Crosslinker-free collagen gelation for corneal regeneration

Development of an artificial cornea can potentially fulfil the demand of donor corneas for transplantation as the number of donors is far less than needed to treat corneal blindness. Collagen-based artificial corneas stand out as a regenerative option, having promising clinical outcomes. Collagen crosslinked with chemical crosslinkers which modify the parent functional groups of collagen. However, crosslinkers are usually cytotoxic, so crosslinkers need to be removed from implants completely before application in humans. In addition, crosslinked products are mechanically weak and susceptible to enzymatic degradation. We developed a crosslinker free supramolecular gelation strategy using pyrene conjugated dipeptide amphiphile (PyKC) consisting of lysine and cysteine; in which collagen molecules are intertwined inside the PyKC network without any functional group modification of the collagen. The newly developed collagen implants (Coll-PyKC) are optically transparent and can effectively block UV light, are mechanically and enzymatically stable, and can be sutured. The Coll-PyKC implants support the growth and function of all corneal cells, trigger anti-inflammatory differentiation while suppressing the pro-inflammatory differentiation of human monocytes. Coll-PyKC implants can restrict human adenovirus propagation. Therefore, this crosslinker-free strategy can be used for the repair, healing, and regeneration of the cornea, and potentially other damaged organs of the body

Short Peptide-Based Smart Thixotropic Hydrogels

Thixotropy is a fascinating feature present in many gel systems that has garnered a lot of attention in the medical field in recent decades. When shear stress is applied, the gel transforms into sol and immediately returns to its original state when resting. The thixotropic nature of the hydrogel has inspired scientists to entrap and release enzymes, therapeutics, and other substances inside the human body, where the gel acts as a drug reservoir and can sustainably release therapeutics. Furthermore, thixotropic hydrogels have been widely used in various therapeutic applications, including drug delivery, cornea regeneration and osteogenesis, to name a few. Because of their inherent biocompatibility and structural diversity, peptides are at the forefront of cutting-edge research in this context. This review will discuss the rational design and self-assembly of peptide-based thixotropic hydrogels with some representative examples, followed by their biomedical applications

Peptide-Based Low Molecular Weight Photosensitive Supramolecular Gelators

Over the last couple of decades, stimuli-responsive supramolecular gels comprising synthetic short peptides as building blocks have been explored for various biological and material applications. Though a wide range of stimuli has been tested depending on the structure of the peptides, light as a stimulus has attracted extensive attention due to its non-invasive, non-contaminant, and remotely controllable nature, precise spatial and temporal resolution, and wavelength tunability. The integration of molecular photo-switch and low-molecular-weight synthetic peptides may thus provide access to supramolecular self-assembled systems, notably supramolecular gels, which may be used to create dynamic, light-responsive “smart” materials with a variety of structures and functions. This short review summarizes the recent advancement in the area of light-sensitive peptide gelation. At first, a glimpse of commonly used molecular photo-switches is given, followed by a detailed description of their incorporation into peptide sequences to design light-responsive peptide gels and the mechanism of their action. Finally, the challenges and future perspectives for developing next-generation photo-responsive gels and materials are outlined

Aggregation-Induced Emission or Hydrolysis by Water? The Case of Schiff Bases in Aqueous Organic Solvents

In the last two decades, several Schiff bases have been reported as AIEgens which remain nonemissive in organic solvents but show strong fluorescence in the presence of water. A methodical analysis involving 21 Schiff bases, including some of the reported molecules, shows that in the presence of water, the Schiff bases hydrolyze to yield the corresponding starting aldehydes and amines. The observed emission in the presence of water is found to be originated from the aggregation of the fluorogenic aldehydes and not of the original molecules. Thus, while the aggregation-induced emission (AIE) effect is valid for these systems, certainly, these Schiff bases cannot be termed as AIEgens. Notably, the observation that these aldehydes can act as AIEgens through their excimer emission is an important phenomenon with respect to the current understanding of AIEgens

Light-triggered syneresis of a water insoluble peptide-hydrogel effectively removes small molecule waste contaminants.

A short peptide based hydrogel exhibits aqueous insolubility, thixotropy and efficient light induced syneresis. Upon irradiation with UV light, the hydrogel shrinks and expells ∼50% of the solvent. Syneresis is caused by light-triggered trans-cis isomerisation of an azobenzene moiety in the peptide derivative. This expulsion of solvent can be effectively exploited in the removal of low molecular weight contaminants in water

Ultrafast, Highly Sensitive, and Selective Detection of p-Xylene at Room Temperature by Peptide-Hydrogel-Based Composite Material

A peptide/carbon dot (CD) composite xerogel is used as a selective p-xylene VOC (volatile organic compound) sensor. The fiber formation by the peptide allows us to attain a semiconducting property, whereas the presence of the CD amplifies the sensitivity. The selective detection of p-xylene is achieved at a very low concentration (response ≈ 96% for 50 ppm) with an ultrafast response (630 ms) and recovery (540 ms). The sensor is also able to detect p-xylene within crude oil, proving its industrial application. In comparison with the available VOC sensors, this work stands out as a low-cost, sensitive, and selective room-temperature p-xylene sensor with ultrafast sensing ability

Hydrogelation of a Naphthalene Diimide Appended Peptide Amphiphile and Its Application in Cell Imaging and Intracellular pH Sensing

This study reports the self-assembly and application of a naphthalene diimide (NDI)-appended peptide amphiphile (PA). H-bonding among the peptide moiety in conjunction with π-stacking between NDI and hydrophobic interactions within the alkyl chain are the major driving forces behind the stepwise aggregation of the PA to form hydrogels. The PA produced efficient self-assemblies in water, forming a nanofibrous network that further formed a self-supportive hydrogel. The molecule followed a three-step self-assembly mechanism. At a lower concentration (50 μM), it forms extremely small aggregates with a very low population of the molecules. With an increase in concentration, spherical aggregates are formed above 450 μM concentration. Importantly, this water-soluble conjugate was found to be nontoxic, cell permeable, and usable for cell imaging. Moreover, the aggregation process and consequently the emission behavior are highly responsive to the pH of the medium. Thus, the pH responsive aggregation and emission behavior has an extended biological application for assessing intracellular pH. The biocompatibility and intracellular pH determining capability suggest it is a promising candidate for use as a supramolecular material in biomedical applications

DNA-Induced Novel Optical Features of Ethyl Viologen-Tethered Perylenediimide Triad

Perylenediimide (PDI) chromophores with redox-active groups have facilitated the construction of several novel functional materials. For the first time, unusual spectroscopic changes and differential binding behavior of a tetracationic ethyl viologen tethered PDI derivative, PDEV, with calf-thymus DNA (ct-DNA) is reported here and is ascertained as due to the DNA compaction. From an initial aggregation on the DNA backbone, resulting in quenching in the emission intensity (turn off), with increase in ct-DNA concentration, a novel and strong fluorescence band (turn on) emerged in the blue region. Distinct absorption spectral changes having sharp features in vibronic patterns, increased fluorescence lifetime (from 0.33 to 4.5 ns), anisotropy values are also displayed on titration with ct-DNA corroborating the changes in microenvironment of PDEV in commensurate with structural transition from coil state to compact state. The structural changes in the DNA are also established from the circular dichroism spectra, viscosity measurements, and DNA melting data. The compaction is also directly visualized in the atomic force microscopy, scanning electron microscopy, and fluorescence microscopy images. Such compact DNA morphologies with redox-responsive donor–acceptor moieties are potential to create desired nanostructures with stimuli-responsive functionalities such as on–off switch, biosensor, drug delivery, and other optoelectronic devices