Dynamic Aquatic Camouflage System Mimicking Reef Squid Transparency

Animals in nature often employ camouflage for survival. One notable example is disruptive coloration, which breaks up outlines and obscures internal features, primarily utilized by terrestrial species to blend with their surroundings. In contrast, aquatic species often prefer optical transparency for camouflage, which regulates visible light transmission and suppresses light scattering. For instance, the reef squid Sepioteuthis lessoniana can achieve optical camouflage by adjusting the size of its chromatophores to control body translucency, thereby mitigating reflections from light sources such as bioluminescent predators or divers' flashlights. Such aquatic organisms also possess pigments that protect against UV radiation, favoring optical camouflage due to the open ocean environment.<br/>Recent studies on active camouflage systems have focused on materials engineering that adjusts transparency and/or color in response to electrical, chemical, thermal, and humidity triggers. However, many of these camouflage materials are rigid, have fixed control circuits, limited color change capabilities, or are unsuitable for aquatic environments due to issues with heat and humidity. Therefore, this study developed a system that integrates a wireless control system (WCS) and an electrochromic display (ECD) to emulate the dynamic transparency modulation of reef squid, providing effective camouflage suitable for various aquatic conditions.<br/>The ECD utilizes an electrochromic layer of tungsten trioxide (WO3) and a counter electrode of nickel oxide (NiO) filled with a lithium (Li)-based polymeric electrolyte, allowing transparency modulation from clear to dark blue (peak wavelength 438 nm) through electrically induced redox reactions. The response times for bleaching and coloration between 2.0V and -2.0V are 14.2 seconds and 35.2 seconds, respectively, ensuring rapid camouflage under aquatic conditions. The structure, with WO3 and NiO, is sputtered between indium-tin-oxide (ITO) films and filled with Li-based polymeric electrolytes, ensuring the ECD's flexibility. Durability tests showed a stable 35% transmittance change over 100 cycles, and mechanical bending tests with a radius of 20.3 mm over 1000 cycles indicated some optical performance degradation due to electrolyte adhesion weakening. Additionally, we confirmed the system's stability by exposing it to a phosphate-buffered solution at 60°C for 200 hours.<br/>The WCS incorporates near-field communication (NFC) and Bluetooth, enabling remote control of the ECD's optical transparency through these wireless protocols. The wireless module functioned effectively in shallow water conditions (7 cm below the surface) with a high signal-to-noise ratio (126.98) and negligible latency (0.1 seconds) in a phosphate-buffered solution (pH 7.3, room temperature). Using polydimethylsiloxane (PDMS) as a waterproof substrate and encapsulation elastomer, the system maintained flexibility due to the low elastic modulus of the encapsulation elastomer. At the same time, its hydrophobic nature ensured stable aquatic operation. Finite-element analysis indicated that the maximum strain of the electrode at a 6 mm bending radius was 0.54%, well below the fracture limit of 1%, and the copper film and elastomer interface remained stable under a 14.6 mm bending condition.<br/>In summary, the integration of ECD and WCS developed a system capable of effective camouflage in various aquatic environments, mimicking reef squid's natural optical camouflage abilities. This system holds potential applications in aquatic exploration and biological research.