Design of a stretchable, conductive silicone-based biomaterial for vocal fold models

Soft Robotics is based on soft and stretchable organic structures that imitates natural movements and finds widespread use in many biomedical applications: diagnosis, drug delivery, assistive and wearable devices, surgical tools, prostheses or artificial organs. The use of transductors and actuators require that the materials used should also possess adequate electrical conductivity. This thesis originates from the collaboration between Politecnico di Torino (Biomedical Lab) and Scuola Superiore Sant'Anna (Soft Mechatronics for BioRobotics Laboratory of the BioRobotics Lab). The aim of this work is the development of a totally polymeric and conductive material for vocal fold models that satisfies three main requirements: i) stretchability in the physiological range of vocal folds, ii) biocompatibility for potential in vivo applications and iii) electrical conductivity for electroglottography studies as a method of validation of the model. The proposed strategy involves the blending of a silicone elastomer with poly(3,4-ethylenedioxythiophene:poly(styrene sulfonate) (PEDOT:PSS), with the addition of Triton X-100 as surfactant and Ethylene Glycol as dopant. First, PEDOT:PSS was blended with polydimethylsiloxane (PDMS), but results were non homogenous and strong phase separation was observed, supposedly due to the long curing time of PDMS. Another commercial silicone elastomer, Silbione RTV4524 (RTV4524), with shorter curing time was used. Different blends were prepared by mixing RTV4524 with PEDOT:PSS, varying the silicone/conductive polymer ratio. Triton X-100 and Ethylene Glycol were kept constant at 1% v/v and 7% v/v, respectively, with respect to PEDOT:PSS. More homogeneous results were achieved for every formulation tested. Two different formulations of the blend (2/1 and 1.5/1 ratios) were chosen and underwent further characterizations. Surface characterization such as ATR-FTIR analysis revealed the presence of PEDOT:PSS absorption bands in the surface, whereas the blends maintained an hydrophobic behaviour, with water contact angle values comparable to those of the pristine silicone: 109.8° ± 5.2° and 109.7° ± 6.2° for 1.5/1 and 2/1 ratios, respectively). Preliminary results from uniaxial tensile tests at break revealed that the presence of PEDOT:PSS in the blends led to a decrease in mechanical properties compared to pristine RTV4524, however still being able to elongate more than 7 times their original length before break. Uniaxial cyclic mechanical tests (100 cycles, strain range 0-40%) showed that both 2/1 and 1.5/1 formulations were capable of cyclic elastic deformation without any decrease in performances. For each case, the Young’s moduli were comparable to that of natural vocal folds, with values of 85.6 ± 11.8 and 59.9 ± 2.9 KPa for 2/1 and 1.5/1 ratios, respectively. Indirect cytotoxicity test through resazurin assay revealed that both blends showed no cytotoxic effects after 24, 48, and 72h, with more than 85% viability at the last time point, compared to those of the controls. Electrical conductivity of the blends is currently under investigation: electromechanical tests are being conducted in order to evaluate if the blends show an appropriate conductivity that would make them suitable for the specific application.