Elizabeth Voigt

Tissue Engineered Tumor Microvessels to Study the Role of Flow Shear Stress on Endothelial Barrier Function

As solid tumors develop, a variety of physical stresses arise including growth induced compressive force, matrix stiffening due to desmoplasia, and increased interstitial fluid pressure and altered flow patterns due to leaky vasculature and poor lymphatic drainage [1]. These microenvironmental stresses likely contribute to the abnormal cell behavior that drives tumor progression, and have become an increasingly significant area of cancer research. Of particular importance, is the role of flow shear stress on tumor-endothelial signaling, vascular function, and angiogenesis. Compared to normal vasculature, blood vessels in tumors are poorly functional due to dysregulated expression of angiogenic growth factors, such as vascular endothelial growth factor (VEGF) or the angiopoietins. Also, because of the abnormal vessel structure, blood velocities can be an order of magnitude lower than that of normal microvessels. Recently published work utilizing intravital microscopy to measure blood velocities in mouse mammary fat pad tumors, demonstrated for the first time that shear rate gradients in tumors may help guide branching and growth of new vessels [2]. However, much still remains unknown about how shear stress regulates endothelial organization, permeability, or expression of growth factors within the context of the tumor microenvironment.Copyright

Shear Stress Mediates Angiogenic Gene Expression in a Microfluidic Tumor Vascular Model

While research has shown that the fluid mechanics of the tumor vasculature reduce transport and uptake of therapeutics, the underlying role of these stresses in regulating tumor-endothelial cell signaling and neovascularization are not well understood. Understanding the reciprocal interaction between endothelial and tumor cells to mediate angiogenesis, and the effect of fluid shear on this process, may offer insight into the development of improved treatment modalities to control highly vascularized tumors. We have previously shown that breast cancer cells cultured under 2D, static conditions with endothelial cells significantly increase expression of pro-angiogenic factors vascular endothelial growth factor (VEGF) and angiopoietin 2 (ANG2) [1]. These preliminary results motivated the investigation of tumor-endothelial cross-talk under 3D, dynamic co-culture conditions.Copyright

Blood Flow Characterization in a Perfused Collagen Vessel Bioreactor Using X-Ray Micro-PIV

Newly developed cancer therapies must pass through a series of increasingly complex testing regimens before obtaining FDA approval as valid treatments. The costs of these tests increase rapidly as the physiological accuracy of the platform increases, from initial proof-of-concept in static tissue cultures, to treatment of animal models, and ultimately to human clinical trials. Three-dimensional engineered blood-perfused tumor models are becoming increasingly important as intermediate platforms for the study and treatment of cancer, as they are superior to static two-dimensional cultures in their reproduction of relevant physiological conditions and are inexpensive in comparison to animal models. Because of this, the design of well-characterized adaptable in vitro vascular tumor models has become a central objective of the emerging field of tumor engineering. Characterization of the flow within three-dimensional tumor models is critical for quantifying fluid shear stress and determining its role in pivotal tumor development processes such as tumor cell angiogenesis and metastasis. Ultimately, this knowledge will provide new avenues for therapeutic modulation of the tumor microenvironment.© 2012 ASME

Wall Shear Stress Measurements in an Arterial Flow Bioreactor

In vitro arterial flow bioreactor systems are widely used in tissue engineering to investigate response of endothelial cells to shear. However, the assumption that such models reproduce physiological flow has not been experimentally tested. Furthermore, shear stresses experienced by the endothelium are generally calculated using a Poiseuille flow assumption. Understanding the performance of flow bioreactor systems is of great importance, since interpretation of biological responses hinges on the fidelity of such systems and the validity of underlying assumptions. Here we test the physiologic reliability of arterial flow bioreactors and the validity of the Poiseuille assumption for a typical system used in tissue engineering. A particle image velocimetry system was employed to experimentally measure the flow within the vessel with high spatial and temporal resolution. Two types of vessels were considered: first, fluorinated ethylene propylene (FEP) tubing representative of a human artery without cells; and second, FEP tubing with a confluent layer of endothelial cells on the vessel lumen. Instantaneous wall shear stress (WSS), time-averaged WSS, and oscillatory shear index were computed from velocity field measurements and compared between cases. The flow patterns and resulting wall shear were quantitatively determined to not accurately reproduce physiological flow, and that the Poiseuille flow assumption was found to be invalid. This work concludes that analysis of cell response to hemodynamic parameters using such bioreactors should be accompanied by corresponding flow measurements for accurate quantification of fluid stresses.

Development of a 3D Microfluidic Culture Model to Study the Effect of Shear Stress on Tumor Angiogenesis

Current in vitro studies of tumor angiogenesis and metastasis are limited by the use of static 2D culture systems or 3D models that poorly reflect the pathological tumor microenvironment. While these systems have provided insight into tumor-inherent mechanisms of neovascularization, they are unable to couple local cellular response with specific biochemical and mechanical cues [1]. Interstitial flow plays an important role in regulating tumor growth; however, there are currently no in vitro cell culture models specifically designed to investigate the effect of fluid shear on tumorigenesis. By integrating tissue-engineering strategies with microfluidics and particle image velocimetry, we have developed a 3D in vitro cell culture model that allows the relationship between shear stress and tumor-endothelial cell cross-talk to be monitored. This research strategy will greatly improve our understanding of shear-stress mediated angiogenesis.Copyright