Charged Up

By Venkat Gaddam, Senior Electrical Engineer, Medtronic, Minneapolis, U.S.A.

Medtronic ensures the safety of recharging subcutaneous medical devices through simulation.

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Charged Up - Medtronic neurostimulator

Neurostimulator recharge diagram 
Diagram of recharger and neurostimulator

Neurostimulators that are placed under the patient’s skin deliver mild electrical signals to provide pain relief by blocking pain messages before they reach the brain. Unlike oral medications that circulate through the patient’s body, the neurostimulator targets the precise area where pain is experienced. Patients can try a neurostimulator to see if it relieves their pain before committing to longterm therapy; the device can be surgically removed later if the patient decides to pursue a different treatment. The batteries of rechargeable neurostimulators are recharged by low-frequency inductive energy transfer using a recharger that is attached to the patient’s belt. The recharger emits a non-radiating magnetic field ranging from 3 kHz to 300 kHz that penetrates human tissue and the implanted device’s sealed metal enclosure for communication and recharging.

Depending upon the operating configuration, wireless power transfer devices operating at frequencies above 9 kHz are subject to Part 15 and/ or Part 18 of Federal Communications Commission (FCC) rules. Medical device manufacturers routinely file Office of Engineering and Technology Laboratory Division Knowledge Database (KDB) inquiries with the FCC to obtain further guidance for wireless power transfer compliance evaluations. As a result of one such inquiry, Medtronic ― the world’s largest medical technology company ― was asked to demonstrate radio frequency (RF) exposure compliance for a wireless power transmitter.

The cost and time required to build a test rig capable of measuring specific absorption rate (SAR) ― the rate at which energy is absorbed by the human body when exposed to an RF electromagnetic field ― from the recharger is quite high. Medtronic was able to avoid these costs and delays in developing its neurostimulators by using ANSYS Maxwell electromagnetic field simulation software to simulate the operation of the recharger and predict SAR in local body tissues. Simulation showed that SAR generated by the recharger was far below existing FCC limits; the FCC accepted the simulation results for certification of the neurostimulator recharger.

ANSYS Maxwell tissue model
10-gram tissue model in ANSYS Maxwell with recharger coil
Muscle tissue ANSYS Maxwell model
1-gram muscle tissue Maxwell model

Medtronic was able to avoid costs and delays in developing its neurostimulators by using ANSYS Maxwell to simulate the operation of the recharger and predict SAR in local body tissues.

TRANSCUTANEOUS RECHARGE INDUCED SAR

The existing FCC RF exposure requirement prescribed by §2.1093(d)(2) requires a SAR exposure limit of 0.08 W/kg as averaged over the whole body, and a spatial peak SAR not exceeding 1.6 W/kg as averaged over any 1 gram of cube-shaped tissue. SAR is the variable typically used to quantify the effects on tissue exposure to RF signals (defined as the time derivative of the incremental energy absorbed by an incremental mass contained in a volume of given density). Spatial peak SAR is determined by calculating the SAR values in the neighborhood of the electromagnetic source. The domain is then divided into cubes of a given size, and the average SAR value in each cube is evaluated. The peak spatial-averaged SAR is determined by the cube with the highest average SAR value.

Medtronic engineers were confident that their recharging system produced low levels of exposure but needed to measure these levels to obtain approval for a new product. There are a number of FCC-certified testing organizations that perform SAR measurements on a contract basis, but Medtronic engineers soon learned that these organizations were not set up to run tests at frequencies as low as those used by the neurostimulator recharger.

Engineers were confident that the recharging system produced levels of exposure far below the limit specified in the regulation but needed to measure these levels to submit a new product.

ANSYS human body model coil position
Coil positioning in ANSYS human body model

ESTIMATING SAR WITH ANSYS MAXWELL

Medtronic engineers used ANSYS Maxwell to estimate the SAR values generated by the recharger coil, with the expectation that the FCC would accept accurate, validated simulation results in lieu of physical testing. They selected Maxwell because the tool makes it easy to set up the model and mesh, and solution times are relatively short. Medtronic engineers employed human tissue models that are available with Maxwell, including muscle sectioned into 10-gram cubes and muscle sectioned into 1-gram cubes. They also used a section tissue model containing skin, fat, fascia and muscle layers. The engineers specified the strength and geometry of the magnetic field generated by the charger. Maxwell adaptively generated an appropriate mesh for solving the problem and used the finite element method to calculate the quasi-static electromagnetic field throughout the solution domain.

Medtronic engineers worked with ANSYS support engineers to create a simple Visual Basic script that calculated SAR values based on the results of the simulation. To calculate the peak spatially averaged SAR, the script calculated the SAR at every element in a 0.25-meter by 0.19-meter by 0.04-meter tissue section. Each 10-gram cube had 2.15-centimeter edges. The peak SAR values as predicted by Maxwell simulation were much lower than the current FCC limits.

SAR values based on the simulation results
SAR values based on the simulation results

Engineers used three different methods to validate the accuracy of Maxwell’s predictions.

VALIDATING THE SIMULATION

Simulation vs. testing
Validating the model against analytical and physical testing results

Medtronic engineers used three different methods to validate the accuracy of Maxwell’s predictions. First, they created a very simple model and hand-calculated the magnetic field with the Biot–Savart equation ― which relates the magnetic field to the magnitude, direction, length and proximity of the electric current ― and at the same time performed the calculation with Maxwell software. Second, they set up a simple physical test using a NARDA Safety Test Solutions® electric and magnetic field probe analyzer to measure the magnetic fields generated by the recharger, and then compared these measurements to a Maxwell simulation. Finally, engineers used ANSYS HFSS 3-D full-wave electromagnetic field simulator to simulate peak 10-gram–averaged and peak 1-gram–averaged SAR values. The Maxwell simulations generated results within a few percent of those produced by each of the validation methods.

Medtronic included the Maxwell simulations as part of its new product submission to the FCC. Medtronic engineers were further able to demonstrate that the SAR value would not have been any larger if the tissue geometry in the model had been divided into a different set of cubes. This was addressed by moving a cube around the tissue geometry in discrete step sizes and calculating the average SAR value at each possible position for the cube. Sweeping the cubic volume determined the average SAR value of every possible cube within the tissue volume of interest. The results showed that the maximum possible peak average SAR value for a 1-gram cube is 15.68 mW/kg, less than 1 percent higher than the value in the model that was arbitrarily partitioned.

Medtronic determined that ANSYS Maxwell provides a fast and relatively simple method of measuring 10-gram and 1-gram peak spatially averaged SAR as required to comply with FCC regulations for recharging devices. The FCC accepted the simulation results, saving the company a considerable amount of time and money that would have been required to obtain the same data using physical testing.

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