Biodegradable Honeycomb-Mimic Scaffolds Consisting of Nanofibrous Walls

The scaffold is a porous three-dimensional (3D) material that supports cell growth and tissue regeneration. Such 3D structures should be generated with simple techniques and nontoxic ingredients to mimic bio-environment and facilitate tissue regeneration. In this work, simple but powerful techniques are demonstrated for the fabrication of lamellar and honeycomb-mimic scaffolds with poly(L-lactic acid). The honeycomb-mimic scaffolds with tunable pore size ranging from 70 to 160 µm are fabricated by crystal needle-guided thermally induced phase separation in a directional freezing apparatus. The compressive modulus of the honeycomb-mimic scaffold is ≈4 times higher than that of scaffold with randomly oriented pore structure. The fabricated honeycomb-mimic scaffold exhibits a hierarchical structure from nanofibers to micro-/macro-tubular structures. Pre-osteoblast MC3T3-E1 cells cultured on the honeycomb-mimic nanofibrous scaffolds exhibit an enhanced osteoblastic phenotype, with elevated expression levels of osteogenic marker genes, than those on either porous lamellar scaffolds or porous scaffolds with randomly oriented pores. The advanced techniques for the fabrication of the honeycomb-mimic structure may potentially be used for a wide variety of advanced functional materials.

A new insight into the mechanism of the tabletability flip phenomenon

Tabletability is an outcome of interparticulate bonding area (BA) - bonding strength (BS) interplay, influenced by the mechanical properties, size and shape, surface energetics of the constituent particles, and compaction parameters. Typically, a more plastic active pharmaceutical ingredient (API) exhibits a better tabletability than less plastic APIs due to the formation of a larger BA during tablet compression. Thus, solid forms of an API with greater plasticity are traditionally preferred if other critical pharmaceutical properties are comparable. However, the tabletability flip phenomenon (TFP) suggests that a solid form of an API with poorer tabletability may exhibit better tabletability when formulated with plastic excipients. In this study, we propose another possible mechanism of TFP, wherein softer excipient particles conform to the shape of harder API particles during compaction, leading to a larger BA under certain pressures and, hence, better tabletability. In this scenario, the BA-BS interplay is dominated by BA. Accordingly, TFP should tend to occur when API solid forms are formulated with a soft excipient. We tested this hypothesis by visualizing the deformation of particles in a model compressed tablet by nondestructive micro-computed tomography and by optical microscopy when the particles were separated from the tablet. The results confirmed that soft particles wrapped around hard particles at their interfaces, while an approximately flat contact was formed between two adjacent soft particles. In addition to the direct visual evidence, the BA-dominating mechanism was also supported by the observation that TFP occurred in the p-aminobenzoic acid polymorph system only when mixed with a soft excipient.

The ubiquity of the tabletability flip phenomenon

The plasticity of materials plays a critical role in adequate powder tabletability, which is required in developing a successful tablet product. Generally, a more plastic material can develop larger bonding areas when other factors are the same, leading to higher tabletability than less plastic materials. However, it was observed that, for a solid form of a compound with poorer tabletability, a mixture with microcrystalline cellulose (MCC) can actually exhibit better tabletability, a phenomenon termed tabletability flip. Hence, there is a chance that a solid form with poor tabletability could have been erroneously eliminated based on the expected tabletability challenges during tablet manufacturing. This study was conducted to investigate the generality of this phenomenon using two polymorph pairs, a salt and free acid pair, a crystalline and amorphous solid dispersion pair, and a pair of chemically distinct crystals. Results show that tabletability flip occurred in all six systems tested, including five pairs of binary mixtures with MCC and one pair in a realistic generic tablet formulation, suggesting the broad occurrence of the tabletability flip phenomenon, where both compaction pressure and the difference in plasticity between the pair of materials play important roles.

On the use of a volume constraint to account for thermal expansion effects on the low-frequency vibrations of molecular crystals

A volume-constraint method is presented as a means to capture the influence of thermal expansion on the low-frequency vibrations in molecular crystals. In particular, the room-temperature terahertz absorption spectra of L-tartaric acid, α-lactose monohydrate, and α-para-aminobenzoic acid (PABA) have been simulated using dispersion-corrected, solid-state density functional theory (DFT-D). By comparing the normal modes obtained with a unit cell optimised without constraints to those obtained with a unit cell optimised while constrained to keep its experimental volume, wholesale improvements to the resultant spectrum is achieved when using the constrained geometry by inhibiting cell contraction. These improvements are demonstrated over a range of popular density functionals and basis sets up to triple-zeta complexity. A correlation method is then presented as a means to quantitatively compare the vibrational pattern of normal modes obtained from both unit cells. This analysis reveals that thermal expansion can effect the character and relative frequency of normal modes, with the choice of geometry ultimately affecting the assignment of the experimental absorptions. The sensibility of using the experimental volume as an approximation is then discussed, where it is speculated that large basis sets or hybrid functionals are necessary to ensure that the thermal expansion effect is not overestimated. The low-frequency absorption spectrum of PABA is then fully characterised using the PBE-D3BJ/6-311G(2d,2p) method.

A first-order phase transition in Blatter's radical at high pressure

The crystal structure of Blatter's radical (1,3-diphenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl) has been investigated between ambient pressure and 6.07 GPa. The sample remains in a compressed form of the ambient-pressure phase up to 5.34 GPa, the largest direction of strain being parallel to the direction of π-stacking interactions. The bulk modulus is 7.4 (6) GPa, with a pressure derivative equal to 9.33 (11). As pressure increases, the phenyl groups attached to the N1 and C3 positions of the triazinyl moieties of neighbouring pairs of molecules approach each other, causing the former to begin to rotate between 3.42 to 5.34 GPa. The onset of this phenyl rotation may be interpreted as a second-order phase transition which introduces a new mode for accommodating pressure. It is premonitory to a first-order isosymmetric phase transition which occurs on increasing pressure from 5.34 to 5.54 GPa. Although the phase transition is driven by volume minimization, rather than relief of unfavourable contacts, it is accompanied by a sharp jump in the orientation of the rotation angle of the phenyl group. DFT calculations suggest that the adoption of a more planar conformation by the triazinyl moiety at the phase transition can be attributed to relief of intramolecular H...H contacts at the transition. Although no dimerization of the radicals occurs, the π-stacking interactions are compressed by 0.341 (3) Å between ambient pressure and 6.07 GPa.

Pressure-induced superelastic behaviour of isonicotinamide

Dynamic organic crystals have come to the fore as potential lightweight alternatives to inorganic actuators providing high weight-to-force ratios. We have observed pressure-induced superelastic behaviour in Form I of isonicotinamide. The reversible single-crystal to single-crystal transformation exhibited by the system is an important component for functioning actuators. Crucially, our observations have enabled us to propose a mechanism for the molecular movement supported by Pixel energy calculations, that may pave the way for the future design and development of functioning dynamic crystals.

Impact of polymorphism on mechanical properties of molecular crystals: a study of p-amino and p-nitro benzoic acid with nanoindentation

We report on nanoindentation data for two pairs of polymorphic compounds of p-aminobenzoic acid (pABA) and p-nitrobenzoic acid (pNBA) and compare it with existing data in the literature. We also explore on a new parameter, s-PBC, as a tool to estimate hardness.

Transforming Computed Energy Landscapes into Experimental Realities: The Role of Structural Rugosity

We exploit the possible link between structural surface roughness and difficulty of crystallisation. Polymorphs with smooth surfaces may nucleate and crystallise more readily than polymorphs with rough surfaces. The concept is applied to crystal structure prediction landscapes and reveals a promising complementary way of ranking putative crystal structures.

The Eight Hydrates of Strychnine Sulfate

Commercial samples of strychnine sulfate were used as the starting material in crystallization experiments accompanied by stability studies. Eight hydrate forms (HyA-HyG), including five novel hydrates, were verified. The crystal structures of HyA ("pentahydrate") and HyF ("hexahydrate") were determined from single-crystal X-ray diffraction data. HyF was identified as the most stable hydrate at high water activities at room temperature (RT), and HyA and HyC were also found to be stable at ambient conditions. Long-time storage experiments over nearly two decades confirm that these three hydrates are stable at ambient conditions (20-60% relative humidity). The other five hydrates, HyB ("dihydrate"), HyD, HyE, HyG, and HyH, are only observable at the low(est) relative humidity (RH) levels at RT. Some of these latter forms can only exist within a very narrow RH range and are therefore intermediate phases. By applying a range of complementary experimental techniques such as gravimetric moisture sorption analysis, thermal analysis, moisture controlled PXRD measurements, and variable temperature IR spectroscopy in combination with principal component analysis, it was possible to identify the distinct hydrate phases and elucidate their stability and dehydration pathways. The observed (de)hydration routes, HyA ↔ HyB, HyC ↔ HyD ↔ HyE, HyF ↔ HyG ↔ HyH and HyF → HyA ↔ HyB, depended on the initial hydrate form, particle size, and atmospheric conditions. In addition, a transformation from HyC/HyA to HyF occurs at high RH values at RT. The specific moisture and temperature conditions of none of the applied drying regimes yielded a crystalline water-free form, which highlights the essential role of water molecules for the formation and stability of the crystalline strychnine sulfate phases.

Same or different – that is the question: identification of crystal forms from crystal structure data

An analysis of the CSD with structural comparison tools shows that differentiating between polymorphism and redeterminations is not always straight forward and requires of complementary tools at the hands of an expert practitioner.

Can solvated intermediates inform us about nucleation pathways? The case of β-pABA

Using crystallography to search for nucleation pathways: α and β polymorphs of p-aminobenzoic acid.

Discovery and recovery of delta p-aminobenzoic acid

A new high-pressure recoverable form has been observed in the model system, p-aminobenzoic acid.