Hydrodynamic microfabrication via ‘‘on the fly’’ photopolymerization of microscale fibers and tubes Wonje Jeong, a Jeongyun Kim, a Sunjeong Kim, b Sanghoon Lee,* a Glennys Mensing c and David. J. Beebe* c a Department of Biomedical Engineering, Dankook University, San 29, Anseodong Cheonan Chungnam, South Korea. E-mail: dbiomed@dankook.ac.kr b Department of Biomedical Engineering, Hanyang University, 17, Hangdang Seongdonggu, Seoul, S. Korea. E-mail: sjk@hanyan.ac.kr c Department of Biomedical Engineering, University of Wisconsin, Madison, WI 53706, USA. E-mail: djbeebe@wisc.edu Received 23rd July 2004, Accepted 25th October 2004 First published as an Advance Article on the web 11th November 2004 A microfluidic apparatus capable of creating continuous microscale cylindrical polymeric structures has been developed. This system is able to produce microstructures (e.g. fibers, tubes) by employing 3D multiple stream laminar flow and ‘‘on the fly’’ in-situ photopolymerization. The details of the fabrication process and the characterization of the produced microfibers are described. The apparatus is constructed by merging pulled glass pipettes with PDMS molding technology and used to manufacture the fibers and tubes. By controlling the sample and sheath volume flow rates, the dimensions of the microstructures produced can be altered without re-tooling. The fiber properties including elasticity, stimuli responsiveness, and biosensing are characterized. Responsive woven fabric and biosensing fibers are demonstrated. The fabrication process is simple, cost effective and flexible in materials, geometries, and scales. Introduction Strings and tubes are perhaps the most common curved objects in both the natural and man-made world, enabling a plethora of functions across diverse fields. At the microscale, they have many potential uses in chemical, biological, and industrial applications. 1–3 Recent advances in man-made microscale systems have largely relied on planar two- and three- dimensional (3D) geometries inherent to integrated circuit derived processes. Recent soft material-based micro systems have provided alternatives in functionality via elastomeric 4 and stimuli responsive materials, 5 but their origin is still rooted in pseudo 3D constructs. Three-dimensional geometries can be achieved using gray scale lithography, 6 surface tension effects, 7 or multiphoton illumination. 8 For the creation of microscale strings and tubes, extrusion/casting, 9 layering, 10 or fugitive 3 processes have been used. While these approaches have merit, all have limitations associated with solid–solid interaction and extraction effects for the production of micron-scale tubes or fibers in continuous lengths. Previously, we have demonstrated the use of surface tension to create curved micro structures. 11 While the use of surface tension allows for parallel fabrication (e.g. arrays) it precludes continuous sequential production. In nature, spiders elegantly produce micron-scale fibers by generating a liquid that solidifies when exposed to air. Here we present an analogous process replacing spontaneous solidifica- tion via exposure to air with light initiated solidification. In this paper, a microfluidic apparatus has been developed that creates a continuous process for the production of microscale cylindrical polymeric structures (e.g., fibers, tubes) by employing 3D multiple stream laminar flow 12,13 and ‘‘on the fly’’ in-situ photopolymerization. 14 With this apparatus, microstructures having diverse shapes such as fibers or tubes are generated by changing the channel configuration, and their dimensions can be altered by controlling the relative flow rates without re-tooling. The characterization of the properties, performance, and potential application of the microfibers are also addressed. First, the stress to strain relation of the microfiber is measured. Second, the use of stimuli responsive fibers is described. The polymerized fibers respond to pH variation with a volume transition from the collapsed to the expanded state. Stimuli responsive materials have many potential applications as efficient and robust actuating elements via direct chemical to mechanical energy transduction. The dynamic volume change of the fiber to a step pH variation is measured by using a microfluidic chamber. Third, the ability to create biosensing fibers is demonstrated. One important attribute of the method is the ability to immobilize biocatalysts into the fiber to create biosensors. We immobilized glucose reactive enzymes into the microfiber, and evaluated the sensitivity to glucose, demonstrating the mass production of a fiber that can be used as a biosensor component whose shape and size can be easily altered. Finally, other applications made possible via the local control of the fiber’s physical and chemical properties are described for combinatorial sensing. Principles and theory The microfiber fabrication apparatus is fabricated by incor- porating pulled glass micropipettes into a preformed hole in a PDMS substrate containing a microchannel network and its schematic is illustrated in Fig. 1a. Into the center hole, the pulled micropipette (for inlet channel) is incorporated for the delivery of sample fluid, while the normal (non-pulled) micropipette is positioned at the opposite side of the hole as an outlet channel. The photopolymerizable sample fluid (4-hydroxybutyl acry- late (4-HBA) mixed with dye and non-polymerizable sheath fluid (50 vol% polyvinyl alcohol (PVA) 1 50 vol% DI water (DI ~ deionized)) are introduced into the two input ports of the system and combined at the ‘X’ position of the apparatus. Facilitated by phenomena that dominate at the microscale (e.g. laminar flow, diffusion), a 3-D coaxial sheath flow stream around the sample flow is formed at the merging position of both flows. Next, the outlet tube is exposed to ultraviolet (UV) DOI: 10.1039/b411249k 576 Lab Chip , 2004, 4 , 576–580 This journal is ß The Royal Society of Chemistry 2004 MINIATURISATION FOR CHEMISTRY, BIOLOGY & BIOENGINEERING