Ligament-Inspired Tough and Anisotropic Fibrous Gel Belt with Programed Shape Deformations via Dynamic Stretching

Direct 3D printing of freeform anisotropic bioactive structure based on shear-oriented ink system

Various anisotropic tissue structures exist in organisms, including muscle tissue, skin tissue, and nerve tissue. Replicating anisotropic tissue structuresin vitrohas posed a significant challenge. Three-dimensional (3D) printing technology is often used to fabricate biomimetic structures due to its advantages in manufacturing principle. However, direct 3D printing of freeform anisotropic bioactive structures has not been reported. To tackle this challenge, we developed a ternary F/G/P ink system that integrates the printability of Pluronic F127 (F), the robust bioactivity and photocrosslinking properties of gelatin methacryloyl (G), and the shear-induced alignment functionality of high-molecular-weight polyethylene glycol (P). And through this strategic ternary system combination, freeform anisotropic tissue structures can be 3D printed directly. Moreover, these anisotropic structures exhibit excellent bioactivity, and promote orientational growth of different cells. This advancement holds promise for the repair and replacement of anisotropic tissues within the human body.

Stepwisely Assembled Multicomponent Fiber with High Water Content and Superior Mechanical Properties for Artificial Ligament

The ligament, which connects bones at the joints, has both high water content and excellent mechanical properties in living organisms. However, it is still challenging to fabricate fibrous materials that possess high water content and ligament-like mechanical characteristics simultaneously. Herein, the design and preparation of a ligament-mimicking multicomponent fiber is reported through stepwise assembly of polysaccharide, calcium, and dopamine. In simulated body fluid, the resulting fiber has a water content of 40 wt%, while demonstrating strength of ≈120 MPa, a Young's modulus of ≈3 GPa, and a toughness of ≈25 MJ m-3. Additionally, the multicomponent fiber exhibits excellent creep and fatigue resistance, as well as biocompatibility to support cell growth in vitro. These findings suggest that the fiber has potential for engineering high-performance artificial ligament.

Strong, tough and anisotropic bioinspired hydrogels

Soft materials are widely used in tissue engineering, soft robots, wearable electronics, etc. However, it remains a challenge to fabricate soft materials, such as hydrogels, with both high strength and toughness that are comparable to biological tissues. Inspired by the anisotropic structure of biological tissues, a novel solvent-exchange-assisted wet-stretching strategy is proposed to prepare anisotropic polyvinyl alcohol (PVA) hydrogels by tuning the macromolecular chain movement and optimizing the polymer network. The reinforcing and toughening mechanisms are found to be "macromolecule crystallization and nanofibril formation". These hydrogels exhibit excellent mechanical properties, such as extremely high fracture stress (12.8 ± 0.7 MPa) and fracture strain (1719 ± 77%), excellent modulus (4.51 ± 0.76 MPa), high work of fracture (134.47 ± 9.29 MJ m-3), and fracture toughness (305.04 kJ m-2) compared with other strong hydrogels and even natural tendons. In addition, excellent conductivity, strain sensing capability, water retention, freezing resistance, swelling resistance, and biocompatibility can also be achieved. This work provides a new and effective method to fabricate multifunctional anisotropic hydrogels with high tunable strength and toughness with potential applications in the fields of regenerative medicine, flexible sensors, and soft robotics.

Continuous Spinning of High-Tough Hydrogel Fibers for Flexible Electronics by Using Regional Heterogeneous Polymerization

Hydrogel fibers have attracted substantial interest for application in flexible electronics due to their ionic conductivity, high specific surface area, and ease of constructing multidimensional structures. However, universal continuous spinning methods for hydrogel fibers are yet lacking. Based on the hydrophobic mold induced regional heterogeneous polymerization, a universal self-lubricating spinning (SLS) strategy for the continuous fabrication of hydrogel fibers from monomers is developed. The universality of the SLS strategy is demonstrated by the successful spinning of 10 vinyl monomer-based hydrogel fibers. Benefiting from the universality of the SLS strategy, the SLS strategy can be combined with pre-gel design and post-treatment toughening to prepare highly entangled polyacrylamide (PAM) and ionic crosslinked poly(acrylamide-co-acrylic acid)/Fe3+ (W-PAMAA/Fe3+ ) hydrogel fibers, respectively. In particular, the W-PAMAA/Fe3+ hydrogel fiber exhibited excellent mechanical properties (tensile stress > 4 MPa, tensile strain > 400%) even after 120 days of swelling in the pH of 3-9. Furthermore, owing to the excellent multi-faceted performance and one-dimensionality of W-PAMAA/Fe3+ hydrogel fibers, flexible sensors with different dimensions and functions can be constructed bottom-up, including the one-dimensional (1D) strain sensor, two-dimensional (2D) direction sensor, three-dimensional (3D) pressure sensor, and underwater communication sensor to present the great potential of hydrogel fibers in flexible electronics.

Bionic ordered structured hydrogels: structure types, design strategies, optimization mechanism of mechanical properties and applications

Natural organisms, such as lobsters, lotus, and humans, exhibit exceptional mechanical properties due to their ordered structures. However, traditional hydrogels have limitations in their mechanical and physical properties due to their disordered molecular structures when compared with natural organisms. Therefore, inspired by nature and the properties of hydrogels similar to those of biological soft tissues, researchers are increasingly focusing on how to investigate bionic ordered structured hydrogels and render them as bioengineering soft materials with unique mechanical properties. In this paper, we systematically introduce the various structure types, design strategies, and optimization mechanisms used to enhance the strength, toughness, and anti-fatigue properties of bionic ordered structured hydrogels in recent years. We further review the potential applications of bionic ordered structured hydrogels in various fields, including sensors, bioremediation materials, actuators, and impact-resistant materials. Finally, we summarize the challenges and future development prospects of bionic ordered structured hydrogels in preparation and applications.

Tendon-Inspired Anisotropic Hydrogels with Excellent Mechanical Properties for Strain Sensors

Anisotropic conductive hydrogels mimicking the natural tissues with high mechanical properties and intelligent sensing have played an important role in the field of flexible electronic devices. Herein, tensile remodeling, drying, and subsequent ion cross-linking methods were used to construct anisotropic hydrogels, which were inspired by the orientation and functionality of tendons. Due to the anisotropic arrangement of the polymer network, the mechanical performance and electrical conductivity were greatly improved in specific directions. The tensile stress and elastic modulus of the hydrogel along the network orientation were 29.82 and 28.53 MPa, which were higher than those along the vertical orientation, 9.63 and 11.7 MPa, respectively. Moreover, the hydrogels exhibited structure-dependent anisotropic sensing. The gauge factors (GFs) parallel to the prestretching direction were greater than the GF along the vertical direction. Thus, the tendon-inspired conductive hydrogels with anisotropy could be used as flexible sensors for joint motion detection and voice recognition. The anisotropic hydrogel-based sensors are highly expected to promote the great development of emerging soft electronics and medical detection.

Konjac glucomannan/xanthan gum/sodium alginate composite hydrogel simulates fascial tissue by pre-stretching and moisture regulation

Improving the mechanical strength and creating an anisotropic structure of edible macromolecular hydrogels is crucial to accurately simulate the texture of connective tissues. In this study, konjac glucomannan (KGM), xanthan gum (XG), and sodium alginate (SA) were used to construct hydrogels, and the effects of different pre-stretching degrees and moisture control on the composite gels were investigated. The results of the mechanical property tests and microstructure tests indicate that pre-stretching and moisture control can significantly enhance the strength of the gels and induce anisotropic structures. In addition, the feasibility of the composite gel structure in simulating brisket fascia was investigated, and it was concluded that 1.5 × -DR samples were most suitable for simulating connective tissue. This study provides compelling evidence for the potential of macromolecular hydrogels in simulating connective tissue and provides theoretical guidance for regulating gel texture.

Ultrastretchable, Antifreezing, and High-Performance Strain Sensor Based on a Muscle-Inspired Anisotropic Conductive Hydrogel for Human Motion Monitoring and Wireless Transmission

Integrating structural anisotropy, excellent mechanical properties, and superior sensing capability into conductive hydrogels is of great importance to wearable flexible electronics yet challenging. Herein, inspired from the aligned structure of human muscle, we proposed a facile and universal method to construct an anisotropic hydrogel composed of polyacrylamide and sodium alginate by pre-stretching in a confined geometry and subsequent ionic cross-linking. The designed hydrogels showed extraordinary mechanical performances, such as ultrahigh stretchability, a comparable modulus to that of human tissues, and good toughness, ascribed to their anisotropically aligned polymer networks. Additionally, the hydrogel possessed anisotropic conductivity due to the anisotropy in ion transport channels. The hydrogel along the vertical direction was further cut and assembled into a flexible strain sensor, exhibiting a low detection limit (0.1%), wide strain range (1585%), rapid response (123 ms), distinct resilience, good stability, and repeatability, thereby being capable of monitoring and discriminating different human movements. In addition, the relatively high ionic conductivity and superior sensitivity enabled the anisotropic hydrogel sensor to be used for wireless human-machine interaction. More interestingly, the Ca²⁺-cross-linking strategy also endowed the hydrogel sensor with antifreezing ability, further broadening their working temperature. This work is expected to speed up the development of hydrogel sensors in the emerging wearable soft electronics.

Functional biomaterials for tendon/ligament repair and regeneration

With an increase in life expectancy and the popularity of high-intensity exercise, the frequency of tendon and ligament injuries has also increased. Owing to the specificity of its tissue, the rapid restoration of injured tendons and ligaments is challenging for treatment. This review summarizes the latest progress in cells, biomaterials, active molecules and construction technology in treating tendon/ligament injuries. The characteristics of supports made of different materials and the development and application of different manufacturing methods are discussed. The development of natural polymers, synthetic polymers and composite materials has boosted the use of scaffolds. In addition, the development of electrospinning and hydrogel technology has diversified the production and treatment of materials. First, this article briefly introduces the structure, function and biological characteristics of tendons/ligaments. Then, it summarizes the advantages and disadvantages of different materials, such as natural polymer scaffolds, synthetic polymer scaffolds, composite scaffolds and extracellular matrix (ECM)-derived biological scaffolds, in the application of tendon/ligament regeneration. We then discuss the latest applications of electrospun fiber scaffolds and hydrogels in regeneration engineering. Finally, we discuss the current problems and future directions in the development of biomaterials for restoring damaged tendons and ligaments.

Hybrid Nanofibrous Composites with Anisotropic Mechanics and Architecture for Tendon/Ligament Repair and Regeneration

Rupture of tendons and ligaments (T/L) is a major clinical challenge due to T/L possess anisotropic mechanical properties and hierarchical structures. Here, to imitate these characteristics, an approach is presented by fabricating hybrid nanofibrous composites. First, hybrid fiber-reinforced yarns are fabricated via successively electrospinning poly(L-lactide-co-ε-caprolactone) (PLCL) and gelatin (Ge) nanofibers onto polyethylene terephthalate (PET) fibers to improve biodurability and biocompatibility. Then, by comparing different manufacturing methods, the knitted structure succeeds in simulating anisotropic mechanical properties, even being stronger than natural ligaments, and possessing comfort compliance superior to clinically used ligament advanced reinforcement system (LARS) ligament. Moreover, after inoculation with tendon-derived stem cells and transplantation in vivo, hybrid nanofibrous composites are integrated with native tendons to guide surrounding tissue ingrowth due to the highly interconnected and porous structure. The knitted hybrid nanofibrous composites are also ligamentized and remodeled in vivo to promote tendon regeneration. Specifically, after the use of optimized anisotropic hybrid nanofibrous composites to repair tendon, the deposition of tendon-associated extracellular matrix proteins is more significant. Thus, this study indicates a strategy of manufacturing anisotropic hybrid nanofibrous composites with superior mechanical properties and good histocompatibility for clinical reconstruction.

Bioinspired Hydrogels as Platforms for Life-Science Applications: Challenges and Opportunities

Hydrogels, as interconnected networks (polymer mesh; physically, chemically, or dynamic crosslinked networks) incorporating a high amount of water, present structural characteristics similar to soft natural tissue. They enable the diffusion of different molecules (ions, drugs, and grow factors) and have the ability to take over the action of external factors. Their nature provides a wide variety of raw materials and inspiration for functional soft matter obtained by complex mechanisms and hierarchical self-assembly. Over the last decade, many studies focused on developing innovative and high-performance materials, with new or improved functions, by mimicking biological structures at different length scales. Hydrogels with natural or synthetic origin can be engineered as bulk materials, micro- or nanoparticles, patches, membranes, supramolecular pathways, bio-inks, etc. The specific features of hydrogels make them suitable for a wide variety of applications, including tissue engineering scaffolds (repair/regeneration), wound healing, drug delivery carriers, bio-inks, soft robotics, sensors, actuators, catalysis, food safety, and hygiene products. This review is focused on recent advances in the field of bioinspired hydrogels that can serve as platforms for life-science applications. A brief outlook on the actual trends and future directions is also presented.

Skeletal Muscle Fibers Inspired Polymeric Actuator by Assembly of Triblock Polymers

Inspired by the striated structure of skeletal muscle fibers, a polymeric actuator by assembling two symmetric triblock copolymers, namely, polystyrene-b-poly(acrylic acid)-b-polystyrene (SAS) and polystyrene-b-poly(ethylene oxide)-b-polystyrene (SES) is developed. Owing to the microphase separation of the triblock copolymers and hydrogen-bonding complexation of their middle segments, the SAS/SES assembly forms a lamellar structure with alternating vitrified S and hydrogen-bonded A/E association layers. The SAS/SES strip can be actuated and operate in response to environmental pH. The contraction ratio and working density of the SAS/SES actuator are approximately 50% and 90 kJ m^-3 , respectively; these values are higher than those of skeletal muscle fibers. In addition, the SAS/SES actuator shows a "catch-state", that is, it can maintain force without energy consumption, which is a feature of mollusc muscle but not skeletal muscle. This study provides a biomimetic approach for the development of artificial polymeric actuators with outstanding performance.

Muscle-Inspired Anisotropic Hydrogel Strain Sensors

Hydrogel strain sensors have attracted tremendous attention in medical monitoring, flexible wearable devices, and human-machine interfaces. However, traditional hydrogels exhibit isotropic sensing performance based on their isotropic structure. Therefore, it is challenging to fabricate a hydrogel with an anisotropic structure similar to human tissues for achieving anisotropic sensing characteristics. Herein, we proposed a simple and effective method for preparing anisotropic poly(vinyl alcohol) (PVA) conductive hydrogels, which demonstrated anisotropic mechanical properties and anisotropic ion conductivity. The anisotropic hydrogel was successfully constructed through first thermal stretching and then directional freezing. The mechanical strength of hydrogels along the parallel stretching direction (stress of 1596 kPa and toughness of 3.69 MJ/m³) was higher than that of the hydrogels along the vertical stretching direction (stress of 883.1 kPa and toughness of 1.96 MJ/m³). Moreover, the hydrogel showed anisotropic conductivity on the advantage of the different ion channels. The prepared hydrogel sensor exhibited anisotropic sensing for multidirectional stress in the strain range from 0.5 to 100%. The gauge factors (GF) parallel to the stretching direction were greater than the GF vertical to the stretching direction. The anisotropic hydrogel sensors are expected to have broad application prospects in flexible wearable devices and medical monitoring.