Physical Characterization and Platelet Interactions under Shear Flows of a Novel Thermoset Polyisobutylene-based Co-polymer

Abstract

Over the years, several polymers have been developed for use in prosthetic heart valves as alternatives to xenografts. However, most of these materials are beset with a variety of issues, including low material strength, biodegradation, high dynamic creep, calcification, and poor hemocompatibility. We studied the mechanical, surface, and flow-mediated thrombogenic response of poly(styrene-coblock-4-vinylbenzocyclobutene)-polyisobutylene-poly(styrene-coblock-4-vinylbenzocylcobutene) (xSIBS), a thermoset version of the thermoplastic elastomeric polyolefin poly(styrene-block-isobutylene-block-styrene) (SIBS), which has been shown to be resistant to in vivo hydrolysis, oxidation, and enzymolysis. Uniaxial tensile testing yielded an ultimate tensile strength of 35 MPa, 24.5 times greater than that of SIBS. Surface analysis yielded a mean contact angle of 82.05° and surface roughness of 144 nm, which was greater than for poly(ε-caprolactone) (PCL) and poly(methyl methacrylate) (PMMA). However, the change in platelet activation state, a predictor of thrombogenicity, was not significantly different from PCL and PMMA after fluid exposure to 1 dyn/cm(2) and 20 dyn/cm(2). In addition, the number of adherent platelets after 10 dyn/cm(2) flow exposure was on the same order of magnitude as PCL and PMMA. The mechanical strength and low thrombogenicity of xSIBS therefore suggest it as a viable polymeric substrate for fabrication of prosthetic heart valves and other cardiovascular devices.

Keywords: SIBS; hemodynamic shearing device; parallel plate flow chamber; polymers; thrombin.

Figure 1

Schematic of poly(Styrene-coblock-4-vinylbenzocyclobutene)-polyIsoButylene-poly(Styrene-coblock-4-vinylbenzocylcobutene) (“xSIBS”).

Figure 2

Shear exposure in the Hemodynamic Shearing Device (HSD). GFP suspended over the biomaterials were exposed to 1 dyne/cm² and 20 dyne/cm² for 12 min in the HSD, a computer-programmable cone-plate-Couette viscometer that exposes platelets to uniform shear stress and allows for real-time sampling.

Figure 3

Parallel plate flow chamber system for platelet adhesion studies. (A) PRP was exposed to 10 dyne/cm² for 5 min in a custom parallel plate flow chamber, with channel dimensions of 75 × 25 × 0.1 mm. (B) The flow was controlled by a syringe pump with a 10 ml syringe that perfused PRP at 1.125 ml/min through the flow chamber into a reservoir. The flow was monitored via a 420 fps sCMOS camera mounted on a Nikon Eclipse ME600 microscope with 40X lens.

Figure 4

Material and mechanical characterization of xSIBS. (A) ATR-FTIR spectroscopy was performed on samples of xSIBS with 25% styrene content that were either unused or had been exposed to GFP and fluid shear stress of 20 dyne/cm² for 12 min in the HSD. A divergence region at 1160 cm⁻¹ indicates C-O bonds on the used xSIBS sample. (B) Tensile testing results for 6 xSIBS specimens indicate nonlinear hyperelastic behavior, with ultimate tensile strength at approximately 35 MPa.

Figure 5

Surface analysis of xSIBS, PCL, and PMMA. AFM images taken of 20 μm × 20 μm sections of (A) as-cast 25%-styrene xSIBs, (B) platelet-exposed 25%-styrene xSIBs, (C) PCL, and (D) PMMA (5 μm lateral scale bar). Ordered styrene cylinder peaks with globular isobutylene valleys were observed on xSIBS. PMMA exhibited the lowest roughness.

Figure 6

Shear-induced platelet activation in the HSD. GFP were exposed to 1 dyne/cm² or 20 dyne/cm² for 12 min in the HSD, with samples drawn every 4 min for the PAS assay. (A) PAS was observed to increase over the duration of the experiment, with the change in PAS (ΔPAS) over 12 min largest in the high flow rate regimes. (B) While the largest changes in PAS (ΔPAS) were measured for PCL and xSIBS at 20 dyne/cm², no significant difference was observed (p > 0.05). The mean ΔPAS for xSIBS was slightly above the threshold observed for shear-induced platelet sensitization (dashed black line).

Figure 7

SEM images of adhered platelets after 5 min exposure of PRP to 10 dyne/cm² in a parallel plate flow chamber. The materials tested included (A) PCL, (B) PMMA, and (C) xSIBS. (D) xSIBS and PMMA yielded significantly higher platelet adhesion as compared to PCL (*p < 0.05).