Mass uptake study of the diffusion of water and SBF into poly(2-hydroxyethyl methacrylate-co-tetrahydrofurfuryl methacrylate) containing aspirin or vitamin B12

The ingress of water and Kokubo simulated body fluid (SBF) into poly(2-hydroxyethyl methacrylate) (PHEMA), and its co-polymers with tetrahydrofurduryl methacrylate (THFMA), loaded with either one of two model drugs, vitamin B₁₂ or aspirin, was studied by mass uptake over the temperature range 298–318 K. The polymers were studied as cylinders and were loaded with either 5 wt% or 10 wt% of the drugs. From DSC studies it was observed that vitamin B₁₂ behaved as a physical cross-linker restricting chain segmental mobility, and so had a small anti-plasticisation effect on PHEMA and the co-polymers rich in HEMA, but almost no effect on the T _g of co-polymers rich in THFMA. On the other hand, aspirin exhibited a plasticising effect on PHEMA and the co-polymers. All of the polymers were found to absorb water and SBF according to a Fickian diffusion mechanism. The polymers were all found to swell to a greater extent in SBF than in water, which was attributed to the presence of Tris buffer in the SBF. The sorptions of the two penetrants were found to follow Fickian kinetics in all cases and the diffusion coefficients at 310 K for SBF were found to be smaller than those for water, except for the polymers containing aspirin where the diffusion coefficients were higher than for the other systems. For example, for sorption into PHEMA the diffusion coefficient for water was 1.41 × 10^?11 m²/s and for SBF was 0.79 × 10^?11 m²/s, but in the presence of 5 wt% aspirin the corresponding values were 1.27 × 10^?11 m²/s and 1.25 × 10^?11 m²/s, respectively. The corresponding values for PHEMA loaded with 5 wt% B12 were 1.25 × 10^?11 m²/s and 0.74 × 10^?11 m²/s, respectively.     

Allan Hoffman: A 70th Birthday Celebration

Water sorption which is not classically Fickian has been observed in a variety of polymers. Deviation from Fickian kinetics is widely assumed to be caused by rate-limiting polymer relaxation, despite minimal proof of this. To the contrary, the evidence accumulated in this work indicates that water transport in initially glassy poly(2-hydroxyethyl methacrylate) (PHEMA), an important water-swellable biomedical polymer, is controlled by Fickian diffusion. First of all, the fractional water uptake is initially linear and independent of sample thickness when plotted against the square root of time over initial thickness, as expected for a Fickian process. Furthermore, the moving solvent front also advanced with the square root of time. Temperature, polymer thermal history and initial solvent concentration all affected the sorption kinetics of PHEMA in manners consistent with a Fickian process. The invariably Fickian sorption mechanism is believed to be the consequence of the water molecule's small size and affinity for hydrophilic, swellable polymers.     

Isoprene-styrene copolymer elastomer and tetrahydrofurfuryl methacrylate mixtures for soft prosthetic applications

Novel elastomer/methacrylate systems have been developed for potential soft prosthetic applications. Mixtures of varying compositions of an isoprene-styrene copolymer elastomer and tetrahydrofurfuryl methacrylate (SIS/THFMA) formed one-gel systems and were heat cured with a peroxide initiator. The blends were characterised in terms of sorption in deionised water and simulated body fluids (SBF), tensile properties and viscoelastic parameters of storage modulus and tan delta, as well as glass transition temperatures using dynamic mechanical analysis (DMA). DMA data gave two distinct peaks in tan delta, a lower temperature transition due to the isoprene phase in SIS and one at high temperature thought to be a combination of THFMA and the styrene phase in SIS. The tensile data showed a clear phase inversion within the mid range compositions changing from plastic to elastomeric behaviour. The sorption studies in deionised water showed a two stage uptake with an initial Fickian region that was linear to t 1/2 followed by a droplet growth/clustering system. The slope of the linear region was dependent on the composition ratio. The extent of overall uptake was osmotically dependent as all materials equilibrated at a much lower uptake in SBF. The diffusion coefficients were found to be concentration dependent.     

Modifications of the hydrophilicity of heterocyclic methacrylate copolymers for protein release

A series of copolymers comprising ethyl methacrylate (EM) and tetrahydrofurfuryl methacrylate (THFMA) gelled with either THFMA monomer or hydroxyethyl methacrylate (HEMA) monomer have been developed. In this paper, we examine the water uptake characteristics of the polymer systems and address the possibility of increasing the hydrophilicity of the systems by changing the ratios of the copolymers. We have investigated whether protein release from the polymers is related to the composition of the polymer systems. More protein was released from the polymers gelled with the more hydrophilic monomer (HEMA) than with THFMA. This was consistent with the calculated diffusion coefficients, which were 10 times greater for the polymers gelled with HEMA than those gelled with THFMA. Interestingly, the water uptake and protein release profiles were not dependent on the ratio of EM and THFMA in the copolymers. This is probably due to the conflicting roles of THFMA in the copolymer; it is both the more hydrophilic component as well as a cross-linking agent. In addition, it would appear that the structural and surface topography of these polymers had more significant effects on protein release than copolymer composition.     

Sorption kinetics and equilibrium uptake for water vapor in soft-contact-lens hydrogels

A gravimetric-sorption technique was used to obtain kinetic and equilibrium adsorption/desorption data for water vapor in four different soft-contact-lens (SCL) polymers at 35 degrees C. The SCL materials are a conventional hydrogel (polymacon) with a low water content at saturation (<50 wt %); two conventional hydrogels (hilafilcon A and alphafilcon A) with a high water content at saturation (>50 wt %); and a siloxane hydrogel (balafilcon A). Absorption and desorption equilibrium isotherms (water activity versus water weight fraction) overlap at high water contents, whereas significant hysteresis is observed at low water contents. The hysteresis loop is likely due to trapping of water in the polymer during the desorption process because of a rubber-to-glass transition of the SCL-film surfaces. Sorption data were interpreted using Flory-Rehner theory. The positive Zimm and Lundberg cluster function suggests that water tends to cluster in these SCL materials, except at very low water content. For polymacon and hilafilcon A, Fickian diffusion is observed for all activities for both water sorption and desorption. However, for alphafilcon A and balafilcon A, non-Fickian features appear at intermediate/low activities, in particular during water desorption, suggesting coupling of the diffusion process with polymer-matrix relaxation. The diffusion coefficient increases significantly with water concentration for polymacon and hilafilcon A (from approximately 0.3 x 10(-8) to 4.0 x 10(-8) cm2/s) because of augmented mixture free volume induced by water sorption, whereas a more complex composition dependence is observed for alphafilcon A and balafilcon A probably as consequence of a combined effect of polymer relaxation, plasticization, and water clustering.     

Study on blood compatibility with poly(2-methoxyethylacrylate)--relationship between surface structure, water structure, and platelet compatibility in 2-methoxyethylacrylate/2-hydroxyethylmethacrylate diblock copolymer

Diblock copolymers composed of 2-methoxyethylacrylate (MEA) and 2-hydroxyethylmethacrylate (HEMA) were firstly prepared (the composition ratio = 90/10, 79/21, 66/34, and 48/52 mol/mol) by anion living polymerization. ESCA analysis of their surface structures (dry state) revealed that PMEA segment was segregated to the top surface in all of the polymers, whereas the results of contact angle of water (wet state) showed that the surfaces were covered with PHEMA segment. In vitro platelet adhesion test showed that these polymers had the excellent compatibility with platelet compared to PHEMA homopolymer. Water structure in the hydrated copolymers was investigated by DSC and freezing bound water was observed for all the polymers like PMEA homopolymer, whereas it was not found in PHEMA homopolymer. Further investigation of water structure based on the results of DSC and EWCMS (equilibrium water content by moisture sorption) suggested that freezing bound water existed in PHEMA segment in addition to PMEA segment. We have proposed that the water plays a key role in the appearance of good blood compatibility of the copolymer, according to our previous works (Tanaka et al. Biomacromolecules 2002;3:36-41, Tanaka et al. J Biomed Mater Res A 2004;68:684-695).     

Effect of phosphate functional groups on the calcification capacity of acrylic hydrogels

The incorporation of negatively charged groups into the structure of synthetic polymers is frequently advocated as a method for enhancing their calcification capacity required in orthopedic and dental applications. However, the results reported by various research groups are rather contentious, since inhibitory effects have also been observed in some studies. In the present study, phosphate groups were introduced in poly(2-hydroxyethyl methacrylate) (PHEMA) by copolymerization with 10% mol of either mono(2-acryloyloxyethyl) phosphate (MAEP) or mono(2-methacryloyloxyethyl) phosphate (MMEP). Incubation of these hydrogels for determined durations (1-9 weeks) in a simulated body fluid (SBF) solution induced deposition of calcium phosphate (CaP) deposits of whitlockite type. After 9 weeks, the amount of calcium deposited on the phosphate-containing polymers was four times lower than that found on PHEMA, as determined by X-ray photoelectron spectroscopy (XPS). Samples of copolymer HEMA-MAEP were implanted subcutaneously in rats and evaluated after 9 weeks. No CaP deposits could be detected on the copolymer by XPS or energy dispersive X-ray spectroscopy, while PHEMA samples were massively calcified. It was concluded that the presence of phosphate groups decreased the calcification capacity of the hydrogels, and that in the conditions of this study, the phosphate groups had an inhibitory effect on the deposition of CaP phases on HEMA-based hydrogels.     

Random co-polymers based on the poloxamer Bayfit® 10WF15 for biomedical applications

Random co-polymers were prepared from the poloxamer Bayfit(?) 10WF15 and their thermal and biological properties analyzed. The poloxamer was characterized, functionalized with methacrylate groups (Bayfit-MA) and further co-polymerized with 2-hydroxyethyl methacrylate (HEMA) with Bayfit-MA feed contents of 1, 5 and 10 wt%. Co-polymers were partially soluble in organic solvents and exhibited a single glass transition temperature indicative of a random monomer distribution in the macromolecular chains. In thermogravimetric studies the co-polymers showed two degradation stages, around 210 and 350 °C, respectively. The thermosensitive behaviour of the poloxamer was studied by turbidimetry. Cloud point temperatures of aqueous solutions of Bayfit(?) 10WF15 (0.5-5 wt%) ranged from 15 to 18 °C and for Bayfit(?) 10WF15 methacrylate (0.5-1 wt%) from 6 to 7 °C. DSC thermograms of hydrated co-polymers showed the typical endothermic peaks with phase transition temperatures close to that of physiological medium. The biocompatibility of initial poloxamer and derivatives was analyzed with human fibroblasts cultures. The IC(50) value of Bayfit(?) 10WF15 was 1.4 mg/ml. Cellular extracts of the co-polymers were not cytotoxic and cellular proliferation and DNA content depended on co-polymer composition.     

Transmission of water through a biocompatible polyurethane: application to circulatory assist devices

The permeability characteristics of a water-segmented polyurethane (Biomer) system under the conditions encountered in circulatory-assist devices were investigated. A diffusion cell and permeability system providing precise control of membrane boundary conditions and allowing continuous measurement of water vapor transmission was designed. Liquid water at 37 degrees C was used as the donor fluid and the system incorporated a constant-flow nitrogen carrier gas and an optical dew point sensor downstream to determine the water vapor mass flow rate as a function of time. The mass flow rate was then numerically integrated and plotted against time to allow calculation of effective diffusion coefficient (D) by the dynamic time lag method. Steady-state permeabilities were found to be insensitive to donor chamber hydrostatic pressure (50-200 mm Hg) indicating that bulk flow is not a transport mechanism in these membranes. The permeability coefficient (P) was independent of membrane thickness (H) over the four samples tested (0.0102, 0.0148, 0.0269, and 0.0366 cm), with an average value of 3.29 X 10(-4) cm2/s. Thus, diffusion was Fickian with negligible boundary layers. A plot of lag time versus H2 was linear (R = 0.98) yielding a value for D of 2.18 X 10(-7) cm2/s. A water-Biomer partition coefficient was determined for each sample with an average value of 1525, indicating a moderately hydrophilic membrane with a water sorption of 6.3% at 37 degrees C. Since water transport is by Fickian diffusion in the absence of bulk flow, liquid water cannot be expected to accumulate in circulatory-assist devices unless a condensing surface is maintained within the system.     

Characterizing multilaminated hydrogels with spatially varying network structure and solute loading using confocal laser scanning microscopy

Multilaminated controlled release devices were formed through photopolymerization techniques to produce hydrogels with spatially varying solute loadings and network structures composed of poly(hydroxyl ethyl methacrylate) (PHEMA) and poly(ethylene glycol) (PEG). Using low molecular weight fluorescent dyes as model drugs, the distribution profiles were characterized non-invasively in pseudo-real-time with confocal laser scanning microscopy (CLSM) during release studies. For comparison, theoretical modeling based on Fickian diffusion theory was performed in conjunction with experimental work to identify any deviations from expected behavior and to guide in the development of future devices. In multilaminates composed of only PHEMA, the evolution of dye distribution during release and cumulative release profiles agreed well with theoretically predicted data, indicating continuity of diffusion and insignificant interfacial hindrance between layers. However, in devices composed of alternating layers of PHEMA and PEG, differences from predicted behavior were experimentally observed in both concentration profiles and release rates, suggesting interfacial obstruction of diffusion, possibly due to the formation of interpenetrating networks. Finally, the simultaneous release of two dyes at different rates from a PEG/PHEMA multilaminate was monitored to demonstrate the usefulness of CLSM in understanding the complex temporal changes in solute distributions in gel devices.     

Phase transfer and characterization of poly(epsilon-caprolactone) and poly(L-lactide) microspheres

A method suitable for transfer of poly(epsilon-caprolactone) and poly(L-lactide) microspheres (synthesized by pseudoanionic dispersion polymerization of epsilon-caprolactone and L-lactide in heptane-1,4-dioxane mixed solvent) from heptane to water was developed. This method consists of treating the microspheres with KOH-ethanol in the presence of surfactants (nonionic Triton X-405, anionic sodium dodecyl sulfate (SDS), and zwitterionic ammonium sulfobetaine-2 (ASB)). Partial hydrolysis of polyesters results in the formation of hydroxyl and carboxyl groups in the surface layer of microspheres and enhances their stability in water-based media. Minimal concentrations of surfactants, needed to obtain stable suspensions of particles, were equal to 3 x 10(-2) and 6 x 10(-2), and 3 x 10(-2) mol l(-1) for Triton X-405. SDS, and ASB, respectively. In the case of poly(epsilon-caprolactone) microspheres, suspensions in water were stable for all three surfactants for pH values ranging from 3 to 11. Suspensions of poly(L-lactide) were stable in the same range of pH values only for ASB. Surface charge density determined by electrophoretic mobility varied for poly(epsilon-caprolactone) microspheres from 2.6 x 10(-7) to 8.9 x 10(-7) mol m(-2), for particles stabilized with Triton X-405 and ASB. respectively. In the case of poly(L-lactide) microspheres, surface charge density varied from 3.9 x 10(-7) (stabilizer: Triton X-405) to 7.4 x 10(-7) mol m(-2) (stabilizer: ASB). Carboxyl groups located in the surface layer of poly(L-lactide) microspheres were used for covalent immobilization of 6-aminoquinoline, a fluorophore with an amino group. Maximum surface concentration of immobilized 6-aminoquinoline was equal to 1.9 x 10(-6) mol m(-2). Poly(epsilon-caprolactone) microspheres transferred into water were loaded with ethyl salicylate. Loading up to 38% (w/w) was obtained.     

Water transport in resin-modified glass-ionomer dental cement

Water uptake and water loss have been studied in a commercial resin-modified glass-ionomer cement, Fuji II LC, under a variety of conditions. Uptake was generally non-Fickian, but affected by temperature. At room temperature, the equilibrium water uptake values varied from 2.47 to 2.78% whereas at low temperature (12 degrees C), it varied from 0.85 to 1.18%. Cure time affected uptake values significantly. Water uptake was much lower than in conventional glass-ionomer restorative cements exposed to water vapor. Loss of water under desiccating conditions was found to be Fickian for the first 5 h loss at both 22 and 12 degrees C. Diffusion coefficients were between 0.45 and 0.76 x 10( -7) cm(2)/s, with low temperature diffusion coefficients slightly greater than those at room temperature. Plotting water loss as percentage versus s(-(1/2)) allowed activation energies to be determined from the Arrhenius equation and these were found to be 65.6, 79.8, and 7.7 kJ/mol respectively for 30, 20, and 10 s cure times. The overall conclusion is that the main advantage of incorporating HEMA into resin-modified-glass-ionomers is to alter water loss behavior. Rate of water loss and total amount lost are both reduced. Hence, resin-modified glass-ionomers are less sensitive to water loss than conventional glass-ionomers.     

Functionalized bridged silsesquioxane-based nanostructured microspheres: performance as novel drug-delivery devices in bone tissue-related applications

Two kinds of functionalized nanostructured hybrid microspheres, based on the bridged silsesquioxane family, were synthesized by employing the sol-gel method via self-assembly of two different organic-inorganic bridged monomers. The architecture reached at molecular level allowed the incorporation of acetylsalicylic acid (ASA) as an anti-inflammatory model drug. The ASA-functionalized microspheres were characterized as delivery devices in simulated body fluid (SBF). The release behaviors of the synthesized microspheres (Fickian or anomalous diffusion mechanisms) were shown to be dependent on the chemical nature of the bridged monomers employed to synthesize the hybrid materials. The functionalized microspheres were proposed as delivery systems into calcium phosphate cements (CPCs), in order to slow down the characteristic drug-delivery kinetics of this kind of bone tissue-related materials. The incorporation of the new functionalized microparticles into the CPCs represented a viable methodology to modify the ASA-release kinetics in comparison to a conventional CPC containing the drug dispersed into the solid phase. The ASA-delivery profiles obtained from the microsphere-loaded CPCs showed that 40-60% of drug can be released after 2 weeks of testing in SBF. The inclusion of the microspheres into the CPC matrices allowed modification of the release profiles through a mechanism that involved two stages: (1) the diffusion of the drug through the organic-inorganic matrix of the microspheres (according to a Fickian or anomalous diffusion, depending on the nanostructuring) and (2) the subsequent diffusion of the drug through the ceramic matrix of the hardened cements. The release behavior of the composite cements was shown to be dependent on the nanostructuring of the hybrid microspheres, which can be selectively tailored by choosing the desired chemical structure of the bridged precursors employed in the sol-gel synthesis. The obtained results demonstrated the ability of this new class of functionalized hybrid microdevices as delivery systems into calcium phosphate materials with potential bone tissue-related drug-delivery applications.     

Carbon dioxide extraction of residual chloroform from biodegradable polymers

Biodegradable polymeric devices for drug delivery and tissue engineering are often fabricated with the use of organic solvents and may still contain significant amounts of solvent (> 1 wt%) even after aggressive vacuum drying. This excess solvent can interfere with tissue response and the mechanical properties of the devices. The aim of this article is to demonstrate that liquid CO(2) extraction can be used to reduce residual solvent in dense poly(L-lactide-co-glycolide) devices to 50 ppm relatively quickly and with minimal changes in architecture under some conditions. Two liquid CO(2) extraction systems were developed to examine the removal of residual solvents from bar-shaped PLGA devices: (1) a low-pressure (1400 psi) batch system, and (2) a high-pressure (5000 psi) continuous-flow system. Eight hours of extraction in the high-pressure system reduced residual chloroform in 3 mm thick bars below the 50-ppm target. A simple Fickian diffusion model was fit to the extraction results. Diffusion coefficients ranged from 1.10 x 10(-6) cm(2)/s to 2.64 x 10(-6) cm(2)/s. The model predicts that approximately 1 h is needed to dry 1-mm bars to chloroform levels below 50 ppm, and 7 h are needed for 3 mm thick bars. The micro- and macroarchitectures of porous PLGA scaffolds created by particulate leaching were not significantly altered by CO(2) drying if the salt used to make the pores was not removed before drying.     

Synthesis,Characterization and Biocompatibility of Novel Biodegradable Cross-linked Co-polymers Based on Poly(propylene oxide) Diglycidylether and Polyethylenimine

Novel biodegradable cross-linked co-polymers were prepared from poly(propylene glycol) diglycidylether (PPGDGE) and poly(ethylene imine) (PEI). PPGDGE and PEI were mixed at ambient temperature with varying PEI concentrations of 10, 15, 18.5, 25, 30, 40 and 50 wt%; the homogenous PPGDGE/PEI mixtures obtained were cured at elevated temperatures, resulting in formation of PPG–PEI cross-linked co-polymers via ring-opening reaction of PPGDGE with PEI. The physicochemical and biological properties of these co-polymers were dependent on the PEI content and the extent of curing reaction. The glass transition temperature of PPG–PEI cross-linked co-polymers varied in the range from ?14 to +42°C, while the co-polymers displayed composition-dependent mechanical behavior, from brittle to ductile with increasing PEI content from 18.5 wt% to 40 wt%. Chinese hamster ovary (CHO) cells were cultured on the PPG–PEI co-polymers; the MTT assay was used to measure cell viability and determine the cytotoxicity. The cell viability rate, relative to tissue-culture polystyrene (TCPS), increased from 49% to 125% with increasing PEI content from 18.5 wt% to 40 wt%. Although epoxy monomers usually exhibit cytotoxicity, the epoxy groups were exhausted via curing reaction in the fully cross-linked co-polymers. The PEI-cured PPG epoxy resin, i.e., PPG–PEI cross-linked co-polymers obtained in this study, showed excellent biocompatibility.     

Wettability and surface charge of Si3N4-bioglass composites in contact with simulated physiological liquids

Wettability and surface charge studies were performed on a novel Si3N4-30wt% bioglass biocomposite. Contact angle and surface tension variation with time were determined at 25 degrees C, respectively, by the sessile and pendant drop techniques, for distinct testing liquids: water, diiodomethane, simulated body fluid (SBF) and bovine serum albumin (BSA) dissolved in SBF solution. This biocomposite revealed a hydrophilic character (theta = 26.6 +/- 2.0 degrees) and a surface tension value (66.6 mJ m(-2)) comparatively higher than those of the most common bioceramics. An important characteristic is the high work of adhesion towards SBF + BSA (96.4 +/- 0.2mJ m(-2)) that was measured. The Si3N4-bioglass material is negatively surface charged above the pH(IEP) = 2.5 in aqueous SBF + BSA solution, as a result of the presence at the surface of unsaturated Si-O bonds and Si-OH groups. The very high negative zeta potential at pH approximately 7 (-58.6 +/- 5.5mV) influenced albumin adsorption and mechanisms are discussed in terms of entropy and enthalpy gains from conformational unfolding and cations coadsorption.     

聚甲基丙烯酸羟乙酯水凝胶人工晶状体材料的合成与性能

以甲基丙烯酸羟乙酯为原料,过硫酸铵/偏重亚硫酸钠为引发体系,二甲基丙烯酸三乙二醇酯为交联剂,采用溶液聚合法制备了聚甲基丙烯酸羟乙酯水凝胶(PHEMA)人工晶状体材料。系统考察了聚合反应时间、温度及引发剂和交联剂的用量等对该水凝胶材料机械强度、平衡水含量(EWC)的影响,并对PHEMA水凝胶的结构和光学性能进行了表征。实验结果表明,PHEMA水凝胶的最佳合成条件为:引发剂0.5wt%,交联剂1.0wt%,反应温度40℃,反应时间36h。在此条件下制备的PHEMA水凝胶的拉伸强度达到0.57MPa,邵氏A硬度为23.0,平衡含水量超过40%,透光率≥97%。     

Water absorption and surface properties of novel poly(ethylmethacrylate) polymer systems for use in bone and cartilage repair

The surface and bulk properties of novel methacrylate polymers prepared by gelling poly(ethyl methacrylate) (PEMA) powder with different ratios of tetrahydrofurfuryl methacrylate (THFMA) and hydroxyethyl methacrylate (HEMA) monomers were investigated. The water adsorption and desorption characteristics of these polymers were measured in water and phosphate buffered saline (PBS). The desorption diffusion coefficients were higher than the adsorption coefficients in both water and PBS. Linear relationships between the equilibrium mass of water taken up and the mass of water desorbed with the concentration of HEMA in the polymer were established. Polymer surfaces were analysed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Surface features varied with polymer composition; during hydration only selective areas of the surface hydrated indicating a heterogeneous surface. Contact angle data showed no trend between the different polymers indicating that contact angles are not an acceptable method of assessing hydrophobicity/wettability of a material which does not have a homogeneous surface. The effect of these bulk and surface characteristics on biological interactions were examined using bovine chondrocytes and human osteoblast (HOB) cell cultures. Cell attachment decreased when HEMA was present in the copolymer.     

Preparation of novel silica-coated alginate gel beads for efficient encapsulation of yeast alcohol dehydrogenase

Biomimetic formation has undoubtedly inspired the preparation of novel organic-inorganic hybrid composites. In this study, silica-coated alginate gel beads were prepared by coating the surface of alginate gel beads with silica film derived from tetramethoxysilane (TMOS). The composition and structure of the silica film were characterized by FT-IR and SEM equipped with EDX. The swelling behavior of silica-coated alginate gel beads was studied to be more stable against swelling than that of alginate gel beads. The results showed that silica-coated alginate gel beads exhibited appropriate diffusion property. The effective diffusion coefficient (D(e)) of NADH in silica-coated alginate beads was 1.76 x 10(-10) m2/s, while the effective diffusion coefficient in alginate beads was 1.84 x 10(-10) m2/s. The model enzyme yeast alcohol dehydrogenase (YADH) was encapsulated in silica-coated alginate and pure alginate beads, respectively. Enzyme leakage of YADH in alginate gel beads was determined to be 32%, while the enzyme leakage in silica-coated alginate gel beads was as low as 11%. Furthermore, the relative activity of YADH in alginate gel beads decreased almost to zero after 10 recycles, while the relative activity of YADH in silica-coated alginate gel beads was 81.3%. The recycling stability of YADH in silica-coated alginate gel beads was found to be increased significantly mainly due to the effective inhibition of enzyme leakage by compact silica film.     

In vitro behavior of silicate glass coatings on Ti6A14V

The in vitro response in simulated body fluid (SBF) of silicate glass coatings on Ti6A14V was evaluated. Glasses belonging to the SiO2-CaO-MgO-Na2O-K2O-P2O5 system were used to prepare 50-70 m thick coatings on Ti6Al4V, employing a simple enameling technique. Glasses with silica content higher than 55 wt% can be used to prepare coatings that do not crack or delaminate and exhibit good adhesion to the alloy. It has been found that coatings with silica content lower than 60 wt% are more susceptible to corrosion and precipitate carbonated hydroxyapatite on their surface during in vitro tests. However, these coatings have a higher thermal expansion than the metal and are under tension. After 2 months in SBF cracks grow in the coating that reach the glass/metal interface and initiate delamination. Glasses with silica content higher than 60 wt% are more resistant to corrosion and have lower thermal expansion. These coatings do not crack but they do not precipitate apatite even after 2 months in SBF.