The model for the movable components of buildings are cones of coniferous wood, which open (right) or close in dryness due to the different swelling behaviour of their fabric. Image: C Zollfrank/TUM

Smart building skins that can respond to environmental changes without electrical input could be the future of buildings, and German researchers have turned to pine cones to inspire next-generation materials.

Conifer pine cones are known for closing when their tissue becomes wet in order to protect seeds, and opening when dry. Researchers from three German universities – the Technical University of Munich, University of Freiburg and University of Stuttgart – are taking inspiration from such natural phenomena to investigate building facade systems that can respond to climate conditions without external motors or energy sources. 

“Sustainable architecture urgently requires new materials if it is to live up to the high energy efficiency and climate protection requirements,” TUM chemist, forest scientist and materials researcher Professor Cordt Zollfrank said.

Together with architects, civil engineers and botanists, Professor Zollfrank is investigating bio-inspired mechanisms that can improve energy efficiency in buildings. The goal is to develop elements for building facade systems that are able to convert signals into mechanical movements without the use of external energy.

An update on the research was recently published in journal Advanced Materials called Toward a New Generation of Smart Biomimetic Actuators for Architecture.

The researchers said bio-inspired kinetic structures for architecture were still “very scarce”, however there was an “urgent need” for them as the sector worked towards decarbonisation.

“For example, it was calculated that automatic sun shading elements on façades could reduce building heat input by 90 per cent, resulting in 80 Mt of possible CO2 savings per annum.”

Examples of outcomes include the design of smart building components that autonomously and precisely track the sun, or those that avoid it.

“Considering for example biomimetic shading tiles, which move as reactions to solar irradiation, the adjustment of tile orientation in respect to sunlight angle of incidence could be much better fine-tuned and lead to an improved building climate.”

Pine cones provide a good starting point for thinking about new generation materials, with the composition of the cones’ cell walls playing a crucial role in opening and closing. The cones are composed of lignin, which does not swell a lot, and cellulose, which has good swelling properties. Because of the orientation of cellulose fibrils in the scale tissue, they bend inwards and close when humidity is high, and move outwards and open when it is dry.

“The exciting thing about this is that the energy for these movements does not come from metabolic processes, but solely from physical mechanisms and material properties,” Professor Zollfrank said.

Using this as a base, he has developed materials with varying swelling propensities to create biomimetic drive elements called “actuators”.

While this has been demonstrated on a small scale, the major barrier is scalability. The larger the cell or tissue, the longer it would take for water to penetrate. So while a pine cone can respond to changes in an hour or so, a building using the same process would take years to accomplish.

Professor Zollfrank has proposed a “restructuring process” at the material level.

“We decouple the tissue size and take the whole thing to the magnitude of an individual cell,” he said. “Via smart cross-links, a loose cell complex is created whose individual components nevertheless still act like individual cells and absorb water extremely rapidly.

“The question now is how such cross-links can be designed as efficiently as possible and how to create them in any size.”

The researchers suggested that stadiums could be one area where the technology could be viable, with convertible roof technology currently extraordinarily complex and expensive.

“Such an architectural system would neither depend on mechanically movable parts, mechatronic actuation, electronic control, nor the need for operational energy. Therefore, it would be functionally more robust, as well as cost and energy efficient.”