On-chip development of hydrogel microfibers from round to square/ribbon shape† Zhenhua Bai, a Janet M. Mendoza Reyes, b Reza Montazami a and Nastaran Hashemi * a We use a microfluidic approach to fabricate gelatin fibers with controlled sizes and cross-sections. Uniform gelatin microfibers with various morphologies and cross-sections (round and square) are fabricated by increasing the gelatin concentration of the core solution from 8% to 12%. Moreover, the increase of gelatin concentration greatly improves the mechanical properties of gelatin fibers; the Young's modulus and tensile stress at break of gelatin (12%) fibers are raised about 2.2 and 1.9 times as those of gelatin (8%) fibers. The COMSOL simulations indicate that the sizes and cross-sections of the gelatin fibers can be tuned by using a microfluidic device with four- chevron grooves. The experimental results demonstrate that the decrease of the sheath-to-core flow-rate ratio from 150 : 1 to 30 : 1 can increase the aspect ratio and size of ribbon-shaped fibers from 35 mm  60 mm to 47 mm  282 mm, which is consistent with the simulation results. The increased size and shape evolution of the cross- section can not only strengthen the Young's modulus and tensile stress at break, but also significantly enhance the tensile strain at break. The development of biocompatible polymeric bers has received a lot of attention due to their outstanding physical and chemical properties. 1,2 Among the various materials, gelatin is an inexpensive, neutral, water-soluble, non-toxic, and FDA- approved biopolymer with excellent biocompatibility, biode- gradability, and cell adhesiveness, which is extensively used in medical products, such as wound dressings, drug delivery systems, and tissue engineering. 3–12 Until now, gelatin has been fabricated in various forms, e.g., lms, 13 nanoparticles, 14 and porous hydrogels. 15 There are several studies to produce func- tional gelatin bers by electrospinning, because of the high surface area, high porosity, and exibility for surface function- alization of gelatin based bers. 3–5 Various solvent systems have been used to prepare electrospinnable gelatin solutions, such as 2,2,2-triuoroethanol (TFE), formic acid, 1,1,1,3,3,3-hexauoro- 2-propanol (HFP), and acetic acid. 7,9 The diameters of the previously obtained electro-spun gelatin bers were in the range of 100–1900 nm, and there is no report on microbers with larger diameters using the electrospinning method. 10 Further- more, the cross-sectional shape of electro-spun bers is almost exclusively limited to round shape due to interfacial tension between the solvent/ber material solution and air. 12 Although there have been some reports on fabrication of gelatin bers with relatively larger size by gel-spinning, the obtained bers are less-uniform, and this method does not allow for tuning of the cross-section and size. 4,16 It is well known that bers with complex shapes have improved mechanical properties and larger surface area, and are promising materials for biological microreactors, tissue engineering, and controlled release. 17 Therefore, the development of a novel method to fabricate gelatin bers with controlled sizes and shapes is highly demanded. Recently, a microuidic device based fabrication method has been recognized as an efficient method for the fabrication of micron-sized bers due to its low-material consumption, conventional volume and size control, enhanced reaction rate, and inexpensive tooling costs. 18–22 Compared with other ber fabrication methods, the microuidic method has a unique advantage that can create bers with a range of cross-sectional shapes. 18,23 The shape of the ber is inuenced by the ow rates and the types and numbers of shaping elements in the channel walls, such as various grooves. 16 At present, great efforts have been devoted to expand the variety of materials and types of structures which can be successfully fabricated by microuidic devices. 17 For instance, Thangawng et al. produced round PMMA bers with diameters down to 300 nm by varying the ratio between the sheath and core ow rates using a 5-diagonal groove device, and ribbon-shaped bers with submicron thickness were also fabricated using a 7-chevron/5-diagonal groove combination device. 23 Moreover, Boyd et al. succeeded in fabricating “double anchor” shaped thiol-end bers using a two-stage hydrodynamic focusing system. 24 However, to the best a Department of Mechanical Engineering, Iowa State University, Ames, IA 50011, USA. E-mail: nastaran@iastate.edu b Department of Computer Engineering, University of Puerto Rico, 00681, Puerto Rico † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ta14573e Cite this: J. Mater. Chem. A, 2014, 2, 4878 Received 7th November 2013 Accepted 20th January 2014 DOI: 10.1039/c3ta14573e www.rsc.org/MaterialsA 4878 | J. Mater. Chem. A, 2014, 2, 4878–4884 This journal is © The Royal Society of Chemistry 2014 Journal of Materials Chemistry A COMMUNICATION Published on 20 January 2014. Downloaded by Iowa State University on 11/03/2014 17:41:43. View Article Online View Journal | View Issue