Use of a capillary rheometer to evaluate the rheological properties of microcrystalline cellulose and silicified microcrystalline cellulose wet masses

Fibrillated Cellulose via High Pressure Homogenization: Analysis and Application for Orodispersible Films

Powdered cellulose (PC) and microcrystalline cellulose (MCC) are common excipients in pharmaceuticals. Recent investigations imply that particle size is the most critical parameter for the different performance in many processes. High-pressure homogenization (HPH) was used to reduce fiber size of both grades. The effect of the homogenization parameters on suspension viscosity, particle size, and mechanical properties of casted films was investigated. PC suspensions showed higher apparent viscosities and yield stresses under the same process conditions than MCC. SLS reduced shear viscosity and thixotropic behavior of both cellulose grades probably due to increased electrostatic repulsion. Homogenization reduced cellulose particle sizes, but re-agglomeration was too strong to analyze the particle size correctly. MCC films showed a tensile strength of up to 16.0 MPa and PC films up to 4.1 MPa. PC films disintegrated within 30 s whereas MCC films did not. Mixtures of MCC and PC led to more stable films than PC alone, but these films did not disintegrate anymore. Diclofenac sodium was incorporated in therapeutic dose with drug load of 47% into orodispersible PC films. The content uniformity of these films fulfilled requirements of Ph.Eur and the films disintegrated in 12 s. In summary, PC and MCC showed comparable results after HPH and most differences could be explained by the smaller particle size of MCC suspensions. These results confirm the hypothesis that mainly the fiber size during processing is responsible for the existing differences of MCC and PC in pharmaceutical process, e.g., wet-extrusion/spheronization.

The rheology of batch and continuously prepared gluten-free bread dough in oscillatory and capillary shear flow

Reduced elasticity and high stickiness of gluten-free bread doughs are major issues regarding the industrial breadmaking process. In this work, we compared traditional batch mixing with a revised continuous extrusion process and extensively study the rheological properties of both doughs. Shear viscosities were measured offline with a capillary rheometer and inline at the extruder die over a large range of apparent shear rates. Data were corrected for entrance effects, wall slip and non-Newtonian flow behaviour. Good agreement between inline and offline measured viscosities were supplemented by amplitude and frequency sweep tests. The results highlight that this extrusion process fostered the production of gluten-free bread dough. We demonstrated that extrusion processing support the combined mixing, kneading, and moulding of gluten-free dough in one single unit. This fundamental study linked physical dough characterization with applied engineering and yielded the understanding and processing of corresponding products.

Rheology of Lignocellulose Suspensions and Impact of Hydrolysis: A Review

White biotechnologies have several challenges to overcome in order to become a viable industrial process. Achieving highly concentrated lignocellulose materials and releasing fermentable substrates, with controlled kinetics in order to regulate micro-organism activity, present major technical and scientific bottlenecks. The degradation of the main polymeric fractions of lignocellulose into simpler molecules is a prerequisite for an integrated utilisation of this resource in a biorefinery concept. The characterisation methods and the observations developed for rheology, morphology, etc., that are reviewed here are strongly dependent on the fibrous nature of lignocellulose, are thus similar or constitute a good approach to filamentous culture broths. This review focuses on scientific works related to the study of the rheological behaviour of lignocellulose suspensions and their evolution during biocatalysis. In order to produce the targeted molecules (synthon), the lignocellulose substrates are converted by enzymatic degradation and are then metabolised by micro-organisms. The dynamics of the mechanisms is limited by coupled phenomena between flow, heat and mass transfers in regard to diffusion (within solid and liquid phases), convection (mixing, transfer coefficients, homogeneity) and specific inhibitors (concentration gradients). As lignocellulose suspensions consist of long entangled fibres for the matrix of industrial interest, they exhibit diverse and complex properties linked to this fibrous character (rheological, morphological, thermal, mechanical and biochemical parameters). Among the main variables to be studied, the rheological behaviour of such suspensions appears to be determinant for process efficiency. It is this behaviour that will determine the equipment to be used and the strategies applied (substrate and biocatalysis feed, mixing, etc.). This review provides an overview of (i) the rheological behaviour of fibrous materials in suspension, (ii) the methods and experimental conditions for their measurements, (iii) the main models used and (iv) their evolution during biocatalytic reactions with a focus on enzymatic hydrolysis.

Model-based analysis of high shear wet granulation from batch to continuous processes in pharmaceutical production--a critical review

The manufacturing of pharmaceutical dosage forms, which has traditionally been a batch-wise process, is now also transformed into a series of continuous operations. Some operations such as tabletting and milling are already performed in continuous mode, while the adaptation towards a complete continuous production line is still hampered by complex steps such as granulation and drying which are considered to be too inflexible to handle potential product change-overs. Granulation is necessary in order to achieve good flowability properties and better control of drug content uniformity. This paper reviews modelling and supporting measurement tools for the high shear wet granulation (HSWG) process, which is an important granulation technique due to the inherent benefits and the suitability of this unit operation for the desired switch to continuous mode. For gaining improved insight into the complete system, particle-level mechanisms are required to be better understood, and linked with an appropriate meso- or macro-scale model. A brief review has been provided to understand the mechanisms of the granulation process at micro- or particle-level such as those involving wetting and nucleation, aggregation, breakage and consolidation. Further, population balance modelling (PBM) and the discrete element method (DEM), which are the current state-of-the-art methods for granulation modelling at micro- to meso-scale, are discussed. The DEM approach has a major role to play in future research as it bridges the gap between micro- and meso-scales. Furthermore, interesting developments in the measurement technologies are discussed with a focus towards inline measurements of the granulation process to obtain experimental data which are required for developing good models. Based on the current state of the developments, the review focuses on the twin-screw granulator as a device for continuous HSWG and attempts to critically evaluate the current process. As a result, a set of open research questions are identified. These questions need to be answered in the future in order to fill the knowledge gap that currently exists both at micro- and macro-scale, and which is currently limiting the further development of the process to its full potential in pharmaceutical applications.

In situ rheometry of concentrated cellulose fibre suspensions and relationships with enzymatic hydrolysis

This work combines physical and biochemical analyses to scrutinize liquefaction and saccharification of complex lignocellulose materials. A multilevel analysis (macroscopic: rheology, microscopic: particle size and morphology and molecular: sugar product) was conducted at the lab-scale with three matrices: microcrystalline cellulose (MCC), Whatman paper (WP) and extruded paper-pulp (PP). A methodology to determine on-line viscosity is proposed and validated using the concept of Metzner and Otto (1957) and Rieger and Novak's (1973). The substrate suspensions exhibited a shear-thinning behaviour with respect to the power law. A structured rheological model was established to account for the suspension viscosity as a function of shear rate and substrate concentration. The critical volume fractions indicate the transition between diluted, semi-diluted and concentrated regimes. The enzymatic hydrolysis was performed with various solid contents: MCC 273.6 gdm/L, WP 56.0 gdm/L, PP 35.1 gdm/L. During hydrolysis, the suspension viscosity decreased rapidly. The fibre diameter decreased two fold within 2 h of starting hydrolysis whereas limited bioconversion was obtained (10-15%).

The rheological properties of modified microcrystalline cellulose containing high levels of model drugs

The rheological properties of different types of microcrystalline cellulose (MCC) mixed with model drugs and water have been evaluated to identify the influence of sodium carboxymethylcellulose (SCMC) added to the cellulose during preparation. A ram extruder was used as a capillary rheometer. The mixtures consisted of 20% spheronizing agent (standard grade MCC or modified types with 6% or 8% of low viscosity grade SCMC) and 80% of ascorbic acid, ibuprofen or lactose monohydrate. The introduction of SCMC changed all rheological parameters assessed. It produced more rigid systems, requiring more stress to induce and maintain flow. Degree of non-Newtonian flow, angle of convergence, extensional viscosity, yield and die land shear stress at zero velocity, and static wall friction were increased, but recoverable shear and compliance were decreased. The presence of SCMC did not remove the influence of the type of drug. The mixture of ibuprofen and standard MCC had the lowest values for shear stress as a function of the rate of shear, extensional viscosity, and angle of convergence, but the highest values for recoverable shear and compliance. The findings indicate that the system has insufficient rigidity to form pellets.

The Evaluation of Formulations for the Preparation of Pellets with High Drug Loading by Extrusion/Spheronization

A capillary rheometer was used to evaluate rheological properties and the fluid mobility of mixtures with a high drug loading (80%) of three model drugs (ibuprofen, lactose, and ascorbic acid) when extruded. These drugs have a range of solubility in water, with 20% microcrystalline cellulose (MCC) as the spheronization aid, and water, pH 2.0, and pH 10.0 buffer as the binder liquid. The results were compared with the ability of the systems to form spherical pellets by the process of extrusion/spheronization. It was found possible to produce round pellets with a narrow size distribution by the process of extrusion/spheronization for formulations containing 80% of either lactose or ascorbic acid with MCC as the spheronization aid. It was not, however, possible to form pellets containing the same level of ibuprofen. This appears to be associated with the high level of fluid mobility observed when the wet masses were extruded in a ram extruder. A range of rheological characteristics in terms of shear stress, die entry pressure, angles of convergence, extensional flow, and elasticity were determined, but the variations in the values of these, which were observed, did not give an indication of the ability of the wet mass to form spherical pellets when subjected to the spheronization process. This could be associated with the fact that the selection of the conditions necessary to provide a valid quantification of the extrusion process did not truly represent the stability of the systems in terms of the mobility of the fluid when the wet mass was processed. The formulation of a wet mass with limited fluid mobility appears to be the first priority of formulations used in extrusion/spheronization.

The evaluation of the rheological properties of lactose/microcrystalline cellulose and water mixtures by controlled stress rheometry and the relationship to the production of spherical pellets by extrusion/spheronization

The consistency of wet powder masses produced from two ratios (7:3 and 8:2) of alpha-lactose monohydrate (L) and microcrystalline cellulose (MCC) mixed with a range of water contents has been assessed with a parallel plate controlled stress rheometer. The range of water contents, which could be studied, was restricted to those, which could be extruded uniformly by a ram extruder. In the creep mode, the instantaneous compliance increased as the water content increased for both L:MCC ratios illustrating the increasing deformability of the mixtures with increasing water content. The derived apparent viscosity of the mixtures as a function of shear rate, increased as the water content decreased and the values for all the systems fell on a common line. This indicates that the measurements are providing a reliable assessment of the mixtures and that the change in water content and L:MCC ratio provides systems, whose change of viscosity with rate of shear is consistent at low rates of shear. The values of the storage and loss moduli obtained from oscillatory measurements, increased with a decrease in water content but this time the two ratios of L:MCC were not on a common line when related to the water content of the mixtures. There was a range of water levels over which both the values of the storage and loss moduli were approximately constant. This corresponded to the level of water, which produced the pellets of the smallest diameter and range of diameters and were of the most spherical shape when produced by a ram extruder and spheronization. For 8:2 L:MCC ratio, there appeared to be a value for both the storage and the loss moduli above which the wet mass could not produce good pellets. For the 7:3 L:MCC these limiting levels were not achieved before extrusion with steady state conditions could be maintained without the mass being too wet or too dry. Instead, there appeared to be minimum levels of the moduli required to ensure that the mixtures were able to produce good pellets.

Preparation of extruded carbamazepine and PEG 4000 as a potential rapid release dosage form

The aim of this research was to use a ram extruder to prepare directly a fast release dosage form with uniform shape and density, containing carbamazepine (C) as a water-insoluble drug and polyethylene glycol 4000 (PEG) as a low melting binder. The potential inclusion of lactose (L) as a hydrophilic filler was also considered. The temperature suitable to ensure a successful extrusion process of several formulations containing PEG in different percentages was found to be below the melting point of the PEG. The influence of composition on the extrusion process of different ram speeds was checked by measuring the pressure at the steady state, the apparent shear rate and the apparent shear stress of a range of mixtures of drug, lactose and PEG. The physical-mechanical properties of extrudates, including tensile strength and Young's modulus, prepared with different ram velocities were also determined. The solid-state physical structure by differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD) was established. The dissolution of the extrudates and their corresponding physical mixtures were compared. The mixtures were found to be shear thinning when extruded; the tensile strength of extrudates was dependent on the composition but not the extrusion rate, while the value of Young's modulus was strongly influenced by the rate of extrusion, but less affected by the composition of the extrudates. The results of DSC and XRD indicated that the solid structure of the extrudates corresponded to that of a physical mixture of the components, hence there had been no change in the physical form of the drug induced by extrusion. In terms of dissolution, the rate of the extrusion process did not influence the performance of the products, whereas the composition did. The extruded mixtures of an equivalent composition exhibited a more rapid release than a simple physical mixture. The addition of lactose reduced the dissolution rate.