Abstract
Transparent hydrogels are key materials for many applications, such as contact lens, imperceptible soft robotics and invisible wearable devices. Introducing large and engineerable optical anisotropy offers great prospect for endowing them with extra birefringence-based functions and exploiting their applications in see-through flexible polarization optics. However, existing transparent hydrogels suffer from limitation of low and/or non-fine engineerable birefringence. Here, we invent a transparent magneto-birefringence hydrogel with large and finely engineerable optical anisotropy. The large optical anisotropy factor of the embedded magnetic two-dimensional material gives rise to the large magneto-birefringence of the hydrogel in the transparent condition of ultra-low concentration, which is several orders of magnitude larger than usual transparent magnetic hydrogels. High transparency, large and tunable optical anisotropy cooperatively permit the magnetic patterning of interference colours in the hydrogel. The hydrogel also shows mechanochromic and thermochromic property. Our finding provides an entry point for applying hydrogel in optical anisotropy and colour centred fields, with several proof-of-concept applications been demonstrated.
Highlights
Transparent hydrogels are key materials for many applications, such as contact lens, imperceptible soft robotics and invisible wearable devices
The processes include the preparation of 2D cobalt-doped titanium oxide (CTO) aqueous suspension, where 2D CTO materials were exfoliated by using a four-stage method from a layered lepidocrocite-type structure bulk[18] (Methods, Supplementary Fig. 1)
Shape anisotropy of CTO is described by its aspect ratio of l/t, where l and t are average lateral size and thickness of the flake (Fig. 1a), having average values of 1.8 μm and ~1.3 nm, respectively (Supplementary Fig. 2)
Summary
Transparent hydrogels are key materials for many applications, such as contact lens, imperceptible soft robotics and invisible wearable devices. The conventional approaches to fabricating anisotropic hydrogel include preparation of aligned functional units by means of self-assembly, exertion of compressing, stretching, or shearing forces, electric fields, and 3D printing[21–24] These methods usually suffer shortcomings of uneven distribution of force and electric field which make the uniformity and precise tunability of structural anisotropy difficult, and limit programmable control of spatially resolved optical anisotropy[24,25]. The embedded CTO has a large optical anisotropy factor of 2.85 × 10−11 C2 J−1 m−1, which is at least one order of magnitude larger than the highest value in other nanomaterials Such hydrogel demonstrates sensitive magnetic response, large and uniform optical anisotropy as well as the resultant multiple transmitted interference colours. Based on the obtained optical properties, the room-temperature solutionprocessable MB-hydrogel shows great potential for personalized optical applications, such as the optical phase retarder, gradient optical attenuator, magnetic see-through colour imager, and mechano-chromic indicator
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