Photocrosslinkable and elastomeric hydrogels for bone regeneration

Mild Thermotherapy-Assisted GelMA/HA/MPDA@Roxadustat 3D-Printed Scaffolds with Combined Angiogenesis-Osteogenesis Functions for Bone Regeneration

Early reconstruction of the vascular network is a prerequisite to the effective treatment of substantial bone defects. Traditional 3D printed tissue engineering scaffolds designed to repair large bone defects do not effectively regenerate the vascular network, and rely only on the porous structure within the scaffold for nutrient transfer and metabolic waste removal. This leads to delayed bone restoration and hence functional recovery. Therefore, strategies for generation scaffolds with the capacity to efficiently regenerate vascularization should be developed. This study loads roxarestat (RD), which can stabilize HIF-1α expression in a normoxic environment, onto the mesopore polydopamine nanoparticles (MPDA@RD) to enhance the reconstruction of vascular network in large bone defects. Subsequently, MPDA@RD is mixed with GelMA/HA hydrogel bioink to fabricate a multifunctional hydrogel scaffold (GHM@RD) through 3D printing. In vitro results show that the GHM@RD scaffolds achieve good angiogenic-osteogenic coupling by activating the PI3K/AKT/HSP90 pathway in BMSCs and the PI3K/AKT/HIF-1α pathway in HUVECs under mild thermotherapy. In vivo experiments reveal that RD and mild hyperthermia synergistically induce early vascularization and bone regeneration of critical bone defects. In conclusion, the designed GHM@RD drug delivery scaffold with mild hyperthermia holds great therapeutic value for future treatment of large bone defects.

Nanofibrous Material-Reinforced Printable Ink for Enhanced Cell Proliferation and Tissue Regeneration

The three-dimensional (3D) printing of biomaterials, cells, and bioactive components, including growth factors, has gained interest among researchers in the field of tissue engineering (TE) with the aim of developing many scaffolds to sustain size, shape fidelity, and structure and retain viable cells inside a network. The biocompatible hydrogel employed in 3D printing should be soft enough to accommodate cell survival. At the same time, the gel should be mechanically strong to avoid the leakage of cells into the surrounding medium. Considering these basic criteria, researchers have developed nanocomposite-based printable inks with suitable mechanical and electroconductive properties. These nanomaterials, including carbon family nanomaterials, transition metal dichalcogenides, and polymeric nanoparticles, act as nanofillers and dissipate stress across polymeric networks through their electroactive interactions. Nanofiber-reinforced printable ink is one kind of nanocomposite-based ink that comprises dispersed nanofiber components in a hydrogel matrix. In this current review, we compile various TE applications of nanofiber-reinforced printable ink and describe the 3D-printing parameters, classification, and impact of cross-linkage. Furthermore, we discuss the challenges and future perspectives in this field.

Bioactive and biomimetic 3D scaffolds for bone tissue engineering using graphitic carbon nitride as a sustainable visible light photoinitiator

Graphitic carbon nitride (g-C3N4) is explored as a novel sustainable visible light photoinitiator for the preparation of biomimetic 3D hydrogel scaffolds comprising gelatin methacrylamide (GelMA) and dopamine methacrylamide for use in tissue engineering. The initiator efficiency was assessed by comparing the swelling behavior and the stability of photopolymerized hydrogels prepared with GelMA of different degrees of functionalization and different comonomer compositions. Bioactive composite hydrogels with a 50 wt% nanohydroxyapatite (nHAp) content, to closely mimic the actual bone composition, were successfully obtained by the introduction of nHAp in the prepolymer solutions followed by photopolymerization. The composite hydrogels demonstrated enhanced mechanical properties and excellent stability in PBS verifying the preparation of robust 3D scaffolds for use in cancellous or pre-calcified bone tissue engineering applications. The in vitro cell response of the composite scaffolds exhibited high cell viability and enhanced differentiation of pre-osteoblasts to mature osteoblasts, demonstrating their osteogenic potential. This work establishes, for the first time, the excellent properties of g-C3N4 as a sustainable, visible light initiator, fully satisfying the principles of green chemistry, for the preparation of robust and biologically relevant hydrogels, and proposes a new approach to overcome the main challenges of conventional photoinitiators in cell scaffold fabrication, such as photobleaching, high cost and non-scalable synthesis employing toxic organic precursors and solvents.

Bioinspired self-healing injectable nanocomposite hydrogels based on oxidized dextran and gelatin for growth-factor-free bone regeneration

Hydrogels with great biocompatibility, biodegradability, and mechanical properties, combined with osteoconductivity, osteoinductivity, and osteointegration as biomaterials for bone regeneration without adding exogenous growth factors and cells are highly appealing but challenging. Here, inspired by organic-inorganic analogues of natural bone tissue and the adhesion chemistry of mussels, nanocomposite hydrogels with self-healing, injectable, adhesive, antioxidant, and osteoinductive properties (termed GO-PHA-CPs) were constructed by Schiff base cross-linking between dopamine-modified gelatin (Gel-DA) and oxidized dextran (ODex). Furthermore, the hydrogel network was enhanced by the introduction of polydopamine-functionalized nanohydroxyapatite (PHA) by improving the interfacial compatibility between the rigid inorganic particles and the flexible hydrogel matrix. Bioactive cod peptides (CPs) with osteogenic activity from Atlantic cod were further incorporated into the nanocomposite hydrogel. As a result, the multicomponent nanocomposite hydrogel favored the adhesion and spreading of MC3T3-E1 cells. The increased ALP activity suggested that GO-PHA-CPs hydrogels contributed to the osteogenic differentiation of MC3T3-E1 cells. The suitability of GO-PHA-CPs hydrogels for enhancing bone regeneration in vivo was further confirmed by the rat femoral defect model. Our results indicate that the multifunctional GO-PHA-CPs nanocomposite hydrogels without growth factors are a promising and effective candidate material for bone regeneration.

GelMA-based bioactive hydrogel scaffolds with multiple bone defect repair functions: therapeutic strategies and recent advances

Currently, the clinical treatment of critical bone defects attributed to various causes remains a great challenge, and repairing these defects with synthetic bone substitutes is the most common strategy. In general, tissue engineering materials that mimic the structural, mechanical and biological properties of natural bone have been extensively applied to fill bone defects and promote in situ bone regeneration. Hydrogels with extracellular matrix (ECM)-like properties are common tissue engineering materials, among which methacrylate-based gelatin (GelMA) hydrogels are widely used because of their tunable mechanical properties, excellent photocrosslinking capability and good biocompatibility. Owing to their lack of osteogenic activity, however, GelMA hydrogels are combined with other types of materials with osteogenic activities to improve the osteogenic capability of the current composites. There are three main aspects to consider when enhancing the bone regenerative performance of composite materials: osteoconductivity, vascularization and osteoinduction. Bioceramics, bioglass, biomimetic scaffolds, inorganic ions, bionic periosteum, growth factors and two-dimensional (2D) nanomaterials have been applied in various combinations to achieve enhanced osteogenic and bone regeneration activities. Three-dimensional (3D)-bioprinted scaffolds are a popular research topic in bone tissue engineering (BTE), and printed and customized scaffolds are suitable for restoring large irregular bone defects due to their shape and structural tunability, enhanced mechanical properties, and good biocompatibility. Herein, the recent progress in research on GelMA-based composite hydrogel scaffolds as multifunctional platforms for restoring critical bone defects in plastic or orthopedic clinics is systematically reviewed and summarized. These strategies pave the way for the design of biomimetic bone substitutes for effective bone reconstruction with good biosafety. This review provides novel insights into the development and current trends of research on GelMA-based hydrogels as effective bone tissue engineering (BTE) scaffolds for correcting bone defects, and these contents are summarized and emphasized from various perspectives (osteoconductivity, vascularization, osteoinduction and 3D-bioprinting). In addition, advantages and deficiencies of GelMA-based bone substitutes used for bone regeneration are put forward, and corresponding improvement measures are presented prior to their clinical application in near future (created with BioRender.com).

Gelatin Methacryloyl Hydrogel, from Standardization, Performance, to Biomedical Application

Gelatin methacryloyl (GelMA), a photocurable hydrogel, is widely used in 3D culture, particularly in 3D bioprinting, due to its high biocompatibility, tunable physicochemical properties, and excellent formability. However, as the properties and performances of GelMA vary under different synthetic conditions, there is a lack of standardization, leading to conflicting results. In this study, a uniform standard is established to understand and enhance GelMA applications. First, the basic concept of GelMA and the density of the molecular network (DMN) are defined. Second, two properties, degrees of substitution and ratio of solid content, as the main measurable parameters determining the DMN are used. Third, the mechanisms and relationships between DMN and its performance in various applications in terms of porosity, viscosity, formability, mechanical strength, swelling, biodegradation, and cytocompatibility are theoretically explained. The main questions that are answered: what does performance mean, why is it important, how to optimize the basic parameters to improve the performance, and how to characterize it reasonably and accurately? Finally, it is hoped that this knowledge will eliminate the need for researchers to conduct tedious and repetitive pre-experiments, enable easy communication for achievements between groups under the same standard, and fully explore the potential of the GelMA hydrogel.

Gelatin hydrogel reinforced with mussel-inspired polydopamine-functionalized nanohydroxyapatite for bone regeneration

Developing high-strength hydrogels with biocompatibility and bone conductibility is still desirable for bone regeneration. The nanohydroxyapatite (nHA) was incorporated into a dopamine-modified gelatin (Gel-DA) hydrogel system to create a highly biomimetic native bone tissue microenvironment. In addition, to further increase the cross-linking density between nHA and Gel-DA, nHA was functionalized by mussel-inspired polydopamine (PDA). Compared with nHA, adding polydopamine functionalized nHA (PHA) increased the compressive strength of Gel-Da hydrogel from 449.54 ± 180.32 kPa to 611.18 ± 211.86 kPa without affecting its microstructure. Besides, the gelation time of Gel-DA hydrogels with PHA incorporation (GD-PHA) was controllable from 49.47 ± 7.93 to 88.11 ± 31.18 s, contributing to its injectable ability in clinical applications. In addition, the abundant phenolic hydroxyl group of PHA was beneficial to the cell adhesion and proliferation of Gel-DA hydrogels, leading to the excellent biocompatibility of Gel-PHA hydrogels. Notably, the GD-PHA hydrogels could accelerate the bone repair efficiency in the rat model of the femoral defect. In conclusion, our results suggest the Gel-PHA hydrogel with osteoconductivity, biocompatibility, and enhanced mechanical properties is a potential bone repair material.

Human urine-derived stem cell exosomes delivered via injectable GelMA templated hydrogel accelerate bone regeneration

The key to critical bone regeneration in tissue engineering relies on an ideal bio-scaffold coated with a controlled release of growth factors. Gelatin methacrylate (GelMA) and Hyaluronic acid methacrylate (HAMA) have been a novel topic of interest in bone regeneration while introducing appropriate nano-hydroxyapatite (nHAP) to improve its mechanical properties. And the exosomes derived from human urine-derived stem cells (human ^USCEXOs) have also been reported to promote osteogenesis in tissue engineering. The present study aimed to design a new GelMA-HAMA/nHAP composite hydrogel as a drug delivery system. The ^USCEXOs were encapsulated and slow-released in the hydrogel for better osteogenesis. The characterization of the GelMA-based hydrogel showed excellent controlled release performance and appropriate mechanical properties. The in vitro studies showed that the ^USCEXOs/GelMA-HAMA/nHAP composite hydrogel could promote the osteogenesis of bone marrow mesenchymal stem cells (BMSCs) and the angiogenesis of endothelial progenitor cells (EPCs), respectively. Meanwhile, the in vivo results confirmed that this composite hydrogel could significantly promote the defect repair of cranial bone in the rat model. In addition, we also found that ^USCEXOs/GelMA-HAMA/nHAP composite hydrogel can promote the formation of H-type vessels in the bone regeneration area, enhancing the therapeutic effect. In conclusion, our findings suggested that this controllable and biocompatible ^USCEXOs/GelMA-HAMA/nHAP composite hydrogel may effectively promote bone regeneration by coupling osteogenesis and angiogenesis.

Self-Adhesive Hydrogel Biomimetic Periosteum to Promote Critical-Size Bone Defect Repair via Synergistic Osteogenesis and Angiogenesis

The periosteum plays an important role in the regeneration of critical-size bone defects, with functions of recruiting multiple cells, accelerating vascular network reconstruction, and guiding bone tissue regeneration. However, these functions cannot be easily implemented by simply simulating the periosteum via a material structure design or by loading exogenous cytokines. Herein, inspired by the periosteal function, we propose a biomimetic periosteum preparation strategy to enhance natural polymer hydrogel membranes using inorganic bioactive materials. The biomimetic periosteum having bone tissue self-adhesive functions and resembling an extracellular matrix was prepared using dopamine-modified gelatin and oxidized hyaluronan (GA/HA), and micro/nanobioactive glass (MNBG) was further incorporated into the hydrogel to fabricate an organic/inorganic co-crosslinked hydrogel membrane (GA/HA-BG). The addition of MNBG enhanced the stability of the natural polymer hydrogel membrane, resulting in a sustained degradation time, biomineralization, and long-term release of ions. The Ca²⁺ and SiO₄^4- ions released by bioactive glass were shown to recruit cells and promote the differentiation of bone marrow stromal cells into osteoblasts, initiating multicentric osteogenic behavior. Additionally, the bioactive ions were able to continuously stimulate the endogenous expression of vascular endothelial growth factor from human umbilical vein endothelial cells through the PI3K/Akt/HIF-1α pathway, which accelerated vascularization of the defect area and synergistically promoted the repair of bone defects. This organic-inorganic biomimetic periosteum has been proved to be effective and versatile in critical-size bone defect repair and is expected to provide a promising strategy for solving clinical issues.

Self-Healing Injectable Hydrogels for Tissue Regeneration

Biomaterials with the ability to self-heal and recover their structural integrity offer many advantages for applications in biomedicine. The past decade has witnessed the rapid emergence of a new class of self-healing biomaterials commonly termed injectable, or printable in the context of 3D printing. These self-healing injectable biomaterials, mostly hydrogels and other soft condensed matter based on reversible chemistry, are able to temporarily fluidize under shear stress and subsequently recover their original mechanical properties. Self-healing injectable hydrogels offer distinct advantages compared to traditional biomaterials. Most notably, they can be administered in a locally targeted and minimally invasive manner through a narrow syringe without the need for invasive surgery. Their moldability allows for a patient-specific intervention and shows great prospects for personalized medicine. Injected hydrogels can facilitate tissue regeneration in multiple ways owing to their viscoelastic and diffusive nature, ranging from simple mechanical support, spatiotemporally controlled delivery of cells or therapeutics, to local recruitment and modulation of host cells to promote tissue regeneration. Consequently, self-healing injectable hydrogels have been at the forefront of many cutting-edge tissue regeneration strategies. This study provides a critical review of the current state of self-healing injectable hydrogels for tissue regeneration. As key challenges toward further maturation of this exciting research field, we identify (i) the trade-off between the self-healing and injectability of hydrogels vs their physical stability, (ii) the lack of consensus on rheological characterization and quantitative benchmarks for self-healing injectable hydrogels, particularly regarding the capillary flow in syringes, and (iii) practical limitations regarding translation toward therapeutically effective formulations for regeneration of specific tissues. Hence, here we (i) review chemical and physical design strategies for self-healing injectable hydrogels, (ii) provide a practical guide for their rheological analysis, and (iii) showcase their applicability for regeneration of various tissues and 3D printing of complex tissues and organoids.

Wirelessly Powered 3D Printed Hierarchical Biohybrid Robots with Multiscale Mechanical Properties

The integration of flexible and stretchable electronics into biohybrid soft robotics can spur the development of new approaches to fabricate biohybrid soft machines, thus enabling a wide variety of innovative applications. Inspired by flexible and stretchable wireless-based bioelectronic devices, we have developed untethered biohybrid soft robots that can execute swimming motions, which are remotely controllable by the wireless transmission of electrical power into a cell simulator. To this end, wirelessly-powered, stretchable, and lightweight cell stimulators were designed to be integrated into muscle bodies without impeding the robots' underwater swimming abilities. The cell stimulators function by generating controlled monophasic pulses of up to ∼9 V in biological environments. By differentiating induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) directly on the cell stimulators using an accordion-inspired, three-dimensional (3D) printing construct, we have replicated the native myofiber architecture with comparable robustness and enhanced contractibility. Wirelessly modulated electrical frequencies enabled us to control the speed and direction of the biohybrid soft robots. A maximum locomotion speed of ∼580 μm/s was achieved in robots possessing a large body size by adjusting the pacing frequency. This innovative approach will provide a platform for building untethered and biohybrid systems for various biomedical applications.

Injectable hydrogels for bone and cartilage tissue engineering: a review

Tissue engineering, using a combination of living cells, bioactive molecules, and three-dimensional porous scaffolds, is a promising alternative to traditional treatments such as the use of autografts and allografts for bone and cartilage tissue regeneration. Scaffolds, in this combination, can be applied either through surgery by implantation of cell-seeded pre-fabricated scaffolds, or through injection of a solidifying precursor and cell mixture, or as an injectable cell-seeded pre-fabricated scaffold. In situ forming and pre-fabricated injectable scaffolds can be injected directly into the defect site with complex shape and critical size in a minimally invasive manner. Proper and homogeneous distribution of cells, biological factors, and molecular signals in these injectable scaffolds is another advantage over pre-fabricated scaffolds. Due to the importance of injectable scaffolds in tissue engineering, here different types of injectable scaffolds, their design challenges, and applications in bone and cartilage tissue regeneration are reviewed.

Surface-Engineered Hybrid Gelatin Methacryloyl with Nanoceria as Reactive Oxygen Species Responsive Matrixes for Bone Therapeutics

Designing various transplantable biomaterials, especially nanoscale matrixes for bone regeneration, involves precise tuning of topographical features. The cellular fate on such engineered surfaces is highly influenced by many factors imparted by the surface modification (hydrophilicity, stiffness, porosity, roughness, ROS responsiveness). Herein, hybrid matrixes of gelatin methacryloyl (GelMA) decorated with uniform layers of nanoceria (nCe), called Ce@GelMA, were developed without direct incorporation of nCe into the scaffolds. The fabrication involves a simple base-mediated in situ deposition in which uniform nCe coatings were first made on GelMA hydrogels and then nCe layered GelMA scaffolds were made by cryodesiccation. In this hybrid platform, degradable GelMA biopolymer provides the porous microstructure and nCe provides the nanoscaled biointerface. The surface morphology and elemental composition of the matrixes analyzed by field emission scanning electron microscopy (FE-SEM) and energy-dispersive spectroscopy (EDS) show uniform nCe distribution. The surface nanoroughness and chemistry of the matrixes were also characterized using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The presence of nCe on GelMA enhanced its mechanical properties as confirmed by compressive modulus analysis. Substantial bonelike nanoscale hydroxyapatite formation was observed on scaffolds after simulated body fluid (SBF) immersion, which was confirmed by SEM, X-ray diffraction (XRD), and Fourier transform infrared (FT-IR) spectroscopy. Moreover, the developed scaffolds could also be used as an antioxidant matrix owing to the reactive oxygen species (ROS) scavenging property of nCe as assessed by 3,3',5,5'-tetramethylbenzidine (TMB) assay. The enhanced proliferation and viability of rat bone marrow mesenchymal stem cells (rMSCs) on the scaffold surface after 3 days of culture ensures the biocompatibility of the proposed material. Considering all, it is proposed that the micro/nanoscaled matrix could mimic the composition and function of hard tissues and could be utilized as degradable scaffolds in engineering bones.

Multifunctional GelMA platforms with nanomaterials for advanced tissue therapeutics

Polymeric hydrogels are fascinating platforms as 3D scaffolds for tissue repair and delivery systems of therapeutic molecules and cells. Among others, methacrylated gelatin (GelMA) has become a representative hydrogel formulation, finding various biomedical applications. Recent efforts on GelMA-based hydrogels have been devoted to combining them with bioactive and functional nanomaterials, aiming to provide enhanced physicochemical and biological properties to GelMA. The benefits of this approach are multiple: i) reinforcing mechanical properties, ii) modulating viscoelastic property to allow 3D printability of bio-inks, iii) rendering electrical/magnetic property to produce electro-/magneto-active hydrogels for the repair of specific tissues (e.g., muscle, nerve), iv) providing stimuli-responsiveness to actively deliver therapeutic molecules, and v) endowing therapeutic capacity in tissue repair process (e.g., antioxidant effects). The nanomaterial-combined GelMA systems have shown significantly enhanced and extraordinary behaviors in various tissues (bone, skin, cardiac, and nerve) that are rarely observable with GelMA. Here we systematically review these recent efforts in nanomaterials-combined GelMA hydrogels that are considered as next-generation multifunctional platforms for tissue therapeutics. The approaches used in GelMA can also apply to other existing polymeric hydrogel systems.

Promotion of Adrenal Pheochromocytoma (PC-12) Cell Proliferation and Outgrowth Using Schwann Cell-Laden Gelatin Methacrylate Substrate

Peripheral nerve injuries cause different degrees of nerve palsy and function loss. Due to the limitations of autografts, nerve tissue engineering (TE) scaffolds incorporated with various neurotrophic factors and cells have been investigated to promote nerve regeneration. However, the molecular mechanism is still poorly understood. In this study, we co-cultured Schwann cells (SCs) and rat adrenal pheochromocytoma (PC-12) cells on 50% degrees of methacryloyl substitution gelatin methacrylate (GelMA) scaffold. The SCs were encapsulated within the GelMA, and PC-12 cells were on the surface. A 5% GelMA was used as the co-culture scaffold since it better supports SCs proliferation, viability, and myelination and promotes higher neurotrophic factors secretion than 10% GelMA. In the co-culture, PC-12 cells demonstrated a higher cell proliferation rate and axonal extension than culturing without SCs, indicating that the secretion of neurotrophic factors from SCs can stimulate PC-12 growth and axonal outgrowth. The mRNA level for neurotrophic factors of SCs in 5% GelMA was further evaluated. We found significant upregulation when compared with a 2D culture, which suggested that this co-culture system could be a potential scaffold to investigate the mechanism of how SCs affect neuronal behaviors.

Polyethylene glycol diacrylate scaffold filled with cell-laden methacrylamide gelatin/alginate hydrogels used for cartilage repair

Natural cartilage tissue has excellent mechanical properties and has certain cellular components. At this stage, it is a great challenge to produce cartilage scaffolds with excellent mechanical properties, biocompatibility, and biodegradability. Hydrogels are commonly used in tissue engineering because of their excellent biocompatibility; however, the mechanical properties of commonly used hydrogels are difficult to meet the requirements of making cartilage scaffolds. The mechanical properties of high concentration polyethylene glycol diacrylate (PEGDA) hydrogel are similar to those of natural cartilage, but its biocompatibility is poor. Low concentration hydrogel has better biocompatibility, but its mechanical properties are poor. In this study, two different hydrogels were combined to produce cartilage scaffolds with good mechanical properties and strong biocompatibility. First, the PEGDA grid scaffold was printed with light curing 3D printing technology, and then the low concentration GelMA/Alginate hydrogel with chondral cells was filled into the PEGDA grid scaffold. After a series of cell experiments, the filling hydrogel with the best biocompatibility was screened out, and finally the filled hydrogel with cells and excellent biocompatibility was obtained. Cartilage tissue engineering scaffolds with certain mechanical properties were found to have a tendency of cartilage formation in in vitro culture. Compared with the scaffold obtained by using a single hydrogel, this molding method can produce a tissue engineering scaffold with excellent mechanical properties on the premise of ensuring biocompatibility, which has a certain potential application value in the field of cartilage tissue engineering.

Recent trends in gelatin methacryloyl nanocomposite hydrogels for tissue engineering

Gelatin methacryloyl (GelMA), a photocrosslinkable gelatin-based hydrogel, has been immensely used for diverse applications in tissue engineering and drug delivery. Apart from its excellent functionality and versatile mechanical properties, it is also suitable for a wide range of fabrication methodologies to generate tissue constructs of desired shapes and sizes. Despite its exceptional characteristics, it is predominantly limited by its weak mechanical strength, as some tissue types naturally possess high mechanical stiffness. The use of high GelMA concentrations yields high mechanical strength, but not without the compromise in its porosity, degradability, and three-dimensional (3D) cell attachment. Recently, GelMA has been blended with various natural and synthetic biomaterials to reinforce its physical properties to match with the tissue to be engineered. Among these, nanomaterials have been extensively used to form a composite with GelMA, as they increase its biological and physicochemical properties without affecting the unique characteristics of GelMA and also introduce electrical and magnetic properties. This review article presents the recent advances in the formation of hybrid GelMA nanocomposites using a variety of nanomaterials (carbon, metal, polymer, and mineral-based). We give an overview of each nanomaterial's characteristics followed by a discussion of the enhancement in GelMA's physical properties after its incorporation. Finally, we also highlight the use of each GelMA nanocomposite for different applications, such as cardiac, bone, and neural regeneration.

Fabrication and Characterization of Biodegradable Gelatin Methacrylate/Biphasic Calcium Phosphate Composite Hydrogel for Bone Tissue Engineering

In the field of bone tissue, maintaining adequate mechanical strength and tissue volume is an important part. Recently, biphasic calcium phosphate (BCP) was fabricated to solve the shortcomings of hydroxyapatite (HA) and beta-tricalcium phosphate (β-TCP), and it is widely studied in the field of bone-tissue engineering. In this study, a composite hydrogel was fabricated by applying BCP to gelatin methacrylate (GelMA). It was tested by using a mechanical tester, to characterize the mechanical properties of the prepared composite hydrogel. The fabricated BCP was analyzed through FTIR and XRD. As a result, a different characteristic pattern from hydroxyapatite (HA) and beta-tricalcium phosphate (β-TCP) was observed, and it was confirmed that it was successfully bound to the hydrogel. Then, the proliferation and differentiation of preosteoblasts were checked to evaluate cell viability. The analysis results showed high cell viability and relatively high bone differentiation ability in the composite hydrogel to which BCP was applied. These features have been shown to be beneficial for bone regeneration by maintaining the volume and shape of the hydrogel. In addition, hydrogels can be advantageous for clinical use, as they can shape the structure of the material for custom applications.

Effect of kartogenin-loaded gelatin methacryloyl hydrogel scaffold with bone marrow stimulation for enthesis healing in rotator cuff repair

BACKGROUND

Strategies involving microfracture, biomaterials, growth factors, and chemical agents have been evaluated for improving enthesis healing. Kartogenin (KGN) promotes selective differentiation of bone marrow mesenchymal stem cells (BMSCs) into chondrocytes. Gelatin methacryloyl (GelMA) is a promising biomaterial for engineering scaffolds and drug carriers. Herein, we investigated KGN-loaded GelMA hydrogel scaffolds with a bone marrow-stimulating technique for the repair of rotator cuff tear.

METHODS

KGN-loaded GelMA hydrogel scaffolds were obtained by ultraviolet GelMA crosslinking and vacuum freeze-drying. Fifty-four New Zealand rabbits were randomly divided into (1) repair only (control), (2) microfracture + repair (BMS), and (3) microfracture + repair augmentation with a KGN-loaded GelMA hydrogel scaffold (combined) groups. Tendons were repaired by transosseous sutures. The structure, degradation, and in vitro KGN release of the scaffolds were characterized. Animals were euthanized 4, 8, and 12 weeks after repair. Enthesis healing was evaluated by macroscopy, microcomputed tomography, histology, and biomechanical tests.

RESULTS

The KGN-loaded GelMA hydrogel scaffolds are porous with a 60.4 ± 28.2-μm average pore size, and they degrade quickly in 2.5 units/mL collagenase solution. Nearly 81% of KGN was released into phosphate-buffered saline within 12 hours, whereas the remaining KGN was released in 7 days. Macroscopically, the repaired tendons were attached to the footprint. No differences were detected postoperatively in microcomputed tomography analysis among groups. Fibrous scar tissue was the main component at the tendon-to-bone interface in the control group. Disorderly arranged cartilage formation was observed at the tendon-to-bone interface in the BMS and combined groups 4 weeks after repair; the combined group exhibited relatively more cartilage. The combined group showed improved cartilage regeneration 8 and 12 weeks after repair. Similar results were found in tendon maturation scores. The ultimate load to failure and stiffness of the repaired tendon increased in all 3 groups. At 4 weeks after repair, the BMS and combined groups exhibited greater ultimate load to failure than the control group, although there was no difference in stiffness among groups. The BMS and combined groups exhibited greater ultimate load to failure and stiffness than the control group, and the combined group exhibited better values than the BMS group at 8 and 12 weeks after repair.

CONCLUSION

Compared with the bone marrow-stimulating technique, the KGN-loaded GelMA hydrogel scaffold with bone marrow stimulation improved enthesis healing by promoting fibrocartilage formation and improving the mechanical properties.

Biomedical application of photo-crosslinked gelatin hydrogels

Abstract

During the past decades, photo-crosslinked gelatin hydrogel (methacrylated gelatin, GelMA) has gained a lot of attention due to its remarkable application in the biomedical field. It has been widely used in cell transplantation, cell culture and drug delivery, based on its crosslinking to form hydrogels with tunable mechanical properties and excellent bio-compatibility when exposed to light irradiation to mimic the micro-environment of native extracellular matrix (ECM). Because of its unique biofunctionality and mechanical tenability, it has also been widely applied in the repair and regeneration of bone, heart, cornea, epidermal tissue, cartilage, vascular, peripheral nerve, oral mucosa, and skeletal muscle et al. The purpose of this review is to summarize the recent application of GelMA in drug delivery and tissue engineering field. Moreover, this review article will briefly introduce both the development of GelMA and the characterization of GelMA. Finally, we discuss the challenges and future development prospects of GelMA as a tissue engineering material and drug or gene delivery carrier, hoping to contribute to accelerating the development of GelMA in the biomedical field.

Graphical abstract

Current Advances in 3D Bioprinting Technology and Its Applications for Tissue Engineering

Three-dimensional (3D) bioprinting technology has emerged as a powerful biofabrication platform for tissue engineering because of its ability to engineer living cells and biomaterial-based 3D objects. Over the last few decades, droplet-based, extrusion-based, and laser-assisted bioprinters have been developed to fulfill certain requirements in terms of resolution, cell viability, cell density, etc. Simultaneously, various bio-inks based on natural-synthetic biomaterials have been developed and applied for successful tissue regeneration. To engineer more realistic artificial tissues/organs, mixtures of bio-inks with various recipes have also been developed. Taken together, this review describes the fundamental characteristics of the existing bioprinters and bio-inks that have been currently developed, followed by their advantages and disadvantages. Finally, various tissue engineering applications using 3D bioprinting are briefly introduced.

The performance of 3D bioscaffolding based on a human periodontal ligament stem cell printing technique

Bone tissue plays an important role in supporting and protecting the structure and function of the human body. Bone defects are a common source of injury and there are many reconstruction challenges in clinical practice. However, 3D bioprinting of scaffolds provides a promising solution. Hydrogels have emerged as biomaterials with good biocompatibility and are now widely used as cell-loaded materials for bioprinting. This study involved three steps: First, sodium alginate (SA), gelatin (Gel), and nano-hydroxyapatite (na-HA) were mixed into a hydrogel and its rheological properties assessed to identify the optimum slurry for printing. Second, SA/Gel/na-HA bioscaffolds were printed using 3D bioprinting technology and their physical properties characterized for surface morphology, swelling, and mechanical properties. Finally, human periodontal ligament stem cells (hPDLSCs) were mixed with SA/Gel/na-HA printing slurry to create a "bioink" to prepare SA/Gel/na-HA/ hPDLSCs cell bioscaffolds. These were tested for biocompatibility and osteogenic differentiation performance using live/dead cell staining, cell adhesion, cell proliferation, and alkaline phosphatase activity. The SA/Gel/na-HA hydrogel exhibited shear-thinning behavior. The equilibrium swelling of the bioscaffold was 125.9%, the compression stress was 0.671 MPa, and the compression elastic modulus was 8.27 MPa. The SA/Gel/na-HA/hPDLSCs cell bioscaffolds caused effective stimulation of cell survival, proliferation, and osteoblast differentiation. Therefore, the SA/Gel/na-HA/hPDLSCs cell bioscaffolds displayed potential as a material for bone defect reconstruction.

Highly tunable bioactive fiber-reinforced hydrogel for guided bone regeneration

One of the most damaging pathologies that affects the health of both soft and hard tissues around the tooth is periodontitis. Clinically, periodontal tissue destruction has been managed by an integrated approach involving elimination of injured tissues followed by regenerative strategies with bone substitutes and/or barrier membranes. Regrettably, a barrier membrane with predictable mechanical integrity and multifunctional therapeutic features has yet to be established. Herein, we report a fiber-reinforced hydrogel with unprecedented tunability in terms of mechanical competence and therapeutic features by integration of highly porous poly(ε-caprolactone) fibrous mesh(es) with well-controlled 3D architecture into bioactive amorphous magnesium phosphate-laden gelatin methacryloyl hydrogels. The presence of amorphous magnesium phosphate and PCL mesh in the hydrogel can control the mechanical properties and improve the osteogenic ability, opening a tremendous opportunity in guided bone regeneration (GBR). Results demonstrate that the presence of PCL meshes fabricated via melt electrowriting can delay hydrogel degradation preventing soft tissue invasion and providing the mechanical barrier to allow time for slower migrating progenitor cells to participate in bone regeneration due to their ability to differentiate into bone-forming cells. Altogether, our approach offers a platform technology for the development of the next-generation of GBR membranes with tunable mechanical and therapeutic properties to amplify bone regeneration in compromised sites. STATEMENT OF SIGNIFICANCE: In this study, we developed a fiber-reinforced hydrogel platform with unprecedented tunability in terms of mechanical competence and therapeutic features for guided bone regeneration. We successfully integrated highly porous poly(ε-caprolactone) [PCL] mesh(es) into amorphous magnesium phosphate-laden hydrogels. The stiffness of the engineered hydrogel was significantly enhanced, and this reinforcing effect could be modulated by altering the number of PCL meshes and tailoring the AMP concentration. Furthermore, the fiber-reinforced hydrogel showed favorable cellular responses, significantly higher rates of mineralization, upregulation of osteogenic-related genes and bone formation. In sum, these fiber-reinforced membranes in combination with therapeutic agent(s) embedded in the hydrogel offer a robust, highly tunable platform to amplify bone regeneration not only in periodontal defects, but also in other craniomaxillofacial sites.

Conditioning of 3D Printed Nanoengineered Ionic-Covalent Entanglement Scaffolds with iP-hMSCs Derived Matrix

Additive manufacturing is a promising method for producing customized 3D bioactive constructs for regenerative medicine. Here, 3D printed highly osteogenic scaffolds using nanoengineered ionic-covalent entanglement ink (NICE) for bone tissue engineering are reported. This NICE ink consists of ionic-covalent entanglement reinforced with Laponite, a 2D nanosilicate (nSi) clay, allowing for the printing of anatomic-sized constructs with high accuracy. The 3D printed structure is able to maintain high structural stability in physiological conditions without any significant swelling or deswelling. The presence of nSi imparts osteoinductive characteristics to the NICE scaffolds, which is further augmented by depositing pluripotent stem cell-derived extracellular matrix (ECM) on the scaffolds. This is achieved by stimulating human induced pluripotent stem cell-derived mesenchymal stem cells (iP-hMSCs) with 2-chloro-5-nitrobenzanilide, a PPARγ inhibitor that enhances Wnt pathway, resulting in the deposition of an ECM characterized by high levels of collagens VI and XII found in anabolic bone. The osteoinductive characteristics of these bioconditioned NICE (bNICE) scaffolds is demonstrated through osteogenic differentiation of bone marrow derived human mesenchymal stem cells. A significant increase in the expression of osteogenic gene markers as well as mineralized ECM are observed on bioconditioned NICE (bNICE) scaffolds compared to bare scaffolds (NICE). The bioconditioned 3D printed scaffolds provide a unique strategy to design personalized bone grafts for in situ bone regeneration.

Advances in Extrusion 3D Bioprinting: A Focus on Multicomponent Hydrogel-Based Bioinks

3D bioprinting involves the combination of 3D printing technologies with cells, growth factors and biomaterials, and has been considered as one of the most advanced tools for tissue engineering and regenerative medicine (TERM). However, despite multiple breakthroughs, it is evident that numerous challenges need to be overcome before 3D bioprinting will eventually become a clinical solution for a variety of TERM applications. To produce a 3D structure that is biologically functional, cell-laden bioinks must be optimized to meet certain key characteristics including rheological properties, physico-mechanical properties, and biofunctionality; a difficult task for a single component bioink especially for extrusion based bioprinting. As such, more recent research has been centred on multicomponent bioinks consisting of a combination of two or more biomaterials to improve printability, shape fidelity and biofunctionality. In this article, multicomponent hydrogel-based bioink systems are systemically reviewed based on the inherent nature of the bioink (natural or synthetic hydrogels), including the most current examples demonstrating properties and advances in application of multicomponent bioinks, specifically for extrusion based 3D bioprinting. This review article will assist researchers in the field in identifying the most suitable bioink based on their requirements, as well as pinpointing current unmet challenges in the field.

Comparative study of gelatin cryogels reinforced with hydroxyapatites with different morphologies and interfacial bonding

Gelatin cryogels are good candidate scaffolds for tissue engineering because of their interconnected macroporous structure. For bone regeneration, inorganic components are chosen to reinforce gelatin cryogels: (i) to mimic the compositions of natural bone tissue and (ii) to meet the mechanical requirements of bone repairing. Cryogels were prepared from methacrylated gelatin (GelMA) in this study, and hydroxyapatite nanorods (HANRs) with surface-grafted acrylate groups (D-HANRs) were synthesized to reinforce the cryogels, in which, the crosslinking between GelMA and D-HANRs was expected. In parallel, HANRs and hydroxyapatite nanowires (HANWs) were also composited with the GelMA cryogels to investigate the effects of filler morphology and interfacial bonding on the overall properties of the resulting composite cryogels comparatively. All these composite cryogels demonstrated potential as bone repairing materials by displaying excellent performances such as high porosity, appropriate water retention, shape recovery, and fast resilience features, as well as good biocompatibility and cell affinity. In comparison with the HANR composited GelMA cryogel, the HANWs were able to ameliorate the compression and the rheology performances of the resulting composite cryogels more efficiently due to the fact that the one-dimensional HANWs played a bridging role in the gelatin matrix. Among all the preparations, however, it was the D-HANRs that achieved the strongest reinforcement efficiency in mechanical properties because the double bonds on their surface could be photo-crosslinked with GelMA to form interfacial bonding. With these findings, we concluded that it was preferable for inorganic fillers designed for cryogel-type bone repairing materials to be in a one-dimensional morphology with surface functional groups to strengthen their interfacial bonding with the polymeric matrix.

Inorganic Biomaterials for Regenerative Medicine

Regenerative medicine leverages the innate potential of the human body to efficiently repair and regenerate damaged tissues using engineered biomaterials. By designing responsive biomaterials with the appropriate biophysical and biochemical characteristics, cellular response can be modulated to direct tissue healing. Recently, inorganic biomaterials have been shown to regulate cellular responses including cell-cell and cell-matrix interactions. Moreover, ions released from these mineral-based biomaterials play a vital role in defining cell identity, as well as driving tissue-specific functions. The intrinsic properties of inorganic biomaterials, such as the release of bioactive ions (e.g., Ca, Mg, Sr, Si, B, Fe, Cu, Zn, Cr, Co, Mo, Mn, Au, Ag, V, Eu, and La), can be leveraged to induce phenotypic changes in cells or modulate the immune microenvironment to direct tissue healing and regeneration. Biophysical characteristics of biomaterials, such as topography, charge, size, electrostatic interactions, and stiffness can be modulated by addition of inorganic micro- and nanoparticles to polymeric networks have also been shown to play an important role in their biological response. In this Review, we discuss the recent emergence of inorganic biomaterials to harness the innate regenerative potential of the body. Specifically, we will discuss various biophysical or biochemical effects of inorganic-based materials in directing cellular response for regenerative medicine applications.

Hydrogel Bioink Reinforcement for Additive Manufacturing: A Focused Review of Emerging Strategies

Bioprinting is an emerging approach for fabricating cell-laden 3D scaffolds via robotic deposition of cells and biomaterials into custom shapes and patterns to replicate complex tissue architectures. Bioprinting uses hydrogel solutions called bioinks as both cell carriers and structural components, requiring bioinks to be highly printable while providing a robust and cell-friendly microenvironment. Unfortunately, conventional hydrogel bioinks have not been able to meet these requirements and are mechanically weak due to their heterogeneously crosslinked networks and lack of energy dissipation mechanisms. Advanced bioink designs using various methods of dissipating mechanical energy are aimed at developing next-generation cellularized 3D scaffolds to mimic anatomical size, tissue architecture, and tissue-specific functions. These next-generation bioinks need to have high print fidelity and should provide a biocompatible microenvironment along with improved mechanical properties. To design these advanced bioink formulations, it is important to understand the structure-property-function relationships of hydrogel networks. By specifically leveraging biophysical and biochemical characteristics of hydrogel networks, high performance bioinks can be designed to control and direct cell functions. In this review article, current and emerging approaches in hydrogel design and bioink reinforcement techniques are critically evaluated. This bottom-up perspective provides a materials-centric approach to bioink design for 3D bioprinting.

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.

Gelatin methacryloyl (GelMA)-based biomaterials for bone regeneration

Gelatin methacryloyl (GelMA)-based biomaterials have been widely used in various biomedical applications due to their suitable biological properties and tuneable physical characteristics. In particular, GelMA can be used as a versatile matrix for bone tissue engineering scaffolds via various strategies to overcome major obstacles such as insufficient mechanical property and uncontrollable degradation. This review presents the research status of GelMA, its structure and function, GelMA-based biomaterials and the development of methods along with their existing challenges.

Hydroxyapatite nanowire composited gelatin cryogel with improved mechanical properties and cell migration for bone regeneration

Hydrogels are normally not robust enough to meet the repairing requirements of bone defects, therefore, cryogels of higher mechanical properties are developed as the more proper candidates for the purpose. In view of the organic-inorganic composition of natural bone tissues, hydroxyapatite (HA) is envisioned as a good additive for protein cryogels to achieve biomimetic compositions, additionally, as an excellent reinforcement to increase the mechanical properties of cryogels. In this study, methacrylated gelatin (GelMA) was synthesized and corresponding 3D-structured cryogel was fabricated, followed by the incorporation of HA nanowires (HANWs) at different amounts as reinforcements. The results showed that the GelMA/HANW composite cryogels possessed highly porous structure with HANWs being homogeneously distributed. The compressive strengths and mechanical stability of the composite cryogels were improved alongside the increasing contents of HANWs. These composite cryogels were proven non-cytotoxic, able to support cell proliferation and promote osteogenic differentiation of bone mesenchymal stromal cells. More importantly, their porous structure allowed cell migration within the matrix, which was normally hard to be achieved in GelMA hydrogel. With improved performance, GelMA/HANW composite cryogels were thus possibly serving as a new type of bone repair materials.

Micro-Nano Bioactive Glass Particles Incorporated Porous Scaffold for Promoting Osteogenesis and Angiogenesis in vitro

Constructing the interconnected porous biomaterials scaffolds with osteogenesis and angiogenesis capacity is extremely important for efficient bone tissue engineering. Herein, we fabricated a bioactive micro-nano composite scaffolds with excellent in vitro osteogenesis and angiogenesis capacity, based on poly (lactic-co-glycolic acid) (PLGA) incorporated with micro-nano bioactive glass (MNBG). The results showed that the addition of MNBG enlarged the pore size, increased the compressive modulus (4 times improvement), enhanced the physiological stability and apatite-forming ability of porous PLGA scaffolds. The in vitro studies indicated that the PLGA-MNBG porous scaffold could enhance the mouse bone mesenchymal stem cells (mBMSCs) attachment, proliferation, and promote the expression of osteogenesis marker (ALP). Additionally, PLGA-MNBG could also support the attachment and proliferation of human umbilical vein endothelial cells (HUVECs), and significantly enhanced the expression of angiogenesis marker (CD31) of HUVECs. The as-prepared bioactive PLGA-MNBG nanocomposites scaffolds with good osteogenesis and angiogenesis probably have a promising application for bone tissue regeneration.

Understanding the impact of crosslinked PCL/PEG/GelMA electrospun nanofibers on bactericidal activity

Herein, we report the design of electrospun ultrathin fibers based on the combination of three different polymers polycaprolactone (PCL), polyethylene glycol (PEG), and gelatin methacryloyl (GelMA), and their potential bactericidal activity against three different bacteria Staphylococcus aureus (S. aureus), Pseudomonas aeruginosa (P. aeruginosa), and Methicillin-resistant Staphylococcus aureus (MRSA). We evaluated the morphology, chemical structure and wettability before and after UV photocrosslinking of the produced scaffolds. Results showed that the developed scaffolds presented hydrophilic properties after PEG and GelMA incorporation. Moreover, they were able to significantly reduce gram-positive, negative, and MRSA bacteria mainly after UV photocrosslinking (PCL:PEG:GelMa-UV). Furthermore, we performed a series of study for gaining a better mechanistic understanding of the scaffolds bactericidal activity through protein adsorption study and analysis of the reactive oxygen species (ROS) levels. Furthermore, the in vivo subcutaneous implantation performed in rats confirmed the biocompatibility of our designed scaffolds.

Electrospun nanofiber blend with improved mechanical and biological performance

Background

Here, electrospun fibers based on a blend of polycaprolactone (PCL), poly(ethylene glycol) (PEG), and gelatin methacryloyl (GelMA) were developed. The careful choice of this polymer combination allowed for the preparation of a biomaterial that preserved the mechanical strength of PCL, while at the same time improving the hydrophilicity of the blended material and human osteoblast maturation.

Methods

The morphology, chemical structure, wettability, and mechanical properties before and after UV photocrosslinking were evaluated. Furthermore, human osteoblasts (hFOB) were cultivated for up to 21 days on the scaffolds, and their potential to upregulate cell proliferation, alkaline phosphatase (ALP) activity, and calcium deposition were investigated.

Results

Contact angle measurement results showed that the developed scaffolds presented hydrophilic properties after PEG and GelMA incorporation before (25°) and after UV photocross-linking (69°) compared to pure PCL (149°). PCL:PEG:GelMA-UV displayed a slight increase in mechanical strength (elastic modulus ~37 MPa) over PCL alone (~33 MPa). Normally, an increase in strength of fibers leads to a decrease in elongation at break, due to the material becoming less deformable and stiffer, thus leading to breaks at low strain. This behavior was observed by comparing PCL (elongation at break ~106%) and PCL:PEG:GelMA-UV (~50%). Moreover, increases in ALP activity (10-fold at day 14) and calcium deposition (1.3-fold at day 21) by hFOBs were detected after PEG and GelMA incorporation after UV photocross-linking compared to pure PCL. Ultrathin and hydrophilic fibers were obtained after PEG and GelMA incorporation after UV photocrosslinking, but the strength of PCL was maintained. Interestingly, those ultrathin fiber characteristics improved hFOB functions.

Conclusion

These findings appear promising for the use of these electrospun scaffolds, based on the combination of polymers used here for numerous orthopedic applications.

Synthesis and Properties of Gelatin Methacryloyl (GelMA) Hydrogels and Their Recent Applications in Load-Bearing Tissue

Photocrosslinked gelatin methacryloyl (GelMA) hydrogels have attracted great concern in the biomedical field because of their good biocompatibility and tunable physicochemical properties. Herein, different approaches to synthesize GelMA were introduced, especially, the typical method using UV light to crosslink the gelatin-methacrylic anhydride (MA) precursor was introduced in detail. In addition, the traditional and cutting-edge technologies to characterize the properties of GelMA hydrogels and GelMA prepolymer were also overviewed and compared. Furthermore, the applications of GelMA hydrogels in cell culture and tissue engineering especially in the load-bearing tissue (bone and cartilage) were summarized, followed by concluding remarks.

In Vitro Osteogenesis Stimulation via Nano-Hydroxyapatite/Carbon Nanotube Thin Films on Biomedical Stainless Steel

We evaluated the electrophoretic deposition of nanohydroxyapatite/superhydrop hilic multiwalled carbon nanotube composites (nHAp/MWCNT) onto stainless steel biomedical alloys for applications in bone tissue engineering. First, nHAp/MWCNT composites were dispersed into 0.042 mol·L⁻¹ of Ca(NO₃)₂·4H₂O + 0.025 mol·L⁻¹ NH₄H₂PO₄ electrolytes (pH = 4.8) at two different concentrations. Next, a voltage of −2 V was applied using 316L stainless steel as a working electrode (0.27 cm²), a high-purity platinum coil wire was used as the auxiliary electrode, and an Ag/AgCl (3 M) electrode was used as the reference electrode. The nHAp/MWCNT composites were characterized by transmission electron microscopy. The deposited nHAp and nHAp/MWCNT films were characterized by profilometry, scanning electron microscopy, X-ray diffractometry and Raman spectroscopy. Human osteoblast cells were cultivated with the different materials and in vitro cytotoxicity was evaluated using lactate dehydrogenase (LDH) assay. The osteogenesis process was evaluated by mRNA levels of the three genes that are directly related to bone repair: Alkaline Phosphatase, Osteopontin and Osteocalcin. We showed that rough, crystalline apatite thin films containing phases of nHAp were successfully deposited onto 316L stainless steel alloys. Also, we noticed that nHAp/MWCNT thin films deposited onto 316L stainless steel alloys upregulated the expression of important genes related to bone mineralization and maturation. Our results strongly support the possibility of this new alternative to modify the surface of metallic biomedical alloys to promote bone tissue regeneration.

Nanoengineered Ionic-Covalent Entanglement (NICE) Bioinks for 3D Bioprinting

We introduce an enhanced nanoengineered ionic-covalent entanglement (NICE) bioink for the fabrication of mechanically stiff and elastomeric 3D biostructures. NICE bioink formulations combine nanocomposite and ionic-covalent entanglement (ICE) strengthening mechanisms to print customizable cell-laden constructs for tissue engineering with high structural fidelity and mechanical stiffness. Nanocomposite and ICE strengthening mechanisms complement each other through synergistic interactions, improving mechanical strength, elasticity, toughness, and flow properties beyond the sum of the effects of either reinforcement technique alone. Herschel-Bulkley flow behavior shields encapsulated cells from excessive shear stresses during extrusion. The encapsulated cells readily proliferate and maintain high cell viability over 120 days within the 3D-printed structure, which is vital for long-term tissue regeneration. A unique aspect of the NICE bioink is its ability to print much taller structures, with higher aspect ratios, than can be achieved with conventional bioinks without requiring secondary supports. We envision that NICE bioinks can be used to bioprint complex, large-scale, cell-laden constructs for tissue engineering with high structural fidelity and mechanical stiffness for applications in custom bioprinted scaffolds and tissue engineered implants.

3D Printing and Electrospinning of Composite Hydrogels for Cartilage and Bone Tissue Engineering

Injuries of bone and cartilage constitute important health issues costing the National Health Service billions of pounds annually, in the UK only. Moreover, these damages can become cause of disability and loss of function for the patients with associated social costs and diminished quality of life. The biomechanical properties of these two tissues are massively different from each other and they are not uniform within the same tissue due to the specific anatomic location and function. In this perspective, tissue engineering (TE) has emerged as a promising approach to address the complexities associated with bone and cartilage regeneration. Tissue engineering aims at developing temporary three-dimensional multicomponent constructs to promote the natural healing process. Biomaterials, such as hydrogels, are currently extensively studied for their ability to reproduce both the ideal 3D extracellular environment for tissue growth and to have adequate mechanical properties for load bearing. This review will focus on the use of two manufacturing techniques, namely electrospinning and 3D printing, that present promise in the fabrication of complex composite gels for cartilage and bone tissue engineering applications.

Facile modulation of cell adhesion to a poly(ethylene glycol) diacrylate film with incorporation of polystyrene nano-spheres

Poly(ethylene glycol) diacrylate (PEGDA) is a common hydrogel that has been actively investigated for various tissue engineering applications owing to its biocompatibility and excellent mechanical properties. However, the native PEGDA films are known for their bio-inertness which can hinder cell adhesion, thereby limiting their applications in tissue engineering and biomedicine. Recently, nano composite technology has become a particularly hot topic, and has led to the development of new methods for delivering desired properties to nanomaterials. In this study, we added polystyrene nano-spheres (PS) into a PEGDA solution to synthesize a nano-composite film and evaluated its characteristics. The experimental results showed that addition of the nanospheres to the PEGDA film not only resulted in modification of the mechanical properties and surface morphology but further improved the adhesion of cells on the film. The tensile modulus showed clear dependence on the addition of PS, which enhanced the mechanical properties of the PEGDA-PS film. We attribute the high stiffness of the hybrid hydrogel to the formation of additional cross-links between polymeric chains and the nano-sphere surface in the network. The effect of PS on cell adhesion and proliferation was evaluated in L929 mouse fibroblast cells that were seeded on the surface of various PEGDA-PS films. Cells density increased with a larger PS concentration, and the cells displayed a spreading morphology on the hybrid films, which promoted cell proliferation. Impressively, cellular stiffness could also be modulated simply by tuning the concentration of nano-spheres. Our results indicate that the addition of PS can effectively tailor the physical and biological properties of PEGDA as well as the mechanical properties of cells, with benefits for biomedical and biotechnological applications.

Assessment of Local Heterogeneity in Mechanical Properties of Nanostructured Hydrogel Networks

Our current understanding of the mechanical properties of nanostructured biomaterials is rather limited to invasive/destructive and low-throughput techniques such as atomic force microscopy, optical tweezers, and shear rheology. In this report, we demonstrate the capabilities of recently developed dual Brillouin/Raman spectroscopy to interrogate the mechanical and chemical properties of nanostructured hydrogel networks. The results obtained from Brillouin spectroscopy show an excellent correlation with the conventional uniaxial and shear mechanical testing. Moreover, it is confirmed that, unlike the macroscopic conventional mechanical measurement techniques, Brillouin spectroscopy can provide the elasticity characteristic of biomaterials at a mesoscale length, which is remarkably important for understanding complex cell-biomaterial interactions. The proposed technique experimentally demonstrated the capability of studying biomaterials in their natural environment and may facilitate future fabrication and inspection of biomaterials for various biomedical and biotechnological applications.

Nanoreinforced Hydrogels for Tissue Engineering: Biomaterials that are Compatible with Load-Bearing and Electroactive Tissues

Given their highly porous nature and excellent water retention, hydrogel-based biomaterials can mimic critical properties of the native cellular environment. However, their potential to emulate the electromechanical milieu of native tissues or conform well with the curved topology of human organs needs to be further explored to address a broad range of physiological demands of the body. In this regard, the incorporation of nanomaterials within hydrogels has shown great promise, as a simple one-step approach, to generate multifunctional scaffolds with previously unattainable biological, mechanical, and electrical properties. Here, recent advances in the fabrication and application of nanocomposite hydrogels in tissue engineering applications are described, with specific attention toward skeletal and electroactive tissues, such as cardiac, nerve, bone, cartilage, and skeletal muscle. Additionally, some potential uses of nanoreinforced hydrogels within the emerging disciplines of cyborganics, bionics, and soft biorobotics are highlighted.