The majority of METs are primarily developed for water/wastewater applications; thus, different types of reactor architectures can significantly influence the effective flow and hydrodynamics (e.g., mass transfer within biofilms) and thereby influence their performance (Kim et al., 2014; Jiang et al., 2015; Yi et al., 2020). Hence, optimizing the shape of the system, working volumes, fluid inlet/outlet settings are critical (Logan et al., 2015; Massaglia et al., 2017; Yi et al., 2020). For instance, a drop-like-shaped MFC could perform better than a square-shaped one due to the larger effective exposed area (Massaglia et al., 2017). Moreover, the addition of baffles in the anode chamber could result in higher voltage output in MFCs due to reduced dead zones where the flow velocity is <5% of the maximum velocity (Yi et al., 2020). However, fabrication of such designs and configurations can be extremely challenging using the traditional manufacturing technique, especially for the miniaturized METs. Several studies have reported successful utilization of 3DP for precise fabrication of reactor bodies for METs (see Table 1).

Di Lorenzo et al. (2014) applied 3DP for rapid fabrication of a miniaturized (2 cm³) air-cathode MFC biosensor. Their fabricated MFC biosensor exhibited a good detection range (3–164 ppm), high sensitivity (0.5 μA mM⁻¹ cm⁻²), a fast response time (2.8 min), and fast recovery time (12 min) while detecting cadmium ions in water samples. Moreover, Quaglio et al. (2019) developed 3D-printed a MFC biosensor (12.5 ml), which allowed optimizing hydrodynamics (optimal fluid motion) and morphology of anode electrode (porosity) via computational fluid dynamics (CFD) modeling for quantification of sodium acetate in water samples. Although 3DP can allow bulk production of small-scale reactors for applications like biosensing within reasonable timeframes and costs, direct 3D printing of large-scale systems is not yet feasible. Nonetheless, approaches like combining 3DP and CFD modeling can enable low-cost advanced reactor engineering for large-scale field applications (e.g., wastewater treatment) of METs. For instance, governing equations for mass, momentum, and energy conservation to find numerical solutions in CFD modeling are not scale-dependent (Parra-Cabrera et al., 2018). Thus, when combined with CFD, 3D printed lab-scale prototypes can still be used for the design and optimization of large-scale systems.

The development of electrodes that possess highly porous structures for optimal bacterial adhesion and excellent electrochemical performance for high current output has been of great interest to the MET research communities (Zhou et al., 2017; Bian et al., 2018a; Bian et al., 2018b). Notably, complex macro-porous 3D electrode materials have been found suitable for providing a larger surface area for the biofilm developments and thereby improve the electrochemical performance of METs (Hindatu et al., 2017; Zhou et al., 2017). However, the major hurdle is in the precise construction of electrodes with geometrical complexities (e.g., porous electrodes) using the traditional manufacturing method (Zhou et al., 2017; Bian et al., 2018a; Bian et al., 2018b). Recently, researchers have explored 3DP for rapid prototyping of such ideal electrodes METs.

Zhou et al. (2017) reported that 3DP could allow the precise design of porous 3D structured X-shape skeleton electrodes with titanium and stainless steel as printing materials. The further modification of 3D-printed metal electrodes with polyaniline could provide a crumpled and biocompatible surface. Notably, the polyaniline coating of the 3D-printed titanium electrode could increase power density up to 400 times than the uncoated one. Calignano et al. (2015) developed an aluminum alloy (AlSi10 Mg) anode using 3DP, which could enable energy output of up to 3 kWh/m³ per day from an MFC. The authors suggested that the 3D microporous coral skeletal structure could provide surface roughness and excellent biocompatibility to electroactive microbes. However, metal 3D printing is still extremely expensive (Pumera, 2019). Moreover, some metal alloys used for 3DP (e.g., AlSi10 Mg) alloy are prone to corrosion (You et al., 2017; Revilla et al., 2020). Previous studies also demonstrated the feasibility of utilizing polymeric materials, such as polylactic acid (PLA), UV curable resin, for 3D printing of electrodes for METs (see Table 1). However, the electrochemical performance of 3D-printed polymeric electrodes is not always satisfactory. For instance, power generation from the PLA electrode developed by You et al. (2017) was ∼4 times lower than the control (plain carbon veil) with the same geometric structures. Despite conductive properties, thin protective polymer layers on the surface of 3D-printed polymeric electrodes can restrict electrochemical performance, which can be removed by surface modification (Pumera, 2019). Bian et al. (2018a) proposed carbonization of the 3D-printed anode from polymer resin to enhance the biocompatibility, which could produce significantly higher power density than a traditional carbon cloth anode (233.5 vs. 69 mW/m²). Similarly, surface modification of a 3D-printed polymeric electrode with copper could produce 12.3-fold higher power than copper mesh electrodes (Bian et al., 2018b). Most recently, He et al. (2021) developed a 3D-printed graphene oxide (GO) aerogel anode with a customized printing ink combining GO, ferric ions, and magnetite nanoparticles. The hierarchical pores in their GO aerogel electrode could provide effective mass transfer of substrates, leading to 7.9 folds higher volumetric current output than carbon felt anode. Thus, utilizing 3DP for electrode fabrication has shown promising results. While cost and performance are often the primary aspects considered by researchers, ensuring the long-term stability (chemical stability, corrosion resistance, etc.) of 3D-printed electrodes would be critical.

Besides electrodes and other physical components of METs, electroactive microorganisms are vital components affecting METs performance (Koch and Harnisch, 2016). The selection and enrichment of kinetically efficient electroactive bacteria (EAB) are often challenging (Torres et al., 2009; Dhar et al., 2016a). Due to slow growth kinetics, the time required for complete enrichment of anode or cathode biofilms can be significant (e.g., months-years), ultimately prolonging the reactor start-up time (Zakaria and Dhar, 2019). Several studies have utilized the effluent from MET reactors as an inoculum source (Zakaria et al., 2018; Barua et al., 2019; Chung et al., 2020a), which can expedite the enrichment process of electroactive biofilms. However, an innovative approach for the rapid establishment of electroactive biofilms is still required. A recent study by Freyman et al. (2020) suggested that 3DP can provide an attractive solution to such time-consuming biofilms development procedures. They investigated 3DP for constructing a bioanode with Shewanella oneidensis MR-1 as living inks. More importantly, Shewanella oneidensis MR-1 survived during the 3D printing process, and their MFC produced a stable current for almost 93 h. Of note, prior to their study, 3DP was applied to engineer 3D objects and structures with microorganisms, such as Escherichia coli, Pseudomonas putida, Acetobacter xylinum, yeast cells (Lehner et al., 2017; Schaffner et al., 2017; Qian et al., 2019). Particularly, due to the adhesion of cells during the process, objects 3D-printed with microbes could potentially achieve a high cell density. Nonetheless, based on the authors’ knowledge, Freyman et al. (2020) first demonstrated the feasibility of 3D-printed bioanode, potentially opening up new possibilities for high-performance METs.

Ion-exchange membranes (IEM) are imperative, especially for the dual-chamber METs, to facilitate the ion transport between anode and cathode, governing the electroneutrality (Dhar and Lee, 2013; Leong et al., 2013). The IEMs can alter the current output, depending on their relative surface area, which is associated with ionic conductivity and internal resistance of METs (Zuo et al., 2007; Philamore et al., 2015). Hence, increasing the specific ion-exchange area of IEM is great of interest, but it is often a major challenge (Philamore et al., 2015). Notably, commercially available IEMs can be costly (e.g., >700 USD/m² for Nafion) due to the complex manufacturing process (Yee et al., 2012; Dhar and Lee, 2013). Besides, IEMs require pre-treatment processes (Ghasemi et al., 2013; Rahimnejad et al., 2014), which can be time-consuming. Therefore, an efficient and low-cost membrane manufacturing method is greatly needed.

Several studies implemented 3DP as an alternative fabrication method for IEMs (Philamore et al., 2015; You et al., 2017; Theodosiou et al., 2019). Philamore et al. (2015) have manufactured IEMs for METs utilizing 3DP at a much lower cost than commercial CEM ($0.00112–0.00357 for latex and $0.16 for Tangoplus resin vs. $0.22–40 for commercial CEM, per 20 cm², USD). The power generation from an MFC with a 3D-printed latex membrane was comparable to commercial CEM (10.51 vs. 11.39 µW). In contrast, maximum power generation was much lower in the MFC with a 3D-printed Tangoplus membrane (0.92 µW). Nonetheless, the latex membrane was more prone to fouling and almost damaged after 210 days of operation, while the Tangoplus membrane was more resistant to biofouling. In contrast, 3D printed membrane using rubber-elastomeric polymers developed by You et al. (2017) could increase maximum power output up to 1.4 times than commercial CEM; however, the fabrication cost of the 3D-printed membrane was higher than commercial CEM. Thus, these results suggested that selecting low-cost and durable polymeric materials would be necessary for fabricating economic and technically sustainable 3D-printed IEMs. However, previous studies primarily focused on the performance of 3D-printed IEMs in terms of current output and biofouling potential, while limited information provided on other critical features, such as proton permeability, membrane resistance, substrate loss, and oxygen diffusion (in MFCs) (Dhar and Lee, 2013).

The spacing between electrodes is critical for minimizing internal energy losses in METs (Moon et al., 2015b). Therefore, MEA (membrane sandwiched between electrodes) has been considered by many researchers to reduce internal energy losses and thereby improve the performance of METs (Moon et al., 2015b). Typically, MEAs are prepared by pressing membrane and electrodes with or without heat treatment (Theodosiou et al., 2019), which has multiple drawbacks. For instance, a high degree of hydrophilicity can often cause anolyte leakage from the surface of MEA (Mashkour et al., 2021). Furthermore, uneven physical contact between membrane and electrode resulting from the conventional fabrication method can increase internal resistance and ultimately lower the current output (Moon et al., 2015a; Vilela et al., 2020). Therefore, precise fabrication methods for MEA are required. Recently, Theodosiou et al. (2019) compared 3D-printed MEAs with a commercial CEM. The 3D-printed MEAs from air-dry clay materials exhibited up to 50% higher power output (130 vs. 66 µW) for >70 days and provided 34–51 times cheaper production cost. Although the results were promising, future studies should focus on a comprehensive investigation of underlying mechanisms that can provide a superior performance of 3D-printed MEAs over the conventional ones.