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Betulin Self‐Assembly: From High Axial Aspect Crystals to Hedgehog Suprastructures

Betulin is introduced as a building block of supramolecular structures that form by noncovalent interactions. Betulin, an abundant triterpene, is first extracted from birch bark to self‐assemble into hierarchical crystalline structures. Following a direct bottom‐up process, solvatomorphs of betulin‐2‐propanol form high axial aspect microparticles, 100–300 µm in length, and lateral sizes in the 15–40 µm range. Under sonication, the microparticles evolve into 3D hedgehog suprastructures (15–30 µm in size) comprising a core (3–5 µm diameter) with connected spines (200–300 nm in width). Depending on the intensity of the delivered energy, the associated morphogenesis starts with metastable aggregates that undergo dislocation, solvent migration, and radial growth. It is found that the formed spherulite‐like, hedgehog structures are a consequence of residual solvent diffusion, leading to crystallites with oriented attachment on specific planes. These results add to the design of structures from natural building blocks, which are expected to deliver functions encoded in the respective morphology. Owing to the unique features of betulin supraparticles, they develop surface architectures, for instance, in superhydrophobic coatings, which are shown to exhibit exceptional repellency and surface mechanical strength, as tested on various substrates.