Medical Device Link.

MANUFACTURING

An improving technology

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Stents have been used for 20 years as a minimally invasive alternative to coronary bypass surgery to treat blocked arteries. The stent, a wire mesh like tube, is expanded via a balloon catheter in the affected artery to restore blood flow. Over the past five years the deployment of stents has increased significantly as a result of the development of drug eluting technology. This has resulted in lower rates of restenosis or re-clogging of the artery, and eliminates the need for subsequent treatment procedures. The design of the stent itself has also evolved over the years to provide improved flexibility during delivery and better support of the arterial wall, which further increases the use of these devices.

Through the years, the manufacturing process for stents has evolved from electrical discharge machining and etching to laser machining. The laser machining process used today consists of a near infrared laser source in the 1000 nm wavelength range that cuts tubular materials, including stainless steel or memory shaped alloys such as Nitinol (NiTi). Other less common stent manufacturing techniques involve the welding of preformed rings (modular stents) and cutting flat sheet material that is subsequently rolled and welded or mechanically fixed to form a cylinder.

Machine configuration

A laser stent machine requires a linear displacement axis, a rotary displacement axis, the laser with cutting head and specialised fixtures for supporting the tube material during the cutting process. Most machines are built from general purpose, “catalogue type” motion components. Current state of the art in these machines consists of a direct drive linear stage with a linear motor and a direct drive rotary stage.

Additional material handling components such as collets and grippers are also used to automate the feeding of the tube stock material into the laser. Depending on the level of automation present, rotary unions for pneumatic actuation or clutch mechanisms may be used to open/close the tube-holding mechanism. Optional features such as wet cutting may be added to address processing issues such as back wall damage (the laser striking the inside opposite wall of the tube), reduction of the heat-affected zone, and waste material evacuation.

Optimised mechanical structure

Optimisation of the linear and rotary axes for a stent platform is often limited to a top-level analysis of the acceleration and speed capabilities of the stages. The optimisation is complete once a stage is located that meets or exceeds the theoretical anticipated cutting speeds, because most users do not have the capability to design and produce these components for themselves. The disadvantage of this approach is that the interaction of the components, including the fixtures that are added to support material handling, is often overlooked. The lack of system level optimisation of the mechanical components and the material handling system is the single greatest impediment to improving overall system performance.

The goal of mechanical optimisation is to increase the frequency at which the first mechanical system resonance occurs. If this resonance is too low, it will adversely affect servo system response and limit performance. One way to improve system resonance is to ensure that the components and their interconnection points are as stiff as possible, because a stiffer system will have a higher system resonance. However, this will only achieve a limited amount of improvement, because the end user is confined to a building-block approach that consists of separate components bolted together to form a system.

Centres of action

A higher order approach to optimisation is to design a system so that the resonant frequencies are not excited within the structure. This is accomplished by attempting to align the “centres of action” of the mechanical structure with the input force. The centres of action are defined as the points at which, when a force is applied, no rotational forces are created. If no rotational forces are induced, the effect of system resonance can be minimised. The centres of action are a collection of three entities:

• Centre of mass (gravity). Point at which an applied force results in only a linear acceleration (no rotational force).
• Centre of stiffness. The point at which, when a force is applied to a locked-in-place axis, no angular motion of the structure occurs (typically normal to the bearings).
• Centre of friction. The point at which, when a force is applied, no net moment is generated by the resulting static friction forces.

Figure 1: Schematic of nonoptimised (left) and optimised solutions (right). (click image to enlarge)

Figure 1 is a simplified rendition of nonoptimised and optimised solutions. The optimised system places the forcing element, indicated by the concentric circles, and bearings in line with the centre of gravity of the load. Under these conditions, the energy input to the system does not excite the resonant characteristics of the moving mass and thus allows higher positioning loop bandwidth.

Rotary axis optimisation

Locating the moving mass in line with the centre of gravity effectively optimises the linear axis performance. For the rotary axis, the load is inherently centred on the axis of rotation. Therefore, optimisation of the rotary axis involves reducing the inertia of the rotating load and improving the reliability of the material handling assembly.

The traditional approach to integrating this assembly starts with a direct drive rotary axis to which a collet closer system is attached. The collet closer could be an ER collet, C-type collet or a three-jaw gripper with a rotary union and piston or draw tube/yoke assembly to open and close the collet. This approach has multiple bolted connection points and represents a significant inertial load to the axis.

The optimised solution for the rotary axis involves integrating the collet mechanism directly into the rotary axis. The direct integration of the collet into the stage reduces the total number of parts as well as the inertia of the system. An air actuated collet system is preferable to a mechanical actuated device because the total number of parts required for implementation is reduced and there is a corresponding reduction in moving mass for the rotary and linear stages.

Figure 2: Optimised motion platform for stent manufacturing.

Figure 2 shows an optimised motion platform for stent manufacturing. The system was designed taking into account the centres of action optimisation principle. The rotary axis is mounted inline with the bearings and linear motor of the linear displacement axis to improve system stiffness and increase the resonant frequency. Also the rotary axis has an integral pneumatic activated collet system with a seal-less rotary union design. The lack of seals removes a source of friction in the rotary axis and leads to further improvement in positioning performance.

Control system optimisation

Stent profiles can have intricate geometries to provide flexibility for the delivery of the stent into the artery, while also being stiff enough when expanded to support the arterial wall. The wide range of radii present in the profile causes some unique challenges for the control system. Ideally, it would be preferable to set a single cutting velocity for the entire profile and have uniform geometries with consistent tolerances throughout the part. In reality, the changing radii create a wide range of accelerations that can lead to variations in tracking capabilities and nonuniform stent geometry such as changing strut width and overshoot/undershoot on features.

From a servo-loop performance perspective, a good first order approximation is that the peak position error is proportional to the peak acceleration. Therefore, if the peak acceleration is limited while processing the part, then the resulting position error will be limited as well. Curved sections in a stent profile induce acceleration as a function of the programmed radius. The acceleration of an arc in the profile can be calculated from the radius of the arc and the programmed feed rate from the following relationship:

A = V2/R, where A is the acceleration, V is the programmed cutting speed and R is the radius of the arc.

Using this relationship, it is possible to regulate the programmed cutting feed rate as a function of part radius. The optimal control system must be capable of looking forward through the programme and calculating the acceleration in real time. The results of this calculation are used to reduce the velocity to ensure that a preprogrammed acceleration limit is not exceeded. In this manner it is possible to limit the peak position error in the programme. If the observed error is too large, then the acceleration limit is reduced to bring the position error within the required tolerance.

Changing the velocity over the profile to limit the acceleration can create problems with the amount of laser energy introduced into the part. If the repetition rate or power of the laser is unchanged, excessive heat can be introduced to the part as the controller slows down to cut small features. In materials where the thermal input is critical, the controller should be capable of regulating the laser power and/or pulse rate as a function of system velocity. Likewise, the laser itself must be capable of maintaining consistent modal beam quality as the power or repetition rate is modified.

Machine structure optimisation

The machine base and the laser optics support structure can have a significant impact on cut quality. As the speed and acceleration of the process increases, the reactive force of the moving mass in the system will increase as well. Failure to provide a stiff optical support structure will result in part geometry errors that are not observable through the control system. These errors are caused by differential motion between the part and the laser cutting head. To limit this source of error, the unsupported length of the laser cut head should be minimised. The optics support structure should ideally be made of suitably stiff material such as granite. Bolted aluminium extrusion or small cross section welded steel support structures should be avoided. A granite base plate is also recommended to compensate for the reactive forces that are generated by the motion in the linear/rotary subsystem.

Programme optimisation

Stent geometries are made up of a series of repeating profiles referred to as cells. The order in which the cells are cut in the pattern can affect not only the process time, but also the structural integrity of the part. For flexible materials such as NiTi, the order of the cutting process is critical to maintain rotational stiffness and minimise deflection of the part during the machining process.

The convoluted nature of the cells also requires that additional cuts be made to the waste material to ensure that it is easily removed from the part. These cuts are added during the motion programme creation phase. The length and number of required cuts can contribute significantly to the overall processing time of the part.

Laser optimisation

The laser technology plays a critical role in achieving optimal results. The most common lasers used for machining of metallic stents are lamp pumped YAG or fibre lasers. Lamp pumped technology offers higher peak power for faster piercing of material, and fibre lasers offer higher operating frequencies and consistent beam quality over a wider range of operating parameters. Material composition, part complexity and wall thickness affect the choice of laser technology for a particular application. Laser suppliers have conducted a significant amount of research targeted at the machining of stents, which they are eager to share with users. Additional information is also available from laser related technical conferences.

Trends in the stent industry

Much has been made recently on the long term implications of drug eluting technology and the late development of clotting as the drug dissipates over time. To minimise damage to the arterial wall that results from the expansion of the stent, manufacturers are seeking to decrease strut width to minimise the contact surface area. The narrowing of the strut width requires that the control system must hold tighter tolerances during the cutting process, because small deviations from commanded position are a correspondingly larger percentage of the overall strut width. In addition to becoming narrower in width, the stent material is also becoming thinner to improve flexibility. Thinner material can sustain higher cutting speeds, which also results in higher acceleration. An optimised system is required to fully leverage the higher cutting speed while maintaining or improving part tolerance in the presence of the higher acceleration.

Another trend in the industry to reduce arterial wall injury is the development of bioabsorbable stents. These devices are typically made of polymers that dissolve in the body over time. The manufacturing process for polymers uses a different laser wavelength and involves an ablation process where the material is vaporised during the machining process. The ablation process is usually a multiple pass procedure, because an individual pulse from the laser does not penetrate completely through the material. The multiple pass procedure requires that the servo system is capable of repeatable operation so that subsequent passes over the part result in the accurate overlay of laser pulses.

Greater throughput, fewer machines

There is much to be gained from an optimal system design. The application of these design principles has yielded systems that are capable of a fourfold improvement in throughput while maintaining, and in many cases improving, part quality compared with nonoptimal systems. This increase in throughput implies that fewer machines are required to produce the same number of parts per hour. The implications for reduced floor space and staffing levels provide an easily justifiable return on investment for this approach. These design principles also have broad applicability to processes beyond stent manufacturing. Similar results have been obtained in applications such as x/y laser machining and high speed device assembly.

Reprinted, in part, from the proceedings of ALAC 2006, Vol.10, pp.39–41. For more information, visit www.alac-iluc.org or call +1 734 944 5850.

Ron Rekowski is Director of Product Marketing, Laser and Medical Group, Aerotech Inc., 101 Zeta Drive, Pittsburgh, Pennsylvannia, 15238, USA, tel. +1 412 967 6891, e-mail: rrekowski@aerotech.com, www.aerotech.com.

* Correspondence should be sent to Dr Cliff Jolliffe, General Manager at Aerotech Ltd, Jupiter House, Calleva Park, Aldermaston RG7 8NN, UK, tel. +44 118 940 9400, e-mail: cjolliffe@aerotech.co.uk.