Bone‐Adhesive Anisotropic Tough Hydrogel Mimicking Tendon Enthesis

Abstract

consist of soft collagen with a water con tent of around 60-80%, their unique structural characteristics provide high tensile strength and excellent fatigue resis tance. [1][2][3][4] The high strength of tendons is derived from the hierarchical structure and anisotropy of collagen fibers formed by crosslinking collagen molecules and gathered to form larger bundles. Tendon tobone integration, called enthesis, is another important feature of tendons. The enthesis is the part where the tendon and ligament insert into the bone and transmit the mechanical load from muscle to bone. The enthesis is composed of four zones (pure dense fibrous connective tissue, uncalcified fibrocartilage, calcified fibro cartilage, and bone) exhibiting gradients in tissue organization with varying cellular composition (collagentomineral), which leads to high adhesive, mechanical prop erties between the tendon and bone. [5][6][7][8] The strong mechanical properties of anisotropic, hierarchically aligned collagen fibers in tendons and the unique tendonto bone integration enable an effective transfer of high mechan ical stress. In general, this unique and complex interface and the corresponding mechanical properties of tendon and liga ment are difficult to mimic using biomaterials.Recently, there have been significant advances in hydrogels exhibiting high mechanical properties or adhesive characteris tics which are extensively improved compared to that of con ventional hydrogels. However, most reported hydrogels either have high adhesion but significantly inferior mechanical properties than those of tendons [9][10][11][12][13][14][15][16][17][18][19] or have strong mechanical properties but very weak adhesion. [20][21][22][23][24][25][26][27][28][29][30][31][32][33] Very recent studies have attempted to simultaneously achieve high adhesiveness and excellent mechanical properties of hydrogels for bioad hesion; [34][35][36] nevertheless those hydrogels are far softer than the tendon. Also, most of them have focused on the adhe sion to relatively soft tissues such as heart, skin, cartilage, and tendon; [9][10][11][12][13][14][15][16][17][18][19]35,36] there have been no reports on mechanically enhanced hydrogel with strong adhesion to the bone. This is due to the tradeoff between the high modulus or strength of hydrogel and its adhesiveness; it is hard to achieve high adhe sion of stiff and strong hydrogels to a solid surface.Hence, this study proposes an anisotropic, stiff, and strong hydrogel with a high adhesiveness to the bone, mimicking both the mechanical properties of the tendon and the tendon tobone interface. A tough triple network (TN) hydrogel Tendon consists of soft collagen, yet it is mechanically strong and firmly adhered to the bone owing to its hierarchically anisotropic structure and unique tendon-to-bone integration (enthesis), respectively. Despite the recent advances in biomaterials, hydrogels simultaneously providing tendon-like high mechanical properties and strong adhesion to bone-mimicking enthesis is still challenging....

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

“…39,186 Bionic ordered structured hydrogel characteristics can improve their mechanical properties by adjusting the degree of crosslinking. 112,184,185,187 This can be achieved by constructing ordered double network hydrogels, 20 introducing metal ions for coordination (Fig. 5A), 172 adding SiO 2 nanoparticles 188 and other methods.…”

“…This led to the formation of a denser hydrogel network, thereby increasing the mechanical strength. 112,172,189 Despite the simplicity of improving the crosslinking degree, stress deformation during practical applications can cause a decrease in the crosslinking degree of hydrogels, which consequently affects their mechanical strength. 100,190 Additionally, an excessive degree of crosslinking can cause hydrogels to become more brittle and experience a loss of tensile strength.…”

Applications of functionally-adapted hydrogels in tendon repair

“…A triple network hydrogel was primarily manufactured, which was comprised of ionically cross-linked alginate, covalently cross-linked polyacrylamide, and poly (2-hydroxyethyl aspartamide) modified with aminopropyl imidazole (PHEA-API). The triple network hydrogel further underwent stretching and secondary cross-linking to form a anisotropic structure with a high modulus (3.0 MPa) and strength (0.8 MPa) ( Choi et al, 2022 ). Another approach fabricated a scaffold by integration of two-dimensional nanomaterials to enhance the mechanical properties of gelatin hydrogel ( Figure 3B ).…”

In situmolecular permeation of liquid cationic polymers into solid anionic polymer films enabling self-adaptive adhesion of hydrogel biosensors

“…31,32 This leads to an improved affinity of the hydrogel surface to other materials. [33][34][35][36] Such a phenomenon can also occur between cationic and anionic groups. In this work, besides the ionic interaction, the in situ molecular permeation removed air layers from the tested surface, which further improved the adhesion of hydrogel sensors.…”