In the study, the research team developed ultrathin, transparent, and conductive hybrid nanomembranes (NMs) with nanoscale thickness, consisting of an orthogonal silver nanowire array embedded in a polymer matrix.
To demonstrate the NMs, they made it into a loudspeaker which can be attached to almost anything to produce sounds. The researchers also introduced a similar device, acting as a microphone, which can be connected to smartphones and computers to unlock voice-activated security systems.
NMs are molcularly thin seperation layers with nanoscale thickness. Polymer NMs have attracted attention owing to their advantages, such as extreme flexibility, ultralight weight, and excellent adhesibility in that they can be attached directly to almost any surface. The issue lies in the fact that they tear easily and exhibit no electrical conductivity.
However, the UNIST research team claim to have solved such problems by embedding a silver nanowire network within a polymer-based NM. This has enabled the demonstration of skin-attachable and imperceptible loudspeaker and microphone.
"Our ultrathin, transparent, and conductive hybrid NMs facilitate conformal contact with curvilinear and dynamic surfaces without any cracking or rupture," says Saewon Kang of UNIST.
He adds: "These layers are capable of detecting sounds and vocal vibrations produced by the triboelectric voltage signals corresponding to sounds, which could be further explored for various potential applications, such as sound input/output devices."
Using the hybrid NMs, the research team fabricated skin-attachable NM loudspeakers and microphones, which would be unobtrusive in appearance because of their excellent transparency and conformal contact capability. These wearable speakers and microphones are paper-thin, yet still capable of conducting sound signals.
"The biggest breakthrough of our research is the development of ultrathin, transparent, and conductive hybrid NMs with nanoscale thickness, less than 100 nanometers," says Professor Hyunhyub Koof UNIST. "These outstanding optical, electrical, and mechanical properties of NMs enable the demonstration of skin-attachable and imperceptible loudspeaker and microphone."
The skin-attachable NM loudspeakers work by emitting thermoacoustic sound by the temperature-induced oscillation of the surrounding air. The periodic Joule heating that occurs when an electric current passes through a conductor and produces heat leads to these temperature oscillations. It has attracted attention for being a stretchable, transparent, and skin-attachable loudspeaker.
Wearable microphones are sensors, attached to a speaker's neck to even sense the vibration of the vocal folds. This sensor operates by converting the frictional force generated by the oscillation of the transparent conductive nanofibre into electric energy. For the operation of the microphone, the hybrid NM is inserted between elastic films with tiny patterns to 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.
"For the commercial applications, the mechanical durability of NMs and the performance of loudspeaker and microphone should be improved further," says Professor Ko.