Background
Birefringent materials, which generate interference colors by modulating the phase retardation of polarized light, have found widespread applications in information security, flexible optics, functional devices, and artistic design. Owing to its intrinsic birefringence and renewable nature, cellulose is considered an ideal candidate for such applications. To date, most studies have focused on cellulose nanocrystal (CNC) systems, where long-range order is achieved via methods such as uniaxial stretching and evaporation-induced self-assembly (EISA). In contrast, cellulose nanofibrils (CNFs), despite their superior mechanical properties and broad application potential (e.g., high-strength fibers, transparent haze films, and functional gels), contain disordered amorphous regions that weaken their birefringence. Consequently, achieving uniform and controllable interference colors in the nanofibril systems remains a marked challenge.

Research Progress
To address this limitation, research teams led by Prof. Kai Zhang (University of Göttingen) and Prof. Rongxin Su (Tianjin University) have developed an integrated salt-assisted assembly (iSAA) that couples ion crosslinking with organic–inorganic co-assembly to construct nanofibril–crystal (NF-C) composite films exhibiting vivid, tunable interference colors (Fig. 1). The work has been recently published in Research.

1. Broad Tunability of Interference Colors
Inspired by natural microstructures (e.g., marine zooplankton), salt crystals are incorporated into bio-based nanofibrillar matrices to enhance birefringence. The resulting self-organized NF–C composites enable modulation of optical phase retardation from approximately 400 nm to over 2,500 nm, producing vibrant interference colors that span all orders of the Michel–Lévy chart and demonstrate the broad color-tuning capability of the iSAA strategy.

2. Integrated Salt-Assisted Assembly Mechanism
The study proposes that, during the iSAA process, metal ions initially crosslink nanofibrils to form a network. As water evaporates, a moisture gradient drives capillary flow, promoting outward ion migration and accumulation at the edges, where crystallization occurs. The growing salt crystals exert expelling and shear forces on the nanofibrils, enabling stress transfer within the network and triggering the synergistic assembly of the NF–C composite. The resulting structure enhances macroscopic birefringence and introduces thickness variations, giving rise to gradient optical phase retardation and the corresponding colors.

Distinct from conventional stretching or field-induced alignment methods, this strategy enables programmable optical responses via organic–inorganic assembly without requiring complex equipment or substrate deformation. It also redefines salts as active drivers of crosslinking and assembly, rather than passive screening or templating, thereby improving structural order and optical anisotropy.

3. Generality and Spectra-Selective Polychromatic Lighting
The authors further demonstrate the universality of the iSAA strategy by fabricating films from different biopolymer fibrils (e.g., phosphorylated and carboxylated CNF as well as protein amyloid fibrils) on hydrophilic and hydrophobic substrates. Additionally, by coupling transmissive interference colors with forward scattering (optical haze), the birefringent system converts incident white light into uniform, soft multicolor illumination (e.g., green, magenta, and yellow), enabling spectrally selective polychromatic lighting.

Outlook
CNFs, characterized by renewability, biodegradability, and high performance, represent a key materials platform for the circular economy. In this work, the iSAA strategy based on CNFs enables cross-scale coupling between nanostructural control and macroscopic polarization optics, providing a general pathway for the design of sustainable birefringent materials.

From an application perspective, such a birefringent system can harness natural light to reduce the demand for artificial electric lighting (e.g., LEDs), offering a sustainable approach to colorful illumination that may further lower greenhouse gas emissions and decrease reliance on scarce or toxic materials. Potential applications include agriculture, where optimized spectral transmission could improve crop yields, and interior design, where tailored lighting can influence human emotions and cognition. Overall, this work establishes iSAA as a universal strategy for engineering nanofibril-based birefringent materials with programmable polarization optics, thereby enabling advanced optical and energy-efficient applications.

The complete study is accessible via DOI: 10.34133/research.1198