A Novel Approach to Describe the Time-Temperature Conversion among Relaxation Curves of Viscoelastic Materials.

Time and temperature, besides pressure in a lesser extent, represent the most significant variables influencing the rheological behavior of viscoelastic materials. These magnitudes are each other related through the well-known Time-Temperature Superposition (TTS) principle, which allows the master curve referred to relaxation (or creep) behavior to be derived as a material characteristic. In this work, a novel conversion law to interrelate relaxation curves at different temperatures is proposed by assuming they to be represented by statistical cumulative distribution functions of the normal or Gumbel family. The first alternative responds to physical considerations while the latter implies the fulfillment of extreme value conditions. Both distributions are used to illustrate the suitability of the model when applied to reliable derivation of the master curve of Polyvinil-Butyral (PVB) from data of experimental programs. The new approach allows not only the TTS shift factors to be estimated by a unique step, but the whole family of viscoelastic master curves to be determined for the material at any temperature. This represents a significant advance in the characterization of viscoelastic materials and, consequently, in the application of the TTS principle to practical design of viscoelastic components.

Single-chamber ICD, single-zone therapy in primary and secondary prevention patients: the simpler the better?

BACKGROUND: It is now well established that implantable cardioverter defibrillator (ICD) implantation reduces mortality in patients at increased risk of sudden cardiac death. However, the best programming parameters remain controversial. Our traditional policy has followed a simple approach in the vast majority of patients. In accordance with ICD programming in the major randomized clinical trials, we programmed a single high-rate, shock-only therapy zone. We aimed to demonstrate in this observational study that simple programming is not associated with higher shock rates or mortality when compared to other published studies. METHODS: Consecutive patients who underwent single-chamber ICD implantation with single-zone, high-rate programming at our institution between 1993 and 2008 were retrospectively studied. Data were collected prospectively in a database regarding details of ICD implantation, demographic data, and indication. RESULTS: Three hundred thirty-two patients were included in our study, 31 % primary prevention and 68 % secondary prevention. Mean ejection fraction (EF) is 33.7 ± 15.3. Over a mean follow-up period of 62.5 ± 38.1 months, 135 patients experienced ICD shock (annualized event rate 7.7 %); 89 patients (26.8 %) appropriate shock in VT-ventricular fibrillation (VF), 68 patients (20.5 %) inappropriate shocks, and 22 patients (6.6 %) both. Twenty-nine patients (8.7 %) were reprogrammed to additional VT-ATP zones. Twenty-two (6.6 %) patients underwent heart transplantation. Sixty-two patients (18.6 %) died during follow-up, 43.6 % out of them due to cardiac cause, mainly progressive heart failure. CONCLUSION: Our results show that simpler settings with single-zone, high-rate programming is associated with ICD shock rates and long-term mortality that does not appear to be worse when compared with contemporary studies which include multizone ICD programming with antitachycardia pacing activated.

Probabilistic assessment of fatigue initiation data on highly crosslinked ultrahigh molecular weight polyethylenes.

Ultrahigh molecular weight polyethylenes (UHMWPE) showing wear resistance, oxidative stability and good mechanical performance go on being a relevant research area in biomaterials for total joint replacements, where fatigue happens to be a recurrent damage mode that needs to be investigated. While crack propagation lifetime has been extensively studied, fatigue initiation data are scarcely offered in the literature, often due to the higher costs implied in the experimental programs. Moreover, their analysis is not always suitable to obtain reliable guidance. Different deterministic and probabilistic methods, generally resting on empirical bases have been previously used to analyze the fatigue initiation data. In this work, the probabilistic Weibull regression model of Castillo et al., based on both physical and statistical conditions, such as weakest link principle and the necessary compatibility between life-time and stress range distributions, is applied for the first time in the assessment of fatigue results of polymers, particularly to highly crosslinked UHMWPEs (HXLPEs). Accordingly, different published experimental data corresponding to HXLPE stabilized by thermal treatments and with α-tocopherol (vitamin E) are re-analyzed. Additional data are incorporated to assess the influence of notched HXLPE on fatigue performance. New conclusions are drawn from this revision.