Surface-induced crystallization of pharmaceuticals and biopharmaceuticals: A review

Study on the acoustic emission frequency of salicylic acid crystal growth based on beam vibration

This study aims to investigate the relationship between the grain size of salicylic acid crystals and the frequency of acoustic emission signalsduring the crystallization of salicylic acid. During the crystallization process of salicylic acid, an acoustic emission system was utilized to monitor the emitted sound signals. Based on the beam vibration theory, a lateral vibration model of the crystals was constructed to study the correlation between acoustic emission signals and crystal grain size. This process occurs with one end of the crystal acting as a fixed end and the other as a free end. The study found that when the crystal grain size is between 100-300 µm, the characteristic frequency of the crystal is 115-344 kHz. Finite element analysis software was also employed to calculate the characteristic frequency of the crystal grain size, and the simulation results were highly consistent with the experimental data.

A dual-drive strategy for enhanced protein crystallization with sodium alginate/hyaluronic acid film: Protein adsorption and supersaturation regulation

Protein crystallization is essential for determining the three-dimensional structures of biomacromolecules and advancing biopharmaceutical development, yet it remains a major challenge in structural biology due to common issues like slow nucleation rates and inconsistent crystal quality. Herein, a dual-drive crystallization (DDC) strategy, relying on a composite film of sodium alginate (SA) and hyaluronic acid (HA), is reported to synergistically regulate both protein adsorption and solution supersaturation. Driven by the electrostatic interactions of SA and the water absorption properties of HA, the SA/HA film achieves enhanced crystallization efficiency and controlled crystal quality mainly. It significantly reduces lysozyme nucleation time by over 66.0 % and better controls crystal size distribution. Molecular simulations further reveal a strong electrostatic interaction energy of -17.0 kcal·mol-1 between protein and SA, which enhances protein adsorption and then promotes cluster formation, nucleation, and crystal growth. Additionally, the DDC strategy efficiently promotes the crystallization of both thaumatin and proteinase K, enhancing the crystallization success rate for proteins with opposite charges. These results highlight the advantages and promising potential of SA/HA film-assisted protein crystallization for effectively producing protein crystals suitable for diverse applications.

Effects of quenched disorder on the kinetics and pathways of phase transition in a soft colloidal system

Although impurities are unavoidable in real-world and experimental systems, most numerical studies on nucleation focus on pure (impurity-free) systems. As a result, the role of impurities in phase transitions remains poorly understood, especially for systems with complex free energy landscapes featuring one or more intermediate metastable phases. In this study, we employed Monte Carlo simulations to investigate the effects of static impurities (quenched disorder) of varying length scales and surface morphologies on the crystal nucleation mechanism and kinetics in the Gaussian core model system-a representative model for soft colloidal systems. We first explored how the nucleation free energy barrier and critical cluster size are influenced by the fraction of randomly pinned (or, static) particles (fp) and the size (np) of the pinned region or cluster. Both the nucleation free energy barrier and critical cluster size increase sharply with increasing fp but decrease as np grows for a given fraction of pinned particles, eventually approaching the homogeneous nucleation limit. On examining the impact of impurity's surface morphology on nucleation kinetics, we observed that the nucleation barrier significantly decreases with increasing the impurity (or, seed) size with crystalline surface morphologies with body-centered cubic showing the greatest facilitation. Interestingly, seeds with random surface roughness had little effect on nucleation kinetics. In addition, the polymorphic identity of particles in the final crystalline phase is influenced by both the seed's surface morphology and system size. This study further provides crucial insights into the intricate relationship between surface-induced local structural fluctuations and the selection of the polymorphic identity in the final crystalline phase, which is essential for understanding and controlling crystallization processes in experiments.

Effect of Mixed Surfactant on Evaporation Driven Salt Crystallization Morphology in Sessile Droplets

Extensive studies have been conducted to manipulate the morphology of sodium chloride salt crystals to tailor their physical and chemical properties. Among the myriad factors considered, the effects of the substrate and additives have profound impacts on the types of salt depositions. Surface charge effects and various ionic surfactants influence ion movement, resulting in diverse crystal morphologies. This manuscript aims to provide a consolidated summary by concurrently studying multiple effects to uncover the salt crystal morphology under the influence of two oppositely charged ionic surfactants on charged and neutral surfaces. The cationic surfactant cetyltrimethylammonium bromide induces skeletal crystal growth by retarding salt precipitation until supersaturation is reached. Conversely, the anionic surfactant sodium dodecyl sulfate hinders ion diffusion at the three-phase contact line. Each surfactant effect is dominant at higher molar concentrations. Surface charge affects the amount of surface adsorption and free-moving ions within the saline surfactant droplets, greatly influencing the number of salt crystals formed on the neutral substrate. However, charge neutralization at the highest concentrations of both surfactants nullifies the surface charge effect, resulting in practically indistinguishable salt crystals with similar sizes and numbers, leading to only a small area difference of 1461 μm2. This study provides insights into the kinetics of crystallization under the combined influence of anionic, cationic, and surface charge interactions. The findings can serve as a future reference for predicting and controlling ionic interactions and crystal morphology.

Surface-Enhanced, Low-Frequency Raman Spectroscopy: A Sensitive Screening Tool for Structural Characterization of Pharmaceuticals

Surface-enhanced, low-frequency Raman spectroscopy (SELFRS) was explored for its potential as a structural screening tool within pharmaceutical applications, including facile small-scale multicomponent analysis. Paracetamol was used as the model drug, and its crystallization behavior with or without the presence of a templating agent (benzoic acid) was investigated using commercial silver-based SERS substrates. The Raman imaging was carried out using two different LFR-enabled instruments employing 532 and 785 nm incident lasers, where each of the setups showed certain affinity for differentiating lattice vibrations of the polymorphic forms of interest: form I and form II. A comparison of SELFRS, SERS, and their combination using chemometrics showed the potential for the LFR spectral range to improve surface-enhanced measurements either individually or in combination with the typically-used fingerprint region without the need to alter the experimental configuration. Additionally, the use of crystallization additives that helped to drive the formation of metastable form II was shown using SELFRS to provide additional mechanistic understanding of the template-assisted crystallization processes.

The use of green protic ionic liquids in the crystallization of isoniazid: Evaluation of physicochemical and biological properties of drug

This study evaluated the synthesis of protic ionic liquids (PILs), 2-hydroxy ethylammonium formate (2-HEAF) and 2-hydroxy ethylammonium acetate (2-HEAA), and their applicability in the crystallization process of the active pharmaceutical ingredient isoniazid (INH) as anti-solvent. Isoniazid is an antibiotic used in the treatment of tuberculosis infections, being used as a first-line chemotherapeutic agent against Mycobacterium tuberculosis. Futhermore, this investigation was conducted in order to evaluate how these PILs can influence the habit, solubility, stability, and therapeutic efficiency of the obtained isoniazid crystals. The 2-HEAF and 2-HEAA PILs were easily formed in reactions between ethanolamine and carboxylic acids (formic or acetic acid), and they have no toxicity against Artemia salina. The PILs were able to crystallize isoniazid, influencing the crystal habit and size. The greatest variations in the hydrogen signals of the NH2 and NH groups of the amine and low variations in the chemical shifts of the hydrogens of the cation of the ethanolamine group from 2-HEAA and 2-HEAF indicate that PILs establish possibly weak interactions with INH. The obtained crystals were amorphous and showed higher solubility in water than standard INH. Moreover, these crystals showed therapeutic efficiency inantimycobacterial activity to inhibit the growth of Mycobacterium tuberculosis. The INH:2-HEAF only degraded 5.1 % (w/w), however, INH:2-HEAA degraded 32.8 % (w/w) after 60 days in an accelerated atmosphere. Then, the 2-HEAA and 2-HEAF were able to crystallize isoniazid, being a new application for these PILs. The used PILs also influenced the characteristics of isoniazid crystals.

PEGDA hydrogel microspheres with encapsulated salt for versatile control of protein crystallization

Due to their biocompatibility and adjustable chemical structure and morphology, hydrogels have great potential in many applications, and can be used to enhance protein crystal quality and crystallization efficiency, contributing to biomedicine manufacturing. Monodispersed PEGDA hydrogel microspheres (HMSs) were synthesized using a Lego-inspired microfluidic device. The generated droplets were then UV polymerized, partially hydrolyzed with 0.1 M NaOH solution to improve their absorption capacity, and soaked in a buffer solution containing 0, 0.5, 1, 2, and 4 M NaCl. Salt-loaded HMSs were used as the medium for the enhanced crystallization of hen egg white lysozyme from aqueous solutions. Different supersaturations were achieved in the protein solutions by releasing NaCl of different concentrations from HMSs, as confirmed by electrical conductivity measurements. HMSs with or without NaCl can both provide heterogeneous nucleation sites due to their nano-porous structure and wrinkled surface. The addition of NaCl-loaded HMSs to the protein solution can also increase or decrease the supersaturation in the whole solution or locally near the HMS, leading to controllable nucleation time and crystal size distribution dependent on the NaCl concentration loaded into HMSs.

Competition between homogeneous and heterogeneous crystallization of CaCO3 during water softening

Homogeneous and heterogeneous crystallization of CaCO3 simultaneously occur in seed-induced crystallization during water softening, while suppressing homogeneous crystallization is necessary due to the production of fine particulates that poorly precipitate. However, homogeneous crystallization is difficult to distinguish from heterogeneous crystallization. Consequently, a central focus in improving water softening is understanding their competing activities. In this study, a novel method for distinguishing homogeneous and heterogeneous calcium carbonate crystallization is described that utilizes magnetite as seed particles. Results showed that saturation index (SI) was the primary driver of both homogeneous and heterogeneous crystallizations. Heterogeneous crystallization was preferentially promoted at low SI, while homogeneous crystallization was promoted at high SI. The highest suppression effect to homogeneous crystallization occurred at SI of about 1.01. Seed dosage and mean particle size were the primary parameters related to the competition of the crystallization types. Higher seed dosage and smaller seed particle sizes promoted heterogeneous crystallization and suppressed homogeneous crystallization. Due to the good adaptability of heterogeneous crystallization at low SI, the absorption of CO2 from the air into the solutions also improved the efficiency of hardness removal. The introduction of seed particles did not change crystalline product phases, with calcite being the only observed phase and possessing rhombohedral forms with highly regular and smooth edges. Water softening pilot test results showed that SI of 1.5 was more favorite for CaCO3 layer formation on seed surface and hardness removal in comparison with SI of 1.0 and 2.0. Overall, the results from this study demonstrate that the introduction of seed particles is a promising approach to suppress the homogeneous crystallization of CaCO3. Moreover, these results can serve as a framework for improved seed-induced crystallization during water softening.

Using singular value decomposition to analyze drug/β-cyclodextrin mixtures: insights from X-ray powder diffraction patterns

The article discusses the use of mathematical models and linear algebra to understand the crystalline structures and interconversion pathways of drug complexes with β-cyclodextrin (β-CD). It involved the preparation and analysis of mixtures of indomethacin, diclofenac, famotidine, and cimetidine with β-CD using techniques such as differential scanning calorimetry (DSC), X-ray powder diffraction (XRPD), and proton nuclear magnetic resonance (1H-NMR). Singular value decomposition (SVD) analysis is used to identify the presence of different polymorphs in the mixtures of these drugs and β-CD, determine interconversion pathways, and distinguish between different forms. In general, linear algebra or artificial intelligence (AI) is used to approximate the contribution of distinguishable entities to various phenomena. We expected linear algebra to completely reveal all eight entities present in the diffractogram dataset. However, after performing the SVD procedure, we found that only six independent basis functions were extracted, and the entities of the INM α-form and the CIM B-form were not included. It is considered that this is due to that data processing is limited to revealing only six or seven independent factors, as it is a small world. The authors caution that these may not always reproduce or approach reality in complicated real-world situations.

A workflow for the development of template-assisted membrane crystallization downstream processing for monoclonal antibody purification

Monoclonal antibodies (mAbs) are commonly used biologic drugs for the treatment of diseases such as rheumatoid arthritis, multiple sclerosis, COVID-19 and various cancers. They are produced in Chinese hamster ovary cell lines and are purified via a number of complex and expensive chromatography-based steps, operated in batch mode, that rely heavily on protein A resin. The major drawback of conventional procedures is the high cost of the adsorption media and the extensive use of chemicals for the regeneration of the chromatographic columns, with an environmental cost. We have shown that conventional protein A chromatography can be replaced with a single crystallization step and gram-scale production can be achieved in continuous flow using the template-assisted membrane crystallization process. The templates are embedded in a membrane (e.g., porous polyvinylidene fluoride with a layer of polymerized polyvinyl alcohol) and serve as nucleants for crystallization. mAbs are flexible proteins that are difficult to crystallize, so it can be challenging to determine the optimal conditions for crystallization. The objective of this protocol is to establish a systematic and flexible approach for the design of a robust, economic and sustainable mAb purification platform to replace at least the protein A affinity stage in traditional chromatography-based purification platforms. The procedure provides details on how to establish the optimal parameters for separation (crystallization conditions, choice of templates, choice of membrane) and advice on analytical and characterization methods.

Polymer-mediated and ultrasound-assisted crystallization of ropivacaine: Crystal growth and morphology modulation

The objective of this research was to modify the crystal shape and size of poorly water-soluble drug ropivacaine, and to reveal the effects of polymeric additive and ultrasound on crystal nucleation and growth. Ropivacaine often grow as needle-like crystals extended along the a-axis and the shape was hardly controllable by altering solvent types and operating conditions for the crystallization process. We found that ropivacaine crystallized as block-like crystals when polyvinylpyrrolidone (PVP) was used. The control over crystal morphology by the additive was related to crystallization temperature, solute concentration, additive concentration, and molecular weight. SEM and AFM analyses were performed providing insights into crystal growth pattern and cavities on the surface induced by the polymeric additive. In ultrasound-assisted crystallization, the impacts of ultrasonic time, ultrasonic power, and additive concentration were investigated. The particles precipitated at extended ultrasonic time exhibited plate-like crystals with shorter aspect ratio. Combined use of polymeric additive and ultrasound led to rice-shaped crystals, which the average particle size was further decreased. The induction time measurement and single crystal growth experiments were carried out. The results suggested that PVP worked as strong nucleation and growth inhibitor. Molecular dynamics simulation was performed to explore the action mechanism of the polymer. The interaction energies between PVP and crystal faces were calculated, and mobility of the additive with different chain length in crystal-solution system was evaluated by mean square displacement. Based on the study, a possible mechanism for the morphological evolution of ropivacaine crystals assisted by PVP and ultrasound was proposed.

Enhancing Protein Crystal Nucleation Using In Situ Templating on Bioconjugate-Functionalized Nanoparticles and Machine Learning

Although protein crystallization offers a promising alternative to chromatography for lower-cost protein purification, slow nucleation kinetics and high protein concentration requirements are major barriers for using crystallization as a viable strategy in downstream protein purification. Here, we demonstrate that nanoparticles functionalized with bioconjugates can result in an in situ template for inducing rapid crystallization of proteins at low protein concentration conditions. We use a microbatch crystallization setup to show that the range of successful crystallization conditions is expanded by the presence of functionalized nanoparticles. Furthermore, we use a custom machine learning-enabled emulsion crystallization setup to rigorously quantify nucleation parameters. We show that bioconjugate-functionalized nanoparticles can result in up to a 7-fold decrease in the induction time and a 3-fold increase in the nucleation rate of model proteins compared to those in control environments. We thus provide foundational insight that could enable crystallization to be used in protein manufacturing by reducing both the protein concentration and the time required to nucleate protein crystals.

Role of Heteronucleants in Melt Crystallization of Crystalline Solid Dispersions

Few publications exist concerning polymorphic control during melt crystallization, particularly when employing heteronucleants. Here, the influence of a polymeric thin film (polyethylene terephthalate, PET) on the crystallization from melt of the polymorphic compound acetaminophen (ACM) in polyethylene glycol (PEG) was investigated. Molten ACM-PEG at different compositions was monitored using in situ Raman spectroscopy for nucleation induction time measurements and phase identification. Furthermore, X-ray diffraction (XRD) served to analyze the preferred orientation (PO) of the pastilles (solidified melt droplets) on PET-coated and uncoated substrates. The results indicate that PET-coated substrates qualitatively accelerate the nucleation of ACM form II (ACM II) in PEG compared to uncoated glass substrates. Additionally, the occurrence of ACM II in PEG was increased by an average of 10% when crystallized on PET-coated substrates compared to uncoated substrates. Overall, these results suggest that ACM can interact through hydrogen bonding with the PET-coated substrate, leading to faster nucleation. This investigation illustrates the effect of PET-coated substrates in the selective crystallization of ACM II in PEG as crystalline solid dispersions (CSDs). Ultimately, the results suggest the implementation of polymeric heteronucleants in melt crystallization processes, specifically, in advanced polymer-based formulation processes for the enhanced polymorphic form control of pharmaceutical compounds in CSDs.

Liquid-liquid phase separation drug aggregate: Merit for oral delivery of amorphous solid dispersions

As a promising strategy, amorphous solid dispersion has been extensively employed in improving the oral bioavailability of insoluble drugs. Despite the numerous advantages, the problems associated with supersaturation stability limit its further application. Recently, the formation and stability of the liquid-liquid phase separation drug aggregate (LLPS-DA) have been found to be vital for supersaturation maintenance. An in-depth review of LLPS-DA was required to further explore the supersaturation maintenance mechanism in vivo. Hence, this study aimed to present a short review to introduce the LLPS-DA, highlight the in vivo advantages for oral administration, and discuss the prospects to help understand the in vivo behavior of LLPS-DA.

Physical stability and dissolution behaviors of amorphous pharmaceutical solids: Role of surface and interface effects

Amorphous pharmaceutical solids (APS) are single- or multi-component systems in which drugs exist in high-energy states with long-range disordered molecular packing. APSs have become one of the most effective and widely used pharmaceutical delivery approaches for poorly water-soluble drugs in the last several decades. Considerable efforts have been made to investigate the physical stability and dissolution behaviors of APSs, however, the underlying mechanisms remain imperfectly understood. Recent studies reveal that surface and interface properties of APSs could strongly affect the physical stability and dissolution behaviors. This paper provides a comprehensive overview of recent studies focusing on the physical stability and dissolution behaviors of APSs from both surface and interface perspectives. We highlight the role of surface or interface properties in nucleation, crystal growth, phase separation, dissolution, and supersaturation. Meanwhile, the challenges and scope of research on surface and interface properties in the future are also briefly discussed. This review contributes to a better understanding of the surface- and interface-facilitated processes, which will provide more efficient and rational guidance for the design of APSs.

Template-Assisted Crystallization Behavior in Stirred Solutions of the Monoclonal Antibody Anti-CD20: Probability Distributions of Induction Times

We present a method to determine the template crystallization behavior of proteins. This method is a statistical approach that accounts for the stochastic nature of nucleation. It makes use of batch-wise experiments under stirring conditions in volumes smaller than 0.3 mL to save material while mimicking larger-scale processes. To validate our method, it was applied to the crystallization of a monoclonal antibody of pharmaceutical interest, Anti-CD20. First, we determined the Anti-CD20 phase diagram in a PEG-400/Na₂SO₄/water system using the batch method, as, to date, no such data on Anti-CD20 solubility have been reported. Then, the probability distribution of induction times was determined experimentally, in the presence of various mesoporous silica template particles, and crystallization of Anti-CD20 in the absence of templates was compared to template-assisted crystallization. The probability distribution of induction times is shown to be a suitable method to determine the effect of template particles on protein crystallization. The induction time distribution allows for the determination of two key parameters of nucleation, the nucleation rate and the growth time. This study shows that the use of silica particles leads to faster crystallization and a higher nucleation rate. The template particle characteristics are shown to be critical parameters to efficiently promote protein crystallization.

Modeling of the Crystallization Conditions for Organic Synthesis Product Purification Using Deep Learning

Crystallization is an important purification technique for solid products in a chemical laboratory. However, the correct selection of a solvent is important for the success of the procedure. In order to accelerate the solvent or solvent mixture search process, we offer an in silico alternative, i.e., a never previously demonstrated approach that can model the reaction mixture crystallization conditions which are invariant to the reaction type. The offered deep learning-based method is trained to directly predict the solvent labels used in the crystallization steps of the synthetic procedure. Our solvent label prediction task is a multi-label multi-class classification task during which the method must correctly choose one or several solvents from 13 possible examples. During the experimental investigation, we tested two multi-label classifiers (i.e., Feed-Forward and Long Short-Term Memory neural networks) applied on top of vectors. For the vectorization, we used two methods (i.e., extended-connectivity fingerprints and autoencoders) with various parameters. Our optimized technique was able to reach the accuracy of 0.870 ± 0.004 (which is 0.693 above the baseline) on the testing dataset. This allows us to assume that the proposed approach can help to accelerate manual R&D processes in chemical laboratories.

Revealing the roles of polymers in supersaturation stabilization from the perspective of crystallization behaviors: A case of nimodipine

Formulating drugs into amorphous solid dispersions (ASDs) represents an attractive means to enhance the aqueous solubility of drugs. Furthermore, water-soluble polymers have proven highly advantageous for stabilizing supersaturated solutions of ASDs. However, the performance and mechanism of various polymers in stabilizing supersaturated drug solutions have not been well-studied. The aim of this study was to investigate the effects of different commercial polymers on the dissolution behaviors and supersaturation stabilization of the ASDs and to further explore the mechanism of polymer mediated supersaturation maintenance by studying the crystallization behaviors of the ASDs. In this study, nimodipine (NMD) was used as a model drug because of its poor water-solubility and fast crystallization rate in aqueous solution, and three polymers polyvinylpyrrolidone (PVP), vinylpyrrolidone-vinyl acetate copolymer (PVP VA), and polyvinyl caprolactam-polyvinyl acetate-polyethylene glycol graft co-polymer (Soluplus) was selected as the drug carriers to form the ASDs with NMD. Solid-state characterizations of the ASDs confirmed the amorphous state of the ASD systems. ASD_{PVP VA} demonstrated superior supersaturation maintenance in dissolution experiments compared to the other two ASD systems. Among the polymers tested, PVP VA most efficiently maintained dissolution of NMD and prevented its crystallization from the supersaturated solution. The ability of PVP VA to most-effectively maintain supersaturation of the drug was manifested by inhibition of crystal nucleation rather than inhibition of crystal growth following nucleation. These results suggest that nucleation inhibition was instrumental in enabling the polymer-mediated supersaturation maintenance, at least with NMD.

Classical and Nonclassical Nucleation and Growth Mechanisms for Nanoparticle Formation

All solid materials are created via nucleation. In this evolutionary process, nuclei form in solution or at interfaces and expand by monomeric growth, oriented attachment, and phase transformation. Nucleation determines the location and size of nuclei, whereas growth controls the size, shape, and aggregation of newly formed nanoparticles. These physical properties of nanoparticles can determine their functionalities, reactivities, and porosities, as well as their fate and transport. Recent advances in nanoscale analytical technologies allow in situ real-time observations, enabling us to uncover the molecular nature of nuclei and the critical controlling factors for nucleation and growth. Although a single theory cannot yet fully explain such evolving processes, we have started to better understand how both classical and nonclassical theories can work together, and we have begun to recognize the importance of connecting these theories. This review discusses the recent convergence of knowledge about the nucleation and the growth of nanoparticles. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Recent advances in drug polymorphs: Aspects of pharmaceutical properties and selective crystallization

Drug polymorphism, an established term used to describe the phenomenon that a drug can exist in different crystalline phases, has attracted great interests in pharmaceutical field in consideration of its important role in affecting the pharmaceutical performance of oral formulations. This paper presents an overview of recent advances in the research on polymorphic drug systems including understandings on nucleation, crystal growth, dissolution, mechanical properties, polymorphic transformation, etc. Moreover, new strategies and mechanisms in the control of polymorphic forms are also highlighted in this review. Furthermore, challenges and trends in the development of polymorphic drugs are briefly discussed, aiming at developing effective and efficient pharmaceutical formulations containing the polymorphic drugs.

The role of phosphoric acid in the crystallization of lenalidomide form DH–water system

In this work, we studied the effect of phosphoric acid (0–10 v%), acting as an additive, on the thermodynamics and nucleation kinetics of the lenalidomide (LDM) latest form (DH).

Template design based on molecular and crystal structure similarity to regulate conformational polymorphism nucleation: the case of α,ω-alkanedi-carb-oxy-lic acids

Template design on polymorph control, especially conformational polymorphs, is still in its infancy and the result of polymorph control is often accidental. A method of regulating the crystallization of conformational polymorphs based on the crystal structure similarity of templates and the target crystal form has been developed. Crystal structure similarity was considered to be able to introduce lattice matching (geometric term) with chemical interactions to regulate conformational polymorph nucleation. The method was successfully applied to induce the crystallization of DA7-II [HOOC-(CH₂) _{n -2}-COOH (diacids), named DAn, where n = 7, 9, 15, 17 and II represents the metastable polymorph] on the surface of DA15-II. An analogous two-dimensional plane - the (002) face of both DA15-II and DA7-II - was firstly predicted as the epitaxially attached face with similar lattice parameters and the strongest adsorption energy. The powder DA15-II template with the preferred orientation face in (002) presented much stronger inducing DA7-II ability than the template with other preferred orientation faces. The epitaxial growth of DA7-II on DA15-II through an identical (002) face was clearly observed and verified by the single-crystal inducing experiments. The molecular dynamics simulation results demonstrated that the strong interactions occurred between DA7 molecules and the (002) face of DA15-II. This method has been verified and further applied to the crystallization of DA7-II on the surface of DA17-II and DA9-II on the surface of DA15-II. This study developed a strategy based on structure similarity to regulate the conformational polymorph and verified the significant role of lattice matching and chemical effects on the design and preparation of templates.

Nanostructural Arrangements and Surface Morphology on Ureasil-Polyether Films Loaded with Dexamethasone Acetate

Ureasil-Poly(ethylene oxide) (ureasil-PEO500) and ureasil-Poly(propylene oxide) (u-PPO400) films, unloaded and loaded with dexamethasone acetate (DMA), have been investigated by carrying out atomic force microscopy (AFM), ultrasonic force microscopy (UFM), contact-angle, and drug release experiments. In addition, X-ray diffraction, small angle X-ray scattering, and infrared spectroscopy have provided essential information to understand the films' structural organization. Our results reveal that while in u-PEO500 DMA occupies sites near the ether oxygen and remains absent from the film surface, in u-PPO400 new crystalline phases are formed when DMA is loaded, which show up as ~30-100 nm in diameter rounded clusters aligned along a well-defined direction, presumably related to the one defined by the characteristic polymer ropes distinguished on the surface of the unloaded u-POP film; occasionally, larger needle-shaped DMA crystals are also observed. UFM reveals that in the unloaded u-PPO matrix the polymer ropes are made up of strands, which in turn consist of aligned ~180 nm in diameter stiffer rounded clusters possibly formed by siloxane-node aggregates; the new crystalline phases may grow in-between the strands when the drug is loaded. The results illustrate the potential of AFM-based procedures, in combination with additional physico-chemical techniques, to picture the nanostructural arrangements in polymer matrices intended for drug delivery.

Tuning Transport Phenomena in Agarose Gels for the Control of Protein Nucleation Density and Crystal Form

Agarose gels provide the ideal environment for studying the nucleation step of complex biomacromolecules under diffusion-controlled conditions. In the present paper, we characterized the influence of agarose on the nucleation of three model proteins, i.e., lysozyme, insulin, and proteinase K, as a function of the agarose concentration using a batch method set-up inside flat capillaries. By using this set-up, we were able to directly count the number of crystals in a given volume and correlate it with the amount of agarose and with the average crystal size. We also studied the crystallization behavior of proteinase K with free-interface diffusion so that batch conditions were achieved through slow diffusion of the precipitant. Thanks to the control over the protein mass transport imposed by the network, a previously unknown crystal form, P212121, was obtained, and the three-dimensional structure was determined at a 1.6 Å resolution. Overall, the versatility of agarose gels makes them ideal candidates for the preparation of microcrystalline suspensions of biopharmaceuticals with precise and reproducible crystal attributes or for the exploration of the existence of different polymorphs.

Role of Self-Assembled Surface Functionalization on Nucleation Kinetics and Oriented Crystallization of a Small-Molecule Drug: Batch and Thin-Film Growth of Aspirin as a Case Study

The present paper assesses the heterogeneous nucleation of a small-molecule drug and its relationship with the surface chemistry of engineered heteronucleants. The nucleation of aspirin (ASA) was tuned by different functional groups exposed by self-assembled monolayers (SAMs) immobilized on glass surfaces. Smooth topographies and defect-free surface modification allowed the deconvolution of chemical and topographical effects on nucleation. The nucleation induction time of ASA in batch crystallization was mostly enhanced by methacrylate and amino groups, whereas it was repressed by thiol groups. In this perspective, we also present a novel strategy for the evaluation of surface-drug interactions by confining drug crystallization to thin films and studying the preferential growth of crystal planes on different surfaces. Crystallization by spin coating improved the study of oriented crystallization, enabling reproducible sample preparation, minimal amounts of drug required, and short processing time. Overall, the acid surface tension of SAMs dictated the nucleation kinetics and the extent of relative growth of the ASA crystal planes. Moreover, the face-selective action of monolayers was investigated by force spectroscopy and attributed to the preferential interaction of exposed groups with the (100) crystal plane of ASA.

Stabilization of Metastable Indomethacin α in Cellulose Nanocrystal Aerogel Scaffolds

Indomethacin (IM) is a small molecule active pharmaceutical ingredient (API) that exhibits polymorphism with the γ-form being the most thermodynamically stable form of the drug. The α-form is metastable, but it exhibits higher solubility, making it a more attractive form for drug delivery. As with other metastable polymorphs, α-IM undergoes interconversion to the stable form when subjected to certain stimuli, such as solvent, heat, pH, or exposure to seed crystals of the stable form. In this study, IM was crystallized into cellulose nanocrystal aerogel scaffolds as a mixture of the two polymorphic forms, α-IM and γ-IM. Differential scanning calorimetry (DSC) and Raman spectroscopy were used to quantitatively determine the amount of each form. Our investigation found that the metastable α-IM could be stabilized within the aerogel without phase transformation, even in the presence of external stimuli, including heat and γ-IM seed crystals. Because interconversion is often a concern during production of metastable forms of APIs, this approach has important implications in being able to produce and stabilize metastable drug forms. While IM was used as a model drug in this study, this approach could be expanded to additional drugs and provide access to other metastable API forms.

Surface-Induced Crystallization of Sodium Dodecyl Sulfate (SDS) Micellar Solutions in Confinement

We investigate the role of confinement on the onset of crystallization in subcooled micellar solutions of sodium dodecyl sulfate (SDS), examining the impact of sample volume, substrate surface energy, and surface roughness. Using small angle neutron scattering (SANS) and dynamic light scattering (DLS), we measure the crystallization temperature upon cooling and the metastable zone width (MSZW) for bulk 10-30 wt% SDS solutions. We then introduce a microdroplet approach to quantify the impact of surface free energy (18-65 mN/m) and substrate roughness (R_α ≃ 0-60 μm) on the kinetics of surface-induced crystallization through measurements of induction time (t_i) under isothermal conditions. While t_i is found to decrease exponentially with decreasing temperature (increasing subcooling) for all tested surfaces, increasing the surface energy could cause a significant further reduction of up to ∼40 fold. For substrates with the lowest surface energy and longest t_i, microscale surface roughness is found to enhance crystal nucleation, in particular for R_α ≥ 10 μm. Finally, we demonstrate that tuning the surface energy and microscopic roughness can be effective routes to promote or delay nucleation in bulk-like volumes, thus greatly impacting the stability of surfactant solutions at lower temperatures.

Protein crystallisation with air bubble templates: case of gas–liquid–solid interfaces

Crystal formation on air bubble–liquid interface, as soft template to efficiently prompt nucleation of proteins.

Enhancing the crystallisation of insulin using amino acids as soft-templates to control nucleation

Amino acid as soft templates in promoting nucleation of insulin.

Impact of Surface Roughness on Crystal Nucleation

We examine the effect of rough surfaces on crystal nucleation by means of kinetic Monte Carlo simulations. Our work makes use of three-dimensional kMC models, explicit representation of transport in solution and rough surfaces modeled as randomly varying height fluctuations (roughness) with exponentially decaying correlation length (topology). We use Forward-Flux Sampling to determine the nucleation rate for crystallization for surfaces of different roughness and topology and show that the effect on crystallization is a complex interplay between the two. For surfaces with low roughness, small clusters form on the surface but as clusters become larger they are increasingly likely to be found in the bulk solution while rougher surfaces eventually favor heterogeneous nucleation on the surface. In both cases, the rough surface raises the local supersaturation in the solution thus leading to another mechanism of enhanced nucleation rate.

Combining Surface Templating and Confinement for Controlling Pharmaceutical Crystallization

Poor water solubility is one of the major challenges to the development of oral dosage forms containing active pharmaceutical ingredients (APIs). Polymorphism in APIs leads to crystals with different surface wettabilities and free energies, which can lead to different dissolution properties. Crystal size and habit further contribute to this variability. An important focus in pharmaceutical research has been on controlling the drug form to improve the solubility and thus bioavailability of APIs. In this regard, heterogeneous crystallization on surfaces and crystallization under confinement have become prominent forms of controlling polymorphism and drug crystal size and habits; however there has not been a thorough review into the emerging field of combining these approaches to control crystallization. This tutorial-style review addresses the major advances that have been made in controlling API forms using combined crystallization methods. By designing templates that not only control the surface functionality but also enable confinement of particles within a porous structure, these combined systems have the potential to provide better control over drug polymorph formation and crystal size and habit. This review further provides a perspective on the future of using a combined crystallization approach and suggests that combining surface templating with confinement provides the advantage of both techniques to rationally design systems for API nucleation.

General and adaptive synthesis protocol for high-quality organosilane self-assembled monolayers as tunable surface chemistry platforms for biochemical applications

The controlled modification of surface properties represents a pervasive requirement to be fulfilled when developing new technologies. In this paper, we propose an easy-to-implement protocol for the functionalization of glass with self-assembled monolayers (SAMs). The adaptivity of the synthesis route was demonstrated by the controlled anchoring of thiol, amino, glycidyloxy, and methacrylate groups onto the glass surface. The optimization of the synthetic pathway was mirrored by extremely smooth SAMs (approximately 150 pm roughness), layer thickness comparable to the theoretical molecule length, absence of silane islands along the surface, quasi-unitary degree of packing, and tailored wettability and charge. The functionalization kinetics of two model silanes, 3-mercapto- and 3-amino-propyltrimethoxysilane, was determined by cross-comparing x-ray photoelectron spectroscopy and time of flight secondary ion mass spectrometry data. Our SAMs with tailored physicochemical attributes will be implemented as supports for the crystallization of pharmaceuticals and biomolecules in upcoming studies. Here, the application to a small molecule drug model, namely aspirin, was discussed as a proof of concept.

Parameter Estimation of the Stochastic Primary Nucleation Kinetics by Stochastic Integrals Using Focused-Beam Reflectance Measurements

The kinetic parameters of stochastic primary nucleation were estimated for the batch-cooling crystallization of L-arginine. It is difficult for process analytical tools to detect the first nucleus. In this study, the latent period for the total number of crystals to be increased to a predetermined threshold was repeatedly measured with focused-beam reflectance measurements. Consequently, the latent periods were different in each measurement due to the stochastic behavior of both primary and secondary nucleation. Therefore, at first, the distribution of the latent periods was estimated by a Monte Carlo simulation for some combinations of the kinetic parameters of primary nucleation. In the simulation, stochastic integrals of the population and mass balance equations were solved. Then, the parameters of the distribution of latent periods were estimated and correlated with the kinetic parameters of primary nucleation. The resulting correlation was represented by a mapping. Finally, the parameters of the actual distribution were input into the inverse mapping, and the kinetic parameters were estimated as the outputs. The estimated kinetic parameters were validated using statistical techniques, which implied that the observed distribution function of the latent periods for the thresholds used in the estimation coincided reasonably with the simulated one based on the estimated parameters.

Emerging role of primary heterogeneous nucleation in pharmaceutical crystallization

Crystallization is an important and difficult to control unit operation in the pharmaceutical industry. Crystallization can control molecular (i.e., polymorphism) and particulate (i.e., particle size and crystal habit) properties of active pharmaceutical ingredient (API). Moreover, these molecular and particulate properties govern the manufacturability, stability, and biopharmaceutical performance of the API and drug product. Nucleation is a key step and primary heterogeneous nucleation is a common mode of nucleation during crystallization. Hence, it is important to understand the parameters affecting primary heterogeneous nucleation, to achieve desirable properties in crystalline APIs. Primary heterogeneous crystallization has usually been linked to the surface characteristics like topography and functionality of the heteronucleant. The review outlines recent findings in the primary heterogeneous crystallization with specific emphasis on its pharmaceutical applications including regulatory considerations. Molecular-level mechanisms governing heteronucleation and subsequent outcome in terms of molecular as well as particulate-level properties of API have also been discussed. Moreover, general guidance for the selection of heteronucleant has also been included. Heterogeneous crystallization is a promising tool for efficient crystallization of API having properties for optimal pharmaceutical performance.

Surface Induced Phenytoin Polymorph. 1. Full Structure Solution by Combining Grazing Incidence X-ray Diffraction and Crystal Structure Prediction

Understanding the behavior and properties of molecules assembled in thin layers requires knowledge of their crystalline packing. The drug phenytoin (5,5-diphenylhydantoin) is one of the compounds that can be grown as a surface induced polymorph. By using grazing incidence X-ray diffraction, the monoclinic unit cell of the new form II can be determined, but, due to crystal size and the low amount of data, a full solution using conventional structure solving strategies fails. In this work, the full solution has been obtained by combining computational structure generation and experimental results. The comparison between the bulk and the new surface induced phase reveals significant packing differences of the hydrogen-bonding network, which might be the reason for the faster dissolution of form II with respect to form I. The results are very satisfactory, and the method might be adapted for other systems, where, due to the limited amount of experimental data, one must rely on additional approaches to gain access to more detailed information to understand the solid-state behavior.

Inhibiting or Accelerating Crystallization of Pharmaceuticals by Manipulating Polymer Solubility

Polymers play a central role in controlling the crystallization of pharmaceuticals with effects as divergent as amorphous form stabilization and the acceleration of crystallization. Here, using pyrazinamide and hydrochlorothiazide as model pharmaceuticals, it is demonstrated that the same functional group interactions are responsible for these opposing behaviors and that whether a polymer speeds or slows a crystallization can be controlled by polymer solubility. This concept is applied for the discovery of polymers to maintain drug supersaturation in solution: the strength of functional group interactions between drug and polymer is assessed through polymer-induced heteronucleation, and soluble polymers containing the strongest-interacting functional groups with drug are shown to succeed as precipitation inhibitors.

Conundrum of γ glycine nucleation revisited: to stir or not to stir?

Stirring promotes formation of the metastable α glycine polymorph, whereas stable γ glycine forms under quiescent conditions.

Template-induced nucleation for controlling crystal polymorphism: from molecular mechanisms to applications in pharmaceutical processing

The major factors governing template-induced nucleation of molecular crystals are assessed, highlighting applications in pharmaceutical manufacturing and formulation processes where the templating effect is used to promote crystal nucleation and for controlling crystal polymorphism.