PubMed · 8091927
The source did not provide an abstract. Follow the original record for more information.
Explore connections, maps & timelines
E Berardesca. 1994. Racial differences in skin function.. https://pubmed.ncbi.nlm.nih.gov/8091927/
Cite the original work for its findings. Save a collection to share your selection of sources.
KEEP EXPLORING
Highly plasticized poly(vinyl chloride) (PVC) membranes (200 per hundred resin [phr]) form the basis of one class of ion-selective electrodes (ISEs). In previous work on the mechanical properties of membranes, the optimal ratio of plasticizer level employed to minimal level required for complete plasticization (phr(exp)/phrmin) was found to be 2.0. The current study was designed to determine whether this ratio is necessary or sufficient for proper ISE function. Dynamic mechanical analysis (DMA) was used to examine the effects of five plasticizers on the dynamic mechanical properties of membranes at three frequencies (110, 11.0, and 1.1 Hz) as a function of temperature (-100 degrees C to +100 degrees C); dioctyl sebacate (DOS), epoxidized propylene glycol dioleate (PGDO), ortho-nitrophenyl octyl ether (o-NPOE), epoxidized soybean oil (ESO), and epoxidized linseed oil (ELO). The glass transition temperature of PVC, which was found to be +77.1 degrees C at 11.0 Hz, was depressed by the addition of 200 phr of each plasticizer from a high of -1.4 degrees C (PGDO at 110 Hz) to a low of -70.2 degrees C (DOS at 1.1 Hz). DMA and electromotive-force (EMF) measurements on membranes plasticized with o-NPOE through a range of phr(exp)/phrmin from 0.5 to 9.3 indicated that a "transition window" occurs between phr(exp)/phrmin of 2.0 and 3.3 in which the membranes change from minimally plasticized polymer films to predictable ion-selective membranes, coinciding with the optimal mechanical properties observed previously. Based on dynamic mechanical properties and EMF response data, the optimal phr(exp)/phrmin++ ratios for membranes as a function of plasticizer were proposed: 0.8 for ESO and ELO, 1.3 for PGDO, 1.7 for DOS, and 3.0 for o-NPOE.
Large cyclic movements between the femoral stem and bone during the first weeks after total hip arthroplasty may hamper bone ingrowth and adversely affect the eventual success of the arthroplasty. Little is known, however, about the magnitude of the motions and its relationship to design and surgical factors. A two-dimensional finite element model of a cementless prosthesis inserted into the proximal femur was constructed to study the effects of two mechanical variables--the stiffness of the implant and the coefficient of friction between bone and implant--on the magnitude of the motions. We investigated the influences of these variables on the subsidence of the prosthesis, the magnitudes of the cyclic motions, and the level of the interface stresses. The presence of friction reduced cyclic motions by about 85% compared with a frictionless interface. Once friction was assumed, varying the coefficient of friction had little effect. The effect of friction on the interface stress state and gross subsidence of the prosthesis was not as great as on cyclic motion. Implant stiffness also affected the magnitudes and distributions of the cyclic motions along the interface. A flexible stem generated motions about three to four times larger proximally than those of a stiff stem, which generated larger motions distally. The influence of stem stiffness on interface stresses and prosthetic subsidence was less than on cyclic motion. The location of the peak shear stresses at the interface around a bonded prosthesis corresponded to the location where cyclic interface motion was maximal for an unbonded prosthesis. However, no direct relationship was found between the magnitudes of peak stresses and the amplitudes of cyclic motions.
A differential equation solution method that does not utilize numerical integration is demonstrated on a physiologic model. A previously proposed model of left ventricular ejection incorporating time-varying elastance, internal resistance, aortic inductance, and a three-component windkessel is solved by collocation. Assuming internal resistance to be constant allows simplification to a third order linear differential equation in left ventricular volume. A trigonometric series is used to approximate the solution and the coefficients of the series as well as the duration of ejection and pre-ejection periods are chosen so that the governing differential equation is exactly satisfied at certain times during ejection (the collocation points), as well as the boundary conditions and steady state condition.