Biocompatible and Sustainable Optical Strain Sensors for Large-Area Applications

By a simple two-step procedure, large photonic strain sensors using a biocompatible cellulose derivative are fabricated. Transient color shifts of the sensors are explained by a theoretical model that consideres the deformation of cholesteric domains, which is in agreement with the experimental results. The extremely simple fabrication method is suitable for both miniaturization and large-sale manufacture, taking advantage of inexpensive and sustainable materials.Biotechnology and Biological Sciences Research Council (David Phillips fellowship (Grant ID: BB/K014617/1)), The Isaac Newton Trust Cambridge (Grant ID: 76933), European Research Council (Grant ID: ERC-2014-STG H2020 639088

Invited Article: Chiral optics of helicoidal cellulose nanocrystal films

Cellulose nanocrystals (CNCs) in water suspensions behave as lyotropic liquid crystals forming a chiral nematic phase above a critical concentration. Such organization can be retained in solid films and give rise to an intense colored appearance. Here, we fully characterize their optical response by applying optical and scanning electron microscopy, imaging scatterometry and angle-resolved reflectance measurements. We show that the experimental results are well explained by computational modeling using the finite-difference time-domain method, but slightly less well by Berremann’s analytical model.This work was supported by the EPSRC Cambridge NanoDTC [No. EP/G037221/1 to R.M.], the National Centre of Competence in Research “Bio-InspiredMaterials”, the Ambizione program of the Swiss National Science Foundation [No. 168223 to B.D.W.], the European Research Council [No. ERC-2014-STG H2020 639088 to S.V. and A.G.D.], and the BBSRC David Phillips fellowship [No. BB/K014617/1 to S.V.]

Shape Memory Cellulose-Based Photonic Reflectors.

Biopolymer-based composites enable to combine different functionalities using renewable materials and cost-effective routes. Here we fabricate novel thermoresponsive photonic films combining cellulose nanocrystals (CNCs) with a polydiolcitrate elastomer exhibiting shape memory properties, known as hydroxyl-dominant poly(dodecanediol-co-citrate) (PDDC-HD). Iridescent films of CNCs are first made by evaporation-induced self-assembly, then embedded in the PDDC-HD prepolymer, and finally cured to obtain a cross-linked composite with shape memory properties. The fabricated samples are characterized by polarized optical microscopy, scanning electron microscopy, and thermomechanical cycling. The obtained hybrid material combines both intense structural coloration and shape memory effect. The association of stiff cellulose nanocrystals and soft polydiolcitrate elastomer enhances the overall mechanical properties (increased modulus and reduced brittleness). This hybrid nanocomposite takes advantage of two promising materials and expands their possibilities to cover a wide range of potential applications as multiresponsive devices and sensors. As they perform from room to body temperatures, they could be also good candidates for biomedical applications.EU FP7 NoE Nanophotonics4Energy Grant No. 248855, the Spanish MINECO project MAT2015-68075 (SIFE), and Comunidad de Madrid S2013/MIT-2740 (PHAMA_2.0) program. All the authors acknowledge the Royal Society (2014/R2-IE140719). A.E. was supported by the FPI PhD program from the MICINN. S.V., B.F.P., and A.G.P. are funded by the BBSRC David Phillips fellowship [BB/K014617/1] and the ERC-2014-STG H2020 639088. G.G. acknowledges the EPSRC [1525292]. M.C.S. acknowledges the Instituto de Salud Carlos III of Spain for a Miguel Servet I contract (MS13/00060)

Fractionation of cellulose nanocrystals : enhancing liquid crystal ordering without promoting gelation

Colloids of electrically charged nanorods can spontaneously develop a fluid yet ordered liquid crystal phase, but this ordering competes with a tendency to form a gel of percolating rods. The threshold for ordering is reduced by increasing the rod aspect ratio, but the percolation threshold is also reduced with this change; hence, prediction of the outcome is nontrivial. Here, we show that by establishing the phase behavior of suspensions of cellulose nanocrystals (CNCs) fractionated according to length, an increased aspect ratio can strongly favor liquid crystallinity without necessarily influencing gelation. Gelation is instead triggered by increasing the counterion concentration until the CNCs lose colloidal stability, triggering linear aggregation, which promotes percolation regardless of the original rod aspect ratio. Our results shine new light on the competition between liquid crystal formation and gelation in nanoparticle suspensions and provide a path for enhanced control of CNC self-organization for applications in photonic crystal paper or advanced composites

Recent advances in the biomimicry of structural colours.

Nature has mastered the construction of nanostructures with well-defined macroscopic effects and purposes. Structural colouration is a visible consequence of the particular patterning of a reflecting surface with regular structures at submicron length scales. Structural colours usually appear bright, shiny, iridescent or with a metallic look, as a result of physical processes such as diffraction, interference, or scattering with a typically small dissipative loss. These features have recently attracted much research effort in materials science, chemistry, engineering and physics, in order to understand and produce structural colours. In these early stages of photonics, researchers facing an infinite array of possible colour-producing structures are heavily inspired by the elaborate architectures they find in nature. We review here the recent technological strategies employed to artificially mimic the structural colours found in nature, as well as some of their current and potential applications