polymers Article Mussel-Inspired Anisotropic Nanocellulose and Silver Nanoparticle Composite with Improved Mechanical Properties, Electrical Conductivity and Antibacterial Activity Hoang-Linh Nguyen 1,†, Yun Kee Jo 2,†, Minkyu Cha 3, Yun Jeong Cha 4, Dong Ki Yoon 4, Naresh D. Sanandiya 1, Ekavianty Prajatelistia 5, Dongyeop X. Oh 6,* and Dong Soo Hwang 1,5,7,* 1 Division of Integrative Bioscience and Biotechnology, Pohang University of Science and Technology, Pohang 790-784, Korea; [email protected] (H.-L.N.); [email protected] (N.D.S.) 2 Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea; [email protected] 3 Pohang Jecheol High School, Pohang 790-784, Korea; [email protected] 4 Graduate School of Nanoscience and Technology and KAIST Institute for the NanoCentury (KINC), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea; [email protected] (Y.J.C.); [email protected] (D.K.Y.) 5 School of Interdisciplinary Bioscience and Bioengineering, Pohang University of Science and Technology, Pohang 790-784, Korea; [email protected] 6 Research Center for Industrial Chemical Biotechnology, Korea Research Institute of Chemical Technology (KRICT), Ulsan 44429, Korea 7 School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea * Correspondence: [email protected] (D.X.O.); [email protected] (D.S.H.); Tel.: +82-52-241-6316 (D.X.O.); +82-54-279-9505 (D.S.H.) † These authors contributed equally to this work. Academic Editor: Antonio Pizzi Received: 2 February 2016; Accepted: 16 March 2016; Published: 22 March 2016 Abstract: Materials for wearable devices, tissue engineering and bio-sensing applications require both antibacterial activity to prevent bacterial infection and biofilm formation, and electrical conductivity to electric signals inside and outside of the human body. Recently, cellulose nanofibers have been utilized for various applications but cellulose itself has neither antibacterial activity nor conductivity. Here, an antibacterial and electrically conductive composite was formed by generating catechol mediated silver nanoparticles (AgNPs) on the surface of cellulose nanofibers. The chemically immobilized catechol moiety on the nanofibrous cellulose network reduced Ag+ to form AgNPs on the cellulose nanofiber. The AgNPs cellulose composite showed excellent antibacterial efficacy against both Gram-positive and Gram-negative bacteria. In addition, the catechol conjugation and the addition of AgNP induced anisotropic self-alignment of the cellulose nanofibers which enhances electrical and mechanical properties of the composite. Therefore, the composite containing AgNPs and anisotropic aligned the cellulose nanofiber may be useful for biomedical applications. Keywords: cellulose nanofibers; silver nanoparticles; anisotropic alignment; mechanical properties; electrical conductivities; antibacterial activities 1. Introduction There are increasing interests in biomedical electronics such as electronic skins, wearable healthcare sensors, and wearable human-device interfaces. The base materials for the next-generation Polymers 2016, 8, 102; doi:10.3390/polym8030102 www.mdpi.com/journal/polymers Polymers 2016, 8, 102 2 of 13 Polymers 2016, 8, 102 2 of 12 wearable electronic systems should possess biocompatibility, antibacterial activity, surface electrical conductivity,wearable electronic and appropriate systems should flexibility possess and biocompa toughness.tibility, antibacterial activity, surface electrical conductivity,Recently, with and appropriate the aforementioned flexibility concerns and toughness. of the base materials for human healthcare device, celluloseRecently, has emerged with the as aforementioned a base material concerns because of th ite is base cheap, materials abundant, for human and healthcare has no cytotoxicity. device, Incellulose particular, has a emerged single thread as a base of nanofibrousmaterial because cellulose it is cheap, not only abundant, exhibits and great has mechanical no cytotoxicity. property In ofparticular, ~120 (GPa) a single [1,2], thread it is also of nanofibrous light in weight cellulose [3] comparednot only exhibits to Kevlar great or mechanical steel [4]. property However, of at ~120 (GPa) [1,2], it is also light in weight [3] compared to Kevlar or steel [4]. However, at a nanoscale a nanoscale point of view, cellulose disintegration is an energy-consuming process, due to intra- point of view, cellulose disintegration is an energy-consuming process, due to intra- and interchain and interchain hydrogen bonds between the cellulose chains [5,6]. Carboxylation of cellulose hydrogen bonds between the cellulose chains [5,6]. Carboxylation of cellulose fibrils by (2,2,6,6- fibrils by (2,2,6,6-tetramethyl-piperidin-1-yl)oxyl (TEMPO), a catalyst for selective oxidization, is tetramethyl-piperidin-1-yl)oxyl (TEMPO), a catalyst for selective oxidization, is a facile chemical a facile chemical pretreatment method for physical disintegration of cellulose fibrils [7]. Individual pretreatment method for physical disintegration of cellulose fibrils [7]. Individual carboxylated carboxylatedcellulose nanofibers cellulose (CCNF) nanofibers generated (CCNF) by generated TEMPO-m byediated TEMPO-mediated carboxylation carboxylationhave a high aspect have ratio a high aspectwith ratioan average with an width average of about width 3–5 of nm about and 3–5length nm of and a few length µm [6]. of aCCNF few µ couldm [6]. generate CCNF could transparent generate transparentand flexible and films flexible with filmsabundant with carboxyl abundant and carboxyl polar hydroxyl and polar functional hydroxyl groups, functional rigid groups, and self- rigid andstanding self-standing hydrogels, hydrogels, and aerogels and aerogelswith high with surface high area surface [6]. However, area [6]. poor However, mechanical poor properties mechanical propertiesdue to weak due tointer-fibril weak inter-fibril interaction, interaction, low bacterial low bacterialresistance, resistance, and low andelectrical low electrical conductivity conductivity have haveimpeded impeded the theapplication application of CCNF of CCNF to biomedical to biomedical or orwearable wearable devices. devices. Thus, Thus, many many studies studies have have focusedfocused on on chemical chemical modification modification toto enhanceenhance inter-fibrillar inter-fibrillar interaction interaction [8], [8], biological biological activities activities [9] [ and9] and electricalelectrical conductivity conductivity [ 10[10]] of of CCNF. CCNF. DopamineDopamine (DA), (DA), a a mussel-inspired mussel-inspired building block block that that contains contains the the key key adhesive adhesive chemistry chemistry of of L-3,4-dihydroxyphenylalanineL-3,4-dihydroxyphenylalanine (L -DOPA)(L-DOPA) and and lysine, lysine, is known is known to form to aform material-independent a material-independent adhesive coatingadhesive via coating oxidative via self-polymerizationoxidative self-polymerization under wet under conditions wet conditions [11]. The [11]. catechol The catechol moiety moiety of DA of can alsoDA induce can also mineralization induce mineralization of metal nanoparticlesof metal nanoparticles as a green as reducinga green reducing agent [12 agent–15]. [12–15]. DA can DA be can easily conjugatedbe easily ontoconjugated a CCNF surfaceonto a throughCCNF amidaitionsurface through reaction amidaition using 1-ethyl-3-(3-dimethylaminopropyl) reaction using 1-ethyl-3-(3- carbodiimidedimethylaminopropyl) (EDC) and Ncarbodiimide-hydroxysuccinimide (EDC) and (NHS) N-hydroxysuccinimide coupling catalysts (NHS over the) coupling carboxyl catalysts groups of over the carboxyl groups of CCNF. The catechol moiety of DA can lead to the reinforcement of CCNF. The catechol moiety of DA can lead to the reinforcement of binding between CCNFs by reducing binding between CCNFs by reducing and overcoming electrostatic repulsive forces. In addition, the and overcoming electrostatic repulsive forces. In addition, the catechol moiety can reduce Ag+ and catechol moiety can reduce Ag+ and form silver nanoparticles (AgNPs) on the cellulose nanofiber form silver nanoparticles (AgNPs) on the cellulose nanofiber without additional chemical treatments without additional chemical treatments or heating. AgNPs have been incorporated into various or heating. AgNPs have been incorporated into various cellulose-based materials such as bacterial cellulose-based materials such as bacterial cellulose, filter paper, cotton fabric, and cellulose gels to cellulose,create remarkable filter paper, electrical cotton fabric, conductivity and cellulose and anti gelsbacterial to create activity remarkable [16–18]. electrical However, conductivity additional and antibacterialchemical treatments activity [ 16and/or–18]. However,heating are additional generally chemicalrequired treatmentsfor the Ag+ and/or reduction heating in the are cellulose generally + requiredmaterials. for theIn addition, Ag reduction 3-dimensional in the cellulose architec materials.ture of cellulose In addition, nano 3-dimensionalstructure due architecture to AgNPs of celluloseincorporation nanostructure has been due poorly to AgNPs studied. incorporation has been poorly studied. InIn this this study, study, we we developed developed AgNP-containing AgNP-containing cellulose cellulose nanofiber nanofiber composites composites with with improved improved mechanicalmechanical property,