Cochlear implant science and medical materials and immunity The country has a strict classification for medical devices, and Class III medical devices are the highest level of medical devices and must be strictly controlled. They are medical devices that are implanted in the human body, used to support and sustain life, and are potentially dangerous to the human body, and their safety and effectiveness must be strictly controlled. The cochlear implant in the field of hearing medicine is a representative of implantable artificial organs. The State Food and Drug Administration (SFDA) is responsible for the approval of cochlear implants, and the provincial drug regulatory authorities are not allowed to do so. As an “implantable” medical device, the material safety of the cochlear implant (specifically, the implant) is of primary importance. Therefore, in this article, we will provide a general introduction to the materials involved in cochlear implants. The immune system is our body’s “defense system”. It has three main functions: defense, surveillance and stabilization. It establishes three lines of defense for the body: mucous membranes and secretions in the skin, bactericidal substances and phagocytes in body fluids, and immune organs and cells. Simply put, it destroys the invading harmful substances; clearing itself of tumor cells, senescent cells, dead cells or other harmful components. If the immune system is defeated, you will be seriously ill or even die. The human body’s immune system is still very strong and sophisticated. The reason some children are told by their doctors that they don’t need antibiotics after a cold is that they can recover by relying on the body’s own immune response. And we also see some children who are often given antibiotics or a bottle, have a low autoimmune system and are often sick. In the news, we often see patients needing a kidney transplant or a bone marrow stem cell transplant, and they need to be “matched”. Why is it necessary to match? Because only if the match is successful, the immune system will think that what is implanted into the body is their own, otherwise there will be a “rejection reaction” and the immune system will eliminate what it thinks does not belong to them. With that said, it is clear why the material of the cochlear implant is so important as an implantable medical device. This is because if the material chosen for the implant is not acceptable to the human body, a rejection reaction may occur after subcutaneous implantation, leading to implant failure. The four latest cochlear implant products in the Chinese market are the CS-10A implant from Norcon in China, the HiRes90K from AB Cochlear in the United States, the CI512 from Cochlear in Australia, and the Concerto from MED-EL in Austria. The implant consists of a receiving coil, a receiving decoder, a stimulator, and an electrode array. The coil is made of gold or platinum, the small box where the chip is placed and the electrode array is titanium, and the outermost wrapping is made of silicone. The coil material is gold or platinum. We don’t need to talk about the conductivity and corrosion resistance of gold or platinum here, but the use of gold or platinum in the design of cochlear implant coils is for the “ductility” and “fatigue life” of these two metals. We know that gold and platinum are very ductile, and it has been reported that 28 grams of gold has been used to draw wire up to 65 kilometers long. In cochlear coils, the wire needs to be very thin and not easily broken, so gold or platinum is generally used. It is said that during the development of the domestic cochlear implant, the famous jewelry manufacturer “Laofengxiang” also sent its veteran metal wire drawing workers to help process the cochlear coils. The electrode part of the Australian cochlear implant has 22 tiny titanium alloy electrodes embedded in silicone, one of which is a channel. The outer wall of the human cochlea is covered with auditory hair cells, and different auditory hair cells are responsible for collecting different frequencies of sound. If we imagine the auditory hair cells as a piano, from low to high, there are 88 keys corresponding to 88 frequencies; and now the requirement is still such a large range, but only 24 keys, then the frequency difference between two keys increases and the sound richness is poorer. So the design requirement of cochlear electrodes is to inlay as many electrodes as possible in this short 16mm, and each electrode corresponds to the corresponding frequency to stimulate the auditory nerve. However, due to the level of production, the complexity of wiring and patent protection, and to prevent too many electrodes from interfering with each other, the maximum number of cochlear implants is currently 22 channels. Perhaps with future developments in nanotechnology, more electrodes can be made? This is just a thought for now. However, from some literature, 8 electrodes or more will give a person the sound information needed to listen and speak, and most current cochlear implants are 12 channels or more. In addition, the American cochlea has creatively invented virtual channel technology, which uses one independent power source per electrode, making 8 points of signal intersection between each electrode gap, resulting in 8 x (16-1) = 120 virtual channels. Currently, the encapsulation methods used to encapsulate the implant part of the microprocessor chip are titanium silicone encapsulation and bioceramic encapsulation (an early product in the United States and Austria, which is no longer used). Both materials have their own characteristics, where bioceramics allows the transfer of large amounts of information with only very low energy and light weight. Possible causes of failure include mainly implant leakage due to head trauma caused by impact and shell rupture or inelastic deformation of the implant. However, the ceramic here is not the same as the ceramic used in our mugs in life and will not be so fragile. Ceramics are already used in many fields, and even some engine housings are made of ceramics. Titanium alloy is recognized worldwide as an excellent metal material in the biomedical field. Its inherent good properties such as light weight, low modulus of elasticity, non-magnetic, non-toxic, corrosion resistant and high strength ensure bioadaptability and protection from external extrusion or impact. In addition, the encapsulation process of cochlear ceramics is very demanding, while titanium is easy to encapsulate, making it relatively easy to promote production. Currently, all new cochlear implant manufacturers are using titanium for their new chip packages. For implants, the chip package needs to take into account what happens if the implantee hits his or her head. So on the one hand, it is important to minimize the size of the implant and reduce the amount of surgical bone grinding; on the other hand, it is important to increase the impact resistance of the encapsulation material as much as possible. The impact resistance of the HiRes90K implant is 6 joules, which is the equivalent of a 1-pound iron ball falling from a 1-meter high position without damaging the chip. In the whole implant’s outer layer, medical silicone is wrapped. It has good biocompatibility and the human body rarely shows rejection reaction after use. In cosmetic plastic surgery, medical silicone is widely used as a material for breast augmentation and buttock augmentation.