Molecular mechanisms in deformation of cross-linked hydrogel nanocomposite

Tough, self-healing and injectable dynamic nanocomposite hydrogel based on gelatin and sodium alginate

Biomacromolecules based injectable and self-healing hydrogels possessing high mechanical properties have widespread potential in biomedical field. However, dynamic features are usually inversely proportional to toughness. It is challenging to simultaneously endow these properties to the dynamic hydrogels. Here, we fabricated an injectable nanocomposite hydrogel (CS-NPs@OSA-l-Gtn) stimultaneously possessing excellent autonomous self-healing performance and high mechanical strength by doping chitosan nanoparticles (CS-NPs) into dynamic polymer networks of oxidized sodium alginate (OSA) and gelatin (Gtn) in the presence of borax. The synergistic effect of the multiple reversible interactions combining dynamic covalent bonds (i.e., imine bond and borate ester bond) and noncovalent interactions (i.e., electrostatic interaction and hydrogen bond) provide effective energy dissipation to endure high fatigue resistance and cyclic loading. The dynamic hydrogel exhibited excellent mechanical properties like maximum 2.43 MPa compressive strength, 493.91 % fracture strain, and 89.54 kJ/m3 toughness. Moreover, the integrated hydrogel after injection and self-healing could withstand 150 successive compressive cycles. Besides, the bovine serum albumin embedded in CS-NPs could be sustainably released from the nanocomposite hydrogel for 12 days. This study proposes a novel strategy to synthesize an injectable and self-healing hydrogel combined with excellent mechanical properties for designing high-strength natural carriers with sustained protein delivery.

Towards Polycaprolactone-Based Scaffolds for Alveolar Bone Tissue Engineering: A Biomimetic Approach in a 3D Printing Technique

The alveolar bone is a unique type of bone, and the goal of bone tissue engineering (BTE) is to develop methods to facilitate its regeneration. Currently, an emerging trend involves the fabrication of polycaprolactone (PCL)-based scaffolds using a three-dimensional (3D) printing technique to enhance an osteoconductive architecture. These scaffolds are further modified with hydroxyapatite (HA), type I collagen (CGI), or chitosan (CS) to impart high osteoinductive potential. In conjunction with cell therapy, these scaffolds may serve as an appealing alternative to bone autografts. This review discusses research gaps in the designing of 3D-printed PCL-based scaffolds from a biomimetic perspective. The article begins with a systematic analysis of biological mineralisation (biomineralisation) and ossification to optimise the scaffold's structural, mechanical, degradation, and surface properties. This scaffold-designing strategy lays the groundwork for developing a research pathway that spans fundamental principles such as molecular dynamics (MD) simulations and fabrication techniques. Ultimately, this paves the way for systematic in vitro and in vivo studies, leading to potential clinical applications.

Computational indentation in highly cross-linked polymer networks

Indentation is a common experimental technique to study the mechanics of polymeric materials. The main advantage of using indentation is this provides a direct correlation between the microstructure and the small-scale mechanical response, which is otherwise difficult within the standard tensile testing. The majority of studies have investigated hydrogels, microgels, elastomers, and even soft biomaterials. However, a less investigated system is the indentation in highly cross-linked polymer (HCP) networks, where the complex network structure plays a key role in dictating their physical properties. In this work, we investigate the structure-property relationship in HCP networks using the computational indentation of a generic model. We establish a correlation between the local bond breaking, network rearrangement, and small-scale mechanics. The results are compared with the elastic-plastic deformation model. HCPs harden upon indentation.

Investigations into the role of non-bond interaction on gelation mechanism of silk fibroin hydrogel

Silk fibroin hydrogel not only has biocompatibility, but also has environmental response ability. It plays an important role in the development of material. The gelation mechanism of silk fibroin hydrogel is very important to textile and medicine fields. The molecular dynamics simulation was used to discuss the structure and non-bond interaction of silk fibroin hydrogel. The results show that the non-bond interactions between silk fibroin molecules and water molecules have certain influence on the formation of silk fibroin hydrogel. According to the hydrogen bond analysis, the hydrogen bonds are mainly formed between random coil peptide fragments at the two ends of silk fibroin molecules and residues 252-254 are the key residues. The electrostatic and polar solvation interactions between silk fibroin molecules plays a major role in cross-linking of the coil segments of two silk fibroin molecules.

Study on Large Deformation Behavior of Polyacrylamide Hydrogel Using Dissipative Particle Dynamics

Meso-scale models for hydrogels are crucial to bridge the conformation change of polymer chains in micro-scale to the bulk deformation of hydrogel in macro-scale. In this study, we construct coarse-grain bead-spring models for polyacrylamide (PAAm) hydrogel and investigate the large deformation and fracture behavior by using Dissipative Particle Dynamics (DPD) to simulate the crosslinking process. The crosslinking simulations show that sufficiently large diffusion length of polymer beads is necessary for the formation of effective polymer. The constructed models show the reproducible realistic structure of PAAm hydrogel network, predict the reasonable crosslinking limit of water content and prove to be sufficiently large for statistical averaging. Incompressible uniaxial tension tests are performed in three different loading rates. From the nominal stress-stretch curves, it demonstrated that both the hyperelasticity and the viscoelasticity in our PAAm hydrogel models are reflected. The scattered large deformation behaviors of three PAAm hydrogel models with the same water content indicate that the mesoscale conformation of polymer network dominates the mechanical behavior in large stretch. This is because the effective chains with different initial length ratio stretch to straight at different time. We further propose a stretch criterion to measure the fracture stretch of PAAm hydrogel using the fracture stretch of C-C bonds. Using the stretch criterion, specific upper and lower limits of the fracture stretch are given for each PAAm hydrogel model. These ranges of fracture stretch agree quite well with experimental results. The study shows that our coarse-grain PAAm hydrogel models can be applied to numerous single network hydrogel systems.

Understanding hydrogelation processes through molecular dynamics

Molecular dynamics (MD) is currently one of the preferred techniques employed to understand hydrogelation processes for its ability to include large amounts of atoms in computational calculations, since substantial amounts of solvent molecules are involved in gel formation.

In situ Study Unravels Bio-Nanomechanical Behavior in a Magnetic Bacterial Nano-cellulose (MBNC) Hydrogel for Neuro-Endovascular Reconstruction

Surgical clipping and endovascular coiling are well recognized as conventional treatments of Penetrating Brain Injury aneurysms. These clinical approaches show partial success, but often result in thrombus formation and the rupture of aneurysm near arterial walls. The authors address these challenging brain traumas with a unique combination of a highly biocompatible biopolymer hydrogel rendered magnetic in a flexible and resilient membrane coating integrated to a scaffold stent platform at the aneurysm neck orifice, which enhances the revascularization modality. This work focuses on the in situ diagnosis of nano-mechanical behavior of bacterial nanocellulose (BNC) membranes in an aqueous environment used as tissue reconstruction substrates for cerebral aneurysmal neck defects. Nano-mechanical evaluation, performed using instrumented nano-indentation, shows with very low normal loads between 0.01 to 0.5 mN, in the presence of deionized water. Mechanical testing and characterization reveals that the nano-scale response of BNC behaves similar to blood vessel walls with a very low Young´s modulus, E (0.0025 to 0.04 GPa), and an evident creep effect (26.01 ± 3.85 nm s^-1 ). These results confirm a novel multi-functional membrane using BNC and rendered magnetic with local adhesion of iron-oxide magnetic nanoparticles.

Solvent triggered irreversible shape morphism of biopolymer films

We report the controlled reversible and irreversible folding behavior of a biopolymer film simply by tuning the solvent characteristics. Generally, solvent triggered folding of soft membranes or film is achieved by unfolding. Here, we show that this unfolding behavior can be suppressed/delayed or even completely eliminated by altering the intrinsic nature of the solvent. A reversible folding of biopolymer film is observed in response to water, whereas, an irreversible folding is observed in the presence of an aromatic alcohol (AA) solution of different molar concentrations. The folding and unfolding behavior originates from the coupled deformation-diffusion phenomena. Our study indicates that the presence of an AA influences the relaxation behavior of polymer chains, which in turn affects the release of stored strain energy during folding. Controlling the reversibility as well as the actuation time of the biopolymer film by tuning the solvent is explained in detail at the bulk scale by applying appropriate experimental techniques. The underlying mechanism for the observed phenomena is complemented by performing a simulation study for a single polymer chain at the molecular length scale. Due to the solvent-triggered hygromorphic response, biopolymer films exhibit huge potential as sensors, soft robots, drug delivery agents, morphing medical devices and in biomedical applications. We provide experimental evidence for the weight lifting capacity of permanently folded membranes, amounting to ∼200 times their own weight.

Folding behavior and molecular mechanism of cross-linked biopolymer film in response to water

Water responsive biopolymers are gaining enormous attention in the different areas of research and applications related to self-folding. In this work, we report that cross-linking is an efficient means of modifying a single layer biopolymer film for a controlled and predictable pathway of folding. The initiation of the folding of a film is caused by the diffusion of water molecules along the film thickness. However, this folding is observed to take place in an unpredictable and random fashion with a pristine biopolymer film and a nano-particle reinforced film. The mechanical properties and the diffusion characteristics of the film are strongly interrelated and affect the overall folding behavior. The underlying mechanism behind this relation is appropriately substantiated by an in depth molecular dynamic study. The detailed characterization of the folding shape and material behavior is performed applying suitable experimental techniques. The potential application of the controlled folding of the cross-linked film as a sensor and as a soft crane is demonstrated in this report.