Additive manufacturing technologies are widely gaining more attention, resulting in the development or modification of 3D-printing techniques and materials. At the same time, economical and ecological aspects force the industry to develop materials that are easier and quicker to manufacture, while attaining their desired properties. The electrochemistry field can certainly take advantage of this fabrication tool for sensing and energy-related processes. In particular, the fabrication of flexible sensing devices could extensively benefit from the currently developed 3D-printing techniques, such as fused-deposition modelling (FDM) and stereolithography (SLA), as they provide a good platform for the integration of flexible materials (polymers). Mechanical flexibility of the electrodes is desirable for biomedical sensing applications, as it improves contact between sensor and skin or neural tissue by adopting the shape and decreases mechanical stress.

Boron-doped diamond (BDD) is a popular material for electrodes because it exhibits metal-like conductivity when sufficiently doped with boron atoms. BDD possesses highly desired electrochemical characteristics such as wide potential window and low background current, while attaining diamond’s chemical stability and biocompatibility. The hardness of diamond, however, has hindered its applications in flexible electrodes due to the mechanical property mismatch between diamond and flexible substrates. Moreover, common manufacturing techniques that focus on flexible BDD electrodes are time-consuming and require complex material transferring steps.

In this work, two 3-D printing techniques, FDM and SLA, have been employed to explore the possibility to prepare 3D-printed flexible BDD-based electrodes. The effect of selected 3D printing technique, particle concentration, treatment process on the mechanical, morphological, electrical and electrochemical properties of the developed composites was thoroughly investigated. A common feature for both SLA and FDM fabrication processes, the presence of BDD particles in the polymer composites resulted in enhanced mechanical properties such as Young’s modulus (increased by 230% and 75%, for FDM and SLA, respectively), but caused a reduction in tensile strength and elongation at break. The FDM-based composites additionally allowed higher weight percent of BDD fillers to be introduced, 40 wt.% in contrast to only 12.5 wt.% achieved by SLA. For this reason, further development of composite electrodes was devoted to FDM, which allowed higher weight percentage of fillers to be introduced in the polymer.

Herein, we report, an innovative, flexible, 3D-printed conductive composite was developed through FDM that displayed promising mechanical and electrochemical characteristics. By using a unique combination of a flexible polymer, thermoplastic polyurethane (TPU), and fillers, BDD particles and carbon nanotubes (CNTs), a conductive composite material was fabricated which enabled its use as a flexible electrode. Three different compositions were fabricated that each consisted of TPU, CNTs and BDD, with CNT-to-BDD ratios of 1:0, 1:1, and 1:2. For the TPU/CNT/BDD electrodes, the electrical conductivity was significantly improved with the addition of BDD particles and displayed an increase of over 7 times (up to 1.2 S/m) compared to without BDD. This effect was similarly visible in the electrochemical characterization, and well-developed peaks were observed in the presence of two commonly used redox markers [Fe(CN)6]3−/4− and [Ru(NH3)6]3+/2+ with increasing BDD concentration. Surface treatment of the TPU/CNT/BDD electrodes drastically enhanced electrochemical properties such as double-layer capacitance (Cdl) by 250 times, but also reported significant increase in peak current intensities for both redox markers. A prominent drop in peak-to-peak separation (∆EP) for [Ru(NH3)6]3+/2+] redox marker was noticed when the electrodes were incorporated with BDD particles. From 178 mV for TPU/CNT, it decreased to 110 mV for the highest BDD-loaded electrode (TPU/CNT/BDD(1:2)). The detection of dopamine was successfully achieved through the fabricated BDD-based composite electrodes. This study provides a state-of-the art, novel composite material that is characterized by excellent flexibility, attractive electrical conductivity and promising electrochemical characteristics. It provides insights into the interactions between composite components and their impact on the electrical and electrochemical properties of the 3D-printed surfaces.