• SH-SY5Y neuroblastoma cells
• piezoelectric thin films
• P(VDF-TrFE)/BTNPs
• model of basilar membrane
• electrical stimulation
• cochlea
• cell culture
• tonotopy
• vibration measurement

The cochlea is a spiral shaped duct located in the inner ear and it represents the receptor organ for hearing allowing the conversion of sound-induced vibration into electrical impulses that are transmitted to the central nervous system. The cochlea is composed of three fluid-filled compartments one of which, the scala media, contains sensory cells called hair cells. These cells are sensitive to a mechanical stimulus and they are innervated by afferent nerve fibers. The hair cells are positioned onto a flexible membrane, the basilar membrane, that vibrates as a sound wave reaches the cochlea. The displacement of the membrane causes the receptor cells to change their potential and this produce a stimulation of the synaptic neurons that transmit an electrical signal to the brain. In this way the characteristics of a sound, such us pitch, loudness and timbre, are perceived and recognized.
The cochlear cells survive for life without renewal thus a their damage causes a irreversible loss of the auditory function. In fact if the hair cell are missing, in the presence of an incoming sound, the stimulation of the auditory nerve and the transmission of the electrical signal to the brain does not occur. According to the most recent estimates of the World Health Organization (2011) over 360 million persons (5.3% of the world population) have disabling hearing loss. The main consequences of hearing impairment are the inability to interpret speech sounds that limits the communication with others and causes difficulties in both professional and social life. Furthermore hearing loss also leads to a strong economic impact for a society due to the costs concerning the medical treatments and social services that must be provided by the medical care and welfare system. These represents valid motivations for the search of new medical therapy in order to improve or restore the hearing function of deaf people.
At present, for the most severe cases of hearing loss, the only medical therapy to obtain a partial hearing restoration is the use of cochlear implant which function is to bypass the missing inner hair cells providing, in the presence of a sound, an electrical stimulation to the auditory nerve. Although the cochlear implants increase hearing capabilities these devices have many disadvantages, such as high cost, high complexity from both hardware and software point of view, health risks due to surgical insert of many components, need of a battery for working.
In the recent years, the idea to use smart materials to mimic the function of the inner hair cells has emerged. In particular, a new approach for obtaining the electrical stimulation of the auditory neurons has been proposed based on the use of piezoelectric materials that provide an electrical charge in response to a mechanical stimulus in a wireless modality.
In line with this approach, the study reported in the present thesis work aimed at developing a biomimetic device that could replace a damaged cochlear epithelium and thus provide electrical stimulation of the auditory nerves. To the purpose, thin biocompatible piezoelectric films were prepare having vibrational response similar to the basilar membrane and also capability to generate an electrical signal as a consequence of a deformation provoked by an applied sound.
In particular, a new piezoelectric composite material was obtained by dispersing barium titanate nanoparticles (BTNPs) into a matrix of poly(vinyldene fluoride- trifluoroethylene) (P(VDF-TrFE)). This composite was first used to develop a prototype of the basilar membrane which was mechanically characterized in term of its vibrating properties. Then, the biocompatibility of the material and its capability to induce electric stimulation to neural cells were evaluated with in vitro experiments. More in detail, the piezoelectric composite films were used as active substrates on which SH-SY5Y neuroblastoma cells were cultured as a model of cochlear sensory neurons. The effect of the electrical stimulation was evaluated in terms of neurite emission from the cells.
The results of the work demonstrated the possibility to obtain, through piezoelectric substrates, an electric stimulation of neuronal cells without an external source of electrical energy but simply by applying a mechanical stimulus that in cochlea is provided by an incoming sound. This is a promising characteristic for the future development of self-powered implantable devices that could be used to bypass a damaged cochlear epithelium.