Structuring supramolecular hyaluronan hydrogels via peptide self-assembly for modulating the cell microenvironment

The use of synthetic extracellular matrices (ECMs) in fundamental in vitro cell culture studies has been instrumental for investigating the interplay between cells and matrix components. To provide cells with a more native environment in vitro, it is desirable to design matrices that are biomimetic and emulate compositional and structural features of natural ECMs. Here, the supramolecular fabrication of peptide-hyaluronan (HA) hydrogels is presented as potential ECM surrogates, combining native HA and rationally designed cationic amphipatic peptides [(KI)nK, lysine (K), isoleucine (I), n ​= ​2-6] whose mechanical properties and microstructure are tunable by the peptide sequence. (KI)nK peptides adopt β-sheet configuration and self-assemble into filamentous nanostructures triggered by pH or ionic strength. The self-assembly propensity of (KI)nK peptides increases with the sequence length, forming single phase hydrogels (shorter peptides) or with phase separation (longer peptides) in presence of the anionic polyelectrolyte HA through electrostatic complexations. The gel phase formed in (KI)nK-HA complexes exhibits viscoelastic behavior and triggers the formation of human mesenchymal stem cell (MSC) spheroids which disassemble over the time. It is anticipated that these (KI)nK-HA hydrogels with tunable physical and biochemical properties offer a promising platform for in vitro applications and in stem cell therapy.

Surface Self-Assembly of Dipeptides on Porous CaCO3 Particles Promoting Cell Internalization

The self-assembling behavior of peptides and derivatives is crucial in the natural process to construct various architectures and achieve specific functions. However, the surface or interfacial self-assembly, in particular, on the surface of micro- or nanoparticles is even less systematically investigated. Here, uniform porous CaCO₃ microparticles were prepared with different charged, hydrophobic and hydrophilic surfaces to assess the self-assembling behavior of dipeptides composed of various sequences. Experimental results indicate that dipeptides with a negative charge in an aqueous solution preferred to self-assemble on the hydrophobic and positively charged surface of CaCO₃ particles, which can be ascribed to the electrostatic and hydrophobic interaction between dipeptides and CaCO₃ particles. Meanwhile, the Log p (lipid-water partition coefficient) of dipeptides has a significant effect on the self-assembling behavior of dipeptides on the surface of porous CaCO₃; dipeptides with high Log p preferred to self-assemble on the surface of CaCO₃ particles, resulting in the improved cell internalization efficiency of particles with low cytotoxicity. After loading with a model drug (doxorubicin), the particles show obvious antitumor activity in animal experiments and can reduce Dox side effects effectively.

Time matters for macroscopic membranes formed by alginate and cationic β-sheet peptides

Hierarchically ordered planar and spherical membranes (sacs) were constructed using amphiphilic and cationic β-sheet peptides that spontaneously assembled together with negatively charged alginate solution. The system was found to form either a fully developed membrane structure with three distinct regions including characteristic perpendicular fibers or a non-fully developed contact layer lacking these standing fibers, depending on the peptide age, membrane geometry and membrane incubation time. The morphological differences were found to strongly depend on fairly-long incubation time frames that influenced both the peptide's intrinsic alignment and the reaction-diffusion process taking place at the interface. A three-stage mechanism was suggested and key parameters affecting the development process were identified. Stability tests in biologically relevant buffers confirmed the suitability of these membranes for bio applications.

Lilypad aggregation: localised self-assembly and metal sequestration at a liquid-vapour interface

Spatially resolved soft materials, such as vesicles and microgels, have shown promise as selective adsorbents and microscale reaction vessels. However, spatiotemporal control of aggregation can be difficult to achieve. In this study, nickel(ii) chloride and a dipyridyl oligo(urea) ligand were combined in a vapour-diffusion setup to produce a localised spheroidal aggregate at the liquid-vapour interface. This aggregate forms via the self-assembly and fusion of monodisperse colloids and grows until its weight is no longer counterbalanced by surface tension. A simple physical model reveals that this process, termed lilypad aggregation, is possible only for surface energies that favour neither bulk aggregation nor the growth of an interfacial film. These surface energies dictate the final size and shape of the aggregate and may be estimated through visual monitoring of its changing morphology. Lilypad aggregates sequester metal from the surrounding sol and can be collected manually from the surface of the liquid.

Poly(vinyl alcohol)-induced thixotropy of an l-carnosine-based cytocompatible, tripeptidic hydrogel

The generally poor mechanical stability of hydrogels limits their use as functional materials for many biomedical applications. In this work, a poly(vinyl alcohol) (PVA) embedded hybrid hydrogel of a β-amino acid-containing Fmoc-protected tripeptide was produced at physiological pH (7.4) and room temperature. The hydrogel system was characterized by a number of techniques, including UV-vis, fluorescence, circular dichroism, FT-IR spectroscopy, electron microscopy, and rheology. While the tripeptide-based pure hydrogel was found to be unstable after ca. half an hour, addition of PVA, a water soluble polymer, increased the temporal and mechanical stability of the hydrogel. A rheological step-strain experiment demonstrates that the peptide-polymer hydrogel is thixotropic. Results from a fluorescence probe study and transmission electron microscopy reveal that addition of PVA increases both the fibre diameter and entanglement. Circular dichroism spectra of the hydrogels confirm the formation of aggregates with supramolecular chirality. The thixotropic nature of the hydrogel has been exploited to entrap and release doxorubicin, an anticancer drug, under physiological conditions. Furthermore, an MTT assay of the Fmoc-tripeptide using AH927 cells confirmed its cytocompatibility, which broadens the utility of the hybrid gel for biomedical applications.

One-Step Assembly of a Biomimetic Biopolymer Coating for Particle Surface Engineering

Advances in material design and applications are highly dependent on the development of particle surface engineering strategies. However, few universal methods can functionalize particles of different compositions, sizes, shapes, and structures. The amyloid-like lysozyme assembly-mediated surface functionalization of inorganic, polymeric or metal micro/nanoparticles in a unique amyloid-like phase-transition buffer containing lysozyme are described. The rapid formation of a robust nanoscale phase-transitioned lysozyme (PTL) coating on the particle surfaces presents strong interfacial binding to resist mechanical and chemical peeling under harsh conditions and versatile surface functional groups to support various sequential surface chemical derivatizations, such as radical living graft polymerization, the electroless deposition of metals, biomineralization, and the facile synthesis of Janus particles and metal/protein capsules. Being distinct from other methods, the preparation of this pure protein coating under biocompatible conditions (e.g., neutral pH and nontoxic reagents) provides a reliable opportunity to directly modify living cell surfaces without affecting their biological activity. The PTL coating arms yeasts with a functional shell to protect their adhered body against foreign enzymatic digestion. The PTL coating further supports the surface immobilization of living yeasts for heterogeneous microbial reactions and the sequential surface chemical derivatization of the cell surfaces, e.g., radical living graft polymerization.

Supramolecular Peptide/Polymer Hybrid Hydrogels for Biomedical Applications

Peptides and polymers are the "elite" building blocks in hydrogel fabrication where the typical approach consists of coupling specific peptide sequences (cell adhesive and/or enzymatically cleavable) to polymer chains aiming to obtain controlled cell responses (adhesion, migration, differentiation). However, the use of polymers and peptides as structural components for fabricating supramolecular hydrogels is less well established. Here, the literature on the design of peptide/polymer systems for self-assembly into hybrid hydrogels, as either peptide-polymer conjugates or combining both components individually, is reviewed. The properties (stiffness, mesh structure, responsiveness, and biocompatibility) of the hydrogels are then discussed from the viewpoint of their potential biomedical applications.

Cooperative Assembly of a Peptide Gelator and Silk Fibroin Afford an Injectable Hydrogel for Tissue Engineering

Silk fibroin (SF) from Bombyx mori has received increasing interest in biomedical fields, because of its slow biodegradability, good biocompatibility, and low immunogenicity. Although SF-based hydrogels have been studied intensively as a potential matrix for tissue engineering, weak gelation performance and low mechanical strength are major limitations that hamper their widespread applicability. Therefore, searching for new strategies to improve the SF gelation property is highly desirable in tissue engineering research. Herein, we report a facile approach to induce rapid gelation of SF by a small peptide gelator (e.g., NapFF). Following the simple mixing of SF and NapFF in water, a stable hydrogel of SF was obtained in a short time period at physiological pH, and the minimum gelation concentration of SF can reach as low as 0.1%. In this process of gelation, NapFF not only can behave itself as a gelator for supramolecular self-assembly, but also can trigger the conformational transition of the SF molecule from random coil to β-sheet structure via hydrophobic and hydrogen-bonding interactions. More importantly, for the generation of a scaffold with favorable cell-surface interactions, a new peptide gelator (NapFFRGD) with Arg-Gly-Asp (RGD) domain was applied to functionalize SF hydrogel with improved bioactivity for cell adhesion and growth. Following encapsulating the vascular endothelial growth factor (VEGF), the SF gel was subcutaneously injected in mice, and served as an effective matrix to trigger the generation of new blood capillaries in vivo.

A fluidic device for the controlled formation and real-time monitoring of soft membranes self-assembled at liquid interfaces

Membrane materials formed at the interface between two liquids have found applications in a large variety of technologies, from sensors to drug-delivery and catalysis. However, studying the formation of these membranes in real-time presents considerable challenges, owing to the difficulty of prescribing the location and instant of formation of the membrane, the difficulty of observing time-dependent membrane shape and thickness, and the poor reproducibility of results obtained using conventional mixing procedures. Here we report a fluidic device that facilitates characterisation of the time-dependent thickness, morphology and mass transport properties of materials self-assembled at fluid-fluid interfaces. In the proposed device the membrane forms from the controlled coalescence of two liquid menisci in a linear open channel. The linear geometry and controlled mixing of the solutions facilitate real-time visualisation, manipulation and improve reproducibility. Because of its small dimensions, the device can be used in conjunction with standard microscopy methods and reduces the required volumes of potentially expensive reagents. As an example application to tissue engineering, we use the device to characterise interfacial membranes formed by supra-molecular self-assembly of peptide-amphiphiles with either an elastin-like-protein or hyaluronic acid. The device can be adapted to study self-assembling membranes for applications that extend beyond bioengineering.

Multicomponent self-assembly as a tool to harness new properties from peptides and proteins in material design

Nature is enriched with a wide variety of complex, synergistic and highly functional protein-based multicomponent assemblies.

Nanostructured interfacial self-assembled peptide-polymer membranes for enhanced mineralization and cell adhesion

Soft interfacial materials, such as self-assembled polymer membranes, are gaining increasing interest as biomaterials since they can provide selective barriers and/or controlled affinity interactions important to regulate cellular processes. Herein, we report the design and fabrication of multiscale structured membranes integrating selective molecular functionalities for potential applications in bone regeneration. The membranes were obtained by interfacial self-assembly of miscible aqueous solutions of hyaluronan and multi-domain peptides (MDPs) incorporating distinct biochemical motifs, including mineralizing (EE), integrin-binding (RGDS) and osteogenic (YGFGG) peptide sequences. Circular dichroism and Fourier transform infrared spectroscopy analyses of the MDPs revealed a predominant β-sheet conformation, while transmission electron microscopy (TEM) showed the formation of fibre-like nanostructures with different lengths. Scanning electron microscopy (SEM) of the membranes showed an anisotropic structure and surfaces with different nanotopographies, reflecting the morphological differences observed under TEM. All the membranes were able to promote the deposition of a calcium-phosphate mineral on their surface when incubated in a mineralizing solution. The ability of the MDPs, coated on coverslips or presented within the membranes, to support cell adhesion was investigated using primary adult periosteum-derived cells (PDCs) under serum-free conditions. Cells on the membranes lacking RGDS remained round, while in the presence of RGDS they appear to be more elongated and anchored to the membrane. These observations were confirmed by SEM analysis that showed cells attached to the membrane and exhibiting an extended morphology with close interactions with the membrane surface. We anticipate that these molecularly designed interfacial membranes can both provide relevant biochemical signals and structural biomimetic components for stem cell growth and differentiation and ultimately promote bone regeneration.

Protein microgels from amyloid fibril networks

Nanofibrillar forms of proteins were initially recognized in the context of pathology, but more recently have been discovered in a range of functional roles in nature, including as active catalytic scaffolds and bacterial coatings. Here we show that protein nanofibrils can be used to form the basis of monodisperse microgels and gel shells composed of naturally occurring proteins. We explore the potential of these protein microgels to act as drug carrier agents, and demonstrate the controlled release of four different encapsulated drug-like small molecules, as well as the component proteins themselves. Furthermore, we show that protein nanofibril self-assembly can continue after the initial formation of the microgel particles, and that this process results in active materials with network densities that can be modulated in situ. We demonstrate that these materials are nontoxic to human cells and that they can be used to enhance the efficacy of antibiotics relative to delivery in homogeneous solution. Because of the biocompatibility and biodegradability of natural proteins used in the fabrication of the microgels, as well as their ability to control the release of small molecules and biopolymers, protein nanofibril microgels represent a promising class of functional artificial multiscale materials generated from natural building blocks.

Functional supramolecular polymers for biomedical applications

As a novel class of dynamic and non-covalent polymers, supramolecular polymers not only display specific structural and physicochemical properties, but also have the ability to undergo reversible changes of structure, shape, and function in response to diverse external stimuli, making them promising candidates for widespread applications ranging from academic research to industrial fields. By an elegant combination of dynamic/reversible structures with exceptional functions, functional supramolecular polymers are attracting increasing attention in various fields. In particular, functional supramolecular polymers offer several unique advantages, including inherent degradable polymer backbones, smart responsiveness to various biological stimuli, and the ease for the incorporation of multiple biofunctionalities (e.g., targeting and bioactivity), thereby showing great potential for a wide range of applications in the biomedical field. In this Review, the trends and representative achievements in the design and synthesis of supramolecular polymers with specific functions are summarized, as well as their wide-ranging biomedical applications such as drug delivery, gene transfection, protein delivery, bio-imaging and diagnosis, tissue engineering, and biomimetic chemistry. These achievements further inspire persistent efforts in an emerging interdisciplin-ary research area of supramolecular chemistry, polymer science, material science, biomedical engineering, and nanotechnology.

Stable emulsions formed by self-assembly of interfacial networks of dipeptide derivatives

We demonstrate the use of dipeptide amphiphiles that, by hand shaking of a biphasic solvent system for a few seconds, form emulsions that remain stable for months through the formation of nanofibrous networks at the organic/aqueous interface. Unlike absorption of traditional surfactants, the interfacial networks form by self-assembly through π-stacking interactions and hydrogen bonding. Altering the dipeptide sequence has a dramatic effect on the properties of the emulsions formed, illustrating the possibility of tuning emulsion properties by chemical design. The systems provide superior long-term stability toward temperature and salts compared to with sodium dodecyl sulfate (SDS) and can be enzymatically disassembled causing on-demand demulsification under mild conditions. The interfacial networks facilitate highly tunable and stable encapsulation and compartmentalization with potential applications in cosmetics, therapeutics, and food industry.

Cooperative self-assembly of peptide gelators and proteins

Molecular self-assembly provides a versatile route for the production of nanoscale materials for medical and technological applications. Herein, we demonstrate that the cooperative self-assembly of amphiphilic small molecules and proteins can have drastic effects on supramolecular nanostructuring of resulting materials. We report that mesoscale, fractal-like clusters of proteins form at concentrations that are orders of magnitude lower compared to those usually associated with molecular crowding at room temperature. These protein clusters have pronounced effects on the molecular self-assembly of aromatic peptide amphiphiles (fluorenylmethoxycarbonyl- dipeptides), resulting in a reversal of chiral organization and enhanced order through templating and binding. Moreover, the morphological and mechanical properties of the resultant nanostructured gels can be controlled by the cooperative self-assembly of peptides and protein fractal clusters, having implications for biomedical applications where proteins and peptides are both present. In addition, fundamental insights into cooperative interplay of molecular interactions and confinement by clusters of chiral macromolecules is relevant to gaining understanding of the molecular mechanisms of relevance to the origin of life and development of synthetic mimics of living systems.

Supramolecular Nanofibers of Peptide Amphiphiles for Medicine

Peptide nanostructures are an exciting class of supramolecular systems that can be designed for novel therapies with great potential in advanced medicine. This paper reviews progress on nanostructures based on peptide amphiphiles capable of forming one-dimensional assemblies that emulate in structure the nanofibers present in extracellular matrices. These systems are highly tunable using supramolecular chemistry, and can be designed to signal cells directly with bioactive peptides. Peptide amphiphile nanofibers can also be used to multiplex functions through co-assembly and designed to deliver proteins, nucleic acids, drugs, or cells. We illustrate here the functionality of these systems describing their use in regenerative medicine of bone, cartilage, the nervous system, the cardiovascular system, and other tissues. In addition, we highlight recent work on the use of peptide amphiphile assemblies to create hierarchical biomimetic structures with order beyond the nanoscale, and also discuss the future prospects of these supramolecular systems.

Fabrication of phospholipid-xanthan microcapsules by combining microfluidics with self-assembly

We report the synthesis of an amphiphilic polysaccharide, a phospholipid (1,2-dioleoyl-sn-glycero-phosphoetilamine, DOPE) conjugated with the anionic xanthan gum, and its ability to spontaneously self-assemble under mild aqueous conditions. This work also aimed to apply a microfluidic platform that can precisely fabricate microsized and monodispersed capsules for cell encapsulation. Stable hollow capsular structures were obtained by the generation of homogeneous spherical droplets of the self-assembled polymer in the microfluidic device through the formation of a water-in-oil emulsion, followed by the stabilization of the polymer aggregates in a separate collection vessel containing phosphate-buffered saline (physiological ionic strength and pH). The properties (size, morphology, permeability) and performance (stability) of the obtained microcapsules were studied, as well their ability to support the viability, function and proliferation of encapsulated cells. ATDC5 cells were encapsulated within the capsules and shown to remain viable, evidencing increased cellular metabolic activity over 21 days of in vitro culture. By combining microfluidic droplet generation and self-assembly of xanthan-DOPE, we were able to fabricate microcapsules that provided an adequate environment for cells to survive and proliferate.

Self-assembly of biomolecular soft matter

Self-assembly programmed by molecular structure and guided dynamically by energy dissipation is a ubiquitous phenomenon in biological systems that build functional structures from the nanoscale to macroscopic dimensions. This paper describes examples of one-dimensional self-assembly of peptide amphiphiles and the consequent biological functions that emerge in these systems. We also discuss here hierarchical self-assembly of supramolecular peptide nanostructures and polysaccharides, and some new results are reported on supramolecular crystals formed by highly charged peptide amphiphiles. Reflecting on presentations at this Faraday Discussion, the paper ends with a discussion of some of the future opportunities and challenges of the field.

Advances in cryogenic transmission electron microscopy for the characterization of dynamic self-assembling nanostructures

Elucidating the structural information of nanoscale materials in their solvent-exposed state is crucial, as a result, cryogenic transmission electron microscopy (cryo-TEM) has become an increasingly popular technique in the materials science, chemistry, and biology communities. Cryo-TEM provides a method to directly visualize the specimen structure in a solution-state through a thin film of vitrified solvent. This technique complements X-ray, neutron, and light scattering methods that probe the statistical average of all species present; furthermore, cryo-TEM can be used to observe changes in structure over time. In the area of self-assembly, this tool has been particularly powerful for the characterization of natural and synthetic small molecule assemblies, as well as hybrid organic-inorganic composites. In this review, we discuss recent advances in cryogenic TEM in the context of self-assembling systems with emphasis on characterization of transitions observed in response to external stimuli.

Microfluidic fabrication of self-assembled peptide-polysaccharide microcapsules as 3D environments for cell culture

We report a mild cell encapsulation method based on self-assembly and microfluidics technology. Xanthan gum, an anionic polysaccharide, was used to trigger the self-assembly of a positively charged multidomain peptide. The self-assembly resulted in the formation of a nanofibrous matrix and using a microfluidic device, microcapsules with homogeneous size were fabricated. The properties and performance of xanthan-peptide microcapsules were optimized by changing peptide/polysaccharide ratio and their effects on the microcapsule permeability and mechanical stability were analyzed. The effect of microcapsule formulation on viability and proliferation of encapsulated chondrocytic (ATDC5) cells was also investigated. The encapsulated cells were metabolically active, showing an increased viability and proliferation over 21 days of in vitro culture, demonstrating the long-term stability of the self-assembled microcapsules and their ability to support and enhance the survival of encapsulated cells over a prolonged time. Self-assembling materials combined with microfluidics demonstrated to be an innovative approach in the fabrication of cytocompatible matrix for cell microencapsulation and delivery.

Functional supramolecular polymers

Supramolecular polymers can be random and entangled coils with the mechanical properties of plastics and elastomers, but with great capacity for processability, recycling, and self-healing due to their reversible monomer-to-polymer transitions. At the other extreme, supramolecular polymers can be formed by self-assembly among designed subunits to yield shape-persistent and highly ordered filaments. The use of strong and directional interactions among molecular subunits can achieve not only rich dynamic behavior but also high degrees of internal order that are not known in ordinary polymers. They can resemble, for example, the ordered and dynamic one-dimensional supramolecular assemblies of the cell cytoskeleton and possess useful biological and electronic functions.

Imaging enzyme-triggered self-assembly of small molecules inside live cells

Self-assembly of small molecules in water to form nanofibres, besides generating sophisticated biomaterials, promises a simple system inside cells for regulating cellular processes. But lack of a convenient approach for studying the self-assembly of small molecules inside cells hinders the development of such systems. Here we report a method to image enzyme-triggered self-assembly of small molecules inside live cells. After linking a fluorophore to a self-assembly motif to make a precursor, we confirmed by (31)P NMR and rheology that enzyme-triggered conversion of the precursor to a hydrogelator results in the formation of a hydrogel via self-assembly. The imaging contrast conferred by the nanofibres of the hydrogelators allowed the evaluation of intracellular self-assembly, the dynamics and the localization of the nanofibres of the hydrogelators in live cells. This approach explores supramolecular chemistry inside cells and may lead to new insights, processes or materials at the interface of chemistry and biology.

Self-assembling peptide scaffolds for regenerative medicine

Biomaterials made from self-assembling, short peptides and peptide derivatives have great potential to generate powerful new therapies in regenerative medicine. The high signaling capacity and therapeutic efficacy of peptidic scaffolds has been established in several animal models, and the development of more complex, hierarchical structures based on peptide materials is underway. This highlight discusses several classes of self-assembling peptide-based materials, including peptide amphiphiles, Fmoc-peptides, self-complementary ionic peptides, hairpin peptides, and others. The self-assembly designs, bioactive signalling strategies, and cell signalling capabilities of these bioactive materials are reported. The future challenges of the field are also discussed, including short-term goals such as integration with biopolymers and traditional implants, and long term goals, such as immune system programming, subcellular targeting, and the development of highly integrated scaffold systems.