Feb 3 2008
A surgeon operating with an endoscope has to rely on the images it displays. For it to deliver reliable images, adhesive bonds must not come apart even under extreme conditions – for instance during sterilization. Researchers have now optimized the bonded layers.
A tiny incision in the skin is all it takes: Through it, the surgeon inserts the endoscope into the patient’s knee. The rod-like instrument shows the doctor images from the inside of the joint and contains the tools required for the upcoming meniscus operation. Just like any other surgical instrument, the endoscope has to be sterilized after each operation. This is done by rapidly heating it to 134°C and then rapidly cooling it.
This course of action places considerable strain on the material: The endoscope’s lens system is made of several consecutively bonded lenses, each with a diameter of less than two millimeters. When the lens system is heated, the lenses expand to different degrees. This causes stress in the bonded layer, whose thickness of only five to ten micrometers is less than that of a human hair. The adhesive bond may separate, resulting in blisters and stains which distort the image and irritate the doctor. These problems sometimes occur after as few as ten sterilizations, although most devices ought to survive about a thousand cycles.
Scientists at the Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research IFAM in Bremen have now optimized the bonding process so that the adhesive layers can withstand the temperature shocks unharmed. The researchers found that adhesives cured first with ultraviolet light and then with heat are particularly suitable. “What’s more, we have improved the entire bonding process,” says IFAM microadhesion specialist Dr. Thomas Gesang. “The adhesives have to withstand two different types of stress: One kind are the internal stresses that occur when curing the adhesive, and the other are external stresses that are caused by the temperature shock on sterilization, for instance.
We have optimized the curing process for the bonded layers in such a way that fewer internal stresses occur. This enables the layer to withstand greater external stresses.” The optimization measures included adjusting the ultraviolet and thermal curing and selecting suitable adhesives. When curing the adhesives with UV light, it is also advantageous for the light to be of the same intensity at all points – this is the only way to ensure that the layer will be homogeneous and stress-free. The method has proved successful: Endoscopes that have been bonded using the new technique have already withstood over a thousand test sterilizations without damage.