Preparation of fast responsive macroporous hydrogels from frozen solutions

Bu çalışmada, 2-akrilamido-2-metilpropan sülfonik asit sodyum tuzu’nun (AMPS) sulu ortamlarda N,N-metilenbisakrilamid (BAAm) çapraz bağlayıcısı ile kopolimerizasyonu incelenmiştir. AMPS monomeri ile BAAm çapraz bağlayıcısının sulu çözeltileri başlatıcı amonyum persülfat (APS) ile hızlandırıcı N,N,N’,N’-tetraetilmetilendiamin (TEMED) redoks başlatıcı sistemi ile polimerizasyon sisteminin donma noktasının altında ve üzerinde gerçekleştirilmiştir. Elde edilen hidrojellerin şişmiş durumdaki elastik modülleri, sudaki şişme oranları, aseton ve sudaki büzülme ve şişme hızları ölçülmüştür. Taramalı elektron mikroskobu yardımı ile polimer ağyapılarının morfolojileri incelenmiştir. Hidrojellerin şişme ve elastik özelliklerinin -8ºC gibi bir jel hazırlama sıcaklığında belirgin bir şekilde değiştiği gözlenmiştir. -8ºC’nin altında sentezlenen hidrojelleri oluşturan ağyapıların gözenekli olduğu ve gözenek çaplarının 30-50 mm arasında değiştiği buna karşılık -8ºC ve üzerinde sentezlenen ağyapıların gözenek içermediği görülmüştür. Hazırlanan Poli(AMPS)(PAMPS) hidrojellerinin mikroyapısının, karnıbahara benzeyen ve değişik büyüklüklerde agregatlar içeren, reaksiyona bağlı faz ayrılma mekanizmasıyla oluşan makrogözenekli ağyapılardan belirgin bir şekilde farklı oldukları ortaya çıkarılmıştır. Bununla birlikte, -8ºC’nin altında sentezlenen PAMPS hidrojelleri için, suda ve asetonda tamamen tersinir şişme-büzülme eğrileri elde edilmiştir. -8ºC’nin altında oluşan jeller, denge şişme hacimlerine 30 saniyede ulaşırken, yüksek sıcaklıklarda oluşan jellerin suda denge durumlarına ulaşmaları için 1 saat gerekmiştir. Ayrıca, şişmiş PAMPS jelleri asetona daldırıldığında, -8ºC’nin altında hazırlanan jeller 5 ile 10 dakika arasında büzülerek dengeye ulaşırken, yüksek sıcaklıklarda sentezlenenler ise hacim değişimlerine dayanamayarak parçalanmıştır. Böylelikle, jel hazırlama sıcaklığının -8ºC’nin altına indirilmesi ile hacim değişimlerine karşı dayanıklı süper hızlı şişen PAMPS hidrojelleri elde edilmiştir. Anahtar Kelimeler: Hidrojel, şişme, donma, makrogözenek. Responsive hydrogels are smart materials, capable of changing volume in response to specific external stimuli, such as the temperature, solvent quality, pH, electric field, etc. These properties of hydrogels received considerable interest in last three decades and, a large number of hydrogel based devices have been proposed, including artificial organs, actuators, and on-off switches. Important parameters of hydrogels, which to a large extent determine their performance, are an improved mechanical property as well as a fast swelling-deswelling rate to the external stimuli.  However, in some applications, such as artificial organs, the conventional hydrogel with slow response rate cannot satisfy this requirement. Therefore, the slow response rate of the hydrogels limits their technological applications. Increasing the response rate of hydrogels has been one of the challenging problems in the last 25 years. In this regard, several strategies have been proposed. A widely used approach to obtain fast responsive hydrogels is to create voids (pores) inside the hydrogel matrix, so that the response rate becomes a function of the microstructure rather than the size or the shape of the gel samples. A new and contradictory method to produce macroporous hydrogels and thus, increasing their response rate is conducting the polymerization reactions below the freezing point of the reaction system. This low temperature gelation so-called cryogelation was used to create an interconnected pore structure within the polymer networks. In this cryogelation process, the reaction system is partially frozen after the onset of polymerization reaction. The ice crystals formed after partial freezing, act as a template, while the dissolved monomer, cross-linker and initiator are concentrated in a small fraction of an unfrozen liquid phase. Although the frozen monomer system appears homogenous, it is composed of two phases: the solid phase and the unfrozen liquid phase where the polymerization takes place. After polymerization and after melting of ice, many pores are formed in the spaces that were originally occupied by solvent crystal. The morphology of the networks consists of polyhedral pores and the network chains building the pore walls. The gels formed under these conditions were named as cryogels. In this study, by applying the cryogelation technique to the present gelling system, cryogels exhibiting superfast swelling-deswelling kinetics were obtained from 2-acrylamido-2-methylpropane sulfonic acid (AMPS) monomer and N,N-methylenebis (acrylamide) (BAAm) crosslinker  in aqueous solution at 5 w/v % initial monomer concentration. The crosslinker BAAm content in the monomer mixture was set to 17 mol %. The gel preparation temperature, Tprep, was varied between -22ºC and 25ºC. The swelling, swelling/deswelling kinetics, the morphologies as well as the elastic properties of the gels formed at various temperatures were studied. Tprep was found to be a significant parameter affecting the properties of the hydrogels. Depending on Tprep, two different regimes were observed from the experiments. At Tprep=-8ºC or above, Poly(AMPS) (PAMPS)  gels exhibit relatively high swelling ratios Veq of the order of 101, and low moduli of elasticity G in the range of 102-103 Pa. However, decreasing Tprep below -8ºC results in a tenfold decrease in the swelling ratio and about tenfold increase in the elastic modulus of gels. Thus, the swelling and elastic properties of PAMPS drastically change as Tprep is decreased below -8ºC. SEM images of PAMPS networks formed at various Tprep, indicated that all of the polymer samples formed below -8ºC have a porous structure with pore sizes of 30-50 µm while those formed at or above -8ºC exhibit a continuous morphology. Completely reversible swelling-deswelling cycles were obtained using PAMPS cryogels prepared below -8ºC. Gels formed below -8ºC attain their equilibrium swollen volumes in less than 30 sec, while those formed at higher temperatures require about 1 h to reach their equilibrium state in water. Moreover, if swollen PAMPS gels are immersed in acetone, those prepared below -8ºC attain their equilibrium collapsed state in 5 to 10 min, while the hydrogels formed at higher temperatures are too weak to withstand the volume changes. Thus, decreasing Tprep below -8oC results in the formation of superperfast-responsive PAMPS hydrogels, which are also stable against volume changes. Keywords: Hydrogel, swelling, freezing, macroporous

Volatility Spillovers in Emerging Markets During the Global Financial Crisis: Diagonal BEKK Approach

The fundamental aim of the paper is to analyze the presence and magnitude of the volatility transmissions in emerging markets, namely India, Hungary, Poland, Turkey and Brazil prior to, and during the latest financial turmoil. Using weekly returns of stock market indices from 2005 to 2011, the study applies Multivariate BEKK Methodology. The empirical results indicate that there exist significant volatility spillover effects for all five countries, though the spillovers are not homogeneous across the pairs. Results exhibit very large GARCH and relatively low ARCH effects. The study provides evidence of high level of financial integration in emerging markets. From an investor perspective, one important implication is that adding stocks from different emerging markets to a porfolio does not lead to risk reduction

Trophic Characteristics of the Sapanca Lake (Turkey)

Systematic measurements of optical properties, concentrations of major and minor chemicals and primary production throughout 1989-1992 enabled us to identify the oligotrophic state of Sapanca lake, Marmara region, Turkey. Lake waters which have low concentrations of major anions and cations (total = 49 μM) overturn every February-March, ventilating the bottom waters and enriching the surface waters with nutrients. Surface waters cool down to 6.5 °C by late winter and then warm steadily to 26 °C by late summer, while temperatures in deep waters range between 6.5 and 10.0 °C throughout the year. When the seasonal thermocline develops, the dissolved oxygen profiles exhibit a subsurface maximum in the thermocline, while in the hypolimnion water, the content varies seasonally from 11.5-12.0 ppm (350-375 μM) in March to 0.5-1.7 ppm (16-56 μM) in late autumn. Surface nitrate concentrations vary markedly with season, from < 0.15 μM in summer to 5.7 μM in early March, whilst the bottom water concentrations range from 13.5-14.0 μM in late autumn to 5.7 μM after the winter overturn. Phosphate concentrations are always less than 0.1 μM throughout the entire water column. Subsurface chlorophyll-a maximum descends in late summer to 20-25 m depths, where the light intensity is less than 1% of the surface value. Below 10-15 m depths, corresponding to the upper thermocline, primary productivity is very low. The range was from 35 to 93 mg C/ m2 day-1 during 1989-1991, consistent with the values in other oligotrophic lakes

Trophic Characteristics of the Sapanca Lake (Turkey)

Systematic measurements of optical properties, concentrations of major and minor chemicals and primary production throughout 1989-1992 enabled us to identify the oligotrophic state of Sapanca lake, Marmara region, Turkey. Lake waters which have low concentrations of major anions and cations (total = 49 μM) overturn every February-March, ventilating the bottom waters and enriching the surface waters with nutrients. Surface waters cool down to 6.5 °C by late winter and then warm steadily to 26 °C by late summer, while temperatures in deep waters range between 6.5 and 10.0 °C throughout the year. When the seasonal thermocline develops, the dissolved oxygen profiles exhibit a subsurface maximum in the thermocline, while in the hypolimnion water, the content varies seasonally from 11.5-12.0 ppm (350-375 μM) in March to 0.5-1.7 ppm (16-56 μM) in late autumn. Surface nitrate concentrations vary markedly with season, from < 0.15 μM in summer to 5.7 μM in early March, whilst the bottom water concentrations range from 13.5-14.0 μM in late autumn to 5.7 μM after the winter overturn. Phosphate concentrations are always less than 0.1 μM throughout the entire water column. Subsurface chlorophyll-a maximum descends in late summer to 20-25 m depths, where the light intensity is less than 1% of the surface value. Below 10-15 m depths, corresponding to the upper thermocline, primary productivity is very low. The range was from 35 to 93 mg C/ m2 day-1 during 1989-1991, consistent with the values in other oligotrophic lakes

Mutations in LAMB1 Cause Cobblestone Brain Malformation without Muscular or Ocular Abnormalities

Cobblestone brain malformation (COB) is a neuronal migration disorder characterized by protrusions of neurons beyond the first cortical layer at the pial surface of the brain. It is usually seen in association with dystroglycanopathy types of congenital muscular dystrophies (CMDs) and ocular abnormalities termed muscle-eye-brain disease. Here we report homozygous deleterious mutations in LAMB1, encoding laminin subunit beta-1, in two families with autosomal-recessive COB. Affected individuals displayed a constellation of brain malformations including cortical gyral and white-matter signal abnormalities, severe cerebellar dysplasia, brainstem hypoplasia, and occipital encephalocele, but they had less apparent ocular or muscular abnormalities than are typically observed in COB. LAMB1 is localized to the pial basement membrane, suggesting that defective connection between radial glial cells and the pial surface mediated by LAMB1 leads to this malformation

CLP1 Founder Mutation Links tRNA Splicing and Maturation to Cerebellar Development and Neurodegeneration

SummaryNeurodegenerative diseases can occur so early as to affect neurodevelopment. From a cohort of more than 2,000 consanguineous families with childhood neurological disease, we identified a founder mutation in four independent pedigrees in cleavage and polyadenylation factor I subunit 1 (CLP1). CLP1 is a multifunctional kinase implicated in tRNA, mRNA, and siRNA maturation. Kinase activity of the CLP1 mutant protein was defective, and the tRNA endonuclease complex (TSEN) was destabilized, resulting in impaired pre-tRNA cleavage. Germline clp1 null zebrafish showed cerebellar neurodegeneration that was rescued by wild-type, but not mutant, human CLP1 expression. Patient-derived induced neurons displayed both depletion of mature tRNAs and accumulation of unspliced pre-tRNAs. Transfection of partially processed tRNA fragments into patient cells exacerbated an oxidative stress-induced reduction in cell survival. Our data link tRNA maturation to neuronal development and neurodegeneration through defective CLP1 function in humans

Fatih’te Bulunan Klasik Bir Osmanlı Yapısı: Medresesinin Rehberliğinde Haydarpaşa Külliyesi

[No Abstract Available

Triassic limestone, turbidites and serpentinite-the Cimmeride orogeny in the Central Pontides

The basement of the Central Pontides, and by implication that of Crimea, consists of pre-Permian low-grade metaclastic rocks intruded by latest Permian-Early Carboniferous (305290 Ma) granitoids. Further up in the stratigraphic sequence are Triassic limestones, which are now preserved as olistoliths in the deformed Upper Triassic turbidites. New conodont and foraminifera data indicate an Anisian to Carnian (Middle to Late Triassic) age for these hemi-pelagic Hallstatt-type limestones. The siliciclastic turbidites surrounding the Triassic limestone contain the Norian (Late Triassic) bivalve Monotis salinaria; the same species is also found in the Tauric series in Crimea. The Upper Triassic flysch in the Central Pontides is locally underlain by basaltic pillow lavas and includes kilometre-size tectonic slices of serpentinite. Both the flysch and the serpentinite are cut by an undeformed acidic intrusion with an Ar-Ar biotite age of 162 +/- 4 Ma (Callovian-Oxfordian). This indicates that the serpentinite was emplaced into the turbidites before Middle Jurassic time, most probably during latest Triassic or Early Jurassic time, and that the deformation of the Triassic sequence pre-dates the Middle Jurassic. Regional geological data from the circum-Black Sea region, including widespread Upper Triassic flysch, Upper Triassic eclogites and blueschists of oceanic crustal affinity, and apparent absence of a 'Cimmerian continent' between the Cretaceous and Triassic accretionary complexes indicate that the latest Triassic Cimmeride orogeny was accretionary rather than collisional and is probably related to the collision and accretion of an oceanic plateau to the southern active margin of Laurasia

A Case of High-Grade Basilar Artery Stenosis with Spontaneous Recovery

High-grade basilar artery stenosis was detected in a 31-year-old man that presented with right hemiparesis and dysarthria. The exact etiology of the stenosis could not be determined. The patient was treated medically and remained asymptomatic during a 6-months follow-up period, and follow-up angiograms showed a dramatic improvement in the high-grade stenosis. This report aimed to draw attention to the importance of follow-up for high-grade basilar artery stenoses with medical therapy prior to using endovascular app- roaches, especially in young patients with a stable clinical course and unknown etiolog