4.1 Introduction

The cochlear implant (CI) has been the top of the line and the gold standard used to aid in the treatment of deafness in children and adults. CIs are also used in cases where infants are born deaf. These implants are one of the greatest successes in the field of neurobionic prostheses because they are technically highly sophisticated and effective [1]. Since they are used so frequently and are implanted into a diversely aged population, the biomaterials of the implant and surface materials used are regarded as highly important [2]. The biocompatibility of these materials is crucial to ensure that the risk of bacteria spreading into the cochlea is mitigated. These implants are within the area of the middle‐ear mucosa in conjunction with the perilymph of the cochlea so if there is any bacteria or the material is not biocompatible, there is a risk of an infection forming and the implant being rejected by the body [3]. CIs have evolved from a single‐electrode device to a multielectrode device with digital signal processing which is much more complex. Modern CI devices can now provide not just sound awareness for deaf people but can also provide good speech comprehension and allow the ability to speak on the telephone [4, 5]. These devices can also be implanted in pediatric population and can even be implanted in children less than 12 months of age [5, 6]. However, there are some eligibility requirements that need to be met in order to qualify for CI devices, and currently, children with inner ear malformation (IEM) are not eligible because the cochlear nerve and the inner need to be normal in order for these devices to work [7]. However, with new flexible biomaterials, it could be possible to treat this population group as well [7]. Just 50 years ago, no effective treatments for deafness and severe losses and hearing existed, and within this time, the immense development of CI devices has been rapid and effective. Early CI devices have been met with great skepticism and highly criticized in the past; however, they have become life‐changing for the deaf population. For children who were born deaf, these devices can give them a better quality of life from the beginning [8, 9]. Although the materials and design are already highly sophisticated, there can always be improvements made, which can increase the effectiveness of these devices [3]. The surface component that communicates with the tissue and the material of the surface influences if the device successfully functions or not. This material has to be very strong, light, and resistant to corrosion because it will be in contact with body fluids for all of the patients’ life. Since a foreign body response is common when it comes to any implants in the body, the goal that is needed to be achieved by the use of biomaterials is to reduce the bodily response to foreign materials in the body by making them as biocompatible and bioinert as possible for patients of all ages [10, 11]. The material used for CI implant electrode contacts is platinum. The casing is made of titanium, and the coating of the electrode is made of silicone. Enhancements of these materials can be made to make improvements to the implants [10, 12]. Some of these enhancements include replacing the materials with biomaterials, enhancing existing materials, and coating the surface.

Surface biotechnology has been a recent, highly successful advancement in the design and functionality of these devices. Irrespective of the actual material of the device, surface modifications are what allow an efficient performance of the device. The surface modifications allow the surface of the implant to initiate reactions after implementation, communicate with the living tissue surrounding it, and ensure proper tissue formation. The study and use of surface modifications have been researched very thoroughly in the past; however, neuro‐stimulative restoration and audio regeneration have been just recently begun research and implementation. There are four ways to enhance CIs, and they are to substitute materials that are currently being used with superior biomaterials, modify the current implant surface to be more bio‐functional at the material–tissue interface so that modulating biological responses can occur without having to change the material attributes, introduce a coding which is normally made of a different material from the underlying material, and introduce local and sustainable drug delivery [13, 14].

The materials currently used in CI devices are platinum, ceramics, titanium, and silicone. All of these materials are known to be biocompatible, and the main reason these materials are used is due to the low corrosion rate; the strength of the materials is known to be adequate for this application and can function for years without needing a revision or replacement surgery. Another very important necessity is that these materials need to be as hypoallergenic as possible. Some of the clinical requirements to choose these materials were that it must be biocompatible, the surgical technique should be as noninvasive as possible, the electrode array should not cause any additional damage when being inserted and after implementation, efficient electrical stimulation of the auditory nerve should be ensured to be non‐damaging, and there should not be an increased risk of infection caused by the implant [15, 16]. There are biological, mechanical, and electrical aspects to determining the materials used for CI devices [2].

4.2 Biological Requirements

One of the most important concerns with any implantable device is the possibility of developing an infection that could potentially spread through the body. CIs are located in the inner ear with the possibility of contact between the implant electrode arrays and bacteria. Bacteria can travel down the electrode arrays and spread into the inner ear, causing an infection. Pathogens can come in contact with the middle‐ear region from the electrode array, and the risk of biofilm forming is present. This risk can be mitigated by completely and securely sealing the cochleostomy. In very extreme cases, meningitis can occur when bacteria spread from the middle ear to the inner ear or cerebrospinal fluid. Fibrous tissue growth is also possible when inserting a CI device [17]. These risks can have damaging consequences, especially since the implantation could occur in childhood and last all the way into older adulthood [2, 18]. Another major concern with any kind of implant is the need to control the interaction between the implant’s surface material and the tissue surrounding the site of implantation. Adverse effects could occur, such as inflammation and fibrosis, when nonspecific proteins and cell adhesion occurs [19]. Due to these concerns, it is crucial that the chosen biomaterials for these implants have the ability to repel nonspecific proteins and do not allow biofilm to form. A study showed that bacterial biofilm can form on the surface of the electrode array because of device failure and not due to infections. Device failure could occur due to faulty electrodes, body fluids enter the device, and kinking of the electrodes [20].

4.3 Electrical Requirements

CIs use electrical stimulation to provide functional hearing. Due to the electrical components of these implants, it is crucial that the appropriate biomaterial should be used to optimize the function of the implant and minimize the side effects that occur when implanting the device [16, 21]. One of the early mechanisms used in CIs was related to the electric activation of the auditory nerve, and studies have shown that patients reported that they heard a noise‐like sound when exposed to sinusoidal electrical stimulation [22]. They also experienced occasional involuntary movement or activation of facial nerves. The electrical stimulation of the auditory nerve causes a hearing sensation in deaf patients, and because electrical stimulation is an important part in the functionality of these devices, special care is taken to find biomaterials that enhance the electrode of the device [23, 24]. Currents are delivered through the electrodes that are placed in the scala tympani (ST), which is one of the three fluid‐filled chambers of the cochlea [22]. Poly(dimethyl siloxane), also known as PDMS, is a biomaterial that was bulk modified in a study. It was used to develop an intracochlear electrode that fit better along the inner wall of the ST, and it was a hydrophilic material and improved the functionality of the electrode because it allowed a closer attachment to the ST [25, 26]. Another interesting study showed that there are electrodes that can be deeply inserted into the epical regions of the cochlea. Typically, an electrode array extends to 1–1.5 turns from the basal cochleostomy; however, one manufacturer (MED‐EL GmbH) used a much longer electrode that extended the electrode out to the apex of the cochlea. This study showed that it is no benefit to having longer electrodes and it could potentially cause trauma to the intracochlear region [27]. The electrical component of these devices is very important to ensure that the device does not fail and there is no intracochlear damage done to the user and to improve the functionality of the device so that the user can hear a broader range of auditory signals.

4.4 Mechanical Requirements

CI devices have an electrode that is embedded with silicone which is inserted into the cochlea. The insertion process of this electrode can cause damage because of the pressure and shock of the procedure. This electrode is made up of different types of medical grade silicone materials that is biocompatible; however, silicone in the inner ear has not been fully tested [28]. These electrodes are typically made of elastic materials because they are stiff enough, so they can be inserted and flexible enough so that no trauma occurs during implantation. This silicone material is used for the electrode because it is flexible and long and can act as a biocompatible sheath that encases the ultrathin platinum wires. Silicones are polymers that are commonly used in medical devices because of the desirable material properties; however, they can be improved in CI devices because after implantation, the “aging process” starts to occur [29]. Silicone deteriorates within the internal environment of the body due to the constant contact with bodily fluids. There is a potential improvement that can be made to the silicone material to prevent the aging process from occurring within the body.

A mechanical requirement for the implants outer casing is that it should be stable and big enough that it can hold all the electronics, and no fluid should be able to enter the device. Currently, titanium is used as the implant outer casing that holds all the electronic components because they are leak resistant, unlike ceramic materials that were initially used. Another component of the implant, the electrode array, has to be flexible and also mechanically stable so that there is no risk of leakage. The material that the electrode array is made of should not allow any breaks in the cable when the array is bent. This can cause short‐circuiting to occur and cause damage to the inner ear [2].

Material selection is mostly dependent on the design of the electrodes because they are the most crucial component of CI devices. Some new designs of electrodes are now including a soft material to cover the tip of the electrodes to reduce the pressure against the basilar membrane and the outer wall of the cochlea [2, 30].

4.5 New Electrode Biomaterials

4.5.1 Dexamethasone (DEX)

The efficiency of CIs is based on the formation of connective tissue surrounding the electrode array. This tissue grows postoperatively and is thought to increase the impedance of the CI device. Dexamethasone (DEX) is a corticosteroid drug used to treat inflammatory and other disorders [17]. DEX (1% and 10% w/w concentration) was incorporated into the silicone sheath of the electrodes and were implanted into guinea pig cochleae, and the synergistic effect of electrical stimulation was observed. The impedances before and after the electrical stimulation were measured, and auditory brainstem responses when acoustic signals were observed were recorded before and after the implantations, along with certain other days during the study. DEX‐eluting CIs show that it can reduce the connective tissue reaction and can significantly lower the fibrous tissue around the electrode array. This leads to the implantation impedance levels being significantly lower than the control group who did not have the DEX‐eluted CI device implanted. The 10% DEX group showed a greater effect in terms of implantation impedance levels, and the 1% DEX group also showed an improvement when compared to the control group; however, it was not as significant as the 10% DEX group results. Figure 4.1 shows the hearing thresholds in dB SPL (sound pressure level) at the frequencies tested. The frequencies tested were between 1 and 40 kHz. It can be observed that the day 0 post‐OP 10% DEX group performed higher in the hearing threshold throughout all frequencies observed. Figure 4.2 shows the connective tissue formation in the ST with the three groups: 0% DEX (control), 1% DEX, and 10% DEX. Figure 4.2 shows the relationship between the connective tissue % and how the three groups performed. There were significant differences observed in the groups with 1% and 10% DEX. 10% DEX performed the best with the least percentage of connective tissue growth. Through this study, it was determined that DEX‐eluting CIs can reduce the fibrous tissue surrounding the implantation and can lower the impedance levels of the attenuating electrode, thus improving the functionality of the device [31].

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