Heon Sang Lee: “The e-skin ​is sensitive enough to detect one hair strand and acoustic sounds”

Heon Sang Lee is a professor in the Department of Chemical Engineering at Dong-A University. Serious Science has asked Prof. Lee to speak on the subject of electronic skins sensitive to pressure, temperature and even sound.

What is your laboratory working on?

Our major research interest is polymer-nanomaterials. We are developing nanocomposites for many applications, such as transparent conductive films, transparent gas barrier films, ferroelectric films and engineering plastics. In particular, the composites we are developing are potential materials for the parts of display (OLED) and mobile phones. We are performing theoretical and experimental research on tailoring the crystalline structure of nanocomposites, mesoscopic dynamics of polymer solutions or polymer melts, dynamic light scattering, statistical thermodynamics for hybrid materials, characterization of carbon nanotubes and graphene oxides.

One of your recent developments is the artificial skin sensitive to pressure and temperature. What does it look like and how does it work?

A ferroelectric material generates electricity by external stimuli such as temperature change and pressure change. We developed a ferroelectric nanocomposite of PVDF (Polyvinylidene fluoride) and reduced graphene oxide which maintains its ferroelectric properties even after processing (solution casting or melt processing) without any further poling process. A planar film made of the composite can sense temperature and pressure due to ferroelectricity. The ferroelectricity is also maintained when the composite is molded into micro-embossing (microdome) film. The pressure sensitivity is extremely improved when the e-skin is ridged similar​ly to a fingerprint and consists of two sandwiched layers of micro-embossing (microdome) ferroelecetric film, resulting in multifunctionality like the one of a human fingertip, sensing dynamic pressure, static pressure and temperature simultaneously. The e-skin is sensitive enough to detect one hair strand and acoustic sounds.

How is ferroelectricity generated?

We tailored the crystalline structure of PVDF to be polar phases in PVDF/graphene nanocomposites. In a ferroelectric nanocomposite, the electric charge can be accumulated in the polar phase (ferroelectric) in response to external stress. The generated voltage is given by the ratio of the generated charge to capacitance. We mimicked the fingerprint pattern and interlocked epidermal-dermal structure of human skin. The inner dermis layer has mechanical receptors that detect sustained pressure and others that sense changing pressure and vibrations. As it was said before, the pressure sensitivity of a ferroelectric film can be improved when the e-skin is ridged akin to a fingerprint and contains two sandwiched layers of micro-embossing (microdome) ferroelecetric film. This is ascribed to the fact that the local pressure is larger at the point of contact between the upper layer- and the lower layer-microdomes. Also, the ridges on the surface make the e-skin sensitive to texture and sound. Moreover, the resistance of the e-skin is sensitively changed by the static pressure loading due to the change of contact area between the upper layer- and the under layer-micromdomes. The ferroelectric e-skin responses to temperature change in a similar way, since the electric charge can also be accumulated in the polar phase in response to temperature change. Also, the resistance of our composite film is reduced by changing temperature which can be attributed to the change in the contact resistance between the rGO sheets by thermo-mechanical variation in the composites. So, our e-skin can have such multifunctional performance.

Why weren’t the previous groups able to make a skin that was sensitive to both temperature and pressure?

Our nanocomposites have reduced electrical resistance due to the dispersion of rGO in PVDF, holding ferroelectric properties. So, the composites can be used as piezoresistive sensors as well as ferroelectric sensors. Our nanocomposte is a thermoplastic, so it can be easily molded into any shape such as micromoded structure (by a simple solution casting or melt processing), maintaining its ferroelectric/piezoresistive properties. So we can fabricate the interlocked microstructures in ferroelectric films, which can enhance the piezoelectric, pyroelectric, and piezoresistive sensitivity to static and dynamic mechano-thermal signals. Other researchers have also reported skins that can sense the temperature and pressure by using a field effect transistor, or a passive graphene foam, or a poled ceramic-polymer nanocomposistes. These materials cannot be easily molded into a film of microdomed-structure by the simple processing with maintaining of ferroelectric properties.

You also mentioned that the e-skin is capable of ‘sensing’ acoustic sounds.

Sound is no more than a pressure fluctuations. The e-skin is very sensitive to pressure fluctuation, so it detects the magnitude and frequency patterns of sound by generating electricity. The sensitivity of e-skin is much higher than a conventional microphone.

What has been achieved in the field in general?

In the recent few years, many research groups have demonstrated flexible electronic skins (e-skins) with high tactile sensitivities that are capable of mimicking the tactile sensing capabilities of the human skin. Functions of e-skins are limited to sensing one or two of the following three: static pressure, dynamic pressure, and temperature – until our e-skin has developed.

You mentioned that you also develop materials that can be used for displays.
Lifespan of OLED display is reduced seriously by oxygen and water vapor. So, a transparent gas barrier film is one of the most important technologies for commercial displays. Polymer composite with graphene can be a good candidate for a transparent barrier film, since graphene itself is impermeable to all gases. In a touch screen a transparent electrical conductive film is generally employed. A carbon nanotube and/or graphene composite can also be good candidates for this application.

What is the future of the field?

The e-skins will have applications in humanoid robotics, skin prosthetics, wearable health monitoring devices, and internet of things (IoT). For robotics, we will make e-skin more durable to very high pressure, holding minute pressure sensitivity. For prosthetics, a method to transfer signals to the brain will be required.