Impact of fill-level in twin-screw granulation on critical quality attributes of granules and tablets

Towards the prediction of barrel fill level in twin-screw wet granulation

The barrel fill level is defined as the fraction of the free available volume for a given screw configuration that is occupied by the wet material and is an interplay of the material throughput, screw speed, screw setup, barrel length of the twin-screw granulator used and the properties of the starting material. The fill level has a major impact on mixing and densification of the wetted mass and thus on the granules produced. It influences the twin-screw granulation process accordingly. In the current study, a model has been developed which is predictive in terms of material hold-ups in the barrel at various process settings by considering the geometries of the different screw elements in a configuration and the conveying velocity of the wet mass through the barrel. The model was checked on two granulators of different dimensions with various screw configurations, different materials and at different process settings. The model represents a step forward in predicting the barrel fill level but further research with a broader spectrum of materials, screw configurations and process settings is still needed and additional twin-screw granulators of other dimensions must be investigated.

Mechanistic modeling of twin screw wet granulation for pharmaceutical formulations: Calibration, sensitivity analysis, and model-driven workflow

Wet granulation, a particle size enlargement process, can significantly enhance the critical quality attributes of powders and improve the ability to form tablets in pharmaceutical manufacturing. In this study, a mechanistic-based population balance model is applied to twin screw wet granulation. This model incorporated a recently developed breakage kernel specifically designed for twin screw granulation, along with nucleation, layering, and consolidation. Calibration and validation were performed on Hydrochlorothiazide and Acetaminophen formulations, which exhibit different particle size and wettability characteristics. Utilizing a compartmental experimental dataset, a comprehensive global sensitivity analysis identified critical inputs impacting quality attributes. The study revealed that the nucleation rate process model, effectively represented particle size distributions for both formulations. Adjustments to nucleation and breakage rate parameters, influenced by material properties and screw configuration, improved the model's accuracy. A model-driven workflow was proposed, offering step-by-step guidelines and facilitating PBM model usage, providing essential details for future active pharmaceutical ingredient (API) formulations.

Cracking the code: Spatial heterogeneity as the missing piece for modeling granular fluidized bed drying

Pharmaceutical twin-screw wet granulation is a multifaceted and intricate process pivotal to drug product development. Accurate modeling of this process is indispensable for optimizing manufacturing parameters and ensuring product quality. The fluid bed dryer, an integral component of this granulation process, significantly influences the granular critical quality attributes. This study builds upon prior research by integrating experimental findings on granule segregation during fluid bed drying into an existing compartmental model, enhancing its predictive capabilities. An additional model layer on granule segregation behavior is composed and integrated into the existing model structure in this study. The added model compartment describes probability distributions on the vertical position of granules within each granule size class considered. To beware of overfitting, predictions of both the moisture content after drying and the granule bed temperature throughout drying are discussed in this study relative to experimental data from earlier published studies. These independent analyses demonstrated a marked improvement in prediction accuracy compared to earlier published model structures. The refined model accurately predicts the residual moisture content after drying for an untrained formulation. Moreover, it simultaneously makes accurate predictions of the granular bed temperature, which emboldens its structural correctness. This advancement makes it a powerful tool for predicting the behavior of the pharmaceutical fluid bed drying, which holds significant promise to facilitate pharmaceutical product development.

Investigating the Effects of Mixing Dynamics on Twin-Screw Granule Quality Attributes via the Development of a Physics-Based Process Map

Twin-screw granulation (TSG) is an emerging continuous wet granulation technique that has not been widely applied in the industry due to a poor mechanistic understanding of the process. This study focuses on improving this mechanistic understanding by analyzing the effects of the mixing dynamics on the granule quality attributes (PSD, content uniformity, and microstructure). Mixing is an important dynamic process that simultaneously occurs along with the granulation rate mechanisms during the wet granulation process. An improved mechanistic understanding was achieved by identifying and quantifying the physically relevant intermediate parameters that affect the mixing dynamics in TSG, and then their effects on the granule attributes were analyzed by investigating their effects on the granulation rate mechanisms. The fill level, granule liquid saturation, extent of nucleation, and powder wettability were found to be the key physically relevant intermediate parameters that affect the mixing inside the twin-screw granulator. An improved geometrical model for the fill level was developed and validated against existing experimental data. Finally, a process map was developed to depict the effects of mixing on the temporal and spatial evolution of the materials inside the twin-screw granulator. This process map illustrates the mechanism of nucleation and the growth of the granules based on the fundamental material properties of the primary powders (solubility and wettability), liquid binders (viscosity), and mixing dynamics present in the system. Furthermore, it was shown that the process map can be used to predict the granule product quality based on the granule growth mechanism.

Scale-up in twin-screw wet granulation: impact of formulation properties

The focus of this study was to investigate the sensitivity of different drug formulations to differences in process parameters based on previously developed scale-up strategies. Three different formulations were used for scale-up experiments from a QbCon® 1 with a screw diameter of 16 mm and a throughput of 2 kg/h to a QbCon® 25 line with a screw diameter of 25 mm and a throughput of 25 kg/h. Two of those formulations were similar in their composition of excipients but had a different API added to the blend to investigate the effect of solubility of the API during twin-screw wet granulation, while the third formulation was based on a controlled release formulation with different excipients and a high fraction of HPMC. The L/S-ratio had to be set specifically for each formulation as depending on the binder and the overall composition the blends varied significantly in their response to water addition and their overall granulation behavior. Before milling there were large differences in granule size distributions based on scale (Earth Mover's Distance 140-1100 µm, higher values indicating low similarity) for all formulations. However, no major differences in granule properties (e.g. Earth Mover's Distance for GSDs: 23-88 µm) or tablet tensile strength (> 1.8 MPa at a compaction pressure of 200 MPa for all formulations with a coefficient of variation < 0.1, indicating high robustness for all formulations) were observed after milling, which allowed for a successful scale-up independent of the selected formulations.

Integrated Continuous Wet Granulation and Drying: Process Evaluation and Comparison with Batch Processing

The pharmaceutical industry is in the midst of a transition from traditional batch processes to continuous manufacturing. However, the challenges in making this transition vary depending on the selected manufacturing process. Compared with other oral solid dosage processes, wet granulation has been challenging to move towards continuous processing since traditional equipment has been predominantly strictly batch, instead of readily adapted to material flow such as dry granulation or tablet compression, and there have been few equipment options for continuous granule drying. Recently, pilot and commercial scale equipment combining a twin-screw wet granulator and a novel horizontal vibratory fluid-bed dryer have been developed. This study describes the process space of that equipment and compares the granules produced with batch high-shear and fluid-bed wet granulation processes. The results of this evaluation demonstrate that the equipment works across a range of formulations, effectively granulates and dries, and produces granules of similar or improved quality to batch wet granulation and drying.

Comparison of scale-up strategies in twin-screw wet granulation

The aim of this study was to compare different scale-up strategies in twin-screw wet granulation and investigate the impact of the selected strategy on granule and tablet properties for a defined formulation. For the scale-up, a granulation process was transferred from a QbCon® 1 with a screw diameter of 16mm to a QbCon® 25 line with a screw diameter of 25mm. Three different scale-up strategies were introduced based on differences in process parameters and their resulting effects on various aspects. such as the powder feed number as a surrogate for the barrel fill level or the circumferential speed. Both are highly dependent on screw diameter and screw speed (SS), while the barrel fill level also depends on the overall throughput. Granules produced on the larger scale were significantly larger due to the larger gap size in the granulator, however, these differences were eliminated after milling. Despite major differences in powder feed number, circumferential speed, overall throughput and SS, product properties for both tablets and granules were strikingly similar after milling on both scales and with all applied strategies. For the selected formulation the effect of varying liquid to solid ratio at the same scale was much higher than the differences between scale-up strategies. The results of this study are promising for future process scale-up from lab scale to production scale in twin-screw wet granulation, as they are indicating towards a robust granulation process leading to similar tablet properties afterwards.

Evaluation of the influence of material properties and process parameters on granule porosity in twin-screw wet granulation

In recent years, continuous twin-screw wet granulation (TSWG) is gaining increasing interest from the pharmaceutical industry. Despite the many publications on TSWG, only a limited number of studies focused on granule porosity, which was found to be an important granule property affecting the final tablet quality attributes, e.g. dissolution. In current study, the granule porosity along the length of the twin-screw granulator (TSG) barrel was evaluated. An experimental set-up was used allowing the collection of granules at the different TSG compartments. The effect of active pharmaceutical ingredient (API) properties on granule porosity was evaluated by using six formulations with a fixed composition but containing APIs with different physical-chemical properties. Furthermore, the importance of TSWG process parameters liquid-to-solid (L/S) ratio, mass feed rate and screw speed for the granule porosity was evaluated. Several water-related properties as well as particle size, density and flow properties of the API were found to have an important effect on granule porosity. While the L/S ratio was confirmed to be the dictating TSWG process parameter, granulator screw speed was also found to be an important process variable affecting granule porosity. This study obtained crucial information on the effect of material properties and process parameters on granule porosity (and granule formation) which can be used to accelerate TSWG process and formulation development.

Dry granulation of vitamin D3 and iron in corn starch matrix: Powder flow and structural properties

In this work, a twin-screw dry granulation (TSDG) was adopted to produce vitamin D3 (VD3) and iron blended dry granules using corn starch as an excipient. Response surface methodology was applied to determine the effect of the formulation compositions (VD3 and iron) on granule properties [tapped bulk density, oil holding capacity, and volumetric mean particle size (D_v50)]. Results indicated that the model fitted well, and responses, in particular flow properties, were significantly affected by the composition. The D_v50 was only influenced by the addition of VD3. The flow properties were characterized by the Carr index and Hausner ratio, which indicated very poor flow of the granules. Scanning electron microscopy with energy dispersive spectroscopy confirm the presence and distribution of Fe⁺⁺ and VD3 in the granules. Overall, TSDG was proven to be a simple alternative method for the preparation of dry granules of VD3 and iron in a blend.

Continuous integrated production of glucose granules with enhanced flowability and tabletability

Glucose is widely used in both the food and pharmaceutical industry. However, the application of industrially crystallized glucose in solid dosage forms is challenged by its poor flowability and tabletability. To improve these characteristics continuous twin-screw granulation was tested, which has the potential to be integrated into the continuous production of solid glucose from corn starch. A completely continuous manufacturing line (including drying and milling) was developed and the different production steps were examined and synchronized. Our line was supplemented with an in-line applicable near-infrared spectroscopic probe to monitor the moisture content of the milled granules in real-time. The flowability and tabletability of the powder improved significantly, and tablets with acceptable breaking force (greater than 100 N) could be prepared from the granules. The developed continuous line can be easily installed into the industrial solid glucose production process resulting in pure glucose granules with adequate flow properties and tabletability in a simple, continuous and efficient way.

Risk Assessment for a Twin-Screw Granulation Process Using a Supervised Physics-Constrained Auto-encoder and Support Vector Machine Framework

Quality risk management is an important task when it pertains to the pharmaceutical industry, as this is directly related to product performance. With the ICH Q9 guidelines, several regulatory bodies have encouraged the pharmaceutical industry to implement risk management plans using scientific and systemic approaches such as quality-by-design to asses product quality. However, the implementation of such methods has been challenging as assessment of risks requires accurate quantitative models to predict changes in quality when variations occur. This study describes a framework that quantitatively assesses risk for a twin screw wet granulation process. This framework consists of a physics-constrained autoencoder system, whose outputs are constrained using physics-based boundary conditions. The latent variables obtained from the auto-encoder are used in a support vector machine-based classifier to understand the granule growth behavior occurring within the system. This framework is able to predict the process outcomes with 86% accuracy and classify the granule growth regimes with a true positive rate of 0.73. Based on the classification the risk associated with the process can be estimated.

Faster to First-time-in-Human: Prediction of the liquid solid ratio for continuous wet granulation

In early development, when active pharmaceutical ingredient (API) is in short supply, it would be beneficial to reduce the number of experiments by predicting a suitable L/S ratio before starting the product development. The aim of the study was to decrease development time and the amount of API needed for the process development of high drug load formulations for continuous twin-screw wet granulation (TSWG). Mixer torque rheometry was used as a pre-formulation tool to predict the suitable L/S ratios for granulation experiments. Three different values that were based on the MTR curves, were determined and assessed for their ability to predict the suitable L/S ratio for TSWG. Three APIs (allopurinol, paracetamol and metformin HCl) were used as model substances in high drug load formulations containing 60% drug substance. The MCC-mannitol ratio was varied to assess the optimal composition for the high-dose formulations. The API solubility affected the mixer torque rheometer (MTR) curves and the optimum L/S ratio for TSWG. The highly soluble metformin needed a much lower L/S ratio compared with allopurinol and paracetamol. A design space was determined for each API based on granule flowability and tablet tensile strength. The flowability of the granules and tensile strength of the tablets improved with an increasing L/S ratio. The MCC-mannitol filler ratio had a significant effect on tabletability for paracetamol and metformin, and these APIs having poor compaction properties needed higher MCC ratios to achieve the 2 MPa limit. The MCC-mannitol ratio had no effect on the granule flow properties. Instead, API properties had the largest influence on both granule flow properties and tensile strength. Based on this study, both the L/S ratio and MCC-mannitol ratio are crucial in controlling the critical quality attributes in high drug load formulations processed by TSWG. The optimum flow and tablet mechanical properties were achieved when using 75:25 MCC-mannitol ratio. Both start of the slope and 2/3 of the L/S ratio at the maximum torque in MTR provided a solid guideline to aim for in a TSWG experiment.

The Effect of Binder Types on the Breakage and Drying Behavior of Granules in a Semi-Continuous Fluid Bed Dryer after Twin Screw Wet Granulation

Current study investigated the effect of different binder types on the granule drying process and the granule breakage behavior in a semi-continuous fluid bed dryer integrated in the C25 ConsiGma-system. The studied binders (i.e. hydroxypropyl pea starch, hydroxypropyl methylcellulose E15, polyvinylpyrrolidone K12, and starch octenyl succinate CO 01) required different liquid amounts to produce similar granule quality. These different liquid requirements were translated into different drying conditions for each binder to result in sufficiently dry granules at the end of a drying cycle. By comparing the size distribution of the granules before entering and after exiting the fluid bed dryer, granule breakage could be evaluated. No effect of the binder type on the granule breakage during drying was observed. However, differences in granule breakage were observed for the binders when processed with the horizontal set-up of the C25 system, as granule breakage during pneumatic transport depended on the binder type. Only one binder (hydroxypropyl pea starch) allowed to avoid granule breakage during the entire process. Furthermore, this research showed that the drying process was mainly steered by the liquid requirements for granulation, and that these liquid requirements depended on the binder used.

Metamorphosis of Twin Screw Extruder-Based Granulation Technology: Applications Focusing on Its Impact on Conventional Granulation Technology

In order to be at pace with the market requirements of solid dosage forms and regulatory standards, a transformation towards systematic processing using continuous manufacturing (CM) and automated model-based control is being thought through for its fundamental advantages over conventional batch manufacturing. CM eliminates the key gaps through the integration of various processes while preserving quality attributes via the use of process analytical technology (PAT). The twin screw extruder (TSE) is one such equipment adopted by the pharmaceutical industry as a substitute for the traditional batch granulation process. Various types of granulation techniques using twin screw extrusion technology have been explored in the article. Furthermore, individual components of a TSE and their conjugation with PAT tools and the advancements and applications in the field of nutraceuticals and nanotechnology have also been discussed. Thus, the future of granulation lies on the shoulders of continuous TSE, where it can be coupled with computational mathematical studies to mitigate its complications.

Indirect monitoring of ultralow dose API content in continuous wet granulation and tableting by machine vision

This paper presents new machine vision-based methods for indirect real-time quantification of ultralow drug content during continuous twin-screw wet granulation and tableting. Granulation was performed with a solution containing carvedilol (CAR) as API in the ultralow dose range (0.05w/w% in the granule) and the addition of riboflavin (RI) as a coloured tracer. An in-line calibration in the range of 0.047-0.058 w/w% was prepared for the measurement of CAR concentration using colour analysis (CA) and particle size analysis (PSA), and the validation with HPLC resulted in respective relative errors of 2.62% and 2.30% showing great accuracy. To improve the technique, a second in-line calibration was conducted in a broader CAR concentration range of 0.039-0.063 w/w% utilizing only half the amount of RI (0.045 w/w%), while doubling the output of the granulation line to 2 kg/h, producing a relative error of 4.51% and 4.29%, respectively. Finally, it was shown that the CA technique can also be carried on to monitor the CAR content of tablets in the 42-62 μg dose range with a relative error of 5.20%. Machine vision was proven to be a potent indirect method for the in-line, determination and monitoring of ultralow API content during continuous manufacturing.

Process Analytical Technology Tools for Monitoring Pharmaceutical Unit Operations: A Control Strategy for Continuous Process Verification

Various frameworks and methods, such as quality by design (QbD), real time release test (RTRT), and continuous process verification (CPV), have been introduced to improve drug product quality in the pharmaceutical industry. The methods recognize that an appropriate combination of process controls and predefined material attributes and intermediate quality attributes (IQAs) during processing may provide greater assurance of product quality than end-product testing. The efficient analysis method to monitor the relationship between process and quality should be used. Process analytical technology (PAT) was introduced to analyze IQAs during the process of establishing regulatory specifications and facilitating continuous manufacturing improvement. Although PAT was introduced in the pharmaceutical industry in the early 21st century, new PAT tools have been introduced during the last 20 years. In this review, we present the recent pharmaceutical PAT tools and their application in pharmaceutical unit operations. Based on unit operations, the significant IQAs monitored by PAT are presented to establish a control strategy for CPV and real time release testing (RTRT). In addition, the equipment type used in unit operation, PAT tools, multivariate statistical tools, and mathematical preprocessing are introduced, along with relevant literature. This review suggests that various PAT tools are rapidly advancing, and various IQAs are efficiently and precisely monitored in the pharmaceutical industry. Therefore, PAT could be a fundamental tool for the present QbD and CPV to improve drug product quality.

Continuous Melt Granulation for Taste-Masking of Ibuprofen

Taste-masking of drugs, particularly to produce formulations for pediatric patients, can be challenging and require complex manufacturing approaches. The objective of this study was to produce taste-masked ibuprofen granules using a novel process, twin-screw melt granulation (TSMG). TSMG is an emerging, high-productivity, continuous process. Granules of ibuprofen embedded in a lipid matrix were produced across a range of process conditions, resulting in a range of output granule particle sizes. The ibuprofen appeared to be miscible with the lipid binder though it recrystallized after processing. The ibuprofen melt granules were tested in simulated saliva using a novel, small-volume dissolution technique with continuous acquisition of the ibuprofen concentration. The ibuprofen release from the granules was slower than the neat API and physical blend, beyond the expected residence time of the granules in the mouth. The ibuprofen release was inversely related to the granule size. A Noyes–Whitney dissolution model was used and the resulting dissolution rate constants correlated well with the granule size.

TPLS as predictive platform for twin-screw wet granulation process and formulation development

In recent years, the interest in continuous manufacturing techniques, such as twin-screw wet granulation, has increased. However, the understanding of the influence of the combination of raw material properties and process settings upon the granule quality attributes is still limited. In this study, a T-shaped partial least squares (TPLS) model was developed to link raw material properties, the ratios in which these raw materials were combined and the applied process parameters for the twin-screw wet granulation process with the granule quality attributes. In addition, the predictive ability of the TPLS model was used to find a suitable combination of formulation composition and twin-screw granulation process settings for a new API leading to desired granule quality attributes. Overall, this study helped to better understand the link between raw material properties, formulation composition and process settings on granule quality attributes. Moreover, as TPLS can provide a reasonable starting point for formulation and process development for new APIs, it can reduce the experimental development efforts and consequently the consumption of expensive (and often limited available) new API.

Evaluation of torque as an in-process control for granule size during twin-screw wet granulation

The potential of torque as in-process control (IPC) to monitor granule size in twin-screw wet granulation (TSG) was investigated. An experimental set-up allowing the collection of granules at four different locations (i.e., in the wetting zone, after the first and second kneading zone and at the end of the granulator) of the granulator screws was used to determine the change in granule size, granule temperature and the contribution of each compartment to the overall torque for varying screw speed, mass feed rate and liquid-to-solid ratio. The only observed correlation was between the granule size and torque increase after the first kneading zone because the torque increase was an indication of the degree in granule growth which was consistently observed with all applied granulation process parameters. No correlation was observed in the other locations as changes of torque were accompanied to either granule breakage and/or growth. Moreover, torque increase was correlated to higher granule temperature, suggesting that energy put into the granulator was partly used to heat up the material being processed and explains additionally the lack of correlation between granule size and torque. Therefore, this study showed that torque could not be used as IPC to monitor granule size during TSG.

Continuous Twin Screw Granulation: A Review of Recent Progress and Opportunities in Formulation and Equipment Design

Continuous twin screw wet granulation is one of the key continuous manufacturing technologies that have gained significant interest in the pharmaceutical industry as well as in academia over the last ten years. Given its considerable advantages compared to wet granulation techniques operated in batch mode such as high shear granulation and fluid bed granulation, several equipment manufacturers have designed their own manufacturing setup. This has led to a steep increase in the research output in this field. However, most studies still focused on a single (often placebo) formulation, hence making it difficult to assess the general validity of the obtained results. Therefore, current review provides an overview of recent progress in the field of continuous twin screw wet granulation, with special focus on the importance of the formulation aspect and raw material properties. It gives practical guidance for novel and more experienced users of this technique and highlights some of the unmet needs that require further research.

Parametric Study of Residence Time Distributions and Granulation Kinetics as a Basis for Process Modeling of Twin-Screw Wet Granulation

Twin-screw wet granulation is a crucial unit operation in shifting from pharmaceutical batch to continuous processes, but granulation kinetics as well as residence times are yet poorly understood. Experimental findings are highly dependent on screw configuration as well as formulation, and thus have limited universal validity. In this study, an experimental design with a repetitive screw setup was conducted to measure the effect of specific feed load (SFL), liquid-to-solid ratio (L/S), and inclusion of a distributive feed screw on particle size distribution (PSD) and shape as well as residence time distribution of a hydrophilic lactose/microcrystalline cellulose based formulation. An intermediate sampling point was obtained by changing inlet ports along the screw axis. Camera-based particle size analysis (QICPIC) indicated no significant change of PSD between the first and second kneading section, except for low L/S and low SFL where fines increase. Mean residence time was approximated as a bilinear fit of L/S and SFL. Moreover, large mass flow pulsations were observed by continuous camera measurements of residence time distribution and correlated to hold-up of the twin-screw granulator. These findings indicate fast granulation kinetics and process instabilities for high mean residence times, questioning current standards of two kneading compartments for wet granulation. The present study further underlines the necessity of developing a multiscale simulation approach including particle dynamics in the future.

Continuous twin screw granulation: Impact of microcrystalline cellulose batch-to-batch variability during granulation and drying - A QbD approach

Despite significant advances in the research domain of continuous twin screw granulation, limited information is currently available on the impact of raw material properties, especially considering batch-to-batch variability. The importance of raw material variability and subsequent mitigation of the impact of this variability on the manufacturing process and drug product was recently stressed in the Draft Guidance for Industry on Quality Considerations for Continuous Manufacturing by the U.S. Food and Drug Administration (FDA). Therefore, this study assessed the impact of microcrystalline cellulose (MCC) batch-to-batch variability and process settings in a continuous twin screw wet granulation and semi-continuous drying line. Based on extensive raw material characterization and subsequent principal component analysis, raw material variability was quantitatively introduced in the design of experiments approach by means of t1 and t2 scores. L/S ratio had a larger effect on critical granule attributes and processability than screw speed and drying time. A large impact of the t1 and t2 scores was found, indicating the importance of raw material attributes. For the studied formulation, it was concluded that MCC batches with a low water binding capacity, low moisture content and high bulk density generated granules with the most desirable quality attributes. Additionally, an innovative and quantitative approach towards mitigating batch-to-batch variability of raw materials was proposed, which is also applicable for additional excipients and APIs.

Evaluation of Binders in Twin-Screw Wet Granulation

The binders povidone (Kollidon 30), copovidone (Kollidon VA64), hypromellose (Pharmacoat 606), and three types of hyprolose (HPC SSL‑SFP, HPC SSL, and HPC SL‑FP) were evaluated regarding their suitability in twin-screw wet granulation. Six mixtures of lactose and binder as well as lactose without binder were twin-screw granulated with demineralized water at different barrel fill levels and subsequently tableted. A screening run with HPC SSL determined the amount of water as an influential parameter for oversized agglomerates. Subsequent examination of different binders, especially Kollidon 30 and Kollidon VA64 resulted in large granules. All binders, except Pharmacoat 606, led to a reduction of fines compared to granulation without a binder. The molecular weight of applied hyproloses did not appear as influential. Tableting required an upstream sieving step to remove overlarge granules. Tableting was possible for all formulations at sufficient compression pressure. Most binders resulted in comparable tensile strengths, while Pharmacoat 606 led to lower and lactose without a binder to the lowest tensile strength. Tablets without a binder disintegrated easily, whereas binder containing tablets of sufficient tensile strength often nearly failed or failed the disintegration test. Especially tablets containing Pharmacoat 606 and HPC SL‑FP disintegrated too slowly.

A review of twin screw wet granulation mechanisms in relation to granule attributes

Due to the trend of continuous pharmaceutical manufacturing, twin screw wet granulation (TSWG), a continuous process, has gained increased research interest as a potential substitution of traditional batch granulation processes. Despite the complex nature of TSWG, its mechanisms have been gradually unveiled with the aid of innovative research strategies. This review synthesizes these recent findings to provide a comprehensive and mechanistic understanding of TSWG. We explain the impact of screw profiles (i.e. conveying, kneading, turbine mixing, and screw mixing elements) and process conditions (i.e. screw speed, feed rate, and liquid-to-solid ratio) on TSWG mixing performance and granule growth along the barrel, both of which ultimately affect critical granule attributes such as content uniformity, size distribution, strength, and compaction properties.

Identifying Critical Binder Attributes to Facilitate Binder Selection for Efficient Formulation Development in a Continuous Twin Screw Wet Granulation Process

The suitability of pharmaceutical binders for continuous twin-screw wet granulation was investigated as the pharmaceutical industry is undergoing a switch from batch to continuous manufacturing. Binder selection for twin-screw wet granulation should rely on a scientific approach to enable efficient formulation development. Therefore, the current study identified binder attributes affecting the binder effectiveness in a wet granulation process of a highly soluble model excipient (mannitol). For this formulation, higher binder effectiveness was linked to fast activation of the binder properties (i.e., fast binder dissolution kinetics combined with low viscosity attributes and good wetting properties by the binder). As the impact of binder attributes on the granulation process of a poorly soluble formulation (dicalcium phosphate) was previously investigated, this enabled a comprehensive comparison between both formulations in current research focusing on binder selection. This comparison revealed that binder attributes that are important to guide binder selection differ in function of the solubility of the formulation. The identification of critical binder attributes in the current study enables rational and efficient binder selection for twin-screw granulation of well soluble and poorly soluble formulations. Binder addition proved especially valuable for a poorly soluble formulation.

Complete two dimensional population balance modelling of wet granulation in twin screw

In this study, a complete two dimensional (internal coordinates) population balance model (2D-PBM) is developed, calibrated and validated as a predictive tool for predicting the particle size and the liquid content distribution of the granules produced from twin screw granulation (TSG). The model is calibrated and validated using experimental distributions for the two internal coordinates that are captured using image processing. Granulation runs are conducted at multiple liquid to solid (L/S) ratios and liquid binder viscosities, and then used to calibrate and validate the 2D-PBM. The mathematical model accounts for aggregation and breakage of the particles occurring in three zones of the TSG with inhomogeneous screw configurations (2 conveying zones and 1 kneading zone). A Madec aggregation kernel, and a linear breakage selection function are used in the 2D-PBM and finite volume numerical approximation is used for solving the model. The calibrated model shows that the aggregation rate in the conveying elements is higher than in the kneading elements while the breakage rate in the kneading elements is much higher than in the conveying elements. Also, the increase in L/S ratio and liquid viscosity leads to higher aggregation rates and lower breakage rates.

Twin screw granulation: A simpler re-derivation of quantifying fill level

The fill level is defined as the volume occupied by the powder and granules inside the twin-screw granulator in proportion to the maximum barrel channel void 'free' volume. In literature, the fill level is one of the key factors that determine the final granule properties as it relies on several factors such as the screw speed, screw element geometry, mass flow rate and barrel length. However, quantitative prediction of the fill level in twin-screw granulation (TSG) is still a developing area, which is required to enable effective development of process design space, to yield a product with desired quality attributes for all process scale levels (small to large equipment). In this study, a simple geometrical model is presented that predicts the barrel channel fill level in TSG. This model relates the volumetric flow rate to the forward volumetric conveying rate of the screws when they advance in the axial direction. Experimentation was conducted to validate the model by analytically measuring mass hold-up, the amount of material remaining in the barrel after steady state was reached, as the fill level is proportional to mass hold-up. Furthermore, the trends in the extent of granulation with the proposed model were investigated. Good agreement was found between the proposed fill level model and the mass hold-up for various screw elements, therefore the model provides a more practical measure of the fill level in TSG.

Continuous twin screw granulation and fluid bed drying: A mechanistic scaling approach focusing optimal tablet properties

This study provides the results of investigation on scaling approaches for three differently-sized continuous granulation lines, each consisting of a twin screw wet granulation process and a continuous fluid bed drying process. To check the initial scaling approach with regard to granule and tablet properties, a process parameter Design of experiment (DoE) was performed on each of the three equipment scales. The processed formulation did not contain cellulose to allow a high overall flowrate through the directly connected granulation and drying sections. Enhanced scaling aspects showed the influence of Froude number [-] at different twin screw granulator scales and screw speeds on the overgranulated particle fraction [% (V/V] as well as on the scale-dependent drying performance of the continuous fluid bed dryers. Scale-independent, specification limits of the two granule material attributes particle fine fraction [%] and residual water content [%] could be defined, resulting in high tableting performance in terms of tabletability and compressibility. Based on these specification limits and the statistical evaluation of the process parameter DoE, a process design space for the continuous granulation and drying process for each scale was calculated. It came up, that this process design space was decreasing in range with increasing equipment scale. The applicability of the presented scaling approach in terms of granule and tablet properties could successfully be demonstrated by three control experiments performed on the different equipment scales. In sum, this work delivers a basis for a smooth transition of scales within process development on the investigated continuous twin screw granulation and drying lines.

Continuous twin screw granulation: Robustness of lactose/MCC-based formulations

In recent years, significant progress has been made in the field of continuous twin screw granulation. However, only limited knowledge is currently available on the impact of active pharmaceutical ingredient (API) properties on granule quality and processability. In this study, the response behavior of four formulations containing APIs (5-10% drug load) with diverse characteristics was compared to the behavior of the corresponding placebo formulation consisting of lactose, microcrystalline cellulose (MCC) and hydroxypropylmethylcellulose (HPMC). API selection was based on extensive material characterization, combining conventional testing with in silico descriptors. For each formulation, a design of experiments was set up, evaluating the impact of liquid to solid (L/S) ratio and screw speed. Response ranges, response behavior and processability of each of the four formulations proved very similar to the placebo formulation when an appropriate center point L/S ratio was chosen. Hence, this robust placebo formulation could prove useful by decreasing drug product development time and consequently providing patients with a faster access to innovative medicine. Additionally, APIs with similar properties exhibited highly comparable response behavior at similar L/S ratios, indicating the potential use of surrogate APIs in novel drug product development.

Hot-Melt Extrusion Process Fluctuations and their Impact on Critical Quality Attributes of Filaments and 3D-printed Dosage Forms

Fused deposition modeling (FDM^TM) is a 3D-printing technology of rising interest for the manufacturing of customizable solid dosage forms. The coupling of hot-melt extrusion with FDM^TM is favored to allow the production of pharma-grade filaments for the printing of medicines. Filament diameter consistency is a quality of great importance to ensure printability and content uniformity of 3D-printed drug delivery systems. A systematical process analysis referring to filament diameter variations has not been described in the literature. The presented study aimed at a process setup optimization and rational process analysis for filament fabrication related to influencing parameters on diameter inhomogeneity. In addition, the impact of diameter variation on the critical quality attributes of filaments (mechanical properties) and uniformity of mass of printed drug-free dosage forms was investigated. Process optimization by implementing a winder with a special haul-off unit was necessary to obtain reliable filament diameters. Subsequently, the optimized setup was used for conduction of rational extrusion analysis. The results revealed that an increased screw speed led to diameter fluctuations with a decisive influence on the mechanical resilience of filaments and mass uniformity of printed dosage forms. The specific feed load was identified as a key parameter for filament diameter consistency.

Delta-mannitol to enable continuous twin-screw granulation of a highly dosed, poorly compactable formulation

In current study, it was investigated if the moisture-mediated polymorphic transition from δ- to β-mannitol during twin screw granulation (TSG) also took place in high drug loaded formulations and if the specific granule morphology associated with the polymorphic transition could enable tableting of granules comprising 75% paracetamol, a poorly compactable drug. Experiments were performed on an integrated continuous manufacturing line, including a twin screw granulator, fluid bed dryer, mill and tablet press. The polymorphic transition of δ- to β-mannitol was observed during twin screw granulation and granules exhibited the needle-shaped morphology, typical of this transition. TSG at low liquid-to-solid (L/S) ratios and use of polyvinylpyrrolidone or hydroxypropylmethylcellulose as binders inhibited the polymorphic transition, whereas screw speed, drying time, drying temperature and airflow did not affect the solid state of mannitol in the granules. Without binder and despite the high paracetamol drug load in the formulation, limited breakage and attrition was observed during drying and milling. In contrast to granules manufactured from a formulation containing paracetamol/β-mannitol which could not be tableted due to extensive capping, granules prepared from a paracetamol/δ-mannitol formulation showed good tabletability. In conclusion, δ-mannitol is a promising TSG excipient, especially for high drug-loaded formulations with poor tabletability.

Continuous twin screw granulation: Influence of process and formulation variables on granule quality attributes of model formulations

In recent years, continuous manufacturing techniques, such as twin screw wet granulation, have gained significant momentum. Due to the large diversity of the (model) formulations and equipment, it is often difficult to generalize conclusions about the importance of process settings. As only limited knowledge is available on the importance of formulation variables, this study focused on the systematic quantification of both process as formulation effects on critical quality attributes of granules from several model formulations. Apart from conventional process and formulation variables, also non-conventional process factors such as nozzle diameter, nozzle orientation and inclusion of a new type of size control elements were evaluated using a Plackett-Burman screening design. Although effects were often formulation-dependent, liquid-to-solid ratio proved the most influential variable for all formulations. Furthermore, binder concentration had a clear effect on granule characteristics, whereas barrel fill level and barrel temperature were less influential. The novel type of size control elements improved granule size distribution and density. The impact of nozzle diameter and wet binder addition proved negligible towards granule properties. Overall it was apparent that lactose/MCC-based formulations correlated better than lactose-based formulations, indicating the possible process robustness of the first filler combination to accommodate API and excipient variability and to handle APIs with different characteristics.

Native starch as in situ binder for continuous twin screw wet granulation

The use of native starch as in situ binder in a continuous twin screw wet granulation process was studied. Gelatinization of pea starch occurred in the barrel of the granulator using a poorly soluble excipient (anhydrous dicalcium phosphate), but the degree of gelatinization depended on the liquid-to-solid ratio, the granule heating and the screw configuration. Furthermore, the degree of starch gelatinization was correlated with the granule quality: higher binder efficiency was observed in runs where starch was more gelatinized. SEM and PLOM images showed experimental runs which resulted in completely gelatinized starch. Other starch types (maize, potato and wheat starch) could also be gelatinized when processed above a critical barrel temperature for gelatinization. This barrel temperature was different for all starches. In situ starch gelatinization was also investigated in combination with a highly soluble excipient (mannitol). The lower granule friability observed using pure mannitol compared to a mannitol/starch mixture indicated that starch did not contribute to the binding, hence starch did not gelatinize during processing. The study showed that native starch can be considered as a promising in situ binder for continuous twin screw wet granulation of a poorly soluble formulation.

Optimisation of an in-line Raman spectroscopic method for continuous API quantification during twin-screw wet granulation and its application for process characterisation

In a previous publication, the development of an in-line Raman spectroscopic method for continuous API quantification during twin-screw wet granulation was presented. An in-line method was developed successfully and the developed method showed an acceptable prediction error. A disadvantage of the developed method was that a measurement was only possible in the dark since light influenced the Raman spectra and made a data interpretation impossible. Therefore, the measurement setup for the implementation of the Raman probe was optimised in the present study to allow a measurement in interior light and to further improve the predictive performance. With the optimised setup, two different calibration models were developed and compared. For the first calibration model, spectra were collected in the dark as before and for the second in interior light. The dark calibration model was able to predict the API content with an RMSEP of 0.31% and the light model with an RMSEP of 0.29%. Thus, both PLS models showed prediction errors in the same order. Consequently, it was possible to evaluate Raman spectra which were collected in interior light. Further, the previous prediction error of 0.60% could be clearly decreased. The optimised Raman method was applicable to evaluate the mixing efficiency of the twin-screw granulator during a split feeding process. The quality of the mixture was monitored behind different barrel sections by Raman spectroscopy and the corresponding API concentrations were predicted by the developed calibration model. For a screw length of 40 D and a screw configuration with two kneading blocks a good mixing ability was observed. For a screw length of 20 D and one kneading block the mixing efficiency was largely acceptable whereas a broad scattering of the API content was observed when no kneading blocks were used. In a second part, an experimental design was performed for each screw configuration to evaluate the influence of the barrel-fill level and screw speed on the mixing efficiency. The quality of the mixture using the entire barrel length was minimally influenced by the fill-level. For the other two positions, the screw speed influenced the quality of the mixture slightly. Thus, for an appropriate mixing, a certain barrel length and a screw configuration with two kneading blocks were necessary.