Marine-Derived Hydrogels for Biomedical Applications

Marine organisms provide novel and broad sources for the preparations and applications of biomaterials. Since the urgent requirement of bio-hydrogels to mimic tissue extracellular matrix (ECM), the natural biomacromolecule hydrogels derived from marine sources have received increasing attention. Benefiting from their outstanding bioactivity and biocompatibility, many attempts have been made to reconstruct ECM components by applying marine-derived natural hydrogels. Moreover, marine hydrogels have been successfully applied in biomedicine by means of microfluidics, electrospray, and bioprinting. In this review, the classification and characteristics of marine-derived hydrogels are summarized. In particular, their role in the development of biomaterials is also introduced. Then, the recent advances in bio-fabrication strategies for various hydrogel materials are focused upon. Besides, the influences of hydrogel types on their functions in biomedical applications are discussed in depth. Finally, critical reflections on the limitations and future development of marine-derived hydrogels are presented.     

Stimuli-Responsive Nanocomposite Hydrogels for Biomedical Applications

The complex tissue-specific physiology that is orchestrated from the nano- to the macroscale, in conjugation with the dynamic biophysical/biochemical stimuli underlying biological processes, has inspired the design of sophisticated hydrogels and nanoparticle systems exhibiting stimuli-responsive features. Recently, hydrogels and nanoparticles have been combined in advanced nanocomposite hybrid platforms expanding their range of biomedical applications. The ease and flexibility of attaining modular nanocomposite hydrogel constructs by selecting different classes of nanomaterials/hydrogels, or tuning nanoparticle-hydrogel physicochemical interactions widely expands the range of attainable properties to levels beyond those of traditional platforms. This review showcases the intrinsic ability of hybrid constructs to react to external or internal/physiological stimuli in the scope of developing sophisticated and intelligent systems with application-oriented features. Moreover, nanoparticle-hydrogel platforms are overviewed in the context of encoding stimuli-responsive cascades that recapitulate signaling interplays present in native biosystems. Collectively, recent breakthroughs in the design of stimuli-responsive nanocomposite hydrogels improve their potential for operating as advanced systems in different biomedical applications that benefit from tailored single or multi-responsiveness.     

DNA Hydrogels as Functional Materials and Their Biomedical Applications

With the remarkable development of DNA nanotechnology, interest in DNA molecules has expanded beyond its biological role to building blocks in materials science. As a unique branch of DNA-based materials, DNA hydrogels have exhibited many fascinating characteristics, including broad biocompatibility, precise programmability, convenient modification, and tunable mechanical properties, which make DNA hydrogels ideal biomaterials. Moreover, by combining with functional nucleic acids, such as aptamers, i-motif nanostructures, CpG oligodeoxynucleotides, and DNAzymes, DNA hydrogels can be further tailored to provide additional target recognition, therapeutic potential, and catalytic activities, allowing them to play important roles in biosensing and medical applications. This review, aims to provide readers with an up-to-date overview of the important developments of biomedical DNA hydrogels. First, it introduces different synthetic strategies of hydrogels that utilize DNA as building materials and functional units within the hydrogel networks and discuss their advantages in biomedical applications. Subsequently, new approaches and applications of biomedical DNA hydrogels in the recent years are highlighted, such as therapeutic systems, cell culture platforms, tissue engineering materials, and biosensors. Finally, future perspectives and remaining challenges of DNA hydrogels in biomedicine are presented.     

Functionalized TiO2 Based Nanomaterials for Biomedical Applications

As baby boomers age, diabetes mellitus, cancer, osteoarthritis, cardiovascular diseases, and orthopedic disorders are more widespread and the demand for better biomedical devices and functional biomaterials is increasing rapidly. Owing to the good biocompatibility, chemical stability, catalytic efficiency, plasticity, mechanical properties, as well as strength‐to‐weight ratio, titanium dioxide (TiO₂) based nanostructured materials are playing important roles in tissue reconstruction and diagnosis of these diseases. Here, recent advance in the research of nanostructured TiO₂ based biomaterials pertaining to bone tissue engineering, intravascular stents, drug delivery systems, and biosensors is described.     

Recent Advances in Design of Functional Biocompatible Hydrogels for Bone Tissue Engineering

Bone related diseases have caused serious threats to human health owing to their complexity and specificity. Fortunately, owing to the unique 3D network structure with high aqueous content and functional properties, emerging hydrogels are regarded as one of the most promising candidates for bone tissue engineering, such as repairing cartilage injury, skull defect, and arthritis. Herein, various design strategies and synthesis methods (e.g., 3D-printing technology and nanoparticle composite strategy) are introduced to prepare implanted hydrogel scaffolds with tunable mechanical strength, favorable biocompatibility, and excellent bioactivity for applying in bone regeneration. Injectable hydrogels based on biocompatible materials (e.g., collagen, hyaluronic acid, chitosan, polyethylene glycol, etc.) possess many advantages in minimally invasive surgery, including adjustable physicochemical properties, filling irregular shapes of defect sites, and on-demand release drugs or growth factors in response to different stimuli (e.g., pH, temperature, redox, enzyme, light, magnetic, etc.). In addition, drug delivery systems based on micro/nanogels are discussed, and its numerous promising designs used in the application of bone diseases (e.g., rheumatoid arthritis, osteoarthritis, cartilage defect) are also briefed in this review. Particularly, several key factors of hydrogel scaffolds (e.g., mechanical property, pore size, and release behavior of active factors) that can induce bone tissue regeneration are also summarized in this review. It is anticipated that advanced approaches and innovative ideas of bioactive hydrogels will be exploited in the clinical field and increase the life quality of patients with the bone injury.     

Biomedical Applications of DNA‐Based Hydrogels

Nucleic acids are gaining significant attention as versatile building blocks for the next generation of soft materials. Due to significant advances in the chemical synthesis and biotechnological production, DNA becomes more widely available enabling its usage as bulk material in various applications. This has prompted researchers to actively explore the unique features offered by DNA‐containing materials like hydrogels. In this review article, recent developments in the field of hydrogels that feature DNA as a component either in the construction of the material or as functional unit within the construct and their biomedical applications are discussed in detail. First, different synthetic approaches for obtaining DNA hydrogels are summarized, which allows classification of DNA materials according to their structure. Then, new concepts, properties, and applications are highlighted such as DNA‐based biosensor devices, drug delivery platforms, and cell scaffolds. With the 2018 Nobel Prize in Physiology or Medicine being awarded to cancer immunotherapy underscoring the importance of this therapy, DNA hydrogel systems designed to modulate the immune system are introduced. This review aims to give the reader a timely overview of the most important and recent developments in this emerging class of therapeutically useful materials of DNA‐based hydrogels.     

2D Covalent Organic Frameworks for Biomedical Applications

Covalent organic frameworks (COFs) are an emerging class of organic crystalline polymers with well‐defined molecular geometry and tunable porosity. COFs are formed via reversible condensation of lightweight molecular building blocks, which dictate its geometry in two or three dimensions. Among COFs, 2D COFs have garnered special attention due to their unique structure composed of two‐dimensionally extended organic sheets stacked in layers generating periodic columnar π‐arrays, functional pore space, and their ease of synthesis. These unique features in combination with their low density, high crystallinity, large surface area, and biodegradability have made them an excellent candidate for a plethora of applications ranging from energy to biomedical sciences. In this article, the evolution of 2D COFs is briefly discussed in terms of different types of chemical linkages, synthetic strategies of bulk and nanoscale 2D COFs, and their tunability from a biomedical perspective. Next, the biomedical applications of 2D COFs specifically for drug delivery, phototherapy, biosensing, bioimaging, biocatalysis, and antibacterial activity are summarized. In addition, current challenges and emerging approaches in designing 2D COFs for advanced biomedical applications are discussed.     

3D Patterning within Hydrogels for the Recreation of Functional Biological Environments

Biological structures are inherently complex in nature. Structural hierarchy, chemical anisotropy, and compositional heterogeneity are ubiquitous in biological systems and play a key role in the functionality of living systems. For decades, methods such as soft lithography have enabled recreation of such arrangements through precise spatial control of molecular patterns in 2D. With technological advances and increasing understanding of molecular and structural biology, there has been an increasing interest in recreating such spatial organizations in 3D. In this review, a comprehensive summary of the latest technologies being used to create 3D patterns of functional molecules within hydrogels for tissue engineering applications is presented. The review is divided into five groups of technologies defined according to the main driving force used to fabricate the patterns including light, precise chemical design, microfluidics, 3D printing, and non-contact forces (i.e. electric, magnetic, or acoustic fields and self-assembly).     

Magnetic Hydrogels and Their Potential Biomedical Applications

Hydrogels find widespread applications in biomedical engineering due to their hydrated environment and tunable properties (e.g., mechanical, chemical, biocompatible) similar to the native extracellular matrix (ECM). However, challenges still exist regarding utilizing hydrogels in applications such as engineering 3D tissue constructs and active targeting in drug delivery, due to the lack of controllability, actuation, and quick‐response properties. Recently, magnetic hydrogels have emerged as a novel biocomposite for their active response properties and extended applications. In this review, the state‐of‐the‐art methods for magnetic hydrogel preparation are presented and their advantages and drawbacks in applications are discussed. The applications of magnetic hydrogels in biomedical engineering are also reviewed, including tissue engineering, drug delivery and release, enzyme immobilization, cancer therapy, and soft actuators. Concluding remarks and perspectives for the future development of magnetic hydrogels are addressed.     

Growth‐Factor Free Multicomponent Nanocomposite Hydrogels That Stimulate Bone Formation

Synthetic osteo‐promoting materials that are able to stimulate and accelerate bone formation without the addition of exogenous cells or growth factors represent a major opportunity for an aging world population. A co‐assembling system that integrates hyaluronic acid tyramine ( HA‐Tyr ), bioactive peptide amphiphiles ( GHK‐Cu²⁺ ), and Laponite ( Lap ) to engineer hydrogels with physical, mechanical, and biomolecular signals that can be tuned to enhance bone regeneration is reported. The central design element of the multicomponent hydrogels is the integration of self‐assembly and enzyme‐mediated oxidative coupling to optimize structure and mechanical properties in combination with the incorporation of an osteo‐ and angio‐promoting segments to facilitate signaling. Spectroscopic techniques are used to confirm the interplay of orthogonal covalent and supramolecular interactions in multicomponent hydrogel formation. Furthermore, physico‐mechanical characterizations reveal that the multicomponent hydrogels exhibit improved compressive strength, stress relaxation profile, low swelling ratio, and retarded enzymatic degradation compared to the single component hydrogels. Applicability is validated in vitro using human mesenchymal stem cells and human umbilical vein endothelial cells, and in vivo using a rabbit maxillary sinus floor reconstruction model. Animals treated with the HA‐Tyr‐HA‐Tyr‐GHK‐Cu²⁺ hydrogels exhibit significantly enhanced bone formation relative to controls including the commercially available Bio‐Oss.     

Ascidian‐Inspired Fast‐Forming Hydrogel System for Versatile Biomedical Applications: Pyrogallol Chemistry for Dual Modes of Crosslinking Mechanism

Exploitation of unique biochemical and biophysical properties of marine organisms has led to the development of functional biomaterials for various biomedical applications. Recently, ascidians have received great attention, owing to their extraordinary properties such as strong underwater adhesion and rapid self‐regeneration. Specific polypeptides containing 3,4,5‐trihydroxyphenylalanine (TOPA) in the blood cells of ascidians are associated with such intrinsic properties generated through complex oxidative processes. In this study, a bioinspired hydrogel platform is developed, demonstrating versatile applicability for tissue engineering and drug delivery, by conjugating pyrogallol (PG) moiety resembling ascidian TOPA to hyaluronic acid (HA). The HA–PG conjugate can be rapidly crosslinked by dual modes of oxidative mechanisms using an oxidant or pH control, resulting in hydrogels with different mechanical and physical characteristics. The versatile utility of HA–PG hydrogels formed via different crosslinking mechanisms is tested for different biomedical platforms, including microparticles for sustained drug delivery and tissue adhesive for noninvasive cell transplantation. With extraordinarily fast and different routes of PG oxidation, ascidian‐inspired HA–PG hydrogel system may provide a promising biomaterial platform for a wide range of biomedical applications.     

Biomaterials that regulate growth factor activity via bioinspired interactions

Growth factor activity is localized within the natural extracellular matrix (ECM) by specific non-covalent interactions with core ECM biomolecules, such as proteins and proteoglycans. Recently, these interactions have inspired us and others to develop synthetic biomaterials that can non-covalently regulate growth factor activity for tissue engineering applications. For example, biomaterials covalently or non-covalently modified with heparin glycosaminoglycans can augment growth factor release strategies. In addition, recent studies demonstrate that biomaterials modified with heparin-binding peptides can sequester cell-secreted heparin proteoglycans and, in turn, sequester growth factors and regulate stem cell behavior. Another set of studies show that modular versions of growth factor molecules can be designed to interact with specific components of natural and synthetic ECMs, including collagen and hydroxyapatite. In addition, layer-by-layer assemblies of GAGs and other natural polyelectrolytes retain growth factors at a cell-material interface via specific non-covalent interactions. This review will detail the various bioinspired strategies being used to non-covalently localize growth factor activity within biomaterials, and will highlight in vivo examples of the efficacy of these materials to promote tissue regeneration.     

Injectability of Biosynthetic Hydrogels: Consideration for Minimally Invasive Surgical Procedures and 3D Bioprinting

Injectable hydrogels are often preferred when designing carriers for cell therapy or developing new bio-ink formulations. Biosynthetic hydrogels, which are a class of materials made with a hybrid design strategy, can be advantageous for endowing injectability while maintaining biological activity of the material. The chemical modification required to make these gels injectable by specific crosslinking pathways can be challenging and also make the hydrogels inhospitable to cells. Therefore, most efforts to functionalize biosynthetic hydrogel precursors toward injectability in the presence of cells try to balance between chemical and biological functionality, in order to preserve cell compatibility while addressing the injectability design challenges. Accordingly, hydrogel crosslinking strategies have evolved to include the use of photoinitiated “click” chemistry or bio-orthogonal reactions with rapid gelation kinetics and minimal cyto-toxicity required when working with cell-compatible hydrogel systems. With many new injectable biosynthetic materials emerging, their impact in cell-based regenerative medicine and bioprinting is also becoming more apparent. This review covers the main strategies that are used to endow biosynthetic polymers with injectability through rapid, cyto-compatible physical or covalent crosslinking and the main considerations for using the resulting injectable hydrogels in cell therapy, tissue regeneration, and bioprinting.     

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

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

Functional Human Vascular Network Generated in Photocrosslinkable Gelatin Methacrylate Hydrogels

The generation of functional, 3D vascular networks is a fundamental prerequisite for the development of many future tissue engineering-based therapies. Current approaches in vascular network bioengineering are largely carried out using natural hydrogels as embedding scaffolds. However, most natural hydrogels present a poor mechanical stability and a suboptimal durability, which are critical limitations that hamper their widespread applicability. The search for improved hydrogels has become a priority in tissue engineering research. Here, the suitability of a photopolymerizable gelatin methacrylate (GelMA) hydrogel to support human progenitor cell-based formation of vascular networks is demonstrated. Using GelMA as the embedding scaffold, it is shown that 3D constructs containing human blood-derived endothelial colony-forming cells (ECFCs) and bone marrow-derived mesenchymal stem cells (MSCs) generate extensive capillary-like networks in vitro. These vascular structures contain distinct lumens that are formed by the fusion of ECFC intracellular vacuoles in a process of vascular morphogenesis. The process of vascular network formation is dependent on the presence of MSCs, which differentiate into perivascular cells occupying abluminal positions within the network. Importantly, it is shown that implantation of cell-laden GelMA hydrogels into immunodeficient mice results in a rapid formation of functional anastomoses between the bioengineered human vascular network and the mouse vasculature. Furthermore, it is shown that the degree of methacrylation of the GelMA can be used to modulate the cellular behavior and the extent of vascular network formation both in vitro and in vivo. These data suggest that GelMA hydrogels can be used for biomedical applications that require the formation of microvascular networks, including the development of complex engineered tissues.     

Hydrophobized Hydrogels: Construction Strategies,Properties, and Biomedical Applications

Hydrogels, as 3D networks containing huge amount of water, display similarity to soft tissues, and thus they are of wide interest in tissue engineering. Hydrogels, due to biocompatibility and porous structure, are valuable therapeutic platforms for hydrophilic drugs. Over the last decade, there has been a strong emphasis on the development of hydrogel platforms with the ability to increase the solubility of hydrophobic drugs. However, the pronounced discrepancy between the hydrophilic character of hydrogels and the hydrophobic nature of numerous pharmacologically active compounds is problematic. In recent years, different strategies are applied using special polymer constructs or composite materials exploiting the advanced scientific knowledge in the area of polymer and lipid-based nano- and microcarriers hydrophobization of the hydrogel turns out to be not only valuable in terms of achieving the ability to dissolve poorly soluble drugs in water, but also proves to be crucial in obtaining bioadhesion in wet conditions, but also, unexpected abnormal water swelling behavior, as well as in mechanical properties such as the dissipation mechanism and self-healable hydrogel properties. This review is mainly focused on recent advances in the usage of hydrophobized hydrogels in biomedical applications.     

Integration of Self‐Assembled Microvascular Networks with Microfabricated PEG‐Based Hydrogels

Despite tremendous efforts, tissue engineered constructs are restricted to thin, simple tissues sustained only by diffusion. The most significant barrier in tissue engineering is insufficient vascularization to deliver nutrients and metabolites during development in vitro and to facilitate rapid vascular integration in vivo. Tissue engineered constructs can be greatly improved by developing perfusable microvascular networks in vitro in order to provide transport that mimics native vascular organization and function. Here a microfluidic hydrogel is integrated with a self‐assembling pro‐vasculogenic co‐culture in a strategy to perfuse microvascular networks in vitro. This approach allows for control over microvascular network self‐assembly and employs an anastomotic interface for integration of self‐assembled microvascular networks with fabricated microchannels. As a result, transport within the system shifts from simple diffusion to vessel supported convective transport and extra‐vessel diffusion, thus improving overall mass transport properties. This work impacts the development of perfusable prevascularized tissues in vitro and ultimately tissue engineering applications in vivo.     

Hydrogel Scaffolds of Amphiphilic and Acidic β‐Sheet Peptides

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