A magnetically coupled resonant passive micro-magnetic stimulator based on ex vivo hippocampal brain slices

Abstract

The present invention discloses a magnetically coupled resonant passive micro-magnetic stimulator based on ex vivo hippocampal brain slices, belonging to the field of biomedical engineering. The energy of the transmitting coil is coupled into the micro-coil, realizing cell-level micro-magnetic stimulation, and the transmission efficiency of the system reaches 39.58%. The steps are as follows: building the device of the passive micro-magnetic stimulator, establishing the theory of magnetically coupled resonant wireless energy transmission and the finite element analysis model of the micro-magnetic stimulator, studying the signal-to-noise ratio of the magnetic field intensity in the hippocampal brain slice region and the transmission efficiency of the system, determining the working parameters of the micro-magnetic stimulator, and experimentally verifying the effectiveness of the micro-magnetic stimulator.

Description

Technical Field

[0001] The present invention belongs to the technical field of biomedical engineering, and particularly relates to a passive micro-magnetic stimulator based on magnetic coupling resonance for ex vivo hippocampal brain slices. Background Art

[0002] At present, there are mainly two ways of micro-magnetic stimulation devices: external stimulation and in vivo implantation. Due to the low quality factor brought by the small volume of the micro-magnetic coil, the current mainly uses the wired power supply method. In 2007, the Massachusetts Institute of Technology first proposed the magnetic coupling resonance wireless energy transmission technology, which uses two resonance objects with the same operating frequency to transmit electrical energy, and has the advantages of long transmission distance, high efficiency, and high power. This technology can be used in wireless implantable medical devices such as health monitoring, disease prevention, drug delivery, and bionic prosthetics. In 2009, Zhang et al. built a frequency tunable platform using circular coils with diameters of 60 cm and 4.1 cm respectively to provide energy for medical implant sensors. However, due to the large size of the coil device, it is limited by the implanted object

[14] . In 2012, Li et al. designed a wireless energy transmission system for the power supply of a micro-implantable medical sensor based on magnetic resonance coupling technology. The diameters of the transmitting coil and the receiving coil are both 1.9 cm. At a frequency of 742 kHz, the distance between the two coils is 1.5 cm, and the transmission efficiency is 85%. In 2012, Ramrakhyani et al. proposed a three-coil WPT system for implanting in the eye. The transmitting coil and the receiving coil are wound with AWG44 Litz wire, with diameters of 3.6 cm and 1.5 cm respectively. At a resonance frequency of 3.37 MHz, the distance between the two coils is 1 cm, and the transmission efficiency of the system is 62.5%. In 2016, Swain et al. proposed a resonance-based wireless power supply system to power medical implantable electronic devices through resonant electromagnetic coupling. The transmitting coil uses a circular spiral coil, and the receiving coil uses a planar circular spiral coil. When the operating frequency is 562 kHz and the distance between the two coils is 3 cm, the maximum transmission efficiency is 26%.

[0003] The above research shows that centimeter-scale coils are widely used in implanted medical devices. However, for millimeter-scale coils, due to their large internal resistance and inductance in the nH range, it is the main reason for the low transmission efficiency and large losses in implanted applications. In 2017, Mirbozorgi et al. proposed a WPT device for millimeter-scale coils implanted in the brain, which adopted a three-coil structure, namely the transmitting coil, the resonant coil, and the implanted coil. The coil diameters were 45 mm, 20 mm, and 1 mm respectively. At a working frequency of 60 MHz, the transmission efficiency was only 2.4%. In 2018, Sun et al. discussed WPT based on magnetic resonance coupling, designed to achieve wireless power transmission using planar spiral coils. The size of the implanted coil was 11 mm, and at a frequency of 39.86 MHz, the power transmission efficiency was 47.2%. In 2018, Basar et al. proposed a four-coil inductive wireless power supply system for capsule endoscopes. The transmitting and receiving coils were circular coils with diameters of 35 cm and 9.5 mm respectively. When the transmission power was at least 758 mW, the power transmission efficiency of this system was only 8.21%. In 2020, Kim et al. proposed an implantable electrocardiogram monitor based on WPT. Both the transmitting and implanted coils used planar square spiral coils. The size of the implanted coil was 24×27×8 mm, and the size of the transmitting coil was 4 times that of the implanted coil. At a working frequency of 6.78 MHz, the effectiveness of this device was verified through animal experiments on a rat model, and the transmission efficiency was only 10%. In 2021, Lee et al. proposed a multi-channel electrophysiological sensor and electrical stimulator using wireless power transmission: with a size of 650×650×250 μm, at a working frequency of 1 GHz, the maximum transmission efficiency was 36%. Researchers have tried to solve the problem of low energy transmission efficiency by increasing the resonant frequency, optimizing the structure of micro-coils, finding the optimal load, and adding intermediate-stage resonators, etc.

[0004] Although there have been some implanted applications for millimeter-scale coils, how to improve the transmission efficiency of millimeter-scale coils has always been a difficult problem. Therefore, this paper innovatively designs a passive micro-magnetic stimulator for ex vivo hippocampal brain slices, theoretically elaborates in detail the design method for improving the wireless energy transmission efficiency of micro-coils and the signal-to-noise ratio of magnetic field strength, measures the transmission efficiency of the system, and experimentally verifies the feasibility of the device for targeted magnetic stimulation applications, which has important research significance. Summary of the Invention

[0005] The object of the present invention is to perform magnetic stimulation on ex vivo hippocampal brain slices, and a magnetically coupled resonant passive micro-magnetic stimulator for ex vivo hippocampal brain slices is proposed.

[0006] The technical solution of the invention is as follows:

[0007] A passive micro-magnetic stimulator based on magnetic coupling resonance, mainly used for magnetic stimulation of ex vivo hippocampal brain slices, the method is as follows:

[0008] (1) System Design of a Magnetically Coupled Resonant Passive Micromagnetic Stimulator Based on Ex Vivo Hippocampal Brain Slices

[0009] The magnetically coupled resonant passive micromagnetic stimulator based on ex vivo hippocampal brain slices mainly consists of two parts: a transmitting module and a stimulating module. Specifically:

[0010] A. Transmitting module

[0011] It mainly includes: an SDG1020 signal source, an FPA301 power amplification module, a heat dissipation resistor, a transmitting coil, and a resonant capacitor. The transmitting coil is a circular spiral coil wound with a wire of 0.7 mm in diameter, with an inner diameter of 2.5 cm, 22 turns, and a height of 5.5 mm. Its inductance is L1 = 16.981 μH, and the DC resistance is R1 = 1.702 Ω.

[0012] B. Stimulating module

[0013] The stimulating module includes: a microcoil and a resonant capacitor. The microcoil is a planar square spiral coil with a size of 3.66×3.66 mm, in a series of 4 layers, with 8 turns in each layer. During the design of the microcoil structure, it is ensured that the current directions of each layer of the coil are the same to increase the inductance of the microcoil. The inductance of the microcoil is L2 = 1.72 μH, and the DC resistance is R2 = 3.44 Ω. The microcoil is insulated by parylene in a vacuum, with a coating thickness of 5 μm, ensuring good biocompatibility and waterproofness.

[0014] (2) Construction of a Wireless Energy Transfer Theoretical Model for a Magnetically Coupled Resonant Passive Micromagnetic Stimulator Based on Ex Vivo Hippocampal Brain Slices

[0015] The experimental device of the passive micromagnetic stimulator is as shown in Figure A in Figure 1 . The transmitting coil is fixed on the outer periphery of the glass electrode of the multi - electrode array system, using a circular spiral coil ( Figure 1 , Figure D in Figure 1 ), and the microcoil uses a planar square spiral coil ( Figure 1 , Figure E in Figure 1 ), placed above the hippocampal brain slice of the glass electrode. At the same time, the glass electrode is filled with artificial cerebrospinal fluid (ACSF), as shown in Figure B in Figure 1 . Since the magnetic field generated by the transmitting coil is coupled to the microcoil through the ACSF, in the solution of eddy current loss, the ACSF needs to be equivalent to a circuit in series with a resistor R0 and an inductor L0. The transmitting coil is placed in the air. Since the conductivity of the air is very small, the equivalent impedance in the air can be ignored. Therefore, the equivalent circuit of the wireless power transfer model designed in this paper is as shown in Figure C in Figure 1 . Among them, is the system sinusoidal signal voltage, R1 and R2 are the self-resistances of the external coil and the micro-coil respectively, L1 and L2 are the inductance values of the external coil and the micro-coil respectively, M is the mutual inductance between the external coil and the micro-coil, M0 is the mutual inductance between the cerebrospinal fluid equivalent model and the micro-coil, k is the coupling coefficient between the external coil and the micro-coil, and C1 and C2 are the resonant capacitors of the transmitting loop and the receiving loop respectively.

[0016] Let the current at the transmitting end be The current at the receiving end is which is also called the induced current. The current reference direction is as shown in Figure 1 Figure C in. Kirchhoff's voltage law (KVL) equations can be listed:

[0017]

[0018] The primary loop current is obtained from equation (1) and the secondary loop current

[0019]

[0020] where is the impedance of the external coil, is the impedance of the micro-coil, and Z0 = R0 + jωL0 is the cerebrospinal fluid equivalent impedance. When the circuit is in the resonant state, the external coil and the micro-coil resonate at the same operating frequency, and the link energy transfer efficiency is the highest. At this time, the impedances of both the primary loop and the secondary loop are purely resistive: Z1 = R1, Z2 = R2. Equation (2) is simplified to:

[0021]

[0022] where is the radiation resistance of the cerebrospinal fluid equivalent circuit coupled to the secondary loop. The formula for the radiation impedance of a single-turn closed coil in a conductive medium is:

[0023]

[0024] where α is the radius of the closed coil, ω is the angular frequency, μ is the relative magnetic permeability, σ is the conductivity of the medium, and β = (μωσ / 2) 1 / 2 .

[0025] The wireless micro-magnetic stimulation device applies the energy stored in the inductance of the micro-coil for magnetic stimulation. The output power P2 is the power across WL2 of the micro-coil. The input power P1 and the output power P2 can be obtained from equation (3):

[0026]

[0027]

[0028] The transmission efficiency formula obtained from formulas (5) and (6) is as follows:

[0029]

[0030] (3) Determination of the operating parameters of the magnetically coupled resonant passive micro-magnetic stimulator based on ex vivo hippocampal brain slices

[0031] For the determination of the operating parameters of the magnetically coupled resonant passive micro-magnetic stimulator based on ex vivo hippocampal brain slices, a magnetically coupled resonant wireless energy transfer model of finite element Comsol is mainly used to quantitatively evaluate the distribution of the magnetic field intensity and the signal-to-noise ratio at the stimulation target point of the hippocampal brain slice, meeting the following magnetic stimulation conditions: (1) The average value of the magnetic field intensity generated when the transmitting coil is placed in the middle 8×8 mm area is less than 0.5 mT; (2) The micro-coil is placed 0.5 mm in the Z direction of the hippocampal brain slice, and the average value of the magnetic field intensity generated within the 2.5×2.5 mm area of the hippocampal brain slice is not less than 1 mT.

[0032] Through model analysis, it is known that: to ensure that the maximum value of the magnetic field intensity generated in the middle area of 8×8 mm of the transmitting coil is less than 0.5 mT, the input signal amplitude is less than or equal to 0.5 A. The maximum value of the magnetic field intensity, the average value of the magnetic field intensity, and the percentage of the data volume with B>1 mT in the 2.5×2.5 mm area of the hippocampal brain slice were studied when a signal of 0.5 A and 620 kHz - 920 kHz was applied to the transmitting coil. Finally, the operating parameters of the passive micro-magnetic stimulator were obtained as follows: The excitation signal of the transmitting coil selects a sinusoidal signal with a frequency of 920 kHz and an amplitude of 0.5 A: In the 2.5×2.5 mm area of the hippocampal region of the brain slice, the maximum value of the magnetic field intensity is 1.42 mT, the average value of the magnetic field intensity is 1.17 mT, and the percentage of the magnetic field intensity exceeding 1 mT is 86.96%.

[0033] Under these operating parameters, the transmission efficiency of the passive micro-magnetic stimulator was measured. The experimental results show that: the transmission efficiency of the system is 39.53%, the output power is 787 mV, and the induced current in the secondary circuit is 281.47 mA.

[0034] (4) Experimental verification of the magnetically coupled resonant passive micro-magnetic stimulator based on ex vivo hippocampal brain slices

[0035] The magnetically coupled resonant passive micro-magnetic stimulator based on ex vivo hippocampal brain slices was verified, mainly using the Comsol finite element model and biological experiments for verification.

[0036] A. Verification of the stimulation effect of the passive micro-magnetic stimulator based on the Comsol finite element model

[0037] Based on the working parameters of the passive micro-magnetic stimulator determined above, simulate the experimental environment of online micro-magnetic stimulation based on the Comsol model: Figure 2 In Figure A, it shows the construction of the brain slice and the position of the recording electrode; Figure 2 In Figure B, the brain slice and the microcoil are placed at the middle position of the bottom of the glassware. The secondary coil is located 0.5 mm directly above the brain slice. ACSF is placed in the glassware. A sinusoidal signal with a frequency of 920 kHz and a current of 0.5 A is applied to the transmitting coil. The magnetic field distribution map of magnetic stimulation on the hippocampal brain slice is as Figure 2 shown in Figure C and Figure 2 Figure D: The maximum magnetic field intensity in the 2.5×2.5 mm area of the hippocampal brain slice is 1.51 mT, the average magnetic field intensity is 1.15 mT, and the percentage of data with a magnetic field intensity B > 1 mT is 83.43%.

[0038] B. Biological experiment verification

[0039] To verify the feasibility of the application of the passive micro-magnetic stimulator designed in this paper based on the magnetic coupling resonance wireless power transfer technology, male SD rats aged 14 - 18 days were anesthetized, and acute posterior components were obtained to get hippocampal brain slices. Subsequently, they were incubated in ACSF at 33°C for more than 1 hour. The brain slices were transferred into the glass electrodes of a multi-electrode array recording system (MEA2100 - 60, Reutlingen, Germany). The transmitting coil was tightly wound around the glass electrodes, and the micro-magnetic stimulation coil was placed above the Schaffer-CA1 neural pathway of the hippocampal brain slice.

[0040] Hippocampal brain slices with intact Schaffer-CA1 were selected for the experiment. Through microscopic observation, the stimulating electrode of the glass electrode was adjusted to correspond to the radiation layer of pyramidal cells in the CA1 region, and the recording electrode corresponded to the pyramidal cell layer. In the experiment, the fEPSPs baseline was first recorded for 10 min. High-frequency stimulation (HFS) with a frequency of 100 Hz, 100 stimulating pulses, and a duration of 1 s was used to induce LTP. After successful induction, fEPSPs were continuously recorded for another 20 min. The magnetic stimulation experiment was divided into three parts. Exp1 (control group): The microcoil was not placed in the glass electrode, and only a sinusoidal current with a frequency of 920 kHz and a current of 0.5 A was applied to the transmitting coil for magnetic stimulation for 10 min. Exp2 (sham exposure group): The microcoil was placed in the glass electrode, but no sinusoidal current was applied to the transmitting coil. Exp3 (experimental group): The microcoil was placed in the glass electrode, and a sinusoidal current with a frequency of 920 kHz and a current of 0.5 A was applied to the transmitting coil, and a continuous sinusoidal magnetic field intensity was coupled into the microcoil for magnetic stimulation for 10 min. After the stimulation, fEPSPs were continuously recorded for another 20 min. During the entire recording process of fEPSPs, 2 recording points were sampled per minute. Here, with magnetic stimulation as the dividing line, 10 - 30 min was defined as LTP1, and 40 - 60 min was defined as LTP2. The experimental results are as Figure 3As shown in Figure (a), the statistical results are as Figure 3 shown in Figure (b).

[0041] From the statistical analysis of the experimental data of Exp1, it can be seen that the magnetic stimulation of the transmitting coil did not cause changes in LTP, determining that the magnetic field intensity generated by the transmitting coil has no effect on the hippocampal slices. From the analysis of the experimental results of Exp2, it is determined that the sham exposure group did not cause changes in LTP either. From the analysis of the experimental results of Exp3, it shows that the magnetic field intensity generated by the power coupled from the transmitting coil to the microcoil is greater than the threshold of neuronal electrical activity, enhancing LTP.

[0042] The advantages and positive effects of the present invention are:

[0043] In this paper, a passive micro-magnetic stimulator for ex vivo hippocampal slices was designed, a magnetic coupling resonance wireless power transmission theory and a finite element analysis model of the micro-magnetic stimulator were established, the transmission efficiency and mutual inductance coefficient of the system at different resonance frequencies were measured, and the magnetic field intensity and distribution law of the stimulation target of the hippocampal slices were analyzed. At a resonance frequency of 920 kHz, the wireless power transmission efficiency of the system reached 39.58% while ensuring an effective magnetic field intensity threshold of 1 mT for the microcoil. Finally, biological experiments were carried out based on ex vivo hippocampal slices for verification, and the results showed that the passive micro-magnetic stimulator has an enhancing effect on the synaptic plasticity LTP of the hippocampal CA1 region. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 is the experimental device of the passive micro-magnetic stimulator

[0045] Figure 2 is the magnetic field distribution map of the passive micro-magnetic stimulator stimulating the hippocampal slices

[0046] Figure 3 is the experimental result diagram of the Schaffer-CA1 synaptic plasticity LTP after passive wire micro-magnetic stimulation DETAILED DESCRIPTION OF THE INVENTION

[0047] The present invention will be further described below with reference to the drawings and embodiments.

[0048] Embodiment 1:

[0049] Step 1: Determine the circuit composition of the transmitting module and the stimulating module of the passive micro-magnetic stimulator;

[0050] Step 2: Determine the number of turns, wire diameter of the transmitting coil, and wire diameter of the selected winding wire; determine the structure and geometric parameters of the microcoil;

[0051] Step 3: Establish a theoretical model and a Comsol finite element model of the passive micro-magnetic stimulation;

[0052] Step 4: Based on the theoretical model of passive micro-magnetic stimulation, determine the parameters affecting the system transmission efficiency and derive the formula for the system transmission efficiency;

[0053] Step 5: Based on the Comsol finite element model, determine the maximum value of the magnetic field intensity in the 2.5×2.5 mm area of the hippocampal slice, the average value of the magnetic field intensity, and the percentage of the data volume with B>1 mT;

[0054] Step 6: Measure the transmission efficiency of the passive micro-magnetic stimulator at different resonant frequencies;

[0055] Step 7: According to the results of Step 5 and Step 6, determine the operating parameters of the passive micro-magnetic stimulation, the frequency and amplitude of the excitation source input;

[0056] Step 8: According to the operating parameters determined in Step 7, with the help of the Comsol finite element simulation model, determine the magnetic stimulation effect of the passive micro-magnetic stimulator on the hippocampal slice;

[0057] Step 9: According to the results of Step 7 and Step 8, build an experimental platform to verify the effectiveness of the passive micro-magnetic stimulator, with the target being the CA1 area of the hippocampal slice.

Claims

1. A magnetically coupled resonant passive micro-magnetic stimulator based on ex vivo hippocampal brain slices, characterized in that, The micro-magnetic stimulator consists of two parts: a transmitting module and a stimulating module. The transmitting module provides energy for the stimulating module through the principle of magnetic coupling resonance to complete the targeted magnetic stimulation of neurons in the isolated hippocampal slices. The method for the magnetically coupled resonance passive micro-magnetic stimulator based on isolated hippocampal slices is as follows: (1) System design of the magnetically coupled resonance passive micro-magnetic stimulator based on isolated hippocampal slices The system design of the magnetically coupled resonance passive micro-magnetic stimulator based on isolated hippocampal slices is carried out, which consists of two parts: a transmitting module and a stimulating module. Specifically: Transmitting module: It consists of an SDG1020 signal source, an FPA301 power amplification module, a transmitting coil, a resonant capacitor, and a heat dissipation resistor; the transmitting coil is a circular spiral coil: the inner diameter is 2.5 cm, the outer diameter is 2.7 cm, the coil height is 5.5 mm, the number of turns is 22, and the diameter of the wire winding the coil is 0.7 mm; the inductance of the transmitting coil is 16.9811 μH, and the heat dissipation resistor is 3 Ω, 50 W. Stimulating module: It consists of a micro-coil and a resonant capacitor; the micro-coil is a planar square spiral coil with a size of 3.66×3.66 mm, the number of turns of the coil is 8, the line width is 110 μm, the line spacing is 70 μm, the inductance is 1.762 μH, and it is processed by a 4-layer series flexible circuit board; the micro-coil is insulated by parylene in vacuum, and the coating thickness is 5 μm to ensure good biocompatibility and waterproofness. (2) Construction of the wireless energy transfer theoretical model of the magnetically coupled resonance passive micro-magnetic stimulator based on isolated hippocampal slices A wireless energy transfer theoretical model is established for the magnetically coupled resonance passive micro-magnetic stimulator based on isolated hippocampal slices, and the transmission efficiency formula of the passive micro-magnetic stimulator in the resonant state is deduced: Among them, P1 represents the input power, P2 represents the output power, L2 represents the inductance of the micro-coil, ω represents the angular frequency, R1 represents the DC resistance of the primary coil, R2 represents the DC resistance of the micro-coil, M represents the mutual inductance between the transmitting coil and the micro-coil, and R ACSF is the radiation resistance; (3) Determination of the working parameters of the magnetically coupled resonance passive micro-magnetic stimulator based on isolated hippocampal slices The working parameters of the magnetically coupled resonance passive micro-magnetic stimulator based on isolated hippocampal slices are determined, and the following two conditions need to be met: a) The average value of the magnetic field intensity generated by the transmitting coil placed in the middle 8×8 mm area is less than 0.5 mT; b) The micro-coil is placed 0.5 mm in the Z direction of the hippocampal slice, and the average value of the magnetic field intensity generated in the 2.5×2.5 mm area of the hippocampal slice is not less than 1 mT; With the help of the wireless transmission model of the passive micro-magnetic stimulator established by Comsol, under the conditions of meeting a) and b), the working parameters of the passive micro-magnetic stimulator are determined as follows: the excitation signal of the transmitting coil is 920 kHz, 0.5 A, and the resonant capacitor of the micro-coil circuit is 15.82 nF; the maximum value of the magnetic field intensity generated by the micro-coil in the 2.5×2.5 mm area of the hippocampal slice reaches 1.42 mT, the average value of the magnetic field intensity is 1.17 mT, and the percentage of the data with the average magnetic field intensity exceeding 1 mT is 86.96%; under these working parameters, the transmission efficiency of the passive micro-magnetic stimulator is measured to be 39.95%. (4) Experimental verification of the magnetically coupled resonance passive micro-magnetic stimulator based on isolated hippocampal slices The magnetic coupling resonance passive micro-magnetic stimulator based on ex vivo hippocampal brain slices was experimentally verified. When the excitation signal of the transmitting coil was 920 kHz and 0.5 A, the average magnetic field intensity generated by the transmitting coil in the middle 8×8 mm area was less than 0.5 mT, which had no effect on the hippocampal brain slices; the average magnetic field intensity generated by the micro-coil in the 2.5×2.5 mm area of the hippocampal brain slices was greater than 1 mT, enhancing LTP and reaching the neuronal response threshold in the CA1 region of the hippocampal brain slices.

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