Biomedical subjects

H Ralph Rawls

Publications and source records attributed to H Ralph Rawls.

4 recordsLinked to original sources

PubMed2005

Synthesis of low-shrinkage polymerizable methacrylate liquid-crystal monomers.

As a part of the continuous pursuit to develop an ideal resinous dental restorative material for use in large posterior cavity restorations, this article reports the easy, high-yield synthesis and the incredibly low polymerization shrinkage property of a new bifunctional liquid crystal (LC) monomer. This new polymerizable nematic liquid crystal is the next higher homolog of the acrylate monomer reported in a previous work.1 It remains liquid crystalline between room and mouth temperatures and can be polymerized to isotropic polymer with the use of the same visible light inhibitor system as used with conventional monomers. The structure of this new monomer has been confirmed to be 2-(t-butyl), 1,4-bis-[4-(6-methacryloxy-hexan-1-oxy)-benzoyloxy]-benzene. Unlike the synthesis of its acrylate homolog, when the same procedure is adopted for the synthesis of this compound, the recovery of the product is not split by a sizable amount of the by-product. Therefore, the reaction is cleaner, with high yield and a less labor-intensive purification procedure. Thus, the synthetic methodology has the potential for easy commercial scale-up. The monomer (V) polymerizes at room temperature with a shrinkage of about 2 vol %, as compared to > 8 vol % for conventional control (GTE), at similar degrees of conversion.

Methacrylates↗

PubMed2004

Synthesis of low-shrinkage polymerizable liquid-crystal monomers.

Polymerization shrinkage remains a major barrier to the universal use of resin restorative in large posterior cavity preparations. A new bifunctional liquid crystal (LC) monomer, 2-(t-butyl), 1,4-bis-[4-(6-acryloxy-hexane-1-oxy)-benzoyloxy] benzene, with exceptionally low polymerization shrinkage, has recently been discovered. The purpose of this communication is to report a new, easy, high-yield synthetic route to synthesize this compound in comfortable larger batches. Synthetic and isolation details, chemical characteristics, and the polymerizable properties of a new structurally related by-product monomer, namely, 2-(t-butyl),1-[4-(6-acryloxy-hexane-1-oxy)-benzoyloxy], 4-[4-[6-(3-acryl oxy-propionoxy)-hexane-1-oxy]-benzoyloxy]-benzene, is also reported. The structural confirmation of this by-product indicates that it resulted from the Michael-type addition of acrylate ion on one of the terminal acryloxy groups of 2-(t-butyl), 1,4-bis-[4-(6-acryloxy-hexane-1-oxy)-benzoyloxy] benzene. The by-product itself, as well as the natural blend of the aforesaid both products as formed in the reaction mixture, also polymerized at room temperature with lesser volume shrinkage as compared to the conventional control (GTE) at similar degrees of conversion.

Biopolymers↗

PubMed2002

Collagen and bone viscoelasticity: a dynamic mechanical analysis.

The purpose of this study was to explore the effects of changes in Type I collagen on the viscoelasticity of bone. Bone coupons were heated at either 100 or 200 degrees C to induce the thermal denaturation of Type I collagen. Half of these specimens were rehydrated after heat treatment; the other half were tested in a dry condition. The degree of denatured collagen (DC%) was analyzed by a selective digestion technique with the use of alpha-chymotrypsin. Isothermal (37 degrees C) and variable temperature tests (scans from 35 to 200 degrees C) were performed with the use of a dynamic mechanical analyzer to evaluate changes in bone viscoelastic properties as a function of collagen damage, specifically, changes in the loss factor (tan delta) and storage modulus (E') were assessed. Significant collagen denaturation occurred only when bone was heated at 200 degrees C irrespective of the hydration condition. Also, DC% did not show a significant effect on tan delta. However, higher values of tan delta were observed in wet samples compared to dry specimens. The temperature-scan tests revealed that the hydration condition, but not DC%, significantly affected the behavior of tan delta. However, E' was not strongly influenced either by DC% or by water content. These results suggest that at a constant frequency the denaturation of collagen triple-helical molecules may have few effects on the viscoelasticity of bone, but moisture may play a prominent role in determining this property.

Biomechanical Phenomena↗

PubMed2002

Growth of calcium phosphate on poling treated ferroelectric BaTiO3 ceramics.

Barium titanate (BaTiO3; BTO) is ferroelectric and piezoelectric after poling treatment. In this study, the bioactivity of BTO was investigated after a poling treatment by examining the formation of crystal growth on specimen surfaces in vitro. Negatively charged BTO surfaces showed calcium phosphate (Ca-P) crystal growth, while deposition of sodium chloride was observed on the positively charged BTO surfaces. After 30 days immersion in Eagle's MEM, the thickness of Ca-P crystal on negatively charged BTO surfaces increased to 0.8-0.9 microm. These data indicate that incorporating selectively polarized BTO on implant surfaces is a promising means for improving the bioactivity of implant materials.

Barium Compounds↗