Effect of two-year degradation on mechanical interaction between a bioresorbable scaffold and blood vessel.

This paper aims to evaluate the mechanical behaviour of a bioresorbable polymeric coronary scaffold using finite element method, focusing on scaffold-artery interaction during degradation and vessel remodelling. A series of nonlinear stress-strain responses was constructed to match the experimental measurement of radial stiffness and strength for polymeric scaffolds over 2-year in-vitro degradation times. Degradation process was modelled by incorporating the change of material property as a function of time. Vessel remodelling was realised by changing the size of artery-plaque system manually, according to the clinical data in literature. Over degradation times, stress on the scaffold tended to increase firstly and then decreased gradually, corresponding to the changing yield stress of the scaffold material; whereas the stress on the plaque and arterial layers showed a continuous decrease. In addition, stress reduction was also observed for scaffold, plaque and artery in the simulations with the consideration of vessel remodelling. For the first time, the work offered insights into mechanical interaction between a bioresorbable scaffold and blood vessel during two-year in-vitro degradation, which has significance in assisting with further development of bioresorbable implants for treating cardiovascular diseases.

Computational analysis of mechanical stress-strain interaction of a bioresorbable scaffold with blood vessel.

Crimping and deployment of bioresorbable polymeric scaffold, Absorb, were modelled using a finite element method, in direct comparison with Co-Cr alloy drug eluting stent, Xience V. Absorb scaffold has an expansion rate lower than Xience V stent, with a less outer diameter achieved after balloon deflation. Due to the difference in design and material properties, Absorb also shows a higher recoiling than Xience V, which suggests that additional post-dilatation is required to achieve effective treatment for patients with calcified plaques and stiff vessels. However, Absorb scaffold induces significantly lower stresses on the artery-plaque system, which can be clinically beneficial. Eccentric plaque causes complications to stent deployment, especially non-uniform vessel expansion. Also the stress levels in the media and adventitia layers are considerably higher for the plaque with high eccentricity, for which the choice of stents, in terms of materials and designs, will be of paramount importance. Our results imply that the benefits of Absorb scaffolds are amplified in these cases.

Molecular weight dependence of calcification of polyethylene glycol hydrogels.

In vivo calcification of polyethylene glycol diacrylate (PEG DA) hydrogels of molecular weight (MW) 400, 1000, 4000, 6000 and 10,000 and polyethylene glycol tetraacrylate (PEG TA) of MW 18,500 was investigated using a rat subcutaneous model. This study was performed in 4-wk-old rats for durations of 1, 3, 6 and 8 wk. The results indicate a strong dependence of calcification upon the MW of the PEG precursor or the MW between crosslinks. Results for gels implanted for 6 wk show that calcification was maximal at a PEG MW of 1000 (224 mg/g +/- 12.8, n = 4) (mean +/- SEM) with less at MW = 400 (23.0 mg/g +/- 9.30, n = 4) and considerably less at higher MWs, e.g. for MW = 10,000 (0.23 mg/g +/- 0.01, n = 4). Results for other time periods indicate a similar calcification trend. The extent of calcification of gels from PEG TA (MW = 18,500) was intermediate (1.09 mg/g +/- 0.43, n = 3) between PEG DA (MW = 6000) (1.39 mg/g +/- 0.42, n = 6) and PEG DA (MW = 10,000) at 6 wk, i.e. calcification depended upon the PEG MW between crosslinks. When composite gels were implanted, such that a highly calcifying gel (MW = 400 or 1000) was encapsulated within a gel of low calcification (MW = 4000), the gel inside calcified to at least the same extent as if it had not been encapsulated. Thus, direct contact with tissues is apparently not necessary for calcification to occur. Energy dispersive X-ray spectroscopy was performed on the mineral deposits in the gels and a P:Ca ratio of 0.67 +/- 0.04 (95% confidence interval) for MW 1000 gels and 0.60 +/- 0.07 for MW 400 gels was found to be consistent with deposition of Ca3(PO4)2.

Surface-immobilized polyethylene oxide for bacterial repellence.

Polyethylene terephthalate films were surface-modified with polyethylene oxide (18,500 g/mol) using a solution technique described previously. These films were investigated for their resistance to bacterial adhesion. Three bacterial strains most commonly associated with implant infections, Staphylococcus epidermidis, Staphylococcus aureus and Pseudomonas aeruginosa, were cultured in tryptic soya broth, human plasma and human serum on the polymeric substrates. Significant reductions (between 70 and 95%) in adherent bacteria were observed on the polyethylene oxide-modified substrates compared to the untreated control polyethylene terephthalate. Surface modification with polyethylene oxide may reduce the risk of implant-associated infections. Plasma fibrinogen was observed to play an important role in the adhesion of all three of these species on both the polyethylene oxide-modified and control polyethylene terephthalate materials.