Polymer hydrogels are attractive materials due to the wide range of applications exploiting their capability to retain and release large amounts of solvent. Both the equilibrium and kinetics of swelling is well understood for the case of spherical hydrogels, as the sphere has obvious symmetries and allows for relatively simple treatments of the problem. In contrast, much little is known for hydrogels with different topology, such as toroidal hydrogels. In fact, the generation of toroidal fluid ligaments has remained a challenge until recently [E. Pairam, A. Fernandez-Nieves, Physical Review Letters 102, 234501 (2009)]. The purpose of this proposal is to study the generation of toroidal droplets in the presence of electric fields and to use these droplets as templates for the generation of toroidal hydrogels of controlled aspect ratio. These materials can then be used to study how swelling occurs in this geometry and how it differs from the swelling of spherical hydrogels. We anticipate novel results along the way to achieve our goals.
We will build on our experience to make toroidal droplets to generate and study charged toroidal droplets. The method consists in extruding a liquid through a needle in the presence of a rotating continuous phase, as shown in the figure. The idea for generating charged droplets consists in using electrically conducting liquids and a metallic needle set to a nonzero electric potential. Integration of the current through the circuit allows the precise calculation of the charge, which as for any electric conductor, will be located at the surface. Preliminary results confirm that the method indeed results in charged toroidal droplets.
The generation of toroidal hydrogels will follow from the generation of charged toroidal droplets consisting of a mixture of monomers that will be UV-polymerized. We will use thermosentive polymers such as poly-(N-isopropylacrylamide), since their cross-linking and polymerization reactions are well understood. In addition, this choice allows swelling studies via temperature changes, which are easy to promote. By changing the aspect ratio of the hydrogels, we will explore how the toroidal geometry affects swelling.
Toroidal hydrogels generated in this way with the right chemistry will be used as substrates for mammalian cells to study how curvature affects cell behavior. As a whole, the hydrogel-cell construct constitutes a new biomaterial that could find a wide range of applications. Hence the work performed in the context of the project will potentially contribute to the development of new intellectual property, in addition to contributing new and fascinating fundamental science.
The participating student will learn the fluid mechanics, electrostatics and polymer physics required to perform the experiments and will be trained in the mathematical modeling and simulation required to interpret and understand the experimental results. These are all fundamental skills in modern science.