Glutathione-triggered formation of molecular hydrogels for 3D cell culture

Self-Assembled Peptide Hydrogel for Accelerated Wound Healing: Impact of N-Terminal and C-Terminal Modifications

Wound dressings are required to provide a moist environment for wounds, protect against invading infections, expedite tissue regeneration, and improve wound healing efficiency. Developing biomaterials with all aforesaid properties is still a big challenge. However, peptide-based hydrogels have the potential to overcome these challenges as they are biocompatible, biodegradable as well as have the ability to mimic the extracellular matrix and provide an appropriate moist environment which is important for wound healing. With this in mind, we report the preparation and comparison of three hexapeptide-based hydrogels, LIVAGD, with the aim to understand the importance of the N-terminal protecting group as well as the C-terminal amino acid substitution on its various biological efficacies. Fmoc and acetyl groups were used for N-terminal peptide protection, while aspartic acid was substituted with lysine at the C-terminus. The resulting peptide-based hydrogels were compared. Fmoc peptide-based hydrogels exhibited efficient anti-inflammatory action along with improved biocompatibility while lysine provided enhanced antibacterial effect to the hydrogel. Additionally, in vivo efficacy was examined using a mouse model, and Fmoc hydrogels demonstrated an improved wound healing ability with ∼40 % faster healing rate in comparison to the reported acetylated peptide hydrogels.

Enzyme-instructed self-assembly (EISA) assists the self-assembly and hydrogelation of hydrophobic peptides

Enzyme-instructed self-assembly (EISA) has several advantages in the preparation of supramolecular self-assembly materials for biomedical applications. In this study, we demonstrated that the enzyme-instructed self-assembly (EISA) strategy could assist the self-assembly and hydrogelation of two hydrophobic and bioactive peptides, tyroservatide (YSV) and laminin pentapeptide (YIGSR). We first synthesized the peptide derivatives of Nap-GFFYSV (peptide 1) and Nap-GFFYIGSR (peptide 2) and found that both peptides could not self-assemble into hydrogels due to their poor solubility. We therefore designed the phosphorylated precursors of the two hydrophobic peptides, Nap-GFFpYSV (precursor 1) and Nap-GFFpYIGSR (precursor 2), respectively, which had good solubility and can be dephosphorylated by alkaline phosphatase (ALP) to form supramolecular hydrogels. In addition, we found that the EISA could also occur on the surface of cells that overexpress ALP. The EISA strategy was a powerful method to generate hydrogels of hydrophobic compounds. We envision the big promise of the strategy in the preparation of biomaterials and nanomaterials of hydrophobic bioactive molecules.

Peptide hydrogel with self-healing and redox-responsive properties

We have rationally designed a peptide that assembles into a redox-responsive, antimicrobial metallohydrogel. The resulting self-healing material can be rapidly reduced by ascorbate under physiological conditions and demonstrates a remarkable 160-fold change in hydrogel stiffness upon reduction. We provide a computational model of the hydrogel, explaining why position of nitrogen in non-natural amino acid pyridyl-alanine results in drastically different gelation properties of peptides with metal ions. Given its antimicrobial and rheological properties, the newly designed hydrogel can be used for removable wound dressing application, addressing a major unmet need in clinical care.

Double-Network Heparin Dynamic Hydrogels: Dynagels as Anti-bacterial 3D Cell Culture Scaffolds

Double cross-linked dynamic hydrogels, dynagels, have been prepared through reversible imine bonds and supramolecular interactions, which showed good pH responsiveness, injectability, self-healing property and biocompatibility. With the further encapsulation of heparin, the obtained hydrogels exhibited good anti-bacterial activity and promotion effects for 3D cell culture.

Characteristics and therapeutic applications of antimicrobial peptides

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

Magneto- and photo-responsive hydrogels from the co-assembly of peptides, cyclodextrins, and superparamagnetic nanoparticles

Dual response to external non-invasive stimuli, such as light and magnetic field, is a highly desirable property in soft nanomaterials with potential applications in soft robotics, tissue engineering, and life-like materials. Within this class of materials, hydrogels obtained from the self-assembly of low molecular weight gelators (LMWGs) are of special interest due to their ease of preparation and modification. Herein, we report a modular co-assembly strategy for a magneto- and photo-responsive supramolecular hydrogel based on the arylazopyrazole (AAP) modified pentapeptide gelator Nap-GFFYS, and β-cyclodextrin vesicles (CDVs) with superparamagnetic cobalt ferrite nanoparticles embedded in their membranes. Upon application of a magnetic field, a reversible increase in the storage modulus is observed during rheological measurements. Additionally, a gel rod could be manipulated with a weak permanent magnet, resulting in macroscopic bending of the rod. Furthermore, through irradiation with UV and visible light, respectively, the host-guest interaction between the AAP moiety and the hydrophobic cavity of the β-CD can be deactivated on demand, thus lowering the stiffness of the hydrogel reversibly.

Beyond Fmoc: a review of aromatic peptide capping groups

Self-assembling short peptides have attracted widespread interest due to their tuneable, biocompatible nature and have potential applications in energy materials, tissue engineering, sensing and drug delivery. The hierarchical self-assembly of these peptides is highly dependent on the selection of not only amino acid sequence, but also the capping group which is often employed at the N-terminus of the peptide to drive self-assembly. Although the Fmoc (9H-fluorenylmethyloxycarbonyl) group is commonly used due to its utility in solid phase peptide synthesis, many other aromatic capping groups have been reported which yield functional, responsive materials. This review explores recent developments in the utilisation of functional, aromatic capping groups beyond the Fmoc group for the creation of redox-responsive, fluorescent and drug delivering hydrogel scaffolds.

Bioinspired Ice Growth Inhibitors Based on Self-Assembling Peptides

Antifreeze proteins (AFPs) are widely found in organisms living in subzero environments. Their strong ability to inhibit ice growth and recrystallization have inspired considerable bioinspired efforts to engineer artificial ice growth inhibitors for cryopreservation. However, it remains challenging to engineer biocompatible and cost-effective synthetic ice growth inhibitors to meet the increasing needs of cryoprotectants in biomedical research and industry. Here we report the design of artificial ice growth inhibitors based on self-assembling peptides. We demonstrate the importance of threonine residues as well as their spatial arrangement for effective ice binding. The engineered self-assembling ice growth inhibiting peptides show moderate ice inhibiting activity including suppression of ice growth rates and retardation of recrystallization of ice crystals. The applications of these peptides in cryopreservation of enzymes and cells were also demonstrated.

Self-assembled GFFYK peptide hydrogel enhances the therapeutic efficacy of mesenchymal stem cells in a mouse hindlimb ischemia model

Mesenchymal stem cell (MSC) transplantation has emerged as a very promising strategy for the treatments of peripheral artery disease (PAD). However, MSC-based therapies are limited by low cell retention and survival rate in the ischemic zone. Small molecular (SM) hydrogels have shown attractive abilities to enhance the therapeutic effects of human MSCs via promoting their proliferation or maintaining their differentiation potential. Here, we designed and synthesized a new bioactive and biocompatible hydrogel, Nap-GFFYK-Thiol, using disulfide bonds as cleavable linkers to control the molecular self-assembly and we hypothesized this hydrogel could enhance the retention and engraftment of human placenta-derived MSCs (hP-MSCs) in a mouse ischemic hindlimb model. In vitro results demonstrated that the Nap-GFFYK-Thiol hydrogel increased cell viability through paracrine effects. Moreover, it enhanced the proangiogenic and anti-apoptotic effects of hP-MSCs. In vivo, Nap-GFFYK-Thiol hydrogel improved the hP-MSC retention in the murine ischemic hindlimb model as visualized by bioluminescence imaging. Furthermore, cotransplantation of hP-MSCs with hydrogel improved blood perfusion, leading to superior limb salvage. These therapeutic effects may attribute to reduced inflammatory cell infiltration, enhanced angiogenesis as well as suppressed collagen deposition. In conclusion, the Nap-GFFYK-Thiol hydrogel fabricated using disulfide bonds as cleavable linkers serves as an artificial niche for promoting hP-MSC survival and proangiogenic factor secretion in PAD therapy and thereby provide an alternative strategy for PAD therapy. STATEMENT OF SIGNIFICANCE: Although several phase I/II clinical trials of MSC-based treatments for critical limb ischemia (CLI) are ongoing, MSC-based therapies are still challenged by the low quality and quantity of cells in the ischemic zone, especially in cases of extensive or irreversible damage. Hydrogels have favorable biocompatibility and safety records in the medical field. In the current study, we engineered a new bioactive and biocompatible hydrogel, Nap-GFFYK-Thiol, using disulfide bonds as cleavable linkers to enhance the therapeutic efficacy of human placenta-derived MSCs (hP-MSCs) in mouse limb ischemia model. Notably, Nap-GFFYK-Thiol hydrogel acts as an artificial niche for promoting hP-MSC survival and proangiogenic factor secretion in PAD therapy, which further promoted the restoration of blood perfusion and regeneration of muscle cells. Considering the proangiogenic effect of Nap-GFFYK-Thiol on hP-MSCs, our results may provide a new strategy for the treatment of PAD.

Stimuli-Responsive Supramolecular Hydrogels and Their Applications in Regenerative Medicine

Supramolecular hydrogels are a class of self-assembled network structures formed via non-covalent interactions of the hydrogelators. These hydrogels capable of responding to external stimuli are considered to be smart materials due to their ability to undergo sol-gel and/or gel-sol transition upon subtle changes in their surroundings. Such stimuli-responsive hydrogels are intriguing biomaterials with applications in tissue engineering, delivery of cells and drugs, modulating tissue environment to promote innate tissue repair, and imaging for medical diagnostics among others. This review summarizes the recent developments in stimuli-responsive supramolecular hydrogels and their potential applications in regenerative medicine. Specifically, various structural aspects of supramolecular hydrogelators involved in self-assembly, the role of external stimuli in tuning/controlling their phase transitions, and how these functions could be harnessed to advance applications in regenerative medicine are focused on. Finally, the key challenges and future prospects for these versatile materials are briefly described.

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

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

Enzyme-Instructed Self-Assembly of Peptides Containing Phosphoserine to Form Supramolecular Hydrogels as Potential Soft Biomaterials

Enzyme-instructed self-assembly (EISA) offers a facile approach to explore the supramolecular assemblies of small molecules in cellular milieu for a variety of biomedical applications. One of the commonly used enzymes is phosphatase, but the study of the substrates of phosphatases mainly focuses on the phosphotyrosine containing peptides. In this work, we examine the EISA of phosphoserine containing small peptides for the first time by designing and synthesizing a series of precursors containing only phosphoserine or both phosphoserine and phosphotyrosine. Conjugating a phosphoserine to the C-terminal of a well-established self-assembling peptide backbone, (naphthalene-2-ly)-acetyl-diphenylalanine (NapFF), affords a novel hydrogelation precursor for EISA. The incorporation of phosphotyrosine, another substrate of phosphatase, into the resulting precursor, provides one more enzymatic trigger on a single molecule, and meanwhile increases the precursors' propensity to aggregate after being fully dephosphorylated. Exchanging the positions of phosphorylated serine and tyrosine in the peptide backbone provides insights on how the specific molecular structures influence self-assembling behaviors of small peptides and the subsequent cellular responses. Moreover, the utilization of D-amino acids largely enhances the biostability of the peptides, thus providing a unique soft material for potential biomedical 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.

Amino Acids and Peptide-Based Supramolecular Hydrogels for Three-Dimensional Cell Culture

Supramolecular hydrogels assembled from amino acids and peptide-derived hydrogelators have shown great potential as biomimetic three-dimensional (3D) extracellular matrices because of their merits over conventional polymeric hydrogels, such as non-covalent or physical interactions, controllable self-assembly, and biocompatibility. These merits enable hydrogels to be made not only by using external stimuli, but also under physiological conditions by rationally designing gelator structures, as well as in situ encapsulation of cells into hydrogels for 3D culture. This review will assess current progress in the preparation of amino acids and peptide-based hydrogels under various kinds of external stimuli, and in situ encapsulation of cells into the hydrogels, with a focus on understanding the associations between their structures, properties, and functions during cell culture, and the remaining challenges in this field. The amino acids and peptide-based hydrogelators with rationally designed structures have promising applications in the fields of regenerative medicine, tissue engineering, and pre-clinical evaluation.

Redox-responsive supramolecular hydrogel based on 10-hydroxy camptothecin-peptide covalent conjugates with high loading capacity for drug delivery

A redox-responsive supramolecular hydrogel system was developed for delivering 10-hydroxy camptothecin (HCPT). The hydrogel was formed by cleaving disulfide bond. The combination of hydrophobic HCPT with hydrogel was a simple and effective way to improve the solubility of HCPT and the drug loading capacity of delivery system. The transmission electron microscopy (TEM) image revealed the self-assembled hydrogel was long and thin nanofibers with a width of <10nm. Rheological test verified the hydrogel had fine physical properties. In vitro release experiment showed that the accumulative releasing percentages within 72h of HCPT-peptide hydrogels at 3.0%, 4.0%, 5.0% were 16.8%, 21.3%, and 26.8% respectively, which indicated the HCPT-peptide hydrogels had a significantly sustained-release characteristic. Besides, in vitro anticancer assay showed that HCPT-peptide hydrogels possessed a favorable anticancer efficacy. These results indicated that HCPT-peptide hydrogel had great potential for cancer treatment as a novel injectable drug delivery system.

D-amino acid-containing supramolecular nanofibers for potential cancer therapeutics

Nanostructures formed by peptides that self-assemble in water through non-covalent interactions have attracted considerable attention because peptides possess several unique advantages, such as modular design and easiness of synthesis, convenient modification with known functional motifs, good biocompatibility, low immunogenicity and toxicity, inherent biodegradability, and fast responses to a wide range of external stimuli. After about two decades of development, peptide-based supramolecular nanostructures have already shown great potentials in the fields of biomedicine. Among a range of biomedical applications, using such nanostructures for cancer therapy has attracted increased interests since cancer remains the major threat for human health. Comparing with L-peptides, nanostructures containing peptides made of D-amino acid (i.e., D-peptides) bear a unique advantage, biostability (i.e., resistance towards most of endogenous enzymes). The exploration of nanostructures containing D-amino acids, especially their biomedical applications, is still in its infancy. Herein we review the recent progress of D-amino acid-containing supramolecular nanofibers as an emerging class of biomaterials that exhibit unique features for the development of cancer therapeutics. In addition, we give a brief perspective about the challenges and promises in this research direction.

Temperature and ion dual responsive biphenyl-dipeptide supramolecular hydrogels as extracellular matrix mimic-scaffolds for cell culture applications

Illustration of the gelation process of a new aromatic short peptide gelator based on biphenyl and its application in cell culture.

Peptide-Based Supramolecular Hydrogels for Delivery of Biologics

The demand for therapeutic biologics has rapidly grown over recent decades, creating a dramatic shift in the pharmaceutical industry from small molecule drugs to biological macromolecular therapeutics. As a result of their large size and innate instability, the systemic, topical, and local delivery of biologic drugs remains a highly challenging task. Although there exist many types of delivery vehicles, peptides and peptide conjugates have received continuously increasing interest as molecular blocks to create a great diversity of supramolecular nanostructures and hydrogels for the effective delivery of biologics, due to their inherent biocompatibility, tunable biodegradability, and responsiveness to various biological stimuli. In this context, we discuss the design principles of supramolecular hydrogels using small molecule peptides and peptide conjugates as molecular building units, and review the recent effort in using these materials for protein delivery and gene delivery.

Ultrafine Au and Ag Nanoparticles Synthesized from Self-Assembled Peptide Fibers and Their Excellent Catalytic Activity

The self-assembly of an amphiphilic peptide molecule to form nanofibers facilitated by Ag(+) ions was investigated. Ultrafine AgNPs (NPs=nanoparticles) with an average size of 1.67 nm were synthesized in situ along the fibers due to the weak reducibility of the -SH group on the peptide molecule. By adding NaBH4 to the peptide solution, ultrafine AgNPs and AuNPs were synthesized with an average size of 1.35 and 1.18 nm, respectively. The AuNPs, AgNPs, and AgNPs/nanofibers all exhibited excellent catalytic activity toward the reduction of 4-nitrophenol, with turnover frequency (TOF) values of 720, 188, and 96 h(-1) , respectively. Three dyes were selected for catalytic degradation by the prepared nanoparticles and the nanoparticles showed selective catalysis activity toward the different dyes. It was a surprising discovery that the ultrafine AuNPs in this work had an extremely high catalytic activity toward methylene blue, with a reaction rate constant of 0.21 s(-1) and a TOF value of 1899 h(-1) .

Supramolecular Hydrogelators and Hydrogels: From Soft Matter to Molecular Biomaterials

In this review we intend to provide a relatively comprehensive summary of the work of supramolecular hydrogelators after 2004 and to put emphasis particularly on the applications of supramolecular hydrogels/hydrogelators as molecular biomaterials. After a brief introduction of methods for generating supramolecular hydrogels, we discuss supramolecular hydrogelators on the basis of their categories, such as small organic molecules, coordination complexes, peptides, nucleobases, and saccharides. Following molecular design, we focus on various potential applications of supramolecular hydrogels as molecular biomaterials, classified by their applications in cell cultures, tissue engineering, cell behavior, imaging, and unique applications of hydrogelators. Particularly, we discuss the applications of supramolecular hydrogelators after they form supramolecular assemblies but prior to reaching the critical gelation concentration because this subject is less explored but may hold equally great promise for helping address fundamental questions about the mechanisms or the consequences of the self-assembly of molecules, including low molecular weight ones. Finally, we provide a perspective on supramolecular hydrogelators. We hope that this review will serve as an updated introduction and reference for researchers who are interested in exploring supramolecular hydrogelators as molecular biomaterials for addressing the societal needs at various frontiers.

Enzymatic formation of a meta-stable supramolecular hydrogel for 3D cell culture

A meta-stable supramolecular hydrogel triggered by phosphatase allows separation of cells post culture by simply pipetting and centrifugation.

Supramolecular nanofibers of self-assembling peptides and proteins for protein delivery

Supramolecular nanofibers of proteins and peptides could be used for intracellular protein delivery.

A genetically modified protein-based hydrogel for 3D culture of AD293 cells

Hydrogels have strong application prospects for drug delivery, tissue engineering and cell therapy because of their excellent biocompatibility and abundant availability as scaffolds for drugs and cells. In this study, we created hybrid hydrogels based on a genetically modified tax interactive protein-1 (TIP1) by introducing two or four cysteine residues in the primary structure of TIP1. The introduced cysteine residues were crosslinked with a four-armed poly (ethylene glycol) having their arm ends capped with maleimide residues (4-armed-PEG-Mal) to form hydrogels. In one form of the genetically modification, we incorporated a peptide sequence ‘GRGDSP’ to introduce bioactivity to the protein, and the resultant hydrogel could provide an excellent environment for a three dimensional cell culture of AD293 cells. The AD293 cells continued to divide and displayed a polyhedron or spindle-shape during the 3-day culture period. Besides, AD293 cells could be easily separated from the cell-gel constructs for future large-scale culture after being cultured for 3 days and treating hydrogel with trypsinase. This work significantly expands the toolbox of recombinant proteins for hydrogel formation, and we believe that our hydrogel will be of considerable interest to those working in cell therapy and controlled drug delivery.

Glutathione-triggered formation of a Fmoc-protected short peptide-based supramolecular hydrogel

A biocompatible method of glutathione (GSH) catalyzed disulfide bond reduction was used to form Fmoc-short peptide-based supramolecular hydrogels. The hydrogels could form in both buffer solution and cell culture medium containing 10% of Fetal Bovine Serum (FBS) within minutes. The hydrogel was characterized by rheology, transmission electron microscopy, and fluorescence emission spectra. Their potential in three dimensional (3D) cell culture was evaluated and the results indicated that the gel with a low concentration of the peptide (0.1 wt%) was suitable for 3D cell culture of 3T3 cells. This study provides an alternative candidate of supramolecular hydrogel for 3D cell culture and cell delivery.

Supramolecular hydrogels made of basic biological building blocks

As a consequence of the self-assembly of small organic molecules in water, supramolecular hydrogels are evolving from serendipitous events during organic synthesis to become a new type of materials that hold promise for applications in biomedicine. In this Focus Review, we describe recent advances in the use of basic biological building blocks for creating molecules that act as hydrogelators and the potential applications of the corresponding hydrogels. After introducing the concept of supramolecular hydrogels and defining the scope of this review, we briefly describe the methods for making and characterizing supramolecular hydrogels. We then discuss representative hydrogelators according to the categories of their building blocks, such as amino acids, nucleobases, and saccharides, and highlight the applications of the hydrogels when necessary. Finally, we offer our perspective and outlook on this fast-growing field at the interface of organic chemistry, materials, biology, and medicine. By providing a snapshot for chemists, engineers, and medical scientists, we hope that this Focus Review will contribute to the development of multidisciplinary research on supramolecular hydrogels for a wide range of applications in different fields.

An enzyme-assisted nanoparticle crosslinking approach to enhance the mechanical strength of peptide-based supramolecular hydrogels

In this work we reported an enzyme-assisted nanoparticle crosslinking (EANC) strategy to enhance the mechanical stability of peptide-based supramolecular hydrogels by more than 3000 times.