Additively Manufactured 3D Micro-bioelectrodes for Enhanced Bioelectrocatalytic Operation

Abstract

The drive toward miniaturization of enzyme-based bioelectronics established a need for three-dimensional (3D) microstructured electrodes, which are difficult to implement using conventional manufacturing processes. Additive manufacturing coupled with electroless metal plating enables the production of 3D conductive microarchitectures with high surface area for potential applications in such devices. However, interfacial delamination between the metal layer and the polymer structure is a major reliability concern, which leads to device performance degradation and eventually device failure. This work demonstrates a method to produce a highly conductive and robust metal layer on a 3D printed polymer microstructure with strong adhesion by introducing an interfacial adhesion layer. Prior to 3D printing, multifunctional acrylate monomers with alkoxysilane (-Si-(OCH₃)₃) were synthesized via the thiol-Michael addition reaction between pentaerythritol tetraacrylate (PETA) and 3-mercaptopropyltrimethoxysilane (MPTMS) with a 1:1 stoichiometric ratio. Alkoxysilane functionality remains intact during photopolymerization in a projection micro-stereolithography (PμSLA) system and is utilized for the sol-gel reaction with MPTMS during postfunctionalization of the 3D printed microstructure to build an interfacial adhesion layer. This leads to the implementation of abundant thiol functional groups on the surface of the 3D printed microstructure, which can act as a strong binding site for gold during electroless plating to improve interfacial adhesion. The 3D conductive microelectrode prepared by this technique exhibited excellent conductivity of 2.2 × 10⁷ S/m (53% of bulk gold) with strong adhesion between a gold layer and a polymer structure even after harsh sonication and an adhesion tape test. As a proof-of-concept, we examined the 3D gold diamond lattice microelectrode modified with glucose oxidase as a bioanode for a single enzymatic biofuel cell. The lattice-structured enzymatic electrode with high catalytic surface area was able to generate a current density of 2.5 μA/cm² at 0.35 V, which is an about 10 times increase in current output compared to a cube-shaped microelectrode.

Keywords: additive manufacturing; electroless metal plating; enzymatic fuel cells; microelectrodes; surface functionalization.

Figure 1

Fabrication of robust 3D conductive microstructures: (A) 3D printing of a functional photocurable resin using PμSLA, (B) introduction of an interfacial adhesion layer by surface functionalization with MPTMS, and (C) electroless gold plating of a 3D printed polymer microstructure. (D) SEM image of the 3D printed BCC lattice structure. (E) Magnified SEM image of 3D printed BCC showing the thickness of a single printed layer.

Figure 2

XPS analysis of the 3D printed polymer with thiol functionalization (SG-MP polymer). (A) High-resolution S2p core-level spectrum of the SG-MP polymer prepared at t_func = 3 h. (B) Atomic percentage of S2p of the SG-MP polymer after thiol functionalization at different t_func. (C) High-resolution C1s core-level spectrum of the SG-MP polymer prepared at t_func = 3 h. (D) The proportion of teh O–C=O moiety to other C1s chemical states of the SG-MP polymer at different t_func. The error bars represent the standard deviation of at least three separate tests.

Figure 3

SEM images of electroless gold-plated 3D microstructures: (A) diamond, (B) gyroid, and (C) primitive. (D) FIB-SEM analysis on the cross section of the electroless plated sample prepared from the SG-MP polymer at t_func = 3 h. Inset shows the magnified image showing the average gold thickness of 456 ± 49 nm. (E) Effect of thiol functionalization time on the electrical conductivity of gold deposited samples (cube structure with dimension of 2 × 2 × 0.5 mm). The reference sample (0 h) for conductivity measurement was electroless plated using a gentler mixing method without sonication because the gold did not adhere to the surface during sonication. The error bars show the standard deviation of at least three independent experiments.

Figure 4

Effect of thiol functionalization on interfacial adhesion between the deposited gold and the polymer surface: (A) the percentage of gold that remained on the polymer surface after adhesion tape test. (B) EDX Au mapping data before and after adhesion tape test. Scale bar represents 500 μm. The error bars show the standard deviation of at least three separate analyses.

Figure 5

Interfacial chemical composition analysis on gold deposited sample (t_func = 4 h) using ToF-SIMS analysis. (A) ToF-SIMS depth profiles of the sample. Cross-section maps along the XZ direction of (B) Au^3–, (C) CHO, (D) SiO₂, and (E) AuS and (F) overlay of the signals for panels B–E (red: Au^3–, yellow: AuS, blue: SiO₂, and green: CHO).

Figure 6

Application of 3D printed microelectrodes as an enzymatic anode: (A) schematic representation of the enzymatic glucose oxidation process inside an electrochemical cell, including the counter electrode (CE), reference electrode (RE), and 3D printed gold working electrode (WE). Cyclic voltammetry curves of glucose oxidation with microelectrodes of (B) cube and (C) diamond lattice form.