Thermoresponsive and Mechanical Properties of Poly(L-proline) Gels

Clickable Polyprolines from Azido-proline N-Carboxyanhydride

Polyproline is a material of great interest in biomedicine due to its helical scaffold of structural importance in collagen and mucins and its ability to gel and to change conformations in response to temperature. Appending of function-modulating chemical groups to such a material is desirable to diversify potential applications. Here, we describe the synthesis of high-molecular-weight homo, block, and statistical polymers of azide-functionalized proline. The azide groups served as moieties for highly efficient click-grafting, as stabilizers of the polyproline PPII helix, and as modulators of thermoresponsiveness. Saccharides and ethylene glycol were utilized to explore small-molecule grafting, and glutamate polymers were utilized to form polyelectrolyte bottlebrush architectures. Secondary structure effects of both the azide and click modifications, as well as lower critical solution temperature behavior, were characterized. The polyazidoprolines and click products were well tolerated by live human cells and are expected to find use in diverse biomedical applications.

Peptide hydrogels: Synthesis, properties, and applications in food science

Due to the unique and excellent biological, physical, and chemical properties of peptide hydrogels, their application in the biomedical field is extremely wide. The applications of peptide hydrogels are closely related to their unique responsiveness and excellent properties. However, its defects in mechanical properties, stability, and toxicity limit its application in the food field. In this review, we focus on the fabrication methods of peptide hydrogels through the physical, chemical, and biological stimulations. In addition, the functional design of peptide hydrogels by the incorporation with materials is discussed. Meanwhile, the excellent properties of peptide hydrogels such as the stimulus responsiveness, biocompatibility, antimicrobial properties, rheology, and stability are reviewed. Finally, the application of peptide hydrogel in the food field is summarized and prospected.

Memory Effect in Thermoresponsive Proline-based Polymers

We report that synthetic polymers consisting of L-proline monomer units exhibit temperature-driven aggregation in water with unprecedented hysteresis. This protein-like behavior is robust and governed by the chirality of the proline units. It paves the way to new processes, driven by either temperature or ionic strength changes, such as a simple "with memory" thermometer.

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.

Water-Assisted and Protein-Initiated Fast and Controlled Ring-Opening Polymerization of Proline N-Carboxyanhydride

The production of polypeptides via the ring-opening polymerization (ROP) of N-carboxyanhydride (NCA) is usually conducted under stringent anhydrous conditions. The ROP of proline NCA (ProNCA) for the synthesis of poly-_L-proline (PLP) is particularly challenging due to the premature product precipitation as polyproline type I helices, leading to slow reactions for up to one week, poor control of the molar mass and laborious workup. Here, we report the unexpected water-assisted controlled ROP of ProNCA, which affords well-defined PLP as polyproline II helices in 2–5 minutes and almost-quantitative yields. Experimental and theoretical studies together suggest the as-yet-unreported role of water in facilitating proton shift, which significantly lowers the energy barrier of the chain propagation. The scope of initiators can be expanded from hydrophobic amines to encompass hydrophilic amines and thiol-bearing nucleophiles, including complex biomacromolecules such as proteins. Protein-mediated ROP of ProNCA conveniently affords various protein-PLP conjugates via a grafting-from approach. PLP modification not only preserves the biological activities of the native proteins, but also enhances their resistance to extreme conditions. Moreover, PLP modification extends the elimination half-life of asparaginase (ASNase) 18-fold and mitigates the immunogenicity of wt ASNase >250-fold (ASNase is a first-line anticancer drug for lymphoma treatment). This work provides a simple solution to a long-standing problem in PLP synthesis, and offers valuable guidance for the development of water-resistant ROP of other proline-like NCAs. The facile access to PLP can greatly boost the application potential of PLP-based functional materials for engineering industry enzymes and therapeutic proteins.

Rethinking Transition Metal Catalyzed N-Carboxyanhydride Polymerization: Polymerization of Pro and AcOPro N-Carboxyanhydrides

Polyproline (PP) based polypeptides have broad applications as protein mimics, ordered materials, hydrogels, and surface coatings. However, a lack of rapid and efficient preparatory methods has challenged synthesis of well-defined high molecular weight materials. Here, we report facile and high-yielding methods for preparation and polymerization of Pro and trans-4-acetoxy-Pro N-carboxyanhdrides (NCAs). For decades, transition metal initiators of NCA polymerization were assumed to be nonstarters with Pro due to the lack of an amide NH proton. We carefully considered the known steps in the initiation mechanism and applied a Ni initiator that intercepts an intermediate and does not require an NH group. This initiator efficiently catalyzes controlled, living polymerization of Pro NCAs, revealing that routes alternate to the previously proposed mechanism must be at play. We also found Co species can catalyze Pro NCA polymerization, and we improved the synthetic methods to prepare the NCA monomers. Our methods are high-yielding and rapid and give tunable, end-functional PP-based homo, statistical, and block polypeptides. We characterized the conformation of PP and trans-4-hydroxy-PP by CD and confirmed the time scale for quantitative conversion from PPI to PPII helices. Overall, our data shed light on the general propagation mechanism of transition metal catalyzed NCA polymerization and have opened the door for efficient preparation of a desirable class of biomaterials.

A review on recent advances in polymer and peptide hydrogels

In this review, we focus on the very recent developments on the use of the stimuli responsive properties of polymer hydrogels for targeted drug delivery, tissue engineering, and biosensing utilizing their different optoelectronic properties. Besides, the stimuli-responsive hydrogels, the conducting polymer hydrogels are discussed, with specific attention to the energy generation and storage behavior of the xerogel derived from the hydrogel. The electronic and ionic conducting gels have been discussed that have applications in various electronic devices, e.g., organic field effect transistors, soft robotics, ionic skins, and sensors. The properties of polymer hybrid gels containing carbon nanomaterials have been exemplified here giving attention to applications in supercapacitors, dye sensitized solar cells, photocurrent switching, etc. Recent trends in the properties and applications of some natural polymer gels to produce thermal and acoustic insulating materials, drug delivery vehicles, self-healing material, tissue engineering, etc., are discussed. Besides the polymer gels, peptide gels of different dipeptides, tripeptides, oligopeptides, polypeptides, cyclic peptides, etc., are discussed, giving attention mainly to biosensing, bioimaging, and drug delivery applications. The properties of peptide-based hybrid hydrogels with polymers, nanoparticles, nucleotides, fullerene, etc., are discussed, giving specific attention to drug delivery, cell culture, bio-sensing, and bioimaging properties. Thus, the present review delineates, in short, the preparation, properties, and applications of different polymer and peptide hydrogels prepared in the past few years.

Reduced Graphene Oxide Incorporated GelMA Hydrogel Promotes Angiogenesis For Wound Healing Applications

PURPOSE

Non-healing or slow healing chronic wounds are among serious complications of diabetes that eventually result in amputation of limbs and increased morbidities and mortalities. Chronic diabetic wounds show reduced blood vessel formation (lack of angiogenesis), inadequate cell proliferation and poor cell migration near wounds. In this paper, we report the development of a hydrogel-based novel wound dressing material loaded with reduced graphene oxide (rGO) to promote cell proliferation, cell migration and angiogenesis for wound healing applications.

METHODS

Gelatin-methacryloyl (GelMA) based hydrogels loaded with different concentrations of rGO were fabricated by UV crosslinking. Morphological and physical characterizations (porosity, degradation, and swelling) of rGO incorporated GelMA hydrogel was performed. In vitro cell proliferation, cell viability and cell migration potential of the hydrogels were analyzed by MTT assay, live/dead staining, and wound healing scratch assay respectively. Finally, in vivo chicken embryo angiogenesis (CEO) testing was performed to evaluate the angiogenic potential of the prepared hydrogel.

RESULTS

The experimental results showed that the developed hydrogel possessed enough porosity and exudate-absorbing capacity. The biocompatibility of prepared hydrogel on three different cell lines (3T3 fibroblasts, EA.hy926 endothelial cells, and HaCaT keratinocytes) was confirmed by in vitro cell culture studies (live/dead assay). The GelMA hydrogel containing 0.002% w/w rGO considerably increased the proliferation and migration of cells as evident from MTT assay and wound healing scratch assay. Furthermore, rGO impregnated GelMA hydrogel significantly enhanced the angiogenesis in the chick embryo model.

CONCLUSION

The positive effect of 0.002% w/w rGO impregnated GelMA hydrogels on angiogenesis, cell migration and cell proliferation suggests that these formulations could be used as a functional wound healing material for the healing of chronic wounds.

Secondary structures of synthetic polypeptide polymers

Synthetic peptide-based polymers can fold into different secondary structures in the same way as do proteins. This review article presents how tuning the polypeptide secondary structure could be a key step to modulate various properties in advanced polymeric materials (size, rigidity, self-assembly,etc.).

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.