Heliotracking Device using Liquid Crystalline Elastomer Actuators

Many living organisms in nature respond to light stimulus and track the light source. Inspired by this, and to maximize the light-harvesting ability of solar cells, here, a spontaneous helio-tracking device based on the differential light-induced actuation of liquid crystalline elastomers (LCEs) is demonstrated. The synthesis of the actuator material involves a robust thiol “click” polymerization, while the addition of indocyanine green (ICG) dye imparts the sensitivity to broad-spectrum and near-infrared light. Highly reproducible thermal and photo-induced linear actuation is demonstrated. The device is based on a freely pivoting payload platform held in place by several linear LCE actuators around the 360° circumference. The side of the device, which is exposed to light, has the actuators contracting and tilting the platform toward the light source. As the light source (e.g., the Sun) is moving around the device, the platform tilt followed, always exposing the payload face to the light; in the dark, the device recovers its neutral position

Elasticity and Relaxation in Full and Partial Vitrimer Networks

We develop a continuum model of dynamic-mechanical response of vitrimers, where the elastic energy of the material accounts for the conserved number of the crosslinks in the network. We also prepare partial vitrimer networks, which consist of variable fractions of transient network based on boronic ester bond-exchange, and of a permanent polymer network. By fitting the theory to our experimental data on stress relaxation, the bond-exchange rate and the fraction of the permanent elastic network are obtained with a linear relationship between the fraction of the transient polymer network and the ratio between the boronic ester and the flexible spacer among the chain-extending thiols. For a 100% vitrimer undergoing a ramp deformation, the stress of the material fi rst increases and then decreases, where the yield time decreases with the increasing strain rate. A partial vitrimer can behave as a pure elastic material without yielding at low strain rates or show a non-monotonic `S-shaped' stress-strain relationship at high strain rates.This work was supported by the European Research Council grant No: 786659. F. M. acknowledges Humboldt Foundation for the support

Transesterification in Epoxy-Thiol Exchangeable Liquid Crystalline Elastomers

The incorporation of vitrimer bond-exchange chemistry into liquid crystalline elastomer networks produces ‘exchangeable liquid crystal elastomers’ (xLCE). These materials offer a facile method of material re-shaping and alignment post-polymerisation via the application of a mechanical stress above the temperature of activation of bond exchange. We use di-epoxy mesogenic monomers with thiol-terminated spacers and crosslinker to investigate a range of resulting xLCE. The “click” chemistry of thiols results in good control over the network topology, and low glass transition in this family of materi-als. By combining different spacers, we were able to obtain smectic and nematic phases, and adjust the liquid crystal to iso-tropic phase transition between 42 and 140 °C, while the elastic-plastic transition temperature was maintained close to 200 °C. The broad gap between these temperatures ensures that thermally actuating uniformly aligned elastomers are stable and show no residual plastic creep, making epoxy-thiol xLCE promising for a range of engineering applications.ERC H202

Liquid Crystalline Vitrimers with Full or Partial Boronic-Ester Bond Exchange

In this manuscript, a new vitrimer chemistry strategy (boronic transesterification) is introduced into liquid crystal elastomers (LCEs) to allow catalyst-free bond exchange to enable processing (director alignment, remolding, and welding) in the liquid crystalline (nematic) phase. Additionally, the concept of partial vitrimer network is explored, where a percolating fraction of the network remains permanently cross-linked, hence preserving the integrity of the materials and preventing large creep. This combined strategy allows one to avoid the shortcomings of current methods of aligning LCE, especially in complex shapes. Thiol-acrylate Michael addition reaction is used to produce uniform polymer networks with controllable thermomechanical response and local plasticity. Control of the plasticity is achieved by varying the fractions of permanent and exchangeable network, where a material “sweet spot” with an optimum elastic/plastic balance is identified. Such exchangeable LCE (xLCE) allows postpolymerization processing, while also minimizing unwanted creep during actuation. Moreover, conjoining multiple materials (isotropic and liquid-crystalline) in a single covalently bonded composite structure results in a variety of smart morphing systems that adopt shapes with complex curvature. Remolding and welding xLCEs may enable the applications of these materials as mechanical actuators in reversibly folding origami, in vivo artificial muscles, and in soft robotics

Dynamic Manipulation of Friction in Smart Textile Composites of Liquid-Crystal Elastomers

Smart surfaces that reversibly change the interfacial friction coefficients in response to external stimuli enable a wide range of applications, such as grips, seals, brake pads, packaging films, and fabrics. Here a new concept of such a smart frictional system is reported: a composite film of a plain-weave polyester textile sheet, and a thermo-responsive nematic liquid crystalline elastomer (LCE). The composite is deployed with retractable micro-undulations of the elastomer inside each weave mesh, enabling dramatic changes of the contact interface with the opposing surface on LCE actuation, which is induced e.g. by a change in temperature (T). At ambient T, the protruding viscoelastic parts of LCE in the nematic phase make contact with the opposing flat surface, resulting in a very high friction. At an elevated T (50C, isotropic phase), the undulations of LCE surface are retracted within the thickness of the textile, and the contacts are limited to small regions around overlapping textile fibers, lowering the friction dramatically. This effect is fully reversible on heating/cooling cycles. The surface undulations are spontaneous, i.e. fabricated without any lithographic or alignment techniques. The present composite opens a new way to practical uses of sheets/films with switchable friction enabled by stimuli-responsive LCEs.ERC H2020 AdG No. 786659 JSPS KAKENHI under Grant No. JP17K1886

Polymerase δ replicates both strands after homologous recombination-dependent fork restart

To maintain genetic stability DNA must be replicated only once and replication completed even when individual replication forks are inactivated. Because fork inactivation is common, the passive convergence of an adjacent fork is insufficient to rescue all inactive forks. Thus, eukaryotic cells have evolved homologous recombination-dependent mechanisms to restart persistent inactive forks. Completing DNA synthesis via Homologous Recombination Restarted Replication (HoRReR) ensures cell survival, but at a cost. One such cost is increased mutagenesis caused by HoRReR being more error prone than canonical replication. This increased error rate implies that the HoRReR mechanism is distinct from that of a canonical fork. Here we exploit the fission yeast Schizosaccharomyces pombe to demonstrate that a DNA sequence duplicated by HoRReR during S phase is replicated semi-conservatively, but that both the leading and lagging strands are synthesised by DNA polymerase delta

Reducing the environmental impact of surgery on a global scale: systematic review and co-prioritization with healthcare workers in 132 countries

Abstract Background Healthcare cannot achieve net-zero carbon without addressing operating theatres. The aim of this study was to prioritize feasible interventions to reduce the environmental impact of operating theatres. Methods This study adopted a four-phase Delphi consensus co-prioritization methodology. In phase 1, a systematic review of published interventions and global consultation of perioperative healthcare professionals were used to longlist interventions. In phase 2, iterative thematic analysis consolidated comparable interventions into a shortlist. In phase 3, the shortlist was co-prioritized based on patient and clinician views on acceptability, feasibility, and safety. In phase 4, ranked lists of interventions were presented by their relevance to high-income countries and low–middle-income countries. Results In phase 1, 43 interventions were identified, which had low uptake in practice according to 3042 professionals globally. In phase 2, a shortlist of 15 intervention domains was generated. In phase 3, interventions were deemed acceptable for more than 90 per cent of patients except for reducing general anaesthesia (84 per cent) and re-sterilization of ‘single-use’ consumables (86 per cent). In phase 4, the top three shortlisted interventions for high-income countries were: introducing recycling; reducing use of anaesthetic gases; and appropriate clinical waste processing. In phase 4, the top three shortlisted interventions for low–middle-income countries were: introducing reusable surgical devices; reducing use of consumables; and reducing the use of general anaesthesia. Conclusion This is a step toward environmentally sustainable operating environments with actionable interventions applicable to both high– and low–middle–income countries

Main-Chain Nematic Side-Chain Smectic Composite Liquid Crystalline Elastomers

A composite liquid crystalline elastomer is designed, combining main-chain and side-chain mesogenic polymers in the network, and resulting in micro-phase separated regions of nematic and smectic ordering in the macroscopically homogeneous elastomer. A range of different fractions of the components is explored, from fully nematic main-chain system, across to fully smectic side-chain elastomer. Thermal phase transitions of both phases coexisting in the material are detected by calorimetry, and the nematic/smectic structure investigated by X-ray scattering. The tensile stress-strain data reveals the key effect of such a multi-phase composite, where the nematic fraction adds ductility while the smectic fraction increases the modulus and mechanical stiffness. Varying the composition, we were able to optimize the mechanical properties of this material type

Scalable upcycling of thermoplastic polyolefins into vitrimers through transesterification

Converting commodity structural thermoplastics into dynamically crosslinked vitrimers.European Research Council H202

Scalable upcycling of thermoplastic polyolefins into vitrimers through transesterification

About 150 million tons of disposed plastic is accumulated each year globally. A massive challenge will be addressed even if a fraction of this amount is reclaimed as relevant feedstock for innovative materials, provided this transformation is accomplished through an affordable process with minimal resources in a high-throughput manner. Vitrimers, the dynamic networks enabled by an associative covalent bond exchange, are an emerging class of materials that combine the best of thermoplastic and thermoset characteristics. Here we report that high performance vitrimers can be produced through chemical transformation of commodity thermoplastic polyolefins (TPOs) in a simple and economical way. Polypropylene (PP) and polyethylene (PE) retrieved from recycling have been converted into permanently crosslinked networks that are rubber-elastic above the melting point, and are capable of bond exchange at a further elevated temperature. We find that the dynamically crosslinked network shows thermally triggered shape-memory behaviour with 90% recovery after multiple fixity-recovery cycles. With superior mechanical stability compared to the precursor TPO, dynamic networks can establish interfacial covalent bonding to assemble objects of complex shapes through welding. The developed method can be applied to a wide range of TPOs without prior knowledge of their precise composition. It suggests a new direction towards recovery of 'smart' materials for sustainable and affordable technologies from plastic recycling, using conventionally operated instruments, without the need to upgrade the infrastructure of the polymer processing industry