Composite Hydrogels with Tunable Anisotropic Morphologies and Mechanical Properties

Abstract

Fabrication of anisotropic hydrogels exhibiting direction-dependent structure and properties have attracted great interest in biomimicking, tissue engineering and bioseparation. Herein, we report a single-step freeze casting-based fabrication of structurally and mechanically anisotropic aerogels and hydrogels composed of hydrazone cross-linked poly(oligoethylene glycol methacrylate) (POEGMA) and cellulose nanocrystals (CNCs). We show that by controlling the composition of the CNC/POEGMA dispersion and the freeze casting temperature, aerogels with fibrillar, columnar, or lamellar morphologies can be produced. Small-angle X-ray scattering experiments show that the anisotropy of the structure originates from the alignment of the mesostructures, rather than the CNC building blocks. The composite hydrogels show high structural and mechanical integrity and a strong variation in Young's moduli in orthogonal directions. The controllable morphology and hydrogel anisotropy, coupled with hydrazone cross-linking and biocompatibility of CNCs and POEGMA, provide a versatile platform for the preparation of anisotropic hydrogels.

Structured Macroporous Hydrogels: Progress, Challenges, and Opportunities

“…[109] Our group has shown that the incorporation of rigid rod-like CNCs increased the compressive modulus of freeze-casted macroporous poly(oligoethylene glycol methacrylate) (POEGMA) hydrogels by nearly one order of magnitude as the CNC concentration was increased from 0.4 to 1.2 wt%. [113] Finally, Khademhosseini and co-workers have shown that the incorporation of CNT into photocrosslinkable gelatin methacrylate (GelMa) hydrogels led to a >threefold increase in compressive modulus (Figure 3). [112] When seeded with cardiomyocytes, these hydrogel scaffolds show promise as functional cardiac patches.…”

“…The incorporation of nanoparticles such as graphene oxide (GO), [108,109] silver nanoparticles, [110] hydroxyapatite nanoparticles, [111] carbon nanotubes (CNT), [112] cellulose nanocrystals (CNCs), [35,113,114] and others have been used as reinforcing agents in macroporous hydrogels, with varying amounts of success. Notably, Huang et al demonstrated that strong intermolecular interactions between GO and chitosan leads to the self-assembly of supramolecular macroporous composite hydrogels demonstrating enhanced mechanical performance versus chitosan-only gels.…”

“…[123] Applications for such hydrogels have included 3D bacterial immobilization/culture for bioremediation and/or biocatalysis, [127] mammalian cell growth/guidance, [119] filtration, [128] and high surface area nanoparticle catalyst carriers. [110] The use of "click-chemistry" in preparing freeze casted hydrogels has also been demonstrated, [113,129] which eliminates the need for additional crosslinking steps. Our group has shown that varying the overall solids content and/or the weight ratio of aldehyde-functionalized CNCs to hydrazide-functionalized POEGMA can lead to structured macroporous hydrogels with tunable pore morphologies (e.g., fibrillar, columnar, or lamellar) via directional freeze casting (Figure 5).…”

Bioinspired Nanocomposite Hydrogels with Highly Ordered Structures

“…• Time/energy-consuming during freezing [94][95][96][97][98] Self-assembly Nanoparticles (0D) Nanotubes (1D) Nanosheets (2D) Nanolamellae (2D)…”

“…For example, cellulose nanocrystals (CNCs) have attracted attention as 1D nanocomponents due to their mechanical strength, commercial availability, and biocompatibility. [92][93][94] By controlling the composition of the CNC/polymer dispersion and the freeze-casting temperature, hydrogels with fibrillar, columnar, or lamellar internal morphologies can be obtained; these morphologies are analogous to the directional structures of biological soft tissues. Thus, fabricated composite hydrogels with ordered CNC structures exhibit high mechanical anisotropy.…”