Three-Dimensional Conformal Coatings through the Entrapment of Polymer Membrane Precursors Du T. Nguyen, ‡ Maya Kleiman, † Keun Ah Ryu, † Stanley Hiew, † Kyle Brubaker, † Rafik Mughnetsyan, † Richard Truong, † Benjamin Dolan, § Edward Tackett, § and Aaron P. Esser-Kahn* ,† † Department of Chemistry, ‡ Department of Physics and Astronomy, and § Rapid Tech, University of California, Irvine, California 92697, United States * S Supporting Information ABSTRACT: We report a technique to coat polymers onto 3D surfaces distinct from traditional spray, spin, or dip coating. In our technique, the surface of a template structure composed of poly(lactic acid) swells and entraps a soluble polymer precursor. Once entrapped, the precursor is cured, resulting in a thin, conformal membrane. The thickness of each coating depends on the coating solution composition, residence time, and template size. Thicknesses ranged from 400 nm to 4 μm within the experimental conditions we explored. The coating method was compatible with a range of polymers. Complicated 3D structures and microstructures of 10 μm thickness and separation were coated using this technique. The templates can also be selectively removed, leaving behind a hollow membrane structure in the shape of the original printed, extruded, or microporous template structures. This technique may be useful in applications that benefit from three-dimensional membrane topologies, including catalysis, separations, and potentially tissue engineering. KEYWORDS: coating, polymer membranes, entrapment, three-dimensional, microstructures ■ INTRODUCTION The ability to coat and deposit materials onto surfaces is one of the most widely used techniques in materials science. Coatings can improve the properties of many materials by changing their surface properties, such as in applications ranging from anticorrosion to semiconducting. 1-5 Processes such as chemical vapor deposition, atomic layer deposition, solution deposition, and layer-by-layer coatings allow for nanoscale depositions. 6-9 Other techniques such as spin coating, dip coating, and electroplating can form thicker coatings. 10-17 For separations applications involving polymeric membranes, a common method is hollow fiber spinning to template the polymeric membrane using solvent dissolution. 18-21 However, few techniques exist that form polymeric membranes with complicated 3D topologies. Here we report a technique to fabricate microvascular membrane structures through the entrapment of a polymer membrane into a poly(lactic acid) (PLA) template structure. No direct modification of the surface is necessary, making this technique applicable to a wide variety of materials. To coat complicated and fine structures, we sought a technique that did not require thin layer coatings, deposition, or reactivity at the surface. We found that the entrapment of biomolecules and catalysts onto the surfaces of poly(lactic acid) by selectively swelling the surface to incorporate the materials had the potential to develop into a new coating method. 22-24 This procedure was modified to accommodate a variety of polymeric membrane materials. In the templating process, a swollen PLA template surface is used to entrap a membrane precursor from solution. Once entrapped, the precursor is cured to form a membrane conformed to the polymer surface. The polymer template can then be removed, leaving behind the newly formed hollow membrane in the same topology as the original template structure. For this process, we have characterized the membrane thickness as a function of coating solution composition, structure sizes, and residence times and found that each variable can be used to control membrane thickness ranging from 400 nm to 4 μm. These studies were primarily performed using polydimethylsiloxane (PDMS) as the membrane material, but we also found that many commercial polymers are compatible with the entrapment technique. We demonstrated its application in various 3D structures including 3D printed templates and phase separated microstructures with feature sizes ranging from the millimeter range to the micrometer range. The membranes are strong enough to retain liquids and have porosities suitable for the selective separation of gases. To perform an entrapment, an appropriate coating solution must be comprised of four elements: template (PLA), template solvent, template nonsolvent, and membrane precursor (Figure 1). A prefabricated microstructure is placed into the coating solution, which swells the PLA template without fully dissolving Received: November 27, 2013 Accepted: January 17, 2014 Published: January 17, 2014 Research Article www.acsami.org © 2014 American Chemical Society 2830 dx.doi.org/10.1021/am4053943 | ACS Appl. Mater. Interfaces 2014, 6, 2830-2835