Quantification of Tribocharging of Pharmaceutical Powders in V-Blenders: Experiments, Multiscale Modeling, and Simulations

Mitigation of Tribocharging in Pharmaceutical Powders using Surface Modified V-Blenders

INTRODUCTION

The pharmaceutical industry involves handling of powders on a large scale for manufacturing of solid dosage forms such as tablets and capsules constituting about 85% of the dosage forms. During this manufacturing process, powders get electrostatically charged due to numerous particle-particle and particle-equipment wall collisions. Most of the pharmaceutical powders are insulators in nature and they accumulate charge for longer durations making it difficult to dissipate the generated charge. In this study, a surface modified blender has been used to analyze tribocharging in pharmaceutical powders.

METHODS

The surface modified blender has been fabricated using two types of materials, an insulator, and a conductor. The conductor or the metal arm induces charge of opposite polarity to that of the charge induced by the insulator arm and the overall charge on the powder decreases during the tumbling motion of the blender. Ibuprofen was used as the model drug and processed in aluminum, polyvinyl chloride (PVC), stainless steel, surface modified aluminum-PVC (Al-PVC) and surface modified stainless steel- PVC (SS-PVC) blender at 20% RH for different blending times such as 2, 10, 20, 30 and 40 min. To better understand the tribocharging phenomenon in surface modified V blenders, an experimentally validated computational model was developed using Discrete Element Method (DEM) modeling.

RESULTS

Significant reduction (> 50%) in electrostatic charge was observed for Ibuprofen using surface modified blenders in comparison to metal only and insulator only V blenders. Additionally, an identical charging trend was observed between the simulation and experimental data.  CONCLUSION: It was established that careful selection of equipment materials could significantly reduce the electrostatic charging of pharmaceutical powders and DEM model could be a really useful tool in assessing the applicability of the modified V blenders.

Prediction of the Effective Work Function of Aspirin and Paracetamol Crystals by Density Functional Theory-A First-Principles Study

Crystals of active pharmaceutical ingredients (API) are prone to triboelectric charging due to their dielectric nature. This characteristic, coupled with their typically low density and often large aspect ratio, poses significant challenges in the manufacturing process. The pharmaceutical industry frequently encounters issues during the secondary processing of APIs, such as particle adhesion to walls, clump formation, unreliable flow, and the need for careful handling to mitigate the risk of fire and explosions. These challenges are further intensified by the limited availability of powder quantities for testing, particularly in the early stages of drug development. Therefore, it is highly desirable to develop predictive tools that can assess the triboelectric propensity of APIs. In this study, Density Functional Theory calculations are employed to predict the effective work function of different facets of aspirin and paracetamol crystals, both in a vacuum and in the presence of water molecules on their surfaces. The calculations reveal significant variations in the work function across different facets and materials. Moreover, the adsorption of water molecules induces a shift in the work function. These findings underscore the considerable impact of distinct surface terminations and the presence of molecular water on the calculated effective work function of pharmaceuticals. Consequently, this approach offers a valuable predictive tool for determining the triboelectric propensity of APIs.

Discrete Element Method Evaluation of Triboelectric Charging Due to Powder Handling in the Capsule of a DPI

The generation and accumulation of an electrostatic charge from handling pharmaceutical powders is a well-known phenomenon, given the insulating nature of most APIs (Active Pharmaceutical Ingredients) and excipients. In capsule-based DPIs (Dry Powder Inhalers), the formulation is stored in a gelatine capsule placed in the inhaler just before inhalation. The action of capsule filling, as well as tumbling or vibration effects during the capsule life cycle, implies a consistent amount of particle-particle and particle-wall contacts. A significant contact-induced electrostatic charging can then take place, potentially affecting the inhaler's efficiency. DEM (Discrete Element Method) simulations were performed on a carrier-based DPI formulation (salbutamol-lactose) to evaluate such effects. After performing a comparison with the experimental data on a carrier-only system under similar conditions, a detailed analysis was conducted on two carrier-API configurations with different API loadings per carrier particle. The charge acquired by the two solid phases was tracked in both the initial particle settling and the capsule shaking process. Alternating positive-negative charging was observed. Particle charging was then investigated in relation to the collision statistics, tracking the particle-particle and particle-wall events for the carrier and API. Finally, an analysis of the relative importance of electrostatic, cohesive/adhesive, and inertial forces allowed the importance of each term in determining the trajectory of the powder particles to be estimated.

Band Bending and Ratcheting Explain Triboelectricity in a Flexoelectric Contact Diode

Triboelectricity was recognized millennia ago, but the fundamental mechanism of charge transfer is still not understood. We have recently proposed a model where flexoelectric band bending due to local asperity contacts drives triboelectric charge transfer in non-metals. While this ab initio model is consistent with a wide range of observed phenomena, to date there have been no quantitative analyses of the proposed band bending. In this work we use a Pt_0.8Ir_0.2 conductive atomic force microscope probe to simultaneously deform a Nb-doped SrTiO₃ sample and collect current-bias data. The current that one expects based upon an analysis including the relevant flexoelectric band bending for a deformed semiconductor quantitively agrees with the experiments. The analysis indicates a general ratcheting mechanism for triboelectric transfer and strong experimental evidence that flexoelectric band bending is of fundamental importance for triboelectric contacts.

Emerging Landscape of Computational Modeling in Pharmaceutical Development

Computational chemistry applications have become an integral part of the drug discovery workflow over the past 35 years. However, computational modeling in support of drug development has remained a relatively uncharted territory for a significant part of both academic and industrial communities. This review considers the computational modeling workflows for three key components of drug preclinical and clinical development, namely, process chemistry, analytical research and development, as well as drug product and formulation development. An overview of the computational support for each step of the respective workflows is presented. Additionally, in context of solid form design, special consideration is given to modern physics-based virtual screening methods. This covers rational approaches to polymorph, coformer, counterion, and solvent virtual screening in support of solid form selection and design.

Loss-in-weight feeding, powder flow and electrostatic evaluation for direct compression hydroxypropyl methylcellulose (HPMC) to support continuous manufacturing

Minimizing variability in the feeding process is important for continuous manufacturing since materials are fed individually and can impact the final product. This study demonstrates the importance of measuring powder properties and highlights the need to characterize the feeding performance both offline with multiple refills and in the intended configuration for the continuous manufacturing equipment. The standard grade hydroxypropyl methylcellulose (HPMC) had material buildup on the loss-in-weight feeder barrel from triboelectric charging and resulted in more mass flow excursions and failed refills which were not observed with the direct compression grades. The location of the electrostatic buildup changed when the feeder was connected to a hopper instead of feeding offline into a collection bucket. Overall, the direct compression HPMC exhibited better flow which resulted in more accurate loss-in-weight feeding with less excursions from the target mass flow and all refills were completed in the first attempt. The improvements with the direct compression HPMC would be beneficial when running any continuous process (wet granulation, roller compaction, or direct compression) or other processes where loss-in-weight feeding is utilized, such as melt extrusion or twin screw granulation.

Discrete Element Modeling (DEM) based investigation of tribocharging in the pharmaceutical powders during hopper discharge

Triboelectric charging is defined as the phenomenon of charge transfer between two different material surfaces when they are brought into contact and separated. The focus of this research is the development of a Discrete Element Method (DEM) based simulation model to predict tribocharging during hopper discharge. Due to decreased particle-wall interactions and reduced particle wall contact times, net charges generated during hopper discharge are low. The simulation model confirmed this effect and was implemented to predict the triboelectric behavior of glass beads and MCC particles during hopper flow, along with the prediction of percent charged and uncharged particles. Approximately one-third of the particles were predicted to remain uncharged during the hopper discharge simulations for mono-dispersed particles, thus rendering a comparatively high amount of charge distribution into a small concentration of materials. The DEM model acted as a tool to predict charges that can be generated during hopper discharge at a specified geometry, with a potential to mitigate particle charging, when used for appropriate selection of hopper angles, and hopper wall materials.

A Simplex Centroid Design to Quantify Triboelectric Charging in Pharmaceutical Mixtures

The present study focuses on the implementation of a modified simplex centroid statistical design to predict the triboelectrification phenomenon in pharmaceutical mixtures. Two drugs (Ibuprofen and Theophylline), 2 excipients (lactose monohydrate and microcrystalline cellulose/MCC), and 2 blender wall materials (aluminum and poly-methyl methacrylate) were studied to identify the trends in charge transfer in pharmaceutical blends. The statistical model confirmed the excipient-drug interactions, irrespective of the blender wall materials, as the most significant factor leading to reduced charging. Also, lactose monohydrate was able to explain the charge variability more consistently compared with MCC powders when used as secondary material. The ratio of the individual components in the blends explained almost 80% of the bulk charging for Ibuprofen mixtures and 70% for Theophylline mixtures. The study also explored the potential lack of efficacy of lactose-MCC as a combination in ternary systems when compared with binary mixtures, for impacts on charge variability in pharmaceutical blends.

Quantification of Moisture-Induced Cohesion in Pharmaceutical Mixtures

Moisture-induced flow variabilities in pharmaceutical blends lead to multiple impediments during manufacturing of solid dosage formulations. Processing and storage humidity conditions both govern the moisture contents of the pharmaceutical mixtures and bear significant impact on the final product quality. In this study, experimentally validated discrete element method-based computational models along with statistical formalism have been implemented to develop a predictive tool for moisture-induced cohesion in binary and tertiary mixtures. V-blending was applied to prepare the pharmaceutical blends, and mixing characterization was performed using a Raman PhAT probe. Optimum fill volume was established for the mixing conditions to minimize static charging due to blender wall interactions on the pharmaceutical powders. A simplex-centroid (augmented) design for 3-component system was implemented to predict and quantify the nonlinear behavior of moisture-induced cohesion between the pharmaceutical powders based on their systematic hopper discharge studies (experiments and simulations). A methodical implementation of these quantification tools was hence performed to validate a design space that enables an approach to the appropriate selection of blend concentrations that achieve minimum mixture flow variability across different humidity conditions.