Experimental investigation and computational modeling of hydrodynamics in bifurcating microchannels

Lysyl Oxidase Production by Murine C3H10T1/2 Mesenchymal Stem Cells Is Increased by TGFβs and Differentially Modulated by Mechanical Stimuli

Tendons are frequently injured and have limited regenerative capacity. This motivates tissue engineering efforts aimed at restoring tendon function through strategies to direct functional tendon formation. Generation of a crosslinked collagen matrix is paramount to forming mechanically functional tendon. However, it is unknown how lysyl oxidase (LOX), the primary mediator of enzymatic collagen crosslinking, is regulated by stem cells. This study investigates how multiple factors previously identified to promote tendon formation and healing (transforming growth factor [TGF]β1 and TGFβ2, mechanical stimuli, and hypoxia-inducible factor [HIF]-1α) regulate LOX production in the murine C3H10T1/2 mesenchymal stem cell (MSC) line. We hypothesized that TGFβ signaling promotes LOX activity in C3H10T1/2 MSCs, which is regulated by both mechanical stimuli and HIF-1α activation. TGFβ1 and TGFβ2 increased LOX levels as a function of concentration and time. Inhibiting the TGFβ type I receptor (TGFβRI) decreased TGFβ2-induced LOX production by C3H10T1/2 MSCs. Low (5 mPa) and high (150 mPa) magnitudes of fluid shear stress were applied to test impacts of mechanical stimuli, but without TGFβ2, loading alone did not alter LOX levels. Low loading (5 mPa) with TGFβ2 increased LOX at 7 days greater than TGFβ2 treatment alone. Neither HIF-1α knockdown (siRNA) nor activation (CoCl2) affected LOX levels. Ultimately, results suggest that TGFβ2 and appropriate loading magnitudes contribute to LOX production by C3H10T1/2 MSCs. Potential application of these findings includes treatment with TGFβ2 and appropriate mechanical stimuli to modulate LOX production by stem cells to ultimately control collagen matrix stiffening and support functional tendon formation.

Endothelial mechanobiology in atherosclerosis

Cardiovascular disease (CVD) is a serious health challenge, causing more deaths worldwide than cancer. The vascular endothelium, which forms the inner lining of blood vessels, plays a central role in maintaining vascular integrity and homeostasis and is in direct contact with the blood flow. Research over the past century has shown that mechanical perturbations of the vascular wall contribute to the formation and progression of atherosclerosis. While the straight part of the artery is exposed to sustained laminar flow and physiological high shear stress, flow near branch points or in curved vessels can exhibit 'disturbed' flow. Clinical studies as well as carefully controlled in vitro analyses have confirmed that these regions of disturbed flow, which can include low shear stress, recirculation, oscillation, or lateral flow, are preferential sites of atherosclerotic lesion formation. Because of their critical role in blood flow homeostasis, vascular endothelial cells (ECs) have mechanosensory mechanisms that allow them to react rapidly to changes in mechanical forces, and to execute context-specific adaptive responses to modulate EC functions. This review summarizes the current understanding of endothelial mechanobiology, which can guide the identification of new therapeutic targets to slow or reverse the progression of atherosclerosis.

Intimal thickening at coronary bifurcations in pediatric heart transplant recipients

Heart transplant recipients are at increased risk for atherosclerosis and cardiac allograft vasculopathy, both initially presenting as intimal thickening. We aimed to determine the presence, extent, and anatomical characteristics of intimal thickness at coronary bifurcations in children using OCT. We measured the intimal thickness of coronary arteries in pediatric transplant recipients using OCT during routine cardiac catheterization. Intimal thickening was defined as (i) a percent change in contralateral intimal thickness greater than 50% when comparing the thickness at the bifurcation to the baseline thickness, and (ii) greater than 0.1 mm. We evaluated 153 unique coronary bifurcations in 31 children (58% boys, median 12.7 years). Intimal thickening was almost exclusively observed in the left coronary system (22 of 67 bifurcations) and rare in the right coronary system (2 of 86 bifurcations; P < .001). There was a positive association between the relative size of the side branch and contralateral intimal thickening at coronary bifurcations (P = .009). Intimal thickening at coronary bifurcations is already present in the left coronary system in many pediatric transplant recipients. The correlation between intimal thickening and side branch size suggests that low shear stress and oscillating shear stress may have an important role in the development of intimal thickening at coronary bifurcations.

Numerical Simulation of Bubble Transport in a Bifurcating Microchannel: A Preliminary Study

In this paper, we present the computational fluid dynamics (CFD) simulations of bubble transport in a first generation bifurcating microchannel. In the present study, the human arteriole is modeled as a two-dimensional (2D) rectangular bifurcating microchannel. The microchannel is filled with blood and a single perfluorocarbon (PFC) bubble is introduced in the parent channel. The simulations are carried out to identify the lodging and dislodging pressures for two nondimensional bubble sizes, Ld (ratio of the dimensional bubble length to the parent tube diameter), that is for Ld = 1 and Ld = 2. Subsequently, the bubble transport and splitting behavior due to the presence of symmetry and asymmetry in the daughter channels of the microchannel is studied for these bubble sizes. The splitting behavior of the bubble under the effect of gravity is also assessed and reported here. For the symmetric bifurcation model, the splitting ratio (SR) (ratio of bubble volume in bottom daughter channel to bubble volume in top daughter channel), of the bubble was found to be 1. For the asymmetric model, the splitting ratio was found to be less than 1. The loss in the bubble volume in the asymmetric model was attributed to surface tension effects and the resistance offered by the flow, which led to the bubble sticking and sliding along the walls of the channel. With the increase in roll angle, Φ (angle which the plane makes with the horizontal to study the effects of gravity), there was a decline in the splitting ratio.

COMPUTATIONAL FLUID DYNAMICS FOR TISSUE ENGINEERING APPLICATIONS

Hydrodynamic cellular environment plays an important role in translating engineered tissue constructs into clinically useful grafts. However, the cellular fluid dynamic environment inside bioreactor systems is highly complex and it is normally impractical to experimentally characterize the local flow patterns at the cellular scale. Computational fluid dynamics (CFD) has been recognized as an invaluable and reliable alternative to investigate the complex relationship between hydrodynamic environments and the regeneration of engineered tissues at both the macroscopic and microscopic scales. This review describes the applications of CFD simulations to probe the hydrodynamic environment parameters (e.g., flow rate, shear stress, etc.) and the corresponding experimental validations. We highlight the use of CFD to optimize bioreactor design and scaffold architectures for improved ex-vivo hydrodynamic environments. It is envisioned that CFD could be used to customize specific hydrodynamic cellular environments to meet the unique requirements of different cell types in combination with advanced manufacturing techniques and finally facilitate the maturation of tissue-engineered constructs.

Conversion of mechanical force into TGF-β-mediated biochemical signals

Mechanical forces influence homeostasis in virtually every tissue [1, 2]. Tendon, constantly exposed to variable mechanical force, is an excellent model in which to study the conversion of mechanical stimuli into a biochemical response [3-5]. Here we show in a mouse model of acute tendon injury and in vitro that physical forces regulate the release of active transforming growth factor (TGF)-β from the extracellular matrix (ECM). The quantity of active TGF-β detected in tissue exposed to various levels of tensile loading correlates directly with the extent of physical forces. At physiological levels, mechanical forces maintain, through TGF-β/Smad2/3-mediated signaling, the expression of Scleraxis (Scx), a transcription factor specific for tenocytes and their progenitors. The gradual and temporary loss of tensile loading causes reversible loss of Scx expression, whereas sudden interruption, such as in transection tendon injury, destabilizes the structural organization of the ECM and leads to excessive release of active TGF-β and massive tenocyte death, which can be prevented by the TGF-β type I receptor inhibitor SD208. Our findings demonstrate a critical role for mechanical force in adult tendon homeostasis. Furthermore, this mechanism could translate physical force into biochemical signals in a much broader variety of tissues or systems in the body.

Lung assist device technology with physiologic blood flow developed on a tissue engineered scaffold platform

There is no technology available to support failing lung function for patients outside the hospital. An implantable lung assist device would augment lung function as a bridge to transplant or possible destination therapy. Utilizing biomimetic design principles, a microfluidic vascular network was developed for blood inflow from the pulmonary artery and blood return to the left atrium. Computational fluid dynamics analysis was used to optimize blood flow within the vascular network. A micro milled variable depth mold with 3D features was created to achieve both physiologic blood flow and shear stress. Gas exchange occurs across a thin silicone membrane between the vascular network and adjacent alveolar chamber with flowing oxygen. The device had a surface area of 23.1 cm(2) and respiratory membrane thickness of 8.7 ± 1.2 μm. Carbon dioxide transfer within the device was 156 ml min(-1) m(-2) and the oxygen transfer was 34 ml min(-1) m(-2). A lung assist device based on tissue engineering architecture achieves gas exchange comparable to hollow fiber oxygenators yet does so while maintaining physiologic blood flow. This device may be scaled up to create an implantable ambulatory lung assist device.

Principles of biomimetic vascular network design applied to a tissue-engineered liver scaffold

Branched vascular networks are a central component of scaffold architecture for solid organ tissue engineering. In this work, seven biomimetic principles were established as the major guiding technical design considerations of a branched vascular network for a tissue-engineered scaffold. These biomimetic design principles were applied to a branched radial architecture to develop a liver-specific vascular network. Iterative design changes and computational fluid dynamic analysis were used to optimize the network before mold manufacturing. The vascular network mold was created using a new mold technique that achieves a 1:1 aspect ratio for all channels. In vitro blood flow testing confirmed the physiologic hemodynamics of the network as predicted by computational fluid dynamic analysis. These results indicate that this biomimetic liver vascular network design will provide a foundation for developing complex vascular networks for solid organ tissue engineering that achieve physiologic blood flow.