Bridging the Modeling Gap: Huygens' Principle for Brain Implants
- Dr. Cheng Yang, Institut für Theoretische Elektrotechnik, Hamburg University of Technology -
Human brain implants that can monitor or partially control neuronal activity have been designed and deployed for many years, such as clinic treatments using deep brain stimulation (DBS) and Vagus nerve stimulation for Parkinson's and depression. In the foreseeable future their usage quite likely will increase to a point where brain implants become ubiquitous: not only for changing patients’ life, but also for advancing normal human beings’ thinking and living. With time brain implants will become highly integrated and intelligent devices of ultra-small size, low-power consumption but full abilities of sensing, controlling, data processing, wireless data and power transmission. Because of its importance to human life and health, it is therefore foreseeable that both the electromagnetic (EM) emission from implants into the surrounding brain tissue and the electromagnetic interference with other implants will have to be tightly controlled.
Within the framework of the i3-LAB HELIOS (Hamburg Electronics Lab for Integrated Optoelectronic Systems), the Institut für Theoretische Elektrotechnik has been working the Institut für Integrierte Schaltkreise on developing novel EM compatible brain implants which have optical interfaces to the nervous tissue. In order to reliably predict the bio-EM compatibility of brain implants, the interaction between implants and the human brain is identified by full-wave simulations which is found to be most accurate. However, common full-wave simulation methods all face enormous numerical challenges due to the multi-scale nature of the problem (smallest dimension depending on the approach <10-6 m, largest dimension 10-1 m) as well as the relatively wide and deep frequency band (<1 MHz to 1000 MHz).
To overcome these challenges, we plan to develop an efficient and flexible approach that combines our own full-wave simulation method with Huygens’ Principle which is well-known in optics. The Huygens’ principle states that every point on a wave-front can be considered itself as a source of wavelets and the superposition of all wavelets forms the new wave-front. In other words, any EM field outside of a volume that encloses sources can be determined from equivalent sources on the surface of that volume. In the context of this project the Huygens’ principle would allow to bridge the huge differences in scale between the modeling of brain implants and the human brain, making it at least partially possible to simulate EM effects of implants in the near and far field region.
Figure 1: Full-wave simulation of human brain implants using the Huygens’ principle.
The source replacement approach has been verified in a realistic human head model using a in-house method of moments (MoM) solver. Both the EM emissions and interferences of implants were examined, showing good accuracy and superior efficiency (Figure 1). Future work will focus on realistic brain implant modeling and simulation to measurement correlations using a high-resolution near field scanning technology, hopefully paving the way for the bio-EM engineering of brain implants in a hierarchical, anisotropic, dispersive human body environment.
 HELIOS project, Technische Universität Hamburg, Hamburg, Germany. [online] https://www.tuhh.de/helios  Huygens–Fresnel principle, wikipedia. [online] https://en.wikipedia.org/wiki/Huygens-Fresnel_principle  CONCEPT-II, Institut für Theoretische Elektrotechnik, Hamburg, Germany. [online] https://www.tet.tuhh.de/en/concept-2  C. Yang, M. Schierholz, E. Trunczik, L.M. Helmich, H.D. Brüns, and C. Schuster, “Efficient and Flexible Huygens’ Source Replacement of mm-scale Human Brain Implant”, Joint IEEE International Symposium on Electromagnetic Compatibility, Signal & Power Integrity, EMC Europe, 2021.
Dr. Cheng Yang Institut für Theoretische Elektrotechnik Technische Universität Hamburg (TUHH) Blohmstraße 15, 21079 Hamburg, Germany