Piezoelectric Nano-Biomaterials for Next Generation Cochlear Implants

The Cochlear Implant (CI) is the only available therapeutic solution for people affected by profound or severe sensorineural hearing loss (SNHL), who have hearing thresholds > 65 dB, mostly due to irreversibly damaged sensory epithelium. The conventional CI is a multi-component electronic device implanted via a surgical procedure, which replaces the whole ear function. Although CI recipient numbers are growing, the use of CI technology is still limited due to cost-benefit issues. Conventional CIs suffer from some unsurpassed disadvantages, limiting the patients’ quality of life and the hearing quality. The damaged ear sensory cells, i.e., hair cells, act as biological transducers by converting mechanical vibration of the basilar membrane to electric signals for the peripheral dendrites of the auditory neurons. As such, nanostructured piezoelectric biomaterials in contact with the basilar membrane could replace hair cell function by exploiting the mechanical tonotopy still present in non-viable cochleae. Moreover, piezoelectric CIs would work in a biomimetic fashion, be self-powered and fully implantable, less expensive (material-based) than bionic CIs and a-magnetic (i.e., MRI-compatible).
Developing a nanostructured piezoelectric device designed under a tissue engineering paradigm, if properly implanted, will establish direct contact with spiral ganglion neural dendrites, thus decreasing the required electrical output for neural stimulation and finally achieving the needed sensitivity via piezoelectric materials. Specifically, we move forward the primitive concept of bulk-structured (i.e., films) piezoelectric CIs by designing a new device based on ultrafine piezoelectric fibers produced via electrospinning to be precisely delivered and efficiently integrated with the biophysical cochlear microenvironment. The intrinsically enhanced surface-to-volume ratio, structural flexibility and piezoelectric response of the ultrafine fibers will allow a piezoelectric multifiber filament with improved biophysical properties to be obtained.
We develop ultrafine piezoelectric fibers made of: (1) polyvinylidene fluoride (PVDF) and its copolymer (polyvinylidene fluoride tri-fluoro ethylene; PVDF-TrFE) and, (2) polyacrylonitrile (PAN), whose piezoelectric properties are studied by assessing the piezoelectric coefficients, as well as via piezoresponse force microscopy in the constant-excitation frequency-modulation mode. The influence played by the used solvents, i.e., methyl ethyl ketone or dimethylformamide/acetone, and the effect of rotating velocity of the electrospinning collector on fiber morphology, mechanical and piezoelectric properties are investigated. To the improve piezoelectric properties, piezoceramic nanomaterials, like BaTiO₃ and LiNbO₃ nanoparticles, are added to obtain polymer nanocomposite fibers. Aligned composite fibers at 20% (w/w%) of LiNbO₃ showed an enhanced piezoelectric response of 90 ± 2 mV, with respect to their random counterparts (40 ± 4 mV), under an applied load of 2.12 N. These structures support SHSY-5Y neural cell growth in vitro. To gain multifunctionality, we study immunomodulatory and antibacterial properties of nanoceramics. In fact, reducing the fibrotic tissue formation would allow the best electric delivery by the piezoelectric materials. The obtained fibrous materials resulted otocompatible in vitro with OC-k3 inner ear cells, showed enhanced indirect and direct antibacterial activity against P. aeruginosa. The LiNbO₃ modulated the expression of pro-inflammatory interleukins, which could help reducing the extent or duration of a post-implant inflammatory response.
Piezoelectric nanostructured materials combining nanotechnology and tissue engineering show promise for cochlear stimulation and otoprotection.

Funded by European Union–Next-Generation EU via the Italian Ministry of University and Research (MUR), PRIN 2022 program (PROMISE project, CUP I53D23004700006).