Thermodynamic analysis of energy dissipation by pharmaceutical tablets during stress unloading

The effects of spray drying, HPMCAS grade, and compression speed on the compaction properties of itraconazole-HPMCAS spray dried dispersions

Spray dried dispersions (SDDs) have the potential to dramatically improve the oral bioavailability of drugs with poor water solubility. However, SDDs tend to have material attributes, such as small particle size, low bulk density, and poor flowability, which are undesirable for downstream processing such as tableting. The objective was to perform a comprehensive compaction characterization of both physical mixtures and SDDs consisting of itraconazole (ITZ) and hypromellose acetate succinate (HPMCAS) to elucidate process and material influences on compressibility and compactibility. We fabricated SDDs with 20% ITZ as a model BCS Class 2 drug and 80% HPMCAS as a polymer carrier. Results indicate that SDDs, as well physical mixtures of ITZ and HPMCAS, were easily deformable with similar compressibility profiles across all compression speeds. Analysis of Heckel plots revealed that yield pressures were fairly low for both physical mixtures and SDDs (43.97-59.75 MPa), indicative of ductile materials. SDDs had a much greater propensity to laminate, especially at higher compression speeds, compared to physical mixtures. This difference is likely due to the higher elastic recovery of SDDs. However, for intact tablets, the mechanical strength of compacts from SDDs tended to be higher than those produced from physical mixtures, likely due to the much smaller particle size of the SDDs. Importantly, examination of the compacts with differential scanning calorimetry did not detect any drug crystallization as a result of compaction. In conclusion, while spray drying did not significantly alter the compressibility of binary mixtures ITZ and HPMCAS, it dramatically impacted compactibility and tabletability, increasing elastic recovery, and making the mixtures more prone to lamination. However, at low compression speeds, SDDs produced tablets with higher tensile strength than physical mixtures.

Transient Temperature Monitoring of Pharmaceutical Tablets During Compaction Using Infrared Thermography

Manufacturing of pharmaceutical tablets from powders is always accompanied by the conversion of irreversible mechanical work of compaction into heat. The heat is generated due to friction between powder particles, particles and the die wall, plastic deformation of particles, bonding, and other irreversible processes. The resulting temperature increase potentially might have significant effects on a tablet's mechanical properties, disintegration time, and drug release. In the present work, we show that using infrared thermography as a nondestructive and noncontact process analytical technology (PAT) tool to measure the tablet's rate of cooling, in contrast to the temperature evolution, can be directly related to the tablet's thermal diffusivity. Results show the potential capabilities of this technique to discriminate and toward predicting tensile strength of tablets between same formulations produced at same compaction force but experienced different process shear conditions. Correlation of the tablet's tensile strength, relative density, and rate of cooling at regular regime with respect to different process shear is also discussed.

Powder densification. 1. Particle-particle basis for incorporation of viscoelastic material properties

The present investigation was undertaken to examine the basic unit of densification: the particle-particle indentation. The true interparticle contact area that is established during densification ultimately determines the quality of the tablet compact. By examining the interfacial contact between mutually indenting viscoelastic particles, the process of contact evolution may be represented in mathematical form through extension of the classical Hertzian elastic contact description to encompass material viscoelastic terms. In this way, the time-dependent response of materials to applied loads may be addressed explicitly. The effects of rates of applied loading and maximum load levels were also considered. This analysis was based on viscoelastic stress data collected using an instrumented Instron analyzer during the densification of PMMA/coMMA, a pharmaceutical polymeric coating material. A crossed cylinder matrix compaction geometry was used to simulate the geometry of two mutually indenting spherical particles. Numerical and graphical solutions delineating the relationship between contact area evolution and the prescribed loading force are presented. This particle-particle description of the contacting interface serves as a unit basis for describing the entire powder bed. The powder bed may ultimately be modeled as a collection of these particles in contact.

Nondestructive viscoelastic analysis of anisotropy in compressed tablets

A nondestructive procedure for determining anisotropy levels in compressed tablets was developed based on dynamic stress/strain measurements of viscoelasticity in the axial and radial directions. An instrument was designed and built capable of 10 nm resolution of strain and operating over the frequency range 1-64 Hz. Tablets of 11 pharmaceutical excipients and drugs, representing both plastic and brittle materials, were found to behave as three-parameter viscoelastic solids in both the axial and radial directions. Elastic and viscous parameters were computed, based on frequency dependence over the range studied, and were found to differ greatly in the axial and radial directions. Degrees of anisotropy, defined as the ratio of the uncoupled spring parameters in the radial vs the axial direction, were found to range from 2- to 12-fold over the materials studied. These levels of anisotropy are thought to be typical of tablets manufactured in a rotary press under normal conditions. Measurements of this type can be easily made routinely as a further means of quality control and reflect an important feature of the internal structure of compressed tablets.

Characterization of compressibility and compactibility of poly(ethylene oxide) polymers for modified release application by compaction simulator

Poly(ethylene oxide) polymers (PEO) appear to have great potential for controlled release applications. These polymers are hydrophilic with good water solubility, low toxicity, and high swelling capacity. As part of formulation optimization for a large-scale solid dosage form production, physicomechanical characterization of PEO was undertaken using a compaction simulator. Heckel plots for all PEOs were constructed, and yield pressures (Py) at different punch velocities were calculated from the linear portion of the plots. Low Py values, increase of Py with increasing punch speed, upward curvature of the plot, and strain rate sensitivity values indicate that the densification process and consolidation mechanism for PEOs of various molecular weights (0.2 x 10(6) to 7 x 10(6)) are identical and follow plastic deformation. PEOs have a high degree of crystallinity (57-85%) and show significant axial recovery (15-25%) upon decompression and ejection. The low Py values (58-78 MPa) and low mean compaction pressures demonstrate that volume reduction (compressibility) under pressure is excellent. However, due to viscoelastic behavior and large axial expansion, tablets of relatively low tensile strength are produced. These observations suggest the need to blend PEO with highly compactible excipients in order to produce tables on a high-speed production press.

Deformation of the Stokes B2 rotary tablet press: quantitation and influence on tablet compaction

Deformations that affect the vertical punch displacement of a Stokes B2 rotary tablet press were characterized with a cathetometer. The press deformation was found to be elastic for both the upper and lower compression roller assemblies. However, the upper and lower compression roller assemblies have different Hookian spring constants: 8.58 x 10(4) and 5.18 x 10(4) kN/m for the upper and lower assemblies, respectively. Using two-way analysis of variance, the Hookian spring constants were shown to be independent of compaction phases and lower punch penetration setting. To study the influence of press deformation on tablet compaction, the Hookian spring constants were factored into the caculation of the incremental work of compaction for dibasic calcium phosphate dihydrate and microcrystalline cellulose. As the peak compression pressure increases, the force-displacement work done on the tablet during the loading phase decreases relative to calculations that neglect press deformation. This decrease in force-displacement work was attributed to elastic press deformation, which absorbs energy during the loading phase and then releases this energy later in the compaction cycle, altering the punch-displacement profile. The rate at which the press stores and releases elastic energy depends in part upon the viscoelastic properties of the tablet. Based upon these results, the coupling between press elasticity and a tablet's viscoelastic properties should be accounted for when analyzing tablet compaction or trying to simulate the punch-displacement profile of a tablet press that deforms during compaction.