Mechanical properties of powders for compaction and tableting: an overview

Critical review on the role of excipient properties in pharmaceutical powder-to-tablet continuous manufacturing

INTRODUCTION

The pharmaceutical industry is gradually changing batch-wise manufacturing processes to continuous manufacturing processes, due to the advantages it has to offer. The final product quality and process efficiency of continuous manufacturing processes is among others impacted by the properties of the raw materials. Existing knowledge on the role of raw material properties in batch processing is however not directly transferable to continuous processes, due to the inherent differences between batch and continuous processes.

AREAS COVERED

A review is performed to evaluate the role of excipient properties for different unit operations used in continuous manufacturing processes. Unit operations that will be discussed include feeding, blending, granulation, final blending, and compression.

EXPERT OPINION

Although the potency of continuous manufacturing is widely recognized, full utilization still requires a number of challenges to be addressed effectively. An expert opinion will be provided that discusses those challenges and potential solutions to overcome those challenges. The provided overview can serve as a framework for the pharmaceutical industry to push ahead process optimization and formulation development for continuous manufacturing processes.

A Method for the Tensile Strength Prediction of Tablets with Differing Powder Plasticities

Tablets are the most commonly used dosage form in the pharmaceutical industry, and their properties such as disintegration, dissolution, and portability are influenced by their strength. However, in industry, the mixing fraction of powders to obtain a tablet compact with sufficient strength is determined based on empirical rules. Therefore, a method for predicting tablet strength based on the properties of a single material is required. The objective of this study was to quantitatively evaluate the relationship between the compression properties and tablet strength of powder mixtures. The compression properties of the powder mixtures with different plasticities were evaluated based on the force-displacement curves obtained from the powder compression tests. Heckel and compression energy analyses were performed to evaluate compression properties. During the compression energy analysis, the ratio of plastic deformation energy to elastic deformation energy (Ep/Ee) was assumed to be the plastic deformability of the powder. The quantitative relationship between the compression properties and tensile strength of the tablets was investigated. Based on the obtained relationship and the compression properties of a single material, a prediction equation was put forward for the compression properties of the powder mixture. Subsequently, a correlation equation for tablet strength was proposed by combining the values of K and Ep/Ee obtained from the Heckel and compression energy analyses, respectively. Finally, by substituting the compression properties of the single material and the mass fraction of the plastic material into the proposed equation, the tablet strength of the powder mixture with different plastic deformabilities was predicted.

Application of co-precipitated glutinous rice starch as a multifunctional excipient in direct compression tablets

Two key properties of excipients for inclusion in direct compression tablets are flowability and compactibility. Glutinous rice starch (GRS) has poor flowability, which limits its use in direct compression tablets. This study aimed to create a multifunctional direct compression excipient (filler binder disintegrant) with improved flowability from GRS by the co-precipitation method. The physicochemical and pharmaceutical properties of the co-precipitated GRS (cpGRS) were investigated. The optimum conditions for producing cpGRS (0.43 M sodium hydroxide solution, 7.09% PVP K30, 14.02% calcium carbonate, 95 min of mixing time and pH of 6.97) resulted in 68.80% yield, fair to good flowability, acceptable tablet strength, and fast disintegration. The FT-IR spectra of cpGRS showed no significant shifts in the key peaks, which indicates that there was an absence of chemical interactions within cpGRS. X-ray diffractograms also showed no significant changes, indicating that the GRS granules, calcium carbonate, and PVP K30 components remained unaltered during co-precipitation. cpGRS also demonstrated a dilution capacity of 50% when paracetamol was used as model drug. When cpGRS was combined with domperidone or propranolol hydrochloride it showed a better deformation capability than the physical mixtures. Although cpGRS was sensitive to lubricant, the hardness and tensile strength were higher than common strength for general purpose use in tablets. When compared to the physical mixture, pregelatinized starch and directly compressible calcium carbonate, the results showed that cpGRS tablets of both model drugs passed the friability test, demonstrated the best disintegration property, provided the fastest and highest drug release profile for propranolol, and improved the drug release profile for domperidone. For propranolol-cpGRS tablets, dissolution medium at different pH did not affect the dissolution profile. For domperidone-cpGRS tablets, the pH of dissolution medium did affect the dissolution profile of the tablets. This was according to the API solubility. These results reveal that cpGRS is an excellent multifunctional i.e., filler, binder, and disintegrant excipient suitable for direct compression tablets. The main component is natural. The preparation method is simple, quick, and efficient. This method does not produce harmful waste and requires only basic equipment, and affordable reactants and devices.

Effect of process parameters in high shear granulation on characteristics of a novel co-processed mesoporous silica material

In this study, insights into the development and optimization of a co-processed excipient based on mesoporous silica are presented. The main advantage of such a material is that it is appropriate for direct tablet compression and has a sufficiently large specific surface area to be suitable for potential subsequent drug loading and formulation of (amorphous) solid dispersions. Our aim was to use a Design of Experiments approach to investigate which process parameters in high shear granulation affect the characteristics of such a co-processed material. The parameters included were the amount of binder (isomalt), the amount of water (granulation liquid), the water addition rate and the speed of the impeller. The responses evaluated and modelled were particle size and its distribution, specific surface area, bulk density, flowability, compressibility and compactibility. The models obtained showed good quality in terms of goodness of fit and predictive power. Active effects were identified for all responses, giving a thorough insight into factors affecting the material characteristics. Optimization experiments resulted in products with the desired characteristics (high specific surface area, large particle size, good flow and compression properties) and confirmed the validity of the generated models.

Multivariate data analysis to evaluate commonly used compression descriptors

Numerous studies elucidated material behavior based on compression analyses. Especially compressibility, compactibility and tabletability were in the focus of these investigations. In the present study, a comprehensive multivariate data analysis was performed using principal component analysis method. Twelve pharmaceutically used excipients were selected for direct compression tableting and subsequent evaluation of several compression analyses. Material properties, tablet properties, tableting parameters and parameters from compression analyses were used as input variables. The materials could successfully be grouped using principal component analysis. Of the tableting parameters, the compression pressure showed the greatest influence on the results. The tabletability was found to be the most important compression analysis in the material characterization. Compressibility and compactibility only played a minor role in the evaluation. Some important insights have been gained for a deeper understanding of the tableting process using the multivariate approach to evaluate the variety of compression data.

Reduced Fine API Agglomeration After Dry Coating for Enhanced Blend Uniformity and Processability of Low Drug Loaded Blends

PURPOSE

The impact of dry coating on reduced API agglomeration remains underexplored. Therefore, this work quantified fine cohesive API agglomeration reduction through dry coating and its impact on enhanced blend uniformity and processability, i.e., flowability and bulk density of multi-component blends API loading as low as 1 wt%.

METHODS

The impact of dry coating with two different types and amounts of silica was assessed on cohesion, agglomeration, flowability, bulk density, wettability, and surface energy of fine milled ibuprofen (~ 10 µm). API agglomeration, measured using Gradis/QicPic employing gentler gravity-based dispersion, resulted in excellent size resolution. Multi-component blends with fine-sized excipients, selected for reduced segregation potential, were tested for bulk density, cohesion, flowability, and blend content uniformity. Tablets formed using these blends were tested for tensile strength and dissolution.

RESULT

All dry coated ibuprofen powders exhibited dramatic agglomeration reduction, corroborated by corresponding decreased cohesion, unconfined yield strength, and improved flowability, regardless of the type and amount of silica coating. Their blends exhibited profound enhancement in flowability and bulk density even at low API loadings, as well as the content uniformity for the lowest drug loading. Moreover, hydrophobic silica coating improved drug dissolution rate without appreciably reducing tablet tensile strength.

CONCLUSION

The dry coating based reduced agglomeration of fine APIs for all three low drug loadings improved overall blend properties (uniformity, flowability, API release rate) due to the synergistic impact of a minute amount of silica (0.007 wt %), potentially enabling direct compression tableting and aiding manufacturing of other forms of solid dosing.

Compression Modulus and Apparent Density of Polymeric Excipients during Compression-Impact on Tabletability

The present study focuses on the compaction behavior of polymeric excipients during compression in comparison to nonpolymeric excipients and its consequences on commonly used Heckel analysis. Compression analysis at compaction pressures (CPs) from 50 to 500 MPa was performed using a compaction simulator. This study demonstrates that the particle density, measured via helium pycnometer (ρpar), of polymeric excipients (Kollidon®VA64, Soluplus®, AQOAT®AS-MMP, Starch1500®, Avicel®PH101) was already exceeded at low CPs (<200 MPa), whereas the ρpar was either never reached for brittle fillers such as DI-CAFOS®A60 and tricalcium citrate or exceeded at CPs above 350 MPa (FlowLac®100, Pearlitol®100SD). We hypothesized that the threshold for exceeding ρpar is linked with predominantly elastic deformation. This was confirmed by the start of linear increase in elastic recovery in-die (ERin-die) with exceeding particle density, and in addition, by the applicability in calculating the elastic modulus via the equation of the linear increase in ERin-die. Last, the evaluation of “density under pressure” as an alternative to the ρpar for Heckel analysis showed comparable conclusions for compression behavior based on the calculated yield pressures. However, the applicability of Heckel analysis for polymeric excipients was questioned in principle. In conclusion, the knowledge of the threshold provides guidance for the selection of suitable excipients in the formulation development to mitigate the risk of tablet defects related to stored elastic energy, such as capping and lamination.

Investigation of the Compressibility Characteristics of Paracetamol using "Compaction Simulator"

Objectives

This study was performed to understand the behavior of poorly compressible paracetamol powder using a compaction simulator (CS), equipment that records data during the compaction process. The aim was to investigate the compressibility of paracetamol tablets using a dry granulation (slugging) process, with different formulation compositions.

Materials and Methods

Formulations were prepared to observe the effect on compressibility with two different lactose-based fillers, Flowlac^®100 and Granulac^®70, and a binder, Kollidon^® K90. In each combination, a total of four formulations were prepared with paracetamol to filler ratios of 1:1 and 0.8:1. Tablets were produced by single punch (11.28 mm) CS at six different pressures (152, 210, 263, 316, 400, and 452 MPa). During compression, upper punch displacement and force data were produced by the CS equipment. The compressed tablets were tested for hardness, thickness, and weight variation and compared with each other.

Results

All formulations reached maximum tensile strength at compaction pressures between 263 and 316 MPa. In the formulations without binder, those containing Granulac^®70 had higher tensile strength than those containing Flowlac^®100 at both filler ratios. The results obtained indicated that the addition of binder to the formulations (F-45-1, F-45-2, F-50-3, and F-50-4) improved the compressibility of paracetamol. Formulation F-45-2, containing Flowlac^®100 and binder, showed better compressibility at 2.9 MPa tensile strength. Data from the CS were used to compare Young's modulus and work of compaction on selected formulations (F-45-1 and F-45-2).

Conclusion

The proposed lactose-based filler, Flowlac^®100, with low pressure can be successfully applied for improving the compressibility of paracetamol. An optimum formulation can be designed with smaller amounts of materials using a compaction simulator.

Impact of die wall material on the mechanical properties of paracetamol tablets

Tablet quality can be affected by material, configuration and design of the tooling which comprise punches and dies. Much research attention had centred around punches, with very little reported on the dies. Dies with modified bore lining materials or inserts are available for special applications. However, the influence of such die types on tablet properties and the compaction process has not been well studied. Often, the reason for selecting dies with harder lining material is for improved wear resistance. In this study, flat-faced and convex-faced tablets were produced from paracetamol granules using dies with different bore inserts. Tablet properties and response parameters of the compaction process were evaluated to understand the impact of die mechanical and surface properties on the compacts formed. Compaction pressure was found to have the greatest impact on tablet elastic recovery (r ≥ 0.96, p < 0.01 in all bivariate correlations) and thus affecting the tensile strength. Choice of die inserts could impact the mechanical properties of convex-faced paracetamol tablets, particularly at high compaction pressures.

Tablet capping predictions of model materials using multivariate approach

The present study demonstrated the prediction of predominant root causes of capping behavior as a function of the powder rheological and the mechanical behavior of Acetaminophen (APAP) and Ibuprofen (IBU). The authors analyzed powder rheological properties for powder blend permeability, pressure drop, and cohesion. The measured deformation properties were compact porosity, internal air pressure, Brinell hardness, and tensile strength. The data were evaluated qualitatively and quantitatively using multivariate techniques, such as principal component analysis (PCA) and principal component regression (PCR) models, respectively, to identify the effect of powder air entrapment efficiency and mechanical behavior on the tablet capping score. The PCA model indicated that pressure drop, cohesion, API amount, and compression pressure correlated positively, whereas permeability, porosity, internal air pressure, Brinell hardness, and tensile strength correlated negatively to the capping potential. APAP and IBU also showed two independent mechanisms as a function of their amount on the capping score at all compression pressures. APAP and IBU followed an exponential and linear relationship, respectively. Furthermore, the dominant powder rheological and deformation behavior affecting the capping score of each material was identified and quantified using two separate PCR models. These models showed that APAP capping was predominantly dependent on its powder properties, while that of IBU was predominantly based on its deformation properties. In conclusion, APAP and IBU compacts capping had respective air induced and deformation induced capping behavior. The proposed approach can aid in understanding the underlying mechanisms of capping and developing an effective, optimized strategy to ensure tablet quality.

Predicting tablet tensile strength with a model derived from the gravitation-based high-velocity compaction analysis data

In the present study, a model was developed to estimate tablet tensile strength utilizing the gravitation-based high-velocity (G-HVC) method introduced earlier. Three different formulations consisting of microcrystalline cellulose (MCC), dicalcium phosphate dihydrate (DCP), hydroxypropyl methylcellulose (HPMC), theophylline and magnesium stearate were prepared. The formulations were granulated using fluid bed granulation and the granules were compacted with the G-HVC method and an eccentric tableting machine. Compaction energy values defined from G-HVC data predicted tensile strength of the tablets surprisingly well. It was also shown, that fluid bed granulation improved the compaction energy intake of the granules in comparison to respective physical mixtures. In addition, general mechanical properties and elastic recovery were also examined for all samples. In this study it was finally concluded, that the data obtained by the method was of practical relevance in pharmaceutical formulation development.

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.

Pharmaceutical cocrystals, salts and multicomponent systems; intermolecular interactions and property based design

As small molecule drugs become harder to develop and less cost effective for patient use, efficient strategies for their property improvement become increasingly important to global health initiatives. Improvements in the physical properties of Active Pharmaceutical Ingredients (APIs), without changes in the covalent chemistry, have long been possible through the application of binary component solids. This was first achieved through the use of pharmaceutical salts, within the last 10-15years with cocrystals and more recently coamorphous systems have also been consciously applied to this problem. In order to rationally discover the best multicomponent phase for drug development, intermolecular interactions need to be considered at all stages of the process. This review highlights the current thinking in this area and the state of the art in: pharmaceutical multicomponent phase design, the intermolecular interactions in these phases, the implications of these interactions on the material properties and the pharmacokinetics in a patient.

Characterization of new functionalized calcium carbonate-polycaprolactone composite material for application in geometry-constrained drug release formulation development

A new mineral-polymer composite (FCC-PCL) performance was assessed to produce complex geometries to aid in development of controlled release tablet formulations. The mechanical characteristics of a developed material such as compactibility, compressibility and elastoplastic deformation were measured. The results and comparative analysis versus other common excipients suggest efficient formation of a complex, stable and impermeable geometries for constrained drug release modifications under compression. The performance of the proposed composite material has been tested by compacting it into a geometrically altered tablet (Tablet-In-Cup, TIC) and the drug release was compared to commercially available product. The TIC device exhibited a uniform surface, showed high physical stability, and showed absence of friability. FCC-PCL composite had good binding properties and good compactibility. It was possible to reveal an enhanced plasticity characteristic of a new material which was not present in the individual components. The presented FCC-PCL composite mixture has the potential to become a successful tool to formulate controlled-release dosage solid forms.

Aggregate Elasticity, Crystal Structure, and Tableting Performance for p-Aminobenzoic Acid and a Series of Its Benzoate Esters

The tableting performance for p-aminobenzoic acid (PABA) and a series of its benzoate esters with increasing alkyl chain length (methyl-, ethyl-, and n-butyl) was determined over a broad range of compaction pressures. The crystalline structure of methyl benzoate (Me-PABA) exhibits no slip systems and does not form viable compacts under any compaction pressure. The ethyl (Et-PABA) and n-butyl (Bu-PABA) esters each have a similar, corrugated-layer structure that displays a prominent slip plane and improves material plasticity at low compaction pressure. The compact tensile strength for Et-PABA is superior to that for Bu-PABA; however, neither material achieved a tensile strength greater than 2 MPa over the compression range studied. Complementary studies with powder Brillouin light scattering (BLS) show the maxima of the shear wave, acoustic frequency distribution red shift in an order consistent with both the observed tabletability and attachment energy calculations. Moreover, zero-porosity aggregate elastic moduli are determined for each material using the average acoustic frequency obtained from specific characteristics of the powder BLS spectra. The Young's moduli for Et- and Bu-PABA is significantly reduced relative to PABA and Me-PABA, and this reduction is further evident in tablet compressibility plots. PABA, however, is distinct with high elastic isotropy as interpreted from the narrow and well-defined powder BLS frequency distributions for both the shear and compressional acoustic modes. The acoustic isotropy is consistent with the quasi-isotropic distribution of hydrogen bonding for PABA. At low compaction pressure, PABA tablets display the lowest tensile strength of the series; however, above a compaction pressure of ca. 70 MPa PABA tablet tensile strength continues to increase while that for Et- and Bu-PABA plateaus. PABA displays lower plasticity relative to either ester, and this is consistent with its crystalline structure and high yield pressure determined from in-die Heckel analysis. Overall the complementary approach of using both structural and the acoustic inputs uniquely provided from powder BLS is anticipated to expand our comprehension of the structure-mechanics relationship and its role in tableting performance.

Relationship between mechanical properties and crystal structure in cocrystals and salt of paracetamol

Objectives were to study mechanical properties of various solid forms of paracetamol and relate to their crystal structures. Paracetamol form I (PRA), its cocrystals with oxalic acid (PRA-OXA) and 4,4-bipyridine (PRA-BPY) and hydrochloride salt (PRA-HCL) were selected. Cocrystals and salt were scaled-up using rational crystallization methods. The resulting materials were subjected to different solid-state characterizations. The powders were sieved and 90-360 µm sieve fraction was considered. These powders were examined by scanning electron microscopy (SEM) and densities were determined. Tablets were made at applied pressures of 35-180 MPa under controlled conditions and the tablet height, diameter and hardness were measured. Tensile strength and porosity of the tablets were estimated using well known models. Crystal structures of these systems were visualized and slip planes were identified. Cocrystal and salt of PRA were physically pure. Sieved powders had comparable morphologies and particle size. The apparent and theoretical densities of powders were similar, but no clear trends were observed. The tensile strengths of these compacts were increased with increasing pressure whereas tabletability decreased in the order oxalic acid > PRA-HCL ≈ PRA-OXA > BPY > PRA-BPY. Tablet tensile strength decreases exponentially with increasing porosity with the exception of PRY-BPY and BPY. Slip plane prediction based on attachment energies may not be independently considered. However, it was possible to explain the improved mechanical properties of powders based on the crystal structure. Cocrystallization and salt formation have introduced structural features that are responsible for improved tableting properties of PRA.

Visualization and quantification of deformation behavior of clopidogrel bisulfate polymorphs during tableting

The deformation behavior of particles under pressure dominates the mechanical properties of solid dosage forms. In this study, the in situ 3D deformation of two polymorphs (I and II) of clopidogrel bisulfate (CLP) was determined to illustrate pressure distribution profiles within the tablet by the deformation of the crystalline particles for the first time. Synchrotron radiation X-ray computed microtomography (SR-μCT) was utilized to visualize and quantify the morphology of thousands crystalline particles of CLP I and CLP II before and after compression. As a result, the deformation was examined across scale dimensions from microns to the size of the final dosage form. Three dimensional parameters such as volume, sphericity, oblate and prolate of individual particle and distributions were computed and analyzed for quantitative comparison to CLP I and CLP II. The different degrees of deformation under the same compression conditions of CLP I and CLP II were observed and characterized quantitatively. The map of deformation degrees within the tablet illustrated the heterogeneous pressure distribution in various regions of the compacted tablet. In conclusion, the polymorph deformation behaviors demonstrated by SR-μCT quantitative structure analysis deepen understanding of tableting across dimensions from microns to millimeters for the macrostrcuture of tablet.

The importance of binder moisture content in Metformin HCL high-dose formulations prepared by moist aqueous granulation (MAG)

The aim of this study was to evaluate binders to improve the flowability of granulates and compactibility of Metformin HCL (Met) using the moist aqueous granulation (MAG) process. The effect of the binder moisture content on granulate and tablet quality was also evaluated. Vinylpyrrolidone-vinyl acetate copolymer (Kollidon VA64 fine: VA64), polyvidone (Povidone K12: PVP), hydroxypropyl cellulose (HPC SSL SF: HPC) and hydroxypropyl methylcellulose (Methocel E5 LV: HPMC) were evaluated as binders. These granulates, except for HPMC, had a lower yield pressure than Met active pharmaceutical ingredient (API). HPMC Met was not sufficiently granulated with low water volume. No problems were observed with the VA64 Met granulates during the tableting process. However, HPC Met granulates had a bowl-forming tendency, and PVP Met granulates had the tendency to stick during the tableting process. These bowl-forming and sticking tendencies may have been due to the low moisture absorbency of HPC and the high volume of bound water of PVP, respectively. VA64 Met granulates had the highest ambient moisture content (bulk water, bound water) and moisture absorbency. It was concluded that the type of binder used for the Met MAG process has an impact on granulate flow and compactibility, as well as moisture absorbency and maintenance of moisture balance.

The impact of hot melt extrusion and spray drying on mechanical properties and tableting indices of materials used in pharmaceutical development

The impact of melt extrusion (HME) and spray drying (SD) on mechanical properties of hypromellose acetate succinate (HPMCAS), copovidone, and their formulated blends was studied and compared with that of reference excipients. Tensile strength (TS), compression pressure (CP), elastic modulus (E), and dynamic hardness (Hd ) were determined along with Hiestand indices using compacts prepared at a solid fraction of ∼0.85. HPMCAS and copovidone exhibited lower Hd , lower CP, and lower E than the reference excipients and moderate TS. HPMCAS was found to be highly brittle based on brittle fracture index values. The CP was 24% and 61% higher for HPMCAS after SD and HME, respectively, than for unprocessed material along with a higher Hd . Furthermore, the TS of HPMCAS and copovidone decreased upon HME. Upon blending melt-extruded HPMCAS with plastic materials such as microcrystalline cellulose, the TS increased. These results suggest that SD and HME could impact reworkability by reducing deformation of materials and in case of HME, likely by increasing density due to heating and shear stress in a screw extruder. A somewhat similar effect was observed for the dynamic binding index (BId ) of the excipients and formulated blends. Such data can be used to quantitate the impact of processing on mechanical properties of materials during tablet formulation development.