Wireless Power Transfer (WPT) enables the transfer of electrical energy without the need for physical connectors or wires. This technology holds significant potential in various fields, especially in healthcare, where it can be applied to power implantable medical devices such as pacemakers, neurostimulators, and drug delivery systems. By eliminating the need for wired connections, WPT can enhance the comfort and longevity of medical devices, reduce the risk of infections, and provide more flexibility for patients.
However, with the introduction of wireless power to medical devices, ensuring safety becomes crucial. One critical factor to consider is the Specific Absorption Rate (SAR), which measures the amount of electromagnetic energy absorbed by the human body. In the case of implantable medical devices, it is essential to evaluate the SAR to ensure that the wireless power transfer does not exceed safe limits, potentially causing harm to tissues or organs.
Simulating both Wireless Power Transfer and SAR is essential to optimize the performance of implantable medical devices while ensuring patient safety. Ansys HFSS addresses these challenges with its advanced electromagnetic simulation capabilities, enabling engineers to model and analyze implantable devices in detail. By accurately simulating power transfer efficiency and SAR levels, HFSS provides critical insights that support the design of safer, more efficient, and reliable medical devices.
In this blog, we will explore how Ansys HFSS is used to simulate Wireless Power Transfer and SAR for implantable medical devices.
Co-Simulation Workflow:
SAR quantifies the rate at which electromagnetic energy is absorbed by a unit mass of tissue. It is expressed in watts per kilogram (W/kg) and is given by:
Where:
- σ = Conductivity of the tissue (S/m)
- |E| = Electric field strength (V/m)
- ρ = Mass density of the tissue (kg/m³)
In the United States and Canada, SAR limits are set at 1.6 W/kg, averaged over 1 g of tissue. In the European Economic Area, the limit is higher at 2 W/kg, averaged over a larger volume of 10 g of tissue.
In the following example, a WPT system operating at 15 MHz is simulated with the Ansys head model.
HFSS model of the WPT device and Ansys head model
Simulating the transmitter/receiver coil by itself in HFSS helps determine its inductance, which is then used to calculate the necessary capacitance to achieve resonance at the desired operating frequency. The following coil was created for this example and shows an inductance of 0.238 μH at 15 MHz.
Transmitter coil
Transmitter coil inductance in microhenries
Additionally, the coil's self-resonance can be plotted as shown below:
Transmitter Z Parameter
Once the HFSS simulation of the coil is complete, it is linked to Circuit, where tuning capacitors are added and tuned to achieve resonance at 15 MHz. The circuit schematic with the tuned capacitors is shown below:
The plotted S-parameters show excellent resonance, with a return loss better than -30 dB at 15 MHz.
Return loss
In the full HFSS model, a similar coil was used for the receiver, with the addition of a plastic enclosure.
The model, including the transmitter, receiver, and head, is initially simulated in HFSS and then dynamically linked to Circuit, where tuning capacitors are added to the design. After tuning the capacitors in Circuit, the updated excitation is pushed back to HFSS, allowing for the visualization of electromagnetic fields with the integrated circuit elements and enabling SAR calculations. The circuit schematic with the tuned capacitors is shown below:
Co-simulation allows the circuit model to control excitations for post-processing of the EM solution. To set 1W of input power in HFSS with a 50-ohm load, the ACMAG in the voltage source properties of Port 1 should be set to 20V, according to the following relationship:
The WPT S-parameters show good resonance at 15 MHz, with a return loss better than -19 dB and an insertion loss of -0.75 dB.
S-Parameters
The corresponding WPT efficiency, calculated as the magnitude of S21 squared, is 84%.
WPT efficiency
Once the updated excitation is pushed back to HFSS, the SAR maximum value and distribution are calculated. First, an object list is created, including the skin and skull objects. Then, it is selected in the SAR settings window, where the mass of tissue and voxel size are specified. With the following settings, the SAR averaged over 1 gram of tissue is calculated using the IEC/IEEE 62704-4 method.
The input power can be verified in HFSS through the Edit Sources window. In this case, the input power is 1 W.
With 1 W of input power and 1 g of tissue mass, the simulated SAR maximum value is 0.4 W/kg. This corresponds to an input power of 4 W to achieve the US limit of 1.6 W/kg.
The simulated SAR maximum value averaged over 10 g is 0.082 W/kg, which corresponds to an input power of 24.4 W to achieve the EU limit of 2 W/kg.
Below is an overview video of the WPT and SAR simulation workflow using Ansys HFSS:
Ozen Engineering Expertise
Ozen Engineering Inc. leverages its extensive consulting expertise in CFD, FEA, optics, photonics, and electromagnetic simulations to achieve exceptional results across various engineering projects, addressing complex challenges like wireless power transfer and SAR simulation for implantable devices.
We offer support, mentoring, and consulting services to enhance the performance and reliability of your electronic system. Trust our proven track record to accelerate projects, optimize performance, and deliver high-quality, cost-effective results for both new and existing systems. For more information, please visit https://ozeninc.com.