## 1 Introduction  
The ability of a cell to sense and respond to mechanical cues from its microenvironment is a fundamental process known as mechanotransduction. This intricate phenomenon plays a crucial role in a wide range of cellular functions, including cell adhesion, migration, differentiation, tissue development, and homeostasis [1]. Cellular mechanotransduction is further involved in several pathophysiological processes and is being considered for its potential as a therapeutic target [2]. In the last two decades, researchers have focused on how the mechanical properties of biological tissues and their extracellular matrixes (ECMs) influence a cell's response, investigating mostly their elastic properties [3; 4; 5; 6; 7; 8; 9; 10]. However, soft biological tissues exhibit a time-dependent mechanical behavior characterized by significant stress relaxation in response to a prescribed deformation [11], and the viscous properties of these tissues have emerged recently as key elements that influence cellular behavior [12; 13; 14; 15]. Soft biological tissues show a viscoelastic nature that arises from the combined effects of their elastic and viscous properties, characterizing their instantaneous and time-dependent responses, respectively. The global mechanical response of soft biological tissues is due to micro-scale reactions and rearrangements of a cell's cytoskeleton and ECM biopolymers, and it is influenced by their unique molecular compositions, microstructures, and hydration levels. Thus, in view of investigating the activated signaling pathways and related cellular responses involved in mechanotransduction processes, it is crucial to characterize the viscoelastic behaviors of soft biological tissues at the cell-length scale and fabricate materials mimicking their mechanical features. In this context, several materials and substrates have been developed, and it has been demonstrated that by tailoring their characteristics (e.g., stiffness, surface, chemistry, and topography), it is possible to influence cultured cells and direct their development in a desirable way [16; 17; 18]. For example, polyacrylamide (PAM) gels and polydimethylsiloxane (PDMS) substrates, functionalized with ECM proteins, have been widely adopted in mechanotransduction studies because of their well-defined manufacturing protocols, which guarantee the robust and reproducible control of the substrate's stiffness [14; 19; 20]. Hydrogels, composed of cross-linked polymer networks that can absorb and retain water [12; 21; 22; 23], have emerged as promising materials for replicating a cell's microenvironment for mechanotransduction studies and tissue engineering applications [24; 25]. However, their viscoelastic properties have not been fully investigated [13; 14], and in order to mimic the native viscoelasticity, new design guidelines for the next-generation manufacture of hydrogels are needed. Therefore, it has become crucial to develop a robust methodological approach for characterizing the viscoelastic properties of soft biological tissues and hydrogels. In clinical scenarios, several approaches have been developed for assessing the stiffness and viscoelasticity of biological tissues in a non-invasive way, including ultrasound elastography [26; 27; 28; 29], magnetic resonance elastography (MRE) [30; 31], and optical coherence elastography (OCE) [32; 33; 34]. On the other hand, in experimental lab settings, techniques such as tensile and compression testing [35; 36], dynamic mechanical analysis (DMA), and nanoindentation [37; 38; 39; 11; 40] have been adopted to quantitatively characterize the viscoelastic behaviors of biological soft tissues and hydrogels. In particular, nanoindentation has emerged as a powerful tool as it offers the advantage of characterizing mechanical properties at the cell-length scale with high precision and spatial resolution. Furthermore, nanoindentation allows for a localized property assessment, which is particularly relevant given the hierarchical and heterogenous natures of biological tissues [41; 42].