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Alwin Kienle: an eye for the whole picture

Harvest time has now come for physicist Alwin Kienle. In the next year or two, the results of his basic research will result in the establishment of an applied centre focusing on the determination of the optical characteristics of scattering media. The deputy director of the Ulm-based ILM hopes that the new optical competence centre will lead to attractive solutions for the pharmaceutical, medical device and nutrition sectors.

Prof. Dr. Alwin Kienle

Alwin Kienle, 43, has been working on the propagation of light in biological tissue since 1991. Back then, the ILM was extremely interested in the subject of Kienle's doctoral thesis and, for almost three years, gave him the scope and freedom he needed for his research. A three-month stay at the Canadian Hamilton Regional Cancer Centre in the laboratory of Prof. Patterson was another huge step forward for Kienle; he then continued his research at the same centre and at the École Polytechnique Fédérale de Lausanne after finishing his doctorate.

Back in Ulm, the DFG scholar commenced work on his habilitation on experimental physics, specifically concentrating on the "Determination of light absorption and scattering in biological tissue". He habilitated in 2000, quickly advancing to become the head of the tissue optics research group, which is now called the "Material optics and imaging" group.

In May 2008, Kienle became the deputy director of the institute where he has been in charge of research ever since. In addition, he has been associate professor at the University of Ulm since January 2009. His group of researchers has grown to 16 staff members. However, due to his many duties, the passionate researcher now has to put his own research on the backburner during his working day and spend his evenings doing the research he enjoys so much.

The question to end all questions: light propagation

Many biological tissues such as the cylinder-shaped dentin tubules shown in the photo have an aligned microstructure. © ILM


When material is irradiated with light, the initial focus is on how the light propagates in the specific material being studied. Since practically all materials that occur in nature scatter or diffract light, both the absorption and the scattering of the light needs to be taken into account. This is the basic physical phenomenon upon which many scientists worldwide base their work.

At the time Kienle was doing his doctorate, the ILM was just five years old and believed that the laser would have a promising medical future. This is why the issue that Kienle was working on was considered by the ILM to be of key importance.

Many different theories have been put forward to describe the propagation of light in scattering media. The three most important are - with decreasing accuracy - Maxwell's, the transport and the diffusion theory. Before achieving a precise understanding of multiple scattering, it is important to understand the scattering behaviour at the individual scatterers. This is possible for simple geometries, which can be described accurately by solving Maxwell's equations. More complicated geometries require the numerical treatment of Maxwell's equations.

Once the propagation of light is understood in a specific medium, additional investigations can focus on further effects, such as the warming of the medium, the ablation of the medium or photochemical effects. Kienle explains that this is done using modelling and simulation, adding that Maxwell's theory is a microscopic theory, which takes into account the microstructure of the medium under consideration. The transport theory is an approximation of Maxwell's theory. Light is assumed to consist of small energy packages (photons), which move through the medium quasi accidentally. The diffusion theory in turn is an approximation of the transport theory, and is valid for large distances between the point of beam entry and the edge of the scattering medium.

Kienle’s approach comprises all three scales

Electromagnetic wave propagating through a dentin layer consisting of cylindrical tubules. The photo is an FDTD simulation, a method for numerically solving Maxwell’s equations. © ILM

Modern research tends to look at the propagation of light in scattering media almost exclusively by using the transport and diffusion theories because solving Maxwell's equations makes extremely high calculation and storage demands. In addition, the microscopic models and the optical properties required for the scattering media are not available. Effects such as dependent scattering, for example, cannot therefore be described. Kienle also explains that the diffusion theory fails completely when it comes to investigating media with an aligned microstructure structure, i.e. anisotropic media.

Almost half of all biological tissue types (muscles, tendons, ligaments, nervous tissue, dentin) have a very aligned microstructure; although contrary to common belief, they are anisotropic. Evidence for this was already available around ten years ago, although no exact solutions were calculated. A couple of years ago, Kienle succeeded in providing substantial evidence for this structure on the organic material wood.

When red becomes blue

When one has understood how light of different wavelengths propagates, this also generates an understanding of the colour of objects. Gaining such knowledge also has its practical benefits, explains Kienle who has examined and explained why veins are blue although blood is red.

Blood is red because the blue and green proportion of visible light is absorbed more strongly than the red proportion of light. Therefore it is mainly the red proportion – following the scattering of the light in the blood – that is remitted to the eye. In skin with blood vessels, the blue and green light is not affected by the blood vessel due to the shallow penetration depth of blue and green light, whilst the red light is affected because of its greater penetration depth. The remission of red light will be reduced as a result of absorption in the blood vessel: the eye sees the complementary colour, which is blue.

Based on this finding, Kienle carried out simulations and estimated the size and depths of blood vessels and was eventually able to provide an explanation for what everybody can see: small blood vessels at the surface are as red as those in the eye. The colour of blood vessels can principally be used to assess the depth at which the blood vessels are located, something that can be used for example in dermatological treatment using lasers. Depending on the depth, Kienle is now able to change the wavelength of the light in order to provide optimal treatment.

Understanding what the microscope shows

Anisotropic light propagation – vertical irradiation of a dentin cube: almost all of the light is transmitted on one side. © ILM

Kienle's holistic approach involving the investigation of light propagation at the large, medium and small scale along with coupling at the individual levels, is new and requires huge computer capacities in order to convert the fine structure of a certain tissue into Maxwell's equations. It is Kienle's medium-term goal to understand the images of a broad range of microscopes and explain how the structures that are visible in the microscope are created.

This understanding would have considerable consequences. For example, a PhD student at the ILM is currently working on the construction of a scattering light microscope which can be used to identify nanometre-scale alterations, for example in cells or cell organelles using spectral or angle-resolved (goniometric) measurements. Working with engineers at the University of Reutlingen, who discovered that chromosomes can only be differentiated with scattering light, Kienle now plans to test whether this method can be used to measure alterations in the chromosomes. If he succeeds, this would make complex fluorescence measurements obsolete and additionally, the measurements would be much quicker.

Model to explain the method: Light rays, which hit the cylindrical axis at a small angle, are scattered in a cone around the cylinder. The light is multiply scattered by the tubules approximately along the tubules' directions. © ILM

Holistic approach enables early diagnoses

The aforementioned multi-scale approach can be used to analyse the tissue optics on the nanometre scale (such as is required for the early diagnoses of tissue, cancer prestages for example). What was achieved by Kienle’s group of researchers is the deduction of diffusion theory solutions for multilayer tissues, which are currently being used in an EU project focusing on the non-invasive investigation of brain functions.

For the planned centre that will focus on the determination of optical characteristics of scattering media, the engineers and physicists at the ILM are currently developing devices based on Kienle’s research. Scientists that can determine the scattering and absorption across different wavelengths and ranges, can also determine the concentration of active ingredients contained in pharmaceutical products or detect contaminations occurring in the manufacturing process quicker and easier. Angle-resolved (gonometric) measurements can also be used to determine the reflection characteristics of diodes and other light sources.

Alwin Kienle has a lot of interdisciplinary work ahead of him in the near future, including the continuation of his previous research and turning this research into applications. In the meantime, the physicist from Ulm has already filed two patents related to the determination of active drug ingredients and the guiding of light, which is a clear indication for him that he is slowly leaving the area of basic research behind him as he enters the area of application.

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