Genetically Programming Stress-Relaxation Behavior in Entirely Protein-Based Molecular Networks

One-Pot Fabrication of Supramolecular Synthetic Protein Hydrogel with Tissue-like Integrated Dynamic Features

Protein-incorporated soft networks have received remarkable attention during the past several years. They possess desirable properties similar to native tissues and organs and exhibit unique advantages in applications. However, fabrication of protein-based hydrogels usually suffers from complex protein mutation and modification or chemical synthesis, which limited the scale and yield of production. Meanwhile, the lack of rationally designed noncovalent interactions in networks may result in a deficiency of the dynamic features of materials. Therefore, a highly efficient method is needed to include supramolecular interactions into protein hydrogel to generate a highly dynamic hydrogel possessing integrated tissue-like properties. Here, we report the design and construction of native protein-based supramolecular synthetic protein hydrogels through a simple and efficient one-pot polymerization of acrylamide and ligand monomers in the presence of a ligand-binding protein. The supramolecular interactions in the network yield integrated dynamic properties, including remarkable stretchability over 10,000% of their original length, ultrafast self-healing abilities within 3-4 s, tissue-like fast stress relaxation, satisfactory ability of adhesion to different living and nonliving substrates, injectability, and high biocompatibility. Furthermore, this material demonstrated potential as a biosensor to monitor small finger movements. This strategy provides a new avenue for fabricating synthetic protein hydrogels with integrated features.

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.

Woven Polymer Networks: From Crystalline to Elastomeric Materials

Weaving technology has been extensively used for manufacturing macroscopic fabrics and satisfying the artistic demands of humans through the ages. Integrating woven geometries into molecular structures is a persistent pursuit, and yet a significant challenge to chemists, owing to the lack of effective methodologies to guide the regular mutual interlacing of molecular strands. In this Concept article, recent progress and related strategies in constructing woven polymer networks (WPNs) are summarized and discussed. An outlook is then given to highlight the future opportunities and challenges in the development of both molecularly woven structures and molecularly woven functional materials.

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.

B12-induced reassembly of split photoreceptor protein enables photoresponsive hydrogels with tunable mechanics

Although the tools based on split proteins have found broad applications, ranging from controlled biological signaling to advanced molecular architectures, many of them suffer from drawbacks such as background reassembly, low thermodynamic stability, and static structural features. Here, we present a chemically inducible protein assembly method enabled by the dissection of the carboxyl-terminal domain of a B12-dependent photoreceptor, CarHC. The resulting segments reassemble efficiently upon addition of cobalamin (AdoB12, MeB12, or CNB12). Photolysis of the cofactors such as AdoB12 and MeB12 further leads to stable protein adducts harboring a bis-His-ligated B12. Split CarHC enables the creation of a series of protein hydrogels, of which the mechanics can be either photostrengthened or photoweakened, depending on the type of B12. These materials are also well suited for three dimensional cell culturing. Together, this new protein chemistry, featuring negligible background autoassembly, stable conjugation, and phototunability, has opened up opportunities for designing smart materials.

Ligand Mobility-Mediated Cell Adhesion and Spreading

Cells live in a highly dynamic environment where their physical connection and communication with the outside are achieved through receptor-ligands binding. Therefore, a precise knowledge of the interaction between receptors and ligands is critical for our understanding of how cells execute different biological duties. Interestingly, recent evidence has shown that the mobility of ligands at the cell-extracellular matrix (ECM) interface significantly affects the adhesion and spreading of cells, while the underlying mechanism remains unclear. Here, we present a modeling investigation to address this critical issue. Specifically, by adopting the Langevin dynamics, the random movement of ligands was captured by assigning a stochastic force along with a viscous drag on them. After that, the evolution of adhesion and subsequent spreading of cells were analyzed by considering the force-regulated binding/breakage of individual molecular bonds connecting polymerizing actin bundles inside the cell to the ECM. Interestingly, a biphasic relationship between adhesion and ligand diffusivity was predicted, resulting in maximized cell spreading at intermediate mobility of ligand molecules. In addition, this peak position was found to be dictated by the aggregation of ligands, effectively reducing their diffusivity, and how fast bond association/dissociation can occur. These predictions are in excellent agreement with our experimental observations where distinct ligand mobility was introduced by tuning the interactions between the self-assembly polymer coating and the surface.

From 4-arm star proteins to diverse stimuli-responsive molecular networks enabled by orthogonal genetically encoded click chemistries

The integrated use of genetically encoded click chemistries and protein topology engineering enabled the creation of various smart protein hydrogels.

Thermo-Viscoelastic Response of Protein-Based Hydrogels

Because of the bioactivity and biocompatibility of protein-based gels and the reversible nature of bonds between associating coiled coils, these materials demonstrate a wide spectrum of potential applications in targeted drug delivery, tissue engineering, and regenerative medicine. The kinetics of rearrangement (association and dissociation) of the physical bonds between chains has been traditionally studied in shear relaxation tests and small-amplitude oscillatory tests. A characteristic feature of recombinant protein gels is that chains in the polymer network are connected by temporary bonds between the coiled coil complexes and permanent cross-links between functional groups of amino acids. A simple model is developed for the linear viscoelastic behavior of protein-based gels. Its advantage is that, on the one hand, the model only involves five material parameters with transparent physical meaning and, on the other, it correctly reproduces experimental data in shear relaxation and oscillatory tests. The model is applied to study the effects of temperature, the concentration of proteins, and their structure on the viscoelastic response of hydrogels.

Injectable, photoresponsive hydrogels for delivering neuroprotective proteins enabled by metal-directed protein assembly

Axon regeneration constitutes a fundamental challenge for regenerative neurobiology, which necessitates the use of tailor-made biomaterials for controllable delivery of cells and biomolecules. An increasingly popular approach for creating these materials is to directly assemble engineered proteins into high-order structures, a process that often relies on sophisticated protein chemistry. Here, we present a simple approach for creating injectable, photoresponsive hydrogels via metal-directed assembly of His6-tagged proteins. The B12-dependent photoreceptor protein CarHC can complex with transition metal ions through an amino-terminal His6-tag, which can further undergo a sol-gel transition upon addition of AdoB12, leading to the formation of hydrogels with marked injectability and photodegradability. The inducible phase transitions further enabled facile encapsulation and release of cells and proteins. Injecting the Zn2+-coordinated gels decorated with leukemia inhibitory factor into injured mouse optic nerves led to prolonged cellular signaling and enhanced axon regeneration. This study illustrates a powerful strategy for designing injectable biomaterials.

Recent advances in bio-orthogonal and dynamic crosslinking of biomimetic hydrogels

In recent years, dynamic, 'click' hydrogels have been applied in numerous biomedical applications. Owing to the mild, cytocompatible, and highly specific reaction kinetics, a multitude of orthogonal handles have been developed for fabricating dynamic hydrogels to facilitate '4D' cell culture. The high degree of tunability in crosslinking reactions of orthogonal 'click' chemistry has enabled a bottom-up approach to install specific biomimicry in an artificial extracellular matrix. In addition to click chemistry, highly specific enzymatic reactions are also increasingly used for network crosslinking and for spatiotemporal control of hydrogel properties. On the other hand, covalent adaptable chemistry has been used to recapitulate the viscoelastic component of biological tissues and for formulating self-healing and shear-thinning hydrogels. The common feature of these three classes of chemistry (i.e., orthogonal click chemistry, enzymatic reactions, and covalent adaptable chemistry) is that they can be carried out under ambient and aqueous conditions, a prerequisite for maintaining cell viability for in situ cell encapsulation and post-gelation modification of network properties. Due to their orthogonality, different chemistries can also be applied sequentially to provide additional biochemical and mechanical control to guide cell behavior. Herein, we review recent advances in the use of orthogonal click chemistry, enzymatic reactions, and covalent adaptable chemistry for the development of dynamically tunable and biomimetic hydrogels.

Design of Bio-Conjugated Hydrogels for Regenerative Medicine Applications: From Polymer Scaffold to Biomolecule Choice

Bio-conjugated hydrogels merge the functionality of a synthetic network with the activity of a biomolecule, becoming thus an interesting class of materials for a variety of biomedical applications. This combination allows the fine tuning of their functionality and activity, whilst retaining biocompatibility, responsivity and displaying tunable chemical and mechanical properties. A complex scenario of molecular factors and conditions have to be taken into account to ensure the correct functionality of the bio-hydrogel as a scaffold or a delivery system, including the polymer backbone and biomolecule choice, polymerization conditions, architecture and biocompatibility. In this review, we present these key factors and conditions that have to match together to ensure the correct functionality of the bio-conjugated hydrogel. We then present recent examples of bio-conjugated hydrogel systems paving the way for regenerative medicine applications.

Woven Polymer Networks via the Topological Transformation of a [2]Catenane

Weaving technology has been widely used to manufacture macroscopic fabrics to meet the artistic and practical needs of humanity for thousands of years. However, the fabrication of molecular fabrics with fascinating topologies and unique mechanical properties represents a significant challenge. Herein, we describe a topological transformation strategy to construct woven polymer networks (WPNs) at the molecular level via ring-opening metathesis polymerization (ROMP) of a zinc-template [2]catenane. The key feature of this approach is the exploitation of the pre-existing catenane crossing points that maintain the dense woven structure and the flexible alkyl chains on the [2]catenane that synergistically work with the crossing points to modulate the physicochemical and mechanical properties of the woven materials. As a result, the WPN possesses a certain degree of flexibility and stretchability, as well as high thermostability and mechanical robustness. Furthermore, we could remove the zinc ions to endow the WPN with more degrees of freedom and then enhance its mechanical behaviors by remetalation. This study not only provides a novel strategy toward woven materials with intriguing structural features and emergent mechanical adaptivities, but also highlights that mechanically interlocked molecules could offer unique opportunities for the construction of smart supramolecular materials with peculiar interlaced topologies at the molecular scale.

100th Anniversary of Macromolecular Science Viewpoint: Synthetic Protein Hydrogels

Our bodies are composed of soft tissues made of various proteins. In contrast, most hydrogels designed for biological applications are made of synthetic polymers. Recently, it is increasingly recognized that genetically synthesized proteins can be tailored as building blocks of hydrogels with biological, chemical, and mechanical properties similar to native soft tissues. In this Viewpoint, we summarize recent progress in synthetic protein hydrogels. We compare the structural and mechanical properties of different protein building blocks. We discuss various biocompatible cross-linking strategies based on covalent chemical reactions and noncovalent physical interactions. We introduce how stimulus-responsive conformational changes or intermolecular interactions at the molecular level can be used to engineer responsive hydrogels. We highlight that hydrogel network structures are as important as the protein sequences for the properties and functions of protein hydrogels and should be carefully designed. Despite great progress and potentials of synthetic protein hydrogels, there are still quite a few unsettled challenges and unexploited opportunities, providing abundant room for future investigation and development, particularly as this field is quickly expanding beyond its initial stage. We discuss a number of possible directions, including optimizing protein production and reducing cost, engineering anisotropic hydrogels to better mimic native tissues, rationally designing hydrogel mechanical properties, investigating interplays of hydrogels and residing cells for 3D cell culture and organoid construction, and evaluating long-term cytotoxicity and immune response.

A review on recent advances in polymer and peptide hydrogels

In this review, we focus on the very recent developments on the use of the stimuli responsive properties of polymer hydrogels for targeted drug delivery, tissue engineering, and biosensing utilizing their different optoelectronic properties. Besides, the stimuli-responsive hydrogels, the conducting polymer hydrogels are discussed, with specific attention to the energy generation and storage behavior of the xerogel derived from the hydrogel. The electronic and ionic conducting gels have been discussed that have applications in various electronic devices, e.g., organic field effect transistors, soft robotics, ionic skins, and sensors. The properties of polymer hybrid gels containing carbon nanomaterials have been exemplified here giving attention to applications in supercapacitors, dye sensitized solar cells, photocurrent switching, etc. Recent trends in the properties and applications of some natural polymer gels to produce thermal and acoustic insulating materials, drug delivery vehicles, self-healing material, tissue engineering, etc., are discussed. Besides the polymer gels, peptide gels of different dipeptides, tripeptides, oligopeptides, polypeptides, cyclic peptides, etc., are discussed, giving attention mainly to biosensing, bioimaging, and drug delivery applications. The properties of peptide-based hybrid hydrogels with polymers, nanoparticles, nucleotides, fullerene, etc., are discussed, giving specific attention to drug delivery, cell culture, bio-sensing, and bioimaging properties. Thus, the present review delineates, in short, the preparation, properties, and applications of different polymer and peptide hydrogels prepared in the past few years.

Calcium-responsive hydrogels enabled by inducible protein–protein interactions

Creation of an entirely protein-based calcium-responsive hydrogel enabled by genetically encoded click chemistry (GECC) and inducible protein–protein interactions.

An Injectable Self-Healing Protein Hydrogel with Multiple Dissipation Modes and Tunable Dynamic Response

Hydrogels with dynamic mechanical properties are of special interest in the field of tissue engineering and drug delivery. However, it remains challenging to tailor the dynamic mechanical response of hydrogels to simultaneously meet diverse application needs. Here, we report a hetero-coiled-coil complex cross-linked protein hydrogel exhibiting unusual multiple energy dissipation modes and tunable dynamic response. Such unique features confer on the hydrogel responsiveness to mechanical stimuli in a broad range of frequencies. Therefore, the hydrogels are injectable due to their shearing-thinning properties at low shear rates of 0.8 rad s^-1 and can fully recover their mechanical properties within a few seconds due to the intrinsic fast dynamics of the cross-linkers. Moreover, the dynamic response of these hydrogels can be fine-tuned by the temperature and the hydrogel network structures. We anticipate that these hydrogels are promising candidates for delivering therapeutic drugs, biological molecules, and cells in a broad spectrum of biomedical applications.

Cobalt-Cross-Linked, Redox-Responsive Spy Network Protein Hydrogels

Although assembly of recombinant proteins by SpyTag/SpyCatcher chemistry has proven to be a versatile approach for creating bioactive hydrogels, the resulting Spy networks often exhibit weak mechanics due to the poor efficiency of interchain cross-linking. Here we leverage metal/ligand (i.e., cobalt/His6-tag) coordination interactions to modulate the bulk mechanics of the protein networks. The drastic difference between the Co²⁺ and Co³⁺ complexes in thermodynamic and kinetic properties enabled us to regulate the materials' properties and to immobilize and release recombinant proteins in a redox-dependent manner. The resulting hydrogels are capable of not only supporting cell growth and proliferation, but also influencing specific cell signaling via immobilized growth factors such as leukemia inhibitory factor (LIF). The integrated use of stimuli-responsive metal coordination and SpyTag/SpyCatcher chemistry opens up a new dimension for designing bioactive protein materials.

Supramolecular and dynamic covalent hydrogel scaffolds: from gelation chemistry to enhanced cell retention and cartilage regeneration

This review highlights the most recent progress in gelation strategies of biomedical supramolecular and dynamic covalent crosslinking hydrogels and their applications for enhancing cell retention and cartilage regeneration.