Fluorescent protein senses and reports mechanical damage in glass-fiber-reinforced polymer composites

Fracture Detection in Bio-Glues with Fluorescent-Protein-Based Optical Force Probes

Glues are being used to bond, seal, and repair in industry and biomedicine. The improvement of gluing performance is hence important for the development of new glues with better and balanced property spaces, which in turn necessitates a mechanistic understanding of their mechanical failure. Optical force probes (OFPs) allow the observation of mechanical material damage in polymers from the macro- down to the microscale, yet have never been employed in glues. Here, the development of a series of ratiometric OFPs based on fluorescent-protein-dye and protein-protein conjugates and their incorporation into genetically engineered bio-glues is reported. The OFPs are designed to efficiently modulate Förster resonance energy transfer upon force application thereby reporting on force-induced molecular alterations independent of concentration and fluorescence intensity both spectrally and through their fluorescence lifetime. By fluorescence spectroscopy in solution and in the solid state and by fluorescence lifetime imaging microscopy, stress concentrations are visualized and adhesive and cohesive failure in the fracture zone is differentiated.

Fiber-Reinforced Polyester Composites with Photoluminescence Sensing Capabilities for UV Degradation Monitoring

The wide application of fiber-reinforced polymer composite (FRPC) materials has given rise to the problem of their durability and performance over time. These problems are largely associated with their environmental conditions and service procedures, including ultraviolet (UV) irradiation. Here, we propose the production of polyester-based composites with different contents of synthesized Y₃Al₅O₁₂:Ce³⁺,Ga (YAG:Ce,Ga) particles to provide sensing abilities towards material degradation. In this regard, the composites were subjected to UV radiation exposure, and its influence on the morphological, mechanical, and optical properties of the materials was investigated. Our findings reveal the self-sensing capabilities of the developed FRPC. The results indicate the potential of the system for the development of highly effective coatings allowing to detect and monitor UV degradation in composite materials for demanding applications.

Chemical design and synthesis of macromolecular profluorescent nitroxide systems as self-reporting probes

The objective of this mini-review article is to highlight the importance of the chemical design towards the synthesis of polymeric profluorescent nitroxides applicable as self-reporting probes.

From Molecules to Polymers-Harnessing Inter- and Intramolecular Interactions to Create Mechanochromic Materials

The development of mechanophores as building blocks that serve as predefined weak linkages has enabled the creation of mechanoresponsive and mechanochromic polymer materials, which are interesting for a range of applications including the study of biological specimens or advanced security features. In typical mechanophores, covalent bonds are broken when polymers that contain these chemical motifs are exposed to mechanical forces, and changes of the optical properties upon bond scission can be harnessed as a signal that enables the detection of applied mechanical stresses and strains. Similar chromic effects upon mechanical deformation of polymers can also be achieved without relying on the scission of covalent bonds. The dissociation of motifs that feature directional noncovalent interactions, the disruption of aggregated molecules, and conformational changes in molecules or polymers constitute an attractive element for the design of mechanoresponsive and mechanochromic materials. In this article, it is reviewed how such alterations of molecules and polymers can be exploited for the development of mechanochromic materials that signal deformation without breaking covalent bonds. Recent illustrative examples are highlighted that showcase how the use of such mechanoresponsive motifs enables the visual mapping of stresses and damage in a reversible and highly sensitive manner.

An engineer's introduction to mechanophores

Mechanophores (MPs) are a class of stimuli-responsive materials that are of increasing interest to engineers due to their potential applications as stress sensors. These mechanically responsive molecules change color or become fluorescent upon application of a mechanical stimulus as they undergo a chemical reaction when a load is applied. By incorporating MPs such as spirolactam, spiropyran, or dianthracene into a material system, the real-time stress distribution of the matrix can be directly observed through a visual response, ideal for damage and failure sensing applications. A wide array of applications that require continuous structural health monitoring could benefit from MPs including flexible electronics, protective coatings, and polymer matrix composites. However, there are significant technical challenges preventing MP implementation in industry. Effective strategies to quantitatively calibrate the photo response of the MP with applied stress magnitudes must be developed. Additionally, environmental conditions, including temperature, humidity, and ultraviolet light exposure can potentially impact the performance of MPs. By addressing these limitations, engineers can work to move MPs from the synthetic chemistry bench to the field. This review aims to highlight recent progress in MP research, discuss barriers to implementation, and provide an outlook on the future of MPs, specifically focused on polymeric material systems. Although the focus is on engineering MPs for bulk materials, a brief overview of mechanochemistry will be discussed followed by methods for activation and quantification of MP photo response (concentrating specifically on fluorescently active species). Finally, current challenges and future directions in MP research will be addressed.

Fluorescence Self-Reporting Precipitation Polymerization Based on Aggregation-Induced Emission for Constructing Optical Nanoagents

Precipitation polymerization is becoming increasingly popular in energy, environment and biomedicine. However, its proficient utilization highly relies on the mechanistic understanding of polymerization process. Now, a fluorescence self-reporting method based on aggregation-induced emission (AIE) is used to shed light on the mechanism of precipitation polymerization. The nucleation and growth processes during the copolymerization of a vinyl-modified AIEgen, styrene, and maleic anhydride can be sensitively monitored in real time. The phase-separation and dynamic hardening processes can be clearly discerned by tracking fluorescence changes. Moreover, polymeric fluorescent particles (PFPs) with uniform and tunable sizes can be obtained in a self-stabilized manner. These PFPs exhibit biolabeling and photosensitizing abilities and are used as superior optical nanoagents for photo-controllable immunotherapy, indicative of their great potential in biomedical applications.

Selective Immobilization of Fluorescent Proteins for the Fabrication of Photoactive Materials

The immobilization of fluorescent proteins is a key technology enabling to fabricate a new generation of photoactive materials with potential technological applications. Herein we have exploited superfolder green (sGFP) and red (RFP) fluorescent proteins expressed with different polypeptide tags. We fused these fluorescent proteins to His-tags to immobilize them on graphene 3D hydrogels, and Cys-tags to immobilize them on porous microparticles activated with either epoxy or disulfide groups and with Lys-tags to immobilize them on upconverting nanoparticles functionalized with carboxylic groups. Genetically programming sGFP and RFP with Cys-tag and His-tag, respectively, allowed tuning the protein spatial organization either across the porous structure of two microbeads with different functional groups (agarose-based materials activated with metal chelates and epoxy-methacrylate materials) or across the surface of a single microbead functionalized with both metal-chelates and disulfide groups. By using different polypeptide tags, we can control the attachment chemistry but also the localization of the fluorescent proteins across the material surfaces. The resulting photoactive material formed by His-RFP immobilized on graphene hydrogels has been tested as pH indicator to measure pH changes in the alkaline region, although the immobilized fluorescent protein exhibited a narrower dynamic range to measure pH than the soluble fluorescent protein. Likewise, the immobilization of Lys-sGFP on alginate-coated upconverting nanoparticles enabled the infrared excitation of the fluorescent protein to be used as a green light emitter. These novel photoactive biomaterials open new avenues for innovative technological developments towards the fabrication of biosensors and photonic devices.

Bio-Inspired, Self-Toughening Polymers Enabled by Plasticizer-Releasing Microcapsules

A new concept for the design of self-toughening thermoplastic polymers is presented. The approach involves the incorporation of plasticizer-filled microcapsules (MCs) in an intrinsically rigid and brittle matrix polymer. The intriguing adaptability that this simple tactic enables is demonstrated with composites composed of a poly(lactic acid) (PLA) matrix and 5-20% w/w poly(urea-formaldehyde) (PUF) MCs that contained hexyl acetate as plasticizer. At low strain (<1.5%), the glassy PLA/MC composites remain rigid, although the intact MCs reduce the Young's modulus and tensile strength by up to 50%. While the neat PLA shows brittle failure at a strain of around 2.5%, the composites yield in this regime, because the MCs rupture and release their plasticizing cargo. This effect leads up to 25-fold increase of the elongation at break and 20-fold increase of the toughness vis-à-vis the neat PLA, while the impact on modulus and ultimate stress is much smaller. Ballistic impact tests show that the self-toughening mechanism also works at much higher strain rates than applied in tensile tests and the operating mechanism is corroborated through systematic thermomechanical studies that involved dynamic mechanical testing and thermal analysis.

Modular Design of Programmable Mechanofluorescent DNA Hydrogels

Mechanosensing systems are ubiquitous in nature and control many functions from cell spreading to wound healing. Biologic systems typically rely on supramolecular transformations and secondary reporter systems to sense weak forces. By contrast, synthetic mechanosensitive materials often use covalent transformations of chromophores, serving both as force sensor and reporter, which hinders orthogonal engineering of their sensitivity, response and modularity. Here, we introduce FRET-based, rationally tunable DNA tension probes into macroscopic 3D all-DNA hydrogels to prepare mechanofluorescent materials with programmable sacrificial bonds and stress relaxation. This design addresses current limitations of mechanochromic system by offering spatiotemporal resolution, as well as quantitative and modular force sensing in soft hydrogels. The programmable force probe design further grants temporal control over the recovery of the mechanofluorescence during stress relaxation, enabling reversible and irreversible strain sensing. We show proof-of-concept applications to study strain fields in composites and to visualize freezing-induced strain patterns in homogeneous hydrogels.

Self-Calibrating Mechanochromic Fluorescent Polymers Based on Encapsulated Excimer-Forming Dyes

While mechanochemical transduction principles are omnipresent in nature, mimicking these in artificial materials is challenging. The ability to reliably detect the exposure of man-made objects to mechanical forces is, however, of great interest for many applications, including structural health monitoring and tamper-proof packaging. A useful concept to achieve mechanochromic responses in polymers is the integration of microcapsules, which rupture upon deformation and release a payload causing a visually detectable response. Herein, it is reported that this approach can be used to create mechanochromic fluorescent materials that show a direct and ratiometric response to mechanical deformation. This can be achieved by filling poly(urea-formaldehyde) microcapsules with a solution of a photoluminescent aggregachromic cyano-substituted oligo(p-phenylene vinylene) and embedding these particles in poly(dimethylsiloxane). The application of mechanical force by way of impact, incision, or tensile deformation opens the microcapsules and releases the fluorophore in the damaged area. Due to excimer formation, the subsequent aggregation of the dye furnishes a detectable fluorescence color change. With the emission from unopened microcapsules as built-in reference, the approach affords materials that are self-calibrating. This new concept appears to be readily applicable to a range of polymer matrices and allows for the straightforward assessment of their structural integrity.

Self-Reporting Fiber-Reinforced Composites That Mimic the Ability of Biological Materials to Sense and Report Damage

Sensing of damage, deformation, and mechanical forces is of vital importance in many applications of fiber-reinforced polymer composites, as it allows the structural health and integrity of composite components to be monitored and microdamage to be detected before it leads to catastrophic material failure. Bioinspired and biomimetic approaches to self-sensing and self-reporting materials are reviewed. Examples include bruising coatings and bleeding composites based on dye-filled microcapsules, hollow fibers, and vascular networks. Force-induced changes in color, fluorescence, or luminescence are achieved by mechanochromic epoxy resins, or by mechanophores and force-responsive proteins located at the interface of glass/carbon fibers and polymers. Composites can also feel strain, stress, and damage through embedded optical and electrical sensors, such as fiber Bragg grating sensors, or by resistance measurements of dispersed carbon fibers and carbon nanotubes. Bioinspired composites with the ability to show autonomously if and where they have been damaged lead to a multitude of opportunities for aerospace, automotive, civil engineering, and wind-turbine applications. They range from safety features for the detection of barely visible impact damage, to the real-time monitoring of deformation of load-bearing components.

Biobased, Internally pH-Sensitive Materials: Immobilized Yellow Fluorescent Protein as an Optical Sensor for Spatiotemporal Mapping of pH Inside Porous Matrices

The pH is fundamental to biological function and its measurement therefore crucial across all biosciences. Unlike homogenous bulk solution, solids often feature internal pH gradients due to partition effects and confined biochemical reactions. Thus, a full spatiotemporal mapping for pH characterization in solid materials with biological systems embedded in them is essential. In here, therefore, a fully biocompatible methodology for real-time optical sensing of pH within porous materials is presented. A genetically encoded ratiometric pH sensor, the enhanced superfolder yellow fluorescent protein (sYFP), is used to functionalize the internal surface of different materials, including natural and synthetic organic polymers as well as silica frameworks. By using controlled, tailor-made immobilization, sYFP is homogenously distributed within these materials and so enables, via self-referenced imaging analysis, pH measurements in high accuracy and with useful spatiotemporal resolution. Evolution of internal pH is monitored in consequence of a proton-releasing enzymatic reaction, the hydrolysis of penicillin by a penicillin acylase, taking place in solution or confined to the solid surface of the porous matrix. Unlike optochemical pH sensors, which often interfere with biological function, labeling with sYFP enables pH sensing without altering the immobilized enzyme's properties in any of the materials used. Fast response of sYFP to pH change permits evaluation of biochemical kinetics within the solid materials. Thus, pH sensing based on immobilized sYFP represents a broadly applicable technique to the study of biology confined to the internally heterogeneous environment of solid matrices.

A Schiff base/quaternary ammonium salt bifunctional graphene oxide as an efficient adsorbent for removal of Th(IV)/U(VI)

A novel approach for facile covalent functionalization of graphene oxide (GO) was proposed in the present study in order to effectively avoid necessary anhydrous conditions and the usage of harsh reagents during the chemical functionalization of GO. Herein, a GO derivative that was functionalized with a primary amine derivative bearing a positively charged quaternary ammonium group, GO-S, was synthesized through a Schiff base condensation reaction between the amine groups of the primary amine derivative and the aldehyde groups of GO. The introduction of the quaternary ammonium groups can prevent GO from stacking and improve the dispersibility of GO after modification. The formation of imine bonds (NCH) between the primary amine and GO has been confirmed by Fourier transform infrared and X-ray photoelectron spectroscopy. The GO-S demonstrated good dispersion stability in aqueous medium and also exhibited better adsorption performance than GO for Th(IV) and U(VI), with a maximum thorium adsorption capacity of 2.22mmol/g and a maximum uranium adsorption capacity of 0.83mmol/g, suggesting a great potential for the application of graphene oxide-based materials for facilitating the removal of Th(IV) and U(VI) from nuclear waste solutions.

Damage indication of 2′, 7′-dichlorofluorescein for epoxy polymer and the effect of water on its damage indicating ability

Epoxy polymer with damage indicating ability was very usable for ships and bridges to detect the cracks at an early stage and to prevent corrosion. 2′, 7′-dichlorofluorescein (DCF), as a damage indicator, was used to report the mechanical damage of epoxy-amine polymer by a strong color change from a light yellow to bright red due to the molecular structure transition from the acid molecular form to the base ion form. The effect of water on damage indicator and damaged epoxy-amine polymer film was evaluated by an immersion test and the properties were characterized by ultraviolet-visible spectrophotometry (UV-Vis), scanning electronic microscopy (SEM), energy dispersive X-ray spectrometer (EDS), zeta potential and thermal gravimetric analysis (TGA). The results showed that DCF was an easy, stable and permanent indicator for epoxy-amine polymer and the water only had a slight influence on the indication stability of damaged epoxy polymer.

Spin fluorescence silencing enables an efficient thermally driven self-reporting polymer release system

A self-reporting profluorescent release system driven by the thermo-reversible dynamic covalent ligation of chromophores to polymer chain, whose fluorescence is silenced by unpaired spins of nitroxides prior to release is introduced.

Dynamic covalent diarylbibenzofuranone-modified nanocellulose: mechanochromic behaviour and application in self-healing polymer composites

Surface mechanochemistry of nanocelluloses modified with a dynamic covalent mechanophore is investigated, and self-healing composites with the celluloses are developed.

Self-reporting Polymeric Materials with Mechanochromic Properties

The mechanical transduction of force onto molecules is an essential feature of many biological processes that results in the senses of touch and hearing, gives important cues for cellular interactions and can lead to optically detectable signals, such as a change in colour, fluorescence or chemoluminescence. Polymeric materials that are able to visually indicate deformation, stress, strain or the occurrence of microdamage draw inspiration from these biological events. The field of self-reporting (or self-assessing) materials is reviewed. First, mechanochromic events in nature are discussed, such as the formation of bruises on skin, the bleeding of a wound, or marine glow caused by dinoflagellates. Then, materials based on force-responsive mechanophores, such as spiropyrans, cyclobutanes, cyclooctanes, Diels–Alder adducts, diarylbibenzofuranone and bis(adamantyl)-1,2-dioxetane are reviewed, followed by mechanochromic blends, chromophores stabilised by hydrogen bonds, and pressure sensors based on ionic interactions between fluorescent dyes and polyelectrolyte brushes. Mechanobiochemistry is introduced as an important tool to create self-reporting hybrid materials that combine polymers with the force-responsive properties of fluorescent proteins, protein FRET pairs, and other biomacromolecules. Finally, dye-filled microcapsules, microvascular networks, and hollow fibres are demonstrated to be important technologies to create damage-indicating coatings, self-reporting fibre-reinforced composites and self-healing materials.

Highly Sensitive Built-In Strain Sensors for Polymer Composites: Fluorescence Turn-On Response through Mechanochemical Activation

A new class of rationally designed mechanophores is developed for highly sensitive built-in strain sensors in polymer composites. These mechanophores are designed to regenerate the π-conjugation pathway between the electron donor and electron acceptor by force-induced cleavage of the covalent bond to form a fluorescent dipolar dye.

Autonomous Indication of Mechanical Damage in Polymeric Coatings

High-resolution in situ autonomous visual indication of mechanical damage is achieved through a microcapsule-based polymeric material system. Upon mechanical damage, ruptured microcapsules release a liquid indicator molecule. A sharp color change from light yellow to bright red is triggered when the liberated indicator 2',7'-dichlorofluorescein reacts with the polymeric coating matrix.

Design of Self-Assembling Protein-Polymer Conjugates

Protein-polymer conjugates are of particular interest for nanobiotechnology applications because of the various and complementary roles that each component may play in composite hybrid-materials. This chapter focuses on the design principles and applications of self-assembling protein-polymer conjugate materials. We address the general design methodology, from both synthetic and genetic perspective, conjugation strategies, protein vs. polymer driven self-assembly and finally, emerging applications for conjugate materials. By marrying proteins and polymers into conjugated bio-hybrid materials, materials scientists, chemists, and biologists alike, have at their fingertips a vast toolkit for material design. These inherently hierarchical structures give rise to useful patterning, mechanical and transport properties that may help realize new, more efficient materials for energy generation, catalysis, nanorobots, etc.

Protein cages and synthetic polymers: a fruitful symbiosis for drug delivery applications, bionanotechnology and materials science

Protein cages have become essential tools in bionanotechnology due to their well-defined, monodisperse, capsule-like structure. Combining them with synthetic polymers greatly expands their application, giving rise to novel nanomaterials fore.g.drug-delivery, sensing, electronic devices and for uses as nanoreactors.

A dual functional epoxy material with autonomous damage indication and self-healing

Autonomous indication of mechanical damage and self-healing epoxy materials was conducted using 2′,7′-dichlorofluorescein (DCF) and glycidyl methacrylate (GMA) solution.

Mechanochromic Dynamic Covalent Elastomers: Quantitative Stress Evaluation and Autonomous Recovery

Stress evaluation in polymeric materials is important in order to not only spot danger in them before serious failure, but also precisely interpret the destructive mechanism, which can improve the lifetime and durability of polymeric materials. Here, we are able to visualize stress by color changes, as well as quantitatively estimate the stress in situ, in segmented polyurethane elastomers with diarylbibenzofuranone-based dynamic covalent mechanophores. We prepared films of the segmented polyurethanes, in which the mechanophores were incorporated in the soft segments, and efficiently activated them by mechanical force. Cleavage of the mechanophores during uniaxial elongation and their recovery after the removal of the stress were quantitatively evaluated by in situ electron paramagnetic resonance measurements, accompanied by drastic color changes.

Mechanochromic Luminescence of Aggregation-Induced Emission Luminogens

Mechanochromic (MC) luminogens have found promising applications in mechanosensors, security papers, and optical storage for their change in emission behaviors in response to mechanical stimuli. Examples on MC luminescent materials are rare before the discovery of MC luminescence in aggregation-induced emission (AIE) luminogens. The twisted conformations of AIE luminogens (AIEgens) with appropriate crystallization capability afford loosely packing patterns, which facilitates their phase transformation in the solid state. The amorphous films of AIEgens exhibit enhanced emission intensity upon pressurization due to the increased molecular interactions, whereas crystals of AIEgens exhibit MC luminescence due to their amorphization by mechanical stimuli. AIEgens enrich the type of MC luminogens but those showing high emission contrast and multicolor emission switching and those working in a turn-on emission mode are seldom reported. Disclosure of the design strategy of high performance MC luminogens and exploration of their high-tech applications may be the future research directions for MC luminogens.

Interfacial carbonation for efficient flame retardance of glass fiber-reinforced polyamide 6

The interfacial carbonation mode is introduced to solve the high flammability of GF-reinforced polymer composites through interfacial char produced.

Joining forces: integrating the mechanical and optical single molecule toolkits

Combining single molecule force measurements with fluorescence detection opens up exciting new possibilities for the characterization of mechanoresponsive molecules in Biology and Materials Science.