Visible Light-Responsive Dynamic Biomaterials: Going Deeper and Triggering More

Exogenous electron generation techniques for biomedical applications: Bridging fundamentals and clinical practice

Endogenous bioelectrical signals are quite crucial in biological development, governing processes such as regeneration and disease progression. Exogenous stimulation, which mimics endogenous bioelectrical signals, has demonstrated significant potential to modulate complex biological processes. Consequently, increasing scientific efforts have focused on developing methods to generate exogenous electrons for biological applications, primarily relying on piezoelectric, acoustoelectric, optoelectronic, magnetoelectric, and thermoelectric principles. Given the expanding body of literature on this topic, a systematic and comprehensive review is essential to foster a deeper understanding and facilitate clinical applications of these techniques. This review synthesizes and compares these methods for generating exogenous electrical signals, their underlying principles (e.g., semiconductor deformation, photoexcitation, vibration and relaxation, and charge separation), biological mechanisms, potential clinical applications, and device designs, highlighting their advantages and limitations. By offering a comprehensive perspective on the critical role of exogenous electrons in biological systems, elucidating the principles of various electron-generation techniques, and exploring possible pathways for developing medical devices utilizing exogenous electrons, this review aims to advance the field and support therapeutic innovation.

Matrix Stiffness and Biochemistry Govern Colorectal Cancer Cell Growth and Signaling in User-Programmable Synthetic Hydrogels

Colorectal cancer (CRC) studies in vitro have been conducted almost exclusively on 2D cell monolayers or suspension spheroid cultures. Though these platforms have shed light on many important aspects of CRC biology, they fail to recapitulate essential cell-matrix interactions that often define in vivo function. Toward filling this knowledge gap, synthetic hydrogel biomaterials with user-programmable matrix mechanics and biochemistry have gained popularity for culturing cells in a more physiologically relevant 3D context. Here, using a poly(ethylene glycol)-based hydrogel model, we systematically assess the role of matrix stiffness and fibronectin-derived RGDS adhesive peptide presentation on CRC colony morphology and proliferation. Highlighting platform generalizability, we demonstrate that these hydrogels can support the viability and promote spontaneous spheroid or multicellular aggregate formation of six CRC cell lines that are commonly utilized in biomedical research. These gels are engineered to be fully degradable via a "biologically invisible" sortase-mediated reaction, enabling the triggered recovery of single cells and spheroids for downstream analysis. Using these platforms, we establish that substrate mechanics play a significant role in colony growth: soft conditions (∼300 Pa) encourage robust colony formation, whereas stiffer (∼2 kPa) gels severely restrict growth. Tuning the RGDS concentration did not affect the colony morphology. Additionally, we observe that epidermal growth factor receptor (EGFR) signaling in Caco-2 cells is influenced by adhesion ligand identity─whether the adhesion peptide was derived from collagen type I (DGEA) or fibronectin (RGDS)─with DGEA yielding a marked decrease in the level of downstream protein kinase phosphorylation. Taken together, this study introduces a versatile method to culture and probe CRC cell-matrix interactions within engineered 3D biomaterials.

Bioengineered nanomaterials for dynamic diagnostics in vivo

In vivo diagnostics obtains real-time physiological information directly from the site of interest in a patient's body, providing more accurate disease diagnosis compared with ex vivo diagnostics. Particularly, in vivo dynamic diagnostics allows the continuous monitoring of physiological signals over a period of time, offering deeper insights into disease pathogenesis and progression. However, achieving in situ dynamic diagnostics in deep tissues presents challenges related to energy and signal penetration as well as dynamic monitoring. Bioengineered nanomaterials serve as an ideal platform for in vivo dynamic diagnostics, leveraging energy conversion and biofunctionalization to enable continuous acquisition of physiological information across temporal and spatial scales. In this review, with reference to the studies from the last five years, we summarize the fundamental components that are essential for dynamic diagnosis in vivo. Firstly, an input energy source with high tissue penetration is needed, such as near-infrared (NIR) light, X-rays, magnetic field and ultrasound. Secondly, a nanomaterial class that is responsive to such an energy source to provide a readable output signal is chosen. Thirdly, bioengineered nanoprobes are designed to exhibit spatial, temporal or spatiotemporal changes in the output signal. Finally, different methods are used to analyse the output signal of nanoprobes, such as detecting changes in optical, radiation, magnetic and ultrasound signals. This review also discusses the obstacles and potential solutions for advancing these bioengineered nanomaterials toward clinical translational applications.

Mitochondrial Oxygen Consumption and Immunocytochemistry of Human Dental Pulp Stem Cell Following 808 nm PBM Therapy: A 3D Cell Culture Study

This study investigated the impact of 808 nm laser photobiomodulation (PBM) on mitochondrial respiration and osteogenic protein expression (OCN, OPN, ALP, RUNX2, COL-1, BMP-2) in human dental pulp stem cells (hDPSCs) within a 3D hydrogel model. hDPSCs were isolated from third molars and maintained under hypoxic conditions. Cells received PBM at 5 and 15 J/cm2 using an 808 nm diode laser. The study showed that 808 nm PBM can alter mitochondrial respiration, with 5 J/cm2 enhancing osteogenic protein expression (OCN, ALP, OPN, RUNX2) but failing to sustain BMP-2 at 24 h. In contrast, 15 J/cm2 induced stronger upregulation and prolonged BMP-2 expression, suggesting an optimal dose for sustained osteogenic activity. BMP-2 was later downregulated, and COL-1 remained unchanged post-PBM. Importantly, this study indicates the dose-specific PBM modulation of mitochondrial respiration and protein expression, but further research is required to optimize treatment protocols.

Rapid and Inexpensive Image-Guided Grayscale Biomaterial Customization via LCD Printing

Hydrogels are an important class of biomaterials that permit cells to be cultured and studied within engineered microenvironments of user-defined physical and chemical properties. Though conventional 3D extrusion and stereolithographic (SLA) printing readily enable homogeneous and multimaterial hydrogels to be formed with specific macroscopic geometries, strategies that further afford spatiotemporal customization of the underlying gel physicochemistry in a non-discrete manner would be profoundly useful toward recapitulating the complexity of native tissue in vitro. Here, we demonstrate that grayscale control over local biomaterial biochemistry and mechanics can be rapidly achieved across large constructs using an inexpensive (~$300) and commercially available liquid crystal display (LCD)-based printer. Template grayscale images are first processed into a "height-extruded" 3D object, which is then printed on a standard LCD printer with an immobile build head. As the local height of the 3D object corresponds to the final light dosage delivered at the corresponding xy-coordinate, this method provides a route toward spatially specifying the extent of various dosage-dependent and biomaterial, forming/modifying photochemistries. Demonstrating the utility of this approach, we photopattern the grayscale polymerization of poly(ethylene glycol) (PEG) diacrylate gels, biochemical functionalization of agarose- and PEG-based gels via oxime ligation, and the controlled 2D adhesion and 3D growth of cells in response to a de novo-designed α5β1-modulating protein via thiol-norbornene click chemistry. Owing to the method's low cost, simple implementation, and high compatibility with many biomaterial photochemistries, we expect this strategy will prove useful toward fundamental biological studies and functional tissue engineering alike.

Skin‐Compatible Carbopol Electrospun Fiber Membranes with pH‐Dependent Rheological Properties for Biomedical Applications

Properties of pH‐responsive electrospun nanofibers incorporated with biocompatible/degradable Carbopol, commonly used in pharmaceuticals and personal care products, are reported. Sonication of Carbopol dispersions prior to electrospinning leads to uniform incorporation into fibers of the host polymer polyvinylpyrrolidone. The hydration behavior is strongly influenced by pH conditions, forming a viscous network at higher pH. Since Carbopol is more responsive to higher pH, at pH > 6 increasing Carbopol concentration leads to increased uptake volume of buffer solution, faster uptake rate and complete gel formation. The physical spreadability (resulting from a combination of viscoelastic properties and the structural polymer network) of the hydrated fibers is evaluated for multiple Carbopol concentrations and pH conditions. At low starting pH of 4, increasing the Carbopol amount results in slightly increasing viscosity while maintain solution pH. On the other hand, at high starting pH of 8 increasing Carbopol concentrations result in significant reduction in the pH of the buffer solution, which in turn decreases the viscosity of the gel and increases its spreadability. These findings provide guidelines for rational designs of pH responsive Carbopol fibers for various applications, including drug delivery, wound dressing, contraceptive devices, and prevention of sexually transmitted diseases.

Skin-compatible Carbopol® Electrospun Fiber Membranes with pH-Dependent Rheological Properties for Biomedical Applications

Properties of pH-responsive electrospun nanofibers incorporated with biocompatible/degradable Carbopol®, commonly used in pharmaceuticals and personal care products, are reported. Sonication of Carbopol® dispersions prior to electrospinning leads to uniform incorporation into fibers of the host polymer polyvinylpyrrolidone. The hydration behavior is strongly influenced by pH conditions, forming a viscous network at higher pH. Since Carbopol® is more responsive to higher pH, at pH > 6 increasing Carbopol® concentration leads to increased uptake volume of buffer solution, faster uptake rate and complete gel formation. The physical spreadability (resulting from a combination of viscoelastic properties and the structural polymer network) of the hydrated fibers was evaluated for multiple Carbopol® concentrations and pH conditions. At low starting pH of 4, increasing the Carbopol® amount results in slightly increasing viscosity while maintain solution pH. On the other hand, at high starting pH of 8 increasing Carbopol® concentrations result in significant reduction in the pH of the buffer solution, which in turn decreases the viscosity of the gel and increases its spreadability. These findings provide guidelines for rational designs of pH responsive Carbopol® fibers for various applications, including drug delivery, wound dressing, contraceptive devices, and prevention of sexually transmitted diseases.

Recent applications of stimulus-responsive smart hydrogels for osteoarthritis therapy

Osteoarthritis is one of the most common degenerative joint diseases, which seriously affects the life of middle-aged and elderly people. Traditional treatments such as surgical treatment and systemic medication, often do not achieve the expected or optimal results, which leads to severe trauma and a variety of side effects. Therefore, there is an urgent need to develop novel therapeutic options to overcome these problems. Hydrogels are widely used in biomedical tissue repairing as a platform for loading drugs, proteins and stem cells. In recent years, smart-responsive hydrogels have achieved excellent results as novel drug delivery systems in the treatment of osteoarthritis. This review focuses on the recent advances of endogenous stimuli (including enzymes, pH, reactive oxygen species and temperature, etc.) responsive hydrogels and exogenous stimuli (including light, shear, ultrasound and magnetism, etc.) responsive hydrogels in osteoarthritis treatment. Finally, the current limitations of application and future prospects of smart responsive hydrogels are summarized.

Photothrombolytics: A light-driven technology for the targeted lysis of thrombi

Occlusive blood clots remain a significant global health challenge and result in emergencies that are main causes of death and disability worldwide. Thrombolytic agents (including tissue plasminogen activator, tPA) are the only pharmacological means to dissolve blood clots. However, these drugs have modest efficacy and severe safety concerns persist. We have developed light-responsive tPA-loaded red blood cells (tPA-RBCsPhoto) to target thrombolytic activity at the site of a blood clot. Herein, we describe the use of light to control the release of tPA from engineered RBCs and the subsequent degradation of a blood clot ex vivo. Furthermore, we have employed this technology to restore blood flow to an occluded mouse artery in vivo using a targeted dose that is 25 times lower than conventional systemic tPA treatment.

Triple-Combination Therapy with a Multifunctional Yolk-Shell Nanozyme Au@CeO2 Loaded with Dimethyl Fumarate for Periodontitis

Periodontitis, a chronic inflammatory disease, is the leading cause of tooth loss in adults and is one of the most prevalent and complex oral conditions. Oxidative stress induced by the excessive generation of reactive oxygen species (ROS) leads to periodontitis, which is closely associated with pathological processes, including mitochondrial dysfunction of periodontal cells and local immune dysregulation. However, current treatment modalities that target single pathological processes have limited long-term therapeutic effects. Herein, a multifunctional Yolk-Shell nanozyme, Au@CeO2-dimethyl fumarate (DMF), which comprehensively addresses the oxidative stress-induced pathophysiological processes of periodontitis through antioxidant activity, mitochondrial maintenance, and immune modulation mechanisms, is described. For material design logic, functionally complementary Au and CeO2 formed an excellent photothermally regulated high-efficiency nanozyme, which also provided an ideal drug carrier for DMF. As for the therapeutic logic, Au@CeO2-DMF restores mitochondrial dysfunction and immune dysregulation, which also contributes to endogenous ROS elimination, thereby achieving long-term stable therapeutic effects. In a rat model, local Au@CeO2-DMF photothermal therapy effectively alleviated ROS-induced tissue damage and restored periodontal homeostasis. Altogether, this study presents a novel antioxidant nanozyme for managing alveolar bone loss under prolonged oxidative stress and demonstrates the importance of comprehensive intervention in key pathological processes in periodontitis treatment design.

Reversible light-responsive protein hydrogel for on-demand cell encapsulation and release

The design of biomaterials that can reconfigure on-demand in response to external stimuli is an emerging area in materials research. However, achieving reversible assembly of protein-based biomaterials by light input remains a major challenge. Here, we present the engineering of a new protein material that is capable of switching between liquid and solid state reversibly, controlled by lights of different wavelengths. The materials are created by incorporating a light-responsive mutant Dronpa protein domain into the backbone of Elastin-Like Proteins (termed DELPs). We show that the DELP material can respond to light and undergo multiple cycles of switching between hydrogel and solution, outperforming the conventional irreversible materials. Additionally, the material is biocompatible with long-term cell proliferation in both adherent and suspension cells. Building on the reversible assembly of the material, we demonstrate efficient cell encapsulation and release upon light triggers. The design principle of incorporating a light-responsive protein element into a structural protein matrix, as demonstrated in this work enables, a broad range of other applications that require adaptive materials to intelligently interface with dynamic biological systems and environments. STATEMENT OF SIGNIFICANCE: This work generates a new class of "smart" biomaterials that uniquely switches between liquid and gel states in response to light input. Light input can be precisely delivered in space and time, highly tunable through wavelengths, intensities, and durations of light exposure. In prior research, light-responsive biomaterials are mostly irreversible, limiting their use to only uni-directional applications and the materials cannot be re-used. In contrast, this material robustly displays reversible switching between liquid and gel using a light-responsive crosslinker. Furthermore, the material is biocompatible, programmable, and suitable for broad applications including but not limited to cell encapsulation, controlled release, tissue engineering, and cell/tissue mechanobiology.

Physical stimuli-responsive polymeric patches for healthcare

Many chronic diseases have become severe public health problems with the development of society. A safe and efficient healthcare method is to utilize physical stimulus-responsive polymer patches, which may respond to physical stimuli, including light, electric current, temperature, magnetic field, mechanical force, and ultrasound. Under certain physical stimuli, these patches have been widely used in therapy for diabetes, cancer, wounds, hair loss, obesity, and heart diseases since they could realize controllable treatment and reduce the risks of side effects. This review sketches the design principles of polymer patches, including composition, properties, and performances. Besides, control methods of using different kinds of physical stimuli were introduced. Then, the fabrication methods and characterization of patches were explored. Furthermore, recent applications of these patches in the biomedical field were demonstrated. Finally, we discussed the challenges and prospects for its clinical translation. We anticipate that physical stimulus-responsive polymer patches will open up new avenues for healthcare by acting as a platform with multiple functions.

Photoactivable CRISPR for Biosensing and Cancer Therapy

Photoactivable CRISPR technology represents a transformative approach in the field of genome editing, offering unprecedented control over gene editing with high spatial and temporal precision. By harnessing the power of light to modulate the activity of CRISPR components, this innovative strategy enables precise regulation of Cas proteins, guide RNAs, and ribonucleoprotein complexes. Recent advancements in optical control methodologies, including the development of photoactivable nanocarriers, have significantly expanded the potential applications of CRISPR in biomedical fields. This Concept highlights the latest developments in designing photoactivable CRISPR systems and their promising applications in biosensing and cancer therapy. Additionally, the remaining challenges and future trends are also discussed. It is expected that the photoactivable CRISPR would facilitate translating more precise gene therapies into clinical use.

Three-Color Protein Photolithography with Green, Red, and Far-Red Light

Protein photolithography is an invaluable tool for generating protein microchips and regulating interactions between cells and materials. However, the absence of light-responsive molecules that allow for the copatterning of multiple functional proteins with biocompatible visible light poses a significant challenge. Here, a new approach for photopatterning three distinct proteins on a single surface by using green, red, and far-red light is reported. The cofactor of the green light-sensitive protein CarH is engineered such that it also becomes sensitive to red and far-red light. These new cofactors are shown to be compatible with two CarH-based optogenetic tools to regulate bacterial cell-cell adhesions and gene expression in mammalian cells with red and far-red light. Further, by incorporating different CarH variants with varying light sensitivities in layer-by-layer (LbL) multiprotein films, specific layers within the films, along with other protein layers on top are precisely removed by using different colors of light, all with high spatiotemporal accuracy. Notably, with these three distinct colors of visible light, it is possible to incorporate diverse proteins under mild conditions in LbL films based on the reliable interaction between Ni2+- nitrilotriacetic acid (NTA) groups and polyhistidine-tags (His-tags)on the proteins and their subsequent photopatterning. This approach has potential applications spanning biofabrication, material engineering, and biotechnology.

Decoding senescence of aging single cells at the nexus of biomaterials, microfluidics, and spatial omics

Aging has profound effects on the body, most notably an increase in the prevalence of several diseases. An important aging hallmark is the presence of senescent cells that no longer multiply nor die off properly. Another characteristic is an altered immune system that fails to properly self-surveil. In this multi-player aging process, cellular senescence induces a change in the secretory phenotype, known as senescence-associated secretory phenotype (SASP), of many cells with the intention of recruiting immune cells to accelerate the clearance of these damaged senescent cells. However, the SASP phenotype results in inducing secondary senescence of nearby cells, resulting in those cells becoming senescent, and improper immune activation resulting in a state of chronic inflammation, called inflammaging, in many diseases. Senescence in immune cells, termed immunosenescence, results in further dysregulation of the immune system. An interdisciplinary approach is needed to physiologically assess aging changes of the immune system at the cellular and tissue level. Thus, the intersection of biomaterials, microfluidics, and spatial omics has great potential to collectively model aging and immunosenescence. Each of these approaches mimics unique aspects of the body undergoes as a part of aging. This perspective highlights the key aspects of how biomaterials provide non-cellular cues to cell aging, microfluidics recapitulate flow-induced and multi-cellular dynamics, and spatial omics analyses dissect the coordination of several biomarkers of senescence as a function of cell interactions in distinct tissue environments. An overview of how senescence and immune dysregulation play a role in organ aging, cancer, wound healing, Alzheimer's, and osteoporosis is included. To illuminate the societal impact of aging, an increasing trend in anti-senescence and anti-aging interventions, including pharmacological interventions, medical procedures, and lifestyle changes is discussed, including further context of senescence.

Smart Nanocomposite Hydrogels as Next-Generation Therapeutic and Diagnostic Solutions

Stimuli-responsive nanocomposite gels combine the unique properties of hydrogels with those of nanoparticles, thus avoiding the suboptimal results of single components and creating versatile, multi-functional platforms for therapeutic and diagnostic applications. These hybrid materials are engineered to respond to various internal and external stimuli, such as temperature, pH, light, magnetic fields, and enzymatic activity, allowing precise control over drug release, tissue regeneration, and biosensing. Their responsiveness to environmental cues permits personalized medicine approaches, providing dynamic control over therapeutic interventions and real-time diagnostic capabilities. This review explores recent advances in stimuli-responsive hybrid gels' synthesis and application, including drug delivery, tissue engineering, and diagnostics. Overall, these platforms have significant clinical potential, and future research is expected to lead to unique solutions to address unmet medical needs.

Photocrosslinkable triple helical protein with enhanced higher-order formation for biomaterial applications

Bacterial collagen, produced via recombinant DNA methods, offers advantages including consistent purity, customizable properties, and reduced allergy potential compared to animal-derived collagen. Its controlled production environment enables tailored features, making it more sustainable, non-pathogenic, and compatible with diverse applications in medicine, cosmetics, and other industries. Research has focused on the engineering of collagen-like proteins to improve their structure and function. The study explores the impact of introducing tyrosine, an amino acid known for its role in fibril formation across diverse proteins, into a newly designed bacterial collagen-like protein (Scl2), specifically examining its effect on self-assembly and fibril formation. Biophysical analyses reveal that the introduction of tyrosine residues didn't compromise the protein's structural stability but rather promoted self-assembly, resulting in the creation of nanofibrils-a phenomenon absent in the native Scl2 protein. Additionally, stable hydrogels are formed when the engineered protein undergoes di-tyrosine crosslinking under light exposure. The hydrogels, shown to support cell viability, also facilitate accelerated wound healing in mouse fibroblast (NIH/3T3) cells. These outcomes demonstrate that the targeted inclusion of functional residues in collagen-like proteins enhances fibril formation and facilitates the generation of robust hydrogels using riboflavin chemistry, presenting promising paths for research in tissue engineering and regenerative medicine.

Bioluminescence Resonance Energy Transfer (BRET)-Mediated Protein Release from Self-Illuminating Photoresponsive Biomaterials

Phototriggered release of various cargos, including soluble protein factors and small molecules, has the potential to correct aberrant biological events by offering spatiotemporal control over local therapeutic levels. However, the poor penetration depth of light historically limits implementation to subdermal regions, necessitating alternative methods of light delivery to achieve the full potential of photodynamic therapeutic release. Here, we introduce a strategy exploiting bioluminescence resonance energy transfer (BRET)-an energy transfer process between light-emitting Nanoluciferase (NLuc) and a photosensitive acceptor molecule-to drive biomolecule release from hydrogel biomaterials. Through a facile, one-pot, and high-yielding synthesis (60-70%), we synthesized a heterobifunctional ruthenium cross-linker bearing an aldehyde and an azide (CHO-Ru-N3), a compound that we demonstrate undergoes predictable exchange of the azide-bearing ligand under blue-green light irradiation (>550 nm). Following site-specific conjugation to NLuc via sortase-tag enhanced protein ligation (STEPL), the modified protein was covalently attached to a poly(ethylene glycol) (PEG)-based hydrogel via strain-promoted azide-alkyne cycloaddition (SPAAC). Leveraging the high photosensitivity of Ru compounds, we demonstrate rapid and equivalent release of epidermal growth factor (EGF) via either direct illumination or via BRET-based bioluminolysis. As NLuc-originated luminescence can be controlled equivalently throughout the body, we anticipate that this unique protein release strategy will find use for locally triggered drug delivery following systemic administration of a small molecule.

Stimuli-Responsive Polymersomes: Reshaping the Immunosuppressive Tumor Microenvironment

The progression of cancer involves mutations in normal cells, leading to uncontrolled division and tissue destruction, highlighting the complexity of tumor microenvironments (TMEs). Immunotherapy has emerged as a transformative approach, yet the balance between efficacy and safety remains a challenge. Nanoparticles such as polymersomes offer the possibility to precisely target tumors, deliver drugs in a controlled way, effectively modulate the antitumor immunity, and notably reduce side effects. Herein, stimuli-responsive polymersomes, with capabilities for carrying multiple therapeutics, are highlighted for their potential in enhancing antitumor immunity through mechanisms like inducing immunogenic cell death and activating STING (stimulator of interferon genes), etc. The recent progress of utilizing stimuli-responsive polymersomes to reshape the TME is reviewed here. The advantages and limitations to applied stimuli-responsive polymersomes are outlined. Additionally, challenges and future prospects in leveraging polymersomes for cancer therapy are discussed, emphasizing the need for future research and clinical translation.

Stimuli-Responsive Nanocomposite Hydrogels for Oral Diseases

Oral diseases encompassing conditions such as oral cancer, periodontitis, and endodontic infections pose significant challenges due to the oral cavity's susceptibility to pathogenic bacteria and infectious agents. Saliva, a key component of the oral environment, can compromise drug efficacy during oral disease treatment by diluting drug formulations and reducing drug-site interactions. Thus, it is imperative to develop effective drug delivery methods. Stimuli-responsive nanocomposite hydrogels offer a promising solution by adapting to changes in environmental conditions during disease states, thereby enabling targeted drug delivery. These smart drug delivery systems have the potential to enhance drug efficacy, minimize adverse reactions, reduce administration frequency, and improve patient compliance, thus facilitating a faster recovery. This review explores various types of stimuli-responsive nanocomposite hydrogels tailored for smart drug delivery, with a specific focus on their applications in managing oral diseases.

Harnessing the potential of hydrogels for advanced therapeutic applications: current achievements and future directions

The applications of hydrogels have expanded significantly due to their versatile, highly tunable properties and breakthroughs in biomaterial technologies. In this review, we cover the major achievements and the potential of hydrogels in therapeutic applications, focusing primarily on two areas: emerging cell-based therapies and promising non-cell therapeutic modalities. Within the context of cell therapy, we discuss the capacity of hydrogels to overcome the existing translational challenges faced by mainstream cell therapy paradigms, provide a detailed discussion on the advantages and principal design considerations of hydrogels for boosting the efficacy of cell therapy, as well as list specific examples of their applications in different disease scenarios. We then explore the potential of hydrogels in drug delivery, physical intervention therapies, and other non-cell therapeutic areas (e.g., bioadhesives, artificial tissues, and biosensors), emphasizing their utility beyond mere delivery vehicles. Additionally, we complement our discussion on the latest progress and challenges in the clinical application of hydrogels and outline future research directions, particularly in terms of integration with advanced biomanufacturing technologies. This review aims to present a comprehensive view and critical insights into the design and selection of hydrogels for both cell therapy and non-cell therapies, tailored to meet the therapeutic requirements of diverse diseases and situations.

Cyclodextrin 'Chaperones' Enable Quasi-Ideal Supramolecular Network Formation and Enhanced Photodimerization of Hydrophobic, Red-shifted Photoswitches in Water

Precision-engineered light-triggered hydrogels are important for a diversity of applications. However, fields such as biomaterials require wavelength outside the harsh UV regime to prevent photodamage, typically requiring chromophores with extended π-conjugation that suffer from poor water solubility. Herein, we demonstrate how cyclodextrins can be used as auxiliary agents to not only solubilize such chromophores, but even to preorganize them in a 2 : 2 host-guest inclusion complex to facilitate photodimerization. We apply our concept to styrylpyrene-end-functionalized star-shaped polyethylene glycols (sPEGs). We initially unravel details of the host-guest inclusion complex using spectroscopy and mass spectrometry to give clear evidence of a 2 : 2 complex formation. Subsequently, we show that the resultant supramolecularly linked hydrogels conform to theories of supramolecular quasi-ideal model networks, and derive details on their association dynamics using in-depth rheological measurements and kinetic models. By comparing sPEGs of different arm length, we further elucidate the model network topology and the accessible mechanical property space. The photo-mediated dimerization proceeds smoothly, allowing to transform the supramolecular model networks into covalent ones. We submit that our strategy opens avenues for executing hydrophobic photochemistry in aqueous environments with enhanced control over reactivity, hydrogel topology or programmable mechanical properties.

Photoresponsive Hydrogels for Tissue Engineering

Hydrophilic and biocompatible hydrogels are widely applied as ideal scaffolds in tissue engineering. The "smart" gelation material can alter its structural, physiochemical, and functional features in answer to various endo/exogenous stimuli to better biomimic the endogenous extracellular matrix for the engineering of cells and tissues. Light irradiation owns a high spatial-temporal resolution, complete biorthogonal reactivity, and fine-tunability and can thus induce physiochemical reactions within the matrix of photoresponsive hydrogels with good precision, efficiency, and safety. Both gel structure (e.g., geometry, porosity, and dimension) and performance (like conductivity and thermogenic or mechanical properties) can hence be programmed on-demand to yield the biochemical and biophysical signals regulating the morphology, growth, motility, and phenotype of engineered cells and tissues. Here we summarize the strategies and mechanisms for encoding light-reactivity into a hydrogel and demonstrate how fantastically such responsive gels change their structure and properties with light irradiation as desired and thus improve their applications in tissue engineering including cargo delivery, dynamic three-dimensional cell culture, and tissue repair and regeneration, aiming to provide a basis for more and better translation of photoresponsive hydrogels in the clinic.

Stiffness Modulation and Pulsatile Release in Dual Responsive Hydrogels

Inspired by nature, self-regulation can be introduced in synthetic hydrogels by incorporating chemo-mechanical signals or coupled chemical reactions to maintain or adapt the material's physico-chemical properties when exposed to external triggers. In this work, we present redox and light dual stimuli responsive hydrogels capable of rapidly adapting the polymer crosslinking network while maintaining hydrogel stability. Upon irradiation with UV light, polymer hydrogels containing redox responsive disulfide crosslinks and light responsive ortho-nitrobenzyl moieties show a release of payload accompanied by adaptation of the hydrogel network towards higher stiffness due to in situ crosslinking by S-nitrosylation. Whereas the hydrogel design allows the network to either become softer in presence of reducing agent glutathione or stiffer upon UV irradiation, simultaneous application of both stimuli induces network self-regulation resulting in a pulsatile form of payload release from the hydrogel. Finally, adaptive stiffness was used to make tunable hydrogels as substrates for different cell lines.

Biomaterial strategies for regulating the neuroinflammatory response

Injury and disease in the central nervous system (CNS) can result in a dysregulated inflammatory environment that inhibits the repair of functional tissue. Biomaterials present a promising approach to tackle this complex inhibitory environment and modulate the mechanisms involved in neuroinflammation to halt the progression of secondary injury and promote the repair of functional tissue. In this review, we will cover recent advances in biomaterial strategies, including nanoparticles, hydrogels, implantable scaffolds, and neural probe coatings, that have been used to modulate the innate immune response to injury and disease within the CNS. The stages of inflammation following CNS injury and the main inflammatory contributors involved in common neurodegenerative diseases will be discussed, as understanding the inflammatory response to injury and disease is critical for identifying therapeutic targets and designing effective biomaterial-based treatment strategies. Biomaterials and novel composites will then be discussed with an emphasis on strategies that deliver immunomodulatory agents or utilize cell-material interactions to modulate inflammation and promote functional tissue repair. We will explore the application of these biomaterial-based strategies in the context of nanoparticle- and hydrogel-mediated delivery of small molecule drugs and therapeutic proteins to inflamed nervous tissue, implantation of hydrogels and scaffolds to modulate immune cell behavior and guide axon elongation, and neural probe coatings to mitigate glial scarring and enhance signaling at the tissue-device interface. Finally, we will present a future outlook on the growing role of biomaterial-based strategies for immunomodulation in regenerative medicine and neuroengineering applications in the CNS.

Self-Assembling Triple-Helix Recombinant Collagen Hydrogel Enriched with Tyrosine

The self-assembly of collagen within the human body creates a complex 3D fibrous network, providing structural integrity and mechanical strength to connective tissues. Recombinant collagen plays a pivotal role in the realm of biomimetic natural collagen. However, almost all of the reported recombinant collagens lack the capability of self-assembly, severely hindering their application in tissue engineering and regenerative medicine. Herein, we have for the first time constructed a series of self-assembling tyrosine-rich triple helix recombinant collagens, mimicking the structure and functionality of natural collagen. The recombinant collagen consists of a central triple-helical domain characterized by the (Gly-Xaa-Yaa)n sequence, along with N-terminal and C-terminal domains featuring the GYY sequence. The introduction of GYY has a negligible impact on the stability of the triple-helical structure of recombinant collagen while simultaneously promoting its self-assembly into fibers. In the presence of [Ru(bpy)3]Cl2 and APS as catalysts, tyrosine residues in the recombinant collagen undergo covalent cross-linking, resulting in a hydrogel with exceptional mechanical properties. The recombinant collagen hydrogel exhibits outstanding biocompatibility and bioactivity, significantly enhancing the proliferation, adhesion, migration, and differentiation of HFF-1 cells. This innovative self-assembled triple-helix recombinant collagen demonstrates significant potential in the fields of tissue engineering and medical materials.

PhoCoil: An Injectable and Photodegradable Single-component Recombinant Protein Hydrogel for Localized Therapeutic Cell Delivery

Hydrogel biomaterials offer great promise for 3D cell culture and therapeutic delivery. Despite many successes, challenges persist in that gels formed from natural proteins are only marginally tunable while those derived from synthetic polymers lack intrinsic bioinstructivity. Towards the creation of biomaterials with both excellent biocompatibility and customizability, recombinant protein-based hydrogels have emerged as molecularly defined and user-programmable platforms that mimic the proteinaceous nature of the extracellular matrix. Here, we introduce PhoCoil, a dynamically tunable recombinant hydrogel formed from a single protein component with unique multi-stimuli responsiveness. Physical crosslinking through coiled-coil interactions promotes rapid shear-thinning and self-healing behavior, rendering the gel injectable, while an included photodegradable motif affords on-demand network dissolution via visible light. PhoCoil gel photodegradation can be spatiotemporally and lithographically controlled in a dose-dependent manner, through complex tissue, and without harm to encapsulated cells. We anticipate that PhoCoil will enable new applications in tissue engineering and regenerative medicine.

Reversible Intracellular Gelation of MCF10A Cells Enables Programmable Control Over 3D Spheroid Growth

In nature, some organisms survive extreme environments by inducing a biostatic state wherein cellular contents are effectively vitrified. Recently, a synthetic biostatic state in mammalian cells is achieved via intracellular network formation using bio-orthogonal strain-promoted azide-alkyne cycloaddition (SPAAC) reactions between functionalized poly(ethylene glycol) (PEG) macromers. In this work, the effects of intracellular network formation on a 3D epithelial MCF10A spheroid model are explored. Macromer-transfected cells are encapsulated in Matrigel, and spheroid area is reduced by ≈50% compared to controls. The intracellular hydrogel network increases the quiescent cell population, as indicated by increased p21 expression. Additionally, bioenergetics (ATP/ADP ratio) and functional metabolic rates are reduced. To enable reversibility of the biostasis effect, a photosensitive nitrobenzyl-containing macromer is incorporated into the PEG network, allowing for light-induced degradation. Following light exposure, cell state, and proliferation return to control levels, while SPAAC-treated spheroids without light exposure (i.e., containing intact intracellular networks) remain smaller and less proliferative through this same period. These results demonstrate that photodegradable intracellular hydrogels can induce a reversible slow-growing state in 3D spheroid culture.

Organic-inorganic composite hydrogels: compositions, properties, and applications in regenerative medicine

Hydrogels, formed from crosslinked hydrophilic macromolecules, provide a three-dimensional microenvironment that mimics the extracellular matrix. They served as scaffold materials in regenerative medicine with an ever-growing demand. However, hydrogels composed of only organic components may not fully meet the performance and functionalization requirements for various tissue defects. Composite hydrogels, containing inorganic components, have attracted tremendous attention due to their unique compositions and properties. Rigid inorganic particles, rods, fibers, etc., can form organic-inorganic composite hydrogels through physical interaction and chemical bonding with polymer chains, which can not only adjust strength and modulus, but also act as carriers of bioactive components, enhancing the properties and biological functions of the composite hydrogels. Notably, incorporating environmental or stimulus-responsive inorganic particles imparts smartness to hydrogels, hence providing a flexible diagnostic platform for in vitro cell culture and in vivo tissue regeneration. In this review, we discuss and compare a set of materials currently used for developing organic-inorganic composite hydrogels, including the modification strategies for organic and inorganic components and their unique contributions to regenerative medicine. Specific emphasis is placed on the interactions between the organic or inorganic components and the biological functions introduced by the inorganic components. The advantages of these composite hydrogels indicate their potential to offer adaptable and intelligent therapeutic solutions for diverse tissue repair demands within the realm of regenerative medicine.

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.

Tailored Drug Delivery Platforms: Stimulus-Responsive Core-Shell Structured Nanocarriers

Core-shell structured nanocarriers have come into the scientific spotlight in recent years due to their intriguing properties and wide applications in materials chemistry, biology, and biomedicine. Tailored core-shell structures to achieve desired performance have emerged as a research frontier in the development of smart drug delivery system. However, systematic reviews on the design and loading/release mechanisms of stimulus-responsive core-shell structured nanocarriers are uncommon. This review starts with the categories of core-shell structured nanocarriers with different means of drug payload, and then highlights the controlled release mechanism realized through stimulus-response processes triggered under different environments. Finally, some multifaceted perspectives on the design of core-shell structured materials as drug carriers are addressed. This work aims to provide new enlightenments and prospects in the drug delivery field for further developing advanced and smart nanocarriers.

A Novel Z-Scheme Heterostructured Bi2 S3 /Cu-TCPP Nanocomposite with Synergistically Enhanced Therapeutics against Bacterial Biofilm Infections in Periodontitis

Porphyrin-based antibacterial photodynamic therapy (aPDT) has found widespread applications in treating periodontitis. However, its clinical use is limited by poor energy absorption, resulting in limited reactive oxygen species (ROS) generation. To overcome this challenge, a novel Z-scheme heterostructured nanocomposite of Bi2 S3 /Cu-TCPP is developed. This nanocomposite exhibits highly efficient light absorption and effective electron-hole separation, thanks to the presence of heterostructures. The enhanced photocatalytic properties of the nanocomposite facilitate effective biofilm removal. Theoretical calculations confirm that the interface of the Bi2 S3 /Cu-TCPP nanocomposite readily adsorbs oxygen molecules and hydroxyl radicals, thereby improving ROS production rates. Additionally, the photothermal treatment (PTT) using Bi2 S3 nanoparticles promotes the release of Cu2+ ions, enhancing the chemodynamic therapy (CDT) effect and facilitating the eradication of dense biofilms. Furthermore, the released Cu2+ ions deplete glutathione in bacterial cells, weakening their antioxidant defense mechanisms. The synergistic effect of aPDT/PTT/CDT demonstrates potent antibacterial activity against periodontal pathogens, particularly in animal models of periodontitis, resulting in significant therapeutic effects, including inflammation alleviation and bone preservation. Therefore, this design of semiconductor-sensitized energy transfer represents an important advancement in improving aPDT efficacy and the treatment of periodontal inflammation.

Tricolor visible wavelength-selective photodegradable hydrogel biomaterials

Photodynamic hydrogel biomaterials have demonstrated great potential for user-triggered therapeutic release, patterned organoid development, and four-dimensional control over advanced cell fates in vitro. Current photosensitive materials are constrained by their reliance on high-energy ultraviolet light (<400 nm) that offers poor tissue penetrance and limits access to the broader visible spectrum. Here, we report a family of three photolabile material crosslinkers that respond rapidly and with unique tricolor wavelength-selectivity to low-energy visible light (400-617 nm). We show that when mixed with multifunctional poly(ethylene glycol) macromolecular precursors, ruthenium polypyridyl- and ortho-nitrobenzyl (oNB)-based crosslinkers yield cytocompatible biomaterials that can undergo spatiotemporally patterned, uniform bulk softening, and multiplexed degradation several centimeters deep through complex tissue. We demonstrate that encapsulated living cells within these photoresponsive gels show high viability and can be successfully recovered from the hydrogels following photodegradation. Moving forward, we anticipate that these advanced material platforms will enable new studies in 3D mechanobiology, controlled drug delivery, and next-generation tissue engineering applications.

4D Biochemical Photocustomization of Hydrogel Scaffolds for Biomimetic Tissue Engineering

Programmable engineered tissues and the materials that support them are instrumental to the development of next-generation therapeutics and gaining new understanding of human biology. Toward these ends, recent years have brought a growing emphasis on the creation of "4D" hydrogel culture platforms-those that can be customized in 3D space and on demand over time. Many of the most powerful 4D-tunable biomaterials are photochemically regulated, affording users unmatched spatiotemporal modulation through high-yielding, synthetically tractable, and cytocompatible reactions. Precise physicochemical manipulation of gel networks has given us the ability to drive critical changes in cell fate across a diverse range of distance and time scales, including proliferation, migration, and differentiation through user-directed intracellular and intercellular signaling. This Account provides a survey of the numerous creative approaches taken by our lab and others to recapitulate the dynamically heterogeneous biochemistry underpinning in vivo extracellular matrix (ECM)-cell interactions via light-based network (de)decoration with biomolecules (e.g., peptides, proteins) and in situ protein activation/generation. We believe the insights gained from these studies can motivate disruptive improvements to emerging technologies, including low-variability organoid generation and culture, high-throughput drug screening, and personalized medicine. As photolithography and chemical modification strategies continue to mature, access to and control over new and increasingly complex biological pathways are being unlocked. The earliest hydrogel photopatterning efforts selectively encapsulated bioactive peptides and drugs into rudimentary gel volumes. Through continued exploration and refinement, next-generation materials now boast reversible, multiplexed, and/or Boolean logic-based biomolecule presentation, as well as functional activation at subcellular resolutions throughout 3D space. Lithographic hardware and software technologies, particularly those enabling image-guided patterning, allow researchers to precisely replicate complex biological structures within engineered tissue environments. The advent of bioorthogonal click chemistries has expanded 4D tissue engineering toolkits, permitting diverse constructs to be independently customized in the vicinity of any cell that is amenable to hydrogel-based culture. Additionally, the adoption of modern protein engineering techniques including genetic code expansion and chemoenzymatic alteration provides a roadmap toward site-specific modification of nearly any recombinant or isolated protein, affording installation of photoreactive and click handles without sacrificing their bioactivity. While the established bind, release, (de)activate paradigm in hydrogel photolithography continues to thrive alongside these modern engineering techniques, new studies are also demonstrating photocontrol of more complex or nonclassical operations, including engineered material-microorganism interfaces and functional protein photoassembly. Such creative approaches offer exciting new avenues for the field, including spatial control of on-demand biomolecule production from cellular depots and patterned bioactivity using a growing array of split protein pairs. Taken together, these technologies provide the foundation for truly biomimetic photopatterning of engineered tissues.

Circadian Clock Regulation via Biomaterials for Nucleus Pulposus

Circadian clock disorder during tissue degeneration has been considered the potential pathogenesis for various chronic diseases, such as intervertebral disc degeneration (IVDD). In this study, circadian clock-regulating biomaterials (ClockMPs) that can effectively activate the intrinsic circadian clock of nucleus pulposus cells (NPCs) in IVDD and improve the physiological function of NPCs for disc regeneration are fabricated via air-microfluidic technique and the chemical cross-linking between polyvinyl alcohol and modified-phenylboronic acid. In vitro experiments verified that ClockMPs can scavenge reactive oxygen species to maintain a stable microenvironment for the circadian clock by promoting the binding of BMAL1 and CLOCK proteins. ClockMPs can regulate the expression of core circadian clock genes by activating the PI3K-AKT pathway in NPCs to remodel the intrinsic circadian clock and promote extracellular matrix synthesis. Furthermore, in vivo experiments of IVDD treated with ClockMPs proved that ClockMPs can promote disc regeneration by regulating the circadian clock of NPCs. In conclusion, ClockMPs provided a novel and promising strategy for circadian clock regulation during tissue regeneration.

Accelerating aging with dynamic biomaterials: Recapitulating aged tissue phenotypes in engineered platforms

Aging is characterized by progressive decline in tissue function and represents the greatest risk factor for many diseases. Nevertheless, many fundamental mechanisms driving human aging remain poorly understood. Aging studies using model organisms are often limited in their applicability to humans. Mechanistic studies of human aging rely on relatively simple cell culture models that fail to replicate mature tissue function, making them poor surrogates for aged tissues. These culture systems generally lack well-controlled cellular microenvironments that capture the changes in tissue mechanics and microstructure that occur during aging. Biomaterial platforms presenting dynamic, physiologically relevant mechanical, structural, and biochemical cues can capture the complex changes in the cellular microenvironment in a well-defined manner, accelerating the process of cellular aging in model laboratory systems. By enabling selective tuning of relevant microenvironmental parameters, these biomaterials systems may enable identification of new therapeutic approaches to slow or reverse the detrimental effects of aging.

Visible Light Degradable Acridine-Containing Polyurethanes in an Aqueous Environment

Light degradable polymers hold significant promise in a wide range of applications including the fabrication of optically recyclable materials, responsive coatings and adhesives, and controlled drug delivery. Here, we report the synthesis of polyurethanes that can be degraded under irradiation of visible light (≤450 nm) from commercial LED (3-15 W) light sources. The photolysis occurs in an aqueous environment via photocleavage of an acridine moiety incorporated within the backbone of the polymer chains. Analysis of the quantum yield as a function of wavelength reveals highly efficient photoreactivity at up to 440 nm activation, which is red-shifted compared to the UV-vis absorbance of the chromophore. The potential of our chemical system in biomaterials is demonstrated by the fabrication of an in situ forming hydrogel that can be degraded by visible light.

Reaching new lights: a review on photo-controlled nanomedicines and their in vivo evaluation

The selective and efficient delivery of bioactive molecules to sites of interest remains a formidable challenge in medicine. In recent years, it has been shown that stimuli-responsive drug delivery systems display several advantages over traditional drug administration such as an improved pharmacokinetic profile and the desirable ability to gain control over release. Light emerged as one of the most powerful stimuli due to its high biocompatibility, spatio-temporal control, and non-invasiveness. On the road to clinical translation, various chemical systems of high complexity have been reported with the aim to improve efficacy, safety, and versatility of drug delivery under complex biological conditions. For future research on the chemical design of such photo-controlled nanomedicines, it is essential to gain an understanding of their in vivo translation and efficiency. Here, we discuss photo-controlled nanomedicines that have been evaluated in vivo and provide an overview of the state-of-the-art that should guide future research design.

Green Chemistry for Crosslinking Biopolymers: Recent Advances in Riboflavin-Mediated Photochemistry

Riboflavin (RF), which is also known as vitamin B2, is a water-soluble vitamin. RF is a nontoxic and biocompatible natural substance. It absorbs light (at wavelengths of 380 and 450 nm) in the presence of oxygen to form reactive singlet oxygen (¹O₂). The generated singlet oxygen acts as a photoinitiator to induce the oxidation of biomolecules, such as amino acids, proteins, and nucleotides, or to initiate chemical reactions, such as the thiol-ene reaction and crosslinking of tyramine and furfuryl groups. In this review, we focus on the chemical mechanism and utilization of the photochemistry of RF, such as protein crosslinking and hydrogel formation. Currently, the crosslinking method using RF as a photoinitiator is actively employed in ophthalmic clinics. However, a significant broadening is expected in its range of applications, such as in tissue engineering and drug delivery.

Advances in 4D printing: from stimulation to simulation

The advancement of four-dimensional (4D) printing has been fueled by the rise in demand for additive manufacturing and the expansion in shape-memory materials. The printing of smart substances that respond to external stimuli is known as 4D printing. 4D printing allows highly controlled shapes to simulate the physiological milieu by adding time dimensions. The 4D printing is suitable with current progress in smart compounds, printers, and its mechanism of action. The 4D printing paradigm, a revolutionary enhancement of 3D printing, was anticipated by various engineering disciplines. Tissue engineering, medicinal, consumer items, aerospace, and organ engineering use 4D printing technology. The current review mainly focuses on the basics of 4D printing and the methods used therein. It also discusses the time-dependent behavior of stimulus-sensitive compounds, which are widely used in 4D printing. In addition, this review highlights material aspects, specifically related to shape-memory polymers, stimuli-responsive materials (classified as physical, chemical, and biological), and modified materials, the backbone of 4D printing technology. Finally, potential applications of 4D printing in the biomedical sector are also discussed with challenges and future perspectives.

Rapid hydrogel formation via tandem visible light photouncaging and bioorthogonal ligation

The formation of benign polymer scaffolds in water using green-light-reactive photocages is described. These efforts pave an avenue toward the fabrication of synthetic scaffolds that can facilitate the study of cellular events for disease diagnosis and treatment. First, a series of boron dipyrromethene (BODIPY) photocages with nitrogen-containing nucleophiles were examined to determine structure-reactivity relationships, which resulted in a >1,000× increase in uncaging yield. Subsequently, photoinduced hydrogel formation in 90 wt % water was accomplished via biorthogonal carbonyl condensation using hydrophilic polymer scaffolds separately containing BODIPY photocages and ortho-phthalaldehyde (OPA) moieties. Spatiotemporal control is demonstrated with light on/off experiments to modulate gel stiffness and masking to provide <100 μm features. Biocompatability of the method was shown through pre-/post-crosslinking cell viability studies. Short term, these studies are anticipated to guide translation to emergent additive manufacturing technology, which, longer term, will enable the development of 3D cell cultures for tissue engineering applications.

Static and photoresponsive dynamic materials to dissect physical regulation of cellular functions

Recent progress in mechanobiology has highlighted the importance of physical cues, such as mechanics, geometry (size), topography, and porosity, in the determination of cellular activities and fates, in addition to biochemical factors derived from their surroundings. In this review, we will first provide an overview of how such fundamental insights are identified by synchronizing the hierarchical nature of biological systems and static materials with tunable physical cues. Thereafter, we will explain the photoresponsive dynamic biomaterials to dissect the spatiotemporal aspects of the dependence of biological functions on physical cues.

An Overview of the Supramolecular Systems for Gene and Drug Delivery in Tissue Regeneration

Tissue regeneration is a prominent area of research, developing biomaterials aimed to be tunable, mechanistic scaffolds that mimic the physiological environment of the tissue. These biomaterials are projected to effectively possess similar chemical and biological properties, while at the same time are required to be safely and quickly degradable in the body once the desired restoration is achieved. Supramolecular systems composed of reversible, non-covalently connected, self-assembly units that respond to biological stimuli and signal cells have efficiently been developed as preferred biomaterials. Their biocompatibility and the ability to engineer the functionality have led to promising results in regenerative therapy. This review was intended to illuminate those who wish to envisage the niche translational research in regenerative therapy by summarizing the various explored types, chemistry, mechanisms, stimuli receptivity, and other advancements of supramolecular systems.

Porphyrin as a versatile visible-light-activatable organic/metal hybrid photoremovable protecting group

Photoremovable protecting groups (PPGs) represent one of the main contemporary implementations of photochemistry in diverse fields of research and practical applications. For the past half century, organic and metal-complex PPGs were considered mutually exclusive classes, each of which provided unique sets of physical and chemical properties thanks to their distinctive structures. Here, we introduce the meso-methylporphyrin group as a prototype hybrid-class PPG that unites traditionally exclusive elements of organic and metal-complex PPGs within a single structure. We show that the porphyrin scaffold allows extensive modularity by functional separation of the metal-binding chromophore and up to four sites of leaving group release. The insertion of metal ions can be used to tune their spectroscopic, photochemical, and biological properties. We provide a detailed description of the photoreaction mechanism studied by steady-state and transient absorption spectroscopies and quantum-chemical calculations. Our approach applied herein could facilitate access to a hitherto untapped chemical space of potential PPG scaffolds.

Evaluation of PEG-b-polycarbonates self-assemblies containing azobenzene or coumarin moieties as nanocarriers using paclitaxel as a model hydrophobic drug

AIM

The work assesses the performance of nanocarriers from amphiphilic block copolymers with functional azobenzene or coumarin moieties for delivery of paclitaxel.

METHODS

Placlitaxel was encapsulated by the nanoprecipitation method. Characterizations were performed by DLS, TEM, Zeta potential and HPLC. Cell viability was investigated in HeLa and Huh-5-2-cell lines.

RESULTS

Coumarin-containing polymeric micelles (D_h = 26 ± 2 nm, PDI =0.28, ζ = ‒22.9 ± 3.6 mV) with 11.2 ± 0.5%w/w drug loading showed enhanced cytotoxicity in HeLa cells (IC50 < 0.02 nM) compared to free paclitaxel (IC50 = 0.17 ± 0.02 nM). Azobenzene-containing polymeric vesicles (D_h = 390 ± 20 nm, PDI =0.24, ζ = ‒33.2 ± 5.0 mV) with a 6.8 ± 0.4%w/w drug loading showed increased cytotoxicity under 530 nm light (IC50 = 0.0114 ± 0.00033 nM) in HeLa cells due to a stimulated delivery of paclitaxel.

CONCLUSION

Effectivity of these block copolymers as paclitaxel nanovectors and light stimulated release has been demonstrated.

Wavelength-Orthogonal Stiffening of Hydrogel Networks with Visible Light

Herein, we introduce the wavelength-orthogonal crosslinking of hydrogel networks using two red-shifted chromophores, i.e. acrylpyerene (AP, λactivation =410-490 nm) and styrylpyrido[2,3-b]pyrazine (SPP, λactivation =400-550 nm), able to undergo [2+2] photocycloaddition in the visible-light regime. The photoreactivity of the SPP moiety is pH-dependent, whereby an acidic environment inhibits the cycloaddition. By employing a spiropyran-based photoacid generator with suitable absorption wavelength, we are able to restrict the activation wavelength of the SPP moiety to the green light region (λactivation =520-550 nm), enabling wavelength-orthogonal activation of the AP group. Our wavelength-orthogonal photochemical system was successfully applied in the design of hydrogels whose stiffness can be tuned independently by either green or blue light.

Contact-Free Remote Manipulation of Hydrogel Properties Using Light-Triggerable Nanoparticles: A Materials Science Perspective for Biomedical Applications

Considerable progress has been made in synthesizing "intelligent", biodegradable hydrogels that undergo rapid changes in physicochemical properties once exposed to external stimuli. These advantageous properties of stimulus-triggered materials make them highly appealing to diverse biomedical applications. Of late, research on the incorporation of light-triggered nanoparticles (NPs) into polymeric hydrogel networks has gained momentum due to their ability to remotely tune hydrogel properties using facile, contact-free approaches, such as adjustment of wavelength and intensity of light source. These multi-functional NPs, in combination with tissue-mimicking hydrogels, are increasingly being used for on-demand drug release, preparing diagnostic kits, and fabricating smart scaffolds. Here, the authors discuss the atomic behavior of different NPs in the presence of light, and critically review the mechanisms by which NPs convert light stimuli into heat energy. Then, they explain how these NPs impact the mechanical properties and rheological behavior of NPs-impregnated hydrogels. Understanding the rheological behavior of nanocomposite hydrogels using different sophisticated strategies, including computer-assisted machine learning, is critical for designing the next generation of drug delivery systems. Next, they highlight the salient strategies that have been used to apply light-induced nanocomposites for diverse biomedical applications and provide an outlook for the further improvement of these NPs-driven light-responsive hydrogels.

Engineered Tissue Models to Replicate Dynamic Interactions within the Hematopoietic Stem Cell Niche

Hematopoietic stem cells are the progenitors of the blood and immune system and represent the most widely used regenerative therapy. However, their rarity and limited donor base necessitate the design of ex vivo systems that support HSC expansion without the loss of long-term stem cell activity. This review describes recent advances in biomaterials systems to replicate features of the hematopoietic niche. Inspired by the native bone marrow, these instructive biomaterials provide stimuli and cues from cocultured niche-associated cells to support HSC encapsulation and expansion. Engineered systems increasingly enable study of the dynamic nature of the matrix and biomolecular environment as well as the role of cell-cell signaling (e.g., autocrine feedback vs paracrine signaling between dissimilar cells). The inherent coupling of material properties, biotransport of cell-secreted factors, and cell-mediated remodeling motivate dynamic biomaterial systems as well as characterization and modeling tools capable of evaluating a temporally evolving tissue microenvironment. Recent advances in HSC identification and tracking, model-based experimental design, and single-cell culture platforms facilitate the study of the effect of constellations of matrix, cell, and soluble factor signals on HSC fate. While inspired by the HSC niche, these tools are amenable to the broader stem cell engineering community.

A disulfide-based linker for thiol-norbornene conjugation: formation and cleavage of hydrogels by the use of light

Photolabile groups are the key components of photo-responsive polymers, dynamically tunable materials with multiple applications in materials and life sciences. They usually consist of a chromophore and a labile bond and are inherently light sensitive. An exception are disulfides, simple reversible linkages, which become photocleavable upon addition of a photoinitiator. Despite their practical features, disulfides are rarely utilized due to their impractical formation. Here, we report a disulfide-based linker series bearing norbornene terminals for facile crosslinking of thiol-functionalized macromers via light-triggered thiol-ene conjugation (TEC). Besides finding a highly reactive lead compound, we also identify an unexpected TEC-retardation, strongly dependent on the molecular linker structure and affecting hydrogel stability. Finally, we present a useful method for localized disulfide cleavage by two-photon irradiation permitting micropatterning of disulfide-crosslinked networks.