Hydrogels With Non-covalently Cross-linked Network For Tissue Repair

Hydrogels,a family of synthetic materials with crosslinked hydrophilic networks and high content of water,are similar to extracellular matrix and therefore are suitable for a wide range of biomedical applications,including tissue repair and regeneration,tissue engineering and 3D-bioprinting.However,the conventional hydrogels are faced with slow gelation dynamics and self-healing,low stiffness,strength and toughness,and fast creep property.Moreover,they are generally weak and brittle.These issues make hydrogels difficultly meet the mechanical and viscoelastic requirements for cell growth,3D printing and tissue repair.It has demonstrated that introducing non-covalent interactions into hydrogel network is an effective strategy for improving gelation dynamics,mechanical properties and viscoelasticity.However,it remains challenges for the development of bioactive hydrogel materials with reasonably non-covalent interactions in the hydrogel network to adapt the biomedical performance requirements.In this dissertation,we have introduced the non-covalent crosslinking systems including hydrophobic association and hydrogen bonding into polysaccharide-based hydrogels to prepare biocompatible hydrogels with high mechanical properties and controlled viscoelasticity.The hydrogels are clinically promising for 3D printing,cartilage damage repair and skin wound healing.The main contents and conclusions are summarized as follows:(1)Novel hyaluronate hydrogels were fabricated through in-situ photo-initiated free radical copolymerization of methacrylated hyaluronate acid(Me HA)and Pluronic F127 diacrylate.The reversible hydrophobic association of F127DA micelles provides an energy dissipation mechanism to resist deformation.The optimized hydrogel with 15 wt.%of F127DA micelles and1.5 wt.%of Me HA(F₁₅H_1.5)exhibited a high compressive strength(3.44 MPa)and modulus(312k Pa),and strong compressive toughness(407.5 k J·m^-3),which matches those of cartilage tissues.On the other hand,the covalently cross-linked F127DA and Me HA chains improved the stability of the network against swelling.The hydrogels showed a low swelling ratio(1.3 times of volume expansion)in phosphate buffered saline solution.After swelling,the compressive strength(0.59MPa),modulus(55 k Pa)and fracture energy(81.8 k J·m^-3)of hydrogel dramatically decreased but still had excellent anti-fragility and self-recovery capability.Furthermore,the mechanical defects of FH hydrogel caused by swelling could be remedied by introducing organic solvents into the hydrogel.It was found that DMSO can induce direct strong hydrogen bonding interactions among Me HA chains replaced from the secondary structure of Me HA in water with water bridged multiple hydrogen.Therefore,the original soft hydrogel with a high swelling degree gradually became stiff with a decreased swelling degree as the DMSO contents in binary solvents increased.the gels show the maximum strength(10.12 MPa),modulus(106.8 k Pa)and toughness(742.1 k J·m^-3)in DMSO with a volume fraction of around 0.6.Moreover,the FH gels display a rapid recoverability under cyclic loading-unloading stressing particularly in the presence of DMSO within the network due to their dual-dynamic dissipation networks including hydrogen-bonded Me HA and hydrophobically assembled F127DA micelles.The micelle-crosslinked hyaluronate hydrogel is degradable and biocompatible.In vivo studies showed that the implantation of hydrogel in thyroid cartilage defects of rabbit larynx effectively promoted the regeneration of cartilage.(2)The covalently micelle-crosslinked hyaluronate hydrogels do not self-healing after destruction,which limited them for direct 3D printing application.In this part,F127DA micelles were introduced into dynamically hydrazone-crosslinked hyaluronic acid hydrogels.It was found that micelles as a deform-resistant unit not only improved the stiffness of hydrogels,but also shortened the gelation time of hydrogel preparation due to a complex formation between hyaluronic acid and micelles through hydrophilic and hydrophobic interactions.The storage moduli of dynamic hydrogels increase from<60 Pa to 180 Pa and the gelation time decreases from 60?300 s to less than 15 s as the content of F127Da increases from 0 to 7%(wt./v).Moreover,the modulus of the hydrogel can be further increased to 1 k Pa by micelle crosslinking initiated by UV-light.The dynamic hydrazones and hydrophobic associations in the network endowed the hydrogels with reversible gel-sol transitions and rapid self-healing,allowing for direct extrusion-based 3D printing.Moreover,the dynamical hydrogels are biocompatible.They can significantly promote the cartilage regeneration and enhance glycosaminoglycan and collagen-II matrix deposition in vivo,implying that such biocompatible hydrogel has promising applications in cartilage repair and regeneration.(3)The 3D printed fibers of macroscopic hyaluronic acid/micelle hydrogels were distorted,resulting in low precision and resolution.On the other hand,it has been demonstrated that the cell spheroids are better for tissue repair than dissociative cells.However,the above hydrogels cannot regulate cell aggregation.Therefore,novel covalently crosslinked chitosan methacrylate(CHMA)and polyvinyl alcohol(PVA)hybrid hydrogels were developed.This bulk hydrogel was controllably fractured into microparticles.The particles are able to associate into hydrogels through extensive hydrogen bonding between CHMA and PVA chains.Such particulate hydrogels showed a self-supportive yield strength and experienced a plug flow when injected through a syringe and subsequently self-healed into gels as shear forces are removed.Moreover,these gels behaved as a viscoelastic solid with outstanding creep resistance.Those characteristics enabled the particulate hydrogels with excellent printability.Diverse biomimetic constructs with very high aspect ratio including blood vessel,human ear,and rat thigh-bone were directly printed by using the microparticle hydrogels.The CHMA/PVA scaffolds supported the growt.h of bone marrow-derived mesenchymal stem cells and formation of cell spheroids,which are most important for tissue repair and regeneration.(4)In order to further investigate the influence of stiffness on the cytoskeleton of cell spheres and tissue repair,Novel hybrid hydrogels consisting of CHMA and PVA were prepared by two-step crosslinking through photo-polymerization and directional freezing-thawing process.The modulus of CHMA/PVA network can be modulated from 14 k Pa to 61 k Pa.Such CHMA/PVA hydrogels can induce the formation of fibroblast microaggregates in vitro.Importantly,the cytoskeleton adhered on the hydrogels with high stiffness(43-61 k Pa)surface was larger than that adhered on the hydrogel with low stiffness(14-35 k Pa).CHMA/PVA hydrogels are biocompatible in vivo.Both hydrogels with low and high stiffness(14 k Pa and 61 k Pa)as wound dressing could significantly accelerated the re-epithelialization and reduce scar formation but more collagen matured in 61 k Pa hydrogel group.The hydrogel group exhibited 4 d faster wound closure than the control group.We envision that such biocompatible CHMA/PVA hydrogel can be used for regulating cell behaviors and enhancing wound healing.