Tunable Pentapeptide Self-Assembled β-Sheet Hydrogels

Scrambled RGD Hexameric Peptide Hydrogel Supports Efficient Self-Assembly and Cell Activity

The amino acid sequence is crucial in controlling peptide-based hydrogel formation, whereby changing the position of a single amino acid can significantly alter the gel's properties. Herein, we report the gelation kinetics and cell viability of scrFmoc-GFFRDG (where we have scrambled the RGD-based gel hexapeptide; Fmoc-GFFRGD). The scrambled sequence showed improved gelation properties compared to the original Fmoc-GFFRGD sequence, with scrFmoc-GFFRDG forming a gel in under 10 min, significantly faster than the 2-h gelation time, and at a concentration eight times lower than the original Fmoc-GFFRGD sequence. We also examined the combination of the two gelators in a ratio of 1:1, final concentration of 0.4% (w/v). Interestingly, the stiffness of the hybrid hydrogel was ∼3 kPa, whereas individually, neither gelator at the same concentration exceeded 0.5 kPa. The cell-adhesion motif RGD improves the ability of the peptides to promote attachment of cells due to integrin recognition. However, when fibroblasts were cultured on the hydrogels, scrFmoc-GFFRDG yielded a higher level of α-SMA expression in cells than those cultured on Fmoc-GFFRGD, suggesting a microenvironment conducive to myofibroblast transitions. This study provides a new outlook on how a well-known scrambled peptide motif (RDG) can fine-tune hydrogel assembly and cell culture applications.

Engineering the Self-Assembly Pathways of POSS-Peptide Amphiphiles to Form Diverse Cross-β Structures

Cross-β structures are crucial in driving protein folding and aggregation. However, due to their strong aggregating tendency, the precise control of the self-assembly of β-sheet-forming peptides remains a challenge. We propose a molecular geometry strategy to study and control the self-assembly of cross-β structures. We conjugate the peptide with shape-persistent polyhedral oligomeric silsesquioxane (POSS), which acts as a hydrophilic head and senses the solvent environment. The POSS-peptide amphiphiles display two distinct self-assembly pathways: twisted nanoribbons transforming into either nanotubes at low water content or flat nanoribbons at high water content. The peptide packing in flat nanoribbons is predominantly modulated by POSS, diverting the system away from crystal formation, which is the absolute lowest energy state of pure peptide self-assemblies. For the first time, we have demonstrated that POSS can serve as a useful tool to adjust the interactions between cross-β strands, achieving fine-tuning of the pathway complexity (i.e., the kinetic and thermodynamic aspects of peptide self-assembly). With this versatile molecular platform incorporating multiple functionalities of POSS and programable peptide sequences, this study provides a platform to exploit cross-β-based nanomaterials with functional and pathological significance.

RGD-functionalised self-assembling peptide hydrogel induces a proliferative profile in human osteoblasts in vitro

Self-assembling peptide hydrogels (SAPHs) have been used in the past decade as reliable three-dimensional (3D) synthetic scaffolds for the culture of a variety of mammalian cells in vitro. Thanks to their versatile physicochemical properties, they allow researchers to tailor the hydrogel properties, including stiffness and functionality to the targeted cells and cells' behaviour. One of the advantages of using SAPH scaffolds is the ease of functionalisation. In the present work, we discuss the effect that functionalising the FEFEFKFK (F, phenylalanine; K, lysine; and E, glutamic acid) hydrogel scaffold using the cell-binding RGDS (fibronectin - R, arginine; G, glycine; D, aspartic acid; S, serine) epitope affects the material properties as well as the function of encapsulated human osteoblast cells. RGDS functionalisation resulted in cells adopting an elongated morphology, suggesting attachment and increased proliferation. While this led to higher cell viability, it also resulted in a decrease in extra-cellular matrix (ECM) protein production as well as a decrease in calcium ion deposition, suggesting lower mineralisation capabilities. The work clearly shows that SAPHs are a flexible platform that allow the modification of scaffolds in a controlled manner to investigate cell-material interactions.

Designing Short Cardin-Motif Peptide and Biopolymer-Based Multicomponent Hydrogels for Developing Advanced Composite Scaffolds for Improving Cellular Behavior

Multicomponent self-assembly represents a cutting-edge strategy in peptide nanotechnology, enabling the creation of nanomaterials with enhanced physical and biological characteristics. This approach draws inspiration from the highly complex nature of the native extracellular matrix (ECM) constituting multicomponent biomolecular entities. In recent years, the combination of bioactive peptide with polymer has gained significant attention for the fabrication of novel biomaterials due to their inherent specificity, tunable physiochemical properties, biocompatibility, and biodegradability. This advanced strategy can address the limitation of the lower mechanical strength of the individual peptide hydrogel by incorporating the biopolymer, resulting in the formation of a composite scaffold. In this direction, this advanced strategy is explored using noncovalent interactions between cellulose nano-fiber (CNF) and cationic Cardin-motif peptide to develop advanced composite scaffolds. The bioactive cationic peptide otherwise failed to form hydrogel at physiological conditions. Interestingly, the differential mixing ratio of CNF and peptide modulated the surface charge, functionality, and mechanical properties of the composite scaffolds, resulting in diverse cellular responses. 10:1 (w/w) ratio of CNF and peptide-based composite scaffold demonstrates improved cellular survival and proliferation in 2D culture conditions. Notably, in 3D cultures, cell proliferation on the 10:1 matrix is comparable to Matrigel, highlighting its potential for advanced tissue engineering applications.

Influence of Electrostatic Interactions on the Self-Assembly of Charged Peptides

Peptides can be designed to self-assemble into predefined supramolecular nanostructures, which are then employed as biomaterials in a range of applications, including tissue engineering, drug delivery, and vaccination. However, current self-assembling peptide (SAP) hydrogels exhibit inadequate self-healing capacities and necessitate the use of sophisticated printing apparatus, rendering them unsuitable for 3D printing under physiological conditions. Here, we report a precisely designed charged peptide, Z5, with the object of investigating the impact of electrostatic interactions on the self-assembly and the rheological properties of the resulting hydrogels. This peptide displays salt-triggered self-assembly resulting in the formation of a nanofiber network with a high β-sheet content. The peptide self-assembly and the hydrogel properties can be modified according to the ionic environment. It is noteworthy that the Z5 hydrogel in normal saline (NS) shows exceptional self-healing properties, demonstrating the ability to recover its initial strength in seconds after the removal of shear force, thus rendering it an acceptable material for printing. In contrast, the strong salt shielding effect and the ionic cross-linking of Z5 hydrogels in PBS result in the bundling of peptide nanofibers, which impedes the recovery of the initial strength post-destruction. Furthermore, incorporating materials with varied charging properties into Z5 hydrogels can alter the electrostatic interactions among peptide nanofibers, further modulating the rheological properties and the printability of SAP hydrogels.

Multiscale Materials Engineering via Self-Assembly of Pentapeptide Derivatives from SARS CoV E Protein

Short peptide-based supramolecular hydrogels hold enormous potential for a wide range of applications. However, the gelation of these systems is very challenging to control. Minor changes in the peptide sequence can significantly influence the self-assembly mechanism and thereby the gelation propensity. The involvement of SARS CoV E protein in the assembly and release of the virus suggests that it may have inherent self-assembling properties that can contribute to the development of hydrogels. Here, three pentapeptide sequences derived from C-terminal of SARS CoV E protein are explored with same amino acid residues but different sequence distributions and discovered a drastic difference in the gelation propensity. By combining spectroscopic and microscopic techniques, the relationship between peptide sequence arrangement and molecular assembly structure are demonstrated, and how these influence the mechanical properties of the hydrogel. The present study expands the variety of secondary structures for generating supramolecular hydrogels by introducing the 310-helix as the primary building block for gelation, facilitated by a water-mediated structural transition into β-sheet conformation. Moreover, these Fmoc-modified pentapeptide hydrogels/supramolecular assemblies with tunable morphology and mechanical properties are suitable for tissue engineering, injectable delivery, and 3D bio-printing applications.

Designing highly tunable anion responsive Cardin-motif peptide based self-assembled nanostructures for accessing diverse cellular response

Several anions present in the extracellular matrix (ECM) not only have significant physiological functions in ECM but also play an important role in regulating peptide-based self-assembly. Herein, we have employed a non-conventional approach to overcome the limitations of the positively charged Cardin-motif peptide that failed to self-assemble at physiological pH. We used a simple and elegant strategy by employing different anions such as HPO42-, Cl- and I- to mask the overall surface charge of peptide. Interestingly, these anions were utilized to modulate the nanostructure formation and mechanical stiffness of peptide hydrogels owing to their differential interactions with water molecules according to the Hofmeister series. Interestingly, these anions induced hydrogels showed diverse cellular responses on two different cell lines, fibroblast and neuronal, indicating diverse application potential of the new scaffold. Thus, this study emphasizes the importance of anions to regulate the self-assembly of Cardin-motif peptide and this approach can be utilized in developing the ideal biomimetic model of ECM for futuristic applications.

Recent research of peptide-based hydrogel in nervous regeneration

Neurological disorders exert significantly affect the quality of life for patients, necessitating effective strategies for nerve regeneration. Both traditional autologous nerve transplantation and emerging therapeutic approaches encounter scientific challenges due to the complex nature of the nervous system and the unsuitability of the surrounding environment for cell transplantation. Tissue engineering techniques offer a promising path for neurotherapy. Successful neural tissue engineering relies on modulating cell differentiation behavior and tissue repair by developing biomaterials that mimic the natural extracellular matrix (ECM) and establish a three-dimensional microenvironment. Peptide-based hydrogels have emerged as a potent option among these biomaterials due to their ability to replicate the structure and complexity of the ECM. This review aims to explore the diverse range of peptide-based hydrogels used in nerve regeneration with a specific focus on dipeptide hydrogels, tripeptide hydrogels, oligopeptide hydrogels, multidomain peptides (MDPs), and amphiphilic peptide hydrogels (PAs). Peptide-based hydrogels offer numerous advantages, including biocompatibility, structural diversity, adjustable mechanical properties, and degradation without adverse effects. Notably, hydrogels formed from self-assembled polypeptide nanofibers, derived from amino acids, show promising potential in engineering neural tissues, outperforming conventional materials like alginate, poly(ε-caprolactone), and polyaniline. Additionally, the simple design and cost-effectiveness of dipeptide-based hydrogels have enabled the creation of various functional supramolecular structures, with significant implications for nervous system regeneration. These hydrogels are expected to play a crucial role in future neural tissue engineering research. This review aims to highlight the benefits and potential applications of peptide-based hydrogels, contributing to the advancement of neural tissue engineering.

Elastin-Derived Peptide-Based Hydrogels as a Potential Drug Delivery System

A peptide-based hydrogel sequence was computationally predicted from the Ala-rich cross-linked domains of elastin. Three candidate peptides were subsequently synthesised and characterised as potential drug delivery vehicles. The elastin-derived peptides are Fmoc-FFAAAAKAA-NH2, Fmoc-FFAAAKAA-NH2, and Fmoc-FFAAAKAAA-NH2. All three peptide sequences were able to self-assemble into nanofibers. However, only the first two could form hydrogels, which are preferred as delivery systems compared to solutions. Both of these peptides also exhibited favourable nanofiber lengths of at least 1.86 and 4.57 µm, respectively, which are beneficial for the successful delivery and stability of drugs. The shorter fibre lengths of the third peptide (maximum 0.649 µm) could have inhibited their self-assembly into the three-dimensional networks crucial to hydrogel formation.

Designing Pathway-Controlled Multicomponent Ultrashort Peptide Hydrogels with Diverse Functionalities at the Nanoscale for Directing Cellular Behavior

Tuning self-assembling pathways by implementing different external stimuli has been extensively studied, owing to their effective control over structural and mechanical properties. Consequently, multicomponent peptide hydrogels with high structural tunability and stimuli responsiveness are crucial in dictating cellular behavior. Herein, we have implemented both coassembly approach and pathway-dependent self-assembly to design nonequilibrium nanostructures to understand the thermodynamic and kinetic aspects of peptide self-assembly toward controlling cellular response. Our system involved an ultrashort peptide gelator and a hydrophilic surfactant which coassembled through different pathways, i.e., heat-cool and sonication methods with variable energy input. Interestingly, it was possible to access diverse structural and mechanical properties at the nanoscale in a single coassembled system. Further, the hydrophilic surfactant provided additional surface functionalities, thus creating an efficient hydrophilic matrix for cellular interaction. Such diverse functionalities in a single coassembled system could lead to the development of advanced scaffolds, with applications in various biomedical fields.

Fast Catalyst-Free Synthesis of Stereoselective Polypeptides via Hierarchical Chiral Assembly

Understanding how life's essential homochiral biopolymers arose from racemic precursors is a challenging quest. Herein, we present a groundbreaking approach involving hierarchical chiral assembly-driven stereoselective ring-opening polymerization of ε-benzyloxycarbonyl-l/d-lysine N-carboxyanhydrides assisted by ultrasonication in an aqueous medium. This method enabled a narrow dispersity within a few minutes and the achievement of high molecular weight for polypeptides, employing a living polymerization mechanism. The polymerization of l and d enantiomers yielded predominantly right- and left-handed superhelical assemblies in a one-pot preparation, respectively. Notably, stereoselective polypeptide segments were efficiently prepared through hierarchical assembly-driven polymerization of racemic monomers in the absence of a catalyst. This research offers an innovative strategy for the convenient preparations of stereoenriched polypeptides and, more importantly, sheds light on the plausible emergence of homochiral peptides in the origin of life.

Harnessing the Potential of Self-Assembled Peptide Hydrogels for Neural Regeneration and Tissue Engineering

Spinal cord injury, traumatic brain injury, and neurosurgery procedures usually lead to neural tissue damage. Self-assembled peptide (SAP) hydrogels, a type of innovative hierarchical nanofiber-forming peptide sequences serving as hydrogelators, have emerged as a promising solution for repairing tissue defects and promoting neural tissue regeneration. SAPs possess numerous features, such as adaptable morphologies, biocompatibility, injectability, tunable mechanical stability, and mimicking of the native extracellular matrix. This review explores the capacity of neural cell regeneration and examines the critical aspects of SAPs in neuroregeneration, including their biochemical composition, topology, mechanical behavior, conductivity, and degradability. Additionally, it delves into the latest strategies involving SAPs for central or peripheral neural tissue engineering. Finally, the prospects of SAP hydrogel design and development in the realm of neuroregeneration are discussed.

In Situ Synthesis and Self-Assembly of Peptide-PEG Conjugates: A Facile Method for the Construction of Fibrous Hydrogels

Peptide-based hydrogels have gained considerable attention as a compelling platform for various biomedical applications in recent years. Their attractiveness stems from their ability to seamlessly integrate diverse properties, such as biocompatibility, biodegradability, easily adjustable hydrophilicity/hydrophobicity, and other functionalities. However, a significant drawback is that most of the functional self-assembling peptides cannot form robust hydrogels suitable for biological applications. In this study, we present the synthesis of novel peptide-PEG conjugates and explore their comprehensive hydrogel properties. The hydrogel comprises double networks, with the first network formed through the self-assembly of peptides to create a β-sheet secondary structure. The second network is established through covalent bond formation via N-hydroxysuccinimide chemistry between peptides and a 4-arm PEG to form a covalently linked network. Importantly, our findings reveal that this hydrogel formation method can be applied to other peptides containing lysine-rich sequences. Upon encapsulation of the hydrogel with antimicrobial peptides, the hydrogel retained high bacterial killing efficiency while showing minimum cytotoxicity toward mammalian cells. We hope that this method opens new avenues for the development of a novel class of peptide-polymer hydrogel materials with enhanced performance in biomedical contexts, particularly in reducing the potential for infection in applications of tissue regeneration and drug delivery.

Exploring a Solvent Dependent Strategy to Control Self-Assembling Behavior and Cellular Interaction in Laminin-Mimetic Short Peptide based Supramolecular Hydrogels

Self-assembled hydrogels, fabricated through diverse non-covalent interactions, have been extensively studied in regenerative medicines. Inspired from bioactive functional motifs of ECM protein, short peptide sequences have shown remarkable abilities to replicate the intrinsic features of the natural extracellular milieu. In this direction, we have fabricated two short hydrophobic bioactive sequences derived from the laminin protein i. e., IKVAV and YIGSR. Based on the substantial hydrophobicity of these peptides, we selected a co-solvent approach as a suitable gelation technique that included different concentrations of DMSO as an organic phase along with an aqueous solution containing 0.1 % TFA. These hydrophobic laminin-based bioactive peptides with limited solubility in aqueous physiological environment showed significantly enhanced solubility with higher DMSO content in water. The enhanced solubility resulted in extensive intermolecular interactions that led to the formation of hydrogels with a higher-order entangled network along with improved mechanical properties. Interestingly, by simply modulating DMSO content, highly tunable gels were accessed in the same gelator domain that displayed differential physicochemical properties. Further, the cellular studies substantiated the potential of these laminin-derived hydrogels in enhancing cell-matrix interactions, thereby reinforcing their applications in tissue engineering.

Biostable hydrogels consisting of hybrid β-sheet fibrils assembled by a pair of enantiomeric peptides

The assembly of chiral peptides facilitates the formation of diverse supramolecular structures with unique physicochemical and biological properties. However, the effects of chirality on peptide assembly and resulting hydrogel properties remain underexplored. In this study, we systematically investigated the assembly propensity, morphology, and biostability of mixture of a pair of enantiomeric peptides LELCLALFLF (ECF-5) and DEDCDADFDF (ecf-5) at various ratios. Results indicate the development of β-sheet fibrils, ultimately leading to the formation of self-supporting hybrid hydrogels. The hydrogel formed at a ratio of 1:1 exhibits a significantly lower storage modulus (G') than of the ratios of 0:1, 1:3, 3:1 and 1:0 (nD/nL; same below). Kink-separated fragments of approximately 100 nm in length predominate at ratios of 1:3 and 3:1, compared with the smooth fibrils at other ratios, probably attributed to an alternating arrangement of the co-assembled and self-assembled peptide fragments. The introduction of ecf-5 to the hybrid hydrogels improves resistance to proteolytic digestion and maintains commendable biocompatibility in both MIN6 and HUVECs cells. These findings provide valuable insights into the development of hydrogels with tailored properties, positing them potential scaffolds for 3D cell culture and tissue engineering.

Water-Based Synthesis of β-Sheet-Like Supramolecular Metallohydrogel Organized by Using a Native Ultrashort Peptide Sequence

Designing supramolecular hydrogels using short peptides is challenging. To control self-assembly, a certain amount of organic solvent is typically added to the system, or the short peptide is modified with a functional group that is hydrophobic, hydrophilic, or highly coordinative. We discovered that l-His-l-Ile-l-Thr (HIT), a very short unmodified "native" tripeptide, selectively responds to Cu2+ ions in pure water to form a transparent supramolecular metallohydrogel. Circular dichroism analysis revealed that Cu2+ ions, but no other metal species, caused HIT to change from a random-coil-like to a β-sheet-like structure. Other spectroscopic methods were used to characterize the properties of the supramolecular metallohydrogel. These results are expected to facilitate the development of native short peptides as advanced functional biomaterials.

Stimuli-responsive peptide hydrogels for biomedical applications

Stimuli-responsive hydrogels can respond to external stimuli with a change in the network structure and thus have potential application in drug release, intelligent sensing, and scaffold construction. Peptides possess robust supramolecular self-assembly ability, enabling spontaneous formation of nanostructures through supramolecular interactions and subsequently hydrogels. Therefore, peptide-based stimuli-responsive hydrogels have been widely explored as smart soft materials for biomedical applications in the last decade. Herein, we present a review article on design strategies and research progress of peptide hydrogels as stimuli-responsive materials in the field of biomedicine. The latest design and development of peptide hydrogels with responsive behaviors to stimuli are first presented. The following part provides a systematic overview of the functions and applications of stimuli-responsive peptide hydrogels in tissue engineering, drug delivery, wound healing, antimicrobial treatment, 3D cell culture, biosensors, etc. Finally, the remaining challenges and future prospects of stimuli-responsive peptide hydrogels are proposed. It is believed that this review will contribute to the rational design and development of stimuli-responsive peptide hydrogels toward biomedical applications.

Engineered assemblies from isomeric pentapeptides augment dry eye treatment

Changing positions of amino acid residues in the peptide sequence alters the peptide' s assembly behaviors, affording various nanostructures. However, it remains elusive that how subtle changes in the peptide sequence influence the in vivo bioactivity of peptide-based nanocarriers, further impacting the efficacy of the encapsulated drugs. We report here a class of isomeric pentapeptide amphiphiles that associate into filaments with different dimensions, which were further used as carriers of Diquafosol tetrasodium (DQS), for the treatment of dry eye disease. Our results suggest that subtle changes in peptide sequences resulted in dramatically different molecular packings and distinct morphologies, which were verified by molecular dynamics simulations. In vivo results show that the drug retention time could be prolonged by the peptidic nanostructures on the ocular surface but were highly morphological-dependent. The longer retention time promised better therapeutic efficacy. In terms of facile synthesis and good biocompatibility, we believe that these peptides could be used for eye disease treatments or other related areas.

pH-sensitive peptide hydrogel encapsulating the anti-angiogenesis drug conbercept and chemotherapeutic drug dox as a combination therapy for retinoblastoma

Retinoblastoma (RB) is a malignant tumor originating from the retina.

Mechanical Properties of Low-Molecular-Weight Peptide Hydrogels Improved by Thiol-Ene Click Chemistry

Low-molecular-weight peptide hydrogels can be formed by self-assembly through weak interactions, but the application of the hydrogel is influenced by its weak mechanical properties. Therefore, it is important to construct low-molecular-weight peptide hydrogels with excellent mechanical properties. In this work, we designed the pentapeptide molecule Fmoc-FFCKK-OH (abbreviated as FFCKK) with a sulfhydryl group, and another low-molecular-weight cross-linker N,N'-methylenebis(acrylamide) (MBA) was introduced to construct a hydrogel with excellent mechanical properties. The secondary structure change process of FFCKK and the assembly mechanism of hydrogel were analyzed using theoretical calculations and experimental characterizations. The occurrence of thiol-ene click chemistry provides covalent interaction in the hydrogel, and the synergistic effect ofcovalent interaction and hydrogen bonding improves the mechanical properties of the hydrogel by nearly 10-fold. The hydrogel was observed to be able to withstand a stress of 368 Pa and to break in a layer-by-layer manner by compression testing. The micromechanics of the hydrogels were characterized, and the excellent mechanical properties of the hydrogels were confirmed. The synergistic approach provides a new idea for the preparation of low-molecular-weight peptide hydrogels and facilitates the expansion of their potential applications in biomedical fields.

Biomedical applications of supramolecular hydrogels with enhanced mechanical properties

Supramolecular hydrogels bound by hydrogen bonding, host-guest, hydrophobic, and other non-covalent interactions are among the most attractive biomaterials available. Supramolecular hydrogels have attracted extensive attention due to their inherent dynamic reversibility, self-healing, stimuli-response, excellent biocompatibility, and near-physiological environment. However, the inherent contradiction between non-covalent interactions and mechanical strength makes the practical application of supramolecular hydrogels a great challenge. This review describes the mechanical strength of hydrogels mediated by supramolecular interactions, and focuses on the potential strategies for enhancing the mechanical strength of supramolecular hydrogels and illustrates their applications in related fields, such as flexible electronic sensors, wound dressings, and three-dimensional (3D) scaffolds. Finally, the current problems and future research prospects of supramolecular hydrogels are discussed. This review is expected to provide insights that will motivate more advanced research on supramolecular hydrogels.

Unbiased in silico design of pH-sensitive tetrapeptides

We used coarse-grain molecular dynamics simulations to screen all possible histidine-bearing tetrapeptide sequences, finding novel peptide sequences with pH-tunable assembly properties. These tetrapeptides could be used for various biological applications, such as triggered delivery of bioactive molecules.

Amyloid peptide hydrogels via formation of coordination polymers with Ag+ by its core peptide equipped with a C-cysteine

We report that the core sequence of amyloid β (Aβ) peptide, KLVFF, when equipped with a C-terminal cysteine residue, exhibited an extremely low minimum hydrogelation concentration of 0.05 wt% in the presence of Ag+ in pH 5 buffer, with this concentration 2 orders of magnitude lower than that of the pentapeptide itself. The CD signal of the Ag+-L-KLVFFC hydrogel was observed to be sensitive to the early-stage aggregation of amyloid β peptide.

Designing ECM-inspired supramolecular scaffolds by utilizing the interactions between a minimalistic neuroactive peptide and heparin

Short bioactive peptide-based supramolecular hydrogels are emerging as interesting candidates for developing scaffolds for tissue engineering applications. However, proteins and peptides represent only a single class of molecules present in the native ECM, thus, recapitulating the complete ECM microenvironment via only peptide-based biomaterials is extremely challenging. In this direction, complex multicomponent-based biomaterials have started gaining importance for achieving the biofunctional complexity and structural hierarchy of the native ECM. Sugar-peptide complexes can be explored in this direction as they provide essential biological signaling required for cellular growth and survival in vivo. In this direction, we explored the fabrication of an advanced scaffold by employing heparin and short bioactive peptide interactions at the molecular level. Interestingly, the addition of heparin into the peptide has significantly modulated the supramolecular organization, nanofibrous morphology and the mechanical properties of the scaffold. Additionally, the combined hydrogels demonstrated superior biocompatibility as compared to the peptide counterpart at certain ratios. These newly developed scaffolds were also observed to be stable under 3-D cell culture conditions and supported cellular adhesion and proliferation. Most importantly, the inflammatory response was also minimized in the case of combined hydrogels as compared to heparin. We expect that this approach of using simple non-covalent interactions between the ECM-inspired small molecules to fabricate biomaterials with improved mechanical and biological properties could advance the current knowledge on designing ECM mimetic biomaterials. Such an attempt would create a novel, adaptable and simplistic bottom-up strategy for the invention of new and more complex biomaterials of ECM origin with advanced functions.

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.

Self-healing cyclic peptide hydrogels

Hydrogels are soft materials of great interest in different areas such as chemistry, biology, and therapy. Gels made by the self-assembly of small molecules are known as supramolecular gels. The modulation of their properties by monomer molecular design is still difficult to predict due to the potential impact of subtle structural modifications in the self-assembly process. Herein, we introduce the design principles of a new family of self-assembling cyclic octapeptides of alternating chirality that can be used as scaffolds for the development of self-healing hydrogelator libraries with tunable properties. The strategy was used in the preparation of an amphiphilic cyclic peptide monomer bearing an alkoxyamine connector, which allowed the insertion of different aromatic aldehyde pendants to modulate the hydrophobic/hydrophilic balance and fine-tune the properties of the resulting gel. The resulting amphiphiles were able to form self-healable hydrogels with viscoelastic properties (loss tangent, storage modulus), which were strongly dependent on the nature and number of aromatic moieties anchored to the hydrophilic peptide. Structural studies by SEM, STEM and AFM indicated that the structure of the hydrogels was based on a dense network of peptide nanotubes. Excellent agreement was established between the peptide primary structure, nanotube length distributions and viscoelastic behaviour.

Emerging 3D bioprinting applications in plastic surgery

Plastic surgery is a discipline that uses surgical methods or tissue transplantation to repair, reconstruct and beautify the defects and deformities of human tissues and organs. Three-dimensional (3D) bioprinting has gained widespread attention because it enables fine customization of the implants in the patient's surgical area preoperatively while avoiding some of the adverse reactions and complications of traditional surgical approaches. In this paper, we review the recent research advances in the application of 3D bioprinting in plastic surgery. We first introduce the printing process and basic principles of 3D bioprinting technology, revealing the advantages and disadvantages of different bioprinting technologies. Then, we describe the currently available bioprinting materials, and dissect the rationale for special dynamic 3D bioprinting (4D bioprinting) that is achieved by varying the combination strategy of bioprinting materials. Later, we focus on the viable clinical applications and effects of 3D bioprinting in plastic surgery. Finally, we summarize and discuss the challenges and prospects for the application of 3D bioprinting in plastic surgery. We believe that this review can contribute to further development of 3D bioprinting in plastic surgery and provide lessons for related research.

Machine learning overcomes human bias in the discovery of self-assembling peptides

Peptide materials have a wide array of functions, from tissue engineering and surface coatings to catalysis and sensing. Tuning the sequence of amino acids that comprise the peptide modulates peptide functionality, but a small increase in sequence length leads to a dramatic increase in the number of peptide candidates. Traditionally, peptide design is guided by human expertise and intuition and typically yields fewer than ten peptides per study, but these approaches are not easily scalable and are susceptible to human bias. Here we introduce a machine learning workflow-AI-expert-that combines Monte Carlo tree search and random forest with molecular dynamics simulations to develop a fully autonomous computational search engine to discover peptide sequences with high potential for self-assembly. We demonstrate the efficacy of the AI-expert to efficiently search large spaces of tripeptides and pentapeptides. The predictability of AI-expert performs on par or better than our human experts and suggests several non-intuitive sequences with high self-assembly propensity, outlining its potential to overcome human bias and accelerate peptide discovery.

Light-Triggered Self-Assembly of Peptide Nanoparticles into Nanofibers in Living Cells through Molecular Conformation Changes and H-Bond Interactions

Controlled self-assembly has attracted extensive interest in biological and nanotechnological applications. Enzymatic or biocatalytic triggered self-assembly is widely used for the diagnostic and prognostic marker in different pathologies because of their nanostructures and biological effects. However, it remains a great challenge to control the self-assembly of peptides in living cells with a high degree of spatial and temporal precision. Here we demonstrate a light-triggered platform that enables spatiotemporal control of self-assembly from nanoparticles into nanofibers in living cells through subtle molecular conformational changes and internal H-bonding interactions. The platform contained 3-methylene-2-(quinolin-8-yl) isoindolin-1-one, which acts as the light-controlled unit to disrupt the hydrophilic/lipophilic balance through the change of molecular conformation, and a peptide that can be a faster recombinant to assemble via H-bonding interactions. The process has good biocompatibility because it does not involve waste generation or oxygen consumption; moreover, the assembly rate constant was fast and up to 0.17 min^-1. It is applied to the regulation of molecular assembly in living cells. As such, our findings demonstrate that light-triggered controllable assembly can be applied for initiative regulating cellular behaviors in living systems.

Peptide-Based Hydrogels: New Materials for Biosensing and Biomedical Applications

Peptide-based hydrogels have attracted increasing attention for biological applications and diagnostic research due to their impressive features including biocompatibility and biodegradability, injectability, mechanical stability, high water absorption capacity, and tissue-like elasticity. The aim of this review will be to present an updated report on the advancement of peptide-based hydrogels research activity in recent years in the field of anticancer drug delivery, antimicrobial and wound healing materials, 3D bioprinting and tissue engineering, and vaccines. Additionally, the biosensing applications of this key group of hydrogels will be discussed mainly focusing the attention on cancer detection.

Self-assembled benzoselenadiazole-capped tripeptide hydrogels with inherent in vitro anti-cancer and anti-inflammatory activity

Self-assembled benzoselenadiazole (BSe)-capped tripeptide based nanofibrillar hydrogels have been developed with inherent anticancer and anti-inflammatory activity.

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.

Supramolecular Hydrogels: Design Strategies and Contemporary Biomedical Applications

Self-assembly of supramolecular hydrogels is driven by dynamic, non-covalent interactions between molecules. Considerable research effort has been exerted to fabricate and optimise supramolecular hydrogels that display shear-thinning, self-healing, and reversibility, in order to develop materials for biomedical applications. This review provides a detailed overview of the chemistry behind the dynamic physicochemical interactions that sustain hydrogel formation (hydrogen bonding, hydrophobic interactions, ionic interactions, metal-ligand coordination, and host-guest interactions). Novel design strategies and methodologies to create supramolecular hydrogels are highlighted, which offer promise for a wide range of applications, specifically drug delivery, wound healing, tissue engineering and 3D bioprinting. To conclude, future prospects are briefly discussed, and consideration given to the steps required to ultimately bring these biomaterials into clinical settings.

Control over Multiple Nano- and Secondary Structures in Peptide Self-Assembly

Herein, we report the rich morphological and conformational versatility of a biologically active peptide (PEP-1), which follows diverse self-assembly pathways to form up to six distinct nanostructures and up to four different secondary structures through subtle modulation in pH, concentration and temperature. PEP-1 forms twisted β-sheet secondary structures and nanofibers at pH 7.4, which transform into fractal-like structures with strong β-sheet conformations at pH 13.0 or short disorganized elliptical aggregates at pH 5.5. Upon dilution at pH 7.4, the nanofibers with twisted β-sheet secondary structural elements convert into nanoparticles with random coil conformations. Interestingly, these two self-assembled states at pH 7.4 and room temperature are kinetically controlled and undergo a further transformation into thermodynamically stable states upon thermal annealing: whereas the twisted β-sheet structures and corresponding nanofibers transform into 2D sheets with well-defined β-sheet domains, the nanoparticles with random coil structures convert into short nanorods with α-helix conformations. Notably, PEP-1 also showed high biocompatibility, low hemolytic activity and marked antibacterial activity, rendering our system a promising candidate for multiple bio-applications.

Peptide framework for screening the effects of amino acids on assembly

Discovery of peptide domains with unique intermolecular interactions is essential for engineering peptide-based materials. Rather than attempting a brute-force approach, we instead identify a previously unexplored strategy for discovery and study of intermolecular interactions: “co-assembly of oppositely charged peptide” (CoOP), a framework that “encourages” peptide assembly by mixing two oppositely charged hexapeptides. We used an integrated computational and experimental approach, probed the free energy of association and probability of amino acid contacts during co-assembly with atomic-resolution simulations, and correlated them to the physical properties of the aggregates. We introduce CoOP with three examples: dialanine, ditryptophan, and diisoleucine. Our results indicated that the opposite charges initiate the assembly, and the subsequent stability is enhanced by the presence of an undisturbed hydrophobic core. CoOP represents a unique, simple, and elegant framework that can be used to identify the structure-property relationships of self-assembling peptide-based materials.

Advanced Methods for the Characterization of Supramolecular Hydrogels

With the increased research on supramolecular hydrogels, many spectroscopic, diffraction, microscopic, and rheological techniques have been employed to better understand and characterize the material properties of these hydrogels. Specifically, spectroscopic methods are used to characterize the structure of supramolecular hydrogels on the atomic and molecular scales. Diffraction techniques rely on measurements of crystallinity and help in analyzing the structure of supramolecular hydrogels, whereas microscopy allows researchers to inspect these hydrogels at high resolution and acquire a deeper understanding of the morphology and structure of the materials. Furthermore, mechanical characterization is also important for the application of supramolecular hydrogels in different fields. This can be achieved through atomic force microscopy measurements where a probe interacts with the surface of the material. Additionally, rheological characterization can investigate the stiffness as well as the shear-thinning and self-healing properties of the hydrogels. Further, mechanical and surface characterization can be performed by micro-rheology, dynamic light scattering, and tribology methods, among others. In this review, we highlight state-of-the-art techniques for these different characterization methods, focusing on examples where they have been applied to supramolecular hydrogels, and we also provide future directions for research on the various strategies used to analyze this promising type of material.

Peptide-Based Supramolecular Systems Chemistry

Peptide-based supramolecular systems chemistry seeks to mimic the ability of life forms to use conserved sets of building blocks and chemical reactions to achieve a bewildering array of functions. Building on the design principles for short peptide-based nanomaterials with properties, such as self-assembly, recognition, catalysis, and actuation, are increasingly available. Peptide-based supramolecular systems chemistry is starting to address the far greater challenge of systems-level design to access complex functions that emerge when multiple reactions and interactions are coordinated and integrated. We discuss key features relevant to systems-level design, including regulating supramolecular order and disorder, development of active and adaptive systems by considering kinetic and thermodynamic design aspects and combinatorial dynamic covalent and noncovalent interactions. Finally, we discuss how structural and dynamic design concepts, including preorganization and induced fit, are critical to the ability to develop adaptive materials with adaptive and tunable photonic, electronic, and catalytic properties. Finally, we highlight examples where multiple features are combined, resulting in chemical systems and materials that display adaptive properties that cannot be achieved without this level of integration.

Self-Assembling Hydrogel Structures for Neural Tissue Repair

Hydrogel materials have been employed as biological scaffolds for tissue regeneration across a wide range of applications. Their versatility and biomimetic properties make them an optimal choice for treating the complex and delicate milieu of neural tissue damage. Aside from finely tailored hydrogel properties, which aim to mimic healthy physiological tissue, a minimally invasive delivery method is essential to prevent off-target and surgery-related complications. The specific class of injectable hydrogels termed self-assembling peptides (SAPs), provide an ideal combination of in situ polymerization combined with versatility for biofunctionlization, tunable physicochemical properties, and high cytocompatibility. This review identifies design criteria for neural scaffolds based upon key cellular interactions with the neural extracellular matrix (ECM), with emphasis on aspects that are reproducible in a biomaterial environment. Examples of the most recent SAPs and modification methods are presented, with a focus on biological, mechanical, and topographical cues. Furthermore, SAP electrical properties and methods to provide appropriate electrical and electrochemical cues are widely discussed, in light of the endogenous electrical activity of neural tissue as well as the clinical effectiveness of stimulation treatments. Recent applications of SAP materials in neural repair and electrical stimulation therapies are highlighted, identifying research gaps in the field of hydrogels for neural regeneration.