Biomimetic Polymer Composites with Piezoelectric and Conductive Particles for Tissue Engineering

Electroactive polymers emerged as a proper route to develop novel scaffolds for tissue engineering bio-mimicking the intrinsic electrostimulation occurring in our organism, such as in bone and cardiac tissues. Different approaches can be found to design these electroactive polymers, such as the use of intrinsic conductive biocompatible polymers (i.e., polypyrrole), and mixing a bioactive polymer with conductive nanoparticles (such as carbon derivatives including nanotubes and graphene). Nanogenerators had recently emerged in this context as the proper transducer technology able to deliver directly an electrical signal onto a tissue using a mechanical stimulus (for instance from biomechanical movements or a remote device). As cited by Wang, “nanogenerator is a field that uses the Maxwell’s displacement current as the driving force for converting mechanical energy into electric power using either piezoelectric or triboelectric effect, whether we use nanomaterials or not”. In particular, piezoelectric nanogenerators (PENG) are based on a phenomenon associated with the intrinsic property of some materials having non-centrosymmetric structures to create a polarization under mechanical strain. Beside the need to enhance the voltage/current output of PENG, today the main challenge for tissue engineering is to increase their bioactivity, flexibility, and mechanical stability, motivating the development of polymer films containing piezoelectric particles. This strategy allows the design of novel multifunctional PENG devices with a synergy effect arising from the properties of the polymer matrix (i.e., biocompatibility and processability) and from the piezoelectric filler. During the last years relevant evidence had shown that the presence of conductive nanoparticles in polymer/piezoelectric composites can drastically improve the piezoelectric behavior. For instance, conductive particles can overcome issues related with the high permittivity from the charge density at the polymer/piezoelectric interface generated by the Maxwell-Wagner-Sillar effect. Indeed, conductive fillers at low amount can easily propagate the charges generated on the surface of individual piezoelectric particles through the whole material.<br/><br/>In this contribution, 3D printed TENG scaffolds based on polycaprolactone (PCL) having piezoelectric Zinc Oxide (ZnO) rods and electrically conductive Thermally Reduced Graphene Oxide (TrGO) particles were developed and characterized as a biomimetic material for bone tissue engineering. Our results show that the ternary PCL/ZnO/TrGO composites presented a much higher dielectric permittivity and piezoelectric coefficient than binary PCL/ZnO composites, that results in a higher voltage generation. These results show that the ZnO-TrGO interaction forms a micro-capacitor network, boosting dielectric properties by charge accumulation at these interfaces. Simultaneously, TrGO creates conductive pathways for efficient piezoelectric charge transport, achieving bone-like properties that make scaffolds suitable for bone tissue engineering. Under 1 MHz ultrasound stimulation at 0.4 W/cm2, the peak-to-peak voltage generation of PCL/ZnO increased from 41.0 ± 3.7 mV to 105.3 ± 7.4 mV due to TrGO. Scaffolds containing ZnO demonstrated accelerated hydrolytic degradation, while both binary and ternary composites showed good MC3T3 cell adhesion, viability, and enhanced ALP activity under both non-ultrasound and ultrasound conditions. 3D printed PENG scaffolds further exhibited improved hMSCs adhesion and viability. Other examples showing that conductive particles can increase the output of PENG based on polymers having piezoelectric particles will be further presented.<br/><br/>Acknowledgment:<br/>The authors gratefully acknowledge the financial support of ANID under the project ANID-Basal Center of Interventional Medicine for Precision and Advanced Cellular Therapy, IMPACT, # FB210024, and under the project EXPLORACION 13220007.