Bongyoung Yoo

Human extracellular matrix (ECM) powders for injectable cell delivery and adipose tissue engineering.

Here, we present extracellular matrix (ECM) powders derived from human adipose tissue as injectable cell delivery carriers for adipose tissue engineering. We postulate that human adipose tissue may provide an ideal biomaterial because it contains large amounts of ECM components including collagen. Fresh human adipose tissue was obtained by a simple surgical operation (liposuction). After removing blood and oil components, the tissue was homogenized, centrifuged, freeze-dried, and ground to powders by milling. In an in vitro study, the human ECM powders were highly effective for promotion of cell attachment and proliferation for three-dimensional (3D) cell culture. In in vivo studies, suspensions of human ECM powders containing human adipose-derived stem cells (hASCs) were subcutaneously injected into nude mice. At eight weeks post-injection, numerous blood vessels were observed and the newly formed tissue exhibited adipogenesis with accumulated intracellular small lipid droplets. Overall, the grafts showed well-organized adipose tissue constructs without any signs of tissue necrosis, cystic spaces, or fibrosis. We believe that human ECM powders could act as efficient injectable biomaterials for tissue engineering and have great potential for meeting clinical challenges in regenerative medicine, particularly in relation to adipose tissue engineering.

Biogenic formation of photoactive arsenic-sulfide nanotubes by Shewanella sp. strain HN-41

Microorganisms facilitate the formation of a wide range of minerals that have unique physical and chemical properties as well as morphologies that are not produced by abiotic processes. Here, we report the production of an extensive extracellular network of filamentous, arsenic-sulfide (As-S) nanotubes (20–100 nm in diameter by ≈30 μm in length) by the dissimilatory metal-reducing bacterium Shewanella sp. HN-41. The As-S nanotubes, formed via the reduction of As(V) and S2O32−, were initially amorphous As2S3 but evolved with increasing incubation time toward polycrystalline phases of the chalcogenide minerals realgar (AsS) and duranusite (As4S). Upon maturation, the As-S nanotubes behaved as metals and semiconductors in terms of their electrical and photoconductive properties, respectively. The As-S nanotubes produced by Shewanella may provide useful materials for novel nano- and opto-electronic devices.

Peptide-mediated shape- and size-tunable synthesis of gold nanostructures.

While several biological processes have been shown to be useful for the production of well-designed, inorganic nanostructures, the mechanism(s) controlling the size and shape of nano and micron size particles remains elusive. Here we report on the controlled size- and shape-specific production of gold nanostructures under ambient reaction conditions using a dodecapeptide, Midas-2, originally selected from a phage-displayed combinatorial peptide library. Single amino acid changes in Midas-2 greatly influence the size (a few nanometers to approximately 100 microm) and shape (nanoparticles, nanoribbons, nanowires and nanoplatelets) of the gold nanostructures produced, and these are controllable by adjusting the solution pH and gold ion concentration. The ability to control the shape and size of the gold nanostructures by changing the peptide structure and reaction conditions will lead to many potential applications, including nanoelectronics, sensors and optoelectronics, because of their unique size- and shape-dependent optical and electrical properties.

In-situ TEM observation of repeating events of nucleation in epitaxial growth of nano CoSi2 in nanowires of Si.

The formation of CoSi and CoSi2 in Si nanowires at 700 and 800 degrees C, respectively, by point contact reactions between nanodots of Co and nanowires of Si have been investigated in situ in a ultrahigh vacuum high-resolution transmission electron microscope. The CoSi2 has undergone an axial epitaxial growth in the Si nanowire and a stepwise growth mode was found. We observed that the stepwise growth occurs repeatedly in the form of an atomic step sweeping across the CoSi2/Si interface. It appears that the growth of a new step or a new silicide layer requires an independent event of nucleation. We are able to resolve the nucleation stage and the growth stage of each layer of the epitaxial growth in video images. In the nucleation stage, the incubation period is measured, which is much longer than the period needed to grow the layer across the silicide/Si interface. So the epitaxial growth consists of a repeating nucleation and a rapid stepwise growth across the epitaxial interface. This is a general behavior of epitaxial growth in nanowires. The axial heterostructure of CoSi2/Si/CoSi2 with sharp epitaxial interfaces has been obtained. A discussion of the kinetics of supply limited and source-limited reaction in nanowire case by point contact reaction is given. The heterostructures are promising as high performance transistors based on intrinsic Si nanowires.

Lossless hybridization between photovoltaic and thermoelectric devices

The optimal hybridization of photovoltaic (PV) and thermoelectric (TE) devices has long been considered ideal for the efficient harnessing solar energy. Our hybrid approach uses full spectrum solar energy via lossless coupling between PV and TE devices while collecting waste energy from thermalization and transmission losses from PV devices. Achieving lossless coupling makes the power output from the hybrid device equal to the sum of the maximum power outputs produced separately from individual PV and TE devices. TE devices need to have low internal resistances enough to convey photo-generated currents without sacrificing the PV fill factor. Concomitantly, a large number of p-n legs are preferred to drive a high Seebeck voltage in TE. Our simple method of attaching a TE device to a PV device has greatly improved the conversion efficiency and power output of the PV device (~30% at a 15°C temperature gradient across a TE device).

A gas nanosensor unaffected by humidity

The fabrication of a gas nanosensor for detecting ammonia gas in air that is unaffected by humidity is demonstrated. On functionalizing single-walled carbon nanotube (SWNT) networks with a precise amount of camphorsulfonic-acid-doped polyaniline (PANI(CSA)), the opposite electrical resistance responses of the CSA-doped PANI and SWNTs to humid air effectively canceled each other, thereby eliminating any effects of humidity during sensing experiments. The approach should be widely applicable for eliminating undesirable interferences for gas nanosensors.

Electrodeposited Ni1 − x Co x Nanocrystalline Thin Films Structure–Property Relationships

Electrodeposition of nanocrystalline Ni and Ni 1-x Co x thin films from chloride baths was systematically investigated by varying the electrodeposition parameters including electrolyte composition (i.e., Co 2+ ion concentration), additive (i.e., saccharin), solution pH, and current density. Their effects on the film growth mechanism, film composition, residual stress, microstructure, grain size, and surface morphology were studied. Ni 1-x Co x thin films electrodeposited from the baths without the addition of saccharin always showed tensile stress mode (145-367 MPa) with varying Co 2+ ion concentration, solution pH, and current density. In the presence of saccharin, the Ni 1-x CO x thin films showed either tensile stress or compressive stress mode, depending on the electrodeposition conditions. Especially, it was observed from a cross-sectional TEM observation that Ni thin film electrodeposited from the bath containing saccharin exhibited the formation of an amorphous Ni layer (about 300 nm thick) at the initial stage of the film growth. Also, Ni 1-x Co x thin films electrodeposited from the bath with/without the addition of saccharin showed the formation of the interface phase layer (about 10-110 nm thick), which has the chemical composition of 50 atom % Ni and 50 atom % Co.

Effect of Complex Agent on Characteristics of Copper Conductive Pattern Formed by Ink-jet Printing

In this study, Cu ion complex ink was successfully synthesized by a modified electrolysis method in which the Cu ions generated from bulk metal plates by an electric field were coordinated with complex agents. The synthesized ink was ink-jet-printed on a flexible substrate and converted to a dense Cu pattern after sintering at 250 °C. The pattern was characterized by X-ray diffractometry, field emission scanning electron microscope, and four-point probe method to confirm the crystal structure, microstructure, and electrical conductivity, respectively. The effect of the type of complex agent on the characteristics of a Cu conductive pattern was also determined using the analysis results. Finally, we conducted the direct writing of conductive dots and lines using the Cu ion complex ink, and confirmed that fine patterning for application in electronics is possible with the Cu ion complex ink.

Magnetic and Microwave Properties of NiFe Nanowires Embedded in Anodized Aluminum Oxide (AAO) Templates

Ferromagnetic NiFe nanowire arrays were fabricated on porous alumina templates by an electrochemical method. The nanowires were controlled to have different diameter (100, 200 nm) and length (10, 20, 30 ¿m) in order to understand the magnetostatic interactions between nanowires. Through the control of geometry of the arrays, the magnetization direction was shown to change from parallel to perpendicular to wire direction. The anisotropy field of these arrays was measured much higher than 1 kOe, which resulted in excellent microwave properties, particularly for noise suppression properties over 10 GHz.

Ultra-long bismuth telluride nanoribbons synthesis by lithographically patterned galvanic displacement

We demonstrated the wafer level batch synthesis and fabrication of single semiconducting thermoelectric nanoribbon based devices by Lithographically Patterned Galvanic Displacement (LPGD). The shape, composition, and dimension of nanoribbons were tailored by adjusting deposition conditions. High resolution TEM images with fast Fourier transform (FFT)-converted selected area electron diffraction (SAED) patterns confirmed the formation of polycrystalline Bi2Te3 intermetallic compound with a rhombohedral structure without elemental Te and Bi. The thickness dependent electrical resistivity of BixTey nanoribbons shows a classic size effect due to the increase in surface boundary scattering. The as-synthesized nanoribbons were n-type semiconductors with no clear trend between field effect carrier mobility and composition, which might be attributed to the trapped charges at the interface between the channel and dielectric layer. The preliminary results on thermoelectric properties (i.e. Seebeck coefficient and power factor) show that the Seebeck coefficient of as-synthesized 0.1 µm thick Bi30Te70 nanoribbon is comparable with bulk counterparts, however, the power factor was lower because of poor crystallinity which leads to higher resistivity.