Changing compliance of poly(vinyl alcohol) tubular scaffold for vascular graft applications through modifying interlayer adhesion and crosslinking density

Abstract

Poly(vinyl alcohol) (PVA) is a water-soluble polymer and forms a hydrogel that has been studied as a potential small-diameter (<6 mm) vascular graft implant. The PVA hydrogel crosslinked using sodium trimetaphosphate (STMP) has been shown to have many beneficial properties such as bioinert, low-thrombogenicity, and easy surface modification. Compared to conventional synthetic vascular graft materials, PVA has also shown to possess better mechanical properties; however, the compliance and other mechanical properties of PVA grafts are yet to be optimized compared to the native blood vessels. Mechanical compliance has been an important parameter to be studied for small-diameter vascular grafts, as compliance has been proposed to play an important role in intimal hyperplasia formation. PVA grafts are made using dip-casting a cylindrical mold into crosslinking solution. The number of dipping can be used to control the wall thickness of the resulting PVA grafts. In this study, we hypothesized that the number of dip layer and wall thickness, the chemical crosslinking, and interlayer adhesive strength could be important parameters in the fabrication process that would affect compliance. This work provides the relationship between the wall thickness, burst pressure, and compliance of PVA. Furthermore, our data showed that interlayer adhesion as well as chemical and physical crosslinking density can increase the compliance of PVA grafts.

Keywords: Vascular graft; compliance; crosslinking density; dip-casting; interlayer adhesion; poly(vinyl alcohol).

Figure 1.

Standard graft fabrication protocol and description of the varied parameter for each experimental groups. (A) standard protocol, (B) representative images of the fabricated grafts for each experimental group, and (C) abbreviations and descriptions of the varied parameter for each experimental group. Samples for all groups were fabricated following the standard protocol except for the described parameter.

Figure 2.

Physical and mechanical characterization of grafts with different number of layers. (A) cross-sectional images. Red arrows used to identify difference level of transparency between layers within a sample. (B) Wall thickness of the samples for different number of layers. (C) Burst pressure plotted against the wall thickness. (D) Compliance plotted against the wall thickness. * = p < 0.05 between indicated groups. n = 9 for all groups. Error bar represents standard deviation.

Figure 3.

Physical characterization of the experimental groups. (A) Cross-sectional images. Red arrows are used to identify difference level of transparency between layers within a sample. (B) Wall thickness of the samples for different number of layers. (B) wall thickness of the samples. * indicates p< 0.05 with respect to control group. # indicates p<0.05 between the groups being compared. n = 9 for all groups.

Figure 4.

Bulk properties of the samples. (A) Compliance of each group. (B) Compliance of each group plotted against their respective wall thickness. Average values of compliance and wall thickness of the internal replicates used to plot the graph. (C) Burst pressure of each group. (D) Burst pressure of each group plotted against their respective wall thickness. Average values of burst pressure and wall thickness of the internal replicates used to plot the graph. (E) Swelling ratio of each group, and (F) swelling ratio plotted with respect to the wall thickness of each group. * indicates p < 0.05 with respect to control group. # indicates p < 0.05 between the groups being compared. n = 9 for all groups.

Figure 5.

Functional groups and STMP crosslinking density in PVA films. (A) Quantification of phosphate content for the samples to characterize covalent crosslinking by STMP. * indicates p< 0.05 between the indicated groups. n = 6 for all groups. C PBS was control sample rehydrated in PBS. C, LS, 15W, 30W, 60–2D, and 18–2D were rehydrated in NaCl solution. * indicates p < 0.05 with respect to the control groups. # indicates p < 0.05 between the groups being compared. (B) Fourier-transform infrared spectroscopy data. Data was normalized to PVA films. n=3 for all samples.

Figure 6.

Degree of crosslinking. Differential Scanning Calorimetry (DSC) data. (A) DSC of (i) control, (ii) 60–2D, and (iii) 18–2D. Data is shown for the second heating and cooling cycle. (B) Heat of fusion and fractional crystallinity of the samples. n=1 for all groups.

Figure 7.

Interfacial energy. (A) the adhesive strength of groups. 15W had p=0.0571 with respect to the control group, and p=0.0495 with respect to the 30W. (B) height map of the samples after adhesive shear test. (C)-(J) height profile of the samples. * indicates p< 0.05 with respect to control group. # indicates p<0.05 between the groups being compared. n=3 for all groups.

Figure 8.

Principle component analysis (PCA) for the identification of significant components. (A) PCA plot of component 1 vs. component 3. (B) PCA plot of component 2 vs. component 3.

Figure 9.

Additional bulk mechanical properties of the selected groups. (A) Longitudinal tensile curve. (B) Circumferential tensile curve. (C) Longitudinal ultimate tensile strength and elastic modulus for each groups. (D) Circumferential ultimate tensile stress and elastic modulus. (E) Suture retention strength. n=6 for tensile test results, and n=9 for suture retention strength for all groups. * indicates p<0.05 with respect to the control group.