PEG-l-PAF and PEG-d-PAF: Comparative Study on Thermogellation and Biodegradation

Autophagy inhibition mediated via an injectable and NO-releasing hydrogel for amplifying the antitumor efficacy of mild magnetic hyperthermia

While mild hyperthermia holds great potential in the treatment of solid tumors, the thermal stress-triggered self-repairing autophagy significantly compromises its efficacy. To circumvent this obstacle, an injectable hydrogel (NO-Gel) composed of thermosensitive poly(ethylene glycol)-polypeptide copolymers modified with abundant NO donors on their side chains is developed. Meanwhile, ferrimagnetic Zn0.5Fe2.5O4 magnetic nanoparticles (MNPs) with high magnetic-heat conversion efficiency are synthesized and loaded into NO-Gel to obtain MNPs@NO-Gel. The MNPs@NO-Gel system exhibits a sol-gel transition upon heating, and has the ability to perform multiple magnetic hyperthermia therapy (MHT) after only one administration due to the even distribution and strong immobilization of MNPs in NO-Gel. NO can be continuously liberated from NO-Gel and this process is markedly accelerated by MHT. Additionally, MNPs@NO-Gel maintains its integrity in vivo for over one month and the released MNPs are metabolized by the spleen. After a single administration of MNPs@NO-Gel at the tumor site, three mild MHT treatments with similar effects are fulfilled, and the sufficient supply of NO effectively inhibits MHT-induced autophagic flux via blocking the formation of autophagosomes and synchronously destroying lysosomes, thereby substantially boosting the efficacy of mild MHT. As a consequence, CT-26 colon tumors are completely eliminated without causing severe side-effects.

Effects of chain lengths and backbone chirality on the bone-targeting ability of poly(glutamic acid)s

Anionic synthetic polypeptides are promising candidates as standalone bone-targeting drug carriers. Nevertheless, the structure-property relationship of the bone-targeting ability of polypeptides remains largely unexplored. Herein we report the optimization of the in vitro and in vivo bone-targeting ability of poly(glutamic acid)s (PGAs) by altering their chain lengths and backbone chirality. PGA 100-mers exhibited higher hydroxyapatite affinity in vitro, but their rapid macrophage clearance limited their targeting ability. Shorter PGA was therefore favored in terms of in vivo bone targeting. Meanwhile, the backbone chirality showed less significant impact on the in vitro and in vivo targeting behavior. This study highlights the modulation of structural parameters on the bone-targeting performance of anionic polypeptides, shedding light on the future design of polypeptide-based carriers.

Mechanisms and influencing factors of peptide hydrogel formation and biomedicine applications of hydrogels

Self-assembled peptide-based hydrogels have shown great potential in bio-related applications due to their porous structure, strong mechanical stability, high biocompatibility, and easy functionalization. Herein, the structure and characteristics of hydrogels and the mechanism of action of several regular secondary structures during gelation are investigated. The factors influencing the formation of peptide hydrogels, especially the pH responsiveness and salt ion induction are analyzed and summarized. Finally, the biomedical applications of peptide hydrogels, such as bone tissue engineering, cell culture, antigen presentation, antibacterial materials, and drug delivery are reviewed.

Interface Chirality: From Biological Effects to Biomedical Applications

Chiral surface is a critical mediator that significantly impacts interaction with biological systems on regulating cell behavior. To better understand how the properties of interfacial Chirality affect cell behavior and address the limitations of chiral materials for biomedical applications, in this review, we mainly focus on the recent developments of chiral bio-interfaces for the controllable and accurate guidance of chiral biomedical phenomena. In particular, we will discuss how cells or organisms sense and respond to the chiral stimulus, as well as the chirality mediating cell fate, tissue repair, and organism immune response will be reviewed. In addition, the biological applications of chirality, such as drug delivery, antibacterial, antivirus and antitumor activities, and biological signal detection, will also be reviewed. Finally, the challenges of chiral bio-interfaces for controlling biological response and the further application of interface chirality materials for biomedical will be discussed.

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.

Effect of Polymer Topology and Residue Chirality on Biodegradability of Polypeptide Hydrogels

Polypeptide-based injectable hydrogels have attracted the attention of biomedical researchers due to their unique biocompatibility and biodegradability, tunable residue chirality, and secondary conformation of polypeptide chains. In the present study, four types of poly(ethylene glycol)-block-poly(glutamic acid)s with different topological structures and residue chirality of polypeptide segments were developed, which were grafted with tyramine side groups for further cross-linking. The results demonstrated that the covalent conjugation between the tyramine groups in the presence of horseradish peroxidase and hydrogen peroxide could form porous hydrogels rapidly. Additionally, the gelation time and mechanical strength of the hydrogels were measured. All the polymer precursors and hydrogels exhibited good cytocompatibility in vitro. Further assessment of the enzymatic degradability of the hydrogels and copolymers in vitro revealed that the degradation rate was influenced by the adjustment of polymer topology or residue chirality of polypeptide copolymers. Subsequently, the effect of copolymer topology and polypeptide chirality on in vivo biodegradability and biocompatibility was assessed. This study will provide insights into the relationship between copolymer structures and hydrogel properties and benefit future polypeptide-based hydrogel studies in biomedical applications.

An oxygen-enriched thermosensitive hydrogel for the relief of a hypoxic tumor microenvironment and enhancement of radiotherapy

The rapid proliferation of tumor cells and tortuous vasculature in solid tumors often bring about a hypoxic tumor microenvironment, which renders tumor cells more resistant to many cancer treatments, including radiotherapy. In this study, an injectable and thermosensitive composite hydrogel composed of perfluorooctanoic acid (PFOA) modified monomethoxy poly(ethylene glycol)-poly(D,L-lactide-co-glycolide) (mPEG-PLGA-PFOA) and perfluorooctyl bromide (PFOB) that presented a thermoreversible sol-gel transition upon heating was developed to deliver exogenous oxygen for the relief of tumor hypoxia and enhancement of radiotherapy. The fluorinated modification of copolymers significantly increased the stability of PFOB in the mPEG-PLGA-PFOA aqueous solution owing to the fluorophilic interaction between PFOB and PFOA-modified copolymers. The introduction of PFOB not only efficiently heightened the oxygen loading capacity of the composite hydrogel, but also endowed it with excellent X-ray opacity, allowing the visual observation of the hydrogel via micro-CT imaging. After peritumoral injection of the oxygen-enriched composite hydrogel, the continuous supply of oxygen effectively relieved tumor hypoxia and down-regulated the expression of hypoxia-inducible factor-1α. Profiting from this, the hyposensitivity of tumor cells to radiation was successfully reversed, and the tumor growth in mice was significantly suppressed and the survival of mice was prolonged when combined with multiple X-ray exposure. As a result, the oxygen-enriched composite hydrogel shows a great potential for radiosensitization to improve the radiotherapeutic efficacy.

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.

PEG-based thermosensitive and biodegradable hydrogels

Injectable thermosensitive hydrogels are free-flowing polymer solutions at low or room temperature, making them easy to encapsulate the therapeutic payload or cells via simply mixing. Upon injection into the body, in situ forming hydrogels triggered by body temperature can act as drug-releasing reservoirs or cell-growing scaffolds. Finally, the hydrogels are eliminated from the administration sites after they accomplish their missions as depots or scaffolds. This review outlines the recent progress of poly(ethylene glycol) (PEG)-based biodegradable thermosensitive hydrogels, especially those composed of PEG-polyester copolymers, PEG-polypeptide copolymers and poly(organophosphazene)s. The material design, performance regulation, thermogelation and degradation mechanisms, and corresponding applications in the biomedical field are summarized and discussed. A perspective on the future thermosensitive hydrogels is also highlighted. STATEMENT OF SIGNIFICANCE: Thermosensitive hydrogels undergoing reversible sol-to-gel phase transitions in response to temperature variations are a class of promising biomaterials that can serve as minimally invasive injectable systems for various biomedical applications. Hydrophilic PEG is a main component in the design and fabrication of thermoresponsive hydrogels due to its excellent biocompatibility. By incorporating hydrophobic segments, such as polyesters and polypeptides, into PEG-based systems, biodegradable and thermosensitive hydrogels with adjustable properties in vitro and in vivo have been developed and have recently become a research hotspot of biomaterials. The summary and discussion on molecular design, performance regulation, thermogelation and degradation mechanisms, and biomedical applications of PEG-based thermosensitive hydrogels may offer a demonstration of blueprint for designing new thermogelling systems and expanding their application scope.

Improvement of Therapeutic Index by the Combination of Enhanced Peptide Cationicity and Proline Introduction

Antimicrobial peptides (AMPs) are promising candidates for new therapeutics to combat the emergence of an increasing number of multidrug-resistant pathogens. However, a major obstacle to the systemic application of AMPs is their possible toxicity. In this study, we improved the therapeutic index of the typical AMP F5W-magainin 2 by simultaneously introducing positive charges (+9-+10) and Pro residues. The former and latter contributed to enhanced antimicrobial activity and reduced cytotoxicity, respectively. The results were sensitive to the positions of Pro substitution. The antimicrobial mechanism was considered to involve both membrane permeabilization and DNA binding. The latter was affected by the peptide charge but not the presence of Pro. The neutralization of lipopolysaccharides, another important role of AMPs, was not very sensitive to either the peptide charge or Pro introduction. This strategy using intrinsic amino acids is also promising from the viewpoints of the economic mass production of AMPs and safety of metabolized peptides.

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.

Chiral Polypeptide Thermogels Induce Controlled Inflammatory Response as Potential Immunoadjuvants

The in vivo implanted biomaterials are known to induce inflammatory response and recruit immune cells, which could be used as robust adjuvants for immunotherapy. However, the degree of inflammatory response induced by the implanted biomaterials is hard to control. In this work, we reported the application of three kinds of thermogels from the polypeptide methoxy poly(ethylene glycol)-polyalanine (mPEG-PAla) with various chiralities to regulate the levels of inflammatory responses in vivo. The mPEG-PLAla (EG₄₅LA₂₈) and mPEG-PDAal (EG₄₅DA₂₇) thermogels exhibited comparable storage modulus ( G') and loss modulus ( G″), both of which were about two times higher than the values of the racemic mPEG-PAla (EG₄₅RA) thermogel. The component d-alanine in the polypeptide thermogels led to controlled tissue inflammation after subcutaneous injection, and the content of d-alanine could adjust the level of inflammation. The expression of tumor necrosis factor (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6) in subcutaneous tissue around the injected thermogel EG₄₅DA₂₇ were 3.62, 1.52, and 4.55 times the levels of those after EG₄₅RA thermogel injection and 4.52, 7.38, and 7.96 times the levels of those after EG₄₅LA₂₈ injection, respectively. The results indicated that the chiral polypeptide thermogels could induce a controllable inflammatory response in vivo and exhibit great potential as an efficient adjuvant for immunotherapy.

Component effect of stem cell-loaded thermosensitive polypeptide hydrogels on cartilage repair

Biophysical properties of the desired biomimetic scaffolds, such as porosity and elasticity, have been proven associated with the efficacy of cartilage regeneration. In this work, the copolymers of poly(l-alanine)-block-poly(ethylene glycol)-block-poly(l-alanine) (PA-PEG-PA) and poly(l-alanine-co-l-phenylalanine)-block-poly(ethylene glycol)-block-poly(l-alanine-co-l-phenylalanine) (PAF-PEG-PAF) with different ratios of alanine to phenylalanine were synthesized. The introduction of a hydrophobic amino acid, i.e., phenylalanine, into polyalanine-based thermosensitive hydrogel led to the enhanced gelation behaviors and upregulated mechanical properties. Moreover, the increase of phenylalanine content resulted in the enlarged pore size and enhanced mechanical strength of PAF-PEG-PAF thermogel, followed by the regeneration of hyaline-like cartilage with reduced fibrous tissue formation in vivo. The findings indicated the great potential of thermosensitive polypeptide hydrogels in cartilage tissue engineering.

STATEMENT OF SIGNIFICANCE

Articular cartilage defect has limited self-repair ability due to the lack of blood supply and innervation, which may lead to knee osteoarthritis afterwards. Injectable hydrogels are demonstrated possessing outstanding properties as biomimetic scaffolds in cartilage tissue engineering, while the effect of biophysical properties on the efficacy of cartilage regeneration has not been revealed. Herein, the poly(ethylene glycol)-polypeptide triblock copolymers with different ratios of alanine to phenylalanine were synthesized. The sol-to-gel transition temperature and the critical gelation concentration decreased as the increased amount of phenylalanine unit, resulting in the enlarged pore size and enhanced mechanical strength. These features lead to better regeneration of hyaline-like cartilage with reduced fibrous tissue formation, indicating great potential of thermosensitive polypeptide hydrogels for efficient cartilage repair.

Synthesis of Hybrid-Polypeptides m-PEO-b-poly(His-co-Gly) and m-PEO-b-poly(His-co-Ala) and Study of Their Structure and Aggregation. Influence of Hydrophobic Copolypeptides on the Properties of Poly(L-histidine)

The highly diverse and sophisticated action of proteins results from their equally diverse primary structure, which along with the nature of interactions between the amino acids, defines the higher self-assembly of proteins. The interactions between amino acids can be very complicated, and their understanding is necessary in order to elucidate the protein structure-properties relationship. A series of well-defined hybrid-polypeptidic diblock copolymers of the type m-PEO-b-poly(His-co-Gly) and m-PEO-b-poly(His-co-Ala) was synthesized through the ring opening polymerization of the N-carboxyanhydrides of the corresponding amino acids, with a molar ratio of the hydrophobic peptide to histidine at 10%, 20% and 40%. The excellent purity of the monomers combined with the high vacuum techniques resulted in controlled polymerization with high molecular and compositional homogeneity. FT-IR, as well as circular dichroism, were employed to investigate the secondary structure of the polymers, while DLS, SLS and ζ-potential were utilized to study the aggregates formed in aqueous solutions, as well as their pH responsiveness. The results revealed that the randomly distributed monomeric units of glycine or alanine significantly influence L-histidine’s structure. Depending on the pH, aggregates with a different structure, different molecular characteristics and a different surface charge are formed, potentially leading to very interesting bioapplications.

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.

Thermo-sensitive polypeptide hydrogel for locally sequential delivery of two-pronged antitumor drugs

In the synergistic treatment with cytotoxic drug and vascular disrupting agent, the order of drug release shows great importance to enhance the antitumor efficacy. When vascular disrupting agent is firstly administrated, the reduced blood supply and overexpressed hypoxia-inducible factor-1α greatly limit the efficiency of chemotherapy. In this work, an injectable thermo-sensitive polypeptide hydrogel was firstly developed for the locally sequential delivery of hydrophilic doxorubicin (DOX, a cytotoxic agent) and hydrophobic combretastatin A4 (CA4, a vascular disrupting drug). The aqueous solution of polypeptide at low temperature transformed into hydrogel under the body temperature after subcutaneous injection and completely degraded after four weeks with excellent biocompatibility. DOX and CA4 were co-loaded into the hydrogel, and the release of DOX showed much faster than that of CA4 due to their difference in water solubility. The superior inhibition of tumor volume after treatment with DOX and CA4 co-loaded hydrogel occurred in the treatment of grafted mouse U14 cervical tumor compared with both free drugs and single drug-loaded hydrogels. In addition, the co-loaded hydrogel obtained enhanced apoptosis of tumor cells, significant shutdown of blood vessels, and wholly regional tumor apoptosis, which indicated the eradication of solid tumor. Moreover, treatments with the drug-loaded hydrogels showed negligible damage to normal tissues, suggesting their low systemic toxicity. The locally sequential delivery system had great potential for in situ synergistic chemotherapy.

STATEMENT OF SIGNIFICANCE

The release order makes great difference in the synergistic efficacies of cytotoxic drug and vascular disrupting agent. When cytotoxic drug is administrated before vascular disrupting agent, an eradication of tumor might be obtained. On the contrary, the antitumor efficiency will be greatly hindered by limited penetration of later cytotoxic drug and drug resistant induced by vascular disrupting agent. Therefore, the adjustment of the delivery behaviors of such two-pronged agents in one platform was significant for their efficiently synergistic chemotherapy. The present study originally provides a convenient strategy and an advanced sample for sequential administration of cytotoxic drug and vascular disrupting agent in one platform based on their water solubility to achieve upregulated efficacy and safety.

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.

Non-invasive monitoring of in vivo degradation of a radiopaque thermoreversible hydrogel and its efficacy in preventing post-operative adhesions

In vivo behavior of hydrogel-based biomaterials is very important for rational design of hydrogels for various biomedical applications. Herein, we developed a facile method for in situ fabrication of radiopaque hydrogel. An iodinated functional diblock copolymer of poly(ethylene glycol) and aliphatic polyester was first synthesized by coupling the hydroxyl end of the diblock copolymer with 2,3,5-triiodobenzoic acid (TIB) and then a radiopaque thermoreversible hydrogel was obtained by mixing it with the virgin diblock copolymer. A concentrated aqueous solution of the copolymer blend was injectable at room temperature and spontaneously turned into an in situ hydrogel at body temperature after injection. The introduction of TIB moieties affords the capacity of X-ray opacity, enabling in vivo visualization of the hydrogel using Micro-CT. A rat model with cecum and abdominal defects was utilized to evaluate the efficacy of the radiopaque hydrogel in the prevention of post-operative adhesions, and a significant reduction of the post-operative adhesion formation was confirmed. Meanwhile, the maintenance of the radiopaque hydrogel in the abdomen after administration was non-destructively detected via Micro-CT scanning. The reconstructed three-dimensional images showed that the radiopaque hydrogel with an irregular morphology was located on the injured abdominal wall. The time-dependent profile of the volume of the radiopaque hydrogel determined by Micro-CT imaging was well consistent with the trend obtained from the dissection observation. Therefore, the radiopaque thermoreversible hydrogel can serve as a potential visualized biomedical implant and this practical mixing approach is also useful for further extension into the in vivo monitoring of other biomaterials.

STATEMENT OF SIGNIFICANCE

While a variety of biomaterials have been extensively studied, it is rare to monitor in vivo degradation and medical efficacy of a material after being implanted deeply into the body. Herein, the radiopaque thermoreversible hydrogel developed by us not only holds desirable performance on the prevention of post-operative abdominal adhesions, but also allows non-invasive monitoring of its in vivo degradation with CT imaging in a real-time, quantitative and three-dimensional manner. The methodology based on CT imaging provides important insights into the in vivo fate of the hydrogel after being deeply implanted into mammals for different biomedical applications and significantly reduces the amount of animals sacrificed.

Hydrogels from amphiphilic star block copolypeptides

Star-shaped amphiphilic block copolymers form hydrogels as opposed to their linear counterparts.

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.

Biodegradable thermogelling polymers: working towards clinical applications

As society ages, aging medical problems such as organ damage or failure among senior citizens increases, raising the demand for organ repair technologies. Synthetic materials have been developed and applied in various parts of human body to meet the biomedical needs. Hydrogels, in particular, have found extensive applications as wound healing, drug delivery and controlled release, and scaffold materials in the human body. The development of the next generation of soft hydrogel biomaterials focuses on facile synthetic methods, efficacy of treatment, and tunable multi-functionalities for applications. Supramolecular 3D entities are highly attractive materials for biomedical application. They are assembled by modules via various non-covalent bonds (hydrogen bonds, p-p stacking and/or van der Waals interactions). Biodegradable thermogels are a class of such supramolecular assembled materials. Their use as soft biomaterials and their related applications are described in this Review.

Poly(lactic acid-co-glycolic acid)-poly(ethylene glycol)-poly(lactic acid-co-glycolic acid) thermogel as a novel submucosal cushion for endoscopic submucosal dissection

Endoscopic submucosal dissection (ESD) is a clinical therapy for early stage neoplastic lesions in the gastrointestinal tract. It is, however, faced with a crucial problem: the high occurrence of perforation. The formation of a submucosal fluid cushion (SFC) via a fluid injection is the best way to avoid perforation, and thus an appropriate biomaterial is vital for this minimally invasive endoscopic technique. In this study, we introduced an injectable thermogel as a novel submucosal injection substance in ESD. The hydrogel synthesized by us was composed of poly(lactic acid-co-glycolic acid)-poly(ethylene glycol)-poly(lactic acid-co-glycolic acid) (PLGA-PEG-PLGA) triblock copolymers. The polymer/water system was a low-viscosity fluid at room temperature and thus easily injected, and turned into a non-flowing gel at body temperature after injection. The submucosal injection of the thermogel to create SFCs was performed in both resected porcine stomachs and living minipigs. High mucosal elevation with a clear margin was maintained for a long duration. Accurate en bloc resection was achieved with the assistance of the thermogel. The mean procedure time was strikingly reduced. Meanwhile, no obvious bleeding, perforation and tissue damage were observed. The application of the thermogel not only facilitated the ESD procedure, but also increased the efficacy and safety of ESD. Therefore, the PLGA-PEG-PLGA thermogel provides an excellent submucosal injection system, and has great potential to improve the ESD technique significantly.

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.

Responsive organogels formed by supramolecular self assembly of PEG-block-allyl-functionalized racemic polypeptides into β-sheet-driven polymeric ribbons

A chemically reactive hybrid diblock polypeptide gelator poly(ethylene glycol)-block-poly(dl-allylglycine) (PEG-b-PDLAG) is an exceptional material, due to the characteristics of thermo-reversible organogel formation driven by the combination of a hydrophilic polymer chain linked to a racemic oligomeric homopeptide segment in a range of organic solvents. One-dimensional stacking of the block copolymers is demonstrated by ATR-FTIR spectroscopy, wide-angle X-ray scattering to be driven by the supramolecular assembly of β-sheets in peptide blocks to afford well-defined fiber-like structures, resulting in gelation. These supramolecular interactions are sufficiently strong to achieve ultra low critical gelation concentrations (ca. 0.1 wt%) in N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and methanol. The critical gel transition temperature was directly proportional to the polymer concentration, so that at low concentrations, thermoreversibility of gelation was observed. Dynamic mechanical analysis studies were employed to determine the organogel mechanical properties, having storage moduli of ca. 15.1 kPa at room temperature.

Conformation-specific self-assembly of thermo-responsive poly(ethylene glycol)-b-polypeptide diblock copolymer

Poly(ethylene glycol)-block-poly(γ-(2-methoxyethoxy)esteryl-L-glutamate) (PEG-b-poly-L-EG2Glu) was synthesized via ring-opening polymerization (ROP) of L-EG2Glu N-carboxyanhydride (NCA) using PEG-NH2 as macroinitiator. This diblock contained a thermo-responsive poly-L-EG2Glu block, which adopted primarily helical conformation in pristine aqueous solution. We found that PEG-b-poly-L-EG2Glu diblock can display two levels of self-assembly behaviors associated with hydrophobic interactions and conformation-specific reassembly, respectively. Upon temperature increase, the PEG-b-poly-L-EG2Glu diblock formed wormlike micelles, in which the poly-L-EG2Glu formed the micelle core and maintained helical conformation. However, extension of thermal annealing time drove the secondary structure transformation of the poly-L-EG2Glu block from helical conformation to β-sheet, which accounted for an assembly structure transition from wormlike micelles to nanoribbons. The critical factor was that poly-L-EG2Glu block can undergo thermo-induced hydrophobicity and conformation transformation, which offered an additional parameter to tune the nature of molecular interactions, i.e., intermolecular versus intramolecular hydrogen bonding interactions. The corresponding conformation and assembly structure changes were characterized using FTIR and electron microscopy, respectively.