Strong and Biostable Hyaluronic Acid-Calcium Phosphate Nanocomposite Hydrogel via in Situ Precipitation Process

A 3D-Printed Biomaterial Scaffold Reinforced with Inorganic Fillers for Bone Tissue Engineering: In Vitro Assessment and In Vivo Animal Studies

This research aimed to substantiate the potential practicality of utilizing a matrix-like platform, a novel 3D-printed biomaterial scaffold, to enhance and guide host cells' growth for bone tissue regeneration. The 3D biomaterial scaffold was successfully printed using a 3D Bioplotter^® (EnvisionTEC, GmBH) and characterized. Osteoblast-like MG63 cells were utilized to culture the novel printed scaffold over a period of 1, 3, and 7 days. Cell adhesion and surface morphology were examined using scanning electron microscopy (SEM) and optical microscopy, while cell viability was determined using MTS assay and cell proliferation was evaluated using a Leica microsystem (Leica MZ10 F). The 3D-printed biomaterial scaffold exhibited essential biomineral trace elements that are significant for biological bone (e.g., Ca-P) and were confirmed through energy-dispersive X-ray (EDX) analysis. The microscopy analyses revealed that the osteoblast-like MG63 cells were attached to the printed scaffold surface. The viability of cultured cells on the control and printed scaffold increased over time (p < 0.05); however, on respective days (1, 3, and 7 days), the viability of cultured cells between the two groups was not significantly different (p > 0.05). The protein (human BMP-7, also known as growth factor) was successfully attached to the surface of the 3D-printed biomaterial scaffold as an initiator of osteogenesis in the site of the induced bone defect. An in vivo study was conducted to substantiate if the novel printed scaffold properties were engineered adequately to mimic the bone regeneration cascade using an induced rabbit critical-sized nasal bone defect. The novel printed scaffold provided a potential pro-regenerative platform, rich in mechanical, topographical, and biological cues to guide and activate host cells toward functional regeneration. The histological studies revealed that there was progress in new bone formation, especially at week 8 of the study, in all induced bone defects. In conclusion, the protein (human BMP-7)-embedded scaffolds showed higher regenerative bone formation potential (week 8 complete) compared to the scaffolds without protein (e.g., growth factor; BMP-7) and the control (empty defect). At 8 weeks postimplantation, protein (BMP-7) significantly promoted osteogenesis as compared to other groups. The scaffold underwent gradual degradation and replacement by new bones at 8 weeks in most defects.

Microcarriers in application for cartilage tissue engineering: Recent progress and challenges

Successful regeneration of cartilage tissue at a clinical scale has been a tremendous challenge in the past decades. Microcarriers (MCs), usually used for cell and drug delivery, have been studied broadly across a wide range of medical fields, especially the cartilage tissue engineering (TE). Notably, microcarrier systems provide an attractive method for regulating cell phenotype and microtissue maturations, they also serve as powerful injectable carriers and are combined with new technologies for cartilage regeneration. In this review, we introduced the typical methods to fabricate various types of microcarriers and discussed the appropriate materials for microcarriers. Furthermore, we highlighted recent progress of applications and general design principle for microcarriers. Finally, we summarized the current challenges and promising prospects of microcarrier-based systems for medical applications. Overall, this review provides comprehensive and systematic guidelines for the rational design and applications of microcarriers in cartilage TE.

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This review summarized fabrication techniques and cartilage repaired application of microcarriers.

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The appropriate materials and design principle for microcarriers in cartilage tissue engineering are discussed.

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Promising future perspectives and challenges in microcarriers fields are outlined.

Magnesium Oxide Nanoparticle Coordinated Phosphate-Functionalized Chitosan Injectable Hydrogel for Osteogenesis and Angiogenesis in Bone Regeneration

Natural polysaccharide (NPH)-based injectable hydrogels have shown great potential for critical-sized bone defect repair. However, their osteogenic, angiogenic, and mechanical properties are insufficient. Here, MgO nanoparticles (NPs) were incorporated into a newly synthesized water-soluble phosphocreatine-functionalized chitosan (CSMP) water solution to form an injectable hydrogel (CSMP-MgO) via supramolecular combination between phosphate groups in CSMP and magnesium in MgO NPs to circumvent these drawbacks of chitosan-based injectable hydrogels. Water-soluble chitosan deviate CSMP was first synthesized by grafting methacrylic anhydride and phosphocreatine into a chitosan chain in a one-step lyophilization process. The phosphocreatine in this hydrogel not only provides sites to combine with MgO NPs to form supramolecular binding but also serves as the reservoir to control Mg²⁺ release. As a result, the lyophilized CSMP-MgO hydrogels presented a porous structure with some small holes in the pore wall, and the pore diameters ranged from 50 to 100 μm. The CSMP-MgO injectable hydrogels were restricted from swelling in DI water (lowest swelling ratio was 16.0 ± 1.1 g/g) and presented no brittle failure during compression even at a strain above 85% (maximum compressive strength was 195.0 kPa) versus the control groups (28.0 and 41.3 kPa for CSMP and CSMP-MgO (0.5) hydrogels), with regulated Mg²⁺ release in a stable and sustained manner. The CSMP-MgO injectable hydrogels promoted in vitro calcium phosphate (hydroxyapatite (HA) and tetracalcium phosphate (TTCP)) deposition in supersaturated calcium phosphate solution and presented no cytotoxicity to MC3T3-E1 cells; the CSMP-MgO hydrogel promoted MC3T3-E1 cell osteogenic differentiation with upregulation of BSP, OPN, and Osterix osteogenic gene expression and mineralization and HUVEC tube formation. Among them, CSMP-MgO (5) presented most of these properties. Moreover, this hydrogel (CSMP-MgO (5)) showed an excellent ability to promote new bone formation in critical-sized calvarial defects in rats. Thus, the CSMP-MgO injectable hydrogel shows great promise for bone regeneration.

In situ mineralized PLGA/zwitterionic hydrogel composite scaffold enables high-efficiency rhBMP-2 release for critical-sized bone healing

Osteoconductive and osteoinductive scaffolds are highly desirable for functional restoration of large bone defects. Here, we report an in situ mineralized poly(lactic-co-glycolic acid)/poly[2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide hydrogel (PLGA/PSBMA) scaffold as a novel high-efficiency carrier for recombinant human bone morphogenetic protein-2 (rhBMP-2) for bone tissue regeneration. By virtue of the oppositely charged structure, the zwitterionic PSBMA component is able to template well-integrated dense mineralization of calcium phosphate throughout the PLGA/PSBMA scaffold. The high affinity between rhBMP-2 and the mineralized matrix, combined with the capability of the zwitterionic hydrogel to sequester and to enable sustained release of ionic proteins, endows the mineralized PLGA/PSBMA scaffolds with high-efficiency sustained release of rhBMP-2 (only 1.7% release within 35 days), thus enabling robust healing of critical-sized (5 mm) nonunion calvarial defects in rats at an ultralow dosage of rhBMP-2 (150 ng per scaffold), at which level successful healing of critical-sized bone defects has never been reported. These findings show that the mineralized PLGA/PSBMA scaffold is promising for bone defect repair.

Hyaluronic-Acid-Based Organic-Inorganic Composites for Biomedical Applications

Applications of natural hyaluronic acid (HYH) for the fabrication of organic-inorganic composites for biomedical applications are described. Such composites combine unique functional properties of HYH with functional properties of hydroxyapatite, various bioceramics, bioglass, biocements, metal nanoparticles, and quantum dots. Functional properties of advanced composite gels, scaffold materials, cements, particles, films, and coatings are described. Benefiting from the synergy of properties of HYH and inorganic components, advanced composites provide a platform for the development of new drug delivery materials. Many advanced properties of composites are attributed to the ability of HYH to promote biomineralization. Properties of HYH are a key factor for the development of colloidal and electrochemical methods for the fabrication of films and protective coatings for surface modification of biomedical implants and the development of advanced biosensors. Overcoming limitations of traditional materials, HYH is used as a biocompatible capping, dispersing, and structure-directing agent for the synthesis of functional inorganic materials and composites. Gel-forming properties of HYH enable a facile and straightforward approach to the fabrication of antimicrobial materials in different forms. Of particular interest are applications of HYH for the fabrication of biosensors. This review summarizes manufacturing strategies and mechanisms and outlines future trends in the development of functional biocomposites.

Injectable silanized hyaluronic acid hydrogel/biphasic calcium phosphate granule composites with improved handling and biodegradability promote bone regeneration in rabbits

Biphasic calcium phosphate (BCP) granules are osteoconductive biomaterials used in clinics to favor bone reconstruction. Yet, poor cohesivity, injectability and mechanical properties restrain their use as bone fillers. In this study, we incorporated BCP granules into in situ forming silanized hyaluronic acid (Si-HA) and hydroxypropylmethylcellulose (Si-HPMC) hydrogels. Hydrogel composites were shown to be easily injectable (F < 30 N), with fast hardening properties (<5 min), and similar mechanical properties (E∼ 60 kPa). In vivo, both hydrogels were well tolerated by the host, but showed different biodegradability with Si-HA gels being partially degraded after 21d, while Si-HPMC gels remained stable. Both composites were easily injected into critical size rabbit defects and remained cohesive. After 4 weeks, Si-HPMC/BCP led to poor bone healing due to a lack of degradation. Conversely, Si-HA/BCP composites were fully degraded and beneficially influenced bone regeneration by increasing the space available for bone ingrowth, and by accelerating BCP granules turnover. Our study demonstrates that the degradation rate is key to control bone regeneration and that Si-HA/BCP composites are promising biomaterials to regenerate bone defects.

Constructing a biomimetic nanocomposite with the in situ deposition of spherical hydroxyapatite nanoparticles to induce bone regeneration

Inspired by the nanostructure of bone, biomimetic nanocomposites comprising natural polymers and inorganic nanoparticles have gained much attention for bone regenerative applications. However, the mechanical and biological performances of nanocomposites are largely limited by the inhomogeneous distribution, uncontrolled size and irregular morphology of inorganic nanoparticles at present. In this work, an innovative in situ precipitation method has been developed to construct a biomimetic nanocomposite which consists of spherical hydroxyapatite (HA) nanoparticles and gelatin (Gel). The homogeneous dispersion of HA nanoparticles in nHA-Gel endowed it with a low swelling ratio, enhanced mechanical properties and slow degradation. Moreover, strontium (Sr) was incorporated into HA nanoparticles to further enhance the bioactivity of nanocomposites. In vitro experiments suggested that nHA-Gel and Sr-nHA-Gel facilitated cell spreading and promoted osteogenic differentiation of bone-marrow-derived mesenchymal stem cells (BMSCs) as compared to pure Gel and mHA-Gel conventional composites developed by mechanical mixing. In vivo rat critical-sized calvarial defect repair further confirmed that nHA-Gel and Sr-nHA-Gel possessed relatively effective bone regenerative abilities among the four groups. Collectively, the biomimetic nanocomposites of nHA-Gel and Sr-nHA-Gel have good efficacy in inducing bone regeneration and would be a promising alternative to bone grafts for clinical applications.

Interface design of carbon filler/polymer composites for electromagnetic interference shielding

The main three methods of interface design for carbon/polymer composites for different carbon materials.

Hyaluronic acid and chondroitin sulfate (meth)acrylate-based hydrogels for tissue engineering: Synthesis, characteristics and pre-clinical evaluation

Hydrogels based on photocrosslinkable Hyaluronic Acid Methacrylate (HAMA) and Chondroitin Sulfate Methacrylate (CSMA) are presently under investigation for tissue engineering applications. HAMA and CSMA gels offer tunable characteristics such as tailorable mechanical properties, swelling characteristics, and enzymatic degradability. This review gives an overview of the scientific literature published regarding the pre-clinical development of covalently crosslinked hydrogels that (partially) are based on HAMA and/or CSMA. Throughout the review, recommendations for the next steps in clinical translation of hydrogels based on HAMA or CSMA are made and potential pitfalls are defined. Specifically, a myriad of different synthetic routes to obtain polymerizable hyaluronic acid and chondroitin sulfate derivatives are described. The effects of important parameters such as degree of (meth)acrylation and molecular weight of the synthesized polymers on the formed hydrogels are discussed and useful analytical techniques for their characterization are summarized. Furthermore, the characteristics of the formed hydrogels including their enzymatic degradability are discussed. Finally, a summary of several recent applications of these hydrogels in applied fields such as cartilage and cardiac regeneration and advanced tissue modelling is presented.

Physical and Chemical Factors Influencing the Printability of Hydrogel-based Extrusion Bioinks

Bioprinting researchers agree that "printability" is a key characteristic for bioink development, but neither the meaning of the term nor the best way to experimentally measure it has been established. Furthermore, little is known with respect to the underlying mechanisms which determine a bioink's printability. A thorough understanding of these mechanisms is key to the intentional design of new bioinks. For the purposes of this review, the domain of printability is defined as the bioink requirements which are unique to bioprinting and occur during the printing process. Within this domain, the different aspects of printability and the factors which influence them are reviewed. The extrudability, filament classification, shape fidelity, and printing accuracy of bioinks are examined in detail with respect to their rheological properties, chemical structure, and printing parameters. These relationships are discussed and areas where further research is needed, are identified. This review serves to aid the bioink development process, which will continue to play a major role in the successes and failures of bioprinting, tissue engineering, and regenerative medicine going forward.

3D Freeform Printing of Nanocomposite Hydrogels through in situ Precipitation in Reactive Viscous Fluid

Composite hydrogels have gained great attention as three-dimensional (3D) printing biomaterials because of their enhanced intrinsic mechanical strength and bioactivity compared to pure hydrogels. In most conventional printing methods for composite hydrogels, particles are preloaded in ink before printing, which often reduces the printability of composite ink with little mechanical improvement due to poor particle-hydrogel interaction of physical mixing. In contrast, the in situ incorporation of nanoparticles into a hydrogel during 3D printing achieves uniform distribution of particles with remarkable mechanical reinforcement, while precursors dissolved in inks do not influence the printing process. Herein, we introduced a "printing in liquid" technique coupled with a hybridization process, which allows 3D freeform printing of nanoparticle-reinforced composite hydrogels. A viscoplastic matrix for this printing system provides not only support for printed hydrogel filaments but also chemical reactants to induce various reactions in printed objects for in situ modification. Nanocomposite hydrogel scaffolds were successfully fabricated through this 3D freeform printing of hyaluronic acid (HAc)-alginate (Alg) hydrogel inks through a two-step crosslinking strategy. The first ionic crosslinking of Alg provided structural stability during printing, while the secondary crosslinking of photo-curable HAc improved the mechanical and physiological stability of the nanocomposite hydrogels. For in situ precipitation during 3D printing, phosphate ions were dissolved in the hydrogel ink and calcium ions were added to the viscoplastic matrix. The composite hydrogels demonstrated a significant improvement in mechanical strength, biostability, as well as biological performance compared to pure HAc. Moreover, the multi-material printing of composites with different calcium phosphate contents was achieved by adjusting the ionic concentration of inks. Our method greatly accelerates the 3D printing of various functional or hybridized materials with complex geometries through the design and modification of printing materials coupled with in situ post-printing functionalization and hybridization in reactive viscoplastic matrices.

3D Freeform Printing of Nanocomposite Hydrogels through in situ Precipitation in Reactive Viscous Fluid

Composite hydrogels have gained great attention as three-dimensional (3D) printing biomaterials because of their enhanced intrinsic mechanical strength and bioactivity compared to pure hydrogels. In most conventional printing methods for composite hydrogels, particles are preloaded in ink before printing, which often reduces the printability of composite ink with little mechanical improvement due to poor particle-hydrogel interaction of physical mixing. In contrast, the in situ incorporation of nanoparticles into a hydrogel during 3D printing achieves uniform distribution of particles with remarkable mechanical reinforcement, while precursors dissolved in inks do not influence the printing process. Herein, we introduced a "printing in liquid" technique coupled with a hybridization process, which allows 3D freeform printing of nanoparticle-reinforced composite hydrogels. A viscoplastic matrix for this printing system provides not only support for printed hydrogel filaments but also chemical reactants to induce various reactions in printed objects for in situ modification. Nanocomposite hydrogel scaffolds were successfully fabricated through this 3D freeform printing of hyaluronic acid (HAc)-alginate (Alg) hydrogel inks through a two-step crosslinking strategy. The first ionic crosslinking of Alg provided structural stability during printing, while the secondary crosslinking of photo-curable HAc improved the mechanical and physiological stability of the nanocomposite hydrogels. For in situ precipitation during 3D printing, phosphate ions were dissolved in the hydrogel ink and calcium ions were added to the viscoplastic matrix. The composite hydrogels demonstrated a significant improvement in mechanical strength, biostability, as well as biological performance compared to pure HAc. Moreover, the multi-material printing of composites with different calcium phosphate contents was achieved by adjusting the ionic concentration of inks. Our method greatly accelerates the 3D printing of various functional or hybridized materials with complex geometries through the design and modification of printing materials coupled with in situ post-printing functionalization and hybridization in reactive viscoplastic matrices.

Polysaccharide-Based Systems for Targeted Stem Cell Differentiation and Bone Regeneration

Bone tissue engineering is an ever-changing, rapidly evolving, and highly interdisciplinary field of study, where scientists try to mimic natural bone structure as closely as possible in order to facilitate bone healing. New insights from cell biology, specifically from mesenchymal stem cell differentiation and signaling, lead to new approaches in bone regeneration. Novel scaffold and drug release materials based on polysaccharides gain increasing attention due to their wide availability and good biocompatibility to be used as hydrogels and/or hybrid components for drug release and tissue engineering. This article reviews the current state of the art, recent developments, and future perspectives in polysaccharide-based systems used for bone regeneration.

Enhanced biolubrication on biomedical devices using hyaluronic acid-silica nanohybrid hydrogels

In this work, highly lubricous hyaluronic acid-silica (HA-SiO₂) nanohybrid coatings were fabricated through a sequential process consisting of a sol-gel followed by electrophoretic deposition (EPD). SiO₂ nanoparticles were uniformly distributed in the coating layers, and the coating thickness was identified as approximately 1-2 μm regardless of the amount of SiO_2. Incorporation of SiO₂ into the HA polymer matrix enhanced the mechanical stability of the nanohybrid coatings, indicating greater interfacial bonding strength compared to HA coating layers alone. In addition, due to improved stability, the nanohybrid coatings showed excellent biolubrication properties, which were evaluated with a tribological experiment. These results indicate that the nanohybrid coatings have great potential to be used in biomedical applications that require superior biolubrication properties.

Coordination of Covalent Cross-Linked Gelatin Hydrogels via Oxidized Tannic Acid and Ferric Ions with Strong Mechanical Properties

The design of gelatin-based hydrogels with high mechanical strength, high gelation temperature, and a rapid self-healing property still presents a challenge to researchers. In the present study, single cross-linked gelatin-oxidized tannic acid (SC-GT/OTA) hydrogels were fabricated through covalent cross-linking between gelatin and tannic acid (TA) oxidized by using sodium periodate (NaIO₄). Double cross-linked gelatin-OTA-FeCl₃·6H₂O (DC-GT/OTA/Fe^III) hydrogels were also created using metal coordination bonds formed between the catechol groups present in OTA and Fe^III in ferric chloride. As a result, the maximum gelling temperature of the SC-GT/OTA hydrogel (37 °C) was considerably higher than that of the pure gelatin hydrogel (15.4 °C). Moreover, the maximum values of compressive stress of SC-GT/OTA hydrogels increased significantly by almost seven times the original value as the molar ratio of NaIO₄ to TA increased from 3:1 to 5:1. When the molar ratio of NaIO₄ to TA was maintained at the constant of 4:1, the storage modulus values of DC-GT/OTA/Fe^III hydrogels with the Fe^III-to-TA molar ratio of 1.5:1 were three to 4 orders of magnitude higher than those of SC-GT/OTA hydrogels in the whole angular frequency range. The double cross-linked gelatin hydrogels developed in this research can be used widely in agriculture and material science fields.

In situ-forming, mechanically resilient hydrogels for cell delivery

Injectable, in situ-forming hydrogels can improve cell delivery in tissue engineering applications by facilitating minimally invasive delivery to irregular defect sites and improving cell retention and survival. Tissues targeted for cell delivery often undergo diverse mechanical loading including high stress, high strain, and repetitive loading conditions. This review focuses on the development of hydrogel systems that meet the requirements of mechanical resiliency, cytocompatibility, and injectability for such applications. First, we describe the most important design considerations for maintaining the viability and function of encapsulated cells, for reproducing the target tissue morphology, and for achieving degradation profiles that facilitate tissue replacement. Models describing the relationships between hydrogel structure and mechanical properties are described, focusing on design principles necessary for producing mechanically resilient hydrogels. The advantages and limitations of current strategies for preparing cytocompatible, injectable, and mechanically resilient hydrogels are reviewed, including double networks, nanocomposites, and high molecular weight amphiphilic copolymer networks. Finally, challenges and opportunities are outlined to guide future research in this developing field.

Hyaluronic Acid-Based Hybrid Hydrogel Microspheres with Enhanced Structural Stability and High Injectability

For hydrogel injection applications, it is important to improve the strength and biostability of the hydrogel as well as its injectability to pass easily through the needle. Making gel microspheres is one approach to achieve these improvements. Granulization of a bulk hydrogel is a common procedure used to form microsized particles; however, the nonuniform size and shape cause an uneven force during injection, damaging the surrounding tissue and causing pain to the patients. In this study, injectable hyaluronic acid (HA)-based hybrid hydrogel microspheres were fabricated using a water-in-oil emulsion process. The injectability was significantly enhanced because of the relatively uniform size and spherical shape of the hydrogel formulates. In addition, the biostability and mechanical strength were also increased owing to the increased cross-linking density compared with that of conventionally fabricated gel microparticles. This tendency was further improved after in situ calcium phosphate precipitation. Our findings demonstrate the great potential of HA-based hydrogel microspheres for various clinical demands requiring injectable biomaterials.

Development of Self-Healing Double-Network Hydrogels: Enhancement of the Strength of Wheat Gluten Hydrogels by In Situ Metal-Catechol Coordination

Wheat gluten, a byproduct of the wheat starch industry, is widely used as a dough strengthener and gelling agent. In this research, we developed novel double-network hydrogels by gelation of gluten using in situ metal-catechol coordination. The first network consisted of physically associated gluten molecules, while the second network consisted of Fe³⁺-cross-linked proanthocyanidins (PACs). Dynamic shear rheology experiments suggested that coordination of Fe³⁺ and PACs greatly enhanced the mechanical properties of the gluten hydrogels. The double-network hydrogels exhibited a 3-fold higher shear modulus than pure gluten hydrogels. The formation of bis- and tris-catechol-Fe³⁺ complexes between Fe³⁺ and PACs in the hydrogels was confirmed by ultraviolet-visible spectrometry and isothermal titration calorimetry (ITC). The ITC measurements of Fe³⁺ binding to PACs indicated a molar stoichiometry of 1:4 and a dissociation constant ( K_D) of 24.9 × 10^-9. When subject to repeated shear deformation-compression cycles, the hydrogels exhibited strong and rapid recovery of their rheological properties. The strong, self-healing characteristics of the double-network gluten hydrogels produced in this study may be useful for certain applications in the food, agriculture, biomedicine, and tissue-engineering industries.

Characterizations and interfacial reinforcement mechanisms of multicomponent biopolymer based scaffold

It is difficult for a single component biopolymer to meet the requirements of scaffold at present. The development of multicomponent biopolymer based scaffold provides an effective method to solve the issue based on the advantages of each kind of the biomaterials. However, the compatibility between different components might be very poor due to the difficulties in forming strong interfacial bonding, and thereby significantly degrading the integrated mechanical properties of the scaffold. In recent years, interface phase introduction, surface modification and in situ growth have been the major strategies for enhancing interfacial bonding. This article presents a comprehensive overview on the research in the area of constructing multicomponent biopolymer based scaffold and reinforcing their interfacial properties, and more importantly, the interfacial bonding mechanisms are systematically summarized. Detailly, interface phase introduction can build a bridge between biopolymer and other components to form strong interface bonding with the two phases under the action of interface phase. Surface modification can graft organic molecules or polymers containing functional groups onto other components to crosslink with biopolymer. In situ growth can directly in situ synthesize other components with the action of nucleating agent serving as an adherent platform for the nucleation and growth of other components to biopolymer surface by chemical bonding. In addition, the mechanical properties (including strength and modulus) and biological properties (including bioactivity, cytocompatibility and biosensing in vitro, and tissue compatibility, bone regeneration capacity in vivo) of multicomponent biopolymer based scaffold after interfacial reinforcing are also reviewed and discussed. Finally, suggestions for further research are given with highlighting the need for specific investigations to assess the interface formation, structure, properties, and more in vivo studies of scaffold before applications.

Nanoparticle-Hydrogel Composites: From Molecular Interactions to Macroscopic Behavior

Hydrogels are materials used in a variety of applications, ranging from tissue engineering to drug delivery. The incorporation of nanoparticles to yield composite hydrogels has gained substantial momentum over the years since these afford tailor-making and extend material mechanical properties far beyond those achievable through molecular design of the network component. Here, we review different procedures that have been used to integrate nanoparticles into hydrogels; the types of interactions acting between polymers and nanoparticles; and how these underpin the improved mechanical and optical properties of the gels, including the self-healing ability of these composite gels, as well as serving as the basis for future development. In a less explored approach, hydrogels have been used as dispersants of nanomaterials, allowing a larger exposure of the surface of the nanomaterial and thus a better performance in catalytic and sensor applications. Furthermore, the reporting capacity of integrated nanoparticles in hydrogels to assess hydrogel properties, such as equilibrium swelling and elasticity, is highlighted.

Improving the Mechanical Rigidity of Hyaluronic Acid by Integration of a Supramolecular Peptide Matrix

Hyaluronic acid (HA), a major component of the extracellular matrix, is an attractive material for various medical applications. Yet, its low mechanical rigidity and fast in vivo degradation hinder its utilization. Here, we demonstrate the reinforcement of HA by its integration with a low-molecular-weight peptide hydrogelator to produce a composite hydrogel. The formulation of HA with the fluorenylmethoxycarbonyl diphenylalanine (FmocFF) peptide, one of the most studied self-assembling hydrogel-forming building blocks, showing notable mechanical properties, resulted in the formation of stable, homogeneous hydrogels. Electron microscopy analysis demonstrated a uniform distribution of the two matrices in the composite forms. The composite hydrogels showed improved mechanical properties and stability to enzymatic degradation while maintaining their biocompatibility. Moreover, the storage modulus of the FmocFF/HA composite hydrogels reached up to 25 kPa. The composite hydrogels allowed sustained release of curcumin, a hydrophobic polyphenol showing antioxidant, anti-inflammatory, and antitumor activities. Importantly, the rate of curcumin release was modulated as a function of the concentration of the FmocFF peptide within the hydrogel matrix. This work provides a new approach for conferring mechanical rigidity and stability to HA without the need of cross-linking, thus potentially facilitating its utilization in different clinical applications, such as sustained drug release.

Fluorine-ion-releasing injectable alginate nanocomposite hydrogel for enhanced bioactivity and antibacterial property

The creation of a moist environment and promotion of cell proliferation and migration together with antibacterial property are critical to the wound-healing process. Alginate (Alg) is an excellent candidate for injectable wound dressing materials because it can form a gel in a mild environment. Taking advantage of its gelation property, an injectable nano composite hydrogel containing nano-sized (about 90 nm) calcium fluoride (CaF₂) particles was developed using in-situ precipitation process. The amount of released fluorine (F^-) ion from the nanocomposite hydrogel increased with increasing CaF₂ content inside the composite hydrogel and the ions stimulated both the proliferation and migration of fibroblast cells in vitro. The antibacterial property of the composite hydrogel against E. coli and S. aureus was confirmed through colony formation test where the number of bacterial colonies significantly decreased compared to Alg hydrogel. The in vivo results based on a full-thickness wound model showed that the nanocomposite hydrogel effectively enhanced the deposition of the extracellular matrix compared to that of the Alg hydrogel. This study demonstrates the potential of this nanocomposite hydrogel as a bioactive injectable wound-dressing material with the ability to inhibit bacterial growth and stimulate cell proliferation and migration for accelerated wound healing.

Effective Wound Healing by Antibacterial and Bioactive Calcium-Fluoride-Containing Composite Hydrogel Dressings Prepared Using in Situ Precipitation

In this study, we report the development of a hyaluronic acid (HA)-based composite hydrogel containing calcium fluoride (CaF₂) with good biocompatibility and antibacterial properties for multifunctional wound dressing applications. CaF₂ was newly selected for incorporation within HA because it can release both Ca²⁺ and F^- ions, which are well-known ions for affecting cell proliferation and inhibiting bacterial growth, respectively. In particular, an in situ precipitation process enables easy control over the released amount of F^- ions by simply adjusting the precursor solutions (calcium chloride (CaCl₂) and ammonium fluoride (NH₄F)) used for the CaF₂ precipitation. CaF₂ particles were uniformly embedded within a HA-based pure hydrogel using an in situ precipitation process. Through variation of the CaCl₂ and NH₄F concentrations used in the precipitation as well as the precipitation time, composite hydrogels with different ion-release profiles were obtained. By controlling the precipitation time, especially for 10 min and after 30 min, large differences in the ion-release profiles as a function of CaF₂ concentration were observed. A shorter precipitation time resulted in faster release of fluoride, whereas for the 30 min and 1 h samples, sustained ion release was achieved. Colony tests and live/dead assays using Escherichia coli and Staphylococcus aureus revealed a lower density of bacteria on the CaF₂ composite hydrogels than on the pure hydrogel for both strains. In addition, improved cellular responses such as cell attachment and proliferation were also observed for the CaF₂ composite hydrogels compared to those for the pure hydrogel. Furthermore, the composite hydrogels exhibited excellent wound healing efficiency, as evidenced by an in vitro cell migration assay. Finally, monitoring of the wound closure changes using a full-thickness wound in a rat model revealed the accelerated wound healing capability of the CaF₂ composite hydrogels compared with that of the pure hydrogel. Based on our findings, these CaF₂ composite hydrogels show great potential for application as advanced hydrogel wound dressings with antibacterial properties and accelerated wound-healing capabilities.

Ultra-tough injectable cytocompatible hydrogel for 3D cell culture and cartilage repair

In this work, we developed a very facile strategy, i.e. dual dynamic crosslinking, to prepare a high performance injectable hydrogel. Poly(vinyl alcohol) (PVA) was crosslinked by 4-carboxyphenylboronic acid (CPBA) through the generation of borate bonding and ionic interaction to bridge the polymer chains in the presence of calcium ions. The dynamic gathering of CPBA could induce a self-reinforcing effect inside the hydrogel matrix, leading to high tensile and compressive moduli of the hydrogel over 1.0 MPa including the highest compressive modulus up to 5.6 MPa. Meanwhile, the mechanical properties of the hydrogel can be broadly and accurately tuned. And owing to the flexible PVA network, the hydrogel is ultra-tough, showing maximum tensile strain, tensile and compressive fracture energies up to 1600%, 600 kJ m^-2 and 25 kJ m^-2, respectively. Besides, the dynamic bonding overcomes the barriers to forming an injected strong hydrogel, e.g. to obtain a modulus and a fracture energy exceeding 1.0 MPa and 40 kJ m^-2, by using a commercial dual-syringe kit under physiological conditions. Such a mild gelation procedure benefits the administration, 3D encapsulation and proliferation of cells of the hydrogels. The application of the PVA hydrogel was demonstrated by effective cartilage repair.

Functionalization of phosphocalcic bioceramics for bone repair applications

This work is devoted to the processing of bone morphogenetic protein (BMP-2) functionalized silicate substituted hydroxyapatite (SiHA) ceramic spheres. The motivation behind it is to develop injectable hydrogel/bioceramic composites for bone reconstruction applications. SiHA microspheres were shaped by spray drying and thoroughly characterized. The silicate substitution was used to provide preferred chemical sites at the ceramic surface for the covalent immobilization of BMP-2. In order to control the density and the release of the immobilized BMP-2, its grafting was performed via ethoxysilanes and polyethylene glycols. A method based on Kaiser's test was used to quantify the free amino groups of grafted organosilanes available at the ceramic surface for BMP-2 immobilization. The SiHA surface modification was investigated by means of X-ray photoelectron spectroscopy, Fourier transformed infrared spectroscopy and thermogravimetry coupled with mass spectrometry. The BMP-2 bioactivity was assessed, in vitro, by measuring the luciferase expression of a stably transfected C3H10 cell line (C3H10-BRE/Luc cells). The results provided evidence that the BMP-2 grafted onto SiHA spheres remained bioactive.

3D printing of hydrogel composite systems: Recent advances in technology for tissue engineering

Three-dimensional (3D) printing of hydrogels is now an attractive area of research due to its capability to fabricate intricate, complex and highly customizable scaffold structures that can support cell adhesion and promote cell infiltration for tissue engineering. However, pure hydrogels alone lack the necessary mechanical stability and are too easily degraded to be used as printing ink. To overcome this problem, significant progress has been made in the 3D printing of hydrogel composites with improved mechanical performance and biofunctionality. Herein, we provide a brief overview of existing hydrogel composite 3D printing techniques including laser based-3D printing, nozzle based-3D printing, and inkjet printer based-3D printing systems. Based on the type of additives, we will discuss four main hydrogel composite systems in this review: polymer- or hydrogel-hydrogel composites, particle-reinforced hydrogel composites, fiber-reinforced hydrogel composites, and anisotropic filler-reinforced hydrogel composites. Additionally, several emerging potential applications of hydrogel composites in the field of tissue engineering and their accompanying challenges are discussed in parallel.

Preparation of a Strong Gelatin-Short Linear Glucan Nanocomposite Hydrogel by an in Situ Self-Assembly Process

Gelatin hydrogels exhibit excellent biocompatibility, nonimmunogenicity, and biodegradability, but they have limited applications in the food and medical industries because of their poor mechanical properties. Herein, we first developed an in situ self-assembly process for the preparation of gelatin-short linear glucan (SLG) nanocomposite hydrogels with enhanced mechanical strength. The microstructure, dynamic viscoelasticity, compression behavior, and thermal characteristics of the gelatin-SLG nanocomposite hydrogels were determined using scanning electron microscopy (SEM), dynamic rheological experiments, compression tests, and texture profile analysis tests. The SEM images revealed that nanoparticles were formed by the in situ self-assembly of SLG in the gelatin matrix and that the size of these nanoparticles ranged between 200 and 600 nm. The pores of the nanocomposite hydrogels were smaller than those of the pure gelatin hydrogels. Transmission electron microscopy images and X-ray diffraction further confirmed the presence of SLG nanoparticles with spherical shapes and B-type structures. Compared with pure gelatin hydrogels, the nanocomposite hydrogels exhibited improved mechanical behavior. Notably, the hardness and maximum values of the compressive stress of gelatin-SLG nanocomposites containing 5% SLG increased by about 2-fold and 3-fold, respectively, compared to the corresponding values of pure gelatin hydrogels.

Naturally derived proteins and glycosaminoglycan scaffolds for tissue engineering applications

Tissue engineering (TE) aims to mimic the complex environment where organogenesis takes place using advanced materials to recapitulate the tissue niche. Cells, three-dimensional scaffolds and signaling factors are the three main and essential components of TE. Over the years, materials and processes have become more and more sophisticated, allowing researchers to precisely tailor the final chemical, mechanical, structural and biological features of the designed scaffolds. In this review, we will pose the attention on two specific classes of naturally derived polymers: fibrous proteins and glycosaminoglycans (GAGs). These materials hold great promise for advances in the field of regenerative medicine as i) they generally undergo a fast remodeling in vivo favoring neovascularization and functional cells organization and ii) they elicit a negligible immune reaction preventing severe inflammatory response, both representing critical requirements for a successful integration of engineered scaffolds with the host tissue. We will discuss the recent achievements attained in the field of regenerative medicine by using proteins and GAGs, their merits and disadvantages and the ongoing challenges to move the current concepts to practical clinical application.

Enhancing the self-recovery and mechanical property of hydrogels by macromolecular microspheres with thermal and redox initiation systems

A redox initiation system was used to efficiently enhance the mechanical behavior of macromolecular microsphere hydrogels.

Repair of volumetric bone defects with a high strength BMP-loaded-mineralized hydrogel tubular scaffold

A high strength and BMP-2-loaded tubular scaffold was engineered by in situ mineralization of a supramolecular hydrogel. This tubular scaffold could lead to an efficient volumetric bone repair.

Long-lasting and bioactive hyaluronic acid-hydroxyapatite composite hydrogels for injectable dermal fillers: Physical properties and in vivo durability

Hyaluronic acid (HAc)–hydroxyapatite (HAp) composite hydrogels were developed to improve the biostability and bioactivity of HAc for dermal filler applications. Two kinds of HAc-HAp composite fillers were generated: HAcmicroHAp and HAc-nanoHAp composites. HAc-microHAp was fabricated by mixing HAp microspheres with HAc hydrogels, and HAc-nanoHAp was made by in situ precipitation of nano-sized HAp particles in HAc hydrogels. Emphasis was placed on the effect of HAp on the durability and bioactivity of the fillers. Compared with the pure HAc filler, all of the HAc-HAp composite fillers exhibited significant improvements in volumetric maintenance based on in vivo tests owing to their reduced water content and higher degree of biointegration between the filler and surrounding tissues. HAc-HAp composite fillers also showed noticeable enhancement in dermis recovery, promoting collagen and elastic fiber formation. Based on their long-lasting durability and bioactivity, HAc-HAp composite fillers have great potential for soft tissue augmentation with multifunctionality.