Research fields
1. Electric field guided assembly of carbon nanotubes for biosensors (2001~2012):
My early research at Northwestern University offered a stepping stone for massive assembly of carbon nanotubes and DNA molecules using an electric field. The dielectrophoretic assembly method demonstrated high yield and precise placement of carbon nanotubes and DNA molecules across electrodes. Such electric forces have been comprehensively studied in follow-up publications. The impact has been widely recognized through an introductory article at Nature Materials. Toward low cost biosensors, the knowledge and experience about the electric field assembly was innovatively combined to create a glucose sensor, an amperometric immunosensor, and a gas sensor. Multiple patents (Micro/nano-fabricated Glucose Sensors Using Single-Walled Carbon Nanotubes, US2005124020; Methods and Related Systems for Carbon Nanotube Deposition, US 7381316; Method and System for Concentrating Particles from a Solution, International Patent Application No. PCT/US2009/046652, WO2009149467; Hybrid Fibers, Devices using Hybrid Fibers, and Methods for Making Hybrid Fibers, US 8940092 B1) were generated on the basis of an electric field-induced assembly, which aim for low-cost and highly sensitive biosensors.

2. Biomimetic cilia for microfluidic and biological applications (2006~2013):
Biological cilia are hair-like structures whose rhythmic beating provides motility for cells and micro-organisms. Cilia can offer a clue to address a current challenge of dexterous manipulation of small particles in microfluidics. Microfluidic manipulation is not efficient because of the fluid's high viscosity, low inertia, and low molecular diffusivity, in particular, in a small volume. To overcome the challenge, a biomimetic approach that imitates the properties and behaviors of biological cilia has been intensively studied. In the project, an underwater fabrication method to manufacture highly compliant cilia was developed to physically resonate cilia in a liquid medium. The resonating mechanism using an added mass effect was investigated through fluid and structure interaction. The mixing and reaction efficiencies of cilia were characterized by experiment, analytical equations and numerical approach. 6 Journal papers were published with a patent entitled with Devices, Apparatus, and Methods Employing Biomimetic Cilia for Microfluidic Manipulation (US 8,778,666 B1, 2014).

3. Point-of-care (POC) diagnostic sensor of tuberculosis (2006~present):
The enrichment of low abundance molecules is a crucial step toward disease diagnosis, drug- delivery and discovery, and environmental monitoring. However, the efficiency of current molecular enrichment methods is very limited due to extremely small mass and dimension. The challenge was addressed by using a dendritic nanotip to generate numerous small tips for efficient enrichment of molecules. The innovation of the tip enrichment system is in the dielectrophoretic attraction of target biomarkers that is induced by a dendritic nanotip with a high- frequency electric field. Unlike other electric field based enrichment methods, the dielectrophoretic force using a dendritic nanotip is greater than that of planar electrodes by a few orders of magnitude because of the intensified e-field strength on a nanotip. This transformative enrichment platform can concentrate molecules as small as 2nm, which has been validated through transmission electron microscope study using 2nm gold nanospheres. To my knowledge, this is the smallest, individual nanoparticles that are enriched by an electric field. Considering the dimensions of most biomolecules greater than 2nm, the nanotip enrichment system can enrich most biomarkers present in body fluids. The enrichment performance was analyzed and applied for detection of low-concentration molecules, which was filed as a patent (Method and System for Concentrating Particles from a Solution, International Patent Application No. PCT/US2009/046652, WO2009149467). The enrichment mechanism was further developed for a point-of-care sensor of tuberculosis diagnosis.

4. Wearable sensors using carbon nanotubes (2014~present):
A wearable sensor can be a platform that can monitor human behavior, physiological parameters and biomarkers for disease diagnosis and cure. Toward light-weight, inexpensive and disposable sensors, two kinds of sensing platform is being investigated. One is carbon nanotube-paper composite (CPC) sensor and the other is carbon nanotube-printed sensors. Using a carbon nanotube-paper composite (CPC), various sensors can be developed, such as bio/chemical/physical sensors. To monitor human behavior, a fracture-induced mechano-electrical sensitivity of a CPC is being investigated for wearable sensing applications. With precise control of the applied strain under uni-axial load to a CPC, the tensile directional fibers coated with CNTs are fractured, and the cellulose fibers inclined or orthogonal to the tension are reoriented to form crossbar junctions near a crack. The junctions create highly sensitive resistive and capacitive responses for measuring strain, force, and non-contact displacement. The novel manufacturing process allows the integration of flexible sensors in low-cost tissue paper, which is easily adapted to human body for behavior monitoring. For transparent and lightweight sensors, a noncontact capillary pen printing method has been investigated. The printing is conducted by the formation of a nanoink bridge between the nib of a capillary pen and a polyethylene terephthalate (PET) film. Since the printing is continuous without contact, various sensors can be fabricated with a single writing unlike inkjet printing. The noncontact printing method is being used for a POC diagnostic sensor of tuberculosis.