*Article* **Characterization of Bio-Inspired Electro-Conductive Soy Protein Films**

**Pedro Guerrero 1,2,\* , Tania Garrido <sup>1</sup> , Itxaso Garcia-Orue 3,4 , Edorta Santos-Vizcaino 3,4 , Manoli Igartua 3,4 , Rosa Maria Hernandez 3,4 and Koro de la Caba 1,2,\***


**Abstract:** Protein-based conductive materials are gaining attention as alternative components of electronic devices for value-added applications. In this regard, soy protein isolate (SPI) was processed by extrusion in order to obtain SPI pellets, subsequently molded into SPI films by hot pressing, resulting in homogeneous and transparent films, as shown by scanning electron microscopy and UV-vis spectroscopy analyses, respectively. During processing, SPI denatured and refolded through intermolecular interactions with glycerol, causing a major exposition of tryptophan residues and fluorescence emission, affecting charge distribution and electron transport properties. Regarding electrical conductivity, the value found (9.889 <sup>×</sup> <sup>10</sup>−<sup>4</sup> S/m) is characteristic of electrical semiconductors, such as silicon, and higher than that found for other natural polymers. Additionally, the behavior of the films in contact with water was analyzed, indicating a controlled swelling and a hydrolytic surface, which is of great relevance for cell adhesion and spreading. In fact, cytotoxicity studies showed that the developed SPI films were biocompatible, according to the guidelines for the biological evaluation of medical devices. Therefore, these SPI films are uniquely suited as bioelectronics because they conduct both ionic and electronic currents, which is not accessible for the traditional metallic conductors.

**Keywords:** soy protein; film; semiconductor; biomaterial
