Understanding the inelastic response of collagen fibrils: A viscoelastic-plastic constitutive model

Enhancing biofabrication: Shrink-resistant collagen-hyaluronan composite hydrogel for tissue engineering and 3D bioprinting applications

Biofabrication represents a promising technique for creating tissues for regeneration or as models for drug testing. Collagen-based hydrogels are widely used as suitable matrix owing to their biocompatibility and tunable mechanical properties. However, one major challenge is that the encapsulated cells interact with the collagen matrix causing construct shrinkage. Here, we present a hydrogel with high shape fidelity, mimicking the major components of the extracellular matrix. We engineered a composite hydrogel comprising gallic acid (GA)-functionalized hyaluronic acid (HA), collagen I, and HA-coated multiwall carbon nanotubes (MWCNT). This hydrogel supports cell encapsulation, exhibits shear-thinning properties enhancing injectability and printability, and importantly significantly mitigates shrinkage when loaded with human fibroblasts compared to collagen I hydrogels (∼20 % vs. > 90 %). 3D-bioprinted rings utilizing human fibroblast-loaded inks maintain their shape over 7 days in culture. Furthermore, inclusion of HAGA into collagen I hydrogels increases mechanical stiffness, radical scavenging capability, and tissue adhesiveness. Notably, the here developed hydrogel is also suitable for human induced pluripotent stem cell-derived cardiomyocytes and allows printing of functional heart ventricles responsive to pharmacological treatment. Cardiomyocytes behave similar in the newly developed hydrogels compared to collagen I, based on survival, sarcomere appearance, and calcium handling. Collectively, we developed a novel material to overcome the challenge of post-fabrication matrix shrinkage conferring high shape fidelity.

Softening of elastic and viscoelastic properties is independent of overstretch rate in cerebral arteries

Collagenous soft tissues are frequently injured by supraphysiologic mechanical deformation, leading to measurable changes in both extra-cellular matrix (ECM) structure and mechanical properties. While each of these alterations has been well studied following quasi-static deformation, little is known about the influence of high strain rate. Previous investigations of high-rate ECM alterations found tropocollagen denaturation and fibrillar kinking to be rate dependent. Given these observations of rate dependence in microstructure alterations, the present work evaluated if the rate and magnitude of overstretch affect the baseline viscoelastic properties of porcine middle cerebral arteries (MCAs). Changes in tissue response were assessed using a series of harmonic oscillations before and after sub-failure overstretches across a large range of rates and magnitudes. We used collagen-hybridizing peptide (CHP) to evaluate the role of tropocollagen denaturation in mechanical softening. Experiments show that softening is dependent on overstretch magnitude but is independent of overstretch rate. We also note that softening progresses at the same rate for both equilibrium (quasi-static) and non-equilibrium (high-rate) properties. Finally, results suggest that tropocollagen denaturation is not the source of the observed sub-yield softening behavior. This study expands fundamental knowledge on the form-function relationship of constituents in collagen fibrils and clarifies material behavior following sub-failure overstretch across a range of strain rates.

Collagen and Its Derivatives Serving Biomedical Purposes: A Review

Biomaterials have been the subject of extensive research, and their applications in medicine and pharmacy are expanding rapidly. Collagen and its derivatives stand out as valuable biomaterials due to their high biocompatibility, biodegradability, and lack of toxicity and immunogenicity. This review comprehensively examines collagen from various sources, its extraction and processing methods, and its structural and functional properties. Preserving the native state of collagen is crucial for maintaining its beneficial characteristics. The challenges associated with chemically modifying collagen to tailor its properties for specific clinical needs are also addressed. The review discusses various collagen-based biomaterials, including solutions, hydrogels, powders, sponges, scaffolds, and thin films. These materials have broad applications in regenerative medicine, tissue engineering, drug delivery, and wound healing. Additionally, the review highlights current research trends related to collagen and its derivatives. These trends may significantly influence future developments, such as using collagen-based bioinks for 3D bioprinting or exploring new collagen nanoparticle preparation methods and drug delivery systems.

Dissipation and recovery in collagen fibrils under cyclic loading: A molecular dynamics study

The hysteretic behavior exhibited by collagen fibrils, when subjected to cyclic loading, is known to result in both dissipation as well as accumulation of residual strain. On subsequent relaxation, partial recovery has also been reported. Cross-links have been considered to play a key role in overall mechanical properties. Here, we modify an existing coarse-grained molecular dynamics model for collagen fibril with initially cross-linked collagen molecules, which is known to reproduce the response to uniaxial strain, by incorporating reformation of cross-links to allow for possible recovery of the fibril. Using molecular dynamics simulations, we show that our model successfully replicates the key features observed in experimental data, including the movement of hysteresis loops, the time evolution of residual strains and energy dissipation, as well as the recovery observed during relaxation. We also show that the characteristic cycle number, describing the approach toward steady state, has a value similar to that in experiments. We also emphasize the vital role of the degree of cross-linking on the key features of the macroscopic response to cyclic loading.

Recent advances in electronic skins: material progress and applications

Electronic skins are currently in huge demand for health monitoring platforms and personalized medicine applications. To ensure safe monitoring for long-term periods, high-performance electronic skins that are softly interfaced with biological tissues are required. Stretchability, self-healing behavior, and biocompatibility of the materials will ensure the future application of electronic skins in biomedical engineering. This mini-review highlights recent advances in mechanically active materials and structural designs for electronic skins, which have been used successfully in these contexts. Firstly, the structural and biomechanical characteristics of biological skins are described and compared with those of artificial electronic skins. Thereafter, a wide variety of processing techniques for stretchable materials are reviewed, including geometric engineering and acquiring intrinsic stretchability. Then, different types of self-healing materials and their applications in electronic skins are critically assessed and compared. Finally, the mini-review is concluded with a discussion on remaining challenges and future opportunities for materials and biomedical research.