In last month’s edition of Holography News®, we explored how bio-inspired security threads, showcased at the Global Currency Forum in Oman, are translating the visual complexity of butterflies, chameleons, oceans, and meadows into security print features. From Louisenthal’s micro-mirror innovations to Kurz’s meadow-inspired threads, industry leaders demonstrated how themes from nature are creating new opportunities in optical security.
In this follow-on article, Francis Tuffy examines the technical foundations underlying these bio- inspired optical designs and how advances in nanotechnology are being employed to create cutting-edge anti- counterfeiting devices.
The foundation of bio-inspired optical security often lies in structural colour. Unlike pigments, which produce colour through chemical absorption and reflection, structural colour is created by micro- and nanoscale physical structures that interact with light to produce vibrant hues. This method of colour generation offers a unique advantage for security applications: the colours produced are angle-dependent and almost impossible to replicate without the same level of nanoscale precision, providing a visually dynamic and highly secure feature.
Pastel Fountain Fill (© Demax).
For instance, Demax’s e-LIDOGRAM® security feature uses engineered nanostructures that produce a ‘Pastel Fountain Fill’ effect, displaying a distinct colour switch when tilted. This effect is created through precision crafted nanostructures designed to reflect specific wavelengths of light, creating the characteristic shift in hue that conventional inks cannot achieve (see HN, July 2023).
Similarly, a team from MIT has developed an elastic material that changes colour when stretched. Here, nanostructures on the material alter the path of light as it reflects, enabling wavelength shifts that create an illusion of dynamic colour change (see HN, August 2023).
These examples illustrate how precise manipulation of nanoscale structures can achieve complex optical effects that add both aesthetic appeal and a layer of counterfeiting resistance to secure documents.
So where are these effortlessly creative optical engineers in nature? Here are a few examples:
Morpho butterflies: The iridescent blue of Morpho butterfly wings results from nanoscale structures that create multilayer interference. The layered, quasi-ordered scales reflect light, producing an intense blue that shifts in hue with changes in viewing angle.
Chameleon skin: Chameleons achieve colour change by manipulating nanocrystals in their skin. By expanding or contracting the distance between these nanocrystals, they control the wavelengths of light reflected, thus altering their skin colour dynamically. This adaptability has influenced materials designed for colour- shifting effects in response to stimuli like pressure or light changes, creating interactive security features that are both functional and visually engaging.
Pollia condensata fruit: Known for its vivid blue appearance, the Pollia fruit achieves structural colour through highly organized layers of cellulose that principally reflect blue light. Notably, this colour remains vibrant and stable over time, making the fruit visually striking, helping it attract animals and insects for seed dispersal despite having no nutritional value.
Photographs of Pollia condensata fruits (© PNAS).
Feathers and shells: Peacock feathers and certain beetle shells achieve iridescence through layered microstructures that cause light to reflect at different angles, producing colour variations based on viewing perspective. This phenomenon is replicated in security devices through multi-layered nanostructures that create angledependent colour shifts, adding a dynamic element to optical security features.
The 2023 Optical & Digital Document Security™ (ODDS) conference showcased industry applications of bio-inspired designs, demonstrating the rapid pace of innovation in this field. Eduard Miloglyadov from OVD Kinegram presented security features that produce structural colours through sub-wavelength cross gratings, introducing new dimensions of colour complexity. By carefully arranging the grating’s nanoscale features, the device achieves precise control over light reflection.
Similarly, Chromatir’s Caleb Meredith demonstrated optical variable devices (OVDs) using concave microstructures to generate cascading reflections, creating the type of dynamic, angle-dependent colour shifts inspired by the iridescence seen in certain beetles and fish scales (see HN, May 2023).
Whilst paying homage to the legacy of bio-inspiration garnered from materials in nature that are structurally engineered, there are other optical security features that take their inspiration not just from the behaviour of light as it interacts with a surface or through a volume but from a complete optical system.
Compound vision, found in insects such as flies, bees and dragonflies, is an evolutionary marvel. Unlike the single-lens structure in human eyes, compound eyes consist of numerous small visual units (ommatidia), each with its own lens and photoreceptor cells. These ommatidia form a multi-lensed eye structure that grants insects unique visual capabilities, such as an expansive field of view, awareness of motion, and sensitivity to various wavelengths.
In compound eyes, each ommatidium captures a small section of the insect’s view. The brain then assembles these multiple segments into a single, composite image. This segmented, mosaic-like approach offers a basis for creating intricate, micro-patterned optical security features that would be difficult to replicate. By applying similar segmentation principles, security devices can incorporate micro-optic elements that produce unique visual effects based on a user’s angle of view.
Such segmentation even enables variable visual information across different angles, adding complexity that poses a significant challenge to counterfeiters.
Whilst not being a part of a fully functioning visual processing system, here are some security features that utilise lens arrays to create unique visual effects:
KINEGRAM DYNAMIC®
(HN Dec 2021) – This technology from KURZ uses micro-lenses arranged in a free-form array, allowing each lens’s surface to be customised to the customer’s specific requirements. This level of customisation enables a wide range of visual effects, enhancing the security and aesthetic appeal of the feature.
Hyperfocal Lens Array (HL)
Developed by Wavefront Technology, the HL combines a 1D lens array with a patterned microstructure [HN Aug 2022]. This combination produces a high- contrast image with depth that can flip, shift, and animate, making it difficult to replicate and providing a strong visual deterrent to counterfeiters.
SUSI Optics®
Applied to the window of polymer banknotes, SUSI Optics is a micro-lens-based print/ foil combination feature [HN Jul 2022]. This feature offers a range of up to 20 different visual effects depending on the viewing angle and which side of the banknote is being viewed.
These examples of structured colour and lens arrays highlight the versatility and effectiveness of bio-inspired optical security features.
The ability to engineer the width, depth and spacing of lines and grooves along with control of the shape and arrangement of lenses offer a broad range of possibilities for enhancing document and product security.
