In addition, the researchers opted for nanowires because the other materials like graphene and carbon nanotubes are not as mechanically strong at nanometer-scale thickness when in free-standing form, according to Hyunhyub Ko, an associate professor at UNIST and co-author of the research. It is this thickness that is the critical element of the material.

“The biggest breakthrough of our research is the development of ultrathin, transparent, and conductive hybrid nanomembranes with nanoscale thickness, less than 100 nanometers,” said Ko. “These outstanding optical, electrical, and mechanical properties of nanomembranes enable the demonstration of skin-attachable and imperceptible loudspeaker and microphone.”

The nanomembrane loudspeaker operates by emitting thermoacoustic sound through the oscillation of the surrounding air brought on by temperature differences. The periodic Joule heating that occurs when an electric current passes through a conductor and produces heat leads to these temperature oscillations.

For the operation of the microphone, the hybrid nanomembrane is sandwiched between elastic films with tiny patterns. In this way, the nanomembrane can precisely detect the sound and the vibration of the vocal cords based on a triboelectric voltage that results from the contact with the elastic films. In these loudspeakers and microphones, the silver nanowires enable both the electrical conductivity and give the nanomembranes their free-standing strength.

While the researchers demonstrated the technology by applying a thin film of the nanomembrane on skin, this may not turn out to be a practical application of the technology, according to the researchers. This is because the performance of the thermoacoustic loudspeaker is proportional to the speaker size and temperature change of the speaker.

If it were directly attached to the skin, the input power level per unit area would increase too much for the generation of a large sound.

Ko added: “For the commercial applications, the mechanical durability of nanomebranes and the performance of loudspeaker and microphone should be improved further.”