Localized potentials in electrical impedance tomography

In this work we study localized electric potentials that have an arbitrarily high energy on some given subset of a domain and low energy on another. We show that such potentials exist for general L-infinity-conductivities (with positive infima) in almost arbitrarily shaped subregions of a domain, as long as these regions are connected to the boundary and a unique continuation principle is satisfied. From this we deduce a simple, but new, theoretical identifiability result for the famous Calderon problem with partial data. We also show how to construct such potentials numerically and use a connection with the factorization method to derive a new non-iterative algorithm for the detection of inclusions in electrical impedance tomography.

Factorization method and irregular inclusions in electrical impedance tomography

In electrical impedance tomography, one tries to recover the conductivity inside a physical body from boundary measurements of current and voltage. In many practically important situations, the investigated object has known background conductivity but it is contaminated by inhomogeneities. The factorization method of Andreas Kirsch provides a tool for locating such inclusions. Earlier, it has been shown that under suitable regularity conditions positive (or negative) inhomogeneities can be characterized by the factorization technique if the conductivity or one of its higher normal derivatives jumps on the boundaries of the inclusions. In this work, we use a monotonicity argument to generalize these results: We show that the factorization method provides a characterization of an open inclusion (modulo its boundary) if each point inside the inhomogeneity has an open neighbourhood where the perturbation of the conductivity is strictly positive (or negative) definite. In particular, we do not assume any regularity of the inclusion boundary or set any conditions on the behaviour of the perturbed conductivity at the inclusion boundary. Our theoretical findings are verified by two-dimensional numerical experiments.

Electrical scanning probe microscopy on organic optoelectronic structures

In the field of organic optoelectronics, the nanoscale structure of the materials has huge im-pact on the device performance. Here, scanning force microscopy (SFM) techniques become increasingly important. In addition to topographic information, various surface properties can be recorded on a nanometer length scale, such as electrical conductivity (conductive scanning force microscopy, C-SFM) and surface potential (Kelvin probe force microscopy, KPFM).rnrnIn the context of this work, the electrical SFM modes were applied to study the interplay be-tween morphology and electrical properties in hybrid optoelectronic structures, developed in the group of Prof. J. Gutmann (MPI-P Mainz). In particular, I investigated the working prin-ciple of a novel integrated electron blocking layer system. A structure of electrically conduct-ing pathways along crystalline TiO2 particles in an insulating matrix of a polymer derived ceramic was found and insulating defect structures could be identified. In order to get insights into the internal structure of a device I investigated a working hybrid solar cell by preparing a cross cut with focused ion beam polishing. With C-SFM, the functional layers could be identified and the charge transport properties of the novel active layer composite material could be studied. rnrnIn C-SFM, soft surfaces can be permanently damaged by (i) tip induced forces, (ii) high elec-tric fields and (iii) high current densities close to the SFM-tip. Thus, an alternative operation based on torsion mode topography imaging in combination with current mapping was intro-duced. In torsion mode, the SFM-tip vibrates laterally and in close proximity to the sample surface. Thus, an electrical contact between tip and sample can be established. In a series of reference experiments on standard surfaces, the working mechanism of scanning conductive torsion mode microscopy (SCTMM) was investigated. Moreover, I studied samples covered with free standing semiconducting polymer nano-pillars that were developed in the group of Dr. P. Theato (University Mainz). The application of SCTMM allowed non-destructive imag-ing of the flexible surface at high resolution while measuring the conductance on individual pillarsrnrnIn order to study light induced electrical effects on the level of single nanostructures, a new SFM setup was built. It is equipped with a laser sample illumination and placed in inert at-mosphere. With this photoelectric SFM, I investigated the light induced response in function-alized nanorods that were developed in the group of Prof. R. Zentel (University Mainz). A block-copolymer containing an anchor block and dye moiety and a semiconducting conju-gated polymer moiety was synthesized and covalently bound to ZnO nanorods. This system forms an electron donor/acceptor interface and can thus be seen as a model system of a solar cell on the nanoscale. With a KPFM study on the illuminated samples, the light induced charge separation between the nanorod and the polymeric corona could not only be visualized, but also quantified.rnrnThe results demonstrate that electrical scanning force microscopy can study fundamental processes in nanostructures and give invaluable feedback to the synthetic chemists for the optimization of functional nanomaterials.rn

Misregulation of the tumour suppressor gene CYLD drives hyperactivation of T cells leading to intestinal pathology

The tumour suppressor gene cyld is mutated in familial cylindromatosis, an autosomal-dominant condition that predisposes to multiple skin tumours. The deubiquitinase CYLD acts as a negative regulator of NF-κB signaling. To analyse the function of CYLD in vivo we used the CYLDex7/8 mice, which are characterized by loss of the full-length transcript and overexpression of a short splice variant of CYLD (sCYLD). In CYLDex7/8 mice the overexpression of sCYLD results in splenomegaly and lymphadenopathy. Additionally, the B cell population in spleen and lymph nodes is increased at the expense of T cells. Analysis of CYLDex7/8 T cells showed a significant reduction of CD4 single positive (SP) and CD8 SP T cells in the thymus and in the periphery. By investigating the impact of sCYLD in TCR signaling in thymocytes, we could demonstrate that sCYLD partially inhibited the activation of Zap70 and thereby negatively regulated TCR signaling. In vitro as well as in vivo we could show that CD4+ T cells displayed a hyperactive phenotype, proliferated to a better extent than WT cells and expressed high amounts of inflammatory cytokines such as IL-6 and IL-17A. Western Blots of steady state thymocytes and peripheral CD4+ T cells were performed, showing that the noncanonical pathway was highly upregulated visualized by the expression levels of RelB and p100 leading to a hyperactive phenotype of CD4+ T cells. In order to investigate the contribution of sCYLD in positive and negative selection in the thymus in vivo, the HY-TCR transgene (HYtg) was crossed to CYLDex7/8 mice. The analysis of CYLDex7/8 HYtg males revealed an increase in CD4+CD8+ DP as well as in CD8+ SP thymocytes, suggesting a less pronounced negative selection in CYLD mutant mice compared to HYtg control mice. Interestingly, the impaired negative selection in the thymus was accompanied by a strong colitis phenotype at early ages (4 weeks). Since medullary TECs (mTECs) play an important role in the late stage of T cell development by negatively selecting autoreactive thymocytes, the levels of mTECs in the medullary compartment was investigated. Of note, low numbers of mTECs were observed, combined with decreased expression levels of the mTEC markers UEA-1, keratin-5, claudin-3 and claudin-4. The reduction of mTECs in the medullary compartment could explain the inflammatory phenotype of CD4+ T cells in CYLDex7/8 mice leading to the severe intestinal pathology observed in these mice. Taken together, these results show an important role of sCYLD in T cell development and function as well as in NF-кB signaling of T cells.

Studies of degradation of organic solar cell materials using electrical scanning probe microscopy

Intense research is being done in the field of organic photovoltaics in order to synthesize low band-gap organic molecules. These molecules are electron donors which feature in combination with acceptor molecules, typically fullerene derivarntives, forming an active blend. This active blend has phase separated bicontinuous morphology on a nanometer scale. The highest recorded power conversionrnefficiencies for such cells have been 10.6%. Organic semiconductors differ from inorganic ones due to the presence of tightly bonded excitons (electron-hole pairs)resulting from their low dielectric constant (εr ≈2-4). An additional driving force is required to separate such Frenkel excitons since their binding energy (0.3-1 eV) is too large to be dissociated by an electric field alone. This additional driving force arises from the energy difference between the lowest unoccupied molecular orbital (LUMO) of the donor and the acceptor materials. Moreover, the efficiency of the cells also depends on the difference between the highest occupied molecular orbital (HOMO) of the donor and LUMO of the acceptor. Therefore, a precise control and estimation of these energy levels are required. Furthermore any external influences that change the energy levels will cause a degradation of the power conversion efficiency of organic solar cell materials. In particular, the role of photo-induced degradation on the morphology and electrical performance is a major contribution to degradation and needs to be understood on a nanometer scale. Scanning Probe Microscopy (SPM) offers the resolution to image the nanometer scale bicontinuous morphology. In addition SPM can be operated to measure the local contact potential difference (CPD) of materials from which energy levels in the materials can be derived. Thus SPM is an unique method for the characterization of surface morphology, potential changes and conductivity changes under operating conditions. In the present work, I describe investigations of organic photovoltaic materials upon photo-oxidation which is one of the major causes of degradation of these solar cell materials. SPM, Nuclear Magnetic Resonance (NMR) and UV-Vis spectroscopy studies allowed me to identify the chemical reactions occurring inside the active layer upon photo-oxidation. From the measured data, it was possible to deduce the energy levels and explain the various shifts which gave a better understanding of the physics of the device. In addition, I was able to quantify the degradation by correlating the local changes in the CPD and conductivity to the device characteristics, i.e., open circuit voltage and short circuit current. Furthermore, time-resolved electrostatic force microscopy (tr-EFM) allowed us to probe dynamic processes like the charging rate of the individual donor and acceptor domains within the active blend. Upon photo-oxidation, it was observed, that the acceptor molecules got oxidized first preventing the donor polymer from degrading. Work functions of electrodes can be tailored by modifying the interface with monomolecular thin layers of molecules which are made by a chemical reaction in liquids. These modifications in the work function are particularly attractive for opto-electronic devices whose performance depends on the band alignment between the electrodes and the active material. In order to measure the shift in work function on a nanometer scale, I used KPFM in situ, which means in liquids, to follow changes in the work function of Au upon hexadecanethiol adsorption from decane. All the above investigations give us a better understanding of the photo-degradation processes of the active material at the nanoscale. Also, a method to compare various new materials used for organic solar cells for stability is proposed which eliminates the requirement to make fully functional devices saving time and additional engineering efforts.

Long-term potentiation and neural network activity in the neonatal rat cerebral cortex in vivo

Long-term potentiation in the neonatal rat rnbarrel cortex in vivo rnLong-term potentiation (LTP) is important for the activity-dependent formation of early cortical circuits. In the neonatal rodent barrel cortex LTP has been so far only studied in vitro. I combined voltage-sensitive dye imaging with extracellular multi-electrode recordings to study whisker stimulation-induced LTP for both the slope of field potential and the number of multi-unit activity in the whisker-to-barrel cortex pathway of the neonatal rat barrel cortex in vivo. Single whisker stimulation at 2 Hz for 10 min induced an age-dependent expression of LTP in postnatal day (P) 0 to P14 rats with the strongest expression of LTP at P3-P5. The magnitude of LTP was largest in the stimulated barrel-related column, smaller in the surrounding septal region and no LTP could be observed in the neighboring barrel. Current source density analyses revealed an LTP-associated increase of synaptic current sinks in layer IV / lower layer II/III at P3-P5 and in the cortical plate / upper layer V at P0-P1. This study demonstrates for the first time an age-dependent and spatially confined LTP in the barrel cortex of the newborn rat in vivo. These activity-dependent modifications during the critical period may play an important role in the development and refinement of the topographic map in the barrel cortex. (An et al., 2012)rnEarly motor activity triggered by gamma and spindle bursts in neonatal rat motor cortexrnSelf-generated neuronal activity generated in subcortical regions drives early spontaneous motor activity, which is a hallmark of the developing sensorimotor system. However, the neuronal activity patterns and functions of neonatal primary motor cortex (M1) in the early movements are still unknown. I combined voltage-sensitive dye imaging with simultaneous extracellular multi-electrode recordings in the neonatal rat S1 and M1 in vivo. At P3-P5, gamma and spindle bursts observed in M1 could trigger early paw movements. Furthermore, the paw movements could be also elicited by the focal electrical stimulation of M1 at layer V. Local inactivation of M1 could significantly attenuate paw movements, suggesting that the neonatal M1 operates in motor mode. In contrast, the neonatal M1 can also operate in sensory mode. Early spontaneous movements and sensory stimulations of paw trigger gamma and spindle bursts in M1. Blockade of peripheral sensory input from the paw completely abolished sensory evoked gamma and spindle bursts. Moreover, both sensory evoked and spontaneously occurring gamma and spindle bursts mediated interactions between S1 and M1. Accordingly, local inactivation of the S1 profoundly reduced paw stimulation-induced and spontaneously occurring gamma and spindle bursts in M1, indicating that S1 plays a critical role in generation of the activity patterns in M1. This study proposes that both self-generated and sensory evoked gamma and spindle bursts in M1 may contribute to the refinement and maturation of corticospinal and sensorimotor networks required for sensorimotor coordination.rn