A contrastive analysis of laser heating between the human and guinea pig cochlea by numerical simulations
© The Author(s). 2016
Received: 30 January 2016
Accepted: 11 May 2016
Published: 23 May 2016
The photo-thermal effect has been hypothesised to be one of the most possible biophysical mechanisms for laser-cochlea stimulation. However, there is a lack of studies to date for direct assessing laser heating in humans due to the large body of evidence required to demonstrate safety and efficacy. Instead, the majority focus on animals like the guinea pig, from which a number of valuable results have been gained. However, in light of the increasing need to improve laser safety, it has became necessary to find out whether studies on animals can shed light on safe laser parameters in the human cochlea. Hence, we conducted this contrastive analysis of laser heating between the human and guinea pig cochlea with the aim of assisting further investigations in this field.
In this work, a 3D symmetrical model was adopted to simplify the spiraled cochlea. With attention focused on the effect of heat conduction, the time-dependent heat equation was solved using finite element method with the COMSOL Script. In the simulations, cochleae with different sizes and various boundary thermal conditions were utilized.
Laser heating in both cochleae has a similar trend. In the first stage, or at the beginning of the laser heating, both cochleae increased their temperatures rapidly. In the second stage in which the laser heating reached a quasi-steady stage, the peak temperatures began to rise slowly as more laser pulses were applied. However, three differences of the laser heating were observed. The first is regarding the temperature rise. The results show that laser heating in guinea pig is higher than that in human under the same laser parameters. The second difference is the fluctuation of temperature rise at the center of the modiolus. There is a larger fluctuation of temperature rise in the guinea pig cochlea, compared with that in the human cochlea. The third one is the time for reaching a steady thermal state. The results show that the guinea pig cochlea takes longer time to reach a steady thermal state than the human cochlea. Those differences are mainly attributed to the distinctive thermal boundaries and the various sizes of the two cochleae.
This study finds that the laser heating in the guinea pig cochlea is higher than that in the human cochlea under the condition of the same laser parameters. However, laser stimulation still displays a high spatial selectivity in both cochleae despite the effects of heat conduction. The results indicate that experimental studies on the guinea pig could appropriately be an alternative model for the sake of laser safety.
In recent years, laser light with a wide range of wavelengths has been used successfully to stimulate auditory responses of gerbils, mice, guinea pigs and cats [1–5]. Besides experiments in animals, Fishman et al.  conducted a pioneering study of optical stimulation of the auditory nerve in a patient who required of surgical removal of a large meningioma. Although many questions in research remain to be answered in terms of laser stimulation, the initial results are promising beginnings. Among various biophysical mechanisms for the laser stimulation, three types of hypotheses have been proposed. The first one is the optophonic effect. A rapid local increase in temperature deriving from water absorption of photons produces a transient acoustic wave which can trigger depolarizing response of hair cells [7–9]. The second hypothesis is the photothermal activation of heat-sensitive ionic channels in the membrane of spiral ganglion cells, such as TRPV4 channels . The third is that rapid local heating by laser can alter the electrical capacitance of the nerve membrane to evoke nerve excitability . However, all the three mechanisms are related to photothermal effects [12, 13]. While, applying photothermal effects in the cochlea bears a potential risk of thermal damage. In future clinical applications, it is important to set safe parameters to have a good spatial selectivity and avoid excessive heating in the human cochlea.
In the human cochlea, spiral ganglion cells in the first turn of the coiled duct which accounts for high auditory frequency are located in the external part of the modiolus, while the ganglion cells that account for low frequency in the second and the third turn are located in the internal part of the modiolus. In the approach of photothermal stimulation, heat diffusion from the laser irradiated zone to the modiolus may reduce the laser selectivity. Therefore, it is necessary to take a quantitative overview of the temperature variations in the spatial domain and assess the effects of the heat diffusion on the laser selectivity. However, there is a lack of studies to date into directly assessing laser heating in humans due to the large body of evidence required to demonstrate safety and efficacy.
Fortunately, the theoretical modeling offers an appropriate method to analyze the temperature variation in the spatial and temporal domain [14–16]. For instance, in the model developed by Thompson et al. [14, 15] the cochlea is represented by a three-layer system: perilymph, nerve tissue, and a bone layer between nerve and perilymph. A range of fiber numerical apertures and light wavelengths were compared regarding stimulation of nerves in the cochlea. Zhang et al.  simplified the spiraled cochlea to a rotational symmetrical structure, and simulated infrared laser heating of the human cochlea for a range of laser pulse energy and repetition rates. These studies confirmed that laser heating in the cochlea can be controlled by properly adjusting laser parameters.
Until now, most of the research in this field focused the attention on animals, which has produced a number of valuable results. The research on animals is essential to explore the mechanism of laser stimulation and can be a useful reference for future investigations of laser application in the human cochlea. From the perspective of laser safety, it is interesting to examine if studies on animals can offer a clue to the safe laser parameters in the human cochlea. Therefore it is useful to make a contrastive analysis of laser heating in the cochlea of animals and human beings.
Considering that guinea pigs have been widely used in experiments, this work also chose the guinea pig as the contrastive object. The temperature variation in the guinea pig and human cochlea was simulated with a 3D model solved with the finite element method. In particular, the difference of laser heating in the human and guinea pig cochlea was analyzed.
Physical properties of cochlear tissues
Heat capacity (J/kg/ °C)
Heat conductivity (W/m/ °C/)
Absorption coefficient (mm−1)
Scattering coefficient (mm−1)
3.60 × 103
1.05 × 103
4.18 × 103
1.00 × 103
1.30 × 103
1.90 × 103
The modeling of laser heating in the cochlea of the guinea pig and human beings prominently differs in their boundary conditions. The human cochlea is located in the skull which is considered as a heat reservoir with a constant body temperature. On the contrary, the guinea pig cochlea is located in the temporal bone filled with air. Thus, we assume that the guinea pig cochlea loses heat mainly via air convection. Another difference between the two cochleae is the cochlear size. The diameter of the cochlear shell is 6 mm for humans and 3 mm for guinea pigs. The diameter of the cochlear nerve core is 2 mm for humans and 0.8 mm for guinea pigs.
The laser wavelength is set to be 1900 nm, and the laser pulse energy and pulse length are kept at 45 μJ and 100 μs respectively, as these laser parameters have been generally utilized in a number of studies [4, 15, 22–24].
For the given laser parameters, the model (Eq. 1) was solved by means of the finite element method with the COMSOL Script 1.3. The mesh elements are set in a tetrahedron shape with different sizes which are set to be small in the laser irradiated zones and slowly increased as the region moves far away from the laser stimulated sites. In total, the 3D model is divided into approximately 40,000 elements and 8000 mesh points.
When laser pulses are applied to stimulate the cochlea, the spiral ganglion cells absorb photons and become hot. Three typical sites in the human cochlea and two sites in the guinea pig cochlea were chosen, aiming to show how temperature changes if giving laser heating. One site, called A, represents the nerve layer 100 μm underneath the osseous spiral lamina, one site, called O, represents the center of the modiolus, as illustrated in Fig. 1. And the last site, called Oh, is located between site A and O in the nerve layer of the human cochlea and has the same distance from the fiber as site O is in the guinea pig. To present a simpler illustration, the site Oh is not displayed in Fig. 1.
Results and discussions
The second difference is the fluctuation of temperature rise at site O. There is a greater fluctuation of temperature rise in the guinea pig cochlea, compared with that in the human cochlea as reported in our previous work . In the cochlea of the guinea pig, the nerve tissue at site O is located in the laser beam and directly absorbs heat from laser pulse trains. Therefore its temperature oscillates following the laser repetition rate. However, the nerve tissue at site O in the human cochlea is far from the laser irradiated zone, it receives heat via heat diffusion from the laser heated zone, which is a slow process. As a consequence, no obvious fluctuation in the temperature is observed.
The differences between the laser heating in the guinea pig cochlea and the human cochlea mainly result from two things. One is the effect of the different thermal boundaries of the two cochleae. The human cochlea is located in the temporal bone where there are plenty of blood capillaries, making its temperature more or less the same as body temperature. Thus, the temperature at the boundary of the human cochlea is set to be constant. However, there is an air gap between the guinea pig cochlea and the closed bulla. Through the air gap, there are two ways transferring heat from the cochlea to the bulla: radiation and convection. By assuming a temperature difference of 1 °C across the gap, an estimation of the power radiated from the guinea pig cochlea can be obtained as being 7 W/m2, and the power loss via air convection is calculated as being about 20 W/m2 by applying Newton’s law of cooling . However, if the air gap was filled with bone, the heat transfer would be dominated by heat conduction. For the same temperature difference of 1 °C, the power loss via heat conduction can be estimated as being 300 W/m2 following the Fourier’s law of heat conduction . Therefore, in a situation of low temperature, heat flows from the cochlea of the guinea pig to its bulla is a slower process compared to the heat conduction to the temporal bone in the human cochlea. The second reason for the difference induced is the effect of the cochlear size. Because the guinea pig cochlea looks smaller than the human cochlea, with the same laser energy, the guinea pig cochlea can justifiably be hotter than the human cochlea.
In summary, as reported in our previous work , the laser-affected zone is larger than the laser illumination due to heat diffusion, but our results show that: (1) The temperature rise in the modiolus is less than that in the laser-targeted ganglion cells; (2) Laser heating in the cochlea of the guinea pig is higher than that in the human cochlea; (3) the laser heating in both cochleae is confined mainly to the laser target region. Our results indicate that the cochlea of guinea pig could be an acceptable model for the sake of laser safety. Therefore, future experimental studies in the guinea pig cochlea could help to set proper laser parameters for laser stimulation of the human cochlea.
Due to the lack of information detailing physical properties of the cochlear tissues, including heat conductivity, heat capacity and optical absorption coefficient, in this work, the physical properties were acquired by analyzing the data of similar tissues described in literature. In addition, the spiraled cochlea was simplified as a four-layer cylinder. In future studies, the modeling of laser heating can be improved with more precise setting of the physical parameters and more careful consideration of the spiraled cochlear structure, as well as much attention on the influence of thermal convection via blood flow and other fluids flow on laser heating.
Infrared laser heating in the cochlea of the guinea pig and human was investigated by applying a 3D four-layer cylindrical model. Two stages in the laser heating were observed. In the first stage, the two cochleae display a sharp increase in temperature. In the next stage, the cochleae enter a quasi-steady stage in which the peak temperature rise changes slowly as more laser pulses are applied. Moreover, the temperature in the cochlea of guinea pig is higher than that in the human cochlea. It is a result of the difference in the cochlear size and the boundary thermal conditions for the cochlea of the guinea pig and humans. This study indicates that, from the perspective of laser safety, future experimental studies in the guinea pig cochlea could help set proper laser parameters for laser stimulation of the human cochlea.
KZ contributed in conception of the project, carried out the literature study, and drafted the manuscript. YZ performed numerical simulations. JL contributed in analysis of results and preparation of the figures. QW contributed in conception of the project, reviewed and revised the manuscript critically. All authors read and approved the final manuscript, and agreed to be accountable for all aspects of the work.
This work was supported by the National Natural Science Foundation of China (Grants no. 51271059), the International Science & Technology Cooperation Program of Anhui (Grant no. 1403062027), the Anhui Provincial Natural Science Foundation (Grant no. 1608085MA10), the Anhui Provincial Quality Project in Universities (Grant no. KJ2011ZD07, 2013ZYJS04, 2014zy047, gxfxZD2016166). The authors express great gratitude for Shengnan Wang and Asempah Isacc for careful checking on the English writing.
The authors declare that they have no competing interests.
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