Engines of change2 May 2018
Brushless motors are replacing their conventional counterparts in many applications, including medical devices. David Callaghan reports on this revolution in electric-powered moving parts.
Brushless DC (BLDC) motors are being used in fully electric and hybrid vehicles, and cordless tools such as leaf blowers, as well as heating, ventilation and refrigeration appliances. While brush motors have already largely been replaced by AC ‘synchronous’ motors, some have remained in use in low-power, DC-only environments. Brushless motors – which are also known as synchronous DC motors or electronically commutated motors (ECMs) – are now steadily replacing traditional types of motor in such places.
In medical settings, these motors have been adopted in patient lifts, powered beds, and mobility devices. They are also used in blowers, compressors, pumps, surgical instruments and equipment for oxygen concentration and generation.
Air pumps in medical socks for blood clot prevention are often powered by BLDC motors, as are airway respirators used for sleep apnoea, and medical analysers to assess blood and urine samples, as well as chromatographs.
A particular type of brushless motor, known as a ‘slotless’ motor, is used in angioplasty and dentistry procedures where automation is needed.
BLDC motors have several advantages over the brushed variety, which was first used in the 19th century. Smaller and longer-lasting, they run cooler and are quieter, as well as being more efficient, powerful and reliable. As the only part that usually wears out is the bearings, the motors last for around 10,000 hours of use, compared with the 2,500 hours or so that older motors could manage before replacement brushes were required.
The main drawback of the BLDC motors is that they are more expensive to produce and purchase, so there are cost implications for cash-strapped hospitals and medical centres.
Quick and quiet
When patients are suffering from sleep apnoea, equipment has to operate quickly, but quietly, and this is one area in which the brushless motors are particularly effective. A motor with brushes is considerably noisier and would not be very useful to a patient with sleep problems. The positive airway pressure (PAP) respirators used to treat sleep apnoea have a blower fan powered by a BLDC motor that responds to the patient’s breathing by widening or decreasing their airway pressure.
In the field of sample analysis, ‘stepper’ motors, which are considered a type of BLDC, are being replaced with an improved model of BLDC. They use the increased torque of BLDC apparatus, and combine longlasting performance with good heattransfer qualities. With the drive for more compact equipment, it is important that the motor can dissipate heat, and this is possible because its windings are fixed to the housing on the outside of the motor. Brushless motors are also being used in heart assistance apparatus, such as pacemakers and pumps.
Heart failure patients can benefit from a catheter-deployed circulatory assist device developed by Procyrion and Maxon. The ‘Aortix’ provides a minimally invasive treatment option for the more than two million chronic heart-failure patients in the US. This device, which is powered by a miniature brushless motor, can help younger patients before progressive damage occurs.
The intra-aortic pump, which measures approximately 6mm in diameter and 6.5cm in length, can be inserted by a catheter in the femoral artery to the thoracic aorta. Once the catheter sheath is retracted, the self-expanding nickel-titanium ‘anchors’ kick in to affix the pump to the aortic wall.
Aortix increases blood flow to organs such as the kidneys. Also, in a model of chronic heart failure, Aortix decreased the heart’s energy consumption by 39%.
The device can operate for more than eight hours on a single battery pack. The external battery pack and control unit have been designed to be ‘hot swappable’, which means the battery can be changed without needing to stop the device. Procyrion has also built a ‘TET’ charging system that enables the battery to be charged wirelessly, which could significantly reduce the risk of infection.
Abiomed and Faulbaher’s Impella pump consists of a brushless DC motor with a diameter of just 4mm in the case of the smallest pump and 6.4mm in the case of the larger, and respective lengths of 12 and 18mm. The pumps are available in different sizes for various capacities.
The pumping performance of the heart is supported with a speed of up to 51,000rpm. The drive is based on a self-contained coil, and consists of a three-phase winding and a bipolar permanent magnet. The position of the rotor is detected by measuring and evaluating the retroactive generator voltage.
Go with the flow
Using a guide wire inserted via the femoral artery, the Impella pump is positioned in the left ventricle and ascending aorta. The blood is transported through the inlet area by the cannula towards the outlet, following the natural flow direction of the blood. The 4mm diameter in the Impella 2.5 model means that the aortic valve leaflets can fully close around the cannula.
The Impella pump is controlled using an external console, which is also used to optimise settings and to monitor the effectiveness of the pump.
Depending on the model, the pump can support the heart for up to ten days, giving the heart time to regenerate. The preparation time required before the pump can be put into action has been reduced to less than three minutes.
Apart from compact dimensions and reliability, a comparatively favourable cost/performance ratio is another important factor, because each Impella pump can only be used once.
Brushless motors are proving they have a crucial role to play in the future development of medical devices, including the incredibly delicate area of supporting patients with heart problems.
The unique qualities of lasting longer and being able to operate without getting hot, mean BLDC motors have distinct advantages over their predecessors, and can be used in very sensitive settings. For medical uses, these qualities are essential, and enable new devices that provide life-saving treatment to be produced. As long as the cost of the new devices can be met, brushless motors can clearly be an invaluable tool.