Tuning Hydrogel Mechanics Using the Hofmeister Effect

The mechanical properties of hydrogels are commonly modified by changing the concentration of the molecular components. This approach, however, does not only change hydrogel mechanics, but also the microstructure, which in turn alters the macroscopic properties of the gel. Here, the Hofmeister effect is used to change the thermoresponsiveness of polyisocyanide hydrogels. In contrast to previous Hofmeister studies, the effect is used to change the phase transition temperatures and to tailor the mechanics of the thermoresponsive (semiflexible) polymer gels. It is demonstrated that the gel stiffness can be manipulated over more than two orders of magnitude by the addition of salts. Surprisingly, the microstructure of the gels does not change upon salt addition, demonstrating that the Hofmeister effect provides an excellent route to change the mechanical properties without distorting other influential parameters of the gel.     

Salt‐Assisted Toughening of Protein Hydrogel with Controlled Degradation for Bone Regeneration

Recently, strong polymer‐based hydrogels have been intensively investigated. However, the development of tough protein hydrogels with controlled degradation for bone regeneration has rarely been reported. Here, regenerated silk fibroin/gelatin (RSF/G) hydrogels with both strength and controlled degradation are prepared via physically and chemically double‐crosslinked networks. As a representative example, the 9%RSF/3%G hydrogel shows approximately 80% elongation and a compressive and tensile modulus of up to 0.25 and 0.21 MPa, respectively. It also shows a degradation rate that can be adjusted to approximately three months in vivo, a value between that of the rapidly degrading gelatin hydrogel and the slowly degrading RSF hydrogel. The 9%RSF/3%G hydrogel has good biocompatibility and promotes the proliferation and differentiation of bone marrow–derived stem cells compared with the control and pure RSF hydrogels. At 12 weeks after implantation of the gel in a calvarial defect, micro‐computed tomography shows greater bone volume and bone mineral density in the 9%RSF/3%G group. More importantly, histology reveals more mineralization and enhancements in the quality and rate of bone regeneration with less of a tissue response in the 9%RSF/3%G group. These results indicate the promising potential of this tough protein hydrogel with controlled degradation for bone regeneration applications.     

Super Bulk and Interfacial Toughness of Physically Crosslinked Double‐Network Hydrogels

Conventional design wisdom prevents both bulk and interfacial toughness to be presented in the same hydrogel, because the bulk properties of hydrogels are usually different from the interfacial properties of the same hydrogels on solid surfaces. Here, a fully‐physically‐linked agar (the first network)/poly(N ‐hydroxyethyl acrylamide) (pHEAA, the second network), where both networks are physically crosslinked via hydrogen bonds, is designed and synthesized. Bulk agar/pHEAA hydrogels exhibit high mechanical properties (2.6 MPa tensile stress, 8.0 tensile strain, 8000 J m^?2 tearing energy, 1.62 MJ m^?3 energy dissipation), high self‐recovery without any external stimuli (62%/30% toughness/stiffness recovery), and self‐healing property. More impressively, without any surface modification, agar/pHEAA hydrogels can be easily and physically anchored onto different nonporous solid substrates of glass, titanium, aluminum, and ceramics to produce superadhesive hydrogel–solid interfaces (i.e., high interfacial toughness of 2000–7000 J m^?2). Comparison of as‐prepared and swollen gels in water and hydrogen‐bond‐breaking solvents reveals that strong bulk toughness provides a structural basis for strong interfacial toughness, and both high toughness mainly stem from cooperative hydrogen bonds between and within two networks and between two networks and solid substrates. This work demonstrates a new gel system to achieve superhigh bulk and interfacial toughness on nonporous solid surfaces.     

Protein‐Release Behavior of Self‐Assembled PEG–β‐Cyclodextrin/PEG–Cholesterol Hydrogels

This paper reports on the degradation and protein release behavior of a self‐assembled hydrogel system composed of β‐cyclodextrin‐ (βCD) and cholesterol‐derivatized 8‐arm star‐shaped poly(ethylene glycol) (PEG₈). By mixing βCD‐ and cholesterol‐derivatized PEG₈ (molecular weights 10, 20 and 40 kDa) in aqueous solution, hydrogels with different rheological properties are formed. It is shown that hydrogel degradation is mainly the result of surface erosion, which depends on the network swelling stresses and initial crosslink density of the gels. This degradation mechanism, which is hardly observed for other water‐absorbing polymer networks, leads to a quantitative and nearly zero‐order release of entrapped proteins. This system therefore offers great potential for protein delivery.     

Tissue Engineering: Scaffold‐Mediated 2D Cellular Orientations for Construction of Three Dimensionally Engineered Tissues Composed of Oriented Cells and Extracellular Matrices (Adv. Funct. Mater. 7/2009)

Various hydrogels, such as poly(γ‐glutamic acid) (γ‐PGA), gelatin (GT), alginic acid (Alg), and agarose (Aga), with 3D interconnected and oriented fibrous pores (OP gels) are prepared for 3D polymeric cellular scaffolds by using silica fiber cloth (SC) as template. After the preparation of these hydrogels with the SC templates, the latter are subsequently removed by washing with hydrofluoric acid solution. Scanning electron microscopy (SEM) clearly shows OP structures in the hydrogels. These various types of OP gels are successfully prepared in this way, independently of the crosslinking mechanism, such as chemical (γ‐PGA or GT), coordinate‐bonded (Alg), or hydrogen‐bonded (Aga) crosslinks. SEM, confocal laser scanning microscopy, and histological evaluations clearly demonstrate that mouse L929 fibroblast cells adhere to and extend along these OP structures on/in γ‐PGA hydrogels during 3D cell culture. The L929 cells that adhere on/in the oriented hydrogel are viable and proliferative. Furthermore, 3D engineered tissues, composed of the oriented cells and extracellular matrices (ECM) produced by the cells, are constructed in vitro by subsequent decomposition of the hydrogel with cysteine after 14 days of cell culture. This novel technology to fabricate 3D‐engineered tissues, consisting of oriented cells and ECM, will be useful for tissue engineering.     

Alginate‐Boronic Acid: pH‐Triggered Bioinspired Glue for Hydrogel Assembly

The development of bioadhesives has become an emerging research field for tissue sealants, wound dressings, and hemostatic agents. However, assembling hydrogels using bioadhesive‐mediated attachment remains a challenging task. Significantly high water content (>90%) in hydrogels compared to that of biological tissues is the main cause of failure. Considering that hydrogels are primary testing scaffolds mimicking in vivo environments, developing strategies to assemble hydrogels that exhibit diverse properties is important. Self‐healing gels have been reported, but such gels often lack biocompatibility, and two gel pieces should be identical in chemistry for assembly, thus not allowing co‐existence of diverse biological environments. Herein, a mussel‐mimetic cis‐diol‐based adhesive, alginate‐boronic acid, that exhibits pH‐responsive curing from a viscoelastic solution to soft gels is developed. Associated mechanisms are that 1) polymeric diffusion occurs at interfaces utilizing intrinsic high water content; 2) the conjugated cis‐diols strongly interact/entangle with hydrogel chains; 3) curing processes begin by a slight increase in pH, resulting in robust attachment of diverse types of hydrogel building blocks for assembly. The findings obtained with alginate‐boronic acid glues suggest a rational design principle to attach diverse hydrogel building blocks to provide platforms mimicking in vivo environments.     

Evolution‐Based Design of an Injectable Hydrogel

A new class of simple, linear, amphiphilic peptides are developed that have the ability to undergo triggered self‐assembly into self‐supporting hydrogels. Under non‐gelling aqueous conditions, these peptides exist in a random coil conformation and peptide solutions have the viscosity of water. On the addition of a buffered saline solution, the peptides assemble into a β‐sheet rich network of fibrils, ultimately leading to hydrogelation. A family of nine peptides is prepared to study the influence of peptide length and amino acid composition on the rate of self‐assembly and hydrogel material properties. The amino acid composition is modulated by varying residue hydrophobicity and hydrophilicity on the two opposing faces of the amphiphile. The conformation of peptides in their soluble and gel state is studied by circular dichroism (CD), while the resultant material properties of their gels is investigated using oscillatory sheer rheology. One weight percent gels formed under physiological conditions have storage modulus (G′) values that vary from ≈20 to ≈800 Pa, with sequence length and hydrophobic character playing a dominant roll in defining hydrogel rigidity. Based on the structural and functional data provided by the nine‐peptide family members, an optimal sequence, namely LK13, is evolved. LK13 (LKLKLKLKLKLKL‐NH₂) undergoes triggered self‐assembly, affording the most rigid gel of those studied (G′=797 ± 105). It displays shear thin‐recovery behavior, allowing its delivery by syringe and is cytocompatibile as assessed with murine C3H10t1/2 mesenchymal stem cells.     

Dipole–Dipole and H‐Bonding Interactions Significantly Enhance the Multifaceted Mechanical Properties of Thermoresponsive Shape Memory Hydrogels

High strength hydrogels were previously constructed based on dipole–dipole and hydrogen bonding reinforcement. In spite of the high tensile and compressive strengths achieved, the fracture energy of the hydrogels strengthened with sole noncovalent bondings was rather low due to the lack in energy dissipating mechanism. In this study, combined dipole–dipole and hydrogen bonding interactions reinforced (DHIR) hydrogels are synthesized by one‐step copolymerization of three feature monomers, namely acrylonitrile (AN, dipole monomer), acrylamide (AAm, H‐bonding monomer), and 2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid (AMPS, anionic monomer) in the presence of PEGDA575, a hydrophilic crosslinker. The electrostatic repulsion from PAMPS allows the gel network to absorb water readily, and meanwhile the synergistic effect of dipole–dipole and H‐bonding interactions enable the DHIR hydrogel to withstand up to 8.3 MPa tensile stress, 4.8 MPa compressive stress and 140–716% elongation at break with the fracture energy reaching as high as 5500 J/m². In addition, this DHIR hydrogel exhibits reversible mechanical properties after undergoing cyclic loading and unloading. Interestingly, the DHIR hydrogels with appropriate compositions demonstrate temperature‐tunable mechanical properties as well as accompanied shape memory effect. The dual noncovalent bonding strengthening mechanism reported here offers a universal strategy for significantly enhancing the comprehensive mechanical properties of hydrogels.     

A Novel Design Strategy for Fully Physically Linked Double Network Hydrogels with Tough,Fatigue Resistant,and Self‐Healing Properties

Double network (DN) hydrogels with two strong asymmetric networks being chemically linked have demonstrated their excellent mechanical properties as the toughest hydrogels, but chemically linked DN gels often exhibit negligible fatigue resistance and poor self‐healing property due to the irreversible chain breaks in covalent‐linked networks. Here, a new design strategy is proposed and demonstrated to improve both fatigue resistance and self‐healing property of DN gels by introducing a ductile, nonsoft gel with strong hydrophobic interactions as the second network. Based on this design strategy, a new type of fully physically cross‐linked Agar/hydrophobically associated polyacrylamide (HPAAm) DN gels are synthesized by a simple one‐pot method. Agar/HPAAm DN gels exhibit excellent mechanical strength and high toughness, comparable to the reported DN gels. More importantly, because the ductile and tough second network of HPAAm can bear stress and reconstruct network structure, Agar/HPAAm DN gels also demonstrate rapid self‐recovery, remarkable fatigue resistance, and notable self‐healing property without any external stimuli at room temperature. In contrast to the former DN gels in both network structures and underlying association forces, this new design strategy to prepare highly mechanical DN gels provides a new avenue to better understand the fundamental structure‐property relationship of DN hydrogels, thus broadening current hydrogel research and applications.     

Stimuli-Responsive Peptide Self-Assembly to Construct Hydrogels with Actuation and Shape Memory Behaviors

Hydrogel actuators, capable of generating reversible deformation in response to external stimulus, are widely considered as new emerging intelligent materials for applications in soft robots, smart sensors, artificial muscles, and so on. Peptide self-assembly is widely applied in the construction of intelligent hydrogel materials due to their excellent stimulus response. However, hydrogel actuators based on peptide self-assembly are rarely reported and explored. In this study, a pH-responsive peptide (MA-FIID) is designed and introduced into a poly(N-isopropyl acrylamide) backbone (PNIPAM) to construct bilayer and heterogeneous hydrogel actuators based on the assembly and disassembly of peptide molecules under different pH conditions. These peptide-containing hydrogel actuators can perform controllable bending, bucking, and complex deformation under pH stimulation. Meanwhile, the Hofmeister effect of PNIPAM hydrogels endows these peptide-containing hydrogels with enhanced mechanical strength, ionic stimulus response (CaCl₂), and excellent shape-memory property. This work broadens the application of supramolecular self-assembly in the construction of intelligent hydrogels, and also provides new inspirations for peptide self-assembly to construct smart materials.     

Dual Physical Crosslinking Strategy to Construct Moldable Hydrogels with Ultrahigh Strength and Toughness

A dual physical crosslinking (DPC) strategy is used to construct moldable hydrogels with ultrahigh strength and toughness. First, polyelectrolyte complex (PEC) hydrogels are prepared through the in situ polymerization of acrylic acid monomers in the concentrated chitosan (Ch) solutions. Subsequently, Ag⁺ ions are introduced into the PEC hydrogels to form coordination bonds between ? NH₂ and ? COOH groups. High‐density electrostatic interaction and coordination bonds endow the DPC hydrogels with high strength and toughness. The mechanical properties of the DPC hydrogels strongly depend on the weight ratio of Ch to poly(acrylic acid) and the loading concentration of Ag⁺ ions. When the loading concentration of Ag⁺ ions is in the range of 1.0–1.5 mol L^?1, DPC 0.10–0.25 hydrogels display the maximum tensile strength (24.0 MPa) and toughness (84.7 MJ m^?3) with a strain of more than 600%. Specially, the DPC hydrogels display an excellent moldable behavior due to the reversible properties of the electrostatic interaction and coordination bonds. The DPC strategy can also be applied in various other systems and opens a new avenue to fabricate hydrogels with outstanding mechanical properties and antibacterial activities.     

Rapid Gelation of Tough and Anti-Swelling Hydrogels under Mild Conditions for Underwater Communication

Swelling is ubiquitous for conventional hydrogels but is not favorable for many situations, especially underwater applications. In this study, an anti-swelling and mechanically robust polyacrylic acid (PAAc)/gelatin composite hydrogel is reported with a rapid gelation process (10¹ s) under mild conditions via the synergy of MXene-activated initiation and zirconium ion (Zr⁴⁺)-induced cross-linking, without the requirement of external energy input. The MXene is found efficient to activate the chain initiation, while the Zr⁴⁺ is prone indispensable for facilitating the cross-linking of formed polymer chains. The resulting hydrogel exhibits integration of exceptional anti-swelling properties and high mechanical performance at room temperature, thanks to the dense hydrogen bonds between PAAc and gelatin chains that enable an upper critical solution temperature above room temperature. Also, desirable electrical conductivity emerges in the hydrogel due to the simultaneous contribution of MXene and Zr⁴⁺, allowing stable electrical signal output of the gel upon deformation underwater. As a demonstration, an underwater communicator by harnessing the gel as a sensing module is assembled, which is capable of wirelessly delivering messages to the decoder on the ground via Morse codes. This study provides an exemplary way for the rapid gelation of tough and anti-swelling hydrogels for durable underwater applications.     

Cationic Hybrid Hydrogels from Amino‐Acid‐Based Poly(ester amide): Fabrication,Characterization, and Biological Properties

A family of biodegradable, biocompatible, water soluble cationic polymer precursor, arginine‐based unsaturated poly (ester amide) (Arg‐UPEA), is reported. Its incorporation into conventional Pluronic diacrylate (Pluronic‐DA) to form hybrid hydrogels for a significant improvement of the biological performance of current synthetic hydrogels is shown. The gel fraction (G_f), equilibrium swelling ratio (Q_eq), compressive modulus, and interior morphology of the hybrid hydrogels as well as their interactions with human fibroblasts and bovine endothelial cells are fully investigated. It is found that the incorporation of Arg‐UPEA into Pluronic‐DA hydrogels significantly changes their Q_eq, mechanical strength, and interior morphology. The structure–property relationship of the newly fabricated hybrid hydrogels is studied in terms of the chemical structure of the Arg‐UPEA precursor, i.e., the number of methylene groups in the Arg‐UPEA repeating unit. The results indicate that increasing methylene groups in the Arg‐UPEA repeating unit increases Q_eq and decreases the compressive modulus of hydrogels. When compared with a pure Pluronic hydrogel, the cationic Arg‐UPEAs/Pluronic hybrid hydrogels greatly improve the attachment and proliferation of human fibroblasts on hydrogel surfaces. A bovine aortic endothelial cells (BAEC) viability test in the interior of the hydrogels shows that the positively charged hybrid hydrogels can significantly improve the viability of the encapsulated endothelial cell over a 2 week study period when compared with a pure Pluronic hydrogel.     

Hydrogel Scaffolds of Amphiphilic and Acidic β‐Sheet Peptides

Amphiphilic and acidic β‐sheet‐forming peptides (AAβPs) having the sequence Pro‐Y‐(Z‐Y)₅‐Pro, Y = Glu or Asp and Z = Phe or Leu may assemble into hydrogel structures at near neutral pH values, several units higher than the intrinsic pKa of their acidic amino acid side chains. The bottom‐to‐top design strategy enables the rationally supported association between the peptides' amino acids composition and bulk pH hydrogelation. Hydrogen bonds between the acidic amino acids side chains in the β‐sheet structure are found to contribute substantially to the stabilization of AAβPs hydrogels. The negatively charged peptides are also found to form gels at lower concentration in presence of calcium ions. Bone forming cells may be cultured on two‐dimensional films of AAβPs hydrogels that form at physiological pH values as well as within three dimensional hydrogel matrices. These acidic‐rich peptides hydrogels may become advantageous in applications related to engineering of mineralized tissues providing controllable, multifunctional calcified scaffolds to affect both the biological activity and the inorganic mineralization.     

Cytocompatible Hydrogels Based on Photocrosslinkable Methacrylated O‐Carboxymethylchitosan with Tunable Charge: Synthesis and Characterization

Ionic hydrogels are attractive for the protection, delivery and controlled release of charged biomacromolecules such as proteins, growth factors or DNA. We have prepared and characterized a series of photocrosslinked anionic hydrogels based on water soluble methacrylated (MA) O‐carboxymethylchitosan (OCMCS) and polyethylene glycol (PEG) diacrylate. OCMCS samples with varying degree of substitution of carboxymethyl group ranging from 0.69 to 1.86 were prepared by reacting native chitosan with different amounts of monochloroacetic acid. The OCMCS products demonstrated differences in solubility, zeta potential (–52.7 to –12.8 mV) and thermal decomposition temperature (260 to 283 °C). The OCMCS products were then reacted with glycidyl methacrylate to make ultra‐violet (UV) crosslinkable OCMCS‐MAs which were blended with PEG diacrylate, a photoinitiator and water and successfully photocrosslinked to create OCMCS‐MA/PEG hydrogels. Water uptake of the hydrogels varied between 226 % to 358 % and the porous structures of the prepared OCMCS‐MA/PEG hydrogels could be modulated by the degree of methacrylation. All the OCMCS‐MA/PEG hydrogel substrates similarly supported attachment and proliferation of Smooth Muscle Cells (SMCs). The in vitro biodegradation of these hydrogels, in the presence of SMCs, could be controlled by the degree of methacrylation; weight loss after 9‐week was (15±1) % and (19±2) % using OCMCS 4‐MA (12.7 % MA) and OCMCS 1‐MA (4.6 % MA), respectively. In addition, the hydrogel based on the most anionic OCMCS 1 showed higher adsorption of basic TGF‐β₁ than similarly modified ‐agarose, ‐dextran, and ‐chondroitin sulfate hydrogels.     

Injectable Polypeptide‐Protein Hydrogels for Promoting Infected Wound Healing

Protein is the key composition of all tissues, which has also been widely used in tissue engineering due to its superior biocompatibility and low immunogenicity. However, natural protein usually lacks active functions such as vascularization, osteo‐induction, and neural differentiation, which limits its further applications as a functional biomaterial. Here, based on the mimetic extracellular matrix feature of bovine serum albumin, injectable polypeptide‐protein hydrogels with vascularization and antibacterial abilities are constructed successfully via coordinative cross‐linking of sulfydryl groups with silver ions (Ag+) for the regeneration of infected wound. In this protein hydrogel system, (Ag+), acting as crosslinkers, can not only provide a sterile microenvironment and a strong and robust antibacterial ability but also introduce K₂(SL)₆K₂ (KK) polypeptide, which endows the hydrogel with vascularization behavior. Furthermore, the in vivo data show that the polypeptide‐protein hydrogel has a considerable collagen deposition and vascularization abilities in the early stage of wound healing, resulting in rapid new tissue regeneration featured with newly appeared hair follicles. Altogether, this newly developed multifunctional 3D polypeptide‐protein hydrogel with vascularization, antibacterial properties, and hair follicle promotion can be a promising approach in biomedical fields such as infected wound healing.     

Facile One‐Step Micropatterning Using Photodegradable Gelatin Hydrogels for Improved Cardiomyocyte Organization and Alignment

Hydrogels are often employed as temporary platforms for cell proliferation and tissue organization in vitro. Researchers have incorporated photodegradable (PD) moieties into synthetic polymeric hydrogels as a means of achieving spatiotemporal control over material properties. In this study protein‐based PD hydrogels composed of methacrylated gelatin and a crosslinker containing o‐nitrobenzyl ester groups are developed. The hydrogels are able to degrade rapidly and specifically in response to UV light and can be photopatterned to a variety of shapes and dimensions in a one‐step process. Micropatterned PD hydrogels are shown to improve cell distribution, alignment, and beating regularity of cultured neonatal rat cardiomyocytes. Overall this work introduces a new class of PD hydrogel based on natural and biofunctional polymers as cell culture substrates for improving cellular organization and function.     

Achieving Swollen yet Strengthened Hydrogels by Reorganizing Multiphase Network Structure

Many living tissues, such as muscle, become mechanically stronger with growth. Yet, synthetic hydrogels usually exhibit an opposite size-mechanical property relation, that is, swelling-weakening behavior. Herein, a series of swollen yet strengthened polyampholyte (PA) hydrogels are developed via a simple metal-ion solution soaking strategy. In this strategy, a dynamic PA hydrogel (with ionic bonds) is dialyzed in ZrOCl₂ solutions (Step-I) and deionized water (Step-II) successively to obtain equilibrated hydrogels. Due to the specific Zr⁴⁺ ions and PA network structure, Step-I takes several months with sample size and mechanical performance increasing continuously, while Step-II only needs several days. Through this strategy, the resultant hydrogel networks are reorganized and eventually constructed by ionic and metal ligand bonds, enabling the swelling yet strengthening behavior. A systematic study confirms that dialysis time in Step-I and corresponding ZrOCl₂ concentration can significantly affect the multiphase microstructures of the hydrogels, resulting in different mechanical enhancements. The optimized hydrogel possesses 39.2 MPa of Young's modulus and 3.7 MPa of tensile strength, which are 302 and 5.5 times these of the original PA gel, respectively. Despite distinct swelling, these hydrogels still mechanically surpass many existing high-performance hydrogels. This study opens a novel pathway for fabricating swollen yet strengthened hydrogels.     

Direct 3D Printing of High Strength Biohybrid Gradient Hydrogel Scaffolds for Efficient Repair of Osteochondral Defect

The emerging 3D printing technique allows for tailoring hydrogel‐based soft structure tissue scaffolds for individualized therapy of osteochondral defects. However, the weak mechanical strength and uncontrollable swelling intrinsic to conventional hydrogels restrain their use as bioinks. Here, a high‐strength thermoresponsive supramolecular copolymer hydrogel is synthesized by one‐step copolymerization of dual hydrogen bonding monomers, N‐acryloyl glycinamide, and N‐[tris(hydroxymethyl)methyl] acrylamide. The obtained copolymer hydrogels demonstrate excellent mechanical properties—robust tensile strength (up to 0.41 MPa), large stretchability (up to 860%), and high compressive strength (up to 8.4 MPa). The rapid thermoreversible gel ? sol transition behavior makes this copolymer hydrogel suitable for direct 3D printing. Successful preparation of 3D‐printed biohybrid gradient hydrogel scaffolds is demonstrated with controllable 3D architecture, owing to shear thinning property which allows continuous extrusion through a needle and also immediate gelation of fluid upon deposition on the cooled substrate. Furthermore, this biohybrid gradient hydrogel scaffold printed with transforming growth factor beta 1 and β‐tricalciumphosphate on distinct layers facilitates the attachment, spreading, and chondrogenic and osteogenic differentiation of human bone marrow stem cells (hBMSCs) in vitro. The in vivo experiments reveal that the 3D‐printed biohybrid gradient hydrogel scaffolds significantly accelerate simultaneous regeneration of cartilage and subchondral bone in a rat model.     

Self‐Assembled Injectable Nanocomposite Hydrogels Stabilized by Bisphosphonate‐Magnesium (Mg2+) Coordination Regulates the Differentiation of Encapsulated Stem Cells via Dual Crosslinking

Nanocomposite hydrogels consist of a polymer matrix embedded with nanoparticles (NPs), which provide the hydrogels with unique bioactivities and mechanical properties. Incorporation of NPs via in situ precipitation in the polymer matrix further enhances these desirable hydrogel properties. However, the noncytocompatible pH, osmolality, and lengthy duration typically required for such in situ precipitation strategies preclude cell encapsulation in the resultant hydrogels. Bisphosphonate (BP) exhibits a variety of specific bioactivities and excellent binding affinity to multivalent cations such as magnesium ions (Mg²⁺). Here, the preparation of nanocomposite hydrogels via self‐assembly driven by bisphosphonate‐Mg²⁺ coordination is described. Upon mixing solutions of polymer bearing BPs, BP monomer (Ac‐BP), and Mg²⁺, this effective and dynamic coordination leads to the rapid self‐assembly of Ac‐BP‐Mg NPs which function as multivalent crosslinkers stabilize the resultant hydrogel structure at physiological pH. The obtained nanocomposite hydrogels are self‐healing and exhibit improved mechanical properties compared to hydrogels prepared by blending prefabricated NPs. Importantly, the hydrogels in this study allow the encapsulation of cells and subsequent injection without compromising the viability of seeded cells. Furthermore, the acrylate groups on the surface of Ac‐BP‐Mg NPs enable facile temporal control over the stiffness and crosslinking density of hydrogels via UV‐induced secondary crosslinking, and it is found that the delayed introduction of this secondary crosslinking enhances cell spreading and osteogenesis.