion-modified carbon paste electrode.

A method has been developed for the determination of clenbuterol in bovine urine using differential-pulse voltammetry (DPV), based on the electrochemical behaviour of clenbuterol at a Nafion-modified carbon paste electrode (CPE). Clenbuterol is irreversibly oxidized at high positive potentials, its irreversibility being due to a chemical follow-up reaction which results in a product showing quasi-reversible electrochemical behaviour at much lower potentials. It is the oxidation peak of this product, arising in acidic media at 0.42 V, which was analysed using DPV, again following the accumulation of clenbuterol at the Nafion-modified CPE. Electrode renewal was achieved by holding the potential at -0.6 V for 120 s in 0.1 mol l-1 NaOH. The determination of clenbuterol in the presence of interfering compounds present in bovine urine samples was then carried out after a two-step clean-up of the urine involving liquid-liquid extraction followed by a mixed-mode solid-phase extraction procedure. This allowed clenbuterol to be detected down to a level of 1.02 x 10(-9) mol l-1 in bovine urine extracts.

Electrochemical behaviour of clenbuterol at Nafion-modified carbon-paste electrodes.

A detailed study of the electrochemistry of clenbuterol at bare carbon-paste electrodes (CPEs) has been carried out. Results showed that clenbuterol undergoes an ECE process. This compound is irreversibly oxidised at high potentials, resulting in the formation of a product which demonstrates quasi-reversible electrochemical behaviour at less positive potentials. The amount of this chemical product formed is very pH-dependent. Investigations into the electrochemical behaviour of clenbuterol at Nafion-modified CPEs were also made. The use of a thin Nafion film cast over the CPE resulted in a large increase in peak current over bare electrodes. Linear accumulation occurred with time, the linear range increasing with decreasing concentration. This allowed the detection of low concentrations of clenbuterol. Diffusion proved to be the rate-controlling process of clenbuterol through the Nafion membrane.