Towards Mechanochromic Devices using Biocompatible Hydroxypropyl Cellulose
Hydroxypropyl cellulose (HPC) is a widely utilised, low environmental impact material. When dissolved at high concentrations it forms structural colours via the formation of a cholesteric, liquid crystalline mesophase, termed photonic HPC. Intrinsically biodegradable, edible and mechanochromic, photonic HPC can be manufactured at scale and low-cost in a simple formulation. A high reflectivity also allows for easy integration with common camera technologies. A rich seam of exploration therefore exists for the photonic application that has already begun with the development of various mechanochromic HPC devices. Currently however, its desirable mechanochromic property has only been reported in the liquid state, and despite a good theoretical understanding, an applied characterisation of aqueous HPC’s mechanochromic colour-pressure response is lacking in the literature. In this thesis, a dynamic, structurally coloured HPC-gelatin hydrogel is manufactured using only cost-effective, biocompatible, and widely available raw materials. Combined in a readily scalable formulation process (planetary centrifugal mixing), the mechanochromism of HPC is reported in the viscoelastic solid-like (gel) state for the first time. Mouldable as a continuous unsupported solid, while retaining the shear-thinning non-Newtonian response of HPC, the mechanochromic relaxation time is enhanced over the equivalent HPC-water mesophase via an intrinsic elasticity provided through the addition of gelatin. Then, using soft lithographic techniques, a mechanochromic HPC device is developed to investigate aqueous HPC’s mechanochromic sensitivity, response times (rise time, latency, mechanochromic relaxation time constant) and cycling performance as a function of its thickness. A standardised microactuating pixel of fixed size and shape is tested, before various pixel sizes, shapes, spacings and array configurations are explored. Our HPC-gelatin hydrogel is then included. We show that HPC mechanochromism is dependent on the applied strain, with a strain threshold between different compression regimes that dictates the mechanochromic outcome. We conclude that with application of a variety of factors, this outcome can be controlled to provide specific effects, details or colour. With our work, a novel insight into HPC mechanochromism is provided to support a movement towards mechanochromic displays using biocompatible hydroxypropyl cellulose.
biocompatible, cellulose, device, display, gel, gelatin, HPC, hydroxypropyl cellulose, mechanochromic, mechanochromism, PDMS, photonic, Polydimethylsiloxane, sensor
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