## 4 Discussion
Among applications encompassing non-electroactive cells (i.e., bone, cartilage, skin), the most upcoming for conductive biomaterials appear the ones concerning bone tissue engineering. In these, it is essential to provide proper tensile strength, compressive strength and elastic modulus in order to support the physiological loads. In addition to these mechanical properties, proper biomaterials for bone regeneration should be able to reproduce the microstructure of the host tissue, ensuring a proper mineralization of the structure to be integrated in the host environment. Among the conductive polymers, PPy and PANI, used in combination with other materials such as gelatin and PLA, were shown to enhance bone regeneration by supporting and stimulating cell adhesion and proliferation [65; 114; 115]. Despite its limitation in term of mechanical properties, also PEDOT:PSS was used in combination with gelatin and bioactive glass was shown to ensure the possibility to stimulate the construct. These results represent a step forward in combining the tissue engineering techniques with the method of enhancing the bone healing and mineralization through an external electrical stimulation. Up to now, the majority of the applications here discussed were performed only in vitro. One of the major limitations of many of the conductive biomaterials is represented by their non-biodegradability, which restricts their use for in vivo application; this issue is in fact related to the possibility to cause inflammation at implantation site and thence the implant needs surgical removal [118]. Interestingly, to overcome this problem, conductive polymers can be combined with other biodegradable polymers to introduce degradability within the composite and design a conductive biodegradable polymer. [PERSON] et al. (2002) [119] combined conductive oligomers of pyrrole and thiophene with degradable ester linkages and synthesized conductive biodegradable polymer. In body, these ester linkages can be break down by enzymes and hence resulting in conductive polymer with unique property of biodegradability. Likewise, [PERSON] et al. (2004) [120] also transforms conductive polymers into degradable electroactive polymers by blending them with degradable polymers. [PERSON] et al. (2010) [27] designed a biodegradable and electrically conductive polymer having aniline pentamer with glycine ethyl ester as side chains and assessed its biological compatibility using Schwann cells. The results confirmed their biocompatibility, as there were no signs of inflammation and cytotoxicity and hence indicating their suitability for scaffold preparation and other biological systems that need electroactivity. Furthermore, a different biodegradable conductive scaffold is prepared in [121] by combination of biodegradable chitosan (97.5%) and conductive polypyrrole (PPy, 2.5%). Regarding carbon and metal-based fillers, similar concerns can be highlighted. One of the main challenges preventing the translation in vivo of the promising approaches demonstrated in vitro is the systemic response that fillers release might have in the host. These materials might therefore cause an inflammatory and immune response which might affect the integration of the scaffold in the human tissues. The most interesting approaches that have been evaluated to overcome this issue are the fabrication of novel biodegradable materials, such as metal oxides (e.g., ZnO) which can provide from one side optimal biocompatibility, non-toxicity and degradability, and from the other great electrical properties. Summarizing, from a comparison of all the discussed applications, the most relevant part of available literature was addressed to an evaluation of the different conductive biomaterials in term of electrical properties, mechanical properties and of cytocompatibility. Results obtained in terms of conductivity appear promising to adopt these materials for the development of scaffolds which could directly act as a sensing/stimulating element.