Conventional materials like metals or ceramics have fixed properties, like strength or conductivity, and so there is a limit of what is possible using single components. Materials scientists push this envelope by combining materials together to form composites, which often exhibit unique combinations of properties. For example, concrete is often reinforced with steel to improve its flexibility. Materials science has also seen rapid development of so-called “smart materials” in recent years, which change some property in response to an external signal, like temperature, electricity, or the presence of a chemical compound. Still, both conventional composites and smart materials have their limitations. Now, bioengineers at the University of Texas at Dallas have developed a new kind of smart composite which uses living organisms embedded inside a gel to respond to environmental cues. This new living system offers a robust means of providing a highly specific mechanical response to a variety of triggers that has not been demonstrated with other composites.
In addition to being essential for bakers and brewers, yeast are a commonly used organism throughout biology as they can survive in a wide range of conditions, multiply rapidly, and have well-understood genetics that can be modified. The scientists incorporated yeast into a gel and found that the gel volume expanded greatly (by 200%) over 48 hours when incubated with nutrients needed for yeast growth. The authors confirmed that this increase in volume was not due to the gel swelling but instead to multiplication of the yeast cells. Smart materials, however, are beneficial in that they respond to some specific signal, not grow constantly. The growth of the yeast could be controlled by the presence or absence of a single molecule called L-histidine in the surrounding solution. Even a closely related molecule was insufficient to allow for growth by itself. This sensitivity to individual molecules could be useful in sensing applications, in which the material will only respond if a specific molecule is present in the fluid.
An even more interesting application is the ability to form specific patterns of response with this living composite. By irradiating the yeast with ultraviolet light, their ability to multiply is destroyed, and so the shape of the resulting responsive material can be controlled by preventing cells in undesired regions from multiplying. The authors used such a method to promote the formation of helixes from the composite. Furthermore, yeast genetics can be modified to express a light-sensitive protein that allows for growth when stimulated with blue light. This mechanism of controlling organisms with light is called optogenetics and is commonly used to control the activity of brain cells in neuroscience. Thus, by using combinations of blue and UV light, it is possible to grow complex shapes out of the living gel.
The formation of a living composite represents a novel step forward at the intersection of bioengineering and materials science. New materials are constantly being developed for bioengineering applications, including implants, systems for drug delivery, or biosensor devices. It will be interesting to see what this new composite could offer to these fields. Changes in volume could be used to represent the detection of a specific marker, or release a drug when conditions for expansion are met. Still, incorporating a living organism into a material offers a new set of challenges – the material, including its living components, must be safe and stable over extended periods of time.
Managing Correspondent: Andrew T. Sullivan
Press Articles: “Shape-morphing living composites,” Phys.org
Original Journal Article: “Shape-morphing living composites,” Science Advances
Image Credit: Pixabay