Surface characteristics such as roughness, micro geometry or the thickness of a transparent coating can determine the therapeutic and economic success of a medical device. Optical three-dimensional measurement systems are gaining increasing significance in this area, because they can be flexibly applied to provide fast and meaningful high resolution measurement data. The application examples described here include implants, stents and micro fluidic components.
By: N. Stegmann-Matthews, NanoFocus AG, Oberhausen, Germany
A sharp view of the surface
In medical device manufacturing precise and reliable measuring techniques are required to verify the design of the device, shorten development phases, and control production processes and make them more efficient by reducing scrap rates. Optical inspection systems are being widely used to determine surface characteristics. These three dimensional (3D) measurement systems, which are based on industry proven confocal technology, are deployed to analyse functional surfaces in a laboratory setting, the manufacturing environment, and for inline product inspection.
Confocal measurement technology is well known for offering various unique characteristics such as high resolution, accuracy and reliability while being virtually free of optical artefacts, even when characterising intricate surface geometries. These confocal techniques go beyond the high resolution microscopic inspection and analysis that normally has to take place in the laboratory setting; they allow nondestructive quality control that can take place directly in or at the production process. This versatility contrasts with laboratory limited scanning electron microscopy, in which automated in-line inspection is not possible, and sample preparation, which is essential for measuring nonconductive materials.
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FIGURE 1: The confocal principle is based on having one common focus of light source and a detector. The light from a LED light source is focused onto the object focal plane and reflected back through the same optics into a detector pinhole. This leads to a suppression of light from the out-of-focus planes and results in sharp intensity signals when the confocal condition is fulfilled. The height data of the object can be precisely determined by using fast signal processing for the detection of the intensity peak.
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In addition to standard products, these systems can be tailored to customer- or industry-specific solutions, including hardware and software for analysis and automation. Three types of 3D measurement systems are available: high resolution confocal microscopes (Figure 1), scanning profilometers and inline inspection systems based on a high speed multichannel sensor. The following examples show how these solutions are employed in medical device manufacturing.
Implants
Surface quality is a major consideration with inlays, prostheses and implants. To optimise the healing process, biological acceptance and cellular growth, manufacturers give the surfaces of these devices special treatments. For example, dental implants must have a certain roughness to achieve optimal attachment to the jawbone. To achieve this, techniques such as abrasive blasting and etching are used to produce textures extending to the nanometre range (Figure 2). Implants such as artificial hip joints must be polished by hand, a process in which duration and pressure are decisive factors. Weak polishing yields excessive surface roughness, and overly hard polishing causes removal of material around relatively hard particles to result in the formation of carbides. These elevated points produce undesirable friction and cause material abrasion, which leads to reduced implant lifetime and inflammation in the patient’s body. The surface must be examined for defects such as carbides and scratches to quantify the implant–patient interface surface, including number of defects per unit area, height and spacing. In addition, roughness in the nanometre scale is also an important parameter for the functionality (Figure 3).
A confocal measurement system is able to define production parameters such as roughness during the development phase according to standardised parameters and determine contact areas. The technology is based on optical filtering using a confocal spatial filter (multi-pinhole filter). A precise 3D topography is calculated on the basis of the acquisition of a large number of optically filtered height sections. In contrast with interferometers, disruptive stray light is already blocked out in the optical path. This way the surface structures can be measured and imaged precisely down to the nanometre scale. Typical measuring time using these systems is 2 to 10 seconds.
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FIGURE 2: The surface of a dental implant is optimised for cellular growth. The roughness parameters of the implant´s surface are measured in accordance with ISO standards.
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This fast technology provides accurate results even when capturing the steeply sloped edges on the threaded portion of a dental implant’s bearing surface. In combination with automation software, these systems can also be used for the consecutive inspection of the surface during the manufacturing process to ensure conformance to the product’s allowable tolerances.
Micro fluidic components
Micro fluidic systems are examples of a complex measuring task. These components are employed in lab-on-a-chip applications that are increasingly used for analyses rather than laboratories. In extremely small spaces a micro fluidic system transports, mixes and separates liquids such as blood and gasses. The micro structure of the surface is critical in providing an optimal transport of cells, gasses and liquids and precise flow rate. To reliably judge the quality of the injection-moulded parts, the valves and channels must be inspected with regard to their cross-sectional area, height, width and volume (Figure 4).
Although a 3D confocal microscope can easily determine these micro geometric parameters, a 3D scanning profilometer is often the chosen solution for most micro fluidic applications. These measurement systems capture relatively large surfaces because of their ability to scan quickly and offer high resolution data, which is required for the typical micro fluidic measurement tasks. They capture the topography in a line pattern. The sample is fixtured to the positioning stage and travels with micrometer accuracy beneath the sensor head while the point sensor simultaneously measures the local height of the sample’s surface and stores this in a height profile. Various sensors are available for different application areas with a particularly high vertical resolution of a few nanometres and a measuring range of more than 1 millimetre.
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FIGURE 3: Two different surface treatments of a hip implant show different characteristics both quantitatively and qualitatively.
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To evaluate the measurement data, proprietary automation software, which can be applied to all systems, offers various ways to represent the captured surface data. Visualisation in the form of 3D pictures with height information as a profile or as parameters in a table is also possible. The integrated measurement report generator enables the user to create individual analysis report templates that include all the crucial elements of a specific measurement task. Once done, the height analysis of the surface, the measurement of step height or the roughness of the channel structure can be determined with a few clicks.
Stents
Implants including stents are in direct contact with human tissue, where they support functions indispensable to sustaining life. To increase the acceptance of a stent by the human body, manufacturers add a bioactive coating to the metal surface. Apart from controlling the thickness and consistency of this layer, stents must go through a 100% quality inspection to make sure that each device does not exhibit any surface defects and that they precisely meet product specifications (Figure 5).
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FIGURE 4: 3D image of a microfluidic component used in “lab-on-a-chip” applications.
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Defects at the sub-micron level caused by geometrical deformations such as contaminant abrasion, cutting failures, contact points, islands, insufficient polishing, pitting or deformation can be detected and analysed without human influence by integrating a high speed confocal sensor into the production line. This measuring technique achieves a resolution that is much higher than laser triangulation at a speed that is significantly higher than an interferometer. These multi-channel 3D sensors incorporate a tuning fork that oscillates at a high frequency and a beam splitter that generates multiple (up to 128) laser beams. Consequently, height data from multiple points of the object´s surface can be acquired in parallel. Moreover, the high sample rate allows the stent or the sensor to be moved at a high speed and thus scanning of large areas is possible in a short time. All acquired height and intensity data are merged to a 3D topographical image or an intensity image. The high numerical aperture allows the precise measuring of the outer surface as well the surface of the inner channel of stents. The sensor with standard optics scans the surface at a speed of up to 200 mm/second and has a lateral resolution of 10 microns and a vertical resolution of 0.5 microns. With the ability to take more than 1 million measurements per second, the sensor is ideal for high throughput production lines. To raise the throughput rate, multiple sensors can be used in parallel.
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FIGURE 5: With a high speed multichannel sensor, defects on stents such as a crack in this case, can be automatically detected up to a size of 1 μm.
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The level of precision provided by the confocal measurement technology, combined with the capability of this optical technology to support research and development operations and QC manufacturing processes, and perform in-line product inspection is recognised by industry leaders. The robust advantages of confocal metrology means that they always find new ways to apply this resolute measurement technology to make products safer for the user and more efficient for the manufacturer.
Nina Stegmann-Matthews
is Technical Editor at NanoFocus AG
Lindnerstrasse 98, D-46149 Oberhausen Germany
tel. +49 2086 200 00
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