(23d) ‘Regeneration’ Phenomenon Observed in Pharmaceutical Crystals Post Breakage – a Case Study on Paracetamol and the Effect of Growth Solvents

The crystalline structure of organic molecules has long puzzled scientists. As attractive is the highly ordered nature and purity of the crystal products, it is equally difficult to decipher the crystalline lattice, morphology, shape and size. Approaches to determine crystal morphology have evolved over the years from non-mechanistic theories that depend on solid crystal properties(1) – surface energies, geometry (BFDH theory), attachment energies etc., to kinetic models that take into account the crystal surface as well as solution properties – 2D nucleation and growth, BCF mechanism, rough growth. But it is surprising to see that very little is known and studied about the effect of breakage on the final crystal structure, shape and size.

Although mechanical breakage is a common occurrence in industrial crystallizers, there exists limited knowledge on the consequent growth of broken crystals - especially in the case when a crystal breaks along its cleavage plane(2). Owing to this, process modelling attempts rarely account for specific post-breakage growth kinetics and overlook that the regrowth kinetics of crystals may vary depending on the method they have been fragmented, resulting in unreliable predictions of crystal shape and size. This work reports for the first time an unconventional ‘regeneration’ phenomenon in paracetamol crystals following breakage along the cleavage plane (010) (Figure 1) (3). The broken part was observed to regrow on the parent crystal, restoring the pre-breakage shape prior to demonstrating overall growth, which was otherwise unexpected – similar to the process in which a lizard regrows its tail after autotomy. Recent work has shown a number of such anomalies in organic crystal behavior due to application of stress, such as autonomous repair of broken crystals via compression(4) and elastic deformation of elongated crystalline habits(5). These eccentricities have the potential of changing for the better or worse, the end crystal properties as well as their downstream performance. Considering that breakage along the cleavage plane is most probable when a crystal is under pressure(6), understanding the growth behavior post cleavage is highly crucial to accurately predict the end crystal properties and improve their downstream processibility

This unique behavior was investigated with macroscopic crystals of paracetamol grown using slow evaporation of a saturated ethanolic solution at room temperature and captured using various imaging techniques. A peculiarity exists in the paracetamol crystal where in the cleavage plane (010) exists internally and does not form part of the equilibrium habit of the crystal. To expose the cleavage plane, point pressure was applied on the facet normal to the cleavage plane, (001), which resulted in a self-propagating crack along (010) and resulted in the crystal splitting into two clean halves each exposing facet (010) (Figure 1). These cleaved crystals were suspended in a saturated paracetamol-ethanol solution using Kevlar fibres to observe regeneration. Suspending the crystals allowed them to grow homogeneously without any facet being obstructed by the walls/base of a vial.

The regrowth was recorded using optical microscopy as well as a novel macro photography rig, developed in-house using DSLR cameras to achieve high resolution images. Consequently regrowth kinetics were obtained by measuring the growth rate of two characteristic lengths of the growing crystals, one parallel (length ) and one perpendicular to the cleavage plane (length ) using the image processing software ImageJ. Data was collected over a period of 1-2 weeks depending on the crystal size as two different crystal size groups were investigated: small (3-5mm) and large (7-9mm). Furthermore, the effect of breakage orientation on regeneration was also studied by inducing cuts parallel and perpendicular to (010).

Results showed that the cleaved crystal displayed extrinsic self-healing into its pre breakage shape by rapid growth of facet 010, a mechanism not seen in an uncut crystal. The regeneration process occurred in 2 stages: rapid regeneration followed by sluggish regeneration and once the original shape was restored the crystal demonstrated overall growth (Figure 2)(3). Although the exact growth rates differed for larger crystals, the same trend was observed irrespective of crystal size. In medium crystals, growth of facet 010 (length ) was approximately 1.5 times faster (0.045 mmhr^-1) than the growth in the same direction of a whole crystal (0.024 mmhr^-1) (Figure 2). This behaviour was seen to be dependent on the direction of breakage with reference to the cleavage plane irrespective of the initial crystal size; cuts parallel to the cleavage plane exhibited self-healing as opposed to perpendicular ones. It was interesting to see that when roughly 10% and 25% of the crystal was cut off parallel to the cleavage plane, it still exhibited regeneration in a similar manner to when it was broken halfway (50% cleaved), but after being cut normal to (010), the crystal did not regenerate, rather retained the habit acquired after breakage (Figure 3).

This unprecedented growth of (010) was speculated to be due to the otherwise low energy facet being extremely unstable in solution after being exposed and therefore growing rapidly to allow the crystal to attain a thermodynamically equilibrium shape – however the exact mechanism of the process is still unknown. The regeneration phenomenon contradicts well-established morphology prediction theories that suggest slow growth of low attachment energy facets, usually the cleavage planes. This anomaly in the growth behaviour is postulated to be due to a combination of surface energetics and the surface chemistry of the solid crystal as well as the solute-solvent interactions which need to be investigated further.

Preliminary work on the effect of growth solvents including acetone and Tetrahydrofuran (THF) has shown cleaved paracetamol crystals to regenerate into their solvent specific equilibrium habit. Growth data from initial experiments have shown that the normalized percentage growth of lengths and showed trends similar to Ethanol, where in the first phase of regeneration, facet propagation of (010) was the fastest (Figure 4). No conclusions can be made at the moment however more experimental repeats and molecular dynamic simulations rationalising the observations are currently ongoing. Moreover, the applicability of regeneration to other crystal systems is such as carbamazepine is also currently ongoing, crystals for which have also shown signs of regeneration during initial screening (Figure 3). This phenomenon comes at a time when ‘dynamic’ organic crystals are gaining increasing traction due to their unusual behavioral responses to external stimuli via transitions in their molecular packing, previously only reported in hard materials like polymers. Therefore the growth kinetics and molecular mechanism of the regeneration process can provide a new direction to crystal engineering.

Acknowledgments

The authors thank Dr. Ian Rosbottom for the invaluable contributions in the early development of the experimental work of this research.

References

1. Nguyen TTH, Rosbottom I, Marziano I, Hammond RB, Roberts KJ. Crystal Morphology and Interfacial Stability of RS-Ibuprofen in Relation to Its Molecular and Synthonic Structure. Cryst Growth Des. 2017;17(6):3088–99.
2. SUN CC, KIANG YH. On the Identification of Slip Planes in Organic Crystals Based on Attachment Energy Calculation. J Pharm Sci. 2008;97(8):3456–61.
3. Bade I, Verma V, Rosbottom I, Heng JYY. Crystal regeneration – a unique growth phenomenon observed in organic crystals post breakage. Mater Horiz. 2023;10:1425-1430
4. Commins P, Hara H, Naumov P. Self-Healing Molecular Crystals. Angewandte Chemie - International Edition. 2016;55(42):13028–32.
5. Vreeman G, Wang C, Reddy CM, Sun CC. Exceptional Powder Tabletability of Elastically Flexible Crystals. Crystal Growth & Design. 2021;
6. Heng JYY, Bismarck A, Lee AF, Wilson K, Williams DR. Anisotropic Surface Energetics and Wettability of Macroscopic Form I Paracetamol Crystals. 2006;(21):2760–9.