Challenges and Trends of Implantable Functional Electrical Neural Stimulators: System Architecture and Parameters

Electrical neural stimulators (ENS) play a crucial role in medical applications and have emerged as promising therapeutic techniques for neurological disorders. The purpose of designing this device in medicine and research is to provide therapeutic benefits, aid in rehabilitation, restore sensory perception, advance scientific knowledge, and enable personalized treatment approaches. The utilization of ENS can significantly improve the quality of life for individuals with neurological disorders and contribute to advancements in neuroscience. Recently, there has been a growing inclination towards implantable biomedical microsystems, which has enticed biomedical engineers and researchers to integrate and implement electrical neural stimulators using CMOS technology. Thus, we present this review to provide a comprehensive overview of integrated circuits in the ENS field. The review aims to discuss the operational principles, design considerations, and approaches while highlighting advancements, applications, and future directions. The review begins with an introduction, emphasizing the necessity of stimulation and the fundamental principles of the ENS. It then discusses the various stimulation patterns, exploring how these patterns can modify neural circuits and restore normal function, as well as the circuit implementations involved in generating the stimulation. Design considerations specific to stimulators are also discussed. Furthermore, the review summarizes the state-of-the-art circuits and systems for the ENS, employing a top-down approach that covers specifications, circuits, and system design. Additionally, it briefly provides an overview of experimental approaches and results from various biomedical tests. Finally, this review outlines future directions, trends, and challenges for enhancing precision, safety, and patient outcomes in stimulation therapies.