Photoinduced Dithiolane Crosslinking for Multiresponsive Dynamic Hydrogels

Lanthanide Coordinated Poly(Thioctic Acid) Materials with Enhanced Strength and Room Temperature Self-Healing Performance

Poly(thioctic acid) materials exhibit excellent room-temperature self-healing properties due to their dynamic disulfide-bonded supramolecular network and have been widely used in applications such as wearable devices, adhesives, and wound patches. However, the limited mechanical properties of poly(thioctic acid) materials with dynamic supramolecular networks limit their practical applications. Therefore, there is an urgent need for a low-energy-consuming and facile method to enhance their mechanical strength and maintain their room-temperature self-healing properties. Here, a novel approach is developed by introducing Eu3⁺-coordination into the copolymerization of thioctic acid (TA) and sodium thioctate (ST), forming hierarchical dynamic supramolecular networks. Copolymerization of TA and ST under mild conditions (60 °C in ethanol/water solvent) introduces stable hydrogen-bonding interactions without additional chemical cross-linkers. Further Eu3⁺-coordination increases the mechanical modulus of the films by more than 100-fold while significantly improving toughness and strength. This is attributed to the large ionic radius and high coordination number of Eu ions with carboxylates which significantly enhanced the strength of the crosslinked network. This strategy offers a novel pathway for developing supramolecular materials with an optimal balance of mechanical strength and self-repairing capabilities, advancing their potential in various applications.

Tunable Mechanical Properties in Biodegradable Cellulosic Bioplastics Achieved via Ring-Opening Polymerization

The development of bioplastics is advancing globally to promote a sustainable society. In this study, we designed cellulosic dual-network bioplastics to address the need for sustainable materials with balanced mechanical properties and biodegradability. Cellulose was used as the first network, and the second network was functionalized to enhance mechanical strength while preserving biodegradability. The dynamic covalent moieties within the second network were generated through dithiolane ring-opening polymerization. The ultimate tensile strength and flexural elongation were controlled within 8.8-193 MPa and 3.3-32.5%, respectively, depending on the degree of dynamic bonds. Moreover, the bioplastics exhibited gradual biodegradability, achieving approximately 30% degradation within 2 weeks. Interestingly, our bioplastics demonstrated the ability to coexist with plants, as their degradation did not negatively affect cell viability or plant growth. This study provides a promising approach to developing advanced bioplastics that reach sustainability goals while offering tunable mechanical properties.

Hydrogel-Based Continuum Soft Robots

This paper comprehensively reviews the latest advances in hydrogel-based continuum soft robots. Hydrogels exhibit exceptional flexibility and adaptability compared to traditional robots reliant on rigid structures, making them ideal as biomimetic robotic skins and platforms for constructing highly accurate, real-time responsive sensory interfaces. The article systematically summarizes recent research developments across several key dimensions, including application domains, fabrication methods, actuator technologies, and sensing mechanisms. From an application perspective, developments span healthcare, manufacturing, and agriculture. Regarding fabrication techniques, the paper extensively explores crosslinking methods, additive manufacturing, microfluidics, and other related processes. Additionally, the article categorizes and thoroughly discusses various hydrogel-based actuators responsive to solute/solvent variations, pH, chemical reactions, temperature, light, magnetic fields, electric fields, hydraulic/electro-osmotic stimuli, and humidity. It also details the strategies for designing and implementing diverse sensors, including strain, pressure, humidity, conductive, magnetic, thermal, gas, optical, and multimodal sensors. Finally, the paper offers an in-depth discussion of the prospective applications of hydrogel-based continuum soft robots, particularly emphasizing their potential in medical and industrial fields. Concluding remarks include a forward-looking outlook highlighting future challenges and promising research directions.

Photo-regulated disulfide crosslinking: a versatile approach to construct mucus-inspired hydrogels

The remarkable defensive ability of native mucus against pathogens has encouraged scientists to map its structure--property correlation and its influence on immune defense mechanisms. However, its poorly defined structure, source-dependent composition, and low availability limit the usefulness of native mucus in the laboratory. This gap creates a strong demand for the development of synthetic mucus-mimetic materials. Here, we report a straightforward strategy for constructing mucus-mimetic hydrogels through photo-regulated disulfide crosslinking. Light-responsive 1,2-dithiolane attached to a linear polyglycerol sulfate (lPGS) backbone allows the macromolecular building blocks to crosslink and form the hydrogel, which mirrors the chemistry of native mucus hydrogel formation with its disulfide-linked mucin chains. The viscoelastic properties of the hydrogel can be easily tuned by controlling both the light exposure time and the number of 1,2-dithiolane units within the polymer backbone. Furthermore, localized UV irradiation allows for spatially resolved hydrogel formation. Importantly, this synthetic polymer can directly crosslink with native mucin, bovine submaxillary mucin (BSM), to convert it into a hydrogel at physiological pH. The versatility of this approach - hydrogel formation via photo-regulated disulfide crosslinking - can be used to develop a synthetic mucus model.

Bio-orthogonal tuning of matrix properties during 3D cell culture to induce morphological and phenotypic changes

Described herein is a protocol for producing a synthetic extracellular matrix that can be modified in situ during three-dimensional cell culture. The hydrogel platform is established using modular building blocks employing bio-orthogonal tetrazine (Tz) ligation with slow (norbornene, Nb) and fast (trans-cyclooctene, TCO) dienophiles. A cell-laden gel construct is created via the slow, off-stoichiometric Tz/Nb reaction. After a few days of culture, matrix properties can be altered by supplementing the cell culture media with TCO-tagged molecules through the rapid reaction with the remaining Tz groups in the network at the gel-liquid interface. As the Tz/TCO reaction is faster than molecular diffusion, matrix properties can be modified in a spatiotemporal fashion simply by altering the identity of the diffusive species and the diffusion time/path. Our strategy does not interfere with native biochemical processes nor does it require external triggers or a second, independent chemistry. The biomimetic three-dimensional cultures can be analyzed by standard molecular and cellular techniques and visualized by confocal microscopy. We have previously used this method to demonstrate how in situ modulation of matrix properties induces epithelial-to-mesenchymal transition, elicits fibroblast transition from mesenchymal stem cells and regulates myofibroblast differentiation. Following the detailed procedures, individuals with a bachelor's in science and engineering fields can successfully complete the protocol in 4-5 weeks. This protocol can be applied to model tissue morphogenesis and disease progression and it can also be used to establish engineered constructs with tissue-like anisotropy and tissue-specific functions.

Programmed shape transformations in cell-laden granular composites

Tissues form during development through mechanical compaction of their extracellular matrix (ECM) and shape morphing, processes that result in complex-shaped structures that contribute to tissue function. While observed in vivo, control over these processes in vitro to understand both tissue development and guide tissue formation has remained challenging. Here, we use combinations of mesenchymal stromal cell spheroids and hydrogel microparticles (microgels) with varied hydrolytic stability to fabricate programmable and dynamic granular composites that control compaction and tissue formation over time. Mixed microgel populations of varying stability provide a further handle to alter compaction, and the level of compaction guides the uniformity and level of ECM deposition within tissues. Last, spatially patterned granular composites of varying compaction enable shape transformations (i.e., bending/curvature) that are stable with culture and are predicted by finite element models.

Photoresponsive Chemistries for User-Directed Hydrogel Network Modulation to Investigate Cell-Matrix Interactions

Synthetic extracellular matrix (ECM) engineering is a highly interdisciplinary field integrating materials and polymer science and engineering, chemistry, cell biology, and medicine to develop innovative strategies to investigate and control cell-matrix interactions. Cellular microenvironments are complex and highly dynamic, changing in response to injury and disease. To capture some of these critical dynamics in vitro, biomaterial matrices have been developed with tailorable properties that can be modulated in situ in the presence of cells. While numerous macromolecules can serve as a basis in the design of a synthetic ECM, our group has exploited multi-arm poly(ethylene glycol) (PEG) macromolecules because of the ease of functionalization, many complementary bio-click reactions to conjugate biological signals, and ultimately, the ability to create well-defined systems to investigate cell-matrix interactions. To date, significant strides have been made in developing bio-responsive and transient synthetic ECM materials that degrade, relax stress, or strain-stiffen in response to cell-mediated stimuli through ECM-cleaving enzymes or integrin-mediated ECM adhesions. However, our group has also designed hydrogels incorporating different photoresponsive moieties, and these moieties facilitate user-defined spatiotemporal modulation of the extracellular microenvironment in vitro. The application of light allows one to break, form, and rearrange network bonds in the presence of cells to alter the biomechanical and biochemical microenvironment to investigate cell-matrix interactions in real-time. Such photoresponsive materials have facilitated fundamental discoveries in the biological pathways related to outside-in signaling, which guide important processes related to tissue development, homeostasis, disease progression, and regeneration. This review focuses on the phototunable chemical toolbox that has been used by Anseth and co-workers to modulate hydrogel properties post-network formation through: bond-breaking chemistries, such as o-nitrobenzyl and coumarin methyl ester photolysis; bond-forming chemistries, such as azadibenzocyclooctyne photo-oligomerization and anthracene dimerization; and bond-rearranging chemistries, such as allyl sulfide addition-fragmentation chain transfer and reversible ring opening polymerization of 1,2-dithiolanes. By using light to modulate the cellular microenvironment (in 2D, 3D, and even 4D), innovative experiments can be designed to study mechanosensing of single cells or multicellular constructs, pattern adhesive ligands to spatially control cell-integrin binding or modulate on-demand the surrounding cell niche to alter outside-in signaling in a temporally controlled manner. To date, these photochemically defined materials have been used for the culture, differentiation, and directed morphogenesis of primary cells and stem cells, co-cultured cells, and even multicellular constructs (e.g., organoids).Herein, we present examples of how this photochemical toolbox has been used under physiological reaction conditions with spatiotemporal control to answer important biological questions and address medical needs. Specifically, our group has exploited these materials to study mesenchymal stem cell mechanosensing and differentiation, the activation of fibroblasts in the context of valve and cardiac fibrosis, muscle stem cell response to matrix changes during injury and aging, and predictable symmetry breaking during intestinal organoid development. The materials and reactions described herein are diverse and enable the design and implementation of an array of hydrogels that can serve as cell delivery systems, tissue engineering scaffolds, or even in vitro models for studying disease or screening for new drug treatments.

Design and Fabrication of Viscoelastic Hydrogels as Extracellular Matrix Mimicry for Cell Engineering

The extracellular matrix (ECM) performs both as a static scaffold and as a dynamic, viscoelastic milieu that actively participates in cell signaling and mechanical feedback loops. Recently, biomaterials with tunable viscoelastic properties have been utilized to mimic the native ECM in the fields of tissue engineering and regenerative medicines. These materials can be designed to support cell attachment, proliferation, and differentiation, facilitating the repair or replacement of damaged tissues. Moreover, viscoelasticity modulation of ECM mimicry helps to develop therapeutic strategies for diseases involving altered mechanical properties of tissues such as fibrosis or cancer. The study of biomaterial viscoelasticity thus intersects with a broad spectrum of biological and medical disciplines, offering insights into fundamental cell biology and practical solutions for improving human health. This review delves into the design and fabrication strategies of viscoelastic hydrogels, focusing particularly on two major viscoelastic parameters, mechanical strength and stress relaxation, and how the hydrogel mechanics influence the interactions between living cells and surrounding microenvironments. Meanwhile, this review discusses current bottlenecks in hydrogel-cell mechanics studies, highlighting the challenges in viscoelastic parameter decoupling, long-term stable maintenance of viscoelastic microenvironment, and the general applicability of testing standards and conversion protocols.

Biomass and Transparent Supramolecular Elastomers for Green Electronics Enabled by the Controlled Growth and Self-Assembly of Dynamic Polymer Networks

Determining the optimal method for preparing supramolecular materials remains a profound challenge. This process requires a combination of renewable raw materials to create supramolecular materials with multiple functions and properties, including simple fabrication, sustainability, a dynamic nature, good toughness, and transparency. In this work, a strategy is presented for toughening supramolecular networks based on solid-phase chain extension. This toughening strategy is simple and environmentally friendly. In addition, a series of biobased elastomers are designed and prepared with adjustable performance characteristics. This strategy can significantly improve the transparency, tensile strength, and toughness of the synthesized elastomer. The synthesized biobased elastomers have great ductility, repairability, and recyclability, and they show good adhesion and dielectric properties. A biobased ionic skin is assembled from these biobased elastomers. Assembled ionic skin can sensitively detect external stimuli (such as stretching, bending, compression, or temperature changes) and monitor human movement. The conductive and dielectric layers of the biobased ionic skin are both obtained from renewable raw materials. This research provides novel molecular design approaches and material selection methods for promoting the development of green electronic devices and biobased elastomers.

Synthetic and natural polymer hydrogels: A review of 3D spheroids and drug delivery

This review centers on the synthesis and characterization of both natural and synthetic hydrogels, highlighting their diverse applications across various fields. We will delve into the evolution of hydrogels, focusing on the importance of polysaccharide-based and synthetic variants, which have been particularly chosen for 3D spheroid development in cancer research and drug delivery. A detailed background on the research and specific methodologies, including the in-situ free radical polymerization used for synthesizing these hydrogels, will be extensively discussed. Additionally, the review will explore various applications of these hydrogels, such as their self-healing properties, swelling ratios, pH responsiveness, and cell viability. A comprehensive literature review will support this investigation. Ultimately, this review aims to clearly outline the objectives and significance of hydrogel synthesis and their applications.

Photochemical Control of Network Topology in PEG Hydrogels

Hydrogels are often synthesized through photoinitiated step-, chain-, and mixed-mode polymerizations, generating diverse network topologies and resultant material properties that depend on the underlying network connectivity. While many photocrosslinking reactions are available, few afford controllable connectivity of the hydrogel network. Herein, a versatile photochemical strategy is introduced for tuning the structure of poly(ethylene glycol) (PEG) hydrogels using macromolecular monomers functionalized with maleimide and styrene moieties. Hydrogels are prepared along a gradient of topologies by varying the ratio of step-growth (maleimide dimerization) to chain-growth (maleimide-styrene alternating copolymerization) network-forming reactions. The initial PEG content and final network physical properties (e.g., modulus, swelling, diffusivity) are tailored in an independent manner, highlighting configurable gel mechanics and reactivity. These photochemical reactions allow high-fidelity photopatterning and 3D printing and are compatible with 2D and 3D cell culture. Ultimately, this photopolymer chemistry allows facile control over network connectivity to achieve adjustable material properties for broad applications.

Facile Physicochemical Reprogramming of PEG-Dithiolane Microgels

Granular biomaterials have found widespread applications in tissue engineering, in part because of their inherent porosity, tunable properties, injectability, and 3D printability. However, the assembly of granular hydrogels typically relies on spherical microparticles and more complex particle geometries have been limited in scope, often requiring templating of individual microgels by microfluidics or in-mold polymerization. Here, we use dithiolane-functionalized synthetic macromolecules to fabricate photopolymerized microgels via batch emulsion, and then harness the dynamic disulfide crosslinks to rearrange the network. Through unconfined compression between parallel plates in the presence of photoinitiated radicals, we transform the isotropic microgels are transformed into disks. Characterizing this process, we find that the areas of the microgel surface in contact with the compressive plates are flattened while the curvature of the uncompressed microgel boundaries increases. When cultured with C2C12 myoblasts, cells localize to regions of higher curvature on the disk-shaped microgel surfaces. This altered localization affects cell-driven construction of large supraparticle scaffold assemblies, with spherical particles assembling without specific junction structure while disk microgels assemble preferentially on their curved surfaces. These results represent a unique spatiotemporal process for rapid reprocessing of microgels into anisotropic shapes, providing new opportunities to study shape-driven mechanobiological cues during and after granular hydrogel assembly.

High Refractive Index, Low Birefringence Holographic Materials via the Homopolymerization of 1,2-Dithiolanes

High refractive index, low birefringence photopolymers were created via the radical-mediated, ring opening homopolymerization of 1,2-dithiolane functionalized monomers and were subsequently evaluated as holographic recording media. This investigation systematically characterized the reaction kinetics, thermodynamics, and volume shrinkage of the 1,2-dithiolane homopolymerization as well as the optical transparency, refractive index, birefringence, and holographic performance of multifunctional 1,2-dithiolane functionalized monomers and their resultant polymers. Real-time kinetic and thermodynamic analyses of a monofunctional 1,2-dithiolane monomer, lipoic acid methyl ester (LipOMe), indicated rapid monomer conversion, exceeding 90% in 60 s, with an overall enthalpy of reaction of 18 ± 1 kJ/mol. The ring-opening polymerization resulted in low shrinkage (10.6 ± 0.3 cm3/mol dithiolane) and a significant bulk refractive index increase (0.030 ± 0.003). The resulting photopolymers exhibited high optical transparency, minimal haze, and negligible birefringence, suggesting the potential of 1,2-homopolymers as optical materials. To further explore the specific capabilities for use as high-performance holographic recording applications, several multifunctional monomers were synthesized with the ethanedithiol lipoic acid monomer (EDT-Lip2) selected for experimentation. Holographic diffraction gratings written using this monomer achieved a peak-to-mean refractive index modulation of 0.008 with minimal haze and birefringence.

Independent Configuration and Reprogramming of Porous Characters in Macroporous Hydrogel Enabled by the Orthogonal Dynamic Network

Macroporous hydrogels have attracted much attention in both industry and academia, where the morphological characteristics of pores are essential. Despite significant improvements on regulating porous structures, the independent configuration and reprogramming of porosity and pore size still remain challenging owing to the lack of a chemical design to decouple the mechanism for adjusting each characteristic. Here, we report a strategy to adaptively control porous features relying on an orthogonal dynamic network. Disulfide bonds are employed to relax polymer chains during freezing via UV irradiation, thus, generating pores in hydrogels. On such a basis, the porosity is continuously switched from 0 to 75% by controlling network relaxation ratios. Subsequently, the pore size is further reversibly manipulated through the association or dissociation of dynamic metallic coordination. As a result, the porosity and pore size achieved independent configurations. Meanwhile, the dynamic nature of the network makes it possible to reprogram the porous character of a prepared hydrogel. Beyond these, the photopatterning of pores represents the capability to regulate the third feature. Our strategy provides an effective way to arbitrarily manipulate porous morphologies, which can inspire the design of future functional porous materials.

Multifunctional hydrogels with spatially controlled light activation with photocaged oligonucleotides

Recreating tissue environments with precise control over mechanical, biochemical, and cellular organization is essential for next-generation tissue models for drug discovery, development studies, and the replication of disease environments. However, controlling these properties at cell-scale lengths remains challenging. Here, we report the development of printing approaches that leverage polyethylene glycol diacrylate (PEGDA) hydrogels containing photocaged oligonucleotides to spatially program material characteristics with non-destructive, non-ultraviolet light. We further integrate this system with a perfusion chamber to allow us to alter the composition of PEGDA hydrogels while retaining common light-activatable photocaged DNAs. We demonstrate that the hydrogels can capture DNA functionalized materials, including cells coated with complementary oligonucleotides with spatial control using biocompatible wavelengths. Overall, these materials open pathways to orthogonal capture of any DNA functionalized materials while not changing the sequences of the DNA.

Modular Multiwell Viscoelastic Hydrogel Platform for Two- and Three-Dimensional Cell Culture Applications

Hydrogels have gained significant popularity as model platforms to study reciprocal interactions between cells and their microenvironment. While hydrogel tools to probe many characteristics of the extracellular space have been developed, fabrication approaches remain challenging and time-consuming, limiting multiplexing or widespread adoption. Thus, we have developed a modular fabrication approach to generate distinct hydrogel microenvironments within the same 96-well plate for increased throughput of fabrication as well as integration with existing high-throughput assay technologies. This approach enables in situ hydrogel mechanical characterization and is used to generate both elastic and viscoelastic hydrogels across a range of stiffnesses. Additionally, this fabrication method enabled a 3-fold reduction in polymer and up to an 8-fold reduction in fabrication time required per hydrogel replicate. The feasibility of this platform for two-dimensional (2D) cell culture applications was demonstrated by measuring both population-level and single-cell-level metrics via microplate reader and high-content imaging. Finally, a 96-well hydrogel array was utilized for three-dimensional (3D) cell culture, demonstrating the ability to support high cell viability. Together, this work demonstrates a versatile and easily adaptable fabrication approach that can support the ever-expanding tool kit of hydrogel technologies for cell culture applications.

Dynamic Antimicrobial Poly(disulfide) Coatings Exfoliate Biofilms On Demand Via Triggered Depolymerization

Bacterial biofilms are notoriously problematic in applications ranging from biomedical implants to ship hulls. Cationic, amphiphilic antibacterial surface coatings delay the onset of biofilm formation by killing microbes on contact, but they lose effectiveness over time due to non-specific binding of biomass and biofilm formation. Harsh treatment methods are required to forcibly expel the biomass and regenerate a clean surface. Here, a simple, dynamically reversible method of polymer surface coating that enables both chemical killing on contact, and on-demand mechanical delamination of surface-bound biofilms, by triggered depolymerization of the underlying antimicrobial coating layer, is developed. Antimicrobial polymer derivatives based on α-lipoic acid (LA) undergo dynamic and reversible polymerization into polydisulfides functionalized with biocidal quaternary ammonium salt groups. These coatings kill >99.9% of Staphylococcus aureus cells, repeatedly for 15 cycles without loss of activity, for moderate microbial challenges (≈105 colony-forming units (CFU) mL-1, 1 h), but they ultimately foul under intense challenges (≈107 CFU mL-1, 5 days). The attached biofilms are then exfoliated from the polymer surface by UV-triggered degradation in an aqueous solution at neutral pH. This work provides a simple strategy for antimicrobial coatings that can kill bacteria on contact for extended timescales, followed by triggered biofilm removal under mild conditions.

A Comprehensive Review of Hydrogel-Based Drug Delivery Systems: Classification, Properties, Recent Trends, and Applications

As adaptable biomaterials, hydrogels have shown great promise in several industries, which include the delivery of drugs, engineering of tissues, biosensing, and regenerative medicine. These hydrophilic polymer three-dimensional networks have special qualities like increased content of water, soft, flexible nature, as well as biocompatibility, which makes it excellent candidates for simulating the extracellular matrix and promoting cell development and tissue regeneration. With an emphasis on their design concepts, synthesis processes, and characterization procedures, this review paper offers a thorough overview of hydrogels. It covers the various hydrogel material types, such as natural polymers, synthetic polymers, and hybrid hydrogels, as well as their unique characteristics and uses. The improvements in hydrogel-based platforms for controlled drug delivery are examined. It also looks at recent advances in bioprinting methods that use hydrogels to create intricate tissue constructions with exquisite spatial control. The performance of hydrogels is explored through several variables, including mechanical properties, degradation behaviour, and biological interactions, with a focus on the significance of customizing hydrogel qualities for particular applications. This review paper also offers insights into future directions in hydrogel research, including those that promise to advance the discipline, such as stimuli-responsive hydrogels, self-healing hydrogels, and bioactive hydrogels. Generally, the objective of this review paper is to provide readers with a detailed grasp of hydrogels and all of their potential uses, making it an invaluable tool for scientists and researchers studying biomaterials and tissue engineering.

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.

Advanced materials technologies to unravel mechanobiological phenomena

Advancements in materials-driven mechanobiology have yielded significant progress. Mechanobiology explores how cellular and tissue mechanics impact development, physiology, and disease, where extracellular matrix (ECM) dynamically interacts with cells. Biomaterial-based platforms emulate synthetic ECMs, offering precise control over cellular behaviors by adjusting mechanical properties. Recent technological advances enable in vitro models replicating active mechanical stimuli in vivo. These models manipulate cellular mechanics even at a subcellular level. In this review we discuss recent material-based mechanomodulatory studies in mechanobiology. We highlight the endeavors to mimic the dynamic properties of native ECM during pathophysiological processes like cellular homeostasis, lineage specification, development, aging, and disease progression. These insights may inform the design of accurate in vitro mechanomodulatory platforms that replicate ECM mechanics.

Solvent-Triggered Chemical Recycling of Ion-Conductive and Self-Healable Polyurethane Covalent Adaptive Networks

Given the substantial environmental challenge posed by global plastic waste, recycling technology for thermosetting polymers has become a huge research topic in the polymer industry. Covalent adaptive networks (CANs), which can reversibly dissociate and reconstruct their network structure, represent a key technology for the self-healing, reprocessing, and recycling of thermosetting polymers. In the present study, we introduce a new series of polyurethane CANs whose network structure can dissociate via the self-catalyzed formation of dithiolane from the CANs' polydisulfide linkages when the CANs are treated in N,N-dimethylformamide (DMF) or dimethyl sulfoxide at 60 °C for 1 h. More interestingly, we found that this network dissociation even occurs in tetrahydrofuran-DMF solvent mixtures with low DMF concentrations. This feature enables a reduction in the use of high-boiling, toxic polar aprotic solvents. The dissociated network structure of the CANs was reconstructed under UV light at 365 nm with a high yield via ring-opening polydisulfide linkage formation from dithiolane pendant groups. These CAN films, which were prepared by a sequential organic synthesis and polymerization process, exhibited high thermal stability and good mechanical properties, recyclability, and self-healing performance. When lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt was added to the CAN films, the films exhibited a maximum ion conductivity of 7.48 × 10-4 S cm-1 because of the contribution of the high concentration of the pendant ethylene carbonate group in the CANs. The ion-conducting CAN films also showed excellent recyclability and a self-healing performance.

Implications of Cellular Mechanical Memory in Bioengineering

The ability to maintain and differentiate cells in vitro is critical to many advances in the field of bioengineering. However, on traditional, stiff (E ≈ GPa) culture substrates, cells are subjected to sustained mechanical stress that can lead to phenotypic changes. Such changes may remain even after transferring the cells to another scaffold or engrafting them in vivo and bias the outcomes of the biological investigation or clinical treatment. This persistence─or mechanical memory─was initially observed for sustained myofibroblast activation of pulmonary fibroblasts after culturing them on stiff (E ≈ 100 kPa) substrates. Aspects of mechanical memory have now been described in many in vitro contexts. In this Review, we discuss the stiffness-induced effectors of mechanical memory: structural changes in the cytoskeleton and activity of transcription factors and epigenetic modifiers. We then focus on how mechanical memory impacts cell expansion and tissue regeneration outcomes in bioengineering applications relying on prolonged 2D plastic culture, such as stem cell therapies and disease models. We propose that alternatives to traditional cell culture substrates can be used to mitigate or erase mechanical memory and improve the efficiency of downstream cell-based bioengineering applications.

Crossover of surface waves and capillary-viscous-elastic transition in soft biomaterials detected by resonant acoustic rheometry

Viscoelastic properties of hydrogels are important for their application in science and industry. However, rheological assessment of soft hydrogel biomaterials is challenging due to their complex, rapid, and often time-dependent behaviors. Resonant acoustic rheometry (RAR) is a newly developed technique capable of inducing and measuring resonant surface waves in samples in a non-contact fashion. By applying RAR at high temporal resolution during thrombin-induced fibrin gelation and ultraviolet-initiated polyethylene glycol (PEG) polymerization, we observed distinct changes in both frequency and amplitude of the resonant surface waves as the materials changed over time. RAR detected a series of capillary-elastic, capillary-viscous, and visco-elastic transitions that are uniquely manifested as crossover of different types of surface waves in the temporally evolving materials. These results reveal the dynamic interplay of surface tension, viscosity, and elasticity that is controlled by the kinetics of polymerization and crosslinking during hydrogel formation. RAR overcomes many limitations of conventional rheological approaches by offering a new way to comprehensively and longitudinally characterize soft materials during dynamic processes.

Design Parameters for Injectable Biopolymeric Hydrogels with Dynamic Covalent Chemistry Crosslinks

Dynamic covalent chemistry (DCC) crosslinks can form hydrogels with tunable mechanical properties permissive to injectability and self-healing. However, not all hydrogels with transient crosslinks are easily extrudable. For this reason, two additional design parameters must be considered when formulating DCC-crosslinked hydrogels: 1) degree of functionalization (DoF) and 2) polymer molecular weight (MW). To investigate these parameters, hydrogels comprised of two recombinant biopolymers: 1) a hyaluronic acid (HA) modified with benzaldehyde and 2) an elastin-like protein (ELP) modified with hydrazine (ELP-HYD), are formulated. Several hydrogel families are synthesized with distinct HA MW and DoF while keeping the ELP-HYD component constant. The resulting hydrogels have a range of stiffnesses, G' ≈ 10-1000 Pa, and extrudability, which is attributed to the combined effects of DCC crosslinks and polymer entanglements. In general, lower MW formulations require lower forces for injectability, regardless of stiffness. Higher DoF formulations exhibit more rapid self-healing. Gel extrusion through a cannula (2 m length, 0.25 mm diameter) demonstrates the potential for minimally invasive delivery for future biomedical applications. In summary, this work highlights additional parameters that influence the injectability and network formation of DCC-crosslinked hydrogels and aims to guide future design of injectable hydrogels.

Emerging Trends in the Chemistry of End-to-End Depolymerization

Over the past couple of decades, polymers that depolymerize end-to-end upon cleavage of their backbone or activation of a terminal functional group, sometimes referred to as "self-immolative" polymers, have been attracting increasing attention. They are of growing interest in the context of enhancing polymer degradability but also in polymer recycling as they allow monomers to be regenerated in a controlled manner under mild conditions. Furthermore, they are highly promising for applications as smart materials due to their ability to provide an amplified response to a specific signal, as a single sensing event is translated into the generation of many small molecules through a cascade of reactions. From a chemistry perspective, end-to-end depolymerization relies on the principles of self-immolative linkers and polymer ceiling temperature (Tc). In this article, we will introduce the key chemical concepts and foundations of the field and then provide our perspective on recent exciting developments. For example, over the past few years, new depolymerizable backbones, including polyacetals, polydisulfides, polyesters, polythioesters, and polyalkenamers, have been developed, while modern approaches to depolymerize conventional backbones such as polymethacrylates have also been introduced. Progress has also been made on the topological evolution of depolymerizable systems, including the introduction of fully depolymerizable block copolymers, hyperbranched polymers, and polymer networks. Furthermore, precision sequence-defined oligomers have been synthesized and studied for data storage and encryption. Finally, our perspectives on future opportunities and challenges in the field will be discussed.

Polymer-Based Hydrogels Applied in Drug Delivery: An Overview

Polymer-based hydrogels are hydrophilic polymer networks with crosslinks widely applied for drug delivery applications because of their ability to hold large amounts of water and biological fluids and control drug release based on their unique physicochemical properties and biocompatibility. Current trends in the development of hydrogel drug delivery systems involve the release of drugs in response to specific triggers such as pH, temperature, or enzymes for targeted drug delivery and to reduce the potential for systemic toxicity. In addition, developing injectable hydrogel formulations that are easily used and sustain drug release during this extended time is a growing interest. Another emerging trend in hydrogel drug delivery is the synthesis of nano hydrogels and other functional substances for improving targeted drug loading and release efficacy. Following these development trends, advanced hydrogels possessing mechanically improved properties, controlled release rates, and biocompatibility is developing as a focus of the field. More complex drug delivery systems such as multi-drug delivery and combination therapies will be developed based on these advancements. In addition, polymer-based hydrogels are gaining increasing attention in personalized medicine because of their ability to be tailored to a specific patient, for example, drug release rates, drug combinations, target-specific drug delivery, improvement of disease treatment effectiveness, and healthcare cost reduction. Overall, hydrogel application is advancing rapidly, towards more efficient and effective drug delivery systems in the future.