Characterization of anisotropic poly(vinyl alcohol) hydrogel by small- and ultra-small-angle neutron scattering

Abstract

Poly͑vinyl alcohol͒ ͑PVA͒ hydrogels are formed from PVA solution when physical cross-links form during freeze/thaw cycling. By applying a stress during the freeze/thaw process, PVA hydrogels with anisotropic mechanical properties are produced. We have used small-and ultra-small-angle neutron scattering to study the structure at length scales of 2 nm to 10 m. By supplementing the neutron data with data from atomic force microscopy, we have probed a large range of length scales within which structural changes responsible for bulk anisotropy occur. We model the gel as interconnected PVA blobs of size 20-50 nm arranged in fractal aggregates extending to micrometers or tens of micrometers. Bulk mechanical anisotropy appears to be due to the alignment of blobs and connections between blobs. This information is essential for tailoring mechanical properties for applications where anisotropy is desirable such as to match the properties of natural tissue in coronary grafts and to control diffusive properties in active wound dressings.

Poly(Vinyl Alcohol) Cryogels for Biomedical Applications

“…This changed the microstructure of the PVA-C, causing the crystallites to orient in the direction of the stress. Details of the nanostructure of the anisotropic PVA-C have been studied using SANS and USANS [11]. Differences for tensile properties between the longitudinal and perpendicular directions increased as the initial strain applied after the first cycle was increased.…”

“…The solution is then thawed back to room temperature leading to the formation of a solid gel-PVA-C. The micro-/ nanostructure of PVA-C has been examined through several techniques including transmission electron microscopy (TEM) [9], small angle X-ray scattering (SAX) [9] and small angle and ultrasmall angle neutron scattering (SANS and USANS) [10,11]. Observations conclude that the first freeze-thaw cycle (FTC) produces polymer-rich regions due to the formation of ice crystals in the amorphous regions [10,12].…”

Synthesis of Anisotropic Hydrogels and Their Applications

“…[80][81][82][83] In some cases,the compression-or stretching-induced deformation remains in place without in situ polymerization owing to the plastic nature of the system. [84][85][86][87] As al imitation of this method, the precursor isotropic hydrogels must withstand large forces so that their gel networks undergo sufficient deformation.…”

Structured Macroporous Hydrogels: Progress, Challenges, and Opportunities

“…That being said, the use of hydrogels as cryoprotectants for cells in conjunction with cryogelation processes has been demonstrated to successfully create cell-loaded macroporous gel scaffolds, albeit with some loss of cell activity following the freezing process. [70] Furthermore, although the rates of heating and cooling and the number of freeze/thaw cycles conducted can at least in part be used to rationally control the pore size and distribution generated, [71,72] the generated pores still typically have a broad pore size distribution and minimal if any directionality. Tam et al have demonstrated the possible benefits of adding typical cryoprotectant carbohydrates such as d-galactose, d-glucose, d-trehalose, or d-sucrose to the pre-gel solution, as all these sugars interact with water to alter the nature of ice crystallization and thus create more transparent hydrogels with improved pore size control.…”