Conformable Electronics With Conductive Silver Structures by Electrohydrodynamic Printing

Recent advances in research and fabrication of flexible, stretchable, or rather conformable electronics with printed conductive structures have enabled a wide range of applications. Various fields such as consumer electronics or wearable devices for health monitoring are affected by these achievements. Owing to gradually increasing demands on enhanced functionalities and an excellent deformability of such electronics, an investigation of appropriate hyperelastic materials and progressive manufacturing techniques are mandatory. In this article, a cost-efficient approach for fabrication of conformable electronics based on vacuum thermoforming with printed microscaled silver structures is presented. The patterns in form of conductive line arrays and meanders are realized by the emerging electrohydrodynamic printing (EHD) technique which constitutes a promising alternative to established additive technologies due to the applicability of various printing media as well as its high material compatibility. Moreover, hyperelastic material models comprising the Mooney-Rivlin, Ogden, neo-Hookean as well as the Yeoh model for description of stretchable thermoplastic polyurethane (TPU) during deformation are contrasted and general capabilities for design optimization of conductive structures are derived by means of numerical simulations. Based on the EHD-printed metallic silver patterns on TPU with a subsequent transfer of the flat 100- $\mu $ m thick matrix toward a 3D-shaped electronic device by thermoforming, first demonstrators with a degree of deformation up to 57% are realized.

I. Introduction

Printed conductive structures [1] are crucial components for the fabrication of flexible, stretchable, or rather conformable electronic devices and have gained enormous attention over the last years. In particular, the patterning of electrodes for healthcare applications by screen printing, inkjet printing, or other established additive techniques is subject of a multitude of research activities. Dong et al. [2] have realized stretchable metal electrode arrays for in vivo neural recording applications on flexible polydimethylsiloxane (PDMS). This elastic substrate is also utilized for patterning of electrocardiogram electrodes based on conductive silver nanowires [3]. In the latest work of Rauf et al. [4], silver nanowire ink is also applied for fabrication of electrodes for cardiac diagnosis. Another medical application by patterning dry electrodes on a compressive textile for a neuromuscular electrical stimulation is introduced by Merhi et al. [5]. Such progressive applications for additive fabrication of conductive structures have paved the way to real time monitoring of human physiological parameters by means of entire wearable devices like self-powered piezoelectric biomedical sensors [6] and highly stretchable pressure sensors addressing tactile sensing or serving as artificial skin [7]. Beside this beneficial accomplishments, the fabrication of flexible and conformable electronics by conventional printing techniques is still confined to a small range of printing media and substrate materials. Another limiting factor is the diverging material behavior of hyperelastic substrates and conductive materials which leads to delamination between both materials and cracking of the metallic structures during deformation [8]. To overcome these restrictions, investigations concerning the material behavior of elastic substrates are indispensable. In this work, thermoplastic polyurethane (TPU) is chosen as substrate for fabrication of a flexible 3-D demonstrator with electrohydrodynamically printed microscaled silver structures due to its reasonable physio-chemical behavior, the adequate processability and an excellent biocompatibility.