Prof. Esen, you and your team develop tiny components that can be assembled using laser beams to create larger structures. What is the intended application for these components?
Prof. Esen: There is a multitude of application possibilities. We are currently focusing on the field of microfluidics. You could build valves for example that are able to control fluid flow rates. It could also be possible to move liquids using micro turbines. This lets you measure how much liquid is moving. This is comparable to a conventional water meter in any household. Thanks to the motion, a wheel also measures usage. We are able to copy this process with very small amounts of liquids.
What materials are being used?
Esen: So far, we are primarily working with polymers. Adding nanoparticles to polymers is another possibility; this enables you to change the magnetic properties for example. Adding silver and gold, for instance, can affect the electrical properties. There is a lot of leeway for different experiments.
How do you produce these components?
Esen: The technique is called two-photon polymerization. We take a drop of polymer, which can be compared to liquid adhesives or dental fillings. In their normal state, both are in liquid form and harden. The polymer is being irradiated with a laser beam; an ultrashort-pulse laser in our specific case. The process only takes place in a small area of the focused spot diameter. In doing so, the polymer material only hardens at the current position of the beam. A two-dimensional structure is created by scanning a layer. In the third dimension, we subsequently guide the beam up to be able to generate the three-dimensional structure. The components are subsequently being “flushed out” and everything that was not exposed is flushed away. The exposed parts remain as a solid structure. What’s more, we can use several beams in parallel, thereby creating several equal components.
How big are the components?
Esen: The range of the beam that creates these components is approximately 200 nanometers. The finished structures we are currently building measure about 10 micrometers or less. Needless to say, there is room to grow and the option of creating larger structures. However, we are explicitly dealing with microscopic components.
The actual process can take anywhere from several minutes to several hours. But since we are working with several laser beams in parallel, we are able to produce several components at the same time.
To assemble the components, you use so-called optical tweezers. How do they work?
Esen: Since the existing structures are so tiny, you can no longer use the classic connection methods. With optical tweezers, you have a laser beam that is passed through a 100x microscope objective lens and strongly focused. If you use round transparent particles, for instance, the light is being reflected and refracted. This causes the beam to change direction. It ensures that forces are transmitted to the object that is being irradiated. If you now move the beam, you virtually take the microstructure along with you so that you are able to move three-dimensionally. Just like with the polymerization process, you can move several of these beams in parallel. With a computer-controlled modulator, the particle positions can be defined in any order and the particles moved or connected relative to one another.
The two-photon polymerization technique and the optical tweezers are designed to be combined with nanodoping. What exactly is the latter technique?
Esen: In the case of nanodoping, you add something to the polymer, for instance, nanoparticles for their magnetic or electrical properties. We have also used titanium dioxide nanoparticles. These particles can be found in anything with a white color – for instance, in toothpaste or in paint. If you add them in different areas in varying doses, you can change the refractive index. This is how micro optical components can be generated. These can serve as lenses or beam splitters.
What possibilities would you like to see from combining the three techniques?
Esen: We are globally one of the few laboratories that are able to use all three techniques at the same time. We are able to generate and assemble any type of structure. This does not work with the individual techniques. The number of possibilities is infinite. We are currently working on combining all three techniques in one smaller, more compact device to specifically target the industrial sector in the field of micro-engineering.
Where could they be utilized in medical technology?
Esen: One possibility would be to build a micro-robot to introduce to the human body and which is controlled from outside. The robot continuously sends signals and information to attending physicians and is able to conduct an analysis on location. An application in the field of microfluidics is also conceivable. For example, if you want to extract, analyze and later re-administer very small amounts of liquids to a patient.