The European Space Agency (ESA) has been researching for years how structures in space can be lighter, more autonomous, and more resilient, but the latest results from the passive solar tracker project have gone a step further than initial expectations. Instead of classic mechanical systems with motors, gearboxes, and sensors, researchers have developed a new generation of composite metastructures that change shape themselves when environmental conditions change, relying on the principles of 4D printing and inspiration from biology.

This is an approach in which a construction is not viewed merely as a static object, but as a material system with "embedded" behavior. In this concept, the dimension of time – the fourth dimension relative to classic 3D printing – becomes as important as geometry. A structure that is completely flat in cold, dark conditions can, for example, gradually bend towards a source of heat and radiation as the temperature rises, and then return to its initial position when conditions change.

The research is led by a team from the Université de Bretagne Sud (UBS) and the Bionics Group research unit, in collaboration with ESA and industrial partners. Their work on bio-inspired 4D printed tubular and helicoidal metacomposites, published in the prestigious journal Advanced Materials Technologies, has shown that such structures can be precisely programmed to move without any additional energy source, solely due to changes in temperature and solar radiation.

From concept to patent: how the autonomous adaptive structure was created

Within ESA's initiative to discover and test radically new ideas, researchers first developed a theoretical model and numerical simulations of the behavior of composite tubes with a specially designed internal architecture. They analyzed how the direction, density, and arrangement of fibers in layers affect twisting, bending, or torsion when the material is heated. Such an approach made it possible to predict how much the construction would move at a given temperature change already in the design phase.

The results exceeded initial expectations: it turned out that the energy density of these metacomposites – when normalized to their stiffness – can be measured against the capabilities of classic shape-memory alloys, but with a simpler construction and without complex actuator systems. This combination of theoretical elaboration and practical experiments was convincing enough for ESA to protect the technology with a patent for autonomous tubular adaptive composite structures, primarily intended for space applications.

The activity leader at ESA, Ugo Lafont, emphasizes that the results obtained went beyond initial expectations and that precisely the combination of academic research and space application made the concept mature enough for patent protection.

The patent covers a new way of assembling 4D printed "building blocks" into modular assemblies that can be passively reconfigured. Each block is a tubular metastructure in itself, but only their spatial arrangement allows for sophisticated deformations of the entire assembly – for example, the joint rotation of the platform on which a solar panel is mounted, without a single motor or gear.

Such an approach changes the paradigm of classic engineering: instead of subsequently adding actuators, bearings, and shafts to the structure, the function is "inscribed" into the material itself. The construction thus becomes both load-bearing and active, which is especially important in space where every additional component is a launch load and a potential point of failure.

Rotoprinting: 4D printing that programs behavior within the architecture itself

The key technological breakthrough of the project is the development of a new technique of composite additive manufacturing called "rotoprinting." At first glance, the process resembles classic filament winding of composite tubes: continuous fibers and matrix are applied to a rotating body. However, unlike standard winding, rotoprinting allows for the layered deposition of fibers in complex helicoidal patterns, where the angle, sequence, and thickness of each layer are precisely controlled.

It is precisely through this "digital weaving" that the future behavior of the construction is defined. The ratio of fibers laid at small and large angles, the way the layers cross, and the transitions between zones of different fiber density determine whether the tube will primarily bend, twist, or combine both effects when heated. Thus, printing does not only create geometry, but a deformation plan is written into the material architecture.

Researchers have shown that by combining different tube sections – each with its own deformation "program" – a longer structure can be built whose behavior changes along its length. One segment can react strongly to small temperature differences, another activates only at higher temperatures, and a third remains almost neutral. In this way, one continuous composite element can contain multiple functional zones without mechanical joints.

Another advantage of this approach is the possibility of using continuous fibers and robotic arms to produce very long structures. While laboratory prototypes are currently on the order of a meter, the same principle is, with equipment scaling, applicable to structures several meters high or more – which opens the way for application in solar farms on planetary surfaces, as well as in space antennas or modules that need to be passively deployed after arriving in orbit.

Sunflower-inspired solar tracker: how the structure "follows" the Sun itself

The most attractive demonstrator of the developed technology is the passive solar tracker. Instead of a series of motors and control computers, this system consists of a platform on which a solar panel is mounted and a complex set of 4D printed tubular metastructures assembled beneath it. When the Sun heats one side of the construction, the internal architecture of the composite causes the platform to rotate towards the heat source, similar to the way a sunflower follows it during the day.

As the Sun moves across the sky, the temperature distribution in the structure changes, and with it, the position of the platform. When conditions change – for example, when the panel enters the shade or night falls – the composite gradually returns to a more neutral position. Everything happens continuously, without sudden movements or the noise produced by motors, and without a single watt of additional energy consumption.

In addition to energy savings, the passive approach also brings a significant simplification of the system. There are no cables, power supplies, gears, or bearings that need to be lubricated and serviced. This reduces mass, but also the risk of failure – an extremely important factor for equipment that must operate reliably in remote locations, whether in polar regions on Earth or at lunar or Martian bases where servicing is not an option.

Laboratory prototypes have shown that it is possible to achieve significant platform rotation in realistic temperature gradients, making the passive tracker competitive with classic solutions at least for certain scenarios. Although it is a demonstrator at an early level of technological development, the concept has clearly shown that "intelligent" behavior can be obtained through metastructure design, rather than through adding electronics.

Basalt fibers and lunar regolith: building from local resources

One of the most interesting aspects of the project is its connection to research on using local resources on the Moon. Basalt fibers, which form the backbone of many developed metacomposites, can potentially be obtained from lunar regolith, the fine-grained powdery rock mass that covers the Moon's surface. ESA and partner institutions have already conducted studies on the extraction and processing of such fibers for the construction of lunar habitats and infrastructure elements.

If basalt fibers can be produced on the Moon itself and then combined with suitable matrices in geopolymer or other composite materials, the possibility of completely local production of 4D printed structures opens up. Robotic arms could use lunar regolith as raw material, create tubular metastructures, and assemble them into load-bearing but also functionally active elements such as passive solar trackers or adaptive communication antenna mounts.

Such an approach fits perfectly with the principles of the circular economy in a space context: instead of bringing complex mechanical assemblies from Earth, local rock would be used on the Moon, and the resulting structures could be adapted or recycled several times during their lifespan. After completing their primary task, the same tubes and blocks could be incorporated into protective barriers, storage modules, or other components of a future lunar base.

In this process, 4D printing brings an additional advantage: the same material can perform different functions in different phases of the mission. Initially, metastructures would be optimized for deployment and orientation, and later their contribution to mechanical stiffness or radiation protection could be more valued. By changing environmental conditions, the dominant role played by the material's embedded behavior also changes.

Sustainability through simplification of space systems

At the center of the whole story is the idea that sustainability in space is often achieved through radical simplification. Every motor, sensor, or electronic assembly requires energy, redundancy, and additional mass, and every mechanical connection is a potential point of failure. If a function like orientation towards the Sun can be achieved by the structure itself taking it over, the space system becomes lighter, less complex, and more autonomous.

4D printed metastructures fit into this logic: once produced and installed, they need no additional inputs other than natural changes in the environment. In space, this means temperature cycles between the sunny and shaded sides of the orbit or planetary rotations, while on planetary surfaces, seasonal changes also come into play. The material behaves like a "passive computer" that, based on temperature changes, "calculates" how much and in which direction it needs to move.

Such an approach also aligns with ESA's broader strategic goal to increase the level of system autonomy in its future missions and reduce the need for constant intervention from Earth. In conditions of signal delay and limited data flow, any mechanism that "knows" on its own what to do, without active control, brings a direct operational advantage.

At the same time, simpler structures mean simpler maintenance. If damage occurs, replacing one metastructure module can be faster and cheaper than repairing an entire motorized mechanism. The modular approach, as envisioned by the patented concept, allows for gradual upgrading or adaptation of the system depending on mission needs.

From the Moon to Earth: applications in energy and construction

Although the first demonstrators were designed for space, researchers are simultaneously developing terrestrial versions of the technology. Instead of basalt fibers linked to lunar regolith, local natural fibers are used – such as flax, hemp, or other plant raw materials – combined with suitable matrices to create biocomposites with programmed behavior. In this way, the concept of "space" metastructures gains very concrete applications in the context of the green transition on Earth.

Passive solar trackers could be used in small and medium solar installations where the gain from tracking the Sun is significant, but complex mechanics are too expensive. Unlike massive industrial systems, simple 4D printed modules would allow basic tracking of the Sun's position without the need for constant maintenance, which is especially attractive for remote or hard-to-reach areas.

Another potential area of application is facade elements and shades that change shape depending on temperature or radiation intensity. Such structures could let in more solar heat in winter and "close" in summer to reduce overheating of the space, all without sensors and motors. 4D printed biocomposites thus become a tool for passive regulation of thermal flows in buildings and reduction of energy consumption for heating and cooling.

In infrastructure projects, adaptive constructions that dampen vibrations or adapt to loads are also possible, for example in bridges, towers, or equipment supports. Although such applications are still in the conceptual study phase, the common idea is that instead of complex active systems, we rely on "smartly" designed materials.

Industrial partners and a joint laboratory for scaling the technology

To move the technology from the laboratory bench toward industrial application, the Université de Bretagne Sud has established a joint laboratory with Coriolis Composites, one of the leading manufacturers of robotic cells and software for automated composite fiber placement. In this joint laboratory, named CompoMorph, research is being conducted on how to transfer the principles of rotoprinting and 4D printing to equipment and processes that already work in industrial plants today.

Robotic arms, which Coriolis Composites develops for classic composite structures in aviation, energy, and the automotive sector, are gaining a new role as platforms for producing metastructures with variable shapes. Instead of just laying layers to achieve the desired strength and stiffness, they must now precisely follow the "behavior path" defined by the researchers – the geography of the fibers that determines how the structure will change over time.

Collaboration with industry also includes defining future specifications for space missions. Companies from the space sector are interested in systems that could reduce the mass and complexity of space antennas, panels, or structures that are deployed after launch. Passive actuators based on 4D printed composites offer the possibility of transferring some of these functions from the domain of mechanics and electronics to the domain of advanced materials.

Such transfer of knowledge from research laboratories to the industrial context is also crucial for further funding of development. The passive solar tracker project has continued to live through new proposals to ESA's Discovery program, but also through national and European projects focused on 4D printing and sustainable composites.

New research and next steps in 4D composite development

Work on bio-inspired tubular and helicoidal metacomposites did not end with the publication of one scientific paper or the approval of a patent. On the contrary, these results have opened up new questions and prompted a series of further studies. One of them is aimed at the development of sustainable 4D printed biocomposites that combine natural fibers and bio-based matrices, with an emphasis on programmed shape change with a minimal carbon footprint.

A second research area concerns technology scaling and reliability in real conditions. It is necessary to examine in detail how long-term cyclic heating and cooling affect the properties of metastructures, how many times the deformation can be "activated" without noticeable material fatigue, and how local damage affects the global behavior of the construction. These questions are crucial before passive solar trackers or similar systems start to be designed for high-risk missions.

A third line of research deals with the integration of such structures into broader engineering systems. Even when we rely on passive shape change, it is necessary to understand how the 4D metastructure will behave in combination with classic supports, panels, containers, or instruments. Simulation models must cover both thermal and mechanical behavior, as well as interaction with the environment – from vacuum and radiation to the load field during launch.

And finally, research teams continue to work on expanding the design space: new fiber laying patterns, new material combinations, and new geometries of tubular and plate structures are being sought to respond to specific mission challenges. Each new pattern is also a new "word" in the language engineers use to communicate with the material, defining what it should do when conditions change.

Open innovation as an engine for space technology development

The idea of the passive solar tracker and 4D printed metastructures for programmable shape change was originally submitted through ESA's open space innovation platform, the Open Space Innovation Platform (OSIP). This mechanism allows researchers from academia and industry to propose unconventional concepts that traditional programs might not immediately recognize.

After initial selection, the project was funded through the Discovery element of ESA's Basic Activities, which serves as a testing ground for riskier but potentially disruptive research. It was within this framework that the team was given the freedom to experiment with a combination of advanced composite technology, inspiration from biology, and the concept of 4D printing. The result is an example of how a relatively small study can turn into a patent, a series of scientific papers, and an entire new direction of research.

Although it will be some time before passive solar trackers based on 4D metastructures are found on the Moon or in commercial solar fields on Earth, today's results are already changing the way we think about structures. Instead of seeing materials as passive "carriers," we increasingly perceive them as active participants in system operation – from deploying equipment in space to managing energy in our cities.