Next-Generation Wearable fNIRS: A Comprehensive Review of Bio-Instrumentation and Hardware Architectures

Comprehensive monitoring of cerebral hemodynamics has led to significant advances in Functional Near-Infrared Systems (fNIRS), particularly in terms of hardware design and development of wearable platforms. These advancements have established fNIRS devices as valuable tools in research and clinical practices; however, most existing literature focuses predominantly on clinical applications or high-level system performance. This review provides a rigorous, bottom-up analysis of bio-instrumentation architectures, evaluating the low-level trade-offs in component selection and circuit design that define modern wearable fNIRS performance. In this paper, we have identified and compared key hardware components of modern fNIRS technologies, including optical sensors, signal conditioning elements, control units, power systems, and communication modules. Significant progress has been made in terms of optical tomography, head coverage and conformity, multimodal integration, hyperscanning, motion tolerance, user comfort, and miniaturization. The paper underscores how systems may have unique architectures although they follow the same foundational principle. It also aims to identify the trade-offs existing in current fNIRS devices. Overall, this paper presents an overview of where we stand in terms of fNIRS development and attempts to trace an outline of the next generation of devices.