## 3 Typical Requirements of Implant Systems
Relatively low voltages of both spontaneous and evoked signals, as well as those produces by the transducer necessitate particular care when designing methods for signal detection, amplification, modulation and transfer. Spontaneous potentials, such as those detected using electroencephalography, electrooculography, electromyography, or electrocardiography, occur naturally within the body and are typically range from less than one \(\upmu\)V to tens of mV range [62]. Potential amplitudes of evoked responses, _i.e._ event related electrical potentials observed in the central nervous system structures as a consequence of a stimulus, are even lower, falling in the less than one \(\upmu\)V to tens of \(\upmu\)V region. The bandwidth of bioelectrical signals ranges from 0.01 Hz to 15 KHz, with low frequency signal (less than 1 MHz) frequently used to wirelessly power up and transfer data from the external module to the indwelling device [62]. Recently, however, implantable electrical systems that function in the Medical Implants Communication Services (MICS) band (402-405 MHz) are being developed, as this band has been expressly designated for implanted medical devices and is only shared with meteorological aids [37]. Surgical placement, orientation, and extraction of the electrodes is intricate, particular where neural system is concerned, and should be designed to synergistically interact with the available stimulation parameter settings to attain the best remedial outcome for a patient [63]. Indeed, given the difficulty in revising the placement of indwelling electrodes, much care should be given to matching the electrode configuration to the stimulation capacity of the stimuli generating module. In general, the specificity of electrical stimulation is restricted due to electrode scaling and physical placement of these onto the stimulated tissues, with some improvement obtained by manipulating the electrical current applied to the tissues, and the volume of tissue being stimulated [64]. For example, current focusing and current steering approaches in cochlear implant systems and deep brain stimulators employ current-controlled stimulation using several autonomous sources of current to attain control over the volume of tissue receiving stimuli [65,66]. Mechanical shaping and deep reactive ion etching were applied to the implantable silicon-based probes used for neural stimulation to minimize the insertion force when introducing multi-electrode arrays into the brain and spinal cord of the animals used in _in vivo_ study [67]. The sensing and recording quality of these arrays were monitored over time, with neuronal spike activity recorded up to 566 days after implantation. Such prolonged implantation has minimal impact on the tissue architecture, as indicated by histopathology evaluation of neurons and astrocytes. The capacity to simultaneously sense and stimulate is highly desirable, as it enables well-tailored, prompt adaptive therapeutics and contributes to our understanding of natural and evoked neural activity [68]. However, in practice, the ability of closed loop neuromodulation devices to detect brain signals is limited, due relatively high amplitude of the stimulation potential compared to the field potential signals used to sense brain activity [50]. Or, in the case of implantable cardiac fibrillators, pacing at fast rates may delay or hinder detection of ventricular tachyarrhythmias [63]. Furthermore, the processes resulting from the stimulation of neural networks are complex, involving both neural excitation and inhibition. Experiments showed that at high frequencies, electrical stimulation resulted in inhibition of subthalamic nucleus activity, while also directly exciting the cell and/or its axon [69].