**Abstract**

In this chapter, we present two novel optical wireless-based cochlear implant architectures: (i) optical wireless cochlear implant (OWCI) and (ii) all-optical cochlear implant (AOCI). Both the architectures aim to decisively improve the reliability and energy efficiency of hearing restoration devices. To provide design and development guidelines, we document their main components, discuss the particularities of the transdermal optical channel, and provide the analytical framework for their accurate modeling. Building upon this framework, we extract closed-form formulas that quantify the communication, the stimulation, and the overall performance. An overall comparison of OWCI and AOCI, as well as conventional cochlear implants, accompanied by future research directions summarizes this chapter. Our findings reveal that both the OWCI and the AOCI outperform conventional cochlear implant approaches; thus, they are identified as promising architectures for the next generation of cochlear implants.

**Keywords:** all-optical cochlear implants, biomedical applications, cell stimulation, neural stimulation, optical wireless cochlear implants, optical wireless communications, optogenetics

## **1. Introduction**

The healthy ear functions much like a receiver (Rx) of acoustic signals, which can be described as time-varying pressure waves in a specific frequency range (20–20,000 Hz). These signals propagate toward the cochlear, which analyzes them based on their spectral content. Specifically, each pressure wave traveling inside the cochlea not only actuates inner and outer hair cells at different locations along its length based on the frequency components of the wave, but also determines the intensity of the perceived sound according to the amplitude of the wave [1]. The various spiking characteristics of the spiral ganglion neurons, such as spike rate, number, and location, encode the amplitude and frequency of the sound.

The most common sensory defect is hearing loss, which plagues more than 466 million people around the world and is mostly caused by cochlear abnormalities [2]. When unaddressed, hearing loss can negatively impact the quality of life in various

ways, such as social isolation, limited education, and unemployment, which are estimated to cost 980 billion dollars annually. To counterbalance this, substantial research effort has been directed toward neuron regenerative techniques, such as pharmacological, gene, as well as cell therapies [3, 4]. Unfortunately, none of the aforementioned approaches is considered to be close to clinical use. Therefore, the most successful hearing restoration approaches to this day are based on cochlear implants (CIs). Of note, CIs can be used in almost all forms of hearing loss.

Conventional CIs are comprised of two parts: one external and one implanted. The former houses a sound receiver and the processor, while the latter contains the stimulation unit. Specifically, the captured sound signal is decomposed to its major frequency components that are assigned to the corresponding channels of the stimulation unit. Each channel delivers the electrical stimulation signal to the spiral ganglion neurons that match the frequency content of the decomposed electrically encoded sound signal. However, due to the relatively high electrical conductivity of the cochlea, the applied electrical stimulation spreads to nearby spiral ganglion neurons, thus stimulating wider spectral windows than the appropriate one. In conjunction with their low-dynamic range [5], conventional CIs offer limited spectral and intensity sound encoding, which is proven to be detrimental for their hearing restoration capabilities [6].

In this chapter, we introduce the major advances that paved the way for the revolution of CIs and the realization of hearing restoration. Initially, we investigate the current state of the art of hearing restoration through CIs. Next, an in-depth analysis of most promising techniques of light-based hearing restoration is presented. Finally, we offer design guidelines as well as future directions for the next generation of CIs.

## **2. Background**

To aid the reader in understanding the requirements of hearing restoration, we provide some background that covers the CIs' evolution since their conceptualization as well as the current research progresses toward the next generation of CIs (**Figure 1**).

#### **2.1 Evolution of CIs**

The concept of hearing restoration through the electrical stimulation of the auditory nerve was conceived by André Djourno and Charles Eyriés in 1957. In their attempt to restore the functionality of the facial nerve through electrical signal applied via a wire, the deaf patient experienced auditory sensations [7]. Based on these findings, multiple attempts were made around the world to develop the first CI with William House performing the first implantation in 1972 [8, 9]. Moreover, the first cochlear implant manufacturing company was founded in 1982 under the name MedEl Corporation, closely followed by Cochlear Limited in 1984, and Advanced Bionics in 1996.

Since their creation, CI companies have iteratively updated their architecture designs, hardware, and optimizing stimulation techniques. The first generation of CIs was released in the early 1980s and included Nucleus 22 and Comfort CI, combined analog signal processing strategies with a multichannel stimulation unit that housed 22 and 4 channels, respectively. These designs were followed by the initial model of Advanced Bionics called Clarion in 1996 that was encased in a ceramic case, contained *Hearing Restoration through Optical Wireless Cochlear Implants DOI: http://dx.doi.org/10.5772/intechopen.104622*

**Figure 1.**

*The evolution of CIs from 1982, when the first CI manufacturing company was founded, until the current stateof-the-art research that validated the feasibility of optogenetics-enabled optical CIs.*

eight channels, and used rechargeable batteries. The second generation included Clarion II, Nucleus 24 Contour, and Combi 40+. These were introduced in the market with 24 electrodes and new sound processors with novel features such as precurved electrode arrays, backward compatibility, frequency modulation capabilities, dual electrodes, and behind-the-ear external components. However, in the early 2000s, completely redesigned highly customizable CI models, namely, Freedom, Pulsar, and HiRes90k, were developed. Their modularity and customization options were the distinguishing factors for these new models that were available in straight or precurved, standard, medium, condensed, and split electrode array architectures, based on the individual particularities of the cochlea of each patient. Moreover, these electrodes were encased in flexible plastic and housed a plastic tip that enabled nontraumatic implantation. In the era after 2010, the latest iterations of CIs have been focused toward higher fidelity sound that enhances the perception of music through state-of-the-art sound processing, wireless control, and software-enabled programability, as well as waterproof designs.

#### **2.2 What the next-generation CI should be?**

The utilization of light-based communications and stimulation has been proposed as a promising alternative for electrical hearing restoration techniques. The superior communication performance of optical wireless communications in trandermal applications revealed the benefits that can be achieved by utilizing light for the communication between the external and implanted components of CIs [10, 11]. Moreover, optogenetics was initially reported by Izzo et al. [12] and has been proven achieve more efficient coding of the spectral information of sound due to its higher temporal confinement compared to the electronic stimulation techniques [13–15]. Although optical stimulation has great potential, it exhibits increased energy requirements for achieving the actuation of spiral ganglion neurons, and thus, future research is necessary for developing more energyefficient techniques [16]. Finally, the combination of optogenetics and optical wireless

communications offers great promise for the realization of an all-optical CI architecture capable of achieving unprecedented performance [17].
