Biomimetic Approach for Liquid Encapsulation with Nanofibrillar Cloaks Elisa Mele,* ,† Ilker S. Bayer, † Gabriele Nanni, † Jose ́ Alejandro Heredia-Guerrero, † Roberta Ruffilli, ‡ Farouk Ayadi, † Lara Marini, † Roberto Cingolani, § and Athanassia Athanassiou † † Nanophysics, and ‡ Nanochemistry, Istituto Italiano di Tecnologia (IIT), Via Morego 30, 16163 Genoa, Italy § Istituto Italiano di Tecnologia (IIT), Via Morego 30, 16163 Genoa, Italy * S Supporting Information ABSTRACT: Technologies that are able to handle micro- volumes of liquids, such as microfluidics and liquid marbles, are attractive for applications that include miniaturized biological and chemical reactors, sensors, microactuators, and drug delivery systems. Inspired from natural fibrous envelopes, here, we present an innovative approach for liquid encapsulation and manipulation using electrospun nanofibers. We demonstrated the realization of non-wetting soft solids consisting of a liquid core wrapped in a hydrophobic fibrillar cloak of a fluoroacrylic copolymer and cellulose acetate. By properly controlling the wetting and mechanical properties of the fibers, we created final architectures with tunable mechanical robustness that were stable on a wide range of substrates (from paper to glass) and floated on liquid surfaces. Remarkably, the realized fiber-coated drops endured vortex mixing in a continuous oil phase at high stirring speed without bursting or water losses, favoring mixing processes inside the entrapped liquid volume. Moreover, the produced cloak can be easily functionalized by incorporating functional particles, active molecules, or drugs inside the nanofibers. ■ INTRODUCTION Systems for liquid manipulation at the microscale, such as digital microfluidic devices 1 and liquid marbles, 2-6 are emerging as miniaturized platforms for chemical and biological processes. They offer advantages in terms of reduced amounts of reagents, shortened reaction velocity, enhanced efficiency, and flexibility. In particular, liquid marbles, namely, drops encapsulated with hydrophobic particles or powders, have been proposed for applications that include gas and liquid sensing, 7 microreactors, 8 and water pollution detection. 9 Because of the great potentialities of these non-wetting droplets, recently, there is a growing interest in improving their mechanical properties that are still unsatisfactory 10 or developing analogous systems with higher robustness. The fibrous three-dimensional membranes that animals and vegetables use to encapsulate functional liquids are an alternative and interesting way for bio-inspired fabrication. In fact, capsule-like structures consisting of bundles of collagen fibers exist at the freely movable synovial joints of the human body (such as the hip and knee). 11 They allow for a large range of movements by reducing the friction between the articular surfaces and delivering nutrients to the cartilages. On the other hand, the fluid-filled volume of each cell in a plant, alga, or fungus is contained in a wall mostly composed of a network of polysaccharide microfibrillars (such as cellulose, mannan, and chitin) that are stabilized in a matrix of proteins and additional polysaccharides. 12 Cell walls are fundamental structures for cell viability, because they provide mechanical support, shape definition, protection against pathogens, and active nutrient regulation. Here, inspired by the aforementioned fibrous envelopes, we present the realization of globular structures consisting of a liquid core trapped in a shell of nanofibers and characterized by low friction and mechanical robustness. To this aim, we exploit the impact and rolling of a water drop on a mat of electrospun fibers with engineered wetting properties and morphology. When the chemical composition of the fibers and the impact velocity of the water drops are controlled, the entire liquid volume is wrapped in a nanofibrous cloak. The use of a continuous network of nanofibers instead of particles improves the mechanical resistance of the final system, creating novel architectures mimicking the natural structures. ■ RESULTS AND DISCUSSION The main element of the fiber-coated drops is a mat of hydrophobic polymer blend nanofibers obtained by electro- spinning. 13 The nanofibers consist of a fluoroacrylic copolymer (Capstone ST100) and cellulose acetate (CA). Capstone is an Received: December 16, 2013 Revised: February 17, 2014 Published: February 24, 2014 Article pubs.acs.org/Langmuir © 2014 American Chemical Society 2896 dx.doi.org/10.1021/la4048177 | Langmuir 2014, 30, 2896-2902