FABRICATION AND CHARACTERATION OF NANOPOROUS SILICA FILM

Colloidal silica sol is formed by a novel hydrolyzing procedure of tetraethyl orthosilicate(TEOS) catalyzing with NH 3 ·H 2 O in aqueous mediums. Glycerol, combining with the hydrolyzed intermediates of TEOS, controls growing of the silica particles; poly(vinyl vinyl alcohol makes the colloidal silica sol with polymeric structure and spinning, thermal strain makes the gel silica film changed into a nanoporous structure with diameter ranging 50-150 nm. Morphologies of the nanoporous silica film have been characterized; the porosities (%) is 32-64; the average dielectric constant at 1MHz region is 2.0 and 2.1; the thermal conductivity is less than 0.8. Chemical mechanism of the sol gel process is discussed.

A review on heat transfer in nanoporous silica aerogel insulation materials and its modeling

Silica aerogels are widely used in many fields for thermal insulation,such as building insulation,electric power energy,energy storage systems,and high-temperature thermal protection due to their excellent insulation per-formance.Therefore,the heat transfer in silica aerogels and its modeling in recent years have attracted much attention and many valuable achievements have been acquired.The heat transfer in nano-porous silica aerogels and its modeling methods are reviewed in this work.This review starts with a brief introduction of heat transfer characteristics in silica aerogels,including the multi-component and multi-mode coupling effect,size effect,and multiscale effect.Then the heat transfer mechanism of each mode,including heat transfer via gas phase,solid phase,and thermal radiation,is reviewed,and the models for predicting the gaseous thermal conductivity in nanoscale pores,gas-contributed thermal conductivity,the apparent thermal conductivity of solid skeleton,and finally the effective thermal conductivity are collected and discussed in details.Besides,modeling of transient heat transfer in silica aerogels is also briefly introduced.Finally,the conclusions and some problems which need to be further investigated in the future are provided.

HIGH DIELECTRIC CONSTANT COMPOSITE BASED ON CHLOROPHYLL A ENTRAPPED NANOPOROUS SILICA GEL

Chlorophyll a(naturally occurring Mg porphyrene)has been entrapped in nano/porous silica gel using sol-gel method at room temperature,producing a stable composite.HR TEM observationreveals regular nanoscale[around 15-20 nm diameter]distribution of aggregated polycrystalinechlorophylla within porous silica matrix.UV-vis study also corroborates the presence of variousaggregated chlorophyll a species within the system.Low field measurement shows almost 400times enhancement of dielectric constant(1700)with incorporation of only 0.125 mg/ml of chlorophyll and the loss is 0.5 at room temperature at 100 Hz.The dielectric constant of the composite reaches 2500 as chlorophyll concentration becomes 1 mg/ml.Observed strong space charge response to the external field and strong frequency dispersion of the dielectric properties ofthe composite can be attributed to the long-range electron delocalization[nomadic polarization]in chlorophyl a aggregates.The electric modulus(M*)formalism used in this study enabled us todistinguish and separate various relaxation processes.It is found that with increasing chlorophyll concentration D.C.relaxation time decreases exponentially at room temperature.It is shown that observed relaxations do not perfectly follow the Debye response in high frequency region due toheterogeneous distribution of chlorophyll aggregates.The low values of room temperature acti-vation energy calculated from Arrhenius plot reveal that polaronic hopping phenomena is absent at grain-interfacial region due to low thermal energy.

Chemically induced hypoxia by dimethyloxalylglycine(DMOG)-loaded nanoporous silica nanoparticles supports endothelial tube formation by sustained VEGF release from adipose tissue-derived stem cells

Inadequate vascularization leading to insufficient oxygen and nutrient supply in deeper layers of bioartificial tissues remains a limitation in current tissue engineering approaches to which prevascularization offers a promising solution.Hypoxia triggering pre-vascularization by enhanced vascular endothelial growth factor(VEGF)expression can be induced chemically by dimethyloxalylglycine(DMOG).Nanoporous silica nanoparticles(NPSNPs,or mesoporous silica nanoparticles,MSNs)enable sustained delivery of molecules and potentially release DMOG allowing a durable capillarization of a construct.Here we evaluated the effects of soluble DMOG and DMOG-loaded NPSNPs on VEGF secretion of adipose tissue-derived stem cells(ASC)and on tube formation by human umbilical vein endothelial cells(HUVEC)-ASC co-cultures.Repeated doses of 100 mM and 500 mM soluble DMOG on ASC resulted in 3-to 7-fold increased VEGF levels on day 9(P<0.0001).Same doses of DMOG-NPSNPs enhanced VEGF secretion 7.7-fold(P<0.0001)which could be maintained until day 12 with 500 mM DMOG-NPSNPs.In fibrin-based tube formation assays,100 mM DMOG-NPSNPs had inhibitory effects whereas 50 mM significantly increased tube length,area and number of junctions transiently for 4 days.Thus,DMOG-NPSNPs supported endothelial tube formation by upregulated VEGF secretion from ASC and thus display a promising tool for prevascularization of tissue-engineered constructs.Further studies will evaluate their effect in hydrogels under perfusion.