Rheological Properties of Blends of Isotactic Polystyrene–Isotactic Poly(para-methylstyrene) Films Derived from a Comparative Dewetting Study

Macromolecules, 2020, DOI:
Austine A. Mulama, Konstantinos Roumpos, L. Pradipkanti, Andrew O. Oduor, and Günter Reiter

We examined changes in rheological properties of nonequilibrated spin-coated thin films of isotactic polystyrene (iPS) and isotactic poly(para-methylstyrene) and blends thereof by dewetting experiments. For 200 nm thick films and a systematic variation of dewetting temperature, we focused on changes in the amount of residual stresses and the corresponding relaxation times as a function of iPS content. We found that both parameters increased in an approximately logarithmic fashion with iPS content, suggesting an increased tendency for the formation of transient clusters in blends with increased iPS content. Assuming linear elasticity, we determined a nonequilibrium shear modulus and a nonequilibrium viscosity, which we compared with published data of the equilibrium properties. Particularly for the viscosity, huge deviations from published values were observed. We provide a tentative explanation.

Silk Sericin-Polylactide Protein-Polymer Conjugates as Biodegradable Amphiphilic Material and Its Application in Drug Release Systems

Bioconjugate Chemistry, 2020, DOI:
Kanittha Boonpavanitchakul, Livia K. Bast, Nico Bruns and Rathanawan Magaraphan

Silk sericin (SS) is a by-product of silk production. In order to transform it into value-added products, sericin can be used as biodegradable and pH-responsive building block in drug delivery materials. To this end, amphiphilic substances were synthesized via the conjugation of hydrophobic polylactide (PLA) to the hydrophilic sericin using a bis-aryl hydrazone linker. PLA was esterified with terephthalaldehydic acid to obtain aromatic aldehyde terminated PLA (PLA-CHO). In addition, lysine groups of SS were modified with the linker succinimidyl-6-hydrazino-nicotinamide (S-HyNic). Then, both macromolecules were mixed to form the amphipilic protein-polymer conjugate in buffer-DMF solution. The formation of bis-aryl hydrazone linkages was confirmed and quantified by UV-Vis spectroscopy. SS-PLA conjugates self-assembled in water into spherical multicompartment micelles with a diameter of around 100 nm. Doxorubicin (DOX) was selected as a model drug for studying the pH-dependent drug release from SS-PLA nanoparticles. The release rate of the encapsulated drug was slower than that of the free drug and dependent on pH; faster at pH 5.0 and resulted in a larger cumulative amount of drug released than at physiological pH of 7.4. The SS-PLA conjugate of high PLA branches showed smaller particle size and lower loading capacity than the one with low PLA branches. Both SS-PLA conjugates had negligible cytotoxicity whereas, after loading with DOX, the SS-PLA micelles were highly toxic for the human liver carcinoma immortalized cell line HepG2. Therefore the SS-based biodegradable amphiphilic material showed great potential as a drug carrier for cancer therapy.

Complex photonic response reveals three-dimensional self-organization of structural coloured bacterial colonies

Journal of the Royal Society Interface, 2020, DOI:
Lukas Schertel, Gea T. van de Kerkhof, Gianni Jacucci, Laura Catón, Yu Ogawa, Bodo D. Wilts, Colin J. Ingham, Silvia Vignolini and Villads E. Johansen

Vivid colours found in living organisms are often the result of scattering from hierarchical nanostructures, where the interplay between order and disorder in their packing defines visual appearance. In the case of Flavobacterium IR1, the complex arrangement of the cells in polycrystalline three-dimensional lattices is found to be a distinctive fingerprint of colony organization. By combining analytical analysis of the angle-resolved scattering response of in vivo bacterial colonies with numerical modelling, we show that we can assess the inter-cell distance and cell diameter with a resolution below 10 nm, far better than what can be achieved with conventional electron microscopy, suffering from preparation artefacts. Retrieving the role of disorder at different length scales from the salient features in the scattering response enables a precise understanding of the structural organization of the bacteria.

Polymerization‐Induced Wrinkled Surfaces with Controlled Topography as Slippery Surfaces for Colorado Potato Beetles

Advanced Materials Interfaces, 2020, DOI:
Johannes B. Bergmann, Dafni Moatsou, Venkata A. Surapaneni, Marc Thielen, Thomas Speck, Bodo D. Wilts and Ullrich Steiner

Controlling the interaction of insect populations with their host plants has recently received renewed attention in the light of pest control. One way to modify the interaction of insects with their host plants in a non‐chemical way is through influence of their de/attachment. Insect detachment has been observed for textured biological and structured artificial surfaces with morphologies ranging from nano‐ to micrometers (0.3–1.5 µm). Here, the formation of design surfaces is investigated through plasma‐induced polymerization of acrylates. This produces pronounced surface wrinkles that are tunable by the manufacturing process. For certain parameters, the wrinkles resemble those of the adaxial side of rubber tree (Hevea brasiliensis) leaves, a natural example of particularly low friction. Traction force measurements on the bio‐inspired surfaces show significantly impacted insect attachment compared to flat surfaces of silica and polymeric materials, opening a pathway to the controlled manufacture of bio‐inspired slippery surfaces for insects that could potentially find use in advanced materials such as wall coatings.

Disordered wax platelets on Tradescantia pallida leaves create golden shine

Faraday Discussions, 2020, DOI:
Gea Theodora van de Kerkhof, Lukas Schertel, Rebecca Poon, Gianni Jacucci, Beverley Jane Glover and Silvia Vignolini

Plants have various strategies to protect themselves from harmful light. An example of such a protective mechanism is the growth of epicuticular nanostructures, such as a layer of hair or wax crystals. Most nanostructures are optimised to screen UV radiation, as UV light is particularly damaging for cellular tissue. We find that, contrary to the commonly found UV reflectance, the epicuticular wax crystals on Tradescantia leaves reflect strongly in the higher visible wavelength regime. Thus, they give the leaves a golden shine. We characterize the optical appearance of Tradescantia pallida ‘purpurea’ leaves by angularly resolved spectroscopy and compare the results to finite difference time domain simulations. We find that it is the disordered assembly of the wax platelets that is the crucial parameter to obtain the observed reflected intensity increase for higher wavelengths.

Visual Appearance of Chiral Nematic Cellulose‐Based Photonic Films: Angular and Polarization Independent Color Response with a Twist

Advanced Materials, 31:52, DOI:
Chun Lam Clement Chan, Mélanie M. Bay, Gianni Jacucci, Roberto Vadrucci, Cyan A. Williams, Gea T. van de Kerkhof, Richard M. Parker, Kevin Vynck, Bruno Frka‐Petesic and Silvia Vignolini

Hydroxypropyl cellulose (HPC) is a biocompatible cellulose derivative capable of self‐assembling into a lyotropic chiral nematic phase in aqueous solution. This liquid crystalline phase reflects right‐handed circular polarized light of a specific color as a function of the HPC weight fraction. Here, it is demonstrated that, by introducing a crosslinking agent, it is possible to drastically alter the visual appearance of the HPC mesophase in terms of the reflected color, the scattering distribution, and the polarization response, resulting in an exceptional matte appearance in solid‐state films. By exploiting the interplay between order and disorder, a robust and simple methodology toward the preparation of polarization and angular independent color is developed, which constitutes an important step toward the development of real‐world photonic colorants.

Dewetting Rheology for Determining Viscoelastic Properties of Nonequilibrated Thin Polymer Films

Macromolecules, 52, DOI:
Austine A. Mulama, Sivasurender Chandran, Konstantinos Roumpos, Andrew O. Oduor and Günter Reiter

We performed systematic dewetting experiments on isotactic poly(para-methylstyrene) (iPpMS) films to explore the temperature dependence of the viscoelastic behavior of these films. We quantified the amount of residual stresses σres induced through film preparation by spin-coating. As anticipated, σres was found to be independent of the temperature Tdew at which dewetting was done. A particular focus was on the temperature dependence of the relaxation time τ of σres, which was measured with the help of three independent dewetting parameters. Within error, all three values of τ were identical and followed an Arrhenius behavior yielding an activation energy of 60 ± 10 kJ/mol. The initial dewetting velocity, being proportional to the ratio of surface tension of iPpMS and the corresponding viscosity, increased significantly with Tdew. Assuming a linear stress–strain response, we deduced that the elastic deformation responsible for the maximum height of the dewetting rim increased with temperature, although σres did not vary with temperature. Correspondingly, the shear modulus of iPpMS films was found to decrease monotonically with increasing temperature. Using a Maxwell-type model, the corresponding viscosity of the film showed the expected decrease with increasing temperature. Our experiments suggest that preparation-induced residual stresses affect material properties such as elastic modulus or viscosity of iPpMS as a function of temperature.

Replicating the complexity of natural surfaces: Technique validation and applications for biomimetics, ecology and evolution

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences, 377:2138, DOI:
Charchit Kumar , Alejandro Palacios , Venkata A. Surapaneni, Georg Bold, Marc Thielen, Erik Licht, Timothy E. Higham, Thomas Speck and Vincent Le Houérou

The surfaces of animals, plants and abiotic structures are not only important for organismal survival, but they have also inspired countless biomimetic and industrial applications. Additionally, the surfaces of animals and plants exhibit an unprecedented level of diversity, and animals often move on the surface of plants. Replicating these surfaces offers a number of advantages, such as preserving a surface that is likely to degrade over time, controlling for non-structural aspects of surfaces, such as compliance and chemistry, and being able to produce large areas of a small surface. In this paper, we compare three replication techniques among a number of species of plants, a technical surface and a rock. We then use two model parameters (cross-covariance function ratio and relative topography difference) to develop a unique method for quantitatively evaluating the quality of the replication. Finally, we outline future directions that can employ highly accurate surface replications, including ecological and evolutionary studies, biomechanical experiments, industrial applications and improving haptic properties of bioinspired surfaces. The recent advances associated with surface replication and imaging technology have formed a foundation on which to incorporate surface information into biological sciences and to improve industrial and biomimetic applications. This article is part of the theme issue ‘Bioinspired materials and surfaces for green science and technology’.

The cellular and genetic basis of structural colour in plants

Current Opinion in Plant Biology, 47, DOI:
Chiara A. Airoldi, Jordan Ferria and Beverley J. Glover

While the pathways that produce plant pigments have been well studied for decades, the use by plants of nanoscale structures to produce colour effects has only recently begun to be studied. A variety of plants from across the plant kingdom have been shown to use different mechanism to generate structural colours in tissues as diverse as leaves, flowers and fruits. In this review we explore the cellular mechanisms by which these nanoscale structures are built and discuss the first insights that have been published into the genetic pathways underpinning these traits.

Effects of Silk Degumming Process on Physicochemical, Tensile, and Optical Properties of Regenerated Silk Fibroin

Macromolecular Materials and Engineering, 303:12, DOI:
Kira Nultsch, Livia K. Bast, Muriel Näf, Salima El Yakhlifi, Nico Bruns and Oliver Germershaus

Sericin removal from silk (degumming) affects material characteristics of silk fibroin (SF). Sodium carbonate is most commonly used for degumming, but numerous alternative methods are available. Herein, a systematic comparison of degumming methods is provided. Sodium carbonate, sodium oleate, trypsin, and ionic liquid are used, and materials are characterized regarding mass loss, SF content, molecular integrity of SF, refractive index, and tensile properties. Complete degumming is achieved within 30 min of using sodium carbonate, but results in significant reduction of molecular weight, shift toward less acidic charge variants, and reduction of yield‐ and rupture force. Sodium oleate and trypsin are inefficient and negatively affect tensile properties, while ionic liquid shows good efficiency and marginal degradation of SF but also reduced yield‐ and rupture force. Refractive index is not affected by degumming. These results allow rational selection of the degumming method and tuning of SF properties for biomedical applications.

Ultrastructure and optics of the prism-like petal epidermal cells of Eschscholzia californica (California poppy)

New Phytologist, 219:3, DOI:
Bodo D. Wilts, Paula J. Rudall, Edwige Moyroud, Tom Gregory, Yu Ogawa, Silvia Vignolini, Ullrich Steiner and Beverley J. Glover

The petals of Eschscholzia californica (California poppy) are robust, pliable and typically coloured intensely orange or yellow owing to the presence of carotenoid pigments; they are also highly reflective at certain angles, producing a silky effect. To understand the mechanisms behind colour enhancement and reflectivity in California poppy, which represents a model species among early‐divergent eudicots, we explored the development, ultrastructure, pigment composition and optical properties of the petals using light microscopy and electron microscopy combined with both spectrophotometry and goniometry. The elongated petal epidermal cells each possess a densely thickened prism‐like ridge that is composed primarily of cell wall. The surface ridges strongly focus incident light onto the pigments, which are located in plastids at the cell base. Our results indicate that this highly unusual, deeply ridged surface structure not only enhances the deep colour response in this desert species, but also results in strongly angle‐dependent ‘silky’ reflectivity that is anisotropic and mostly directional.

advanced materials special issue

Special Issue: Bio-Inspired Materials

Bioinspiration is a powerful concept to develop novel functional materials. This Special Issue, edited by Silvia Vignolini and Nico Bruns, presents bioinspired materials across all length scales, such as wood‐based water‐purification systems, mussel‐protein‐inspired glues, diatoms that encapsulate drugs, fiber‐reinforced composites that indicate damage by bleeding, helical cellulose nanocrystals for optical applications, pigments and coatings that mimic the structural color of insects and plants, architecture inspired by the moving parts of plants, and many more.

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

Advanced Materials, 30:19, DOI:
Omar Rifaie‐Graham, Edward A. Apebende, Livia K. Bast and Nico Bruns

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.

Disorder in convergent floral nanostructures enhances signalling to bees

Nature, 550, DOI:
Edwige Moyroud, Tobias Wenzel, Rox Middleton, Paula J. Rudall, Hannah Banks, Alison Reed, Greg Mellers, Patrick Killoran, M. Murphy Westwood, Ullrich Steiner, Silvia Vignolini and Beverley J. Glover

Diverse forms of nanoscale architecture generate structural colour and perform signalling functions within and between species. Structural colour is the result of the interference of light from approximately regular periodic structures; some structural disorder is, however, inevitable in biological organisms. Is this disorder functional and subject to evolutionary selection, or is it simply an unavoidable outcome of biological developmental processes? Here we show that disordered nanostructures enable flowers to produce visual signals that are salient to bees. These disordered nanostructures (identified in most major lineages of angiosperms) have distinct anatomies but convergent optical properties; they all produce angle-dependent scattered light, predominantly at short wavelengths (ultraviolet and blue). We manufactured artificial flowers with nanoscale structures that possessed tailored levels of disorder in order to investigate how foraging bumblebees respond to this optical effect. We conclude that floral nanostructures have evolved, on multiple independent occasions, an effective degree of relative spatial disorder that generates a photonic signature that is highly salient to insect pollinators.