Optical Imaging of Tissue-Engineered Vascular Grafts

Introduction

Tissue-engineered vascular grafts (TEVGs) can be created with a variety of scaffolds and cell types. One option consists of a polymer-based tube with an inner endothelial cell lining. Because this cellular lining makes the graft more biocompatible, a considerable amount of clinical interest exists for developing these tissue-engineered grafts. Another area of interest is the use of these vascular grafts as an in situ model system. Such a model would give researchers the ability to perform numerous cardiovascular-related experiments on a simple blood vessel mimic – prior to animal or clinical studies.

In order to develop such an in situ model system, a set of diagnostic tools are needed to characterize the cellular layer within the vascular graft. Optical coherence tomography (OCT) is one such technology that provides the necessary capabilities to identify structural attributes of the cellular layer. An additional technology known as laser-induced fluorescence (LIF) can provide bio-chemical measures to complement the structural information acquired by OCT.

OCT of Vascular Grafts

Figure 1 is a simple schematic showing the orientation of the OCT images with respect to the vascular graft. The endoscope is inserted into the center of the vascular graft. OCT is then used to form an image where the top of the image represents the luminal (interior) and the bottom represents the abluminal (exterior) portions of the graft.

The cellular layer is often represented by a bright layer near the top of the OCT image. The polymer on which the cells are grown, ePTFE, is shown as a less intense layer towards the bottom of the OCT image. Small white reflections can be seen within the ePTFE - we believe these are air bubbles that have infiltrated the porous polymer.

Figure 2 shows an OCT image of a tissue-engineered vascular graft. The image is 20-mm long and 1.4-mm wide with an axial and lateral resolution of approximately 16-um .


Figure 1. Explanation of Vascular Graft OCT Images.
Grafts developed by Kristen O'Halloran and Stuart Williams, Ph.D.


Figure 2. OCT Image of Vascular Grafts.

Fluorescence of Vascular Grafts

Figure 3. An Excitation-Emission Matrix.
EEM Development with Ned Kirkpatrick and Urs Utzinger, Ph.D.  

OCT gives structural information about the vascular graft. However, we also use laser-induced fluorescence (LIF) to obtain biochemical information of known fluorophores from within the TEVG.

Figure 3 shows an excitation-emission matrix of ePTFE - a porous polymer commonly used for the scaffold of tissue engineered vascular grafts. An excitation-emission matrix shows the emitted wavelengths resulting from a particular excitation wavelength. The shapes of this contour mapping can be used to extract bio-chemical information of the TEVG.

Technological Developments

The tissue-engineered vascular grafts are developed within a device known as a bioreactor. Slight modifications to our previous endoscope designs were made to image vertically within the sterile environment of the bioreactor. An upgrade to include endoscopic and LIF capability is also being performed on our relatively fast time-domain OCT system. Additionally, an extensive optical characterization of the materials within the bioreactor is currently being conducted in an effort to identify future developments that would be particularly useful for imaging TEVG's in situ.

Researchers

Primary: Garret Bonnema and Kristen O'Halloran, Stuart Williams, and Jennifer Barton

Additional: Ned Kirkpatrick, Urs Utzinger, and the members of the Tissue Optics and Cardiovascular Research Laboratories

Funding

NIH Biomedical Engineering Cardiovascular Training Grant (Garret Bonnema)

NSF Graduate Research Fellowship (Kristen O'Halloran)

 

For additional information contact Garret Bonnema.
Email Dr. Barton
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