Uniform Reversible Buckling in Highly Hydrated Spherical Ultrathin Hydrogel Shells.

Elastic colloids with reversible shape transformations have various applications in cellular mimicry, controlled release, targeted therapy, responsive biosensors, and adaptive optics. Herein, a reversible volume reduction is explored in ultrathin spherical hydrogel shells made of poly(methacrylic acid) in response to osmotic pressure changes in solution. The 4-µm pH-responsive hydrogel shells are synthesized via polymer multilayer assembly on sacrificial spherical microparticles to produce nanostructured hydrogel shells with varied thicknesses. The degree of hydrogel shell volume reduction and rapid recovery of its spherical shape in response to osmotically induced deformation and after stress removal are studied in highly hydrated shells. The spherical hydrogel shape uniformly deforms inward by producing a dimple at critical osmotic pressures (1-15 kN m^-2), followed by the formation of a soft half-shell. These deformations are entirely uniform and are rapidly reversed upon stress removal. These large shape deformations and quick recovery are due to the shell's low elasticity of 4.0 ± 0.1 MPa, characteristic of elastomers. This study demonstrates the potential of the ultrathin highly hydrated hydrogel microshells for extending the class of elastomeric colloids with programmable compressibility and flow properties, leading to new fundamental and applied knowledge about elastic non-spherical hydrogels.