## 1 Introduction  
In recent years, intensive research has been aimed at using renewable materials to manufacture components for smart microelectronic applications. In particular, this concerns flexible electronic solutions. Substrates made of poly(ethylene terephthalate) (PET) [1], polyurethane [2], elastomers [3], poly(dimethylsiloxane) (PDMS) [4], polyimine [5] or fabrics [6] are used for this purpose. There has been a growing interest observed in the use of naturally occurring materials such as sponge [7], a composition of chitin and lignin [8], or cellulose [9] as substrates. Among the previously mentioned materials, cellulose is the most commonly used in the form of fibers [10] and nanofibers [11] based on which the substrate is formed. In the latter case, the substrate is characterized by improved transparency [12]. Filter paper [13] and coffee filters [14] are also frequently used materials for such technical solutions. Using cellulose ensures that forming the final product is easy with regards to shape formation and flexibility. Some other advantages can be negligible toxicity, uncomplicated disposal, and low weight. Due to its widespread availability and relatively low price, cellulose is a promising component for flexible electronic solutions. The limitation of cellulose is its low moisture and heat resistance, poor mechanical strength,and low thermal and electrical conductivity. Therefore, as mentioned earlier, this material is often combined with some other components to eliminate or at least minimize the drawbacks. When combined with an electrically conductive material such as silver [15], copper nanowires [16], CNTs [17], graphene [18], electrically conductive polymers [19; 20; 21] or their compositions [22], new technical possibilities can result, which provide a broader application in the fabrication of advanced solutions for electro-smog protection [23], chemical species-based sensors and actuators [24; 25], consumer electronics [26], low-temperature heat sources [27], or energy recovery solutions [28]. In the solutions mentioned above, cellulose can be used as a substrate onto which an electrically conductive material is applied in the form of conductive pathways. Depending on the product's subsequent application, the process can be carried out using drop, paint, spray, dip, print coating, or hand-drawing techniques [29]. In an alternative approach, the electrically conductive material is implemented directly into the pulp bulk structure during its manufacturing stage to form a so-called hybrid paper [30]. Products manufactured this way can also be laminated using appropriate polymer resins, such as PDMS [10]. This treatment makes it possible to provide electrical insulation, simultaneously improving the mechanical and functional properties of the hybrid paper obtained this way. Among the many mechanical strain sensor technical solutions, a relatively small group are those based on cellulose (or its derivatives) and AgNWs [31]. In solutions of this type, the system's response to external tensions is converted into an electrical signal, processed and then recorded in the form of time-dependent changes in relative resistance. Resistance in the electroconductive areas used in sensors can change due to the following three mechanisms: crack propagation, tunnelling, and disconnection-reconnection [15]. The latter mechanism prevails when 1D-type materials such as AgNPs or CNTs are in use. These conductors overlap to form some conductive networks based on conductive paths. As a result of the superimposed forces, these paths can undergo reversible disconnections and reconnections, which are reflected in related changes in the sensor's resistance. Owing to their unique and versatile properties, technical solutions based on hybrid paper can also be used to construct multi-functional sensors for temperature measurement [32] or proximity sensing [33]. Such multi-task sensors may also perform additional duties, such as heating [34] or functioning as actuators [35].