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Nerve stimulation gets infrared light treatment

02 Apr 2007

Stimulating nerves with pulses of infrared light could help to restore balance, intensify hearing implants and lead to more advanced prosthetic limbs. James Tyrrell talks with Mark Bendett to discover more about Aculight's role in this up-and-coming research area.

Rather than popping pills, patients of the future may find themselves relying on infrared (IR) nerve stimulation to tackle a range of medical conditions. Increasingly, scientists are looking at the body's neural network as a way of providing localized treatment. The concept may sound far-fetched, but vagus nerve stimulation is available today as an alternative to Prozac for managing depression.

Neurons are typically activated using electricity, but researchers have recently discovered that pulses of IR light can also stimulate nerves. To help drive this work, Aculight has teamed its optics know-how with the research community's medical knowledge and come up with an investigative tool for use in the laboratory.

In simple terms, the fibre-coupled laser device can be thought of as a neurological circuit tester. "You stimulate the nerve at one location and observe what happens at the other end to see if the pathway is open," Mark Bendett, Aculight's director of product development, told OLE. "In a Petri dish you can see how a signal propagates through adjacent cells."

The firm is collaborating with Northwestern University, US, on a programme to optically stimulate the cochlear nerve, which plays a key role in hearing. Today, cochlear implants are electrically activated and feature approximately 10 channels that help to recreate the sensation of hearing, but the technology appears to have hit a barrier. "Right now in a cochlear implant, the real limitation is current-spreading," said Bendett. "Even if you put 100 electrodes into the cochlea, most of them would crosstalk and you wouldn't gain any more fidelity."

It turns out that one of the big advantages of optical stimulation is the technique's high spatial accuracy. "You don't have the problem of current spreading because the process involves photons hitting the target and not electrons, which are conducted by water in the body," he added.

The team expects IR stimulation to offer similar benefits when applied to the ear's vestibular system. "As we get older our vestibular system degrades, which can lead to a lack of balance," explained Bendett. "I think that this could be one of the largest complaints that we will see with an aging population."

Origins of the technology
Researchers at Vanderbilt University, US, were some of the first to investigate the effect of pulsed IR light on neural activity. "They demonstrated the basic concept using a free-electron laser," explained Bendett. "You want a wavelength that is absorbed sufficiently to stimulate the nerve without ablating the tissue and it turns out that 1.85 µm is one of those wavelengths."

The group approached Aculight to provide a portable light source. Bendett and his colleagues responded with an engineering prototype based on a diode laser array. Two years later, Aculight released its production version dubbed Capella that could be used straight out of the box. "The current model has a tremendously rich set of features," said Bendett. "Users can switch between trains of pulses and single and continuous formats using front knob controls."

One of the challenges that the designers faced was the need to balance coupling efficiency with the amount of power on target. "We want to maximize the efficiency of the coupling from the laser to the fibre, but at the same time we need to contain the light to a sub-cellular level of about 10 µm or so," revealed Bendett. "This can be done using a variety of special optics or tapered fibres, but the trick is to do it cost-effectively."

Aculight offers the Capella in both high- and low-power configurations. "As you would expect, it's a trade-off between pulse energy and speed," said Bendett. "We can actually semi-customize the unit to match the application, so if you are working on a smaller nerve and need less optical power then we can give you sharper rise times."

Although currently an investigative tool, the firm expects the technology to evolve into a fully implantable device. "We believe that this will end up in curative applications," Bendett said. "It's further down the road, but we can see uses in prosthetic limbs where you might do the stimulation optically and then provide feedback electric-ally to make the unit touch-sensitive."

With so many applications on their hands, Bendett and his co-workers are busy trying to figure out which ones will really benefit from optical stimulation. "I was at a neural interface conference last year where a Parkinson's patient with an electrical deep-brain stimulator stood up in front of the audience," he recalled. "The patient could turn up the stimulator so that it completely stopped their tremors, but at this level they were then unable to speak." This side-effect is a symptom of electrical crosstalk. "The current is hitting the cells that it is supposed to, but it's also hitting those that it shouldn't," explained Bendett. "It's exactly this kind of application where we see optical systems coming into play."

Aculight's business strategy for nerve stimulation is based on intellectual property. "We are positioning ourselves to be a sort of clearing house for the technology," said Bendett. "We have an exclusive licence from Vanderbilt University and would like to be involved as an OEM [original equipment manufacturer] for the hardware."

The firm is keen to get its optical stimulator into the hands of researchers. "We would like to disseminate the technology across as many clinical platforms as possible," said Bendett. "And the easiest way for us to do that is to put it out to dozens of research groups who can then work with us on the technology."

The firm already has National Institutes of Health grants with the Northwestern and Vanderbilt universities, and aims to be flexible in terms of providing users with the technology. "Currently we have a total of six universities testing different nerves and techniques with our laser," explained Bendett. "Ultimately, to get a good idea of the application landscape we expect three to four times this number of institutions to trial the device."

Today, the unit is aimed at the research market and is registered as a laboratory instrument. "As soon as we start working on humans we will have to go through the US Food and Drug Administration approval process," said Bendett. "Realistically, we will probably do this in conjunction with other companies in the medical field rather than on our own."

• For more information contact mark.bendett@aculight.com.

• This article originally appeared in the March 2007 issue of Optics & Laser Europe magazine.

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