Photoresponsive hydrogel friction.

Photoresponsive hydrogels are an emerging class of stimuli-responsive materials that exhibit changes in physical or chemical properties in response to light. Previous investigations have leveraged photothermal mechanisms to achieve reversible changes in hydrogel friction, although few have focused on photochemical means. To date, the tribological properties of photoswitchable hydrogels (e.g., friction and lubrication) have remained underexplored. In this work, we incorporated photoresponsive methoxy-spiropyran-methacrylate monomers (methoxy-SP-MA) into a hydrogel network to form a copolymerized system of poly(N-isopropylacrylamide-co-2-acrylamido-2-methylpropane sulfonic acid-co-methoxy-spiropyran-methacrylate) (p(NIPAAm-co-AMPS-co-SP)). We demonstrated repeatable photoresponsive changes to swelling, friction, and stiffness over three light cycles. Our findings suggest that volume changes driven by the decreased hydrophilicity of the methoxy-SP-MA upon light irradiation are responsible for differences in the mechanical and tribological properties of our photoresponsive hydrogels. Our results could inform future designs of photoswitchable hydrogels for applications ranging from biomedical applications to soft robotics.

Synthesis, mol-ecular and crystal structures of 4-amino-3,5-di-fluoro-benzo-nitrile, ethyl 4-amino-3,5-di-fluoro-benzoate, and diethyl 4,4'-(diazene-1,2-di-yl)bis-(3,5-di-fluoro-benzoate).

The crystal structures of two inter-mediates, 4-amino-3,5-di-fluoro-benzo-nitrile, C7H4F2N2 (I), and ethyl 4-amino-3,5-di-fluoro-benzoate, C9H9F2NO2 (II), along with a visible-light-responsive azo-benzene derivative, diethyl 4,4'-(diazene-1,2-di-yl)bis-(3,5-di-fluoro-benzoate), C18H14F4N2O4 (III), obtained by four-step synthetic procedure, were studied using single-crystal X-ray diffraction. The mol-ecules of I and II demonstrate the quinoid character of phenyl rings accompanied by the distortion of bond angles related to the presence of fluorine substituents in the 3 and 5 (ortho) positions. In the crystals of I and II, the mol-ecules are connected by N-H⋯N, N-H⋯F and N-H⋯O hydrogen bonds, C-H⋯F short contacts, and π-stacking inter-actions. In crystal of III, only stacking inter-actions between the mol-ecules are found.

Reversible and size-controlled assembly of reflectin proteins using a charged azobenzene photoswitch.

Disordered proteins often undergo a stimuli-responsive, disorder-to-order transition which facilitates dynamic processes that modulate the physiological activities and material properties of cells, such as strength, chemical composition, and reflectance. It remains challenging to gain rapid and spatiotemporal control over such disorder-to-order transitions, which limits the incorporation of these proteins into novel materials. The reflectin protein is a cationic, disordered protein whose assembly is responsible for dynamic color camouflage in cephalopods. Stimuli-responsive control of reflectin's assembly would enable the design of biophotonic materials with tunable color. Herein, a novel, multivalent azobenzene photoswitch is shown to be an effective and non-invasive strategy for co-assembling with reflectin molecules and reversibly controlling assembly size. Photoisomerization between the trans and cis (E and Z) photoisomers promotes or reduces Coulombic interactions, respectively, with reflectin proteins to repeatedly cycle the sizes of the photoswitch-reflectin assemblies between 70 nm and 40 nm. The protein assemblies formed with the trans and cis isomers show differences in interaction stoichiometry and secondary structure, which indicate that photoisomerization modulates the photoswitch-protein interactions to change assembly size. Our results highlight the utility of photoswitchable interactions to control reflectin assembly and provide a tunable synthetic platform that can be adapted to the structure, assembly, and function of other disordered proteins.

Synthesis and Thermo-Selective Recycling of Diels-Alder Cyclopentadiene Thermoplastics.

Catalyst-free and reversible step-growth Diels-Alder (DA) polymerization has a wide range of applications in polymer synthesis and is a promising method for fabricating recyclable thermoplastics. The effectiveness of polymerization and depolymerization relies on the chemical building blocks, often utilizing furan as the diene and maleimide as the dienophile. Compared to the traditional diene-dienophile or two-component approach that requires precise stoichiometry, cyclopentadiene (Cp) can serve dual roles via self-dimerization. This internally balanced platform offers a route to access high-molecular-weight polymers and a dynamic handle for polymer recycling, which has yet to be explored. Herein, through a reactivity investigation of different telechelic Cp derivatives, the uncontrolled cross-linking of Cp was addressed, revealing the first successful DA homopolymerization. To demonstrate the generality of our methodology, we synthesized and characterized six Cp homopolymers with backbones derived from common thermoplastics, such as poly(dimethylsiloxane), hydrogenated polybutadiene, and ethylene phthalate. Among these materials, the hydrogenated polybutadiene-Cp analog can be thermally depolymerized (Mn = 68 to 23 kDa) and repolymerized to the parent polymer (Mn = 68 kDa) under solvent- and catalyst-free conditions. This process was repeated over three cycles without intermediate purification, confirming the efficient thermo-selective recyclability. The varied degradable properties of the other four Cp-incorporated thermoplastics were also examined. Overall, this work provides a general methodology for accessing a new class of reversible homopolymers, potentially expanding the design and construction of sustainable thermoplastics.

Photoactivation of Millimeters Thick Liquid Crystal Elastomers with Broadband Visible Light Using Donor-Acceptor Stenhouse Adducts.

Light-responsive liquid crystal elastomers (LCEs) are stimuli-responsive materials that facilitate the conversion of light energy into a mechanical response. In this work, a novel polysiloxane-based LCE with donor-acceptor Stenhouse adduct (DASA) side-chains is synthesized using a late-stage functionalization strategy. It is demonstrated that this approach does not compromise the molecular alignment observed in the traditional Finkelmann method. This easy, single-batch process provides a robust platform to access well-aligned, light-responsive LCE films with thickness ranging from 400 µm to a 14-layer stack that is 5 mm thick. Upon irradiation with low-intensity broadband visible light (100-200 mW cm-2), these systems undergo 2D planar actuation and complete bleaching. Conversely, exposure to higher-intensity visible light induces bending followed by contraction (300 mW cm-2). These processes are repeatable over several cycles. Finally, it is demonstrated how light intensity and the resulting heat generation influences the photothermal stationary state equilibrium of DASA, thereby controlling its photoresponsive properties. This work establishes the groundwork for advancement of LCE-based actuators beyond thin film and UV-light reliant systems.

Dynamic Manipulation of Droplets on Liquid-Infused Surfaces Using Photoresponsive Surfactant.

Fast and programmable transport of droplets on a substrate is desirable in microfluidic, thermal, biomedical, and energy devices. Photoresponsive surfactants are promising candidates to manipulate droplet motion due to their ability to modify interfacial tension and generate "photo-Marangoni" flow under light stimuli. Previous works have demonstrated photo-Marangoni droplet migration in liquid media; however, migration on other substrates, including solid and liquid-infused surfaces (LIS), remains an outstanding challenge. Moreover, models of photo-Marangoni migration are still needed to identify optimal photoswitches and assess the feasibility of new applications. In this work, we demonstrate 2D droplet motion on liquid surfaces and on LIS, as well as rectilinear motion in solid capillary tubes. We synthesize photoswitches based on spiropyran and merocyanine, capable of tension changes of up to 5.5 mN/m across time scales as short as 1.7 s. A millimeter-sized droplet migrates at up to 5.5 mm/s on a liquid, and 0.25 mm/s on LIS. We observe an optimal droplet size for fast migration, which we explain by developing a scaling model. The model also predicts that faster migration is enabled by surfactants that maximize the ratio between the tension change and the photoswitching time. To better understand migration on LIS, we visualize the droplet flow using tracer particles, and we develop corresponding numerical simulations, finding reasonable agreement. The methods and insights demonstrated in this study enable advances for manipulation of droplets for microfluidic, thermal and water harvesting devices.

Tethered together: DASA design towards aqueous compatibility.

Donor-acceptor Stenhouse adducts (DASAs) are an exciting class of photoswitches due to their facile tunability, visible light absorbance, and negative photochromism. While they have shown use in a variety of applications, to date all reported DASA derivatives have low equilibrium and/or poor photoswitching in polar protic solvents, which is vital for moving towards applications in biological systems. We demonstrate a strategy to introduce a substitution on the DASA triene that results in derivatives that are stable and have high dark equilibrium of the open form in polar protic solvents. Decreasing the charge separation of these new derivatives also allows for reversible switching in polar and protic solvents including THF : water mixtures.

Visible light-responsive materials: the (photo)chemistry and applications of donor-acceptor Stenhouse adducts in polymer science.

Donor-acceptor Stenhouse adduct (DASA) photoswitches have gained a lot of attention since their discovery in 2014. Their negative photochromism, visible light absorbance, synthetic tunability, and the large property changes between their photoisomers make them attractive candidates over other commonly used photoswitches for use in materials with responsive or adaptive properties. The development of such materials and their translation into advanced technologies continues to widely impact forefront materials research, and DASAs have thus attracted considerable interest in the field of visible-light responsive molecular switches and dynamic materials. Despite this interest, there have been challenges in understanding their complex behavior in the context of both small molecule studies and materials. Moreover, incorporation of DASAs into polymers can be challenging due to their incompatibility with the conditions for most common polymerization techniques. In this review, therefore, we examine and critically discuss the recent developments and challenges in the field of DASA-containing polymers, aiming at providing a better understanding of the interplay between the properties of both constituents (matrix and photoswitch). The first part summarizes current understanding of DASA design and switching properties. The second section discusses strategies of incorporation of DASAs into polymers, properties of DASA-containing materials, and methods for studying switching of DASAs in materials. We also discuss emerging applications for DASA photoswitches in polymeric materials, ranging from light-responsive drug delivery systems, to photothermal actuators, sensors and photoswitchable surfaces. Last, we summarize the current challenges in the field and venture on the steps required to explore novel systems and expand both the functional properties and the application opportunities of DASA-containing polymers.

Controlled-Radical Polymerization of α-Lipoic Acid: A General Route to Degradable Vinyl Copolymers.

Here, we present the synthesis and characterization of statistical and block copolymers containing α-lipoic acid (LA) using reversible addition-fragmentation chain-transfer (RAFT) polymerization. LA, a readily available nutritional supplement, undergoes efficient radical ring-opening copolymerization with vinyl monomers in a controlled manner with predictable molecular weights and low molar-mass dispersities. Because lipoic acid diads present in the resulting copolymers include disulfide bonds, these materials efficiently and rapidly degrade when exposed to mild reducing agents such as tris(2-carboxyethyl)phosphine (Mn = 56 → 3.6 kg mol-1). This scalable and versatile polymerization method affords a facile way to synthesize degradable polymers with controlled architectures, molecular weights, and molar-mass dispersities from α-lipoic acid, a commercially available and renewable monomer.

Tailoring Writability and Performance of Star Block Copolypeptides Hydrogels through Side-Chain Design.

Shear-recoverable hydrogels based on block copolypeptides with rapid self-recovery hold potential in extrudable and injectable 3D-printing applications. In this work, a series of 3-arm star-shaped block copolypeptides composed of an inner hydrophilic poly(l-glutamate) domain and an outer ß-sheet forming domain is synthesized with varying side chains and block lengths. By changing the ß-sheet forming domains, hydrogels with diverse microstructures and mechanical properties are prepared and structure-function relationships are determined using scattering and rheological techniques. Differences in the properties of these materials are amplified during direct-ink writing with a strong correlation observed between printability and material chemistry. Significantly, it is observed that non-canonical ß-sheet blocks based on phenyl glycine form more stable networks with superior mechanical properties and writability compared to widely used natural amino acid counterparts. The versatile design available through block copolypeptide materials provides a robust platform to access tunable material properties based solely on molecular design. These systems can be exploited in extrusion-based applications such as 3D-printing without the need for additives.

Molecular-scale substrate anisotropy, crowding and division drive collective behaviours in cell monolayers.

The ability of cells to reorganize in response to external stimuli is important in areas ranging from morphogenesis to tissue engineering. While nematic order is common in biological tissues, it typically only extends to small regions of cells interacting via steric repulsion. On isotropic substrates, elongated cells can co-align due to steric effects, forming ordered but randomly oriented finite-size domains. However, we have discovered that flat substrates with nematic order can induce global nematic alignment of dense, spindle-like cells, thereby influencing cell organization and collective motion and driving alignment on the scale of the entire tissue. Remarkably, single cells are not sensitive to the substrate's anisotropy. Rather, the emergence of global nematic order is a collective phenomenon that requires both steric effects and molecular-scale anisotropy of the substrate. To quantify the rich set of behaviours afforded by this system, we analyse velocity, positional and orientational correlations for several thousand cells over days. The establishment of global order is facilitated by enhanced cell division along the substrate's nematic axis, and associated extensile stresses that restructure the cells' actomyosin networks. Our work provides a new understanding of the dynamics of cellular remodelling and organization among weakly interacting cells.

Design, Synthesis, and Application of a Water-soluble Photocage for Aqueous Cyclopentadiene-based Diels-Alder Photoclick Chemistry in Hydrogels.

Spatiotemporally functionalized hydrogels have exciting applications in tissue engineering, but their preparation often relies on radical-based strategies that can be deleterious in biological settings. Herein, the computationally guided design, synthesis, and application of a water-soluble cyclopentadienone-norbornadiene (CPD-NBD) adduct is disclosed as a diene photocage for radical-free Diels-Alder photopatterning. We show that this scalable CPD-NBD derivative is readily incorporated into hydrogel formulations, providing gels that can be patterned with dienophiles upon 365 nm uncaging of cyclopentadiene. Patterning is first visualized through conjugation of cyanine dyes, then biological utility is highlighted by patterning peptides to direct cellular adhesion. Finally, the ease of use and versatility of this CPD-NBD derivative is demonstrated by direct incorporation into a commercial 3D printing resin to enable the photopatterning of structurally complex, printed hydrogels.

Tailored Polypeptide Star Copolymers for 3D Printing of Bacterial Composites Via Direct Ink Writing.

Hydrogels hold much promise for 3D printing of functional living materials; however, challenges remain in tailoring mechanical robustness as well as biological performance. In addressing this challenge, the modular synthesis of functional hydrogels from 3-arm diblock copolypeptide stars composed of an inner poly(l-glutamate) domain and outer poly(l-tyrosine) or poly(l-valine) blocks is described. Physical crosslinking due to ß-sheet assembly of these star block copolymers gives mechanical stability during extrusion printing and the selective incorporation of methacrylate units allows for subsequent photocrosslinking to occur under biocompatible conditions. This permits direct ink writing (DIW) printing of bacteria-based mixtures leading to 3D objects with high fidelity and excellent bacterial viability. The tunable stiffness of different copolypeptide networks enables control over proliferation and colony formation for embedded Escherichia coli bacteria as demonstrated via isopropyl ß-d-1-thiogalactopyranoside (IPTG) induction of green fluorescent protein (GFP) expression. This translation of molecular structure to network properties highlights the versatility of these polypeptide hydrogel systems with the combination of writable structures and biological activity illustrating the future potential of these 3D-printed biocomposites.

Controlled Diels-Alder "Click" Strategy to Access Mechanically Aligned Main-Chain Liquid Crystal Networks.

Aligned liquid crystal polymers are materials of interest for electronic, optic, biological and soft robotic applications. The manufacturing and processing of these materials have been widely explored with mechanical alignment establishing itself as a preferred method due to its ease of use and widespread applicability. However, the fundamental chemistry behind the required two-step polymerization for mechanical alignment has limitations in both fabrication and substrate compatibility. In this work we introduce a new protection-deprotection approach utilizing a two-stage Diels-Alder cyclopentadiene-maleimide step-growth polymerization to enable mild yet efficient, fast, controlled, reproducible and user-friendly polymerizations, broadening the scope of liquid crystal systems. Thorough characterization of the films by DSC, DMA, POM and WAXD show the successful synthesis of a uniaxially aligned liquid crystal network with thermomechanical actuation abilities.

Design of Surface-Aligned Main-Chain Liquid-Crystal Networks Prepared under Ambient, Light-Free Conditions Using the Diels-Alder Cycloaddition.

Surface-aligned liquid-crystal networks (LCNs) offer a solution for developing functional materials capable of performing a range of tasks, including actuation, shape memory, and surfaces patterning. Here we show that Diels-Alder cycloaddition can be used to prepare the backbone of planar aligned LCNs under mild ambient conditions without the addition of additives or UV irradiation. The mechanical properties of the networks have robust viscoelastic modulus and stiffness with a reversible local free volume change upon physical aging. This study shows new opportunities to design surface-aligned LCNs based on additive free step-growth Diels-Alder polymerization and enables the potential to incorporate a wider range of photochromic materials into LCNs.

Triboelectric Nanogenerators: Enhancing Performance by Increasing the Charge-Generating Layer Compressibility.

Triboelectric nanogenerators (TENGs) have received significant attention for next-generation wearable electronics due to their simple device structure and low cost. Although the performance of TENGs is intimately tied to compressibility effects in the charge-generating layer, achieving high compressibility with conventional elastomers is challenging because molecular entanglements place a lower bound on the softness of cross-linked networks. Here, we demonstrate that bottlebrush elastomers are efficient charge-generating layers that improve the output performance of TENGs, including voltage, current, and surface potential, by minimizing entanglements and decreasing the compressive modulus (E). For example, a cross-linked bottlebrush with poly(dimethylsiloxane) side chains yielded TENGs with an output voltage (120 V) more than two times larger than a linear PDMS network (55 V). In conclusion, this study highlights the advantage of designing new charge-generating layers with improved compressibility to enhance TENG performance.

Copper-Mediated Single-Electron Approach to Indoline Amination: Scope, Mechanism, and Total Synthesis of Asperazine A.

Pyrroloindolines bearing a C3-N linkage comprise the core of many biologically active natural products, but many methods toward their synthesis are limited by the sterics or electronics of the product. We report a single electron-based approach for the synthesis of this scaffold and demonstrate high-yielding aminations, regardless of electronic or steric demands. The transformation uses copper wire and isopropanol to promote the reaction. The broad synthetic utility of this heterogeneous copper-catalyzed approach to access pyrroloindolines, diketopiperazine, furoindoline, and (+)-asperazine is included, along with experiments to provide insight into the mechanism of this new process.

A multi-stage single photochrome system for controlled photoswitching responses.

The ability of molecular photoswitches to convert on/off responses into large macroscale property change is fundamental to light-responsive materials. However, moving beyond simple binary responses necessitates the introduction of new elements that control the chemistry of the photoswitching process at the molecular scale. To achieve this goal, we designed, synthesized and developed a single photochrome, based on a modified donor-acceptor Stenhouse adduct (DASA), capable of independently addressing multiple molecular states. The multi-stage photoswitch enables complex switching phenomena. To demonstrate this, we show spatial control of the transformation of a three-stage photoswitch by tuning the population of intermediates along the multi-step reaction pathway of the DASAs without interfering with either the first or final stage. This allows for a photonic three-stage logic gate where the secondary wavelength solely negates the input of the primary wavelength. These results provide a new strategy to move beyond traditional on/off binary photochromic systems and enable the design of future molecular logic systems.

Rational mechanochemical design of Diels-Alder crosslinked biocompatible hydrogels with enhanced properties.

An important but often overlooked feature of Diels-Alder (DA) cycloadditions is the ability for DA adducts to undergo mechanically induced cycloreversion when placed under force. Herein, we demonstrate that the commonly employed DA cycloaddition between furan and maleimide to crosslink hydrogels results in slow gelation kinetics and "mechanolabile" crosslinks that relate to reduced material strength. Through rational computational design, "mechanoresistant" DA adducts were identified by constrained geometries simulate external force models and employed to enhance failure strength of crosslinked hydrogels. Additionally, utilization of a cyclopentadiene derivative, spiro[2.4]hepta-4,6-diene, provided mechanoresistant DA adducts and rapid gelation in minutes at room temperature. This study illustrates that strategic molecular-level design of DA crosslinks can provide biocompatible materials with improved processing, mechanical durability, lifetime, and utility.

Depinning of Multiphase Fluid Using Light and Photo-Responsive Surfactants.

The development of noninvasive and robust strategies for manipulation of droplets and bubbles is crucial in applications such as boiling and condensation, electrocatalysis, and microfluidics. In this work, we realize the swift departure of droplets and bubbles from solid substrates by introducing photoresponsive surfactants and applying asymmetric illumination, thereby inducing a "photo-Marangoni" lift force. Experiments show that a pinned toluene droplet can depart the substrate in only 0.38 s upon illumination, and the volume of an air bubble at departure is reduced by 20%, indicating significantly faster departure. These benefits can be achieved with moderate light intensities and dilute surfactant concentrations, without specially fabricated substrates, which greatly facilitates practical applications. Simulations suggest that the net departure force includes contributions from viscous stresses directly caused by the Marangoni flow, as well as from pressure buildup due to flow stagnation at the contact line. The manipulation scheme proposed here shows potential for applications requiring droplet and bubble removal from working surfaces.