A structural approach including the behavior of collagen cross-links to model patient-specific human carotid arteries

The final publication is available at Springer via http://dx.doi.org/10.1007/s10439-014-0995-7The objective of this work is to develop a remodeling model for biological matter coupling two different processes in a 3D framework: reorientation of the preferential direction of a given fibered structure and reorientation of the fibrils or filaments that make up such a structure. This work uses the microsphere-based approach to take into account the micro mechanics involved in biological fibered structures regarding both their passive behavior and the reorientation of their micro constituents. Moreover, the macro behavior of the material as a whole is obtained by means of homogenizing the underlying micro response. We associate the orientation space of the integration directions to the physical space of micro-fibrils. To approximate the directional distribution of the fibrils within each fiber bundle, a Bingham probability orientation density function is introduced into the Helmholtz energy function. With all these assumptions, the problem is studied from an energetic point of view, describing the dissipation inherent to remodeling processes, and the evolution equations for both reorientations (change in preferential direction of the network and change in shape of the fibril distribution) re obtained. The model is included in a finite element code which allows computing different geometries and boundary value problems. This results in a complete methodology for characterizing the reorientation evolution of different fibered biological structures, such as cells. Our results show remodeling of fibered structures in two different scales, presenting a qualitatively good agreement with experimental findings in cell mechanics. Hierarchical structures align in the direction of the maximum principal direction of the considered stimulus and narrow in the perpendicular direction. The dissipation rates follows predictable trends although there are no experimental findings to date for comparison. The incorporation of metabolic processes and an insight into cell-oriented mechano-sensing processes can help to overcome the limitations involved.Peer ReviewedPostprint (author's final draft

Excavaciones arqueológicas en el sitio de Buenavista (Lanzarote): Nuevos datos para el estudio de la colonización protohistórica del archipiélago

The archaeological works carried out at Buenavista (Lanzarote) have shown a big rectangular structure inside an stratigraphic sequence with a number of artifacts and chronology fluctuating between 10th Century BC and 3rd Century AD. The dates are new for Canarian protohistory, as well as the fact that organic remains associated to Phoeno-Punic materials (pottery, copper and bronze objects and a glass seed) have been found. The goal of this paper is to present a synthesis of the archaeological works at Buenavista showing stratigraphy, chronology and material record.Los trabajos arqueológicos desarrollados en el sitio de Buenavista (Lanzarote) han evidenciado la presencia de una amplia estructura constructiva de planta rectangular, inmersa en una secuencia estratigráfica en la que se ha registrado un numeroso contexto artefactual repartido en varias áreas funcionales y en un marco cronológico que discurre entre los siglos X a.C. y III d.C. Si ya de por sí esas fechas suponen una novedad con respecto a las cronologías que conocíamos para la Protohistoria canaria, no es menos novedoso el hecho de que se hayan obtenido de restos orgánicos asociados a diferentes registros materiales de procedencia cultural fenicio-púnica, en concreto cerámicas modeladas a torno, objetos fabricados en cobre y bronce o una cuenta vítrea. Este trabajo pretende dar a conocer una visión sintética de las intervenciones arqueológicas realizadas en Buenavista, efectuando una valoración general de los resultados más destacados a nivel estratigráfico, cronológico y del registro material

MODELING ELECTRICITY PRICES: INTERNATIONAL EVIDENCE

This paper analyses the evolution of electricity prices in deregulated markets. We present a general model that simultaneously takes into account the possibility of several factors: seasonality, mean reversion, GARCH behaviour and time-dependent jumps. The model is applied to equilibrium spot prices of electricity markets from Argentina, Australia (Victoria), New Zealand (Hayward), NordPool (Scandinavia), Spain and U.S. (PJM) using daily data. Six different nested models were estimated to compare the relative importance of each factor and their interactions. We obtained that electricity prices are mean-reverting with strong volatility (GARCH) and jumps of time-dependent intensity even after adjusting for seasonality. We also provide a detailed unit root analysis of electricity prices against mean reversion, in the presence of jumps and GARCH errors, and propose a new powerful procedure based on bootstrap techniques.

Computational modeling of hypertensive growth in the human carotid artery

The final publication is available at Springer via http://dx.doi.org/10.1007/s00466-013-0959-zArterial hypertension is a chronic medical condition associated with an elevated blood pressure. Chronic arterial hypertension initiates a series of events, which are known to collectively initiate arterial wall thickening. However, the correlation between macrostructural mechanical loading, microstructural cellular changes, and macrostructural adaptation remains unclear. Here, we present a microstructurally motivated computational model for chronic arterial hypertension through smooth muscle cell growth. To model growth, we adopt a classical concept based on the multiplicative decomposition of the deformation gradient into an elastic part and a growth part. Motivated by clinical observations, we assume that the driving force for growth is the stretch sensed by the smooth muscle cells. We embed our model into a finite element framework, where growth is stored locally as an internal variable. First, to demonstrate the features of our model, we investigate the effects of hypertensive growth in a real human carotid artery. Our results agree nicely with experimental data reported in the literature both qualitatively and quantitatively.Peer ReviewedPostprint (author's final draft

Mathematical modeling of collagen turnover in biological tissue

The final publication is available at Springer via http://dx.doi.org/10.1007/s00285-012-0613-yWe present a theoretical and computational model for collagen turnover in soft biological tissues. Driven by alterations in the mechanical environment, collagen fiber bundles may undergo important chronic changes, characterized primarily by alterations in collagen synthesis and degradation rates. In particular, hypertension triggers an increase in tropocollagen synthesis and a decrease in collagen degradation, which lead to the well-documented overall increase in collagen content. These changes are the result of a cascade of events, initiated mainly by the endothelial and smooth muscle cells. Here, we represent these events collectively in terms of two internal variables, the concentration of growth factor TGF-$\beta$ and tissue inhibitors of metalloproteinases TIMP. The upregulation of TGF-$\beta$ increases the collagen density. The upregulation of TIMP also increases the collagen density through decreasing matrix metalloproteinase MMP. We establish a mathematical theory for mechanically-induced collagen turnover and introduce a computational algorithm for its robust and efficient solution. We demonstrate that our model can accurately predict the experimentally observed collagen increase in response to hypertension reported in literature. Ultimately, the model can serve as a valuable tool to predict the chronic adaptation of collagen content to restore the homeostatic equilibrium state in vessels with arbitrary micro-structure and geometry.Peer ReviewedPostprint (author's final draft

Energy efficiency evaluation of a hybrid energy system: Analiz

This paper deals with the analysis of a specific hybrid energy system (HYS) from an overall energy efficiency aspect and also from an economical aspect. The hybrid energy system was purposely assembled from standard, market available energy technologies that are presently used in majority of building facilities, particularly in residential ones. Based on the previous aspect, the HYS was assembled using a standard split heat pump system (air-conditioning unit) with an integrated accumulation boiler for hot water preparation and using a small photovoltaic (PV) system. An overall energy efficiency analysis showed the HYS system to be highly energy efficient on average as overall energy efficiency ranged from 50% up to over 300% (the heat pump system acts as a kind of efficiency booster). An economic analysis showed that the HYS produced energy cost ranged from 0.035€/kWh up to 0.15 €/kWh, in investment conditions. A detailed energy efficiency and feasibility analysis showed that the herein analyzed HYS can be a viable option for small or medium building applications in mild climates

Scaling-up a resource efficient economy: embedding biorefineries in the industry of the future through agile supply chains

During recent years the design of industry supply chain has experimented significant changes through the establishment of collaborative networks. Nowadays, there is a strong necessity for reducing costs and offering the maximum agility in terms of product availability and rapid respond in front of unanticipated changes, facts that are being achieved by implementing decentralized supply chain structures. Making reference to the article field of study, the biorefineries supply chain, some of their inefficiencies may be solved by adapting them to decentralised superstructures. Nevertheless, regarding the chemical and energy industry, these new guidelines in supply chain design are not yet full established due to the complications that may appear in terms of storage and transportation of intermediate products. There is a large amount of literature working on the objective to manage uncertainty in the biomass supply chain (Yue et al., 2014). As a concise summary, the main problems to face along the biomass supply chain consist on feedstock uncertainty, quality variability, demand variability, time dependence, storage, transportation and flexibility (Miret et al., 2016). The aim of this document is to evaluate different feasible decentralized supply chain scenarios for the particular case of biorefineries, focusing on bio-ethanol production. The new scenarios are defined to struggle against the mentioned problems and provide more agility to the biomass supply chain. The article will continue by introducing the current biorefinery concept, in order to define a starting point for the development of new scenarios. Next, a research on the state of the art of decentralised biorefineries will be presented. Right after, the KPIs selected to evaluate the advantages and disadvantages of the different scenarios will be introduced. Finally, a discussion of each scenario will be conducted for taking out the conclusions

Fifteen years follow-up of photorefractive keratectomy up to 10 D of myopia: outcomes and analysis of the refractive regression

PURPOSE: To evaluate outcomes of photorefractive keratectomy up to -10.00 D of myopia and -4.50 of astigmatism and to develop a predictive model for the refractive changes in the long term. SETTING: Vissum Corporation and Miguel Hernandez University (Alicante, Spain). DESIGN: Retrospective-prospective observational series of cases. METHODS: This study included 33 eyes of 33 patients aged 46.79±7.04 years (range 40-57) operated with the VISX 20/20 excimer laser with optical zones of 6 mm. No mitomycin C was used in any of these cases. The minimum follow-up was 15 years. The main outcome measures were: uncorrected and corrected distance visual acuity, manifest refraction and corneal topography. Linear regression models were developed from the observed refractive changes over time. RESULTS: Safety and efficacy indexes at 15 years were 1.18 and 0.83, respectively. No statistically significant differences were detected for any keratometric variable during the follow-up (p≥0.103). 15 years after the surgery 54.55% of the eyes were within ±1.00 D of spherical equivalent and 84.85% within ±2.00 D. The uncorrected distance visual acuity at 15 years was 20/25 or better in 60.6% of the eyes and 20/40 or better in 72.73% of the eyes. The correlation between the attempted and the achieved refractions was r=0.948 (p<0.001) at 1 year, and r=0.821 (p<0.001) at 15 years. No corneal ectasia was detected in any case during the follow-up. CONCLUSIONS: Photorefractive keratectomy is a safe refractive procedure in the long term within the range of myopia currently considered suitable for its use, although its efficacy decreases with time, especially, in high myopia. The model developed predicts a myopic regression of 2.00 D at 15 years for an ablation depth of 130 µm

Modeling electricity prices: international evidence

This paper analyses the evolution of electricity prices in deregulated markets. We present a general model that simultaneously takes into account the possibility of several factors: seasonality, mean reversion, GARCH behaviour and time-dependent jumps. The model is applied to equilibrium spot prices of electricity markets from Argentina, Australia (Victoria), New Zealand (Hayward), NordPool (Scandinavia), Spain and U.S. (PJM) using daily data. Six different nested models were estimated to compare the relative importance of each factor and their interactions. We obtained that electricity prices are mean-reverting with strong volatility (GARCH) and jumps of time-dependent intensity even after adjusting for seasonality. We also provide a detailed unit root analysis of electricity prices against mean reversion, in the presence of jumps and GARCH errors, and propose a new powerful procedure based on bootstrap techniques