LED-Based optical neural implants

Up to the present, there have been a variety of attempts to both monitor and control the signals in the nerve systems to investigate the brain functions as well as to treat the neurological diseases. Although electrical methods have been widely employed for both the detection of neural activity and the stimulation of the nerves, various other methods have also been proposed, which include optical, mechanical, thermal, chemical, and magnetic neural stimulation and detection. Optogenetics is a field of research that studies the control and probing of neural activities using light stimulation, which gained research momentum in the early 2000s. Among the various techniques for neural modulation, optogenetics has become one of the most powerful research tools to selectively control or modulate the activity of specific types of neurons, which has been impossible for conventional electrical methods. Due to the unique and powerful characteristics such as cell-type selectivity and biosafety, optogenetics and optical neural interfaces have gained increasing interest as a potential solution for brain studies and human neurological diseases, with its application expanding to a greater diversity of areas. In this chapter, we review various optical neural interfaces for implantable systems by considering the device configurations such as type of light source, presence of waveguides, and optical characteristics such as light power and coupling efficiency. Considering the interest for an untethered implantable neural interfaces and recent trend of utilizing LED (light-emitting diode) as an alternative light source in place of laser, two different types of LED-based systems are compared. Systems which use LED for direct illumination of the nerve cells and systems using LED coupled with waveguides are those. Several different approaches are compared in terms of efficiency, fabrication complexity, multifunctionality, and biosafety. In addition to current relevant technologies, existing challenges are discussed to achieve the ideal implantable optical system for practical purposes.