Photo-cross-linking approach to engineering small tyrosine-containing peptide hydrogels with enhanced mechanical stability

Photocuring 3D Printing of Hydrogels: Techniques, Materials, and Applications in Tissue Engineering and Flexible Devices

Photocuring 3D printing of hydrogels, with sophisticated, delicate structures and biocompatibility, attracts significant attention by researchers and possesses promising application in the fields of tissue engineering and flexible devices. After years of development, photocuring 3D printing technologies and hydrogel inks make great progress. Herein, the techniques of photocuring 3D printing of hydrogels, including direct ink writing (DIW), stereolithography (SLA), digital light processing (DLP), continuous liquid interface production (CLIP), volumetric additive manufacturing (VAM), and two photon polymerization (TPP) are reviewed. Further, the raw materials for hydrogel inks (photocurable polymers, monomers, photoinitiators, and additives) and applications in tissue engineering and flexible devices are also reviewed. At last, the current challenges and future perspectives of photocuring 3D printing of hydrogels are discussed.

Photo-responsive metal-organic gels of rigid phenylene-1,3-di-substituted angular dienes with metal halides: gel-to-gel transformations triggered by [2 + 2] polymerization

Herein, the first report on gel-to-gel transformations via [2 + 2] photopolymerization in MOGs of metal halides and rigid dienes is presented. The MOGs and their xerogels show exceptional ability to undergo [2 + 2] polymerisation upon UV irradiation. Gel-to-gel transformations are very rare as the post-modification of gelators weakens the gel and transforms it to a sol. Such transformations change the molecular assemblies into gels with altered mechanical and chemical properties. These phenomena pave the way to synthesize new MOGs with improved rigidity that cannot be synthesized otherwise.

The Design of a Participatory Peptide Nucleic Acid Duplex Crosslinker to Enhance the Stiffness of Self-Assembled Peptide Gels

Herein, peptide nucleic acids (PNAs) are employed in the design of a participatory duplex PNA-peptide crosslinking agent. Biophysical and mechanical studies show that crosslinkers present during peptide assembly leading to hydrogelation participate in the formation of fibrils while simultaneously installing crosslinks into the higher-order network that constitutes the peptide gel. The addition of 2 mol % crosslinker into the assembling system results in a ~100 % increase in mechanical stiffness without affecting the rate of peptide assembly or the local morphology of fibrils within the gel network. Stiffness enhancement is realized by only affecting change in the elastic component of the viscoelastic gel. A synthesis of the PNA-peptide duplex crosslinkers is provided that allows facile variation in peptide composition and addresses the notorious hydrophobic content of PNAs. This crosslinking system represents a new tool for modulating the mechanical properties of peptide-based hydrogels.

Self-propelled assembly of nanoparticles with self-catalytic regulation for tumour-specific imaging and therapy

Targeted assembly of nanoparticles in biological systems holds great promise for disease-specific imaging and therapy. However, the current manipulation of nanoparticle dynamics is primarily limited to organic pericyclic reactions, which necessitate the introduction of synthetic functional groups as bioorthogonal handles on the nanoparticles, leading to complex and laborious design processes. Here, we report the synthesis of tyrosine (Tyr)-modified peptides-capped iodine (I) doped CuS nanoparticles (CuS-I@P1 NPs) as self-catalytic building blocks that undergo self-propelled assembly inside tumour cells via Tyr-Tyr condensation reactions catalyzed by the nanoparticles themselves. Upon cellular internalization, the CuS-I@P1 NPs undergo furin-guided condensation reactions, leading to the formation of CuS-I nanoparticle assemblies through dityrosine bond. The tumour-specific furin-instructed intracellular assembly of CuS-I NPs exhibits activatable dual-modal imaging capability and enhanced photothermal effect, enabling highly efficient imaging and therapy of tumours. The robust nanoparticle self-catalysis-regulated in situ assembly, facilitated by natural handles, offers the advantages of convenient fabrication, high reaction specificity, and biocompatibility, representing a generalizable strategy for target-specific activatable biomedical imaging and therapy.

Photoresponsive peptide materials: Spatiotemporal control of self-assembly and biological functions

Peptides work as both functional molecules to modulate various biological phenomena and self-assembling artificial materials. The introduction of photoresponsive units to peptides allows the spatiotemporal remote control of their structure and function upon light irradiation. This article overviews the photoresponsive peptide design, interaction with biomolecules, and applications in self-assembling materials over the last 30 years. Peptides modified with photochromic (photoisomerizable) molecules, such as azobenzene and spiropyran, reversibly photo-controlled the binding to biomolecules and nanostructure formation through self-assembly. Photocleavable molecular units irreversibly control the functions of peptides through cleavage of the main chain and deprotection by light. Photocrosslinking between peptides or between peptides and other biomolecules enhances the structural stability of peptide assemblies and complexes. These photoresponsive peptides spatiotemporally controlled the formation and dissociation of peptide assemblies, gene expressions, protein-drug interactions, protein-protein interactions, liposome deformation and motility, cytoskeleton structure and stability, and cell functions by appropriate light irradiation. These molecular systems can be applied to photo-control biological functions, molecular robots, artificial cells, and next-generation smart drug delivery materials.

Clinical Applicability of Visible Light-Mediated Cross-linking for Structural Soft Tissue Reconstruction

Visible light-mediated cross-linking has utility for enhancing the structural capacity and shape fidelity of laboratory-based polymers. With increased light penetration and cross-linking speed, there is opportunity to extend future applications into clinical spheres. This study evaluated the utility of a ruthenium/sodium persulfate photocross-linking system for increasing structural control in heterogeneous living tissues as an example, focusing on unmodified patient-derived lipoaspirate for soft tissue reconstruction. Freshly-isolated tissue is photocross-linked, then the molar abundance of dityrosine bonds is measured using liquid chromatography tandem mass spectrometry and the resulting structural integrity assessed. The cell function and tissue survival of photocross-linked grafts is evaluated ex vivo and in vivo, with tissue integration and vascularization assessed using histology and microcomputed tomography. The photocross-linking strategy is tailorable, allowing progressive increases in the structural fidelity of lipoaspirate, as measured by a stepwise reduction in fiber diameter, increased graft porosity and reduced variation in graft resorption. There is an increase in dityrosine bond formation with increasing photoinitiator concentration, and tissue homeostasis is achieved ex vivo, with vascular cell infiltration and vessel formation in vivo. These data demonstrate the capability and applicability of photocrosslinking strategies for improving structural control in clinically-relevant settings, potentially achieving more desirable patient outcomes using minimal manipulation in surgical procedures.

Empowering canonical biochemicals with cross-linked novelty: Recursions in applications of protein cross-links

Diversity in the biochemical workhorses of the cell-that is, proteins-is achieved by the innumerable permutations offered primarily by the 20 canonical L-amino acids prevalent in all biological systems. Yet, proteins are known to additionally undergo unusual modifications for specialized functions. Of the various post-translational modifications known to occur in proteins, the recently identified non-disulfide cross-links are unique, residue-specific covalent modifications that confer additional structural stability and unique functional characteristics to these biomolecules. We review an exclusive class of amino acid cross-links encompassing aromatic and sulfur-containing side chains, which not only confer superior biochemical characteristics to the protein but also possess additional spectroscopic features that can be exploited as novel chromophores. Studies of their in vivo reaction mechanism have facilitated their specialized in vitro applications in hydrogels and protein anchoring in monolayer chips. Furthering the discovery of unique canonical cross-links through new chemical, structural, and bioinformatics tools will catalyze the development of protein-specific hyperstable nanostructures, superfoods, and biotherapeutics.

Peptide- and Metabolite-Based Hydrogels: Minimalistic Approach for the Identification and Characterization of Gelating Building Blocks

Minimalistic peptide- and metabolite-based supramolecular hydrogels have great potential relative to traditional polymeric hydrogels in various biomedical and technological applications. Advantages such as remarkable biodegradability, high water content, favorable mechanical properties, biocompatibility, self-healing, synthetic feasibility, low cost, easy design, biological function, remarkable injectability, and multi-responsiveness to external stimuli make supramolecular hydrogels promising candidates for drug delivery, tissue engineering, tissue regeneration, and wound healing. Non-covalent interactions such as hydrogen bonding, hydrophobic interactions, electrostatic interactions, and π-π stacking interactions play key roles in the formation of peptide- and metabolite-containing low-molecular-weight hydrogels. Peptide- and metabolite-based hydrogels display shear-thinning and immediate recovery behavior due to the involvement of weak non-covalent interactions, making them supreme models for the delivery of drug molecules. In the areas of regenerative medicine, tissue engineering, pre-clinical evaluation, and numerous other biomedical applications, peptide- and metabolite-based hydrogelators with rationally designed architectures have intriguing uses. In this review, we summarize the recent advancements in the field of peptide- and metabolite-based hydrogels, including their modifications using a minimalistic building-blocks approach for various applications.

Peptide-Based Hydrogels: Template Materials for Tissue Engineering

Tissue and organ regeneration are challenging issues, yet they represent the frontier of current research in the biomedical field. Currently, a major problem is the lack of ideal scaffold materials' definition. As well known, peptide hydrogels have attracted increasing attention in recent years thanks to significant properties such as biocompatibility, biodegradability, good mechanical stability, and tissue-like elasticity. Such properties make them excellent candidates for 3D scaffold materials. In this review, the first aim is to describe the main features of a peptide hydrogel in order to be considered as a 3D scaffold, focusing in particular on mechanical properties, as well as on biodegradability and bioactivity. Then, some recent applications of peptide hydrogels in tissue engineering, including soft and hard tissues, will be discussed to analyze the most relevant research trends in this field.

Advances in Peptide-Based Hydrogel for Tissue Engineering

The development of peptide-based materials has emerged as one of the most challenging aspects of biomaterials in recent years. It has been widely acknowledged that peptide-based materials can be used in a broad range of biomedical applications, particularly in tissue engineering. Among them, hydrogels have been attracting considerable interest in tissue engineering because they mimic tissue formation conditions by providing a three-dimensional environment and a high water content. It has been found that peptide-based hydrogels have received more attention due to mimicking proteins, particularly extracellular matrix proteins, as well as the wide variety of applications they are capable of serving. It is without a doubt that peptide-based hydrogels have become the leading biomaterials of today owing to their tunable mechanical stability, high water content, and high biocompatibility. Here, we discuss in detail various types of peptide-based materials, emphasizing peptide-based hydrogels, and then we examine in detail how hydrogels are formed, paying particular attention to the peptide structures that are incorporated into the final structure. Following that, we discuss the self-assembly and formation of hydrogels under various conditions, as well as the parameters to be considered as critical factors, which include pH, amino acid composi- tion within the sequence, and cross-linking techniques. Further, recent studies on the development of peptide-based hydrogels and their applications in tissue engineering are reviewed.

Self-Assembly, Bioactivity, and Nanomaterials Applications of Peptide Conjugates with Bulky Aromatic Terminal Groups

The self-assembly and structural and functional properties of peptide conjugates containing bulky terminal aromatic substituents are reviewed with a particular focus on bioactivity. Terminal moieties include Fmoc [fluorenylmethyloxycarbonyl], naphthalene, pyrene, naproxen, diimides of naphthalene or pyrene, and others. These provide a driving force for self-assembly due to π-stacking and hydrophobic interactions, in addition to the hydrogen bonding, electrostatic, and other forces between short peptides. The balance of these interactions leads to a propensity to self-assembly, even for conjugates to single amino acids. The hybrid molecules often form hydrogels built from a network of β-sheet fibrils. The properties of these as biomaterials to support cell culture, or in the development of molecules that can assemble in cells (in response to cellular enzymes, or otherwise) with a range of fascinating bioactivities such as anticancer or antimicrobial activity, are highlighted. In addition, applications of hydrogels as slow-release drug delivery systems and in catalysis and other applications are discussed. The aromatic nature of the substituents also provides a diversity of interesting optoelectronic properties that have been demonstrated in the literature, and an overview of this is also provided. Also discussed are coassembly and enzyme-instructed self-assembly which enable precise tuning and (stimulus-responsive) functionalization of peptide nanostructures.

Magnetic Resonance Imaging: Time-Dependent Wetting and Swelling Behavior of an Auxetic Hydrogel Based on Natural Polymers

A time-dependent understanding of swelling characteristics and external stimuli behavior is crucial for the development and understanding of functional hydrogels. Magnetic resonance imaging (MRI) offers the opportunity to study three-dimensional (3D) soft materials nondestructively. This technique is already widely used as an image-based medical diagnostic tool and is applied here to evaluate complex structures of a hydrogel-a double network of chemically crosslinked casein enhanced with alginate-fabricated by 3D printing. When hydrogel disks immersed in four different liquid systems were analyzed, the material exhibited distinct system-dependent behavior characterized by rheological and mechanical measurements. Further material functionalization was achieved by macroscopic structuring of the hydrogel as an auxetic material based on a re-entrant honeycomb structure. MRI offers the advantage of monitoring overall changes in the area of the analyzed specimen and internal structural changes simultaneously. To assess the behavior of this complex structure, a series of short MRI measurements, each lasting 1.7 min, captured liquid diffusion and thus structural swelling behavior. A clear dependence of external and internal structural changes as a function of liquid properties causing these changes was observed. In conclusion, this approach might pave the way for prospective applications to monitor liquid diffusion into (e.g., vascularization) and swelling behavior of functional hydrogels.

Changing mechanical properties of photopolymerized, dityrosine-crosslinked protein-based hydrogels

Hydrogels based on renewable resources are a promising class of materials for future applications in pharmaceutics, drug delivery and personalized medicine. Thus, optional adjustments of mechanical properties such as swelling behavior, elasticity and network strength are desired. In this context, hydrogels based on the biological raw materials bovine serum albumin and casein were prepared by dityrosine-crosslinking of their tyrosine residues through visible light-induced photopolymerization. Changing the tyrosine accessibility by urea addition before photopolymerization increased the storage modulus of the hydrogels by 650% while simultaneously being more elastic. Furthermore, contributions of the buffer system composition, variation of protein concentration and storage medium towards mechanical properties of the hydrogel such as storage moduli, elasticity, fracture strain, compressive strength and relative weight swelling ratio are discussed. It could be shown, that changes in precursor solution and storage medium characteristics are crucial parameters towards tuning the mechanical properties of protein-based hydrogels.

Chemical crosslinking in 'reactive' multicomponent gels

We show that the hydrolysis of EDC can be used to construct a reactive system to trigger permanent covalent crosslinking between the components in multicomponent gels comprising gelators with a carboxylic acid and amine group yielding an amide functionalized gel with enhanced mechanical properties.

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

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

Supramolecular Peptide Nanofiber Hydrogels for Bone Tissue Engineering: From Multihierarchical Fabrications to Comprehensive Applications

Bone tissue engineering is becoming an ideal strategy to replace autologous bone grafts for surgical bone repair, but the multihierarchical complexity of natural bone is still difficult to emulate due to the lack of suitable biomaterials. Supramolecular peptide nanofiber hydrogels (SPNHs) are emerging biomaterials because of their inherent biocompatibility, satisfied biodegradability, high purity, facile functionalization, and tunable mechanical properties. This review initially focuses on the multihierarchical fabrications by SPNHs to emulate natural bony extracellular matrix. Structurally, supramolecular peptides based on distinctive building blocks can assemble into nanofiber hydrogels, which can be used as nanomorphology-mimetic scaffolds for tissue engineering. Biochemically, bioactive motifs and bioactive factors can be covalently tethered or physically absorbed to SPNHs to endow various functions depending on physiological and pharmacological requirements. Mechanically, four strategies are summarized to optimize the biophysical microenvironment of SPNHs for bone regeneration. Furthermore, comprehensive applications about SPNHs for bone tissue engineering are reviewed. The biomaterials can be directly used in the form of injectable hydrogels or composite nanoscaffolds, or they can be used to construct engineered bone grafts by bioprinting or bioreactors. Finally, continuing challenges and outlook are discussed.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Enhancement of the mechanical properties of lysine-containing peptide-based supramolecular hydrogels by chemical cross-linking

Exposure of lysine-containing peptide-based gelators to the cross-linking agent glutaraldehyde allows tuning of gel mechanical properties. The effect of cross-linking depends on the position of the lysine residue in the peptide chain, the concentration of gelator and the conditions under which cross-linking takes place. Through control of these factors, cross-linking leads to increased gel strength.

Self-Assembled Nanomaterials for Chronic Skin Wound Healing

Significance: Chronic wounds are one of the major burdens of the U.S. health care system with an annual cost of $31.7 billion and affecting an estimated 2.4-4.5 million people. Several underlying molecular and cellular pathophysiological mechanisms, including poor vascularization, excessive extracellular matrix (ECM) degradation by proteases, decreased growth factor activity, and bacterial infection can lead to chronic wounds. More effective wound therapies need to address one or more of these mechanisms to significantly advance wound care. Recent Advances: Self-assembled nanomaterials may provide new therapeutic options for chronic wound healing applications as those materials generally exhibit excellent biocompatibility and can bear multiple functionalities, such as ECM-mimicking properties, drug delivery capabilities, and tunable mechanics. Furthermore, self-assembled nanomaterials can be produced at low cost, and owing to their ability to self-organize, generate complex multifunctional structures that can be tailored to the varying sizes and shapes of chronic wounds. Self-assembled nanomaterials have been engineered to serve as wound dressings, growth factor delivery systems, and antimicrobials. Critical Issues: As there are many different types of self-assembled nanomaterials, which in turn have different mechanisms of self-assembly and physiochemical properties, one type of self-assembled nanomaterials may not be sufficient to address all underlying mechanisms of chronic wounds. However, self-assembled nanomaterials can be easily tailored, and developing multifunctional self-assembled nanomaterials that can address various targets in chronic wounds will be needed. Future Directions: Future studies should investigate combinations of various self-assembled nanomaterials to take full advantage of their multifunctional properties.

Stimuli responsive dynamic transformations in supramolecular gels

Supramolecular gels are formed by the self-assembly of small molecules under the influence of various non-covalent interactions. As the interactions are individually weak and reversible, it is possible to perturb the gels easily, which in turn enables fine tuning of their properties. Synthetic supramolecular gels are kinetically trapped and usually do not show time variable changes in material properties after formation. However, such materials potentially become switchable when exposed to external stimuli like temperature, pH, light, enzyme, redox, and chemical analytes resulting in reconfiguration of gel matrix into a different type of network. Such transformations allow gel-to-gel transitions while the changes in the molecular aggregation result in alteration of physical and chemical properties of the gel with time. Here, we discuss various methods that have been used to achieve gel-to-gel transitions by modifying a pre-formed gel material through external perturbation. We also describe methods that allow time-dependent autonomous switching of gels into different networks enabling synthesis of next generation functional materials. Dynamic modification of gels allows construction of an array of supramolecular gels with various properties from a single material which eventually extend the limit of applications of the gels. In some cases, gel-to-gel transitions lead to materials that cannot be accessed directly. Finally, we point out the necessity and possibility of further exploration of the field.

Characteristics and therapeutic applications of antimicrobial peptides

The demand for novel antimicrobial compounds is rapidly growing due to the phenomenon of antibiotic resistance in bacteria. In response, numerous alternative approaches are being taken including use of polymers, metals, combinatorial approaches, and antimicrobial peptides (AMPs). AMPs are a naturally occurring part of the immune system of all higher organisms and display remarkable broad-spectrum activity and high selectivity for bacterial cells over host cells. However, despite good activity and safety profiles, AMPs have struggled to find success in the clinic. In this review, we outline the fundamental properties of AMPs that make them effective antimicrobials and extend this into three main approaches being used to help AMPs become viable clinical options. These three approaches are the incorporation of non-natural amino acids into the AMP sequence to impart better pharmacological properties, the incorporation of AMPs in hydrogels, and the chemical modification of surfaces with AMPs for device applications. These approaches are being developed to enhance the biocompatibility, stability, and/or bioavailability of AMPs as clinical options.

Utilizing Frémy's Salt to Increase the Mechanical Rigidity of Supramolecular Peptide-Based Gel Networks

Peptide-based supramolecular gels are an important class of biomaterials that can be used for biomedical applications ranging from drug delivery to tissue engineering. Methodology that allows one to readily modulate the mechanical properties of these gels will allow yet even a broader range of applications. Frémy's salt is an inorganic salt and long-lived free radical that is known to oxidize phenols. Herein, we show that Frémy's salt can be used to dramatically increase the mechanical rigidity of hydrogels formed by tyrosine-containing self-assembling β-hairpin peptides. When Frémy's salt is added to pre-formed gels, it converts tyrosine residues to o-quinones that can subsequently react with amines present within the lysine side chains of the assembled peptide. This results in the installation of chemical crosslinks that reinforce the gel matrix. We characterized the unoxidized and oxidized gel systems using UV-Vis, transmission electron microscopy and rheological measurements and show that Frémy's salt increases the gel rigidity by nearly one order of magnitude, while retaining the gel's shear-thin/recovery behavior. Thus, Frémy's salt represents an on-demand method to modulate the mechanical rigidity of peptide-based self-assembled gels.

Oxidative Photocatalytic Homo- and Cross-Coupling of Phenols: Nonenzymatic, Catalytic Method for Coupling Tyrosine

The oxidative photocatalytic method for phenol-phenol homo-coupling and cross-coupling is described and isolated yields of 16-97% are obtained. Measured oxidation potentials and computed nucleophilicity parameters support a mechanism of nucleophilic attack of one partner onto the oxidized neutral radical form of the other partner. Understanding of this model permitted development of cross-coupling reactions between nucleophilic phenols/arenes and easily oxidized phenols with high selectivity and efficiency. A highlight of this method is that one equivalent of each coupling partner is utilized. Building on these findings, a non-enzymatic, catalytic method for coupling tyrosine was also developed.

Synthetic peptide hydrogels as 3D scaffolds for tissue engineering

The regeneration of tissues and organs poses an immense challenge due to the extreme complexity in the research work involved. Despite the tissue engineering approach being considered as a promising strategy for more than two decades, a key issue impeding its progress is the lack of ideal scaffold materials. Nature-inspired synthetic peptide hydrogels are inherently biocompatible, and its high resemblance to extracellular matrix makes peptide hydrogels suitable 3D scaffold materials. This review covers the important aspects of peptide hydrogels as 3D scaffolds, including mechanical properties, biodegradability and bioactivity, and the current approaches in creating matrices with optimized features. Many of these scaffolds contain peptide sequences that are widely reported for tissue repair and regeneration and these peptide sequences will also be discussed. Furthermore, 3D biofabrication strategies of synthetic peptide hydrogels and the recent advances of peptide hydrogels in tissue engineering will also be described to reflect the current trend in the field. In the final section, we will present the future outlook in the design and development of peptide-based hydrogels for translational tissue engineering applications.

Ultrashort Peptide Self-Assembly: Front-Runners to Transport Drug and Gene Cargos

The translational therapies to promote interaction between cell and signal come with stringent eligibility criteria. The chemically defined, hierarchically organized, and simpler yet blessed with robust intermolecular association, the peptides, are privileged to make the cut-off for sensing the cell-signal for biologics delivery and tissue engineering. The signature service and insoluble network formation of the peptide self-assemblies as hydrogels have drawn a spell of research activity among the scientists all around the globe in the past decades. The therapeutic peptide market players are anticipating promising growth opportunities due to the ample technological advancements in this field. The presence of the other organic moieties, enzyme substrates and well-established protecting groups like Fmoc and Boc etc., bring the best of both worlds. Since the large sequences of peptides severely limit the purification and their isolation, this article reviews the account of last 5 years' efforts on novel approaches for formulation and development of single molecule amino acids, ultra-short peptide self-assemblies (di- and tri- peptides only) and their derivatives as drug/gene carriers and tissue-engineering systems.

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.

Programmable enzymatic oxidation of tyrosine-lysine tetrapeptides

The ability to control the response of self-assembled systems upon exposure to external stimuli has been a long-standing goal of supramolecular chemistry. Short peptides are an attractive platform to realise this objective due to their chemical diversity and modular nature. Here, we synthesise a library of Fmoc-capped tetrapeptides, each containing two tyrosine and two lysine residues and varying in their amino acid sequence. Despite having similar secondary structure, these tetrapeptides form structures which are highly sequence dependent, yielding aggregates, nanofibres or monomers. This in turn highly affects the rate and degree of oxidative polymerisation by the enzyme tyrosinase, with self-assembled nanofibres exhibiting a greater degree of polymerisation. We monitor the formation of tyrosine oxidation products over time, finding that the precipitation of polymers is driven by quinone-based species. This affects the electrochemical properties of the oxidised peptide polymers, as determined through electrical impedance spectroscopy. Finally, intrinsic fluorescence microscale thermophoresis studies confirm that the degree of oxidative polymerisation is highly dependent on tyrosine solvent accessibility and the presence of peptide monomers. The ability to tune the kinetics of enzymatically active substrates and understand their polymerisation pathways on a molecular level is important for the creation of programmable, enzyme responsive biomaterials.

Non-cytotoxic Dityrosine Photocrosslinked Polymeric Materials With Targeted Elastic Moduli

Controlling mechanical properties of polymeric biomaterials, including the elastic modulus, is critical to direct cell behavior, such as proliferation and differentiation. Dityrosine photocrosslinking is an attractive and simple method to prepare materials that exhibit a wide range of elastic moduli by rapidly crosslinking tyrosyl-containing polymers. However, high concentrations of commonly used oxidative crosslinking reagents, such as ruthenium-based photoinitiators and persulfates, present cytotoxicity concerns. We found the elastic moduli of materials prepared by crosslinking an artificial protein with tightly controlled tyrosine molarity can be modulated up to 40 kPa by adjusting photoinitiator and persulfate concentrations. Formulations with various concentrations of the crosslinking reagents were able to target a similar material elastic modulus, but excess unreacted persulfate resulted in cytotoxic materials. Therefore, we identified a systematic method to prepare non-cytotoxic photocrosslinked polymeric materials with targeted elastic moduli for potential biomaterials applications in diverse fields, including tissue engineering and 3D bioprinting.

Photo-crosslinkable elastomeric protein-derived supramolecular peptide hydrogel with controlled therapeutic CO-release

As an attempt to establish a method for efficient and safe administration of therapeutic carbon monoxide (CO) to the human body, supramolecular nanoplatforms incorporated with CO-releasing molecules (CORMs) have recently been developed. In particular, hydrogel scaffolds have attracted considerable attention due to the possibility of site-specific and controlled liberation of CO. However, it would be greatly beneficial to enhance the mechanical strength of hydrogels to widen their applicability in biomedical, pharmaceutical, and surgical sectors. Herein, we report a visible light-mediated crosslinkable supramolecular CO-releasing hydrogel (CORH), based on the fibrillar assembly of elastomeric protein-derived tyrosine-containing short peptides. A photo-driven dimerization of tyrosine moieties located on the fibrillar surface of CORH, accelerated by a Ru-based catalyst, results in the entanglement and bundling of nanofibrils that significantly increases the mechanical strength and stability of the CORH, which allows prolonged CO-liberation through limiting the contact of CORMs with water molecules. The contact probability of a CORM with water determined by the spatial position of the CORM on the fibrils containing a crosslinkable tyrosine moiety that affects CO-releasing behavior was confirmed by adjusting the CORM position closer to or farther from the tyrosine in the peptide sequence. A bulky CORM closely located to the tyrosine in a peptide inhibited the effective dityrosine formation of tyrosine on the fibril surface, resulting in loose bundling of nanofibrils in the CORH and facilitating the release of CO through the exchange with water. The photo-crosslinked CORH demonstrated a potent cytoprotective effect on oxidatively stressed cardiomyocytes, as expected. This work could provide a useful insight for the practical application of gasotransmitters as functional nanomaterials in pharmaceutical and biomedical fields.

Preparation and applications of peptide-based injectable hydrogels

In situ injectable hydrogels have shown tremendous potential application in the biomedical field due to their significant drug accumulation at lesion sites, sustained release and markedly reduced systemic side effects. Specifically, peptide-based hydrogels, with unique biodegradation, biocompatibility, and bioactivity, are attractive molecular skeletons. In addition, peptides play a prominent role in normal metabolism, mimicking the natural tissue microenvironment and responding to stimuli in the lesion environment. Their advantages endow peptide-based hydrogels with great potential for application as biomedical materials. In this review, the fabrication and production of peptide-based hydrogels are presented. Several promising candidates, which are smart and environment-sensitive, are briefly reviewed. Then, the recent developments of these hydrogels for biomedical applications in tissue engineering, as drug/gene vehicles, and anti-bacterial agents are discussed. Finally, the development of peptide-based injectable hydrogels for biomedical applications in the future is surveyed.

Recent advances of self-assembling peptide-based hydrogels for biomedical applications

Peptide-based hydrogels have been proven to be preeminent biomedical materials due to their high water content, tunable mechanical stability, great biocompatibility and excellent injectability. The ability of peptide-based hydrogels to provide extracellular matrix-mimicking environments opens up opportunities for their biomedical applications in fields such as drug delivery, tissue engineering, and wound healing. In this review, we first describe several methods commonly used for the fabrication of robust peptide-based hydrogels, including spontaneous hydrogelation, enzyme-controlled hydrogelation and cross-linking-enhanced hydrogelation. We then introduce some representative studies on their applications in drug delivery and antitumor therapy, antimicrobial and wound healing materials, and 3D bioprinting and tissue engineering. We hope that this review facilitates the advances of hydrogels in biomedical applications.

Tyrosine-Rich Peptides as a Platform for Assembly and Material Synthesis

The self-assembly of biomolecules can provide a new approach for the design of functional systems with a diverse range of hierarchical nanoarchitectures and atomically defined structures. In this regard, peptides, particularly short peptides, are attractive building blocks because of their ease of establishing structure-property relationships, their productive synthesis, and the possibility of their hybridization with other motifs. Several assembling peptides, such as ionic-complementary peptides, cyclic peptides, peptide amphiphiles, the Fmoc-peptide, and aromatic dipeptides, are widely studied. Recently, studies on material synthesis and the application of tyrosine-rich short peptide-based systems have demonstrated that tyrosine units serve as not only excellent assembly motifs but also multifunctional templates. Tyrosine has a phenolic functional group that contributes to π-π interactions for conformation control and efficient charge transport by proton-coupled electron-transfer reactions in natural systems. Here, the critical roles of the tyrosine motif with respect to its electrochemical, chemical, and structural properties are discussed and recent discoveries and advances made in tyrosine-rich short peptide systems from self-assembled structures to peptide/inorganic hybrid materials are highlighted. A brief account of the opportunities in design optimization and the applications of tyrosine peptide-based biomimetic materials is included.

Photochromic, organogelating and self-sorting behaviour of di-(dithienylethene) derivatives

They wouldn't mix! – Narcissistic self-sorting was found to exist in a 1 : 1 mixture of the two photoisomers of a di-(dithienylethene)-containing organgelator.

Silk fibroin hydrogels for potential applications in photodynamic therapy

In this study, we prepared translucid hydrogels with different concentrations of silk fibroin, extracted from raw silk fibers, and used them as a matrix to incorporate the photosensitizer 5-(4-aminophenyl)-10,15,20-tris-(4-sulphonatophenyl) porphyrin trisodium for application in photodynamic therapy (PDT). The hydrogels obtained were characterized by rheology, spectrophotometry, and scattering techniques to elucidate the factors involved in the formation of the hydrogel, and to characterize the behavior of silk fibroin (SF) after incorporating of the porphyrin to the matrix. The rheology results demonstrated that the SF hydrogels had a shear thinning behavior. In addition, we were able to verify that the structure of the material was able to be recovered over time after shear deformation. The encapsulation of porphyrins in hydrogels leads to the formation of self-assembled peptide nanostructures that prevent porphyrin aggregation, thereby greatly increasing the generation of singlet oxygen. Also, our findings suggest that porphyrin can diffuse out of the hydrogel and permeate the outer skin layers. This evidence suggests that SF hydrogels could be used as porphyrin encapsulation and as a drug carrier for the sustained release of photosensitizers for PDT.

Responsive peptide-based supramolecular hydrogels constructed by self-immolative chemistry

Peptide-based supramolecular hydrogels that are stimuli-responsive under aqueous conditions have many potential biological applications, including drug delivery and sensing. Herein, we reported a series of responsive peptide-based supramolecular hydrogels that respond to glutathione (GSH), nitric oxide (NO) and hydrogen sulfide (H₂S), which are biologically important signaling molecules. The responsive hydrogelators were designed by "self-immolative" chemistry and constructed by using self-immolative groups to modify short peptides. The self-immolative capping group could be removed in the presence of a corresponding trigger, thus causing gel-sol phase transitions. The potential of our responsive hydrogels for drug release was also demonstrated in this study. Our study offered several candidates of responsive hydrogels for sensing and drug delivery.

Multiple Cross-Linking of a Small Peptide to Form a Size Tunable Biopolymer with Efficient Cell Adhesion and Proliferation Property

Development of biocompatible polymeric systems capable of cell adhesion and proliferation is a challenging task. Proper cross-linking of small cell adhesive peptide sequences is useful in this respect as it provides the inherent nontoxic environment as well as the cross-linked polymeric network to the cells for adhesion and proliferation. A multiple cross-linking strategy is applied to create a peptide-based cross-linked polymer. Covalent linkage through disulfide bond formation, supramolecular linkage using homoternary complexation by CB[8], and enzymatic cross-linking by HRP-mediated dimerization of tyrosine are used to prepare the cross-linked, peptide-based polymer decorated with cell-adhesive RGDS sequence. The supramolecular cross-linking via CB[8] provided stability as well as brings the RGDS sequences at the surface of the polymer particles. The order of cross-linking allowed to fine-tune the particle size of the polymer and polymer particles of wide range (200-1000 nm) can be prepared by varying the order. The cross-linked polymer particles (P1 and P2) were found to be stable at wide range of temperature and pH. Moreover, as intended, the polymer was noncytotoxic in nature and showed efficient cell adhesion and proliferation property, which can be used for further biological applications.

Minimalist Prion-Inspired Polar Self-Assembling Peptides

Nature provides copious examples of self-assembling supramolecular nanofibers. Among them, amyloid structures have found amazing applications as advanced materials in fields such as biomedicine and nanotechnology. Prions are a singular subset of proteins able to switch between a soluble conformation and an amyloid state. The ability to transit between these two conformations is encoded in the so-called prion domains (PrDs), which are long and disordered regions of low complexity, enriched in polar and uncharged amino acids such as Gln, Asn, Tyr, Ser, and Gly. The polar nature of PrDs results in slow amyloid formation, which allows kinetic control of fiber assembly. This approach has been exploited for fabrication of multifunctional materials because in contrast to most amyloids, PrDs lack hydrophobic stretches that can nucleate their aggregation, their assembly depends on the establishment of a large number of weak interactions along the complete domain. The length and low complexity of PrDs make their chemical synthesis for applied purposed hardly affordable. Here, we designed four minimalist polar binary patterned peptides inspired in PrDs, which include the [Q/N/G/S]-Y-[Q/N/G/S] motif frequently observed in these domains: NYNYNYN, QYQYQYQ, SYSYSYS, and GYGYGYG. Despite their small size, they all recapitulate the properties of full-length PrDs, self-assembling into nontoxic amyloids under physiological conditions. Thus, they constitute small building blocks for the construction of tailored prion-inspired nanostructures. We exploited Tyr residues in these peptides to generate highly stable dityrosine cross-linked assemblies for the immobilization of metal nanoparticles in the fibrils surface and to develop an electrocatalytic amyloid scaffold. Moreover, we show that the shorter and more polar NYNNYN, QYQQYQ, and SYSSYS hexapeptides also self-assemble into amyloid-like structures, consistent with the presence of these tandem motifs in human prion-like proteins.

Photocrosslinked tyramine-substituted hyaluronate hydrogels with tunable mechanical properties improve immediate tissue-hydrogel interfacial strength in articular cartilage

Articular cartilage lacks the ability to self-repair and a permanent solution for cartilage repair remains elusive. Hydrogel implantation is a promising technique for cartilage repair; however for the technique to be successful hydrogels must interface with the surrounding tissue. The objective of this study was to investigate the tunability of mechanical properties in a hydrogel system using a phenol-substituted polymer, tyramine-substituted hyaluronate (TA-HA), and to determine if the hydrogels could form an interface with cartilage. We hypothesized that tyramine moieties on hyaluronate could crosslink to aromatic amino acids in the cartilage extracellular matrix. Ultraviolet (UV) light and a riboflavin photosensitizer were used to create a hydrogel by tyramine self-crosslinking. The gel mechanical properties were tuned by varying riboflavin concentration, TA-HA concentration, and UV exposure time. Hydrogels formed with a minimum of 2.5 min of UV exposure. The compressive modulus varied from 5 to 16 kPa. Fluorescence spectroscopy analysis found differences in dityramine content. Cyanine-3 labelled tyramide reactivity at the surface of cartilage was dependent on the presence of riboflavin and UV exposure time. Hydrogels fabricated within articular cartilage defects had increasing peak interfacial shear stress at the cartilage-hydrogel interface with increasing UV exposure time, reaching a maximum shear stress 3.5× greater than a press-fit control. Our results found that phenol-substituted polymer/riboflavin systems can be used to fabricate hydrogels with tunable mechanical properties and can interface with the surface tissue, such as articular cartilage.

Controlling gelation with sequence: Towards programmable peptide hydrogels

The self-assembling peptide IKHLSVN, inspired by inspection of a protein-protein interface, has previously been reported as one of a new class of bio-inspired peptides. Here the peptide, dubbed littleSven, and modifications designed to probe the resilience of the sequence to self-assembly, is characterised. Although the parent peptide did not form a hydrogel, small modifications to the sequence (one side chain or an N-terminus modification) led to hydrogels with properties (eg. gelation time and rheology) that could be tuned by these small alterations. The results suggest that peptides derived from protein-protein interfaces are resilient to changes in sequence and can be harnessed to form hydrogels with controlled properties.

STATEMENT OF SIGNIFICANCE

Natural occurring self-assembly peptides are attractive building blocks for engineered bionanomaterials due to their biocompatibility and biodegradability. The bio-inspired self-assembly peptide, IKHLSVN, was used as a template to design peptides that readily formed hydrogels. The peptide sequence was specifically tuned to create a bionanomaterial with different properties that could be exploited downstream for a broad range of applications: nanowires, drug release, vaccine adjuvant, tissue engineering. We describe how small modifications to the parent peptide alter the amyloid-like characteristics and gel strength for each peptide.

Electroresponsive Supramolecular Graphene Oxide Hydrogels for Active Bacteria Adsorption and Removal

Bacteria contamination in drinking water and medical products can cause severe health problems. However, currently available sterilization methods, mainly based on the size-exclusion mechanism, are typically slow and require the entire contaminated water to pass through the filter. Here, we present an electroresponsive hydrogel based approach for bacteria adsorption and removal. We successfully engineered a series of graphene oxide hydrogels using redox-active ruthenium complexes as noncovalent cross-linkers. The resulting hydrogels can reversibly switch their physical properties in response to the applied electric field along with the changes of oxidation states of the ruthenium ions. The hydrogels display strong bacteria adsorbing capability. A hydrogel of 1 cm(3) can adsorb a maximum of 1 × 10(8) E. coli. The adsorbed bacteria in the hydrogels can then be inactivated by a high voltage electric pulse and removed from the hydrogels subsequently. Owing to the high bacteria removal rate, reusability, and low production cost, these hydrogels represent promising candidates for the emergent sterilization of medical products or large-scale purification of drinking water.

Photoresponsive gelators

We review the different approaches that have been used to form low molecular weight gels that respond to light.