Propolis is a sticky substance used by bees to seal their hive and protect the colony against pathogens. Its main components are plant resins, beeswax, essential oils, pollen, and other organic substances. The chemical and medicinal properties of propolis have been extensively studied, but little is known about its physical and especially adhesive properties. To gain a better understanding of propolis and its potential for adhesive applications, we performed several experiments, including adhesion tests with propolis in different conditions and on various substrates, differential scanning calorimetry analysis, and compression tests. Propolis shows clear viscoelastic behavior and temperature-dependent mechanical properties. Our results demonstrate that propolis adheres well to a wide range of substrates from glass to PTFE, but also enables stronger adhesion at higher temperatures and longer contact times. Even underwater, in wet conditions, quite a substantial adhesion was measured. The data are interpreted from a biomechanical point of view, and the significance of the obtained results for bee biology is discussed.
In a biomimetic top-down process, challenging the problem of resin deposition on woodworking machine tools, an adequate biological model was sought, which hypothetically could have developed evolutionary anti-adhesive strategies. The honeybee (Apis mellifera) was identified as an analogue model since it collects and processes propolis, which largely consists of collected tree resin. Propolis is a sticky substance used by bees to seal their hive and protect the colony against pathogens. In spite of its stickiness, honeybees are able to handle and manipulate propolis with their mandibles. We wanted to know if beneficial anti-adhesive properties of bee mandibles reduce propolis adhesion. The anatomy of bee mandibles was studied in a (cryo-)scanning electron microscope. Adhesion experiments were performed with propolis on bee mandibles to find out if bee mandibles have anti-adhesive properties that enable bees to handle the sticky material. A scale-like pattern was found on the inside of the mandible. Fresh mandibles were covered with a seemingly fluid substance that was at least partially removed during the washing process. Propolis adhesion on bee mandibles was measured to be 1 J/m2 and was indeed significantly lower compared to five technical materials. Propolis adhesion was higher on mandibles that were washed compared to fresh, unwashed mandibles. Results indicate that the medial surface of the mandible is covered with a fluid substance that reduces propolis adhesion. First results suggested that the surface pattern does do not have a direct effect on propolis adhesion.
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