Inductive wireless power transfer (WPT) can be used to power implanted as well as wearable medical devices, such as a percutaneous auricular vagus nerve stimulation device. This device is placed on the neck of the patient and is connected to needle electrodes in the auricle. With regard to WPT, limitations on exposure to electric and magnetic fields should not be exceeded. Furthermore, these fields should not interfere with the therapeutic goal of stimulation, i.e., with unintended peripheral nerve stimulation in the auricle. These effects are investigated by numerical simulation of induced internal fields in the head and neck and, for the first time, subsequent neuronal simulations, quantifying the potential of neuronal excitation by the fields in the auricle in particular. Internal electric field values were in the range of 1\%-5\% of the ICNIRP 2010 basic restrictions, and current densities were in the range of 30\%-45\% of the ICNIRP 1998 basic restrictions, indicating that all tested configurations are conform the guidelines. Basic restrictions on heating of tissue turned out not to be of relevance for this application. Thresholds for neuronal stimulation were two orders of magnitude higher than the induced fields, suggesting that there is almost no risk for unintended stimulation

Joseph, Wout

Kampusch, Stefan

Kaniusas, Eugenijus

Martens, Luc

Razlighi, Babak Dabiri

Szeles, Jozsef C.

Tanghe, Emmeric

Van de Steene, Tom

English

Ghent University Academic Bibliography

Exposure and Neuronal Excitation byWireless Power Transfer forAuricular Vagus Nerve StimulationT. Van de Steene, E. Tanghe, L. Martens, W. JosephDepartment of Information TechnologyGhent University/IMECGhent, Belgiumtom.vandesteene@ugent.beS. Kampusch, B. Dabiri Razlighi, E. KaniusasInstitute of Electrodynamics, Microwave and Circuit EngineeringVienna University of TechnologyVienna, AustriaJ. C. Sze´lesDepartment of SurgeryMedical University ViennaVienna, AustriaAbstract—Inductive wireless power transfer (WPT) can beused to power implanted as well as wearable medical devices,such as a percutaneous auricular vagus nerve stimulation device.This device is placed on the neck of the patient and is connected toneedle electrodes in the auricle. With regard to WPT, limitationson exposure to electric and magnetic fields should not beexceeded. Furthermore, these fields should not interfere with thetherapeutic goal of stimulation, i.e., with unintended peripheralnerve stimulation in the auricle. These effects are investigated bynumerical simulation of induced internal fields in the head andneck and, for the first time, subsequent neuronal simulations,quantifying the potential of neuronal excitation by the fields inthe auricle in particular. Internal electric field values were inthe range of 1%-5% of the ICNIRP 2010 basic restrictions, andcurrent densities were in the range of 30%-45% of the ICNIRP1998 basic restrictions, indicating that all tested configurationsare conform the guidelines. Basic restrictions on heating of tissueturned out not to be of relevance for this application. Thresholdsfor neuronal stimulation were two orders of magnitude higherthan the induced fields, suggesting that there is almost no riskfor unintended stimulation.Index Terms—wireless power transfer (WPT), magneticfield exposure, percutaneous auricular vagus nerve stimulation(pVNS), ICNIRP guidelinesI. INTRODUCTIONWearable medical devices are playing an increasingly im-portant role in healthcare today. As with any other electricaldevice, sufficient power is needed to keep the device running,while delivering and storing this energy might be a challenge.Wireless power transfer (WPT) by means of inductive couplinghas a number of advantages that often make it the best suitedmethod for several medical devices.In particular, WPT can be used to power percutaneousauricular vagus nerve stimulation (pVNS) devices such asin [1] and [2]. These devices are placed on the head orneck as shown in Fig. 1, and are connected to a numberof needle electrodes which are placed in the auricle. Whilethis is not an implanted device, WPT offers a number ofbenefits over a wired connection for power supply. Since nophysical connection is needed for power transfer and/or datatransfer, the device design can be optimized towards a greentechnology.However, a possible side effect to take into account, isthe generated magnetic fields. The resulting induced electricfields and circular currents might lead to peripheral nervestimulation and to a lesser extent, heating, although the latteris not expected to be an important factor at the frequencies ofWPT (100 kHz - 200 kHz). Limits on the specific absorptionrate (SAR), which are intended to restrict heating in the body,have to be considered at frequencies between 100 kHz and10 GHz [3]. However, limitations on current densities (whichhave to be considered in the range 1 Hz - 10 MHz [3]) aremore likely to be critical than limitations on SAR.Numerical exposure studies have already been conductedfor active implantable medical devices and WPT in the prox-imity of the body in humans and animals [4], [5], [6], [7].These studies have not yet combined an exposure assessmentwith subsequent neuronal simulations, directly quantifying theeffect on peripheral nerves.This study aims to quantify the electric and magnetic fieldsin the head and neck, and more specifically in the auricle bymeans of numerical simulations using a heterogeneous humanhead model. Subsequently, these values will be compared tothe applicable limitations on exposure of humans to electro-magnetic fields as defined by the International Commission onNon-Ionizing Radiation Protection (ICNIRP) [3], [8]. Finally,the actual potential of these fields to stimulate neurons in theauricle will be investigated. This is particularly interesting inthe case of pVNS due to the proximity of the target neuronsin the auricle to the coils used for WPT. Of course it shouldbe avoided that WPT has an influence on the fields in theauricle in such a way that the intended stimulation by needleelectrodes is disturbed with potentially altered therapeuticeffect as a result.Moreover, it is interesting to compare the ICNIRP basicrestrictions (BR), which are meant among others to limitperipheral nerve stimulation, with actual stimulation thresholdsfor the case of WPT for pVNS.II. MODELS AND METHODSThe electric and magnetic fields, exposure and neural acti-vation resulting from WPT to transfer energy to a wearablepVNS device are investigated [9]. Numerical models of boththe pVNS device and a human head are used in combinationwith simulation software to obtain the internal fields.A. Model of pVNS deviceThe model of the pVNS device, wherein WPT takes place,consists of a plastic housing, a transmitter coil with 10 turnsattached to a 0.5 mm circular ferrite plate and a receiver coilwith two layers of 11 turns each, a battery with metal housing,and fiberglass PCBs.The magnetic field of the coils is excited by a source currentin the primary coil which is limited to 2 A. Simulations arecarried out at three different frequencies: 130 kHz, which isthe intended frequency of use, 100 kHz and 200 kHz, whichdefine a usual range for WPT applications, according to [10].B. Anatomical modelsFor the human head, the MIDA-model [11] was used,which is a high resolution model containing 153 structuresof the head. This model is used to assess the exposure andmaximum field values in the different tissues, which are tobe compared to the exposure guidelines [3], [8]. Values ofelectrical and magnetic properties of the different tissues areincluded in the model. Only the conductivity of the skin waschanged. This value was set to 0.2 S/m, according to [12]instead of 0.00045 S/m (100 kHz), 0.0006 S/m (130 kHz)and 0.001 S/m (200 kHz) as defined by the original model[11]. In order to be able to investigate the effect of the fieldsin the auricle with sufficient detail, a high resolution and morerealistic model of the auricle by [2], especially composedfor the investigation of pVNS, was added to the simulations.This anatomical model contains neurons corresponding to thelocations of the auricular branch of the vagus nerve. Boththe auricular cartilage and skin have conductivity values of0.2 S/m [2]. The model and locations of neurons is illustratedin Fig. 2.For the reason of possible variations in the placement of thedevice, four different configurations are defined. In all fourcases, the device is placed behind and slightly below the ear,but small variations in position and rotation angle exist. Theseconfigurations are called C1 - C4, and are shown in Fig 1. ForC4 the auricle is closer to the coils than practical with respectto the housing dimensions, and could therefore be consideredas a worst-case scenario.(a) (b)(c) (d)Fig. 1. (a) - (d) Placement of the essential components for WPT (white) inthe pVNS device behind the auricle for the different configurations C1 - C4respectively.Fig. 2. Detailed view of the 3D model of the auricle and neuron locationsfor neuronal simulations [2].C. Electromagnetic simulationsSimulations are carried out using Sim4Life [13], accordingto the numerical methodology of a similar study by [7].The quasi-static electromagnetic approximation was used. Thefirst step is to calculate the magnetic fields arising from thecurrents in transmitting and receiving coils. For this case, theMagnetic Vector Potential solver of the software is used, whichcalculates the H field, the B field and magnetic vector poten-tial A for a given source current. The computational domaincan consist of materials with variable magnetic permeability,perfect magnetic conductors and perfect electric conductors.The transmitting coil was operated with the maximum currentof 2 A. The receiving coil is modeled as a perfect electricalconductor.Secondly, the Magneto Quasi-Static solver is used to calcu-late the electric field and current density field in the lossydomain (σ 6= 0), assuming that ohmic currents dominatedisplacement currents. From these fields, the quantities ofinterest for the ICNIRP BRs [3], [8] can be automaticallycalculated: For the electric field, the 99% value is taken foreach tissue while current densities are averaged over a 1 cm2circular area perpendicular to the direction of the current. TheSAR is calculated as well, averaging over 10 g of tissue.D. Neuronal simulationsFor the neuronal simulations, the internal fields in theanatomical model of the human auricle of [2] are used asan input. This model contains seven axons, located at regionsof the auricle where the vagus nerve is prominent. The usedneuronal model is the spatially extended non-linear nodemodel (SENN-model) [14]. In order to simulate the worst casescenario, the axon diameter was set to 12 µm, which is thelargest of possible diameters of the neurons of the vagus nervein the auricle [15] and are therefore the most easily excitable[16]. The titration factor, which is the factor by which thestimulus (in this case the fields generated during WPT) has tobe multiplied in order to induce spiking of axons, is calculatedby the software. A titration factor larger than unity means thatneurons will not be stimulated during WPT.III. RESULTS AND DISCUSSIONA. Electric and current density field distributionsFrom the simulations for each of the configurations C1-C4and each of the investigated frequencies (100 kHz - 130 kHz- 200 kHz), the electric field and current density field in thehead and neck were obtained. Fig. 3 (a) and Fig. 3 (b) showthe resulting fields for configuration C1 at 100 kHz.The highest values for the electric field as well as forthe current density are found in the skin of the ear and/orclose to the device, depending on the configuration. Theinfluence of varying tissue conductivity is visible as well,especially for the current density field. The 99% electric fieldvalue was extracted for each individual tissue and surface-averaged current density values were calculated for the wholemodel. Maximum values are shown in Table I and Table IIrespectively, together with the ICNIRP BR.(a) (b)Fig. 3. (a) Electric field normalized to 0.7 V/m and (b) current density fieldnormalized to 0.1 A/m2 in a coronal slice of the head through the ear.Higher values are found at higher frequencies, which is inaccordance with Faraday’s law of induction. When comparingdifferent configurations, it can be seen that the maximumelectric field values can differ by a factor three. ConfigurationC2 results in the lowest and C4 in the highest values for everyfrequency.B. Compliance with guidelinesFrom the results in section III-A, it is clear that the relevantmaximum values for both the electric field and current densityfield are below the ICNIRP BR. This is visualized in Fig. 4 andFig. 5, which show the maximum field values as a percentageof the ICNIRP BR for each frequency. The values of theelectric field are between 1% and 5% of the BR. This meansthat it is highly unlikely that the limits will be exceeded due toWPT by the device. For the current density, values are betweenTABLE I99% ELECTRIC FIELD VALUES FOR VARYING FREQUENCY ANDCONFIGURATION (V/m)FrequencyConfiguration 100 kHz 130 kHz 200 kHzC1 0.30 0.39 0.61C2 0.21 0.27 0.42C3 0.34 0.44 0.68C4 0.59 0.76 1.17ICNIRP BR 13.50 17.55 27.00TABLE IIMAXIMUM SURFACE-AVERAGED CURRENT DENSITY FIELD VALUES FORVARYING FREQUENCY AND CONFIGURATION (A/m2)FrequencyConfiguration 100 kHz 130 kHz 200 kHzC1 0.08 0.10 0.15C2 0.06 0.08 0.12C3 0.07 0.09 0.13C4 0.09 0.11 0.18ICNIRP BR 0.20 0.26 0.40Fig. 4. 99% Electric field values for different configurations and frequenciesnormalized to the ICNIRP BR.Fig. 5. Maximum current density values for different configurations andfrequencies normalized to the ICNIRP BR.30% and 45% of the BR. While this is considerably higherthan for the electric field, there is still more than a factor twoof margin. It is also seen that variation of maximum currentdensities is lower than for electric field values (III-A). Thislower variation suggests that other configurations than the fourconfigurations tested, are less likely to induce current densitiesexceeding the limits, although only additional simulations canconfirm this presumption.The SAR was calculated as well, and all values werebetween 0.00021% (C2 at 100 kHz) and 0.00168% (C4 at200 kHz) of the 2 W/kg BR [3]. This was as expected at therelatively low investigated frequencies.C. Neuronal excitation thresholdsThe electric fields in the auricle were used to simulate theresponse of seven neurons by calculation of the titration factor.The results are summarized in Fig. 6. For each combinationof configuration number and frequency, the minimum valuefrom all seven neurons was taken.Fig. 6 shows that the titration factors are very high. Thisindicates that the needed electric field for stimulation of anyof the neurons needs to be more than two orders of magnitudehigher than the actual field. It is very unlikely that WPTwill result in neuronal stimulation and therefore interfere withthe therapeutic goal of the device. These results are expectedsince the field values are below the ICNIRP BR. The lowesttitration factors were obtained for neurons N6 and N7 (276and 342 respectively). These are the closest to the device andcorrespond with the locations of high electric fields as well.While these results show that no neural stimulation will beseen, there is still a large variation of the results (a factor36 between N3 and N6) for only a limited total number ofFig. 6. Minimum values of the titration factor for varying frequency andconfiguration, indicating that the investigated axons will not be stimulated asa result of WPT.neurons. Therefore, investigation of more neurons with varyinggeometries and locations on the auricle, might add to thestatistical significance of the results.IV. CONCLUSIONThe exposure of patients to the induced electric fields andcurrent density fields by WPT was investigated by numericalsimulations at different frequencies and different configura-tions, covering inter-user variability. The electric fields werein the range of 1% - 5% of the ICNIRP BR and the currentdensity fields were in the range of 30% - 45%, indicating thatthe device is conform the guidelines for the tested setups. SARvalues, which are four orders of magnitude or more below thelimits, turned out to be irrelevant for this application at theinvestigated frequencies.For the first time, the generated fields by WPT were directlyused for neuronal simulations, which is especially interestingfor pVNS, due to the proximity of the WPT device and thestimulation region. These simulations indicated that there isalmost no risk for interference between WPT and pVNS, sincestimulation thresholds were two orders of magnitude or morehigher than the induced fields.Seeing the variation between different configurations, inves-tigation of additional configurations and non-sinusoidal wave-forms/harmonics might be needed to confirm these results.Comparison of numerical simulations to measured field valuesaround the device would add to the validity of the results aswell.ACKNOWLEDGMENTThis work was supported by the Research Foundation -Flanders (FWO-V) under grant agreement No G003415N.REFERENCES[1] S. Kampusch, E. Kaniusas, F. Thu¨rk, D. Felten, I. Hofmann,and J. C. Sze´les, “Device development guided by user satisfactionsurvey on auricular vagus nerve stimulation,” Current Directionsin Biomedical Engineering, vol. 2, no. 1, jan 2016. [On-line]. Available: https://www.degruyter.com/view/j/cdbme.2016.2.issue-1/cdbme-2016-0131/cdbme-2016-0131.xml[2] A. M. Samoudi, S. Kampusch, E. Tanghe, J. C. Sze´les, L. Martens,E. Kaniusas, and W. Joseph, “Numerical modeling of percutaneousauricular vagus nerve stimulation: a realistic 3D model to evaluatesensitivity of neural activation to electrode position,” Medical andBiological Engineering and Computing, 2017.[3] International Commission on Non-Ionizing Radiation Protection,“Guidelines for limiting exposure to time-varying electric, magnetic,and electromagnetic fields (up to 300 GHz),” Health physics,vol. 74, no. 4, pp. 494–522, apr 1998. [Online]. Available:http://www.ncbi.nlm.nih.gov/pubmed/9525427[4] T. Campi, S. Cruciani, F. Palandrani, V. De Santis, A. Hirata, andM. Feliziani, “Wireless power transfer charging system for AIMDs andpacemakers,” IEEE Transactions on Microwave Theory and Techniques,2016.[5] X. L. Chen, A. E. Umenei, D. W. Baarman, N. Chavannes, V. D.Santis, J. R. Mosig, and N. Kuster, “Human Exposure to Close-RangeResonant Wireless Power Transfer Systems as a Function of DesignParameters,” IEEE Transactions on Electromagnetic Compatibility,vol. 56, no. 5, pp. 1027–1034, oct 2014. [Online]. Available:http://ieeexplore.ieee.org/document/6766202/[6] F. Wen and X. Huang, “Human Exposure to Electromagnetic Fieldsfrom Parallel Wireless Power Transfer Systems.” International journalof environmental research and public health, vol. 14, no. 2, 2017.[Online]. Available: http://www.ncbi.nlm.nih.gov/pubmed/28208709http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=PMC5334711[7] S. Benaissa, A. M. Samoudi, D. Plets, G. Vermeeren,L. Verloock, B. Minnaert, N. Stevens, L. Martens, F. A.Tuyttens, B. Sonck, and W. Joseph, “Numerical assessmentof EMF exposure of a cow to a wireless power transfersystem for dairy cattle,” Computers and Electronics inAgriculture, vol. 151, pp. 219–225, aug 2018. [Online]. Available:https://linkinghub.elsevier.com/retrieve/pii/S0168169917314898[8] International Commission on Non-Ionizing Radiation Protection,“Guidelines for limiting exposure to time-varying electricand magnetic fields (1 Hz - 100 kHz),” Health physics,vol. 99, no. 6, pp. 818–836, 2010. [Online]. Available:http://www.icnirp.org/cms/upload/publications/ICNIRPLFgdl.pdf[9] “SzeleSTIM,” 2018. [Online]. Available: www.szelestim.com[10] Wireless Power Consortium, “System description wireless power trans-fer,” Tech. Rep. October, 2010.[11] M. I. Iacono, E. Neufeld, E. Akinnagbe, K. Bower, J. Wolf,I. Vogiatzis Oikonomidis, D. Sharma, B. Lloyd, B. J. Wilm, M. Wyss,K. P. Pruessmann, A. Jakab, N. Makris, E. D. Cohen, N. Kuster,W. Kainz, and L. M. Angelone, “MIDA: A Multimodal Imaging-Based Detailed Anatomical Model of the Human Head and Neck,”PLOS ONE, vol. 10, no. 4, pp. 1–35, 2015. [Online]. Available:https://doi.org/10.1371/journal.pone.0124126[12] V. De Santis, X. L. Chen, I. Laakso, and A. Hirata, “An equivalentskin conductivity model for low-frequency magnetic field dosimetry,”Biomedical Physics & Engineering Express, 2015.[13] Zurich MedTech, “Sim4Life,” 2018. [Online]. Available:zmt.swiss/sim4life/[14] J. P. Reilly, V. T. Freeman, and W. D. Larkin, “Sensory effects oftransient electrical stimulation–evaluation with a neuroelectric model,”IEEE Trans Biomed Eng, vol. 32, no. 12, pp. 1001–1011, dec 1985.[15] S. Safi, J. Ellrich, and W. Neuhuber, “Myelinated Axons in the AuricularBranch of the Human Vagus Nerve,” Anatomical Record, vol. 299, no. 9,pp. 1184–1191, 2016.[16] A. M. Samoudi, “Electromagnetic Modelling and Optimizationfor SPECT-MRI and Auricular Vagus Nerve Stimulation,”Ph.D. dissertation, Ghent University, 2017. [Online]. Available:https://biblio.ugent.be/publication/8560652/file/8560653

Exposure and neuronal excitation by wireless power transfer for auricular vagus nerve stimulation

Steene, Tom Van de

Dabiri Razlighi, Babak

Szeles, J.Constantin

reposiTUm (TUW Vienna)

Exposure and Neuronal Excitation by Wireless Power Transfer for Auricular Vagus Nerve Stimulation

Crossref

https://doi.org/10.1109/ismict.2019.8743928

Exposure and neuronal excitation by wireless power transfer for auricular vagus nerve stimulation

Abstract

Similar works

Full text

Available Versions

Ghent University Academic Bibliography

reposiTUm (TUW Vienna)

Crossref