Compact, high-torque motors power handheld, automated biopsy system for speed of sampling, ease of use, and faster patient recovery.
Breast biopsies have come a long way from the open surgical procedures of decades ago. In today’s procedures, technicians remove the tissue using a several-mm-diameter cannula inserted in the breast. While a significant improvement over the traditional method, this approach can still require multiple insertions of the cannula for optimal sampling, which increases tissue trauma and recovery time. When medical device manufacturer SenoRx decided to develop an automated, handheld system capable of harvesting multiple samples with a single insertion, they turned to compact, accurate, high-torque motors from MICROMO.
In the SenoRx EnCor automated biopsy system, the tip of the cannula is a solid, sharp tip; the sample is harvested through a roughly 0.25 cm ´ 2 cm aperture that opens up in the side. Vacuum pulls the tissue into this opening. A cutter within the cannula slides up to sever the tissue, which is drawn by vacuum into a tissue chamber in the device. The cannula then rotates to capture tissue at multiple other angular positions—for example, 0˚, 60˚, 120˚, etc.—without the need to remove and reinsert the cannula.
The EnCor is a two-part device consisting of a disposable probe that integrates the cannula and the tissue chamber, and a driver unit that powers rotate the cutter, one to rotate the outer cannula of the disposable in sync with it, and one to translate the cutter along the axis of the cannula.
The device is designed as a handheld unit, which means that real estate is at a premium. “Size is the most important thing, so I chose the smallest motors that had the torque we needed to perform the biopsies,” says senior staff engineer Martin Shabaz of SenoRx. “MICROMO’s motors provide high torque in a small package.”
Engineering the System
To build the prototype, the SenoRx team worked with off-the-shelf motors, which allowed them to take advantage of MICROMO’s Express Prototyping Program. “They deliver more power, higher torque when you have brushes on them. MICROMO provided the DC Micromotors that were exactly what we were looking for.”
In addition to the encoder, each motor has a planetary gearhead with a 64:1 reduction ratio. The probe and driver interface via idler gears. Backlash isn’t an issue for this application, Shabaz says. “We require precise movements and fortunately the backlash in these gears is so minimal compared to these gross movements that there is no negative effect.”
Standard products weren’t enough to do the job, though. To meet size specs for the driver, the team worked with MICROMO to develop a custom multi-conductor cable that would fit inside the housing. Not only did it satisfy form factor constraints, it featured spare conductors that allowed Shabaz to ground the encoder case through the cable to aid in meeting the International Electrotechnical Commission (IEC) susceptibility and a custom-designed microcontroller in the control module handles motion commands as well as ancillary tasks like operation of lights, switches, and the vacuum system. “We made it totally programmable,” says Shabaz. “We wanted to come up with a control system that we can simply upgrade in the future as our product evolves.”
That flexibility allows the user to operate the unit using preprogrammed sampling patterns, like automatically harvesting tissue every 60°; or to operate the device manually during the biopsy procedure.
Biopsies by definition involve contact with tissues and bodily fluids. To protect both technician and electronics, the EnCor was designed as a closed system. The tissue remains in the probe and fluids are removed via vacuum. The driver is splash-resistant and can be cleaned with germicidal wipes. The only elements exposed are the interface gears, which are protected with a plastic dam. As a result, the unit as a whole is contamination resistant.
Solving the challenges
The EnCor systems are designed to last a minimum of three years. Because of the high duty cycles involved five or more biopsies per day in some facilities the SenoRx team put the drivers through stringent lifecycle testing to simulate three years of normal use. “We conduct load and burn-in tests on every motor to ensure that they perform to our stringent specifications,” Shabaz notes.
The team also put the units through shock and vibration testing; here a problem cropped up. The units are handheld, and thus subject to being dropped. In shock tests to simulate operating conditions, Shabaz discovered that the interface between the motor and the encoders was being compromised. In some cases, the encoders were merely dislodged so that they generated an error code; in others, they broke right off.
To solve the issue, MICROMO’s dedicated machine shop strengthened the encoder-motor interface by lengthening the overlap between the two cases and reinforcing the joint with epoxy. Meanwhile, Shabaz increased the length of the frame holding the motors so that it would encompass the encoders, as well. The result is a much more durable unit.
As with all medical devices, the EnCor is subject to stringent safety regulations. On start up, the driver goes through a self-test routine for both motors and encoders. Once the probe is attached to the driver, the system goes through a sequence to calibrate the probe. The process doesn’t end when the biopsy starts. Every movement is restricted to a specific window; if the unit attempts motion outside those windows, the microcontroller will generate an appropriate error message and shut down.
Shabaz, for one, has been quite satisfied with performance, so much so that he’s already planning to integrate a new MICROMO encoder in his next-generation EnCor device. As to the current design, feedback has been uniformly positive. “[The device] works great,” he says. “It's been very well received.” So well that about a thousand EnCor systems have been deployed globally. “MICROMO’s motors and their knowledgeable staff assisted us in our effort to get our products to market as quickly as possible.”