The EnCor automated biopsy device harvests multiple samples at different radial positions with the need to remove and reinsert the device.

Breast biopsies have come a long way from the open surgical procedures practiced decades ago. Today, technicians remove the tissue using a cannula inserted in the breast. While this is 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.

In response to this challenge, medical device manufacturer SenoRx engineered its EnCor automated, handheld system for harvesting multiple samples with a single insertion. Critical to the success of this device is the attention paid to the motors and motor control. Irvine, CA-based SenoRx relies on compact, high-torque motors by MIC ROMO, Clearwater, FL.

The EnCor system harvests biopsy samples through an approximate 0.25 cm × 2 cm aperture. Vacuum suction pulls the tissue into this opening. A cutter within the cannula slides up to sever the tissue, which is drawn 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 automated biopsy device consists of a driver unit that contains the motors (pale blue) and a disposable probe.

The two-part device consists of a disposable probe that integrates the cannula and the tissue chamber, and a driver unit that powers the probe. The driver includes three motors: one to 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 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 employ MicroMo’s Express Prototyping Program. Shabaz chose brushed DC motors with encoders because “they deliver more power— higher torque when you have brushes on them.”

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 multiconductor 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 susceptibility and emissions requirements.

The tissue chamber/ probe module snaps onto the driver unit.

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.