Factors influencing the compressive strength of an injectable calcium sulfate–hydroxyapatite cement

A biphasic injectable bone substitute, suitable for filling bone defects, that sets in the body, based on calcium sulfate and hydroxyapatite (HA), is presented. For applications in bone defects the compressive strength is important to assure support of the defect site during loading when the patient is weight bearing. To control the strength, the influence of four different factors; the liquid-to-powder (L/P) ratio, the HA particle morphology, the HA content and the amount of accelerator, were investigated. -Calcium sulfate hemihydrate (CSH) and four different HA powders (three sintered and one spray-dried) were used. All differed in size and morphology. CSH and each HA powder were mixed together with distilled water to form the bone substitute. An accelerator, in form of calcium sulfate dihydrate, was added to the powder phase to obtain an adequate setting time. Cylindrical specimens were compression tested. A lower L/P-ratio gave stronger cement, but was more difficult to inject. The shape and the morphology of the HA particles influenced the strength, and reducing the amount of HA increased the strength. The amount of accelerator (calcium sulfate dihydrate) had no influence.     

Impact behavior of hydroxyapatite reinforced polyethylene composites

Hydroxyapatite particulate reinforced high density polyethylene composite (HA-HDPE) has been developed as a bone replacement material. The impact behavior of the composites at 37 °C has been investigated using an instrumented falling weight impact testing machine. The fracture surfaces were examined using SEM and the fracture mechanisms are discussed. It was found that the fracture toughness of HA-HDPE composites increased with HDPE molecular weight, but decreased with increasing HA volume fraction. Examination of fracture surfaces revealed weak filler/matrix interfaces which can debond easily to enable crack initiation and propagation. Increasing HA volume fraction increases the interface area, and more cracks can form and develop, thus decreasing the impact resistance of the composites. Another important factor for the impact behavior of the composites is the matrix. At higher molecular weight, HDPE is able to sustain more plastic deformation and dissipates more impact energy, hence improving the impact property.     

Unemployment rates, single parent density, and indices of child poverty: their relationship to different categories of child abuse and neglect

An arrangement of the seven transmembrane alpha-helices of the G protein-coupled Secretin receptor family is proposed. The helices of 27 homologous receptor sequences were plotted as helical wheels. The solvent inaccessible portion of each helix was used to assign relative orientations. They were arranged according to two criteria: 1) conserved, hydrophilic residues and aligned positions with restricted volume changes face the other helices and 2) aligned positions with low identity and large volume change face the lipid. The positive inside rule confirms the assumption that loops connecting transmembrane helices I-II, III-IV, V-VI and the C-terminal part of the receptors are intracellular. Our model approach was tested using the Bacteriorhodopsin family. The use of volume changes at each position in the transmembrane helix was crucial for the good correlation of the orientation of the helices using the model approach and the structure of bacteriorhodopsin solved by electron microscopy [Grigorieff N, Ceska TA, Downing KH, Baldwin JM, and Henderson R (1996) J Mol Biol 259 393-421]. The tests of our modelling approach showed that six helices were within a 15 degrees derivation in the orientation and five helices were within a horizontal derivation of two residues. The largest orientational derivations of a helix were 40 degrees and the largest horizontal displacement was four residues. A long stretch of side chains predicted to possess low resistance to movement in helix V of the Secretin receptor family suggests an involvement in receptor activation. Comparison of the Secretin receptor family and the larger G protein-coupled Rhodopsin family showed many similarities, despite the lack of obvious sequence identity.     

Pattern synthesis for small linear arrays with known element pattern and scattering matrix

A pattern synthesis for linear arrays which takes account of measured element pattern and coupling matrix of the array mounted in its environment and additional constraints for the power feeding network has been implemented. Results are shown for a realized six element dipole array.     

A mixing model for diagnosing reacting tracers and tracing reactants in chemical and biochemical processes

It has previously been shown (Tanner et al., 1985) that biochemical and chemical reaction processes of the Type A → B → C can lead not only to kinetic hysteresis between the rate of formation of C and the concentration of species B in batch processes, but also hysteresis between those variables in a closed, imperfectly mixed (two zone) batch reactor. Furthermore, crossplotting the intermediate reactant, B, in one region of the poorly mixed reactor against B from the other major region leads to a clockwise hysteresis curve which is defined by both its area and the coordinate phase angle. This paper shows that the earlier analysis (Tanner et al., 1985) can be extended to the more general system A → B, where A in one region of the vessel is crossplotted against A measured in the other region. With an initial concentration of A* in one zone equal to its highest concentration, the inscribed area double-valued crossplot of A, uniquely defines the system in terms of the inter-vessel flowrate to reaction rate constant ratio, D/k.     

Computer study of tweed as a precursor to a martensitic transformation of a Bcc lattice

Using molecular dynamics computer simulations we have studied a bcc crystal with an interatomic potential having a second derivative which passes through zero near second neighbor distances. This causes an inherent tetragonal instability which manifests itself as random dynamic tetragonal distortions at high temperature (“dynamic tweed”), quasi-static and somewhat spatially correlated tetragonal distortions at intermediate temperatures (“static tweed”), and an apparent martensitic transformation to a close-packed structure at low temperatures. The effect of simple defects (vacancies and interstitials) on the development and spatial localization of the pretransformation structure (tweed) and the subsequent martensitic transformation will also be discussed. This paper is based on a presentation made in the symposium “Pretransformation Behavior Related to Displacive Transformation in Alloys” presented at the 1986 annual AIME meeting in New Orleans, March 2–6, 1986, under the auspices of the ASM-MSD Structures Committee.     

A trial of the synthetic malaria vaccine SPf66 in Tanzania: rationale and design

By using acetyl strophanthidin (AS) 0.2 mumol/L, the delayed after-depolarization (DAD) was induced in sheep cardiac Purkinje fibers and recorded with intra-cellular microelectrode. When beta-adrenoceptor was blocked by propranolol 1.0 mumol/L, phenylephrine 1.0 mumol/L increased the amplitude of DAD from 8.1 +/- 2.2 mV to 9.5 +/- 2.8 mV, prolonged the duration of DAD from 240 +/- 47 ms to 273 +/- 47 ms (n = 13, P < 0.01) and increased the up rising velocity of DAD from 0.039 +/- 0.023 V/s to 0.051 +/- 0.026 V/s (n = 13, P < 0.05). The DAD occurred earlier for 30 +/- 47 ms to preceding action potential (n = 13, P < 0.05). When triggered action potentials were induced by norepinephrine 1.0 mumol/L on the basis of DAD, propranolol 1.0 mumol/L could suppress the triggered beats while phentolamine 1.8 mumol/L showed little effect. The above results indicate that excitation of alpha-receptor had only slight augmentation effect on DAD. However, for the triggered activity induced by DAD, the inhibitory effect of beta-blockers are stronger than that of alpha-blockers.