Researchers at Drexel University have developed a novel building material that could revolutionise the way structures maintain comfortable temperatures.

Inspired by the heat-regulating ears of jackrabbits and elephants, the team created a cement-based material embedded with a network of tiny channels, or vasculature, filled with paraffin. This setup allows the material to passively heat or cool surfaces such as walls, floors, and ceilings. 

The study aims to address the massive energy demands of buildings, which account for nearly 40% of total energy use. Of that, about half is spent on heating or cooling. 

“Architecturally, it looks nice to have a lot of window area on a building, but this also results in diminished insulation properties,” said Rhythm Osan, an undergraduate student in Drexel’s College of Engineering and a co-author of the study.

“In an ideal world, a building wouldn’t lose any heat, but from a realistic constructability standpoint, issues like thermal bridging, air leakage from ducts, material performance and joint detailing will always pose some heat loss.”

Rather than fight against the reality of thermal leakage, the team decided to turn the surfaces themselves into active temperature regulators.

How paraffin and concrete work together

The innovation comes from combining a specially printed polymer matrix and concrete to form a vascular system within the surface. This internal network is then filled with paraffin, a phase-change material (PCM) similar to what is used in candles.

Phase-change materials are ideal for this application because they absorb and release heat during their transitions between solid and liquid. When the temperature drops and the paraffin solidifies, it releases heat. When it warms up and melts, it absorbs heat – producing a cooling effect.

“We have previously used paraffin-based material as the phase-change ingredient for self-warming concrete, so we knew that it was a reliable, natural substance that could affect the surface temperature of concrete building materials,” said Robin Deb, PhD, a research scientist in Drexel’s Advance Infrastructure Materials (AIM) Lab and co-author of the study.  

“For this application, we selected a phase-change material with a melting temperature at about 18 degrees Celsius, a relatively low melting point, to test its effectiveness in cold climates. But this system would allow for tailoring the phase-change material to be responsive in warmer climates as well.”

The concept is similar to how humans and animals regulate their body temperature. “Look at the way our circulatory system is used to regulate temperature,” elaborated Amir Farnam, PhD, an associate professor and project lead.

“When it’s hot out, blood runs to the surface – we might get a little red in the face and begin to sweat through our glands, and this cools us down through a phase-change process – sweat evaporation. This is a very effective, natural process that we wanted to replicate in building materials.” 

Three-dimensional X-ray of vascular building materials developed by Drexel College of Engineering researchers. Image: Drexel University

Testing the bio-inspired cement

To test their design, the researchers created multiple cement samples, each with different patterns and arrangements of channels: single, multiple, parallel, diagonal, and diamond-shaped grids. The channels ranged from three to eight millimetres thick.

They then filled the channels with paraffin and ran tests to evaluate both the mechanical strength of the samples and how effectively they could slow temperature changes. Among the various designs, the diamond-shaped channel grid delivered the best combination of strength and thermal performance. 

This version withstood stretching and compression while regulating surface temperature, slowing heating or cooling by 1 to 1.25°C per hour.

“We found, perhaps not surprisingly, that more vasculature surface area equates to better thermal performance. This observation is similar to physiology of elephant and jackrabbit ears, which contain extensive areas of vasculature to help regulate their body temperature,” said Deb.

“We believe that our vascular materials could play a similar role in a building by helping to offset temperature shifts and reduce energy demand from HVAC to maintain thermal comfort.”

Towards stronger, smarter, and greener buildings

Despite the hollow channels, the material stayed strong for practical use. Adding fine aggregates improved its durability without affecting the vascular system.

“While this study was intended to show a proof of concept, these results are promising and something we can build on,” said Farnam. 

“This shows both the effectiveness of this method for regulating surface temperature in cementitious materials, as well as a simple and cost-effective method for producing them. With additional testing and scaling we believe this has the potential to make a significant contribution to the many ongoing efforts to improve the energy efficiency of buildings.” 

Next, the team plans to experiment with different kinds of phase-change materials, alternate channel designs, and larger building material samples. These trials will take place over longer periods and in more varied environmental conditions to assess long-term performance.

The study was published in the Journal of Building Engineering.