High‐Strength,Durable All‐Silk Fibroin Hydrogels with Versatile Processability toward Multifunctional Applications

Hydrogels are the focus of extensive research due to their potential use in fields including biomedical, pharmaceutical, biosensors, and cosmetics. However, the general weak mechanical properties of hydrogels limit their utility. Here, pristine silk fibroin (SF) hydrogels with excellent mechanical properties are generated via a binary‐solvent‐induced conformation transition (BSICT) strategy. In this method, the conformational transition of SF is regulated by moderate binary solvent diffusion and SF/solvent interactions. β‐sheet formation serves as the physical crosslinks that connect disparate protein chains to form continuous 3D hydrogel networks, avoiding complex chemical and/or physical treatments. The Young's modulus of these new BSICT–SF hydrogels can reach up to 6.5 ± 0.2 MPa, tens to hundreds of times higher than that of conventional hydrogels (0.01–0.1 MPa). These new materials fill the “empty soft materials' space” in the elastic modulus/strain Ashby plot. More remarkably, the BSICT–SF hydrogels can be processed into different constructions through different polymer and/or metal‐based processing techniques, such as molding, laser cutting, and machining. Thus, these new hydrogel systems exhibit potential utility in many biomedical and engineering fields.     

Protein Fragment Reconstitution as a Driving Force for Self‐Assembling Reversible Protein Hydrogels

Due to their potential biomedical applications, protein‐based hydrogels have attracted considerable interest. Although various methods have been developed to engineer self‐assembling, physically‐crosslinked protein hydrogels, exploring novel driving forces to engineer such hydrogels remains challenging. Protein fragment reconstitution, also known as fragment complementation, is a self‐assembling mechanism by which protein fragments can reconstitute the folded conformation of the native protein when split into two halves. Although it has been used in biophysical studies and bioassays, fragment reconstitution has not been explored for hydrogel construction. Using a small protein GL5 as a model, which is capable of fragment reconstitution to reconstitute the folded GL5 spontaneously when split into two halves, GN and GC, we demonstrate that protein fragment reconstitution is a novel driving force for engineering self‐assembling reversible protein hydrogels. Fragment reconstitution between GN and GC crosslinks GN and GC‐containing proteins into self‐assembling reversible protein hydrogels. These novel hydrogels show temperature‐dependent reversible sol‐gel transition, and excellent property against erosion in water. Since many proteins can undergo fragment reconstitution, we anticipate that such fragment reconstitution may offer a general driving force for engineering protein hydrogels from a variety of proteins, and thus significantly expanding the ‘toolbox’ currently available in the field of biomaterials.     

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.     

Reversible Programing of Soft Matter with Reconfigurable Mechanical Properties

Biology uses various cross‐linking mechanisms to tailor material properties, and this is inspiring technological efforts to couple independent cross‐linking mechanisms to create hydrogels with complex mechanical properties. Here, it is reported that a hydrogel formed from a single polysaccharide can be triggered to reversibly switch cross‐linking mechanisms and switch between elastic and viscoelastic properties. Specifically, the pH‐responsive self‐assembling aminopolysaccharide chitosan is used. Under acidic conditions, chitosan is polycationic and can be electrostatically cross‐linked by sodium dodecyl sulfate (SDS) micelles to confer viscoelastic and self‐healing properties. Under basic conditions, chitosan becomes neutral, the electrostatic SDS–chitosan interactions are no longer operative, and chitosan chains can self‐assemble to form crystalline network junctions that serve as strong physical cross‐links that confer elastic properties. Mechanical measurements performed in water demonstrate these different mechanical behaviors and the repeated pH‐induced switching between these behaviors. Printing of SDS micelles onto a neutral chitosan film allows the cross‐linking mechanisms to be spatially programed to confer anisotropic mechanical properties. The reversibility of these cross‐linking mechanisms allows the patterned films to be erased and reprogramed with reconfigured mechanical properties. Potentially, the ability to reversibly program hydrogel networks enables fabrication of the dynamically reconfigurable networks required for soft machines.     

Biodegradable Polymer Crosslinker: Independent Control of Stiffness,Toughness, and Hydrogel Degradation Rate

Hydrogels are being increasingly studied for use in various biomedical applications including drug delivery and tissue engineering. The successful use of a hydrogel in these applications greatly relies on a refined control of the mechanical properties including stiffness, toughness, and the degradation rate. However, it is still challenging to control the hydrogel properties in an independent manner due to the interdependency between hydrogel properties. Here it is hypothesized that a biodegradable polymeric crosslinker would allow for decoupling of the dependency between the properties of various hydrogel materials. This hypothesis is examined using oxidized methacrylic alginate (OMA). The OMA is synthesized by partially oxidizing alginate to generate hydrolytically labile units and conjugating methacrylic groups. It is used to crosslink poly(ethylene glycol) methacrylate and poly(N‐hydroxymethyl acrylamide) to form three‐dimensional hydrogel systems. OMA significantly improves rigidity and toughness of both hydrogels as compared with a small molecule crosslinker, and also controls the degradation rate of hydrogels depending on the oxidation degree, without altering their initial mechanical properties. The protein‐release rate from a hydrogel and subsequent angiogenesis in vivo are thus regulated with the chemical structure of OMA. Overall, the results of this study suggests that the use of OMA as a crosslinker will allow the implantation of a hydrogel in tissue subject to an external mechanical loading with a desired protein‐release profile. The OMA synthesized in this study will be, therefore, highly useful to independently control the mechanical properties and degradation rate of a wide array of hydrogels.     

Regulation of Stem Cell Fate in a Three‐Dimensional Micropatterned Dual‐Crosslinked Hydrogel System

Micropatterning technology is a powerful tool for controlling the cellular microenvironment and investigating the effects of physical parameters on cell behaviors, such as migration, proliferation, apoptosis, and differentiation. Although there have been significant developments in regulating the spatial and temporal distribution of physical properties in various materials, little is known about the role of the size of micropatterned regions of hydrogels with different crosslinking densities on the response of encapsulated cells. In this study, a novel alginate hydrogel system that can be micropatterned three‐dimensionally is engineered to create regions that are crosslinked by a single mechanism or dual mechanisms. By manipulating micropattern size while keeping the overall ratio of single‐ to dual‐crosslinked hydrogel volume constant, the physical properties of the micropatterned alginate hydrogels are spatially tunable. When human adipose‐derived stem cells (hASCs) are photoencapsulated within micropatterned hydrogels, their proliferation rate is a function of micropattern size. Additionally, micropattern size dictates the extent of osteogenic and chondrogenic differentiation of photoencapsulated hASC. The size of 3D micropatterned physical properties in this new hydrogel system introduces a new design parameter for regulating various cellular behaviors, and this dual‐crosslinked hydrogel system provides a new platform for studying proliferation and differentiation of stem cells in a spatially controlled manner for tissue engineering and regenerative medicine applications.     

Radiopaque Highly Stiff and Tough Shape Memory Hydrogel Microcoils for Permanent Embolization of Arteries

Transcatheter arterial embolization of aneurysm with metal microcoils is notoriously prone to recanalization arising from the low filling ratio due to their extreme rigidity. Smart hydrogel microcoils with tunable modulus may essentially significantly improve the therapeutic efficacy. Here, a radiopaque highly stiff body‐temperature‐triggered shape memory (SM) hydrogel is fabricated for the first time by introducing reversible hydrophobic dipole pairing microdomains in the flexibly crosslinked network, followed by BaSO₄ precipitation. This radiopacification does not affect their mechanical performances as well as the SM effect. It is demonstrated that the mechanical properties of SM hydrogels are comparable to those of rubbers and can be modulated by adjusting temperature ranging from 20 to 40 °C. Benefiting from the thermoresponsive mechanical properties, the stiff radiopaque hydrogel strip can easily pass through a catheter under the protection of cool saline for delivery into pig's renal artery, and spontaneously and rapidly transformed into a microcoil upon contacting blood. Real‐time angiogram reveals that continuous delivery of several hydrogel microcoils can efficiently occlude the blood supply. The kidneys are atrophied considerably over three month postoperative follow‐up, and no recanalization occurs throughout the experimental time. These novel hydrogel microcoils are promising to be developed as novel permanent embolic agents for treating aneurysm.     

Supertough Hybrid Hydrogels Consisting of a Polymer Double‐Network and Mesoporous Silica Microrods for Mechanically Stimulated On‐Demand Drug Delivery

Despite their potential in various fields of bioapplications, such as drug/cell delivery, tissue engineering, and regenerative medicine, hydrogels have often suffered from their weak mechanical properties, which are attributed to their single network of polymers. Here, supertough composite hydrogels are proposed consisting of alginate/polyacrylamide double‐network hydrogels embedded with mesoporous silica particles (SBA‐15). The supertoughness is derived from efficient energy dissipation through the multiple bondings, such as ionic crosslinking of alginate, covalent crosslinking of polyacrylamide, and van der Waals interactions and hydrogen bondings between SBA‐15 and the polymers. The superior mechanical properties of these hybrid hydrogels make it possible to maintain the hydrogel structure for a long period of time in a physiological solution. Based on their high mechanical stability, these hybrid hydrogels are demonstrated to exhibit on‐demand drug release, which is controlled by an external mechanical stimulation (both in vitro and in vivo). Moreover, different types of drugs can be separately loaded into the hydrogel network and mesopores of SBA‐15 and can be released with different speeds, suggesting that these hydrogels can also be used for multiple drug release.     

Complex Tuning of Physical Properties of Hyperbranched Polyglycerol‐Based Bioink for Microfabrication of Cell‐Laden Hydrogels

Microfabrication technology has emerged as a valuable tool for fabricating structures with high resolution and complex architecture for tissue engineering applications. For this purpose, it is imperative to develop “bioink” that can be readily converted to a solid structure by the modus operandi of a chosen apparatus, while optimally supporting the biological functions by tuning their physicochemical properties. Herein, a photocrosslinkable hyperbranched polyglycerol (acrylic hyperbranched glycerol (AHPG)) is developed as a crosslinker to fabricate cell‐laden hydrogels. Due to its hydrophilicity as well as numerous hydroxyl groups for the conjugation of reactive functional groups (e.g., acrylate), the mechanical properties of resulting hydrogels could be controlled in a wide range by tuning both molecular weight and degree of acrylate substitution of AHPG. The control of mechanical properties by AHPG is highly dependent on the type of monomer, due to the hydrophilic/hydrophobic balance of polyglycerol backbone and acrylate as well as the dynamic conformational flexibility based on the molecular weight of polyglycerol. The cell encapsulation studies demonstrate the biocompatibility of the AHPG‐linked hydrogels. Eventually, the AHPG‐based hydrogel precursor solution is employed as a bioink for a digital light processing based printing system to generate cell‐laden microgels with various shapes and sizes for tissue engineering applications.     

A Novel Double‐Crosslinking‐Double‐Network Design for Injectable Hydrogels with Enhanced Tissue Adhesion and Antibacterial Capability for Wound Treatment

Most photocrosslinkable hydrogels have inadequacy in either mechanical performance or biodegradability. This issue is addressed by adopting a novel hydrogel design by introducing two different chitosan chains (catechol‐modified methacryloyl chitosan, CMC; methacryloyl chitosan, MC) via the simultaneous crosslinking of carbon–carbon double bonds and catechol‐Fe³⁺ chelation. This leads to an interpenetrating network of two chitosan chains with high crosslinking‐network density, which enhances mechanical performance including high compressive modulus and high ductility. The chitosan polymers not only endow the hydrogels with good biodegradability and biocompatibility, they also offer intrinsic antibacterial capability. The quinone groups formed by Fe³⁺ oxidation and protonated amino groups of chitosan polymer further enhance antibacterial property of the hydrogels. Serving as one of the two types of crosslinking mechanisms, the catechol‐Fe³⁺ chelation can covalently link with amino, thiol, and imidazole groups, which substantially enhance the hydrogel's adhesion to biological tissues. The hydrogel's adhesion to porcine skin shows a lap shear strength of 18.1 kPa, which is 6‐time that of the clinically established Fibrin Glue's adhesion. The hydrogel also has a good hemostatic performance due to the superior tissue adhesion as demonstrated with a hemorrhaging liver model. Furthermore, the hydrogel can remarkably promote healing of bacteria‐infected wound.     

Soft Robotics Programmed with Double Crosslinking DNA Hydrogels

DNA nanotechnology is developed for decades to construct dynamic responsive systems in optics, quantum electronics, and therapeutics. While DNA nanotechnology is a powerful tool in nanomaterials, it is rare to see successful applications of DNA molecules in the macroscopic regime of material sciences. Here, a novel strategy to magnify the nanometer scale DNA self‐assembly into a macroscopic mechanical responsiveness is demonstrated. By incorporating molecularly engineered DNA sequences into a polymeric network, a new type of responsive hydrogel (D‐gel), whose overall morphology is dynamically controlled by DNA hybridization‐induced double crosslinking is able to be created. As a step toward manufacturing, the D‐gel in combination with a bottom‐up 3D printing technology is employed to rapidly create modular macroscopic structures that feature programmable reconfiguration and directional movement, which can even mimic the complex gestures of human hands. Mechanical operations such as catch and release are demonstrated by a proof‐of‐concept hydrogel palm, which possessed great promise for future engineering applications. Compared with previously developed DNA hydrogels, the D‐gel features an ease of synthesis, faster response, and a high degree of programmable control. Moreover, it is possible to scale up the production of D‐gel containing responsive devices through direct 3D printing.     

Synthesizing Organo/Hydrogel Hybrids with Diverse Programmable Patterns and Ultrafast Self‐Actuating Ability via a Site‐Specific “In Situ” Transformation Strategy

A simple yet robust strategy called “‘in situ' transformation” is developed to prepare organo/hydro binary gels based on the aminolysis of poly(pentafluorophenyl acrylate) (pPFPA). Treated with desired hydrophilic, oleophilic alkylamines, and their mixtures, pPFPA‐based organogels can be thoroughly transformed to targeted hydrogels, organogels, and even organohydrogels with outstanding mechanical properties. Further, relying on programed aminolysis procedures, site‐specific “in situ” transformation can be realized, giving rise to organo/hydro binary gels with diverse patterns and morphologies, such as macroscopic layered organo/hydrogel with a smooth‐transitioned yet mechanically robust interface, reconfigurable microscale organo/hydrogel hybrids with a high spatial‐resolution pattern capable of reversibly transforming between 2D sheets and 3D helixes with controlled chirality in different solvents, and core–shell structured organo/hydrogel hybrids with readily adjustable core/shell dimensions, tunable internal stress, and transparency. Finally, an oscillator based on a bilayered organo/hydrogel hybrid is developed. Attributing to the synergistic effect of organogel expansion and hydrogel contraction, as well as the robust interfacial mechanical properties, this oscillator is capable of ultrafast self‐actuating through harvesting surrounding chemical and thermal energy. This work provides new design principles and highly efficient synthetic strategy for organo/hydro binary gels, and expands their potential applications in soft robotics.     

Functional Nanomaterials on 2D Surfaces and in 3D Nanocomposite Hydrogels for Biomedical Applications

An emerging approach to improve the physicobiochemical properties and the multifunctionality of biomaterials is to incorporate functional nanomaterials (NMs) onto 2D surfaces and into 3D hydrogel networks. This approach is starting to generate promising advanced functional materials such as self‐assembled monolayers (SAMs) and nanocomposite (NC) hydrogels of NMs with remarkable properties and tailored functionalities that are beneficial for a variety of biomedical applications, including tissue engineering, drug delivery, and developing biosensors. A wide range of NMs, such as carbon‐, metal‐, and silica‐based NMs, can be integrated into 2D and 3D biomaterial formulations due to their unique characteristics, such as magnetic properties, electrical properties, stimuli responsiveness, hydrophobicity/hydrophilicity, and chemical composition. The highly ordered nano‐ or microscale assemblies of NMs on surfaces alter the original properties of the NMs and add enhanced and/or synergetic and novel features to the final SAMs of the NM constructs. Furthermore, the incorporation of NMs into polymeric hydrogel networks reinforces the (soft) polymer matrix such that the formed NC hydrogels show extraordinary mechanical properties with superior biological properties.     

Elastomeric Fibrous Hybrid Scaffold Supports In Vitro and In Vivo Tissue Formation

Biomimetic materials with biomechanical properties resembling those of native tissues while providing an environment for cell growth and tissue formation, are vital for tissue engineering (TE). Mechanical anisotropy is an important property of native cardiovascular tissues and directly influences tissue function. This study reports fabrication of anisotropic cell‐seeded constructs while retaining control over the construct's architecture and distribution of cells. Newly synthesized poly‐4‐hydroxybutyrate (P4HB) is fabricated with a dry spinning technique to create anelastomeric fibrous scaffold that allows control of fiber diameter, porosity, and rate ofdegradation. To allow cell and tissue ingrowth, hybrid scaffolds with mesenchymalstem cells (MSCs) encapsulated in a photocrosslinkable hydrogel were developed. Culturing the cellularized scaffolds in a cyclic stretch/flexure bioreactor resulted in tissue formation and confirmed the scaffold's performance under mechanical stimulation. In vivo experiments showed that the hybrid scaffold is capable of withstanding physiological pressures when implanted as a patch in the pulmonary artery. Aligned tissue formation occurred on the scaffold luminal surface without macroscopic thrombus formation. This combination of a novel, anisotropic fibrous scaffold and a tunable native‐like hydrogel for cellular encapsulation promoted formation of 3D tissue and provides a biologically functional composite scaffold for soft‐tissue engineering applications.     

Digital Plasmonic Patterning for Localized Tuning of Hydrogel Stiffness

The mechanical properties of the extracellular matrix (ECM) can dictate cell fate in biological systems. In tissue engineering, varying the stiffness of hydrogels—water‐swollen polymeric networks that act as ECM substrates—has previously been demonstrated to control cell migration, proliferation, and differentiation. Here, “digital plasmonic patterning” (DPP) is developed to mechanically alter a hydrogel encapsulated with gold nanorods using a near‐infrared laser, according to a digital (computer‐generated) pattern. DPP can provide orders of magnitude changes in stiffness, and can be tuned by laser intensity and speed of writing. In vitro cellular experiments using A7R5 smooth muscle cells confirm cell migration and alignment according to these patterns, making DPP a useful technique for mechanically patterning hydrogels for various biomedical applications.     

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.     

Bioinspired Anisotropic Hydrogel Actuators with On–Off Switchable and Color‐Tunable Fluorescence Behaviors

An effective approach to develop a novel macroscopic anisotropic bilayer hydrogel actuator with on–off switchable fluorescent color‐changing function is reported. Through combining a collapsed thermoresponsive graphene oxide‐poly(N‐isopropylacrylamide) (GO‐PNIPAM) hydrogel layer with a pH‐responsive perylene bisimide‐functionalized hyperbranched polyethylenimine (PBI‐HPEI) hydrogel layer via macroscopic supramolecular assembly, a bilayer hydrogel is obtained that can be tailored and reswells to form a 3D hydrogel actuator. The actuator can undergo complex shape deformation caused by the PNIPAM outside layer, then the PBI‐HPEI hydrogel inside layer can be unfolded to trigger the on–off switch of the pH‐responsive fluorescence under the green light irradiation. This work will inspire the design and fabrication of novel biomimetic smart materials with synergistic functions.     

A Printable Photopolymerizable Thermosensitive p(HPMAm‐lactate)‐PEG Hydrogel for Tissue Engineering

Bioprinting is a new technology in regenerative medicine that allows the engineering of tissues by specific placement of cells in biomaterials. Importantly, the porosity and the relatively small dimensions of the fibers allow rapid diffusion of nutrients and metabolites. This technology requires the availability of hydrogels that ensure viability of encapsulated cells and have adequate mechanical properties for the preparation of structurally stable and well‐defined three‐dimensional constructs. The aim of this study is to evaluate the suitability of a biodegradable, photopolymerizable and thermosensitive A–B–A triblock copolymer hydrogel as a synthetic extracellular matrix for engineering tissues by means of three dimensional fiber deposition. The polymer is composed of poly(N‐(2‐hydroxypropyl)methacrylamide lactate) A‐blocks, partly derivatized with methacrylate groups, and hydrophilic poly(ethylene glycol) B‐blocks of a molecular weight of 10 kDa. Gels are obtained by thermal gelation and stabilized with additional chemical cross‐links by photopolymerization of the methacrylate groups coupled to the polymer. A power law dependence of the storage plateau modulus of the studied hydrogels on polymer concentration is observed for both thermally and chemically cross‐linked hydrogels. The hydrogels demonstrated mechanical characteristics similar to natural semi‐flexible polymers, including collagen. Moreover, the hydrogel shows suitable mechanical properties for bioprinting, allowing subsequent layer‐by‐layer deposition of gel fibers to form stable constructs up to at least 0.6 cm (height) with different patterns and strand spacing. The resulting constructs have reproducible vertical porosity and the ability to maintain separate localization of encapsulated fluorescent microspheres. Moreover, the constructs show an elastic modulus of 119 kPa (25 wt% polymer content) and a degradation time of approximately 190 days. Furthermore, high viability is observed for encapsulated chondrocytes after 1 and 3 days of culture. In summary, we conclude that the evaluated hydrogel is an interesting candidate for bioprinting applications.     

Bioorthogonal Strategies for Engineering Extracellular Matrices

Hydrogels are commonly used as engineered extracellular matrix (ECM) mimics in applications ranging from tissue engineering to in vitro disease models. Ideal mechanisms used to crosslink ECM‐mimicking hydrogels do not interfere with the biology of the system. However, most common hydrogel crosslinking chemistries exhibit some form of crossreactivity. The field of bioorthogonal chemistry has arisen to address the need for highly specific and robust reactions in biological contexts. Accordingly, bioorthogonal crosslinking strategies are incorporated into hydrogel design, allowing for gentle and efficient encapsulation of cells in various hydrogel materials. Furthermore, the selective nature of bioorthogonal chemistries can permit dynamic modification of hydrogel materials in the presence of live cells and other biomolecules to alter matrix mechanical properties and biochemistry on demand. This review provides an overview of bioorthogonal strategies used to prepare cell‐encapsulating hydrogels and highlights the potential applications of bioorthogonal chemistries in the design of dynamic engineered ECMs.