## 3 Results
### Conductive Fillers
#### 3.2.2 Metal-Based Solutions  
The metal-based solutions are mainly founded on the use of noble metals, which are resistant to corrosion and oxidation in moist air [73; 74].  
* _Gold_--An interesting metal which has attracted attention in fields ranging from drug delivery, cancer therapies and regenerative medicine is gold, specifically gold nanoparticles (AuNPs). Gold is noble metal characterized by fundamental properties for tissue engineering like: biocompatibility, electrical, optical and catalytic properties, ease of functionalization, higher mechanical properties. It has been proven that AuNPs are capable to enhance intercellular electrical communications, augment cardiomyocyte function and cardiac differentiation of MSC. They have been used to produce a unique extracellular milieu that couples tunable conductivity and elasticity for cardiac tissue engineering (CTE) [92; 93; 94]. AuNPs have been employed together with different polymers to obtain conductive scaffolds evidencing an outgrowth process, where neurites originate from the soma, extend and branch, developing into a complex dendritic tree [95].
* _Platinum_--Platinum Nanoparticles (PtNPs) have been indicated as promising biocompatible metal, which, when incorporated within scaffold, produce a positive influence both on cell growth and conductivity of the overall scaffold; thus, ensuring the possibility of an electric-based monitoring of cellular activities. Additionally, they protect cells from oxidation-induced inflammation which inhibits pulmonary inflammation and induced bone loss by decreasing osteoclastogenesis. PtNPs also assisted cell growth and could promote cell proliferation [96].  
Figure 2: **(a)** Impedance spectrum (real part) of a typical membrane coated with collagen and immersed in medium at 0, 6, 24, and 44 h. Note that the 24 and 44 h spectra are coincident. (**b**) Impedance magnitude change, \(\mid\)Zp \(\mid\), for the same membrane with respect to time 0 h. (**c**) Plot of real impedance values for a CNT/PLLA membrane with C3A cells as a function of time after cell seeding, (**d**) magnitude of impedance change for the same membrane. (Reprinted with copyright permission [78] Copyright © 2011 Elsevier).  
* _Silver_--Silver (Ag) is a well-known material already used in coatings for biomedical devices and in wound care products thanks to its antibacterial properties. Since it is a metal, it shows electrical conductivity and charge storage capacity [97; 98]. It must be underlined that this metal is under debate regarding the aspect of biocompatibility: Ag antibacterial properties are associated to the release of Ag\({}^{+}\) ions and their release rate can be affected by silver nanoparticles (AgNPs) surface to volume ratio and capping agents, thus leading in toxicity problems [98].
* _Zinc_--Complex tissue regeneration of peripheral nervous system (PNS) and, particularly central nervous system (CNS) (no spontaneous regeneration due to inhibitory cues connected to astroglial cells) suggest the usage of nanoparticles due to their ability to reduce activity of undesirable cells. In order to enhance neural tissue regeneration in electrical fields through conductive materials, like piezoelectric materials, zinc is a promising candidate in this field. This metal presents important properties like biodegradability (lower degradation rate than iron and magnesium (0.018-0.145 mm/year)), biocompatibility, saturability, non-toxic behavior and rapid healing capacity [99; 100].  
The main characteristics of metal-based fillers and the corresponding manufacturing techniques used in developing scaffolds, are reported in Table 3.