Cyclodextrin 'Chaperones' Enable Quasi-Ideal Supramolecular Network Formation and Enhanced Photodimerization of Hydrophobic, Red-shifted Photoswitches in Water.

Precision-engineered light-triggered hydrogels are important for a diversity of applications. However, fields such as biomaterials require wavelength outside the harsh UV regime to prevent photodamage, typically requiring chromophores with extended π-conjugation that suffer from poor water solubility. Herein, we demonstrate how cyclodextrins can be used as auxiliary agents to not only solubilize such chromophores, but even to preorganize them in a 2 : 2 host-guest inclusion complex to facilitate photodimerization. We apply our concept to styrylpyrene-end-functionalized star-shaped polyethylene glycols (sPEGs). We initially unravel details of the host-guest inclusion complex using spectroscopy and mass spectrometry to give clear evidence of a 2 : 2 complex formation. Subsequently, we show that the resultant supramolecularly linked hydrogels conform to theories of supramolecular quasi-ideal model networks, and derive details on their association dynamics using in-depth rheological measurements and kinetic models. By comparing sPEGs of different arm length, we further elucidate the model network topology and the accessible mechanical property space. The photo-mediated dimerization proceeds smoothly, allowing to transform the supramolecular model networks into covalent ones. We submit that our strategy opens avenues for executing hydrophobic photochemistry in aqueous environments with enhanced control over reactivity, hydrogel topology or programmable mechanical properties.

An Opto- and Thermal-Rewrite PCM/CNF-IR 780 Energy Storage Nanopaper with Mechanical Regulated Performance.

In spite of efforts to fabricate self-assembled energy storage nanopaper with potential applications in displays, greenhouses, and sensors, few studies have investigated their multiple stimuli-sensitivities. Here, an opto- and thermal-rewrite phase change material/cellulose nanofibril (PCM/CNF) energy storage nanopaper with mechanical regulated performance is facilely fabricated, through 5 min sonication of PCMs and CNFs in an aqueous system. The combination of PCM and CNF not only guarantees the recyclability of PCM without leakage, but also offers nanopaper adaptive properties by leveraging the mobility and optical variation accompanying solid-to-liquid transition of PCM. Besides, trace near-infrared (NIR) dye (IR 780) in it imparts a PCM-embedded nanopaper photothermal effect to modulate the local transparency via time- and position-controlled laser exposure, leading to a reusable opto-writing nanopaper. Furthermore, since the synergistic effect of stick-and-slip function attributes from PCMs and pore structures are produced by calcium ions, the PCM/CNF energy storage nanopaper exhibits excellent mechanically regulated performance from rigid to flexible, which greatly enriches their application in energy-efficient smart buildings and displays.

Efficient Softening and Toughening Strategies of Cellulose Nanofibril Nanocomposites Using Comb Polyurethane.

Cellulose nanofibrils (CNFs) have attracted attention as building blocks for sustainable materials owing to their high performance and the advantages of their abundant natural resources. Bioinspired CNF/polymer nanocomposites, consisting of a soft polymer phase and a high fraction (>50 wt %) of CNF reinforcement, have been focused on excellent mechanical properties, including Young's modulus, mechanical strength, and toughness, mimicking the energy dissipation system in nature. However, efficient softening and toughening with a small amount of the soft phase is still a challenge because a large amount of the polymer phase (nearly 50%) is still required to provide ductility and toughness. Here, we describe a topological strategy in the polymer phase for efficient toughening of bioinspired CNF nanocomposites with a water-soluble comb polyurethane (PU). The comb PU provided higher elongation at break and more efficient flexibility for the nanocomposite than the linear PU, even at a small content. Moreover, CNF nanocomposites with 30 wt % of PU content and tetrabutylammonium as bulky counterions showed enhanced toughness (180% higher) and strain at break (250% higher) when compared to pure CNF due to the promotion of slippage between nanofibrils. Scanning electron microscopy (SEM) images of the fracture surface for CNF/comb PU nanocomposites displayed the pull-out of mesoscale layers and nanofibrils, supporting that the comb topology promotes the slippage between fibrils. Furthermore, the rheological study revealed that the comb PU has an entanglement plateau modulus lower than linear PU by 1 order of magnitude, related to the loosened entanglements. Our study establishes an efficient softening and toughening strategy while using small amounts of polymer phase addition, promoting interfibrillar slippage with the loosely entangled comb PU phase.

Self-Assembled Bioinspired Nanocomposites.

Bioinspired materials engineering impacts the design of advanced functional materials across many domains of sciences from wetting behavior to optical and mechanical materials. In all cases, the advances in understanding how biology uses hierarchical design to create failure and defect-tolerant materials with emergent properties lays the groundwork for engaging into these topics. Biological mechanical materials are particularly inspiring for their unique combinations of stiffness, strength, and toughness together with lightweightness, as assembled and grown in water from a limited set of building blocks at room temperature. Wood, nacre, crustacean cuticles, and spider silk serve as some examples, where the correct arrangement of constituents and balanced molecular energy dissipation mechanisms allows overcoming the shortcomings of the individual components and leads to synergistic materials performance beyond additive behavior. They constitute a paradigm for future structural materials engineering-in the formation process, the use of sustainable building blocks and energy-efficient pathways, as well as in the property profiles-that will in the long term allow for new classes of high-performance and lightweight structural materials needed to promote energy efficiency in mobile technologies.This Account summarizes our efforts of the past decade with respect to designing self-assembling bioinspired materials aiming for both mechanical high-performance structures and new types of multifunctional property profiles. The Account is set out to first give a definition of bioinspired nanocomposite materials and self-assembly therein, followed by an in-depth discussion on the understanding of mechanical performance and rational design to increase the mechanical performance. We place a particular emphasis on materials formed at high fractions of reinforcements and with tailor-made functional polymers using self-assembly to create highly ordered structures and elucidate in detail how the soft polymer phase needs to be designed in terms of thermomechanical properties and sacrificial supramolecular bonds. We focus on nanoscale reinforcements such as nanoclay and nanocellulose that lead to high contents of internal interfaces and intercalated polymer layers that experience nanoconfinement. Both aspects add fundamental challenges for macromolecular design of soft phases using precision polymer synthesis. We build upon those design criteria and further develop the concepts of adaptive bioinspired nanocomposites, whose properties are switchable from the outside using molecularly defined triggers with light. In a last section, we discuss how new types of functional properties, in particular flexible and transparent gas barrier materials or fire barrier materials, can be reached on the basis of the bioinspired nanocomposite design strategies. Additionally, we show new types of self-assembled photonic materials that can even be evolved into self-assembling lasers, hence moving the concept of mechanical nanocomposite design to other functionalities.The comparative discussion of different bioinspired nanocomposite architectures with nematic, fibrillar, and cholesteric structures, as based on different reinforcing nanoparticles, aims for a unified understanding of the design principles and shall aid researchers in the field in the more elaborate design of future bioinspired nanocomposite materials based on molecular control principles. We conclude by addressing challenges, in particular also the need for a transfer from fundamental molecular materials science into scalable engineering materials of technological and societal relevance.

Wavelength-Gated Adaptation of Hydrogel Properties via Photo-Dynamic Multivalency in Associative Star Polymers.

Responsive materials, such as switchable hydrogels, have been largely engineered for maximum changes between two states. In contrast, adaptive systems target distinct functional plateaus between these maxima. Here, we demonstrate how the photostationary state (PSS) of an E/Z-arylazopyrazole photoswitch can be tuned by the incident wavelength across a wide color spectrum, and how this behavior can be exploited to engineer the photo-dynamic mechanical properties of hydrogels based on multivalent photoswitchable interactions. We show that these hydrogels adapt to the wavelength-dependent PSS and the number of arylazopyrazole units by programmable relationships. Hence, our material design enables the facile adjustment of the mechanical properties without laborious synthetic efforts. The concept goes beyond the classical switching from state A to B, and demonstrates pathways for a truly wavelength-gated adaptation of hydrogel properties potentially useful to engineer cell fate or in soft robotics.

Glass Transition Temperature Regulates Mechanical Performance in Nacre-Mimetic Nanocomposites.

Although research in bioinspired nanocomposites is delivering mechanically superior nanocomposite materials, there remain gaps in understanding some fundamental design principles. This article discusses how the mechanical properties of nacre-mimetic polymer/nanoclay nanocomposites with nanoconfined polymer layers are controlled by the thermo-mechanical polymer properties, that is, glass transition temperature, Tg, using a series of poly(ethylene glycol methyl ether methacrylate-co-N,N-dimethylacrylamide) copolymers with tunable Tg from 130 to -55 °C. It is elucidated that both the type of copolymer and the nanoconfined polymer layer thickness control energy dissipation and inelastic deformation at high fractions of reinforcements in such bioinspired nanocomposites.

Sustainable Chitin Nanofibrils Provide Outstanding Flame-Retardant Nanopapers.

Sustainable polysaccharide nanofibrils formed from chitin or cellulose are emerging biobased nanomaterials for advanced materials requiring high mechanical performance, barrier properties, for bioactive materials, or other functionalities. Here, we demonstrate a single-step, waterborne approach to prepare additive-free flame-retardant and self-extinguishing, mechanical high-performance nanopapers based purely on surface-deacetylated chitin nanofibrils (ChNFs). We show that the flammability can be critically reduced by exchanging the counterions, e.g. to the phosphate type, using the respective acid providing electrostatic stabilization in the preparation of the ChNFs. This exchange renders beneficial elemental combinations of high contents of N/P (nitrogen/phosphorus) in the final nanopapers, known to provide outstanding performance in halogen- and heavy metal-free flame-retardant materials. Full fire barrier nanopapers can even be obtained by hybridizing the ChNF with nanoclay. Comprehensive fire retardancy tests, including vertical and horizontal flame tests and microscale cone combustion calorimetry, as well as fire breakthrough tests elucidate excellent flame-retardant properties and high structural integrity when being burned. The intrinsic elemental composition of chitin, containing nitrogen, and the simple modification of the counterions to include phosphorus provides key advantages over related, but flammable nanocellulose materials that often require significant chemical modifications and additives to become fire-retardant. By activating a global food waste, this study presents a critical advance for bioinspired, green, and mechanical high-performance materials with extraordinary flame-retardant and fire barrier properties based on sustainable feedstock, using benign water-based room temperature processing, and by avoiding heavy metals and halogen atoms in their composition.

Preparation of Highly Monodisperse Monopatch Particles with Orthogonal Click-Type Functionalization and Biorecognition.

Patchy particles are next generation colloidal building blocks for self-assembly and find further use as (bio) sensors. Progress in this direction crucially depends on developing straightforward preparation pathways able to provide patchy particles with highest uniformity and integrating precise, orthogonal, and spatially localized functionalizations to mediate interaction patterns. This continues to be one of the great challenges in colloid science. Herein, a method is shown utilizing functionalized random and block copolymers as microcontact printing inks to prepare patchy particles with outstanding control over patch size and quality. The polymeric nature and tight covalent attachment of the ink prevents flow of the ink over the particle during printing. This minimizes patch broadening and yields very small and extremely uniform patches, which is especially challenging for particle sizes below 10 µm. Click-type (amine/active ester, alkyne/azide, biotin/avidin) reactions can be performed selectively on the patch or on the particle body, rendering the particles interesting for application in imaging, biomolecular detection, and as advanced precision colloid-based building blocks.

Scalable Approach to Molecular Motor-Polymer Conjugates for Light-Driven Artificial Muscles.

The integration of molecular machines and motors into materials represents a promising avenue for creating dynamic and functional molecular systems, with potential applications in soft robotics or reconfigurable biomaterials. However, the development of truly scalable and controllable approaches for incorporating molecular motors into polymeric matrices has remained a challenge. Here, it is shown that light-driven molecular motors with sensitive photo-isomerizable double bonds can be converted into initiators for Cu-mediated controlled/living radical polymerization enabling the synthesis of star-shaped motor-polymer conjugates. This approach enables scalability, precise control over the molecular structure, block copolymer structures, and high-end group fidelity. Moreover, it is demonstrated that these materials can be crosslinked to form gels with quasi-ideal network topology, exhibiting light-triggered contraction. The influence of arm length and polymer structure is investigated, and the first molecular dynamics simulation framework to gain deeper insights into the contraction processes is developed. Leveraging this scalable methodology, the creation of bilayer soft robotic devices and cargo-lifting artificial muscles is showcased, highlighting the versatility and potential applications of this advanced polymer chemistry approach. It is anticipated that the integrated experimental and simulation framework will accelerate scalable approaches for active polymer materials based on molecular machines, opening up new horizons in materials science and bioscience.

Recyclable and Light-Adaptive Vitrimer-Based Nacre-Mimetic Nanocomposites.

Nacre's natural design consists of a perfect hierarchical assembly that resembles a brick-and-mortar structure with synergistic stiffness and toughness. The field of bioinspired materials often provides attractive architecture and engineering pathways which allow to explore outstanding property areas. However, the study of nacre-mimetic materials should not be limited to the design of its architecture but ought to include the understanding, operation, and improvement of internal interactions between their components. Here, we introduce a vitrimer prepolymer system that, once integrated into the nacre-mimetic nanocomposites, cures and cross-links with the presence of Lewis acid catalyst and further manifests associative dynamic exchange reactions. Bond exchanges are controllable by molecular composition and catalyst content and characterized by creep, shear-lag, and shape-locking tests. We exploit the vitrimer properties by laminating ca. 70 films into thick bulk materials, and characterize the flexural resistance and crack propagation. More importantly, we introduce recycling by grinding and hot-pressing. The recycling for highly reinforced nacre-mimetic nanocomposites is critically enabled by the vitrimer chemistry and improves the sustainability of bioinspired nanocomposites in cyclic economy. Finally, we integrate photothermal converters into the structures and use laser irradiation as external trigger to activate the vitrimer exchange reactions.

Electrical switching of high-performance bioinspired nanocellulose nanocomposites.

Nature fascinates with living organisms showing mechanically adaptive behavior. In contrast to gels or elastomers, it is profoundly challenging to switch mechanical properties in stiff bioinspired nanocomposites as they contain high fractions of immobile reinforcements. Here, we introduce facile electrical switching to the field of bioinspired nanocomposites, and show how the mechanical properties adapt to low direct current (DC). This is realized for renewable cellulose nanofibrils/polymer nanopapers with tailor-made interactions by deposition of thin single-walled carbon nanotube electrode layers for Joule heating. Application of DC at specific voltages translates into significant electrothermal softening via dynamization and breakage of the thermo-reversible supramolecular bonds. The altered mechanical properties are reversibly switchable in power on/power off cycles. Furthermore, we showcase electricity-adaptive patterns and reconfiguration of deformation patterns using electrode patterning techniques. The simple and generic approach opens avenues for bioinspired nanocomposites for facile application in adaptive damping and structural materials, and soft robotics.

Modular functionalization and hydrogel formation via red-shifted and self-reporting [2+2] cycloadditions.

We present a modularly applicable, red-shifted and self-reporting photodynamic covalent crosslinker, abbreviated qStyPy, that performs [2+2] cycloadditions upon irradiation with 470 nm in water. The rational design of qStyPy increases its hydrophilicity due to a permanent charge and features a broad emission in the far-red/near-infrared regime as a readout for the cycloadduct formation, rendering qStyPy suitable for biomedical applications.

Development of Bioinspired Functional Chitosan/Cellulose Nanofiber 3D Hydrogel Constructs by 3D Printing for Application in the Engineering of Mechanically Demanding Tissues.

Soft tissues are commonly fiber-reinforced hydrogel composite structures, distinguishable from hard tissues by their low mineral and high water content. In this work, we proposed the development of 3D printed hydrogel constructs of the biopolymers chitosan (CHI) and cellulose nanofibers (CNFs), both without any chemical modification, which processing did not incorporate any chemical crosslinking. The unique mechanical properties of native cellulose nanofibers offer new strategies for the design of environmentally friendly high mechanical performance composites. In the here proposed 3D printed bioinspired CNF-filled CHI hydrogel biomaterials, the chitosan serves as a biocompatible matrix promoting cell growth with balanced hydrophilic properties, while the CNFs provide mechanical reinforcement to the CHI-based hydrogel. By means of extrusion-based printing (EBB), the design and development of 3D functional hydrogel scaffolds was achieved by using low concentrations of chitosan (2.0-3.0% (w/v)) and cellulose nanofibers (0.2-0.4% (w/v)). CHI/CNF printed hydrogels with good mechanical performance (Young's modulus 3.0 MPa, stress at break 1.5 MPa, and strain at break 75%), anisotropic microstructure and suitable biological response, were achieved. The CHI/CNF composition and processing parameters were optimized in terms of 3D printability, resolution, and quality of the constructs (microstructure and mechanical properties), resulting in good cell viability. This work allows expanding the library of the so far used biopolymer compositions for 3D printing of mechanically performant hydrogel constructs, purely based in the natural polymers chitosan and cellulose, offering new perspectives in the engineering of mechanically demanding hydrogel tissues like intervertebral disc (IVD), cartilage, meniscus, among others.

Hierarchical cross-linking for synergetic toughening in crustacean-mimetic nanocomposites.

The twisted plywood structure as found in crustacean shells possesses excellent mechanical properties with high stiffness and toughness. Synthetic mimics can be produced by evaporation-induced self-assembly of cellulose nanocrystals (CNCs) with polymer components into bulk films with a cholesteric liquid crystal structure. However, these are often excessively brittle and it has remained challenging to make materials combining high stiffness and toughness. Here, we describe self-assembling cholesteric CNC/polymer nanocomposites with a crustacean-mimetic structure and tunable photonic band gap, in which we engineer combinations of thermo-activated covalent and supramolecular hydrogen-bonded crosslinks to tailor the energy dissipation properties by precise molecular design. Toughening occurs upon increasing the polymer fractions in the nanocomposites, and, critically, combinations of both molecular bonding mechanisms lead to a considerable synergetic increase of stiffness and toughness - beyond the common rule of mixtures. Our concept following careful molecular design allows one to enter previously unreached areas of mechanical property charts for cholesteric CNC-based nanocomposites. The study shows that the subtle engineering of molecular energy dissipation units using sophisticated chemical approaches enables efficient enhancing of the properties of bioinspired CNC/polymer nanocomposites, and opens the design space for future molecular enhancement using tailor-made interactions.

Biodegradable Laser Arrays Self-Assembled from Plant Resources.

The transition toward future sustainable societies largely depends on disruptive innovations in biobased materials to substitute nonsustainable advanced functional materials. In the field of optics, advanced devices (e.g., lasers or metamaterial devices) are typically manufactured using top-down engineering and synthetic materials. This work breaks with such concepts and switchable lasers self-assembled from plant-based cellulose nanocrystals and fluorescent polymers at room temperature and from water are shown. Controlled structure formation allows laser-grade cholesteric photonic bandgap materials, in which the photonic bandgap is matched to the fluorescence emission to function as an efficient resonator for low threshold multimode lasing. The lasers can be switched on and off using humidity, and can be printed into pixelated arrays. Additionally, the materials exhibit stiffness above typical thermoplastic polymers and biodegradability in soil. The concept showcases that highly advanced functions can be encoded into biobased materials, and opens the design space for future sustainable optical devices of unprecedented function.

Biodegradation of Crystalline Cellulose Nanofibers by Means of Enzyme Immobilized-Alginate Beads and Microparticles.

Recent advances in nanocellulose technology have revealed the potential of crystalline cellulose nanofibers to reinforce materials which are useful for tissue engineering, among other functions. However, the low biodegradability of nanocellulose can possess some problems in biomedical applications. In this work, alginate particles with encapsulated enzyme cellulase extracted from Trichoderma reesei were prepared for the biodegradation of crystalline cellulose nanofibers, which carrier system could be incorporated in tissue engineering biomaterials to degrade the crystalline cellulose nanoreinforcement in situ and on-demand during tissue regeneration. Both alginate beads and microparticles were processed by extrusion-dropping and inkjet-based methods, respectively. Processing parameters like the alginate concentration, concentration of ionic crosslinker Ca2+, hardening time, and ionic strength of the medium were varied. The hydrolytic activity of the free and encapsulated enzyme was evaluated for unmodified (CNFs) and TEMPO-oxidized cellulose nanofibers (TOCNFs) in suspension (heterogeneous conditions); in comparison to solubilized cellulose derivatives (homogeneous conditions). The enzymatic activity was evaluated for temperatures between 25-75 °C, pH range from 3.5 to 8.0 and incubation times until 21 d. Encapsulated cellulase in general displayed higher activity compared to the free enzyme over wider temperature and pH ranges and for longer incubation times. A statistical design allowed optimizing the processing parameters for the preparation of enzyme-encapsulated alginate particles presenting the highest enzymatic activity and sphericity. The statistical analysis yielded the optimum particles characteristics and properties by using a formulation of 2% (w/v) alginate, a coagulation bath of 0.2 M CaCl2 and a hardening time of 1 h. In homogeneous conditions the highest catalytic activity was obtained at 55 °C and pH 4.8. These temperature and pH values were considered to study the biodegradation of the crystalline cellulose nanofibers in suspension. The encapsulated cellulase preserved its activity for several weeks over that of the free enzyme, which latter considerably decreased and practically showed deactivation after just 10 d. The alginate microparticles with their high surface area-to-volume ratio effectively allowed the controlled release of the encapsulated enzyme and thereby the sustained hydrolysis of the cellulose nanofibers. The relative activity of cellulase encapsulated in the microparticles leveled-off at around 60% after one day and practically remained at that value for three weeks.

Facile and On-Demand Cross-Linking of Nacre-Mimetic Nanocomposites Using Tailor-Made Polymers with Latent Reactivity.

The development of on-demand cross-linking strategies is a key aspect in promoting mechanical properties of high-performance bioinspired nanocomposites. Here, we embed styrene sulfonyl azide groups with latent chemical reactivity into water-soluble copolymers and assemble those with high-aspect-ratio synthetic nanoclays to generate well-defined layered polymer/nanoclay nacre-mimetics. A considerable stiffening and strengthening occurs upon activation of the covalent cross-linking using simple heating. Varying the amount of cross-linkable units allows molecular control of mechanical properties from ductile to stiff and strong. Moreover, the covalent cross-linking enhances the moisture stability of water-borne nacre-mimetics. The strategy is facile and versatile allowing for a transfer into applications.

Switchable Supracolloidal Coassembly of Microgels Mediated by Host/Guest Interactions.

Supramolecular engineering of multibody colloidal systems provides flexible ways of manipulating superstructures and material properties. We investigate a coassembling microgel (MG) system, in which host- and guest-modified MG partners coassemble by molecular recognition, and show in detail how electrostatic repulsion needs to be balanced for the supramolecular recognition to take place. We observe a gradual change from repellent MGs to stable clusters and ordered flocculates upon decreasing electrostatic repulsion. The adaptive nature of the multivalent interactions embedded in the soft MG shell leads to kinetically trapped scenarios and fibril formation from spherical building blocks.