Templating polyacrylamide hydrogel for interconnected microstructure and improved performance Maria Bassil , 1 Georges El Haj Moussa, 1 Mario El Tahchi 1,2 1 LBMI, Department of Physics, Lebanese University - Faculty of Sciences 2, PO Box 90656, Jdeidet, Lebanon 2 Department of Bioengineering, University of California, Los Angeles, 570 Westwood plaza, 90095 CA Correspondence to: M. Bassil (E - mail: maria.bassil@ul.edu.lb) ABSTRACT: In this study, the pressure and temperature are monitored during acrylamide polymerization and their effects on the mechanical properties and swelling of the resulting hydrogel are investigated. The polymerization kinetic and network formation mechanism are correlated to the environmental thermodynamic conditions under which the hydrogels are polymerized. Then, the swelling and Young’s modulus are measured and shown to be tunable along a wide range of values. The swelling ratio varies between 50 and 2262 while Young’s modulus varies between 10.99 and 40.70 kPa. In addition, the formation of macroporous hydrogel with channel like structures along the vacuum direction under a reduced pressure of 5 mbar is reported. The macroporous hydrogel has a modulus of 40.70 kPa and shrink approximatively three times faster than the hydrogel polymerized under normal pressure and has a modulus of 10.99 kPa. Hence, this interconnected network can overcome the fluid diffusion limitations of bulk hydrogels without compromising the mechanical properties. V C 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 136, 46205. KEYWORDS: mechanical properties; polyelectrolytes; porous materials; stimuli-sensitive polymers; swelling Received 15 September 2017; accepted 23 December 2017 DOI: 10.1002/app.46205 INTRODUCTION Hydrolyzed polyacrylamide (PAAM) hydrogels are a class of bio- materials that have demonstrated great potential for tissue engi- neering and regenerative medicine applications. They are defined as crosslinked macromolecular networks formed by hydrophilic polymers that absorb large amounts of water and biological fluid while maintaining their three-dimensional stability. 1–3 These soft, smart, and stimuli-responsive materials are chemosensitive, 4–7 electroactive under low driving voltage, 8–10 and biocompatible. 11 They are extensively used in the development of artificial implants such as artificial muscles or soft actuators 3,9,12,13 and scaffolds that mimic the native extracellular matrices. 14–17 The two features of hydrogels that all applications capitalize upon are the high porosity to enable rapid solute transfer within the material and controllable mechanical properties to fit a wide range of applications. Various methods like solvent casting/particle leaching, gas foam- ing, fiber bonding, 3D printing, and freeze-drying are used to generate high porosity. 18–21 However, the stiffness of the hydrogel tends to decrease as the porosity of the hydrogel increases 22–24 which is an obstacle for their broad applications. Therefore, developing hydrogels of superior mechanical properties and high porosity is a common challenge of major interest. Extensive work has been reported on the sensing properties and actuation mechanism of PAAM hydrogels as on their applica- tion as scaffolds for tissue engineering. 1–17 However, few of them are concerned about the effect of the thermodynamic parameters like temperature and pressure, applied during the free radical polymerization, on the properties of the resulting hydrogel. 25–27 Therefore, understanding the network formation mechanism under various experimental conditions is crucial to predict and then tailor the hydrogel physical properties. The results show that the polymerization conditions can be tuned to achieve desired properties of the material which is important in engineering new hydrogel based system. A wide range of swell- ing capability ranking between 50 and 2262 and a controlled stiff- ness ranking between 10.99 and 40.70 kPa are obtained. In addition, we succeeded to template the hydrogels macromo- lecular network under reduced pressure of 5 mbar. The original- ity of this work lies in the use of a simple method characterized by the ease of processability and the cost competitiveness over others techniques. 18,19 The resulting PAAM hydrogel, that has a V C 2018 Wiley Periodicals, Inc. J. APPL. POLYM. SCI. 2018, DOI: 10.1002/APP.46205 46205 (1 of 6)