Printed Structurally Colored Cellulose Sensors and Displays

Abstract

of these processes include response to changes in the environment for concealment, communication, and temperature management. A well-known example is the ability of chameleons to change the color of their skin forming a protective tint to ward off predators and protect themselves. [1] To achieve this function, chameleons can tighten or relax their skin to change the crystalline structure under the skin's surface, causing a wavelength shift of reflected light and ultimately a color change. [2][3][4] This structural color is a typical occurrence in nature and a significant determinant of life activities. Physically, structural color is a phenomenon caused by Bragg reflection of light in materials that are organized in a periodic pattern whose repeating distance times the refractive index falls within the wavelength of the visible spectrum. [5] Liquid crystals are one class of materials capable of presenting structural colors. Colloids or mesogens arrange in a layered nematic or a twisted so-called chiral nematic or cholesteric arrangement with a helicoidal twist. [5] In the latter, each layer of the nematic phase is slightly twisted with respect to the following layer, while the distance between each layer defines the wavelength of the reflected light. Artificial structural color films have been successfully manufactured utilizing self-assembly and applied at a large scale using roll-to-roll or layer-by-layer methods. [6][7][8] However, the color portrayed in these films is generally fixed due to the stable and rigid structure that they form. [9][10][11] Materials that dynamically change color in response to an external stimulus, such as electrochromic, photochromic, and thermochromic materials, have also emerged. [12][13][14] Although they provide easily tunable color, these materials typically rely on complex and expensive chemical synthesis and are hazardous or non-environmentally friendly. Recently, scientists have developed flexible materials that can be stretched to change color with mechanical stimuli. Even though they provide easily tunable color, these materials are still constrained by the difficulties of their preparation procedures and are not ideal for mass manufacturing. [15][16][17] In search of materials that are non-toxic, biodegradable, and cost-effective, hydroxypropyl cellulose (HPC) is a cellulose derivative emerging as an attractive candidate for stimuli-responsive colored materials. [18] HPC spontaneously forms cholesteric liquid crystal in an aqueous solution and allows Bragg reflection of circularly polarized light in the visible range. [19][20][21] Furthermore, HPC concentration, temperature and additives can affect the strucural color, opening up for the design of multifunctional materials responding to more than one stimuli. [22][23][24][25]

Unraveling the Governing Mechanisms Behind the Chiral Nematic Self-Assembly of Cellulose-Based Polymers

“…Once a cholesteric liquid crystalline phase has developed, the spacing between each plane of the helicoidal arrangement is dependent on the fraction of polymer in the mesophase. Generally, a higher concentration leads to a smaller pitch and a blue-shift of the reflection spectrum of the chains as more closely packed. ,,,,,− ,,,,,,,, Specifically, a power law dependence between the pitch p (units of μm) and concentration ϕ p (units of wt %) is repeatedly reported like in eq . ,, …”

“…The inclusion of mesophase additives may also affect the pitch and reflection wavelength. In some cases, the incorporation of these species monotonically decreases the pitch, attributed to a reduction in the free volume, or monotonically increase the pitch, ascribed to their ability to expand the space between cholesteric layers. , Additives may disturb the intermolecular interactions of the polymer–solvent system, which endow more complex changes to the helical structure. For example, the addition of LiSCN salt to aqueous HPC mesophases increases the cholesteric pitch, while the opposite effect occurs with more concentrated LiCl suspensions .…”

“…Generally, a higher concentration leads to a smaller pitch and a blue-shift of the reflection spectrum of the chains as more closely packed. 9,16,17,21,25,[28][29][30]32,33,37,42,43,45,55,69 Specifically, a power law dependence between the pitch p (units of μm) and concentration ϕ p (units of wt %) is repeatedly reported like in eq 7. 39,43,44 = p p p m 0 (7) For polypeptide liquid crystals, in which this relationship was first observed, the exponent controlling this relationship is between −1 and −2, while a value near −3 is more commonly observed for cholesteric mesophases of HPC.…”

Exploiting the Thermotropic Behavior of Hydroxypropyl Cellulose to Produce Edible Photonic Pigments

“…[15] As such, the photonic applications of HPC have typically focused upon sealing an aqueous mesophase within nonpermeable media, [16][17][18] for use as, e.g., a mechanochromic sensor. [3,17,19,20] However, to enable its use as a colorant it is preferable to employ HPC in the solid state, where the pitch is fixed to reflect a specific color. To date, several methods…”

Mechanochromic and Conductive Chiral Nematic Nanostructured Film for Bioinspired Ionic Skins

“…Recently, researchers have attempted to endow cellulosic photonic materials with electrical conductivity to fabricate optical/electrical dual-signal smart materials. − For instance, raw HPC was directly mixed with carbon nanotubes in water and then self-assembled to fabricate electronic conductive photonic films. The resulting films can be used as optical/electrical dual-signal electronic skins to quantitatively sense the external stimuli through electrical resistance and visually map the stimulating sites by color variation. − However, the method of directly mixing with conductive agents before self-assembly is not suitable for the preparation of conductive CNC-based photonic materials. The self-assembly process of CNCs can easily be disturbed by charged conductive agents because their self-assembly is usually governed by the balance between the orientation-dependent attractive interactions of CNCs and repulsive interactions of negatively charged sulfate ester groups on CNC surfaces .…”