Electromagnetic simulations guide electrode design in novel cancer treatment

Feb. 7, 2002
The vector plot is a cross section of a four-electrode array.

The vector plot is a cross section of a four-electrode array. Two needles are at a high potential and two at low potential. Cosmos/EMS calculates field strength for a range of applied voltages.

The color contours provide another way to look at the vector plot of the fourelectrode array. The EMS software lets users select a point in the analysis and report on its field strength.

Cosmos/EMS generates an electric-field strength contour plot of a six-electrode array with two needles at high potential, two at a low potential, and two dormant.

A process called electroporation opens pores in human and other mammalian tissue cells using electric fields, thereby providing the basis for a new cancer treatment. In the process, chemotherapeutic agents are first applied directly to cancerous tumor cells. Then the electric field is applied. This causes an increased uptake of the agent and results in a more focused application. Traditional applications might inject agents systematically, spreading them throughout the body. The new treatment looks most promising for head and neck tumors.

Genetronics Biomedical Ltd., San Diego, one developer of this technology, has also tested the technique in gene therapy as a way to deliver DNA into cells. According to the company, medical studies indicate less patient discomfort and faster recovery than traditional treatments.

One challenge in optimizing the system is designing electrodes that can generate required field strengths around cancerous tissue. Engineers at Genetronics have turned to Cosmos/EMS, finite-elementbased software from SRAC, Los Angeles, to calculate field strengths based on electrode placement and applied voltage.

"A treatment might involve two, four, or six electrodes, with each pair pulsed in sequence or simultaneously," says Chris Andre, a senior mechanical engineer with Genetronics. "After modeling electrode depth and location, we analyze the field strengths created through the defined volume. Research thus far has shown that a minimum of 600 V/cm is required to increase drug delivery. This applied voltage results in a current of about 5 to 85 amps during treatment," says Andre.

To determine or analyze electric-field strengths, users need only the electrostatic module in the EMS software. It assumes objects are either perfect conductors or perfect insulators. "I usually create electrodes and their array in a solid modeler, define the space around the array, and then mesh it," says Andre. "Inputting a permittivity value lets the software model human tissue," he adds. Analysis after meshing takes about five minutes on a computer with a 933-MHz processor, 256-Mbyte RAM, and a 64-Mbyte video card. Because the analysis is so brief, Andre assigns the finest mesh possible, sometimes using 100,000 or more elements in the region under study.

An electroporation pulse lasts less than a second. The equipment can vary the number of pulses, but generally it's low frequency. Clinical studies have determined the appropriate pulse parameters for optimal drug delivery.

Genetronics' electroporation instrument, called MedPulser, does not require a physician to set up pulse characteristics. The device is preset to deliver a required voltage across the electrodes. "The equipment's pulse generator defines pulse amplitude, duration, and frequency," he adds.

For electrostatic studies in the medical arena, Andre suggests obtaining a good understanding of the required field strength. "Once we have that, we can work toward defining the electrodes and their arrangement." Andre also suggests learning to use an integrated FEA and CAD modeling system. "Then it's easy to go back to the modeler and change geometry when necessary, and it will be necessary." Andre's FEA-EMS software works with SolidWorks from SolidWorks Inc., Concord, Mass.

Don't be bashful asking the FEA developer for assistance, adds Andre. "Working through tutorials and speaking to the software developer's support staff shortens the learning curve, so following iterations of design to analysis become faster as you do more."

Field generators online

To get an idea of how electric fields form with regard to electrode placement, dial in to www.its.caltech.edu/ ~phys1/java/phys1/ EField/EField.html. Viewers can place electrodes in a plane, identify a relative charge and polarity, and watch the software define fields and potentials. A similar generator with a bit more explanation can be found at www.slcc.edu/schools. hum_sci/physics/tutor/ 2220/e_fields/java/.

For permittivity values of human tissue, Andre suggests referring to CRC Handbook of Biological Effects of Electromagnetic by Fields, Polk, Charles, and Elliot Postow, eds. Boca Raton: CRC Press, 1988.

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