Oral: Mechanical Responses of Nanothin Hydrogel Microcapsules to Osmotic Pressure-induced Deformations

Shape transformations in stimuli-responsive hydrogel microsystems remain attractive for biomimetics and controlled delivery applications. Herein, we studied the relationship between physicochemical properties and shape deformations in poly(methacrylic acid) (PMAA) multilayer hydrogel capsules in response to poly(styrene sulfonate sodium salt) (PSS)-induced osmotic pressure changes. Ultrathin capsules with varied shell thicknesses were synthesized using a templated multilayer assembly of hydrogen-bonded polymer template coatings on 4-μm nonporous sacrificial inorganic microparticles. The pH responses of obtained PMAA multilayer hydrogel microcapsules were assessed in solutions across acidic and basic pH. The surface charge of the hydrogel capsules shifted from neutral to negative values at pH>4. The capsule shape deformations via inward shell buckling were observed at or above the critical PSS concentration. The critical osmotic pressure leading to the capsule shape change was used to determine the bulk modulus of the hydrogel capsule. Recovery of the hydrogel shape was investigated in response to environmental changes, including solution acidity and PSS concentration. Our study brings a fundamental understanding of the shape transformation in hollow nanothin hydrogels, which can be helpful in the development of adaptable microsystems for targeted and controlled delivery applications.