Summary of the thesis

The mechanical properties of biological tissue describe its behavior under pressure or force. These intrinsic tissue properties can often be related to function. For instance, the spine provides strength and structural support and the ribcage and skull offer protection for delicate internal organs such as the lungs and the brain. On a smaller scale, it has been shown that cells and tissues feel and respond to the mechanical properties of their surroundings. The micro-mechanical environment of a tissue thus has a direct influence on its functioning and vice versa. Despite the diagnostic opportunities as well as a clear relation to function, very few methods are available to accurately assess tissue mechanical properties on the microscale. Most clinical methods to measure tissue stiffness rely on large scale imaging techniques that lack the resolution necessary to differentiate on the micro-level, while material science approaches such as the atomic force microscope are focused on the nano-level and fail to capture the broader view. In this thesis a novel approach for micro-indentation is presented based on ferrule-top technology. A ferrule-top sensor consists of a cantilever accurately positioned above a single mode optical fiber. By coupling monochromatic laser light through the fiber, the displacement of the cantilever with respect to its neutral position can be monitored very accurately. Traditionally, ferrule-top probes are fabricated out of borosilicate parts and their diameter is around 3 mm. The cantilever can be equipped with various tips or coatings depending on the application. In order to perform accurate force measurements with a ferrule-top device, the spring constant of the cantilever has to be determined. As part of this thesis an experimental calibration method for force transducers with interferometric readout has been developed. The method relies on the idea of mounting the sensor on a calibrated piezoelectric translation stage, which is then used to push the free handing end of the cantilever against the pan of a weighing scale. The displacement of the translation stage is regulated by a high gain negative feedback loop designed to keep the bending of the cantilever equal to a multiple of the wavelength of the readout laser (i.e. in the maximum of the interference pattern). At the end of the integration time, the transducer is forced to move to the next maximum of interference, where it is again locked into position. Repeating a similar procedure for a series of consecutive maximum-to-maximum steps, one can finally plot the weight indicated by the scale as a function of the displacement of the cantilever, and, from there, extract its spring constant. Using a calibrated ferrule-top sensor, the stiffness of soft biological tissue can be determined by means of micro-indentation. This technique is based on measurements of displacement of the tissue (i.e. indentation) as a response of a force applied by