Microscopes find a niche inspecting and measuring stents, products where a matter of microns could be the difference between an acceptable outcome and a life threatening situation
Hundreds of thousands of patients currently live healthy lives due to the development of balloon angioplasty and stenting.
A stent is an expandable metal mesh which is inserted in to an artery, with the purpose of providing a scaffold to the vessel to prevent collapse.
As technology progresses, these stents become ever more advanced.
Most recently, drug coated stents have improved patient recovery rates further.
Stents are typically introduced to the body through an artery and guided in to position by a catheter, where they are inflated by a balloon. This procedure eliminates the need for major surgery with its associated risks.
A stent is a tiny, precision engineered component that will be placed inside a human body.
It is designed to withstand considerable mechanical and biological forces when located at the site of a damaged artery wall.
In addition, the stent must be manoeuvred into position and expanded by the balloon catheter without causing any damage to the patient or the inflation balloon.
To achieve all of these goals, the stent has to be manufactured and inspected to the very highest tolerances. Any defects to the structure or surface of the stent could reduce the operational strength and effectiveness of the device, immediately upon implantation or perhaps more seriously, after the surgical procedure is completed.
There can be few other products where a matter of microns could be the difference between acceptable product and a life threatening situation.
One challenge facing stent manufacturers is the nature of the finished product.
Typically, a stent will be around 15mm long, with an unexpanded diameter of 2mm.
Each web in the structure is less than 200um wide, with a material thickness of around 100um. The structure has been produced to allow the stent to flex and be expanded by the balloon catheter.
Consequently, the device is not a rigid tube, but more of a flexible, alloy mesh.
This makes conventional gauging virtually impossible as the stent is extremely lightweight and will be distorted and possibly damaged under mechanical contact.
A further challenge is the surface finish and material of the device, usually being a bright reflective metal finish with multiple facets and a curved surface.
These considerations all make inspection and gauging challenging using conventional measuring machines and inspection microscopes.
Most of the major international stent manufacturers are currently using inspection and measurement microscopes from Vision Engineering with more than 200 of these systems are currently used for these difficult applications.
There are two basic variations available - stereo inspection and non-contact measurement.
All of the microscopes developed by Vision Engineering have unique optical advantages that make them ideal for stent applications.
Inspectors are presented with a true, optical image which has not been digitised or distorted.
Stent images are projected from a large display head, allowing the operators to assume a comfortable body posture and reducing the eyestrain associated with conventional microscope use.
This is particularly important when the stents are 100% checked by the operators throughout an entire eight hour shift.
When inspecting a component as critical as a stent, any improvements to operator efficiency will have a dramatic impact on product quality, and thus patient safety.
Non-contact measuring systems allow accurate measurements to be made without any mechanical contact to the component.
The systems from Vision Engineering do this by producing an optical view of the stent.
Unlike a stereo system, there is only one optical path used, looking straight down onto a viewing stage, perpendicular to the component.
A single optical path removes the perception of depth and parallax error - vital if an accurate measurement is required.
This presents a field of view that is crisply focussed in one plane, allowing for accurate gauging of the image.
The XY stage has 0.5um encoders, so it can be moved underneath the optics, measuring precisely the movement as it does so.
The operator views the image through the display head and is able to locate key features and edges, using a crosshair, fixed in the centre of the view.
The quality of the image means that these measurements can be accurately and repeatably carried out, where other gauging techniques would fail to locate these features.
When connected to a microprocessor, this allows for multiple measurements to be made over the entire length of the stent.
The continuing development of stent technology, combined with an ageing population ensures that stent production volumes will continue their growth.
This is good news for the manufacturers of non-contact measuring systems as optical technology seems to provide the optimum solution for this vital inspection and measurement task.
