Tensile Fatigue of Poly(Vinyl Alcohol) Hydrogels with Bio‐Friendly Toughening Agents

Abstract

Inspired by the avoidance of toxic chemical crosslinkers and harsh reaction conditions, this work describes a poly(vinyl alcohol)‐based (PVA) double‐network (DN) hydrogel aimed at maintaining biocompatibility through the combined use of bio‐friendly additives and freezing–thawing cyclic processing for the application of synthetic soft‐polymer implants. This DN hydrogel is studied using techniques that characterize both its chemical and mechanical behavior. A variety of bio‐friendly additives are screened for their effectiveness at improving the toughness of the PVA hydrogel system in monotonic tension. Starch is selected as the best additive for further tensile testing as it brings about a near 30% increase in ultimate tensile strength and maintains ease of processing. This PVA–starch DN sample is then studied for its tensile fatigue properties through cyclic, strain‐controlled testing to develop a fatigue life curve. Though an increase in monotonic tensile strength is observed, the PVA–starch DN hydrogel does not bring about an improvement in the fatigue behavior as compared to the control. Although synthetic hydrogel reinforcement is widely researched, this work presents the first fatigue analysis of its kind and it is intended to serve as a guide for future fatigue studies of reinforced hydrogels.

Flaw Sensitivity and Tensile Fatigue of Poly(Vinyl Alcohol) Hydrogels

“…Using methods from prior work, [ 27 ] homogenous solutions consisting of PVA (16 wt% with respect to the total weight of deionized water combined with PVA) were obtained with a high degree of processability. The screening process of FT cycles provided a quantitative tracking of the improvement of mechanical properties through crystalline domain development and non‐covalent interactions.…”

“…Adjustment of the shape of the sample was necessary for the following three reasons: 1) the Type V sample had a gauge section that was too small to accommodate larger cracks and cylindrical holes, 2) the Type V samples had a tendency to buckle when tested cyclically in strain‐controlled tension, and 3) the Type V‐shaped samples had previous issues with failure near the grip section instead of the gauge section when stretched in a cyclic fashion. [ 27 ] With these reasons in mind, the same customized fatigue shape previously developed in our work was utilized for fatigue testing herein. [ 27 ] This difference in sample geometry of a thicker, shorter gauge section can be seen in comparing Figure 2A to b and c.…”

“…[ 27 ] With these reasons in mind, the same customized fatigue shape previously developed in our work was utilized for fatigue testing herein. [ 27 ] This difference in sample geometry of a thicker, shorter gauge section can be seen in comparing Figure 2A to b and c.…”

“…Analogous to our prior work, [ 27 ] PVA hydrogels were prepared by dissolving the polymer in deionized water and pouring the solution into a designated mold. Subsequently, freezing and thawing the solution was used to physically crosslink the polymer network through crystalline domains, thus creating the hydrogel.…”

“…[ 28,29 ] The starting point of this study took the form of investigating the mechanical properties of the PVA hydrogel by systematically varying the number of FT cycles from one to eight. As opposed to the previous work of utilizing two FT cycles for both control and all bio‐friendly toughening agents, [ 27 ] this work sought to understand the impact of the number of FT cycles. The hydrogel samples were tested for their monotonic tension properties using the ASTM D638‐14 Type V shape to obtain the top‐performing FT cycle number based solely on “flaw‐free” toughness, which is once again defined as the area under the stress–strain curve.…”