Chinese scientists develop film to power devices using body heat

Forget searching for a power socket; the heat from your wrist might soon be enough to keep your smartwatch running indefinitely. Once a concept confined to science fiction, this idea is moving closer to reality.

A team of Chinese scientists has developed a groundbreaking plastic film capable of converting body heat — or even the warmth from a car engine — into electricity with record-breaking performance. The study, published Friday in the journal Science, addresses a major challenge facing next-generation wearable technology and Internet of Things sensors: sustainable power generation.

With the widespread use of smartwatches, fitness trackers, and medical patches, the need for daily charging has become a significant inconvenience. To address this, scientists have been exploring ways to harness heat from the body or the environment to generate electricity using thermoelectric materials.

Thermoelectric materials function through the Seebeck effect, where a temperature gradient across the material causes electrons to move from the hot side to the cold side, generating an electric current. Traditionally, the most efficient thermoelectric materials have been inorganic semiconductors, which are usually rigid, heavy, and often contain toxic elements. While flexible organic polymers offer advantages such as being lightweight, flexible, and well-suited for large-area printing, they have struggled to achieve comparable performance.

Liu Liyao, an author of the study and an associate professor at the Institute of Chemistry, Chinese Academy of Sciences, pointed out that the challenge lies in the intrinsic molecular-level trade-off in organic thermoelectric materials.

"High-performance thermoelectric materials should satisfy two conflicting requirements: on one hand, they need to minimize thermal conductivity to maintain a sustained temperature gradient across the material; on the other hand, they require high electrical conductivity to enable efficient charge carrier transport," Liu said, noting that for most materials, these two properties go hand-in-hand, opposite to the optimal performance.

After years of dedicated research in thermoelectric materials, the team developed a polymer film with a novel hierarchical porous structure. This structure features pores of varying sizes, ranging from nanometers to micrometers, as well as irregular shapes and a disordered spatial distribution.

Liu said that the chaotic, hole-filled structure acts like a rugged mountain range for heat, making it difficult for heat waves to bounce around. Meanwhile, the narrow channels between the pores constrain the polymer molecules to line up in neat, orderly rows, creating low-resistance pathways for electron transport, like cars effortlessly driving on highways in mountainous areas.

This design has achieved a 72-percent reduction in thermal conductivity while increasing carrier mobility by 52 percent, effectively decoupling these traditionally competing properties.

The new plastic film also demonstrates the best performance among flexible thermoelectric materials at room temperature, achieving a record-high thermoelectric figure-of-merit — an indicator used to describe thermoelectric efficiency, with a higher score representing better performance — of 1.64 at around 70C, a temperature relevant for many industrial and body-heat applications. This shatters the previous record for flexible materials at 1.4.

A laboratory experiment has verified the feasibility of the structure. Attaching the integrated film, which measures 10 centimeters in length and 8 centimeters in width, to the human body can generate an output voltage of 9 millivolts. It is estimated that when scaled to wall-sized dimensions, this material could potentially power ultra-low-power applications, such as wireless sensors.

"This structure is compatible with industrial printing techniques, allowing for the potential large-scale production of this 'power-generating plastic'," Liu said.

With further optimization, including cost reduction and performance stabilization, it could expand applications not only to wearable electronics but also to distributed green energy systems and even space-based power generation, she added.