Hydrogel fibers for ACL prosthesis: Design and mechanical evaluation of PVA and PVA/UHMWPE fiber constructs.

J Biomech. 2013 Apr 4;

Authors: Bach JS, Detrez F, Cherkaoui M, Cantournet S, Ku DN, Corté L

Abstract
Prosthetic devices for anterior cruciate ligament (ACL) reconstruction have been unsuccessful due to mechanical failure or chronic inflammation. Polymer hydrogels combine biocompatibility and unique low friction properties; however, their prior use for ligament reconstruction has been restricted to coatings due to insufficient tensile mechanics. Here, we investigate new constructs of polyvinyl alcohol (PVA) hydrogel fibers. In water, these fibers swell to an equilibrium water content of 50% by weight, retaining a tensile modulus greater than 40MPa along the fiber axis at low strain. Rope constructs were assembled for ACL replacement and mechanical properties were compared with data from the literature. Pure PVA hydrogel constructs closely reproduce the non-linear tensile stiffness of the native ACL with an ultimate strength of about 2000N. An additional safety factor in tensile strength was achieved with composite braids by adding ultrahigh molecular weight polyethylene (UHMWPE) fibers around a core of PVA cords. Composition and braiding angle are adjusted to produce a non-linear tensile behavior within the range of the native ligament that can be predicted by a simple rope model. This design was found to sustain over one million cycles between 50 and 450N with limited damage and less than 20% creep. The promising mechanical performances of these systems provide justification for more extensive in vivo evaluation.

PMID: 23562622 [PubMed - as supplied by publisher]

A low-volume, single pass in-vitro system of high shear thrombosis in a stenosis.

Thromb Res. 2013 Mar 24;

Authors: Para AN, Ku DN

Abstract
INTRODUCTION: Arterial thrombosis leading to heart attack and stroke requires the rapid accumulation of millions of platelets under pathologically high shear. Previous in vitro systems studying platelets typically use endpoints that emphasize platelet-surface effects rather than large-scale platelet-platelet accumulation that precedes occlusion. Further, most platelet tests do not recreate shear rates present during arterial occlusion. We present an alternative flow system to study large thrombus formation under pathologic shear conditions in an anatomic stenosis with reasonable volumes of human blood. MATERIALS AND METHODS: An in-vitro system using a syringe pump was created to subject low volume (<30mLs), whole blood samples to very high shear rates (>3,500s(-1)) through a stenosis. Thrombus was quantified using an optical microscope from initial deposition to large scale accumulation. Images were taken using a high definition camera in real time. RESULTS AND CONCLUSIONS: Occlusive thrombus blocks the collagen-coated lumen with millions of platelets using human whole, heparinized blood. Rapid Platelet Accumulation rates in human blood are 4.5±2.4μm(3)/μm(2)/min (n=21). There is an initial lag time of 7.4±3.8min (n=21) before the onset of large scale thrombosis. The rates of platelet accumulation in vitro are consistent with the clinical timescale of coronary or carotid artery occlusion. Porcine blood has a faster accumulation rate of 9.6±6.1μm(3)/μm(2)/min (n=7, p<0.05) and a shorter lag time of 2.7±0.5min (n=7, p<0.05). The long lag time for large thrombus formation suggests that some in-vitro assays will miss the main mechanism creating thrombotic occlusion.

PMID: 23535566 [PubMed - as supplied by publisher]

Correlation of thrombosis growth rate to pathological wall shear rate during platelet accumulation.

Biotechnol Bioeng. 2012 Oct;109(10):2642-50

Authors: Bark DL, Para AN, Ku DN

Abstract
Local hemodynamics may strongly influence atherothrombosis, which can lead to acute myocardial infarction and stroke. The relationship between hemodynamics and thrombosis during platelet accumulation was studied through an in vitro flow system consisting of a stenosis. Specifically, wall shear rates (WSR) ranging from 0 to 100,000 s(-1) were ascertained through computations and compared with thrombus growth rates found by image analysis for over 5,000 individual observation points per experiment. A positive correlation (P < 0.0001) was found between thrombus accumulation rates and WSR up to 6,000 s(-1), with a decrease in growth rates at WSR >6,000 s(-1) (P < 0.0001). Furthermore, growth rates at pathological shear rates were found to be two to four times greater than for physiological arterial shear rates below 400 s(-1). Platelets did not accumulate for the first minute of perfusion. The initial lag time, before discernible thrombus growth could be found, diminished with shear (P < 0.0001). These studies show the quantitative increase in thrombus growth rates with very high shear rates in stenoses onto a collagen substrate.

PMID: 22539078 [PubMed - indexed for MEDLINE]