Cheng Jen Chuong

Orthotropic Mechanical Properties of Chemically Treated Bovine Pericardium

AbstractTo 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.

Effect of Cell Migration on the Maintenance of Tension on a Collagen Matrix

AbstractAlthough it is known that cells promote structural reorganization of the collagen architecture, how individual cells exert mechanical tension on the matrix is not clearly understood. In the present study we have investigated the mechanical interaction of individual corneal fibroblasts with a collagen matrix using an improved version of our previously described in vitro force-measurement system (Roy, P. et al. Exp. Cell Res. 232:106–117, 1997). The elastic distortion of the collagen matrix exerted by cells was temporally recorded and analyzed using a two-dimensional finite-element model to quantify the forces exerted on the matrix. Time-lapse videomicroscopy of serum-cultured cells on the matrix for up to 6 h revealed that individual fibroblasts generated measurable tension on the matrix during pseudopodial extension and slow retraction. Fast retraction, an event observed during active cell migration, was associated with dramatic release of tension on the matrix. An apparent inverse correlation was observed between cell translocation and maintenance of matrix tension. Additional experiments with cells under serum-free conditions revealed that these cells fail to generate any detectable tension on the matrix despite undergoing filopodial extension and retraction. Since serum-free cells do not form focal adhesions or stress fibers, these experimental data suggest that contractility of nonmotile cells, coupled with strong cell–matrix adhesion, is the most favorable mechanism of generating and maintaining tension on the extracellular matrix.

The influence of thermal treatment on the mechanical characteristics of a PLLA coiled stent.

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.

Viscoelastic properties measurement of the prolapsed anterior vaginal wall: a patient-directed methodology

OBJECTIVE In-vivo measurement of the viscoelastic properties of the prolapsed anterior vaginal wall (AVW) in post-menopausal women undergoing cystocele repair. STUDY DESIGN A BTC-2000 cutometer-like instrument was introduced during vaginal repair of symptomatic stage 2-3 AVW prolapse. Under anesthesia, 10-mm orifice probe was applied to the AVW at the level of the bladder neck. A suction pressure ramp (0 to -147 mmHg in 6s) was delivered causing tissue uplift, followed by immediate release to 0 mmHg, measuring tissue relaxation for 20s. Similar measurements were performed over the suprapubic region (SP) for comparison purpose. The rate of tissue recovery was obtained by fitting a Voigt model to the data and expressing results as the ratio E/η [(spring modulus E)/(dashpot viscosity η)]. The effective strain energy (SE) was calculated from the pressure-uplift data and evaluated from initiation to: (1) maximum storage in tissue at peak vacuum; (2) tissue recovery after vacuum release; (3) net SE loss over the entire loading-unloading cycle. RESULTS In 22 women, higher AVW peak and residual tissue uplift values, and lower E/η ratios were found compared with SP results. The AVW stored less elastic strain energy at peak vacuum than did the SP, and AVW net energy loss over the uplift-recovery cycle was greater than for SP controls. Not only was the AVW more compliant than the SP, with higher viscous damping, but the tissue was also less able to store recoverable energy upon distension. CONCLUSION Such in-vivo measurements quantify the biomechanical properties of the prolapsed AVW and may assist in its management.

Thermal treatment effects on a PLLA bioresorbable stent

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

Exploring biomechanical methods to study the human vaginal wall.

To critically review studies of the biomechanical properties of connective tissue in the normal and prolapsed human vaginal wall and to identify criteria that are suitable for in vivo measurements which could improve patient management.

Range of Thermal Treatment Upon the Mechanical Characteristics of PLLA Coiled Stents

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

Thermal Treatment Effects Upon the Degradation Characteristics of PLLA Coiled Stents

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