**5. Discussion**

Several challenges must be solved in open and closed-loop IEMDs, from the point of view of microelectronics engineering. Limitations are imposed from the technical and engineering perspectives, and are combined with limitations imposed by medical regulations, methods and ethical concerns. Some of the relevant open issues are summarized in the following.

• Size: the most important challenge of an implantable system is the size. Any implantable medical device (IEMD) is composed of several electrical modules. Some of these modules consist of off-chip components such as wireless powering modules or the wireless data transmitter. The specification of these modules should be defined in a way that the IEMD system presents an acceptable size. The IEMD weight is a related parameter. Increasing the size and the weight of IEMDs also increases the complexity of the surgery. Hence, for the convenience of the patients, IEMDs should have a minimum number of off-chip components in order to present a minimum size and weight.

A solution for decreasing the size of an implant is to integrate the active circuits as close as possible to the electrode. One method to realize this solution consists of fabricating a silicon-based electrode which allows the active circuits to be implemented on the same silicon or by attaching the active circuitry to the silicon-based electrode using post-CMOS processes.


for Class I devices (lowest risk). Furthermore, biocompatibility is the major part of acquiring an ISO (International Organization for Standardization) standard such as ISO 10993.

• Data storage: in order to increase the accuracy of the seizure detection as well as to provide freedom and autonomy to the patient during the recording period, the implant should store data over a few hours. This feature is important since patients should not have to wear any bulky holder of an external unit (helmet, belt) during some specific activities or during sleeping. Hence, the system should save the recorded data on an implanted memory. If the IEMD is powered by a rechargeable battery, the IEMD should save the recorded data on a non-volatile memory since the IEMD may be turned off by the under-voltage lockout detection circuit.

While many of the aforementioned challenges find solutions at circuit and system levels, we observe a trend aiming at efficiently solving these issues considering the circuit and the system as a whole, i.e., some part of the trade-offs is solved exploiting specific circuit characteristics, while other parts of the trade-offs find solutions exploiting algorithms or taking benefits of system-level improvements.
