Sensors and Actuators B 206 (2015) 456–462 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical jo u r nal homep age: www.elsevier.com/locate/snb Modulating the movement of hydrogel actuator based on catechol–iron ion coordination chemistry Bruce P. Lee a,∗ , Meng-Hsien Lin a , Ameya Narkar a , Shari Konst b , Randall Wilharm b a Department of Biomedical Engineering, Michigan Technological University, Houghton, MI 49931, USA b Department of Chemistry, Michigan Technological University, Houghton, MI 49931, USA a r t i c l e i n f o Article history: Received 10 June 2014 Received in revised form 21 September 2014 Accepted 23 September 2014 Available online 2 October 2014 Keywords: Mussel adhesive protein Dopamine Hydrogel actuator Iron ion pH responsive a b s t r a c t Hydrogel actuators were prepared by combining ionoprinting technique with reversible metal ion coor- dination chemistry found in mussel adhesive proteins. Hydrogels were formulated with dopamine methacrylamide (DMA), which contains a catechol side chain that forms strong complexes with ferric (Fe 3+ ) ions. Catechol–Fe 3+ ion complexation increased local crosslinking density, which induced hydrogel bending at the site of ionoprinting. The effect of multiple factors on the dynamic response of hydrogel actuation was tracked by following the bending curvature at the ionoprinting site. In general, the extent and rate of hydrogel actuation increased with increasing pH, deposited Fe 3+ ion content, and DMA content but was inversely proportional to hydrogel thickness. The ability to modulate hydrogel actuation using multiple parameters is potentially useful in controlling the actuator movements. Additionally, Fe 3+ ion- containing bulk hydrogels demonstrated significant reduction in molecular weight between crosslinks as well as elevated storage and loss modulus values based on oscillatory rheometry when compared to those of Fe 3+ -free control. These differences in physical and viscoelastic properties contributed to the actuation of ionoprinted samples. Specifically, conditions that promoted a large crosslinking differential between the ionoprinted region and the bulk hydrogel (i.e., outside of the ionoprinted region) contributed to increased rate and extent of hydrogel folding. Faster actuation at elevated pH levels was attributed to the formation of complexes with higher catechol:Fe 3+ ion stoichiometric ratios. Hydrogel actuation and deswelling were also observed at pH of 3.5 although to a lesser degree, potentially due to a stronger affinity between network-bound catechol and Fe 3+ ions as compared to complexes formed in a dilute solution. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Hydrogels are three-dimensional polymer networks with water contents as high as over 99% [1,2]. The physical, chemical, and bio- logical properties of these materials can be easily controlled by fabrication methods and chemical compositions. Hydrogels also exhibit excellent biocompatibility and structural similarity to nat- ural extracellular matrices. As such, hydrogels have emerged as Abbreviations: EDTA, ethylenediaminetetraacetic acid; EWC, equilibrium water content; Fe 3+ ion, ferric ion; DMA, dopamine methacrylamide; DMPA, 2,2-dimethoxy-2-phenylacetophenone; DOPA, 3,4-dihydroxyphenylalanine; HEA, N-hydroxyethyl acrylamide; MAP, mussel adhesive protein; MBAA, N,N ′ - methylene-bisacrylamide; Mc , average molecular weight between crosslinks; pHEA, poly(N-hydroxyethyl acrylamide); R ′ , rate of change in the bending curvature; R0, initial bending curvature; Rmax, maximum bending curvature. ∗ Corresponding author. Tel.: +1 906 487 3262. E-mail address: bplee@mtu.edu (B.P. Lee). promising biomaterials for applications ranging from scaffolds for tissue engineering and repair [3,4], drug delivery [5,6], and artifi- cial connective tissues [7], to tissue adhesives [8–10]. Hydrogels that can change their shape and physical properties in response to various environmental stimuli (e.g., temperature, pH, humidity) are being explored as actuators for applications such as soft robotic components, biosensors, artificial muscle tissues, and controlled drug delivery [11–14]. Recently, our lab exploited the reversible metal coordination chemistry found in mussel adhesive proteins (MAPs) to create a novel pH-responsive hydrogel actuator [15]. Hydrogels were prepared with network-bound catechol through photo-initiated polymerization of dopamine methacrylamide (DMA, Fig. 1). DMA mimics the adhesive catechol moiety, 3,4-dihydroxyphenylalanine (DOPA), which accounts for as much as 25 mol% in MAPs [16]. DOPA and other catechol derivatives (e.g., dopamine, 3,4- dihydroxyhydrocinnamic acid) have demonstrated remarkable water-resistant adhesive properties to a wide range of surfaces http://dx.doi.org/10.1016/j.snb.2014.09.089 0925-4005/© 2014 Elsevier B.V. All rights reserved.