The scientists Ali Khaheshi and Dr Hamed Rajabi from the Functional Morphology and Biomechanics working group explore the structure of insect wings to improve man-made constructions.
Flexible yet robust – in many fields such as medicine, space and aviation or robotics, these two properties are core requirements for the mechanisms and systems that are installed. Whether wing or turbine, prosthesis or protector: they are all most efficient when they are both flexible and robust. However, what seems so obvious has posed a huge challenge to engineering for decades. Because by their very nature, these two attributes are mutually exclusive. "The more you increase one of the properties in a construction, the more the other one moves in the opposite direction," explained doctoral researcher Ali Khaheshi. If a material has maximum flexibility and is therefore durable, although it is adaptable it does not provide sufficient stability to bear large loads, for example. In turn, if it has a high load-bearing capacity, and can literally bear a heavy load, it usually does not have enough flexibility to withstand heavy wear and tear. "The simplest example is a rubber band," explained Dr Hamed Rajabi, "it cannot carry much weight, but it can be easily deformed and remains hard-wearing for a long time. A counter-example is chalk: it can withstand a lot of pressure, but at a certain point it suddenly breaks." As so often in life, it comes down to the right mixture.
Striking the perfect balance between both properties represents a pinnacle of engineering. The two scientists from the Functional Morphology and Biomechanics working group of Professor Stanislav N. Gorb are dedicated to pursuing this. Dr Hamed Rajabi and Ali Khaheshi work in the multidisciplinary research group at the Zoological Institute, and are part of the priority research area Kiel Nano, Surface and Interface Science (KiNSIS), which lies at the interface between engineering, biology and physics. Their solution approach is to be inspired by biological systems, especially insect wings. "We know that insect wings have a very high load-bearing capacity," said Rajabi, who has been conducting detailed research into the fine wing structures for almost ten years. "If you closely observe insects in flight, you see that the wings can withstand extreme deformation. It makes you wonder how such flexible systems can simultaneously bear such a heavy load?" Their secret is a mechanically stable vein structure that permeates the delicate wing membrane. Their soft cross-connections allow maximum deformation of the wings under aerodynamic conditions. They also have spikes, fixed thorn-like structures which limit the radius of movement and "lock" the joint at high loads, in order to maintain load-bearing capacity and thus the ability to fly. According to Rajabi, the dragonfly is a particularly good example of this.
In order to be able to use the structural and functional principle of the dragonfly wings for technical applications, the researchers first had to determine the right balance between the components, the compromise between flexibility and stability. "That was the difficult part," said Rajabi with a chuckle. "With the help of a 3D printer, we created nine different sets of these joint-like connections and tested them repeatedly under different stress scenarios," explained Khaheshi. They then tested the best formula on a 3D-printed kite – the first of its kind. Usually a toy for children, the diamond-shaped kite with the crossed struts provided the perfect basis for a test run in the air. The results are highly promising: the spike construction attached to the kite line was able to withstand stormy gusts of 80 kilometres per hour on Eckernförde beach well over 20 times, without any problems. The lightweight kite from the 3D printer can thus withstand much higher loads than conventional models.
Khaheshi and Rajabi are now applying their technology to many wide-ranging areas of life and research. They recently developed a medical splint for the human wrist following a similar approach, with which the angle of movement can be precisely controlled during sport or after an injury. They have already applied for a patent.
Author: Anna-Kristina Pries