In Situ Fabrication of Fiber Reinforced Three-Dimensional Hydrogel Tissue Engineering Scaffolds

Toward morphologically relevant extracellular matrix: nanofiber-hydrogel composites for tumor cell culture

The natural extracellular matrix (ECM) consists of a continuous integrated fibrin network and a negatively charged proteoglycan-based matrix. In this work, we report a novel three-dimensional nanofiber hydrogel composite that mimics the natural ECM structure, exhibiting both degradability and mechanical characteristics comparable to that of tumor tissue. The embedded nanofiber improves the hydrogel mechanical properties, and varying the fiber density can match the elastic modulus of different tumor tissues (1.51-10.77 kPa). The degradability of the scaffold gives sufficient space for tumor cells to secrete and remodel the ECM. The expression levels of cancer stem cell markers confirmed the development of aggressive and metastatic phenotypes of prostate cancer cells in the 3D scaffold. Similar results were obtained in terms of anticancer resistance of prostate cancer cells in 3D scaffolds showing stem cell-like properties, suggesting that the current bionic 3D scaffold tumor model has broad potential in the development of effective targeted agents.

Tough physically crosslinked poly(vinyl alcohol)-based hydrogels loaded with collagen type I to promote bone regeneration in vitro and in vivo

Poly(vinyl alcohol) (PVA) hydrogels exhibit great potential as ideal biomaterials for tissue engineering, owing to their non-toxicity, high water content, and strong biocompatibility. However, limited mechanical strength and low bioactivity have constrained their application in bone tissue engineering. In this study, we have developed a tough PVA-based hydrogel using a facile physical crosslinking method, comprising of PVA, tannic acid (TA), and hydroxyapatite (HA). Systematic experiments were conducted to examine the physicochemical properties of PVA/HA/TA hydrogels, including their compositions, microstructures, and mechanical and rheological properties. The results demonstrated that the PVA/HA/TA hydrogels possessed the porous microstructures and excellent mechanical properties. Furthermore, collagen type I (ColI) was used to further improve the biocompatibility and bioactivity of PVA/HA/TA hydrogels. In vitro experiments revealed that PVA/HA/TA/COL hydrogel could offer a suitable microenvironment for the growth of MC3T3-E1 cells and promote their osteogenic differentiation. Meanwhile, the PVA/HA/TA/COL hydrogel demonstrated the ability to promote bone regeneration and osteointegration in a rat femoral defect model. This study provides a potential strategy for the use of PVA-based hydrogels in bone tissue engineering.

Emerging Technology of Nanofiber-Composite Hydrogels for Biomedical Applications

Hydrogels and nanofibers have been firmly established as go-to materials for various biomedical applications. They have been mostly utilized separately, rarely together, because of their distinctive attributes and shortcomings. However, the potential benefits of integrating nanofibers with hydrogels to synergistically combine their functionalities while attenuating their drawbacks are increasingly recognized. Compared to other nanocomposite materials, incorporating nanofibers into hydrogel has the distinct advantage of emulating the hierarchical structure of natural extracellular environment needed for cell and tissue culture. The most important technological aspect of developing "nanofiber-composite hydrogel" is generating nanofibers made of various polymers that are cross-linked and short enough to maintain stable dispersion in hydrated environment. In this review, recent research efforts to develop nanofiber-composite hydrogels are presented, with added emphasis on nanofiber processing techniques. Several notable examples of implementing nanofiber-composite hydrogels for biomedical applications are also introduced.

Chitosan and cellulose-based composite hydrogels with embedded titanium dioxide nanoparticles as candidates for biomedical applications

Hydrogel based matrices and titanium dioxide (TiO₂) nanoparticles (NPs) are well established materials in bone tissue engineering. Nevertheless, there is still a challenge to design appropriate composites with enhanced mechanical properties and improved cell growth. Progressing in this direction, we synthesized nanocomposite hydrogels by impregnating TiO₂ NPs in a chitosan and cellulose-based hydrogel matrix containing polyvinyl alcohol (PVA), to enhance the mechanical stability and swelling capacity. Although, TiO₂ has been incorporated into single and double component matrix systems, it has rarely been combined with a tri-component hydrogel matrix system. The doping of NPs was confirmed by Fourier transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy and small- and wide-angle X-ray scattering. Our results showed that incorporation of TiO₂ NPs improved the tensile properties of the hydrogels significantly. Furthermore, we performed biological evaluation of scaffolds, swelling degree, bioactivity assessment, and hemolytic tests to prove that all types of hydrogels were safe for use in the human body. The culturing of human osteoblast-like cells MG-63 on hydrogels showed better adhesion of cells in the presence of TiO₂ and showed increasing proliferation with increasing amount of TiO₂. Our results showed that the sample with the highest TiO₂ concentration, CS/MC/PVA/TiO₂ (1 %) had the best biological properties.

Utilizing 4D Printing to Design Smart Gastroretentive, Esophageal, and Intravesical Drug Delivery Systems

The breakthrough of 3D printing in biomedical research has paved the way for the next evolutionary step referred to as four dimensional (4D) printing. This new concept utilizes the time as the fourth dimension in addition to the x, y, and z axes with the idea to change the configuration of a printed construct with time usually in response to an external stimulus. This can be attained through the incorporation of smart materials or through a preset smart design. The 4D printed constructs may be designed to exhibit expandability, flexibility, self-folding, self-repair or deformability. This review focuses on 4D printed devices for gastroretentive, esophageal, and intravesical delivery. The currently unmet needs and challenges for these application sites are tried to be defined and reported on published solution concepts involving 4D printing. In addition, other promising application sites that may similarly benefit from 4D printing approaches such as tracheal and intrauterine drug delivery are proposed.

Marrying the incompatible for better: Incorporation of hydrophobic payloads in superhydrophilic hydrogels

HYPOTHESIS

The entrapment of lyophobic in superhydrophilic hydrogels is challenging because of the intrinsic incompatibility between hydrophobic and hydrophilic molecules. To achieve such entrapment without affecting the hydrogel's formation, the electrospinning of nanodroplets or nanoparticles with a water-soluble polymer could reduce the incompatibility through the reduction of interfacial tension and the formation of a barrier film preventing coalescence or aggregation.

EXPERIMENTS

Nanodroplets or nanoparticles dispersion are electrospun in the presence of a hydrophilic polymer in hydrogel precursors. The dissolution of the hydrophilic nanofibers during electrospinning allows a redispersion of emulsion droplets and nanoparticles in the hydrogel's matrix.

FINDINGS

Superhydrophilic hydrogels with well-distributed hydrophobic nanodroplets or nanoparticles are obtained without detrimentally imparting the viscosity of hydrogel's precursors and the mechanical properties of the hydrogels. Compared with the incorporation of droplets without electrospinning, higher loadings of hydrophobic payload are achieved without premature leakage. This concept can be used to entrap hydrophobic agrochemicals, drugs, or antibacterial agents in simple hydrogels formulation.

Droplet-based bioprinting enables the fabrication of cell–hydrogel–microfibre composite tissue precursors

Composites offer the option of coupling the individual benefits of their constituents to achieve unique material properties, which can be of extra value in many tissue engineering applications. Strategies combining hydrogels with fibre-based scaffolds can create tissue constructs with enhanced biological and structural functionality. However, developing efficient and scalable approaches to manufacture such composites is challenging. Here, we use a droplet-based bioprinting system called reactive jet impingement (ReJI) to integrate a cell-laden hydrogel with a microfibrous mesh. This system uses microvalves connected to different bioink reservoirs and directed to continuously jet bioink droplets at one another in mid-air, where the droplets react and form a hydrogel that lands on a microfibrous mesh. Cell–hydrogel–fibre composites are produced by embedding human dermal fibroblasts at two different concentrations (5 × 106 and 30 × 106 cells/mL) in a collagen–alginate–fibrin hydrogel matrix and bioprinted onto a fibre-based substrate. Our results show that both types of cell–hydrogel–microfibre composite maintain high cell viability and promote cell–cell and cell–biomaterial interactions. The lower fibroblast density triggers cell proliferation, whereas the higher fibroblast density facilitates faster cellular organisation and infiltration into the microfibres. Additionally, the fibrous component of the composite is characterised by high swelling properties and the quick release of calcium ions. The data indicate that the created composite constructs offer an efficient way to create highly functional tissue precursors for laminar tissue engineering, particularly for wound healing and skin tissue engineering applications.

Graphic abstract

Mechanical Properties of Bioengineered Corneal Stroma

For the majority of patients with severe corneal injury or disease, corneal transplantation is the only suitable treatment option. Unfortunately, the demand for donor corneas greatly exceeds the availability. To overcome shortage issues, a myriad of bioengineered constructs have been developed as mimetics of the corneal stroma over the last few decades. Despite the sheer number of bioengineered stromas developed , these implants fail clinical trials exhibiting poor tissue integration and adverse effects in vivo. Such shortcomings can partially be ascribed to poor biomechanical performance. In this review, existing approaches for bioengineering corneal stromal constructs and their mechanical properties are described. The information collected in this review can be used to critically analyze the biomechanical properties of future stromal constructs, which are often overlooked, but can determine the failure or success of corresponding implants.

Biofabrication of vasculature in microphysiological models of bone

Bone contains a dense network of blood vessels that are essential to its homoeostasis, endocrine function, mineral metabolism and regenerative functions. In addition, bone vasculature is implicated in a number of prominent skeletal diseases, and bone has high affinity for metastatic cancers. Despite vasculature being an integral part of bone physiology and pathophysiology, it is often ignored or oversimplified inin vitrobone models. However, 3D physiologically relevant vasculature can now be engineeredin vitro, with microphysiological systems (MPS) increasingly being used as platforms for engineering this physiologically relevant vasculature. In recent years, vascularised models of bone in MPSs systems have been reported in the literature, representing the beginning of a possible technological step change in how bone is modelledin vitro. Vascularised bone MPSs is a subfield of bone research in its nascency, however given the impact of MPSs has had inin vitroorgan modelling, and the crucial role of vasculature to bone physiology, these systems stand to have a substantial impact on bone research. However, engineering vasculature within the specific design restraints of the bone niche is significantly challenging given the different requirements for engineering bone and vasculature. With this in mind, this paper aims to serve as technical guidance for the biofabrication of vascularised bone tissue within MPS devices. We first discuss the key engineering and biological considerations for engineering more physiologically relevant vasculaturein vitrowithin the specific design constraints of the bone niche. We next explore emerging applications of vascularised bone MPSs, and conclude with a discussion on the current status of vascularised bone MPS biofabrication and suggest directions for development of next generation vascularised bone MPSs.

Nanofiber-reinforced bulk hydrogel: preparation and structural, mechanical, and biological properties

Alginate-based hydrogels are increasingly being used as biomaterials for tissue engineering, drug carriers, and wound dressing; however, their poor mechanical strength limits their applications. Nanofiber reinforcement is an effective method for increasing the mechanical strength of hydrogels. However, the macro preparation of nanofiber-reinforced hydrogels with a bulk structure is challenging. Herein, we describe the fabrication of nanofiber-reinforced bulk alginate hydrogel composites. The mechanical properties of hydrogels were significantly improved, and the reinforcement law of nanofiber was systematically studied. The maximum tensile stress (0.76 MPa) was obtained with 30% nanofiber content, which was 87% higher than that of pure alginate hydrogel. The compressive stress of the composite hydrogel exhibited "J-curve" behavior with gradually increasing nanofiber content, which indicated that the composited hydrogels were suitable as biomaterials. Furthermore, in 2 h, the hydrogels killed more than 90% of the bacteria that were present, and the bacteriostatic rate reached 100% after 12 h of treatment. More importantly, the sterile environment continued to be maintained, and the composited hydrogel also had satisfactory cytocompatibility and cell adhesion. Compared with pure alginate hydrogel, the roughness of the composited hydrogel surface was increased, which resulted in stronger cell adhesion. Therefore, the composite hydrogel demonstrated improved mechanical and biological properties, and exhibited the potential for clinical application.

Recent Advances in Fiber-Hydrogel Composites for Wound Healing and Drug Delivery Systems

In the last decades, much research has been done to fasten wound healing and target-direct drug delivery. Hydrogel-based scaffolds have been a recurrent solution in both cases, with some reaching already the market, even though their mechanical stability remains a challenge. To overcome this limitation, reinforcement of hydrogels with fibers has been explored. The structural resemblance of fiber-hydrogel composites to natural tissues has been a driving force for the optimization and exploration of these systems in biomedicine. Indeed, the combination of hydrogel-forming techniques and fiber spinning approaches has been crucial in the development of scaffolding systems with improved mechanical strength and medicinal properties. In this review, a comprehensive overview of the recently developed fiber-hydrogel composite strategies for wound healing and drug delivery is provided. The methodologies employed in fiber and hydrogel formation are also highlighted, together with the most compatible polymer combinations, as well as drug incorporation approaches creating stimuli-sensitive and triggered drug release towards an enhanced host response.

In Situ Formation of 3D Conductive and Cell-Laden Graphene Hydrogel for Electrically Regulating Cellular Behavior

Electroconductive and injectable hydrogels are attracting increasing attention owing to the needs of electrically induced regulation of cell behavior, tissue engineering of electroactive tissues, and achieving minimum invasiveness during tissue repair. In this study, a novel in situ formed 3D conductive and cell-laden hydrogel is developed, which can be broadly used in bioprinting, tissue engineering, neuroengineering etc. An instantaneous, uniform spatial distribution and encapsulation of cells can be achieved as a result of hydrogen bonding induced hydrogel formation. Particularly, the cell-laden hydrogel can be easily obtained by simply mixing and shaking the polydopamine (PDA) functionalized rGO (rGO-PDA) with polyvinyl alcohol (PVA) solution containing cells. Graphene oxide is reduced and functionalized by dopamine to restore the electrical conductivity, while simultaneously enhancing both hydrophilicity and biocompatibility of reduced graphene oxide. In vitro culture of PC12 cells within the cell-laden hydrogel demonstrates its biocompatibility, noncytotoxicity as well as the ability to support long-term cell growth and proliferation. Enhanced neuronal differentiation is also observed, both with and without electrical stimulation. Overall, this 3D conductive, cell-laden hydrogel holds great promise as potential platform for tissue engineering of electroactive tissues.

Enhancement of the Mechanical Properties of Hydrogels with Continuous Fibrous Reinforcement

Reinforcing mechanically weak hydrogels with fibers is a promising route to obtain strong and tough materials for biomedical applications while retaining a favorable cell environment. The resulting hierarchical structure recreates structural elements of natural tissues such as articular cartilage, with fiber diameters ranging from the nano- to microscale. Through control of properties such as the fiber diameter, orientation, and porosity, it is possible to design materials which display the nonlinear, synergistic mechanical behavior observed in natural tissues. In order to fully exploit these advantages, it is necessary to understand the structure-property relationships in fiber-reinforced hydrogels. However, there are currently limited models which capture their complex mechanical properties. The majority of reported fiber-reinforced hydrogels contain fibers obtained by electrospinning, which allows for limited spatial control over the fiber scaffold and limits the scope for systematic mechanical testing studies. Nevertheless, new manufacturing techniques such as melt electrowriting and bioprinting have emerged, which allow for increased control over fiber deposition and the potential for future investigations on the effect of specific structural features on mechanical properties. In this review, we therefore explore the mechanics of fiber-reinforced hydrogels, and the evolution of their design and manufacture from replicating specific features of biological tissues to more complex structures, by taking advantage of design principles from both tough hydrogels and fiber-reinforced composites. By highlighting the overlap between these fields, it is possible to identify the remaining challenges and opportunities for the development of effective biomedical devices.

Specialty Tough Hydrogels and Their Biomedical Applications

Hydrogels have long been explored as attractive materials for biomedical applications given their outstanding biocompatibility, high water content, and versatile fabrication platforms into materials with different physiochemical properties and geometries. Nonetheless, conventional hydrogels suffer from weak mechanical properties, restricting their use in persistent load-bearing applications often required of materials used in medical settings. Thus, the fabrication of mechanically robust hydrogels that can prolong the lifetime of clinically suitable materials under uncompromising in vivo conditions is of great interest. This review focuses on design considerations and strategies to construct such tough hydrogels. Several promising advances in the proposed use of specialty tough hydrogels for soft actuators, drug delivery vehicles, adhesives, coatings, and in tissue engineering settings are highlighted. While challenges remain before these specialty tough hydrogels will be deemed translationally acceptable for clinical applications, promising preliminary results undoubtedly spur great hope in the potential impact this embryonic research field can have on the biomedical community.

Smart Hydrogels in Tissue Engineering and Regenerative Medicine

The field of regenerative medicine has tremendous potential for improved treatment outcomes and has been stimulated by advances made in bioengineering over the last few decades. The strategies of engineering tissues and assembling functional constructs that are capable of restoring, retaining, and revitalizing lost tissues and organs have impacted the whole spectrum of medicine and health care. Techniques to combine biomimetic materials, cells, and bioactive molecules play a decisive role in promoting the regeneration of damaged tissues or as therapeutic systems. Hydrogels have been used as one of the most common tissue engineering scaffolds over the past two decades due to their ability to maintain a distinct 3D structure, to provide mechanical support for the cells in the engineered tissues, and to simulate the native extracellular matrix. The high water content of hydrogels can provide an ideal environment for cell survival, and structure which mimics the native tissues. Hydrogel systems have been serving as a supportive matrix for cell immobilization and growth factor delivery. This review outlines a brief description of the properties, structure, synthesis and fabrication methods, applications, and future perspectives of smart hydrogels in tissue engineering.

Hierarchical assembly in PLA-PEO-PLA hydrogels with crystalline domains and effect of block stereochemistry

Understanding the development of microstructure (e.g., structures with length scales roughly 0.5-500 μm) in hydrogels is crucial for their use in several biomedical applications. We utilize ultra-small-angle neutron scattering (USANS) and confocal microscopy to explore microstructure of poly(lactide)-poly(ethylene oxide)-poly(lactide) (PLA-PEO-PLA) triblock copolymer hydrogels with varying l/d-lactide ratio. We have previously found that these polymers self-assemble on the nanoscale into micelles. Here, we observe large-scale structures with diverse morphologies, including highly porous self-similar networks with characteristic sizes spanning approximately 120 nm-200 μm. These structural features give rise to power-law scattering indicative of fractal structures in USANS. Mass fractal and surface fractal structures are found for gels with l/d ratios of 80/20 and 50/50, respectively. Confocal microscopy shows microscale water-filled channels and pores that are more clearly evident in gels with a higher fraction of l-lactide in the PLA block as compared to the 50/50 hydrogels. Tuning block stereochemistry may provide a means of controlling the self-assembly and structural evolution at both the nanoscale and microscale, impacting application of these materials in tissue engineering and drug delivery.

Incorporating 4D into Bioprinting: Real-Time Magnetically Directed Collagen Fiber Alignment for Generating Complex Multilayered Tissues

In vitro multilayered tissues with mimetic architectures resembling native tissues are valuable tools for application in medical research. In this study, an advanced bioprinting strategy is presented for aligning collagen fibers contained in functional bioinks. Streptavidin-coated iron nanoparticles are embedded in printable bioinks with varying concentrations of low gelling temperature agarose and type I collagen. By applying a straightforward magnetic-based mechanism in hydrogels during bioprinting, it is possible to align collagen fibers in less concentrated hydrogel blends with a maximum agarose concentration of 0.5 w/v%. Conversely, more elevated concentrations of agarose in printable blends show random collagen fiber distribution. Interestingly, hydrogel blends with unidirectionally aligned collagen fibers show significantly higher compression moduli compared to hydrogel blends including random fibers. Considering its application in the field of cartilage tissue engineering, bioprinted constructs with alternating layers of aligned and random fibers are fabricated. After 21 days of culture, cell-loaded constructs with alternating layers of aligned and random fibers express markedly more collagen II in comparison to solely randomly oriented fiber constructs. These encouraging results translate the importance of the structure and architecture of bioinks used in bioprinting in light of their use for tissue engineering and personalized medical applications.

Dual Suppression Effect of Magnetic Induction Heating and Microencapsulation on Ice Crystallization Enables Low-Cryoprotectant Vitrification of Stem Cell-Alginate Hydrogel Constructs

Stem cells microencapsulated in hydrogel as stem cell-hydrogel constructs have wide applications in the burgeoning cell-based medicine. Due to their short shelf life at ambient temperature, long-term storage or banking of the constructs is essential to the "off-the-shelf" ready availability needed for their widespread applications. As a high-efficiency, easy-to-operate, low-toxicity, and low-cost method for long-term storage of the constructs, low-cryoprotectant (CPA) vitrification has attracted tremendous attention recently. However, we found many cells in the stem cell-alginate constructs (∼500 μm in diameter) could not attach to the substrate post low-CPA vitrification with ∼2 M penetrating CPAs. To address this problem, we introduced nanowarming via magnetic induction heating (MIH) of Fe3O4 nanoparticles to minimize recrystallization and devitrification during the warming step of the low-CPA vitrification procedure. Our results indicate that high-quality stem cell-alginate hydrogel constructs with an intact microstructure, high immediate cell survival (>80%), and greatly improved attachment efficiency (by nearly three times, 68% versus 24%) of the encapsulated cells could be obtained post-cryopreservation with nanowarming. Moreover, the cells encapsulated in the cell-hydrogel constructs post-cryopreservation maintained normal proliferation under 3D culture and retained intact biological function of multilineage differentiation. This novel low-CPA vitrification approach for cell cryopreservation enabled by the combined use of alginate hydrogel microencapsulation and Fe3O4 nanoparticles-mediated nanowarming may be valuable in facilitating the widespread application of stem cells in the clinic.