**3. Future directions**

It would be expected that future devices will meet the existing basic requirements which include wearing comfort, cosmetic appeal, long battery life and customizable frequency-cum-level dependent gains for ideal hearing rehabilitation. There is extensive ongoing research in different aspects of hearing technology, some of which are described here. The micro-electrical–mechanical systems (MEMS) microphone allows for a further reduction in size as well as the possibility of multiple microphones on a single device. This will be very useful for noisy environments for selecting a target source while rejecting other competing sounds thus improving directionality of the hearing device [34]. Silicon microfabrication of microphones making them more sensitive and with lower thermal noise was inspired by the ears of the parasitoid fly, *Ormia ochracea* and is another unique approach [35]. As the size of the microphone progressively reduces, self-calibrating devices are likely to become more common in the future. Such systems can help in achieving target frequency-and-level dependent gains at the initial fitting and can greatly speed up the initial fitting process. Initiation of the hearing device could itself trigger a self-adjustment procedure to ensure the device overcomes daily variations [36]. DSPs have progressively shrunk in size and power requirement which together with improvements in battery technology will contribute to increased intervals between battery recharging. Developments in both battery chemistry and internal components such has anodes will likely result in longer battery life, faster recharging, smaller size and increased voltage that will enable increased dynamic range and DSP processor speed [36].

Bluetooth is one such technology that hearing device manufacturers are increasingly making available in their devices. This makes the hearing device compatible with any Bluetooth enabled technology across manufacturers allowing for handsfree usage of phone and other sound devices. Future hearing devices may contain sensors already existent in smart watches and other wearable devices which may collect information and present it via an auditory speech signal tailored to the wearer. There is evidence that auditory evoked electrical responses change depending on which sound source an individual is attending to [37]. This has been referred to as cognitively controlled hearing aids and research is ongoing in this field at present. The possibilities for improving the bone conduction device in the future are endless. A better housing design and improved transducer technology can improve conventional BCDs. This would be a significant advantage since they are non-invasive and a lower-cost alternative. Expanding surgical criteria to children below a certain age with smaller devices would likely offer them better hearing at a younger age and improved socio-linguistic development. Development of more patient optimized and powerful transducers will expand current indications for implantation.

A variety of approaches to regeneration or repair of auditory sensory and related structures are being investigated to restore hearing. Future hearing devices may be developed consisting of an acoustic hearing aid and a linked implanted component capable of eluting chemicals or signals for enhancement of this regenerative process. Drug delivery would be enabled through the implant, to preserve neurons or even promote the growth of neurites (toward the electrode array) from existing neurons or regeneration of neurons and associated structures [38, 39]. In 2002, the Free Electron Laser was used to conduct initial experiments on optical stimulation of a peripheral nerve [40]. This concept was later transferred to the cochlea and stimulation units have been further miniaturized and an implantable unit for stimulation in cat models is currently under work [41]. Necessary safety studies are underway that has led to the development of an optogenetic implant unit that

#### *Audio-Vestibular Neurosensory Prosthetics: Origins, Expanding Indications and Future Directions DOI: http://dx.doi.org/10.5772/intechopen.95592*

is being used in clinical trials. The feasibility of supplementing the ABI array of surface electrodes with penetrating microstimulating electrodes has been demonstrated in animal studies and human trials involving using this combination is currently ongoing. The initial results of the first AMI patients have been encouraging in terms of the ability to implant the array into the auditory midbrain safely and to restore some hearing function. Future work in this area will needed to overcome the primary limitation of midbrain stimulation which is optimal placement of the electrode array. Research is also currently underway on creating a totally implantable vestibular implant in which all the components of the implant could be internalized. Advances in miniaturization, microelectromechanical systems (MEMS), nanotechnology, battery technology and circuit energy efficiency could help realize this goal. A vestibular brainstem implant could prove helpful for bilateral vestibular hypofunction and such a device could be potentially combined with an auditory brainstem device [29].

Thus, there are new development efforts in this field that will either significantly improve prosthetic performance or change the face of auditory prosthesis altogether. There is also the possibility that auditory prostheses will be integrated with other peripheral and central prostheses (eg. vestibular and deep brain implants) to treat not just one symptom but to address its whole spectrum. Finally, the progress in neuroscience, particularly in non-invasive brain monitoring will allow a full account of individual variability in both the optimization & the performance of prosthetic hearing devices.
