Alison Sweeney (Seed)
Figure 1. Images formed by a single-index spherical lens (left) vs. a graded index lens (right).
Figure 2. SAXS-determined structure factors of the radial gradient of squid lens material and Chinese “century” egg. Panels 1-4: structure factor of intact squid lens using protein dimer atomic coordinates as form factor. Panel 5: structure factor of intact, preserved egg using raw egg protein as form factor.
To see underwater, where there is no air/water interface at the surface of the eye, all the optical power of the visual system must come from the lens. Therefore, to construct a visual system that is both sensitive and appropriately scaled, an aquatic lens must have very high optical power, which in turn necessitates a graded refractive index to avoid spherical aberration (figure 1). Squid lenses assemble from a mixture of closely evolutionarily related isoforms of one protein comprising the large majority of the mass of the lens. These lens proteins are called S-crystallins. Remarkably, squid lens proteins are capable of forming a thermodynamically stable, transparent material at all possible material densities: the refractive index at the center of the lens requires nearly dry protein, while the periphery of the lens has an index close to water. Our work has shown that proteins in the squid lens can self-assemble into networks of variable density that also encode the optical properties required for acute underwater vision. These new insights into squid lens assembly could provide a path forward for making new graded-index materials for inexpensive and sophisticated optics and photonics.