Demand for medical devices is increasing. As a result, there is pressure from health authorities to get device manufacturers to cut costs. Simon Smith of motion company Aerotech describes the measures his company has taken to improve manufacturing in this field.
One of the Western World's most common killers is coronary heart disease - with angioplasty being the most common preventative and remedial treatment. In this treatment, a tubular device called a stent is inserted into arteries to relieve blockages. Stents are precision devices with diameters ranging between 1 and 5mm (although rarely more than 2mm in diameter). They are typically 15mm in length and tube thicknesses vary from 30 to 600microns.
In use, a stent is collapsed to a small diameter and placed over a balloon on the end of a catheter. Once in place within the artery, the balloon is inflated, expanding the stent, this then locks into place - forming a scaffold. The complicated geometry of the spiral shapes cut into the device poses many manufacturing problems, especially when it comes to clamping and machining. Stents are typically made from stainless steel, titanium, or nitinol (NiTi) tubes, but the most commonly used is 316L stainless steel, which is also non-ferro-magnetic.
Increases in cases of heart disease and the fact that stents are typically expensive have placed greater emphasis on finding improvements to both the stent itself and the manufacturing process. One of the big problems is the tendency for scar tissue to build around stents leading to remedial treatment or bypass surgery. This scarring is exacerbated by mechanical anomalies - such as burring or uneven surfaces - caused in the manufacturing processes. One remedy has been the development of drug-coated stents, the first of which was implanted on the same day in April that the US Food and Drug Administration approved the devices. The drug coating prevents clogging, but the cost of the stent was £2,250! Even bare metal stents cost nearly £600. So the emphasis is on making them quicker, better and cheaper.
In general, medical devices are small for two basic reasons: they often have to fit into small spaces; or they are made of expensive materials, thus the need for cost-effective miniaturisation. Lasers, which can work in areas that require tolerances of a few microns, are ideal solutions for the cost-effective manufacture of such devices.
It was inevitable that better machining technology would be employed and laser micromachining techniques were developed to cut the tiny tubes. The push to decrease cost, maintain accuracy and reduce mechanical deficiencies meant that better positioning stages, motion controllers, programming methods and laser controls had to be developed. Any solution would have to lower system inertia, to allow for better dynamics, while at the same time provide the necessary system stiffness or bandwidth to enable the intricate shapes to be tracked at higher speeds.
Another big problem is the curves and shapes exhibited by the stents. These present programmers with a problem on how to optimise the accelerations and feeds to suit the differing contours. In the past, this could often only be achieved by trial and error, increasing the time required to setup a machine for different devices. Programmers also had to consider the combination of laser settings and speed setting, making programs very complex and hard to understand.
Aerotech has engineered a solution to address these specific problems. Using direct-drive linear stages and slotless rotary motors, coupled with a unique low-inertia spindle design, with a built in air operated collet, a number of important advantages for stent makers have been realised. Aerotech's solutions are faster, more accurate and, because of their non-contact operation, they are inherently wear and maintenance free.
Coupled with the development of more productive, higher accuracy stages, the company also had to address the problem associated with laser machining. When the cutting beam negotiates a corner, there was a tendency for burning to occur at the points where the motion is slowed to allow corners to be taken. Given the power of modern lasers, heat is a major issue that has an affect on the surrounding material. Aerotech's engineers therefore reasoned that it was desirable to control the laser power in tandem with the motion of the cutting path.
To achieve this, the built in functionality of the motion controller is used. The user can program a specific pulse width and distance between pulses, and then relate this to the vectored contoured move. In practice, a constant pulse train of fixed width is output from the controller when running at constant speed - this is used to control the laser power. As the controller slows down to negotiate the corner, so does the rate that the pulses are fired, due to the increase in time between the programmed fire distances - the laser power output is reduced relative to this.
Aerotech's laser control enables cut kerf and weld width to be controlled concurrently with the motion axes control. The company calls this position synchronised output (PSO); it provides on-the-fly control of the laser pulse and power outputs. All of which is achieved within the hardware and so is much faster that background tasking methods such as polling; and there is no impact on motion such as that encountered when using block processing.
Aerotech has also developed a software-only controller that runs off any PC and uses FireWire (IEEE-1394) for its interconnectivity. This software motion control opened up a number of possibilities to the company, including the ability to control the laser pulses in synchronisation with the motion commands.
The software motion control is compatible with traditional CNC techniques, in that it interfaces with existing CAD/CAM systems and uses familiar G codes. Features such as multi-block look ahead - whereby blocks of CNC code are identified before the point in the program is reached - enables movement to be decelerated automatically before reaching a critical move such as a small radius. The multi-block look ahead function also allows the user to slow down the cutting speed and laser power automatically so that the accelerations and decelerations needed during contouring small radii in the parts do not exceed the stage's dynamic capabilities. In reality this will cut down the time spent getting new jobs running on the machine and allows for an optimal feed rate to be programmed into the program only once - thereafter the motion controller will work out how fast the moves should be to optimise the desired profiles.
In trials, Aerotech has been able to achieve dramatic cycle time reductions while maintaining or improving upon the quality of the finished stents. In comparative tests, using the best equipment currently available, the company doubled the production throughput. It likens the use of metal cutting lathes and milling machines for medical device manufacturing as using the proverbial sledgehammer to crack a nut. A comprehensive catalogue is available, from the company, detailing its extensive range of equipment for the medical, life sciences and laser markets.
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