Effect of molecular weight and concentration of poly(acrylic acid) on the formation of a polymeric calcium phosphate cement

Previous investigations have noted that the tetracalcium phosphate (TTCP)/dicalcium phosphate anhydrous (DCPA) apatite forming calcium phosphate cement (CPC) possesses many favorable properties from a biomaterials standpoint. Despite these positive properties various shortcomings have limited clinical usage of these materials and fostered investigations into the effect of numerous additives. The present study concerns the effect of poly(acrylic acid) (PAA) addition and the influence of factors such as molecular weight and concentration of the additive on the properties of the set cement. One-way ANOVA was conducted using all results obtained, to firstly derive the influence of concentration within each molecular weight group, and secondly to derive the influence of molecular weight within each concentration group. All investigated mechanical properties were influenced by both molecular weight and concentration of the additive. Higher molecular weights tended to result in cements with shorter setting times and higher compressive, diametral and biaxial flexural strengths than their lower molecular weight counterparts. The effect of concentration on the properties of the set cement however was somewhat more complex, a negative correlation was observed between the initial setting time and PAA concentration. In regards to the final setting time, any correlation with concentration was difficult to derive as a consequence of the highly brittle nature of cements made with low concentrations. In regard to mechanical properties, intermediate concentrations tended to give higher strengths than both their higher and lower counterparts, however the exact pattern was largely specific to the mechanical strength test employed. We conclude that molecular weight and concentration of PAA influence the setting behavior and final mechanical properties of the TTCP/DCPA cement, and that selection of an appropriate PAA solution can lead to the production of cements with properties superior to those formed in the absence of the polymer.     

Variations in the compressive strength of dental cements stored in ionic or acidic solutions

The compressive strengths of various dental cements (a zinc polycarboxylate, a zinc phosphate, a glass-ionomer and two resin-modified glass ionomers, RMGICs) have been determined following storage in pure water, 0.9% sodium chloride solution or 20 mmol dm^–3 lactic acid solution for periods of time ranging from 24 h to 3 months. The glass-ionomer cement showed no differences between different storage solutions or at different storage times, whereas the zinc polycarboxylate, zinc phosphate and the resin-modified glass ionomer cements showed significant differences following storage in the solutions for 24 h compared with pure water. The zinc polycarboxylate cement was significantly weaker at 24 h in 0.9% NaCl and lactic acid than in pure water, whereas most of the other cements were significantly stronger in both 0.9% NaCl and lactic acid. One of the RMGICs (Vitremer luting, ex. 3M), however, was significantly stronger only in the NaCl solution, not in the lactic acid. In general, by 1 week, the strengths all reverted to being essentially the same as for specimens stored in pure water for most subsequent storage times, and did not change significantly on storage for up to 3 months. This effect of storage medium on the early strength has not been reported previously and since the media were chosen to model certain characteristics of natural saliva, the changes observed seem likely to occur in vivo. It is concluded that pure water is not the best medium for storing these cements if they are to behave as they do under clinical conditions. © 2001 Kluwer Academic Publishers     

Harmonization of legislation and regulations to achieve food safety: US and Canada perspective

Trade in food and food ingredients among the nations of the world is rapidly expanding and, with this expansion, new supply chain partners, from globally disparate geographic regions, are being enrolled. Food and food ingredients are progressively sourced more from lesser developed nations. Food safety incidents in the USA and Canada show a high unfavorable correlation between illness outbreaks and imported foods. In the USA, for example, foodborne disease outbreaks caused by imported food appeared to rise in 2009 and 2010, and nearly half of the outbreaks, associated with imported food, implicated foods imported from areas which previously had not been associated with outbreaks. Projecting supply chains into new geographical regions raises serious questions about the capacity of the new supply chain partners to provide the requisite regulatory framework and sufficiently robust public health measures for ensuring the safety of the foods and foodstuffs offered for international trade. The laws, regulation and legislation among the many nations participating in the global food trade are, at best, inconsistent. These inconsistencies frequently give rise to trade disputes and cause large quantities of food to be at risk of destruction on the often dubious pretext that they are not safe. Food safety is often viewed through a political or normative lens. Often as not, this lens has been wrought absent scientific precision. Harmonization of food safety legislation around sound scientific principles, as advocated by the US Food Safety Modernization Act (FSMA), would ultimately promote trade and likely provide for incremental improvement in public health. Among the priority roles of most national governments are the advancement of commerce and trade, preservation of public health and ensuring domestic tranquility. Achieving these priorities is fundamental to creating and preserving the wealth of nations. Countries such as the Netherlands, Canada, Germany, Japan and the USA, for example, have very stable governments, are leaders in trade and commerce and enjoy high standards of public health. It is not by accident or coincidence that these nations are also among the world's wealthiest. Attainment of national priorities, especially those related to promoting trade in foodstuffs and also in preserving public health (food safety), would benefit greatly from international efforts in harmonizing food safety regulations and legislation. © 2013 Society of Chemical Industry     

Relationship of a dystrophin-associated glycoprotein to junctional acetylcholine receptor clusters in rat skeletal muscle

The relationship of a member of the transmembrane dystrophin-associated glycoprotein (DAG) complex to acetylcholine receptors (AChRs) was investigated using immunofluorescence techniques at rat neuromuscular junctions (NMJs) viewed en face. These results were compared with those from a similar previous study of dystrophin and an autosomal homologue (utrophin or dystrophin-related protein, DRP) (Bewick et al. Neuro Report 1992; 3: 857-860). The region of highest 43 K DAG (43DAG) labelling projected beyond the AChRs by approximately 0.3 microns, as does that for dystrophin. By contrast DRP labelling precisely co-localizes with the AChRs. These results suggest that at the NMJ, the region of high 43DAG concentration encompasses the area of highest intensity labelling for both DRP and dystrophin.     

Resistance to Fracture of a Partially Stabilized Zirconia/β-Alumina Composite

Partially stabilized zirconia (PSZ) /β-alumina composites exhibit extensive non-elastic deformation during fracture. Repeated loading/stable fracture/unloading experiments were performed on chevron-notched four-point-bend specimens of the composite and pure PSZ. The energy consumption during the propagation of long cracks in the composite (∼500 J/m²) is 1 order of magnitude larger than for PSZ (∼50 J/m²). Breaking strengths of 127 MPa were obtained with a Weibull modulus of 43.     

Caspases cleave focal adhesion kinase during apoptosis to generate a FRNK-like polypeptide

Focal adhesion kinase (Fak) is a non-receptor protein-tyrosine kinase that stimulates cell spreading and motility by promoting the formation of contact sites between the cell and the extracellular matrix (focal adhesions). It suppresses apoptosis by transducing survival signals that emanate from focal adhesions via the clustering of transmembrane integrins by components of the extracellular matrix. We demonstrate that Fak is cleaved by caspases at two distinct sites during apoptosis. The sites were mapped to DQTD772, which was preferentially cleaved by caspase-3, and VSWD704, which was preferentially cleaved by caspase-6 and cytotoxic T lymphocyte-derived granzyme B. The cleavage of Fak during apoptosis separates the tyrosine kinase domain from the focal adhesion targeting (FAT) domain. The carboxyl-terminal fragments that are generated suppress phosphorylation of endogenous Fak and thus resemble a natural variant of Fak, FRNK, that inhibits Fak activity by preventing the localization of Fak to focal adhesions. The cleavage of Fak by caspases may thus play an important role in the execution of the suicide program by disabling the anti-apoptotic function of Fak. Interestingly, rodent Fak lacks an optimal caspase-3 consensus cleavage site although it is cleaved in murine cells undergoing apoptosis at an upstream site. This appears to be the first example of a caspase substrate where the cleavage sites are not conserved between species.     

Glass-ionomer cements in restorative dentistry

This article reviews the current status and future prospects for glass-ionomer materials. These materials are of two chemical types: the older, self-hardening cements, which set by an acid-base neutralization reaction to give relatively brittle materials; and the newer, resin-modified cements, which set partly by polymerization and partly by neutralization. Compared with the self-hardening cements, the latter materials have improved esthetics, improved resistance to moisture, and greater toughness. Both types of glass-ionomer cement bond well to enamel and dentin and release a clinically useful amount of fluoride. They have been used in a variety of applications: as liners or bases, for luting of stainless steel crowns, for Class V restorations in permanent teeth, and for Class II and Class III restorations in primary teeth. The resin-modified glass-ionomers are particularly promising for these latter uses, although it is too early to be sure whether their long-term durability is sufficient. Self-hardening glass-ionomer materials are likely to retain specific niches of clinical application, including in their metal-reinforced and cermet-containing forms.