Hairy Gels: Mimicking the Hair-Covered Architecture and Responses of Animal and Plant Tissues

Many animal and plant tissues are covered with slender hairs or filaments (called cilia, villi, tentacles, etc.). The hairs have important functions, which include protection from external elements, adhesion to prey, or increased absorption of nutrients. Here, we present a technique to create soft materials covered by hairs. Both the base and the hairs are <i>hydrogels</i> (i.e., aqueous polymer networks). The hairs are grown from the base by an "<i>inside-out</i>" polymerization guided by a template. By carefully selecting the monomers and cross-linkers for the polymerization, we control the chemistry as well as the mechanical properties of the base and the hairs. Moreover, we can tune all geometric parameters, including the hair diameter, length, and spacing. Hairs increase the surface area of the base gel by 10-fold, allowing a hairy gel to absorb solutes much faster than a bare gel. We also create patterns of different hairs on the same surface, including hairs that respond to stimuli such as magnetic fields. Lastly, we induce hair-covered gels to <i>fold into tubes</i> upon exposure to a stimulus (<i>viz</i>. a change in solvent quality). In the folded tube, the hairs can decorate the outer or inner surface. Folding of hairy gels mimics the folding of sundew plant leaves to trap prey.