Physical processes of tableting

Elucidation of the capping mechanism during the high-speed tableting process based on FEM simulation and fracture mechanics analysis

Capping is a phenomenon in which the cap of a tablet separates from the bonding area, thereby presenting a significant problem in tablet production. The compression speed is an important factor that affects the occurrence of capping. Capping is a phenomenon in which the tablet cap separates from the band of the tablet. However, the effect of compression speed on capping has not yet been fully elucidated. In this study, numerical analyses were performed using the finite element method (FEM) with the Drucker-Prager cap and Perzyna models at 40 mm/s to further understand the effect of compression speed on capping occurrence. The calculated loading curves agreed with the experimental curves at different tableting speeds. Moreover, the simulation results confirmed that the stress distribution inside the tablet became more non-uniform with increased compression speed. Additionally, we aimed to develop a prediction method for capping tendency using FEM simulation. Predicting the capping tendency by comparing the residual stress and tablet strength at the edge of the tablet is challenging. On the other hand, accurate predictions were achieved by comparing the stress intensity factor with the fracture toughness of the tablets, based on fracture mechanics principles. Our concept of predicting capping tendencies offers valuable insights for advancing research on the tableting process.

Impact of solid content on the bulk properties of lyophilized powders

Interest in oral delivery of biological drug products, commonly prepared through lyophilization, is surging. Typically, low solid content solutions are employed for lyophilization to enhance mass transfer and minimize drying time. Yet, this approach often results in lyophilized powders with low bulk density and poor flowability, challenging downstream processing steps that are required for oral product development. Increasing solid content in a starting solution can, in theory, increase the density of lyophilized cakes and powders post-milling. However, the effectiveness of improving powder density and flowability using a higher solid content has not been experimentally verified. In addition, the impact of using a higher solid content on other physicochemical properties of lyophilized materials remains uncertain. To address the knowledge gaps, we lyophilized three common bulk cryoprotectants at two different solid contents (5% and 10%) and systematically evaluated their solid-state properties, bulk density, flowability, compaction characteristics, and physical stability. We found that powders prepared at a higher solid content (10%) exhibited higher bulk density, but they still failed to meet the requirements for easy oral product development. A change in solid content also leads todistinct solid-state properties, compaction behaviors, and stability, highlighting the importance of thorough characterization of lyophilized materials when solid content is changed in the course of oral solid dosage formulation development.

Investigations on the Impacts of Drugs or Excipients with Different Physicochemical and Compaction Properties on the Disintegration Behavior of Kollidon®SR-Based Binary Controlled Release Matrix Tablets

The objective of this study was to examine the impact of the physicochemical properties of the loaded drug or excipient, the concentration of Kollidon®SR (KSR), and the mechanical characteristics of KSR compacts on their disintegration times. Using disintegration apparatus, a two-hour constraint was chosen as the process's end point. Lactose-KSR compacts subjected to the highest compression pressure and Microcrystalline cellulose-KSR compacts with KSR concentrations exceeding 30% exhibited disintegration times of less than ten minutes. Likewise, compacts containing Diltiazem HCl-KSR demonstrated brief disintegration times across all tested KSR concentrations and compression pressures. Compacts of Modafinil, Metformin HCl, and Ascorbic acid-KSR displayed disintegration times ranging from fast to moderate, contingent upon the levels of KSR and compression pressure applied. Compacts containing KSR with Aspirin, Salicylic acid, or Ibuprofen did not exhibit significant disintegration even at minimal amounts of KSR (0.5%). Theophylline-KSR tablets also showed prolonged dissolution times, even at very low concentrations of KSR. The disintegration times of Dic-KSR tablets were roughly close to an hour and were predominantly unaffected by varying KSR levels and only marginally influenced by compression pressures. It is possible to draw the conclusion that different drugs or excipients have different minimum KSR requirements to resist compacts' disintegration process. Compounds that demonstrate low solubility in water can result in extended disintegration times for KSR compacts. The melting points of these compounds, in conjunction with the Py values of the compacts and their compaction properties, could affect the disintegration process, although a precise evaluation is necessary.

The effect of unloading and ejection conditions on the properties of pharmaceutical tablets

This study investigates decompression and ejection conditions on tablet characteristics by comparing compact densities and tensile strengths made using regular rigid dies and custom-built die systems that enable triaxial decompression. Die-wall pressure evolution during decompression and ejection stresses did not meaningfully impact the density and tensile strength of the materials tested: microcrystalline cellulose, crystalline lactose monohydrate, and mannitol. Furthermore, the apparent differences in tensile strength between rectangular cuboids and cylindrical compacts are unrelated to decompression and ejection conditions, but rather a consequence of their shapes and of the test configurations. This suggests that elastic and plastic deformations that may occur during decompression and ejection are not significantly influenced by die-wall pressure evolution. We thus conclude that while triaxial decompression and constraint-free ejection may allow the production of defect-free compacts for materials that otherwise are defect prone using a rigid die, they seem to pose no benefits when the materials already produce defect-free compacts using a rigid die.

Tableting behavior of freeze and spray-dried excipients in pharmaceutical formulations

Most of biopharmaceuticals, in their liquid form, are prone to instabilities during storage. In order to improve their stability, lyophilization is the most commonly used drying technique in the pharmaceutical industry. In addition, certain applications of biopharmaceutical products can be considered by oral administration and tablets are the most frequent solid pharmaceutical dosage form used for oral route. Thus, the tableting properties of freeze-dried products used as cryo and lyoprotectant could be a key element for future pharmaceutical developments and applications. In this study, we investigated the properties that might play a particular role in the specific compaction behavior of freeze-dried excipients. The tableting properties of freeze-dried trehalose, lactose and mannitol were investigated and compared to other forms of these excipients (spray-dried, commercial crystalline and commercial crystalline milled powders). The obtained results showed a specific behavior in terms of compressibility, tabletability and brittleness for the amorphous powders obtained after freeze-drying. The comparison with the other powders showed that this specific tableting behavior is linked to both the specific texture and the physical state (amorphization) of these freeze-dried powders.

A strategy to optimize precompression pressure for tablet manufacturing based on in-die elastic recovery

A precompression pressure optimization strategy using in-die elastic recovery was developed to effectively address tablet lamination caused by air entrapment. This strategy involves exacerbating the air entrapment issue using high tableting speeds and main compaction pressures and collecting in-die elastic recovery data as a function of precompression pressure. The optimized precompression pressure, which corresponds to the minimum elastic recovery, is most effective at eliminating air from the powder bed prior to the main compression. When the optimized precompression pressure was employed, intact tablets of a model blend prone to lamination due to air entrapment could be produced over a wide range of high main compaction pressures, while tablets without precompression laminated immediately after ejection at equivalent main compaction pressures. This optimization strategy is effective for addressing lamination issues due to air entrapment using precompression. An advantage of this strategy is that intact tablets are not required to identify an optimized precompression pressure since elastic recovery measurements occur in-die.

Tableting properties of freeze-dried trehalose: Physico-chemical and mechanical investigation

Freeze-drying of biopharmaceutical products is the method of choice in order to improve their stability and storage conditions. Such freeze-dried products are usually intended for parenteral route administration. However, many biopharmaceutical materials administered by parenteral route are used to treat local diseases particularly in the gastro-intestinal tract. Therefore, many studies concentrate nowadays their effort on developing alternative dosage forms to deliver biopharmaceutical molecules by the oral route. Tablets are the most popular solid pharmaceutical dosage form used for oral administration since they present many advantages, but poor informations are available on the possibility of tableting freeze-dried powders. In this study, we evaluate the compaction behavior of freeze-dried trehalose powder since trehalose is one of the most used cryo and lyoprotectant for the lyophilisation of biopharmaceutical entities. Results show that freeze-dried trehalose powder can be tableted while remaining amorphous and the obtained compacts present very specific properties in terms of compressibility, tabletability, brittleness and viscoelasticity compared to the crystalline trehalose and compared to classical pharmaceutical excipients.

Investigations on Compaction Behavior of Kollidon®SR-Based Multi-component Directly Compressed Tablets for Preparation of Controlled Release Diclofenac Sodium

The physics of tablets mixtures has gained much attention lately. The purpose of this work is to evaluate the compaction properties of Kollidon® SR (KSR) in the presence of different excipients such as Microcrystalline cellulose (MCC), Monohydrous lactose (MH Lactose), Poly (vinyl acetate) (PVA100), and a water-soluble drug Diclofenac sodium (DNa) to prepare once daily formulation. Tablets were prepared using direct compression and were compressed into flat-faced tablets using hydraulic press at various pressures. The combination of MCC and KSR in the tablets showed reduced porosity, and almost constant low Py values as KSR levels increased; also, KSR-DNa tablets had higher percentage porosity and crushing strength values than KSR-MH Lactose tablets. The crushing strengths of KSR-MCC tablets were larger than those of KSR-DNa tablets. Ternary mixture tablets comprised of KSR-MCC-DNa showed decreased porosities and low Py values as the percentage of KSR increased especially at high compression pressures but had higher crushing strengths compared to KSR-DNa or MCC-DNa binary tablets. KSR-MH Lactose-DNa ternary tablets experienced lower porosities and crushing strengths compared to KSR-MCC-DNa tablets. Quaternary tablets of KSR-PVA100-MCC-DNa showed lower porosity and Py values than quaternary tablets obtained using similar proportion of MH Lactose instead of MCC. In conclusion, optimum quaternary tablets were obtained with optimum crushing strengths, relatively low Py, and moderate percentage porosities among all prepared quaternary tablets. The drug release of the optimum quaternary tablets demonstrated similar in vitro release profile compared to that of the marketed product with a mechanism of release that follows Korsmeyer-Peppas model.

Prediction of the tensile strength of tablets using LSTM networks on compression profiles

This publication's objective was to predict the tensile strength of tablets using an analysis of process data comprising compression pressure, sampling timestamps, and punch positions. A recurrent neuronal network, specifically designed with Long Short-Term Memory layers, was utilized to accommodate the time-series characteristics of the data. A dataset from 344 tablet compression cycles was employed for model training, after which the model demonstrated a predictive ability with a coefficient of determination of 0.954 on test data from 804 tableting cycles. The foundational database incorporated data from both pure substances and mixtures consisting of up to four components compressed at various compression pressures and with three different tablet masses. Interestingly, the prediction errors did not exhibit any significant correlation with specific materials, mixtures, maximum compression pressures, or tablet weights. With the aid of the model, it was possible to calculate the entire tabletability profile of twelve substances from just a single compression process each. Models of this nature bear promising potential for future application in the research and development of formulations as well as in production processes to predict tensile strength.

Effect of geometrical features on the capping behavior of biconvex tablets

Capping is a common industrial issue during the manufacturing of pharmaceutical tablets. It is influenced by both process and formulation parameters. In this work, a systematic study of the influence of the geometrical features of biconvex tablets on capping occurrence was performed on a model formulation, using a design of experiment. Capping was characterized by the pressure at which half of the produced tablets were capped. The influence of the tablet geometry was assessed by varying three parameters: the diameter (D), the band thickness (W) and the ratio between the radius of curvature (R) and the diameter, i.e. R/D. Results showed that having a large diameter, a low band thickness and a high curvature (i.e. a low R/D) favored capping occurrence. Moreover, the effects are not independent as cross-effect were detected. Finally, even for homothetic tablets (i.e. same R/D and W/D) it is shown that a large diameter increases capping occurrence. These results could be used in the future as a guideline for punch selection during tablet development.

Indices for the brittleness of pharmaceutical tablets: A reassessment

Brittleness is an important mechanical property. In the classical sense, a material is considered brittle if, during loading, it behaves elastically until failure. Nevertheless, it is also sometimes understood as the fact to be resistant to breakage. In the case of pharmaceutical tablets, three different indices have been defined to measure brittleness: the brittle fracture index (BFI), the brittle/ductile index (BDI) and the tablet brittleness index (TBI). The aim of this work was to reassess the meaning of the different indices that are known to give contradictory results. Using theoretical considerations, numerical modelling and experiments, it was possible to show that the only index that unequivocally measures the brittleness of the tablet understood as elastic until failure is the BFI. If the other two indices can be useful, for example to assess the friability of the tablet in the case of the TBI, they do not make it possible to measure tablet brittleness in the classical sense, i.e. as opposed to ductility.

Comparing failure tests on pharmaceutical tablets: interpretation using experimental results and a numerical approach with cohesive zone models

The mechanical strength is an important quality attribute of pharmaceutical tablets. It can be determined using different failure tests like the Brazilian test or the three-point bending test. Nevertheless, literature shows that different failure tests often give conflicting values of tensile strengths (TS), which are generally calculated using the maximum stress criterion as a failure criterion. This work started from the hypothesis that these discrepancies are in fact due to the application of this criterion which is not suited to study pharmaceutical tablets, first due to heterogeneity of the stress distributions during the tests and second due to the quasi-brittle nature of pharmaceutical tablets. As an alternative, a numerical fracture criterion which is known to be well-suited for quasi-brittle solids (cohesive zone model, CZM) was used and calibrated using experiments. Using this approach, the breaking forces obtained numerically were shown to be in fair agreement with the experimental ones. Above all, the numerical results made it possible to catch the trends when comparing the different failure tests one to another. Especially, the model made it possible to retrieve the factor 2 between the TS obtained by three-point bending and by diametral compression found in the literature.

Effects of Alkaline Solvent Type and pH on Solid Physical Properties of Carrageenan from Eucheuma cottonii

The effects of alkali type and pH on the physical properties of carrageenan have been extensively studied. However, their effects on certain characteristics of solid-state properties of carrageenan have not been identified. This research aimed to investigate the effect of alkaline solvent type and pH on the solid physical properties of carrageenan isolated from Eucheuma cottonii. Carrageenan was extracted from the algae using NaOH, KOH, and Ca(OH)₂ at pHs of 9, 11, and 13. Based on the results of preliminary characterization, including yield, ash content, pH, sulphate content, viscosity, and gel strength, it was found that all samples followed Food and Agriculture Organization (FAO) specifications. The swelling capacity of carrageenan based on the type of alkali was KOH > NaOH > Ca(OH)₂. The FTIR spectra of all samples were in agreement with that of standard carrageenan. The molecular weight (MW) of carrageenan using KOH as the alkali followed the order pH 13 > pH 9 > pH 11, while using NaOH, the order was pH 9 > pH 13 > pH 11, and while using Ca(OH)₂, the order was pH 13 > pH 9 > pH 11. The results of the solid-state physical characterization of carrageenan with the highest MW in each type of alkali showed that the morphology of carrageenan using Ca(OH)₂ has a cubic shape and is more crystal-like. The order of crystallinity of carrageenan using different types of alkali was Ca(OH)₂ (14.44%) > NaOH (9.80%) > KOH (7.91%), while the order of density was Ca(OH)₂ > KOH > NaOH. The order of solid fraction (SF) of the carrageenan was KOH > Ca(OH)₂ > NaOH, while the tensile strength when using KOH was 1.17, when using NaOH it was 0.08, and while using Ca(OH)₂, it was 0.05. The bonding index (BI) of carrageenan using KOH = 0.04, NaOH = 0.02, and Ca(OH)₂ = 0.02. The brittle fracture index (BFI) of the carrageenan was KOH = 0.67, NaOH = 0.26, and Ca(OH)₂ = 0.04. The order of carrageenan solubility in water was NaOH > KOH > Ca(OH)₂. These data can be used as the basis for the development of carrageenan for excipients in solid dosage forms.

Study of Diminutive Granules as Feed Powders for Manufacturability of High Drug Load Minitablets

The maximal amount of drug contained in a minitablet is limited. To reduce the total number of minitablets in a single dose, high drug load minitablets can be prepared from high drug load feed powders by various pharmaceutical processing techniques. Few researchers have however examined the influence of pharmaceutical processing techniques on the properties of high drug load feed powders, and consequently the manufacturability of high drug load minitablets. In this study, silicification of the high drug load physical mix feed powders alone did not yield satisfactory quality attributes and compaction parameters to produce good quality minitablets. The abrasive nature of fumed silica increased ejection force and damage to the compaction tools. Granulation of fine paracetamol powder was crucial for the preparation of good quality high drug load minitablets. The diminutive granules had superior powder packing and flow properties for homogenous and consistent filling of the small die cavities when preparing minitablets. Compared to the physical mix feed powders for direct compression, the granules which possessed higher plasticity, lower rearrangement and elastic energies, yielded better quality minitablets with high tensile strength and rapid disintegration time. High shear granulation demonstrated greater process robustness than fluid bed granulation, with less discernment on the quality attributes of feed powder. It could proceed without fumed silica, with the high shear forces reducing interparticulate cohesivity. An in-depth understanding on the properties of high drug load feed powders with inherently poor compactability and poor flowability is important for the manufacturability of high drug load minitablets.

Evaluation of Alternative Metallic Stearates as Lubricants in Pharmaceutical Tablet Formulation

Magnesium stearate (MgSt) is perhaps one of the most frequently used lubricants in tablet formulation due to its superior lubrication capacity, yet it could also negatively affect the critical quality attributes of pharmaceutical products. Therefore, we provided a rather comprehensive evaluation of another two FDA-approved metallic stearates, sodium stearate (NaSt) and calcium stearate (CaSt), as alternative tablet lubricants. The primary objective of the present study is to comparatively evaluate the physicochemical properties and lubrication efficiency of the three metallic stearates. In addition, it was also aimed to specify the most influential factor for ranking and differentiating the lubricity of various lubricants using principal component analysis. Unit ejection force could be used herein as a simple and the most powerful parameter to evaluate the lubrication performance instead of the friction coefficient. The results suggested that CaSt, MgSt, and NaSt had similar impacts on the mechanical strength of tablets. However, CaSt exhibited insufficient lubrication effects as the formulations containing CaSt showed low pressure transmission ratios, high unit ejection forces, and high friction coefficients. In contrast, both MgSt and NaSt displayed satisfactory lubrication efficiency without negatively impacting tabletability. Notably, the lubrication performance of the formulation containing 0.5 wt% NaSt was almost identical to that of the formulation with 1 wt% MgSt, indicating that NaSt had a remarkable lubrication capability probably due to its high specific surface area. In summary, the findings of this investigation should provide practical information and feasible methodologies to readily determine the lubricity and to sensibly select alternative lubricants for pharmaceutical tablet formulations.

A Multivariate Formulation and Process Development Platform for Direct Compression

The efficient development of robust tableting processes is challenging due to the lack of mechanistic understanding on the impact of raw material properties and process parameters on tablet quality. The experimental determination of the effect of process and formulation parameters on tablet properties and subsequent optimization is labor-intensive, expensive and time-consuming. The combined use of an extensive raw material property database, process simulation tools and multivariate modeling allows more efficient and more optimized development of the direct compression (DC) process. In this study, key material attributes and in-process mechanical properties with a potential effect on tablet processability and tablet properties were identified. In a first step, an extensive characterization of 55 raw materials (over 100 material descriptors) (Van Snick et al., 2018) and 26 formulation blends (31 material descriptors) (Dhondt et al., 2022) was performed. These blends were subsequently compacted on a compaction simulator under multiple process conditions through a design of experiments (DoE) approach. A T-shaped partial least squares (T-PLS) model was established which correlates tablet quality attributes with process settings, raw material properties and blend ratios. During future development of the DC formulation and process for a new active pharmaceutical ingredient (API), this model can then be used to provide a preliminary formulation and compaction process settings as starting point to be further optimized during development trials based on well-defined raw material characteristics and compaction tests. This study hence contributes to a better understanding on the impact of raw material properties and process settings on a DC process and final properties of the produced tablets; and provides a platform allowing a more efficient and more optimized development of a robust tableting process.

On the complexity of predicting tablet capping

Predicting tablet defects, such as capping, that might occur during manufacturing, is a challenge in the pharmaceutical industry. In the literature, different parameters were presented to predict capping but no general consensus seems to have been reached yet. In this article, we chose to study a wide range of products (18 formulations, 8 of which presenting capping) to predict capping on biconvex tablets using the properties characterized on defect-free flat-faced tablets (tensile strength, solid fraction, elastic recovery, etc.), made using the same process parameters. Single parameters and predictive indices presented in the literature were evaluated on this set of formulations and were found not suitable to predict capping. A predictive model was then developed using a decision tree analysis and was found to depend only on three in-die tablet properties: the plastic energy per volume, the in-die elastic recovery and the residual die-wall pressure. This model was tested on another set of 13 formulations chosen to challenge it. The capping behavior of 29 out of the 31 formulations studied in total was well estimated using the developed model with only two products which were predicted to cap and did not. This shows the potential of the used approach in terms of risk analysis and assessment for capping occurrence.

Impact of unloading kinematics on the occurrence of capping during the production of pharmaceutical tablets

Capping is a common defect that can occur during the manufacturing of pharmaceutical tablets. Several studies showed that decreasing the unloading speed of the manufacturing cycle plays a role in the occurrence of such defects. Following this idea, we study in this work the influence of the unloading step on capping using a compaction simulator. Measuring the die wall pressure made it possible to detect precisely that tablets capped just after the unloading (some milliseconds only). To evaluate the impact of the unloading speed on capping, we developed a two-step unloading phase controlled by three manufacturing parameters. It was possible to mitigate capping by decreasing the speed at which the contact between the punches and the tablet was lost. Capping seemed due to dynamical effects related to the release of the axial pressure. The modification of the unloading step to mitigate capping led to significant changes in tablet density but no clear trends were found for the residual die-wall pressure and tablet strength. This work made it possible to improve the understanding of capping. Moreover, the two-step unloading cycle gave a new idea for possible modifications that could be done on rotary presses in order to mitigate capping.

Air entrapment during tablet compression - diagnosis, impact on tableting performance, and mitigation strategies

Air entrapment during powder compression, a phenomenon that can cause tablet defects upon decompression and ejection, was diagnosed for celecoxib powder by comparing its in-die elastic recovery profiles with and without precompression prior to the main compression. Without precompression, the elastic recovery of celecoxib compacts significantly increased from ∼4% at a main compaction pressure of 150 MPa to ∼14% at and above 200 MPa. The large increase in elastic recovery is eliminated when a precompression step is employed. The deaeration of powder by precompression resulted in higher tablet strength, accompanied by lower tablet porosity. Thus, precompression is an effective strategy to mitigate the deleterious effects of air entrapment in tablet manufacturing. We also found that, although entrapped air caused significantly higher elastic recovery, it does not affect the plasticity parameter derived from an in-die Heckel analysis.

Beyond Brittle/Ductile Classification: Applying Proper Constitutive Mechanical Metrics to Understand the Compression Characteristics of Pharmaceutical Materials

Active pharmaceutical ingredients (API) and excipients are often classified as 'brittle' or 'ductile' based on their yield pressure determined through the Heckel analysis. Such a brittle/ductile classification is often correlated to some measure of elasticity, die-wall stresses, and brittle fracture propensities from studies performed with a handful of model excipients. This subsequently gives rise to the presumption that all ductile materials behave similarly to microcrystalline cellulose (MCC) and that all brittle materials to lactose, mannitol, or dicalcium phosphate. Such a 'one-size-fits-all' approach can subsequently lead to inaccurate classification of APIs, which often behave very differently than these model excipients. This study compares the commonly reported mechanical metrics of two proprietary APIs and two classical model excipients. We demonstrate that materials classified as 'ductile' by Heckel's 'standards' may behave very differently than MCC and in some cases may even have a propensity for brittle failure. Our data highlight the complexity of APIs and the need to evaluate a set of mechanical metrics, instead of binary assignments of ductility or brittleness based on quantities that do not fully capture the tableting process, to truly optimize a tablet formulation as part of the overall target product profile.

Characterization of Tableting Speed-Dependent Deformation Properties of Active Pharmaceutical Ingredients in Powder Mixtures Using Out-of-Die Method

A quantitative evaluation method for determining the effect of tableting speed on the compression properties of pharmaceutical powders was investigated in this study. Cilostazol and ibuprofen were used as active pharmaceutical ingredients (APIs) and mixed with lactose monohydrate and microcrystalline cellulose. Viscoelasticity was examined to evaluate the raw material, and stress relaxation tests were conducted to determine the apparent viscosity and elasticity coefficients of the placebo and two APIs. Tablets were prepared using a compaction simulator and a rotary tablet press at the tableting speeds ranging from laboratory to commercial. The in-die or out-of-die strain rate sensitivity (SRS) indices were determined as a measure of the compressibility and compactibility. The results showed that the sensitivity of the out-of-die SRS was higher than that of the in-die SRS. The out-of-die SRS of ibuprofen 20% powder, which showed high elasticity and low viscosity, was 13.3-47.9%, whereas that of the placebo and cilostazol 20% (w/w) powder was <7.5%. A peripheral speed difference of more than 300 mm/s during the out-of-die SRS was sensitive enough to detect the capping tendency. Cilostazol, which has lower elasticity and higher viscosity than ibuprofen, was tested using powder mixtures with the API concentrations of 5-40%; the compressibility SRS was <5% for all API concentrations. In contrast, the compressibility SRS of ibuprofen increased from 4.8 to 81% depending on the API concentration. Using the compressibility SRS as an index, it was possible to extract the tableting speed-dependent compressibility characteristics of API from the powder mixtures containing API.

Use of Atomic Force Microscopy (AFM) to monitor surface crystallization in caffeine-oxalic acid (CAFOXA) cocrystal compacts

Our objective was to monitor the surface crystallization in disordered caffeine-oxalic acid (CAFOXA) cocrystals following exposure to elevated water vapor pressure. This was accomplished using atomic force microscopy (AFM). Disorder was induced in the cocrystal particles by the common pharmaceutical unit operations of milling and compaction. The 'activated' solid, upon exposure to elevated water vapor pressure, had a high propensity to sorb water. This led to a rise in molecular mobility and the surface underwent rapid crystallization to form needle shaped crystals of CAFOXA. Using AFM height and phase imaging, we were able to directly visualize phase transformations on the compact surface. The milled compacts exhibited higher processing induced disorder than the unmilled compacts, thereby accelerating the surface recrystallization.

Lamination of Pharmaceutical Tablets: Classification and Influence of Process Parameters

Lamination is a common industrial problem during the production of pharmaceutical tablets. It corresponds to a failure of the tablet in one or several planes parallel to the surface and passing through the tablet band. But different kinds of lamination exist, and a classification of the different cases is proposed in this work. Type 1 corresponds to a multiple fracture caused by air entrapment. Type 2 occurs because of the shear stresses developing when the tablet goes out of the die. Type 3, which is limited to convex tablets, is due to a tensile stress developing at the center of the tablet at the end of the unloading that further propagates toward the band. One case of each type was studied experimentally in order to test three solutions classically used at the industrial level: slowing down the press, using a precompression and using a tapered die. Results shows that, in coherence with the proposed mechanisms, lamination type 1 can be mitigated by slowing down the press or using a precompression. For type 2, only the tapered die solution stopped lamination. None of the solutions completely solved lamination type 3. Nevertheless, the use of a tapered die decreased the severity of the problem avoiding the propagation of the crack until the surface.

The use of X-ray microtomography to investigate the microstructure of pharmaceutical tablets: Potentials and comparison to common physical methods

Within this study, tablets microstructure was investigated by X-ray microtomgraphy. The aim was to gain information about their microstructure, and thus, derive deeper interpretation of tablet properties (mechanical strength, component distribution) and qualified property functions. Challenges in image processing are discussed for the correct identification of solids and voids. Furthermore, XMT measurements are critically compared with complementary physical methods for characterizing active pharmaceutical ingredient (API) content and porosity and its distribution (mercury porosimetry, calculated tablet porosity, Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM)). The derived porosity by XMT is generally lower than the calculated porosity based on geometrical data due to the resolution of the XMT in relation to the pore sizes in tablets. With rising compactions stress and API concentration, deviations between the actual and the calculated API decrease. XMT showed that API clusters are present for all tablets containing >1 wt% of ibuprofen. The 3D orientation of the components is assessable by deriving cord lengths along all dimensions of the tablets. An increasing compaction stress leads to rising cord lengths, showing higher connectivity of the respective material. Its lesser extent in the z-direction illustrates the anisotropy of the compaction process. Additionally, cracks in the fabric are identified in tablets without visible macroscopic damage. Finally, the application of XMT provides valuable structural insights if its limitations are taken into account and its strengths are fostered by advanced pre- and post-processing.

Use of impulse excitation technique for the characterization of the elastic anisotropy of pharmaceutical tablets

Capping and lamination are common defects occurring during the manufacturing of pharmaceutical tablets. Several studies showed that tablet anisotropy can play a role in the occurrence of such defects. In this work, we propose a new and easy methodology to characterize the anisotropy of flat-faced cylindrical tablets, which are considered as transversally isotropic due to the process, through the study of their elastic properties using impulse excitation technique and finite-element method (FEM) simulations. The study was performed for tablets with a thickness-to-diameter-ratio between 0.160 and 0.222. FEM simulations showed that it was possible to determine three out of the five elastic constants of the tablet using the first three natural vibration modes. An anisotropic index was then built as the ratio of the two apparent shear moduli. Moreover, in order to simplify the estimation of tablet anisotropy and to avoid the systematic use of FEM simulations, an analytical model was also developed. It only requires the measurement of the tablet dimensions and of the first three natural frequencies. Using this technique, experimental measurements on tablets made of classical pharmaceutical excipients were done and found coherent with the existing literature. This indicates thus that this methodology is a quick, easy and reliable characterization method in order to access tablet anisotropy.

How can single particle compression and nanoindentation contribute to the understanding of pharmaceutical powder compression?

The deformation behaviour of a powder and, thus, of the individual particles is a crucial parameter in powder compaction and affects powder compressibility and compactibility. The classical approach for the characterization of the deformation behaviour is the performance of powder compression experiments combined with the application of mathematical models, such as the Heckel-Model, for the derivation of characteristic compression parameters. However, the correlation of these parameters with the deformation behaviour is physically often not well understood. Single particle compression and nanoindentation enables the in-depth investigation of the deformation behaviour of particulate materials. In this study, single particle compression experiments were performed for the characterization of the deformation behaviour of common pharmaceutical excipients and active pharmaceutical ingredients (APIs) with various, irregular particle morphologies of industrial relevance and the findings are compared with the results from powder compression. The technique was found useful for the characterization and clarification of the qualitative deformation behaviour. However, the derivation of a quantitative functional relationship between single particle deformation behavior and powder compression is limited. Nanoindentation was performed as complementary technique for the characterization of the micromechanical behavior of the APIs. A linear relationship between median indentation hardness and material densification strength as characteristic parameter derived by in-die powder compression analysis is found.

An insight into predictive parameters of tablet capping by machine learning and multivariate tools

Capping is the frequently observed mechanical defect in tablets arising from the sub-optimal selection of the formulation composition and their robustness of response toward process parameters. Hence, overcoming capping propensity based on the understanding of suitable process and material parameters is of utmost importance to expedite drug product development. In the present work, 26 diverse formulations were characterized at commercial tableting condition to identify key tablet properties influencing capping propensity, and a predictive model based on threshold properties was established using machine learning and multivariate tools. It was found that both the compaction parameters (i.e., compaction pressure, radial stress transmission characteristics, and Poisson's ratio), and the material properties, (i.e., brittleness, yield strength, particle bonding strength and elastic recovery) strongly dictate the capping propensity of a tablet. In addition, ratio of elastic modulus in the orthogonal direction in a tablet and its variation with porosity were notable quantitative metrics of capping occurrence.

The influence of particle size on the application of compression and compaction models for tableting

The physical characteristics of raw materials determine powder compression and compaction performance as relevant in pharmaceutical processes. For instance, the influence of initial particle size on powder compression and the resulting strength of specimen are highly complex and are still not sufficiently understood. Existing studies are often limited to materials with well-defined deformation behaviour, such as purely brittle or ductile. However, the deformation behaviour of active pharmaceutical ingredients (APIs) is often more complex. In this study, the influence of initial particle size on powder compressibility and compactibility is systematically characterized by consideration of in-die compressibility, specific energies, quick elastic recovery, tablet porosity and, tensile strength for the binder microcrystalline cellulose and three APIs. The decrease of particle size leads to an increase of the resistance against compression by trend and probably to a different contribution of the acting deformation mechanisms. The compactibility is increased with decreasing particle size because of the increasing number of bonds in a cross-sectional area of the tablet, as found by the application of the model of Rumpf. Furthermore, it is found that the model of Rumpf combined with the JKR model provides a meaningful property function to estimate tablet tensile strength.

Tableting of mini-tablets in comparison with conventionally sized tablets: A comparison of tableting properties and tablet dimensions

Mini-tablets are solid dosage forms with increasing interest for pharmaceutical industry due to clinical and biopharmaceutical benefits. But technological aspects on mini-tableting are not fully investigated. Therefore, the impact of punch size and tableting pressure for industrially relevant excipients like microcrystalline cellulose, lactose, isomalt and Ludiflash® are investigated using 8 and 11.28 mm punches for conventionally sized tablets and 1,2 and 3 mm punches for mini-tablets. For evaluation of the effect of tablet size on deformation behaviour and mechanical properties, compressibility, compactibility and tabletability plots are created and evaluated. Deformation behaviour is analysed by In-Die Heckel plot and modified Weibull function. Further, specific plastic energy (SPE) profiles are generated out of force-displacement plots. The effect of the adjustment of the aspect ratio towards 1 as in conventionally sized tablets on deformation behaviour and tabletability is analysed. The effect of tablet size on deformation behaviour mainly showed lower yield pressures for conventionally sized tablets, whereas comparable SPEs were obtained with all tablet sizes. Furthermore, mini-tablets indicate better compactibility, as (depending on the excipient) higher tensile strengths were obtained at lower solid fractions. However, no superior tabletability properties are obtained for mini-tablets compared to conventionally sized tablets.

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Mini-tablets show higher compactibility compared to conventionally sized tablets.

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Mini-tablets show specific plastic energy profiles similar to conventional tablets.

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There is no trend for better tabletability for mini-tablets or conventionally sized tablets.

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Highest yield pressures are obtained with mini-tablets.

Characterization and modeling of the viscoelasticity of pharmaceutical tablets

Evolution of the compaction properties of powders with the compaction speed (strain rate sensitivity, SRS) is a common phenomenon during the manufacturing of pharmaceutical tablets. Nevertheless, several different phenomena can be responsible of the SRS like friction, viscoelasticity, viscoplasticity or air entrapment. In this work, an original experimental methodology was developed to characterize specifically the viscoelasticity of tablets using a compaction simulator. After various compressions, tablets were finally loaded elastically at different constant strain rates. This methodology made it possible to measure the apparent bulk and shear moduli as a function of the strain rate. The methodology was successfully applied to microcrystalline cellulose (MCC), Starch, Lactose monohydrate (GLac) and Anhydrous Calcium Phosphate (ACP). No significant evolution of the moduli was found for Lac and ACP as expected. On the contrary, for MCC and Starch, both shear and bulk moduli were found to increase along with the strain rate. The viscoelastic behavior was then successfully modeled using prony series. Assessment of the model parameters was achieved by inverse identification using an analytical model and a finite element analysis.

Roller compaction: Ribbon splitting and sticking

Roller compaction is the main technique employed in dry granulation. Ribbon sticking and splitting are among the major factors that can hinder the use of this process for some formulations. Ribbon splitting can occur either transversally (through the ribbon thickness) or longitudinally (through the ribbon width). It was observed that transverse splitting is commonly associated with sticking of the split ribbons to the rollers and results in an inferior performance of the process. Longitudinal splitting is associated with an across-width distribution of the ribbon density so that there may an adverse effect on the mechanical strength and dissolution properties of the tablets formed from the milled granules. The aim of the current work was to elucidate the mechanisms of splitting by an experimental study involving single component powders with a range of yield strengths, including those that are commonly used as excipients. Both smooth and knurled rollers were employed without and with lubrication by applying magnesium stearate to the rollers. The minimum gap was fixed and the maximum roll stress was varied. The observed trends for the smooth rollers were rationalised in terms of a splitting index, which is a measure of the residual stresses driving crack growth relative to the tensile strength of the ribbons. There was a lower limit at which splitting was observed but the occurrence of transverse splitting decreased and that for longitudinal splitting increased with increasing values of the index, which was accompanied by an increase in mixed transverse-longitudinal splitting. Transverse splitting was always associated with sticking to the rollers and was prevented by external lubrication. The main difference with the knurled rollers was that in some cases transverse splitting occurred without sticking to the rollers. A detailed discussion of the mechanisms involved is presented.

A Mathematical Approach to Consider Solid Compressibility in the Compression of Pharmaceutical Powders

In-die compression analysis is an effective method for the characterization of powder compressibility. However, physically unreasonable apparent solid fractions above one or apparent in-die porosities below zero are often calculated for higher compression stresses. One important reason for this is the neglect of solid compressibility and hence the assumption of a constant solid density. In this work, the solid compressibility of four pharmaceutical powders with different deformation behaviour is characterized using mercury porosimetry. The derived bulk moduli are applied for the calculation of in-die porosities. The change of in-die porosity due to the consideration of solid compressibility is for instance up to 4% for microcrystalline cellulose at a compression stress of 400 MPa and thus cannot be neglected for the calculation of in-die porosities. However, solid compressibility and further uncertainties from, for example the measured solid density and from the displacement sensors, are difficult or only partially accessible. Therefore, a mathematic term for the calculation of physically reasonable in-die porosities is introduced. This term can be used for the extension of common mathematical models, such as the models of Heckel and of Cooper & Eaton. Additionally, an extended in-die compression function is introduced to precisely describe the entire range of in-die porosity curves and to enable the successful differentiation and quantification of the compression behaviour of the investigated pharmaceutical powders.

Fifty-Eight Years and Counting: High-Impact Publishing in Computational Pharmaceutical Sciences and Mechanism-Based Modeling

With this issue of the Journal of Pharmaceutical Sciences, we celebrate the nearly 6 decades of contributions to mechanistic-based modeling and computational pharmaceutical sciences. Along with its predecessor, The Journal of the American Pharmaceutical Association: Scientific Edition first published in 1911, JPharmSci has been a leader in the advancement of pharmaceutical sciences beginning with its inaugural edition in 1961. As one of the first scientific journals focusing on pharmaceutical sciences, JPharmSci has established a reputation for publishing high-quality research articles using computational methods and mechanism-based modeling. The journal's publication record is remarkable. With over 15,000 articles, 3000 notes, and more than 650 reviews from industry, academia, and regulatory agencies around the world, JPharmSci has truly been the leader in advancing pharmaceutical sciences.

Effect of friction between powder and tooling on the die-wall pressure evolution during tableting: Experimental and numerical results for flat and concave punches

Tablet final properties are mainly determined during the compaction process by the evolution of the stresses applied to the powder. Any process or product parameter that may influence this stress evolution may have a direct impact on the tablet final properties. In this article, we studied the influence of the friction between the tooling and the powder on the evolution of the die-wall pressure during compaction using flat and concave punches. Experimental studies were performed on microcrystalline cellulose as well as numerical studies using finite element method (FEM) simulation. Both methodologies indicate that increasing the friction between the powder and the tooling promotes an increase in the die-wall pressure during tableting. This is in contradiction with results that can be found in the literature. Moreover, the results of this study showed that for flat punches, the stress evolution is mainly driven by the die/powder friction. On the contrary, for concave punches, changing the punches/powder friction have also a consequence in the evolution of the die-wall pressure. This could have practical consequences in sticking situations where, due film formation on the punches, the friction between the punches and the powder may change during tableting.