Bryan J. Norton

Analytical approaches for determining heat distributions and thermal criteria for infrared neural stimulation

Abstract. Infrared neural stimulation (INS) is becoming an important complementary tool to electrical stimulation. Since the mechanism of INS is photothermal, describing the laser-induced heat distribution is fundamental to determining the relationship between stimulation pulses and neural responses. This work developed both a framework describing the time evolution of the heat distribution induced by optical fluence and a new method to extract thermal criteria (e.g., temperature change and rate of change) for neural activation. To solve the general problem of describing the temperature distribution, a Green’s function solution to the heat diffusion equation was determined and convolved with the optical fluence. This provided a solution in the form of a single integral over time, from which closed-form solutions can be determined for special cases. This work also yielded an expression for thermal relaxation time, which provides a rigorous description of thermal confinement for INS. The developed framework was then applied to experimental data from the cochlea to extract the minimum temperature increase and rate of that increase to stimulate the cochlear spiral ganglion. This result, and similar analyses applied to other neural systems, can then shed light on the fundamental mechanism for INS and aid the development of optical neuroprostheses.

Laser source development for infrared neural stimulation

Infrared neural stimulation (INS) has emerged as a complementary technology to electrical stimulation with greater precision and no stimulation artifact. Many studies have been performed with Lockheed Martin Aculights (LMA) Capella laser. To explore all optical parameters useful for INS, LMA has produced alternate versions of Capellas operating at a variety of wavelengths, and initial tests indicate their utility. In an effort to begin miniaturizing laser sources for INS, LMA has developed three channel, wearable laser packs for chronic experiments that have been used for up to three months. To even further miniaturize laser sources toward implantable use in humans, LMA has partnered with Vixar, Inc. to develop vertical cavity surface emitting lasers (VCSELs) operating at 1860nm. To date, these devices have produced over 60 mW from a form factor of a few hundred microns, and have shown a clear path toward achieving parameters needed for use in neuroprostheses.

Green’s function representation of laser induced thermal dynamics and determination of thermal criteria for optically induced neural activation

Infrared nerve stimulation (INS) is rapidly becoming an important tool for basic research and a promising new clinical technology to selectively activate nerves to restore function, map the nervous system, and perform diagnostic procedures. To the best of our understanding, the mechanism of stimulation is photothermal; thus, describing the laserinduced heat distribution is fundamental to determining the relationship between stimulation pulse and neural response. This work develops both a framework describing the time evolution of the heat distribution induced by optical fluence and a novel method to extract thermal criteria for neural activation. We are first concerned with the general problem of describing the temperature distribution in a homogenous medium. To this end, we determine a Green’s function solution to the heat diffusion equation and convolve it with the optical fluence. This provides a general solution to the thermal problem of interest in the form of a single integral over time. Other useful closed form solutions can be determined for interesting special cases. This pursuit also yields an expression for the thermal relaxation time, which provides a rigorous description of thermal confinement for INS applications. The insight we gain from this framework allows us to extract thermal criteria for neural activation from experimental data. Our work provides both insight into the mechanism for stimulation and understanding sufficient to aid in the development of INS devices. Thermal criteria values will prove useful for choosing parameters such as spot size, pulse width, stimulation spacing, and stimulation depth in future INS applications.

Laser induced mortality of Anopheles stephensi mosquitoes

Small, flying insects continue to pose great risks to both human health and agricultural production throughout the world, so there remains a compelling need to develop new vector and pest control approaches. Here, we examined the use of short (<25 ms) laser pulses to kill or disable anesthetized female Anopheles stephensi mosquitoes, which were chosen as a representative species. The mortality of mosquitoes exposed to laser pulses of various wavelength, power, pulse duration, and spot size combinations was assessed 24 hours after exposure. For otherwise comparable conditions, green and far-infrared wavelengths were found to be more effective than near- and mid-infrared wavelengths. Pulses with larger laser spot sizes required lower lethal energy densities, or fluence, but more pulse energy than for smaller spot sizes with greater fluence. Pulse duration had to be reduced by several orders of magnitude to significantly lower the lethal pulse energy or fluence required. These results identified the most promising candidates for the lethal laser component in a system being designed to identify, track, and shoot down flying insects in the wild.

Laser dosimetry for disabling anopheles stephensi mosquitoes in-flight(Conference Presentation)

The Photonic Fence is a system designed to detect mosquitoes and other pestilent flying insects in an active region and to apply lethal doses of laser light to them. Previously, we determined lethal fluence levels for a variety of lasers and pulse conditions on anesthetized Anopheles stephensi mosquitoes. In this work, similar studies were performed while the bugs were freely flying within transparent cages. Dose-response curves were created for various beam diameter, pulse width, and power conditions at 455 nm, 532 nm, 1064nm, and 1540 nm wavelengths. Besides mortality outcomes, the flight behavior of the bugs and the performance of the tracking system were monitored for consistency and to ensure that they had no impact on the mortality outcomes. As in anesthetized experiments, the visible wavelengths required significantly less fluence than near infrared wavelengths to reliably disable bugs. For the visible wavelengths, lethal fluence values were generally equivalent to those found in anesthetized dosing, while near infrared wavelengths required approximately twice the fluence compared with anesthetized experiments. The performance of the optical tracking system remained highly stable throughout the experiments, and it was found not to influence mortality results for pulse widths up to 25 ms. In general, keeping energy constant while decreasing power and increasing pulse width reduced mortality levels. The results of this study further affirm the practicality of using optical approaches to protect people and crops from flying insects.