# Towards Environmentally Friendly Accelerometers Based on Bacterial Cellulose
## 1 Introduction  
The pervasive diffusion of electronics has raised the request for low-cost, smart, small-scale, and miniaturized sensors [1, 2]. A large part of both the present and future markets will be covered by accelerometers, with many consumable applications in fields such automobiles, cellphones, and wearable electronics, just to mention a few [3]. The interest in this class of sensors is reflected in a variety of fabrication processes and working principles, such as resonators and micro-machined devices (MOEMS and MEMS) [4, 5, 6, 7]. As a common trend of the adopted technology, the sensor industry has proposed integrated accelerometers [8]. Moreover, accelerometers exploiting optical, resistive, capacitive, etc., transduction mechanisms have been proposed [6, 7, 9]. The cost of the sensing system is a relevant constraint that can determine the success of technologies, and the examples reported above are affected by the need for a power source. Such a drawback can be overcome if self-generating materials [10], such as piezoelectric and aluminum nitride (AlN) [11, 12, 13], are considered. Though sensors based on AlN have raised scientific and industrial interest, micromachined AlN-based devices are based on silicon-based processes [14]. As such, the production of sensors based on this technology cannot be considered green. Moreover, they need dedicated recycling or disposing processes [15, 16]. As a final remark, silicon-based devices are rigid, while flexibility is requested from next-generation applications [17]. The need for greener sensing devices, i.e., requiring fewer raw materials and less energy and limited amounts of non-renewable matter and producing more easily disposable, recyclable, or even biodegradable devices, imposes the development of new technologies based on more sustainable and greener materials. A trend is occurring in the field of polymeric materials for sensing applications as suitable candidates for realizing next-generation sensing systems. Such an evolution willrequire developing new composites in order to improve their performance and produce greener devices. A scheme of the evolution that is occurring in the field of polymeric composites is reported in Figure 1. More specifically, the figure shows a possible evolution from nonpolymeric and un-green devices to polymeric greener ones [18; 19; 20; 21; 22; 23]. It is worth saying that being polymeric is not a guarantee of greener devices. This is why two directions of possible evolutions are shown in the figure (numbers reported in square brackets indicate papers reported in the References section). The box at the bottom right corner in the figure refers to the final result of the indicated path, i.e., all-polymeric and green sensing devices. Cellulose is among the most promising base materials for the realization of next-generation flexible and sensing systems, and many applications have been proposed so far [24; 25]. More recently, bacterial cellulose (BC), also known as microbial cellulose or bio-cellulose, has been proposed as a greener alternative to plant-derived cellulose [26; 27]. It is produced by suitable aerobic bacteria when grown in specific environmental conditions. Although BC has already been proposed in many applications, including electronics [28], sensors [29], and actuators [30], few examples exist on generating sensors based on BC [31]. In this paper, a flexible generating accelerometer, which exploits BC as the base material, is studied, modelled, and characterized. The device consists of a three-layer structure, obtained by impregnating a BC layer with ionic liquids (ILs). The BC layer is further covered with conducting polymers (CPs) [31; 32; 33]. The accelerometer is fabricated, therefore, by using greener base products and an environmentally friendly production process. In [34], a bio-polymer-based accelerometer working in a cantilever configuration has been proposed. That accelerometer uses plant-derived cellulose.