Double-Network Hydrogels Strongly Bondable to Bones by Spontaneous Osteogenesis Penetration

A grape seed protein-tannic acid powder to transform various non-adhesive hydrogels into adhesive gels

Realizing adhesion between wet materials remains challenging because of the interfacial water. Current strategies depend on complicated surface modifications, resulting in limited functions. Herein, a facile strategy based on the powder of grape seed protein and tannic acid (GSP-TA) was reported to endow various non-adhesive hydrogels adhesion without chemical modifications for both hydrogels and adherents. The GSP-TA powder has the capability to absorb interfacial water, form an adhesive layer on the hydrogel surface, diffusion into the underneath hydrogel matrix, and establish the initial adhesion within 5 s. By forming multiple non-covalent interactions between powders and substrates, the GSP-TA powder served as an efficient surface treating agent, enabling robust adhesion to solid substrates (wood, cardboard, glass, iron, and rubber) and wet tissues (pigskin, muscle, liver and heart). The adhesive strength for wood, cardboard, glass, iron, and rubber was 145.92 ± 5.93, 123.93 ± 15.98, 66.24 ± 7.67, 98.22 ± 4.13, and 80.83 ± 7.48 kPa, respectively. Because of reversible interactions, the adhesion was also repeatable. Due to the merits of grape seed protein and plant polyphenol, it could be completely degraded within 11 days. Bearing several merits, this strategy has promising applications in wound patches, tissue repair, and sensors.

Anisotropic Liesegang pattern for the nonlinear elastic biomineral-hydrogel complex

The Liesegang pattern is a beautiful natural anisotropic patterning phenomenon observed in rocks and sandstones. This study reveals that the Liesegang pattern can induce nonlinear elasticity. Here, a Liesegang-patterned complex with biomineral-hydrogel repetitive layers is prepared. This Liesegang-patterned complex is obtained only when the biomineralization is performed under the supersaturated conditions. The Liesegang-patterned complex features a nonlinear elastic response, whereas a complex with a single biomineral shell shows a linear behavior, thus demonstrating that the Liesegang pattern is essential in achieving nonlinear elasticity. The stiff biomineral layers have buffered the concentrated energy on behalf of soft hydrogels, thereby exposing the hydrogel components to reduced stress and, in turn, enabling them to perform the elasticity continuously. Moreover, the nonlinear elastic Liesegang-patterned complex exhibits excellent stress relaxation to the external loading, which is the biomechanical characteristic of cartilage. This stress relaxation allows the bundle of fiber-type Liesegang-patterned complex to endure greater deformation.

Sandwich hydrogel to realize cartilage-mimetic structures and performances from polyvinyl alcohol, chitosan and sodium hyaluronate

Developing artificial substitutes that mimic the structures and performances of natural cartilage is of great importance. However, it is challenging to integrate the high strength, excellent biocompatibility, low coefficient of friction, long-term wear resistance, outstanding swelling resistance, and osseointegration potential into one material. Herein, a sandwich hydrogel with cartilage-mimetic structures and performances was prepared to achieve this goal. The precursor hydrogel was obtained by freezing-thawing the mixture of poly vinyl alcohol, chitosan and deionized water three cycles, accompanied by soaking in sodium hyaluronate solution. The top of the precursor hydrogel was hydrophobically modified with lauroyl chloride and then loaded with lecithin, while the bottom was mineralized with hydroxyapatite. Due to the multiple linkages (crystalline domains, hydrogen bonds, and ionic interactions), the compressive stress was 71 MPa. Owing to the synergy of the hydrophobic modification and lecithin, the coefficient of friction was 0.01. Additionally, no wear trace was observed after 50,000 wear cycles. Remarkably, hydroxyapatite enabled the hydrogel osseointegration potential. The swelling ratio of the hydrogel was 0.06 g/g after soaking in simulated synovial fluid for 7 days. Since raw materials were non-toxic, the cell viability was 100 %. All of the above merits make it an ideal material for cartilage replacement.

Thermoresponsive Injectable Hydrogel To Mimic the Heat- and Strain-Stiffening Behavior of Biopolymers toward Muscle Cell Proliferation

Injectable hydrogels with nonlinear mechanical attributes to emulate natural biopolymers hold paramount significance in tissue engineering, offering the potential to create scaffolds that seamlessly mimic the biomechanical intricacies of living tissues. Herein, we unveil a synthetic design strategy employing Schiff base chemistry to furnish a peptide-polymer hierarchical contractile injectable hydrogel network. This innovative design demonstrates cross-linking of supramolecular peptide nanostructures such as nanofibers, 1NF, and twisted bundles, 1TB, with a thermosensitive aldehyde-functionalized polymer, PCHO. These networks exhibit interesting nonlinear mechanical stiffening responses to temperature and external stress. Furthermore, the hydrogels transform into a gel state at physiological temperature to exhibit injectable behavior and demonstrate compression load-bearing capabilities. Finally, the hydrogel network exhibits excellent biocompatibility and cell proliferation toward fibroblast, L929, and myoblast, C2C12, to validate their use as potential extracellular matrix mimetic injectable scaffolds.

Microfabrication of Gelatin Methacrylate/Hydroxyapatite Composites by Utilizing Alternate Soaking Process

To construct a complex three-dimensional (3D) structure mimicking bone microstructure, hydrogel models of polymerized gelatin methacrylate (pGelMA) were fabricated by using stereolithography and modified with hydroxyapatite (HAp) via an alternate soaking process (ASP) using a solution of calcium and phosphate ions. Fabricated pGelMA line models whose widths were designed as 100, 300, and 600 μm were modified with HAp by ASP by changing the immersion time and number of cycles. After ASP, all of the line models with widths of 100, 300, and 600 μm were successfully modified with HAp, and large amounts of HAp were covered with the fabricated models by increasing both the immersion time and the number of cycles in ASP. HAp was observed near the surface of the line model with a width of 600 μm after ASP at an immersion time of 10 s, while the entire model was modified with HAp using ASPs for longer immersion times. The adhesion and spread of mesenchymal stem cells (MSCs) on the pGelMA-HAp discs depended on the ASP conditions. Moreover, the HAp modification of 3D pyramid models without alteration of the microstructure was also conducted. This two-step fabrication method of first fabricating frameworks of hydrogel models by stereolithography and subsequently modifying the fabricated models with HAp will lead to the development of 3D cell culture systems to support bone grafts or to create biological niches, such as artificial bone marrow.

Polydopamine-Functionalized Strontium Alginate/Hydroxyapatite Composite Microhydrogel Loaded with Vascular Endothelial Growth Factor Promotes Bone Formation and Angiogenesis

Critical-size bone defects are a common and intractable clinical problem that typically requires filling in with surgical implants to facilitate bone regeneration. Considering the limitations of autologous bone and allogeneic bone in clinical applications, such as secondary damage or immunogenicity, injectable microhydrogels with osteogenic and angiogenic effects have received considerable attention. Herein, polydopamine (PDA)-functionalized strontium alginate/nanohydroxyapatite (Sr-Alg/nHA) composite microhydrogels loaded with vascular endothelial growth factor (VEGF) were prepared using microfluidic technology. This composite microhydrogel released strontium ions stably for at least 42 days to promote bone formation. The PDA coating can release VEGF in a controlled manner, effectively promote angiogenesis around bone defects, and provide nutritional support for new bone formation. In in vitro experiments, the composite microhydrogels had good biocompatibility. The PDA coating greatly improves cell adhesion on the composite microhydrogel and provides good controlled release of VEGF. Therefore, this composite microhydrogel effectively promotes osteogenic differentiation and vascularization. In in vivo experiments, composite microhydrogels were injected into critical-size bone defects in the skull of rats, and they were shown by microcomputed tomography and tissue sections to be effective in promoting bone regeneration. These findings demonstrated that this novel microhydrogel effectively promotes bone formation and angiogenesis at the site of bone defects.

Journey of Mineral Precursors in Bone Mineralization: Evolution and Inspiration for Biomimetic Design

Bone mineralization is a ubiquitous process among vertebrates that involves a dynamic physical/chemical interplay between the organic and inorganic components of bone tissues. It is now well documented that carbonated apatite, an inorganic component of bone, is proceeded through transient amorphous mineral precursors that transforms into the crystalline mineral phase. Here, the evolution on mineral precursors from their sources to the terminus in the bone mineralization process is reviewed. How organisms tightly control each step of mineralization to drive the formation, stabilization, and phase transformation of amorphous mineral precursors in the right place, at the right time, and rate are highlighted. The paradigm shifts in biomineralization and biomaterial design strategies are intertwined, which promotes breakthroughs in biomineralization-inspired material. The design principles and implementation methods of mineral precursor-based biomaterials in bone graft materials such as implant coatings, bone cements, hydrogels, and nanoparticles are detailed in the present manuscript. The biologically controlled mineralization mechanisms will hold promise for overcoming the barriers to the application of biomineralization-inspired biomaterials.

Occlusive membranes for guided regeneration of inflamed tissue defects

Guided bone regeneration aided by the application of occlusive membranes is a promising therapy for diverse inflammatory periodontal diseases. Symbiosis, homeostasis between the host microbiome and cells, occurs in the oral environment under normal, but not pathologic, conditions. Here, we develop a symbiotically integrating occlusive membrane by mimicking the tooth enamel growth or multiple nucleation biomineralization processes. We perform human saliva and in vivo canine experiments to confirm that the symbiotically integrating occlusive membrane induces a symbiotic healing environment. Moreover, we show that the membrane exhibits tractability and enzymatic stability, maintaining the healing space during the entire guided bone regeneration therapy period. We apply the symbiotically integrating occlusive membrane to treat inflammatory-challenged cases in vivo, namely, the open and closed healing of canine premolars with severe periodontitis. We find that the membrane promotes symbiosis, prevents negative inflammatory responses, and improves cellular integration. Finally, we show that guided bone regeneration therapy with the symbiotically integrating occlusive membrane achieves fast healing of gingival soft tissue and alveolar bone.

Biomimetic Mineralization: From Microscopic to Macroscopic Materials and Their Biomedical Applications

Biomineralization is an attractive pathway to produce mineral-based biomaterials with high performance and hierarchical structures. To date, the biomineralization process and mechanism have been extensively studied, especially for the formation of bone, teeth, and nacre. Inspired by those, abundant biomimetic mineralized materials have been fabricated for biomedical applications. Those bioinspired materials generally exhibit great mechanical properties and biological functions. Nevertheless, substantial gaps remain between biomimetic materials and natural materials, particularly with respect to mechanical properties and mutiscale structures. This Review summarizes the recent progress of micro- and macroscopic biomimetic mineralization from the perspective of materials synthesis and biomedical applications. To begin with, we discuss the progress of biomimetic mineralization at the microscopic level. The mechanical strength, stability, and functionality of the nano- and micromaterials are significantly improved by introducing biominerals, such as DNA nanostructures, nanovaccines, and living cells. Next, numerous biomimetic strategies based on biomineralization at the macroscopic scale are highlighted, including in situ mineralization and bottom-up assembly of mineralized building blocks. Finally, challenges and future perspectives regarding the development of biomimetic mineralization are also presented with the aim of offering insights for the rational design and fabrication of next-generation biomimetic mineralized materials.

Intestinal Villi-Inspired Mathematically Base-Layer Engineered Microneedles (IMBEMs) for Effective Molecular Exchange during Biomarker Enrichment and Drug Deposition in Diversified Mucosa

The mucosa-interfacing systems based on bioinspired engineering design for sampling/drug delivery have manifested crucial potential for the monitoring of infectious diseases and the treatment of mucosa-related diseases. However, their efficiency and validity are severely restricted by limited contact area for molecular transfer and dissatisfactory capture/detachment capability. Herein, inspired by the multilayer villus structure of the small intestine that enables high nutrient absorption, a trigonometric function-based periodic pattern was fabricated and integrated on the base layer of the microneedle patch, exhibiting a desirable synergistic effect with needle tips for deep sample enrichment and promising molecular transfer, significantly improving the device-mucosa bidirectional interaction. Moreover, mathematical modeling and finite element analysis were adopted to visualize and quantify the microcosmic molecular transmission process, guiding parameter optimization in actual situation. Encouragingly, these intestinal villi-inspired mathematically base-layer engineered microneedles (IMBEMs) have demonstrated distinguished applicability among mucosa tissue with varying surface curvatures, tissue toughness, and local environments, and simultaneously, have gained favorable support from healthy volunteers receiving preliminary test of IMBEMs patches. Overall, validated by numerous in vitro and in vivo tests, the IMBEMs were confirmed to act as a promising candidate to facilitate mucosa-based sampling and topical drug delivery, indicating highly clinical translation potential.

Diffusion-Limited Processes in Hydrogels with Chosen Applications from Drug Delivery to Electronic Components

Diffusion is one of the key nature processes which plays an important role in respiration, digestion, and nutrient transport in cells. In this regard, the present article aims to review various diffusion approaches used to fabricate different functional materials based on hydrogels, unique examples of materials that control diffusion. They have found applications in fields such as drug encapsulation and delivery, nutrient delivery in agriculture, developing materials for regenerative medicine, and creating stimuli-responsive materials in soft robotics and microrobotics. In addition, mechanisms of release and drug diffusion kinetics as key tools for material design are discussed.

High-Strength Double-Network Conductive Hydrogels Based on Polyvinyl Alcohol and Polymerizable Deep Eutectic Solvent

Conductive hydrogels feature the flexibility of soft materials plus conductive properties providing functionality for effectively sticking to the epidermis and detecting human activity signals. Their stable electrical conductivity also effectively avoids the problem of uneven distribution of solid conductive fillers inside traditional conductive hydrogels. However, the simultaneous integration of high mechanical strength, stretchability, and transparency through a simple and green fabrication method remains a great challenge. Herein, a polymerizable deep eutectic solvent (PDES) composed of choline chloride and acrylic acid was added to a biocompatible PVA matrix. The double-network hydrogels were then simply prepared by thermal polymerization and one freeze-thaw method. The introduction of the PDES significantly improved the tensile properties (1.1 MPa), ionic conductivity (2.1 S/m), and optical transparency (90%) of the PVA hydrogels. When the gel sensor was fixed to human skin, real-time monitoring of a variety of human activities could be implemented with accuracy and durability. Such a simple preparation method performed by combining a deep eutectic solvent with traditional hydrogels offers a new avenue to construct multifunctional conductive hydrogel sensors with excellent performance.

Multi-thermo responsive double network composite hydrogel for 3D printing medical hydrogel mask

3D printing of multifunctional hydrogels offers great opportunities for developing innovative biomedical technologies as it can provide custom-designed shapes and structures conformal to arbitrary contours. There have been significant improvements of the 3D printing techniques, but the available printable hydrogel materials limit the progress. Here, we investigated the use of a poloxamer diacrylate (Pluronic P123) to augment the thermo-responsive network composed of poly(N-isopropylacrylamide) and develop a multi-thermoresponsive hydrogel for photopolymerization 3D printing. The hydrogel precursor resin was synthesised to be printable with high-fidelity of fine structures and once cured can form a robust thermo-responsive hydrogel. By utilizing N-isopropyl acrylamide monomer and a Pluronic P123 diacrylate crosslinker as 2 separate thermo-responsive components it was found that the final hydrogel displayed 2 distinct lower critical solution temperature (LCST) switches. This enables the loading of hydrophilic drugs at fridge temperature and improving the strength of the hydrogel at room temperature while still maintaining a drug release at body temperature. The thermo-responsive material properties of this multifunctional hydrogel material system were investigated, showing a significant promise as a medical hydrogel mask. Furthermore, it is demonstrated that it can be printed in sizes large enough to fit and adhere to a human face at 1:1 scale with high dimensional accuracy, as well as its ability to load with hydrophilic drugs.

Recent advances in hydrogels for preventing tumor recurrence

Malignant tumors remain a high-risk disease with high mortality all over the world. Among all the cancer treatments, surgery is the primary approach in the clinical treatment of tumors. However, tumor invasion and metastasis pose challenges for complete tumor resection, accompanied by high recurrence rates and reduced quality of life. Hence, there is an urgent need to explore effective adjuvant therapies to prevent postoperative tumor recurrence and relieve the pain of the patients. Nowadays, the booming local drug delivery systems which can be applied as postoperative adjuvant therapies have aroused people's attention, along with the rapid development in the pharmaceutical and biological materials fields. Hydrogels are a kind of unique carrier with prominent biocompatibility among a variety of biomaterials. Due to their high similarity to human tissues, hydrogels which load drugs/growth factors can prevent rejection reactions and promote wound healing. In addition, hydrogels are able to cover the postoperative site and maintain sustained drug release for the prevention of tumor recurrence. In this review, we survey controlled drug delivery hydrogels such as implantable, injectable and sprayable formulations and summarize the properties required for hydrogels used as postoperative adjuvant therapies. The opportunities and challenges in the design and clinical application of these hydrogels are also elaborated.

Engineering of an electrically charged hydrogel implanted into a traumatic brain injury model for stepwise neuronal tissue reconstruction

Neural regeneration is extremely difficult to achieve. In traumatic brain injuries, the loss of brain parenchyma volume hinders neural regeneration. In this study, neuronal tissue engineering was performed by using electrically charged hydrogels composed of cationic and anionic monomers in a 1:1 ratio (C1A1 hydrogel), which served as an effective scaffold for the attachment of neural stem cells (NSCs). In the 3D environment of porous C1A1 hydrogels engineered by the cryogelation technique, NSCs differentiated into neuroglial cells. The C1A1 porous hydrogel was implanted into brain defects in a mouse traumatic damage model. The VEGF-immersed C1A1 porous hydrogel promoted host-derived vascular network formation together with the infiltration of macrophages/microglia and astrocytes into the gel. Furthermore, the stepwise transplantation of GFP-labeled NSCs supported differentiation towards glial and neuronal cells. Therefore, this two-step method for neural regeneration may become a new approach for therapeutic brain tissue reconstruction after brain damage in the future.

Metal-Coordinated Dynamics and Viscoelastic Properties of Double-Network Hydrogels

Biological soft tissues are intrinsically viscoelastic materials which play a significant role in affecting the activity of cells. As potential artificial alternatives, double-network (DN) gels, however, are pure elastic and mechanically time independent. The viscoelasticization of DN gels is an urgent challenge in enabling DN gels to be used for advanced development of biomaterial applications. Herein, we demonstrate a simple approach to regulate the viscoelasticity of tough double-network (DN) hydrogels by forming sulfonate-metal coordination. Owing to the dynamic nature of the coordination bonds, the resultant hydrogels possess highly viscoelastic, mechanical time-dependent, and self-recovery properties. Rheological measurements are performed to investigate the linear dynamic mechanical behavior at small strains. The tensile tests and cyclic tensile tests are also systematically performed to evaluate the rate-dependent large deformation mechanical behaviors and energy dissipation behaviors of various ion-loaded DN hydrogels. It has been revealed based on the systematic analysis that robust strong sulfonate-Zr⁴⁺ coordination interactions not only serve as dynamic crosslinks imparting viscoelastic rate-dependent mechanical performances, but also strongly affect the relative strength of the first PAMPS network, thereby increasing the yielding stress σ_y and the fracture stress at break σ_b and reducing the stretch ratio at break λ_b. It is envisioned that the viscoelasticization of DN gels enables versatile applications in the biomedical and engineering fields.

Polymeric Nanocomposite Hydrogel Scaffolds in Craniofacial Bone Regeneration: A Comprehensive Review

Nanocomposite biomaterials combine a biopolymeric matrix structure with nanoscale fillers. These bioactive and easily resorbable nanocomposites have been broadly divided into three groups, namely natural, synthetic or composite, based on the polymeric origin. Preparing such nanocomposite structures in the form of hydrogels can create a three-dimensional natural hydrophilic atmosphere pivotal for cell survival and new tissue formation. Thus, hydrogel-based cell distribution and drug administration have evolved as possible options for bone tissue engineering and regeneration. In this context, nanogels or nanohydrogels, created by cross-linking three-dimensional polymer networks, either physically or chemically, with high biocompatibility and mechanical properties were introduced as promising drug delivery systems. The present review highlights the potential of hydrogels and nanopolymers in the field of craniofacial tissue engineering and bone regeneration.

Periosteum-inspired in situ CaP generated nanocomposite hydrogels with strong bone adhesion and superior stretchability for accelerated distraction osteogenesis

BACKGROUND

Distraction osteogenesis (DO) is an efficacious but lengthy procedure to reconstruct segmental bone defects under the principle of tension-stress, during which the periosteum-mediated mechanical stimulation plays a pivotal role. Inspired by the dynamic process of DO and the mechanical stimulation of periosteum, a new design of bionic periosteum was developed to simulate the mechanical transduction of natural periosteum for the application in DO procedure.

METHODS

In this study, an injectable organic-inorganic hybrid hydrogel was developed based on a novel combination of the PEGylated poly (glycerol sebacate) (PEGS) polymer network and in situ formed CaP nanoparticles (ICPNs). Rat bone marrow mesenchymal stem cells (rBMSCs) and human umbilical vein endothelial cells (HUVECs) were cultured and tested in vitro to evaluate biocompatibility, cell adhesion, proliferation, and pro-osteogenic and pro-angiogenic activity. In vivo experiments were conducted in the rat tibial model of distraction osteogenesis.

RESULTS

The developed nanocomposite hydrogels exhibited excellent injectability, robust bone adhesion, superior stretchability, and enhanced osteogenic activity. The results of in vitro and in vivo studies showed that PEGS/ICPN hydrogels could promote new bone formation and mineralization during the dynamic distraction process through the synergistic effects of angiogenesis and osteogenesis.

CONCLUSIONS

This periosteum-inspired nanocomposite hydrogel represents a mechanobiology approach for effectively restoring large bone defects through the dynamic DO process.

Small Extracellular Vesicles Released from Bioglass/Hydrogel Scaffold Promote Vascularized Bone Regeneration by Transferring miR-23a-3p

BACKGROUND

The treatment of critical-size bone defect is a great difficulty in orthopedics. Osteogenesis and angiogenesis are critical issue during the process of bone repair and remodeling. Mesenchymal stem cells (MSCs)-derived exosomes have the same therapeutic effect to MSCs-based therapies. The effect of human umbilical cord MSCs-derived sEVs (hUC-MSCs-sEVs) on vascularized bone regeneration and the potential mechanism remains to be investigated. Herein, we aimed to explore the therapeutic effect and the mechanism of hUC-MSCs-sEVs on critical-size bone defect.

METHODS

To investigate the potential osteogenesis and angiogenesis effects of sEVs in vitro, we extracted sEVs from hUC-MSCs, and then sEVs were co-incubated with BMSCs and HUVECs. We next investigated the effect and potential mechanism of sEVs on the effects of osteogenesis and angiogenesis. We fabricated 3D-printed bioglass scaffold with Gelma/nanoclay hydrogel coatings to load sEVs (BG-gel-sEVs) to ensure in vivo sustained efficacy of sEVs. Finally, the skull defect model was used to evaluate the capacity of vascularized bone regeneration of the composited scaffolds.

RESULTS

hUC-MSCs-sEVs facilitated calcium deposition and the endothelial network formation, inducing osteogenic differentiation and angiogenesis by delivering miR-23a-3p to activate PTEN/AKT signaling pathway. Additionally, the BG-gel-sEVs composited scaffold achieved vascularized bone regeneration in vivo.

CONCLUSION

This finding illuminated that hUC-MSCs-sEVs promoted osteogenesis and angiogenesis by delivering miR-23a-3p to activate PTEN/AKT signaling pathway, achieving vascularized bone regeneration.

PVA/pectin composite hydrogels inducing osteogenesis for bone regeneration

Hydrogels composed from biomolecules have gained great interests as biomaterials for tissue engineering. However, their poor mechanical properties limit their application potential. Here, we synthesized a series of tough composite hydrogels from poly (vinyl alcohol) (PVA) and pectin for bone tissue engineering. With a balance of scaffold stiffness and pore size, PVA-Pec-10 hydrogel enhanced adhesion and proliferation of osteoblasts. The hydrogel significantly promoted osteogenesis in vitro by improving the alkaline phosphates (ALP) activity and calcium biomineralization, as well as upregulating the expressions of osteoblastic genes. The composite hydrogel also accelerated the bone healing process in vivo after transplantation into the femoral defect. Additionally, our study demonstrated that pectin and its Ca²⁺ crosslinking network play a crucial role of inducing osteogenesis through regulating the Ca²⁺/CaMKII and BMP-SMAD1/5 signaling. The optimized structure composition and multifunctional properties make PVA-Pec hydrogel highly promising to serve as a candidate for bone tissue regeneration.

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Recoverable PVA-Pec hydrogel is prepared by the freezing-thawing process.

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PVA-Pec-10 hydrogel display well attachment and osteogenesis capacity.

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PVA-Pecl-10 hydrogel enhanced osteogenesis by Ca²⁺/CaMKII and BMP-SMAD1/5 signaling.

A bio-inspired, ultra-tough, high-sensitivity, and anti-swelling conductive hydrogel strain sensor for motion detection and information transmission

Conductive hydrogels are excellent candidates for the next-generation wearable materials and are being extensively investigated for their potential use in health monitoring devices, human-machine interfaces, and other fields. However, their relatively low mechanical strength and performance degradation due to swelling have presented challenges in their practical application. Inspired by the multiscale heterogeneous architecture of biological tissue, a dynamic cross-linked, ultra-tough, and high-sensitivity hydrogel with a swelling resistance characteristic was fabricated by the principle of multiple non-covalent interaction matching and a step-by-step construction strategy. A heterogeneous structure was constructed by the combination of a 'soft' hydrophobic-conjugated micro-region structural domain with inter/intra-molecular hydrogen bonding and π-π stacking along with 'rigid' cross-linking via strong ionic coordination interactions. Reversible cross-linking synergies and variations in the content of rigid and flexible components guaranteed the hydrogel to undergo flexible and efficient modulation of the structures and gain excellent mechanics, including elongation at break (>2000%), toughness (∼60 MJ m^-3), and recovery (>88%). Notably, hydrogels displayed good anti-swelling properties even in solutions with different pH (pH 2-11) and solvents. Moreover, the hydrogel further exhibited fast response (47.4 ms) and high sensitivity due to the presence of dynamic ions (Fe³⁺, Na⁺, and Cl^-); therefore, it was assembled into a sensor to detect various human motions and used as a signal transmitter for the encryption and decryption of information according to Morse code. This study provides basis for the development of a variety of robust and flexible conductive hydrogels with multifunctional sensing applications in next-generation wearable devices.

Fabrication of biphasic cartilage-bone integrated scaffolds based on tissue-specific photo-crosslinkable acellular matrix hydrogels

The fabrication of biphasic cartilage-bone integrated scaffolds is an attractive alternative for osteochondral repair but has proven to be extremely challenging. Existing three-dimensional (3D) scaffolds are insufficient to accurately biomimic the biphasic cartilage-bone integrated microenvironment. Currently, photo-crosslinkable hydrogels based on tissue-specific decellularized extracellular matrix (dECM) have been considered as an important technique to fabricate biomimetic scaffolds, but so far there has been no breakthrough in the photo-crosslinkable hydrogel scaffolds with biphasic cartilage-bone biomimetic microenvironment. Here, we report a novel strategy for the preparation of biomimetic cartilage-bone integrated scaffolds based on photo-crosslinkable cartilage/bone-derived dECM hydrogels, which are able to reconstruct biphasic cartilage-bone biomimetic microenvironment. The biphasic cartilage-bone integrated scaffolds provided a 3D microenvironment for osteochondral regeneration. The cartilage biomimetic scaffolds, consisting of cartilage-derived dECM hydrogels, efficiently regulated chondrogenesis of bone marrow mesenchymal stem cells (BMSCs). The bone biomimetic scaffolds, composed of cartilage/bone-derived dECM hydrogels, first regulated chondrogenesis of BMSCs, followed by endochondral ossification over time. Taken together, the biphasic cartilage-bone integrated tissue could be successfully reconstructed by subcutaneous culture based on cartilage-bone bilayered structural design. Furthermore, the biphasic cartilage-bone biomimetic scaffolds (cell-free) achieved satisfactory cartilage-bone integrated regeneration in the osteochondral defects of rabbits’ knee joints.

Simultaneously stimulated osteogenesis and anti-bacteria of physically cross-linked double-network hydrogel loaded with MgO-Ag2O nanocomposites

Hydrogels, with a three-dimensional network of water-soluble polymer and water, could simulate the critical properties of extracellular matrix, which has been widely used in bone tissue engineering. However, most of conventional hydrogels for bone regeneration are fragile and have poor osteogenic activity, which restricts their applications. In this work, a novel nanoparticle-hydrogel composite consisting of physically cross-linked double-network loaded with MgO-Ag₂O nanocomposites was developed by the sol-gel method. The Mg²⁺ released from MgO-Ag₂O nanocomposites was used as an ionic cross-linking site of sodium alginate (SA), while the hydrophobic micelles in the polyacrylamide (PAAM) network is acted as another crosslinking point. The results indicated that the novel nanoparticle-hydrogel composites had good self-recovery ability and excellent mechanical properties compared with the conventional sodium alginate (SA)/polyacrylamide (PAAM) hydrogels. Additionally, it showed a slow release of Mg and Ag ions due to the dual function of the embedding effect of hydrogels and the increasing pH of the solution induced by the hydrolysis of sodium alginate. In terms of in vitro tests, the nanoparticle-hydrogel composites showed significantly stimulatory effects on the proliferation and differentiation of SaOS-2 cells. In addition, the antibacterial effects of the nanoparticle-hydrogel composites were gradually enhanced with the increase of MgO-Ag₂O content.

Matrix-Directed Mineralization for Bulk Structural Materials

Mineral-based bulk structural materials (MBSMs) are known for their long history and extensive range of usage. The inherent brittleness of minerals poses a major problem to the performance of MBSMs. To overcome this problem, design principles have been extracted from natural biominerals, in which the extraordinary mechanical performance is achieved via the hierarchical organization of minerals and organics. Nevertheless, precise and efficient fabrication of MBSMs with bioinspired hierarchical structures under mild conditions has long been a big challenge. This Perspective provides a panoramic view of an emerging fabrication strategy, matrix-directed mineralization, which imitates the in vivo growth of some biominerals. The advantages of the strategy are revealed by comparatively analyzing the conventional fabrication techniques of artificial hierarchically structured MBSMs and the biomineral growth processes. By introducing recent advances, we demonstrate that this strategy can be used to fabricate artificial MBSMs with hierarchical structures. Particular attention is paid to the mass transport and the precursors that are involved in the mineralization process. We hope this Perspective can provide some inspiring viewpoints on the importance of biomimetic mineralization in material fabrication and thereby spur the biomimetic fabrication of high-performance MBSMs.

Janus Hydrogel to Mimic the Structure and Property of Articular Cartilage

Designing hydrogels with adequate strength, remarkable swelling resistance, low friction coefficient, excellent biocompatibility, and osseointegration potential is essential for replacing articular cartilage. However, it remains challenging to integrate all these properties into one material. In this work, a Janus hydrogel was prepared from polyvinyl alcohol, chitosan, and sodium hyaluronate, followed by a one-sided dipping in situ precipitation mineralization to form a layer of hybridized hydroxyapatite (HAp), wherein the two surfaces had distinct compositions and functions. Because of the negative carboxyl groups from sodium hyaluronate, the top surface possessed a friction coefficient as low as 0.024. On account of the HAp mineralized layer, the bottom side had osteogenesis potential. Owing to the synergy of physical linkages, the hydrogel displayed compressive strength as high as 78 MPa. Furthermore, it demonstrated remarkable swelling resistance with strength retention near 100% even after soaking in PBS solution at 37 °C for 7 days. The absence of toxic chemicals maintained the merits of starting polymers and resulted in excellent biocompatibility (cell viability ≈ 100%), making it an ideal substitute for articular cartilage.

Recent Advances of Natural Polysaccharide-based Double-network Hydrogels for Tissue Repair

Natural polysaccharide hydrogels have been extensively explored for many years due to their outstanding biocompatibility and biodegradability, which are very promising candidates as artificial soft materials for biomedical applications. However, their inferior mechanical performances greatly limited their applications. Introduction of double-network (DN) structure has been well documented to be an efficient strategy for significant improvement of the mechanical property of hydrogels. Here, recent progress of natural polysaccharide-based DN hydrogels is reviewed from the perspective of fundamental concepts on both design rationale and preparation strategies to biomedical application in tissue repair. Retrospect of the DN-strengthened polysaccharide hydrogels can give a deep insight into the fundamental relationship of such bio-based hydrogels among structural design, mechanical properties and practical demands, thereby prompting their translation to clinical application prospects.

Engineering DNA-Guided Hydroxyapatite Bulk Materials with High Stiffness and Outstanding Antimicrobial Ability for Dental Inlay Applications

Programmable base pair interactions at the nanoscale make DNA an attractive scaffold for forming hydroxyapatite (HAP) nanostructures. However, engineering macroscale HAP mineralization guided by DNA molecules remains challenging. To overcome this issue, a facile strategy is developed for the fabrication of ultrastiff DNA-HAP bulk composites. The electrostatic complexation of DNA and a surfactant with a quaternary ammonium salt group enables the formation of long-range ordered scaffolds using electrospinning. The growth of 1D and 2D HAP minerals is thus realized by this DNA template at a macroscale. Remarkably, the as-prepared DNA-HAP composites exhibit an ultrahigh Young's modulus of ≈25 GPa, which is comparable to natural HAP and superior to most artificial mineralized composites. Furthermore, a new type of dental inlay with outstanding antibacterial properties is developed using the stiff DNA-HAP. The encapsulated quaternary ammonium group within the dense HAP endows the composite with long-lasting and local antibacterial activity. Therefore, this new type of super-stiff biomaterial holds great potential for oral prosthetic applications.

Thermosensitive P(AAc-co-NIPAm) hydrogels display enhanced toughness and self-healing via ion-ligand interactions

Hydrogels containing thermosensitive polymers such as poly(N-isopropylacrylamide) (P(NIPAm)) may contract during heating and show great promise in fields ranging from soft robotics to thermosensitive biosensors. However, these gels often exhibit low stiffness, tensile strength, and mechanical toughness, limiting their applicability. Through copolymerization of P(NIPAm) with poly(Acrylic acid) (P(AAc)) and introduction of ferric ions (Fe³⁺ ) that coordinate with functional groups along the P(AAc) chains, we here introduce a thermoresponsive hydrogel with significantly enhanced mechanical extensibility, strength, and toughness. Using both experimentation and constitutive modeling, we find that increasing the ratio of m(AAc):m(NIPAm) in the prepolymer decreases strength and toughness but improves extensibility. In contrast, increasing Fe³⁺ concentration generally improves strength and toughness with little decrease in extensibility. Due to reversible coordination of the Fe³⁺ bonds, these gels display excellent recovery of mechanical strength during cyclic loading and self-healing ability. While thermosensitive contraction imbued by the underlying P(NIPAm) is reduced slightly with increased Fe³⁺ concentration, the temperature transition range is widened and shifted upwards towards that of human body temperature (between 30 and 40°C), perhaps rendering these gels suitable as in vivo biosensors. Finally, these gels display excellent adsorptive properties with a variety of materials, rendering them possible candidates in adhesive applications. This article is protected by copyright. All rights reserved.

Biomimetic Scaffolds with a Mineral Gradient and Funnel-Shaped Channels for Spatially Controllable Osteogenesis

A facile method is described herein for generating a mineral gradient in a biodegradable polymer scaffold. The gradient is achieved by swelling a composite film made of polycaprolactone (PCL) and hydroxyapatite (HAp) nanoparticles with a PCL solution. During the swelling process, the solvent and PCL polymer chains diffuse into the composite film, generating a gradient in HAp density at their interface. The thickness of the mineral gradient can be tuned by varying the extent of swelling to match the length scale of the natural tendon-to-bone attachment (20-60 µm). When patterned with an array of funnel-shaped channels, the mineral gradient presents stem cells with spatial gradations in both biochemical cues (e.g., osteoinductivity and conductivity associated with the HAp nanoparticles) and mechanical cues (e.g., substrate stiffness) to stimulate their differentiation into a graded distribution of cell phenotypes. This new class of biomimetic scaffolds holds great promise for facilitating the regeneration of the injured tendon-to-bone attachment by stimulating the formation of a functionally graded interface.

Anti-Swelling, Robust, and Adhesive Extracellular Matrix-Mimicking Hydrogel Used as Intraoral Dressing

Due to the wet and dynamic environment of the oral cavity, the healing of intraoral wounds, such as tooth extraction wounds, requires stable and firm wound dressings. In clinical practice, cotton balls and gauzes, sponge plugs, or sutures are used to treat extraction wounds, but none of these means can continuously isolate the wound from the intraoral environment and facilitate ideal healing conditions. Herein, inspired by the natural extracellular matrix, a family of wound dressings is developed for intraoral wound repair. Infiltrating a ductile long-chain hydrogel network into a prefabricated, sturdy macromolecular meshwork and in situ crosslinking endowed the composite hydrogel with controllable swelling behaviors and robust mechanical properties. The macromolecular meshwork functioned as the backbone to support the composite and restricts the swelling of the long-chain hydrogel network. In vitro tests verified that this wound dressing can provide durable protection for intraoral wounds against complex irritations. Furthermore, accelerated wound healing occurred when the wound dressing is applied in vivo on a canine tooth extraction model, due to the effective reduction of acute inflammation. These results suggest that this family of bioinspired hydrogels has great potential for application as intraoral wound dressing.

Bamboo-Based Biomaterials for Cell Transportation and Bone Integration

The construction of hierarchical porous structure in biomaterials is of great significance for improving nutrient transport and biological performance. However, it is still challenging to design porous bone substitutes with high strength and biological properties, which limits their clinical applications in load-bearing bone regeneration. Herein, based on hierarchical porous structure of renewable bamboo, the mineralized calcium phosphate/bamboo composite scaffolds with high strength and excellent transport performance are successfully prepared in combination of biotemplated approach and biomimetic mineralization. The mineralized biomaterials have simultaneously achieved high mechanical strength and low modulus, similar to those of cortical bone. Furthermore, the mineralized biomaterials exhibit good liquid transport capacity and can transport cells along anti-gravity direction. Based on density functional theory (DFT) calculations, the mineralized calcium phosphate reveals the optimal H₂ O adsorption energy (-0.651 eV) and low diffusion energy barrier (0.743 eV), which is conducive to enhance hydrophilicity and liquid transport performance. Moreover, owing to the synergistic effect of the porous structure of biotemplate and bioactive mineralized components, the mineralized biomaterials possess enhanced bone integration and osteoconduction properties. The present study shed light on deeper understanding of mineralized biosourced materials, offering a strategy of combining green chemistry with tissue engineering to prepare eco-friendly biomaterials.

Visible-Light-Mediated Nano-biomineralization of Customizable Tough Hydrogels for Biomimetic Tissue Engineering

Biomineralized tough hydrogels (BTHs) have advanced applications in the fields of soft bioelectronics and biomimetic tissue engineering. But the development of rapid and general photomineralization strategies for one-step fabrication of customizable BTHs is still a challenging task. Here we report a straightforward, low-cost visible-light-mediated nano-biomineralization (VLMNB) strategy via a rational design of a phosphate source and efficient ruthenium photochemistry. Multinetwork tough hydrogels are simultaneously constructed under the same condition. Therefore, BTHs are rapidly prepared in a short time as low as ∼60 s under visible light irradiation. The in situ formation of calcium phosphate particles can improve BTHs' mechanical and biological properties and reduce the friction coefficient with bones. Furthermore, fast biomineralization and solidification processes in these BTHs benefit their injectable and highly flexible customizable design, showing applications of promoting customizable skin repair and bone regeneration.

Tough, Flexible, and Bioactive Amphoteric Copolymer-Based Hydrogel for Bone Regeneration without Encapsulation of Seed Cells/Simulating Cues

Bone tissue scaffolds with good bulk or surface osteoconductivity are always pursued by biomaterial scientists. In this paper, we design a tough and flexible amphoteric copolymer-based (AC) hydrogel with bioactive groups for bone regeneration. In detail, our hydrogels are copolymerized with N-acyl glycinamide (NAGA), anionic acrylate alendronate (AcAln), and cationic (2-(acryloyloxy)ethyl) trimethyl ammonium chloride (DMAEA-Q) by free radical polymerization. There are three kinds of synergetic physical cross-links among our polyamphion hydrogels: (1) double hydrogen bonds between amide groups in NAGA to provide toughness, (2) hydrogen bonds between dual bisphosphite groups in AcAln, and (3) weak ionic pairs between the anionic bisphosphite groups and the cationic quaternary ammonium groups in DMAEA-Q to offer flexibility. The AC hydrogel shows osteoid-like viscoelasticity, which makes the AC hydrogel osteogenesis inductive. During the repairing process, the bioactive bisphosphite groups accelerate the calcium fixation to expedite the mineralization of the new-formed bone. At the same time, the surface charge property of AC hydrogels also prevents fibrous cyst formation, thus guaranteeing osseointegration. Our in vitro data strongly demonstrate that the AC hydrogel is an excellent matrix to induce osteogenesis of rat bone marrow mesenchymal stem cells. More importantly, the following in vivo experiments further prove that the AC hydrogel can reach satisfactory bone regeneration without encapsulation of seed cells or application of external simulating cues. These exciting results demonstrate that our AC hydrogel is a promising scaffold for bone regeneration. Our work can also inspire the constituent and structure design of biomaterial scaffolds for tissue regeneration.

Loose Pre-Cross-Linking Mediating Cellulose Self-Assembly for 3D Printing Strong and Tough Biomimetic Scaffolds

The lack of an effective printable ink preparation method and the usual mechanically weak performance obstruct the functional 3D printing hydrogel exploitation and application. Herein, we propose a gentle pre-cross-linking strategy to enable a loosely cross-linked cellulose network for simultaneously achieving favorable printability and a strong hydrogel network via mediating the cellulose self-assembly. A small amount of epichlorohydrin is applied to (i) slightly pre-cross-link the cellulose chains for forming the percolating network to regulate the rheological properties and (ii) form the loosely cross-linked points to mediate the cellulose chains' self-assembly for achieving superior mechanical properties. The fabrication of the complex 3D structures verifies the design flexibility. The printed cellulose hydrogels exhibit a biomimetic nanofibrous topology, remarkable tensile and compressive strength (5.22 and 11.80 MPa), as well as toughness (1.81 and 2.16 MJ/m³). As a demonstration, a bilayer scaffold (mimicking the osteochondral structure) consisting of a top pristine cellulose and a bottom cellulose/bioactive glass hydrogel is printed and exhibits superior osteochondral defect repair performance, showing a potential in tissue engineering. We anticipate that our loose pre-cross-linking 3D printing ink preparation concept can inspire the development of other polymeric inks and strong 3D printing functional hydrogels, eventually spreading the applications in diverse fields.

In Situ Supramolecular Gel Formed by Cyclohexane Diamine with Aldehyde Derivative

Low-molecular-weight gels have great potential for use in a variety of fields, including petrochemicals, healthcare, and tissue engineering. These supramolecular gels are frequently metastable, implying that their properties are kinetically controlled to some extent. Here, we report on the in situ supramolecular gel formation by mixing 1,3-cyclohexane diamine (1) and isocyanate derivative (2) without any catalysis at room temperature in various organic solvents. A mixture of building blocks 1 and 2 in various organic solvents, dichloromethane, tetrahydrofuran, chloroform, toluene, and 1,4-dioxane, resulted in the stable formation of supramolecular gel at room temperature within 60–100 s. This gel formation was caused by the generation of urea moieties, which allows for the formation of intermolecular hydrogen-bonding interactions via reactions 1 and 2. In situ supramolecular gels demonstrated a typical entangled fiber structure with a width of 600 nm and a length of several hundred μm. In addition, the supramolecular gels were thermally reversible by heating and cooling. The viscoelastic properties of supramolecular gels in strain and frequency sweets were enhanced by increasing the concentration of a mixed 1 and 2. Furthermore, the supramolecular gels displayed a thixotropic effect, indicating a thermally reversible gel.

An injectable hydroxypropyl-β-cyclodextrin cross-linked gelatin-based hydrogel loaded bone mesenchymal stem cell for osteogenic and in vivo bone regeneration of femoral head necrosis

An injectable hydroxypropyl-β-cyclodextrin (HPβCD) cross-linking of gelatin (Gel) based hydrogel was embedded with BMSC in vivo bone regeneration of femoral head necrosis. This HPβCD-Gel hydrogel possesses quick gelation within 6 min; a high-water uptake resulted in faster biodegradation, high swelling, and a 3D porous network that strengthened its mechanical, surface, and morphological properties. The results indicated that BMSC showed high cell viability (>90%) during measurement; HPβCD-Gel hydrogels induced BMSC differentiation into osteocytes within 14 days more efficiently than the osteogenic medium. The HPβCD-Gel/BMSC hydrogels that were injected into the necrosis site of the femoral head in the vessels were measured for 2 weeks. In addition, the vessel density and mean vessel diameters increased in the next 2-8 weeks followed by increased new bone formation, according to the in vivo analysis. Overall, our findings show that this method is a promising strategy for improving femoral head necrosis bone regeneration.

Machine Learning-Driven Biomaterials Evolution

Biomaterials is an exciting and dynamic field, which uses a collection of diverse materials to achieve desired biological responses. While there is constant evolution and innovation in materials with time, biomaterials research has been hampered by the relatively long development period required. In recent years, driven by the need to accelerate materials development, the applications of machine learning in materials science has progressed in leaps and bounds. The combination of machine learning with high-throughput theoretical predictions and high-throughput experiments (HTE) has shifted the traditional Edisonian (trial and error) paradigm to a data-driven paradigm. In this review, each type of biomaterial and their key properties and use cases are systematically discussed, followed by how machine learning can be applied in the development and design process. The discussions are classified according to various types of materials used including polymers, metals, ceramics, and nanomaterials, and implants using additive manufacturing. Last, the current gaps and potential of machine learning to further aid biomaterials discovery and application are also discussed.

Evaluation of biological responses to micro-particles derived from a double network hydrogel

The double network hydrogels (DN gels) composed of poly (2-acrylamido-2-methyl propanesulfonic acid) (PAMPS) as the brittle first network and poly (N,N-dimethylacrylamide) (PDMA) as the ductile second network have been proven...

Self-Organized Spatiotemporal Mineralization of Hydrogel: A Simulant of Osteon

Nature creates fascinating self-organized spatiotemporal patterns through the delicate control of reaction-diffusion dynamics. As the primary unit of cortical bone, osteon has concentric lamellar architecture, which plays a crucial role in the mechanical and physiological functions of bone. However, it remains a great challenge to fabricate the osteon-like structure in a natural self-organization way. Taking advantage of the nonequilibrium reaction in hydrogels, a simple mineralization strategy to closely mimic the formation of osteon in a mild physiological condition is developed. By constructing two reverse concentration gradients of ions from periphery to interior of cylindrical hydrogel, spatiotemporal self-organization of calcium phosphate in concentric rings is generated. It is noteworthy that minerals in different layers possess diverse contents and crystalline phases, which further guide the adhesion and spread of osteoblasts on these patterns, resembling the architecture and cytological behavior of osteon. Besides, theoretical data indicates the predominate role of ion concentrations and pH values of solution, in good accordance with experimental results. Independent of precise instruments, this lifelike method is easily obtained, cost-efficient, and effectively imitates the mineral deposition in osteon from a physiochemical view. The strategy may be expanded to develop other functional material patterns via spatiotemporal self-organization.

Asymmetric barrier membranes based on polysaccharide micro-nanocomposite hydrogel: Synthesis, characterization, and their antibacterial and osteogenic activities

Guided tissue regeneration (GTR) strategies enable periodontal tissue regeneration, generally by providing barrier membranes. However, currently available membranes have limited osteoconductive and antibacterial potential. To address these challenges, we fabricated a new asymmetric barrier membrane. Agarose hydrogel functions as the main body of the barrier membrane. Hollow carbonated hydroxyapatite (CHA) prepared by hydrothermal method, was sedimented in agarose to exhibit an asymmetrical structure. And ε-poly-lysine (ε-PLL) was chosen as an antimicrobial agent to equip the membrane with long-lasting antibacterial activity. With the increased dose of CHA addition, the barrier membrane shows better biocompatibility, and higher mechanical properties. We demonstrated the osteoconductivity and antibacterial properties of the membrane in vitro and in vivo. In summary, our findings suggest that the barrier membrane has good osteoconductive and antibacterial properties, indicating its potential for periodontal tissue engineering.

Large-sized bone defect repair by combining a decalcified bone matrix framework and bone regeneration units based on photo-crosslinkable osteogenic microgels

Physiological repair of large-sized bone defects is great challenging in clinic due to a lack of ideal grafts suitable for bone regeneration. Decalcified bone matrix (DBM) is considered as an ideal bone regeneration scaffold, but low cell seeding efficiency and a poor osteoinductive microenvironment greatly restrict its application in large-sized bone regeneration. To address these problems, we proposed a novel strategy of bone regeneration units (BRUs) based on microgels produced by photo-crosslinkable and microfluidic techniques, containing both the osteogenic ingredient DBM and vascular endothelial growth factor (VEGF) for accurate biomimic of an osteoinductive microenvironment. The physicochemical properties of microgels could be precisely controlled and the microgels effectively promoted adhesion, proliferation, and osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) in vitro. BRUs were successfully constructed by seeding BMSCs onto microgels, which achieved reliable bone regeneration in vivo. Finally, by integrating the advantages of BRUs in bone regeneration and the advantages of DBM scaffolds in 3D morphology and mechanical strength, a BRU-loaded DBM framework successfully regenerated bone tissue with the desired 3D morphology and effectively repaired a large-sized bone defect of rabbit tibia. The current study developed an ideal bone biomimetic microcarrier and provided a novel strategy for bone regeneration and large-sized bone defect repair.

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The photo-crosslinkable microgels contained both osteogenic ingredient DBM powders and angiogenic growth factor VEGF.

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The photo-crosslinkable microgels effectively promote adhesion, proliferation, and osteogenic differentiation of BMSCs in vitro.

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Bone regeneration units (BRUs) successfully achieve reliable bone regeneration in vivo.

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The combination of DBM scaffold and BRUs successfully regenerate bone tissue with the desired 3D morphology and repair large-sized bone defect of rabbit tibia.

Biomimetic cuttlebone polyvinyl alcohol/carbon nanotubes/hydroxyapatite aerogel scaffolds enhanced bone regeneration

Inspired by the ordered porous nanostructure of bone, biomimetic functionalization porous biomaterial could be considered as promising substitutes for bone regeneration. To realize the relevant biomimetic porous structure, polyvinyl alcohol (PVA)-based biomimetic cuttlebone aerogel scaffold which simultaneously contained modified carbon nanotubes (MCNTs) and hydroxyapatite (HAP) was first prepared using a one-step rapid freeze-drying method. By adjusting the MCNTs contents, both the surface hydrophilicity and the mechanical properties of the scaffold could be improved concurrently. Besides, the PVA/MCNTs/HAP enhanced the adhesion, differentiation and gene expression of osteogenic markers performances of MC3T3-E1 cells. Furthermore, the aerogel scaffolds were implanted into the calvarial defect model of SD IGS Rat to evaluate osteogenic performance in vivo. The Micro-CT characterization and bone content theoretical analysis after 8 weeks together indicated that the PVA/MCNTs/HAP aerogel scaffolds could accelerate bone regeneration without the contribution of endogenous cytokines. The unique biomimetic porous structure, superior mechanical properties and excellent bone regeneration capacity of PVA/MCNTs/HAP aerogel scaffolds made them potential materials for bone regeneration.

Mechanical Integrity in a Dynamic Interpenetrating Hydrogel Network of Supramolecular Peptide-Polysaccharide Supports Enhanced Chondrogenesis

Tissue engineering demands intelligently designed scaffolds that encompass the properties of the target tissues in terms of mechanical and bioactive properties. An ideal scaffold for engineering a cartilage tissue should provide the chondrocytes with a favorable 3D microarchitecture apart from possessing optimal mechanical characteristics such as compressibility, energy dissipation, strain stiffening, etc. Herein, we used a unique design approach to develop a hydrogel having a dynamic interpenetrating network to serve as a framework to support chondrocyte growth and differentiation. An amyloid-inspired peptide amphiphile (1) was self-assembled to furnish kinetically controlled nanofibers and incorporated in a dynamic covalently cross-linked polysaccharide network of carboxymethyl cellulose dialdehyde (CMC-D) and carboxymethyl chitosan (CMCh) using Schiff base chemistry. The dynamic noncovalent interaction played a pivotal role in providing the desired modulation in the structure and mechanical properties of the double-network hydrogels that are imperative for cartilage scaffold design. The adaptable nature supported shear-induced extrusion of the hydrogel and facilitated various cellular functions while maintaining its integrity. The potential of the as-developed hydrogels to support in vitro chondrogenesis was explored using human chondrocytes. Evidence of improved cell growth and cartilage-specific ECM production confirmed the potential of the hydrogel to support cartilage tissue engineering while reaffirming the significance of mimicking the biophysical microenvironment to induce optimal tissue regeneration.

Hydroxyapatite-hybridized double-network hydrogel surface enhances differentiation of bone marrow-derived mesenchymal stem cells to osteogenic cells

Recently, we have developed a hydroxyapatite (HAp)-hybridized double-network (DN) hydrogel (HAp/DN gel), which can robustly bond to the bone tissue in the living body. The purpose of this study is to clarify whether the HAp/DN gel surface can differentiate the bone marrow-derived mesenchymal stem cells (MSCs) to osteogenic cells. We used the MSCs which were harvested from the rabbit bone marrow and cultured on the polystyrene (PS) dish using the autogenous serum-supplemented medium. First, we confirmed the properties of MSCs by evaluating colony forming unit capacity, expression of MSC markers using flow cytometry, and multidifferential capacity. Secondly, polymerase chain reaction analysis demonstrated that the HAp/DN gel surface significantly enhanced mRNA expression of the eight osteogenic markers (TGF-β1, BMP-2, Runx2, Col-1, ALP, OPN, BSP, and OCN) in the cultured MSCs at 7 days than the PS surfaces (p < 0.0001), while the DN gel and HAp surfaces provided no or only a slight effect on the expression of these markers except for Runx2. Additionally, the alkaline phosphatase activity was significantly higher in the cells cultured on the HAp/DN gel surface than in the other three material surfaces (p < 0.0001). Thirdly, when the HAp/DN gel plug was implanted into the rabbit bone marrow, MSC marker-positive cells were recruited in the tissue generated around the plug at 3 days, and Runx2 and OCN were highly expressed in these cells. In conclusion, this study demonstrated that the HAp/DN gel surface can differentiate the MSCs into osteogenic cells.

Smart and Biomimetic 3D and 4D Printed Composite Hydrogels: Opportunities for Different Biomedical Applications

In recent years, smart/stimuli-responsive hydrogels have drawn tremendous attention for their varied applications, mainly in the biomedical field. These hydrogels are derived from different natural and synthetic polymers but are also composite with various organic and nano-organic fillers. The basic functions of smart hydrogels rely on their ability to change behavior; functions include mechanical, swelling, shaping, hydrophilicity, and bioactivity in response to external stimuli such as temperature, pH, magnetic field, electromagnetic radiation, and biological molecules. Depending on the final applications, smart hydrogels can be processed in different geometries and modalities to meet the complicated situations in biological media, namely, injectable hydrogels (following the sol-gel transition), colloidal nano and microgels, and three dimensional (3D) printed gel constructs. In recent decades smart hydrogels have opened a new horizon for scientists to fabricate biomimetic customized biomaterials for tissue engineering, cancer therapy, wound dressing, soft robotic actuators, and controlled release of bioactive substances/drugs. Remarkably, 4D bioprinting, a newly emerged technology/concept, aims to rationally design 3D patterned biological matrices from synthesized hydrogel-based inks with the ability to change structure under stimuli. This technology has enlarged the applicability of engineered smart hydrogels and hydrogel composites in biomedical fields. This paper aims to review stimuli-responsive hydrogels according to the kinds of external changes and t recent applications in biomedical and 4D bioprinting.

Load-bearing hydrogels ionically reinforced through competitive ligand exchanges

Fast advances in soft robotics and tissue engineering demand for new soft materials whose mechanical properties can be interchangeably and locally varied, thereby enabling, for example, the design of soft joints within an integral material. Inspired by nature, we introduce a competitive ligand-mediated approach to selectively and interchangeably reinforce metal-coordinated hydrogels. This is achieved by reinforcing carboxylate-containing hydrogels with Fe³⁺ ions. Key to achieving a homogeneous, predictable reinforcement of the hydrogels is the presence of weak complexation agents that delay the formation of metal-complexes within the hydrogels, thereby allowing a homogeneous distribution of the metal ions. The resulting metal-reinforced hydrogels show a compressive modulus of up to 2.5 MPa, while being able to withstand pressures as high as 0.6 MPa without appreciable damage. Competitive ligand exchanges offer an additional advantage: they enable non-linear compositional changes that, for example, allow the formation of joints within these hydrogels. These features open up new possibilities to extend the field of use of metal reinforced hydrogels to load-bearing applications that are omnipresent for example in soft robots and actuators.

Hydrogel Composites with Different Dimensional Nanoparticles for Bone Regeneration

The treatment of large segmental bone defects and complex types of fractures caused by trauma, inflammation, or tumor resection is still a challenge in the field of orthopedics. Various natural or synthetic biological materials used in clinical applications cannot fully replicate the structure and performance of raw bone. This highlights how to endow materials with multiple functions and biological properties, which is a problem that needs to be solved in practical applications. Hydrogels with outstanding biocompatibility, for their casting into any shape, size, or form, are suitable for different forms of bone defects. Therefore, they have been used in regenerative medicine more widely. In this review, versatile hydrogels are compounded with nanoparticles of different dimensions, and many desirable features of these materials in bone regeneration are introduced, including drug delivery, cell factor vehicle, cell scaffolds, which have potential in bone regeneration applications. The combination of hydrogels and nanoparticles of different dimensions encourages better filling of bone defect areas and has higher adaptability. This is due to the minimally invasive properties of the material and ability to match irregular defects. These biological characteristics make composite hydrogels with different dimensional nanoparticles become one of the most attractive options for bone regeneration materials.