## 3 Typical Requirements of Implant Systems  
When designing an implantable electronic system, several general requirements are to be addressed, namely minimal size and weight, low power consumption, good reliability, high biocompatibility and minimal toxicity, high data rate and data latency. As the case with any commercial product, the design of the implantable devices is heavily influenced by the demands and preferences of their consumers. In addition to being less invasive to the body of the patient during the implantation, smaller and lighter devices are likely to result in less pain and discomfort to the host during healing and use. The excessive size and weight may be detrimental to the healing process by putting pressure on the adjacent tissues that have already been damaged as a result of surgery, contributing to the inflammatory processes within the peri-implant space. Small and light devices are less restrictive in terms of normal level of human activity, and thus afford better quality of life to the patients. The power source and encapsulation components remain the major contributors to the overall weight and size of the device, whereas the electric circuitry components have decreased dramatically with the advancements in MEMS and nanotechnology. Coupling capacitors used to ensure charge-balance and effectively minimize current leakage may further increase the volume of the implantable module [54; 55]. Lower power consumption is important in terms of both the long-term performance of the device and the safety to the patient. Close proximity of the electrodes to living tissues places firm restrictions on the amount of dissipation in power an implanted electronic system should not exceed, as extensive dissipation may inflict damage onto these soft tissues [56]. In addition to thermally-induced damage [41], the electrical stimulation-induced tissue injury (overstimulation) and damage due to the electrochemical products released into physiological medium as a result of electrode corrosion should be considered [54]. The energy use by interface electronics should also be minimized to ensure longevity of the implants with single-use batteries, as the replacement of such a device would require a costly and invasive surgical procedure [57]. Although using a rechargeable battery may address the need for battery replacement surgical intervention, the need for frequent charging may be inconvenient, time- and resource-consuming activity. For battery-less devices powered by an RF link, the low power restriction is also applied to ensure the electromagnetic energy radiated or backscattered by the device during wireless communication is in line with the IEEE human tissue exposure standards [29]. Excessive electromagnetic fields can potentially undermine correct device functioning, leading to temporary device malfunction or permanent damage. Indeed, device reliability is paramount, as failure may not only cause discomfort, pain, or local damage to the peri-implant space, but may in some cases result in the irreversible damage or death of the patient. Considering that many implants are introduced deep into the tissues and cavities of the body, device maintenance is complicated, with risks to the health of the patient. It is important to note that the presence of a neurostimulation system may limit the electromagnetic diagnostics and treatment, e.g., magnetic resonance imaging (MRI), to which the patient can be exposed [58]. Heating, magnetic field interactions, induced currents, and interference with correct functioning of the implanted modules may result in considerable temporary or permanent damage, e.g., transient dystonia, paralysis, coma, or death [59; 60; 61]. The electrode material and structure should be selected so that during stimulation, sufficient change can be injected to elicit the desired response, and that the level of products from irreversible Faradaic reactions that result from this stimulation are sufficiently low as not to damage the surrounding tissues and the electrode itself [54; 56].