A Revolutionary Customizable Crafting Platform for Electronic Skins (with Videos)


February 03, 2022

(Spotlight on Nanowerk) For years, thin-film electronics have shown their superior performance over conventional electronic devices in many applications. Therefore, the demand for thin film devices continues to grow at a rapid rate.

Thin-film electronics are mostly attached to something, such as flexible sensors on the skin, but the specific characteristics of the target location cannot always be determined. In this case, conventional simulation technology loses its edge as a tool to optimize device performance and reduce prototyping costs.

Conventional skin electronics is a fixed design that does not consider the individual characteristics of the user or the ability to actively adapt to varying user specifications.

In order to handle different sensor specifications and body impedances, design adjustments are inevitable when manufacturing these devices. Unfortunately, conventional manufacturing methods can be expensive and time-consuming, even when it comes to making small changes to existing circuit designs.

To solve this problem, researchers at Seoul National University have developed a real-time fabrication platform that can adapt to arbitrary environments by freely drawing or erasing paths in a circuit board. The system uses a laser-based process in which focused light sinters or ablates metallic nanoparticles onto a flexible, ultra-thin substrate.

Illustration of sensor impedance and adaptation in situ and in operando (SOA). The device is actively adapted to various user demands and corresponding mounting locations through impedance and SOA sensor. (Image: Applied Nano and Thermal Science Laboratory, Seoul National University) (click image to enlarge)

This research is described in an article Advanced functional materials (“Evolveable Skin Electronics by In Situ and In Operando Adaptation”). This revolutionary customizable electronics manufacturing platform allows for simultaneous evolutionary design modification and the addition of new features to the original skin electronics while they are in operation.

“Our findings allow us to easily draw and erase wearable device electronics in real time based on user needs,” said Professor Seung Hwan Ko of the Applied Nano and Thermal Science (ANTS) lab at Seoul National University, Nanowerk. “We found that controlling the source and power of laser light could anneal metallic nanoparticles to create conductive paths, and also to selectively erase existing circuits. Additionally, we can stack multiple layers and selectively create interconnections between them.

Researchers call this process in situ and in operando adaptation, or SOA. SOA allows instant adaptation of electronic skin devices for arbitrary users and placement. For example, the team demonstrated that an electronic skin device originally designed to monitor temperature can be extended to include UV and humidity sensors. Additionally, the device’s wireless power transmission is enabled when attached to the user’s skin through real-time antenna modification. This is crucial because electromagnetic properties vary from user to user and body part to body part.

For example, the team demonstrated that an electronic skin device originally designed to monitor temperature can be extended to include UV and humidity sensors. Additionally, the wireless power transmission of the device, when attached to human skin, has been enabled through real-time antenna modification.

In situ and in operando adaptation (SOA) is based on a rewrite scheme with two main features: 1) additive manufacturing by visible wavelength laser sintering of metallic nanoparticles, and 2) subtractive manufacturing by laser ablation ultraviolet. Each laser source selectively converts silver nanoparticles into electrically connected layers or removes existing electrical patterns. (Video: Applied Nano and Thermal Science Laboratory, Seoul National University)

They describe in their article how this new platform enables immediate response to diverse user specifications, providing simultaneous customization for various portable and wireless applications. The system can be applied in various potential applications with wireless/battery-free operation, such as skin condition monitoring, human physiological measurements and VR application.

The system is even sufficient to operate an electronic circuit remotely. The researchers successfully operated the circuit to control a vehicle up to 50 cm away from the transmitter using a long dipole antenna.

A dipole antenna system attached to a paper controller used to remotely control a vehicle with a controller without a battery. (Video: Applied Nano and Thermal Science Laboratory, Seoul National University)

The SOA process can be repeated as many times as needed to add or remove features such as skin condition monitoring, human physiological measurements, motion tracking, and more.

“Since SOA is performed by lasers, we need to understand some characteristics of laser systems,” Ko explains. “There may be alternative parameters, but we operated a continuous laser with a wavelength of 532 nm (green) for circuit drawing and a pulsed laser with a wavelength of 355 nm (ultraviolet) for circuit erasing.Theoretically, the two lasers can replace each other, but the speed and quality of the processes are significantly different.

The laser waveform as a function of time has a crucial effect on heat dissipation at the target. The pulsed laser provides high energy for a very short time, so the heat supplied to the spot does not have enough time to dissipate. As a result, the narrow but highly heated region evaporates.

While the CW laser delivers constant power to the target, the heat generated at the spot therefore has a better chance of dissipating to the nearby region than is the case with the pulsed laser. As a result, the metallic nanoparticles melt rather than evaporate and the molten metal becomes a metallic film that forms as the laser passes. The wavelength of the CW laser is related to the absorbance of the material, and 532 nm is the wavelength that metallic nanoparticles absorb the most.

As the team points out, at the initial stage of the process, the erasing process is the trickiest part of SOA because it might erase the upper circuit layer, but might also damage the lower circuit layer. Since the insulation layer is only 2 to 3 μm thick, the operating conditions of the erasing laser must be fine tuned.

Although low power conditions can prevent damage, they inevitably lead to a slow process, which would limit the practical use of SOA. By using colorless polyimide as the ultra-thin substrate, the low absorbance of this insulating material mitigates damage to the insulating layer, enabling reasonable erasing speed.

The developed sensor is used to control a virtual hand in real time. (Video: Applied Nano and Thermal Science Laboratory, Seoul National University)

As a next step, the team plans to develop an automatic optimization algorithm for analog SOA.

“Existing simulation tools for analog systems support parametric study on sets of geometric properties – such as length, width, number of certain models – that determine the performance of an analog system,” notes Ko. “Our current SOA process only conducts parametric studies for a single parameter such as the length of an antenna, so we need a new algorithm that determines which of several parameters to change for a given state and estimates optimal state during the manufacturing process.”

He adds that this approach has enough potential as a next-generation optimization tool for various analog systems, including antennas and metamaterials.

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Michael is the author of three books published by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology, Nanotechnology: The Future is Tiny and Nanoengineering: The Skills and Tools Making Technology Invisible Copyright ©




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