Reduced Graphene Oxide‐GelMA Hybrid Hydrogels as Scaffolds for Cardiac Tissue Engineering

Biomaterials currently used in cardiac tissue engineering have certain limitations, such as lack of electrical conductivity and appropriate mechanical properties, which are two parameters playing a key role in regulating cardiac cell behavior. Here, the myocardial tissue constructs are engineered based on reduced graphene oxide (rGO)‐incorporated gelatin methacryloyl (GelMA) hybrid hydrogels. The incorporation of rGO into the GelMA matrix significantly enhances the electrical conductivity and mechanical properties of the material. Moreover, cells cultured on composite rGO‐GelMA scaffolds exhibit better biological activities such as cell viability, proliferation, and maturation compared to ones cultured on GelMA hydrogels. Cardiomyocytes show stronger contractility and faster spontaneous beating rate on rGO‐GelMA hydrogel sheets compared to those on pristine GelMA hydrogels, as well as GO‐GelMA hydrogel sheets with similar mechanical property and particle concentration. Our strategy of integrating rGO within a biocompatible hydrogel is expected to be broadly applicable for future biomaterial designs to improve tissue engineering outcomes. The engineered cardiac tissue constructs using rGO incorporated hybrid hydrogels can potentially provide high‐fidelity tissue models for drug studies and the investigations of cardiac tissue development and/or disease processes in vitro.     

Hydrogel machines

As polymer networks infiltrated with water, hydrogels constitute the major components of the human body; and hydrogels have been widely used in applications that closely interact with biological organisms, such as tissue engineering, drug delivery, and biological research. More recently, owing to their superior softness, wetness, responsiveness, biocompatibility, and bioactivity, hydrogels are being intensively investigated for versatile functions in devices and machines including sensors, actuators, coatings, optics, electronics, and water harvesters. A nascent field named hydrogel machines rapidly evolves, exploiting hydrogels as key components for devices and machines. While there are reviews on individual categories of hydrogel machines in literature, a comprehensive discussion on various categories of hydrogel machines that systematically correlate hydrogels’ properties and machines’ functions is still missing in the field. This review is aimed to provide such a panoramic overview. We first classify various hydrogel machines into a number of categories according to their applications. For each category, we discuss (i) the working principles of the hydrogel machines, (ii) the specific properties of hydrogels that enable the key functions of the machines, and (iii) challenges faced by hydrogel machines and recent developments to address them. The field of hydrogel machines will not only translate fundamental understanding of hydrogels into new applications, but also shift the paradigm in machine design by integrating hydrogels that can potentially minimize physical and physiological mismatches with biological organisms.     

Self‐Healing Hydrogels

Over the past few years, there has been a great deal of interest in the development of hydrogel materials with tunable structural, mechanical, and rheological properties, which exhibit rapid and autonomous self‐healing and self‐recovery for utilization in a broad range of applications, from soft robotics to tissue engineering. However, self‐healing hydrogels generally either possess mechanically robust or rapid self‐healing properties but not both. Hence, the development of a mechanically robust hydrogel material with autonomous self‐healing on the time scale of seconds is yet to be fully realized. Here, the current advances in the development of autonomous self‐healing hydrogels are reviewed. Specifically, methods to test self‐healing efficiencies and recoveries, mechanisms of autonomous self‐healing, and mechanically robust hydrogels are presented. The trends indicate that hydrogels that self‐heal better also achieve self‐healing faster, as compared to gels that only partially self‐heal. Recommendations to guide future development of self‐healing hydrogels are offered and the potential relevance of self‐healing hydrogels to the exciting research areas of 3D/4D printing, soft robotics, and assisted health technologies is highlighted.     

25th Anniversary Article: Engineering Hydrogels for Biofabrication

With advances in tissue engineering, the possibility of regenerating injured tissue or failing organs has become a realistic prospect for the first time in medical history. Tissue engineering – the combination of bioactive materials with cells to generate engineered constructs that functionally replace lost and/or damaged tissue – is a major strategy to achieve this goal. One facet of tissue engineering is biofabrication, where three‐dimensional tissue‐like structures composed of biomaterials and cells in a single manufacturing procedure are generated. Cell‐laden hydrogels are commonly used in biofabrication and are termed “bioinks”. Hydrogels are particularly attractive for biofabrication as they recapitulate several features of the natural extracellular matrix and allow cell encapsulation in a highly hydrated mechanically supportive three‐dimensional environment. Additionally, they allow for efficient and homogeneous cell seeding, can provide biologically‐relevant chemical and physical signals, and can be formed in various shapes and biomechanical characteristics. However, despite the progress made in modifying hydrogels for enhanced bioactivation, cell survival and tissue formation, little attention has so far been paid to optimize hydrogels for the physico‐chemical demands of the biofabrication process. The resulting lack of hydrogel bioinks have been identified as one major hurdle for a more rapid progress of the field. In this review we summarize and focus on the deposition process, the parameters and demands of hydrogels in biofabrication, with special attention to robotic dispensing as an approach that generates constructs of clinically relevant dimensions. We aim to highlight this current lack of effectual hydrogels within biofabrication and initiate new ideas and developments in the design and tailoring of hydrogels. The successful development of a “printable” hydrogel that supports cell adhesion, migration, and differentiation will significantly advance this exciting and promising approach for tissue engineering.     

Stabilization of Polymer‐Hydrogel Capsules via Thiol–Disulfide Exchange

Polymer hydrogels are used in diverse biomedical applications including drug delivery and tissue engineering. Among different chemical linkages, the natural and reversible thiol–disulfide interconversion is extensively explored to stabilize hydrogels. The creation of macro‐, micro‐, and nanoscale disulfide‐stabilized hydrogels commonly relies on the use of oxidizing agents that may have a detrimental effect on encapsulated cargo. Herein an oxidization‐free approach to create disulfide‐stabilized polymer hydrogels via a thiol–disulfide exchange reaction is reported. In particular, thiolated poly(methacrylic acid) is used and the conditions of polymer crosslinking in solution and on colloidal porous and solid microparticles are established. In the latter case, removal of the core particles yields stable, hollow, disulfide‐crosslinked hydrogel capsules. Further, a procedure is developed to achieve efficient disulfide crosslinking of multilayered polymer films to obtain stable, liposome‐loaded polymer‐hydrogel capsules that contain functional enzymatic cargo within the liposomal subcompartments. This approach is envisaged to facilitate the development of biomedical applications of hydrogels, specifically those including fragile cargo.     

Skin‐Inspired Multifunctional Autonomic‐Intrinsic Conductive Self‐Healing Hydrogels with Pressure Sensitivity,Stretchability, and 3D Printability

The advent of conductive self‐healing (CSH) hydrogels, a class of novel materials mimicking human skin, may change the trajectory of the industrial process because of their potential applications in soft robots, biomimetic prostheses, and health‐monitoring systems. Here, the development of a mechanically and electrically self‐healing hydrogel based on physically and chemically cross‐linked networks is reported. The autonomous intrinsic self‐healing of the hydrogel is attained through dynamic ionic interactions between carboxylic groups of poly(acrylic acid) and ferric ions. A covalent cross‐linking is used to support the mechanical structure of the hydrogel. Establishing a fair balance between the chemical and physical cross‐linking networks together with the conductive nanostructure of polypyrrole networks leads to a double network hydrogel with bulk conductivity, mechanical and electrical self‐healing properties (100% mechanical recovery in 2 min), ultrastretchability (1500%), and pressure sensitivity. The practical potential of CSH hydrogels is further revealed by their application in human motion detection and their 3D‐printing performance.     

Synthesis, characterization, and in vitro evaluation of a hydrogel-based corneal onlay

Blindness due to opacity of the cornea is treated by corneal transplantation with donor tissue. Due to the limited supply of suitable donor corneas, the need for synthetic corneal equivalents is clear. Herein we report the design and in vitro characterization of a hydrogel-based implant; this implant will serve as a permanent, transparent, space-filling onlay with a two-layer design that mimics the native corneal stratification to support surface epithelialization and foster integration with the surrounding tissue. The top layer of the implant was composed of a 2-hydroxyethylmethacrylate hydrogel containing methacrylic acid as the co-monomer (HEMA-co-MAA) with tunable dimensions and compressive modulus ranging from 700-1000 kPa. The bottom layer, which constitutes the bulk of the implant and is designed to provide integration with the corneal stroma, is a dendrimer hydrogel with high water content and compressive modulus ranging from 500-1200 kPa. Both hydrogels were found to possess optical and diffusion properties similar to those of the human cornea. In addition, composite implants with uniform and structurally sound interfaces were formed when the gels were sequentially injected and cross-linked in the same mold. HEMA-co-MAA hydrogels were covalently modified with type I collagen to enable corneal epithelial cell adhesion and spreading that was dependent upon the collagen coating density but independent of hydrogel stiffness. Similarly, dendrimer hydrogels supported the adhesion and spreading of corneal fibroblasts upon modification with the adhesion ligand arginine-glycine-aspartic acid (RGD). Fibroblast adhesion was not dependent upon dendrimer hydrogel stiffness for the formulations studied and, after in vitro culture for 4 weeks, fibroblasts remained able to adhere to and conformally coat the hydrogel surface. In conclusion, the tunable physical properties and structural integrity of the laminated interface suggests that this design is suitable for further study. The judicious tuning of material properties and inclusion of bioactive moieties is a promising strategy for promotion of implant epithelialization and tissue integration.     

Creating Stiff,Tough, and Functional Hydrogel Composites with Low‐Melting‐Point Alloys

Reinforcing hydrogels with a rigid scaffold is a promising method to greatly expand the mechanical and physical properties of hydrogels. One of the challenges of creating hydrogel composites is the significant stress that occurs due to swelling mismatch between the water‐swollen hydrogel matrix and the rigid skeleton in aqueous media. This stress can cause physical deformation (wrinkling, buckling, or fracture), preventing the fabrication of robust composites. Here, a simple yet versatile method is introduced to create “macroscale” hydrogel composites, by utilizing a rigid reinforcing phase that can relieve stress‐induced deformation. A low‐melting‐point alloy that can transform from a load‐bearing solid state to a free‐deformable liquid state at relatively low temperature is used as a reinforcing skeleton, which enables the release of any swelling mismatch, regardless of the matrix swelling degree in liquid media. This design can generally provide hydrogels with hybridized functions, including excellent mechanical properties, shape memory, and thermal healing, which are often difficult or impossible to achieve with single‐component hydrogel systems. Furthermore, this technique enables controlled electrochemical reactions and channel‐structure templating in hydrogel matrices. This work may play an important role in the future design of soft robots, wearable electronics, and biocompatible functional materials.     

Adaptable Hydrogel Networks with Reversible Linkages for Tissue Engineering

Adaptable hydrogels have recently emerged as a promising platform for three‐dimensional (3D) cell encapsulation and culture. In conventional, covalently crosslinked hydrogels, degradation is typically required to allow complex cellular functions to occur, leading to bulk material degradation. In contrast, adaptable hydrogels are formed by reversible crosslinks. Through breaking and re‐formation of the reversible linkages, adaptable hydrogels can be locally modified to permit complex cellular functions while maintaining their long‐term integrity. In addition, these adaptable materials can have biomimetic viscoelastic properties that make them well suited for several biotechnology and medical applications. In this review, an overview of adaptable‐hydrogel design considerations and linkage selections is presented, with a focus on various cell‐compatible crosslinking mechanisms that can be exploited to form adaptable hydrogels for tissue engineering.     

Fabrication of chitosan/poly(ε-caprolactone) composite hydrogels for tissue engineering applications

The aim of this study was to fabricate three-dimensional (3D) porous chitosan/poly(ε-caprolactone) (PCL) hydrogels with improved mechanical properties for tissue engineering applications. A modified emulsion lyophilisation technique was developed to produce 3D chitosan/PCL hydrogels. The addition of 25 and 50 wt% of PCL into chitosan substantially enhanced the compressive strength of composite hydrogel 160 and 290%, respectively, compared to pure chitosan hydrogel. The result of ATR–FTIR imaging corroborated that PCL and chitosan were well mixed and physically co-existed in the composite structures. The composite hydrogels were constructed of homogenous structure with average pore size of 59.7 ± 14 μm and finer pores with average size of 4.4 ± 2 μm on the wall of these larger pores. The SEM and confocal laser scanning microscopy images confirmed that fibroblast cells were attached and proliferated on the 3D structure of these composite hydrogels. The composite hydrogels acquired in this study possessed homogeneous porous structure with improved mechanical strength and integrity. They may have a high potential for the production of 3D hydrogels for tissue engineering applications.     

Highly Elastic and Ultratough Hybrid Ionic–Covalent Hydrogels with Tunable Structures and Mechanics

Hybrid ionically–covalently crosslinked double‐network (DN) hydrogels are attracting increasing attention on account of their self‐recovery ability and fatigue resistance, but their relative low mechanical strength and tedious performance adjustment severely limit their applications. Herein, a new strategy to concurrently fabricate hybrid ionic–covalent DN hydrogels and modulate their structures and mechanics is reported, in which an in situ formed chitosan ionic network is incorporated by post‐crosslinking the chitosan‐based composite hydrogel using multivalent anions solutions. The obtained hybrid DN hydrogels exhibit predominant mechanical properties including superior elastic modulus, high tensile strength, and ultrahigh fracture energy because of the more efficient energy dissipation of rigid short‐chain chitosan network. Notably, the swollen hydrogels still remain mechanically strong and tough even after immersion in water for 24 h. More significantly, simply changing the post‐crosslinking time can vary the compactness and rigidity of the chitosan network in situ, achieving flexible and efficient modulation of the structures and mechanics of the hybrid DN hydrogels. This study opens up a new horizon in the preparation and regulation of DN hydrogels for promising applications in tissue scaffolds, actuators, and wearable devices.     

Vacancy‐Driven Gelation Using Defect‐Rich Nanoassemblies of 2D Transition Metal Dichalcogenides and Polymeric Binder for Biomedical Applications

A new approach of vacancy‐driven gelation to obtain chemically crosslinked hydrogels from defect‐rich 2D molybdenum disulfide (MoS₂) nanoassemblies and polymeric binder is reported. This approach utilizes the planar and edge atomic defects available on the surface of the 2D MoS₂ nanoassemblies to form mechanically resilient and elastomeric nanocomposite hydrogels. The atomic defects present on the lattice plane of 2D MoS₂ nanoassemblies are due to atomic vacancies and can act as an active center for vacancy‐driven gelation with a thiol‐activated terminal such as four‐arm poly(ethylene glycol)–thiol (PEG‐SH) via chemisorption. By modulating the number of vacancies on the 2D MoS₂ nanoassemblies, the physical and chemical properties of the hydrogel network can be controlled. This vacancy‐driven gelation process does not require external stimuli such as UV exposure, chemical initiator, or thermal agitation for crosslinking and thus provides a nontoxic and facile approach to encapsulate cells and proteins. 2D MoS₂ nanoassemblies are cytocompatible, and encapsulated cells in the nanocomposite hydrogels show high viability. Overall, the nanoengineered hydrogel obtained from vacancy‐driven gelation is mechanically resilient and can be used for a range of biomedical applications including tissue engineering, regenerative medicine, and cell and therapeutic delivery.     

Mechanical properties and in vitro behavior of nanofiber-hydrogel composites for tissue engineering applications

Hydrogel-based biomaterial systems have great potential for tissue reconstruction by serving as temporary scaffolds and cell delivery vehicles for tissue engineering (TE). Hydrogels have poor mechanical properties and their rapid degradation limits the development and application of hydrogels in TE. In this study, nanofiber reinforced composite hydrogels were fabricated by incorporating electrospun poly(ε-caprolactone) (PCL)/gelatin 'blend' or 'coaxial' nanofibers into gelatin hydrogels. The morphological, mechanical, swelling and biodegradation properties of the nanocomposite hydrogels were evaluated and the results indicated that the moduli and compressive strengths of the nanofiber reinforced hydrogels were remarkably higher than those of pure gelatin hydrogels. By increasing the amount of incorporated nanofibers into the hydrogel, the Young's modulus of the composite hydrogels increased from 3.29 ± 1.02 kPa to 20.30 ± 1.79 kPa, while the strain at break decreased from 66.0 ± 1.1% to 52.0 ± 3.0%. Compared to composite hydrogels with coaxial nanofibers, those with blend nanofibers showed higher compressive strength and strain at break, but with lower modulus and energy dissipation properties. Biocompatibility evaluations of the nanofiber reinforced hydrogels were carried out using bone marrow mesenchymal stem cells (BM-MSCs) by cell proliferation assay and immunostaining analysis. The nanocomposite hydrogel with 25 mg ml(-1) PCL/gelatin 'blend' nanofibers (PGB25) was found to enhance cell proliferation, indicating that the 'nanocomposite hydrogels' might provide the necessary mechanical support and could be promising cell delivery systems for tissue regeneration.     

Tough,Self‐Healing Hydrogels Capable of Ultrafast Shape Changing

Achieving multifunctional shape‐changing hydrogels with synergistic and engineered material properties is highly desirable for their expanding applications, yet remains an ongoing challenge. The synergistic design of multiple dynamic chemistries enables new directions for the development of such materials. Herein, a molecular design strategy is proposed based on a hydrogel combining acid–ether hydrogen bonding and imine bonds. This approach utilizes simple and scalable chemistries to produce a doubly dynamic hydrogel network, which features high water uptake, high strength and toughness, excellent fatigue resistance, fast and efficient self‐healing, and superfast, programmable shape changing. Furthermore, deformed shapes can be memorized due to the large thermal hysteresis. This new type of shape‐changing hydrogel is expected to be a key component in future biomedical, tissue, and soft robotic device applications.     

Inside Front Cover: Rigid,Self‐Assembled Hydrogel Composed of a Modified Aromatic Dipeptide (Adv. Mater. 11/2006)

Formation of a self‐assembled hydrogel with remarkable mechanical rigidity using a very simple building block, 9‐fluorenylmethoxycarbonyl‐diphenylalanine peptide, is reported by Gazit and co‐workers on p. 1365. The hydrogel forms under mild conditions in aqueous solution, using a much shorter peptide than previously reported, and has physical properties exceeding those of hydrogels formed by much longer polypeptides, as previously reported for diphenylalanine nanotubes. The rigidity is likely facilitated by the aromatic nature of the peptide building block. The hydrogel is stable under extreme conditions, and can be shaped in accordance to the vessel it is assembled in, making it useful for a variety of applications.     

Biodegradable and semi-biodegradable composite hydrogels as bone substitutes: morphology and mechanical characterization

Biodegradable and semi-biodegradable composite hydrogels are proposed as bone substitutes. They consist of an hydrophilic biodegradable polymer (HYAFF 11) as matrix and two ceramic powders (α-TCP and HA) as reinforcement. Both components of these composites have been of great interest in biomedical applications due to their excellent biocompatibility and tissue interactions, however they have never been investigated as bone substitute composites. Morphological and mechanical analysis have shown that the two fillers behave in a very different way. In the HYAFF 11/α-TCP composite, α-TCP is able to hydrolyze in contact with water while in the HYAFF 11 matrix. As a result, the composite sets and hardens, and entangled CDHA crystals are formed in the hydrogel phase and increases in the mechanical properties are obtained. In the HYAFF11/HA composite the ceramic reinforcement acts as inert phase leading to lower mechanical properties. Both mechanical properties and microstructure analysis have demonstrated the possibility to design hydrophilic biodegradable composite structures for bone tissue substitution applications.     

Differential physical, rheological, and biological properties of rapid in situ gelable hydrogels composed of oxidized alginate and gelatin derived from marine or porcine sources

Marine derived gelatin is not known to associate with any communicable diseases to mammals and could be a reasonable substitute for gelatin derived from either bovine or porcine sources. The low melting point of marine gelatin (8°C) also offers greater formulation flexibility than mammalian derived gelatins. However, the sub-optimal physical properties of marine gelatin generally limit the interest to further develop it for biomedical applications. This study aimed at investigating the feasibility of using oxidized alginate (Oalg) as a high activity macromolecular crosslinker of marine gelatin to formulate in situ gelable hydrogels with the goal of enhancing the latter’s physical properties. The performance of Oalg/marine gelatin hydrogel was compared to Oalg/porcine gelatin hydrogel; in general, the physicomechanical properties of both hydrogels were comparable, with the hydrogels containing porcine gelatin exhibiting moderately higher mechanical strengths with shorter gelation times, smaller size pores, and higher swelling ratios. On the contrary, the biological performances of the two hydrogels were significantly difference. Cells cultured in the marine gelatin derived hydrogel grew significantly faster, with greater than 60% more cells by 7 days and they exhibited more spread-out conformations as compared those cultured in the porcine derived hydrogel. Production of ECM by cells cultured in the Oalg/marine gelatin hydrogel was up to 2.4 times greater than that of in the Oalg/porcine gelatin hydrogel. The biodegradation rate of the hydrogel formulated from marine gelatin was greater than its counterpart prepared from porcine gelatin. These differences have important implications in the biomedical applications of the two hydrogels. Huijuan Liao and Hanwei Zhang are the first authors.     

Graphite/poly (vinyl alcohol) hydrogel composite as porous ringy skirt for artificial cornea

Graphite/poly (vinyl alcohol) (PVA) hydrogel composites, which were designed as the porous ringy skirt surrounding the transparent core of a novel artificial cornea, were prepared by using the freeze/thawing process and the particle-leaching technique. The properties of the composites, including the water content, the mechanical strength, the porous architecture and the interactions between the graphite and PVA, were investigated. The tissue responses to the composite and pure PVA hydrogel were studied by in vivo implantation in the dorsal muscles of mice. The results showed that chemical interactions were present between the graphite and PVA in the composite, which benefited the combination of the two phases and enhanced the uniform distribution of graphite particles in the PVA matrix. However, the present of graphites in the PVA hydrogels reduced the tensile strength, elongation at break and water content of the composite. Moreover, the porous graphite/PVA hydrogel composite had interconnective pore structure with high porosity and enough mechanical strength. According to the histological analysis of 1 week and 12 weeks post-implantation, the graphite/PVA hydrogel composites showed less inflammatory reactions than the PVA hydrogels at the 1 week post-implantation. Moreover, compared to pure PVA hydrogel, the graphite/PVA hydrogel composite exhibited enhanced migration and infiltration of cells, and more neovascularization and tissue ingrowth. These in vivo characteristics will be beneficial for the long-term biofixation of artificial cornea. Therefore, the porous graphite/PVA hydrogel composite has a potential to be used as novel artificial cornea skirt.     

Ultrastretchable and Wireless Bioelectronics Based on All‐Hydrogel Microfluidics

Hydrogel bioelectronics that can interface biological tissues and flexible electronics is at the core of the growing field of healthcare monitoring, smart drug systems, and wearable and implantable devices. Here, a simple strategy is demonstrated to prototype all‐hydrogel bioelectronics with embedded arbitrary conductive networks using tough hydrogels and liquid metal. Due to their excellent stretchability, the resultant all‐hydrogel bioelectronics exhibits stable electrochemical properties at large tensile stretch and various modes of deformation. The potential of fabricated all‐hydrogel bioelectronics is demonstrated as wearable strain sensors, cardiac patches, and near‐field communication (NFC) devices for monitoring various physiological conditions wirelessly. The presented simple platform paves the way of implantable hydrogel electronics for Internet‐of‐Things and tissue–machine interfacing applications.     

Microstructural and mechanical characteristics of PHEMA-based nanofibre-reinforced hydrogel under compression

Natural network-structured hydrogels (e.g. bacterial cellulose (BC)) can be synthesised with specific artificial hydrogels (e.g. poly(2-hydroxyethyl methacrylate) (PHEMA)) to form a tougher and stronger nanofibre-reinforced composite hydrogel, which possesses micro- and nano-porous structure. These synthetic hydrogels exhibit a number of advantages for biomedical applications, such as good biocompatibility and better permeability for molecules to pass through. In this paper, the mechanical properties of this nanofibre-reinforced hydrogel containing BC and PHEMA have been characterised in terms of their tangent modulus and fracture stress/strain by uniaxial compressive testing. Numerical simulations based on Mooney-Rivlin hyperelastic theory are also conducted to understand the internal stress distribution and possible failure of the nanofibre-reinforced hydrogel under compression. By comparing the mechanical characteristics of BC, PHEMA, and PHEMA-based nanofibre reinforced hydrogel (BC-PHEMA) under the compression, it is possible to develop a suitable scaffold for tissue engineering on the basis of fundamental understanding of mechanical and fracture behaviours of nanofibre-reinforced hydrogels.