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Wall injury is observed during stent expansion within atherosclerotic arteries, related in part to stimulation of the inflammatory process. Wall stress and strain induced by stent expansion can be closely examined by finite element analysis (FEA), thus shedding light on procedure-induced sources of inflammation. The purpose of this work was to use FEA to examine the interaction of a coiled polymer stent with a plaque-containing arterial wall during stent expansion. An asymmetric fibrotic plaque-containing arterial wall model was created from intravascular ultrasound (IVUS) images of a diseased artery. A 3D model for a coil stent at unexpanded state was generated in SolidWorks. They were imported into ANSYS for FEA of combined stent expansion and fibrotic plaque-distortion. We simulated the stent expansion in the plaqued lumen by increasing balloon pressure from 0 to 12 atm in 1 atm step. At increasing pressure, we examined how the expanding stent exerts forces on the fibrotic plaque and vascular wall components, and how the latter collectively resist and balance the expansive forces from the stent. Results show the expanding coiled stent creates high stresses within the plaque and the surrounding fibrotic capsule. Lower stresses were observed in adjacent medial and adventitial layers. High principal strains were observed in plaque and fibrotic capsule. The results suggest fibrotic capsule rupture might occur at localized regions. The FEA/IVUS method can be adapted for routine examination of the effects of the expansion of selected furled stents against IVUS-reconstructed diseased vessels, to improve stent deployment practices.

To investigate the influence of elevated annealing temperature (70–90 °C) on the mechanical properties of coiled helical PLLA stents. PLLA 0.10 mm fibers and O3 mm × 12 mm stents were fabricated and annealed at 70, 80 and 90 °C for 25 min. The mechanical properties of the fibers and the functional characteristics of the stents were measured and compared to a control group processed at 21 °C. The stents were mounted, expanded and relaxed using an O3 mm × 2 cm balloon catheter to a maximum balloon pressure of 12 atm. Measurements of stent diameter, length, and the balloon pressure were used to determine the effective circumferential strain, incremental stiffness, elastic recoil, and lengthening of the stents. Stents exhibited progressively higher incremental stiffness with annealing temperature, higher collapse resistance and a reduction in elastic recoil vs. controls. Single fiber mechanical properties decreased as annealing temperature increased. Differential scanning calorimetry revealed crystallinity increased within thermally annealed stent fibers compared with controls. SEM examination indicated thermally annealed stents underwent less twisting than controls during balloon-induced unfurling. Thermal annealing of PLLA fibers and stents between 70 and 90 °C induced changes in crystalline structure, thereby favorably influencing fiber stress–strain behavior and stent expansion characteristics.

We studied the effects of thermal treatment on the expansive characteristics of a coil-within-coil Poly(L-lactic acid) (PLLA) fiber stent developed at our institution to improve its mechanical performance and reproducibility. Following fabrication, furled stents were thermally treated at 62 degrees C for 25 min. The mechanical characteristics were measured compared with those of untreated stents when both were expanded via sequential balloon catheter pressure loading up to 12 atm. Treated stents reached full diameter at 3 atm and maintained that diameter despite further pressure increases. Using measurements of pressure, diameter, and axial length, we calculated the sequential mechanical work required to unfurl the stent. The mechanical work for complete unfurling of treated stents was significantly less than that required for untreated controls. Little axial dimensional change was observed for treated stents. Treated stents exhibited higher stiffness than controls at all pressure levels and also demonstrated higher resistance to external pressure-induced collapse, as measured in a special apparatus developed in our laboratory. Differential scanning calorimetry measurements indicated higher crystallinity values for fibers used in treated stents compared with controls. SEM examination of striations revealed that treated stents underwent less twist than controls following balloon-induced unfurling. The results indicate that, thermal treatment improves the reorientation and realignment of fiber crystalline structure, and favorably influences on the fiber stress-strain behavior and the expansive mechanical characteristics of the PLLA fiber stents.

Deposition of thin polymeric films on nanoporous membranes is shown to provide a mechanically simple and inexpensive approach to regulate trans-membrane gas flows. For this purpose, polymeric films were deposited on polycarbonate track-etched (PCTE) membranes of 50 nm and 100 nm pore size. The films were generated by low-pressure glow discharge plasma polymerization of vinyl acetic acid (CH2 CHCH2COOH) or perfluorohexane (n-C6F14) monomers. The plasma discharge was operated under pulsed conditions to provide better control of the polymeric film chemistry along with improved control of film thickness. The gradual reduction in average membrane pore sizes with increasing film thickness resulted in controlled decreases in the permeation rates of O2 and CO2 gases. In addition to film thickness, the permeation rates were observed to be functions of the specific composition and cross-link density of the polymer films deposited, as well as the nature of the permeant gases. It was determined that the plasma deposition process employed provides sufficient permeation control for use in applications requiring fine modulation of gas permeation rate such as those encountered in development of a blood oxygenator, a major motivation for the present study.

Polymeric vascular stents must employ other strategies than malleable deformation, as generally practiced with metal stents, to expand and withstand compressive stresses in situ. The stent expansion strategy must further consider induced flow perturbations and wall stresses that may injure the vessel wall and promote thrombogenesis. Analyzing the stresses furled stents undergo during balloon-assisted expansion is an important first step in achieving a better understanding of stent-wall mechanical interactions, thereby to improve stent function. To this end, we performed finite element (FE) analysis of the balloon-induced unfurling of an internally coiled, bioresorbable polymeric stent employing a 3D FE solid model of a 120 degrees symmetric stent segment and a large deformation finite strain formulation. Uni-axial tensile testing of stent fiber elastic to plastic yielding provided the mechanical property information, and the von Mises criterion was employed to establish the elastic-plastic transition in the FE model. The model was validated with pressure and deformation measurements obtained during stent expansion tests. The internal coils of this inner coil-outer coil design twisted as the stent expanded, leading to plastic yielding at the point of tangency of the inner and outer coils. The remaining stent fiber portions underwent elastic bending. Cross-sections revealed only the outside surface layer of the coiled fiber underwent plastic yielding. The interior elastic fiber was supported by this plastic shell. The analysis suggests that during balloon-induced expansion, local plastic yielding in torsion "sets" the stent fibers, imparting high radial collapse resistance. The results further suggest that the stent exerts non-uniform mechanical forces on the vessel wall during expansion.

To facilitate bioprosthetic heart valve design, especially in the use of novel antimineralization chemical technologies, a thorough understanding of the multiaxial mechanical properties of chemically treated bovine pericardium (BP) is needed. In this study, we utilized a small angle light scattering based tissue pre-sorting procedure to select BP specimens with a high degree of structural uniformity. Both conventional glutaraldehyde (GL) and photo-oxidation (PO) chemical treatment groups were studied, with untreated tissue used as the control group. A second set of GL and PO groups was prepared by prestretching them along the preferred fiber direction during the chemical treatment. An extensive biaxial test protocol was used and the resulting stress-strain data fitted to an exponential strain energy function. The high structural uniformity resulted in both a consistent mechanical response and low variability in the material constants. For free fixed tissues, the strain energy per unit volume for GL treated BP was ∼ 2.8 times that of PO treated BP at an equibiaxial Green’s strain level of 0.16. Prestretched tissues exhibited a profound increase in both stiffness and the degree of anisotropy, with the GL treatment demonstrating a greater effect. Thus, structural control leads to an improved understanding of chemically treated BP mechanical properties. Judicious use of this knowledge can facilitate the design and enhanced long-term performance of bioprosthetic heart valves. © 1998 Biomedical Engineering Society. PAC98: 8790+y, 8745Bp, 8780+s

Cryopreservation of engineered tissue equivalents (TE) has achieved limited success. Successful preservation of a functional TE or tissue could provide prolonged transplantation time windows or off-the-shelf availability of engineered tissues. A quantitative understanding of the freezing-induced cell-fluid-matrix mechanical interaction is critical towards successful preservation of functional tissue. Using quantum dot-mediated cell image deformetry, Teo et al have experimentally mapped the instantaneous deformation of a TE during freezing process and showed its resulting volume change after thawing due to fluid loss [1]. To further our understanding of the freezing-induced fluid-matrix interaction, we have developed a biphasic model that simulates the transient of thermal load, its resulting TE elastic deformation, the corresponding pressure redistribution within, and the fluid movement in the TE.Copyright © 2011 by ASME

Vascular smooth muscle cells (VSMCs) are constantly exposed to cyclic stretch in the body, which makes it beneficial to study the effects of cyclic stretch on VSMCs. In this study, we developed a poly(dimethyl siloxane) (PDMS) compact six-well device that can be used to study the combined effect of cyclic strain and various growth factors on cultured VSMCs. Cell adhesion, alignment, and proliferation under 10% or 20% cyclic strain at 1 Hz were studied using this surface-enhanced PDMS device. The combined effects of cyclic strain with either transforming growth factor-β, vascular endothelial growth factor, fibroblast growth factor, or epidermal growth factor on VSMC proliferation was also examined. Results showed that VSMCs adhered well on the surface-enhanced multiwell device and they aligned perpendicularly to the direction of the cyclic strain. Cell proliferation was inhibited by 10% cyclic strain at 1 Hz compared with static control. The mitogenic effects of the growth factor were less potent under either 10% or 20% cyclic strain. With simple modification to accommodate more wells, this device could potentially be a useful tool for more economical, high throughput screening application.

We applied the pseudostrain energy function of Humphrey et al. [J. biomech. Engng 112, 333-346 (1990a, b)] to characterize the passive biaxial mechanical properties of the right ventricle free wall myocardium [Sacks and Chuong, J. biomech. Engng 115, 202-205 (1993)]. The myocardium was assumed to be incompressible, pseudoelastic, and transversely isotropic, with transmural variations in fiber orientation within test specimens accounted for by the strain energy function. Using nonlinear regression, material constants were determined for the right ventricle free wall myocardium from the sinus and conus regions. The pseudostrain energy function was found to model the biaxial mechanical data well (r20.99). Transmural variations in Cauchy stresses, as well as the magnitude of the in-plane shear stress, were found to be small. Although comparisons with the left ventricle midwall myocardium data [Humphrey et al., J. biomech. Engng 112, 340-346 (1990b)] show clear quantitative differences, there is an overall qualitative similarity in the mechanical behavior of ventricular myocardium.

Stents have been used as a medical device to restore blood flow in stenosed vessels. During this process of stent expansion, the balloon and stent could damage the endothelial lining of the vascular wall [Grewe, Farb] and alter the mechanical stress states of various tissue components. The subsequent tissue re-growth and vascular remodeling have been implicated among the causes leading to restenosis. Stent designs vary over a wide range from materials used, geometric shapes, fabrication methods, treatments, among many other parameters. There are many different stent designs. We have developed an internally coiled, furled helical PLLA fiber stent at our institution [Su 03, Zilberman 02, 04, 05]. We have previously shown the effect of 62°C thermal annealing on the expansive characteristics of this stent [Welch 08].Copyright © 2009 by ASME

Stents are a medical device used to restore blood flow in stenosed vessels. During the process of stent expansion, the balloon and stent could damage the endothelial lining of the vascular wall and alter the mechanical stress states of various tissue components. The evolution of stents has progressed from bare metal stents to drug eluting stents or stents coated with drugs such as Sirolimus. Drug eluting stents are currently used to address the restenosis event that occurs after implantation. These stents have been effective but late stent thrombosis has been an emerging issue with drug eluting stents. Bioresorbable stents have emerged as a potential candidate for drug eluting stents. Su 03 has shown drug-loading PLLA with curcumin can be used as an anti-inflammatory agent. We have developed an internally coiled, furled helical PLLA fiber stent at our institution [Su 03, Zilberman 02, 04, 05]. We have previously shown the effect of 62°C thermal annealing on the expansive characteristics of this stent [Welch 08].Copyright © 2009 by ASME

Nanostructured membranes with precisely engineered nanopores were fabricated on a thin silicon nitride membrane, using a combination of bulk micromachining and focused-ion-beam drilling. These membranes are designed to preserve microscale blood channel dimensions, thereby permitting the red cell shape change that enhances gas exchange in the pulmonary capillary. The membranes were tested for their mechanical stability and the results were verified with finite element analysis. Initial studies have proven the membranes to be robust, and capable of withstanding pressures typically experienced in blood oxygenator channels. A novel MEMS-based blood oxygenation system employing the nanoporous membranes is also presented. The oxygenation system is designed to have controlled blood and gas volumes for efficient blood oxygenation.

Stent navigation and expansion may injure vascular endothelium, including vulnerable plaque lesions. Balloon expansion and deployment of a stent can lead to injury or the endothelial lining and stretching of the arterial wall [1]. Understanding the traction forces an expanding stent imparts on the vascular wall at the endothelial surface, the underlying plaque lesions and other tissue components during expansion is an important step in improving short term stent-wall mechanics. More importantly, the long term influence of stent-vascular wall mechanical interactions in restenosis remains unknown, and this analysis may shed light on the process.Copyright © 2007 by ASME

To study the interaction between a stone and a cavitation jet during extracorporeal shock-wave lithotripsy (ESWL), we developed a model based on geometrical acoustics describing the impingement of a cavitation microjet on an elastic solid boundary and the subsequent propagation of the resultant shock waves in the solid. The model incorporated the acoustic and mechanical properties of different stone types to calculate the jet impact pressure at the stone surface and the stress and strain at the propagating shock fronts. This model was validated using stone phantoms in slab configuration fabricated to have comparable acoustic and mechanical properties to renal calculi. This chapter summarizes our efforts, both analytical and experimental, towards the understanding of stone fragmentation in extracorporeal shock wave lithotripsy.

Advances in tissue engineering have led to the development of artificially grown dermal tissues for use in burn and ulcer treatments. An example of such an engineered tissue is Dermagraft™, which is grown using human neonatal fibroblasts on rectangular sheets of biodegradable mesh. Using small angle light scattering (SALS), we quantified the collagen fiber architecture of Dermagraft with the mesh scaffold contributions removed through the use of a structurally based optical model. Dermagraft collagen fibers were found to have a preferred direction nearly parallel to the long dimension of the kiteshaped mesh opening with small spatial variations over the mesh. This study demonstrated the utility of SALS as a rapid and inexpensive technique for the evaluation of gross collagen fiber architecture in engineered tissues.

The biaxial mechanical properties of right ventricular free wall (RVFW) myocardium were studied. Tissue specimens were obtained from the sub-epicardium of potassium-arrested hearts and different stretch protocols were used to characterize the myocardium’s mechanical response. To assess regional differences, we excised tissue specimens from the conus and sinus regions. The RVFW myocardium was found to be consistently anisotropic, with a greater stiffness along the preferred (or averaged) fiber direction. The anisotropy in the conus region was more pronounced than in the sinus region. A comparison with studies of left ventricle (LV) midwall myocardium revealed that, 1) the fiber direction stiffnesses are greater in the RVFW than in the LV, 2) the degree of anisotropy is greater in the RVFW than in the LV.