The ability to analyze a large field of parameters (<<100) from a fairly large population of cells (1000) on a cell-by-cell basis quickly and accurately, presents a major challenge for the fields of both cellular biology and analytical biochemistry. This ability, however, would give the researcher the means to identify and study variations within a given populations. This knowledge of sub-populations has profound implications in fields such as cancer biology', and cell differentiation.
We demonstrate the potential of a forward-looking Doppler optical coherence tomography (OCT) probe for color flow imaging in several commonly seen narrowed artery morphologies. As a proof of concept, we present imaging results of a surgically exposed thrombotic occlusion model that was imaged superficially to demonstrate that Doppler OCT can identify flow within the recanalization channels of a blocked artery. We present Doppler OCT images in which the flow is nearly antiparallel to the imaging direction. These images are acquired using a flexible 2.2-mm-diam catheter that used electrostatic actuation to scan up to 30 deg ahead of the distal end. Doppler OCT images of physiologically relevant flow phantoms consisting of small channels and tapered entrance geometries are demonstrated.
In this paper conducting polymer based active catheters are presented. Design considerations along with the promise and
challenges associated with conducting polymer driven devices are discussed. A conducting polymer driven intravascular
catheter is described briefly and its design challenges such as structural rigidity and angle of bending are studied. Then a
detailed description of a polypyrrole based active catheter that is ultimately intended for in-vivo imaging applications
will be presented. The active catheter contains an optical fibre and is designed to scan the fibre in two dimensions at a
speed of 30 Hz to provide real time imaging. The preliminary design was realized by fabricating polypyrrole actuators
on a commercially available catheter and patterning the polymer using laser machining technique. The initial device was
tested at lower speeds and an image was taken using optical coherence tomography (OCT). The primary challenge to
achieving an effective polypyrrole driven catheter for real time imaging is to demonstrate high speed actuation with
reasonable liftetime. According to our model, electrochemical characteristics of the conducting polymer such as
electronic conductivity, ionic conductivity and electrochemical strain need to be improved to achieve the desired catheter
scanning speed.
We measure the tumor vascular response to varying irradiance rates during photodynamic therapy (PDT) in a Dunning rat prostate model with interstitial Doppler optical coherence tomography (IS-DOCT). Rats are given a photosensitizer drug, Photofrin, and the tumors are exposed to light (635 nm) with irradiance rates ranging from 8 to 133 mW/cm2 for 25 min, corresponding to total irradiance of 12 to 200 J/cm2 (measured at surface). The vascular index computed from IS-DOCT results shows the irradiance rate and total irradiance dependent microvascular shutdown in the tumor tissue during PDT. While faster rates of vascular shutdown were associated with increasing PDT irradiance rate and total irradiance, a threshold effect was observed as irradiance rates above 66 mW/cm2 (surface), where no further increase in vascular shutdown rate was detected. The maximum post-treatment vascular shutdown (81%) without immediate microcirculatory recovery was reached with the PDT condition of 33 mW/cm2 and 50 J/cm2. Control groups without Photofrin show no significant microvascular changes. Microvascular shutdown occurs at different rates and shows correlation with PDT total irradiance and irradiance rates. These dependencies may play an important role in PDT treatment planning, feedback control for treatment optimization, and post-treatment assessment.
Actuation of polypyrrole in aqueous sodium hexafluorophosphate solution has been shown to produce relatively large
strains. However little has been published on appropriate potential range of actuation in this electrolyte. This information
is clearly crucial for applications. Our particular interest is in disposable applications where a relatively small number of
cycles are needed, and maximum strain is desired. The electrochemical degradation as a function of voltage is
investigated by cycling the film between fixed voltages and measuring the charge transfer. The experiment was done on
a glassy carbon substrate in order to reduce effects of change in resistance with oxidation state, preventing actuation. The
dependence of charging on voltage and the rate of reduction in the extent of charging are measured. The voltage range
for effective operation of the device was found to be -0.4 V to 0.8 V versus a Ag/AgCl reference electrode in order to
achieve stable performance over at least 30 minutes. The mechanisms of degradation at potentials beyond 0.8 V appear
to be the substitution of hydroxyl ions in the polymer backbone, as suggested in reports on degradation of polypyrrole in
other electrolytes. An observed reduction in charge transfer rate at potentials lower than -0.4 V is consistent with a
reduction in ionic conductivity at highly reduced states, as has also been suggested in the literature.
Microcirculatory changes, such as vascular shutdown, may be a predictor to the therapeutic efficacy of
photodynamic therapy (PDT). The aim of this study was to measure the tumour vascular response to varying irradiance
rates during PDT deep within prostate tumour xenograft, via interstitial Doppler optical coherence tomography
(DOCT).
DOCT provides micron-scale spatial resolution allowing visualization of structures at near histological levels,
and yields flow velocity resolution of ~20 μm/s. Current in vivo DOCT imaging probes are limited to intraluminal and
near-surface sites. To improve the accessibility of DOCT to anatomically relevant sites deep within the body (e.g.,
prostate), an interstitial (IS) needle (~700μm diameter) probe was developed for minimally invasive monitoring of the
microvascular response to PDT (irradiance administered superficially) within tumour tissue. Rats were given a
photosensitizer drug, Photofrin, and 20-24 h later the tumours were exposed to light (635nm) with an irradiance rate of
8-133 mW/cm2 for 25 minutes to a total irradiance of 12-200 J/cm2. Results illustrated different rates of vascular
shutdown within the tumour as imaged by IS-DOCT, related to the administered PDT irradiance rate and total
irradiance. Controls (probe only, probe + light) showed no significant microvascular changes.
IS-DOCT was able to detect and monitor microvascular changes during PDT. Microvascular shutdown
occurred at different rates and showed correlation with PDT light dose and irradiance rate. These dependencies may
play an important role in PDT treatment planning, feedback control for treatment optimization, and post treatment
assessment.
Time domain Doppler optical coherence tomography (DOCT) is a promising non-invasive imaging system with high spatial (~20μm) and velocity resolution (~20μm/s) that can image microvascular blood flow. It is important to understand and account for the complicated 3D nature of small blood vessels. To address this problem, two realistic flow phantoms were designed with known geometries -- an occluded flow path to model vessel narrowing, and a Y-bifurcation to simulate vessel branching. The current DOCT system produces 2D images, which when stacked sequentially can yield 3D images of microstructure and perfusion-level blood flow. 3D reconstructions allow the investigation of internal flow profiles, including an abrupt stenosis in the occluded phantom. This research will help guide our image interpretation of in-vivo DOCT studies, including treatment response monitoring in animal tumours and endoscopic assessment of the human GI tract.
Chronic total occlusions (CTOs) are defined as complete occlusions of an artery older than one month. Minimally invasive catheter-based interventions commonly employed for partial occlusions (e.g., balloon angioplasty followed by stenting) are problematic in CTOs because of the phycisian's inability to pass the device through the occlusion without a significant risk of arterial wall perforations. Furthermore, successfully treated CTOs exhibit a high re-occlusion rate. As a result, these cases are mostly sent to bypass surgery. With the advent of drug-eluting stents that reduce the incidence of re-occlusion, and thus, eliminating the second problem, new devices have begun to emerge that aim to recanalize CTOs without the cost and trauma of bypass surgery. These devices, however, need effective image guidance methods to ensure successful crossing of the CTOs.
Optical coherence tomography (OCT) is being evaluated as an intravascular imaging modality for guiding catheter-based interventions of CTOs. Occluded ex vivo human arterial samples were used to produce longitudinal cross-sections using an OCT system. These OCT images were compared with histology to assess OCT's ability to identify different components of the occluded artery, evaluate the imaging depth, and determine the ability to detect the underlying vessel wall.
Given the inherent difficulties of creating a mechanically scanning OCT probe in the distal tip of a catheter for use in a stenotic artery, we directed our initial efforts towards developing a "motionless" fiber based OCT system using a single mode fiber array. We discuss design considerations for implementing a forward viewing intravascular OCT probe.
Doppler optical coherence tomography (OCT) can image tissue structure and blood flow at micrometer-scale resolution, with limited imaging penetration depth. We report a novel linear-scanning needle-based Doppler OCT system using angle-polished gradient-index or ball-lensed fibers. A prototype system using a 19-guage (diameter ~0.9 mm) echo-genic needle is constructed and demonstrates in vivo imaging of bidirectional blood flow in rat leg and abdominal cavity. To our knowledge this is the first demonstration of Doppler OCT through a needle probe in interstitial applications to visualize deeply situated microcirculation.
Microfluidic devices, or "lab-on-a-chip" systems for single cell analysis represent a new field of micro-total analysis systems (μTAS) that could not only perform a task quicker, and more accurately than conventional methods, but could also incorporate additional tools to the study of biological variability in a population by allowing researchers to directly examine the contents of a multitude of single cells from the population under study. Realizing such a device, presents several engineering challenges to the fields of micro-machining, micromanipulation and analytical bio-chemistry. The device needs to be able to accurately and efficiently select, manipulate and analyze volumes represented by a single cell without diluting the contents. For this purpose, optical tweezers and scissors were implemented to select single cells on a microchip, bring the cell to a desired location, and lyse the cell using the optical scissors. Channels were engineered in the device using a molecular fluorine (F2) laser. Each channel’s cross-section is approximately the size of an individual cell (10μm wide and deep). This paper, describes the manipulation of cells on a microchip using optical tweezers and the injection of the cellular contents by optical scissors from a single cell into 10μm channels.
Ongoing efforts to engineer a system capable of selecting and labeling single cells using optical micromanipulation tools and performing electrophoretic separation on the contents of a single cell using the 'lab-on-a-chip' format are presented. At the heart of this design, are channels with 10micrometers diameter cross-sections, etched using a molecular fluorine laser. Individual cells are moved on the microchip using optical tweezers. These single cells are brought into contact with a liposome containing fluorescent tags. The liposome and cell are fused using optical scissors; resulting in a cell with labeled components. This cell is lysed using the optical scissors, and high voltage is applied to separate the contents. This design will allow us to directly look at protein and mRNA expression from a single cell without amplifying the contents of interest, as well as to obtain the population averages and their variations from the analysis of a sufficient number of individual cells.
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