Geometry-controlled rigidity in non-spherical hydrogel capsules

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NON-TECHNICAL SUMMARYThe goal of this project is to create hollow micro-sized hydrogel particles (microcapsules) with geometrically controlled rigidity and to provide fundamental understanding of their shape transformations under pressure. Particle rigidity and shape are crucial parameters that dictate specific biological functions, and controlling them could allow comprehensive optimization of capsules for delivery of medications. To date, regulating rigidity (stiffness) of hydrogel particles by varying their shapes has presented a major challenge due to the lack of mechanical stability of shape-defined hydrogel structures. The key task of this research is to understand how stiffness of microcapsules can be controlled by structural characteristics including non-spherical curvature, edge and facet number, and overall shape. The proposed research will impact the development of micro-sized hydrogels of complex shapes that can provide a powerful means to mimic key properties of biological systems and hold potential as cell-mimicking particles to be developed into artificial cells and multifunctional delivery carriers. The design of these capsules also offers prospects for developing materials with unique actuation characteristics having applications in polymer and materials science, biomedical engineering, and chemical biology. The educational objective of the project is to enhance a discovery-driven multidisciplinary polymer science program at UAB, to promote student training from the high school through graduate-level in modern aspects of chemistry, polymer science, and materials science, and to broaden recruitment of historically underrepresented populations. These efforts will also help to increase public awareness of nanoscale polymeric materials and to train the next generation of the STEM workforce in the southeast. TECHNICAL SUMMARY:The goal of this research is to test the hypothesis of geometry-controlled rigidity in hollow hydrogel microcapsules and to gain fundamental understanding of pressure-induced capsule shape transitions (buckling) at a molecular-level. Current approaches aim to control the stiffness of hydrogel particles mainly by choice of polymer, fluid content, and crosslink density. The objectives of the proposed study are: (i) to determine how pressure-induced shape transformations are affected by physicochemical properties of the capsule wall (shell) in vertices-free non-spherical capsules; (ii) to understand how pressure-induced shape transitions are influenced by the physicochemical properties of the capsule shell in vertices-reinforced capsules; and (iii) to discover how capsule shell rigidity can be controlled via the internal structure of the multilayer hydrogel shell. Dimensional and shape changes defined by negative volume changes (shrinkage and buckling instability) in response to increasing osmotic pressure will be investigated using an osmotic pressure difference method. The impacts of crosslink density, chain persistence length, shell hydration and architecture on the hydrogel stability against osmotic pressure-induced buckling (shape loss) and shape recovery after stress removal will be studied. The fundamental relationships between the geometry-controlled rigidity and pressure-induced shape transitions will be explored using a combination of in-situ techniques including atomic force and confocal microscopy, ellipsometry, nanoindentation, neutron scattering, and ATR-FTIR. The expected collaborative effort with the UAB medical community will provide valuable opportunities for two-way feedback between the research on fundamental material properties and testing materials performance for biomedical applications.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.