title: Heating during infrared neural stimulation.
abstract: OBJECTIVE
  Infrared neural stimulation (INS) has recently evoked interest as an alternative to electrical stimulation. The mechanism of activation is the heating of water, which induces changes in cell membrane potential but may also trigger heat sensitive receptors. To further elucidate the mechanism, which may be dependent on cell type, a detailed description of the temperature distribution is necessary. A good control of the resulting temperature during INS is also necessary to avoid excessive heating that may damage the cells. Here we present a detailed model for the heating during INS and apply it for INS of in vitro neural networks of rat cerebral cortex neurons.
  
  
  METHODS
  A model of the heating during INS of a cell culture in a non-turbid media was prepared using multiphysics software. Experimental parameters such as initial temperature, beam distribution, pulse length, pulse duration, frequency and laser-cell distance were used. To verify the model, local temperature measurements using open pipette resistance were conducted. Furthermore, cortical neurons in culture were stimulated by a 500  mW pulsed diode laser (wavelength 1,550  nm) launched into a 200  µm multimodal optical fiber positioned 300  µm from the glass surface. The radiant exposure was 5.2 J/cm(2) .
  
  
  RESULTS
  The model gave detailed information about the spatial and temporal temperature distribution in the heated volume during INS. Temperature measurements using open pipette resistance verified the model. The peak temperature experienced by the cells was 48°C. Cortical neurons were successfully stimulated using the 1,550  nm laser and single cell activation as well as neural network inhibition were observed.
  
  
  CONCLUSIONS
  The model shows the spatial and temporal temperature distribution in the heated volume and could serve as a useful tool for future studies of the heating during INS.
authors.affiliation: Royal Institute of Technology, School of Technology and Health, Alfred Nobels Allé 10, 14152, Huddinge, Sweden.
authors.forename: Rickard
authors.initials: R
authors.lastname: Liljemalm
authors.forename: Tobias
authors.initials: T
authors.lastname: Nyberg
authors.forename: Hans
authors.initials: H
authors.lastname: von Holst
mesh_terms: D000818:Animals; D001831:Body Temperature / Q000528:radiation effects*; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000166:cytology / Q000502:physiology / Q000528:radiation effects*; D004594:Electrophysiology; D007259:Infrared Rays; D054023:Lasers, Semiconductor; D009474:Neurons / Q000502:physiology / Q000528:radiation effects*; D051381:Rats
keywords: INS; cortical neurons; heat transfer; laser; modeling; optical stimulation; simulation