Photographs, Metals, and Gels

The word "photograph" is a combination of "photo" (light) and "graph" (drawing).

By the time many of today's university students were born, digital cameras were already commercially available. While photographs taken with digital cameras and smartphones are the norm today, there was an era of film photography. Film photography requires not only image projection techniques, such as the camera obscura that Johannes Vermeer may have used, but also technology using photosensitive agents to fix the projected image onto film. In the 18th century, Johann Heinrich Schulze discovered that a slurry of silver nitrate and chalk turned black when exposed to light. This was followed by the introduction of the daguerreotype and the calotype, and later the development of the gelatin dry plate by Richard Leach Maddox, which used a mixture of silver bromide and gelatin.

Regarding this reaction where silver turns black when exposed to light, some of you may have conducted a silver mirror test in high school. Since silver has a low ionization tendency, it readily precipitates as a metal. Silver ions are reduced by light to become silver, which appears black. In film photography, this corresponds to the creation of a negative through exposure. In essence, a substance that darkens when exposed to light is coated onto the film, and a light image is projected onto it, causing a site-selective reduction reaction that precipitates the metal.

This reduction reaction can be induced not on the two-dimensional surface of a film, but three-dimensionally inside an object. In our laboratory, we are conducting research on creating metal microstructures inside hydrogels using ultrashort pulse lasers. A hydrogel is something that contains water internally, like jelly, soft contact lenses, konjac, and pudding. Maddox used it in a dry state, but gelatin is also a hydrogel. When femtosecond laser pulses are focused inside a hydrogel, nonlinear interactions cause metal ions to be reduced and the metal to precipitate only at the focal point where the photon density is high. The metal structures have various optical and electrical properties that change in response to the deformation, volume expansion, or contraction of the hydrogel. By developing this technology, it is possible, for example, to combine two types of metals to control the bending direction of a hydrogel, like a soft robot, using light.

With the interaction of light and matter as the core of our research, we use lasers to draw metal structures that serve optical and electrical roles inside soft materials, similar to the human body. Can you imagine the human-like devices and robots that may be created in the future?