## 1 Introduction  
The term bioelectrochemical systems (BESs) encompasses a group of technologies that have aroused a great interest in both industrial and academic spheres in recent years [1]. BESs, which can be seen as a combination of electrochemical and biological processes, hold significant potential for practical application across different industrial sectors, spanning energy production and storage, wastewater treatment, biosynthesis and biosensing, among others [2; 3; 4]. However, BESs have not yet been developed to a practical scale. One of the biggest challenges is related to their scalability [1], as the performance of the electrodes worsens rapidly with size [5; 6]. In this regard, an efficient BES design requires electrodes capable of delivering large current densities (to minimize the size of the electrode) at low overpotentials (to optimize energy usage) [7]. Both current density and overpotential depend on many factors, the most important ones being perhaps the microbial activity and metabolism of the electroactive biofilm, the interaction between the biofilm and the electrode (i.e., a transfer of electrons) and the electrode material [8; 9]. Several strategies have been adopted to optimize these factors, including the use of different sources of inoculum, [10] optimization of the electrode potential, [11] electrode surface modifications [12] and the use of different electrode materials [13]. The latter has been a hot topic in recent years, [14] and the latest advances include the incorporation of common nanomaterials, Mxenes (two-dimensional inorganic compounds along with atomically thin layers of transition metal compounds), metal-organic frameworks or conductive composites, all of which have the aim of improving conductivity and the active surface area of the electrode [13]. The use of conductive polymers (in combination with additive manufacturing techniques) as a matrix for the development of bio-electrodes in BESs is becoming increasingly common [15]. Poly-lactic acid (PLA)-based conductive composites represent a suitable alternative thanks to their relatively low cost, biocompatibility and capacity to produce custom-shaped electrode geometries [16; 17; 18; 19; 20; 21]. Nevertheless, the electrochemical performance of PLA-based bioelectrodes is not always acceptable [22] because of the presence of non-conductive polymer layers on the surface of the electrode. To address this issue, surface modification techniques [23] are needed to remove part of those non-conductive layers.  
In this paper, we explore the potential to enhance the performance of 3D-printed conductive planar monolithic electrodes made of PLA and TPU (thermoplastic polyurethane). Our approach involves modifying the microtopography of the electrode surfaces by creating a triangular pattern with varying depths. This modification aims to achieve two main objectives: first, to improve current production by increasing the total surface area of the electrode, and second, to promote the development and adhesion of an electroactive biofilm. Additionally, we aim to further enhance the electrochemical performance of the PLA-based electrodes by boosting surface conductivity through the electrodeposition of graphene oxide.