Squid reveal the secret to a “perfect” lens

Original paper: Eye patches: Protein assembly of index-gradient squid lenses

Evolution usually solves challenges differently than human engineers—something easy for biology is often difficult for us, and vice versa. Learning from biology can help us solve difficult challenges more easily. One example of this is making complex optical lenses.

When light enters a new material it refracts, changing velocity and direction. A lens uses geometry and material properties to direct light on a specific path. You can see in Figure 1 how the shape and refractive index of a biconvex lens combine to direct light at a single spot. In biology, complex eyes like those found in most vertebrates and in squid have a lens that directs light onto the retina at the back of the eye, forming an image to be processed by the brain. Squid use spherical lenses to do this, but spherical lenses have a problem. As you can see in Figure 2, if you make a spherical lens out of one material (like glass), the light rays overlap after exiting the lens and the resulting image is blurry. This is called “spherical aberration.” Human engineers use spherical lenses a lot, and we correct for spherical aberration by combining multiple lenses. Squid, on the other hand, have evolved a lens that self-corrects for this distortion.

We know, in theory, how a squid might do this. In 1854, the famous physicist James Clerk Maxwell mathematically designed a spherical lens with “perfect” focus. He showed that if the density of the lens changes along the radius, forming a density gradient that he called a “perfect medium,” then the lens will produce a clear image. Today engineers can make gradient index lenses like this, but the process is difficult and energy intensive. Squid evolved to grow them easily. Could understanding how squid make these lenses help human engineers learn to do the same thing? This question inspired Dr. Jing Cai and Prof. Alison Sweeney to study the structure of the squid lens.

You can see in Figure 3 that when you crack open the lens in a squid eye you find rings like the inside of a tree trunk. If each of these rings has a slightly different density, they could combine to create a perfect medium. The lens of an eye is made out of proteins called crystallins, which fold into individual particles before linking together into a single material. Cai and her collaborators discovered that the lenses of the Longfin inshore squid (Doryteuthis pealeii) use 53 different crystallin proteins of different sizes. They also found that the different proteins are used in different parts of the lens, and each layer of the lens has a slightly different structure. As you can see in Figure 4, small proteins at the center of the lens are densely packed together so that each protein is connected to six other proteins. However, the larger proteins at the edge of the lens have more space between them, and each protein only touches two others.

This makes sense when you think about the cells that make these proteins. Cells rely on diffusion to bring building blocks to the right place for protein assembly and to send each assembled protein out to where it’s needed. When finished proteins link together to grow the lens, they disrupt this diffusion and stop protein production. By growing from dense to less dense and using so many different proteins (53 in the Longfin inshore squid), the cells are able to start and stop the growth of different layers while maintaining a single particle network. No part of the lens separates out or turns opaque, but there are still large enough regions with different densities to diffract light into alignment.

Cai and her collaborators showed that squid lenses definitely use a density gradient similar to Maxwell’s perfect medium to correct for spherical aberration. It’s likely that this density gradient not only creates a perfect medium, but also helps control lens assembly. Now that we know how squid build a perfect spherical lens, it is easier to envision how human engineers could grow our own complex optical materials.