Self-healing of hierarchical materials

On the utility of hierarchical self-healing fiber bundle materials under different environments

Bio-materials use a hierarchical structure to optimize their self-healing mechanical behavior. However, the utility may be restricted by different environments. In this paper, based on the previous work in Ji and Li (J Mater Sci 53:14858-14870, 2018) on the constitutive relation of hierarchical self-healing fiber bundle materials (FBMs), the stability is investigated for the mechanical-environmental interaction system established in Ji and Li (Int J Fract 212:105-112, 2018). With the principle of total potential, the stability criterion is proposed. The critical environment stiffness is derived and the system is therefore classified into the absolutely stable one and the conditionally stable one. For the conditionally stable system, the applied strain of FBMs reduces to be [0,[Formula: see text]], where [Formula: see text] is the cutoff strain. Finally, the toughness of hierarchical self-healing FBMs is studied for different interaction systems. The results show that in the absolutely stable system there exists a critical healing rate across which the toughness benefits from a hierarchical structure. In the conditionally stable system, the toughness is significantly affected by the environment stiffness, i.e., the toughness of a FBM increases with a rising hierarchical level, whereas it decreases with a rising healing rate. Moreover, the critical healing rate for toughness becomes greater compared to that in an absolutely stable system.

An Analytic Model of Tissue Self-Healing and Its Network Implementation: Application to Fibrosis and Aging

Here we present a model capable of self-healing and explore its ability to resolve pathological alterations in biological tissue. We derive a simple analytic model consisting of an agent representing a cell that exhibits anabolic or catabolic activity, and which interacts with its tissue substrate according to tissue stiffness. When perturbed, this system returns toward a stable fixed point, a process corresponding to self-healing. We implemented this agent-substrate mechanism numerically on a hexagonal elastic network representing biological tissue. Agents, representing fibroblasts, were placed on the network and allowed to migrate around while they remodeled the network elements according to their activity which was determined by the stiffnesses of network elements that each agent encountered during its random walk. Initial injury to the network was simulated by increasing the stiffness of a single central network element above baseline. This system also exhibits a fixed point represented by the uniform baseline state. During the approach to the fixed point, interactions between the agents and the network create a transient spatially extended halo of stiffer network elements around the site of initial injury, which aids in overall injury repair. Non-equilibrium constraints generated by persistent injury prohibit the network to return to baseline and results in progressive stiffening, mimicking the development of fibrosis. Additionally, reducing anabolic or catabolic rates delay self-healing, reminiscent of aging. Our model thus embodies what may be the simplest set of attributes required of a spatiotemporal self-healing system, and so may help understand altered self-healing in chronic fibrotic diseases and aging.

Mechanical and Self-Healing Behavior of Matrix-Free Polymer Nanocomposites Constructed via Grafted Graphene Nanosheets

Through molecular dynamics (MD) simulation, the structure and mechanical properties of matrix-free polymer nanocomposites (PNCs) constructed via polymer-grafted graphene nanosheets are studied. The dispersion of graphene sheets is characterized by the radial distribution function (RDF) between graphene sheets. We observe that a longer polymer chain length L_g leads to a relatively better dispersion state attributed to the formation of a better brick-mud structure, effectively screening the van der Waals interactions between sheets. By tuning the interaction strength ε_end-end between end functional groups of grafted chains, we construct physical networks with various robustness characterized by the formation of the fractal clusters at high ε_end-end values. The effects of ε_end-end and L_g on the mechanical properties are examined, and the enhancement of the stress-strain behavior is observed with the increase of ε_end-end and L_g. Structural evolution during deformation is quantified by calculating the orientation of the graphene sheets and their distribution, the stress decomposition, and the size of the clusters formed between end groups and their distribution. Then, we briefly study the effects of time and temperature on the self-healing behavior of these unique PNCs in the rubbery state. Lastly, the self-healing kinetics is quantitatively analyzed. In general, this work can provide some rational guidelines to design and fabricate matrix-free PNCs with both excellent mechanical and self-healing properties.