Sponsored Content

Electric motors have become so commonplace in daily life that we may not realize how many of them surround us and provide functions that make tasks easier, more efficient or more convenient. Beyond those motors, though, there are others we don’t notice unless we look deep and hard for them. These provide benefits and features that were unthinkable just a few years ago, as they are small yet powerful, precise and waterproof, to cite just a few of their attributes.

Today’s reality is that these tiny, precision motor implementations (plus associated gearheads and encoders) are enabling design-in of these motion sources in ultraclean applications such as medical devices, including some that operate entirely inside the human body. For maxon precision motors, more than 60 percent of their products go into the medical market, and that percentage is even higher for their very small motors.

Two examples demonstrate the extraordinary capabilities of these implanted motors. In the first application, the challenge was to develop a pump to provide heart support to boost blood flow. Among the many project challenges was providing a pump that operates while flooded with blood, and creating a motor impeller (provided by the pump-system vendor) that will not destroy red blood cells.

This project places many demands on size versus power tradeoffs, as well as the integrity of the sealing system (also provided by the system vendor). The implantable pump needs to be biocompatible and sterilizable, and there are other safety concerns as well.

How maxon Did It

Figure 1: The cardiac-assist pump is attached to the aortic wall via built-in anchors, which engage after the catheter sheath is retracted. Source: maxon precision motorsFigure 1: The cardiac-assist pump is attached to the aortic wall via built-in anchors, which engage after the catheter sheath is retracted. Source: maxon precision motors

The solution was an implantable heart-support system with a motor from maxon medical (a division of maxon precision motors) at its core. The pump, which is just 6 mm in diameter and 65 mm long, accelerates a portion of blood flow and thus increases the aggregate blood flow.

The pump is not the only design issue, of course: overall packaging is also critical to functionality and suitability. The motor is delivered to the heart via a catheter placed in the femoral artery to the descending thoracic artery. A catheter sheath is then retracted and built-in anchors expand to attach to the arterial wall (Figure 1).

Figure 2: The entire aortic-implanted cardiac pump system, including motor and impeller, is a mere 6 mm (0.6 cm) long. Source: maxon precision motorsFigure 2: The entire aortic-implanted cardiac pump system, including motor and impeller, is a mere 6 mm (0.6 cm) long. Source: maxon precision motors

The objective was to make the pump durable and biocompatible. maxon began with their standard EC6 motor (Figure 2), then added customized electrical leads, shaft length and bearing assemblies to meet the application requirements. Power comes from leads that go to an external site or transcutaneous energy-transfer system for no external power leads.

The pump’s impeller is magnetically coupled to the motor shaft—a separation and isolation technique used for large pumps (such as those of oil derricks), which was adapted for this miniature pump by maxon’s customer. The design also required a high-efficiency motor core for both extended battery life and reduced self-heating (which would damage blood). The successful project took two years and yielded significant results, with the heart’s energy consumption reduced by 39 percent.

maxon’s Non-Rotary Motor Solution

Figure 3: The active drug-delivery system is implanted, in the lower abdomen, directly underneath the skin. Source: maxon precision motorsFigure 3: The active drug-delivery system is implanted, in the lower abdomen, directly underneath the skin. Source: maxon precision motorsA second example centered on the need for an implantable drug pump, used to deliver accurately dosed pain meds and other medicine (Figure 3). This pump configuration is preferred because implanting it allows for highly localized delivery along with lower dosages. The final unit was to have a wireless, programmable interface to adjust dosages, with an internal medicine reservoir to be refilled as needed. The overall viable system lifetime of such a pump is several years, limited primarily by battery life.

Figure 4: The maxon reciprocating piston pump used in the implanted-drug delivery system generates a linear movement that results in a minute, precisely controllable liquid pumping. Source: maxon precision motors Figure 4: The maxon reciprocating piston pump used in the implanted-drug delivery system generates a linear movement that results in a minute, precisely controllable liquid pumping. Source: maxon precision motors While the initial sense is to use a suitable rotary motor (these inherently provide torque and rotational speed), a detailed analysis by maxon’s specialists showed that linear motion of a piston pump was a better approach (Figure 4). Using such a pump with an outer diameter of just 12 mm and with only 0.2 W of power consumption resulted in a system with pumping precision in the order of a microliter per piston stroke. In addition, it includes integrated force sensors to detect occlusions (blockages) and positions sensors to provide redundancy in overseeing operations, so any issues with the piston pump function or drug delivery is immediately recognized.

Materials, Manufacturing Expertise Bolster Design Innovation

Developing these implantable medical pumps takes much more than on-paper design innovation. Translating a viable concept and design into a reliable, effective physical device requires a blend of multiple skills and experience. Material selection, fabrication techniques and test and inspection must conform to the highest standards and are often supplemented by proprietary practices for additional benefits and consistency.

Titanium is used extensively due to its biocompatibility, so machining and laser welding this difficult-to-work-with material are key competencies. Welds are tested for integrity via penetration depth, micro hardness and density tests. All tests are done in compliance with industry best practices, documentation, inspection and test standards, along with adherence to industry and regulatory mandates. A clean-room environment and carefully controlled manufacturing process are also integral aspects of the production system.

A finished product is more than the metal and manufacturing, of course. On the design side, expertise in efficient drive trains, temperature management and low-noise design are also critical, as is experience in techniques for sealing sensors, cabling and contacts.

Non-Implantable Motors Also Demonstrate Innovation

Figure 5: The fully sterilizable ENX EASY encoder is part of a complete system, including BLDC motor, gearhead and sensor that withstand more than 1000 autoclave cycles. Source: maxon precision motorsFigure 5: The fully sterilizable ENX EASY encoder is part of a complete system, including BLDC motor, gearhead and sensor that withstand more than 1000 autoclave cycles. Source: maxon precision motorsIn addition to the implantable motors, maxon is driving innovation and advances in related areas such as medical power tools. For example, the company has developed ENX EASY, the world's first steam-sterilizable encoder (Figure 5), which is rated for 1,000 autoclave cycles. It can be combined with matching brushless DC (BLDC) motors and planetary gearheads, and the encoder can be integrated into the brushless drives without any increase in length. The unit is specified for operation up to 120,000 rpm and a 104 W power rating.

Summary

Motor drivers—the electronics that deliver control power to the motor—get much of the attention, but their features and progress are only part of the story. In fact, while motors may be considered mature, the reality is that there are many new, innovative and carefully targeted products.

Translating these innovative ideas into reality calls for an exceptional combination of skills, experience and expertise in design, materials, manufacturing and standards. That’s why maxon precision motors is opening up new solutions to long-standing problems, such as advanced, implantable, motion-driven medical devices. To discover what is possible, ask the experts!

Resources

Maxon—Circulatory Support without Surgery for Heart Failure Patients