Influence of mechanical properties on impact fracture: Prediction of the milling behaviour of pharmaceutical powders by nanoindentation

Crystal structure-mechanical property relationship in succinic acid and L- alanine probed by nanoindentation

Establishing structure-mechanical property relationships is crucial for understanding and engineering the performance of pharmaceutical molecular crystals. In this study, we employed nanoindentation, a powerful technique that can probe mechanical properties at the nanoscale, to investigate the hardness and elastic modulus of single crystals of succinic acid and L-alanine. Nanoindentation results reveal distinct mechanical behaviors between the two compounds, with L-alanine exhibiting significantly higher hardness and elastic modulus compared to succinic acid. These differences are attributed to the underlying variations in molecular crystal structures - the three-dimensional bonding network and high intermolecular interaction energies of L-alanine molecules leads to its stiffness compared to the layered and weakly bonded crystal structure of succinic acid. Furthermore, the anisotropic nature of succinic acid is reflected in the directional dependence of the mechanical responses where it has been found that the (111) plane is more resistant to indentation than (100). By directly correlating the nanomechanical properties obtained from nanoindentation with the detailed crystal structures, this study provides important insights into how differences in molecular arrangements can translate into different macroscopic mechanical performance. These findings have implications on the selection of molecular crystals for optimized drug manufacturability.

Recent Advances in Nanomechanical Measurements and Their Application for Pharmaceutical Crystals

Mechanical behavior of pharmaceutical crystals directly impacts the formulation development and manufacturing of drug products. The understanding of crystal structure-mechanical behavior of pharmaceutical and molecular crystals has recently gained substantial attention among pharmaceutical and materials scientists with the advent of advanced nanomechanical testing instruments like nanoindentation. For the past few decades, instrumented nanoindentation was a popular technique for measuring the mechanical properties of thin films and small-length scale materials. More recently it is being implemented to investigate the mechanical properties of pharmaceutical crystals. Integration of correlative microscopy techniques and environmental control opened the door for advanced structure-property correlation under processing conditions. Preventing the degradation of active pharmaceutical ingredients from external factors such as humidity, temperature, or pressure is important during processing. This review deals with the recent developments in the synchronized nanomechanical measurements of pharmaceutical crystals toward the fast and effective development of high-quality pharmaceutical drug products. This review also summarizes some recent reports to intensify how one can design and control the nanomechanical properties of pharmaceutical solids. Measurement challenges and the scope for studying nanomechanical properties of pharmaceutical crystals using nanoindentation as a function of crystal structure and in turn to develop fundamental knowledge in the structure-property relationship with the implications for drug manufacturing and development are discussed in this review. This review further highlights recently developed capabilities in nanoindentation, for example, variable temperature nanoindentation testing, in situ imaging of the indented volume, and nanoindentation coupled Raman spectroscopy that can offer new quantitative details on nanomechanical behavior of crystals and will play a decisive role in the development of coherent theories for nanomechanical study of pharmaceutical crystal.

An extended macroindentation method for determining the hardness of poorly compressible materials

Indentation hardness, H, is an important mechanical property that quantifies the resistance to deformation by a material. For pharmaceutical powders, H can be determined using a macroindentation method, provided they can form intact tablets suitable for testing. This work demonstrates a method for determining the hardness of problematic materials that cannot form suitable tablets for macroindentation. The method entails predicting the hardness of a given powder at zero porosity (H₀) from those of microcrystalline cellulose and its binary mixture with the test compound using a power law mixing rule based on weight fraction. This method was found suitable for 13 binary mixtures. In addition, the H₀ values derived by this method could capture changes due to different particle sizes of sucrose and sodium chloride. Furthermore, the derived H₀ reasonably agreed with the single crystal indentation hardness of a set of 16 crystals when accounting for the effect of indentation condition and structural anisotropy. The mixture method thus extends the use of macroindentation for predicting indentation hardness of powders that cannot form intact tablets and, hence, their plasticity.

Review of nanosuspension formulation and process analysis in wet media milling using microhydrodynamic model and emerging characterization methods

Wet media milling is a popular technology used to prepare nanosuspensions. However, the theories and methods to guide the research on the formulation and process affecting wet media milling remain limited. The research on wet media milling follows a "black box" approach to a certain extent. This review focuses on exploring the formulation and process parameters factors in wet media milling. The formulation factors include the concentration, hydrophilicity/hydrophobicity, and structure of the drug and stabilizer, whereas the milling process parameters include the milling speed, milling time, and material, size, and filling volume of milling beads. Contrary to other reviews, this review attempts to quantify and visualize these factors by combining a microhydrodynamic model with emerging characterization methods to provide a scientific basis for the selection of nanosuspension formulations and process parameters, as opposed to the conventional trial-and-error approach.

Mechanical Characterization of Pharmaceutical Powders by Nanoindentation and Correlation with Their Behavior during Grinding

Controlling the size of powder particles is pivotal in the design of many pharmaceutical forms and the related manufacturing processes and plants. One of the most common techniques for particle size reduction in the process industry is powder milling, whose efficiency relates to the mechanical properties of the powder particles themselves. In this work, we first characterize the elastic and plastic responses of different pharmaceutical powders by measuring their Young modulus, the hardness, and the brittleness index via nano-indentation. Subsequently, we analyze the behavior of those powder samples during comminution via jet mill in different process conditions. Finally, the correlation between the single particle mechanical properties and the milling process results is illustrated; the possibility to build a predictive model for powder grindability, based on nano-indentation data, is critically discussed.

Nanomechanical testing in drug delivery: Theory, applications, and emerging trends

Mechanical properties play a central role in drug formulation development and manufacturing. Traditional characterization of mechanical properties of pharmaceutical solids relied mainly on large compacts, instead of individual particles. Modern nanomechanical testing instruments enable quantification of mechanical properties from the single crystal/particle level to the finished tablet. Although widely used in characterizing inorganic materials for decades, nanomechanical testing has been relatively less employed to characterize pharmaceutical materials. This review focuses on the applications of existing and emerging nanomechanical testing methods in characterizing mechanical properties of pharmaceutical solids to facilitate fast and cost-effective development of high quality drug products. Testing of pharmaceutical materials using nanomechanical techniques holds potential to develop fundamental knowledge in the structure-property relationships of molecular solids, with implications for solid form selection, milling, formulation design, and manufacturing. We also systematically discuss pitfalls and useful tips during sample preparation and testing for reliable measurements from nanomechanical testing.

Mechanical Anisotropy and Tabletability of Famotidine Polymorphs

In the drug development process, early characterization of solid forms can help to envisage the bulk processability of a powder, which should assist in selecting an optimal solid form. In...

Sonofragmentation of Organic Molecular Crystals vs Strength of Materials

Mechanochemistry, the interface between the chemical and the mechanical worlds, includes the relationship between the chemical and mechanical properties of solids. In this work, fragmentation of organic molecular crystals during ultrasonic irradiation of slurries has been quantitatively investigated. This has particular relevance to nucleation processes during sonocrystallization, which is increasingly used in the processing and formulation of numerous pharmaceutical agents (PAs). We have discovered that the rates of sonofragmentation are very strongly correlated with the strength of the materials (as measured by Vickers hardness and Young's modulus). This is a mechanochemical extension of the Bell-Evans-Polanyi Principle or Hammond's Postulate: the kinetics (i.e., rates) of solid fracture correlate with thermodynamic properties of solids (e.g., Young's modulus). The mechanism of the particle breakage is consistent with a direct interaction between the shockwaves or localized microjets created by the ultrasound (through acoustic cavitation) and the solid particles in the slurry. Comparisons of the sonofragmentation patterns of ionic and molecular crystals showed that ionic crystals are more sensitive to sonofragmentation than molecular crystals for a given Young's modulus. The rates of sonofragmentation are proposed to correlate with the types and densities of imperfections in the crystals.

Exploring space-energy matching via quantum-molecular mechanics modeling and breakage dynamics-energy dissipation via microhydrodynamic modeling to improve the screening efficiency of nanosuspension prepared by wet media milling

Introduction: The preparation of nanosuspensions by wet media milling is a promising technique that increases the bioavailability of insoluble drugs. The nanosuspension is thermodynamically unstable, where its stability might be influenced by the interaction energy between the stabilizers and the drugs after milling at a specific collision energy. However, it is difficult to screen the stabilizers and the parameters of milling accurately and quickly by using traditional analysis methods. Quantum-molecular mechanics and microhydrodynamic modeling can be applied to improve screening efficiency.Areas covered: Quantum-molecular mechanics model, which includes molecular docking, molecular dynamics simulations, and data on binding energy, provides insights into screening stabilizers based on their molecular behavior at the atomic level. The microhydrodynamic model explores the mechanical processes and energy dissipation in nanomilling, and even combines information on the mechanical modulus and an energy vector diagram for the milling parameters screening of drug crystals.Expert opinion: These modeling methods improve screening efficiency and support screening theories based on thermodynamics and physical dynamics. However, how to reasonably combine different modeling methods with their theoretical characteristics and further multidimensional and cross-scale simulations of nanosuspension formation remain challenges.

Insights into the ameliorating ability of mesoporous silica in modulating drug release in ternary amorphous solid dispersion prepared by hot melt extrusion

In this work, the application of various mesoporous silica grades in the preparation of stabilized ternary amorphous solid dispersions of Felodipine using hot melt extrusion was explored. We have demonstrated the effectiveness of mesoporous silica in these dispersions without the need for any organic solvents i.e., no pre-loading or immersion steps required. The physical and chemical properties, release profiles of the prepared formulations and the surface concentrations of the various molecular species were investigated in detail. Formulations containing 25 wt% and 50 wt% of Felodipine demonstrated enhanced stability and solubility of the drug substance compared to its crystalline counterpart. Based on the Higuchi model, ternary formulations exhibited a 2-step or 3-step release pattern which can be ascribed to the release of drug molecules from the organic polymer matrix and the external silica surface, followed by a release from the silica pore structure. According to the Korsmeyer-Peppas model, the release rate and release mechanism are governed by a complex quasi-Fickian release mechanism, in which multiple release mechanisms are occurring concurrently and consequently. Stability studies indicated that after 6 months storage of all formulation at 30% RH and 20 °C, Felodipine in all formulations remained stable in its amorphous state except for the formulation comprised of 40 wt% Syloid AL-1FP with a 50 wt% drug load.

Modeling Food Particle Systems: A Review of Current Progress and Challenges

For many years, food engineers have attempted to describe physical phenomena such as heat and mass transfer in food via mathematical models. Still, the impact and benefits of computer-aided engineering are less established in food than in most other industries today. Complexity in the structure and composition of food matrices are largely responsible for this gap. During processing of food, its temperature, moisture, and structure can change continuously, along with its physical properties. We summarize the knowledge foundation, recent progress, and remaining limitations in modeling food particle systems in four relevant areas: flowability, size reduction, drying, and granulation and agglomeration. Our goal is to enable researchers in academia and industry dealing with food powders to identify approaches to address their challenges with adequate model systems or through structural and compositional simplifications. With advances in computer simulation capacity, detailed particle-scale models are now available for many applications. Here, we discuss aspects that require further attention, especially related to physics-based contact models for discrete-element models of food particle systems.

Particle breakability assessment using an Aero S disperser

Milling is widely used in various industries to tailor the particle size distribution for desired attributes. The ability to predict milling behaviour by testing the breakability of a small quantity of material is of great interest. In this paper, a widely available aerodynamic dispersion method, i.e. the Aero S disperser of Malvern Mastersizer 3000 has been evaluated for this purpose. This device is commonly used for dispersion of fine and cohesive powders, as the particles are accelerated and impacted at a bend, but here its use for assessing particle breakability is explored. The fluid flow field is modelled using one-way coupled Computational Fluid Dynamics approach, as the particle concentration is low, following which the particle impact velocity is calculated by Lagrangian tracking and used in the analysis of particle breakage. Experimental work on the breakability is carried out using aspirin, paracetamol, sucrose and α-lactose monohydrate particles. The relative shift in the specific surface area is determined and together with the calculated particle impact velocity and physical properties, they are used to calculate the breakability index. A good agreement is obtained with the single particle impact testing and aerodynamic dispersion by Scirocco disperser, indicating the breakability could also be inferred from this method.

A Multi-variate Mathematical Model for Simulating the Granule Size Distribution in Roller Compaction-Milling Process

Granule size distribution (GSD) is one of the critical quality attributes in the roller compaction (RC) process. Determination of GSD for newly developed pharmaceutical compounds with unknown ribbon breakage behaviors at the RC milling step requires a quantitative insight into process parameters and ribbon attributes. Despite its pivotal role in mapping the process operating conditions to achieve desired granule size, limited work has been presented in literature with a focus on RC-milling modeling. In this study, a multi-variate mathematical model is presented to simulate the full size-distribution of granulated ribbons as a function of ribbon mechanical properties. Experimental data with a lab-scale oscillating milling apparatus were generated using ribbons made of various powder compositions. Model parameters were determined by fitting it to experimental data sets. Parameters obtained from the first step were correlated to ribbon Young's modulus. The model was validated by predicting GSD of data that were excluded in model development step. Predictive capabilities of the developed model were further explored by simulating GSD profiles of a granulated pharmaceutical excipient obtained at three different conditions of a real-scale Gerteis RC system. While maintaining the milling operating conditions similar to the lab-scale apparatus (i.e., screen size and spacing, and low rotor speed), the proposed modeling approach successfully predicted the GSD of roller compacted MCC powder as the model compound. This model can be alternatively utilized in conjunction with an RC model in order to facilitate the process understanding to obtain granule attributes as part of Quality-by-Design paradigm.

Density functional theory predictions of the mechanical properties of crystalline materials

The DFT-predicted mechanical properties of crystalline materials are crucial knowledge for their screening, design, and exploitation.

Modeling and Simulation of Process Technology for Nanoparticulate Drug Formulations-A Particle Technology Perspective

Crystalline organic nanoparticles and their amorphous equivalents (ONP) have the potential to become a next-generation formulation technology for dissolution-rate limited biopharmaceutical classification system (BCS) class IIa molecules if the following requisites are met: (i) a quantitative understanding of the bioavailability enhancement benefit versus established formulation technologies and a reliable track record of successful case studies are available; (ii) efficient experimentation workflows with a minimum amount of active ingredient and a high degree of digitalization via, e.g., automation and computer-based experimentation planning are implemented; (iii) the scalability of the nanoparticle-based oral delivery formulation technology from the lab to manufacturing is ensured. Modeling and simulation approaches informed by the pharmaceutical material science paradigm can help to meet these requisites, especially if the entire value chain from formulation to oral delivery is covered. Any comprehensive digitalization of drug formulation requires combining pharmaceutical materials science with the adequate formulation and process technologies on the one hand and quantitative pharmacokinetics and drug administration dynamics in the human body on the other hand. Models for the technical realization of the drug production and the distribution of the pharmaceutical compound in the human body are coupled via the central objective, namely bioavailability. The underlying challenges can only be addressed by hierarchical approaches for property and process design. The tools for multiscale modeling of the here-considered particle processes (e.g., by coupled computational fluid dynamics, population balance models, Noyes–Whitney dissolution kinetics) and physiologically based absorption modeling are available. Significant advances are being made in enhancing the bioavailability of hydrophobic compounds by applying innovative solutions. As examples, the predictive modeling of anti-solvent precipitation is presented, and options for the model development of comminution processes are discussed.

Determining the Impact of Roller Compaction Processing Conditions on Granule and API Properties

The attrition of drug particles during the process of dry granulation, which may (or may not) be incorporated into granules, could be an important factor in determining the subsequent performance of that granulation, including key factors such as sticking to punches and bio-performance of the dosage form. It has previously been demonstrated that such attrition occurs in one common dry granulation process train; however, the fate of these comminuted particles in granules was not determined. An understanding of the phenomena of attrition and incorporation into granule will improve our ability to understand the performance of granulated systems, ultimately leading to an improvement in our ability to optimize and model the process. Unique feeding mechanisms, geometry, and milling systems of roller compaction equipment mean that attrition could be more or less substantial for any given equipment train. In this work, we examined attrition of API particles and their incorporation into granule in an equipment train from Gerteis, a commonly used equipment train for dry granulation. The results demonstrate that comminuted drug particles can exist free in post-milling blends of roller compaction equipment trains. This information can help better understand the performance of the granulations, and be incorporated into mechanistic models to optimize such processes.

Aggregate Elasticity and Tabletability of Molecular Solids: a Validation and Application of Powder Brillouin Light Scattering

Describing the elastic deformation of single-crystal molecular solids under stress requires a comprehensive determination of the fourth-rank stiffness tensor (C_ijkl). Single crystals are, however, rarely utilized in industrial applications, and thus averaging techniques (e.g., the Voigt or Reuss approach) are employed to reduce the C_ijkl (or its inverse S_ijkl) to polycrystalline aggregate mechanical moduli. With increasing elastic anisotropy, the Voigt and Reuss-averaged aggregate moduli can diverge dramatically and, provided that drug molecules almost exclusively crystallize into low-symmetry space groups, warrants a significant need for accurate aggregate mechanical moduli. This elasticity data, which currently is largely absent for pharmaceutical materials, is expected to aid understanding how materials respond to direct compression and tablet formation. Powder Brillouin light scattering (p-BLS) has recently demonstrated facile access to porosity-independent, aggregate mechanical moduli. In this study, we extend our previous p-BLS model for obtaining mechanical properties and validate our approach against a broad library of molecular solids with diverse intermolecular interaction topologies and with previously determined C_ijkl which permits benchmarking our results. Our Young's and shear moduli determined with p-BLS strongly correlate, with limited bias (i.e., a near 1:1 relation), with the Voigt-averaged Young's and shear moduli determined using the C_ijkl. Through follow-on tabletability studies, we introduce initial classifications of tabletability behavior based on the results of our p-BLS studies and the apparent elastic anisotropy. With further development, this approach represents a robust and novel method to potentially identify materials for optimum tabletability at early developmental stages.

Nanomilling of Drugs for Bioavailability Enhancement: A Holistic Formulation-Process Perspective

Preparation of drug nanoparticles via wet media milling (nanomilling) is a very versatile drug delivery platform and is suitable for oral, injectable, inhalable, and buccal applications. Wet media milling followed by various drying processes has become a well-established and proven formulation approach especially for bioavailability enhancement of poorly water-soluble drugs. It has several advantages such as organic solvent-free processing, tunable and relatively high drug loading, and applicability to a multitude of poorly water-soluble drugs. Although the physical stability of the wet-milled suspensions (nanosuspensions) has attracted a lot of attention, fundamental understanding of the process has been lacking until recently. The objective of this review paper is to present fundamental insights from available published literature while summarizing the recent advances and highlighting the gap areas that have not received adequate attention. First, stabilization by conventionally used polymers/surfactants and novel stabilizers is reviewed. Then, a fundamental understanding of the process parameters, with a focus on wet stirred media milling, is revealed based on microhydrodynamic models. This review is expected to bring a holistic formulation-process perspective to the nanomilling process and pave the way for robust process development scale-up. Finally, challenges are indicated with a view to shedding light on future opportunities.