Flow Function of Pharmaceutical Powders Is Predominantly Governed by Cohesion, Not by Friction Coefficients

A novel mixing rule model to predict the flowability of directly compressed pharmaceutical blends

In the pharmaceutical industry, powder flowability is an essential manufacturability attribute to consider when selecting the suitable manufacturing route and formulation. The selection of the formulation is usually based on the physical and chemical properties of the Active Pharmaceutical Ingredient (API) under consideration. Current industrial practice heavily relies on experimental work, which often results in significant labor and API consumption that results in higher costs. In this study we describe the development of a mixing rule to predict powder blend flowability from the flow properties of the individual components for industrial formulations manufactured via Direct Compression (DC). The mixing rule assumes that the granular solids' interactions are dominated by cohesive forces but are pragmatic to calibrate from the perspective of the typical data collated in an industrial environment. The proposed model was validated using a range of different APIs and the results show that the model can effectively predict the flowability properties of any formulation across the space of DC-relevant formulation compositions. Finally, a connection between the model and APIs properties (shape and size) was investigated via a linear correlation between the API particle properties and interparticle forces.

A concise summary of powder processing methodologies for flow enhancement

The knowledge of powder properties has been highlighted since the 19th century since most formulations focus on solid dosage forms, and powder flow is essential for various manufacturing operations. A poor powder flow may generate problems in the manufacturing processes and cause the plant's malfunction. Hence these problems should be studied and rectified beforehand by various powder flow techniques to improve and enhance powder flowability. The powder's physical properties can be determined using compendial and non-compendial methods. The non-compendial practices generally describe the powder response under the stress and shear experienced during their processing. The primary interest of the current report is to summarize the flow problems and enlist the techniques to eliminate the issues associated with the powder's flow properties, thereby increasing plant output and minimizing the production process inconvenience with excellent efficiency. In this review, we discuss powder flow and its measurement techniques and mainly focus on various approaches to improve the cohesive powder flow property.

Reviewing the Impact of Powder Cohesion on Continuous Direct Compression (CDC) Performance

The pharmaceutical industry is undergoing a paradigm shift towards continuous processing from batch, where continuous direct compression (CDC) is considered to offer the most straightforward implementation amongst powder processes due to the relatively low number of unit operations or handling steps. Due to the nature of continuous processing, the bulk properties of the formulation will require sufficient flowability and tabletability in order to be processed and transported effectively to and from each unit operation. Powder cohesion presents one of the greatest obstacles to the CDC process as it inhibits powder flow. As a result, there have been many studies investigating potential manners in which to overcome the effects of cohesion with, to date, little consideration of how these controls may affect downstream unit operations. The aim of this literature review is to explore and consolidate this literature, considering the impact of powder cohesion and cohesion control measures on the three-unit operations of the CDC process (feeding, mixing, and tabletting). This review will also cover the consequences of implementing such control measures whilst highlighting subject matter which could be of value for future research to better understand how to manage cohesive powders for CDC manufacture.

Starch flow behavior alone and under different glidants action using the shear cell method

The objective of this work was to analyze the flow behavior of a commonly used filler (pregelatinised starch) and the effect of two of the most used lubricants (talc and colloidal silicon dioxide). The studies were carried out according to the conventional methods (Angle of Repose, Bulk and Tapped densities and from these the Compressibility Index) and shear cell methods (Brookfield Powder Flow Tester apparatus) described in European Pharmacopeia (Ph. Eur.). The results showed some surprising and unexpected values for the flow behavior of this filler under influence of the methods and the used glidants. Regarding pure starch and mixtures containing talc, the flow behavior was similar between them and the Flow Index (ff_c) values varied between 1.8 and 4 (very cohesive and cohesive) as consolidation stress (σ₁) increased. In this case, the glidant effect was not observed. However, for the mixtures of starch with colloidal silicon dioxide this effect was observed providing Flow Index (ff_c) values between 2.6 and 8.9 (cohesive and easy-flowing) as consolidation stress (σ₁) increased. Other parameters that are also used to characterize flow properties, more specifically, within silos, chutes and hoppers, such as effective angle of internal friction (φe), effective angle of wall friction (φx), critical arching and critical rathole values, provided similar information. Based in the obtained results from all tests it can be said that the talc did not induce improvement on the starch flow behavior in the used conditions in opposition to the effect produced by colloidal silicon dioxide.HighlightsExample 1. A good flowability of powders is needed in order to be compressed/filled;Example 2. The overcome the poor flow it is usual to use glidants;Example 3. CSD improved the pregelatinised starch (Starch 1500®) flow;Example 4. Talc do not have relevant effect in the pregelatinised starch (Starch 1500®) flow;Example 5. Powder FlowTester method showed more complete and consistent results.

Simplifying Johanson's roller compaction model to build a "Virtual Roller Compactor" as a predictive tool - Theory and practical application

The purpose of the study is to build a "virtual roller compactor" as a predictive tool to assess the roll force (RF)-maximum pressure (P_max) and RF-ribbon density relationship for pharmaceutical roller compaction. We provided a theoretical basis to demonstrate that, there exists a critical nip angle for a pharmaceutical powder, beyond which the RF-P_max relationship is insensitive to wall friction angle or effective angle of internal friction. We showed that for most pharmaceutical roller compaction, the critical nip angle is lower than 17 degree, and can be exceeded via wall friction elevation, using rolls with non-smooth surface. Under this condition, the original Johanson model can be substantially simplified to a single equation requiring only one material property (compressibility). By performing manufacturing-scale roller compaction using materials with diverse compressibility, we showed that the simplified, friction angle-free model performed similar to the original Johanson model. It can predict the RF-P_max and RF-ribbon density relationship well after applying a correction factor. The predictive tool, in the form of a user-friendly graphical user interface, was created based on the simplified model. The tool was adopted for in-house, bench-scale formulation development and scale-up because of its ease-of-use, good predicting capability, and very low material demand.

QbD approach to downstream processing of spray-dried amorphous solid dispersions - a case study

In the current study, we demonstrate a structured approach to downstream process development for spray dried amorphous solid dispersions. Direct compression is generally not suitable due to typically poor flow of spray dried powders in tablets. Roller compaction (RC) is therefore the method of choice to enable spray dried dispersion downstream processing. Here, a structured experimental design of RC process parameters was used. The objective was to identify process conditions that lead to improved powder flow without compromising tablet robustness. Ten blends were compacted using different process parameters, and subsequently compressed into tablets. The impact of process parameters on granules and tablet properties was analyzed. We demonstrate that compaction force, gap and mesh aperture have major impact on RC outcomes. A combination of large gap and low force was identified as optimum combination of RC process parameters leading to powder flow improvement that could guarantee low tablet weight variation and at the same prevented loss of blend compressibility.

Flow Function of Pharmaceutical Powders at Low-Stress Conditions Can Be Inferred Using a Simple Flow-Through-Orifice Device

Multiple pharmaceutical powder processes operate at stresses lower than utilized in typical lab-scale shear cell testing. To bridge this gap, we developed a method to determine intrinsic powder flow properties, in particular, flow function (FFc), under such low stresses. A simple, commercially-available flow-through-orifice device (Flodex™ apparatus) was selected. By developing a theoretical framework using Jenike's radial stress field analysis, the major principal stress and FFc of the tested powder can be derived from the otherwise empirical "Flodex" experiment. This method was applied to 10 distinct pharmaceutical powders. The major principal stresses associated with the test were estimated to be in the order of 100 Pa, significantly lower than what is achievable using shear cell-based methods. The resulting FFc values are generally consistent with the data extrapolated from ring shear testing. We showed that for pharmaceutical powders, FFc decreases with decreasing consolidation stress, but the values are always greater than 1. Therefore, the threshold for poor/acceptable flowability (by FFc) should be used with caution at low-stress conditions. Through this work, we showed that by integrating the radial stress field theory with a simple flow-through-orifice experiment, intrinsic powder flow properties under low stresses could be reliably determined.

Lactose monohydrate flow characterization using shear cell method

The flow properties of pharmaceutical powders have a great importance in the manufacturing of solid dosage forms. In order to ensure the performance in the production line this parameter must be determined. There are several methods described in European Pharmacopeia that are used to measure these properties. Some of them were used in this study and the results obtained from conventional methods (Conv) and shear cell using the powder flow tester (PFT) showed differences that were more evident in fractions with smaller particle size (F < 63) and for bulk powder (F_Total). The various powder behaviors showed to be related with the size of the particles. An increase of the ff_c (Flow Index) was observed with the increase of the particle size. It was also found for the different fractions that the ff_c always increases with increasing major principal consolidation stress (σ₁). This study shown to be predictive because it also allowed the behavior profiles of other LactMN fractions to be known by interpolation of the median size (Dv50) or σ₁ values ranged between the studied intervals. Furthermore, it was also observed that ff_c of the F_Total was similar to the F < 63, showing the same behavior under σ₁. The occurrence of caking was not observed.

Metformin Hydrochloride and Sitagliptin Phosphate Fixed-Dose Combination Product Prepared Using Melt Granulation Continuous Processing Technology

The development of oral solid dosage forms, such as tablets that contain a high dose of drug(s), requires polymers and other additives to be incorporated at low levels as possible, to keep the final tablet weight low, and, correspondingly, the dosage form size small enough to be acceptable from a patient perspective. Additionally, a multi-step batch-based manufacturing process is usually required for production of solid dosage forms. This study presents the development and production, by twin-screw melt granulation technology, of a high-dose immediate-release fixed-dose combination (FDC) product of metformin hydrochloride (MET) and sitagliptin phosphate (SIT), with drug loads of 80% w/w and 6% w/w, respectively. For an 850/63 mg dose of MET/SIT, the final weight of the caplets was approximately 1063 mg compared with 1143 mg for the equivalent dose in Janumet®, the marketed product. Mixtures of the two drugs and polymers were melt-granulated at temperatures below the individual melting temperatures of MET and SIT (231.65 and 213.89°C, respectively) but above the glass transition temperature or melting temperature of the binder(s) used. By careful selection of binders, and processing conditions, direct compressed immediate-release caplets with desired product profiles were successfully produced. The melt granule formulations before compression showed good flow properties, were larger in particle size than individual starting API materials and were easily compressible. Melt granulation is a suitable platform for developing direct compressible high-dose immediate-release solid dosage forms of FDC products.

Flow characterization of a pharmaceutical excipient using the shear cell method

The powders used in the production of solid dosage forms must have ability to flow that allows their industrial processing. Although this property has been studied for most of the powders, in this study non-expected flow behaviors were observed for the model excipient used, Microcrystalline Cellulose (MCC). Several fractions with different sizes were fractioned by sieving of the model excipient and its flow behaviors were analyzed by different methods. The shear cell results showed an increase of the flowability index (ff_c) with the increase of the particle size and consolidation stress. Some related information has been referenced in the literature, however, in this work it was shown for different size fractions that the ff_c decreased above a certain consolidation stress value (2000-4000 N/m²). The explanation of this phenomenon is based on the increase of cohesion. Furthermore, it was also observed that the fractions with sizes between 125-180 µm present a ff_c higher than bulk powder (F_Total) with similar percentile (D_V50) indicating that this index is dependent on the size of the particles and also on its size distribution range. Thus, it can be affirmed that more homogeneous samples in size and with a narrower distribution present a better ff_c.

Inhaled mucoactive particles with tailored architecture for enhanced aerodynamicity, stability and efficacy

In respiratory and genetic disorders such as asthma, chronic obstructive pulmonary disease (COPD), chronic bronchitis and cystic fibrosis (CF), the lungs produce excess mucus, resulting in a thickened mass, which clogs up the airways and reduces airflow. Consequently, breathing becomes more difficult. Medications that break down the structure of mucus will be especially useful in managing the early symptoms of these diseases and preventing their progression into the more severe forms. This work therefore seeks to develop an inhaled mucoactive dry powder formulation that is efficacious on multiple fronts. As an innovative step, sodium chloride was used to tailor the surface architecture of ambroxol hydrochloride particles, such that the resulting angular features on the surfaces contributed to the creation of corrugated particles with enhanced aerodynamicity. The optimized spray-dried powder particles were of respirable-size (d₅₀ of 2.85 ± 0.15 μm) and moderately corrugated. When the crystalline powder was dispersed via an Aerolizer® inhaler at 60 L/min, it gave a fine particle fraction (FPF) of ~31%, which was a ten-fold improvement over the unmodified species (i.e. ambroxol hydrochloride alone). Tests on artificial sputum medium (ASM) showed that the optimized formulation was potentially useful in liquefying the mucus, which favorably pointed towards the effectiveness of the formulation. In addition, the formulation was also stable to moisture ingress (up to ~60% RH) and had good flowability. Hence, the advent of angular adjuvant sodium chloride particles in a mucoactive formulation conferred a three-fold benefit to the product: (1) Improved aerodynamicity and flowability, (2) Enhanced moisture stability and (3) Synergistic mucolytic properties.

Proportionality between powder cohesion and unconfined yield strength from shear cell testing

From an analysis of the geometry of the yield locus and the Mohr's circle for determining unconfined yield strength (f c) in shear cell testing, it has been shown that powder cohesion is proportional to f c, where the proportionality constant is a function of angle of linearized yield locus, (1-sinθ)/(2cosθ). While both parameters are routinely included in shear cell data, only one parameter is needed to characterize flow properties of a new powder.

On the mechanism of colloidal silica action to improve flow properties of pharmaceutical excipients

The mechanism of colloidal silica action to improve flow properties of pharmaceutical powders is known to be based on inter-particle force disruption by silica particles adhered to the particle surface. In the present article, the kinetic aspects of this action are investigated, focusing on non-spherical particles of different size. Blends comprising microcrystalline cellulose or calcium hydrogen phosphate dihydrate and colloidal silica were examined using powder rheometer. The blends were formulated to represent effects of particle size, surface texture, colloidal silica loading, and mixing time. Pre-conditioning, shear testing, compressibility, and flow energy measurements were used to monitor flow properties. Components and blends were analyzed using particle size analysis and scanning electron microscopy (SEM), using energy dispersive spectroscopy (EDS) and back-scattered electron (BSE) detection to determine surface particle arrangement. All studied parameters were found to have substantial effects on flow properties of powder blends. Those effects were explained by identifying key steps of colloidal silica action, which were found to proceed at substantially different rates, causing the flow properties change over time being dependent on the blend formulation and the component properties.

Characterization of mannitol granules and powder: A comparative study using two flowability testers

In the manufacture of tablets, especially in direct tableting processes, the flowability of excipient powders and formulated powders is one of the most important characteristics. In the past two decades, orally disintegrating tablets (ODTs) have been prepared as popular solid dosage forms for elderly patients. Many types of mannitol granules have been developed and marketed as new pharmaceutical excipients for ODTs, owing to the solubility and palatability of mannitol. Characterizing the flow behaviors of these mannitol granules is essential to their use. The flowability of mannitol excipients was the focus of the present study. A fine crystalline mannitol powder, eight commercial types of mannitol granules and four types of mannitol mixture granules were evaluated. Two flowability testers were used for comparing and analyzing the samples' flowabilities. A variety of methodologies were used: an assessment using Carr's index, a shear test and a dynamic flow test. Mannitol powder showed the lowest Carr's index, meaning the lowest flowability. Spherical mannitol granules showed the lowest angle of internal friction in the shear test and extremely low basic flow energy in the dynamic flow test. Larger granules showed relatively high values for Carr's index, but also a relatively high total flow energy.

Impact of Shear History on Powder Flow Characterization Using a Ring Shear Tester

In this study, we have investigated the impact of repeated shear displacement on powder flow properties. We show that when multiple yield loci are obtained using the same bulk solid specimen by stepping through different stress levels (i.e., stress walk [SW]), the shear deformation of the powder in a rotational shear cell, that is, Schulze Ring Shear Tester, is maximized, reducing the powder shear strength. This approach is material and time sparing; however, it imprecisely predicts better powder flowability. The magnitude of the change in the unconfined yield strength, σ_c, due to this prolonged shear displacement appears to be material-dependent, being less impactful for free-flowing powders. Using the SW and the individual yield loci-generated flow properties, we have demonstrated that in hopper design, the shear displacement effect impacts the computed critical arching diameter more than the critical mass flow angle. This knowledge of powder flow properties highlights limitations associated with the SW. An exponential function was found to describe the relationship between the change in σ_c at the highest major principal stress and the density weighted flowability, ff_ρ, with an R² of 0.98. Such a model could be a valuable tool for correcting shear strength results obtained from SW, saving time, and material.

Influence of Material Properties on the Effectiveness of Glidants Used to Improve the Flowability of Cohesive Pharmaceutical Powders

This study investigated the effect of material properties, primarily particle size and surface energy, on the effectiveness of glidants used for the purpose of flowability enhancement. Three pharmaceutical grade glidants (Aerosil 200, Aerosil R972, and Cab-O-Sil M5P) were evaluated and blended with various pharmaceutical actives as well as cohesive excipients common to capsule and tablet formulation. Flowability enhancement was characterized by the flow function coefficient (ff _c ). An industry-relevant mixer (Turbula mixer) and a highly efficient and effective mixer (LabRAM vibratory mixer) were used to further understand the effect of material properties on glidant effectiveness. While concepts of inter-particle cohesion and interaction strength were applied to evaluate their usefulness in understanding and predicting flowability enhancement, theoretical expectations did not fully explain the behavior of all three glidants. However, the study suggests that the low surface energy and optimal particle size of Aerosil R972 relative to the other glidants results in lower inter-particle force and consequently better flowability. Aerosil R972 was also shown to be more effectively utilized in the Turbula mixing process particularly for larger (d₅₀ > 40 μm) and less cohesive (ff _c  > 3) materials. This may be due to its lower surface energy and hydrophobic surface which allows it to disperse easily. Overall, this study provides useful insight into the material properties which influence the effectiveness of glidants used in formulation development.