## 1 Introduction  
Scaffold-based 3D cell cultures are highly investigated for many applications in pharmacology, drug development, diagnostics, tissue engineering, and regenerative medicine. Their main value relies on the capability to reproduce an environment which mimics the in vivo conditions actually undergone by cells. This approach improves reliability and usefulness of the results obtained in in vitro experiments--in terms of cell response to drugs, nanomaterials and biomolecules--addressing the translation to in vivo applications [1]. Nowadays, traditional analytical evaluations, considered the reliable \"gold standard\", are performed by using dyes, DNA sequencing, immune-based assays, or fluorescence tags, which are often invasive, sample destructive and therefore prevent continuous monitoring. In this picture, one of the most critical hurdle is the possibility to evaluate specific cell functions without affecting their developmental processes. This would ensure a fundamental leap towards an improved and interactive monitoring of cell cultures [2; 3]. Several efforts have been performed starting from 1970 to implement non-invasive strategies for monitoring cells and interacting with them by means of different physical cues (e.g., optical, electrical, or magnetic) [4]. Among all, specific techniques based on the application of an electrical field are attractive in terms of non-invasiveness, accuracy, sensitivity, low-cost, and time effectiveness. In this light, researchers started to investigate several methods to interact with cell development by recording or stimulating them by using specific electrical signals [5]. Electrical-based techniques for cell monitoring can be mainly classified in potential- and impedance-based ones. The first one--applicable to electrically active cells--usually gives the possibility to record/stimulate cell growth by means of a planar array of micrometric metallic electrode [6]. The latter one, suitable for monitoring also non-electrically active cells, relates cell behavior to measurable changes of the electric impedance, due to their inherent structure and morphology [7]. Throughout the years, both these techniques have become widespread methods for pharmacological drug development, biomarkers screening and in vitro pathology modeling [8; 9; 10; 11; 12; 13; 14]. In the last decades, a huge interest has been addressed toward to possibility to translate both these techniques from 2D to 3D sensing [15; 16]. This further methodological exploitation may ensure the monitoring of cells growth or differentiation with improved reliability, for both pharmacology and regenerative medicine purposes. The main issues introduced when shifting this approach towards 3D environment are related with the distance between cells and electrodes and the electrical properties of the scaffold materials, both of which might affect the overall reliability in the measurements. Regarding a potential-based approach, the contact between electrodes and cells is considered primarily necessary in order to get a proper interaction both during recording and stimulating phases. By using a traditional 3D structure with non-conducting material, the contact between cells and microelectrodes is certainly impaired by the presence of the scaffold itself. Thus, the possibility to improve the overall conductivity of the scaffold is of primary interest [17; 18; 19]. Regarding impedance-based solutions, when switching from 2D to 3D environment, the main issue is represented by the electrical properties of the material used to embed and sustain cell growth, which has a predominant role in the measurement of the overall system (i.e., cells + scaffold) impedance [20]. The introduction of conductive materials--as alternative or combined with the traditional polymer/natural non-conductive matrices--thus represents a promising strategy to improve the interaction between sensors and cells and avoid the need of external invasive device [21; 22].