Novel wavelengths for laser nerve stimulation.

Beckman Laser Institute and Medical Clinic, University of California, Irvine, California 92612Background: Laser light is known to stimulate nerves.This study investigated alternative wavelengths for nervestimulation.Materials and Methods: The sciatic nerves of rats wereirradiatedwithfourdifferentlasers—aHo:YAG(2100nm),a Yb:glass ﬁber laser (1495nm) and diode lasers (1450nmand 1540nm).Results:Alllasersevokedavisiblelegtwitchresponse,andelectromyography conﬁrmed muscle activation. TheYb:glass laser at 1495nm delivered through a single modeﬁber was found to be the most effective stimulus. Thestimulation threshold for a 2 millisecond pulse from theYb:glass laser was determined to be 3.7 22.8mJ/cm .Conclusions: The Yb:glass laser has the potential for usein neurostimulation, as an alternative to electrical stim-ulation. Lasers Surg. Med. 42:69–75, 2010. 2009 Wiley-Liss, Inc.Key words: nerve stimulation; laserINTRODUCTIONLaser light has been shown to be an effective nervestimulus. For example, Wells et al. [1–4] have stimulatedrat peripheral nerves, Izzo et al. [5–8] have stimulatedgerbil cochlea and Fried et al. [9] have stimulated the ratcavernous nerve. Although electrical stimulation is thestandard method of neural excitation, laser stimulationoffers some signiﬁcant advantages. Electrical stimulationrequires an electrode to be in contact with the neuron,whichmaydamagethetissueunderinvestigation.Also,theelectrical ﬁeld will spread from the point of contact, so thatneighboring nerves may also be stimulated and thestimulus may also be recorded by the detecting device. Anoptical stimulus does not have these issues—it is non-contact,spatiallyselective,andthereisnostimulusartifactin neural recordings.The thermal gradient created in the nerve ﬁber byabsorption of the laser pulse is thought to stimulate thenerve. It has been suggested that the Ho:YAG laseremitting at 2.1mm, has an ideal penetration depth intissue for nerve stimulation [2]. This is based on acompromise between enough absorption to generate athermal effect, but not so much as to damage the tissue.Aculight produces a nerve stimulator (Capella R-1850)emitting at 1.85mm [7] which has similar absorptioncharacteristics to a Ho:YAG laser. Assuming water to bethe main chromophore, laser wavelengths in the region of1.5mm will result in comparable absorption, as can be seenin Figure 1. This article investigates 1,450, 1,495, and1,540nmlasersfornervestimulation.The1,450–1,540nmwavelengthsareabsorbedmorethan1.85mmlaser,butlessthan 2.1mm, so offer alternative penetration depths intissue. They, therefore, have the potential to provide achoice for matching the laser to the thickness of the nerveﬁber requiring stimulation. The 1,500nm region is com-monly used in the telecommunications industry and,therefore, beneﬁts from cheap and readily availabletechnology which can be used for nerve stimulationprocedures. The focus of this investigation is to determinethe feasibility for stimulation of some of the lasers in thiswavelength region. Direct comparison between lasers wasnotmadesincetheyhaddifferentpoweranddelivery(beamdiameter)characteristics. Indeed,theeffectofpulselengthor beam diameter on nerve stimulation is not systemati-cally investigated in this article.MATERIALS AND METHODSSprague–Dawley rats (n¼10, 300g) were used for invivo sciatic nerve stimulation. All experiments wereconducted in compliance with Institutional Animal Careand Use Committee (IACUC) guidelines. Prior to surgery,the rats were anesthetized by an intraperitoneal injectionof ketamine (100mg/kg) and xylazine (13mg/kg). Addi-tional doses were administered as needed to maintainsedation. The hair was shaved from the upper area of thehindlegs.Theratswereplacedontheirventralsurfaceandan incision made in the skin from the lumbar vertebralcolumninthemidlineinalateraldirectionalongthelateralaspectofthethigh.Thesciaticnervewasexposedthroughagluteal muscle splitting incision. Both hind legs were usedto give a total sample size of n¼20.A stainless steelneedle electrode (Jari ElectrodeSupply,Gilroy,CA)wasinsertedintothebicepsfemorismuscleandagroundsurfaceelectrodeattachedtotheskinontheupperthigh. electromyograph (EMG) signals were recorded via adata acquisition module (NI-USB 6211, National Instru-ments, Austin, TX) with a 10kHz sampling frequency andﬁltered between 20 and 4,999Hz. EMG’s were recordedwithoutlaserﬁring,andwithlaserﬁringbutnotdirectedtothe nerve, to ensure recorded signals had no artifacts. Aholmium:YAG laser at 2,100nm (Newstar, Irvine, CA), aytterbium:glassﬁberlaserat1,495nm(IPGPhotonics,Oxford,