# Design of a Fully Integrated Inductive Coupling System: A Discrete Approach Towards Sensing Ventricular Pressure

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## Abstract

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^{2}area. This work considers a robust magnetic coupling between an external reading coil and the implantable module: a three-dimensional inductor and a touch mode capacitive pressure sensor (TMCPS) set. In this approach, the coupling modules were modelled as RCL circuits tuned at a 13.56 MHz frequency. The analytical design was validated by means of Comsol Multiphysics, CoventorWare, and ANSYS HFSS software tools. A power transmission efficiency (PTE) of 94% was achieved through a 3.5 cm-thick biological tissue, based on high magnitudes for the inductance (L) and quality factor (Q) components. A specific absorption rate (SAR) of less than 1.6 W/Kg was attained, which suggests that this IPT system can be implemented in a safe way, according to IEEE C95.1 safety guidelines. The set of inductor and capacitor integrated arrays were designed over a very thin polyimide film, where the 3D coil was 18 mm in diameter and approximately 50% reduced in size, considering any conventional counterpart. Finally, this new approach for the IMD was under development using low-cost thin film manufacturing technologies for flexible electronics. Meanwhile, as an alternative test, this novel system was fabricated using a discrete printed circuit board (PCB) approach, where preliminary electromagnetic characterization demonstrates the viability of this bidirectional IPT design.

## 1. Introduction

## 2. Theoretical Inductive Power Transfer Design

^{2}, which demands new design features for the telemetric design. The design considers that the external coil is located on the outer surface of the body and the implantable coil is placed inside the left ventricle at a depth of 3.5 cm. This distance corresponds to the combined thickness of the skin, fat, and muscle, separating both coils. The two RCL circuits are tuned to a resonance frequency of 13.56 MHz to attain the best PTE. According to the ISO 14,117 standard, this frequency does not induce tissue damage by radiation and heating, nor cause interference with other implantable medical devices. Table 1 shows the constitutive parameters of the biological tissue used as the magnetic core.

#### 2.1. Mechanical Sensing Action

#### 2.2. Inductive Power Calculation

#### 2.3. Electromagnetic Flux Coupling

#### 2.4. Electromagnetic Flux Calculation

#### 2.4.1. RCL Implantable Set

#### 2.4.2. RCL External Device

#### 2.5. Results

## 3. Electromagnetic Flux Simulation

## 4. Experimental Coupling Evaluation

#### 4.1. Experimental Methodology

#### 4.1.1. Implantable Coil Over PCB

#### 4.1.2. The External Module

#### 4.2. Experimental Results

#### 4.2.1. Inductive Coupling

#### 4.2.2. Capacitance Factor

#### 4.2.3. Misalignment Response

#### 4.2.4. Combined Electromagnetic Core

_{i}was performed as follows. The B graph represents both coils separated by a container with Hartmann solution as the magnetic core, realized in steps of 0.5 cm. The A graph represents the synthetic tissue, increasing from 2 mm to 3.5 cm thickness, and then the rest of distance separation was increased step by step across air. The A+B graph represents the synthetic tissue, from 2 mm to 3.5 cm thickness, and then the rest of distance was stepped with the Hartmann solution. The A+B+Synthetic skin graph represents the 3.5 cm synthetic tissue added to another polymer named synthetic skin with 2 mm thickness, and then the full distance was filled with Hartmann solution. As can be seen, the best transmitting signal corresponds to the A graph and the worst transmitting signal condition is across the full Hartmann solution array. This is expected because of the aqueous ionic composition. It is worthwhile mentioning the transmitted strength of the A signal, which is very near to that of the B graph, clearly surpassed the required 3.5 cm of biological tissue for effective coupling. The other two graphs are intermediate and very similar because the magnetic core contained a mixed synthetic material and the aqueous ionic liquid. The inductive coupling across the full-ionic solution, as the worst transmitting condition, shows excellent electromagnetic signal transmission. With this experimental work, we are then able to demonstrate the feasibility of our proposed IPT module.

## 5. Discussion

^{2}implanting area, is discussed, attending the proposal for implanting into the left ventricle. Several details for biocompatibility are considered and fulfill regulations, according to IEEE C95.1 guidelines. This novel system includes the following: an integrated internal capacitive sensor and inductor array and an external reading module.

_{i}was obtained to feed the capacitive array and generate a good enough electromagnetic response. However, for 4 cm and higher misalignments, the lateral displacement played an important role, since the reduced V

_{i}signal could be confused with the reference signal of the measuring equipment, which was approximately 20 mV. Therefore, we can conclude that this discrete-like IPT system can transmit receiving data through two coils under a transverse misalignment no greater than 3.5 cm, which is a high window for possible misaligning conditions.

## 6. Conclusions

^{2}internal area, was discussed, attending the proposal for implanting into the left ventricle. Several details for biocompatibility were considered and fulfilled regulations according to IEEE C95.1 guidelines.

## Author Contributions

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 2.**Inductive-coupling array (Maxwell–Wien bridge circuit) to perform transfer power and indirect ventricular pressure measurements.

**Figure 4.**The intensity of field $\mathrm{H}$ for the external coil as a function of external radius ${\mathrm{R}}_{\mathrm{out}.\mathrm{r}}$ considering three radiation distances $\mathrm{X}$.

**Figure 6.**The relationship of both parameters: (

**a**) the coupling factor and (

**b**) the mutual inductance as a function of distance separation between the two coils.

**Figure 7.**(

**a**) The quality factor as a function of the capacitance variations in the touch mode capacitive pressure sensor (TMCPS). (

**b**) The relationship between the internal quality factor and the power transmission efficiency (PTE) system.

**Figure 8.**(

**a**) Inductance characteristic of the internal coil; L = 25.6 μH, R = 280.8 Ω, Q = 12.4. (

**b**) Quality factor of the internal coil, (

**c**) inductance of the external coil; L = 20.18 μH, R = 6.61 Ω, Q = 325.4. (

**d**) Quality factor of the external coil.

**Figure 9.**The intensity of the magnetic flux for the inductive power transfer (IPT) system at 13.56 MHz. The bottom plane represents the external coil (radiating source).

**Figure 10.**Variation of the magnetic flux density at (

**a**) 4.5 MHz, (

**b**) 7 MHz, (

**c**) 9.5 MHz, and (

**d**) 12 MHz.

**Figure 12.**The specific absorption rate for the bidirectional IPT system at the operating frequency of 13.56 MHz.

**Figure 13.**Modules manufactured on PCB-FR4 substrate. (

**a**) Bridge circuit and (

**b**) internal and external coils. (

**c**) Configuration for measuring the coupling at 3.5 cm distance between the coils.

**Figure 14.**Induced voltage as a function of the separation distance between the coils for (

**a**) air and (

**b**) synthetic tissue.

**Figure 15.**Experimental output voltage ${\mathrm{V}}_{\mathrm{out}}$ as a function of capacitance and frequency at the implantable set, using a synthetic tissue as the magnetic core.

**Figure 16.**Characterization of induced voltage under transverse misalignment for different radiation distances through (

**a**) air and (

**b**) synthetic tissue.

**Figure 17.**Induced voltage as a function of the separation distance between the coils combining different materials as the electromagnetic core.

**Table 1.**Constitutive parameters of human biological tissue at a frequency of 13.56 MHz [17].

Model | Thickness [cm] | Conductivity [Sm^{−1}] | Relative Permittivity | Wavelength [m] |
---|---|---|---|---|

Skin | 0.5 | 0.38421 | 177.13 | 2.87 |

Fat | 1 | 0.030354 | 11.827 | 11.11 |

Muscle | 2 | 0.62818 | 138.44 | 3.24 |

Quantity | Symbol | Internal Coil | External Coil |
---|---|---|---|

Internal diameter | Din | 2 mm | 2 mm |

External diameter | Dout | 18 mm | 8 cm |

Width | w | 160 μm | 700 μm |

Thickness | h | 1.5 μm and 1 μm | 35 μm |

Number of turns | N | 28 each loop | 27 |

length | l | 1.14 m | 1.77 m |

Self-inductance | L | 20.98 μH | 21.24 μH |

Electrical resistance | R | 171.86 Ω | 5.6 Ω |

Quality factor | Q | 11.5 | 354 |

Operating frequency | f_{s} | 13.56 MHz | |

Radiation distance | X | 3.5 cm | |

Coupling coefficient | k | 0.16 | |

Mutual inductance | M | 3.79 μH | |

Power transmission efficiency | η | 92.%–74.5% |

Parameter | Symbol | Magnitude |
---|---|---|

Internal diameter | Din | 2 mm |

External diameter | Dout | 3.8 cm |

Width | w | 250 µm |

Separation | S | 250 µm |

Thickness | h | 35 µm |

Number turns | N | 37 |

Length | l | 1.3 m |

Self-inductance | L | 21.29 µH |

Resistance | R | 6.9 Ω |

Component | Magnitude | Units | |
---|---|---|---|

Resistances | ${\mathrm{R}}_{\mathrm{r}}{,\mathrm{R}}_{1}$ | $4.5$ | $\Omega $ |

${\mathrm{R}}_{2}{,\mathrm{R}}_{3}$ | $47$ | $\Omega $ | |

Capacitances | ${\mathrm{C}}_{1}$ | $6.8$ | $\mathrm{pF}$ |

${\mathrm{C}}_{\mathrm{r}}$ | $50$ | $\mathrm{pF}$ | |

Inductances | ${\mathrm{L}}_{1},{\mathrm{L}}_{\mathrm{r}}$ | $21.29$ | $\mathrm{uH}$ |

${\mathrm{L}}_{\mathrm{s}}$ | $22.5$ | $\mathrm{uH}$ |

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**MDPI and ACS Style**

Hernández Sebastián, N.; Villa Villaseñor, N.; Renero-Carrillo, F.-J.; Díaz Alonso, D.; Calleja Arriaga, W. Design of a Fully Integrated Inductive Coupling System: A Discrete Approach Towards Sensing Ventricular Pressure. *Sensors* **2020**, *20*, 1525.
https://doi.org/10.3390/s20051525

**AMA Style**

Hernández Sebastián N, Villa Villaseñor N, Renero-Carrillo F-J, Díaz Alonso D, Calleja Arriaga W. Design of a Fully Integrated Inductive Coupling System: A Discrete Approach Towards Sensing Ventricular Pressure. *Sensors*. 2020; 20(5):1525.
https://doi.org/10.3390/s20051525

**Chicago/Turabian Style**

Hernández Sebastián, Natiely, Noé Villa Villaseñor, Francisco-Javier Renero-Carrillo, Daniela Díaz Alonso, and Wilfrido Calleja Arriaga. 2020. "Design of a Fully Integrated Inductive Coupling System: A Discrete Approach Towards Sensing Ventricular Pressure" *Sensors* 20, no. 5: 1525.
https://doi.org/10.3390/s20051525