Optical Systems Improve Nerve Stimulation

Aculight Corporation, Bothell, Washington

The stimulation of nerve tissue is a technique that is used in both research and clinical applications. Neuroscientists use nerve stimulation to study the fundamental principles of the nervous system and to research Parkinson’s disease, Alzheimer’s disease, and nerve regeneration, among others. Medical professionals use nerve stimulation for everything from pain and depression management to brain mapping. Today’s stimulators use electrical current to stimulate nerves, resulting in significant limitations. Thanks to a novel optical stimulation technique pioneered by Vanderbilt University, Aculight has developed a compact, laser-based neural stimulator that overcomes these obstacles.

Figure 1. Spatial Selectivity in Electrical vs. Optical Stimulation. a) With conventional electrical stimulation all fascicles within the nerve bundle are stimulated simultaneously; b) Optical stimulation can selectively stimulate an individual fascicle.

Vanderbilt University researchers originally conducted optical nerve stimulation studies using a Free Electron Laser (FEL). The FEL allowed broad wavelength studies between 2 and 9 μm, and ultimately led to an optimal stimulation wavelength of 2.1 μm. Although the FEL work helped target the wavelength, the high cost and large size of the FEL prevented its use for research or clinical applications. Vanderbilt researchers then used a lamp-pumped Ho:YAG laser for generating 2.1 μm. While this allowed them to further test the optical stimulation technique, the Ho:YAG required a chiller and a large amount of space compared to existing electrical stimulators. Vanderbilt then approached Aculight to develop a low-cost, compact laser source that could be used in a research or clinical setting.

Figure 2. The Wavelength Dependence of Tissue Absorption and the safety ratio, which is defined as the ablation threshold divided by the stimulation threshold.

The prototype optical nerve stimulator now mimics today’s electrical stimulators in size and operating requirements, and unlike alternative laser sources such as lamp-pumped solid-state lasers, the unit’s air-cooled architecture does not require the bulk or high cost of a water cooling system. In addition, the prototype optical stimulator allows users to control laser pulse width, repetition rate, output power, and wavelength. This wavelength control enables varying penetration depth in nerve tissue, which is a unique feature specific to optical nerve stimulation.

Optical nerve stimulation offers several advantages over traditional electrical nerve stimulation, including the absence of an electrical artifact, spatial specificity, and the ability to stimulate nerves without physically touching them. Specifically, with electrical stimulation, the applied current that stimulates the nerve also conducts along the nerve for some length. This current is known as an electrical artifact and it prevents accurate monitoring of compound nerve action potentials (CNAP) close to the point of stimulation. With photonic stimulation, only the CNAP is present when monitoring the nerve response.

Optical nerve stimulation also allows users to select individual nerve bundles within a nerve trunk. For example, the sciatic nerve has several bundles that control individual leg movements from the hamstring muscle to the toes. Unlike electrical nerve stimulation, optical stimulation can individually target nerve bundles to induce movements in a small subset of muscles rather than the entire leg. Peripheral nerve mapping can lead to benefits for researchers and clinicians in targeting the deficient nerves more quickly and precisely.

Moreover, electrical stimulation requires touching a nerve to administer the current. This can result in accidental nerve damage or, when done repeatedly, unintentional nerve stimulation. Optical nerve stimulation is a non-contact technique, making it safer and more accurate.

Aculight plans to develop the optical nerve stimulator first for research and then for clinical applications. The first clinical application likely will be nerve monitoring for ear, nose, and throat (ENT) surgeries conducted near critical nerves. Long-term applications for optical nerve stimulation could include implantable devices for therapeutic needs. For example, implantable optical nerve stimulator devices could significantly improve the resolution of cochlear implants, enabling the deaf to hear in much greater detail.

This article was written by Jim Webb, project manager, medical products, for Aculight Corporation. For more information, contact Mr. Webb at 425-482-1100, via e-mail at This email address is being protected from spambots. You need JavaScript enabled to view it..