Mechanism of Mechanical Training-Induced Self-Reinforced Viscoelastic Behavior of Highly Hydrated Silk Materials

Self-Reinforcing Ionogel Bioadhesive Interface for Robust Integration and Monitoring of Bioelectronic Devices with Hard Tissues

Integrating bioelectronic devices with hard tissues, such as bones and teeth, is essential for advancing diagnostic and therapeutic technologies. However, stable and durable adhesion in dynamic, moist environments remains challenging. Traditional bioadhesives often fail to maintain strong bonds, especially when interfacing with metal electrodes and hard tissues. This study introduces a self-reinforcing ionogel bioadhesive interface (IGBI) combining silk fibroin and calcium ions, designed to provide robust and conductive integration of bioelectronic devices with hard tissues. The IGBI exhibits strong adhesion (up to 186 J m-2) and undergoes mechanical self-reinforcement through a structural transition in silk fibroin under physiological conditions. In vivo experiments demonstrate the IGBI's effectiveness in repairing bone defects and reimplanting teeth, with the added capability of wireless, real-time monitoring of bone healing. This approach allows for continuous tracking of tissue regeneration without a second invasive surgery for device removal. The IGBI represents a significant advancement in bioelectronic integration, offering a durable and versatile solution for challenging environments. Such unique self-reinforcing properties make the IGBI a promising material for biomedical applications where traditional adhesives are insufficient.

Strain learning in protein-based mechanical metamaterials

Mechanical deformation of polymer networks causes molecular-level motion and bond scission that ultimately lead to material failure. Mitigating this strain-induced loss in mechanical integrity is a significant challenge, especially in the development of active and shape-memory materials. We report the additive manufacturing of mechanical metamaterials made with a protein-based polymer that undergo a unique stiffening and strengthening behavior after shape recovery cycles. We utilize a bovine serum albumin-based polymer and show that cyclic tension and recovery experiments on the neat resin lead to a ~60% increase in the strength and stiffness of the material. This is attributed to the release of stored length in the protein mechanophores during plastic deformation that is preserved after the recovery cycle, thereby leading to a "strain learning" behavior. We perform compression experiments on three-dimensionally printed lattice metamaterials made from this protein-based polymer and find that, in certain lattices, the strain learning effect is not only preserved but amplified, causing up to a 2.5× increase in the stiffness of the recovered metamaterial. These protein-polymer strain learning metamaterials offer a unique platform for materials that can autonomously remodel after being deformed, mimicking the remodeling processes that occur in natural materials.

Embedding Living Cells with a Mechanically Reinforced and Functionally Programmable Hydrogel Fiber Platform

Living materials represent a new frontier in functional material design, integrating synthetic biology tools to endow materials with programmable, dynamic, and life-like characteristics. However, a major challenge in creating living materials is balancing the tradeoff between structural stability, mechanical performance, and functional programmability. To address this challenge, a sheath-core living hydrogel fiber platform that synergistically integrates living bacteria with hydrogel fibers to achieve both functional diversity and structural and mechanical robustness is proposed. In the design, microfluidic spinning is used to produce hydrogel fiber, which offers advantages in both structural and functional designability due to their hierarchical porous architectures that can be tailored and their mechanical performance that can be enhanced through a variety of post-processing approaches. By introducing living bacteria, the platform is endowed with programmable functionality and life-like capabilities. This work reconstructs the genetic circuits of living bacteria to express chromoproteins and fluorescent proteins as two prototypes that enable the coloration of living fibers and sensing water pollutants by monitoring the amount of fluorescent protein expressed. Altogether, this study establishes a structure-property-function optimized living hydrogel fiber platform, providing a new tool for accelerating the practical applications of the emerging living material systems.

Tendon Repair and Regeneration Using Bioinspired Fibrillation Engineering That Mimicked the Structure and Mechanics of Natural Tissue

Replicating the controlled nanofibrillar architecture of collagenous tissue represents a promising approach in the design of tendon replacements that have tissue-mimicking biomechanics─outstanding mechanical strength and toughness, defect tolerance, and fatigue and fracture resistance. Guided by this principle, a fibrous artificial tendon (FAT) was constructed in the present study using an engineering strategy inspired by the fibrillation of a naturally spun silk protein. This bioinspired FAT featured a highly ordered molecular and nanofibrillar architecture similar to that of soft collagenous tissue, which exhibited the mechanical and fracture characteristics of tendons. Such similarities provided the motivation to investigate FAT for applications in Achilles tendon defect repair. In vitro cellular morphology and expression of tendon-related genes in cell culture and in vivo modeling of tendon injury clearly revealed that the highly oriented nanofibrils in the FAT substantially promoted the expression of tendon-related genes combined with the Achilles tendon structure and function. These results provide confidence about the potential clinical applications of the FAT.

Design and Synthesis of Stem Cell-Laden Keratin/Glycol Chitosan Methacrylate Bioinks for 3D Bioprinting

With the advancements in tissue engineering and three-dimensional (3D) bioprinting, physiologically relevant three-dimensional structures with suitable mechanical and bioactive properties that mimic the biological tissue can be designed and fabricated. However, the available bioinks are less than demanded. In this research, the readily available biomass sources, keratin and glycol chitosan, were selected to develop a UV-curable hydrogel that is feasible for the 3D bioprinting process. Keratin methacrylate and glycol chitosan methacrylate were synthesized, and a hybrid bioink was created by combining this protein-polysaccharide cross-linked hydrogel. While human hair keratin could provide biological functions, the other composition, glycol chitosan, could further enhance the mechanical strength of the construct. The mechanical properties, degradation profile, swelling behavior, cell viability, and proliferation were investigated with various ratios of keratin methacrylate to glycol chitosan methacrylate. The composition of 2% (w/v) keratin methacrylate and 2% (w/v) chitosan methacrylate showed a significantly higher cell number and swelling percentage than other compositions and was designated as the bioink for 3D printing afterward. The feasibility of stem cell loading in the selected formula was examined with an extrusion-based bioprinter. The cells and spheroids can be successfully printed with the synthesized bioink into a specific shape and cultured. This work provides a potential option for bioinks and delivers insights into personalization research on stem cell-laden biofabricated hydrogels in the future.

Mechanical Training-Driven Structural Remodeling: A Rational Route for Outstanding Highly Hydrated Silk Materials

Highly hydrated silk materials (HHSMs) have been the focus of extensive research due to their usefulness in tissue engineering, regenerative medicine, and soft devices, among other fields. However, HHSMs have weak mechanical properties that limit their practical applications. Inspired by the mechanical training-driven structural remodeling strategy (MTDSRS) in biological tissues, herein, engineered MTDSRS is developed for self-reinforcement of HHSMs to improve their inherent mechanical properties and broaden potential utility. The MTDSRS consists of repetitive mechanical training and solvent-induced conformation transitions. Solvent-induced conformation transition enables the formation of β-sheet physical crosslinks among the proteins, while the repetitive mechanical loading allows the rearrangement of physically crosslinked proteins along the loading direction. Such synergistic effects produce strong and stiff mechanically trained-HHSMs (MT-HHSMs). The fracture strength and Young's modulus of the resultant MT-HHSMs (water content of 43 ± 4%) reach 4.7 ± 0.9 and 21.3 ± 2.1 MPa, respectively, which are 8-fold stronger and 13-fold stiffer than those of the as-prepared HHSMs, as well as superior to most previously reported HHSMs with comparable water content. In addition, the animal silk-like highly oriented molecular crosslinking network structure also provides MT-HHSMs with fascinating physical and functional features, such as stress-birefringence responsibility, humidity-induced actuation, and repeatable self-folding deformation.