It is becoming evident that tissue-engineered constructs adapt to altered mechanical loading, and that specific combinations of multidirectional loads appear to have a synergistic effect on the remodeling. However, most studies of mechanical stimulation of engineered vascular tissue engineering employ only uniaxial stimulation. Here we present a novel computer-controlled bioreactor and biomechanical testing device designed to precisely and simultaneously control mean and cyclic values of transmural pressure (at rates up to 1 Hz and ranges of 40 mmHg), luminal flow rate, and axial length (or load) applied to gel-derived, scaffold-derived, and selfassembly-derived tissue-engineered blood vessels during culture, while monitoring vessel geometry with a resolution of 6.6 mm. Intermittent monitoring of the extracellular matrix and cells is accomplished on live tissues using multi-photon confocal microscopy under unloaded and loaded conditions at multiple time-points in culture (on the same vessel) to quantify changes in cell and extracellular matrix content and organization. This same device is capable of performing intermittent cylindrical biaxial biomechanical testing at multiple time-points in culture (on the same vessel) to quantify changes in the mechanical behavior during culture. Here we demonstrate the capabilities of this new device on self-assembly-derived and collagen-gel-derived tissue-engineered blood vessels.

Along with insights into the potential for graft success, knowledge of biomechanical properties of small diameter tissue-engineered blood vessel (TEBV) will enable designers to tailor the vessels' mechanical response to closer resemble that of native tissue. Composed of two layers that closely mimic the native media and adventitia, a tissue-engineered vascular adventitia (TEVA) is wrapped around a tissue-engineered vascular media (TEVM) to produce a self-assembled tissue-engineered media/adventia (TEVMA). The current study was undertaken to characterize the biaxial biomechanical properties of TEVM, TEVA and TEVMA under physiological pressures as well as characterize the stress-free reference configuration. It was shown that the TEVA had the greatest compliance over the physiological loading range while the TEVM had the lowest compliance. As expected, compliance of the SA-TEBV fell in between with an average compliance of 2.73 MPa 21 . Data were used to identify material parameters for a microstructurally motivated constitutive model. Identified material parameters for the TEVA and TEVM provided a good fit to experimental data with an average coefficient of determination of 0.918 and 0.868, respectively. These material parameters were used to develop a two-layer predictive model for the response of a TEVMA which fit well with experimental data.

There is a great unmet clinical need to develop small diameter tissue engineered blood vessels (TEBV) with low thrombogenicity and immune response, suitable mechanical properties, and a capacity to remodel to their environment [2, 3]. Development of a clinically useful small diameter TEBV will surely rely on techniques from a wide variety of disciplines, ranging from molecular and cell biology and biochemistry to material science and biomechanics. With regard to the latter, biomechanical stimuli, such as cyclic strain, have been shown to stimulate remodeling of collagen gel-derived TEBVs to greatly improve their mechanical behavior [5]. In native blood vessels, remodeling mechanisms appear to be aimed towards maintaining the local, 3-D mechanical environment (i.e., the local stresses or strains). It is becoming increasingly obvious that tissue engineered constructs also adapt to altered mechanical loading, and specific combinations of multidirectional loads appear to have a synergistic effect on the remodeling. Tissue engineered heart valve constructs exposed to cyclic flexure and shear stress, for example, exhibit a five-fold increase in production of extracellular matrix (ECM) constituents compared to constructs exposed to cyclic flexure or shear stress alone [1]. A critical gap remains, however, in understanding the role of both unidirectional and multidirectional loading on TEBV remodeling. Towards this end, we have developed theoretical and experimental frameworks to study remodeling of collagen and fibrin gel-derived TEBVs.

Coronary artery disease remains to be the leading cause of morbidity and mortality in industrialized nations. Current treatments for small diameter grafts are limited by the availability of suitable autologous vessels and high thrombogenic potential of synthetic grafts. There is a clinical need to development of tissue engineered blood vessels (TEBV) suitable for vascular by pass grafting.