Anton Evgenievich Chernyakov

Purpose . To improve the accuracy of diagnosing microcirculation disorders in the intestinal wall tissues in small laboratory animals by introducing hyperspectral imaging in combination with a controlled polychrome LED light source. Materials and methods. 10 clinically healthy Wistar rats were used in the study. Intestinal ischemia was modeled by the ligation of mesenteric vessels after laparotomy under inhalation anesthesia. Relaparotomy was performed in 1, 6 and 12 hours. The viability of intestinal tissues and the identifi cation of secondary pathological changes were assessed with the visual Kerte method and a polychrome LED light source providing contrast visualization of biological tissues. Intestinal wall perfusion was assessed by the hyperspectral imaging system. Intestinal resection followed by the sampling for morphological examination was made according to time intervals established by the examination protocol. Results. Spectral composition of the light source, which reliably detects the intestinal wall necrosis, includes two spectral lines with maxima of λpeak = 503 nm and λpeak = 594 nm at 2:1 intensity ratio. Hyperspectral imaging allowed to get two-dimensional maps of tissue saturation with the following values: 66 ± 2 % (intact tissue), 42 ± 5 % (1 hour), 26 ± 3 % (6 hours) and 21 ± 3 % (12 hours). The performed morphological analysis revealed three key ischemic intervals: 1 hour (ischemia onset), 6 hours (reversible ischemia) and 12 hours (irreversible necrosis). The obtained data confi rm the signifi cant decrease in saturation which correlates with morphological changes, and the effectiveness of the discussed technique for assessing the viability of intestinal tissues. Conclusion . In the course of the study, the authors have developed a technology for optimizing the lighting during surgery so as to improve contrast imaging of ischemic and necrotic tissue changes. Hyperspectral imaging promotes non-invasive and objective intraoperative assessment of the ischemic damage in the intestinal wall without any exogenous fl uorescent drugs. The obtained results also demonstrate the prospects of introducing optimized lighting and hyperspectral imaging into clinical practice so as to improve diagnosis and surgical treatment of ischemic intestinal lesions.

Introduction. Diseases accompanied by a violation of the blood supply to the intestinal wall occupy one of the main places in urgent surgery of the abdominal organs. Intraoperative assessment of intestinal viability is one of the most difficult tasks and plays a leading role in determining the volume of surgical aid, predicting the course of the postoperative period.Aim. To study the possibility of using contrast imaging using a controlled polychrome LED light source to assess the viability of the intestinal wall of a model animal in conditions of acute ischemia.Materials and methods. The work is based on the results of experimental studies conducted on 15 clinically healthy sexually mature laboratory rats. The simulation of acute small intestine ischemia lasting from 15 minutes to 12 hours was performed by ligation of the major vessels. Each animal underwent a relaparotomy after a corresponding time interval. The intestine was extracted from the abdominal cavity and visual parameters of wall necrosis were assessed using the Kerte method and using a polychrome LED light source for contrast imaging of biological tissues during surgery. After determining the visual signs of necrosis, intestinal fragments were submitted for pathomorphologic examination. The study was ended by removing the animal from the experiment according to the protocol approved by the Ethics Committee.Results. The spectral composition of the light source providing the most reliable detection of necrosis of the intestinal wall is represented by two spectral bands with maximum wavelengths of peak = 503 nm, peak = 594 nm and an approximate ratio of band intensities of 2:1. By morphological study, the following intervals were found to be significant when simulating small intestinal ischemia in the experiment: 1 hour after ligation - time of onset of ischemia, 6 hours - time when ischemia is reversible, and 12 hours - time when small intestine necrosis is recorded. Conclusions. The use of a controlled shadowless semiconductor light source for contrast imaging of biological tissues during surgery in the selected mode improves the definition of visual parameters of intestinal viability.

The current dependences of the power and spectral characteristics, including their distribution (mapping) over the emitting surface, of AlInGaN LEDs of a “vertical” design have been studied in a wide range of operating currents up to ~70 A. It was found that, starting from a certain level of excitation, it is incorrect to use the concept of the average current density when analyzing the efficiency drop. The main factor in reducing the internal quantum efficiency and the radiation output coefficient, which limits the energy capabilities of the LED, is the effect of current crowding to the contacts.

The thermal properties, including thermal resistance R th , capacitance C th and time constant τ th of highpower AlGaInN LEDs have been investigated and analyzed. The transient thermal behavior was expressed as a multiexponential function with time constants associated with a specific R th,i and C th,i components of LED structure.

The driving current, chip area of high-power AlGaInN light emitting diodes (LEDs) and the level of integration of LED arrays are continuously increased to provide ever higher output light flux. The new developments require more attention to pay to the thermal management of LEDs, commonly assessed in terms of the thermal resistance. Temperature distribution in a LED array and its effect on the chip thermal resistance has been studied both theoretically and experimentally. The thermal measurements were performed with the help of a temperature-sensitive parameter - forward voltage drop on the p-n junction under the action of heating current. Two methods of heat exciting were used: step-like or harmonically pulse-width modulated heating current. Analysis of forward voltage relaxation at transient thermal processes allows determination of thermal impedance components corresponding to the structural elements of the LEDs and arrays. The temperature distribution in the LED array predicted by coupled simulations of the heat transfer agrees well with the experimental measured temperature mapping by the IR-microscopy.

The results of the degradation study of commercial InGaN/GaN LEDs with the external quantum efficiency (EQE) ~ 40-50 % at 450-460 nm are presented. It has been clarified that one of the mechanisms responsible for EQE degradation and the unpredictable failure of LEDs is the multiphonon recombination of carriers. The distorted forward branch of I-V characteristics at U <; 2V and the appearance of the SI ~j 4 section on the current spectral noise density dependences on current density in LEDs before or after 100 hours of aging test are the criteria identifying an unpredictable failure.

A high-power AlGaInN light-emitting flip-chip crystal with a new configuration of contact pads is developed and fabricated. The implementation of a two-level metallization scheme with a dielectric insulating interlayer significantly improves the active-to-total area ratio of the heterostructure (to 78%). Numerical simulation of the current spread, employed when developing the chip topology, makes it possible to achieve high uniformity of the current distribution over the active-region area and to obtain small values of the differential resistance of the chip (0.3 Ω). Light-emitting diodes with the maximum external quantum efficiency (60%) and output optical power (542 mW) at a working pump current of 350 mA are fabricated on the basis of crystals developed in the study.