Vacuum-induced Surface Freezing Research Articles

Combining Mathematical Modeling and Thermal Infrared Data in the Freezing of Pharmaceutical Liquid Formulations

Infrared-based (IR) thermal imaging data was combined here with mathematical modeling to describe the freezing process of a pharmaceutical formulation being lyophilized using two different loading configurations; (i) vials in direct contact with the shelf and (ii) vials suspended over it. In all the experiments, the nucleation event was trigged at a specific time instant using the vacuum induced surface freezing (VISF) method. The IR thermal data was given as input to three different mathematical models for freezing and used to estimate the resulting cake’s pore size (dp) distribution. The resulting dp values were then compared to experimental data obtained through SEM images coupled with an image segmentation tool. The supersaturation model showed the best agreement between the estimated dp and experimental values, while minor discrepancies were shown by the other two models. Nonetheless, the outcomes of these last two models, given as inputs to a mathematical model for the primary drying phase, resulted in satisfactory predictions of the product temperature at the moving front, the product resistance to vapor flow, and the primary drying end point. It follows that the combination of the IR thermocamera and freezing modeling is a promising tool for the in-line monitoring and optimization of a freeze-drying cycle.

Surface Treatment of Glass Vials for Lyophilization: Implications for Vacuum-Induced Surface Freezing

Freeze-drying is commonly used to increase the shelf-life of pharmaceuticals and biopharmaceuticals. Freezing represents a crucial phase in the freeze-drying process, as it determines both cycle efficiency and product quality. For this reason, different strategies have been developed to allow for a better control of freezing, among them, the so-called vacuum-induced surface freezing (VISF), which makes it possible to trigger nucleation at the same time in all the vials being processed. We studied the effect of different vial types, characterized by the presence of hydrophilic (sulfate treatment) or hydrophobic (siliconization and TopLyo Si–O–C–H layer) inner coatings, on the application of VISF. We observed that hydrophobic coatings promoted boiling and blow-up phenomena, resulting in unacceptable aesthetic defects in the final product. In contrast, hydrophilic coatings increased the risk of fogging (i.e., the undesired creeping of the product upward along the inner vial surface). We also found that the addition of a surfactant (Tween 80) to the formulation suppressed boiling in hydrophobic-coated vials, but it enhanced the formation of bubbles. This undesired bubbling events induced by the surfactant could, however, be eliminated by a degassing step prior to the application of VISF. Overall, the combination of degasification and surfactant addition seems to be a promising strategy for the successful induction of nucleation by VISF in hydrophobic vials.

Investigation of the Freezing Phenomenon in Vials Using an Infrared Camera.

The freezing phenomenon has a dramatic impact on the quality of freeze-dried products. Several freezing models applied to solutions in vials have been proposed to predict the resulting product morphology and describe heat transfer mechanisms. However, there is a lack of detailed experimental observations of the freezing phenomenon in vials in the literature. Thus, the present work offers new experimental observations of the freezing phenomenon in vials by infrared (IR) thermography. IR imaging allowed each vial’s whole axial temperature profile to be collected during freezing, providing significant insights into the process. Spontaneous nucleation and vacuum-induced surface freezing (VISF), as a controlled nucleation technique, are investigated. Batches having vials in direct contact with the shelf (exchanging heat mainly through conduction) as well as suspended (exchanging heat mainly through natural convection and radiation) were tested. The study used three solutions: sucrose 5%, mannitol 5%, and dextran 10%. SEM images coupled with an automated image segmentation technique were also performed to examine possible correlations between the freezing observations and the resulting pore size distributions. IR thermography was found to be a promising tool for experimentally predicting the resulting product morphology in-line.

Controlling Ice Nucleation during Lyophilization: Process Optimization of Vacuum-Induced Surface Freezing

Biopharmaceuticals are often lyophilized to improve their storage stability. Controlling ice nucleation during the freezing step of the lyophilization process is desired to increase homogeneity of product properties across a drug product batch and shorten the primary drying time. The present communication summarizes the process optimization of the freezing process when using vacuum-induced surface freezing to control ice nucleation, in particular for amorphous samples. We characterized freeze-dried samples for solid state properties, and compared these to uncontrolled nucleated samples using bovine serum albumin (BSA) as a model protein. Freezing parameters were optimized to obtain complete nucleation, adequate cake resistance during the subsequent lyophilization cycle, and elegant cakes. We highlight the challenges associated with vacuum-induced surface freezing and propose optimized freezing parameters to control ice nucleation, enabling manufacturing of amorphous samples.

Design of Vacuum-Induced Freezing Protocols for High Fill Volume Formulations in Freeze-Drying: A Strategic Approach

This case study proposes a development strategy for the SynchroFreeze vacuum-induced surface freezing technology for challenging high fill volume model systems. Critical steps during the development of a nucleation protocol are discussed as an example approach for implementing vacuum-induced surface freezing for high fill volume products. Slow pressure ramps and hold steps at adequate pressures have been found to be crucial for avoiding defects caused by either excessive outgassing or incomplete degassing. The evaporative mass loss during the SynchroFreeze procedure is characterized and thermal gradients during nucleation for several model systems with concentrations in the 50–400 mg/mL range are analyzed. The technology results in a measurable mass loss that may be relevant for low fill volume formulations. Thermal data show a pronounced temperature gradient throughout the entire product solution during nucleation by vacuum-induced surface freezing. The formulation composition, concentration, and shelf temperature have been shown to influence this temperature gradient. Reliable nucleation was achieved for sucrose formulations with concentrations up to 200 mg/mL at shelf temperatures minimally below the equilibrium freezing point.

Investigation of Two Different Pressure-Based Controlled Ice Nucleation Techniques in Freeze-Drying: The Integral Role of Shelf Temperature After Nucleation in Process Performance and Product Quality

The purpose of this study was to investigate the impact of shelf temperature modifications during application of controlled ice nucleation techniques on process data and critical product quality attributes for a challenging, high-concentration and high-fill volume amorphous model system. Different freezing programs were applied and compared for the mechanistically different depressurization and vacuum-induced surface freezing techniques. Critical process data, such as product temperature and drying time, were analyzed. The final products were characterized with a focus on product morphology, residual moisture, reconstitution time and stability. The shelf temperature directly after primary nucleation showed a major influence on process performance and product quality attributes, with an isothermal hold step at an intermediate temperature leading to optimal results in terms of homogeneity and reduction of product temperatures and drying time for the model system used. The different controlled ice nucleation techniques led to significantly different results in terms of product morphology and process data, showing that the two mechanistically different controlled nucleation techniques are not interchangeable.

Vacuum-Induced Surface Freezing for the Freeze-Drying of the Human Growth Hormone: How Does Nucleation Control Affect Protein Stability?

In the present work, the effect of controlled nucleation on the stability of human growth hormone (hGH) during freeze-drying has been investigated. More specifically, the vacuum-induced surface freezing technique has been compared to conventional freezing, both with and without an annealing step. Size exclusion chromatography and cell-based potency assays have been used to characterize the formation of soluble aggregates and the biological activity of hGH, respectively. The results obtained indicate that controlled nucleation has a positive effect on both cycle performance and product homogeneity because of the formation of bigger ice crystals, and characterized by a narrower dimensional distribution. From the point of view of hGH stability, we observed that vacuum-induced surface freezing is not detrimental to the biological activity of the protein, or aggregate formation. In addition, the effect of 2 different formulations, including trehalose or cellobiose, on protein preservation was also considered for this study.

Tuning, measurement and prediction of the impact of freezing on product morphology: A step toward improved design of freeze-drying cycles

The control of freezing is crucial as it can influence the freeze-drying operation as well as the chemical and physical properties of the lyophilized product such as the specific surface area, dissolution rate, and drug stability. In this review article, we focus on two areas related to lyophilization in the pharmaceutical industry: (1) advances in our understanding of the relationship between the freezing conditions, product morphology and drying efficiency, and (2) the control of nucleation in solutions to be frozen. The final objective is to accelerate the transition toward a more robust design of freeze-drying cycles, based on deep knowledge of the phenomena involved rather than on empirical observations. While these topics are not new, they are areas of significant current interest to the community engaged in drying in the pharmaceutical industry.Abbreviations: FDM: freeze-drying microscope; GMP: good manufacturing practice; INA: ice nucleating agent; QbD: quality by design; PRT: pressure rise test; SSA: specific surface area, m2; UIIN: ultrasound-induced ice nucleation; VIN: vacuum induced nucleation; VISF: vacuum induced surface freezing.

Vacuum Induced Surface Freezing as an effective method for improved inter- and intra-vial product homogeneity

The stochastic nature of nucleation makes it difficult to control batch homogeneity in conventional freezing, and this lack of control is in contrast with the current emphasis on Quality by Design. Among the techniques which have been developed to overcome this problem, Vacuum Induced Surface Freezing is probably the most promising for application in manufacturing, because it does not require additional equipment and can be scaled-up more easily than other proposed approaches. In this work, we summarize the impact of Vacuum Induced Surface Freezing on product morphology, and the efficiency of the subsequent drying steps as well. We will show that this controlled freezing approach is extremely beneficial for both the efficiency of the freeze-drying process, and the quality and homogeneity of the final product. The hope is that this work could contribute to the commercial implementation of controlled nucleation technology, overcoming the final resistance to its widespread use. It is our opinion that this would be a substantial improvement, beneficial for both the pharmaceutical industry and the end users.

Optimization of freezing parameters for freeze-drying mango (Mangifera indica L.) slices

ABSTRACTToday, there is an increasing trend toward consuming healthful food products, and dried fruits are one option for obtaining desirable nutrition in dehydrated products. Proper drying method selection is important for minimizing quality losses. Lyophilization (freeze-drying), a technique that removes water by sublimation at low temperature, can produce excellent dried products. Fast vacuum induction during freezing has been proposed as a way to control ice nucleation and to obtain enhanced lyophilized pharmaceutical products. However, this technique has not yet been applied to food freeze-drying. The aim of this work was to evaluate the effects of the freezing rate (0.4, 0.25, and 0.1°C/min), the minimum chamber pressure during vacuum-induced surface freezing (900, 700, and 500 mTorr) and the sample temperature at which the induced vacuum was applied (0, −2, and −4°C) on the total process time, final moisture content, rehydration capacity, total color difference, and total polyphenol content during the lyophilization of mango (Mangifera indica L.) slices. A Box–Behnken design with three factors at three levels was used to design the experiments, to generate the polynomial equations relating the dependent and independent variables, and to determine the optimal operational conditions. The results highlight the reduction of total process time (30%) at high freezing rates and the influence of the tested operating conditions on freeze-drying optimization. It was found that the optimal conditions that satisfy commercial quality goals were pressure between 500 and 650 mTorr, temperature between −2 and 0°C, and freezing rate values close to 0.4°C/min.

Prediction of product morphology of lyophilized drugs in the case of Vacuum Induced Surface Freezing

In pharmaceutical industry, freeze-drying is often used to produce protein therapeutics that are stable during shipping and long-term storage. The design of adequate freezing process is a crucial aspect to be considered as influences the final morphology and, hence, physicochemical properties of the lyophilized product. In this study, a mechanistic model is developed for better understanding the relationship between morphology of lyophilized samples and freezing conditions. More specifically, we focus on Vacuum Induced Surface Freezing that is a recent, promising technology that allows a precise control of temperature of nucleation. Model simulations were validated by comparison with the lyophilized product morphology as observed by Scanning Electron Microscopy. The model was found to give accurate results for two liquid formulations, containing mannitol and sucrose, and varying nucleation temperature.

Tortuosity of Aligned Channels in Alumina Membranes Produced by Vacuum-Induced Surface Directional Freezing.

Vacuum-induced surface freezing of colloidal alumina was used to produce membranes that have elongated, aligned channels and, hence, are tortuous in the direction perpendicular to ice crystal growth. The effective tortuosity of the membranes was measured by steady-state diffusion of a solute, methylene blue. The resulting diffusion profiles show an initial step-increase in amount of dye reaching the acceptor that is caused by capillarity drawing the donor solution through any non-wetted channels in the membrane. This is followed by a linear steady-state phase whose flux is proportional to dye concentration in the donor and inversely proportional to the colloid’s volume fraction of dispersed phase. From the steady-state flux, the effective tortuosity, τ* = (α/τ)−1, was calculated. This is the reciprocal quotient of the reduced available area for diffusion within the membrane, = A*/A, where A* is the available area and A is the cross-sectional area of the membrane, and the increased mean diffusional path length, i.e., tortuosity , where L* is the mean path length and L is the membrane thickness. The values of τ* lie in the range of 2–38 and increase as the volume fraction of dispersed phase is larger. This latter effect indicates that τ* > 1 results, to a larger extent, from the reduced available diffusion area, α, than from the lengthened pathway, τ, in these aligned porous membranes.

Freezing kinetics of vacuum-induced surface directional freezing in a glass vial

Directional freezing is used to prepare aligned porous materials made from ceramic dispersions. The alternate procedure of vacuum-induced surface-freezing uses a reduction in pressure to achieve freezing rather than the use of a cryogen-driven cold-finger. In this work the rate of directional freezing that occurs during vacuum-induced surface freezing of alumina dispersions in glass vials was measured using a video camera technique. The rate of advancement of the freezing front is broadly similar to that seen with conventional freeze casting: initially 900µm/s caused by spontaneous freezing of a surface layer at 250mTorr, followed by steady state at approximately 12–15µm/s during directional freezing. The steady state phase could be described by Stefan’s equation where the ice layer thickness grows with the square root of time. The temperature differences across the sample necessary to give the measured rates of advancement of the freezing front were calculated to be 18.8°C, 13.9°C and 9.1°C for volume fractions of dispersed phase of 0.032, 0.064 and 0.128, respectively. The temperature of the slurry below the freezing front declined from 0°C at the start of directional freezing to around −23°C at its conclusion. Stefan’s equation predicts that the temperature of the cold source, i.e. a sublimation front, is −14°C declining to −37°C, which are plausible values. The mechanism of vacuum-induced surface directional freezing is suggested to be undercooling of the slurry surface to below −10°C at reduced pressure by evaporative water loss. After rapid formation of a continuous, frozen surface-layer, sublimation starts and the enthalpy of sublimation assumes the role of a cold source for directional freezing. A sublimation front moves down through the frozen region behind the freezing front and has a rapidly declining temperature down to below −35°C. This is the cold source for directional freezing.

Vacuum-Induced Surface Freezing to Produce Monoliths of Aligned Porous Alumina.

Vacuum-induced surface freezing has been used to produce uni-directional freezing of colloidal aluminum oxide dispersions. It leads to zones of different structure within the resulting sintered monoliths that are highly similar to those known for freeze casting using a cryogen cold source. A more-or-less dense surface layer and a cellular sub-surface region are formed, beneath which is a middle region of aligned lamellae and pores that stretches through most of the depth of the monolith. This is the case even at a volume fraction of dispersed phase as low as 0.032. A more-dense but still porous base layer is formed by accumulation of rejected nanoparticles preceding the freezing front and differs from previous reports in that no ice lenses are observed. X-ray micro-computed tomography reveals a uniform aligned pore structure vertically through the monolith. The pores close to the periphery are oriented radially or as chords, while the center region contains domains of parallel pores/lamellae. The domains are randomly oriented to one another, as already reported for regular freeze casting. This technique for directional freezing is convenient and easy to perform, but requires further refinement in that the temperature gradient and freezing rates remain yet to be measured. Also, control of the temperature gradient by varying chamber vacuum and shelf temperature needs to be evaluated.

Impact of vacuum-induced surface freezing on inter- and intra-vial heterogeneity

This paper aimed to study the impact of freezing on both within-batch (inter-vial) and within-product (intra-vial) heterogeneity. This analysis has been carried out using two freezing protocols, the conventional shelf-ramped method and the Vacuum Induced Surface Freezing, and placebo formulations containing both crystallizing (mannitol) and amorphous (lactose and sucrose) excipients. The freezing conditions (i.e., the temperature of freezing, the temperature and time of the equilibration phase, and the filling volume) were found to have a dramatic impact on both the within-batch and the within-product homogeneity. Overall, we observed that the control of freezing can effectively minimize the variability in product characteristics, and moisture content, within the same batch. In addition to more uniform production, the control of freezing was found to be fundamental to achieve a more uniform product than that produced by the shelf-ramped freezing method. The influence of the freezing protocol on the crystallization process of mannitol was also investigated, showing that the temperature of freezing plays a key role in the formation of the mannitol polymorphs.