What if the materials we wore and built with could shape-shift? Responsive materials are already here thanks to new research and they could soon be hitting the market.
Shape-shifting constructions
One of the amazing things about plants is their ability to change shape in response to environmental stimuli such as light and moisture, even without a brain or central nervous system.
Now, a new generation of biomaterials is similarly able to respond to environmental cues. Importantly, these materials are sensitive to changes happening around them because of their molecular structure alone, not aided by an external power source or a digitally programmed set of instructions.
Construction is a promising application for this type of biomaterial. German researchers published a paper last year on weather-responsive building shades that could keep the sun out or the heat in through subtle material shifts.
The research, which resulted from collaboration between the University of Freiburg and the University of Stuttgart, started off with cellulose – a material found in plants that changes size in line with humidity levels around it. They used a 3D printer to shape cellulose fibres until they closely resembled the structure of cellulose within living plants.
The result was an engineered biomaterial that unfurls under high humidity, creating an umbrella-like shading, then contracts in low humidity to let in heat and light. Some refer to this as ‘passively responsive bioclimatic architecture’: an architecture that responds to environmental cues of its own accord. The team has demonstrated the technology by installing it at a research building at the University of Freiburg.
In terms of borrowing from nature, this adaptive material ticks all the boxes. First, it is made from plant raw materials. Cellulose is an abundant natural material found in the cell walls of plants, fruits, leaves, and vegetables. Second, the cellulose is processed in ways that are inspired by the makeup of biological organisms. In short, it is both a biomaterial (made from biological stuff) and a bio-inspired material (designed based on biological mechanisms).
Building with weather-responsive biomaterials could be useful for adapting our built environment to the extreme impacts of climate change. However, materials like this can also help limit global warming if more widely adopted by the building industry.
Right now, major construction materials such as concrete or steel are huge contributors to global carbon emissions. Using responsive biomaterials in building work alongside carbon-intensive mineral-based materials would cut the impacts on the building industry, particularly as these materials can switch between multiple functions without needing any additional energy.
Sustainable simplicity
In general, the simplicity of biomaterials gives it an edge when it comes to building climate-responsive structures.
Conventional ‘smart buildings’ rely on complex networks of sensors and computer systems that allow them to become sensitive to the environment. These technologically intensive projects present a number of practical issues since motor-controlled mechanical parts and intricate moving mechanisms can make for frequent technical failures and high maintenance costs.
Relying on the natural responsiveness of modified biomaterials is a potentially much lower-cost way to make buildings more climate-resilient, cutting down the number of moving parts involved. As Professor Achim Menges from the University of Stuttgart described it, “The biomaterial structure itself is the machine.”
Guided curvature
Other researchers are leaning into low-tech routes for creating weather-responsive architectural biomaterials. University of Wellington researchers have developed a way to capitalise on the bending properties of commercially available plywood.
Unlike the Stuttgart team, this team did not make a responsive biomaterial by altering microscopic structures. They chose to work instead on the visible scale, cutting and shaping the wood in ways that would influence their movement and behaviour in an overall architectural structure.
The researchers iteratively developed plywood tiles, producing various shapes and sizes. These were in effect puzzle pieces that would eventually slot together to form a curved architectural structure.
Each wooden tile is laser-cut with very deliberate grooves. The combination of tile shape and tile groove patterns, the researchers surmised, would determine how the overall structure would be able to expand and contract in response to dryness or humidity.
They demonstrated their material on the beaches of Rio de Janeiro, using it to build an amphitheatre that drastically changed shape in response to temperature throughout the day. In the moisture, cooler mornings, the amphitheatre roof was tightly curled in, forming a closed canopy. Then, its tip would slowly stretch outwards to form an open shade as the beach became drier during the day.
These contracting and expanding movements were guided by the specific ways that the shaped wooden tiles sat together as well as by the grooved shapes bored into their surfaces.
The advantage of this approach is its simplicity: using readily available tools and commercially abundant plywood, the researchers were able to mock up a structure that responded to environmental conditions in controlled and reversible ways.
Liquid crystal adaptations
Textiles are another application for these responsive materials. MIT scientists have developed FibeRobo, a fibre that contracts when hot and opens up when it’s cold.
This type of material takes a cue from natural mechanisms. Scientists call this type of response in biology ‘actuation’ – an example would be the way a pine tree cone responds to surrounding humidity by opening its flaps to release seeds. The air humidity is swelling up layers within the cone.
Just as in architectural applications, the big plus of this responsive material is that there is no need for complicated sensors or hardware embedded into the material: in FiebRobo as in the pinecone, all the mechanisms are there within the fibres. This means that any textile manufacturer can work with a material, which is compatible with existing machinery and manufacturing processes. It is also a more appealing consumer package, eliminating the need for bulky gadgets woven into their clothing.
The fibre used here is an example of a liquid crystal elastomer whose properties change under different conditions. The LCE developed by the MIT team have molecules that become dislodged from their usually orderly crystal arrangements under higher temperatures, making the material contract to keep in heat.
The exact temperatures at which the material changes can be adjusted depending on how the LCE is synthesised. This property was crucial in allowing for its use in textile applications since some LCEs are only sensitive to extreme temperatures.
Biobased LCEs
Right now, the chemicals used to synthesise the LCE textile from MIT are not biobased, although these researchers eventually want to develop recyclable or biodegradable versions.
In Europe, however, a research programme called ALCEMIST will be dedicated to developing biobased liquid crystal elastomers from polysaccharides. Starting in 205, the European Research Council-funded project also prioritised industrial scalability. One of the applications they are exploring is autonomously adjusting ‘kinetic buildings’ like the ones envisaged by the Stuttgart researchers with their responsive building facades.
Another less obvious area of application for biobased LCEs is a reversible glue that is strong to hold while products are used but can easily be ‘switched off’ once it comes to end-of-life disassembly and recycling.
Enhancing wood
Wood is one of the most traditional biomaterials but today, companies are looking to bring it into new applications, including as a substitute for silicate glass. What’s more, certain kinds of engineered wood can take on new properties that are not found in nature, including responsiveness to heat and light.
The obstacle in the way of using wood for construction is its natural tendency to swell or degrade – also known as its hygroscopicity. To overcome these barriers, researchers have gone down a number of routes, including modifying wood surface behaviour using a polymer called PNIPAM (poly(N-isopropylacrylamide)).
When treated with PNIPAM, materials become able to reversibly change from hydrophilic to hydrophobic depending on the surrounding temperatures. This is important in preventing wood swelling, which happens when wood cells absorb moisture from the surrounding environment.
Researchers are also working on creating a low-cost, light-responsive and transparent wood that could replace silicate glass as window material. This is another climate-ready application of responsive biomaterials in building design: wood is more insulating than glass and so could reduce energy consumption in buildings.
In 2022, Chinese scientists published a way to modify wood with a substance called AG-80/DDM epoxy resin. When the wood is dipped in the substance, it begins to change colour in response to light. The resin imparts other properties to the wood too including pH-responsiveness, fluorescence, and weather resistance.
Responsive biomaterials can take on multiple functions and properties depending on different environmental conditions. Their ability to shift into new forms and back again can support sustainable design by cutting the need for carbon-intensive resources. Responsive architecture becomes possible without the use of metal, mineral, and energy-intensive computational hardware.
