A wireless magnetic resonance energy transfer system for micro implantable medical sensors.

Li X, Zhang H, Peng F, Li Y, Yang T, Wang B, Fang D - Sensors (Basel) (2012)

Related In: Results  -  Collection

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f4-sensors-12-10292: (a) Coupling factor k23versus distance (k12 = k34 = 0.5, L2 = 25 μH, L3 = 28 μH); (b) Power efficiency versus distance and frequency.
Mentions: The four coils are set to be resonant at the same frequency, ω = 1/√LnCn, n ∈ {1,2,3,4} and Zmn = Rn. For small driver and load coil inductance and relatively large distance between coil1 and coil4, coil1 and coil3, and coil2 and coil4, the cross coupling factor k13, k14, k24 can be neglected. The current in coil1 (i1) and coil4 (i4) can be calculated from Equation (10) and Qn (Qn = ωLn/Rn). Therefore, the power transfer efficiency can be obtained as [6]:(12)η=i42RLUsi1=(k122Q1Q2)(k232Q2Q3)(k342Q3Q4)[(1+k122Q1Q2)(1+k342Q3Q4)+k232Q2Q3][1+k232Q2Q3+k342Q3Q4]from Equation (12), the coupling factor is the primary factor to improve power transfer efficiency. Based on equivalent circuit model, coupling factor between coil2 and coil3 can be calculated as:(13)k23=M23/√L2L3(14)M23=N2N3M(a2,a3,d)where N2 and N3 are the turns of coil2 and coil3. M(a2,a3,d) is the mutual inductance between two single coils (with radius of a2 and a3, distance d) according to Equation (2). According to the optimized coil geometry parameters according to the theoretical model in Table 1, the model of k23 can be calculated from Equations (13,14) and it is a function of distance. In additon k12, k34 can aslo be derived as 0.5 from Equations (13,14) under certain distance. Figure 4(a) plots the coupling factor k23 as a function of distance (d) between transmitter coils and receiver coils, and Figure 4(b) plots the power efficiency as a function of operating frequency (f) and distance (d) between transmitter coils and receiver coils. It is indicated that the maximum power transfer efficiency is achieved at 650 kHz. Therefore, based on the above optimization and analysis results, the best energy transfer efficiency is obtained at the working frequency of 650 kHz for this four-coil resonant power transfer system.

Bottom Line: The energy transfer efficiency of the four-coil system is greatly improved compared to the conventional two-coil system.In addition, the output current varies with changes in the distance.The whole implanted part is packaged with PDMS of excellent biocompatibility and the volume of it is about 1 cm(3).

View Article: PubMed Central - PubMed

Affiliation: School of Electronics and Information Engineering, Beijing Jiaotong University, Beijing 100044, China. lixiuhan@bjtu.edu.cn

ABSTRACT
Based on the magnetic resonance coupling principle, in this paper a wireless energy transfer system is designed and implemented for the power supply of micro-implantable medical sensors. The entire system is composed of the in vitro part, including the energy transmitting circuit and resonant transmitter coils, and in vivo part, including the micro resonant receiver coils and signal shaping chip which includes the rectifier module and LDO voltage regulator module. Transmitter and receiver coils are wound by Litz wire, and the diameter of the receiver coils is just 1.9 cm. The energy transfer efficiency of the four-coil system is greatly improved compared to the conventional two-coil system. When the distance between the transmitter coils and the receiver coils is 1.5 cm, the transfer efficiency is 85% at the frequency of 742 kHz. The power transfer efficiency can be optimized by adding magnetic enhanced resonators. The receiving voltage signal is converted to a stable output voltage of 3.3 V and a current of 10 mA at the distance of 2 cm. In addition, the output current varies with changes in the distance. The whole implanted part is packaged with PDMS of excellent biocompatibility and the volume of it is about 1 cm(3).

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