Electrosurgical pad eases trauma: whole CAE eases design process.

3M Company, a manufacturer of over 60,000 industrial and consumers products, uses CAE methods extensively in its product and process development efforts. Many of their analyses are performed using ANSYS software. "Our modeling needs are as diverse as our product lines and manufacturing processes," said Dave Lindeman, a senior CAE specialist who works in one of 3M's Central Research laboratories in St Paul, MN.

Within 3M, ANSYS has been used to design and optimize everything from MEMS devices to large-scale capital equipment, and from asthma inhalers to diaper closures. "ANSYS provides us with a powerful tool for addressing our most challenging design problems, whether they involve structural mechanics, heat transfer, fluid flow, electromagnetics, or some combination of the above," Lindeman said.

One recent modeling project involved the development of a revolutionary electrosurgical grounding pad.

Electrosurgery refers to surgical procedures in which high-frequency AC is passed through a surgical instrument to cause cutting and/or coagulation. A complete circuit is formed by the waveform generator, the electrosurgical instrument, and an electrosurgical grounding pad. Historically, large pads were used so that current densities and patient trauma were minimized at the current collection site. However, large pads are more difficult to handle, do not easily adhere to curved surfaces, and restrict flexibility in placement.

The 9100 Series 3M Universal Electrosurgical Pad is smaller and therefore easier to place on patients. The pad can be smaller because it features 3M's Safety Ring technology, a patented coating technology that distributes electrosurgical current more uniformly over the conductive surface of the pad. Safety Ring also permits universal orientation--the pad may be oriented in any direction relative to the surgical site. The 9100 Series pad is smaller, but maintains the safety of traditional pads up to 33% larger, and meets the performance criteria set forth by the Association for the Advancement of Medical Instrumentation (AAMI).

Safety Ring involves the use of a scrim embedded within the adhesive layer of the pad. This scrim locally modifies the effective impedance of the adhesive, and reduces the current density concentrations observed at the corners and edges of traditional pads.

Although discovered experimentally, simulation was critical to the development of Safety Ring technology. In particular, a model of an idealized human thigh was used to evaluate current density levels in the corium and epidermis layers of the skin. A number of design sensitivity studies were performed to determine the dependence of current density levels on pad geometry, scrim impedance distributions, and other variables. The analyses helped identify the optimal pad design, and confirmed experimentally observed behavior. In particular, the analyses confirmed that corner and edge effects could be eliminated through proper pad design.

"The powerful scripting capabilities allowed us to efficiently explore literally tens-of-thousands of design alternatives," said Lindeman. Prototyping and testing on such a scale was clearly infeasible.

"The analyses allowed us to optimize our design quickly," added Sam Netherly, project leader and co-inventor of the Safety Ring technology.

Currently, 3M is using similar technology to develop multi-function cardiac electrodes. These electrodes are used primarily with the portable defibrillators commonly carried in airplanes and emergency vehicles. The electrodes provide the interface for both ECG monitoring and defibrillation. ANSYS is being used to evaluate and optimize the design of the embedded scrim.

In this case, however, a detailed model of a human thorax is being used to evaluate the efficacy of particular electrode designs and placements. The thorax model--derived from the Visible Human Project--includes detailed representations of the major organs, muscles, and skeletal structures.

"These models allow us to predict the current path through a geometrically complex and non-homogenous system, and to determine what percentage of the heart has reached the critical current density level for defibrillation," said Lindeman.

To capture the geometric complexity of the human body accurately, finite-clement models with two to six million degrees of freedom are often required. "The iterative solvers and parallelization of ANSYS allow us to solve these large models efficiently," notes Lindeman. These simulations are being run on 3M's SGI Origin 2000--a shared memory parallel computer configured with twentyeight 500 MHz R14000 processors and 28 GB of RAM.

Current development projects that feature significant modeling efforts at 3M include new library self-check systems, RFID systems, drug delivery systems, and hydrogen fuel cell systems. "ANSYS will help us develop and optimize these products, and many more," Lindeman concludes.

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