Facile fabrication of stable wettability gradients on elastomeric surfaces for applications in water collection and controlled cell adhesion.

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We have developed a simple and effective method to prepare stable wettability gradients on an elastomeric soft substrate, polydimethylsiloxane (PDMS). In our method, a partially cured PDMS film composed of a definite ratio of elastomer and crosslinking agent was heated over a hot surface with a temperature gradient. This causes differential thermal curing of the PDMS film and the water contact angle (wettability) of the resultant surface showed gradual variation across the length. This method allows us to design and fabricate wettability gradients with rationally controlled directionality and shapes (e.g., linear and radial gradients). The stability of the wettability gradients was studied and a chemical treatment method was developed to enhance the stability at room temperature. Stable wettability gradients prepared through this method can find applications as reliable platforms and scaffolds offering controlled or directional wetting and adhesion. We have demonstrated the practical applications of the wettability gradients in directional water collection, controlled crystallization of materials, and controlled cell adhesion of HeLa cells, osteoblasts and NIH/3T3 cells. The multi-functional characteristics of these wettable gradients are expected to be handy in other domains using soft materials and interfaces also.