In brain studies, the function of the cerebrospinal fluid (CSF) awakes growing interest, particularly related to studies of the glymphatic system in the brain, which is connected with the complex system of lymphatic vessels responsible for cleaning the tissues. The CSF is a clear, colourless liquid including water (H2O) approximately with a concentration of 99 %. In addition, it contains electrolytes, amino acids, glucose, and other small molecules found in plasma. The CSF acts as a cushion behind the skull, providing basic mechanical as well as immunological protection to the brain. Disturbances of the CSF circulation have been linked to several brain related medical disorders, such as dementia.
Our goal is to develop an in vivo method for the non-invasive measurement of cerebral blood flow and CSF circulation by exploiting optical and capacitive sensing techniques simultaneously. We introduce a prototype of a wearable probe that is aimed to be used for long-term brain monitoring purposes, especially focusing on studies of the glymphatic system. In this method, changes in cerebral blood flow, particularly oxy- and deoxyhaemoglobin, are measured simultaneously and analysed with the response gathered by the capacitive sensor in order to distinct the dynamics of the CSF circulation behind the skull. Presented prototype probe is tested by measuring liquid flows inside phantoms mimicking the CSF circulation.
A high-speed optical coherence tomography (OCT) with 1-μm axial resolution was applied to assess the thickness of a cell-free layer (CFL) and a spatial distribution of red blood cells (RBC) next to the microchannel wall. The experiments were performed in vitro in a plain glass microchannel with a width of 2 mm and height of 0.2 mm. RBCs were suspended in phosphate buffered saline solution at the hematocrit level of 45%. Flow rates of 0.1 to 0.5 ml/h were used to compensate gravity induced CFL. The results indicate that OCT can be efficiently used for the quantification of CFL thickness and spatial distribution of RBCs in microcirculatory blood flow.
We present the use of sub-micron resolution optical coherence tomography (SMR SD-OCT) in volumetric
characterization of ink- jet printed color filters, aimed for electronic paper display (EPD). The device used in the study is based on supercontinuum light source, Michelson interferometer centered at 600 nm and employs 400-800 nm spectral region. Spectra are acquired at a continuous rate of 140,000 per second. Color filter array of 143 μm x 141 μm sized and 6 rtm deep ink pools was studied. The volumetric OCT reconstruction was done using the experimental SMR SD-OCT device and a commercial SD-OCT imaging system. The ink layer in the pools was estimated to be 2μm thin. The optical profilometer was used for reference measurements.
We present the use of sub-micron resolution optical coherence tomography (OCT) in quality inspection for printed
electronics. The device used in the study is based on a supercontinuum light source, Michelson interferometer
and high-speed spectrometer. The spectrometer in the presented spectral-domain optical coherence tomography
setup (SD-OCT) is centered at 600 nm and covers a 400 nm wide spectral region ranging from 400 nm to 800
nm. Spectra were acquired at a continuous rate of 140,000 per second. The full width at half maximum of the
point spread function obtained from a Parylene C sample was 0:98 m. In addition to Parylene C layers, the
applicability of sub-micron SD-OCT in printed electronics was studied using PET and epoxy covered solar cell,
a printed RFID antenna and a screen-printed battery electrode. A commercial SD-OCT system was used for
reference measurements.
Application of time domain, ultra high resolution optical coherence tomography (UHR-OCT) in evaluation of
microfluidic channels is demonstrated. Presented study was done using experimental UHR-OCT device based on a
Kerr-lens mode locked Ti:sapphire femtosecond laser, a photonic crystal fibre and modified, free-space Michelson
interferometer. To show potential of the technique, microfluidic chip fabricated by VTT Center for Printed
Intelligence (Oulu, Finland) was measured. Ability for full volumetric reconstruction in non-contact manner
enabled complete characterization of closed entity of a microfluidic channel without contamination and harm
for the sample. Measurement, occurring problems, and methods of postprocessing for raw data are described.
Results present completely resolved physical structure of the channel, its spatial dimensions, draft angles and
evaluation of lamination quality.
Doppler Optical Coherence Tomography (DOCT) is a modern technique used for accurate measurements of blood flow
in the superficial layers of human skin, retina or other tissues and their phantoms. In this work, we considered the effect
of both static and dynamic superficial layer of the scattering medium on the measured velocity of a flow located beneath
this layer. In the case of static layer a tissue phantom consisting of a plain glass capillary (inner size 0.3 × 3 mm)
embedded into a slab of Intralipid solution mimicking human skin was designed. Flow velocity profiles were measured
at different embedding depths and Intralipid concentrations. The obtained results show a decrease in the measured peak
velocity value of the flow in the embedded capillary with increasing the embedding depth and/or concentration of the
Intralipid solution in the static layer. A dynamic superficial layer was considered in the case with two plain glass
capillaries (inner size 0.2 × 2 mm) attached together. Flow rate of the lower capillary was fixed to 100 ml/h, while the
parameters of the upper flow were varied (concentration from 1 % to 4 % and flow rate from 0 to 200 ml/h). The results
obtained with the above parameters do not show significant distortions in the measured flow velocity profile, only false
velocity peaks arising at the rear flow boundaries.
Self-mixing interferometry is a promising technique for a variety of measurement applications. Using a laser diode with
an external cavity as interferometer, the technique offers several advantages over traditional interferometric
configurations. This research used a self-mixing interferometer built in our own laboratory. It is based on a blue emitting
GaN laser diode with a wavelength of 405 nm. Light is directed through an optical fiber from which a 1-cm section of
cladding has been removed, and a cuvette for holding the sample is fixed around this part. Interference patterns, created
in the laser cavity, are acquired with a computer-based data acquisition system and later processed using Matlab
software. Since samples with different refractive indices create interference patterns with different phases, even small
changes in sample concentrations can be measured. However, coupling light into a single-mode optical fiber is a very
challenging task, and the setup is very sensitive to external interference like airflows or vibrations. Experiments with the
device showed that, in stability measurements, the standard deviation of the recorded fringe pattern shifts was only 1.7
nm. In sample measurements, the refractive index change in the sample chamber varied from 1.0029 to 1.33,
corresponding to a fringe pattern shift of 297±4 nm.
Doppler Optical Coherence Tomography (DOCT) is a useful technique for flow measurements. Its potential applications
include industrial suspension viscosity measurements and blood flow measurements. In this work, a flow velocity profile
of 1% Intralipid was measured in a capillary with an inner diameter of 0.8 mm and in a microfluidic channel with a
cross-section of 1000 μmx100 μm. Two different DOCT measurement systems were utilized in the experiments: a
commercial conventional OCT system and a laboratory-built DOCT system, intended particularly for flow velocity
measurements. In the laboratory-built DOCT system, depth scanning was achieved by moving the whole measurement
system with the reference mirror fixed. This modification from a conventional OCT system improves lateral resolution
during the scanning process. A syringe pump was used to induce flow in the capillary. Flow velocity was measured with
flow rates from 1 ml/min to 3.33 ml/min using both measurement systems. For a flow rate of 3.33 ml/min, both systems
gave reasonable results. For flow rates lower than 3.33 ml/min, however, the laboratory-built DOCT system gave much
better results. Its mean measurement error was as low as 0.8%, while that of the commercial OCT was 6.8%. Measured
with the laboratory-built DOCT system, capillary force-induced flow velocity in the microfluidic channel was around 2
mm/s. The commercial OCT system, on the other hand, proved unsuitable for flow measurements in the microfluidic
channel due to its high scanning speed.
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