Novel Cochlear Electrode Array Development Using Microfabrication Techniques
Özet
Hearing loss, which is seen worldwide, can affect people of all ages and their quality of life. The cochlear implant (CI) is a medical device used to correct sensorineural (due to dysfunction of the inner hair cells) hearing problems. CI has a scientific history of about 60 years and a commercial history of about 40 years and is recognized as one of the most successful neuro-prostheses applied clinically. A CI replaces the process of capturing and amplifying the mechanical energy of sound in the normal hearing mechanism and creates sound perception by directly electrically stimulating the auditory nerve (cochlear nerve). Although the regions that these systems stimulate, and accordingly the stimulation parts and software they use, differ, the other parts and their general functioning logic are very similar. All of these systems end with an electrically conductive metal composite structure called the electrode array (EA).
Cochlear implants (CIs) are manufactured and sold on the international market by a few companies. The main obstacle to the low-cost production of these implants, which are implanted thousands of times a year in medium-sized countries around the world, is the costly electrode array manufacturing process, which requires intensive labor and precision. In a controlled environment, many quality control steps and tests are required to achieve high quality standards in the production process, which involves soldering electrically conductive wires with a diameter of 20–30 µm onto the stimulation electrodes one by one under a microscope with the dexterity of a specialist worker, and micro-injecting a silicon carrier. This significantly increases the unit cost of the cochlear implant. The cost of the implant therefore limits access to cochlear implants around the world. Today, CIs are accepted as clinically successful. However, the narrow frequency range, inability to distinguish sounds in crowded environments, high cost, and long production process are significant drawbacks of this technology. Moreover, in recent years, there has been a notable advancement in microfabrication and flexible electronics methods, which has led to an enhanced feasibility of flexible thin film fabrications containing electrode arrays. As a result, researchers have turned their attention to developing thin-film electrode arrays (TFEAs), which could provide a cost and performance effective alternative to conventional electrode arrays (CEAs). However, TFEA manufactured using Micro Electro-Mechanical System (MEMS) fabrication techniques has not yet found clinical application, primarily due to significant disparities in their structural and mechanical properties compared to CEAs.
Efforts also have been made to design electrode arrays with features that minimize the forces exerted during insertion inside the cochlea. This includes optimizing the shape and profile of the electrode wires to reduce the risk of damage to the delicate structures of the inner ear. Recently, the cochlear implant producers have released a series of different cochlear arrays where there are variations in the shape of electrode wires, such as incorporating a wavy or curved pattern. Although there are no studies in the literature that address this issue in terms of elasticity and biotribology, recent artificial model and clinical studies have demonstrated that this structure increases the likelihood of atraumatic deep implantation into the cochlea without damaging the basilar membrane or other basic neural structures.
In the framework of the above-mentioned context, this thesis presents the design, fabrication and characterization of a flexible thin film electrode array (TFEA) for cochlear implant, which offers several advantages over the conventional CEA. Moreover, the shape of the electrically conductive wires on the mechanical and tribological properties is also investigated for both CEA and TFEA.
In the first part of the thesis, the CEA was designed as five different composite structures with eight ball-shaped electrodes and different structures (straight/wavy) of the wires inside. While the periods of the wavy wires were changed, their amplitudes were kept constant. CEA designs are made as conical cylinder elastomer body without wire (CEA -1), wavy wire embedded elastomer bodies with different periods of 1.6 mm (CEA-2), 2 mm (CEA-3), 2.4 mm (CEA-4) and straight wire embedded elastomer body (CEA-5). These designs aim to strike a balance between rigidity and flexibility, potentially improving the ease of insertion and contact with cochlear structures. It is also shown that these wavy structures provide additional flexibility with change of the composite stiffness in longitudinal and transverse direction, which can be advantageous in conforming to the curves of the cochlea.
Next, TFEA is designed as eight or twelve electrodes on conical and rectangular thin polymer film, respectively. Since the TFEA has a plane structure initially, different carrier designs have been realized to provide flexibility to the electrode array and to transform it into a three-dimensional form in order to move in the cochlea. Accordingly, half conical cylinder elastomer body (carrier) (TFEA-1), rectangular thin film composite structure mounted on half conical cylinder elastomer body (TFEA-2), straight steel wire embedded in this composite structure (TFEA-3) and sinusoidal wave copper wire embedded (TFEA-4) TFEA models and half tapered cylinder-shaped thin film embedded in full conical cylinder elastomer body (TFEA-5) TFEA model are designed. The dimensions of all design TFEAs are identical in final form, with only the components differing, and are physically compatible with commercially available CEAs.
The mechanical properties of electrode arrays are important for their placement in the cochlea. Therefore, Finite Element Analysis (FEA) is performed to determine the elastic properties of various designs. The displacement behavior of the CEAs and TFEAs under large deformation conditions in both longitudinal and lateral directions is investigated, focusing on each direction separately. The boundary conditions for the FEA analysis are determined as the tip of the EA is guided while the base part is fixed. When the CEA designs are analyzed, the highest stiffness in both directions is observed in CEA-5. Accordingly, when the wire is wavy, it becomes a more flexible structure. As the period of the wavy wires decreases, the stiffness decreases even more. When the designs of TFEA are analyzed, it is observed that adding a dummy wire to the carrier of TFEA increases both longitudinal and lateral stiffness. However, the highest stiffness in both directions is observed in TFEA-5. The increase in the cross-sectional area of the material also increased its stiffness.
CEAs are manufactured with 8 electrodes as in the design and different shapes (straight or wavy) in the wires inside the body. To make the wires wavy, two wavy apparatus with the same period are used. A straight wire is placed between these two parts and the desired wave shape is created by applying constant pressure to the parts. Traditional soft molding method is used for the fabrication of CEA.
TFEAs are fabricated with 8 and 12 metal electrodes, respectively, on the surface of a conical and rectangular planar polymer thin film similar to the design. There are two main microfabrication steps that are applied to obtain the TFEA. In the initial step, a planar thin-film electrode array is created using lithography techniques in a clean room environment. Next, a soft molding process is employed to integrate a carrier with the thin film, transforming it into a three-dimensional shape. This transformation enables the TFEA to navigate effectively within the cochlea. In the first fabrication stage, the polymer film, which will form the substrate of the TFEA, must first be fixed to the Si wafer for microfabrication but must be easily separated at the end of the process. This is one of the most challenging processes as any bubbles or wrinkles between the film and the substrate will result in a failed development process in the lithography step. After obtaining the Titanium (Ti) / platinum (Pt) for the 8-electrode thin film and titanium (Ti) / aluminum (Al) for the 12-electrode thin film, soft molding method is used to manufacture and integrate the carrier part of the TFEA. Two separate processes are used for half and full cylinder carriers, with the full cylinder carrier manufactured in a unique two-stage molding process.
A custom-built experimental setup is employed for the purpose of conducting stiffness measurement tests, which are designed to examine the mechanical properties of the EAs. While CEAs generally produced results similar to those observed in FEA, CEA-3 exhibited an initial increase in force in the horizontal direction followed by a decrease. The TFEA-2 configuration exhibits the highest stiffness in the lateral direction, while the TFEA-5 configuration demonstrates the highest stiffness in the longitudinal direction for the tapered cylinder. It is observed that both the CEA and TFEA exhibit increased flexibility when the wires have a wavy pattern.
The friction dynamics of EAs of cochlear implants are investigated experimentally using novel test-setups. To understand the friction dynamics of electrode arrays with artificial surfaces, flat surface and artificial cochlea models are studied at different preloads, speeds and environments (dry or wet). Firstly, a flat glass surface is used as the contact surface to study the friction characteristics of the tip of the EA between the surface and the contact. The friction loop is used to understand the kinematic and deformation profile of EAs during tribological tests. The experimental evidence demonstrates that the addition of a dummy wire to the carrier of the EA results in enhanced longitudinal and transverse stiffness. Therefore, the reinforcement of the structure enables it to withstand elevated normal loads prior to buckling, thereby postponing its collapse. Furthermore, it delays the initiation of dynamic sliding due to the elevated buckling limit. This delay is particularly important in cases where static friction dominates, as it facilitates a gradual transition to the dynamic shear zone. Consequently, it is demonstrated that this approach strengthens the structure against buckling and optimizes its performance and stability during dynamic sliding inside the cochlea.
As the preload forces between the flat glass surface and the tip of the EA increase, the friction force increases. The highest friction force and insertion forces are found in EAs with straight wires. The waviness of the wire decreases the friction and insertion force. The type of TFEA support is found to affect both normal and frictional forces as well as the coefficient of friction. TFEA-3 is found to have a higher friction force compared to TFEA-2 and TFEA-4. This difference is attributed to the higher hardness and adhesion of TFEA-3. Also, changes in friction speed lead to changes in friction force. TFEAs generally show an increase in friction force with increasing friction speed. Friction forces decreased in wet friction experiments compared to dry conditions.
Subsequently, further experiments were conducted utilizing an artificial cochlea to elucidate the reaction forces exerted on the round window, which is the first contact point of the TFEAs when placed in the cochlea. The experiment is carried out for dry and wet conditions as in the first experiment. Under wet conditions, the insertion force values are lower than under dry conditions. However, a non-linear relationship between insertion speed and insertion force is observed. CEA-5 has the highest insertion force, while CEA-3 and CEA-4 have similar insertion forces. The fact that the steel wire in TFEA-3 is close to the end of the EA prevents it from getting stuck and travelling in the cochlea. TFEA-2 bends easily during insertion in the cochlea and cannot advance. Although TFEA-4 is easier to place than the other two samples, the number of wires in the carrier can be increased to ensure complete placement. TFEA-5 is placed into the cochlea more easily and advanced more than the other TFEA samples. Compared to TFEA-2 with the lowest insertion force, TFEA-4 and TFEA-5 have higher insertion force but close to the values of the CEAs. It is also revealed that the type of TFEA support structure affects both normal and frictional forces, as well as the coefficient of friction.
