Practical examples Medical technology
© joaquincorbalanDamage analysis
on an oxygenator
Problem: Contamination and cracks in the intake manifold of oxygenators
Oxygenators are used – for example during operations – to exchange blood gas in the extracorporeal circuit and to regulate the temperature of arterial blood. These devices are made of highly transparent polycarbonate, among other materials. The oxygenators are rinsed and cleaned with water and certain alcohols after and immediately before use in the operating theater. At a certain point in time, complaints were reported from hospitals about a product series, which quickly became more frequent. The complaints relate to superficial cracks on one of the connection pieces. On the one hand, the resulting risk consists of a mechanical weakening of the material, which is more or less obvious. On the other hand, the risk of microbes embedded in the cracks, which are protected from direct contact with the cleaning media in the crack, is much more subtle. In this way, they can come into contact with the blood.
Analysis method: Polarisation microscope images
An analysis using polarisation microscopy methods shows that the material of the oxygenator housing is under strong tension. This can be seen from the vivid, sharply defined color patterns in the image. The image also shows the contamination in the area of a connection piece.
Result: Adjustment of production parameters necessary after change of supplier
Christoph Zekorn
Head of the Microscopic Analysis Laboratory
Admission test
for therapeutic splint
Problem: Mechanical investigations on a PMMA occlusal splint
For the sale of therapeutic splints on the American market, medical products are subject to approval by the Food and Drug Administration (FDA). The occlusal splints are made of a PMMA material. The greatest danger with this product is that fractures and splinters can occur during sleep due to grinding or unfavorable tilting, which can lead to suffocation. Such environmental conditions, typically 37 °C with high humidity, must be taken into account in the test setup for mechanical tests. In order to obtain approval, mechanical tests are carried out in accordance with the international standard ISO 20795-1 “Dentistry on denture base resins”. Another ISO requirement is a direct comparison with a product already approved on the market.
Analysis method: Determination of material properties according to ISO 20795-1
After preparing suitable test specimens by grinding and milling, the bending properties are tested in a temperature-controlled, aqueous environment (Fig. 2). The properties determined for the new bite splint are compared with the properties determined for the competitor’s product.
Result: Evaluation of the bending properties
Based on the bending stress/strain diagrams determined in the bending test, the characteristic values of the flexural modulus and the flexural strength in water at 37 °C can be derived. Figure 3 shows a good match between two different PMMA bite splint batches and that the properties determined exceed those of the competitor’s product.
Taking into account the standard specification of Efsoll = 2,000 MPa for the flexural modulus, all the bite splints tested were able to exceed the required stiffness. With regard to flexural strength, the behavior is comparable. With a flexural strength of σf = 100 MPa, the two PMMA materials can be rated as equal. Compared to the competitive product with σf ≈ 80 MPa and the standard specification of σfsoll ≥ 65 MPa, these are clearly exceeded.
The comparison with standard specifications and already approved products enables a reliable assessment of the mechanical properties of the “new” bite splint.
Dipl.-Ing. (FH) Christiane Wintgens
Project engineer for dynamic testing