• Lehti-Polojärvi, Mari

Abstract

The research on stem cell-derived cells and tissues is transitioning from traditional two-dimensional (2D) cultures into in vivo mimicking three-dimensional (3D) cell- biomaterial constructs. These 3D cultures enable novel solutions for disease models, toxicology studies, drug development, and precision medicine. However, assessing the state of the live 3D constructs requires advanced measurement and imaging techniques. Optical microscopy techniques are commonly used to image in vitro specimens, but imaging of the mesoscopic 3D constructs is challenging due to their thickness that causes the loss of optical signal. In addition, many optical imaging techniques are phototoxic or require staining/optical clearing of the specimens that renders them unusable for further culturing. Therefore, there is a need for novel 3D imaging techniques.<br/><br/>Multimodal imaging approaches produce multiphysical information of the specimen and enable to combine the strengths of different techniques. The approach of this doctoral thesis work was to develop a novel electrical impedance tomography (EIT) technique and integrate it with an optical projection tomography (OPT) system. EIT is an imaging technique where specimen’s electrical conductivity is reconstructed based on several current injections and voltage measurements on the specimen surface. If these electrical measurements are conducted at different frequencies, the technique is called multifrequency EIT (mfEIT). In OPT, projection images are acquired from many view angles around the specimen and a morphological 3D image is reconstructed.<br/><br/>The aim of this dissertation was to create a novel method for 3D specimen investigation by developing an integrated OPT-mfEIT technique. First, the mfEIT was designed to be suitable for the integration: electrode configurations, imaging chambers, and rotational measurement protocols were developed. In addition, rotational reconstruction algorithms and data fusion techniques were developed to obtain multiphysical image reconstructions. Secondly, the functionality of a new mfEIT device was demonstrated with plant phantoms. The frequency dependent conductivities of the plants were well shown in the reconstructed images. Thirdly, the final developed 3D OPT-mfEIT was experimentally validated with a plant phantom, spheroids, and ex vivo tissues. The resulting images revealed both the specimen’s 3D morphology and conductivity at multiple frequencies. The derived conductivity spectra enabled the detection of stem cells in biomaterial spheroids and the analysis of cell membranes integrity in tissues.<br/><br/>In conclusion, this dissertation presents a new technique for the mesoscopic scale 3D imaging. The developed OPT-mfEIT technique can be used to image and assess various 3D specimens such as spheroids and organoids, that are challenging to image with traditional microscopy techniques. The obtained conductivity spectrum enables novel analysis of the biological specimens because electrical conductivity is not available with traditional microscopy techniques. Applications of in vitro EIT are relatively new in the field of biological specimen imaging, and more validation studies are needed on the correlation of biological phenomena and the resulting conductivity images. OPT-mfEIT provides a tool for such validation studies. Overall, this technique is expected to open new avenues in 3D tissue engineering research and contribute to advance the tissue engineering methods towards clinical applications.

Topics

• impedance spectroscopy
• surface
• tomography
• strength
• two-dimensional
• optical microscopy
• electrical conductivity