The Holten-Andersen Group’s approach to bio-inspired materials
MIT materials science lab tackles sustainability by drawing inspiration from nature
There is no question that nature is the best engineer. As hard as material scientists try, replicating nature’s intricate processes and networks is a holy grail that often seems nearly unattainable. Instead of attempting to copy nature, some scientists draw inspiration from nature’s mechanisms and apply them to the synthesis of goods for human use. The field of producing materials using design principles from nature is known as bio-inspired material research.
This is what inspires Dr. Niels Holten-Andersen. A biologist by training, he sees the ways in which materials synthesis can benefit from the methods of nature. As a PhD student at the University of California, Santa Barbara, he studied how mussels anchor to rocks using non-living fibers that are difficult to break and exhibit self-healing properties. After completing his PhD, he took his interests and formed the Holten-Andersen Group at MIT. His group focuses on applying the mechanisms of the self-healing process in mussel fibers to materials such as hydrogels, with the goal of developing a pliant but self-healing material to improve wound healing in patients.
The mussel has an organ that can sense a surface and secrete fibers one by one onto the surface, anchoring the mussel to the rock. These fibers are made mostly of proteins held together with metal coordination bonds, similar to hemoglobin or snail slime. If the fibers behaved like a rubber band, a mussel would snap back to its surface and potentially suffer damage if pulled from a rock and released by a predator. Instead, the fibers dissipate energy when they are being stretched, so the fibers stretch and relax slowly, and the mussel is able to safely return to its place if disturbed by a predator. Under a microscope, it would seem that the fibers allow this to happen by breaking, but no sustained damage is seen, so the fibers must heal themselves after being stretched. This is surprising, as the fibers themselves have no living cells.
In addition to studying the applications of the self-healing properties of these fibers, Holten-Andersen’s group also studies the use of these metal coordination bonds in self-reporting materials, which contain compounds that automatically report a response to certain stimuli. In these coordination bonds, nature often uses transition metals like iron, but the group studies the effect of using lanthanides instead. These metals fluoresce, so it is possible to make hydrogels that are both self-healing and also emit light. These hydrogels can change color as a function of how hard they are being pulled. Alternatively, a material with these metals can change color as it breaks as a warning that the material needs to be replaced.
Another application of this research draws more inspiration from biology. Mussels create these fibers underwater, so Holten-Andersen’s group is attempting to make hydrogels underwater, combining metal ions with metal binding polymers. The ions and polymers bind quickly underwater, resulting in strong bonds within the hydrogel. The resultant material could have applications in medicine and other fields.
But why emulate the design of nature at all? Holten-Andersen believes that both approaches are important, but a strong argument for the bio-inspired method is that this process is far more sustainable. Biological materials are biodegradable, which makes their impact on the environment much less harmful than many synthetic materials. For example, microbes can break down bio-derived or biodegradable materials but they cannot break down plastics or many other synthetic goods, so a bio-inspired approach favors sustainability and reduces environmental impact. Plastics could be replaced by components of wood, and other tough materials could be replaced by chitin, the material that makes up the shells of insects and fish scales.
Holten-Andersen stressed that biological materials are not perfect, but that they have been optimized over millions of years of evolution to increase functionality over time. A potential downside of a completely bio-inspired synthesis is that the material is inherently biodegradable, which means that the material may be less stable or long-lasting. This would require consumers to find a balance between sustainability and stability in their products. A potential solution to this issue is to create materials that are stable in their functional form but can be triggered to become biodegradable once their efficacy has worn off.
Much of the Holten-Andersen Group’s past work has been focused on soft materials, but Holten-Andersen is extending his group’s research to biomineralization, the process by which nature creates hard, inorganic structures. By placing metal ions into a material (like hydrogel) and inducing growth of metal oxide particles, one can make a material stiffer to the point that the material is almost entirely inorganic. A small amount of ions in a hydrogel material can drastically change the mechanical properties, and depending on the ion, these materials can even be magnetic, which could have applications for robotics.
Despite Holten-Andersen’s years of research and teaching, he believes that what makes MIT is not the research or the innovation or the professors, but rather the students who provide the energy and stamina of the institution. He finds it humbling to spend time with and teach students.