A 2D material-based transparent hydrogel with engineerable interference colours

Hydrogels are water-based, crosslinked polymer networks with tissue-like properties (elasticity, wettability, biocompatibility) enabling broad applications from biomedicine to smart sensing. Recent efforts target transparent hydrogels for see-through electronics, soft robotics, and optical components. Introducing optical anisotropy could add birefringence-based functionalities (e.g., digital coding, sensing, polarization navigation, diagnosis). Notably, sufficiently large birefringence can produce transmitted interference colours with vivid, non-photobleaching characteristics, offering advantages over pigment-based coloration. However, current transparent hydrogels typically enable only black-to-white switching due to insufficient and poorly tunable birefringence. Achieving large, uniform, and controllable optical anisotropy is thus essential to expand hydrogel applications in polarization optics and colour engineering. Conventional routes to anisotropy (mechanical deformation, electric fields, self-assembly, 3D printing) often yield non-uniform fields/forces and limited spatial programmability. Magnetic fields, in contrast, are uniform over large areas, precisely tunable, and contact-free, making them attractive for programmable anisotropy. Yet common magnetic inclusions (e.g., iron oxide nanoparticles) introduce strong optical absorption and low optical anisotropy, limiting both optical path length L and birefringence Δn, preventing the ΔnL > 400 nm condition (Michel-Lévy chart) needed for interference colours. Considering shape, optical, and magnetic anisotropies, wide-bandgap magnetic 2D materials with large aspect ratios offer a promising route. This work reports a magneto-birefringent transparent hydrogel (MB-hydrogel) using paramagnetic cobalt-doped TiO2 (CTO) flakes to achieve large, engineerable optical anisotropy and interference colours under low magnetic fields, enabling see-through polarization optics and colour-centric applications.

Prior approaches to anisotropic hydrogels include shear/strain alignment, electric-field alignment, self-assembly, and 3D printing, but suffer from non-uniform stimuli and limited spatial control. Magnetic alignment offers uniform, precisely tunable, and contactless control over sizable areas. However, typical magnetic additives (e.g., iron oxide) exhibit strong optical absorption and low optical anisotropy, restricting Δn and the allowable optical path length, thus failing to meet ΔnL > 400 nm for transmitted interference colours. Comparisons with prior magnetic hydrogels using 1D rods (fibrinogen, Pb-doped silica, PHBV, agarose, DPPC) and 2D discs (nontronite, polymersomes, rare-earth-chelating bicelles) show markedly smaller birefringence and/or require very high magnetic fields (∼10 T). The authors leverage earlier insights that shape anisotropy in 2D materials strongly couples to optical/magnetic anisotropy, and that wide-bandgap 2D oxides can provide high transparency with large anisotropic refractive indices. This motivates exploring magnetic 2D CTO to overcome absorption and anisotropy limitations of prior systems and to enable interference colour formation at sub-tesla fields.

Synthesis: 2D cobalt-doped TiO2 (CTO) flakes were exfoliated from layered lepidocrocite-type bulk via a four-stage method (per prior work; see Methods, Supplementary Fig. 1). Average lateral size l ≈ 1.8 μm and thickness t ≈ 1.3 nm (Supplementary Fig. 2) yield a large aspect ratio (l/t), exceeding those of typical rod/plate inclusions. Resin preparation: A UV-curable MB-resin was formulated by dispersing 2D CTO in water with poly(ethylene glycol) diacrylate (PEGDA, MW ≈ 700 g/mol) monomer and photoinitiator Irgacure 2959 (MW ≈ 224.25 g/mol). Hydrogelation and magnetic alignment: The MB-resin was subjected to an external magnetic field (H) to align suspended 2D CTO flakes and simultaneously UV-illuminated to cure, thereby freezing the aligned state within the hydrogel. Post-curing, the magnetic-field direction defines the H-axis in the hydrogel. Interface characterization and mechanics: XPS analysis showed a Ti 2p3/2 peak shift from 458.47 eV (pure CTO) to 457.44 eV (PEGDA-modified CTO), indicating hydrogen bonding at the CTO–polymer interface and associated screening effects (reduced binding energy for Ti–O–H vs Ti–O). Mechanical testing revealed enhanced tensile/compressive performance and durability with CTO: at 20% tensile strain, stress increased from 10 kPa (no CTO) to 15 kPa (with CTO); durability retention improved from ∼60% after 14 cycles (no CTO, fractured) to >90% after 50 cycles (with CTO). Alignment assessment: Angle-resolved light scattering with emergent light L verified alignment: scattering intensity increased with angle between L and H-axis, peaking at 90°, consistent with the largest scattering cross-section of 2D CTO when L ⟂ H. At higher CTO concentration (0.2 vol%), macroscopic domains formed and aligned with µ0H = 0.6 T. SEM on freeze-dried samples showed random CTO orientation without field vs alignment along the field at 1 T. Optical anisotropy measurements: A polarizer–sample–crossed analyzer setup with a 450 nm laser measured transmitted intensity vs azimuth θ (angle between input polarization and H-axis) and polarization state (via polarimetry). Intensity followed I = I0 sin^2(2θ) sin^2(δ/2), with δ = 2πΔnL/λ, confirming birefringence; polarization evolved from linear to near-circular and back with θ, with H-axis as slow axis. Magneto-birefringence characterization: A series of hydrogels (No. 4#–12#; L = 2 mm; C = 0.02 vol%) were cured under µ0H = 0–800 mT (100 mT steps). With fixed L and λ, intensity and polarization extrema vs H indicated Δn(H) rising from 0 to ≈2.0 × 10^-4, with saturation behavior at higher fields. Fitting to a standard electro-/magneto-birefringence saturation form yielded a saturation birefringence Δn_s ≈ 2.4 × 10^-4. Using this, the intrinsic optical anisotropy factor of CTO flakes was estimated as Ag ≈ 2.85 × 10^-11 C^2 J^-1 m^-1, independent of concentration. Interference colour generation and tuning: For colour, thicker samples (L = 6 mm; C = 0.02 vol%; No. 13#–24#) were cured under µ0H = 160–1040 mT (80 mT steps). Under white backlighting, each hydrogel exhibited a distinct transmitted interference colour. CIE-1931 colour coordinates and transmission spectra were recorded; the main transmission peak redshifted from ∼450 nm to ∼650 nm as H increased, consistent with λ = 2Δn(H)L. Colour order and gamut were tuned by CTO concentration (0.02–0.1 vol%) and thickness (3–9 mm), accessing first-, second-, and third-order Michel-Lévy regions. Device demonstrations: True zero-order waveplates were fabricated within the first-order regime: quarter-wave at 450 nm (No. 25#) and half-wave at 450 nm (No. 26#) and 650 nm (No. 27#). A gradient optical attenuator (No. 28#) was made by curing in a spatially varying field (0–400 mT), producing position-dependent transmittance (e.g., at 450 nm from 42.3% to 0% along the gradient). Patternable colour imaging was achieved by selective UV exposure under controlled H to lithograph coloured features; multicolour patterns (letters, fish-like patterns) were formed by sequential exposures with varying fields. Mechanochromic and thermochromic responses were demonstrated; pressing altered flake alignment relative to viewing direction, shifting colours consistent with changes in ΔnL (supported by Michel-Lévy chart).

• A magneto-birefringent transparent hydrogel (MB-hydrogel) using 2D cobalt-doped TiO2 (CTO) flakes exhibits large, finely tunable optical anisotropy and transmitted interference colours under sub-tesla magnetic fields.
• High transparency: transmittance >90% across the visible for MB-hydrogel-1; uniform anisotropy with <1% variation over a 6 mm × 6 mm area.
• Mechanical reinforcement via CTO–polymer interfacial hydrogen bonding: tensile stress at 20% strain increased from 10 kPa (no CTO) to 15 kPa (with CTO); durability improved to >90% retention after 50 cycles vs ∼60% after 14 cycles without CTO.
• Large magneto-birefringence: Δn(H) up to ∼2.0 × 10^-4 (for L = 2 mm, C = 0.02 vol%, µ0H ≤ 800 mT); saturation birefringence Δn_s ≈ 2.4 × 10^-4.
• Intrinsic optical anisotropy factor of CTO: Ag ≈ 2.85 × 10^-11 C^2 J^-1 m^-1, exceeding prior nanomaterial values by ≥ an order of magnitude and surpassing high-birefringence liquid crystals.
• Required magnetic field reduced by an order of magnitude compared to prior magnetic hydrogels (from ∼10 T to ≤0.8–1 T), compatible with permanent magnets; unlike Fe-oxide systems, CTO maintains transparency.
• Interference colours enabled by achieving Δn(H)L > 400 nm; main transmission peak redshifted from ~450 nm to ~650 nm as H increased (L = 6 mm, C = 0.02 vol%).
• Colour order/gamut tunable by CTO concentration (0.02–0.1 vol%) and thickness (3–9 mm), accessing first to third Michel-Lévy orders.
• Functional devices demonstrated: true zero-order quarter/half-waveplates (450 nm, 650 nm); gradient optical attenuator with position-dependent transmittance (e.g., at 450 nm from 42.3% to 0%); magnetic see-through colour imaging with lithographically patterned colours; mechanochromic and thermochromic indicators.

The study addresses the central challenge of imparting large, programmable birefringence to transparent hydrogels—previously limited by low optical anisotropy and absorption of magnetic inclusions—by leveraging 2D CTO flakes with exceptionally high shape, optical, and magnetic anisotropy. Magnetic-field alignment of CTO within a UV-cured hydrogel creates a uniform slow axis and large Δn under low fields, preserving optical transparency. This combination overcomes the ΔnL threshold for interference, enabling stable, tunable transmitted colours and polarization control in a soft, transparent medium. The high optical anisotropy factor (Ag) and reduced field requirements broaden the feasibility of practical devices using permanent magnets. Demonstrations of waveplates, gradient attenuators, and magnetically patterned colour images illustrate the hydrogel’s utility in see-through polarization optics and colour-centric applications. The mechanochromic and thermochromic responses further highlight multifunctionality. Overall, the findings substantiate that 2D magnetic oxides can transform hydrogel optics by delivering engineerable anisotropy without sacrificing transparency, opening pathways for wearable/implantable photonics, soft robotics displays, and programmable optical components.

This work introduces a transparent magneto-birefringent hydrogel embedding aligned 2D cobalt-doped TiO2 flakes, achieving record-high magneto-birefringence among transparent magnetic hydrogels under sub-tesla fields. The system maintains high optical clarity, uniform anisotropy, and mechanical robustness. A large intrinsic optical anisotropy factor and tunable Δn enable transmitted interference colours and polarization control, validated through functional prototypes (waveplates, gradient attenuators, magnetically patterned colour images, mechano-/thermochromic indicators). These results provide an entry point for applying hydrogels in optical anisotropy and colour-centered technologies, particularly see-through flexible polarization optics and personalized photonic devices. Future developments could expand patterning resolution, integration with wearable platforms, and exploration of other magnetic 2D materials to further tailor optical and magnetic responses.

The paper does not explicitly detail limitations. Practical considerations may include dependence on magnetic field strength and uniformity during curing, optimization of concentration/thickness for desired colour orders, and potential scalability and long-term environmental stability, but these are not extensively discussed.