Supramolecular Nanofibers of Peptide Amphiphiles for Medicine

Tuning Structural Organization via Molecular Design and Hierarchical Assembly to Develop Supramolecular Thermoresponsive Hydrogels

The cellular microenvironment is composed of a dynamic hierarchical fibrillar architecture providing a variety of physical and bioactive signals to the surrounding cells. This dynamicity, although common in biology, is a challenge to control in synthetic matrices. Here, responsive synthetic supramolecular monomers were designed that are able to assemble into hierarchical fibrous structures, combining supramolecular fiber formation via hydrogen bonding interactions, with a temperature-responsive hydrophobic collapse, resulting in cross-linking and hydrogel formation. Therefore, amphiphilic molecules were synthesized, composed of a hydrogen bonding ureido-pyrimidinone (UPy) unit, a hydrophobic alkyl spacer, and a hydrophilic oligo(ethylene glycol) tail. The temperature responsive behavior was introduced by functionalizing these supramolecular amphiphiles with a relatively short poly(N-isopropylacrylamide) (PNIPAM) chain (M n ∼ 2.5 or 5.5 kg/mol). To precisely control the assembly of these monomers, the length of the alkyl spacer between the UPy moiety and PNIPAM was varied in length. A robust sol-gel transition, with the dodecyl UPy-PNIPAM molecule, was obtained, with a network elasticity enhancing over 2000 times upon heating above room temperature. The UPy-PNIPAM compounds with shorter alkyl spacers were already hydrogels at room temperature. The sol-gel transition of the dodecyl UPy-PNIPAM hydrogelator could be tuned by the incorporation of different UPy-functionalized monomers. Furthermore, we demonstrated the suitability of this system for microfluidic cell encapsulation through a convenient temperature sol-gel transition. Our results indicate that this novel thermoresponsive supramolecular system offers a modular platform to study and guide single-cell behavior.

Navigating the challenges and exploring the perspectives associated with emerging novel biomaterials

The field of biomaterials is a continuously evolving interdisciplinary field encompassing biological sciences, materials sciences, chemical sciences, and physical sciences with a multitude of applications realized every year. However, different biomaterials developed for different applications have unique challenges in the form of biological barriers, and addressing these challenges simultaneously is also a challenge. Nevertheless, immense progress has been made through the development of novel materials with minimal adverse effects such as DNA nanostructures, specific synthesis strategies based on supramolecular chemistry, and modulating the shortcomings of existing biomaterials through effective functionalization techniques. This review discusses all these aspects of biomaterials, including the challenges at each level of their development and application, proposed countermeasures for these challenges, and some future directions that may have potential benefits.

Glucose-Driven Droplet Formation in Complexes of a Supramolecular Peptide and Therapeutic Protein

Biology achieves remarkable function through processes arising from spontaneous or transient liquid-liquid phase separation (LLPS) of proteins and other biomolecules. While polymeric systems can achieve similar phenomena through simple or complex coacervation, LLPS with supramolecular materials has been less commonly shown. Functional applications for synthetic LLPS systems are an expanding area of emphasis, with particular focus on capturing the transient and dynamic state of these structures for use in biomedicine. Here, a net-cationic supramolecular peptide amphiphile building block with a glucose-binding motif is shown that forms LLPS structures when combined with a net-negatively charged therapeutic protein, dasiglucagon, in the presence of glucose. The droplets that arise are dynamic and coalesce quickly. However, the interface can be stabilized by addition of a 4-arm star PEG. When the stabilized droplets formed in glucose are transferred to a bulk phase containing different glucose concentrations, their stability and lifetime decrease according to bulk glucose concentration. This glucose-dependent formation translates into an accelerated release of dasiglucagon in the absence of glucose; this hormone analogue itself functions therapeutically to correct low blood glucose (hypoglycemia). These droplets also offer function in mitigating the most severe effects of hypoglycemia arising from an insulin overdose through delivery of dasiglucagon in a mouse model of hypoglycemic rescue. Accordingly, this approach to use complexation between a supramolecular peptide amphiphile and a therapeutic protein in the presence of glucose leads to droplets with functional potential to dissipate for the release of the therapeutic material in low blood glucose environments.

Tailoring Multicontrolled Supramolecular Assemblies of Stiff-Stilbene Amphiphiles into Macroscopic Soft Scaffolds as Cell-Material Interfaces

Biocompatible synthetic supramolecular systems have shed light on biomedical and tissue-regenerative material applications. The intrinsic functional applicability, tunability, and stimuli-responsiveness of synthetic supramolecular systems allow one to develop various multicontrolled supramolecular assemblies in aqueous media. However, it remains highly challenging to use state-of-the-art supramolecular assemblies of photoresponsive amphiphiles controlled by multiple stimulations in fabricating macroscopic materials. Herein, we demonstrate a stiff-stilbene amphiphile (SA) multicontrolled supramolecular assembling system that comprises two different charged end groups. The excellent photoswitchabilities of SA in both organic and aqueous media are demonstrated. Furthermore, multiple stimuli, i.e., light, pH, and counterions, are applied to control the supramolecular assembling behaviors, which are monitored by circular dichroism spectroscopy and electron microscopies. This multicontrolled supramolecular system can be systematically assembled into macroscopic soft functional scaffolds, whose structural parameters are investigated by electron microscopies and X-ray diffraction techniques, suggesting the large aspect ratio of SA nanostructures assembled into macroscopic soft scaffolds. The fabricated soft functional scaffold is highly biocompatible for photocontrolled biotarget encapsulation/release selectively, as well as a cell-material interface for diverse cells' attachment. This new synthetic multicontrolled soft functional material provides a new strategy toward the development of next-generation controllable and biocompatible soft functional materials.

Lipopeptides as tools in catalysis, supramolecular, materials and medicinal chemistry

Lipopeptides are amphiphilic peptides in which an aliphatic chain is attached to either the C or N terminus of peptides. Their self-assembly - into micelles, vesicles, nanotubes, fibres or nanobelts - leads to applications in nanotechnology, catalysis or medicinal chemistry. Self-organization of lipopeptides is dependent on both the length of the lipid tail and the amino acid sequence, in which the chirality of the peptide sequence can be transmitted into the supramolecular species. This Review describes the use of lipopeptides to design synthetic advanced dynamic supramolecular systems, nanostructured materials or self-responsive delivery systems in the area of medical biotechnology. We examine the influence of external stimuli, the ability of lipopeptide-derived structures to adapt over time and their application as medicinal agents with antibacterial, antifungal, antiviral or anticancer activities. Finally, we discuss the catalytic efficiency of lipopeptides, with the aim of building minimal synthetic enzymes, and recent efforts to incorporate metals into lipopeptide assemblies.

Rational design of peptide-based implants for corneal bioengineering

Regeneration of damaged cornea can save vision for millions of patients. Most of these patients are waiting for transplantation of a donor cornea or suitable substitute to restore vision. Although donor cornea transplantation is the most clinically accepted treatment, shortage of donor cornea results in almost 69 out of every 70 patients untreated with the waiting list for transplantation drastically increasing every year according to a prepandemic estimation. Therefore, corneal replacements are coming up as a cutting-edge alternative strategy. In view of the peptides, especially collagen-like peptides and peptide amphiphiles with bioactive functional motifs demonstrate promising avenue for the corneal tissue engineering and promoting regeneration, by their hierarchical self-assembling propensity to acquire desired nano- to macroscale 3D architecture. Here, we analyze rational peptide designing, self-assembly, and strategies of peptide/peptide-based nanoscale building blocks to create the extracellular matrix mimetic implants for functional regeneration of the cornea.

Biomaterials via peptide assembly: Design, characterization, and application in tissue engineering

A core challenge in biomaterials, with both fundamental significance and technological relevance, concerns the rational design of bioactive microenvironments. Designed properly, peptides can undergo supramolecular assembly into dynamic, physical hydrogels that mimic the mechanical, topological, and biochemical features of native tissue microenvironments. The relatively facile, inexpensive, and automatable preparation of peptides, coupled with low batch-to-batch variability, motivates the expanded use of assembling peptide hydrogels for biomedical applications. Integral to realizing dynamic peptide assemblies as functional biomaterials for tissue engineering is an understanding of the molecular and macroscopic features that govern assembly, morphology, and biological interactions. In this review, we first discuss the design of assembling peptides, including primary structure (sequence), secondary structure (e.g., α-helix and β-sheets), and molecular interactions that facilitate assembly into multiscale materials with desired properties. Next, we describe characterization tools for elucidating molecular structure and interactions, morphology, bulk properties, and biological functionality. Understanding of these characterization methods enables researchers to access a variety of approaches in this ever-expanding field. Finally, we discuss the biological properties and applications of peptide-based biomaterials for engineering several important tissues. By connecting molecular features and mechanisms of assembling peptides to the material and biological properties, we aim to guide the design and characterization of peptide-based biomaterials for tissue engineering and regenerative medicine. STATEMENT OF SIGNIFICANCE: Engineering peptide-based biomaterials that mimic the topological and mechanical properties of natural extracellular matrices provide excellent opportunities to direct cell behavior for regenerative medicine and tissue engineering. Here we review the molecular-scale features of assembling peptides that result in biomaterials that exhibit a variety of relevant extracellular matrix-mimetic properties and promote beneficial cell-biomaterial interactions. Aiming to inspire and guide researchers approaching this challenge from both the peptide biomaterial design and tissue engineering perspectives, we also present characterization tools for understanding the connection between peptide structure and properties and highlight the use of peptide-based biomaterials in neural, orthopedic, cardiac, muscular, and immune engineering applications.

Elongated self-assembled nanocarriers: From molecular organization to therapeutic applications

Self-assembled cylindrical aggregates made of amphiphilic molecules emerged almost 40 years ago. Due to their length up to micrometers, those particles display original physico-chemical properties such as important flexibility and, for concentrated samples, a high viscoelasticity making them suitable for a wide range of industrial applications. However, a quarter of century was needed to successfully take advantage of those improvements towards therapeutic purposes. Since then, a wide diversity of biocompatible materials such as polymers, lipids or peptides, have been developed to design self-assembling elongated drug nanocarriers, suitable for therapeutic or diagnostic applications. More recently, the investigation of the main forces driving the unidirectional growth of these nanodevices allowed a translation toward the formation of pure nanodrugs to avoid the use of unnecessary side materials and the possible toxicity concerns associated.

Realizing tissue integration with supramolecular hydrogels

Biomaterial matrices must permit tissue growth and maturation for the success of tissue regeneration strategies. Naturally, this accommodation is achieved via the dynamic remodeling of a cell's extracellular matrix (ECM). Synthetically, hydrolytic or enzymatic degradation are often engineered into materials for this purpose. More recently, supramolecular interactions have been used to provide a biomimetic and tunable mechanism to facilitate tissue formation via their dynamic and reversible non-covalent interactions. By engineering the mechanical and bioactive properties of a material, supramolecular chemists are able to design permissivity into the construct and facilitate tissue integration in-vivo. Furthermore, via the reversibility of non-covalent interactions, injectability and responsiveness can be designed for enhanced delivery and spatio-temporal control. In this review, we delineate the basic considerations needed when designing permissive supramolecular hydrogels for tissue engineering with an eye toward tissue growth and integration. We highlight three archetypal hydrogel systems that have shown well-documented tissue integration in vivo, and provide avenues to assess tissue in-growth. Careful design and assessment of the biomedical potential of a supramolecular hydrogels can inspire the creation of robust and dynamic implants for new tissue engineering applications.

Engineering the drug carrier biointerface to overcome biological barriers to drug delivery

Micro and nanoscale drug carriers must navigate through a plethora of dynamic biological systems prior to reaching their tissue or disease targets. The biological obstacles to drug delivery come in many forms and include tissue barriers, mucus and bacterial biofilm hydrogels, the immune system, and cellular uptake and intracellular trafficking. The biointerface of drug carriers influences how these carriers navigate and overcome biological barriers for successful drug delivery. In this review, we examine how key material design parameters lead to dynamic biointerfaces and improved drug delivery across biological barriers. We provide a brief overview of approaches used to engineer key physicochemical properties of drug carriers, such as morphology, surface chemistry, and topography, as well as the development of dynamic responsive materials for barrier navigation. We then discuss essential biological barriers and how biointerface engineering can enable drug carriers to better navigate and overcome these barriers to drug delivery.

Networks of High Aspect Ratio Particles to Direct Colloidal Assembly Dynamics and Cellular Interactions

Injectable colloids that self-assemble into three-dimensional networks are promising materials for applications in regenerative engineering, as they create open systems for cellular infiltration, interaction, and activation. However, most injectable colloids have spherical morphologies, which lack the high material-biology contact areas afforded by higher aspect ratio materials. To address this need, injectable high aspect ratio particles (HARPs) were developed that form three-dimensional networks to enhance scaffold assembly dynamics and cellular interactions. HARPs were functionalized for tunable surface charge through layer-by-layer electrostatic assembly. Positively charged Chitosan-HARPs had improved particle suspension dynamics when compared to spherical particles or negatively charged HARPs. Chit-HARPs were used to improve the suspension dynamics and viability of MIN6 cells in three-dimensional networks. When combined with negatively charged gelatin microsphere (GelMS) porogens, Chit-HARPs reduced GelMS sedimentation and increased overall network suspension, due to a combination of HARP network formation and electrostatic interactions. Lastly, HARPs were functionalized with fibroblast growth factor 2 (FGF2) to highlight their use for growth factor delivery. FGF2-HARPs increased fibroblast proliferation through a combination of 3D scaffold assembly and growth factor delivery. Taken together, these studies demonstrate the development and diverse uses of high aspect ratio particles as tunable injectable scaffolds for applications in regenerative engineering.

Self-assembling Peptides in Current Nanomedicine: Versatile Nanomaterials for Drug Delivery

BACKGROUND

The development of modern nanomedicine greatly depends on the involvement of novel materials as drug delivery system. In order to maximize the therapeutic effects of drugs and minimize their side effects, a number of natural or synthetic materials have been widely investigated for drug delivery. Among these materials, biomimetic self-assembling peptides (SAPs) have received more attention in recent years. Considering the rapidly growing number of SAPs designed for drug delivery, a summary of how SAPs-based drug delivery systems were designed, would be beneficial.

METHOD

We outlined research works on different SAPs that have been investigated as carriers for different drugs, focusing on the design of SAPs nanomaterials and how they were used for drug delivery in different strategies.

RESULTS

Based on the principle rules of chemical complementarity and structural compatibility, SAPs such as ionic self-complementary peptide, peptide amphiphile and surfactant-like peptide could be designed. Determined by the features of peptide materials and the drugs to be delivered, different strategies such as hydrogel embedding, hydrophobic interaction, electrostatic interaction, covalent conjugation or the combination of them could be employed to fabricate SAPs-drug complex, which could achieve slow release, targeted or environment-responsive delivery of drugs. Furthermore, some SAPs could also be combined with other types of materials for drug delivery, or even act as drug by themselves.

CONCLUSION

Various types of SAPs have been designed and used for drug delivery following various strategies, suggesting that SAPs as a category of versatile nanomaterials have promising potential in the field of nanomedicine.

Glycosaminoglycan-Inspired Biomaterials for the Development of Bioactive Hydrogel Networks

Glycosaminoglycans (GAG) are long, linear polysaccharides that display a wide range of relevant biological roles. Particularly, in the extracellular matrix (ECM) GAG specifically interact with other biological molecules, such as growth factors, protecting them from proteolysis or inhibiting factors. Additionally, ECM GAG are partially responsible for the mechanical stability of tissues due to their capacity to retain high amounts of water, enabling hydration of the ECM and rendering it resistant to compressive forces. In this review, the use of GAG for developing hydrogel networks with improved biological activity and/or mechanical properties is discussed. Greater focus is given to strategies involving the production of hydrogels that are composed of GAG alone or in combination with other materials. Additionally, approaches used to introduce GAG-inspired features in biomaterials of different sources will also be presented.

Fabrication and Plasma Modification of Nanofibrous Tissue Engineering Scaffolds

This paper provides a comprehensive overview of nanofibrous structures for tissue engineering purposes and the role of non-thermal plasma technology (NTP) within this field. Special attention is first given to nanofiber fabrication strategies, including thermally-induced phase separation, molecular self-assembly, and electrospinning, highlighting their strengths, weaknesses, and potentials. The review then continues to discuss the biodegradable polyesters typically employed for nanofiber fabrication, while the primary focus lies on their applicability and limitations. From thereon, the reader is introduced to the concept of NTP and its application in plasma-assisted surface modification of nanofibrous scaffolds. The final part of the review discusses the available literature on NTP-modified nanofibers looking at the impact of plasma activation and polymerization treatments on nanofiber wettability, surface chemistry, cell adhesion/proliferation and protein grafting. As such, this review provides a complete introduction into NTP-modified nanofibers, while aiming to address the current unexplored potentials left within the field.

Atheroma Niche-Responsive Nanocarriers for Immunotherapeutic Delivery

Nanomedicine is a promising, noninvasive approach to reduce atherosclerotic plaque burden. However, drug delivery is limited without the ability of nanocarriers to sense and respond to the diseased microenvironment. In this study, nanomaterials are developed from peptide amphiphiles (PAs) that respond to the increased levels of matrix metalloproteinases 2 and 9 (MMP2/9) or reactive oxygen species (ROS) found within the atherosclerotic niche. A pro-resolving therapeutic, Ac2-26, derived from annexin-A1 protein, is tethered to PAs using peptide linkages that cleave in response to MMP2/9 or ROS. By adjusting the molar ratios and processing conditions, the Ac2-26 PA can be co-assembled with a PA containing an apolipoprotein A1-mimetic peptide to create a targeted, therapeutic nanofiber (ApoA1-Ac226 PA). The ApoA1-Ac2-26 PAs demonstrate release of Ac2-26 within 24 h after treatment with MMP2 or ROS. The niche-responsive ApoA1-Ac2-26 PAs are cytocompatible and reduce macrophage activation from interferon gamma and lipopolysaccharide treatment, evidenced by decreased nitric oxide production. Interestingly, the linkage chemistry of ApoA1-Ac2-26 PAs significantly affects macrophage uptake and retention. Taken together, these findings demonstrate the potential of PAs to serve as an atheroma niche-responsive nanocarrier system to modulate the inflammatory microenvironment, with implications for atherosclerosis treatment.

Self-assembling injectable peptide hydrogels for emerging treatment of ischemic stroke

Ischaemic stroke remains one of the leading causes of death and disability worldwide, without any long-term effective treatments targeted at regeneration. This has led to developments of novel, biomaterial-based strategies using self-assembling peptide hydrogels.

Supramolecular Packing and Macroscopic Alignment Controls Actuation Speed in Macroscopic Strings of Molecular Motor Amphiphiles

Three-dimensional organized unidirectionally aligned and responsive supramolecular structures have much potential in adaptive materials ranging from biomedical components to soft actuator systems. However, to control the supramolecular structure of these stimuli responsive, for example photoactive, materials and control their actuation remains a major challenge. Toward the design of “artificial muscles”, herein, we demonstrate an approach that allows hierarchical control of the supramolecular structure, and as a consequence its photoactuation function, by electrostatic interaction between motor amphiphiles (MA) and counterions. Detailed insight into the effect of various ions on structural parameters for self-assembly from nano- to micrometer scale in water including nanofiber formation and nanofiber aggregation as well as the packing structure, degree of alignment, and actuation speed of the macroscopic MA strings prepared from various metal chlorides solution, as determined by electronic microscopy, X-ray diffraction, and actuation speed measurements, is presented. Macroscopic MA strings prepared from calcium and magnesium ions provide a high degree of alignment and fast response photoactuation. By the selection of metal ions and chain length of MAs, the macroscopic MA string structure and function can be controlled, demonstrating the potential of generating multiple photoresponsive supramolecular systems from an identical molecular structure.

Peptide Nanomaterials Designed from Natural Supramolecular Systems

Natural supramolecular assemblies exhibit unique structural and functional properties that have been optimized over the course of evolution. Inspired by these natural systems, various bio-nanomaterials have been developed using peptides, proteins, and nucleic acids as components. Peptides are attractive building blocks because they enable the important domains of natural protein assemblies to be isolated and optimized while retaining the original structures and functions. Furthermore, the peptide subunits can be conjugated with exogenous molecules such as peptides, proteins, nucleic acids, and metal nanoparticles to generate advanced functions. In this personal account, we summarize recent progress in the construction of peptide-based nanomaterial designed from natural supramolecular systems, including (1) artificial viral capsids, (2) self-assembled nanofibers, and (3) protein-binding motifs. The peptides inspired by nature should provide new design principles for bio-nanomaterials.

Dynamic Hydrogels from Host-Guest Supramolecular Interactions

Hydrogel biomaterials are pervasive in biomedical use. Applications of these soft materials range from contact lenses to drug depots to scaffolds for transplanted cells. A subset of hydrogels is prepared from physical cross-linking mediated by host-guest interactions. Host macrocycles, the most recognizable supramolecular motif, facilitate complex formation with an array of guests by inclusion in their portal. Commonly, an appended macrocycle forms a complex with appended guests on another polymer chain. The formation of poly(pseudo)rotaxanes is also demonstrated, wherein macrocycles are threaded by a polymer chain to give rise to physical cross-linking by secondary non-covalent interactions or polymer jamming. Host-guest supramolecular hydrogels lend themselves to a variety of applications resulting from their dynamic properties that arise from non-covalent supramolecular interactions, as well as engineered responsiveness to external stimuli. These are thus an exciting new class of materials.

Self-Assembled Micellar Structures of Lipopeptides with Variable Number of Attached Lipid Chains Revealed by Atomistic Molecular Dynamics Simulations

We present atomistic molecular dynamics simulation study of the self-assembly behavior of toll-like agonist lipopeptides (Pam _nCSK4) in aqueous solutions. The variable number of hexadecyl lipid chains ( n = 1, 2, 3) per molecule has been experimentally suggested to have remarkable influence on their self-assembled nanostructures. Starting from preassembled spherical or bilayer configurations, the aggregates of lipopeptides, PamCSK4 and Pam₂CSK4, which contain peptide sequences CSK4 linked to either mono- or dilipid chains (Pam), evolve into spherical-like micelles within 30 ns, whereas the self-assembled structure of trilipidated lipopeptides, Pam₃CSK4, relaxes much slower and reaches an equilibrium state of flattened wormlike micelle with a bilayer packing structure. The geometric shapes and sizes, namely the gyration radii of spherical micelles and thickness of the flattened wormlike micelle, are found to be in good agreement with experimental measurements, which effectively validates the simulation models and employed force fields. Detailed analyses of molecular packing reveal that these self-assembled nanostructures all consist of a hydrophobic core constructed of lipid chains, a transitional layer, and a hydrophilic interfacial layer composed of peptide sequences. The average area per peptide head at the interfaces is found to be nearly constant for all micellar structures studied. The packing parameter of the lipopeptide molecules thus increases with the increase of the number of linked lipid chains, giving rise to the distinct micellar shape transition from spherical-like to flattened wormlike geometry with bilayer stacking, which is qualitatively different from the shape transitions of surfactant micelles induced by variation of concentration or salt type. To facilitate the close-packing of the lipid chains in the hydrophobic core, the lipopeptide molecules typically take the bent conformation with average tilt angles between the peptide sequences and the lipid chains ranging from 110° to 140°. This consequently affects the orientation angles of the lipid chains with respect to the radial or normal direction of the spherical-like or flattened wormlike micelles. In addition, the secondary structures of the peptides may also be altered by the number of lipid chains to which they are linked and the resultant micellar structures. Our simulation results on the microscopic structural features of the lipopeptide nanostructures may provide potential insights into their bioactivities and contribute to the design of bioactive medicines or drug carriers. The force fields built for these lipopeptides and the geometric packing discussions could also be adopted for simulating and understanding the self-assembly behavior of other bioactive amiphiphiles with similar chemical compositions.

Amphiphilic peptides as novel nanomaterials: design, self-assembly and application

Designer self-assembling peptides are a category of emerging nanobiomaterials which have been widely investigated in the past decades. In this field, amphiphilic peptides have received special attention for their simplicity in design and versatility in application. This review focuses on recent progress in designer amphiphilic peptides, trying to give a comprehensive overview about this special type of self-assembling peptides. By exploring published studies on several typical types of amphiphilic peptides in recent years, herein we discuss in detail the basic design, self-assembling behaviors and the mechanism of amphiphilic peptides, as well as how their nanostructures are affected by the peptide characteristics or environmental parameters. The applications of these peptides as potential nanomaterials for nanomedicine and nanotechnology are also summarized.

Functional Assemblies Emerging in Complex Mixtures of Peptides and Nucleic Acid-Peptide Chimeras

Striking synergy between nucleic acids and proteins is exhibited in living cells. Whether such mutual activity can be performed using simple supramolecular nucleic acid-peptide (NA-pep) architectures remains a mystery. To shed light on this question, we studied the emergence of a primitive synergy in assemblies of short DNA-peptide chimeras. Specifically, we characterized multiple structures forming along gradual mixing trajectory, in which a peptide solution was seeded with increasing amounts of NA-pep chimeras. We report on the systematic change from β-sheet-peptide-based fibrillar architectures into the spherical structures formed by the conjugates. Remarkably, we find that through forming onion-like structures, the conjugates exhibit increased DNA hybridization stability and bind small molecules more efficiently than the peptides or DNA alone. A brief discussion highlights the implications of our findings for the production of new materials and for research on the origin of life.

Self-assembly of peptide amphiphiles for drug delivery: the role of peptide primary and secondary structures

Peptide amphiphiles (PAs), functionalized with alkyl chains, are capable of self-assembling into various nanostructures. Recently, PAs have been considered as ideal drug carriers due to their good biocompatibility, specific biological functions, and hypotoxicity to normal cells and tissues. Meanwhile, the nanocarriers formed by PAs are able to achieve controlled drug release and enhanced cell uptake in response to the stimulus of the physiological environment or specific biological factors in the location of the lesion. However, the underlying detailed drug delivery mechanism, especially from the aspect of primary and secondary structures of PAs, has not been systematically summarized or discussed. Focusing on the relationship between the primary and secondary structures of PAs and stimuli-responsive drug delivery applications, this review highlights the recent advances, challenges, and opportunities of PA-based functional drug nanocarriers, and their potential pharmaceutical applications are discussed.

Multicomponent peptide assemblies

Self-assembled peptide nanostructures have been increasingly exploited as functional materials for applications in biomedicine and energy. The emergent properties of these nanomaterials determine the applications for which they can be exploited. It has recently been appreciated that nanomaterials composed of multicomponent coassembled peptides often display unique emergent properties that have the potential to dramatically expand the functional utility of peptide-based materials. This review presents recent efforts in the development of multicomponent peptide assemblies. The discussion includes multicomponent assemblies derived from short low molecular weight peptides, peptide amphiphiles, coiled coil peptides, collagen, and β-sheet peptides. The design, structure, emergent properties, and applications for these multicomponent assemblies are presented in order to illustrate the potential of these formulations as sophisticated next-generation bio-inspired materials.

Collagen mimetic peptide discs promote assembly of a broad range of natural protein fibers through hydrophobic interactions

Collagen mimetic peptides that alone formed two-dimensional nanoscale discs driven by hydrophobic interactions were shown in electron microscopy studies to also co-assemble with natural fibrous proteins to produce discs-on-a-string (DoS) nanostructures. In most cases, peptide discs also facilitated bundling of the protein fibers. This provides insight into how synthetic and natural proteins may be combined to develop multicomponent, multi-dimensional architectures at the nanoscale.

Self-assembly of amphiphilic tripeptides with sequence-dependent nanostructure

Supramolecular chemistry enables the creation of a diversity of nanostructures and materials. Many of these have been explored for applications as biomaterials and therapeutics. Among them, self-assembling peptides have been broadly applied. The structural diversity afforded from the library of amino acid building blocks has enabled control of emergent properties across length-scales. Here, we report on a family of amphiphilic tripeptides with sequence-controlled nanostructure. By altering one amino acid in these peptides, we can produce a diversity of nanostructures with different aspect-ratio and geometry. Peptides that produce high aspect-ratio structures can physically entangle to form hydrogels, which support cell viability in culture. Importantly, in comparison to many other short self-assembling peptide biomaterials, those reported here form filamentous nanostructures in the absence of typical secondary structures (i.e., β-sheet). Thus, we have illustrated a facile way to obtain versatile biomaterials with different nanostructural morphology from short and defined peptide sequences.

Injectable network biomaterials via molecular or colloidal self-assembly

Self-assembly is a powerful tool to create functional materials. A specific application for which self-assembled materials are ideally suited is in creating injectable biomaterials. Contrasting with traditional biomaterials that are implanted through surgical means, injecting biomaterials through the skin offers numerous advantages, expanding the scope and impact for biomaterials in medicine. In particular, self-assembled biomaterials prepared from molecular or colloidal interactions have been frequently explored. The strategies to create these materials are varied, taking advantage of engineered oligopeptides, proteins, and nanoparticles as well as affinity-mediated crosslinking of synthetic precursors. Self-assembled materials typically facilitate injectability through two different mechanisms: i) in situ self-assembly, whereby materials would be administered in a monomeric or oligomeric form and self-assemble in response to some physiologic stimulus, or ii) self-assembled materials that, by virtue of their dynamic, non-covalent interactions, shear-thin to facilitate flow within a syringe and subsequently self-heal into its reassembled material form at the injection site. Indeed, many classes of materials are capable of being injected using a combination of these two mechanisms. Particular utility has been noted for self-assembled biomaterials in the context of tissue engineering, regenerative medicine, drug delivery, and immunoengineering. Given the controlled and multifunctional nature of many self-assembled materials demonstrated to date, we project a future where injectable self-assembled biomaterials afford improved practice in advancing healthcare.

A Supramolecular Vaccine Platform Based on α-Helical Peptide Nanofibers

A supramolecular peptide vaccine system was designed in which epitope-bearing peptides self-assemble into elongated nanofibers composed almost entirely of alpha-helical structure. The nanofibers were readily internalized by antigen presenting cells and produced robust antibody, CD4+ T-cell, and CD8+ T-cell responses without supplemental adjuvants in mice. Epitopes studied included a cancer B-cell epitope from the epidermal growth factor receptor class III variant (EGFRvIII), the universal CD4+ T-cell epitope PADRE, and the model CD8+ T-cell epitope SIINFEKL, each of which could be incorporated into supramolecular multi-epitope nanofibers in a modular fashion.

Enhanced Antibacterial Properties of Self-Assembling Peptide Amphiphiles Functionalized with Heparin-Binding Cardin-Motifs

The emergence of antibiotic resistance in bacteria has caused many healthcare problems and social burdens. In this study, a type of self-assembled peptide amphiphiles (PA) functionalized with a heparin-binding Cardin-motif peptide (sequence (AKKARK)₂) has been designed to combat bacterial drug resistance. Above the critical micelle concentration (CMC) at 45 μM, these amphiphilic Cardin antimicrobial peptide (ACA-PA) can self-assemble into cylindrical supramolecular structures (7-10 nm in diameter) via hydrophobic interactions and β-sheet secondary conformation. The ACA-PA displays excellent antibacterial properties against both Gram-positive and Gram-negative bacteria. This work also demonstrates the effects of molecular self-assembly on antibacterial activity of peptide amphiphiles. The ACA-PA exhibits antibacterial activity on Gram-positive bacteria in a dose-dependent manner, but in the case of Gram-negative bacteria, the antibacterial potency of ACA-PA is remarkably enhanced at concentrations above the CMC. The ACA-PA has been shown to cause bacterial cytoplasmic leakage, causing localized membrane disruption in Gram-positive bacteria and blisters on disorganized membranes of Gram-negative bacteria. Therefore, these peptide-based nanoparticles have promising potential as antimicrobial agents without resorting to the use of antibiotics, and, thus, should be further studied for a wide range of biomaterial applications.

Supramolecular biofunctional materials

This review discusses supramolecular biofunctional materials, a novel class of biomaterials formed by small molecules that are held together via noncovalent interactions. The complexity of biology and relevant biomedical problems not only inspire, but also demand effective molecular design for functional materials. Supramolecular biofunctional materials offer (almost) unlimited possibilities and opportunities to address challenging biomedical problems. Rational molecular design of supramolecular biofunctional materials exploit powerful and versatile noncovalent interactions, which offer many advantages, such as responsiveness, reversibility, tunability, biomimicry, modularity, predictability, and, most importantly, adaptiveness. In this review, besides elaborating on the merits of supramolecular biofunctional materials (mainly in the form of hydrogels and/or nanoscale assemblies) resulting from noncovalent interactions, we also discuss the advantages of small peptides as a prevalent molecular platform to generate a wide range of supramolecular biofunctional materials for the applications in drug delivery, tissue engineering, immunology, cancer therapy, fluorescent imaging, and stem cell regulation. This review aims to provide a brief synopsis of recent achievements at the intersection of supramolecular chemistry and biomedical science in hope of contributing to the multidisciplinary research on supramolecular biofunctional materials for a wide range of applications. We envision that supramolecular biofunctional materials will contribute to the development of new therapies that will ultimately lead to a paradigm shift for developing next generation biomaterials for medicine.

Self-Assembling Multidomain Peptide Nanofibers for Delivery of Bioactive Molecules and Tissue Regeneration

Multidomain peptides (MDPs) are a class of self-assembling peptides that are organized in a β-sheet motif, resulting in a nanofibrous architecture. This structure is stabilized by hydrophobic packing in the fiber core and a hydrogen-bonding network down the fiber long axis. Under easily controllable conditions, regulated by electrostatic interactions between the peptides and the pH and salt composition of the solvent, the nanofiber length can be dramatically extended, resulting in fiber entanglement and hydrogel formation. One of the chief strengths of this supramolecular material is that the design criteria governing its structure and assembly are robust and permit a wide range of modifications without disruption. This allows the MDPs to be tailored to suit a wide range of applications, particularly in biomedical engineering. For example, delivery of small molecules, proteins, and cells is easily achievable. These materials can be trapped within the matrices of the hydrogel or trapped within the hydrophobic core of the nanofiber, depending on the cargo and the design of the MDP. Interactions between the nanofibers and their cargo can be tailored to alter the release profile, and in the most sophisticated cases, different cargos can be released in a cascading time-dependent fashion. The MDP hydrogel and its cargo can be targeted to specific locations, as the thixotropic nature of the hydrogel allows it to be easily aspirated into a syringe and then delivered from a narrow-bore needle. The sequence of amino acids making up the MDP can also be modified to permit cross-linking or enzymatic degradation. Selection of sequences with or without these modifications allows one to control the rate of degradation in vivo from as rapidly as 1 week to well over 6 weeks as the MDP nanofibers are degraded to their amino acid components. MDP sequences can also be modified to add biomimetic sequences derived from growth factors and other signaling proteins. These chemical signals are displayed at a very high density on the fibers’ surface, where they contribute to the modification of cellular behavior. We have used this approach to drive blood vessel formation, which is critical for tissue regeneration generally and more specifically for the treatment of diseases related to poor blood flow. MDPs represent an ideal case of bottom-up design where control of chemical structure leads to control of self-assembly and nanostructure and thereby control of material properties that collectively can control biological function.

Peptide hormones and lipopeptides: from self-assembly to therapeutic applications

This review describes the properties and activities of lipopeptides and peptide hormones and how the lipidation of peptide hormones could potentially produce therapeutic agents combating some of the most prevalent diseases and conditions. The self‐assembly of these types of molecules is outlined, and how this can impact on bioactivity. Peptide hormones specific to the uptake of food and produced in the gastrointestinal tract are discussed in detail. The advantages of lipidated peptide hormones over natural peptide hormones are summarised, in terms of stability and renal clearance, with potential application as therapeutic agents. © 2017 The Authors Journal of Peptide Science published by European Peptide Society and John Wiley & Sons Ltd.

Drug delivery by supramolecular design

Principles rooted in supramolecular chemistry have empowered new and highly functional therapeutics and drug delivery devices. This general approach offers elegant tools rooted in molecular and materials engineered to address the many challenges faced in treating disease.

Engineering responsive supramolecular biomaterials: Toward smart therapeutics

Engineering materials using supramolecular principles enables generalizable and modular platforms that have tunable chemical, mechanical, and biological properties. Applying this bottom-up, molecular engineering-based approach to therapeutic design affords unmatched control of emergent properties and functionalities. In preparing responsive materials for biomedical applications, the dynamic character of typical supramolecular interactions facilitates systems that can more rapidly sense and respond to specific stimuli through a fundamental change in material properties or characteristics, as compared to cases where covalent bonds must be overcome. Several supramolecular motifs have been evaluated toward the preparation of "smart" materials capable of sensing and responding to stimuli. Triggers of interest in designing materials for therapeutic use include applied external fields, environmental changes, biological actuators, applied mechanical loading, and modulation of relative binding affinities. In addition, multistimuli-responsive routes can be realized that capture combinations of triggers for increased functionality. In sum, supramolecular engineering offers a highly functional strategy to prepare responsive materials. Future development and refinement of these approaches will improve precision in material formation and responsiveness, seek dynamic reciprocity in interactions with living biological systems, and improve spatiotemporal sensing of disease for better therapeutic deployment.

A tenascin-C mimetic peptide amphiphile nanofiber gel promotes neurite outgrowth and cell migration of neurosphere-derived cells

Biomimetic materials that display natural bioactive signals derived from extracellular matrix molecules like laminin and fibronectin hold promise for promoting regeneration of the nervous system. In this work, we investigated a biomimetic peptide amphiphile (PA) presenting a peptide derived from the extracellular glycoprotein tenascin-C, known to promote neurite outgrowth through interaction with β1 integrin. The tenascin-C mimetic PA (TN-C PA) was found to self-assemble into supramolecular nanofibers and was incorporated through co-assembly into PA gels formed by highly aligned nanofibers. TN-C PA content in these gels increased the length and number of neurites produced from neurons differentiated from encapsulated P19 cells. Furthermore, gels containing TN-C PA were found to increase migration of cells out of neurospheres cultured on gel coatings. These bioactive gels could serve as artificial matrix therapies in regions of neuronal loss to guide neural stem cells and promote through biochemical cues neurite extension after differentiation. One example of an important target would be their use as biomaterial therapies in spinal cord injury.

STATEMENT OF SIGNIFICANCE

Tenascin-C is an important extracellular matrix molecule in the nervous system and has been shown to play a role in regenerating the spinal cord after injury and guiding neural progenitor cells during brain development, however, minimal research has been reported exploring the use of biomimetic biomaterials of tenascin-C. In this work, we describe a selfassembling biomaterial system in which peptide amphiphiles present a peptide derived from tenascin-C that promotes neurite outgrowth. Encapsulation of neurons in hydrogels of aligned nanofibers formed by tenascin-C-mimetic peptide amphiphiles resulted in enhanced neurite outgrowth. Additionally, these peptide amphiphiles promoted migration of neural progenitor cells cultured on nanofiber coatings. Tenascin-C biomimetic biomaterials such as the one described here have significant potential in neuroregenerative medicine.

Super-resolution microscopy reveals structural diversity in molecular exchange among peptide amphiphile nanofibres

The dynamic behaviour of supramolecular systems is an important dimension of their potential functions. Here, we report on the use of stochastic optical reconstruction microscopy to study the molecular exchange of peptide amphiphile nanofibres, supramolecular systems known to have important biomedical functions. Solutions of nanofibres labelled with different dyes (Cy3 and Cy5) were mixed, and the distribution of dyes inserting into initially single-colour nanofibres was quantified using correlative image analysis. Our observations are consistent with an exchange mechanism involving monomers or small clusters of molecules inserting randomly into a fibre. Different exchange rates are observed within the same fibre, suggesting that local cohesive structures exist on the basis of β-sheet discontinuous domains. The results reported here show that peptide amphiphile supramolecular systems can be dynamic and that their intermolecular interactions affect exchange patterns. This information can be used to generate useful aggregate morphologies for improved biomedical function.

Dynamic behaviour in supramolecular systems is an important aspect of their functionality. Here, the authors use stochastic optical reconstruction microscopy to unveil structural diversity in self-assembled peptide amphiphile nanofibres, with potential relevance to biomedical applications.

Tissue-Factor Targeted Peptide Amphiphile Nanofibers as an Injectable Therapy To Control Hemorrhage

Noncompressible torso hemorrhage is a leading cause of mortality in civilian and battlefield trauma. We sought to develop an i.v.-injectable, tissue factor (TF)-targeted nanotherapy to stop hemorrhage. Tissue factor was chosen as a target because it is only exposed to the intravascular space upon vessel disruption. Peptide amphiphile (PA) monomers that self-assemble into nanofibers were chosen as the delivery vehicle. Three TF-binding sequences were identified (EGR, RLM, and RTL), covalently incorporated into the PA backbone, and shown to self-assemble into nanofibers by cryo-transmission electron microscopy. Both the RLM and RTL peptides bound recombinant TF in vitro. All three TF-targeted nanofibers bound to the site of punch biopsy-induced liver hemorrhage in vivo, but only RTL nanofibers reduced blood loss versus sham (53% reduction, p < 0.05). Increasing the targeting ligand density of RTL nanofibers yielded qualitatively better binding to the site of injury and greater reductions in blood loss in vivo (p < 0.05). In fact, 100% RTL nanofiber reduced overall blood loss by 60% versus sham (p < 0.05). Evaluation of the biocompatibility of the RTL nanofiber revealed that it did not induce RBC hemolysis, did not induce neutrophil or macrophage inflammation at the site of liver injury, and 70% remained intact in plasma after 30 min. In summary, these studies demonstrate successful binding of peptides to TF in vitro and successful homing of a TF-targeted PA nanofiber to the site of hemorrhage with an associated decrease in blood loss in vivo. Thus, this therapeutic may potentially treat noncompressible hemorrhage.

Supramolecular biomaterials

Polymers, ceramics and metals have historically dominated the application of materials in medicine. Yet rationally designed materials that exploit specific, directional, tunable and reversible non-covalent interactions offer unprecedented advantages: they enable modular and generalizable platforms with tunable mechanical, chemical and biological properties. Indeed, the reversible nature of supramolecular interactions gives rise to biomaterials that can sense and respond to physiological cues, or that mimic the structural and functional aspects of biological signalling. In this Review, we discuss the properties of several supramolecular biomaterials, as well as their applications in drug delivery, tissue engineering, regenerative medicine and immunology. We envision that supramolecular biomaterials will contribute to the development of new therapies that combine highly functional materials with unmatched patient- and application-specific tailoring of both material and biological properties.

Cell Interactions with Vascular Regenerative MAA-Based Materials in the Context of Wound Healing

In diabetic patients the development of chronic non-healing wounds is a common complication. A methacrylic acid-based biomaterial is a vascular regenerative material that enhances diabetic healing without the use of cells or growth factors. The bioactive nature of this material is thought to be associated with its anionic charge or surface chemistry. Contact between the methacrylic acid-based biomaterial and tissue begins with protein (including complement) adsorption and is followed by interaction of the biomaterial with resident and infiltrating cells in the wound bed (e.g., macrophages and endothelial cells). This results in changes to their surface receptors to activate phosphorylation cascades that lead to differential activation of signalling pathways such as those involving osteopontin and sonic hedgehog. These changes modulate the phenotype of the cells in the wound bed, eventually improving vessel formation and wound healing. Understanding the molecular and cellular mechanisms will have broad implications for biomaterials, not just the methacrylic acid-based material, and will facilitate the advancement of regenerative biomaterials for diverse applications.

Self-Assembled Proteins and Peptides as Scaffolds for Tissue Regeneration

Self-assembling proteins and peptides are increasingly gaining interest for potential use as scaffolds in tissue engineering applications. They self-organize from basic building blocks under mild conditions into supramolecular structures, mimicking the native extracellular matrix. Their properties can be easily tuned through changes at the sequence level. Moreover, they can be produced in sufficient quantities with chemical synthesis or recombinant technologies to allow them to address homogeneity and standardization issues required for applications. Here. recent advances in self-assembling proteins, peptides, and peptide amphiphiles that form scaffolds suitable for tissue engineering are reviewed. The focus is on a variety of motifs, ranging from minimalistic dipeptides, simplistic ultrashort aliphatic peptides, and peptide amphiphiles to large "recombinamer" proteins. Special emphasis is placed on the rational design of self-assembling motifs and biofunctionalization strategies to influence cell behavior and modulate scaffold stability. Perspectives for combination of these "bottom-up" designer strategies with traditional "top-down" biofabrication techniques for new generations of tissue engineering scaffolds are highlighted.

Modifying a known gelator scaffold for nitrite detection

The process of selecting and modifying a known gelator scaffold to develop a new nitrite-based sensor is described. Five new azo-sulfonate gelators were discovered and characterized. The most promising scaffold exhibits a stable diazonium intermediate, proceeds in a high yield, and gels nitrite-spiked tap, river, and pond water.

Biomimetic approaches in bone tissue engineering: Integrating biological and physicomechanical strategies

The development of responsive biomaterials capable of demonstrating modulated function in response to dynamic physiological and mechanical changes in vivo remains an important challenge in bone tissue engineering. To achieve long-term repair and good clinical outcomes, biologically responsive approaches that focus on repair and reconstitution of tissue structure and function through drug release, receptor recognition, environmental responsiveness and tuned biodegradability are required. Traditional orthopedic materials lack biomimicry, and mismatches in tissue morphology, or chemical and mechanical properties ultimately accelerate device failure. Multiple stimuli have been proposed as principal contributors or mediators of cell activity and bone tissue formation, including physical (substrate topography, stiffness, shear stress and electrical forces) and biochemical factors (growth factors, genes or proteins). However, optimal solutions to bone regeneration remain elusive. This review will focus on biological and physicomechanical considerations currently being explored in bone tissue engineering.

Biocompatibility and characterization of a peptide amphiphile hydrogel for applications in peripheral nerve regeneration

Peripheral nerve injury is a debilitating condition for which new bioengineering solutions are needed. Autografting, the gold standard in treatment, involves sacrifice of a healthy nerve and results in loss of sensation or function at the donor site. One alternative solution to autografting is to use a nerve guide conduit designed to physically guide the nerve as it regenerates across the injury gap. Such conduits are effective for short gap injuries, but fail to surpass autografting in long gap injuries. One strategy to enhance regeneration inside conduits in long gap injuries is to fill the guide conduits with a hydrogel to mimic the native extracellular matrix found in peripheral nerves. In this work, a peptide amphiphile (PA)-based hydrogel was optimized for peripheral nerve repair. Hydrogels consisting of the PA C₁₆GSH were compared with a commercially available collagen gel. Schwann cells, a cell type important in the peripheral nerve regenerative cascade, were able to spread, proliferate, and migrate better on C₁₆GSH gels in vitro when compared with cells seeded on collagen gels. Moreover, C₁₆GSH gels were implanted subcutaneously in a murine model and were found to be biocompatible, degrade over time, and support angiogenesis without causing inflammation or a foreign body immune response. Taken together, these results help optimize and instruct the development of a new synthetic hydrogel as a luminal filler for conduit-mediated peripheral nerve repair.

The powerful functions of peptide-based bioactive matrices for regenerative medicine

In an effort to develop bioactive matrices for regenerative medicine, peptides have been used widely to promote interactions with cells and elicit desired behaviors in vivo. This paper describes strategies that utilize peptide-based molecules as building blocks to create supramolecular nanostructures that emulate not only the architecture but also the chemistry of the extracellular matrix in mammalian biology. After initiating a desired regenerative response in vivo, the innate biodegradability of these systems allow for the natural biological processes to take over in order to promote formation of a new tissue without leaving a trace of the nonnatural components. These bioactive matrices can either bind or mimic growth factors or other protein ligands to elicit a cellular response, promote specific mechano-biological responses, and also guide the migration of cells with programmed directionality. In vivo applications discussed in this review using peptide-based matrices include the regeneration of axons after spinal cord injury, regeneration of bone, and the formation of blood vessels in ischemic muscle as a therapy in peripheral arterial disease and cardiovascular diseases.