## 2 Materials
### Nanofilers  
Different nanofillers have been used in combination with APs to develop new materials with enhanced properties. In this review, we mainly focus on conductive fillers such as carbon nanotubes and bio-based organic fillers as cellulose nanocrystals.  
Carbon nanostructures are some of the most important innovative products of nanotechnology, since the discovery of fullerenes, carbon nanotubes (CNTs), carbon nanofibers (CNFs), graphene, and other carbon related forms [58; 59; 60; 61]. Carbon nanotubes (CNTs) can be viewed as one or more graphene sheet rolled in a cylindrical way. Single walled carbon nanotubes (SWCNTs) have the diameter typically around 1 nm and length until some micrometer and are tubes made of a single sheet of graphene, while multi walled carbon nanotubes (MWCNTs) are made from more graphene sheets, so the diameter increases with respect to SWCNTs, until 10-20 nm [62]. The most important properties of CNTs cannot be attributed to the nanometer dimension, rather to the higher aspect ratio, that is the ratio between the length and the diameter of CNTs. Furthermore, CNTs display extraordinary mechanical, thermal, and electrical properties, they have ordered structure, ultralight weight, high mechanical strength, high electrical conductivity, high thermal conductivity, metallic or semi-metallic behavior, and high surface area, that can be correlated with the morphology and the high aspect ratio. The synthesis methods for carbon nanostructures include laser vaporization, arc discharge, catalytic chemical vapor deposition (CVD), and catalytic plasma-enhanced chemical vapor deposition (PECVD). Pristine CNTs are naturally hydrophobic, hence they lack of solubility in most solvents used with APs. To overcome this problem, CNT functionalization is achieved by adsorption, electrostatic interaction, or covalent bonding of different molecules and chemistries [62]. In combination with APs, CNTs were mainly used to modulate the bulk properties of the polymeric matrix, such as thermal, mechanical, and electrical properties [10; 11; 12; 13].  
Among bio-based organic fillers, cellulose nanostructures are a class of fascinating nanomaterials that recently have received a great interest both in industry and academia research, owing to their unique structural physicochemical properties such as biodegradability, renewability, low density, adaptable surface chemistry, optical transparency, and improved mechanical properties [63]. Cellulose is one of the most ubiquitous and abundant polymers on the planet, given its widespread industrial use in the present age. Cellulose is widely distributed in higher plants, in several marine animals (i.e., tunicates), and to a lesser degree in algae, fungi, bacteria, invertebrates, and even amoeba (protozoa). In general, cellulose is a fibrous, tough, water-insoluble substance that plays an essential role in maintaining the structure of plant cell walls. Cellulose fibers with nanometer dimensions can be isolated from the wood cell wall by using different methods, i.e., by chemical and mechanical treatments. The high elastic modulus of cellulose, together with improved thermal stability, decreased thermal expansion, and increased thermal conductivity, makes cellulose in general, and nanocellulose in particular, highly interesting for use as reinforcement in polymer nanocomposite materials. Cellulose nanostructures can be used to modulate important APs properties such as mechanical behavior, with the aim of developing fully bio-based nanocomposites [14; 15].