Emerging Role of Injectable Dipeptide Hydrogels in Biomedical Applications

Owing to their properties such as biocompatibility, tunable mechanical properties, permeability toward oxygen, nutrients, and the ability to hold a significant amount of water, hydrogels have wide applications in biomedical research. They have been engaged in drug delivery systems, 3D cell culture, imaging, and extracellular matrix (ECM) mimetics. Injectable hydrogels represent a major subset of hydrogels possessing advantages of site-specific conformation with minimal invasive techniques. It preserves the inherent properties of drug/biomolecules and is devoid of any side effects associated with surgery. Various polymeric materials utilized in developing injectable hydrogels are associated with the limitations of toxicity, immunogenicity, tedious manufacturing processes, and lack of easy synthetic tunability. Peptides are an important class of biomaterials that have interesting properties such as biocompatibility, stimuli responsiveness, shear thinning, self-healing, and biosignaling. They lack immunogenicity and toxicity. Therefore, numerous peptide-based injectable hydrogels have been explored in the past, and a few of them have reached the market. In recent years, minimalistic dipeptides have shown their ability to form stable hydrogels through cooperative noncovalent interactions. In addition to inherent properties of lengthy peptide-based injectable hydrogels, dipeptides have the unique advantages of low production cost, high synthetic accessibility, and higher stability. Given the instances of expanding significance of injectable peptide hydrogels in biomedical research and an emerging recent trend of dipeptide-based injectable hydrogels, a timely review on dipeptide-based injectable hydrogels shall highlight various aspects of this interesting class of biomaterials. This concise review that focuses on the dipeptide injectable hydrogel may stimulate the current trends of research on this class of biomaterial to translate its significance as interesting products for biomedical applications.

Peptide-Based Hydrogels: New Materials for Biosensing and Biomedical Applications

Peptide-based hydrogels have attracted increasing attention for biological applications and diagnostic research due to their impressive features including biocompatibility and biodegradability, injectability, mechanical stability, high water absorption capacity, and tissue-like elasticity. The aim of this review will be to present an updated report on the advancement of peptide-based hydrogels research activity in recent years in the field of anticancer drug delivery, antimicrobial and wound healing materials, 3D bioprinting and tissue engineering, and vaccines. Additionally, the biosensing applications of this key group of hydrogels will be discussed mainly focusing the attention on cancer detection.

Hydrogels as Emerging Materials for Cornea Wound Healing

Hydrogel biomaterials have many favorable characteristics including tuneable mechanical behavior, cytocompatibility, optical properties suitable for regeneration and restoration of the damaged cornea tissue. The cornea is a tissue susceptible to various injuries and traumas with a complicated healing cascade, in which conserving its transparency and integrity is critical. Accordingly, the hydrogels' known properties along with the stimulation of nerve and cell regeneration make them ideal scaffold for corneal tissue engineering. Hydrogels have been used extensively in clinical applications for the repair and replacement of diseased organs. The development and optimizing of novel hydrogels to repair/replace corneal injuries have been the main focus of researches within the last decade. This research aims to critically review in vitro, preclinical, as well as clinical trial studies related to corneal wound healing using hydrogels in the past 10 years, as this is considered as an emerging technology for corneal treatment. Several unique modifications of hydrogels with smart behaviors have undergone early phase clinical trials and showed promising outcomes. Financially, this considers a multibillion dollars industry and with huge interest from medical devices as well as pharmaceutical industries with several products may emerge within the next five years.

Hydrogels-based ophthalmic drug delivery systems for treatment of ocular diseases

An increasing number of people worldwide are affected by eye diseases, eventually leading to visual impairment or complete blindness. Conventional treatment involves the use of eye drops. However, these formulations often confer low ocular bioavailability and frequent dosing is required. Therefore, there is an urgent need to develop more effective drug delivery systems to tackle the current limitations. Hydrogels are multifunctional ophthalmic drug delivery systems capable of extending drug residence time and sustaining release of drugs. In this review, common ocular diseases and corresponding therapeutic drugs are briefly introduced. In addition, various types of hydrogels reported for ophthalmic drug delivery, including in-situ gelling hydrogels, contact lenses, low molecular weight supramolecular hydrogels, cyclodextrin/poly (ethylene glycol)-based supramolecular hydrogels and hydrogel-forming microneedles, are summarized. Besides, marketed hydrogel-based opthalmic formulations and clinical trials are also highlighted. Finally, critical considerations regarding clinical translation of biologics-loaded hydrogels are discussed.

Low molecular weight self-assembling peptide-based materials for cell culture, antimicrobial, anti-inflammatory, wound healing, anticancer, drug delivery, bioimaging and 3D bioprinting applications

In this review, we have focused on the design and development of low molecular weight self-assembling peptide-based materials for various applications including cell proliferation, tissue engineering, antibacterial, antifungal, anti-inflammatory, anticancer, wound healing, drug delivery, bioimaging and 3D bioprinting. The first part of the review describes about stimuli and various noncovalent interactions, which are the key components of various self-assembly processes for the construction of organized structures. Subsequently, the chemical functionalization of the peptides has been discussed, which is required for the designing of self-assembling peptide-based soft materials. Various low molecular weight self-assembling peptides have been discussed to explain the important structural features for the construction of defined functional nanostructures. Finally, we have discussed various examples of low molecular weight self-assembling peptide-based materials for cell culture, antimicrobial, anti-inflammatory, anticancer, wound healing, drug delivery, bioimaging and 3D bioprinting applications.

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.

Directing the nanoparticle formation by the combination with small molecular assembly and polymeric assembly for topical suppression of ocular inflammation

In this paper, we presented a simple yet versatile strategy to generate a high drug payload nanoparticles by the combination with small molecular assembly and polymeric assembly for topical suppression of ocular inflammation. Upon physical mixing of the succinated triamcinolone acetonide (TA-SA) supramolecular hydrogel with the poly (ethylene glycol)-poly (ɛ-caprolactone)-poly (ethylene glycol) (PECE) aqueous solution at 37 °C, TA-SA/PECE nanoparticles formed spontaneously and characterized thoroughly by transmission electron microscopy (TEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The formed TA-SA/PECE nanoparticles displayed a comparable in vitro anti-inflammatory efficacy to that of native triamcinolone acetonide (TA), through a significant downregulation of various proinflammatory cytokines levels (e.g., NO, TNF-α) in a lipopolysaccharide (LPS) actived RAW264.7 macrophage. Meanwhile, the enhanced transcorneal drug permeability of TA-SA/PECE nanoparticles over that of TA suspension was clearly observed in an isolated rabbit cornea. Intraocular biocompatibility test demonstrated that TA-SA/PECE nanoparticles presented good biocompatibility after topical instillation during entire study period. More importantly, the TA-SA/PECE nanoparticles displayed superior therapeutic efficacy over that of the TA suspension in the endotoxin-induced uveitis (EIU) rabbit model via decreasing neutrophil infiltration in anterior chamber. Overall, the proposed TA-SA/PECE nanoparticles might be a promising candidate for uveitis therapy.

Supramolecular nanofibers of dexamethasone derivatives to form hydrogel for topical ocular drug delivery

The low bioavailability exhibits by conventional ocular formulation owing to rapid precorneal clearance and lower corneal permeability can be overcame by the application of the gelling system. In the present study, a prodrug supramolecular hydrogel derived from succinated dexamethasone (Dex-SA) was fabricated using a pH hydrolytic strategy and explored as a "self-delivery" system for ophthalmic drugs. The self-assembled Dex-SA supramolecular hydrogel exhibited a typical nano-fibrous microstructure and was thixotropic. Both dexamethasone (Dex) and Dex-SA prodrug sustainably released from Dex-SA supramolecular hydrogel in a period of 120 h in vitro release study, and the initial pH value of hydrogel significantly influence on the release ratio of Dex/Dex-SA. Furthermore, the lyophilized Dex-SA supramolecular hydrogel displayed long-term stability without causing any apparent hydrolysis of Dex-SA at -20 °C over 30 day and quickly re-formed a hydrogel after dissolving into aqueous solution. The formed Dex-SA supramolecular hydrogel had lower cytotoxicity than Dex at drug concentration up to 2.5 mM, and exhibited a comparable anti-inflammatory efficacy to a Dex sodium phosphate (Dexp) aqueous solution in lipopolysaccharide-activated RAW264.7 macrophages. Topical instillation of the Dex-SA supramolecular hydrogel showed excellent intraocular biocompatibility and it was not an irritant in rabbit eyes. More importantly, the Dex-SA supramolecular hydrogel provided a prolonged precorneal retention and significantly enhanced the ocular bioavailability over Dexp aqueous solution after topical instillation. Overall, this work illustrates an effective approach for the development of prodrug supramolecular hydrogels to extend the precorneal retention and enhance ocular bioavailability of drugs after topical instillation.

Self-assembled pH-responsive supramolecular hydrogel for hydrophobic drug delivery

In this study, a novel supramolecular hydrogel system, abbreviated as AGC₁₆/NTS, prepared by molecular self-assembly of cationic gemini surfactant 1,3-bis(N,N-dimethyl-N-cetylammonium)-2-propylacrylatedibromide (AGC₁₆) and anionic aromatic compound trisodium 1,3,6-naphthalenetrisulfonate (NTS), was used to encapsulate hydrophobic model drug curcumin (Cur), constructing a pH-responsive drug delivery system. Cur was effectively encapsulated into the hydrophobic domains of AGC₁₆/NTS through hydrophobic interaction, which was confirmed by ¹H NMR measurement. The effects of Cur on the mechanical strength, phase transition behaviour and morphology of AGC₁₆/NTS were characterized by rheology and cryogenic scanning electron microscopy (cryo-SEM) methods. The pH-responsive release of Cur from AGC₁₆/NTS was obtained and the release amount of Cur ascended with pH value decreasing from 7.4 to 3.0. The hydrodynamic sizes of the released Cur-aggregates determined by dynamic light scattering (DLS) were used to analyse the release process of Cur at different pH. The cell viability assay and cell imaging experiment demonstrated that Cur-loaded hydrogel has much higher cytotoxicity and better cell uptake compared to free Cur. Overall, the AGC₁₆/NTS hydrogel is a prospective material for use in encapsulation and controlled-release of hydrophobic drug molecules.

Supramolecular hydrogel, AGC₁₆/NTS, was used to encapsulate hydrophobic drug curcumin (Cur), constructing a pH-responsive drug delivery system; the uptake of released Cur by cancer cells also occurred.

Diclofenac sodium ion exchange resin complex loaded melt cast films for sustained release ocular delivery

Purpose: The goal of the present study is to develop polymeric matrix films loaded with a combination of free diclofenac sodium (DFS_free) and DFS:Ion exchange resin complexes (DFS:IR) for immediate and sustained release profiles, respectively.

Methods: Effect of ratio of DFS and IR on the DFS:IR complexation efficiency was studied using batch processing. DFS:IR complex, DFS_free, or a combination of DFS_free+DFS:IR loaded matrix films were prepared by melt-cast technology. DFS content was 20% w/w in these matrix films. In vitro transcorneal permeability from the film formulations were compared against DFS solution, using a side-by-side diffusion apparatus, over a 6 h period. Ocular disposition of DFS from the solution, films and corresponding suspensions were evaluated in conscious New Zealand albino rabbits, 4 h and 8 h post-topical administration. All in vivo studies were carried out as per the University of Mississippi IACUC approved protocol.

Results: Complexation efficiency of DFS:IR was found to be 99% with a 1:1 ratio of DFS:IR. DFS release from DFS:IR suspension and the film were best-fit to a Higuchi model. In vitro transcorneal flux with the DFS_free+DFS:IR_(1:1)(1 + 1) was twice that of only DFS:IR_(1:1) film. In vivo, DFS solution and DFS:IR_(1:1) suspension formulations were not able to maintain therapeutic DFS levels in the aqueous humor (AH). Both DFS_free and DFS_free+DFS:IR_(1:1)(3 + 1) loaded matrix films were able to achieve and maintain high DFS concentrations in the AH, but elimination of DFS from the ocular tissues was much faster with the DFS_free formulation.

Conclusion: DFS_free+DFS:IR combination loaded matrix films were able to deliver and maintain therapeutic DFS concentrations in the anterior ocular chamber for up to 8 h. Thus, free drug/IR complex loaded matrix films could be a potential topical ocular delivery platform for achieving immediate and sustained release characteristics.

Aliphatic-Alcohol-Induced Opaque-to-Transparent Transformation and Application of Solubility Theory in a Bis-Dipeptide-Based Supramolecular Gel

A bis-dipeptide supramolecular gelator (DMPV) is prepared, based on l-valine moieties having a pyridinyl group and a long fatty diamine. It is found that the gelator can immobilize organic/water binary mixed solvents, and gel-to-gel transitions with unprecedented opaque-to-transparent transformations are observed upon using aliphatic alcohols such as methanol, ethanol, 1-propyl alcohol, and isopropanol as the organic components. Morphological investigations indicate that a reassembly process occurs, and microstructure evolutions from agglomerates to nanofibers are observed. Opaque and transparent assemblies can interconvert, and respond and restore under mechanical force and pH stimuli. Moreover, Hansen and Flory-Huggins parameters are used to investigate the effect of the solvent on the gelation performance of DMPV. This may facilitate the structure and solvent optimizations and aid the development of advanced gel systems.

Self-Healable Supramolecular Hydrogel Formed by Nor-Seco-Cucurbit[10]uril as a Supramolecular Crosslinker

A supramolecular hydrogel was formed by a simple mixing of solutions of nor-seco-cucurbit[10]uril (NS-CB[10]) and adamantylamine-terminated 4-armed polyethylene glycol (AdA-4-arm-PEG). In the formation of the hydrogel, NS-CB[10] acted as a noncovalent crosslinker to form a ternary complex with two AdA moieties. The dynamic and selective nature of the host-guest interaction between NS-CB[10] and AdA enabled the supramolecular hydrogel to rapidly recover its physical properties after it was damaged. In addition, the recovered hydrogel retained its physical properties with negligible differences from those of the pristine material, even after multiple self-healing cycles. The NS-CB[10]-based hydrogel with the self-healing property may be useful for various biological applications such as drug delivery, cell therapy and tissue engineering.

Enzymatic hydrogelation of self-assembling peptide I4K2and its antibacterial and drug sustained-release activities

I₄K₂hydrogel induced by plasma amine oxidase (PAO) has antibacterial and drug sustained-release properties.