Crystallization is a chemical process by which atoms and molecules in solution are arranged in a highly organized structure (normally solid) called crystal. This precipitation process can be triggered by a number of physical triggers, such as evaporation or freezing, although deposition can also occur directly from gas.

Crystallization occurs in two steps:

  • Nucleation: formation of molecular proto-aggregates, also known as nuclei, seeds, or embryos, which will evolved into larger macroscopic crystals. Nucleation is triggered when the solution in which the atoms or molecules are reaches conditions of supersaturation or supercooling. However, as this process occurs at a molecular level, the characterization of the mechanisms that govern nucleation is not fully understood. This situation is worsened because this first phase can suffer from long induction times. Nucleation can be started by the presence of templates (surface catalysis), physical structures that may resemble the structure of the crystals and can act as an anchor point.
  • Crystal growth: once the crystal seed has formed, new atoms, ions, or molecules can attach, expanding the crystal in an orderly repeating pattern, which extends in all three space dimensions. This second step occurs much faster than the nucleation. Crystals are characterized by structural rigidity and a notable resistance to deformation.

Discovery

The discovery phase in crystallization aims to discover new crystalline forms and understand their properties and applications. This research is crucial in fields such as pharmaceuticals, where different forms of the same crystal can vary therapeutic effectiveness, stability, and even manufacturability. During the discovery phase, a systematic experimental approach is fundamental to understanding the conditions – temperature, solvent, solute concentration, and pressure – that will promote the nucleation and growth of crystals. The primary goal is identifying the optimal conditions that will yield stable and reproducible protocols for producing crystals.
The discovery phase employs analytical techniques, such as X-ray diffraction, thermal analysis, and optical density analysis, to achieve this objective. The insights obtained during this step will provide fundamental information for a smooth transition for the optimization, scale-up, and industrial production phases.

What challenges appear in the discovery phase?

Crystal polymorphism (the precipitation of certain molecules can take different forms, depending on the intermolecular arrangements.1) can result in suboptimal performance. Polymorphs vary in their physico-chemical properties, including solubility and bioavailability, chemical and physical stability, as well as mechanical properties. For this reason, extensive polymorph screening is crucial to identify the optimal structure not only during synthesis but also during use and storage. The complexity of this process and the added lack of knowledge in this field have resulted in the description of crystallization as an art rather than a science.
In silico simulation is used routinely in crystallization to reduce the time and resources used in the discovery phase. However, this methodology also has significant limitations, which can only be addressed by physical testing. Early detection of crystalline forms requires high sensitivity and precision and monitoring multiple parameters simultaneously. This includes close control of factors that impact solubilization processes, such as temperature, and clues about the evolution of the process, such as optical density.

CrystalEYES | Crystallization Monitoring Sensor

CrystalEYES | Crystallization Monitoring Sensor

Easily Determine Solubility… The need to study crystallization is widespread in the che...

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CrystalSCAN | Parallel Crystallization Monitoring Platform

CrystalSCAN | Parallel Crystallization Monitoring Platform

The CrystalSCAN is a bench-top, automated, parallel crystallization monitoring platform, f...

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How can reproducibility be increased in the discovery phase?

Crystallization is a highly complex process impacted by numerous factors. Temperature, for instance, significantly affects solubility, impacting both the nucleation and crystal growth processes. Evaporation and cooling, which control the concentration of solute in the mix, are also very heavily influenced by temperature. Minor deviations from the ideal conditions often result in suboptimal outcomes, challenging consistent replication. Automation can significantly enhance reproducibility by minimizing human error and ensuring precise control over experimental conditions2.
Impurities, even in trace amounts, can significantly affect the crystallization process. They may act as nucleation sites, facilitating seed formation and interfering with crystal growth rates, leading to inconsistent results.
Supersaturation is another critical parameter in crystallization, defined as the point where the solute concentration exceeds what is thermodynamically stable. Accurate control and measure of supersaturation are essential for controlling nucleation rates and ensuring consistent crystal formation.

Solutions

CrystalEYES is H.E.L’s crystallization monitoring sensor, which can detect changes in the solution’s turbidity that indicate the precipitation processes. CrystalEYES provides insights into the process, allowing parameter adjustments to optimize conditions to increase the reproducibility and stability of the desired crystal form.

CrystalSCAN is designed for parallel crystallization monitoring and significantly accelerating the screening of parameters in the discovery phase. This is a powerful tool for identifying the optimal nucleation and crystal growth conditions. CrystalSCAN’s ability to determine solubility curves of metastable zone widths makes it a fantastic instrument for understanding the impact of physicochemical variables in the process.

CrystalEYES | Crystallization Monitoring Sensor

CrystalEYES | Crystallization Monitoring Sensor

Easily Determine Solubility… The need to study crystallization is widespread in the che...

Details
CrystalSCAN | Parallel Crystallization Monitoring Platform

CrystalSCAN | Parallel Crystallization Monitoring Platform

The CrystalSCAN is a bench-top, automated, parallel crystallization monitoring platform, f...

Details

Development

Following the initial discovery of new crystal forms, the development phase shifts the focus from exploration to optimization. This phase aims to refine the conditions identified during the discovery phase to enhance the crystal forms’ yield, purity, and reproducibility. Such improvements are essential for ensuring that the process is efficient enough to be considered at larger scales and that the quality of the product meets the standards for industrial applications. This is particularly important in the pharmaceutical and chemical industries.

During the development phase, the aim is to understand and control the crystal growth and stability mechanism. It is paramount that different variables, such as temperature gradients, solvent mixtures, and seeing techniques, affect the kinetics of both nucleation and crystal growth are studied to achieve this. It usually requires a combination of modeling tools and controlled experiments, enabling the prediction and manipulation of crystal morphology and size distribution.

Additionally, this phase often utilizes scale-up experiments to gain insight into the viability of the process and its replicability on a larger scale. For this, the crystallization process must be economically feasible and adhere to environmental and safety standards. As we can see, the development phase combines the acquisition of technical information and its integration into scalable processes that align with the market needs and regulation requirements.

What are the challenges in optimizing conditions for crystal growth and purity during development?

The main driving force in the crystallization process is the solubility of the compound of interest. The solubility depends on a range of variables:

  • Solvent(s): due to the interaction with the solutes, solvents can stabilize or destabilize different crystal forms. For instance, solvents that strongly interest the molecule of interest can enhance the solubility of the crystal by interfering with the intermolecular bonds within the solute and vice versa. In multi-solvent systems, the solvent ratio can drastically affect the polarity of the medium and consequently affect the solubility, leading to different crystal forms.
  • Temperature: although an increase in temperature generally tends to favor the solubility of solids in liquid, this is not universal. Temperature changes also influence the nucleation and growth rate of crystals. Thermodynamics dictates that at higher temperatures, the molecular motion increases, which, in turn, can drastically affect crystal quality and size distribution.
  • pH: the pH of a solution will impact the charge state of solute molecules, especially in the case of ionic compounds or molecules that contain ionizable groups. Changes in the ionic state will affect the solubility of the molecule. Additionally, pH can affect nucleation processes due to its influence on the electrostatic interaction between atoms and molecules.

CrystalEYES | Crystallization Monitoring Sensor

CrystalEYES | Crystallization Monitoring Sensor

Easily Determine Solubility… The need to study crystallization is widespread in the che...

Details
CrystalSCAN | Parallel Crystallization Monitoring Platform

CrystalSCAN | Parallel Crystallization Monitoring Platform

The CrystalSCAN is a bench-top, automated, parallel crystallization monitoring platform, f...

Details

How can efficiency be increased in this phase?

While significant efforts have been directed toward increasing the throughput of the screening phase, the development phase has not received the same level of attention. In the screening phase, as previously discussed, the primary focus was identifying physical and chemical conditions favorable for crystal formation. However, the focus shifts on this second phase towards refining these conditions to optimize crystal production3,4.

To increase the efficiency of this phase, it is crucial to design matrixes that encompass the conditions that will impact nucleation and crystal growth. This can include different solutions, solvents, pH levels, and temperatures. To make it effective, these designed arrays must systematically modify the physicochemical variables to fine-tune the crystallization environment. Some approaches start with broad, coarse rangers that are adjusted more granularly and precisely until the desired crystal volume and quality are achieved.3

By methodically refining these variables, researchers can develop optimized crystallization processes that yield higher volumes of suitable crystals, thereby enhancing the overall productivity and efficacy of the development phase.

Solutions

While significant efforts have been directed toward increasing the throughput of the screening phase, the development phase has not received the same level of attention. In the screening phase, as previously discussed, the primary focus was identifying physical and chemical conditions favorable for crystal formation. However, the focus shifts on this second phase towards refining these conditions to optimize crystal production3,4.

To increase the efficiency of this phase, it is crucial to design matrixes that encompass the conditions that will impact nucleation and crystal growth. This can include different solutions, solvents, pH levels, and temperatures. To make it effective, these designed arrays must systematically modify the physicochemical variables to fine-tune the crystallization environment. Some approaches start with broad, coarse rangers that are adjusted more granularly and precisely until the desired crystal volume and quality are achieved.3

By methodically refining these variables, researchers can develop optimized crystallization processes that yield higher volumes of suitable crystals, thereby enhancing the overall productivity and efficacy of the development phase.

CrystalEYES | Crystallization Monitoring Sensor

CrystalEYES | Crystallization Monitoring Sensor

Easily Determine Solubility… The need to study crystallization is widespread in the che...

Details
CrystalSCAN | Parallel Crystallization Monitoring Platform

CrystalSCAN | Parallel Crystallization Monitoring Platform

The CrystalSCAN is a bench-top, automated, parallel crystallization monitoring platform, f...

Details

Scale-Up

Scale-up refers to the process of transition crystallization methods from the controlled environment of laboratory benchtop experiments to full industrial-scale production. To ensure smooth scaling up, it is fundamental to understand its most critical parameter: mixing, which will influence the process attributes and performance. Mixing determines the effectiveness of heat and mass transfer, along with the phase dispersion characteristics. Mixing is influenced by several factors: crystallizer geometry, internal geometry and configuration, physiochemical properties of fluids and solids, and operating parameters 5. For a successful scale-up, it is crucial to ensure that local and global mixing conditions can be reliably reproduced as the transition from the laboratory scale to the pilot scale and, ultimately, to commercial production occurs. This replication is essential for maintaining process consistency and quality across different production scales.

What issues arise during scale-up?

The scale-up process often faces challenges rooted in mixing and resultant complications, such as heat transfer, among others. As production moves from bench top towards industrial scale, the homogeneity achieved through mixing becomes more challenging to maintain. This can lead to uneven temperature and solute concentration distribution, which can be detrimental to the nucleation and crystal growth phases. This variability can result in non-uniform particle size distribution and shape anomalies, compromising quality and yield. The change in scale also typically affects heat transfer efficiency, as large volumes add an extra layer of difficulty for how the heat needs to be distributed or removed, complicating the maintenance of the precise temperature required for optimal crystallization.

Other factors impacted by scale-up include fluid dynamics and suspension behavior. This can affect, for example, the frequency at which fresh superheated solution is delivered to critical zones. This can drastically affect the supersaturation levels. These changes require meticulous testing to recalibrate these factors and be able to replicate the conditions achieved on smaller scales. Without careful adjustment and optimization of these factors, scale-up efforts can lead to increased production costs, extended development times, and even failure to meet regulatory and quality standards, posing substantial risks to commercial viability.

What challenges are encountered at a large scale?

In scaling up crystallization to commercial production, the push to maximize efficiency and reduce costs often can lead to variability that can significantly affect the quality and safety of the final product. Large-scale operations may struggle to maintain the precise and controlled conditions required to obtain optimal products, resulting in variations in the physical properties of the crystals. Such alterations not only impact the effectiveness of the products but also result in safety risks – especially when dealing with hazardous or explosive substances 6.

Companies must address these challenges by performing accurate and realistic risk assessments, utilizing robust quality systems, and using advanced monitoring techniques. This proactive approach helps to maintain consistency across batches without compromising health and safety. It is fundamental that the reactors and crystallizers are manufactured with durable and appropriate materials, mitigating the introduction of impurities and preventing equipment degradation. By integrating these strategies, manufacturers can safeguard both the integrity of their products and the health and safety of their operations, ensuring they meet both commercial demands and regulatory standards.

Solutions

In scaling up crystallization to commercial production, the push to maximize efficiency and reduce costs often can lead to variability that can significantly affect the quality and safety of the final product. Large-scale operations may struggle to maintain the precise and controlled conditions required to obtain optimal products, resulting in variations in the physical properties of the crystals. Such alterations not only impact the effectiveness of the products but also result in safety risks – especially when dealing with hazardous or explosive substances 6.

Companies must address these challenges by performing accurate and realistic risk assessments, utilizing robust quality systems, and using advanced monitoring techniques. This proactive approach helps to maintain consistency across batches without compromising health and safety. It is fundamental that the reactors and crystallizers are manufactured with durable and appropriate materials, mitigating the introduction of impurities and preventing equipment degradation. By integrating these strategies, manufacturers can safeguard both the integrity of their products and the health and safety of their operations, ensuring they meet both commercial demands and regulatory standards.

CrystalSCAN | Parallel Crystallization Monitoring Platform

CrystalSCAN | Parallel Crystallization Monitoring Platform

The CrystalSCAN is a bench-top, automated, parallel crystallization monitoring platform, f...

Details

References

1. Ajito, K., Nakamura, M., Tajima, T. & Ueno, Y. Terahertz spectroscopy methods and instrumentation. (2017).

2. Thakur, A. S. et al. Improved success of sparse matrix protein crystallization screening with heterogeneous nucleating agents. PLoS One 2, e1091 (2007).

3. Luft, J. R. et al. Efficient optimization of crystallization conditions by manipulation of drop volume ratio and temperature. Protein Sci. 16, 715–722 (2007).

4. Chayen, N. E. & Saridakis, E. Protein crystallization for genomics: towards high-throughput optimization techniques. Acta Crystallogr. D Biol. Crystallogr. 58, 921–927 (2002).

5. Nere, N. K. et al. CASE STUDIES ON CRYSTALLIZATION SCALE-UP. Chem. Eng. Pharm. Ind. Act. Pharm. Ingred. 617–633 (2019).

6. Ettouney, R. & El-Rifai, M. Explosion of ammonium nitrate solutions, two case studies. Process Saf. Environ. Prot. 90, 1–7 (2012).