We report on the valorization of wood bark components, specifically bioactive betulin, in effort to promote sustainability of forestry resources. Self-assembly of betulin with tailored structures and geometries is shown for establishing and optimizing the functionality of supraparticles. Betulin, an abundant triterpene, is first extracted from birch bark to then follow directed bottom-up self-assembly into hierarchical crystalline structures. In this process, solvatomorphs of betulin-2-propanol firstly form high axial aspect morphologies, 100-300 μm in length and lateral sizes in the 15-40 μm range. Under sonication, the microparticles evolve into 3-D hedgehog suprastructures (15–30 μm in size) comprising a core (3-5 μm diameter) with connected spines (200- 300 nm in width). A key in our strategy is to continuously provide energy input, via sonication. We propose that the formation of betulin superparticles involves four main steps, namely, metastable aggregates, rupture, solvent migration, and regular radial growth, where the formation and the oriented attachment on specific crystal facets occurs via residual solvents from recrystallization and purification of betulin from bark. Our results provide the fundamental basis for designing supramolecular 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.

Particle assembly of betulin extracted from bark and applications

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