IOP PUBLISHING SMART MATERIALS AND STRUCTURES Smart Mater. Struct. 16 (2007) 1614–1620 doi:10.1088/0964-1726/16/5/015 Swelling characteristics of acrylic acid polyelectrolyte hydrogel in a dc electric field Esmaiel Jabbari 1 , Javad Tavakoli and Alireza S Sarvestani Biomimetic Materials and Tissue Engineering Laboratories, Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208, USA E-mail: jabbari@engr.sc.edu Received 10 September 2006, in final form 28 June 2007 Published 3 August 2007 Online at stacks.iop.org/SMS/16/1614 Abstract A novel application of environmentally sensitive polyelectrolytes is in the fabrication of BioMEMS devices as sensors and actuators. Poly(acrylic acid) (PAA) gels are anionic polyelectrolyte networks that exhibit volume expansion in aqueous physiological environments. When an electric field is applied to PAA polyelectrolyte gels, the fixed anionic polyelectrolyte charges and the requirement of electro-neutrality in the network generate an osmotic pressure, above that in the absence of the electric field, to expand the network. The objective of this research was to investigate the effect of an externally applied dc electric field on the volume expansion of the PAA polyelectrolyte gel in a simulated physiological solution of phosphate buffer saline (PBS). For swelling studies in the electric field, two platinum-coated plates, as electrodes, were wrapped in a polyethylene sheet to protect the plates from corrosion and placed vertically in a vessel filled with PBS. The plates were placed on a rail such that the distance between the two plates could be adjusted. The PAA gel was synthesized by free radical crosslinking of acrylic acid monomer with ethylene glycol dimethacrylate (EGDMA) crosslinker. Our results demonstrate that volume expansion depends on the intensity of the electric field, the PAA network density, network homogeneity, and the position of the gel in the field relative to positive/negative electrodes. Our model predictions for PAA volume expansion, based on the dilute electrolyte concentration in the gel network, is in excellent agreement with the experimental findings in the high-electric-field regime (250–300 Newton/Coulomb). 1. Introduction Polyelectrolyte gels are three-dimensional crosslinked poly- meric structures which are able to swell in aqueous physio- logical solution and retain a significant fraction of water in their structure without dissolving [1–3]. The diffusivity of macromolecules and proteins in hydrogels is four to five orders of magnitude higher compared to hydrophobic glassy poly- mers. Polyelectrolyte gels, due to their high water content, exhibit excellent biocompatibility and they can be crosslinked 1 Address for correspondence: Department of Chemical Engineering, Swearingen Engineering Center, Rm 2C11, University of South Carolina, Columbia, SC 29208, USA. with biologically active peptides to fabricate smart environ- mentally responsive hydrogels [4–6]. Hydrogels exhibit large volume changes to many environmental factors including sol- vent composition, pH, salt concentration, temperature, light in- tensity, glucose concentration, antigens, and the application of an external electric field [7, 8]. Polyelectrolyte gels are used extensively in medicine and pharmacy as drug delivery sys- tems, contact lenses, catheters, wound dressings, and biosen- sors [9, 10]. One of the most powerful applications of environmentally sensitive polyelectrolytes is in the fabrication of bio-micro- electro-mechanical (BioMEMS) devices as sensors and actua- tors [11]. BioMEMS-based applications of polyelectrolyte gels 0964-1726/07/051614+07$30.00 © 2007 IOP Publishing Ltd Printed in the UK 1614