ORIGINAL ARTICLE Influence of radiant exposure and repetition rate in infrared neural stimulation with near-infrared lasers Mohammad Javad Alemzadeh-Ansari 1 & Mohammad Ali Ansari 2 & Mahdi Zakeri 2 & Majid Haghjoo 3 Received: 7 February 2017 /Accepted: 30 January 2019 # Springer-Verlag London Ltd., part of Springer Nature 2019 Abstract In this study, we combine heat diffusion equation and modified Hodgkin-Huxley axonal model to investigate how an action potential is generated during infrared neural stimulation. The effects of temporal and spatial distribution of heat induced by infrared pulsed lasers on variation of electrical membrane capacitance are investigated. These variations can lead to depolarize the membrane and generate an action potential. We estimate the threshold values of laser light parameters such as energy density, pulse duration, and repetition rate are needed to trigger an action potential. In order to do it, we present an analytic solution to heat diffusion equation. Then, the analytic results are verified by experimental results. Furthermore, the modified Hodgkin-Huxley axonal model is applied to simulate the generation of action potential during infrared neural stimulation by taking into account the temperature dependence of electrical membrane capacitance. Results show that the threshold temperature increase induced by a train infrared pulse laser can be smaller if repetition rate is higher. These results also indicate that temperature rise time and axon diameter influence on threshold temperature increase. To verify threshold values estimated by the presented method, we use a train infrared pulsed laser (λ = 1450 nm with repetition rate of 3.8 Hz, pulse duration of 18 ms and energy density of 5 J/cm 2 ) to optically pace an adult rat heart, and we are able to successfully pace the rat heart during an open-heart surgery. The presented method can be used to estimate threshold values of laser parameters required for generating an action potential, and it can provide an insight to how the temperature changes lead to neural stimulation during INS. Keywords Infrared neural stimulation . Axon . Heat diffusion equation . Optical pacing Introduction Infrared neural stimulation (INS) is an emerging method that can be applied to stimulate cochlea, vestibular system, peripheral motor nervous, and pacing embryonic and adult heart [1–8]. Jenkins et al. demonstrated that infrared stimula- tion can be used to pace adult heart. They evaluated the fea- sibility of optical pacing of an adult rabbit heart using a pulsed laser (1870 nm) with repetition rate 2.5 Hz, pulse width 8 ms, and energy density 6.3 J/cm 2 . In this study, the heart was excised from a euthanized rabbit before cannulating and per- fusing the heart on a modified Langendorff apparatus [9]. INS provides several advantages over conventional electric stimulation, including contact-free delivery, spatial precision, and lack of stimulation artifact [10, 11]. However, INS has limitations for neural prosthesis applications and damaging thermal effects of the stimulated beam. These limitations deal with heat deposition into tissue; reducing required optical en- ergy while achieving neural stimulation is an important con- sideration [12]. The exact mechanism of INS is under discus- sion in the literature, but some results show that it is photothermal in nature [10–13]. Wells et al. have first demon- strated using pulsed infrared laser light to produce firing of an action potential through a thermal effect [8, 12–14]. Recent Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10103-019-02741-4) contains supplementary material, which is available to authorized users. * Mohammad Ali Ansari m_ansari@sbu.ac.ir 1 Cardiovascular Intervention Research Center, Rajaie Cardiovascular Medical and Research Center, Iran University of Medical Sciences, Tehran, Iran 2 Optical Bio Imaging Laboratory (OBI lab), Laser and Plasma Research Institute, Shahid Beheshti University, Velenjak, Tehran, Iran 3 Cardiac Electrophysiology Research Center, Rajaie Cardiovascular Medical and Research Center, Iran University of Medical Sciences, Tehran, Iran Lasers in Medical Science https://doi.org/10.1007/s10103-019-02741-4