Smart textiles of the future are expected to do more than just warm or cool: they could measure heart rate and blood pressure, power smartphones, or even function as sensors in protective clothing. To make this possible, the fabrics are coated with conductive layers. This presents a challenge: these ultra-thin coatings – only a few dozen nanometers thick and thus a thousand times thinner than a human hair – must adhere uniformly to the flexible, often rough fabric surface. A team in Jena, Marburg, and the USA has now developed a new, contactless method to make material defects in these coatings visible.

“For the first time, our method gives us a detailed, spatially resolved analysis of electrical conductivity on textiles,” explains Dr. Jonathan Plentz, whose Photonic Thin Film Systems group at Leibniz IPHT applied the method to textile substrates for the first time, in collaboration with Philipps University Marburg and Brown University in the USA. “What makes it special is that, while conventional measurement methods are usually in direct contact with the surface  – and can even damage the material – the ­terahertz method works ­contactlessly and non-­destructively. This allows us to detect defects early and specifically improve the development of
smart textiles.”

Terahertz Radiation as a Precise Analytical Tool

Terahertz radiation lies in the electromagnetic spectrum between microwaves and infrared light. It has a special property: it can penetrate many materials without damaging them, while providing valuable structural information. It is already used in medical technology to detect skin cancer, and in the security sector to inspect luggage.
For their measurements, the researchers irradiate the coated textiles with terahertz waves and analyze how these interact with the material. “The way the radiation is reflected or absorbed gives us important clues about the conductivity of the coating,” explains Dr. Maximilian Hupfer from Leibniz IPHT, who verified the electrical conductivity of the samples using four-point probe measurements.
What makes this special: while conventional measurement methods are typically in direct contact with the surface – and can even damage the material – the terahertz method works contactlessly and non-destructively.

Tracking Down Material Defects

To validate their method, the researchers examined two materials frequently used in smart textiles: silver and indium tin oxide (ITO). These are applied to fabrics as conductive and simultaneously ­transparent coatings.
The results show that terahertz radiation can precisely map the conductivity of the coatings –
and detect defects that are ­difficult to spot with other methods. Such irregularities could impair the ­function of smart ­textiles – for example, by causing sensors to operate unreliably or interrupting electrical connections.

Applications in Protective ­Clothing and ­Medical ­Technology

The new measurement method could be an important step in advancing smart textiles.
“Especially in medical technology or protective clothing, it is crucial that these materials function reliably,” says Jonathan Plentz. But other industries could benefit as well: “The technology could also be used in the automotive sector or for flexible electronics.”
“In the future, this method could contribute to quality control in industrial production,” Plentz adds. “This would make the manufacturing of smart textiles more efficient and more sustainable.”