polymers Article Conductive GelMA–Collagen–AgNW Blended Hydrogel for Smart Actuator Jang Ho Ha 1 , Jae Hyun Lim 2 , Ji Woon Kim 2 , Hyeon-Yeol Cho 3 , Seok Geun Jo 4 , Seung Hyun Lee 4 , Jae Young Eom 4 , Jong Min Lee 4, * and Bong Geun Chung 1, *   Citation: Ha, J.H.; Lim, J.H.; Kim, J.W.; Cho, H.-Y.; Jo, S.G.; Lee, S.H.; Eom, J.Y.; Lee, J.M.; Chung, B.G. Conductive GelMA–Collagen–AgNW Blended Hydrogel for Smart Actuator. Polymers 2021, 13, 1217. https:// doi.org/10.3390/polym13081217 Academic Editor: Christine Wandrey Received: 15 March 2021 Accepted: 8 April 2021 Published: 9 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Mechanical Engineering, Sogang University, Seoul 04107, Korea; hajangho1991@naver.com 2 Department of Biomedical Engineering, Sogang University, Seoul 04107, Korea; boy6635@naver.com (J.H.L.); rlawldns68@naver.com (J.W.K.) 3 Department of Bio & Fermentation Convergence Technology, Kookmin University, Seoul 02707, Korea; chohy@kookmin.ac.kr 4 Division of Chemical Industry, Yeungnam University College, Daegu 42415, Korea; 96tjrrms@naver.com (S.G.J.); jirovsvovan@naver.com (S.H.L.); djawodud40@ync.ac.kr (J.Y.E.) * Correspondence: jlee@ync.ac.kr (J.M.L.); bchung@sogang.ac.kr (B.G.C.) Abstract: Blended hydrogels play an important role in enhancing the properties (e.g., mechani- cal properties and conductivity) of hydrogels. In this study, we generated a conductive blended hydrogel, which was achieved by mixing gelatin methacrylate (GelMA) with collagen, and silver nanowire (AgNW). The ratio of GelMA, collagen and AgNW was optimized and was subsequently gelated by ultraviolet light (UV) and heat. The scanning electron microscope (SEM) image of the conductive blended hydrogels showed that collagen and AgNW were present in the GelMA hydro- gel. Additionally, rheological analysis indicated that the mechanical properties of the conductive GelMA–collagen–AgNW blended hydrogels improved. Biocompatibility analysis confirmed that the human umbilical vein endothelial cells (HUVECs) encapsulated within the three-dimensional (3D), conductive blended hydrogels were highly viable. Furthermore, we confirmed that the molecule in the conductive blended hydrogel was released by electrical stimuli-mediated structural deformation. Therefore, this conductive GelMA–collagen–AgNW blended hydrogel could be potentially used as a smart actuator for drug delivery applications. Keywords: conductive blended hydrogel; gelatin methacrylate; collagen; silver nanowire; molecule release 1. Introduction Hydrogels have widely been used in biological applications due to their hydrophilicity and excellent physical–chemical properties [1–3]. The structure and characteristics of hy- drogels are similar to the three-dimensional (3D) microenvironment of living tissues [4–6]. In particular, natural hydrogels (e.g., collagen, hyaluronic acid, and fibrin) made by extra- cellular matrix (ECM) components have good biocompatibility and biodegradability [7]. Moreover, they have only a few side effects, such as immune and inflammatory reactions during transplantation [8–10]. Despite their various advantages, the natural hydrogels still have several limitations, such as weak mechanical properties [11,12], difficulty of control and reproduction due to batch-to-batch variation [13], and expensive production costs [14]. To solve these limitations, the blended hydrogels are fabricated to form a stabilized structure that crosslinks via physicochemical bonds. Gelatin methacrylate (GelMA), which is made by bonding a methacrylate group and an amine-containing gelatin group, shows excellent mechanical properties [15,16] and biocompatibility [17]. In the previous study, GelMA and chitosan hydrogels were mixed to culture bone mesenchymal stem cells [18]. The use of natural hydrogel-based biomaterial (e.g., chitosan, gelatin) is limited as a scaffold due to its poor mechanical properties. The blended GelMA–chitosan hydrogel improved mechanical properties compared to the Polymers 2021, 13, 1217. https://doi.org/10.3390/polym13081217 https://www.mdpi.com/journal/polymers