Unclonable human-invisible machine vision markers leveraging the omnidirectional chiral Bragg diffraction of cholesteric spherical reflectors.

The seemingly simple step of molding a cholesteric liquid crystal into spherical shape, yielding a Cholesteric Spherical Reflector (CSR), has profound optical consequences that open a range of opportunities for potentially transformative technologies. The chiral Bragg diffraction resulting from the helical self-assembly of cholesterics becomes omnidirectional in CSRs. This turns them into selective retroreflectors that are exceptionally easy to distinguish-regardless of background-by simple and low-cost machine vision, while at the same time they can be made largely imperceptible to human vision. This allows them to be distributed in human-populated environments, laid out in the form of QR-code-like markers that help robots and Augmented Reality (AR) devices to operate reliably, and to identify items in their surroundings. At the scale of individual CSRs, unpredictable features within each marker turn them into Physical Unclonable Functions (PUFs), of great value for secure authentication. Via the machines reading them, CSR markers can thus act as trustworthy yet unobtrusive links between the physical world (buildings, vehicles, packaging,…) and its digital twin computer representation. This opens opportunities to address pressing challenges in logistics and supply chain management, recycling and the circular economy, sustainable construction of the built environment, and many other fields of individual, societal and commercial importance.

Chameleon skin-inspired polymeric particles for the detection of toluene vapor.

Inspired by the structural coloring in nature, especially the crystalline ordering and responsive characteristics of those found in chameleon skins, artificial photonic materials for sensor applications were fabricated. Cholesteric liquid crystals (CLCs) were employed in the templated synthesis of polymeric particles with periodic structures that allow visible light to undergo Bragg reflection and their response was tested against volatile organic compounds (VOCs). We demonstrate that the particles were responsive against toluene with detection limits on the order of 100 ppm. Such sensitivity was shown to be achieved due to the critical steps followed during the CLC-templated synthesis of particles that resulted in the storage of elastic energy in the anisotropic glassy polymer network. In addition, the design of particle-assisted sensor chips that allow easy integration into wearable optical devices for reliable, continuous and online tracking of VOC concentrations is presented. These results proved that sensors developed from the CLC-templated particles can be used multiple times without a significant loss of sensitivity and offered rapid, sensitive and battery-free detection.