INTERCONNECTED SELF-PROPAGATING PHOTOPOLYMER WAVEGUIDES : AN ALTERNATIVE TO STEREOLITHOGRAPHY FOR RAPID FORMATION OF LATTICE-BASED OPEN-CELLULAR MATERIALS

Recently, a new technique has been developed to create unique open-cellular materials with micro-scale truss, or lattice features ranging from tens to hundreds of microns. These materials are formed from a three-dimensional, interconnected array of self-propagating photopolymer waveguides. By utilizing this self-propagating effect, three-dimensional open-cellular polymer materials can be formed in seconds. In addition, intrinsic to the process is the ability to control specific micro-lattice parameters which ultimately affect the bulk material properties. Unlike stereolithography, this new fabrication technique is rapid (~ minutes to form an entire part) and relies on a single two-dimensional exposure surface to form three-dimensional structures (thickness > 25 mm possible). This combination of speed and planar scalability opens the possibility for large-scale mass manufacturing. The utility of these new materials range from lightweight energy absorbing structures to thermal management materials to bio-scaffolds. Introduction Cellular materials are materials with significant porosity, and thus have considerably lower bulk density than their solid counterparts [1]. Because of their porosity, this class of materials is commonly utilized in lightweight structural applications, or for applications that can directly benefit from the material porosity, such as energy absorption or thermal management [2-4]. Polymer foams are one of the most common types of cellular materials and can be manufactured with a wide range of average unit cell sizes and structures. However, typical foaming processes result in a stochastic structure that limits the mechanical performance and multifunctional application space [1]. Ordered open-cellular structures have the potential for improved mechanical performance (higher stiffness and strength per unit mass), as well as accessible open volume for unique multifunctional capabilities [5]. Fabrication techniques for ordered open-cellular polymer materials have relied primarily on rapid prototyping processes, such as stereolithography. Although these techniques enable significant flexibility in the design of the cellular architecture, they also restrict the scalability of the material, and ultimately limit their utility for large-scale or high volume applications. A new, scalable technique has been developed for fabricating ordered open-cellular polymer materials with micro-lattice features [6, 7]. These materials are formed by exposing a twodimensional mask with a pattern of circular apertures that is covering a reservoir containing an appropriate photomonomer, as shown in Figure 1. Within the photomonomer, self-propagating photopolymer waveguides originate at each aperture in the direction of the UV collimated beam and polymerize together at points of intersection. By simultaneously forming an interconnected array of these fibers in three-dimensions and removing the uncured monomer, unique threedimensional lattice-based open-cellular polymer materials can be rapidly fabricated. The following sections include a brief overview of the fabrication capabilities of this new process, as well as highlights from ongoing research. In addition, we have included a detailed comparison of this fabrication technology with other approaches that can be used to fabricate ordered opencellular materials, such as stereolithography. Figure 1. Schematic representation of the process used to form micro-lattice structure from a self-propagating polymer waveguides and an example structure formed by this process. Unit Cell Architectures and Feature Size Range Inherent to the photopolymer waveguide process is the ability to control the architectural features of the bulk cellular material by controlling the fiber angle, diameter, and threedimensional spatial location during fabrication. The general unit cell architecture is controlled by the pattern of circular apertures on the mask and the orientation and angle on the collimated incident UV light beams. The unit cell architecture shown in Figure 2a is formed by exposing a mask with a square pattern of equally spaced, identical circular apertures to four collimated beams. The collimated beams are rotated 90 apart with respect to the mask normal and have equal incident angle () on the mask surface. The dimension L represents the node-to-node spacing, while H is the unit cell height [6]. If a hexagonal mask pattern is employed, unit-cell architectures with three-fold and six-fold symmetry are possible. Figure 2b is a unit cell formed from three equally angled exposure beams, where each beam is aligned along the major axis of the hex pattern but rotated 120 apart with respect to the mask normal. Figure 2c is the unit cell that is formed if six angled exposure beams are used. Each of these beams is also aligned with a major axis of the hex pattern but rotated 60 with respect to the mask normal [8]. Figure 2. Example unit cell architecture with (a) 4-fold symmetry, (b) 3-fold symmetry, and (c) six-fold symmetry. H