Multilayered Spheres, Tubes, and Surfaces Synthesized by "Inside-Out" Polymerization
Raghavan, Srinivasa R
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Numerous materials in nature, including eggs, onions, spinal discs, and blood vessels, have multiple layers. Each layer in these materials has a distinct composition and thereby a unique function in the overall material. Our work is motivated by the need to find a simple, versatile route for the synthesis of such multilayered materials. Toward this goal, we have devised a technique termed “inside-out polymerization” to synthesize multilayered materials with precise control over the composition and thickness of each layer. Each layer is a crosslinked polymer gel and it grows from the surface of the previous layer, with this growth being controlled by precursor molecules present in the core of the structure. Using this technique, we synthesize multilayer structures in three different geometries, as described below. First, we outline our technique and use it to create multilayered polymer capsules. In particular, we create interesting capsules with concentric layers of non-responsive and stimuli-responsive polymers. The thickness of the stimuli-responsive layer varies sharply due to the stimulus while the non-responsive layer remains at the same thickness. In addition, the permeability of small molecules through the stimuli-responsive layers is also altered. This means that these multilayered capsules could be used to conduct pulsatile release of solutes such as drugs or other chemicals. In addition, we also show that multilayered capsules exhibit improved mechanical properties compared to those of the fragile core. Next, we extend our technique to the synthesis of multilayered polymer tubes. Our technique provides precise control over the inner diameter of the tube, the number of layers in the tube wall, and the thickness and chemistry of each layer. Tubes can be patterned with different polymers either in the lateral or longitudinal directions. Patterned tubes based on stimuli-responsive polymers exhibit the ability to spontaneously change their lumen diameter in response to stimuli, or to convert from a straight to a curled shape. On the whole, these tubes mimic several features exhibited by blood vessels like veins and arteries. In our last study, we use our technique to create hair-like structures that grow outward from a base polymer gel. The diameter, length, and spacing of hairs can all be tuned. The addition of hairs serves to increase the net surface area of the base gel by nearly 10-fold. This increase is comparable to the surface area increase provided by hairs called “villi” on the inner walls of small intestines. In accordance with the increased surface area, hairy surfaces extract solutes from a solution much faster than a bare surface. We also impart stimuli-responsive properties to the hairs (e.g., magnetic properties), and we show that hairy gels can be induced to fold into tubes with hairs on the outside or inside. The latter mimics the structure of the small intestine.