Organic-inorganic composite hydrogels: compositions, properties, and applications in regenerative medicine

Hydrogels, formed from crosslinked hydrophilic macromolecules, provide a three-dimensional microenvironment that mimics the extracellular matrix. They served as scaffold materials in regenerative medicine with an ever-growing demand. However, hydrogels composed of only organic components may not fully meet the performance and functionalization requirements for various tissue defects. Composite hydrogels, containing inorganic components, have attracted tremendous attention due to their unique compositions and properties. Rigid inorganic particles, rods, fibers, etc., can form organic-inorganic composite hydrogels through physical interaction and chemical bonding with polymer chains, which can not only adjust strength and modulus, but also act as carriers of bioactive components, enhancing the properties and biological functions of the composite hydrogels. Notably, incorporating environmental or stimulus-responsive inorganic particles imparts smartness to hydrogels, hence providing a flexible diagnostic platform for in vitro cell culture and in vivo tissue regeneration. In this review, we discuss and compare a set of materials currently used for developing organic-inorganic composite hydrogels, including the modification strategies for organic and inorganic components and their unique contributions to regenerative medicine. Specific emphasis is placed on the interactions between the organic or inorganic components and the biological functions introduced by the inorganic components. The advantages of these composite hydrogels indicate their potential to offer adaptable and intelligent therapeutic solutions for diverse tissue repair demands within the realm of regenerative medicine.

Bioinspired mineralization of engineered living materials to promote osteogenic differentiation

In this work, Engineered Living Materials (ELMs), based on the combination of genetically-modified bacteria and mineral-reinforced organic matrices, and endowed with self-healing or regenerative properties and adaptation to specific biological environments were developed. Concretely, we produced ELMs combining human mesenchymal stem cells (hMSCs) and Lactococcus lactis (L. lactis), which was specifically programmed to deliver bone morphogenetic protein (BMP-2) upon external stimulation using nisin, into mineralized alginate matrices. The hybrid organic/inorganic matrix was built through a protocol, inspired by bone mineralization, in which alginate (Alg) assembly and apatite (HA) mineralization occurred simultaneously driven by calcium ions. Chemical composition, structure and reologhical properties of the hybrid 3D matrices were dedicately optimized prior the incorportation of the living entities. Then, the same protocol was reproduced in the presence of hMSC and engineered L. lactis that secrete BMP-2 resulting in 3D hybrid living hydrogels. hMSC viability and osteogenic differentiation in the absence and presence of the bacteria were evaluated by live/dead and quantitative real-time polymerase chain reaction (qPCR) and immunofluorescence assays, respectively. Results demonstrate that these 3D engineered living material support osteogenic differentiation of hMSCs due to the synergistic effect between HA and the growth factors BMP-2 delivered by L. lactis.

Chirality-Induced Spin Selectivity in Supramolecular Chirally Functionalized Graphene

Chiral graphene hybrid materials have attracted significant attention in recent years due to their various applications in the areas of chiral catalysis, chiral separation and recognition, enantioselective sensing, etc. On the other hand, chiral materials are also known to exhibit chirality-dependent spin transmission, commonly dubbed "chirality induced spin selectivity" or CISS. However, CISS properties of chiral graphene materials are largely unexplored. As such, it is not clear whether graphene is even a promising material for the CISS effect given its weak spin-orbit interaction. Here, we report the CISS effect in chiral graphene sheets, in which a graphene derivative (reduced graphene oxide or rGO) is noncovalently functionalized with chiral Fmoc-FF (Fmoc-diphenylalanine) supramolecular fibers. The graphene flakes acquire a "conformational chirality" postfunctionalization, which, combined with other factors, is presumably responsible for the CISS signal. The CISS signal correlates with the supramolecular chirality of the medium, which depends on the thickness of graphene used. Quite interestingly, the noncovalent supramolecular chiral functionalization of conductive materials offers a simple and straightforward methodology to induce chirality and CISS properties in a multitude of easily accessible advanced conductive materials.

Short-Peptide Supramolecular Hydrogels for In Situ Growth of Metal-Organic Framework-Peptide Biocomposites

The development of bio-MOFs or MOF biocomposites through the combination of MOFs with biopolymers offers the possibility of expanding the potential applications of MOFs, making use of more environmentally benign processes and reagents and giving rise to a new generation of greener and more bio-oriented composite materials. Now, with the increasing use of MOFs for biotechnological applications, the development of new protocols and materials to obtain novel bio-MOFs compatible with biomedical or biotechnological uses is needed. Herein, and as a proof of concept, we have explored the possibility of using short-peptide supramolecular hydrogels as media to promote the growth of MOF particles, giving rise to a new family of bio-MOFs. Short-peptide supramolecular hydrogels are very versatile materials that have shown excellent in vitro and in vivo biomedical applications such as tissue engineering and drug delivery vehicles, among others. These peptides self-assemble by noncovalent interactions, and, as such, these hydrogels are easily reversible, being more biocompatible and biodegradable. These peptides can self-assemble by a multitude of stimuli, such as changes in pH, temperature, solvent, adding salts, enzymatic activity, and so forth. In this work, we have taken advantage of this ability to promote peptide self-assembly with some of the components required to form MOF particles, giving rise to more homogeneous and well-integrated composite materials. Hydrogel formation has been triggered using Zn²⁺ salts, required to form ZIF-8, and formic acid, required to form MOF-808. Two different protocols for the in situ MOF growth have been developed. Finally, the MOF-808 composite hydrogel has been tested for the decontamination of water polluted with phosphate ions as well as for the catalytic degradation of toxic organophosphate methyl paraoxon in an unbuffered solution.

Magnetite Mineralization inside Cross-Linked Protein Crystals

Crystallization in confined spaces is a widespread process in nature that also has important implications for the stability and durability of many man-made materials. It has been reported that confinement can alter essential crystallization events, such as nucleation and growth and, thus, have an impact on crystal size, polymorphism, morphology, and stability. Therefore, the study of nucleation in confined spaces can help us understand similar events that occur in nature, such as biomineralization, design new methods to control crystallization, and expand our knowledge in the field of crystallography. Although the fundamental interest is clear, basic models at the laboratory scale are scarce mainly due to the difficulty in obtaining well-defined confined spaces allowing a simultaneous study of the mineralization process outside and inside the cavities. Herein, we have studied magnetite precipitation in the channels of cross-linked protein crystals (CLPCs) with different channel pore sizes, as a model of crystallization in confined spaces. Our results show that nucleation of an Fe-rich phase occurs inside the protein channels in all cases, but, by a combination of chemical and physical effects, the channel diameter of CLPCs exerted a precise control on the size and stability of those Fe-rich nanoparticles. The small diameters of protein channels restrain the growth of metastable intermediates to around 2 nm and stabilize them over time. At larger pore diameters, recrystallization of the Fe-rich precursors into more stable phases was observed. This study highlights the impact that crystallization in confined spaces can have on the physicochemical properties of the resulting crystals and shows that CLPCs can be interesting substrates to study this process.

Fmoc-diphenylalanine gelating nanoarchitectonics: A simplistic peptide self-assembly to meet complex applications

9-fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF), has been has been extensively explored due to its ultrafast self-assembly kinetics, inherent biocompatibility, tunable physicochemical properties, and especially, the capability of forming self-sustained gels under physiological conditions. Consequently, various methodologies to develop Fmoc-FF gels and their corresponding applications in biomedical and industrial fields have been extensively studied. Herein, we systemically summarize the mechanisms underlying Fmoc-FF self-assembly, discuss the preparation methodologies of Fmoc-FF hydrogels, and then deliberate the properties as well as the diverse applications of Fmoc-FF self-assemblies. Finally, the contemporary shortcomings which limit the development of Fmoc-FF self-assembly are raised and the alternative solutions are proposed, along with future research perspectives.

Biocompatible Short-Peptides Fibrin Co-assembled Hydrogels

Fibrin hydrogels made by self-assembly of fibrinogen obtained from human plasma have shown excellent biocompatible and biodegradable properties and are widely used in regenerative medicine. The fibrinogen self-assembly process can be triggered under physiological conditions by the action of thrombin, allowing the injection of pregel mixtures that have been used as cell carriers, wound-healing systems, and bio-adhesives. However, access to fibrinogen from human plasma is expensive and fibrin gels have limited mechanical properties, which make them unsuitable for certain applications. One solution to these problems is to obtain composite gels made of fibrin and other polymeric compounds that improve their mechanical properties and usage. Herein, we prepared composite hydrogels made by the self-assembly of fibrinogen together with Fmoc-FF (Fmoc-diphenylalanine) and Fmoc-RGD (Fmoc-arginine-glycine-aspartic acid). We have shown that the mixture of these three peptides co-assembles and gives rise to a unique type of supramolecular fiber, whose morphology and mechanical properties can be modulated. We have carried out a complete characterization of these materials from chemical, physical, and biological points of view. Composite gels have improved mechanical properties compared to pure fibrin gels, as well as showing excellent biocompatibility ex vivo. In vivo experiments have shown that these gels do not cause any type of inflammatory response or tissue damage and are completely resorbed in short time, which would enable their use as vehicles for cell, drug, or growth factor release.

Chirality-Induced Spin Selectivity in Heterochiral Short-Peptide-Carbon-Nanotube Hybrid Networks: Role of Supramolecular Chirality

Supramolecular short-peptide assemblies have been widely used for the development of biomaterials with potential biomedical applications. These peptides can self-assemble in a multitude of chiral hierarchical structures triggered by the application of different stimuli, such as changes in temperature, pH, solvent, etc. The self-assembly process is sensitive to the chemical composition of the peptides, being affected by specific amino acid sequence, type, and chirality. The resulting supramolecular chirality of these materials has been explored to modulate protein and cell interactions. Recently, significant attention has been focused on the development of chiral materials with potential spintronic applications, as it has been shown that transport of charge carriers through a chiral environment polarizes the carrier spins. This effect, named chirality-induced spin selectivity or CISS, has been studied in different chiral organic molecules and materials, as well as carbon nanotubes functionalized with chiral molecules. Nevertheless, this effect has been primarily explored in homochiral systems in which the chirality of the medium, and hence the resulting spin polarization, is defined by the chirality of the molecule, with limited options for tunability. Herein, we have developed heterochiral carbon-nanotube-short-peptide materials made by the combination of two different chiral sources: that is, homochiral peptides (l/d) + glucono-δ-lactone. We show that the presence of a small amount of glucono-δ-lactone with fixed chirality can alter the supramolecular chirality of the medium, thereby modulating the sign of the spin signal from "up" to "down" and vice versa. In addition, small amounts of glucono-δ-lactone can even induce nonzero spin polarization in an otherwise achiral and spin-inactive peptide-nanotube composite. Such "chiral doping" strategies could allow the development of complementary CISS-based spintronic devices and circuits on a single material platform.

Molecularly Imprinted Polymer-Amyloid Fibril-Based Electrochemical Biosensor for Ultrasensitive Detection of Tryptophan

A tryptophan (Trp) sensor was investigated based on electrochemical impedance spectroscopy (EIS) of a molecularly imprinted polymer on a lysozyme amyloid fibril (MIP-AF). The MIP-AF was composed of aniline as a monomer chemically polymerized in the presence of a Trp template molecule onto the AF surface. After extracting the template molecule, the MIP-AF had cavities with a high affinity for the Trp molecules. The obtained MIP-AF demonstrated rapid Trp adsorption and substantial binding capacity (50 µM mg⁻¹). Trp determination was studied using non-Faradaic EIS by drop drying the MIP-AF on the working electrode of a screen-printed electrode. The MIP-AF provided a large linear range (10 pM–80 µM), a low detection limit (8 pM), and high selectivity for Trp determination. Furthermore, the proposed method also indicates that the MIP-AF can be used to determine Trp in real samples such as milk and cancer cell media.