Overview


Our lab designs and manufactures micro-nano devices inspired by living organisms or integrates some biological systems and uses them for material and energy conversion, signal transmission and sensing (sensing and monitoring), and computing. 


Detailed information for our research can be found in the M-Terview.

http://www.materic.or.kr/v2/mp/content.asp?f_id=171&page=1&listType=10&s_kinds=&s_word=&listCnt=







Spatio-temporal Control of Nanostructures


  • Various promising applications using self-assembly of nanostructures


Self-assembly of nanostructures opens up a number of exciting possibilities, including batteries, solar cells, photonic crystals, nanoplasmonics, and so on. When an assembly is highly organized and can be controlled locally and timely, not only is quality performance improved, but new opportunities and functions, such as genotyping, diagnosis, printing, or display, can be realized. 

 


Our approach: Spatio-temporal control of nanostructure self-assembly using microfluidics and electrokinetics


For localized positioning in the microchannel or on the substrate, we use microfluidics and electrokinetics. Microfluidic phenomena, surface functionalization, inkjet printing, electrospinning, and other chemistry, physics, and tools are studied.

These well-organized structures enable the realization of energy harvesting, iontronics, optoelectronics, and biomedical microdevices.



The capillary stop induced by neck pressure is actively used to construct highly organized nanoparticles with the desired region. The nanoparticle-containing suspension is introduced from the reservoir connected to the deep channel and moves via capillary flow until it reaches the interface between the shallow and deep channels. The sudden change in angle at the interface causes neck pressure, allowing the suspension to stop. The solvent in the suspension is then evaporated, drawing the nanoparticles to the interfaces and assembling them into 3D FCC (face-centered cubic) structures, a process known as evaporation-driven self-assembly. The nanopores formed by the in-situ self-assembly of nanoparticles in a specific region can be used to create 3D nanochannel networks membrane (NCNM) with permselectivity through the overlapped electrical double layer (EDL) from polarized nanoparticles. Since the size, polarity, and surface charge of nanoparticles can easily control the characteristics of 3D NCNM, the microfluidic device integrated with 3D NCNM allows for many interesting and challenging applications such as iontronics, energy harvesting and conversion, optoelectronics, and so on.





Structural Color


Colorimetric sensors using Photonic Crystals


Many animals and plants in nature display structural colors in their epidermis that are based on nanostructures and can respond visually to environmental cues. In response to chemical stimuli, photonic crystals (PCs), which alter the inter-distance, have attracted the attention of researchers. Since PCs are constructed from materials with regular nanostructures that are dielectric, light with particular wavelengths is reflected by diffraction and interference, which is based on the Bragg equation, and this is what is perceived as the visible light region. When the distance between periodic nanostructures changes, the bandgap, or the region where light cannot pass, shifts, and this color change may be observed by the human eye.


  • Colorimetric VOCs sensing


The principle and applications for colorimetric VOCs sensing based on photonic crystals can be found in the followings:  



Link1: https://www.youtube.com/watch?v=vwWHVAseRqM&t=625s


  • Virus sensing based on optical guidance using photonic crystals



[Real-time visualization of influenza virus using Qdot-aptamer beacon and photonic crystals for enhancement of fluorescent signal, RSC Advances 2018]


  • Smart Contact Lens using PC-based strain sensor


[The photonic crystal-based smart contact lens for continuous intraocular pressure monitoring, Lab on a Chip 2020]



Actively tunable structural color



Tunable photonic crystals (TPCs) are attractive due to their tunable interactions and ability to actively control light propagation through external stimuli. Based on these properties, TPCs can be applied as intelligent devices, such as color displays, chemical and biological sensors, color-changing paints and inks, and photonic papers. Generally, TPCs are realized according to two phenomena: Changes in (1) the lattice distance between periodic nanostructures or (2) the effective refractive index of the photonic crystals. These changes can be triggered by several stimuli, such as chemical, thermal, humidity, electrical, magnetic, mechanical or light stimuli. Among the TPCs, electrically tunable photonic crystals (ETPCs) are the most promising for practical technological applications due to their precise and easy control, fast response time, and convenient implementation. However, the previously reported ETPCs require specific liquid cells that contain solvent, electrolytes, or liquid crystals. These features result in a long switching time, large hysteresis, long and complex fabrication process, and unstable operation with a short lifetime. 

To address these issues, a new approach of ETPCs, that is, flexible all-solid-state ETPCs, is proposed through chemically induced polymer swelling and lattice control in photonic crystals using dielectric elastomer actuators (DEAs). The all-solid-state ETPCs show a wide range of color changes from red to blue-green and long-term stable operation with low hysteresis.


This work was featured in many news including donga science (link)







Electrokinetics


Iontronics for neuromorphic computing


Iontronics is a new technology that uses ions as signal carriers to connect solid-state electrical devices with biological systems. In an aqueous environment, the multiple ionic carriers are transported and controlled through biological ion channels, which have three main functions: ion selectivity, gating, and ion rectification, all of which are linked to their respective roles in living organisms, such as signal generation, storage, and transmission. Artificial nanochannels or nanopores mimicking the biological ion channels are more stable and robust and are used to develop into ion-selective membranes, diodes, and transistors for a variety of applications, including energy harvesting, desalination, and biosensing. 

An interesting feature related to the current rectifying behavior in nanochannels, which has not been investigated extensively, is the memristive properties of ionic channels. These memristive properties can be observed by applying a periodic potential over the nanochannel. The memristive effects in these nanochannels are of interest because ion channel memristors are essential for generating the action potential in neurons, that is, a key element for the ionic circuits to mimic neuro-biological architectures present in the nervous system (Neuromorphic computing).


  • Brain-inspired computing with fluidic iontronic nanochannels [PNAS 2024]


Significance

The brain’s computing principles (neurons connected by synapses) and information carriers (ions in water) both differ fundamentally from those of conventional computers. Building on this distinction, we present an aqueous memristor that emulates the brain’s short-term synaptic plasticity features through ion transport in water, mirroring the natural processes in the brain. This device, which is inspired by and understood through a theoretical model, is applied as a synaptic element for reservoir computing, a brain-inspired machine learning framework. Thus we implement a brain-inspired computing element in a brain-inspired fluidic medium, representing a considerable step toward computing devices that proverbially both walk and talk like the brain.

The brain’s remarkable and efficient information processing capability is driving research into brain-inspired (neuromorphic) computing paradigms. Artificial aqueous ion channels are emerging as an exciting platform for neuromorphic computing, representing a departure from conventional solid-state devices by directly mimicking the brain’s fluidic ion transport. Supported by a quantitative theoretical model, we present easy-to-fabricate tapered microchannels that embed a conducting network of fluidic nanochannels between a colloidal structure. Due to transient salt concentration polarization, our devices are volatile memristors (memory resistors) that are remarkably stable. The voltage-driven net salt flux and accumulation, that underpin the concentration polarization, surprisingly combine into a diffusion like quadratic dependence of the memory retention time on the channel length, allowing channel design for a specific timescale. We implement our device as a synaptic element for neuromorphic reservoir computing. Individual channels distinguish various time series, that together represent (handwritten) numbers, for subsequent in silico classification with a simple readout function. Our results represent a significant step toward realizing the promise of fluidic ion channels as a platform to emulate the rich aqueous dynamics of the brain.




  • Asymmetric nanochannel network-based bipolar ionic diode [ACS Nano 2022]


A higher rectification degree in ionic diodes is required to achieve better performance in applications. Nonetheless, the active geometrical change that is critical for inducing electrical potential asymmetry is difficult to realize in typical ionic diodes because of the intrinsic limitation of the fabrication method. An asymmetric nanochannel-network-based bipolar diode shows a high rectification degree of ~1600 - the highest value realized until now, to the best of our knowledge. Such high rectification is obtained based on the synergetic effect of the bipolar surface charge and the optimization of the microchannel through experimental study and multiphysics numerical simulation. It induces ion concentrations at the heterogeneous junction based on the accumulation effect under the forward potential bias. In particular, this proposed molecular concentration occurs in the ohmic region without vortex and instability that is inevitable in the conventional nanoelectrokinetic concentration. Combining this accumulation with the horizontally aligned configuration of nanochannel network membrane (NCNM), a highly sensitive and quantitative mercury ion (Hg2+) sensor based on a fluorescent signal is fabricated that allows direct measurement using a general fluorescent microscope. The detection limit of Hg2+ is 10 pM that is ~10 times lower than the best detection limit realized so far (~100 pM) in fluorescent dye-based detection. This demonstrates the potential of asymmetric NCNM for high-performance ion transport in applications such as energy conversion, based on its design and material flexibility.

 




  • High Current Ionic Diode Using Homogeneously Charged Asymmetric Nanochannel Network Membrane [Nano Letters 2016]


An asymmetric nanochannel network membrane (NCNM) constructed by in situ self-assembly of nanoparticles with uniform surface charge is used to create a high current ionic diode. Previously, it was known that the unipolar diode should be formed through the geometry control of nanoscale channels, comparable to an electrical double layer. The unipolar diode is realized with asymmetric geometry in microscale in this work, and high current is achieved thanks to 3D NCNM. Because of the amplified ionic signal, this high current allows for more reliable and quantitative analysis of ionic dynamics, as well as challenging applications like energy harvesting.


 

 



Energy Conversion


Eco-friendly conversion of microplastics using photocatalytic oxidation enhanced by nanoelectrokinetics 


The ion-concentration-polarization-assisted photocatalytic reactor is developed, which generates a nonlinear electric field across the microchannel of this system, allowing for an 85.5% increase in reaction rate compared to a linear potential, and a high reaction rate constant of up to 12.7 min-1. The nonlinear electric field created by concentration polarization, the nanofluidic electrokinetic phenomena, resulting in a significantly higher potential drop across the photocatalyst layer, preventing photoexcited electron and hole recombination. The degradation of the plastic film validates the enhanced photocatalytic activity. Due to its simple setup and fabrication techniques, this proposed enhancing mechanism demonstrates a novel application of nanofluidics for improving the photocatalytic effect, as well as the potential to be a new class of platform for a photocatalytic reactor. [Lab on a Chip 2022]




High-voltage nanofluidic energy generator


A high-voltage nanofluidic energy generator inspired by electrical eel using ion-concentration gradients is developed, which converts Gibbs free energy into electricity without any pollutants. The high voltage can be induced by alternatively multi-stacking cation and anion-exchange nanochannel network membranes (CE-NCNMs and AE-NCNMs) in a confined microscale space. These membranes were constructed by in situ self-assembled nanoparticles with hydroxyl and amine groups, respectively. The multiple stacks of CE-NCNMs & AE-NCNMs were successfully realized by precisely guiding the nanodrops with the suspended positively or negatively charged nanoparticles into the desired positions in the multilayered microchannel platform. The performance of the proposed nanofluidic energy generator was quantitatively investigated by changing nanoparticle species, intermembrane distance (IMD), and environmental temperature. [Nano Energy 2018]


This work was featured in news including YTN (link)




Wearable devices for energy harvesting


  • Safe, Durable, and Sustainable Self-Powered Smart Contact Lenses


Smart contact lenses with fully embedded glucose fuel cells are proposed, which are safe, flexible, and durable against deformations. These fuel cells produced stable power throughout the day or during intermittent use after storage for weeks. When the lenses were exposed to 0.05 mM glucose solution, a steady-state maximum power density of 4.4 μW/cm2 was achieved by optimizing the chemistry and porous structure of the fuel cell components. Additionally, even after bending the lenses in half 100 times, the fuel cell performance was maintained without any mechanical failure. Lastly, when the fuel cells were connected to electroresponsive hydrogel capacitors, we could clearly distinguish between the tear glucose levels under normal and diabetic conditions through the naked eye. [ACS Nano 2022]






A wearable and flexible textile-based biofuel cell is developed using moisture management fabric (MMF) widely used in sportswear as a transport layer for sustainable and high-power energy harvesting. The reduction of PB-modified cathode is driven by the oxidation of glucose catalyzed by GOD-modified anode, and this enables a single-compartment structure where MMF acts as biofuel transport media. MMF made of polyester can naturally induce a continuous, high-speed flow that facilitates molecule transport for efficient chemical reactions without an additional pump. The resulting highly efficient power generation in MMF is explored and verified by comparing it with those of cotton and paper. Additionally, multi-stack biofuel cell in both parallel and series was successfully realized, and the open circuit voltage and maximum power reached 1.08 V and 80.2 μW, respectively. Integrated into a bandage and sportswear, a six-stack biofuel cell was able to generate sufficient electrical power from human sweat and turn on a sports watch directly. Owing to the low-cost and scalable fabrication process, the proposed biofuel cell has great potential to be systematically integrated into clothes, and generate sufficient and sustainable electrical power for wearable electronics using biofuel (e.g. glucose, lactase) from various bodily fluids, like sweat and urine.


This work was featured in many news including Chosun (link)





MEMS-based devices



MEA for Cryotherapy


Chemical anesthetics are commonly utilized in the human body for a variety of skin tissue operations. However, chemical anesthesia is required for a longer period of time than a short procedure duration. Furthermore, depending on the patients' physiological condition or characteristics, there are side effects such as skin allergic reactions and negative impacts on both heart rate and breathing. To address the drawbacks of chemical anesthesia, it is critical to investigate and develop a purely physical and fast cryoanethesia with minimal side effects. There is some research to modulate neuron signal by photothermal stimulation modulation, static low-temperature testing of a single cell using a patch or voltage clamp, and low-temperature activated TRP channel research in neuron cell experiments related to temperature change. However, there is a limit to understanding neuron signal transfer characteristics between neuron networks at low temperatures and gathering data on neuron signal transmission characteristics when temperature changes rapidly. We present a novel cryo-neuromodulation platform made up of a microelectrode array (MEA) system and a high-speed and precise probe-type cooling device that offers the quick and localized cooling temperature. We investigate at the possibilities of cryo-neuromodulation by exploring with temperature parameters (temperature and duration time) that induce neuron signal block and recovery without causing cell damage.





3D Hydrogen sensors


The purpose of this research is to create low-cost, high-sensitivity hydrogen sensors and modules that can be manufactured using microelectromechanical system (MEMS) technology to detect hydrogen in ultra-small and high-concentration ranges while ensuring reliability and productivity.