Engineering Nanofibrillar Hydrogels Mimicking the Nonlinear Mechanical Behavior of Biological Fibrous Networks

Biological polymers such as collagen and fibrin exhibit strong stiffening under shear and elongational strains, but soften under compressional strain. This behavior starkly contrasts with the responses seen in networks formed by flexible-strand molecules, setting the stage for innovative biomaterials that can replicate the intricate mechanical properties of natural tissue. Development of a novel engineered hydrogel with properties that mimic the non-linear mechanical behavior characteristic of natural fibrous networks like those found in biological polymers will have wide medical applications. However, inherent mechanical asymmetry of the constituent filaments within these networks, where the modulus of stretching significantly surpasses the bending modulus, attributing to the nonlinear mechanical behavior observed. Traditionally, studies in this domain have been hampered by the limited control over the network architecture when using hydrogels formed by biological polymers. Addressing this challenge, a new study published in Proceedings of the National Academy of Sciences and led by Elisabeth Prince from Waterloo University and Professor Eugenia Kumacheva from the University of Toronto, the researchers developed nanofibrillar hydrogel that mimics the nonlinear mechanical behavior of biological fibrous networks, such as those found in collagen and fibrin. This endeavor was aimed at understanding the complex biomechanical properties of biological tissues and developing biomimetic materials for applications in tissue engineering and regenerative medicine.

The team first synthesized a novel hydrogel, referred to as EKGel, from cellulose nanocrystals (CNCs) and gelatin. The CNCs were chemically modified to introduce aldehyde groups that facilitate covalent cross-linking with the amino groups of gelatin, forming a fibrous network structure. This approach allowed for controlled architecture of the hydrogel, replicating the fibrous nature of biological tissues. The researchers characterized the hierarchical structure of the EKGel using scanning electron microscopy   and confocal microscopy analysis. They quantified the fiber diameters, lengths, and the distribution of gelatin and CNCs within the fibers. This structural characterization confirmed the fibrous architecture and homogeneous distribution of components within the hydrogel. The authors performed rheological tests to assess the mechanical properties of the EKGel, focusing on its response to shear and compressional strains. The experiments demonstrated that the EKGel exhibits shear-stiffening behavior, where the hydrogel becomes significantly stiffer in response to shear strains, and compression-induced softening, where the hydrogel’s stiffness decreases under compressional strain. These findings are in line with the mechanical behavior observed in biological fibrous networks. Moreover, they compared the mechanical properties of EKGel with those of natural fibrous networks, such as fibrin and collagen gels. The EKGel exhibited similar nonlinear mechanical responses, including shear-stiffening and compression-induced softening, validating its biomimetic properties. To provide a theoretical foundation for the observed mechanical behaviors, the researchers utilized an affine model that considers the mechanical asymmetry of the constituent filaments within the hydrogel. The model accurately predicted the shear-stiffening and compression-induced softening behaviors of the EKGel, further corroborating the experimental results.

The authors found that the engineered EKGel successfully replicated the nonlinear mechanical properties of biological fibrous networks, including shear-stiffening and compression-induced softening. The chemical modification of CNCs and the covalent cross-linking with gelatin allowed for precise control over the hydrogel’s fibrous structure, resembling the architecture of biological tissues. Moreover, the authors experimental findings were in agreement with predictions from the affine model, indicating that the nonlinear mechanical properties are attributable to the mechanical asymmetry of the fibrous structure within the hydrogel.  Although not explicitly detailed in the initial overview, the biocompatible materials (cellulose nanocrystals and gelatin) used in EKGel synthesis suggest potential for applications in tissue engineering and regenerative medicine, given its biomimetic mechanical properties. The engineered hydrogel showcases both shear-stiffening and compression-induced softening behaviors, aligning with the predictions of the affine model and thus providing a robust platform to further understand the biomechanical properties of fibrous networks. These findings are not just academic; they hold profound implications for tissue engineering, where mimicking the native mechanical environment of cells is crucial for the successful integration and function of engineered tissues. The variability in the composition of the hydrogel allows for tuning its mechanical properties, offering a versatile tool for researchers to explore the role of mechanical signals in cell behavior, disease progression, and tissue repair and regeneration.

The implications of the new study is significant in the development of biomimetic materials that can closely replicate the complex mechanical properties of biological tissues. This is particularly crucial in the field of regenerative medicine, where there is a persistent need for materials that can support the growth, differentiation, and function of cells in a manner that mimics their natural environment.  In a nutshell, the work by Professors Elisabeth Prince and Eugenia Kumacheva successfully developed engineered hydrogels that closely mimic the nonlinear mechanical behavior of natural fibrous networks, offering new insights into the structural and mechanical complexities of fibrous networks.  This should be valuable tool for researchers to investigate the fundamental principles of biomechanics and their implications for health and disease. Moreover, these innovative materials are set to play a central role in the development of therapeutic strategies that harness the body’s own regenerative capabilities, marking a new era in regenerative medicine and tissue engineering.