Rationally designed synthetic protein hydrogels with predictable mechanical properties

Unveiling the structure of protein-based hydrogels by overcoming cryo-SEM sample preparation challenges

Protein-based hydrogels have gained significant attention for their potential use in applications such as drug delivery and tissue engineering. Their internal structure is complex, spans across multiple length scales and affects their functionality, yet is not well understood because of folded proteins' sensitivity to physical and chemical perturbations and the high water content of hydrogels. Cryo-scanning electron microscopy (cryo-SEM) has the potential to reveal such hierarchical structure when hydrated hydrogels are prepared with appropriate cryofixation. We show for photochemically cross-linked, folded globular bovine serum albumin (BSA) protein hydrogels that preparation artefacts are reduced by in situ gelation, high pressure freezing (HPF), plasma focused ion beam (pFIB) milling, sublimation, and low dose secondary electron imaging. Cryo-SEM of folded BSA protein hydrogels prepared in this way reveals a heterogeneous network with nanoscale porosity (∼60 nm pores) surrounded by high secondary electron emission regions (∼30 nm diameter) interconnected by narrower, lower emission regions (∼20 nm length). This heterogeneous network structure is consistent with small angle scattering studies of folded protein hydrogels, with fractal-like clusters connected by intercluster regions. We further test the potential of cryo-SEM to detect the impact of protein unfolding on hydrogel network formation and reveal nanoscale differences in cluster sizes consistent with those derived from scattering data. Importantly, cryo-SEM directly images pores for sizing in both systems, with initial results on BSA suggesting protein unfolding induces an increase of ∼10 nm in pore sizes. Our findings on cryo-SEM sample preparation challenges and solutions provide new opportunities to link hydrogel structure to function.

Leveraging Protein-Ligand and DNA Interactions to Control Hydrogel Mechanics

Biomacromolecules can serve as molecularly precise building blocks for hydrogel materials, dictating material properties that depend on the chemical identity and interactions of the individual components. Herein, we introduce biomolecular hydrogels where ligand-functionalized DNA sequences form the hydrogel backbone and multivalent protein-ligand interactions form supramolecular cross-links. In these hydrogels, we can independently leverage the programmable rigidity of DNA (i.e., single-stranded vs double-stranded DNA) and defined protein-ligand binding affinities spanning >10 orders of magnitude to modulate the gel stiffness, stress relaxation, and shear thinning. We learn that (1) double-stranded networks have stiffness values up to 3 orders of magnitude greater than single-stranded networks and exhibit thermoresponsiveness and (2) the protein-ligand binding affinities and dissociation rate constants determine the network topologies and stress relaxation rates of the hydrogels. Finally, the hydrogels exhibit cytocompatibility and cell-type-specific degradation, where cells can migrate through the gels via interactions between the gels and their ligand-binding receptors. Together, this work demonstrates that varying the local chemical interactions of the hydrogel backbone and the supramolecular binding affinity of dynamic cross-links leads to cytocompatible hydrogels with tunable viscoelastic properties for applications in drug delivery and tissue engineering.

Engineered Hydrogels for Organoid Models of Human Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is characterized by increased lipid accumulation and excessive deposition of extracellular matrix (ECM) that results in tissue stiffening. The potential interplay between matrix stiffness and hepatocyte lipid accumulation during NAFLD has not been established. Here, an in vitro NAFLD model is developed using chemically defined, engineered hydrogels and human induced pluripotent stem cell-derived hepatic organoids (HOs). Specifically, dynamic covalent chemistry crosslinking, along with transient small molecule competitors, are used to create dynamic stiffening hydrogels that enable the reproducible culture of HOs. Within matrices that mimic the stiffness of healthy to diseased tissue (≈1-6 kPa), lipid droplet accumulation in HOs is triggered by exposure to an NAFLD-associated free fatty acid. These NAFLD model suggests that higher stiffness microenvironments result in increased hepatic lipid droplet accumulation, increased expression of fibrosis markers, and increased metabolic dysregulation. By targeting the ROCK mechanosignaling pathway, the synergy between matrix stiffness and lipid droplet accumulation is disrupted. The in vitro model of NAFLD has the potential to understand the role of mechanosignaling in disease progression and identify new pathways for therapeutic intervention.

Preparation, characterization, multidimensional applications and prospects of protein bio-based hydrogels: A review

Protein bio-based hydrogels have emerged as a versatile and functional class of biomaterials due to their unique properties, including biocompatibility, biodegradability, tunable mechanical strength, and environmental responsiveness (pH, enzymes, ions and light). This review comprehensively explores the preparation methods, physicochemical properties, and multidimensional applications of protein-based hydrogels. Various fabrication techniques, such as chemical crosslinking, physical gelation, and enzymatic reactions, are discussed, highlighting their impact on the structure and functionality of hydrogels. The intrinsic properties of protein hydrogels are summarized in detail, including mechanical properties, swelling resistance, frost resistance, adhesion, biocompatibility and degradability. Furthermore, this review delves into their diverse applications, which span tissue engineering, drug delivery, wound healing, food preservation, biosensing, and environmental remediation. Finally, the challenges and future prospects of protein-based hydrogels are addressed, emphasizing the necessity for scalable production, enhanced stability, and multifunctional integration to meet the growing demands of advanced biomedical and industrial applications. This review aims to provide a comprehensive understanding of protein-based hydrogels and to inspire innovative research in this rapidly evolving field.

A Facile Strategy to Construct Stretchable and Thermoreversible Double Network Hydrogels with Low Hysteresis and High Toughness Based on Entanglement and Hydrogen Bond Networks

High toughness and low hysteresis are of great significance for stretchable hydrogels to strengthen their reliability and practicability for cycle-loaded applications. Whereas, it is still challenging to simultaneously gain mutually repulsive properties due to the existence of dissipation structure. Here, stretchable and recoverable double network (DN) hydrogels comprising highly entangled network structure and temperature-induced dense hydrogen bond (HB)-associated network structure are synthesized, in which slidable entanglements and dense HBs act as the effective crosslinking in first and second networks, respectively. Moveable entanglements and stable HB structures endow hydrogels with high stretchability (999%), high tensile strength (0.82 MPa) and high fracture toughness (4.51 MJ·m-3) through stress transmission along long chains and HB clusters energy dissipation. Moreover, the imposed energy on hydrogels can saved in oriented polymer chains by disentanglement as an entropy loss, thus ensuring the low hysteresis (6.2% at 100% tensile strain) of hydrogels. In addition, deformed hydrogel can be well remodeled and recovered (stress recovery of 95.5%) by temperature-triggered sol-gel transition of HB network. By means of entanglement and hydrogen bond strategies, stretchable DN hydrogels not only equilibrate the conflict of low hysteresis and high toughness, but also provides a new perspective to design high-property hydrogels for cycle-loaded applications.

Capturing the impact of protein unfolding on the dynamic assembly of protein networks

The rapid assembly of molecular or nanoscale building blocks into extended arrays is crucial to the construction of functional networks in vivo and in vitro and depends on various factors. One factor seldom considered is the dynamic changes of the building block shape. Folded protein building blocks offer a unique system to investigate dynamic shape changes due to their intrinsic ability to change from a compact and specific folded structure to an extended unfolded structure in response to a perturbation such as force. Here, we use photochemically crosslinked folded protein hydrogels constructed from force labile protein building blocks as a model dynamic shape-changing network system and characterise them by combining time-resolved rheology and small-angle X-ray scattering (SAXS). This approach probes both the load-bearing network structures, using rheology, and network architectures, using SAXS, thereby providing a crosslength scale understanding of the network formation. We propose a triple assembly model for the structural evolution of networks constructed from force labile protein building block consisting of: primary formation where monomeric folded proteins create the preliminary protein network scaffold; a subsequent secondary formation phase, where larger oligomers of protein diffuse to join the preliminary network scaffold; and finally in situ unfolding and relaxation which leads to the mature network structure of connected larger and denser fractal-like clusters. The time-resolved SAXS data provides evidence that protein unfolding occurs on the edges of the fractal-like clusters, resulting in a population of unfolded proteins in the space between clusters. Identifying the key stages of assembly in protein networks constructed from force labile proteins provides a greater understanding of the importance of protein unfolding in hierarchical biomechanics in vivo and creates future opportunities to develop bespoke biomaterials for novel biomedical applications.

In situ forming and self-crosslinkable protein hydrogels for localized cancer therapy and topical wound healing

Proteins, inherently biocompatible and biodegradable, face a challenge in forming stable hydrogels without external chemical crosslinkers. Here, we construct a ring-shaped trimeric SpyTag-fused Proliferating Cell Nuclear Antigen Protein (ST-PCNA) as a core protein building block, and a dumbbell-shaped tandem dimeric SpyCatcher (SC-SC) as a bridging component. Self-crosslinked PCNA/SC-SC Protein (2SP) hydrogels are successfully formed by simply mixing the solutions of ST-PCNA and SC-SC, without chemical crosslinkers. During their formation by mixing, various cargo molecules, including anti-cancer drugs, photosensitizers, and functional proteins, are efficiently incorporated, producing cargo@2SP hydrogels. Most of the entrapped cargo molecules gradually release as the hydrogels erode. Anti-cancer drug- or photosensitizer-incorporated 2SP hydrogels are successfully formed through localized injection beneath the 4 T1 tumor in mice. The localized gradual release of drugs in physiological microenvironment substantially suppresses tumor growth, and highly localized photosensitizers retained in the 2SP hydrogels raises the local temperature above 45 °C upon laser irradiation, resulting in a significant suppression of tumor growth. Additionally, the topical administration of growth factor proteins-incorporated 2SP hydrogels to the incision wound area effectively regenerates the skin, with rapid reconstruction of extracellular matrix. The injectable and self-crosslinkable 2SP hydrogels developed here offer promise as novel biocompatible scaffolds for local therapy.

Molecular self-assembly strategy tuning a dry crosslinking protein patch for biocompatible and biodegradable haemostatic sealing

Uncontrolled haemorrhage is a leading cause of trauma-related fatalities, highlighting the critical need for rapid and effective haemostasis. Current haemostatic materials encounter limitations such as slow clotting and weak mechanical strength, while most of bioadhesives compromise their adhesion performance to wet tissues for biocompatibility and degradability. In this study, a molecular self-assembly strategy is proposed, developing a biocompatible and biodegradable protein-based patch with excellent adhesion performance. This strategy utilizes fibrinogen modified with hydrophobic groups to induce self-assembly into a hydrogel, which is converted into a dry patch. The protein patch enhances adhesion performance on the wet tissue through a dry cross-linking method and robust intra/inter-molecular interactions. This patch demonstrates excellent haemostatic efficacy in  both porcine oozing wound and porcine severe acute haemorrhage. It maintains biological functionality, and ensures sustained wound sealing while gradually degrading in vivo, making it a promising candidate for clinical tissue sealing applications.

Capturing Dynamic Assembly of Nanoscale Proteins During Network Formation

The structural evolution of hierarchical structures of nanoscale biomolecules is crucial for the construction of functional networks in vivo and in vitro. Despite the ubiquity of these networks, the physical mechanisms behind their formation and self-assembly remains poorly understood. Here, this study uses photochemically cross-linked folded protein hydrogels as a model biopolymer network system, with a combined time-resolved rheology and small-angle x-ray scattering (SAXS) approach to probe both the load-bearing structures and network architectures respectively thereby providing a cross-length scale understanding of the network formation. Combining SAXS, rheology, and kinetic modeling, a dual formation mechanism consisting of a primary formation phase is proposed, where monomeric folded proteins create the preliminary protein network scaffold; and a subsequent secondary formation phase, where both additional intra-network cross-links form and larger oligomers diffuse to join the preliminary network, leading to a denser more mechanically robust structure. Identifying this as the origin of the structural and mechanical properties of protein networks creates future opportunities to understand hierarchical biomechanics in vivo and develop functional, designed-for-purpose, biomaterials.

Hydrogel-Based Strategies for Liver Tissue Engineering

The liver's role in metabolism, detoxification, and immune regulation underscores the urgency of addressing liver diseases, which claim millions of lives annually. Due to donor shortages in liver transplantation, liver tissue engineering (LTE) offers a promising alternative. Hydrogels, with their biocompatibility and ability to mimic the liver's extracellular matrix (ECM), support cell survival and function in LTE. This review analyzes recent advances in hydrogel-based strategies for LTE, including decellularized liver tissue hydrogels, natural polymer-based hydrogels, and synthetic polymer-based hydrogels. These materials are ideal for in vitro cell culture and obtaining functional hepatocytes. Hydrogels' tunable properties facilitate creating artificial liver models, such as organoids, 3D bioprinting, and liver-on-a-chip technologies. These developments demonstrate hydrogels' versatility in advancing LTE's applications, including hepatotoxicity testing, liver tissue regeneration, and treating acute liver failure. This review highlights the transformative potential of hydrogels in LTE and their implications for future research and clinical practice.

Electric-field assisted cascade reactions to create alginate/carboxymethyl chitosan composite hydrogels with gradient architecture and reconfigurable mechanical properties

Rational designs of polysaccharide-based hydrogels with organ-like three-dimensional architecture provide a great possibility for addressing the shortages of allograft tissues and organs. However, spatial-temporal control over structure in bulk hydrogel and acquire satisfied mechanical properties remain an intrinsic challenge to achieve. Here, we show how electric-field assisted molecular self-assembly can be coupled to a directional reaction-diffusion (RD) process to produce macroscopic hydrogel in a controllable manner. The electrical energy input was not only to generate complex molecule gradients and initiate the molecular self-assembly, but also to guide/facilitate the RD processes for the gel rapid growth via a cascade construction interaction. The hydrogel mechanical properties can be tuned and enhanced by using an interpenetrating biopolymer network and multiple ionic crosslinkers, leading to a wide-range of mechanical modulus to match with biological organs or tissues. We demonstrate diverse 3D macroscopic hydrogels can be easily prepared via field-assisted directional reaction-diffusion and specific joint interactions. The humility-triggered dissipation of functional gradients and antibacterial performance confirm that the hydrogels can serve as an optically variable soft device for wound management. Therefore, this work provides a general approach toward the rational fabrication of soft hydrogels with controlled architectures and functionality for advanced biomedical systems.

Tissue-Adaptive BSA Hydrogel with Dual Release of PTX and bFGF Promotes Spinal Cord Injury Repair via Glial Scar Inhibition and Axon Regeneration

Spinal cord injury (SCI) is a severe clinical disease usually accompanied by activated glial scar, neuronal axon rupture, and disabled motor function. To mimic the microenvironment of the SCI injury site, a hydrogel system with a comparable mechanical property to the spinal cord is desirable. Therefore, a novel elastic bovine serum albumin (BSA) hydrogel is fabricated with excellent adhesive, injectable, and biocompatible properties. The hydrogel is used to deliver paclitaxel (PTX) together with basic fibroblast growth factor (bFGF) to inhibit glial scar formation as well as promote axon regeneration and motor function for SCI repair. Due to the specific interaction of BSA with both drugs, bFGF, and PTX can be controllably released from the hydrogel system to achieve an effective concentration at the wound site during the SCI regeneration process. Moreover, benefiting from the combination of PTX and bFGF, this bFGF/PTX@BSA system significantly aided axon repair by promoting the elongation of axons across the glial scar with reduced reactive astrocyte secretion. In addition, remarkable anti-apoptosis of nerve cells is evident with the bFGF/PTX@BSA system. Subsequently, this multi-functionalized drug system significantly improved the motor function of the rats after SCI. These results reveal that bFGF/PTX@BSA is an ideal functionalized material for nerve repair in SCI.

Injectable, in-situ forming, tunable, biocompatible gelatin hydrogels for biomedical applications

Gelatin hydrogels have drawn attention for their diverse biomedical applications due to their flexible physiochemical properties. However, such gelatin hydrogels are made of toxic crosslinkers and photoinitiators, restricting their non-invasive deep tissue application. The in-situ forming chemical crosslinked without such toxic crosslinker and UV light has not been explored under physiological conditions. This study establishes a simple method to fabricate an injectable click-chemistry-based in-situ forming gelatin hydrogel in a physiological environment (without toxic UV or photoinitiator) with tunable physiochemical properties to modulate cellular response. Using Divinyl Sulfone (DVS) modification, gelatin hydrogel (GelVS) is optimized with tunable degradation properties, moduli (100 Pa -1000 Pa), gelation time, swelling, degradation, and viscoelastic behaviour. The in-vitro results using fibroblast and stem cells show that the hydrogel and its precursors were cytocompatible with diverging feedback of cells as the modulus varies. The in-vivo analysis for injectability, degradation, and biocompatibility of the GelVS hydrogel displays their biocompatible nature and lasts up to 30 days at the injecting site. Overall results indicate that DVS-modified GelVS hydrogel will be a great system with tunable physicochemical properties to modulate favorable cellular response for tissue regeneration and non-invasive deep tissue application.

Mechanical Unfolding of Network Nodes Drives the Stress Response of Protein-Based Materials

Biomaterials synthesized from cross-linked folded proteins have untapped potential for biocompatible, resilient, and responsive implementations, but face challenges due to costly molecular refinement and limited understanding of their mechanical response. Under a stress vector, these materials combine the gel-like response of cross-linked networks with the mechanical unfolding and extension of proteins from well-defined 3D structures to unstructured polypeptides. Yet the nanoscale dynamics governing their viscoelastic response remains poorly understood. This lack of understanding is further exacerbated by the fact that the mechanical stability of protein domains depends not only on their structure, but also on the direction of the force vector. To this end, here we propose a coarse-grained network model based on the physical characteristics of polyproteins and combine it with the mechanical unfolding response of protein domains, obtained from single molecule measurements and steered molecular dynamics simulations, to explain the macroscopic response of protein-based materials to a stress vector. We find that domains are about 10-fold more stable when force is applied along their end-to-end coordinate than along the other tethering geometries that are possible inside the biomaterial. As such, the macroscopic response of protein-based materials is mainly driven by the unfolding of the node-domains and rearrangement of these nodes inside the material. The predictions from our models are then confirmed experimentally using force-clamp rheometry. This model is a critical step toward developing protein-based materials with predictable response and that can enable applications for shape memory and energy storage and dissipation.

Engineered Protein Hydrogels as Biomimetic Cellular Scaffolds

The biochemical and biophysical properties of the extracellular matrix (ECM) play a pivotal role in regulating cellular behaviors such as proliferation, migration, and differentiation. Engineered protein-based hydrogels, with highly tunable multifunctional properties, have the potential to replicate key features of the native ECM. Formed by self-assembly or crosslinking, engineered protein-based hydrogels can induce a range of cell behaviors through bioactive and functional domains incorporated into the polymer backbone. Using recombinant techniques, the amino acid sequence of the protein backbone can be designed with precise control over the chain-length, folded structure, and cell-interaction sites. In this review, the modular design of engineered protein-based hydrogels from both a molecular- and network-level perspective are discussed, and summarize recent progress and case studies to highlight the diverse strategies used to construct biomimetic scaffolds. This review focuses on amino acid sequences that form structural blocks, bioactive blocks, and stimuli-responsive blocks designed into the protein backbone for highly precise and tunable control of scaffold properties. Both physical and chemical methods to stabilize dynamic protein networks with defined structure and bioactivity for cell culture applications are discussed. Finally, a discussion of future directions of engineered protein-based hydrogels as biomimetic cellular scaffolds is concluded.

Advancing Synthetic Hydrogels through Nature-Inspired Materials Chemistry

Synthetic extracellular matrix (ECM) mimics that can recapitulate the complex biochemical and mechanical nature of native tissues are needed for advanced models of development and disease. Biomedical research has heavily relied on the use of animal-derived biomaterials, which is now impeding their translational potential and convoluting the biological insights gleaned from in vitro tissue models. Natural hydrogels have long served as a convenient and effective cell culture tool, but advances in materials chemistry and fabrication techniques now present promising new avenues for creating xenogenic-free ECM substitutes appropriate for organotypic models and microphysiological systems. However, significant challenges remain in creating synthetic matrices that can approximate the structural sophistication, biochemical complexity, and dynamic functionality of native tissues. This review summarizes key properties of the native ECM, and discusses recent approaches used to systematically decouple and tune these properties in synthetic matrices. The importance of dynamic ECM mechanics, such as viscoelasticity and matrix plasticity, is also discussed, particularly within the context of organoid and engineered tissue matrices. Emerging design strategies to mimic these dynamic mechanical properties are reviewed, such as multi-network hydrogels, supramolecular chemistry, and hydrogels assembled from biological monomers.

Mechanical Regulation of Polymer Gels

The mechanical properties of polymer gels devote to emerging devices and machines in fields such as biomedical engineering, flexible bioelectronics, biomimetic actuators, and energy harvesters. Coupling network architectures and interactions has been explored to regulate supportive mechanical characteristics of polymer gels; however, systematic reviews correlating mechanics to interaction forces at the molecular and structural levels remain absent in the field. This review highlights the molecular engineering and structural engineering of polymer gel mechanics and a comprehensive mechanistic understanding of mechanical regulation. Molecular engineering alters molecular architecture and manipulates functional groups/moieties at the molecular level, introducing various interactions and permanent or reversible dynamic bonds as the dissipative energy. Molecular engineering usually uses monomers, cross-linkers, chains, and other additives. Structural engineering utilizes casting methods, solvent phase regulation, mechanochemistry, macromolecule chemical reactions, and biomanufacturing technology to construct and tailor the topological network structures, or heterogeneous modulus compositions. We envision that the perfect combination of molecular and structural engineering may provide a fresh view to extend exciting new perspectives of this burgeoning field. This review also summarizes recent representative applications of polymer gels with excellent mechanical properties. Conclusions and perspectives are also provided from five aspects of concise summary, mechanical mechanism, biofabrication methods, upgraded applications, and synergistic methodology.

Formation of Nanofibrillar Self-Healing Hydrogels Using Antimicrobial Peptides

The rise of drug-resistant microorganisms has prompted the development of innovative strategies with the aim of addressing this challenge. Among the alternative approaches gaining increased attention are antimicrobial peptides (AMPs), a group of peptides with the ability to combat microbial pathogens. Here, we investigated a small peptide, KLVFF, derived from the Alzheimer's amyloid-β (Aβ) protein. While Aβ has been associated with the development of neurodegenerative diseases, the core part of the Aβ protein, namely the Aβ 16-20 fragment, has also been exploited to obtain highly functional biomaterials. In this study we found that KLVFF is capable of self-assembling into a fibrillar network to form a self-healing hydrogel. Moreover, this small peptide can undergo a transition from a gel to a liquid state following application of shear stress, in a reversible manner. As an AMP, this material exhibited both antibacterial and antifungal properties while remaining highly biocompatible and noncytotoxic toward mammalian cells. The propensity of the KLVFF hydrogel to rapidly assemble into highly ordered macroscopic structures makes it an ideal candidate for biomedical applications necessitating antimicrobial activity, such as wound healing.

Mechanochemistry: Fundamental Principles and Applications

Mechanochemistry is an emerging research field at the interface of physics, mechanics, materials science, and chemistry. Complementary to traditional activation methods in chemistry, such as heat, electricity, and light, mechanochemistry focuses on the activation of chemical reactions by directly or indirectly applying mechanical forces. It has evolved as a powerful tool for controlling chemical reactions in solid state systems, sensing and responding to stresses in polymer materials, regulating interfacial adhesions, and stimulating biological processes. By combining theoretical approaches, simulations and experimental techniques, researchers have gained intricate insights into the mechanisms underlying mechanochemistry. In this review, the physical chemistry principles underpinning mechanochemistry are elucidated and a comprehensive overview of recent significant achievements in the discovery of mechanically responsive chemical processes is provided, with a particular emphasis on their applications in materials science. Additionally, The perspectives and insights into potential future directions for this exciting research field are offered.

Stabilization of Non-Native Folds and Programmable Protein Gelation in Compositionally Designed Deep Eutectic Solvents

Proteins are adjustable units from which biomaterials with designed properties can be developed. However, non-native folded states with controlled topologies are hardly accessible in aqueous environments, limiting their prospects as building blocks. Here, we demonstrate the ability of a series of anhydrous deep eutectic solvents (DESs) to precisely control the conformational landscape of proteins. We reveal that systematic variations in the chemical composition of binary and ternary DESs dictate the stabilization of a wide range of conformations, that is, compact globular folds, intermediate folding states, or unfolded chains, as well as controlling their collective behavior. Besides, different conformational states can be visited by simply adjusting the composition of ternary DESs, allowing for the refolding of unfolded states and vice versa. Notably, we show that these intermediates can trigger the formation of supramolecular gels, also known as eutectogels, where their mechanical properties correlate to the folding state of the protein. Given the inherent vulnerability of proteins outside the native fold in aqueous environments, our findings highlight DESs as tailorable solvents capable of stabilizing various non-native conformations on demand through solvent design.

A Handle-Free, All-Protein-Based Optical Tweezers Method to Probe Protein Folding-Unfolding Dynamics

Optical tweezers (OT) have evolved into powerful single molecule force spectroscopy tools to investigate protein folding-unfolding dynamics. To stretch a protein of interest using OT, the protein must be flanked with two double stranded DNA (dsDNA) handles. However, coupling dsDNA handles to the protein is often of low yield, representing a bottleneck in OT experiments. Here, we report a handle-free, all-protein-based OT method for investigating protein folding/unfolding dynamics. In this new method, we employed disordered elastin-like polypeptides (ELPs) as a molecular linker and the mechanically stable cohesin-dockerin (Coh-Doc) pair as the prey-bait system to enable the efficient capture and stretching of individual protein molecules. This novel approach was validated by using model proteins NuG2 and RTX-v, yielding experimental results comparable to those obtained by using the dsDNA handle approach. This new method provides a streamlined and efficient OT approach to investigate the folding-unfolding dynamics of proteins at the single molecule level, thus expanding the toolbox of OT-based single molecule force spectroscopy.

Artificial structural proteins: Synthesis, assembly and material applications

Structural proteins have evolved over billions of years and offer outstanding mechanical properties, such as resilience, toughness and stiffness. Advances in modular protein engineering, polypeptide modification, and synthetic biology have led to the development of novel biomimetic structural proteins to perform in biomedical and military fields. However, the development of customized structural proteins and assemblies with superior performance remains a major challenge, due to the inherent limitations of biosynthesis, difficulty in mimicking the complexed macroscale assembly, etc. This review summarizes the approaches for the design and production of biomimetic structural proteins, and their chemical modifications for multiscale assembly. Furthermore, we discuss the function tailoring and current applications of biomimetic structural protein assemblies. A perspective of future research is to reveal how the mechanical properties are encoded in the sequences and conformations. This review, therefore, provides an important reference for the development of structural proteins-mimetics from replication of nature to even outperforming nature.

Tough Hydrogels with Different Toughening Mechanisms and Applications

Load-bearing biological tissues, such as cartilage and muscles, exhibit several crucial properties, including high elasticity, strength, and recoverability. These characteristics enable these tissues to endure significant mechanical stresses and swiftly recover after deformation, contributing to their exceptional durability and functionality. In contrast, while hydrogels are highly biocompatible and hold promise as synthetic biomaterials, their inherent network structure often limits their ability to simultaneously possess a diverse range of superior mechanical properties. As a result, the applications of hydrogels are significantly constrained. This article delves into the design mechanisms and mechanical properties of various tough hydrogels and investigates their applications in tissue engineering, flexible electronics, and other fields. The objective is to provide insights into the fabrication and application of hydrogels with combined high strength, stretchability, toughness, and fast recovery as well as their future development directions and challenges.

ForceGen: End-to-end de novo protein generation based on nonlinear mechanical unfolding responses using a language diffusion model

Through evolution, nature has presented a set of remarkable protein materials, including elastins, silks, keratins and collagens with superior mechanical performances that play crucial roles in mechanobiology. However, going beyond natural designs to discover proteins that meet specified mechanical properties remains challenging. Here, we report a generative model that predicts protein designs to meet complex nonlinear mechanical property-design objectives. Our model leverages deep knowledge on protein sequences from a pretrained protein language model and maps mechanical unfolding responses to create proteins. Via full-atom molecular simulations for direct validation, we demonstrate that the designed proteins are de novo, and fulfill the targeted mechanical properties, including unfolding energy and mechanical strength, as well as the detailed unfolding force-separation curves. Our model offers rapid pathways to explore the enormous mechanobiological protein sequence space unconstrained by biological synthesis, using mechanical features as the target to enable the discovery of protein materials with superior mechanical properties.

De novo design of modular protein hydrogels with programmable intra- and extracellular viscoelasticity

Relating the macroscopic properties of protein-based materials to their underlying component microstructure is an outstanding challenge. Here, we exploit computational design to specify the size, flexibility, and valency of de novo protein building blocks, as well as the interaction dynamics between them, to investigate how molecular parameters govern the macroscopic viscoelasticity of the resultant protein hydrogels. We construct gel systems from pairs of symmetric protein homo-oligomers, each comprising 2, 5, 24, or 120 individual protein components, that are crosslinked either physically or covalently into idealized step-growth biopolymer networks. Through rheological assessment, we find that the covalent linkage of multifunctional precursors yields hydrogels whose viscoelasticity depends on the crosslink length between the constituent building blocks. In contrast, reversibly crosslinking the homo-oligomeric components with a computationally designed heterodimer results in viscoelastic biomaterials exhibiting fluid-like properties under rest and low shear, but solid-like behavior at higher frequencies. Exploiting the unique genetic encodability of these materials, we demonstrate the assembly of protein networks within living mammalian cells and show via fluorescence recovery after photobleaching (FRAP) that mechanical properties can be tuned intracellularly in a manner similar to formulations formed extracellularly. We anticipate that the ability to modularly construct and systematically program the viscoelastic properties of designer protein-based materials could have broad utility in biomedicine, with applications in tissue engineering, therapeutic delivery, and synthetic biology.

Natural and Engineered Isoforms of the Inflammasome Adaptor ASC Form Noncovalent, pH-Responsive Hydrogels

The protein ASC polymerizes into intricate filament networks to assemble the inflammasome, a filamentous multiprotein complex that triggers the inflammatory response. ASC carries two Death Domains integrally involved in protein self-association for filament assembly. We have leveraged this behavior to create noncovalent, pH-responsive hydrogels of full-length, folded ASC by carefully controlling the pH as a critical factor in the polymerization process. We show that natural variants of ASC (ASC isoforms) involved in inflammasome regulation also undergo hydrogelation. To further demonstrate this general capability, we engineered proteins inspired by the ASC structure that also form hydrogels. We analyzed the structural network of the natural and engineered protein hydrogels using transmission and scanning electron microscopy and studied their viscoelastic behavior using shear rheology. Our results reveal one of the very few examples of hydrogels created by the self-assembly of globular proteins and domains in their native conformation and show that Death Domains can be used alone or as building blocks to engineer bioinspired hydrogels.

Oyster-inspired carbon dots-functionalized silica and dialdehyde chitosan to fabricate a soy protein adhesive with high strength, mildew resistance, and long-term water resistance

Developing multifunctional adhesives with exceptional cold-pressing strength, water resistance, toughness, and mildew resistance remains challenging. Herein, inspired by oysters, a multifunctional organic-inorganic hybrid soybean meal (SM)-based adhesive was fabricated by incorporating amino-modified carbon dots functionalized silica nanoparticles (CDs@SiO2) and dialdehyde chitosan (DCS) into SM matrix. DCS effectively enhanced the interface interactions of organic-inorganic phases and the rigid nanofillers CDs@SiO2 uniformly dispersed in the SM matrix, which provided energy dissipation to improve the adhesive's toughness. Owing to the stiff skeleton structure and enhanced crosslinking density, the crosslinker-modified SM (MSM)/DCS/CDs@SiO2-2 wood adhesive exhibited outstanding cold-pressing strength (0.74 MPa), wet shear strength (1.36 MPa), and long-term water resistance (49 d). Additionally, the resultant adhesive showed superior antimildew and antibacterial properties benefiting from the introduction of DCS. Intriguingly, the fluorescent properties endowed by carbon dots further broadened the application of adhesives for realizing security testing. This study opens a new pathway for the synthesis of multifunctional biomass adhesives in industrial and household applications.

Bivalent Gadolinium Ions Forming Injectable Hydrogels for Simultaneous In Situ Vaccination Therapy and Imaging of Soft Tissue Sarcoma

Doxorubicin (DOX) is the classic soft tissue sarcomas (STS) first-line treatment drug, while dose-dependent myelosuppression and cardiotoxicity limit its application in clinic. This research intends to apply DOX, which is also an inducer of immunogenic cell death as a part for "in situ vaccination" and conjointly uses PD-1 inhibitors to enhance antitumor efficacy. In order to achieve the sustained vaccination effect and real-time monitoring of distribution in vivo, the in situ forming and injectable hydrogel platform with the function of visualization is established for local delivery. The hydrogel platform is synthesized by hyaluronic acid-dopamine coordinated with gadolinium ions (Gd2+ ). Gd2+ provides the ability of magnetic resonance imaging, meanwhile further cross-linking the hydrogel network. Experiments show excellent ability of sustained release and imaging tracking for the hydrogel platform. In mouse STS models, the "in situ vaccination" hydrogels show the best effect of inhibiting tumor growth. Further analysis of tumor tissues show that "in situ vaccination" group can increase T cell infiltration, promote M1-type macrophage polarization and block elevated PD-1/PD-L1 pathway caused by DOX. These results are expected to prove the potential for synthesized hydrogels to achieve a universal platform for "in situ vaccination" strategies on STS treatments.

Self-Assembly of Platelet Lysates Proteins into Microparticles by Unnatural Disulfide Bonds for Bottom-Up Tissue Engineering

There is a demand to design microparticles holding surface topography while presenting inherent bioactive cues for applications in the biomedical and biotechnological fields. Using the pool of proteins present in human-derived platelet lysates (PLs), the production of protein-based microparticles via a simple and cost-effective method is reported, exploring the prone redox behavior of cysteine (Cy-SH) amino acid residues. The forced formation of new intermolecular disulfide bonds results in the precipitation of the proteins as spherical, pompom-like microparticles with adjustable sizes (15-50 µm in diameter) and surface topography consisting of grooves and ridges. These PL microparticles exhibit extraordinary cytocompatibility, allowing cell-guided microaggregates to form, while also working as injectable systems for cell support. Early studies also suggest that the surface topography provided by these PL microparticles can support osteogenic behavior. Consequently, these PL microparticles may find use to create live tissues via bottom-up procedures or injectable tissue-defect fillers, particularly for bone regeneration, with the prospect of working under xeno-free conditions.

Next-generation protein-based materials capture and preserve projectiles from supersonic impacts

Extreme energy-dissipating materials are essential for a range of applications. The military and police force require ballistic armour to ensure the safety of their personnel, while the aerospace industry requires materials that enable the capture, preservation and study of hypervelocity projectiles. However, current industry standards display at least one inherent limitation, such as weight, breathability, stiffness, durability and failure to preserve captured projectiles. To resolve these limitations, we have turned to nature, using proteins that have evolved over millennia to enable effective energy dissipation. Specifically, a recombinant form of the mechanosensitive protein talin was incorporated into a monomeric unit and crosslinked, resulting in a talin shock-absorbing material (TSAM). When subjected to 1.5 km s-1 supersonic shots, TSAMs were shown to absorb the impact and capture and preserve the projectile.

Recent advance in bioactive hydrogels for repairing spinal cord injury: material design, biofunctional regulation, and applications

Functional hydrogels show potential application in repairing spinal cord injury (SCI) due to their unique chemical, physical, and biological properties and functions. In this comprehensive review, we present recent advance in the material design, functional regulation, and SCI repair applications of bioactive hydrogels. Different from previously released reviews on hydrogels and three-dimensional scaffolds for the SCI repair, this work focuses on the strategies for material design and biologically functional regulation of hydrogels, specifically aiming to show how these significant efforts can promoting the repairing performance of SCI. We demonstrate various methods and techniques for the fabrication of bioactive hydrogels with the biological components such as DNA, proteins, peptides, biomass polysaccharides, and biopolymers to obtain unique biological properties of hydrogels, including the cell biocompatibility, self-healing, anti-bacterial activity, injectability, bio-adhesion, bio-degradation, and other multi-functions for repairing SCI. The functional regulation of bioactive hydrogels with drugs/growth factors, polymers, nanoparticles, one-dimensional materials, and two-dimensional materials for highly effective treating SCI are introduced and discussed in detail. This work shows new viewpoints and ideas on the design and synthesis of bioactive hydrogels with the state-of-the-art knowledges of materials science and nanotechnology, and will bridge the connection of materials science and biomedicine, and further inspire clinical potential of bioactive hydrogels in biomedical fields.

Ester Bonds for Modulation of the Mechanical Properties of Protein Hydrogels

Hydrogels are soft materials constructed of physically or chemically crosslinked polymeric net-works with abundant water. The crosslinkers, as the mechanophores that bear and respond to mechanical forces, play a critical role in determining the mechanical properties of hydrogels. Here, we use a polyprotein as the crosslinker and mechanophore to form covalent polymer hydrogels in which the toughness and fatigue fracture are controlled by the mechanical unfolding of polyproteins. The protein Parvimonas sp. (ParV) is super stable and remains folded even at forces > 2 nN; however, it can unfold under loading forces of ~100 pN at basic pH values or low calcium concentrations due to destabilization of the protein structures. Through tuning the protein unfolding by pH and calcium concentrations, the hydrogel exhibits differences in modulus, strength, and anti-fatigue fracture. We found that due to the partially unfolding of ParV, the Young's modulus decreased at pH 9.0 or in the presence of EDTA (Ethylene Diamine Tetraacetic Acid), moreover, because partially unfolded ParV can be further completely unfolded due to the mechanically rupture of ester bond, leading to the observed hysteresis of the stretching and relaxation traces of the hydrogels, which is in line with single-molecule force spectroscopy experiments. These results display a new avenue for designing pH- or calcium-responsive hydrogels based on proteins and demonstrate the relationship between the mechanical properties of single molecules and macroscopic hydrogel networks.

De novo design of modular protein hydrogels with programmable intra- and extracellular viscoelasticity

Relating the macroscopic properties of protein-based materials to their underlying component microstructure is an outstanding challenge. Here, we exploit computational design to specify the size, flexibility, and valency of de novo protein building blocks, as well as the interaction dynamics between them, to investigate how molecular parameters govern the macroscopic viscoelasticity of the resultant protein hydrogels. We construct gel systems from pairs of symmetric protein homo-oligomers, each comprising 2, 5, 24, or 120 individual protein components, that are crosslinked either physically or covalently into idealized step-growth biopolymer networks. Through rheological assessment and molecular dynamics (MD) simulation, we find that the covalent linkage of multifunctional precursors yields hydrogels whose viscoelasticity depends on the crosslink length between the constituent building blocks. In contrast, reversibly crosslinking the homo-oligomeric components with a computationally designed heterodimer results in non-Newtonian biomaterials exhibiting fluid-like properties under rest and low shear, but shear-stiffening solid-like behavior at higher frequencies. Exploiting the unique genetic encodability of these materials, we demonstrate the assembly of protein networks within living mammalian cells and show via fluorescence recovery after photobleaching (FRAP) that mechanical properties can be tuned intracellularly, in correlation with matching formulations formed extracellularly. We anticipate that the ability to modularly construct and systematically program the viscoelastic properties of designer protein-based materials could have broad utility in biomedicine, with applications in tissue engineering, therapeutic delivery, and synthetic biology.

Cartilage-like protein hydrogels engineered via entanglement

Load-bearing tissues, such as muscle and cartilage, exhibit high elasticity, high toughness and fast recovery, but have different stiffness (with cartilage being significantly stiffer than muscle)^1-8. Muscle achieves its toughness through finely controlled forced domain unfolding-refolding in the muscle protein titin, whereas articular cartilage achieves its high stiffness and toughness through an entangled network comprising collagen and proteoglycans. Advancements in protein mechanics and engineering have made it possible to engineer titin-mimetic elastomeric proteins and soft protein biomaterials thereof to mimic the passive elasticity of muscle^9-11. However, it is more challenging to engineer highly stiff and tough protein biomaterials to mimic stiff tissues such as cartilage, or develop stiff synthetic matrices for cartilage stem and progenitor cell differentiation¹². Here we report the use of chain entanglements to significantly stiffen protein-based hydrogels without compromising their toughness. By introducing chain entanglements¹³ into the hydrogel network made of folded elastomeric proteins, we are able to engineer highly stiff and tough protein hydrogels, which seamlessly combine mutually incompatible mechanical properties, including high stiffness, high toughness, fast recovery and ultrahigh compressive strength, effectively converting soft protein biomaterials into stiff and tough materials exhibiting mechanical properties close to those of cartilage. Our study provides a general route towards engineering protein-based, stiff and tough biomaterials, which will find applications in biomedical engineering, such as osteochondral defect repair, and material sciences and engineering.

Natural and engineered isoforms of the inflammasome adaptor ASC form non-covalent, pH-responsive hydrogels

The protein ASC polymerizes into intricate filament networks to assemble the inflammasome, a filamentous multiprotein complex that triggers the inflammatory response. ASC carries two Death Domains integrally involved in protein self-association for filament assembly. We have leveraged this behavior to create non-covalent, pH-responsive hydrogels of full-length, folded ASC by carefully controlling the pH as a critical factor in the polymerization process. We show that natural variants of ASC (ASC isoforms) involved in inflammasome regulation also undergo hydrogelation. To further demonstrate this general capability, we engineered proteins inspired in the ASC structure that successfully form hydrogels. We analyzed the structural network of the natural and engineered protein hydrogels using transmission and scanning electron microscopy, and studied their viscoelastic behavior by shear rheology. Our results reveal one of the very few examples of hydrogels created by the self-assembly of globular proteins and domains in their native conformation and show that Death Domains can be used alone or as building blocks to engineer bioinspired hydrogels.

Anisotropic, Hydrogel Microparticles as pH-Responsive Drug Carriers for Oral Administration of 5-FU

In the last 20 years, the development of stimuli-responsive drug delivery systems (DDS) has received great attention. Hydrogel microparticles represent one of the candidates with the most potential. However, if the role of the cross-linking method, polymer composition, and concentration on their performance as DDS has been well-studied, still, a lot needs to be explained regarding the effect caused by the morphology. To investigate this, herein, we report the fabrication of PEGDA-ALMA-based microgels with spherical and asymmetric shapes for 5-fluorouracil (5-FU) on-demand loading and in vitro pH-triggered release. Due to anisotropic properties, the asymmetric particles showed an increased drug adsorption and higher pH responsiveness, which in turn led to a higher desorption efficacy at the target pH environment, making them an ideal candidate for oral administration of 5-FU in colorectal cancer. The cytotoxicity of empty spherical microgels was higher than the cytotoxicity of empty asymmetric microgels, suggesting that the gel network's mechanical proprieties of anisotropic particles were a better three-dimensional environment for the vital functions of cells. Upon treatment with drug-loaded microgels, the HeLa cells' viability was lower after incubation with asymmetric particles, confirming a minor release of 5-FU from spherical particles.

Single-Chain Inherent Elasticity of Macromolecules: From Concept to Applications

"The Tao begets the One. One begets all things of the world." These words of wisdom from Tao Te Ching are of great inspiration to scientists in polymer materials science and engineering: The "One" means an individual polymer chain while polymer materials consist of numerous chains. The understanding of the single-chain mechanics of polymers is crucial for the bottom-up rational design of polymer materials. With a backbone and side chains, a polymer chain is more complex than a small molecule. Moreover, an individual polymer chain is usually placed in a complicated environment (such as solvent, cosolute, and solid surface), which significantly affects the behaviors of the chain. With all these factors, it is hard to fully understand the elastic behaviors of polymers. Herein, we will first introduce the concept of the single-chain inherent elasticity of polymers, which is a fundamental property determined by the polymer backbone. Then, the applications of inherent elasticity in quantifying the effects of side chains and surrounding environment will be summarized. Finally, the challenges in related fields at present and potential research directions in the future will be discussed.

Biocompatible, Resilient, and Tough Nanocellulose Tunable Hydrogels

Hydrogels have been proposed as potential candidates for many different applications. However, many hydrogels exhibit poor mechanical properties, which limit their applications. Recently, various cellulose-derived nanomaterials have emerged as attractive candidates for nanocomposite-reinforcing agents due to their biocompatibility, abundance, and ease of chemical modification. Due to abundant hydroxyl groups throughout the cellulose chain, the grafting of acryl monomers onto the cellulose backbone by employing oxidizers such as cerium(IV) ammonium nitrate ([NH₄]₂[Ce(NO₃)₆], CAN) has proven a versatile and effective method. Moreover, acrylic monomers such as acrylamide (AM) may also polymerize by radical methods. In this work, cerium-initiated graft polymerization was applied to cellulose-derived nanomaterials, namely cellulose nanocrystals (CNC) and cellulose nanofibrils (CNF), in a polyacrylamide (PAAM) matrix to fabricate hydrogels that display high resilience (~92%), high tensile strength (~0.5 MPa), and toughness (~1.9 MJ/m³). We propose that by introducing mixtures of differing ratios of CNC and CNF, the composite's physical behavior can be fine-tuned across a wide range of mechanical and rheological properties. Moreover, the samples proved to be biocompatible when seeded with green fluorescent protein (GFP)-transfected mouse fibroblasts (3T3s), showing a significant increase in cell viability and proliferation compared to samples comprised of acrylamide alone.

Orchestrating Chemical and Physical Cross-Linking in Protein Hydrogels to Regulate Embryonic Stem Cell Growth

Protein hydrogels are ideal candidates for next-generation biomaterials due to their genetically programmable properties. Herein, we report an entirely protein-based hydrogel as an artificial extracellular matrix (ECM) for regulating the embryonic stem cell growth. A synergy between chemical and physical cross-linking was achieved in one step by SpyTag/SpyCatcher reaction and P zipper association at 37 °C. The hydrogels' stress relaxation behaviors can be tuned across a broad spectrum by single-point mutation on a P zipper. It has been found that faster relaxation can promote the growth of HeLa tumor spheroids and embryonic stem cells, and mechanical regulation of embryonic stem cells occurs via retention of the cells at the G1 phase. The results highlight the promise of genetically encoded protein materials as a platform of artificial ECM for understanding and controlling the complex cell-matrix interactions in a 3D cell culture.

Nanomaterial-based biohybrid hydrogel in bioelectronics

Despite the broadly applicable potential in the bioelectronics, organic/inorganic material-based bioelectronics have some limitations such as hard stiffness and low biocompatibility. To overcome these limitations, hydrogels capable of bridging the interface and connecting biological materials and electronics have been investigated for development of hydrogel bioelectronics. Although hydrogel bioelectronics have shown unique properties including flexibility and biocompatibility, there are still limitations in developing novel hydrogel bioelectronics using only hydrogels such as their low electrical conductivity and structural stability. As an alternative solution to address these issues, studies on the development of biohybrid hydrogels that incorporating nanomaterials into the hydrogels have been conducted for bioelectronic applications. Nanomaterials complement the shortcomings of hydrogels for bioelectronic applications, and provide new functionality in biohybrid hydrogel bioelectronics. In this review, we provide the recent studies on biohybrid hydrogels and their bioelectronic applications. Firstly, representative nanomaterials and hydrogels constituting biohybrid hydrogels are provided, and next, applications of biohybrid hydrogels in bioelectronics categorized in flexible/wearable bioelectronic devices, tissue engineering, and biorobotics are discussed with recent studies. In conclusion, we strongly believe that this review provides the latest knowledge and strategies on hydrogel bioelectronics through the combination of nanomaterials and hydrogels, and direction of future hydrogel bioelectronics.

Protein-engineered biomaterials for cartilage therapeutics and repair

Cartilage degeneration and injury are major causes of pain and disability that effect millions, and yet treatment options for conditions like osteoarthritis (OA) continue to be mainly palliative or involve complete replacement of injured joints. Several biomaterial strategies have been explored to address cartilage repair either by the delivery of therapeutics or as support for tissue repair, however the complex structure of cartilage tissue, its mechanical needs, and lack of regenerative capacity have hindered this goal. Recent advances in synthetic biology have opened new possibilities for engineered proteins to address these unique needs. Engineered protein and peptide-based materials benefit from inherent biocompatibility and nearly unlimited tunability as they utilize the body's natural building blocks to fabricate a variety of supramolecular structures. The pathophysiology and needs of OA cartilage are presented here, along with an overview of the current state of the art and next steps for protein-engineered repair strategies for cartilage.

Directed assembly of genetically engineered eukaryotic cells into living functional materials via ultrahigh-affinity protein interactions

Engineered living materials (ELMs) are gaining traction among synthetic biologists, as their emergent properties and nonequilibrium thermodynamics make them markedly different from traditional materials. However, the aspiration to directly use living cells as building blocks to create higher-order structures or materials, with no need for chemical modification, remains elusive to synthetic biologists. Here, we report a strategy that enables the assembly of engineered Saccharomyces cerevisiae into self-propagating ELMs via ultrahigh-affinity protein/protein interactions. These yeast cells have been genetically engineered to display the protein pairs SpyTag/SpyCatcher or CL7/Im7 on their surfaces, which enable their assembly into multicellular structures capable of further growth and proliferation. The assembly process can be controlled precisely via optical tweezers or microfluidics. Moreover, incorporation of functional motifs such as super uranyl-binding protein and mussel foot proteins via genetic programming rendered these materials suitable for uranium extraction from seawater and bioadhesion, respectively, pointing to their potential in chemical separation and biomedical applications.

Construction of Sol-Gel Phase-Reversible Hydrogels with Tunable Properties with Native Nanofibrous Protein as Building Blocks

Reversible sol-gel transforming behaviors combined with tunable mechanical properties are vital demands for developing biomaterials. However, it remains challenging to correlate these properties with the hydrogels constructed by denatured protein as building blocks. Herein, taking advantage of naturally high-affinity coordination environments consisting of i, i + 4 His-Glu motifs offered by paramyosin, a ubiquitous nanofibrous protein, we found that Zn²⁺ rather than Ca²⁺ or Mg²⁺ has the ability to trigger the self-assembly of native abalone paramyosin (AbPM) into protein hydrogels under benign conditions, while the addition of EDTA induces the hydrogels back into protein monomers, indicative of a reversible process. By using such sol-gel reversible property, the AbPM gels can serve as a vehicle to encapsulate bioactive molecules such as curcumin, thereby protecting it from degradation from thermal and photo treatment. Notably, based on the high conserved structure of native AbPM, the mechanical property and biological activity of the fabricated AbPM hydrogels can be fined-tuned by its noncovalent interaction with small molecules. All these findings raise the possibility that native paramyosin can be explored as a new class of protein hydrogels which exhibit favorable properties that the traditional hydrogels constructed by denatured protein building blocks do not have.

Non-Covalently Associated Streptavidin Multi-Arm Nanohubs Exhibit Mechanical and Thermal Stability in Cross-Linked Protein-Network Materials

Constructing protein-network materials that exhibit physicochemical and mechanical properties of individual protein constituents requires molecular cross-linkers with specificity and stability. A well-known example involves specific chemical fusion of a four-arm polyethylene glycol (tetra-PEG) to desired proteins with secondary cross-linkers. However, it is necessary to investigate tetra-PEG-like biomolecular cross-linkers that are genetically fused to the proteins, simplifying synthesis by removing additional conjugation and purification steps. Non-covalently, self-associating, streptavidin homotetramer is a viable, biomolecular alternative to tetra-PEG. Here, a multi-arm streptavidin design is characterized as a protein-network material platform using various secondary, biomolecular cross-linkers, such as high-affinity physical (i.e., non-covalent), transient physical, spontaneous chemical (i.e., covalent), or stimuli-induced chemical cross-linkers. Stimuli-induced, chemical cross-linkers fused to multi-arm streptavidin nanohubs provide sufficient diffusion prior to initiating permanent covalent bonds, allowing proper characterization of streptavidin nanohubs. Surprisingly, non-covalently associated streptavidin nanohubs exhibit extreme stability, which translates into material properties that resemble hydrogels formed by chemical bonds even at high temperatures. Therefore, this study not only establishes that the streptavidin nanohub is an ideal multi-arm biopolymer precursor but also provides valuable guidance for designing self-assembling nanostructured molecular networks that can properly harness the extraordinary properties of protein-based building blocks.

Acetic Acid Enables Precise Tailoring of the Mechanical Behavior of Protein-Based Hydrogels

Engineering viscoelastic and biocompatible materials with tailored mechanical and microstructure properties capable of mimicking the biological stiffness (<17 kPa) or serving as bioimplants will bring protein-based hydrogels to the forefront in the biomaterials field. Here, we introduce a method that uses different concentrations of acetic acid (AA) to control the covalent tyrosine-tyrosine cross-linking interactions at the nanoscale level during protein-based hydrogel synthesis and manipulates their mechanical and microstructure properties without affecting protein concentration and (un)folding nanomechanics. We demonstrated this approach by adding AA as a precursor to the preparation buffer of a photoactivated protein-based hydrogel mixture. This strategy allowed us to synthesize hydrogels made from bovine serum albumin (BSA) and eight repeats protein L structure, with a fine-tailored wide range of stiffness (2-35 kPa). Together with protein engineering technologies, this method will open new routes in developing and investigating tunable protein-based hydrogels and extend their application toward new horizons.

Protein scaffolds in human clinics

Fundamental clinical areas such as drug delivery and regenerative medicine require biocompatible materials as mechanically stable scaffolds or as nanoscale drug carriers. Among the wide set of emerging biomaterials, polypeptides offer enticing properties over alternative polymers, including full biocompatibility, biodegradability, precise interactivity, structural stability and conformational and functional versatility, all of them tunable by conventional protein engineering. However, proteins from non-human sources elicit immunotoxicities that might bottleneck further development and narrow their clinical applicability. In this context, selecting human proteins or developing humanized protein versions as building blocks is a strict demand to design non-immunogenic protein materials. We review here the expanding catalogue of human or humanized proteins tailored to execute different levels of scaffolding functions and how they can be engineered as self-assembling materials in form of oligomers, polymers or complex networks. In particular, we emphasize those that are under clinical development, revising their fields of applicability and how they have been adapted to offer, apart from mere mechanical support, highly refined functions and precise molecular interactions.

What Is the Force-per-Molecule Inside a Biomaterial Having Randomly Oriented Units?

Both synthetic and natural protein-based materials are made of randomly oriented cross-linked molecules. Here we introduce a coarse-grained approach to estimate the average force-per-molecule for materials made from globular proteins. Our approach has three steps: placement of molecules inside a unit volume, cross-linking, and trimming to remove the protein domains that do not participate to the force response. Following this procedure, we estimate the number of active domains per cross-section area, that allows for a direct calculation of the force-per-domain. Among the variables considered, we found that concentration was the most sensitive parameter. We then synthesized protein hydrogels made from BSA and polyprotein L and measured the stresses that these materials can withstand. We found that forces-per-molecules of up to 17 pN per domain can be obtained experimentally using protein hydrogels. Our approach represents an important step toward understanding the scaling of tension in biomaterials.

Biomimetic Hydrogels in the Study of Cancer Mechanobiology: Overview, Biomedical Applications, and Future Perspectives

Hydrogels are biocompatible polymers that are tunable to the system under study, allowing them to be widely used in medicine, bioprinting, tissue engineering, and biomechanics. Hydrogels are used to mimic the three-dimensional microenvironment of tissues, which is essential to understanding cell–cell interactions and intracellular signaling pathways (e.g., proliferation, apoptosis, growth, and survival). Emerging evidence suggests that the malignant properties of cancer cells depend on mechanical cues that arise from changes in their microenvironment. These mechanobiological cues include stiffness, shear stress, and pressure, and have an impact on cancer proliferation and invasion. The hydrogels can be tuned to simulate these mechanobiological tissue properties. Although interest in and research on the biomedical applications of hydrogels has increased in the past 25 years, there is still much to learn about the development of biomimetic hydrogels and their potential applications in biomedical and clinical settings. This review highlights the application of hydrogels in developing pre-clinical cancer models and their potential for translation to human disease with a focus on reviewing the utility of such models in studying glioblastoma progression.

Tuning Protein Hydrogel Mechanics through Modulation of Nanoscale Unfolding and Entanglement in Postgelation Relaxation

Globular folded proteins are versatile nanoscale building blocks to create biomaterials with mechanical robustness and inherent biological functionality due to their specific and well-defined folded structures. Modulating the nanoscale unfolding of protein building blocks during network formation (in situ protein unfolding) provides potent opportunities to control the protein network structure and mechanics. Here, we control protein unfolding during the formation of hydrogels constructed from chemically cross-linked maltose binding protein using ligand binding and the addition of cosolutes to modulate protein kinetic and thermodynamic stability. Bulk shear rheology characterizes the storage moduli of the bound and unbound protein hydrogels and reveals a correlation between network rigidity, characterized as an increase in the storage modulus, and protein thermodynamic stability. Furthermore, analysis of the network relaxation behavior identifies a crossover from an unfolding dominated regime to an entanglement dominated regime. Control of in situ protein unfolding and entanglement provides an important route to finely tune the architecture, mechanics, and dynamic relaxation of protein hydrogels. Such predictive control will be advantageous for future smart biomaterials for applications which require responsive and dynamic modulation of mechanical properties and biological function.

Intramolecular covalent bonds in Gram-positive bacterial surface proteins

Gram-positive bacteria experience considerable mechanical perturbation when adhering to host surfaces during colonization and infection. They have evolved various adhesion proteins that are mechanically robust to ensure strong surface adhesion. Recently, it was discovered that these adhesion proteins contain rare, extra intramolecular covalent bonds that stabilize protein structures and participate in surface bonding. These intramolecular covalent bonds include isopeptides, thioesters, and ester bonds, which often form spontaneously without the need for additional enzymes. With the development of single-molecule force spectroscopy techniques, the detailed mechanical roles of these intramolecular covalent bonds have been revealed. In this review, we summarize the recent advances in this area of research, focusing on the link between the mechanical stability and function of these covalent bonds in Gram-positive bacterial surface proteins. We also highlight the potential impact of these discoveries on the development of novel antibiotics and chemical biology tools.