White Without Pigment: Kyoto Researchers Borrow Nature’s Design

White is usually sold as something to add: a pigment in a coating, an ingredient in ink, a layer that hides what lies beneath. Researchers in Kyoto are exploring another route. They make a polymer look white by changing the structure inside it, then use related changes at its surface to control how it meets water.

The technology, deep-foam photolithography, or DFP, was announced by Kyoto University’s Institute for Integrated Cell-Material Sciences, iCeMS, on September 10. The work involves Easan Sivaniah’s group and collaborators including Tokyo Metropolitan University and Donghua University. The universities identify September 10, Japan time, as the publication date of the Nature study.^[1]

Its promise is a way to create white patterns and water-repellent surfaces without relying on titanium-dioxide pigment or a PFAS water-repellent coating. Its significance becomes clearer when the optical achievement is separated from the questions still facing a finished material: what it contains, how it is manufactured and how its performance survives use.

The difference between a white substance and a white structure

A transparent material can become white when it contains many interfaces that scatter visible light. Foam offers an everyday example. Repeated scattering redistributes light rather than selectively absorbing the wavelengths that would give a strong hue. Kyoto’s announcement places the new work in that family of natural structural whites.^[1]

This does not mean conventional white pigments work without structure. Their particles and the surrounding medium also determine how effectively light scatters. The change is in where the useful interfaces are created: within a porous polymer instead of through the addition of a separate whitening pigment.

Thickness matters. A sample may appear pale against a light background but fail to hide a dark one. Reflectance, transmission and opacity answer related but different questions. A credible replacement for a commercial coating must be compared at relevant thicknesses and under comparable conditions; a bright photograph alone cannot establish its performance.

Light prepares the polymer; solvent develops the foam

DFP exposes a photosensitized polymer to ultraviolet light, producing both broken chains and a crosslinked network. Subsequent solvent treatment drives swelling and pore formation. The detailed Japanese release describes local phase separation as part of the proposed mechanism. Adjusting exposure and development controls the resulting structure.^[2]

The balance is the interesting part. Crosslinking connects chains; scission cuts them. Their coexistence helps determine what the material does when the solvent enters. This is controlled processing, rather than simply making a plastic cloudy through uncontrolled damage.

The published abstract describes spatial control of foam expansion and collapse, with differences in optical properties and wettability. It also reports applicability to films and fibres. The approach therefore concerns a processing platform, rather than a single new resin with a single possible use.^[3]

Calling the result pigment-free should not be confused with calling it chemistry-free. Light-sensitive ingredients, polymer chemistry and solvent interactions remain central. Physical structure delivers the visible function, but chemical processing makes that structure possible.

What 20,000 dpi tells us—and what it does not

The researchers report approximately 20,000 dots per inch. As a unit conversion, one inch divided by 20,000 is about 1.27 micrometres. That is an illustrative spacing corresponding to the stated resolution, not a claim that every pore has that diameter.^[3]

Patterning adds a different capability from making an entire sheet white. It allows a manufacturer to choose where the optical change occurs. The same general idea of spatial control can also create neighbouring regions that interact differently with liquid.

Resolution, however, is not production speed. A process capable of fine features may still face questions about exposure time, usable area and consistency from batch to batch. Commercial evaluation would need all of those measures, alongside equipment and solvent-handling costs. The dpi figure establishes a scale of patterning, not the price of printing a square metre.

A scientific history written on leaves

Biomimetic water repellency has a substantial experimental history. In a 1997 Planta paper, Wilhelm Barthlott and Christoph Neinhuis investigated how plant-surface roughness, particle adhesion and water repellency interact. Contamination experiments showed rolling droplets removing particles from strongly water-repellent leaves, including lotus leaves.^[9]

The lesson is more subtle than “rough surfaces stay clean.” The authors described both surface microstructure and the hydrophobic character associated with plant-surface chemistry. Geometry and composition work together. Copying the outline of a leaf is not sufficient to reproduce its behaviour.

In the Kyoto process, controlled evolution and collapse of the foam can create finely textured surfaces. The joint release describes flower-like structures with superhydrophobic behaviour. The interior that scatters light and the surface that contacts a droplet are connected parts of the material, but they need different performance tests.^[2]

A beaded droplet, a droplet that rolls away and a fabric that prevents water penetrating under pressure are not interchangeable demonstrations. Oil repellency is another property again. That distinction is crucial when evaluating possible replacements for treatments used on outdoor clothing or packaging.

The long search for useful white

The history of white pigments is also a history of changing industrial requirements. The Museum of Fine Arts, Boston’s CAMEO database describes lead white as a principal pigment from antiquity into the early twentieth century, when zinc and titanium whites increasingly replaced it.^[7]

Titanium-white production developed in the 1910s. Its strong hiding power and the demand for alternatives to toxic lead white helped its adoption. It became useful across many applications because it solved practical problems, not merely because it was available.^[8]

That history argues for careful comparison rather than a simple story of bad old materials and good new ones. EFSA’s 2021 assessment concluded that titanium dioxide could no longer be considered safe as a food additive because genotoxicity concerns could not be excluded. It was an assessment of ingestion as a food additive, not a single verdict on every industrial use.^[10]

A polymer alternative likewise needs assessment for its intended setting. An artistic panel, a garment and a food-contact package have different requirements. Optical performance can justify investigating a substitution; it cannot, by itself, establish suitability for every application.

Reading the PFAS-free claim precisely

Japan’s Environment Ministry defines PFAS as the family of per- and polyfluoroalkyl substances. Its overview highlights the persistence, bioaccumulation and long-range transport associated with PFOS and PFOA. The term covers a broad group, making it important to identify the particular substances and functions being replaced.^[6]

The universities present DFP as avoiding a PFAS water-repellent coating. But the Nature paper’s public Extended Data lists trifluoroacetic acid, a fluorinated solvent, in preparation of one PET example. A route to water repellency without a PFAS coating is therefore not evidence that every experimental manufacturing route avoids fluorinated chemicals.^[1][3]

That observation does not establish residual solvent in the finished sample, nor does it mean all the demonstrated polymers use the same preparation. It establishes a boundary around the claim. Manufacturing choices, solvent recovery and residual analysis belong in an eventual environmental assessment alongside the absence of an added repellent coating.

Japan.co.jp’s analysis is that this boundary makes the research easier to evaluate. Reducing dependence on one class of surface treatment can be worthwhile without proving that an entire product has the lowest environmental footprint. Those propositions require different evidence.

A pattern that survives bending can still contain cracks

The supplementary study provides an initial durability test using a polystyrene-based printed film. After 1,200 bending cycles, the patterned foam remained visible, but microscopy found cracks in both foamed and unfoamed regions. The authors characterize the mechanical work as preliminary.^[4]

That is more informative than either declaring the material durable or assuming it is fragile. Retaining a visible pattern is a useful result. It is not the same as retaining water repellency through washing, abrasion or prolonged weather exposure. The latter questions matter if the eventual application is clothing rather than a protected display.

Different applications would also expose different failure modes. Folding concentrates stress in packaging; rubbing acts directly on a textile’s surface; cleaning introduces chemicals as well as motion. A useful development programme would test the function the customer needs after the stresses the product actually encounters.

Nor does avoiding mineral pigment make a polymer biodegradable. Feedstock, additives, processing, useful lifetime and disposal all contribute to an environmental comparison. A lighter material that must be replaced more often could produce a different result from one that lasts longer. These are evaluation questions, not demonstrated disadvantages of DFP.

From Kyoto’s laboratory to a usable object

The joint release outlines possible collaborations with Kyoto craftspeople and applications including screens, lighting and parasols. These are development ambitions, not announcements that a mass-produced product is already available.^[2]

Tokyo Metropolitan University identifies co-author Taiki Yanagishima as an associate professor working in soft matter and colloid physics. That disciplinary connection is apt: the study concerns how relatively soft materials organize internally, and how those arrangements create behaviour visible at human scale.^[5]

An initial application need not replace the entire white-pigment or water-repellent market. It needs to make a particular combination of appearance, pattern, handling and service life useful enough to justify its production. A limited application can teach researchers what a laboratory image cannot: how the material performs when someone uses it repeatedly.

The strongest idea in this work is the ability to design function into selected regions of a familiar polymer. White becomes a property to engineer through space; wetting becomes another variable in that design. Nature supplies the physical inspiration. Turning it into a reliable product requires the equally demanding work of manufacturing and testing.