Jerry Y. Y. Heng

The use of murine embryonic stem cells, alginate encapsulation, and rotary microgravity bioreactor in bone tissue engineering

The application of embryonic stem cells (ESCs) in bone tissue engineering and regenerative medicine requires the development of suitable bioprocesses that facilitate the integrated, reproducible, automatable production of clinically-relevant, scaleable, and integrated bioprocesses that generate sufficient cell numbers resulting in the formation of three-dimensional (3D) mineralised, bone tissue-like constructs. Previously, we have reported the enhanced differentiation of undifferentiated mESCs toward the osteogenic lineage in the absence of embryoid body formation. Herein, we present an efficient and integrated 3D bioprocess based on the encapsulation of undifferentiated mESCs within alginate hydrogels and culture in a rotary cell culture microgravity bioreactor. Specifically, for the first 3 days, encapsulated mESCs were cultured in 50% (v/v) HepG2 conditioned medium to generate a cell population with enhanced mesodermal differentiation capability followed by osteogenic differentiation using osteogenic media containing ascorbic acid, beta-glycerophosphate and dexamethasone. 3D mineralised constructs were generated that displayed the morphological, phenotypical, and molecular attributes of the osteogenic lineage, as well mechanical strength and mineralised calcium/phosphate deposition. Consequently, this bioprocess provides an efficient, automatable, scalable and functional culture system for application to bone tissue engineering in the context of macroscopic bone formation.

Surface Modification of Natural Fibers Using Bacteria: Depositing Bacterial Cellulose onto Natural Fibers To Create Hierarchical Fiber Reinforced Nanocomposites

Triggered biodegradable composites made entirely from renewable resources are urgently sought after to improve material recyclability or be able to divert materials from waste streams. Many biobased polymers and natural fibers usually display poor interfacial adhesion when combined in a composite material. Here we propose a way to modify the surfaces of natural fibers by utilizing bacteria ( Acetobacter xylinum) to deposit nanosized bacterial cellulose around natural fibers, which enhances their adhesion to renewable polymers. This paper describes the process of modifying large quantities of natural fibers with bacterial cellulose through their use as substrates for bacteria during fermentation. The modified fibers were characterized by scanning electron microscopy, single fiber tensile tests, X-ray photoelectron spectroscopy, and inverse gas chromatography to determine their surface and mechanical properties. The practical adhesion between the modified fibers and the renewable polymers cellulose acetate butyrate and poly(L-lactic acid) was quantified using the single fiber pullout test.

Inverse Gas Chromatographic Method for Measuring the Dispersive Surface Energy Distribution for Particulates

Inverse gas chromatography (IGC) is a widely used method for determining the dispersive component of the surface energy (gamma s (d)) of particulate and fibrous solids. Such measurements are normally conducted at very low solute concentrations (infinite dilution), and they result in a single numerical value of gamma s (d) for homogeneous materials which exhibit Henrys Law adsorption behavior. However, many real solid surfaces are heterogeneous and this may be demonstrated by the nonlinear isotherms obtained at low solute surface coverages resulting in reported gamma s (d) values which are not unique. This paper presents a new method for determining of gamma s (d) distributions as a function of the solute surface coverage using adsorption isosteres for an homologous series of hydrocarbon adsorbates. gamma s (d) distributions reported here were successfully determined using two different solid materials (glass beads and alumina particles) up to typical surface coverages of approximately 10% and clearly show significant variations in gamma s (d) with solute surface coverage. The effects of sample aging and pretreatment also exhibited clear differences in the gamma s (d) distributions obtained. gamma s (d) was determined using both the Dorris-Gray and Schultz methods, with the Dorris-Gray method exhibiting a much lower experimental error. It was established that the errors associated with this statistical measurement of surface energy were strongly dependent on the quality of the experimental data sets obtained. R (2) for the linearity of fit of the retention data to the Dorris-Gray gamma s (d) analysis was found to be a valid criterion for predicting the robustness of gamma s (d) distributions obtained. Detailed discussions of other critical experimental and analysis factors relevant to this methodology, as well as the reproducibility of gamma s (d) profiles are also presented. This paper establishes that IGC can be used for determining the gamma s (d) distributions of particulate solids and is demonstrated that this method is very useful way for studying the surface energy heterogeneity of complex particulate solids.

Determination of the Surface Energy Distributions of Different Processed Lactose

Particulate interactions between drug and lactose carrier in dry powder inhaler formulations are affected by the heterogenous energy distribution on the surface of the individual compounds. A new method based on Inverse Gas Chromatography at finite concentration is applied to study the energy heterogeneity of untreated, milled, and recrystallized lactose of similar particle size distribution. Energy distributions for the dispersive surface energy and the specific free energy of ethanol are obtained. Milling causes an increase in surface energy due to formation of amorphous regions. Untreated and recrystallized materials have similar surface energies at low surface coverages but show clear differences in energy distribution.

Effect of Milling on Particle Shape and Surface Energy Heterogeneity of Needle-Shaped Crystals

PurposeMilling and micronization of particles are routinely employed in the pharmaceutical industry to obtain small particles with desired particle size characteristics. The aim of this study is to demonstrate that particle shape is an important factor affecting the fracture mechanism in milling.MethodsNeedle-shaped crystals of the β polymorph of D-mannitol were prepared from recrystallization in water. A portion of the recrystallized materials was ball-milled. Unmilled and milled sieved fractions of recrystallized D-mannitol were analyzed by dynamic image analysis (DIA) and inverse gas chromatography (IGC) at finite concentration to explain the breakage/fracture behavior.ResultsIn the process of ball-milling, D-mannitol preferentially fractured along their shortest axis, exposing (011) plane with increased hydrophilicity and increased bounding rectangular aspect ratio. This is in contrary to attachment energy modeling which predicts a fracture mechanism across the (010) plane with increased hydrophobicity, and small change in particle shape.ConclusionsCrystal size, and more importantly, crystal shape and facet-specific mechanical properties, can dictate the fracture/cleavage behavior of organic crystalline materials. Thorough understanding of the crystal slip systems, combining attachment energy prediction with particle shape and surface characterization using DIA and IGC, are important in understanding fracture behavior of organic crystalline solids in milling and micronization.

Determination of surface heterogeneity of d-mannitol by sessile drop contact angle and finite concentration inverse gas chromatography

The sensitivity of two techniques in tracking changes in surface energetics was investigated for a crystalline excipient, D-mannitol. Macroscopic crystals of D-mannitol were grown from saturated water solution by slow cooling, and sessile drop contact angle was employed to measure the anisotropic surface energy. The facet-specific surface energy was consistent with localised hydroxyl group concentrations determined by X-ray photoelectron spectroscopy (XPS), and was also in excellent agreement with the surface energy distribution of the powder form of mannitol measured via a new methodology using inverse gas chromatography (IGC) at finite concentrations. The gamma(SV)(d) was found to vary between 39.5 mJ/m(2) and 44.1 mJ/m(2) for contact angle and between 40 mJ/m(2) and 49 mJ/m(2) for IGC measurements. We report here, a high level of surface heterogeneity on the native mannitol crystal surfaces. When the surfaces of both D-mannitol samples (powder and large single crystals) were modified by dichlorodimethylsilane to induce surface hydrophobicity, both IGC and contact angle revealed a homogeneous surface due to functionalisation of mannitol crystal surface with methyl groups resulting in gamma(SV)(d) of approximately 34 mJ/m(2). It was shown that both IGC and contact angle techniques are able to detect surface chemical variations and detailed surface energetic distribution.

Methods to determine surface energies of natural fibres: a review

To tailor the interaction across composite interfaces especially for the development of green composites, i.e. composites made completely from renewable materials, information about the fibre surfaces is required. We review the current state of the art of methods to determine the surface tension of natural fibres and discuss the advantages and disadvantages of techniques used. Although numerous techniques have been employed to characterise surface tension of natural fibres, it seems that commonly used wetting techniques are very much more affected by the non-ideal character of natural fibres. Inverse Gas Chromatography (IGC) is a much better suited technique to determine the surface energetic properties of natural fibres than wetting techniques. The surface tension of natural reinforcements, determined using IGC, was reported for nanosized bacterial cellulose as well as bamboo, cornhusk, flax, hemp and sisal, covering a wide range of cellulose content. The effect of methods to separate/extract fibres from the plants as well as of a few surface modification procedures on the fibre surface properties is also reviewed. The dispersive part of the natural fibre surface tension γ d S varies from 32 to 61 mJ/m2. The fibre surface tension increases with increasing cellulose content of natural fibres. We also found that a higher basicity (Donor Number, K B to Acceptor Number, K Aratio) was observed for fibres containing more cellulose. This may be reflective of higher crystalline cellulose content in the surfaces of the fibres, as only the ether linkage of the cellulose is labile for hydrogen bonding.

Phase behavior of medium and high internal phase water-in-oil emulsions stabilized solely by hydrophobized bacterial cellulose nanofibrils.

Water-in-oil emulsions stabilized solely by bacterial cellulose nanofibers (BCNs), which were hydrophobized by esterification with organic acids of various chain lengths (acetic acid, C2-; hexanoic acid, C6-; dodecanoic acid, C12-), were produced and characterized. When using freeze-dried C6-BCN and C12-BCN, only a maximum water volume fraction (ϕw) of 60% could be stabilized, while no emulsion was obtained for C2-BCN. However, the maximum ϕw increased to 71%, 81%, and 77% for C2-BCN, C6-BCN, and C12-BCN, respectively, 150 h after the initial emulsification, thereby creating high internal phase water-in-toluene emulsions. The observed time-dependent behavior of these emulsions is consistent with the disentanglement and dispersion of freeze-dried modified BCN bundles into individual nanofibers with time. These emulsions exhibited catastrophic phase separation when ϕw was increased, as opposed to catastrophic phase inversion observed for other Pickering emulsions.

Anisotropic surface chemistry of crystalline pharmaceutical solids.

The purpose of this study was to establish the link between the wetting behavior of crystalline pharmaceutical solids and the localized surface chemistry. A range of conventional wetting techniques were evaluated and compared with a novel experimental approach: sessile drop contact angle measurements on the individual facets of macroscopic (>1 cm) single crystals. Conventional measurement techniques for determining surface energetics such as capillary rise and sessile drops on powder compacts were found not to provide reliable results. When the macroscopic crystal approach was used, major differences for advancing contact angles, θa, of 0 waterwere observed—as low as 16° on facet (001) and as high as 68° on facet (010) of form I paracetamol. θa trends were in excellent agreement with X-ray photoelectron spectroscopy surface composition and known crystallographic structures, suggesting a direct relationship to the local surface chemistry. Inverse gas chromatography (IGC) was further used to probe the surface properties of milled and unmilled samples, as a function of particle size. IGC experiments confirmed that milling exposes the weakest attachment energy facet, with increasing dominance as particle size is reduced. The weakest attachment energy facet was also found to exhibit the highest θa for water and to be the 0 most hydrophobic facet. This anisotropic wetting behavior was established for a range of crystalline systems: paracetamol polymorphs, aspirin, and ibuprofen racemates. θa was 0 found to be very sensitive to the local surface chemistry. It is proposed that the hydrophilicity/hydrophobicity of facets reflects the presence of functional groups at surfaces to form hydrogen bonds with external molecules.

Investigation of drug-polymer interaction in solid dispersions by vapour sorption methods.

The objective of this study was to investigate the effect of different polymeric carriers in solid dispersions with an active pharmaceutical ingredient (API) on their water vapour sorption equilibria and the influence of the API-polymer interactions on the dissolution rate of the API. X-ray diffraction, scanning electron microscopy (SEM), moisture sorption analysis, infrared (IR) spectroscopy and dissolution tests were performed on various API-polymer systems (Valsartan as API with Soluplus, PVP and Eudragit polymers) after production of amorphous solid dispersions by spray drying. The interactions between the API and polymer molecules caused the water sorption isotherms of solid dispersions to deviate from those of ideal mixtures. The moisture sorption isotherms were lower in comparison with the isotherms of physical mixtures in all combinations with Soluplus and PVP. In contrast, the moisture sorption isotherms of solid dispersions containing Eudragit were significantly higher than the corresponding physical mixtures. The nature of the API-polymer interaction was explained by shifts in the characteristic bands of the IR spectra of the solid dispersions compared to the pure components. A correlation between the dissolution rate and the water sorption properties of the API-polymer systems has been established.