Soft Materials with Concentric Layers: Drawing Inspiration from Onions, Arteries, Pufferfish and Pearls

In nature, we see many spheroidal structures with concentric layers: eggs, embryos, seeds, vegetables like the onion, and tissues like the spinal discs. These are all synthesized inside-out, i.e., a core is formed initially, followed by a first layer and then additional layers. Inspired by the above, we have developed an inside-out technique to synthesize a variety of layered soft materials (Nature Communications, 2017, 8, 193).

First, we have created onion-like polymeric capsules with multiple concentric layers. Each layer grows from the surface of the previous one, and both layer composition and thickness can be tuned. Using this technique, we have created capsules that inflate due to gas evolved from a reaction in the core; such inflation mimics how pufferfish react to predators. We have also made a hydrophobic skin around a core gel that is hydrophilic: just as in fruit like an orange, the skin prevents the gel from losing water. Lastly, when a thin layer of a thermo-responsive gel covers a gel core, we find that, upon heating, the capsule glows like a pearl. Indeed, the glow (pearlescence) of this soft capsule is as bright as that of a hard, inorganic pearl.

Layered structures also exist in other geometries. Tissues in our body including arteries, veins, and intestines, are all multilayered tubes. We have extended our inside-out technique to cylindrical geometries to create polymeric multilayered tubes. Much like arteries, we can engineer these tubes to constrict or dilate in response to the properties of the flowing liquid. In the case of our intestines, they have an outer layer of hairs called villi. To mimic this aspect, we have made planar gels with a layer of hairs, i.e., tubular protrusions. Both the hair thickness as well as density can be controlled. The hairy surfaces absorb solutes much more efficiently than a bare surface, akin to villi-covered intestines.