Cincinnati, OH —To cut or not to cut? This age-old medical question has gotten easier to answer in recent years with the emergence of minimally invasive surgical procedures. Yet when it comes to diagnosing breast cancer, the surgical removal of golf ball-sized tissue specimens remains all too common—though perhaps not for long. Engineers from Ethicon Endo-surgery recently developed a new system that reduces the physical trauma of breast biopsies.
Called the Mammotome®Hand Held, this alternative to open surgical biopsies consists of a hand-held probe and a cart-mounted module that carries the system's controls, vacuum pumps, and drive motors. Once inserted into the breast through a 1/4-inch incision and positioned via ultrasonic imaging, the Hand Held's needle-like probe collects the samples by means of vacuum aspiration and an internal rotating cutter. The vacuum sucks the sample into an aperture in the probe—within reach of the cutter. The whole procedure can be performed under local anesthesia in a doctor's office or other outpatient facility "in a more human setting than a hospital," says Doug Ladd, director of Ethicon's breast care division.
While advancing the transition from knife to needle is nothing new for Ethicon, the Hand Held's vacuum and control unit does represent the company's first effort at creating such an elaborate control module. "It's really our first time designing a piece of capital equipment," notes John Hibner, staff design engineer.
For the control unit's drive train, which powers the cutter's rotation and length-wise translation within the probe, Ethicon chose two fractional-horsepower DC motors from MicroMo Electronics (Clearwater, FL). The motors connect to the cutter's spur gear and lead screw via speedometer cables that run from the control module to the probe handle. Both motors feature closed-loop PID control and a 300-CPR encoder to track the position of the cutter inside the probe. "We always need to know exactly where the cutter is," Hibner says, explaining that precise position control lets the cutter close the aperture through which the sample enters without bottoming out at the end of the probe. Good position control—within 0.001 inch—also enabled the design team to minimize the length of the cushion, or "dead zone," at the end of the probe, which in turn lets the Hand Held operate closer to the chest cavity wall. "There's less material at the end of the probe to interfere with the chest cavity," Hibner says.
The Hand Held's vacuum pumps also operate under closed-loop control. This "Smart-Vac" technology, for which Ethicon has applied for a patent, lets the vacuum cycle on and off in lock step with the cutter activity. According to Hibner, this feature supports the Hand Held's ability to collect multiple samples with just one insertion of the probe. "Users rotate the probe to take a new sample," he says. Smart-Vac also provides the system with a range of diagnostic capabilities, such as an indication of vacuum-pressure loss.
The probe itself, though considered disposable, had to be engineered more like a precision surgical instrument than a throwaway. As a starting point, the design team loosely based the first probe prototypes on a permanently mounted probe used by the first Mammotome systems—table-based models for use with stereotactic imaging systems. As the Hand Held probe progressed, the team refined the design with some important goals in mind: Minimize weight and cost; maximize precision and reliability. According to Eric Thompson, development engineer, the design team achieved the first two goals by keeping costly and heavy drive components out of the probe body. "We put all the expensive components in the control module," he says, noting that this approach has a side benefit of allowing the same control module to serve as platform for other hand held devices. A switch to plastic gears from the metal ones found in earlier Mammotome probe designs resulted in further weight reductions. To ensure precision, many of the probe's 17 components have dimensional tolerances of 0.001—0.002 inch "because we were concerned about linear tolerance stack-up," reports Thompson, who notes that the cutter positioning has to be within 0.016 inch to reliably close the aperture and not hit the end of the probe. As for reliability, the team designed the probe to keep the fluid path away from the drive components, and added seals not found in the first models, he says.
Also part of the system's "disposable" side are many of the pneumatic components—including vacuum hoses and stopcocks that met tough clearance-versus-torque requirements. "Here we tried to use off-the-shelf components wherever possible," says Thompson. Even some of the key components, including the rotational solenoids used to route the air to the vacuum or atmosphere as the vacuum cycles on or off, are commercially available.
Throughout the project, the design team made liberal use of plastics—in the probe, its holster, and the control cabinet. According to Dave Keilholz, senior development engineer, plastics helped the design team not only meet cost goals but also come up with a probe handle that's ergonomic and lightweight. And because looks really do count in a doctor's office, plastics also helped the team create a cabinet that's more sculptural and colorful than the "microwave oven-like box" that the design team created at the beginning of the project. Among the plastics that saw the most use were 10% glass-filled polycarbonates from Dow Plastics (Midland, MI). Dow also supplied flame-retardant ABS for the unit's LCD, turntable-mounted monitor. Dow was also the source for many of the probe plastics, including radiation-resistant polycarbonate, TPU elastomers for the seals, and liquid crystal polymers. LNP Engineering Thermoplastics Inc. (Exton, PA), meanwhile, contributed the internally lubricated materials used for the cutter gears.
To meet aggressive time-to-market goals of less than one year, Ethicon's development strategy also took advantage of the expertise in its supplier base. Plexus, the contract manufacturing firm that puts the Mammotome together, contributed design-for-assembly advice. And the injection molders that produce the system's many plastic parts, GW Plastics and Phillips Plastics, helped tweak the Hand Held designs for manufacturability.
And from the very beginning of the project, Ethicon began by building prototypes of the Mammotome's components with some help from product design firm Herbst Lazar Bell. Says Keilholz, "Our strategy was build, learn, build, learn, build, learn."