Characterisation of some experimental silicones

Release of antimicrobials/antibacterials like chlorhexidine diacetate (CHD) has proved successful in inhibiting Candidal colonisation of silicone-based biomaterials. However, their addition will increase water uptake and may compromise the mechanical integrity. Two experimental silicones (S1 and S2) differing only in the surface treatment of the filler, were investigated. Ultimate tensile strength (UTS), % elongation at break (Eb), Shore A hardness and, when doped with 1% CHD, water uptake and CHD release were measured. Elastic modulus (E) was calculated from the hardness measurements. There was no significant difference in UTS and Eb between the two materials. However S1 had a higher hardness (30.6 ± 0.97) and thus E (0.76 MPa) than S2 (hardness = 23.8 ± 0.48, E = 0.45 MPa). Water uptake for S2 (0.6%) was higher than for S2 (0.1) and addition of CHD dramatically increased the uptake of both (S1 = 3.1%, S2 = 4.0%). Release of CHD was higher for S2 (30%) than S1 (27%). Equating osmotic pressure within the droplets with elastic restraining force gave an extension ratio of 1.95 for S1 and 5.39 for S2. Thus, addition of a hydrophilic agent can compromise the mechanical integrity of low modulus silicones.     

Elucidation of mitochondrial effects by tetrahydroaminoacridine (tacrine) in rat, dog, monkey and human hepatic parenchymal cells

Tetrahydroaminoacridine (tacrine) causes morphological and functional changes in the endoplasmic reticulum, ribosomes, and mitochondria in the liver of humans and animals. In order to investigate species differences as well as to understand the morphological changes, we examined the effects of tacrine on respiration and electron transport in mitochondria isolated from rat, dog, monkey, and human liver. Tacrine produced significantly decreased respiratory control ratios (RCR) in all species at concentrations ranging from 5 to 25 microg/ml. Human mitochondria were more sensitive to tacrine effects with RCR decreased 24% at 5 microg/ml while other species were unaffected at this concentration. The tacrine effects were characterized by increased hepatic mitochondrial State 4 respiration in rats and decreased State 3 respiration in humans. Mitochondria from aged rats were more sensitive to the effects of tacrine than mitochondria from young animals, with significantly decreased RCR at 10 microg/ml in aged rats while mitochondria from young rats were unaffected at this concentration. Concomitant with the respiratory changes, mitochondrial DNA synthesis was impaired. Since tacrine undergoes extensive biotransformation, we also explored the possibility that metabolites could exert detrimental effects. The ranking order of potency for decreasing RCR caused by monohydroxylated metabolites was: tacrine > 4-OH and 7-OH > 2-OH, 1-OH, and velnacrine with the latter group of metabolites having no effect on mitochondrial respiration at concentrations up to 50 microg/ml. In vivo administration of 20 mg/kg tacrine to rats for up to 20 days caused a paradoxical increase in RCR and P/O on Day 1 and decreased RCR on Days 9 and 20, the later findings being consistent with in vitro data. From these data we propose that tacrine does not necessarily have to be metabolized to exert effects on mitochondria at different sites in the electron transport chain that differ among species. These effects are exacerbated in mitochondria from older animals and humans appear to be more sensitive than the laboratory animals studied.     

Centenary year of scientific papers in the British Dental Journal

This penultimate part of the centenary series looks at E Matthews' paper on "The alginates in dentistry' and is reviewed by Mike Braden. In the paper, Matthews discussed the need to find a suitable alternative to certain dental materials such as agar agar when the main supplier of this, Japan, entered the Second World War.     

Failure to cure Mycobacterium gordonae peritonitis associated with continuous ambulatory peritoneal dialysis

Nontuberculous mycobacteria are increasingly recognized as important pathogens in peritonitis associated with continuous ambulatory peritoneal dialysis (CAPD). Mycobacterium gordonae rarely causes human infection and is the least likely mycobacterium to produce clinical infection in CAPD patients. We describe a patient with persistent M. gordonae peritonitis acquired while undergoing CAPD. During 18 months of treatment, clinical improvement occurred but a microbiological cure could not be achieved. Principles of therapy for mycobacterial peritonitis developing during CAPD are reviewed, and potential explanations for our patient's failure to respond to therapy are discussed.     

Microchip-based purification of DNA from biological samples

A microchip solid-phase extraction method for purification of DNA from biological samples, such as blood, is demonstrated. Silica beads were packed into glass microchips and the beads immobilized with sol-gel to provide a stable and reproducible solid phase onto which DNA could be adsorbed. Optimization of the DNA loading conditions established a higher DNA recovery at pH 6.1 than 7.6. This lower pH also allowed for the flow rate to be increased, resulting in a decrease in extraction time from 25 min to less than 15 min. Using this procedure, template genomic DNA from human whole blood was purified on the microchip platform with the only sample preparation being mixing of the blood with load buffer prior to loading on the microchip device. Comparison between the microchip SPE (microchipSPE) procedure and a commercial microcentrifuge method showed comparable amounts of PCR-amplifiable DNA could be isolated from cultures of Salmonella typhimurium. The greatest potential of the microchipSPE device was illustrated by purifying DNA from spores from the vaccine strain of Bacillus anthracis, where eventual integration of SPE, PCR, and separation on a single microdevice could potentially enable complete detection of the infectious agent in less than 30 min.     

Effect of composition on the mechanical properties and water sorption of a butadiene/styrene copolymer-methacrylate monomer soft lining material

A series of experimental soft lining materials has been produced using a butadiene/styrene copolymer with methacrylate monomers, and assessed in terms of their tensile properties. Three different methacrylate monomers, n-hexyl (HMA), ethyl hexyl (EHMA) and 1-tridecyl (TDMA), and two different initiators, benzoyl peroxide (BP) and lauryl peroxide (LP), were used. Other variables were copolymer/monomer ratio and level of cross-linking. Water sorption studies were also carried out on some of the materials, selected in terms of strength, on the pure copolymer (with and without partitioning agent) and on homopolymers of HMA and EHMA. Generally, the EHMA-based materials had the highest strengths and the TDMA the lowest. The highest tensile strength at 11.36±1.80 MPa was produced by the 50/50, LP initiated EHMA material with 1% cross-linking agent, which also had the lowest water uptake. There appeared to be a relationship between tensile strength and water uptake. Water uptake was found to be governed by the partitioning agent in the copolymer powder.     

Lattice Dynamics of Li-ZrNCl: An Electron doped Layered Superconductor

We report recent results on structural and lattice dynamical investigations of Li_.16ZrNCl and its non superconducting reference ZrNCl using the methods of elastic and inelastic neutron scattering as well as of inelastic light scattering. Comprehensive information is obtained by accompanying lattice dynamical calculations with a Born von Kàrmàn model. Significant spectral shifts which might be connected to strong electron-phonon coupling in the superconducting system are observed.