In this review paper, we focus on the 3D printing technologies that consist of the extruding of fluid material in lines to form structures for electro-and biomechanical applications. Our paper reviews various 3D print technologies, materials, sensing technologies and applications of extrusion-based 3D printing. We also discuss how to overcome some of the challenges with 3D printed sensors, such as the anisotropy of the conductors as well as the drift and nonlinearity of the materials.

Abstract. Current additive manufacturing allows for the implementation of electrically interrogated 3-D printed sensors. In this contribution various technologies, sensing principles and applications are discussed. We will give both an overview of some of the sensors presented in literature as well as some of our own recent work on 3-D printed sensors. The 3-D printing methods discussed include fused deposition modelling (FDM), using multi-material printing and poly-jetting. Materials discussed are mainly thermoplastics and include thermoplastic polyurethane (TPU), both un-doped as well as doped with carbon black, polylactic acid (PLA) and conductive inks. The sensors discussed are based on biopotential sensing, capacitive sensing and resistive sensing with applications in surface electromyography (sEMG) and mechanical and tactile sensing. As these sensors are based on plastics they are in general flexible and therefore open new possibilities for sensing in soft structures, e.g. as used in soft robotics. At the same time they show many of the characteristics of plastics like hysteresis, drift and non-linearity. We will argue that 3-D printing of embedded sensors opens up exciting new possibilities but also that these sensors require us to rethink how to exploit non-ideal sensors.

This paper introduces characterization techniques to investigate electrical properties of 3D-printed conductors. It presents the combination of a physical model to describe frequency dependent electrical properties of 3D-printed conductors; the use of infrared thermography in combination with Joule heating to characterize electrical anisotropy in 3D-printed sheets; and the use of the voltage contrast scanning electron microscopy method (VCSEM) to determine potential distributions in 3D-printed sheets. By means of lock-in thermography, infrared (IR) measurements are improved and amplitude modulation enables lock-in thermography at excitation frequencies above the thermal cut-off frequency. Measurements on sensor samples show the potential of the methods for characterizing sheet-like, conductive structures. The characterization methods allow improvement of 3D-printed sensor designs and exploit electrical properties of 3D-printed conductors.

3D-printing conductive structures have recently been receiving increased attention, especially in the field of 3D-printed sensors. However, the printing processes introduce anisotropic electrical properties due to the infill and bonding conditions. Insights into the electrical conduction that results from the anisotropic electrical properties are currently limited. Therefore, this research focuses on analytically modeling the electrical conduction. The electrical properties are described as an electrical network with bulk and contact properties in and between neighbouring printed track elements or traxels. The model studies both meandering and open-ended traxels through the application of the corresponding boundary conditions. The model equations are solved as an eigenvalue problem, yielding the voltage, current density, and power dissipation density for every position in every traxel. A simplified analytical example and Finite Element Method simulations verify the model, which depict good correspondence. The main errors found are due to the limitations of the model with regards to 2D-conduction in traxels and neglecting the resistance of meandering ends. Three dimensionless numbers are introduced for the verification and analysis: the anisotropy ratio, the aspect ratio, and the number of traxels. Conductive behavior between completely isotropic and completely anisotropic can be modeled, depending on the dimensionless properties. Furthermore, this model can be used to explain the properties of certain 3D-printed sensor structures, like constriction-resistive strain sensors.

This paper introduces a characterization technique to study 3D-printing of conductors and sensors during fabrication. Currently characterization of 3D-printed sensors is done after fabrication. In our novel method, however, the electrical resistance is monitored in-situ by electrically contacting the part in the beginning of the print process. This way, the effect of every additional layer on the total resistance is determined. Our new experimental method opens up ways to study 3Dprinting of sensors in order to better understand the processes at hand, e.g. it may allow distinguishing between bulk and interlayer resistances. FEM simulations and experiments are used to validate the use of this new method.