Biaxial mechanical properties of the inferior vena cava in C57BL/6 and CB-17 SCID/bg mice

Hemodynamics and Wall Mechanics of Vascular Graft Failure

Blood vessels are subjected to complex biomechanical loads, primarily from pressure-driven blood flow. Abnormal loading associated with vascular grafts, arising from altered hemodynamics or wall mechanics, can cause acute and progressive vascular failure and end-organ dysfunction. Perturbations to mechanobiological stimuli experienced by vascular cells contribute to remodeling of the vascular wall via activation of mechanosensitive signaling pathways and subsequent changes in gene expression and associated turnover of cells and extracellular matrix. In this review, we outline experimental and computational tools used to quantify metrics of biomechanical loading in vascular grafts and highlight those that show potential in predicting graft failure for diverse disease contexts. We include metrics derived from both fluid and solid mechanics that drive feedback loops between mechanobiological processes and changes in the biomechanical state that govern the natural history of vascular grafts. As illustrative examples, we consider application-specific coronary artery bypass grafts, peripheral vascular grafts, and tissue-engineered vascular grafts for congenital heart surgery as each of these involves unique circulatory environments, loading magnitudes, and graft materials.

A Systematic Comparison of Normal Structure and Function of the Greater Thoracic Vessels

The greater thoracic vessels are central to a well-functioning circulatory system and are often targeted in congenital heart surgeries, yet the structure and function of these vessels have not been well studied. Here we use consistent methods to quantify and compare microstructural features and biaxial biomechanical properties of the following six greater thoracic vessels in wild-type mice: ascending thoracic aorta, descending thoracic aorta, right subclavian artery, right pulmonary artery, thoracic inferior vena cava, and superior vena cava. Specifically, we determine volume fractions and orientations of the structurally significant wall constituents (i.e., collagen, elastin, and cell nuclei) using multiphoton imaging, and we quantify vasoactive responses and mechanobiologically relevant mechanical quantities (e.g., stress, stiffness) using computer-controlled biaxial mechanical testing. Similarities and differences across systemic, pulmonary, and venous circulations highlight underlying design principles of the vascular system. Results from this study represent another step towards understanding growth and remodeling of greater thoracic vessels in health, disease, and surgical interventions by providing baseline information essential for developing and validating predictive computational models.

Dissociation of pulse wave velocity and aortic wall stiffness in diabetic db/db mice: The influence of blood pressure

Introduction: Vascular stiffness is a predictor of cardiovascular disease and pulse wave velocity (PWV) is the current standard for measuring in vivo vascular stiffness. Mean arterial pressure is the largest confounding variable to PWV; therefore, in this study we aimed to test the hypothesis that increased aortic PWV in type 2 diabetic mice is driven by increased blood pressure rather than vascular biomechanics. Methods and Results: Using a combination of in vivo PWV and ex vivo pressure myography, our data demonstrate no difference in ex vivo passive mechanics, including outer diameter, inner diameter, compliance (Db/db: 0.0094 ± 0.0018 mm²/mmHg vs. db/db: 0.0080 ± 0.0008 mm²/mmHg, p > 0.05 at 100 mmHg), and incremental modulus (Db/db: 801.52 ± 135.87 kPa vs. db/db: 838.12 ± 44.90 kPa, p > 0.05 at 100 mmHg), in normal versus diabetic 16 week old mice. We further report no difference in basal or active aorta biomechanics in normal versus diabetic 16 week old mice. Finally, we show here that the increase in diabetic in vivo aortic pulse wave velocity at baseline was completely abolished when measured at equivalent pharmacologically-modulated blood pressures, indicating that the elevated PWV was attributed to the concomitant increase in blood pressure at baseline, and therefore "stiffness." Conclusions: Together, these animal model data suggest an intimate regulation of blood pressure during collection of pulse wave velocity when determining in vivo vascular stiffness. These data further indicate caution should be exerted when interpreting elevated PWV as the pure marker of vascular stiffness.

In vivo development of tissue engineered vascular grafts: a fluid-solid-growth model

Methods of tissue engineering continue to advance, and multiple clinical trials are underway evaluating tissue engineered vascular grafts (TEVGs). Whereas initial concerns focused on suture retention and burst pressure, there is now a pressing need to design grafts to have optimal performance, including an ability to grow and remodel in response to changing hemodynamic loads. Toward this end, there is similarly a need for computational methods that can describe and predict the evolution of TEVG geometry, composition, and material properties while accounting for changes in hemodynamics. Although the ultimate goal is a fluid-solid-growth (FSG) model incorporating fully 3D growth and remodeling and 3D hemodynamics, lower fidelity models having high computational efficiency promise to play important roles, especially in the design of candidate grafts. We introduce here an efficient FSG model of in vivo development of a TEVG based on two simplifying concepts: mechanobiologically equilibrated growth and remodeling of the graft and an embedded control volume analysis of the hemodynamics. Illustrative simulations for a model Fontan conduit reveal the utility of this approach, which promises to be particularly useful in initial design considerations involving formal methods of optimization which otherwise add considerably to the computational expense.

Nicotine Affects Murine Aortic Stiffness and Fatigue Response During Supraphysiological Cycling

Nicotine exposure is a major risk factor for several cardiovascular diseases. Although the deleterious effects of nicotine on aortic remodeling processes have been studied to some extent, the biophysical consequences are not fully elucidated. In this investigation, we applied quasi-static and dynamic loading to quantify ways in which exposure to nicotine affects the mechanical behavior of murine arterial tissue. Segments of thoracic aortas from C57BL/6 mice exposed to 25 mg/kg/day of subcutaneous nicotine for 28 days were subjected to uniaxial tensile loading in an open-circumferential configuration. Comparing aorta segments from nicotine-treated mice relative to an equal number of control counterparts, stiffness in the circumferential direction was nearly twofold higher (377 kPa ± 165 kPa versus 191 kPa ± 65 kPa, n = 5, p = 0.03) at 50% strain. Using a degradative power-law fit to fatigue data at supraphysiological loading, we observed that nicotine-treated aortas exhibited significantly higher peak stress, greater loss of tension, and wider oscillation band than control aortas (p ≤ 0.01 for all three variables). Compared to simple stress relaxation tests, fatigue cycling is shown to be more sensitive and versatile in discerning nicotine-induced changes in mechanical behavior over many cycles. Supraphysiological fatigue cycling thus may have broader potential to reveal subtle changes in vascular mechanics caused by other exogenous toxins or pathological conditions.

A comparison of passive and active wall mechanics between elastic and muscular arteries of juvenile and adult rats

The elastic abdominal aorta and muscular femoral artery are susceptible to aneurysm and atherosclerosis, respectively. The vessel wall mechanics should be an important element for the difference. The objective of the study is to demonstrate a comparison of vessel wall mechanics between elastic and muscular arteries of juvenile and adult rats to show the changes of mechanical properties relevant to aging. The passive and active mechanical tests, theoretical analysis, and histological evaluation were carried out to investigate mechanical properties of vessel walls in the abdominal aorta and carotid and femoral arteries of young and adult rats. There are stiffening femoral artery, unchanged carotid artery, and distensible abdominal aorta in adult rats as compared with the young. The opening angle has values of 54 ± 13°, 82 ± 13°, and 94 ± 13° in the abdominal aorta and carotid and femoral arteries of adult rats, respectively, as well as 80 ± 22°, 93 ± 19°, and 100 ± 23° in the young. The findings are explained by the significantly reduced width of collagen fibers in the abdominal aorta, relatively unchanged width in the carotid artery, and significantly increased width in the femoral artery of adult rats as compared with the young. In conjunction with available literatures, we concluded that inconsistency for nonlinear age-related changes of artery wall mechanics occurs between arteries of different types, which may be a risk factor for the occurrence of abdominal aorta aneurysm and femoral artery atherosclerosis.

Vascular adaptation in the presence of external support - A modeling study

Vascular grafts have long been used to replace damaged or diseased vessels with considerable success, but a new approach is emerging where native vessels are merely supported, not replaced. Although external supports have been evaluated in diverse situations - ranging from aneurysmal disease to vein grafts or the Ross operation - optimal supports and procedures remain wanting. In this paper, we present a novel application of a growth and remodeling model well suited for parametrically exploring multiple designs of external supports while accounting for mechanobiological and immunobiological responses of the supported native vessel. These results suggest that a load bearing external support can reduce vessel thickening in response to pressure elevation. Results also suggest that the final adaptive state of the vessel depends on the structural stiffness of the support via a mechano-driven adaptation, although luminal encroachment may be a complication in the presence of chronic inflammation. Finally, the supported vessel can stiffen (structurally and materially) along circumferential and axial directions, which could have implications on overall hemodynamics and thus subsequent vascular remodeling. The proposed framework can provide valuable insights into vascular adaptation in the presence of external support, accelerate rational design, and aid translation of this emerging approach.

Venous Mechanical Properties After Arteriovenous Fistulae in Mice

BACKGROUND

An arteriovenous fistula (AVF) exposes the outflow vein to arterial magnitudes and frequencies of blood pressure and flow, triggering molecular pathways that result in venous remodeling and AVF maturation. It is unknown, however, how venous remodeling, that is lumen dilation and wall thickening, affects venous mechanical properties. We hypothesized that a fistula is more compliant compared with a vein because of altered contributions of collagen and elastin to the mechanical properties.

METHODS

Ephb4^+/- and littermate wild-type (WT) male mice were treated with sham surgery or needle puncture to create an abdominal aortocaval fistulae. The thoracic inferior vena cava was harvested 3 wk postoperatively for mechanical testing and histological analyses of collagen and elastin.

RESULTS

Mechanical testing of the thoracic inferior vena cava from Ephb4^+/- and WT mice showed increased distensibility and increased compliance of downstream veins after AVF compared with sham. Although Ephb4^+/- veins were thicker than WT veins at the baseline, after AVF, both Ephb4^+/- and WT veins showed similar wall thickness as well as similar collagen and elastin area fractions, but increased collagen undulation compared with sham.

CONCLUSIONS

Fistula-induced remodeling of the outflow vein results in circumferentially increased distensibility and compliance, likely due to post-translational modifications to collagen.

Biaxial biomechanical properties of the nonpregnant murine cervix and uterus

From a biomechanical perspective, female reproductive health is an understudied area of research. There is an incomplete understanding of the complex function and interaction between the cervix and uterus. This, in part, is due to the limited research into multiaxial biomechanical functions and geometry of these organs. Knowledge of the biomechanical function and interaction between these organs may elucidate etiologies of conditions such as preterm birth. Therefore, the objective of this study was to quantify the multiaxial biomechanical properties of the murine cervix and uterus using a biaxial testing set-up. To accomplish this, an inflation-extension testing protocol (n = 15) was leveraged to quantify biaxial biomechanical properties while preserving native matrix interactions and geometry. Ultrasound imaging and histology (n = 10) were performed to evaluate regional geometry and microstructure, respectively. Histological analysis identified a statistically significant greater collagen content and significantly smaller smooth muscle content in the cervix as compared to the uterus. No statistically significant differences in elastic fibers were identified. Analysis of bilinear fits revealed a significantly stiffer response from the circumferentially orientated ECM fibers compared to axially orientated fibers in both organs. Bilinear fits and a two-fiber family constitutive model showed that the cervix was significantly less distensible than the uterus. We submit that the regional biaxial information reported in this study aids in establishing an appropriate reference configuration for mathematical models of the uterine-cervical complex. Thus, may aid future work to elucidate the biomechanical mechanisms leading to cervical or uterine conditions.

Biomechanical characterization of murine pulmonary arteries

The biomechanical properties of the major pulmonary arteries play critical roles in normal physiology as well as in diverse pathophysiologies and clinical interventions. Importantly, advances in medical imaging enable simulations of pulmonary hemodynamics, but such models cannot reach their full potential until they are informed with region-specific material properties. In this paper, we present passive and active biaxial biomechanical data for the right and left main pulmonary arteries from wild-type mice. We also evaluate the suitability of a four-fiber family constitutive model as a descriptor of the passive behavior. Despite regional differences in size, the biaxial mechanical properties, including passive stiffness and elastic energy storage, the biaxial wall stresses at in vivo pressures, and the overall contractile capacity in response to smooth muscle cell stimulation under in vivo conditions are remarkably similar between the right and left branches. The proposed methods and results can serve as baseline protocols and measurements for future biaxial experiments on murine models of pulmonary pathologies, and the constitutive model can inform computational models of normal pulmonary growth and remodeling. Our use of consistent experimental protocols and data analyses can also facilitate comparative studies in health and disease across the systemic and pulmonary circulations as well as studies seeking to understand remodeling in surgeries such as the Fontan procedure, which involves different types of vessels.

Micromechanics of elastic lamellae: unravelling the role of structural inhomogeneity in multi-scale arterial mechanics

Microstructural deformation of elastic lamellae plays important roles in maintaining arterial tissue homeostasis and regulating vascular smooth muscle cell fate. Our study unravels the underlying microstructural origin that enables elastic lamellar layers to evenly distribute the stresses through the arterial wall caused by intraluminal distending pressure, a fundamental requirement for tissue and cellular function. A new experimental approach was developed to quantify the spatial organization and unfolding of elastic lamellar layers under pressurization in mouse carotid arteries by coupling physiological extension-inflation and multiphoton imaging. Tissue-level circumferential stretch was obtained from analysis of the deformation of a thick-walled cylinder. Our results show that the unfolding and extension of lamellar layers contribute simultaneously to tissue-level deformation. The inner lamellar layers are wavier and unfold more than the outer layers. This waviness gradient compensates the larger tissue circumferential stretch experienced at the inner surface, thus equalizing lamellar layer extension through the arterial wall. Discoveries from this study reveal the importance of structural inhomogeneity in maintaining tissue homeostasis through the arterial wall, and may have profound implications on vascular remodelling in aging and diseases, as well as in tissue engineering of functional blood vessels.

Immuno-driven and Mechano-mediated Neotissue Formation in Tissue Engineered Vascular Grafts

In vivo development of a neovessel from an implanted biodegradable polymeric scaffold depends on a delicate balance between polymer degradation and native matrix deposition. Studies in mice suggest that this balance is dictated by immuno-driven and mechanotransduction-mediated processes, with neotissue increasingly balancing the hemodynamically induced loads as the polymer degrades. Computational models of neovessel development can help delineate relative time-dependent contributions of the immunobiological and mechanobiological processes that determine graft success or failure. In this paper, we compare computational results informed by long-term studies of neovessel development in immuno-compromised and immuno-competent mice. Simulations suggest that an early exuberant inflammatory response can limit subsequent mechano-sensing by synthetic intramural cells and thereby attenuate the desired long-term mechano-mediated production of matrix. Simulations also highlight key inflammatory differences in the two mouse models, which allow grafts in the immuno-compromised mouse to better match the biomechanical properties of the native vessel. Finally, the predicted inflammatory time courses revealed critical periods of graft remodeling. We submit that computational modeling can help uncover mechanisms of observed neovessel development and improve the design of the scaffold or its clinical use.

Gradual loading ameliorates maladaptation in computational simulations of vein graft growth and remodelling

Vein graft failure is a prevalent problem in vascular surgeries, including bypass grafting and arteriovenous fistula procedures in which veins are subjected to severe changes in pressure and flow. Animal and clinical studies provide significant insight, but understanding the complex underlying coupled mechanisms can be advanced using computational models. Towards this end, we propose a new model of venous growth and remodelling (G&R) based on a constrained mixture theory. First, we identify constitutive relations and parameters that enable venous adaptations to moderate perturbations in haemodynamics. We then fix these relations and parameters, and subject the vein to a range of combined loads (pressure and flow), from moderate to severe, and identify plausible mechanisms of adaptation versus maladaptation. We also explore the beneficial effects of gradual increases in load on adaptation. A gradual change in flow over 3 days plus an initial step change in pressure results in fewer maladaptations compared with step changes in both flow and pressure, or even a gradual change in pressure and flow over 3 days. A gradual change in flow and pressure over 8 days also enabled a successful venous adaptation for loads as severe as the arterial loads. Optimization is used to accelerate parameter estimation and the proposed framework is general enough to provide a good starting point for parameter estimations in G&R simulations.

Structural and Functional Differences Between Porcine Aorta and Vena Cava

Elastin and collagen fibers are the major load-bearing extracellular matrix (ECM) constituents of the vascular wall. Arteries function differently than veins in the circulatory system; however as a result from several treatment options, veins are subjected to sudden elevated arterial pressure. It is thus important to recognize the fundamental structure and function differences between a vein and an artery. Our research compared the relationship between biaxial mechanical function and ECM structure of porcine thoracic aorta and inferior vena cava. Our study suggests that aorta contains slightly more elastin than collagen due to the cyclical extensibility, but vena cava contains almost four times more collagen than elastin to maintain integrity. Furthermore, multiphoton imaging of vena cava showed longitudinally oriented elastin and circumferentially oriented collagen that is recruited at supraphysiologic stress, but low levels of strain. However in aorta, elastin is distributed uniformly, and the primarily circumferentially oriented collagen is recruited at higher levels of strain than vena cava. These structural observations support the functional finding that vena cava is highly anisotropic with the longitude being more compliant and the circumference stiffening substantially at low levels of strain. Overall, our research demonstrates that fiber distributions and recruitment should be considered in addition to relative collagen and elastin contents. Also, the importance of accounting for the structural and functional differences between arteries and veins should be taken into account when considering disease treatment options.

Prolonged Mechanical Circumferential Stretch Induces Metabolic Changes in Rat Inferior Vena Cava

OBJECTIVE/BACKGROUND

Circumferential stretch on the vein wall has been suggested as a potential etiological factor in the development of varicose veins. However, the influence of vein wall stretch on vein metabolism has not yet been explored. The aim of this study was to investigate the effect of short and prolonged mechanical stretch on vein wall metabolism.

METHODS

Circular segments of inferior vena cava from male Sprague-Dawley rats were exposed to normal 0.5-g (nonstretched) or high 2-g (stretched) tension for short (4 h) or prolonged (18 h) duration (five vein segments per group). Contraction response to phenylephrine (10^-5 M) and KCl (96 mM) was elicited to observe the effect of circumferential stretch on vein function. The polar and organic metabolites in vein tissue were extracted using a bilayer extraction method. Aqueous and organic extracts were analyzed using nuclear magnetic resonance spectroscopy and ultra performance liquid chromatography coupled to mass spectrometry, respectively. Data acquired from both analytical platforms were analyzed using mathematical modeling.

RESULTS

Increased concentrations of valine (p = .02) and choline (p = .03) metabolites and triglyceride moieties (p = .03) were observed in veins stretched for 18 h compared with the nonstretched/18 h group.

DISCUSSION

Increased concentrations of branched chain amino acid valine and cell membrane constituent choline indicate increased muscle breakdown and increased metabolism of membrane phospholipids under stretch in an ex-vivo model. Increased intensities of triglyceride moieties in stretched vein segments for 18 h suggest that high pressure may induce an inflammatory response.

CONCLUSION

This study has shown that prolonged mechanical circumferential stretch (18 h) alters the metabolic profile of rat inferior vena cava.

Biomechanical diversity despite mechanobiological stability in tissue engineered vascular grafts two years post-implantation

Recent advances in vascular tissue engineering have enabled a paradigm shift from ensuring short-term graft survival to focusing on long-term stability and growth potential. We present the first experimental-computational study of a tissue-engineered vascular graft (TEVG) effectively over the full lifespan of the recipient. We show that grafts implanted within the venous circulation of mice remained patent over 2 years without thrombus, stenosis, or aneurysmal dilatation. Moreover, the gross appearance and mechanical properties of the grafts evolved to be similar to the host vein within 24 weeks, with mean neovessel geometry and properties remaining unchanged thereafter despite a continued turnover of extracellular matrix. Biomechanical diversity manifested after 24 weeks, however, via two subsets of grafts despite all procedures being the same. Computational modeling and associated immunohistological analyses suggested that this diversity likely resulted from a differential ratio of collagen types I and III, with lower I to III ratios promoting grafts having a compliance similar to the native vein. We submit that TEVGs can exhibit the desired long-term mechanobiological stability; hence, we must now focus on evaluating growth potential and optimizing scaffold properties to achieve compliance matching throughout neovessel development.

Computational simulation of the adaptive capacity of vein grafts in response to increased pressure

Vein maladaptation, leading to poor long-term patency, is a serious clinical problem in patients receiving coronary artery bypass grafts (CABGs) or undergoing related clinical procedures that subject veins to elevated blood flow and pressure. We propose a computational model of venous adaptation to altered pressure based on a constrained mixture theory of growth and remodeling (G&R). We identify constitutive parameters that optimally match biaxial data from a mouse vena cava, then numerically subject the vein to altered pressure conditions and quantify the extent of adaptation for a biologically reasonable set of bounds for G&R parameters. We identify conditions under which a vein graft can adapt optimally and explore physiological constraints that lead to maladaptation. Finally, we test the hypothesis that a gradual, rather than a step, change in pressure will reduce maladaptation. Optimization is used to accelerate parameter identification and numerically evaluate hypotheses of vein remodeling.

A hypothesis-driven parametric study of effects of polymeric scaffold properties on tissue engineered neovessel formation

Continued advances in the tissue engineering of vascular grafts have enabled a paradigm shift from the desire to design for adequate suture retention, burst pressure and thrombo-resistance to the goal of achieving grafts having near native properties, including growth potential. Achieving this far more ambitious outcome will require the identification of optimal, not just adequate, scaffold structure and material properties. Given the myriad possible combinations of scaffold parameters, there is a need for a new strategy for reducing the experimental search space. Toward this end, we present a new modeling framework for in vivo neovessel development that allows one to begin to assess in silico the potential consequences of different combinations of scaffold structure and material properties. To restrict the number of parameters considered, we also utilize a non-dimensionalization to identify key properties of interest. Using illustrative constitutive relations for both the evolving fibrous scaffold and the neotissue that develops in response to inflammatory and mechanobiological cues, we show that this combined non-dimensionalization computational approach predicts salient aspects of neotissue development that depend directly on two key scaffold parameters, porosity and fiber diameter. We suggest, therefore, that hypothesis-driven computational models should continue to be pursued given their potential to identify preferred combinations of scaffold parameters that have the promise of improving neovessel outcome. In this way, we can begin to move beyond a purely empirical trial-and-error search for optimal combinations of parameters and instead focus our experimental resources on those combinations that are predicted to have the most promise.

Unrealistic statistics: how average constitutive coefficients can produce non-physical results

The coefficients of constitutive models are frequently averaged in order to concisely summarize the complex, nonlinear, material properties of biomedical materials. However, when dealing with nonlinear systems, average inputs (e.g. average constitutive coefficients) often fail to generate average behavior. This raises an important issue because average nonlinear constitutive coefficients of biomedical materials are commonly reported in the literature. This paper provides examples which demonstrate that average constitutive coefficients applied to nonlinear constitutive laws in the field of biomedical material characterization can fail to produce average stress-strain responses and in some cases produce non-physical responses. Results are presented from a literature survey which indicates that approximately 90% of tissue measurement studies that employ a nonlinear constitutive model report average nonlinear constitutive coefficients. We suggest that reviewers and editors of future measurement studies discourage the reporting of average nonlinear constitutive coefficients. Reporting of individual coefficient sets for each test sample should be considered and discussed as designation for a "best practice" in the field of biomedical material characterization.

Implantation of inferior vena cava interposition graft in mouse model

Biodegradable scaffolds seeded with bone marrow mononuclear cells (BMCs) are often used for reconstructive surgery to treat congenital cardiac anomalies. The long-term clinical results showed excellent patency rates, however, with significant incidence of stenosis. To investigate the cellular and molecular mechanisms of vascular neotissue formation and prevent stenosis development in tissue engineered vascular grafts (TEVGs), we developed a mouse model of the graft with approximately 1 mm internal diameter. First, the TEVGs were assembled from biodegradable tubular scaffolds fabricated from a polyglycolic acid nonwoven felt mesh coated with ε-caprolactone and L-lactide copolymer. The scaffolds were then placed in a lyophilizer, vacuumed for 24 hr, and stored in a desiccator until cell seeding. Second, bone marrow was collected from donor mice and mononuclear cells were isolated by density gradient centrifugation. Third, approximately one million cells were seeded on a scaffold and incubated O/N. Finally, the seeded scaffolds were then implanted as infrarenal vena cava interposition grafts in C57BL/6 mice. The implanted grafts demonstrated excellent patency (>90%) without evidence of thromboembolic complications or aneurysmal formation. This murine model will aid us in understanding and quantifying the cellular and molecular mechanisms of neotissue formation in the TEVG.

Computational model of the in vivo development of a tissue engineered vein from an implanted polymeric construct

Advances in vascular tissue engineering have been tremendous over the past 15 years, yet there remains a need to optimize current constructs to achieve vessels having true growth potential. Toward this end, it has been suggested that computational models may help hasten this process by enabling time-efficient parametric studies that can reduce the experimental search space. In this paper, we present a first generation computational model for describing the in vivo development of a tissue engineered vein from an implanted polymeric scaffold. The model was motivated by our recent data on the evolution of mechanical properties and microstructural composition over 24 weeks in a mouse inferior vena cava interposition graft. It is shown that these data can be captured well by including both an early inflammatory-mediated and a subsequent mechano-mediated production of extracellular matrix. There remains a pressing need, however, for more data to inform the development of next generation models, particularly the precise transition from the inflammatory to the mechanobiological dominated production of matrix having functional capability.