Three-dimensional scaffolds for tissue engineering: the importance of uniformity in pore size and structure

To validate the importance of uniformity in pore size and structure of a scaffold for tissue engineering, we fabricated two types of scaffolds with uniform (inverse opal scaffolds) and nonuniform pore sizes and structures, and then evaluated their properties in terms of diffusion of macromolecules, spatial distribution of fibroblasts, and differentiation of preosteoblasts. Our results confirmed the superior performance of the inverse opal scaffolds due to the uniform pore size, homogeneous environment, and high interconnectivity: a higher diffusion rate, a uniform distribution of cells, and a higher degree of differentiation. In addition, we found that both the differentiation of cells and secretion of extracellular matrix were dependent on the properties of the individual pore to which the cells were attached, rather than the bulk properties of a scaffold. Our results clearly indicate that inverse opal scaffolds could provide a better microenvironment for cells in comparison to a scaffold with nonuniform size and structure.     

Control of pore size and pore uniformity in films based on self‐assembling block copolymers

Blends of self‐assembling polystyrene‐block‐poly(4‐vinyl pyridine) (PS‐b‐P4VP) diblock‐copolymers and poly(4‐vinyl pyridine) (P4VP) homopolymers were used to fabricate isoporous and nanoporous films. Block copolymers (BCP) self‐assembled into a structure where the minority component forms very uniform cylinders, while homopolymers, resided in the core of the cylinders. Selective removal of the homopolymers by ethanol immersion led to the formation of well‐ordered pores. In films without added homopolymer, just immersion in ethanol and subsequent swelling of the P4VP blocks was found to be sufficient to create pores. Pore sizes were tuned between 10 and 50 nm by simply varying the homopolymer content and the molecular weight of the block‐copolymer. Uniformity was lost when the average pore size exceeded 30 nm because of macrophase separation. However, preparation of films from low M_W diblock copolymers showed that it is possible to have excellent pore size control and a high porosity, while retaining a low pore size distribution. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1568–1579     

Effect of membrane parameters on the size and uniformity in preparing agarose beads by premix membrane emulsification

Agarose microbeads were prepared by premix membrane emulsification with Shirasu-Porous Glass (SPG) membrane and Polyethylene (PE) membrane. The effects of membrane parameters, including pore size, pore size distribution, contact angle between membrane surface and the water phase, shape of pore opening and membrane thickness on size and uniformity of agarose beads were investigated in this study. The results showed that pore size distribution and shape of pore opening did not affect the emulsification results apparently within a wide range in premix membrane emulsification, not as the result in general emulsification. The contact angle between the water phase and the membrane surface must be large enough to obtain uniform-sized agarose beads in both direct membrane emulsification and premix membrane emulsification. The results also showed that the membrane pore size and thickness affected the size distribution of emulsion. Thicker membrane resulted in more uniform and smaller emulsion when the number of pass through membrane is controlled. There was a linear relationship between the number average diameter of agarose beads and membranes pores size in premix membrane emulsification. Agarose beads with diameters from 3.06 to 9.02 μm were prepared by controlling membranes pore size. The ratio of the number average diameter of agarose beads to membrane pore diameters was found to be 0.486.     

Atomic force microscope studies of membranes: Surface pore structures of Cyclopore and Anopore membranes

Non-contact atomic force microscopy has been used to investigate the surface pore structure of Cyclopore and Anopore microfiltration membranes in air. Three Cyclopore membranes and three Anopore membranes of different pore sizes were studied. Excellent high resolution images were obtained. Analysis of the images gave quantitative information on the surface pore structure, in particular the pore size distribution. Non-contact AFM is an excellent means of obtaining such information for microfiltration membranes.     

Effect of pore size distribution on the surface diffusivity in activated carbon: Hybrid Dubinin-Langmuir isotherm

The concentration dependence of the observed surface diffusivity for activated carbon due to the pore size distribution is theoretically investigated. The mathematical model is derived based on the assumption of a local hybrid adsorption isotherm (proposed recently by Shethna and Bhatia, 1994) and a local surface diffusive flux for a particular pore of half widthr. Using those local quantities and assuming a Gamma pore size distribution, the observed surface diffusivity is obtained. This observed surface diffusivity was found to increase rapidly with loading if the chemical potential is the driving force for surface flow. Furthermore, this observed surface diffusivity,D/D(0), was found to be the same as the Darken thermodynamic correction factor, using only the macroscopic isotherm information. This indicates that the thermodynamic correction factor contains information on the averaging of the surface heterogeneity.Nomenclature a coefficient for surface diffusivity - A adsorbate molecular area - c affinity parameter of the surface adsorption isotherm - C(P, T) concentration - C _max maximum adsorbed concentration - D _obs observed surface diffusivity - D _s intrinsic surface diffusivity - D _s0 coefficient of intrinsic surface diffusivity - E ₀ characteristic energy of Dubinin isotherm - F(r) pore size distribution - J local flux - J _obs observed flux - k empirical constant of Dubinin isotherm - K Langmuir affinity parameter - K _m Langmuir affinity parameter at maximum micropore half width - m structural parameter defined in Eq. (13) - n Dubinin variable exponent - q structural parameter defined in Eq. (13) - Q function ofr andT defined in Eq. (4) - Q _m function ofn defined in Eq. (8) - P pressure - P ⁰ vapour pressure - P _t threshold pressure defined in Eq. (3) - P ₀,P _L pressure at two end of the slab - P minimum pressure for Dubinin isotherm - P _m threshold pressure at maximum micropore half width - S ₁,S ₂ scaling factors defined in Eq. (15) - r pore half width - r ₀ smallest micropore half width - r _t threshold micropore half width demarcates Dubinin and Langmuir mechanisms - r _m maximum micropore half width - v _M liquid molar volume of adsorbate - V pore volume - R gas constant - T temperature - x axial variable - parameter defined in Eq. (A-7) - affinity factor of Dubinin isotherm - _L constant defined in Eq. (A-7) - small number - pore variance - non dimensional local isotherm - _D non dimensional Dubinin isotherm - _L non dimensional Langmuir isotherm - _S non dimensional surface isotherm - _obs non dimensional overall adsorption isotherm     

Stabilisation of the water permeability of mineral ultrafiltration membranes: An empirical modelling of surface and pore hydration

This work focuses on the study of the hydration phenomenon operating in Na–mordenite membranes during the conditioning step (stabilisation of filtration properties). First, experimental (filtration of pure water) tests are carried out immediately after putting the membrane in its casing and until the stabilization of the membrane permeation flux. The evolution of the hydraulic permeability shows that there are two separate steps during the conditioning of the membrane. A numerical approximation of the hydraulic permeability during the conditioning step was carried out. The first part of the equation expresses a fast decrease in the membrane's permeability during the beginning of the conditioning step (several hours). This behaviour is attributed to a surface hydration of the membrane and also to a modification of the crystalline framework. The second one is a slower phenomenon that takes place until the end of the conditioning step. It is attributed to the (intra-crystalline) hydration of micropores.     

Hydration forces between bilayers in the presence of dissolved or surface-linked sugars

We analyse the experimental evidence of the hydration force near phospholipid bilayers when the “solvent” is a solution of carbohydrates. Two cases must be clearly distinguished: when sugar is dissolved, depletion causes a supplementary attractive force, while in the case of sugar linked to the lipid the contact pressure increases by orders of magnitude. Attractive interaction inferred between bilayers is sometimes derived from indirect evidence, i.e. scattering, attraction between layers adsorbed, shape of phase boundary limits, and without the simultaneous determination of the osmotic compressibility. Generally, water molecules in the first hydration shell of sugar compete with water molecules bound (by more than one kT in free energy) to lipid head-groups. A general result is that the decay length of any repulsive effect remains close to 0.2 nm, even in concentrated sugar solutions. A tentative general explanation of this experimental fact is given together with consequences, such as the possibility of several types of critical points appearing in bilayer stacks. Decay length as well as effective contact pressure is considered with respect to carbohydrate activity.     

Streaming potential and surface charge density of microporous membranes with pore diameter in the range of thickness

A system formed by two phases bathing a microporous membrane is studied considering its behavior as a dynamic system. So, the natural frequencies for each used membrane is determined and then, applying a flow ramp with a rate sufficiently small, the streaming potential can be obtained from the slope of the pressure values versus electrical potential.The determination of the electric potential inside the pores, φ, requires to solve the Poisson–Böltzmann equation in the case of membranes with pore diameter in the range of thickness, for which the radial components of velocity of the fluid must be considered. Since there is no analytical solution, a numerical method was used to obtain φ. The electrical potential value at a distance equal to hydrodynamic radius from pore axis (zeta potential) is used to evaluate the streaming potential by the Helmholtz–Smoluchowsky relation. These values were compared with the experimental data accomplishing the suitable iterations over the surface charge density until coincidence.The values of the surface charge density for the studied membranes show a concentration dependence described by Langmuir’s model for the greatest pore diameters and Freundlich’s model for the smallest pore diameters.     

Quasi-equilibrium nitrogen adsorption gravimetry : comparison with volumetry for the determination of surface areas and pore size distributions

The comparison involves one gravimetric technique and two volumetric ones (either conventional or quasi-static). The samples used are a bronze powder (0.1 m²g⁻¹ only), a graphite powder (c.a. 10 m²g⁻¹, with a phase change of the nitrogen monolayer used as a sensitive thermometer) and two mesoporous silica gels (pore size c.a., 5.0 and 11.5 nm). Relative advantages of either technique are pointed out.     

The use of moment theory to interpret diffusion and sieving measurements in terms of the pore size distribution in heteroporous membranes

Moment theory has been applied to model porous membranes to show that one can place reasonable bounds on the cumulative pore size distribution, the hindered diffusivity or the reflection coefficient of large solutes in a heteroporous membrane by measuring the diffusive permeability to a small solute, the hydraulic permeability and one or two additional transport characteristics. These additional measurements involve either the flux of a small solute at Pe1, the hindered diffusivity of a large solute or the reflection coefficient of a large solute at Peå1. Membrane heteroporosity is incroporated in the predicted bounds without requiring one to make any a priori assumptions about the nature of the pore size distribution. In this paper, the results from calculations performed with different model membranes containing log-normal pore size distributions are reported. A comparison of the results obtained with three different membranes shows that one can distinguish between membranes with the same average pore size but different pore size distributions by measuring either the hindered diffusion coefficient or the reflection coefficient of two different sized solutes. A comparison of the bounds on D and the bounds on σ predicted from different types of transport measurements shows that, under certain conditions, one can place tighter bounds on one transport characteristic by measuring a different one.     

Quantification of the pore size distribution (porosity profiles) in microfiltration membranes by SEM,TEM and computer image analysis

Transport properties of membranes are closely related to morphological properties like surface porosity and variation of their inner pore structure. Scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM) are powerful tools to characterise the microscopical pore structure of membranes in a qualitative manner. In order to provide more quantitative data of surface and cross-sectional pores computer image analysis can be used. Parameters like ‘porous area fraction’ and ‘mean free path length’ have been selected to describe the pore distribution within porosity profiles in order to consider the effect that the pores within the cross-section are connected to each other.     

Effect of surface acidity and pore size of Al-substituted plugs-containing SBA-15 and MCM-41 silicas on the polymerization of THF

We reported here the simultaneous influence of surface acidity and pore size of Al-substituted hexagonal mesoporous silicas(Al-doped plugs-containing SBA-15 and Al-doped MCM-41) on polymerization of THF.These materials were directly synthesized by introduced aluminum isopropoxide into reaction mixture including surfactant and siliceous precursor.Al-doped plugs-containing SBA-15(denotes as PAS) samples not only possess typical two-step desorption isotherms,which implied PAS materials generated plugs in their mesochannel,but also exhibit larger pore size and thicker wall than that of Al-doped MCM-41(denotes as ACM), which implied PAS would have a great advantage on catalytic reaction involving large molecular(e.g.polymer of THF) in industrial point of view.To investigate catalytic activity of PAS and ACM with moderate acidic sites the polymerization of THF in the presence of acetic anhydride was carried out.The results showed PAS exhibiting good performance on polymerization of THF.Such result could be related to the large pore size and moderate acidic sites.     

Application of charged membranes in water softening: modeling and experiments in the presence of polyelectrolytes

In this paper theoretical and experimental results for water softening by bipolar membranes in the presence of polyelectrolytes have been presented. A modified capillary model is introduced for flow of a multiion solution through the membrane. The most important modifications are: an integral boundary condition in Poisson equation and a coupling coefficient in the convective molar flux by using the modified Faxen equation. In addition, electrical permittivity of solution has not been considered as a constant. The predictions of the model were compared well with the experimental data obtained by using bipolar membranes in a recirculating test system in the presence of polyelectrolytes.     

Relative peak broadening and pore size distribution of the stationary phase in exclusion chromatography

Summary It is shown experimentally that in size exclusion chromatography theh-values of a homologous series versus the relative molecular mass of the samples and the pore size distribution of the packing material determined by exclusion chromatography exhibit the same maximum.Part of the Ph.D. Thesis of W. Werner, University of Saarbrücken, Saarbrücken 1976.     

Separation of casein micelles from whey proteins by high shear microfiltration of skim milk using rotating ceramic membranes and organic membranes in a rotating disk module

This paper investigates the microfiltration of skim milk in order to separate caseins micelles from two whey proteins, α-lactalbumin (α-La) and β-lactoglobulin (β-Lg), using a modified dynamic filtration pilot (MSD) consisting in 6 ceramic 9-cm diameter membrane disks of 0.2 μm pores, rotating around a shaft inside cylindrical housing. A comparison was made with another dynamic filtration module consisting in a disk rotating near a fixed PVDF 15.5 cm diameter membrane with 0.15 μm pores. Maximum permeate fluxes were 120 L h⁻¹ m⁻² with the MSD module at 1930 rpm and at 40 °C, and 210 L h⁻¹ m⁻² at 2500 rpm and 45 °C, with the rotating disk module. Casein rejection was around 99% at high speed for both membranes. α-La transmission decreased with increasing transmembrane pressure (TMP) from 75% to 60% for ceramic membranes and from 25% to 10% for the PVDF one. β-Lg transmissions were lower, ranging from 23% to 15% for ceramic membranes and from 20% to 5% for the PVDF one. In a concentration test with the PVDF membrane at 2000 rpm, the flux decayed from 200 L h⁻¹ m⁻² at initial concentration to 80 L h⁻¹ m⁻² at VRR = 3.2 and 22.1% of the initial α-La mass was recovered in the permeate, against 8.1% for β-Lg. Permeate fluxes in the mass transfer limited regime (J_lim) of the MSD and rotating disk module operated at various speeds were well correlated by the equation J_lim = 17.13 V_av where V_av denoted the disk azimuthal velocity averaged over the membrane area. Measurements of J_lim, taken from Ref. [G. Samuelsson, P. Dejlmek, G. Tragardh, M. Paulsson, Minimizing whey protein retention in crossflow microfiltration of skim milk. Int. Dairy J. 7 (1997) 237–242] during MF of skim milk using tubular ceramic membranes at velocities from 1.5 to 8 m s⁻¹ with permeate co-current recirculation were found to obey the same correlation.     

Spectral force analysis using atomic force microscopy reveals the importance of surface heterogeneity in bacterial and colloid adhesion to engineered surfaces

Coatings developed to reduce biofouling of engineered surfaces do not always perform as expected based on their native properties. One reason is that a relatively small number of highly adhesive sites, or the heterogeneity of the coated surface, may control the overall response of the system to initial bacterial deposition. It is shown here using an approach we call spectral force analysis (SFA), based on force volume imaging of the surface with atomic force microscopy, that the behavior of surfaces and coatings can be better understood relative to bacterial adhesion. The application of vapor deposited TiO₂ metal oxide increased bacterial and colloid adhesion, but coating the surface with silica oxide reduced adhesion in a manner consistent with SFA based on analysis of the “stickiest” sites. Application of a TiO₂-based paint to a surface produced a relatively non-fouling surface. Addition of a hydrophilic layer coating to this surface should have decreased fouling. However, it was observed that this coating actually increased fouling. Using SFA it was shown that the reason for the increased adhesion of bacteria and particles to the hydrophilic layer was that the surface produced by this coating was highly heterogeneous, resulting in a small number of sites that created a stickier surface. These results show that while it is important to manufacture surfaces with coatings that are relatively non-adhesive to bacteria, it is also essential that these coatings have a highly uniform surface chemistry.     

Protein adsorption on the mesoporous molecular sieve silicate SBA-15: effects of pH and pore size

A mesoporous molecular sieve silicate, SBA-15, with three pore sizes (38.1 A, 77.3 A, and 240 A) has been synthesized using a non-ionic, tri-block copolymer as a template in a sol-gel method. The effects of synthesis conditions on the pore size and pore-size distribution of this adsorbent have been described. The adsorption of proteins on these crystalline, ordered, materials has been studied. The kinetics of adsorption and equilibrium capacity have been probed with three proteins of different dimensions. The effects of electrostatic interactions and protein size are illustrated. It has been shown that SBA-15 materials can be tailored to show size selectivity for proteins, and very high capacities (450 mg/g) can be obtained. Furthermore, the rates of adsorption are shown to be dependent on the pore size, protein structure and solution pH.     

Details of surface features in aromatic polyamide reverse osmosis membranes characterized by scanning electron and atomic force microscopy

In the present article, some new events on the surface morphology of the aromatic polyamide thin‐film‐composite (TFC) membranes were demonstrated in conjunction with their inherent chemical nature. In addition, the detailed, quantitative understanding of the microscopic surface features was shown to be essential in controlling the water permeability and eventually developing the high performance membranes. The surface roughness and the surface area were mainly affected by the existence or nonexistence of the crosslinking and/or the free amide groups not pertinent to the formation of the hydrogen bonding, which in turn contributed to the water permeability. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1429–1440, 1999     

Kinetics of latex formation of PBMA latex in the presence of alkali soluble resin using atomic force microscopy

 The effect of alkali-soluble resin (ASR), poly(ethylene-co-acrylic acid), EAA, postadded to emulsifier-free monodisperse poly(butyl methacrylate) (PBMA) latexes on the kinetics of film formation was investigated using atomic force microscopy (AFM). Corrugation height of latex particles in films was monitored at various annealing temperatures as a function of annealing time. Enhanced polymer diffusion was found in a latex film containing ASR regardless of anneal-ing temperature. With increasing annealing temperature, a much higher rate of polymer diffusion was found in latex films containing ASR. These results can be interpreted that the low molecular weight and low Tg EAA resin adsorbed at the particle surface is more susceptible to diffusion than that of the PBMA in the film formation stage, thus it enhances the mobility of PBMA polymer. Received: 30 October 1997 Accepted: 20 March 1998     

Spin trapping by 5,5-dimethylpyrroline-N-oxide in Fenton media in the presence of Nafion perfluorinated membranes: limitations and potential

Spin trapping by 5,5-dimethylpyrroline-N-oxide (DMPO) was used for the detection of radicals in Fenton media in the presence and absence of Nafion perfluorinated ionomers. For ethanol as solvent, the same types of spin adducts were detected in the presence or absence of Nafion. Solvent-derived adducts, DMPO/*OC2H5 and DMPO/*CH(OH)CH3, were identified, and their presence was rationalized by Fe(III)-catalyzed nucleophilic addition of ethanol to the spin trap and hydrogen abstraction by *OH radicals; oxygen radical adducts, DMPO/*O2(-) and DMPO/*OOH, were also detected. In Fenton media with methanol as solvent (and no Nafion), the DMPO/*O2(-) adduct dominated immediately after sample preparation, and a mixture consisting of DMPO/*OCH3, DMPO/*CH3, DMPO/*O2(-), and DMPO/*OOH adducts was detected after 30 min. In the presence of Nafion, only the adduct DMPO/*OH was detected. For water as solvent, only the DMPO/*OH adduct was detected, in both the absence and the presence of Nafion. The full hyperfine tensor components of this adduct were determined in Fenton media in the presence of Nafion with water and methanol as solvents. In Nafion/water exposed to the Fenton reagent at 358 K for 3 h, a DMPO adduct of a carbon-centered radical was also identified and assigned to a Nafion-derived fragment; its exact nature is under investigation. Variations of the 14N and Hbeta hyperfine splittings of a given adduct with the local polarity were key to the identification of some DMPO adducts, in particular DMPO/*O2(-). Both *OOH and O2*- adducts, with different 14N and Hbeta splittings, were detected simultaneously in some samples, for the first time in the spin trapping literature. Comparison with the results of a direct electron spin resonance study of Nafion exposed to the Fenton reagent indicated that spin trapping by DMPO can provide complementary information on the type of radicals present during Nafion degradation. The spin trapping approach described in this paper is limited, however, to systems that do not contain organic solvents.