© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2011, XX, 1–4 1 www.advmat.de www.MaterialsViews.com COMMUNICATION wileyonlinelibrary.com D. J. Lorang, D. Tanaka, Prof. J. A. Lewis Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory University of Illinois at Urbana-Champaign 1304 W. Green St, Urbana, IL 61801, USA E-mail: jalewis@illinois.edu C. M. Spadaccini, K. A. Rose Center for Micro and NanoTechnology Lawrence Livermore National Laboratory 7000 East Ave., Livermore, CA 94551, USA N. J. Cherepy Chemical Sciences Division Lawrence Livermore National Laboratory 7000 East Ave., Livermore, CA 94551, USA DOI: 10.1002/adma.201102411 Integrated optical systems require waveguides that can route light along defined pathways with minimal losses and negligible cross- talk. [1–3] In addition to signal transmission, optical waveguides play an important role in the area of sensing. For example, eva- nescent field sensors are applied to the detection of analytes in the body, [4] atmosphere, [5] and liquid solutions. [6] Polymeric and hybrid materials are of increasing interest for these applications due to their low temperature processing. [7] To date, channel waveguides have been fabricated by direct lithographic pat- terning, [8,9] photoresist-templated etching, [10] or soft-lithographic approaches. [11,12] However, these techniques are limited either to in-plane configurations or require repeated developing or etching steps to produce multiple layers of waveguides. Those processing steps often have a deleterious effect on waveguide performance, leading to rough edges and, hence, higher optical loss. [8,11,13] Direct-write assembly of optical waveguides is an alter- native patterning method, in which soft functional materials are deposited in the desired configuration in a single step. For example, Parker et al. [14] recently patterned silk waveguides by printing a concentrated, viscoelastic ink composed of silk fibroin through a fine deposition nozzle. To maintain its fila- mentary form, the ink must be deposited into a methanol-rich reservoir to induce rapid coagulation. This approach is not feasible for patterning optical waveguides from photocurable fluids that lack the inherent rheological properties needed for filament formation. Although high-molecular-weight polymers could be added as viscosifying agents, their presence may give rise to inhomogeneities within the printed waveguide that exac- erbate optical loss. [15] To overcome this difficulty, fugitive mate- rials have been used as templates to create waveguides from photocurable liquids, but this approach has been limited to simple geometries. [16] To create high-quality waveguides from photocurable liquids, new advances are needed to enable direct filamentary patterning of these soft functional materials. Here, we report the fabrication of optical waveguides in arbi- trary planar and nonplanar configurations via photocurable liquid core–fugitive shell printing ( Figure 1). Specifically, a hybrid organic–inorganic core fluid, OrmoClear (Micro Resist Technology, GmbH), is encapsulated within a viscoelastic shell composed of an aqueous triblock copolymer, Pluronic F127 (BASF), solution. The fugitive shell serves as a sacrificial sup- port for the core fluid before it is cured with ultraviolet (UV) light. To coextrude these materials in the desired core–shell configuration, a custom printhead was designed and built con- sisting of two cylindrical nozzles aligned coaxially (see Figure S1 in the Supporting Information). The core fluid and viscoelastic fugitive ink shell are loaded into separate reservoirs and printed simultaneously by applying air pressure to each reservoir. This core–shell geometry produces optical waveguides whose dimen- sions are dictated by the diameter of the inner nozzle as well as their respective applied deposition pressures. OrmoClear is of interest for printed waveguides due to its low optical loss in the visible and NIR wavelengths and near-zero shrinkage upon curing, which inhibits crack formation. [17,18] However, OrmoClear is a Newtonian fluid with a viscosity of 5.0 Pa·s and a low shear elastic modulus ( G′) of 0.35 Pa that is less than its viscous modulus ( G″) of 2.5 Pa ( Figure 2). If this material is deposited alone, it would undergo significant wetting and spreading during waveguide printing. To encapsu- late this fluid, we utilize a fugitive hydrogel shell composed of 35 wt% triblock copolymer composed of poly(ethylene oxide), PEO, and poly(propylene oxide), PPO, blocks (Pluronic F127) in deionized water. Note, this material is highly transparent in the visible and ultraviolet wavelengths, allowing for UV-curing of the waveguide core. This fugitive ink was originally devel- oped for printing self-healing materials and hydrogels with embedded 3D biomimetic microvascular networks. [19,20] Aqueous Pluronic F127 triblock copolymer solutions undergo a phase transition that is both temperature and con- centration dependent. [21–23] Pluronic F127 solutions possess a critical micelle temperature (CMT) of ∼10 °C, [20] in which the PEO–PPO–PEO species form micelles consisting of a PPO core surrounded by a PEO corona. Dehydration of the PPO block leads to pronounced hydrophobic interactions that drive micelle formation. Upon cooling the material below the CMT, the hydrophobic PPO units are hydrated enabling individual PEO–PPO–PEO species to become soluble in water. Under ambient conditions, these solutions exhibit a critical micelle concentration (CMC) of ∼21 w/w%. By exploiting their known phase behavior, we have created a stiff fugitive shell composed David J. Lorang, Douglas Tanaka, Christopher M. Spadaccini, Klint A. Rose, Nerine J. Cherepy, and Jennifer A. Lewis* Photocurable Liquid Core–Fugitive Shell Printing of Optical Waveguides