Spatiotemporally controllable and cytocompatible approach builds 3D cell culture matrix by photo-uncaged-thiol Michael addition reaction

Advanced Multifunctional Hydrogels for Enhanced Wound Healing through Ultra-Fast Selenol-SNAr Chemistry

Fabrication of versatile hydrogels in a facile and effective manner represents a pivotal challenge in the field of biomaterials. Herein, a novel strategy is presented for preparing on-demand degradable hydrogels with multilevel responsiveness. By employing selenol-dichlorotetrazine nucleophilic aromatic substitution (SNAr) to synthesize hydrogels under mild conditions in a buffer solution, the necessity of additives or posttreatments can be obviated. The nucleophilic and redox reactions between selenol and tetrazine culminate in the formation of three degradable chemical bonds-diselenide, aryl selenide, and dearomatized selenide-in a single, expeditious step. The resultant hydrogel manifests exceptional adaptability to intricate environments in conjunction with self-healing and on-demand degradation properties. Furthermore, the resulting material demonstrated light-triggered antibacterial activity. Animal studies further underscore the potential of integrating metformin into Se-Tz hydrogels under green light irradiation, as it effectively stimulates angiogenesis and collagen deposition, thereby fostering efficient wound healing. In comparison to previously documented hydrogels, Se-Tz hydrogels exhibit controlled degradation and drug release, outstanding antibacterial activity, mechanical robustness, and bioactivity, all without the need for costly and intricate preparation procedures. These findings underscore Se-Tz hydrogels as a safe and effective therapeutic option for diabetic wound dressings.

Advances and Trends of Photoresponsive Hydrogels for Bone Tissue Engineering

The development of static hydrogels as an optimal choice for bone tissue engineering (BTE) remains a difficult challenge primarily due to the intricate nature of bone healing processes, continuous physiological functions, and pathological changes. Hence, there is an urgent need to exploit smart hydrogels with programmable properties that can effectively enhance bone regeneration. Increasing evidence suggests that photoresponsive hydrogels are promising bioscaffolds for BTE due to their advantages such as controlled drug release, cell fate modulation, and the photothermal effect. Here, we review the current advances in photoresponsive hydrogels. The mechanism of photoresponsiveness and its advanced applications in bone repair are also elucidated. Future research would focus on the development of more efficient, safer, and smarter photoresponsive hydrogels for BTE. This review is aimed at offering comprehensive guidance on the trends of photoresponsive hydrogels and shedding light on their potential clinical application in BTE.

Dynamic Exchange in 3D Cell Culture Hydrogels Based on Crosslinking of Cyclic Thiosulfinates

Dynamic polymer materials are highly valued substrates for 3D cell culture due to their viscoelasticity, a time-dependent mechanical property that can be tuned to resemble the energy dissipation of native tissues. Herein, we report the coupling of a cyclic thiosulfinate, mono-S-oxo-4-methyl asparagusic acid, to a 4-arm PEG-OH to prepare a disulfide-based dynamic covalent hydrogel with the addition of 4-arm PEG-thiol. Ring opening of the cyclic thiosulfinate by nucleophilic substitution results in the rapid formation of a network showing a viscoelastic fluid-like behaviour and relaxation rates modulated by thiol content through thiol-disulfide exchange, whereas its viscoelastic behaviour upon application as a small molecule linear crosslinker is solid-like. Further introduction of 4-arm PEG-vinylsulfone in the network yields a hydrogel with weeks-long cell culture stability, permitting 3D culture of cell types that lack robust proliferation, such as human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs). These cells display native behaviours such as cell elongation and spontaneous beating as a function of the hydrogel's mechanical properties. We demonstrate that the mode of dynamic cyclic thiosulfinate crosslinker presentation within the network can result in different stress relaxation profiles, opening the door to model tissues with disparate mechanics in 3D cell culture.

Flavin mononucleotide in visible light photoinitiating systems for multiple-photocrosslinking and photoencapsulation strategies

Visible light-induced photocrosslinking techniques have attracted significant attention for their flexibility, controllability, safety, and energy conservation, especially in tissue engineering and biofabrication, compared to UV photocrosslinking. Despite these advantages, current photoinitiators are constrained by various challenges, including inadequate photoinitiation efficiency, low biocompatibility, poor water solubility, and limited compatibility with diverse crosslinking systems. Here, a water-soluble derivative of riboflavin, flavin mononucleotide (FMN-), was used to assess its potential as an initiator in multiple-photocrosslinking systems, including radical photopolymerization, dityrosine, and ditryptophan coupling crosslinking, under blue light irradiation. Blue light irradiation facilitated an efficient electron transfer reaction between FMN- and persulfate, owing to their suitable spectral compatibility and photoactivity. The resulting oxidizing free radicals and excited triplet state of FMN- served as initiating active species for the multiple-photocrosslinking reactions. The combination of FMN- and potassium persulfate (KPS) exhibited exceptional photoinitiation efficiency for various biomaterials, including silk fibroin, gelatin, poly(ethylene glycol) diacrylate, and carboxymethyl cellulose modified with amino acids. Furthermore, the cytocompatibility of the FMN-/KPS photoinitiator was demonstrated by the survival rates of 3T3-LI fibroblasts encapsulated in it, which exceeded 95 % when compared to a commercial initiator. STATEMENT OF SIGNIFICANCE: By introducing persulfate, the photoinitiation efficiency of flavin mononucleotide was significantly improved. The application scenarios of flavin mononucleotide and persulfate combinations were also greatly extended, including radical photopolymerization, dityrosine, diphenylalanine, and ditryptophan coupling crosslinking. Among them, the coupling crosslinking of amino acids (di-phenylalanine, and di-tryptophan) modified carboxymethyl cellulose, to our knowledge, was first reported. The excellent cytocompatibility of cell encapsulation further proved that the combinations of flavin mononucleotide and persulfate have great potential in tissue engineering.

Visible light photocrosslinking of sugar beet pectin for 3D bioprinting applications

Herein, we report the hydrogelation of sugar beet pectin (SBP) via visible light-mediated photocrosslinking and its applications in extrusion-based 3D bioprinting. Rapid hydrogelation (<15 s) was achieved by applying 405 nm visible light to an SBP solution in the presence of tris(bipyridine)ruthenium(II) chloride hexahydrate ([Ru(bpy)₃]²⁺) and sodium persulfate (SPS). The mechanical properties of the hydrogel could be tuned by controlling the visible light irradiation time and concentrations of SBP, [Ru(bpy)₃]²⁺, and SPS. High-fidelity 3D hydrogel constructs were fabricated by extruding inks containing 3.0 wt% SBP, 1.0 mM [Ru(bpy)₃]²⁺, and 1.0 mM SPS. Human hepatoblastoma (HepG2) cells encapsulated in SBP hydrogels remained viable and metabolically active after 14 d of culture. Overall, this study demonstrates the feasibility of applying SBP and a visible light-mediated photocrosslinking system to the 3D bioprinting of cell-laden constructs for tissue engineering applications.

Photoresponsive hydrogel-based soft robot: A review

Soft robots have received a lot of attention because of their great human-robot interaction and environmental adaptability. Most soft robots are currently limited in their applications due to wired drives. Photoresponsive soft robotics is one of the most effective ways to promote wireless soft drives. Among the many soft robotics materials, photoresponsive hydrogels have received a lot of attention due to their good biocompatibility, ductility, and excellent photoresponse properties. This paper visualizes and analyzes the research hotspots in the field of hydrogels using the literature analysis tool Citespace, demonstrating that photoresponsive hydrogel technology is currently a key research direction. Therefore, this paper summarizes the current state of research on photoresponsive hydrogels in terms of photochemical and photothermal response mechanisms. The progress of the application of photoresponsive hydrogels in soft robots is highlighted based on bilayer, gradient, orientation, and patterned structures. Finally, the main factors influencing its application at this stage are discussed, including the development directions and insights. Advancement in photoresponsive hydrogel technology is crucial for its application in the field of soft robotics. The advantages and disadvantages of different preparation methods and structures should be considered in different application scenarios to select the best design scheme.

From Free-Radical to Radical-Free: A Paradigm Shift in Light-Mediated Biofabrication

In recent years, the development of novel photocrosslinking strategies and photoactivatable materials has stimulated widespread use of light-mediated biofabrication techniques. However, despite great progress toward more efficient and biocompatible photochemical strategies, current photoresins still rely on photoinitiators (PIs) producing radical-initiating species to trigger the so-called free-radical crosslinking/polymerization. In the context of bioprinting, where cells are encapsulated in the bioink, the presence of radicals raises concerns of potential cytotoxicity. In this work, a universal, radical-free (RF) photocrosslinking strategy to be used for light-based technologies is presented. Leveraging RF uncaging mechanisms and Michael addition, cell-laden constructs are photocrosslinked by means of one- and two-photon excitation with high biocompatibility. A hydrophilic coumarin-based group is used to cage a universal RF photocrosslinker based on 4-arm-PEG-thiol (PEG4SH). Upon light exposure, thiols are uncaged and react with an alkene counterpart to form a hydrogel. RF photocrosslinker is shown to be highly stable, enabling potential for off-the-shelf products. While PI-based systems cause a strong upregulation of reactive oxygen species (ROS)-associated genes, ROS are not detected in RF photoresins. Finally, optimized RF photoresin is successfully exploited for high resolution two-photon stereolithography (2P-SL) using remarkably low polymer concentration (<1.5%), paving the way for a shift toward radical-free light-based bioprinting.

Reinforced hydrogel network building by a rapid dual-photo-coupling reaction for 3D printing

A facile hydrogel fabrication strategy based on a simultaneous dual-photo-coupling reaction (i.e., photoinduced S-nitrosylation and Schiff base reaction) was reported. This strategy allowed a strengthened three-arm crosslinking network to form in one step and the hydrogels obtained displayed rapid gelation, excellent mechanical strength and biocompatibility for cell encapsulated-3D printing in real time. Our hydrogel fabrication strategy will likely foster advances in biomaterials and the extreme speed and reinforced mechanical strength should significantly benefit 3D printing and related applications.

Spatial and Temporal Modulation of Cell Instructive Cues in a Filamentous Supramolecular Biomaterial

Supramolecular materials provide unique opportunities to mimic both the structure and mechanics of the biopolymer networks that compose the extracellular matrix. However, strategies to modify their filamentous structures in space and time in 3D cell culture to study cell behavior as encountered in development and disease are lacking. We herein disclose a multicomponent squaramide-based supramolecular material whose mechanics and bioactivity can be controlled by light through co-assembly of a 1,2-dithiolane (DT) monomer that forms disulfide cross-links. Remarkably, increases in storage modulus from ∼200 Pa to >10 kPa after stepwise photo-cross-linking can be realized without an initiator while retaining colorlessness and clarity. Moreover, viscoelasticity and plasticity of the supramolecular networks decrease upon photo-irradiation, reducing cellular protrusion formation and motility when performed at the onset of cell culture. When applied during 3D cell culture, force-mediated manipulation is impeded and cells move primarily along earlier formed channels in the materials. Additionally, we show photopatterning of peptide cues in 3D using either a photomask or direct laser writing. We demonstrate that these squaramide-based filamentous materials can be applied to the development of synthetic and biomimetic 3D in vitro cell and disease models, where their secondary cross-linking enables mechanical heterogeneity and shaping at multiple length scales.

Controlling Properties and Functions of Polymer Gels Using Photochemical Reactions

Photoresponsive polymer gels have attracted increasing interest owing to their potential applications in healable materials, drug release systems, and extracellular matrices. Because polymer gels provide suitable environments for photochemical reactions, their properties and functions can be controlled with light with a high spatiotemporal resolution. Herein, the design of photoresponsive polymer gels based on different types of photochemical reactions is introduced. The mechanism and applications of irreversible photoreactions, such as photoinduced free-radical polymerization, photoinduced click reactions, and photolysis, as well as reversible photoreactions such as photoinduced reversible cycloadditions, reversible photosubstitution of metal complexes, and photoinduced metathesis are reviewed. The remaining challenges of photoresponsive polymer gels are also discussed.

Ultrafast, tough, and adhesive hydrogel based on hybrid photocrosslinking for articular cartilage repair in water-filled arthroscopy

A hydrogel scaffold for direct tissue-engineering application in water-irrigated, arthroscopic cartilage repair, is badly needed. However, such hydrogels must cure quickly under water, bind strongly and permanently to the surrounding tissue, and maintain sufficient mechanical strength to withstand the hydraulic pressure of arthroscopic irrigation (~10 kilopascal). To address these challenges, we report a versatile hybrid photocrosslinkable (HPC) hydrogel fabricated though a combination of photoinitiated radical polymerization and photoinduced imine cross-linking. The ultrafast gelation, high mechanical strength, and strong adhesion to native tissue enable the direct use of these hydrogels in irrigated arthroscopic treatments. We demonstrate, through in vivo articular cartilage defect repair in the weight-bearing regions of swine models, that the HPC hydrogel can serve as an arthroscopic autologous chondrocyte implantation scaffold for long-term cartilage regeneration, integration, and reconstruction of articular function.

Determining Michael Acceptor Reactivity from Kinetic, Mechanistic, and Computational Analysis for the Base-catalyzed Thiol-Michael Reaction

A combined experimental and computational study of the reactivities of seven commonly used Michael acceptors paired with two thiols within the framework of photobase-catalyzed thiol-Michael reactions is reported. The rate coefficients of the propagation (kP), reverse propagation (k-P), chain-transfer (kCT), and overall reaction (koverall) were experimentally determined and compared with the well-accepted electrophilicity parameters of Mayr and Parr, and DFT-calculated energetics. Both Mayr's and Parr's electrophilicity parameters predict the reactivities of these structurally varying vinyl functional groups well, covering a range of overall reaction rate coefficients from 0.5 to 6.2 s-1. To gain insight into the individual steps, the relative energies have been calculated using DFT for each of the stationary points along this step-growth reaction between ethanethiol and the seven alkenes. The free energies of the individual steps reveal the underlying factors that control the reaction barriers for propagation and chain transfer. Both the propagation and chain transfer steps are under kinetic control. These results serve as a useful guide for Michael acceptor selection to design and predict thiol-Michael-based materials with appropriate kinetic and material properties.

Photoclick Chemistry: A Bright Idea

At its basic conceptualization, photoclick chemistry embodies a collection of click reactions that are performed via the application of light. The emergence of this concept has had diverse impact over a broad range of chemical and biological research due to the spatiotemporal control, high selectivity, and excellent product yields afforded by the combination of light and click chemistry. While the reactions designated as "photoclick" have many important features in common, each has its own particular combination of advantages and shortcomings. A more extensive realization of the potential of this chemistry requires a broader understanding of the physical and chemical characteristics of the specific reactions. This review discusses the features of the most frequently employed photoclick reactions reported in the literature: photomediated azide-alkyne cycloadditions, other 1,3-dipolarcycloadditions, Diels-Alder and inverse electron demand Diels-Alder additions, radical alternating addition chain transfer additions, and nucleophilic additions. Applications of these reactions in a variety of chemical syntheses, materials chemistry, and biological contexts are surveyed, with particular attention paid to the respective strengths and limitations of each reaction and how that reaction benefits from its combination with light. Finally, challenges to broader employment of these reactions are discussed, along with strategies and opportunities to mitigate such obstacles.

A Self-Catalyzed Visible Light Driven Thiol Ligation

We introduce a highly efficient ligation system based on a visible light-induced rearrangement affording a thiophenol which rapidly undergoes thiol-Michael additions. Unlike conventional light-triggered thiol-ene/yne systems, which rely on the use of photocaged bases/nucleophiles, (organo)-photo catalysts, or radical photoinitiators, our system provides a light-induced reaction in the absence of any additives. The ligation is self-catalyzed via the pyridine mediated deprotonation of the photochemically generated thiophenol. Subsequently, the thiol-Michael reaction between the thiophenol anion and electron deficient alkynes/alkenes proceeds additive-free. Hereby, the underlying photoinduced rearrangement of o-thiopyrinidylbenzaldehyde (oTPyB) generating the free thiol is described for the first time. We studied the influence of various reactions conditions as well as solvents and substrates. We exemplify our findings in a polymer end group modification and obtained macromolecules with excellent end group fidelity.

Heat- and Light-Responsive Materials Through Pairing Dynamic Thiol-Michael and Coumarin Chemistry

Covalent adaptable networks (CANs) based on the thiol-Michael (TM) linkages can be thermal and pH responsive. Here, a new vinyl-sulfone-based thiol-Michael crosslinker is synthesized and incorporated into acrylate-based CANs to achieve stable materials with dynamic properties. Because of the reversible TM linkages, excellent temperature-responsive re-healing and malleability properties are achieved. In addition, for the first time, a photoresponsive coumarin moiety is incorporated with TM-based CANs to introduce light-mediated reconfigureability and postpolymerization crosslinking. Overall, these materials can be on demand dynamic in response to heat and light but can retain mechanical stability at ambient condition.

Red-Light Driven Photocatalytic Oxime Ligation for Bioorthogonal Hydrogel Design

Light-mediated polymer cross-linking is frequently employed for the preparation of hydrogels for biomedical applications. However, most photopolymerization processes require activation by UV light or short wavelength visible light, which are highly absorbed by skin and tissue, limiting their uses in transdermal initiation. Herein, we introduce red light-enabled oxime ligation by the in situ photogeneration of aldehydes, which rapidly react with hydroxylamines. We demonstrate efficient polymer cross-linking behind a dermal tissue model by red light initiation. Optimization of the photopolymerization conditions allows for 3D encapsulation of human foreskin fibroblasts with good cell viability postencapsulation.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Spatiotemporally Controlled Photoresponsive Hydrogels: Design and Predictive Modeling from Processing through Application

Photoresponsive hydrogels (PRHs) are soft materials whose mechanical and chemical properties can be tuned spatially and temporally with relative ease. Both photo-crosslinkable and photodegradable hydrogels find utility in a range of biomedical applications that require tissue-like properties or programmable responses. Progress in engineering with PRHs is facilitated by the development of theoretical tools that enable optimization of their photochemistry, polymer matrices, nanofillers, and architecture. This review brings together models and design principles that enable key applications of PRHs in tissue engineering, drug delivery, and soft robotics, and highlights ongoing challenges in both modeling and application.

Polyvinyl alcohol: a high-resolution hydrogel resist for humidity-sensitive micro-/nanostructure

A high-resolution nanopatterning technique is desirable with the present rapid development of hydrogel nanodevices. Here, we demonstrate that polyvinyl alcohol (PVA), a popular polymeric hydrogel, can function as the negative-tone resist for electron beam lithography (EBL) with a resolution capability as narrow as 50 nm half-pitch. Furthermore, the hydrophilic groups of PVA are stable after EBL exposure, and thus the pattern still shows rapid responsivity to humidity change. An aqueous nanopatterning process including dissolution, spin-coating and development is setup, which is friendly for organic device fabrication free of organic solvent. This high-resolution nanopatterning technique with PVA is helpful for the design and realization of hydrogel-related nanodevices in the future.

Injectable microfluidic hydrogel microspheres based on chitosan and poly(ethylene glycol) diacrylate (PEGDA) as chondrocyte carriers

Chitosan/PEGDA double-network hydrogel microspheres prepared by microfluidic method as chondrocyte carriers for bottom-up cartilage tissue engineering.

The relationship between thiol-acrylate photopolymerization kinetics and hydrogel mechanics: An improved model incorporating photobleaching and thiol-Michael addition

Biocompatible hydrogels with defined mechanical properties are critical to tissue engineering and regenerative medicine. Thiol-acrylate photopolymerized hydrogels have attracted special interest for their degradability and cytocompatibility, and for their tunable mechanical properties through controlling factors that affect reaction kinetics (e.g., photopolymerization, stoichiometry, temperature, and solvent choice). In this study, we hypothesized that the mechanical property of these hydrogels can be tuned by photoinitiators via photobleaching and by thiol-Michael addition reactions. To test this hypothesis, a multiscale mathematical model incorporating both photobleaching and thiol-Michael addition reactions was developed and validated. After validating the model, the effects of thiol concentration, light intensity, and pH values on hydrogel mechanics were investigated. Results revealed that hydrogel stiffness (i) was maximized at a light intensity-specific optimal concentration of thiol groups; (ii) increased with decreasing pH when synthesis occurred at low light intensity; and (iii) increased with decreasing light intensity when synthesis occurred at fixed precursor composition. The multiscale model revealed that the latter was due to higher initiation efficiency at lower light intensity. More broadly, the model provides a framework for predicting mechanical properties of hydrogels based upon the controllable kinetics of thiol-acrylate photopolymerization.

Orthogonal click reactions enable the synthesis of ECM-mimetic PEG hydrogels without multi-arm precursors

Click chemistry reactions have become an important tool for synthesizing user-defined hydrogels consisting of poly(ethylene glycol) (PEG) and bioactive peptides for tissue engineering. However, because click crosslinking proceeds via a step-growth mechanism, multi-arm telechelic precursors are required, which has some disadvantages. Here, we report for the first time that this requirement can be circumvented to create PEG-peptide hydrogels solely from linear precursors through the use of two orthogonal click reactions, the thiol-maleimide Michael addition and thiol-norbornene click reaction. The rapid kinetics of both click reactions allowed for quick formation of norbornene-functionalized PEG-peptide block copolymers via Michael addition, which were subsequently photocrosslinked into hydrogels with a dithiol linker. Characterization and in vitro testing demonstrated that the hydrogels have highly tunable physicochemical properties and excellent cytocompatiiblity. In addition, stoichiometric control over the crosslinking reaction can be leveraged to leave unreacted norbornene groups in the hydrogel for subsequent hydrogel functionalization via bioorthogonal inverse-electron demand Diels-Alder click reactions with s-tetrazines. After selectively capping norbornene groups in a user-defined region with cysteine, this feature was leveraged for protein patterning. Collectively, these results demonstrate that our novel chemical strategy is a simple and versatile approach to the development of hydrogels for tissue engineering that could be useful for a variety of applications.

Red and Near-Infrared Light-Cleavable Polymers

Photocleavable polymers have attracted much attention in drug delivery, photopatterning, and controlling cell behavior. Photolysis is usually induced by UV light. However, UV light cannot penetrate deeply into biological tissue and may damage biological components. Therefore, conventional UV-light-cleavable polymers are problematic for deep-tissue biomedical applications. In this feature article, red and near-infrared light-cleavable polymers are reviewed, and their potential applications are highlighted. The remaining challenges in the field of photocleavable polymers are discussed.

Spatiotemporal hydrogel biomaterials for regenerative medicine

Hydrogels mimic many of the physical properties of soft tissue and are widely used biomaterials for tissue engineering and regenerative medicine. Synthetic hydrogels have been developed to recapitulate many of the healthy and diseased states of native tissues and can be used as a cell scaffold to study the effect of matricellular interactions in vitro. However, these matrices often fail to capture the dynamic and heterogenous nature of the in vivo environment, which varies spatially and during events such as development and disease. To address this deficiency, a variety of manufacturing and processing techniques are being adapted to the biomaterials setting. Among these, photochemistry is particularly well suited because these reactions can be performed in precise three-dimensional space and at specific moments in time. This spatiotemporal control over chemical reactions can also be performed over a range of cell- and tissue-relevant length scales with reactions that proceed efficiently and harmlessly at ambient conditions. This review will focus on the use of photochemical reactions to create dynamic hydrogel environments, and how these dynamic environments are being used to investigate and direct cell behavior.

Non-invasively visualizing cell-matrix interactions in two-photon excited supramolecular hydrogels

Visualizing the role of extracellular matrix (ECM) in cell bioactivities in three-dimensional (3D) view is highly important for in-depth understanding of fundamental physiological issues in various in vitro experiments. Using current designs it is difficult to produce 3D biomimetic ECM with intrinsic fluorescence under non-invasive near-infrared excitation. Herein, we have designed and synthesized a series of non-conventional coumarin-derived hydrogelators, which can self-assemble to form nanofibrous 3D supramolecular hydrogels through C-HO bonds and be excited by two-photon absorption, ensuring the direct and dynamic visualization of cell-matrix interactions with high resolution images in a 3D environment. Real-time monitoring of ECM-regulated dynamic cell behaviours is highly desirable for future basic and applied research.

Versatile Bioorthogonal Hydrogel Platform by Catalyst-Free Visible Light Initiated Photodimerization of Anthracene

Recent developments in photochemistry have introduced new methods to prepare hydrogels initiated by nonharmful light which is essential for encapsulation of cells and bioactive components. However, bioorthogonal photoclick reactions generally requires two components for cross-linking and, in many cases, the formation of a reactive intermediate that may cross-react with nucleophiles in biological media. Here we report the utilization of a visible light triggered dimerization of electron-rich anthracene for polymer cross-linking to form bulk hydrogels and microgels. Incorporation of gelatin within the hydrogel enhanced cell attachment and viability after 7 days of culture and spatiotemporal conjugation of a bioactive component using photochemical dimerization of anthracene was demonstrated. This work therefore introduces a simple yet powerful tool for light modulated bioorthogonal polymer cross-linking, which can be utilized in various bioengineering applications.

Hybrid 3D printing and electrodeposition approach for controllable 3D alginate hydrogel formation

Calcium alginate hydrogels are widely used as biocompatible materials in a substantial number of biomedical applications. This paper reports on a hybrid 3D printing and electrodeposition approach for forming 3D calcium alginate hydrogels in a controllable manner. Firstly, a specific 3D hydrogel printing system is developed by integrating a customized ejection syringe with a conventional 3D printer. Then, a mixed solution of sodium alginate and CaCO₃ nanoparticles is filled into the syringe and can be continuously ejected out of the syringe nozzle onto a conductive substrate. When applying a DC voltage (∼5 V) between the substrate (anode) and the nozzle (cathode), the Ca²⁺ released from the CaCO₃ particles can crosslink the alginate to form calcium alginate hydrogel on the substrate. To elucidate the gel formation mechanism and better control the gel growth, we can further establish and verify a gel growth model by considering several key parameters, i.e., applied voltage and deposition time. The experimental results indicate that the alginate hydrogel of various 3D structures can be formed by controlling the movement of the 3D printer. A cell viability test is conducted and shows that the encapsulated cells in the gel can maintain a high survival rate (∼99% right after gel formation). This research establishes a reliable method for the controllable formation of 3D calcium alginate hydrogel, exhibiting great potential for use in basic biology and applied biomedical engineering.

An aptamer-patterned hydrogel for the controlled capture and release of proteins via biorthogonal click chemistry and DNA hybridization

An aptamer-patterned hydrogel can realize immobilization and controlled release of proteins in a spatiotemporal manner.

An ionic coordination hybrid hydrogel for bioseparation

An ionic coordination hybrid hydrogel is formed with ionic and covalent crosslinked networks via one-step copolymation.

High mechanical strength chitosan-based hydrogels cross-linked with poly(ethylene glycol)/polycaprolactone micelles for the controlled release of drugs/growth factors

Novel chitosan-based hydrogels crosslinked with PECL micelles with high mechanical strength for the controlled release of drugs/growth factors.

Material design and photo-regulated hydrolytic degradation behavior of tissue engineering scaffolds fabricated via 3D fiber deposition

An ideal behavior of a tissue engineering scaffold is that it degrades and reshapes at a rate that matches the formation of new tissues. However, this ideal situation may not occur as the scaffold often undergoes too slow or too fast degradation. To test the promise of the active control of scaffold degradation, in this work, a photo/water dual-degradable porous scaffold was designed and fabricated using a 3D fiber deposition (3DF) system from a linear biopolymer (named PLANB) that combined the o-nitrobenzyl linkages and hydrolysable ester bone in the polymer chains. The chemical structure, molecular weight and polydispersity of PLANB were characterized by IR, NMR, GPC and MALDI-TOF-MS. The thermal properties of PLANB evaluated by DSC and TGA assays enabled a 3DF printing at the temperature around its melting point without chemical changes. The introduction of a real-time IR (RTIR) technique not only facilitated the determination of photolysis kinetics and quantum yield, but also enabled the capture of intermediate products during the photo-cleavage process of PLANB scaffolds. A minute-scale daily photolysis combined with a continuous hydrolysis process was implemented to test the photo-regulated hydrolytic degradation behavior of PLANB scaffolds in vitro, and the results obtained from both the SEM image and the mass loss profile demonstrated a porous-void microstructure along the strands of scaffolds and an apparent increase of the mass loss amount compared with the control group without photo-irradiation. Furthermore, PLANB scaffolds showed low cytotoxicity to L929 cells and performed well in promoting cell adhesion. It can therefore be concluded that such scaffolds have great potential in offering a diverse range of control over degradation kinetics of tissue engineering scaffolds to be tailored to individual tissue regeneration situations.

Transdermal thiol-acrylate polyethylene glycol hydrogel synthesis using near infrared light

Light-induced polymerization has been widely applied for hydrogel synthesis, which conventionally involves the use of ultraviolet or visible light to activate a photoinitiator for polymerization. However, with these light sources, transdermal gelation is not efficient and feasible due to their substantial interactions with biological systems, and thus a high power is required. In this study, we used biocompatible and tissue-penetrating near infrared (NIR) light to remotely trigger a thiol-acrylate reaction for efficient in vivo gelation with good controllability. Our gelation system includes gold nanorods as a photothermal agent, a thermal initiator, diacrylate polyethylene glycol (PEG), and thiolated PEG. Irradiation with a low-power NIR laser (0.3 W cm(-2)) could induce gelation via a mixed-mode reaction with a small increase in temperature (∼5 °C) under the optimized conditions. We also achieved successful transdermal gelation via the NIR-assisted photothermal thiol-acryl reactions. This new type of NIR-assisted thiol-acrylate polymerization provides new opportunities for in situ hydrogel formation for injectable hydrogels and delivery of drugs/cells for various biomedical applications.

Combinatorial Method/High Throughput Strategies for Hydrogel Optimization in Tissue Engineering Applications

Combinatorial method/high throughput strategies, which have long been used in the pharmaceutical industry, have recently been applied to hydrogel optimization for tissue engineering applications. Although many combinatorial methods have been developed, few are suitable for use in tissue engineering hydrogel optimization. Currently, only three approaches (design of experiment, arrays and continuous gradients) have been utilized. This review highlights recent work with each approach. The benefits and disadvantages of design of experiment, array and continuous gradient approaches depending on study objectives and the general advantages of using combinatorial methods for hydrogel optimization over traditional optimization strategies will be discussed. Fabrication considerations for combinatorial method/high throughput samples will additionally be addressed to provide an assessment of the current state of the field, and potential future contributions to expedited material optimization and design.

Tissue-Integratable and Biocompatible Photogelation by the Imine Crosslinking Reaction

A novel photogelling mechanism by the phototriggered-imine-crosslinking (PIC) reaction is demonstrated. Hyaluronic acid grafted with o-nitrobenzene, a photogenerated aldehyde group, can quickly photo-crosslink with amino-bearing polymers or proteins. Once the in situ photogelling on a wound occurs, the PIC gelling process can well integrate a hydrogel with surrounding tissue by covalent bonding, thus making it a powerful tool for tissue engineering and regenerative medicine.

3D printing of functional biomaterials for tissue engineering

3D printing is emerging as a powerful tool for tissue engineering by enabling 3D cell culture within complex 3D biomimetic architectures. This review discusses the prevailing 3D printing techniques and their most recent applications in building tissue constructs. The work associated with relatively well-known inkjet and extrusion-based bioprinting is presented with the latest advances in the fields. Emphasis is put on introducing two relatively new light-assisted bioprinting techniques, including digital light processing (DLP)-based bioprinting and laser based two photon polymerization (TPP) bioprinting. 3D bioprinting of vasculature network is particularly discussed for its foremost significance in maintaining tissue viability and promoting functional maturation. Limitations to current bioprinting approaches, as well as future directions of bioprinting functional tissues are also discussed.