Stop, check!

The rule of three – measurement, verification and quality assurance – underpinning the production of medical technology equipment naturally challenges methodology developers and instrument manufacturers to quantify quality.  This is all the more true when new materials make their appearance.
In many application areas – e.g. high-quality endoprosthetics used as artificial hip and knee joint replacements or plates implanted to stabilise bone fractures – new materials are much in demand.

For example, in some sectors, the very good mechanical, chemical and physical properties of biocompatible synthetics have already wrested over 80% of the market away from titanium. And biodegradable materials, i.e. materials which automatically dissolve in the body to be replaced by bone produced by the body, give every indication of yielding new treatment options.

Implants composed of polymers or compounds of magnesium have already produced very promising and successful results while biodegradable stents open up new treatment prospects. However, the quality of both implants and prosthetics relies not only on choosing the right materials but also the best surface structure. Surgeons know that a key determining factor of biocompatibility is also surface roughness. For example, this applies to dental implants where roughness measured in the nanometer range (one nanometer = one billionth of a metre) is crucial for protein binding capacity and therefore for rapid growth into jaw bone. Also, the topography of implants has a direct bearing on their wetting properties. The faster the blood can wet the implant surface during implantation, the greater is the likelihood that the operation and therefore the healing will be successful.

Alicona Imaging GmbH from Graz also perceives the importance of these interconnections. This company’s problem-solving contribution is to market its innovative 3D surface measurement technology with a vertical resolution of up to 10 nanometers. It is regarded as highly suitable for the surface characterisation of implants. Says CEO Dr. Stefan Scherer, “We are an international supplier of optical 3D surface measurement technology for quality assurance in both laboratory and production. Our core competence is the measurement of form and roughness.”

The InfiniteFocus measurement system enables the cost-effective measurement of even complex, miniaturised surfaces. The cost effectiveness is due in no small part to the system’s ability to combine the features of a roughness and form measurement instrument, thereby delivering all the functions of an optical profilometer and a micro-coordinate machine. As well as the high resolution, this advance in measurement technology also gains from the system’s extremely high measurement point density. Capable of registering over 100 million measurement points, the system’s three-dimensional recording and roughness measurement capacity naturally handles relatively large measurement fields and volumes.

Stefan Scherer again, “Even when scanning complex forms or materials with different properties, users can work across large vertical and lateral ranges to achieve a resolution of up to 10 nanometers.” For a complete survey of a form, there is an optional rotation unit, which rotates the object step by step through 360 degrees. This persuades technicians working in this field, “We know of no comparable optical measurement system which can deliver such reliable statements on roughness even across large measurement ranges,” confirms Dr. Frank Rupp, leader of the ‘Interface Analysis of Medical Materials’ working party at the Tübingen Polyclinic.

Using data provided by the InfiniteFocus measurement system, it will be possible in future to determine the correlation between the topography of an implant and its behaviour in the body with increasing reliability and especially express it in numerical values. And, as a by-product, Dr. Stefan Scherer also forecasts another direct benefit, “In production, the use of 3D surface measurement from Alicona reduces the risk of rejection, which is expensive.”
Dr.-Ing. Andrey Bulavinov from the Fraunhofer Institute for Destruction-free Test Procedures (IZFP) in Saarbrücken has developed a novel technique known as ‘sampling phased array’ in his insitute for the well-known and tested ultrasound procedure. This makes it possible to make authoritative two and three dimensional error pattern reconstructions at high test speeds. And acoustic anisotropic materials – for example carbon fibre materials – which have a great future in medical technology, can now be tested with this procedure.

Avoidance of misinterpretations

But they are classed with the ‘complicated’ materials as regards ultrasonic testing. As Andrey Bulavinov explains on this subject, “During manufacture, a specific material texture is produced which causes the direction of propagation of ultrasonic waves to deviate from the desired angle of incidence. This `acoustic anisotropy’ often used to cause ultrasonic readings to be interpreted falsely.” This is not the case with the procedure he has developed. And the industry can now benefit from this as well. Because in 2009, together with his partner Dr.-Ing. Roman Pinchuk, Andrey Bulavinov founded I-Deal Technologies GmbH as a spin off of the Fraunhofer Institute in Saarbrücken. The two CEO’s and their team now pursue the goal of marketing the scientific expertise in the form of production-ready test systems for industry and the public sector. But the researchers from the Federal Physical and Technical Institute (PTB) in Braunschweig are not that far yet. They are engaged in several working parties on the development of innovative measurement techniques.

meditec questioned Dr. Sai Gao and Dr. Zhi Li about this. The impetus for this was a project by the German Research Community (DFG) to investigate the mechanical properties of thin and ultra-thin films and layers. These terms of reference have now led to the development of some MEMS-based (micro electro mechanical systems) surface measurement instruments. MEMS are extremely small and combine mechanical elements with an electrical drive in one component. These can be used for the precision measurement of small components, including medical technology components, and thereby support quality assurance – for example, the very small, portable nanoindentation instrument. This can be used to make measurements directly on the component.

The PTB offers cooperation

Dr. Sai Gao describes the procedure as follows, “At one and the same time, our system generates the testing force and measures the depth of penetration. The testing force is generated by means of an electrostatic comb drive. The depth is measured by means of capacity change between the teeth of the comb drive. These are designed as two interlocking structures of which one is in a fixed position and the other is mobile.” Dr. Sai Gao continues, “This places this miniature measurement system in a new generation of instruments. They can be produced with a high degree of accuracy and could now cost effectively go into the production line.

This system can measure the hardness, elasticity and plasticity of layers which are thinner than one micrometer. This enables their adhesiveness, durability and resistance to wear to be determined. Says Dr. Sai Gao, “In future such coats can also be used for medical applications, for example in the production of extremely thin catheters.” Depending on the field of application, they will need to satisfy an extremely wide range of requirements. For example, when under stress, they must not break or peel. The MEMS technology was used for the first time in the development of the nanoindentation prototypes. Drawing on this technology, the researchers at the PTB have also developed a lateral-confocal microscope.

It is intended for use in the quality assurance measures for MEMS products themselves. Conventional optical-confocal microscopes reach their limits at sizes of less than half a micrometer. In contrast with this, the principle developed by the PTB which resulted in the lateral-confocal microscope is much more sensitive and also capable of displaying the behaviour of moving components in the microsystems to be tested in a nanometric resolution range. Patents have been applied for for both instruments. The PTB offers cooperation schemes in terms of technology transfer to companies keen to benefit from the latest developments in measurement technology, sensorics and production engineering. A very desirable step. This is because medical technology is also venturing into ever smaller dimensions and so is dependent on reliable measurement technology, which is also forging ahead into these fields.

Robert Wouters


German Summary

Messen, prüfen, Qualität sichern: Dieser eherne Grundsatz der Medizintechnikfertigung fordert naturgemäß die Entwickler von Verfahren und Hersteller von Geräten zur Qualitätsbestimmung heraus. Dies einmal mehr, als neue Werkstoffe am Horizont aufgetaucht sind.  Der deutschsprachige Beitrag ist nachzulesen auf www.meditec-international.com/medi0511qs