Bioinspired and biobased 4D-printing for adaptive building facades

As buildings account for a significant portion of global carbon emissions due to the energy consumed for maintaining occupant comfort, reducing the energy required for active heating, cooling, and ventilation is of crucial importance. In our paper, we show how biobased cellulosic materials and bioinspired 4D-printed bilayers can enable weather-responsive shading. Building upon almost a decade of foundational research on self-shaping mechanisms inspired by pine cone scales, which open and close without consuming any metabolic energy, we address new challenges in addressing real weather conditions and architectural integration. We study the motion response of hygromorphic shading elements and demonstrate, in a real building, an adaptive facade system that autonomously provides shading without relying on electrical operating energy.
Biobased hygromorphic materials and bioinspired 4D-printed structures
Cellulose is a natural, abundant, and renewable material that swells and shrinks with variations in humidity. This property, known as hygromorphism, is frequently observed in nature, such as in the opening and closing of pine cone scales. We leveraged this hygromorphic property by custom-engineering biobased cellulosic filaments (Figure 1) and 4D-printing them in a bilayered structure inspired by the scales of the pine cone.

The so-called “4D-printing” additive manufacturing technique enables material systems to autonomously change shape in response to external stimuli after printing. We developed a computational fabrication method to control the extrusion of cellulosic materials using a standard 3D-printer (Figure 2), making it possible to harness the self-shaping and reversible behavior of the 4D-printed material system. In high humidity, the cellulosic materials absorb moisture and expand, causing the 4D-printed mechanisms to curl. Conversely, in low humidity, the cellulosic materials release moisture and contract, causing the 4D-printed mechanisms to return flat.

Designing hygromorphic material systems for solar shading
We used the livMatS Biomimetic Shell at the University of Freiburg as a case study building for our weather-responsive shading system. Its location in Freiburg, which experiences a temperate climate characterized by hot, dry summers and cold, humid winters, makes heat management a relevant challenge (Figure 3).
We designed our shading system to assist in the climate regulation of the building’s interior by opening in high humidity and low temperatures to allow sunlight in for natural heating, while closing in low humidity and high temperatures to block direct sunlight and minimize excessive radiation. With the aid of environmental analysis integrated into our computational design workflow, we developed a modular design based on a pair of self-shaping bilayer flaps that adjust their curvature to the sun’s movement throughout the day and across seasons.

Evaluation under lab-controlled and real weather conditions
We validated the 4D-printed self-shaping mechanisms by examining their motion response under controlled laboratory conditions, revealing their curvature under varying combinations of temperature and humidity. Further testing also proved that the 4D-printed mechanisms maintain their reversible and repeatable motion response under many cycles of actuation and prolonged UV exposure.
To validate the robustness of the self-shaping shading elements against other daily and seasonal weather effects, we constructed a facade mock-up of the weather-responsive shading system, which simulated the target building in Freiburg through its height and south-facing orientation. Over one year of monitoring the system under real weather conditions showed no noticeable reduction in actuation or mechanical damage from natural weathering, demonstrating the shading system’s responsiveness and reliable performance (Figure 4).
Figure 4: Daily oscillations in temperature and humidity result in the autonomous closure of the 4D-printed shading elements during hot and sunny summer days, shown here from May 31st - June 6th in 2022.
Architectural integration of self-shaping shading elements
With the assurance of our extensive testing, we rented a total of four fused filament fabrication 3D-printers for production. Over the course of 17 days, we fabricated all 424 bespoke self-shaping shading elements for eight geometrically unique windows, which covered an area of 9.37 m² while using only 5.5 kg of cellulosic filament. On average, each shading element took approximately 20 minutes to print, due to their lightweight and material-efficient mesostructure design, and we installed the 4D-printed shading system on the upper facade of the livMatS Biomimetic Shell at the University of Freiburg. The so-called “Solar Gate” autonomously adjusts from cool morning conditions to afternoon heat by changing its shape and shading configuration (Figure 5) – all without using any electrical operating energy.

Powered solely by changes in daily and seasonal weather cycles, the Solar Gate represents an energy-autonomous and resource-efficient alternative to conventional shading systems that rely on complex controls, electrical power, and maintenance – while highlighting the potential of accessible, low-cost technologies such as additive manufacturing and cellulose as an abundant, renewable material for creating sustainable architectural solutions. Through our interdisciplinary collaboration between biologists, material scientists, and architects, we aim to showcase how the hygroscopic properties of cellulose can be leveraged through bioinspired 4D-printing to develop sustainable and highly responsive architectural solutions.
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