Versatile Biofunctionalization of Polypeptide-Based Thermosensitive Hydrogels via Click Chemistry

Thermo-induced physically crosslinked polypeptide-based block copolymer hydrogels for biomedical applications

Stimuli-responsive synthetic polypeptide-containing block copolymers have received considerable attention in recent years. Especially, unique thermo-induced sol-gel phase transitions were observed for elaborately-designed amphiphilic diblock copolypeptides and a range of poly(ethylene glycol) (PEG)-polypeptide block copolymers. The thermo-induced gelation mechanisms involve the evolution of secondary conformation, enhanced intramolecular interactions, as well as reduced hydration and increased chain entanglement of PEG blocks. The physical parameters, including polymer concentrations, sol-gel transition temperatures and storage moduli, were investigated. The polypeptide hydrogels exhibited good biocompatibility in vitro and in vivo, and displayed biodegradation periods ranging from 1 to 5 weeks. The unique thermo-induced sol-gel phase transitions offer the feasibility of minimal-invasive injection of the precursor aqueous solutions into body, followed by in situ hydrogel formation driven by physiological temperature. These advantages make polypeptide hydrogels interesting candidates for diverse biomedical applications, especially as injectable scaffolds for 3D cell culture and tissue regeneration as well as depots for local drug delivery. This review focuses on recent advances in the design and preparation of injectable, thermo-induced physically crosslinked polypeptide hydrogels. The influence of composition, secondary structure and chirality of polypeptide segments on the physical properties and biodegradation of the hydrogels are emphasized. Moreover, the studies on biomedical applications of the hydrogels are intensively discussed. Finally, the major challenges in the further development of polypeptide hydrogels for practical applications are proposed.

Application of Porous Polyetheretherketone Scaffold/ Vancomycin-Loaded Thermosensitive Hydrogel Composites for Antibacterial Therapy in Bone Repair

Polyetheretherketone (PEEK) has been widely used in bone repair, but it often fails due to bacterial infection. Herein, a high-strength porous polyetheretherketone scaffold (ps-PK) loaded with antibacterial drug-loaded hydrogel strategy is proposed. The prepared ps-PK possesses high porosity (30.8%-64.7%) and the compression modulus is between 0.4-0.98 GPa. The interconnected pore-type structure endows it with a drug loading capacity. Poly(_D,L -lactic acid-co-glycolic acid)-b-Poly(ethylene glycol)-b-Poly(_D,L -lactic acid-co-glycolic acid) (PLGA-PEG-PLGA) thermoresponsive hydrogels loaded with vancomycin are used as the drug sustained-release system. The vancomycin-loaded hydrogels in the solution state at a low temperature were filled into a porous polyetheretherketone scaffold (ps-PK-VGel) and formed a gel state after implantation in vivo. The antibacterial rate of ps-PK-VGel against methicillin-resistant staphylococcus aureus (MRSA) in vitro was 99.7% and histological observation in vivo demonstrates that the ps-PK-VGel shows obvious antibacterial activity. Given its excellent antibacterial ability and mechanical properties, the porous PEEK scaffold composite drug-loaded thermosensitive hydrogel has great potential in bone repair surgery applications. This article is protected by copyright. All rights reserved.

dl-Polyalanine as a PEG-Free Thermogel

Thermogelling behavior of aqueous polymer solutions comes from the delicate balance between hydrophilic and hydrophobic moieties of the polymer. Typically, poly(ethylene glycol) (PEG) has been used as a hydrophilic block in most thermogels reported to date. However, recent papers have suggested the potential immunogenicity of PEG-conjugated compounds. Here, we report that aqueous solutions of dl-polyalanine (DL-PA) with a specific molecular weight can exhibit thermogelling behavior. In particular, DL-PA with a molecular weight (M_n) of 6690 Da, DL-PA67, exhibited sol-to-gel transition at the physiologically important temperature range of 30-40 °C. ¹H NMR and FTIR data indicated that the mechanism of thermogelation is related to dehydration and conformational changes of DL-PA67 from random coil to β-sheet structures. Subcutaneous injection of an aqueous DL-PA67 solution into rats confirmed the gel formation and its histocompatibility with mild tissue irritation.

Recent advances in post-polymerization modifications on polypeptides: synthesis and applications

Polypeptides, a kind of very promising biomaterial, have shown a wide range of applications due to their excellent biocompatibility, easy accessibility, and structural variability. To synthesize polypeptides with desired functions, post-polymerization modification (PPM) plays an important role in introducing novel chemical structure on their side-chains. The key of PPM strategy is to develop highly selective and efficient reactions that can couple the additional functional moieties with pre-installed side-chain functionalities on polypeptides. In this minireview, classic PPM reactions and especially their recent progresses are summarized, including different modification approaches for unsaturated alkyl group, oxygen-containing functional group, nitrogen-containing functional group, sulfur-containing functional group and other special functional group on side chains. In addition, this review also highlights the applications of structure-diversified polypeptides generated via PPM strategy in the field of biomaterial.

Crucial Impact of Residue Chirality on the Gelation Process and Biodegradability of Thermoresponsive Polypeptide Hydrogels

Thermosensitive polypeptide hydrogels have gained considerable attention in potential biomedical applications, of which the polymer structure may be tuned by residue chirality. In this study, polypeptide-based block copolymers with different chiralities were synthesized by ring-opening polymerization of γ-ethyl-l-glutamate N-carboxyanhydride and/or γ-ethyl-d-glutamate N-carboxyanhydride using amino-terminated monomethoxy poly(ethylene glycol) as a macroinitiator. All mPEG-polypeptide copolymers underwent sol-gel transition with an increase in temperature. The block copolymers with mixed enantiomeric residues of γ-ethyl-l-glutamate (ELG) and γ-ethyl-d-glutamate (EDG) in the polypeptide blocks exhibited lower critical gelation concentrations and lower critical gelation temperatures compared with those composed of pure ELG or EDG residues. We established that the difference in gelation properties between the copolymers was derived from the distinction of the secondary structures. We further demonstrated the influence of polypeptide chirality on the degradability and biocompatibility of hydrogels in vivo. Our findings provide insights into the design of hydrogels having tailored secondary conformation, gelation property, and biodegradability.

Physiologically relevant pH- and temperature-responsive polypeptide hydrogels with adhesive properties

Physiologically relevant pH- and temperature-responsive polypeptide hydrogels with adhesive properties were developed and characterized.

Functional Glycopolypeptides: Synthesis and Biomedical Applications

Employing natural-based renewable sugar and saccharide resources to construct functional biopolymer mimics is a promising research frontier for green chemistry and sustainable biotechnology. As the mimics/analogues of natural glycoproteins, synthetic glycopolypeptides attracted great attention in the field of biomaterials and nanobiotechnology. This review describes the synthetic strategies and methods of glycopolypeptides and their analogues, the functional self-assemblies of the synthesized glycopolypeptides, and their biological applications such as biomolecular recognition, drug/gene delivery, and cell adhesion and targeting, as well as cell culture and tissue engineering. Future outlook of the synthetic glycopolypeptides was also discussed.

Antibacterial polypeptide/heparin composite hydrogels carrying growth factor for wound healing

We report an efficient growth factor delivering system based on polypeptide/heparin composite hydrogels for wound healing application. Linear and star-shaped poly(l-lysine) (l-PLL and s-PLL) were chosen due to not only their cationic characteristics, facilitating the efficient complexation of negatively charged heparin, but also the ease to tune the physical and mechanical properties of as-prepared hydrogels simply by varying polypeptide topology and chain length. The results showed that polymer topology can be an additional parameter to tune hydrogel properties. Our experimental data showed that these composite hydrogels exhibited low hemolytic activity and good cell compatibility as well as excellent antibacterial activity, making them ideal as wound dressing materials. Unlike other heparin-based hydrogels, these composite hydrogels with heparin densely deposited on the surface can increase the stabilization and concentration of growth factor, which can facilitate the healing process as confirmed by our in vivo animal model. We believe that these PLL/heparin composite hydrogels are promising wound dressing materials and may have potential applications in other biomedical fields.

Self-Assemblable Polymer Smart-Blocks for Temperature-Induced Injectable Hydrogel in Biomedical Applications

Self-assembled temperature-induced injectable hydrogels fabricated via self-assembly of polymer smart-blocks have been widely investigated as drug delivery systems and platforms for tissue regeneration. Polymer smart-blocks that can be self-assembly play an important role in fabrication of hydrogels because they can self-assemble to induce the gelation of their copolymer in aqueous solution. The self-assembly occurs in response to an external stimulus change, such as temperature, pH, glucose, ionic strength, light, magnetic field, electric field, or their combination, which results in property transformations like hydrophobicity, ionization, and conformational change. The self-assembly smart-block based copolymers exist as a solution in aqueous media at certain conditions that are suitable for mixing with bioactive molecules and/or cells. However, this solution turns into a hydrogel due to the self-assembly of the smart-blocks under exposure to an external stimulus change in vitro or injection into the living body for a controllable release of loaded bioactive molecules or serving as a biomaterial scaffold for tissue regeneration. This work reports current scenery in the development of these self-assembly smart-blocks for fabrication of temperature-induced injectable physically cross-linked hydrogels and their potential application as drug delivery systems and platforms for tissue engineering.

Injectable polypeptide hydrogel-based co-delivery of vaccine and immune checkpoint inhibitors improves tumor immunotherapy

Immunotherapy, an attractive option for cancer treatment, necessitates the direct stimulation of immune cells in vivo and the simultaneous effective inhibition of immunosuppressive tumor microenvironments.

Methods: In the present study, we developed an injectable PEG-b-poly(L-alanine) hydrogel for co-delivery of a tumor vaccine and dual immune checkpoint inhibitors to increase tumor immunotherapy efficacy. Tumor cell lysates, granulocyte-macrophage colony stimulating factor (GM-CSF), and immune checkpoint inhibitors (anti-CTLA-4/PD-1 antibody) were readily encapsulated in the porous hydrogel during the spontaneous self-assembly of polypeptide in aqueous solution.

Results: Sustained release of tumor antigens and GM-CSF persistently recruited and activated dendritic cells (DCs) and induced a strong T-cell response in vivo, which was further enhanced by the immune checkpoint therapy. The hydrogel vaccine also upregulated the production of IgG and the secretion of cytokines including IFN-γ, IL-4, and TNF-α. Importantly, the hydrogel-based combination therapy had superior immunotherapy effects against melanoma and 4T-1 tumor in comparison with the vaccine alone or in addition with a single immune checkpoint blockade. In studying the underlying mechanism, we found that the hydrogel-based combinatorial immunotherapy not only significantly increased the activated effector CD8⁺ T cells within the spleens and tumors of vaccinated mice, but also reduced the ratio of Tregs.

Conclusion: Our findings indicate that the polypeptide hydrogel can be used as an effective sustained delivery platform for vaccines and immune checkpoint inhibitors, providing an advanced combinatorial immunotherapy approach for cancer treatment.

Enhanced local cancer therapy using a CA4P and CDDP co-loaded polypeptide gel depot

A CA4P and CDDP co-loaded polypeptide gel depot was prepared for enhanced local colon cancer treatment.

Facile Approach to Covalent Copolypeptide Hydrogels and Hybrid Organohydrogels

Crosslinking of tryptophan (Trp) containing copolypeptides with varying ratios of benzyl-l-glutamate (BLG) and Nα-(carbobenzyloxy)-l-lysine (Z-Lys) is achieved by the selective reaction with hexamethylene-bis-TAD (bisTAD). Conversion of the resulting organogels into biocompatible hydrogels by full BLG or Z-Lys deprotection is demonstrated. Moreover, diffusion controlled deprotection allows the design of macroscopic hybrid organohydrogels comprising hydrophilic as well as hydrophobic regions at a desired ratio and position. FTIR and SEM analysis confirm the coexistence of both hydrophilic and hydrophobic segments in one copolypeptide piece. Selective loading of hydrogel and organogel segments with hydrophilic and hydrophobic dyes, respectively, is observed on macroscopic amphiphilic gels and films. These materials offer significant potential as dual-loaded drug release gels as well as tissue engineering platforms.

Synthesis and gelation of copolypept(o)ides with random and block structure

Copolypept(o)ides of polysarcosine (PSar) and poly(N-isopropyl-L-glutamine) (PIGA) with random and block sequence structures were synthesized by ring-opening polymerization (ROP) of sarcosine N-carboxyanhydrides (Sar-NCA) and γ-benzyl-l-glutamate N-carboxyanhydrides (BLG-NCA) and post modification. With different distribution of Sar along the main chain, H-bonding pattern and secondary structure of polypeptides were turned, as well as aggregation and gelation behavior. Both copolypept(o)ides formed hydrogels above their critical gelation concentrations (CGCs) without thermo-sensitivity, which was normally reserved for PEG copolypeptides (eg, PEG-b-PIGA). In particular, a different mechanism from previously reported micellar percolation or fibrillar entanglement was suggested for gelation of the random copolypept(o)ide. Therefore, hydrogels from copolymers of PSar and PIGA represented a new approach to construct easy-handling, biocompatible, biodegradable and thermo-stable gels that could potentially be applied in biomedical fields.

Multi-responsive polypeptide hydrogels derived from N-carboxyanhydride terpolymerizations for delivery of nonsteroidal anti-inflammatory drugs

A polypeptide-based hydrogel system, when prepared from a diblock polymer with a ternary copolypeptide as one block, exhibited thermo-, mechano- and enzyme-responsive properties, which enabled the encapsulation of naproxen (Npx) during the sol-gel transition and its release in the gel state. Statistical terpolymerizations of l-alanine (Ala), glycine (Gly) and l-isoleucine (Ile) NCAs at a 1 : 1 : 1 feed ratio initiated by monomethoxy monoamino-terminated poly(ethylene glycol) afforded a series of methoxy poly(ethylene glycol)-block-poly(l-alanine-co-glycine-co-l-isoleucine) (mPEG-b-P(A-G-I)) block polymers. β-Sheets were the dominant secondary structures within the polypeptide segments, which facilitated a heat-induced sol-to-gel transition, resulting from the supramolecular assembly of β-sheets into nanofibrils. Deconstruction of the three-dimensional networks by mechanical force (sonication) triggered the reverse gel-to-sol transition. Certain enzymes could accelerate the breakdown of the hydrogel, as determined by in vitro gel weight loss profiles. The hydrogels were able to encapsulate and release Npx over 6 days, demonstrating the potential application of these polypeptide hydrogels as an injectable local delivery system for small molecule drugs.

α,ω-Diphenylalanine-End-Capping of PEG-PPG-PEG Polymers Changes the Micelle Morphology and Enhances Stability of the Thermogel

Pluronics F127 (P, PEG-PPG-PEG triblock copolymer) was coupled with diphenylalanine (FF) to prepare FF-end-capped Pluronics (FFPFF). With increasing temperature from 10 to 60 °C, the FFPFF self-assembled to vesicles in water. The unimer-to-vesicle transition accompanies endothermic enthalpy of 53.9 kcal/mol. Aqueous P and FFPFF solutions exhibited thermogelation in 15.0-24.0 wt %. The gel phase of FFPFF was stable up to 90 °C, whereas that of P turned into a sol again at 55-86 °C, indicating that end-capping with FF improved the gel stability against heat. In addition, the carboxylic acids of the FF end-groups can form coordination bonds with metal ions, and the gel modulus at 37 °C increased from 15-21 KPa (P) to 20-25 KPa (FFPFF) to 24-28 KPa (FFPFF-Zn), and the duration of gel against water-erosion increased from 24 h (P) to 60 h (FFPFF-Zn), leading to a useful biomaterial for sustained drug delivery. The FFPFF-Zn gels implanted in the rats' subcutaneous layer induced a mild inflammatory responses. Contrary to the previous end-capping of Pluronics by poly(lactic acid), polycarprolactone, carboxylic acid, and so on that weakened the gel stability, the diphenylalanine end-capping strengthened the stability of Pluronics gel against heat and water-erosion. This paper suggests that the control of polymer nanoassemblies directed by FF end-groups improves the mechanical properties and stability of the resulting thermogel and, thus, provides a useful drug delivery carrier with prolonged durability.

Multiresponsive and biocompatible self-healing hydrogel: its facile synthesis in water, characterization and properties

Multiresponsive and biocompatible self-healing ε-PL/A-Pul/BPEI hydrogels were prepared in aqueous solution by Schiff base reaction with aldehyded pullulan (A-Pul), ε-poly-l-lysine (ε-PL) and branched polyethyleneimine (BPEI) as materials. The imine bonds were rapidly cross-linked into a hydrogel network within 80 s. Scanning electron microscopy images showed that the hydrogels exhibited a cross-linked structure with the average pore size from 58 to 82 μm. Rheology tests indicated that the hydrogels maintained good mechanical properties. Water contact angles and swelling studies suggested that the hydrogels could swell in water, with a max swell ratio of 1559%, and pH and temperature had an influence on the equilibrium swelling ratio. The hydrogels could be injected either before or after gelation, and they displayed a self-healing process in ddH₂O at room temperature based on the dynamic uncoupling and recoupling of the imine bonds. The MTT assays implied that the hydrogels were non-cytotoxic on mice bone marrow mesenchymal stem cells. Therefore, the hydrogels showed potential application in biomedical fields, and consequently further work was performed using the self-healing hydrogels as drug carriers in in vitro/vivo antitumor studies.

pH-Responsive Peptide Supramolecular Hydrogels with Antibacterial Activity

Smart hydrogels have received increasing attention for many applications. Here, we synthesized a class of cationic peptide amphiphiles that can self-assemble into hydrogels by ring-opening polymerization (ROP) and post-modification strategy. The incorporation of cationic lysine residues suppresses the formation of fibril-like structure and further the gelation ability of the samples. Sodium alginate (SA) is used to enhance the rheology performance of the hydrogels. The hydrogels exhibit pH-dependent self-assembly and the gelation behavior that enables them to be ideal smart hydrogel systems for biomedical applications. Furthermore, the as-prepared hybrid peptide hydrogels show antibacterial activity.

Self-assembled PEG-poly(l-valine) hydrogels as promising 3D cell culture scaffolds

Self-assembled polypeptide aggregates have shown great promise in biomedical fields including drug delivery, tissue regeneration and regenerative medicine. In this study, we report self-assembled hydrogels based on mPEG-block-poly(l-valine) (PEV) copolymers. PEV copolymers with varying poly(l-valine) chain lengths were prepared by the ring-opening polymerization of N-carboxy anhydrides of l-valine using mPEG-NH₂ as the initiator. ¹H NMR and GPC confirmed their well-defined chemical structures. FT-IR analysis and DSC curves indicated the combined α-helix and β-sheet secondary polypeptide conformation and the PEG crystallization microphase in bulk solid state, respectively. Moreover, the poly(l-valine) block restricted the crystallization of PEG segment. DLS, TEM and circular dichroism spectra were employed to study the self-assembly profiles of PEV copolymers in aqueous solution. The results manifested that in diluted solution, PEV copolymers showed a combination of typical β-sheet and α-helical polypeptide structures and self-assembled into nanostructures with diverse morphologies and sizes. For concentrated PEV solutions, clear hydrogel phases were observed and dynamic rheological analyses demonstrated that the hydrogel modulus was sensitive to the polypeptide length, angular frequency, shear strain and temperature. The hydrogel formation was possibly dominated by the physical aggregation of PEV nanoassemblies as well as driven by the formation of particular polypeptide secondary structures. Human fibroblast NIH/3T3 cells were encapsulated and cultured within the hydrogel scaffolds. The encapsulated cells exhibited high viability, suggesting that PEV hydrogels have excellent cytocompatibility and could be used as three-dimensional (3D) cell culture matrices. Collectively, self-assembled PEGylated poly(l-valine) conjugate hydrogels represented a new kind of biomaterial scaffold in biomedical fields including but not limited to 3D cell culture.

Synthetic polypeptides: from polymer design to supramolecular assembly and biomedical application

This review highlights the recent advances in the chemical design, supramolecular assembly, and biomedical application of synthetic polypeptides fromN-carboxyanhydrides.

Orthogonally reactive amino acids and end groups in NCA polymerization

We summarize recent strategies for the synthesis of orthogonally reactive polypeptides and polypeptoids by direct and post-polymerization approaches.

Injectable, Biomolecule-Responsive Polypeptide Hydrogels for Cell Encapsulation and Facile Cell Recovery through Triggered Degradation

Injectable hydrogels have been widely investigated in biomedical applications, and increasing demand has been proposed to achieve dynamic regulation of physiological properties of hydrogels. Herein, a new type of injectable and biomolecule-responsive hydrogel based on poly(l-glutamic acid) (PLG) grafted with disulfide bond-modified phloretic acid (denoted as PLG-g-CPA) was developed. The hydrogels formed in situ via enzymatic cross-linking under physiological conditions in the presence of horseradish peroxidase and hydrogen peroxide. The physiochemical properties of the hydrogels, including gelation time and the rheological property, were measured. Particularly, the triggered degradation of the hydrogel in response to a reductive biomolecule, glutathione (GSH), was investigated in detail. The mechanical strength and inner porous structure of the hydrogel were influenced by the addition of GSH. The polypeptide hydrogel was used as a three-dimensional (3D) platform for cell encapsulation, which could release the cells through triggered disruption of the hydrogel in response to the addition of GSH. The cells released from the hydrogel were found to maintain high viability. Moreover, after subcutaneous injection into rats, the PLG-g-CPA hydrogels with disulfide-containing cross-links exhibited a markedly faster degradation behavior in vivo compared to that of the PLG hydrogels without disulfide cross-links, implying an interesting accelerated degradation process of the disulfide-containing polypeptide hydrogels in the physiological environment in vivo. Overall, the injectable and biomolecule-responsive polypeptide hydrogels may serve as a potential platform for 3D cell culture and easy cell collection.

Hydrogels with smart systems for delivery of hydrophobic drugs

INTRODUCTION

Smart hydrogel systems present opportunities to not only provide hydrophobic molecule encapsulation capability but to also respond to specific delivery routes. Areas covered: An overview of the design principles, preparation methods and applications of hydrogel systems for delivery of hydrophobic drugs is given. It begins with a summary of the advantages of hydrogels as delivery vehicles over other approaches, particularly macromolecular nanocarriers, before proceeding to address the design and preparation strategies and chemistry involved, with a particular focus on the introduction of hydrophobic domains into (naturally) hydrophilic hydrogels. Finally, the applications in different delivery routes are discussed. Expert opinion: Modifications to conventional hydrogels can endow them with the capability to carry hydrophobic drugs but other functions as well, such as the improved mechanical stability, which is important for long-term in vivo residence and/or self-healing properties useful for injectable delivery pathways. These modifications harness hydrophobic-hydrophobic forces, physical interactions and inclusion complexes. The lack of in-depth understanding of these interactions, currently limits more delicate and application-oriented designs. Increased efforts are needed in (i) understanding the interplay of gel formation and simultaneous drug loading; (ii) improving hydrogel systems with respect to their biosafety; and (iii) control over release mechanism and profile.

Recent trends in pH/thermo-responsive self-assembling hydrogels: from polyions to peptide-based polymeric gelators

In this article, we highlight some recent developments in "smart" physical hydrogels achieved by self-assembling of block type macromolecules. More precisely we focus on two interesting types of gelators namely conventional ionic (or ionogenic) block copolymers and peptide-based polymers having as a common feature their responsiveness to pH and/or temperature which are the main triggers used for potential biomedical applications. Taking advantage of the immense skills of conventional block copolymer hydrogelators, namely macromolecular design, self-assembling mechanism, gel rheological properties, responsiveness to various triggers and innovative applications, the development of novel self-assembling gelators, integrating the new knowledge emerging from the peptide-based systems, opens new horizons towards bio-inspired technologies.

Well-defined (co)polypeptides bearing pendant alkyne groups

A novel metal-free strategy, using hydrogen-bonding catalytic ring opening polymerization of alkyne-functionalized N-carboxy anhydrites of α-amino acids, was developed for the synthesis of well-defined polypeptides bearing pendant alkyne groups.

Influence of the secondary structure on the AIE-related emission behavior of an amphiphilic polypeptide containing a hydrophobic fluorescent terminal and hydrophilic pendant groups

AIE-related emission of polypeptide containing an AIE-active terminal is correlated with secondary structures (α-helix, β-sheet and random coil) of the peptide chains.

High performance and reversible ionic polypeptide hydrogel based on charge-driven assembly for biomedical applications

In the pursuit of new strategies for the design and synthesis of high performance, physically associated hydrogels, dynamic materials formed through electrostatic interactions can serve as a powerful model. Here, we introduce a convenient strategy to obtain biodegradable hydrogels from ABA triblock ionic polypeptides formed by mixing poly(L-glutamic acid)-block-poly(ethylene glycol)-block-poly(L-glutamic acid) (PGA-PEG-PGA) with poly(L-lysine)-block-poly(ethylene glycol)-block-poly(L-lysine) (PLL-PEG-PLL). The hydrogels showed tunable physical properties, high strength and reversible response. The reactive function groups in the ionic blocks can conjugate with oppositely charged drugs or proteins and allow for further modification. These ionic ABA triblock polyelectrolytes can also encapsulate intact cells without significantly compromising cell viability, suggesting that the hydrogels have excellent cytocompatibility. In vivo evaluation performed in rats with subcutaneous injection indicated that the gels were formed and degraded, and hematoxylin and eosin staining suggested good biocompatibility in vivo. In addition, these advantages, combined with the synthetic accessibility of the copolymer, make this cross-linking system a flexible and powerful new tool for the development of injectable hydrogels for biomedical applications.

Brush macromolecules with thermo-sensitive coil backbones and pendant polypeptide side chains: synthesis, self-assembly and functionalization

Macromolecular brushes with thermo-sensitive coil backbones and pendant poly(γ-benzyl-l-glutamate) side chains were synthesized by reversible addition–fragmentation chain transfer and ring-opening polymerization. Functionalization and self-assembly of the macromolecules were investigated.

Multi-responsive Hydrogels Derived from the Self-assembly of Tethered Allyl-functionalized Racemic Oligopeptides

A multi-responsive triblock hydrogelator oligo(dl-allylglycine)-block-poly(ethylene glycol)-block-oligo(dl-allylglycine) (ODLAG-b-PEG-b-ODLAG) was synthesized facilely by ring-opening polymerization (ROP) of DLAG N-carboxyanhydride (NCA) with a diamino-terminated PEG as the macroinitiator. This system exhibited heat-induced sol-to-gel transitions and either sonication- or enzyme-induced gel-to-sol transitions. The β-sheeting of the oligopeptide segments was confirmed by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and wide-angle X-ray scattering (WAXS). The β-sheets further displayed tertiary ordering into fibrillar structures that, in turn generated a porous and interconnected hydrogel matrix, as observed via transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The reversible macroscopic sol-to-gel transitions triggered by heat and gel-to-sol transitions triggered by sonication were correlated with the transformation of nanostructural morphologies, with fibrillar structures observed in gel and spherical aggregates in sol, respectively. The enzymatic breakdown of the hydrogels was also investigated. This allyl-functionalized hydrogelator can serve as a platform for the design of smart hydrogels, appropriate for expansion into biological systems as bio-functional and bio-responsive materials.

Molecular tools for the construction of peptide-based materials

Proteins and peptides are fundamental components of living systems where they play crucial roles at both functional and structural level. The versatile biological properties of these molecules make them interesting building blocks for the construction of bio-active and biocompatible materials. A variety of molecular tools can be used to fashion the peptides necessary for the assembly of these materials. In this tutorial review we shall describe five of the main techniques, namely solid phase peptide synthesis, native chemical ligation, Staudinger ligation, NCA polymerisation, and genetic engineering, that have been used to great effect for the construction of a host of peptide-based materials.

Effects of l-lactide and d,l-lactide in poly(lactide-co-glycolide)-poly(ethylene glycol)-poly(lactide-co-glycolide) on the bulk states of triblock copolymers, and their thermogellation and biodegradation in water

In this study, the effects of l-lactide and d,l-lactide on the thermogelling and biodegradation behaviors of PLGA-PEG-PLGA copolymers were revealed.

Catalyst-mediated yet catalyst-free hydrogels formed by interfacial chemical activation

Catalyst-mediated yet catalyst-free catechol-containing adhesive hydrogels.

Tunable mechano-responsive organogels by ring-opening copolymerizations of N-carboxyanhydrides

The simple copolymerization of N-carboxyanhydride (NCA) monomers is utilized to generate copolypeptides having a combination of α-helix and β-sheet sub-structures that, when grown from a solvophilic synthetic polymer block segment, are capable of driving mechano-responsive supramolecular sol-to-gel-to-sol and sol-to-gel-to-gel transitions reversibly, which allow also for injection-based processing and self-healing behaviors. A new type of polypeptide-based organogelator, methoxy poly(ethylene glycol)-block-poly(γ-benzyl-l-glutamate-co-glycine) (mPEG-b-P(BLG-co-Gly)), is facilely synthesized by statistical ring-opening copolymerizations (ROPs) of γ-benzyl-l-glutamate (BLG) and glycine (Gly) NCAs initiated by mPEG-amine. These systems exhibit tunable secondary structures and result in sonication stimulus responsiveness of the organogels with the polypeptide segment variation, controlled by varying the ratio of BLG NCA to Gly NCA during the copolymerizations. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) studies indicate the α-helical component decreases while the β-sheet content increases systematically with a higher mole fraction of Gly in the polypeptide segment. The supramolecular assembly of β-sheet nanofibrils, having a tunable width over the range of 10.4 - 14.5 nm with varied BLG to Gly ratio, are characterized by transmission electron microscopy (TEM). The further self-assembly of these nanostructures into 3-D gel networks within N,N-dimethylformamide (DMF) occurs at low critical gelation concentrations (CGC) (lowest ca. 0.6 wt %). Increased BLG to Gly ratios lead to an increase of the α-helical component in the secondary structures of the polypeptide segments, resulting in wider and more flexible nanofibrils. The presence of α-helical component in the polymers enhances the stability of the organogels against sonication, and instantaneous gel-to-gel transitions are observed as in situ reconstruction of networks occurs within the gelled materials after sonication. In marked contrast, the β-sheet-rich gel, prepared from mPEG-b-PGly, exhibits an instant gel-to-sol transition after sonication is applied. The CGC concentration and stiffness of this mPEG-b-P(BLG-co-Gly) organogel system can be tuned by simply varying the percentages of α-helix and β-sheet in the secondary structures through control of the BLG to Gly ratio during synthesis. The mechanical properties of these organogels are studied by dynamic mechanical analyses (DMA), having storage moduli of ca. 12.1 kPa at room temperature. The injectability and self-healing capabilities are demonstrated by direct observation of the macroscopic self-healing behavior experiment.

Injectable enzymatically crosslinked hydrogels based on a poly(l-glutamic acid) graft copolymer

Enzyme-mediated injectable hydrogels based on a poly(l-glutamic acid) graft copolymer with tunable physicochemical properties, biodegradability and good biocompatibility were developed.