3D Printed Edible Hydrogel Electrodes
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3D Printed Edible Hydrogel Electrodes Alex Keller1, Leo Stevens2, Gordon G. Wallace2 and Marc in het Panhuis1,2 1 Soft Materials Group, School of Chemistry, University of Wollongong, Wollongong, NSW 2522, Australia. 2 Intelligent Polymer Research Institute, ARC Centre of Excellence for Electromaterials Science, AIIM Facility, University of Wollongong, Wollongong, NSW 2522, Australia. ABSTRACT We report on a hand-held reactive printing device used to pattern highly conductive, edible hydrogel wires formed from gellan gum, gelatin, cross-linkers and a common salt (NaCl). The conductivity of the gels when printed (190 ± 20 mS/cm) closely matched the conductivity recorded for cast systems (200 ± 19 mS/cm). Printing was observed to reduce the elastic modulus and failure strains of hydrogels under compression, but printed gels retained sufficient integrity for application as flexible conductive lines. We demonstrate that hand-held printing can utilize to pattern soft conductor elements within a simple electronic circuit. INTRODUCTION Silicon based electronics are largely incompatible with living tissue, having no biodegradation pathways and a mismatch in mechanical compliance. Despite this, silicon based electronics are already in use in a wide variety of medical devices such as pacemakers[1] and cochlear ear implants[2]. These systems rely on encapsulating electronic circuits within soft and impermeable plastics, which provide a physical barrier between the electronics and the body. Because of their construction, the systems never fully integrate with their host tissue, and remain at risk of fibrosis and infection throughout the life of the implant, or until they are surgically removed[3]. Replacing traditional electronic materials with soft and bioresorbable alternatives provides the potential of a new generation of implantable and edible electronics that are ultimately biodegraded with no long-term health consequences. For example, bioelectronics offer a possible solution to pill retention as simple gastrointestinal (GI) monitoring devices can be constructed from bioresobable materials [4- 8]. Hydrogels are hydrophilic polymer networks, able to absorb 99% (w/w) water[9]. Biopolymer-based hydrogels are a promising material for bioelectronics development as they are often biocompatible[10], 3D printable[11] and possess unique chemistries that can be tailored for specific medical applications[12]. Gelatin and gellan gum are two hydrogel-forming biopolymers commonly used as food additives to products such as yogurt, ice cream and jelly[9, 13]. These gels are generally soft and mechanically weak, but may be rendered tough like rubber using the ionic-covalent entanglement (ICE) approach[14, 15]. ICE gels are constructed through the enmeshing of independently cross-linked ionic and covalent networks, resulting in synergistic strengthening[16]. Herein, we describe a simple method of producing simple bioelectronics through the hand-held extrusion of ICE hydrogels. Our gels are formed from a combination of edible and bioresorbab
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