Byung Hwan Chu

The control of cell adhesion and viability by zinc oxide nanorods

The ability to control the behavior of cells that interact with implanted biomaterials is desirable for the success of implanted devices such as biosensors or drug delivery devices. There is a need to develop materials that can limit the adhesion and viability of cells on implanted biomaterials. In this study, we investigated the use of zinc oxide (ZnO) nanorods for modulating the adhesion and viability of NIH 3T3 fibroblasts, umbilical vein endothelial cells, and capillary endothelial cells. Cells adhered far less to ZnO nanorods than the corresponding ZnO flat substrate. The few cells that adhered to ZnO nanorods were rounded and not viable compared to the flat ZnO substrate. Cells were unable to assemble focal adhesions and stress fibers on nanorods. Scanning electron microscopy indicated that cells were not able to assemble lamellipodia on nanorods. Time-lapse imaging revealed that cells that initially adhered to nanorods were not able to spread. This suggests that it is the lack of initial spreading, rather than long-term exposure to ZnO that causes cell death. We conclude that ZnO nanorods are potentially useful as an adhesion-resistant biomaterial capable of reducing viability in anchorage-dependent cells.

Contributions of surface topography and cytotoxicity to the macrophage response to zinc oxide nanorods.

Macrophages associated with implanted biomaterials are primary mediators of chronic inflammation and foreign body reaction to the implant. Hence, various approaches have been investigated to modulate macrophage interactions with biomaterial surfaces to mitigate inflammatory responses. Nanostructured materials possess unique surface properties, and nanotopography has been reported to modulate cell adhesion and viability in a cell type-dependent manner. Zinc oxide (ZnO) has been investigated in a number of biomedical applications and surfaces presenting well-controlled nanorod structures of ZnO have recently been developed. In order to investigate the influence of nanotopography on macrophage adhesive response, we evaluated macrophage adhesion and viability on ZnO nanorods, compared to a relatively flat sputtered ZnO controls and using glass substrates for reference. We found that although macrophages are capable of initially adhering to and spreading on ZnO nanorod substrates, the number of adherent macrophages on ZnO nanorods was reduced compared to ZnO flat substrate and glass. Additionally adherent macrophage number on ZnO flat substrate was reduced as compared to glass. While these data suggest nanotopography may modulate macrophage adhesion, reduced cell viability on both sputtered and nanorod ZnO substrate indicates appreciable toxicity associated with ZnO. Cell death was apparently not apoptotic, given the lack of activated caspase-3 immunostaining. A decrease in viable macrophage numbers when ZnO substrates were present in the same media verified the role of ZnO substrate dissolution, and dissolved levels of Zn in culture media were quantified. In order to determine long-term physiological responses, ZnO nanorod-coated and sputtered ZnO-coated polyethylene terephthalate (PET) discs were implanted subcutaneously in mice for 14 d. Upon implantation, both ZnO-coated discs resulted in a discontinuous cellular fibrous capsule indicative of unresolved inflammation, in contrast to uncoated PET discs, which resulted in typical foreign body capsule formation. In conclusion, although ZnO substrates presenting nanorod topography have previously been shown to modulate cellular adhesion in a topography-dependent fashion for specific cell types, this work demonstrates that for primary murine macrophages, cell adhesion and viability correlate to both nanotopography and toxicity of dissolved Zn, parameters which are likely interdependent.

Randomly oriented, upright SiO2 coated nanorods for reduced adhesion of mammalian cells.

Cell interactions with nanostructures are of broad interest because of applications in controlling tissue response to biomedical implants. Here we show that dense and upright SiO2 coated nanorods nearly eliminate cell adhesion in fibroblasts and endothelial cells. The lack of adhesion is not due to a decrease in matrix protein adsorption on the nanostructures, but rather an inability of cells to assemble focal adhesions. Using spatially patterned nanorods, we show that cells display a preference for flat regions of the surface. Our results support a model in which interfering with nanoscale spacing of ligated integrins results in reduced cell adhesion and subsequent cell death. We propose that dense monolayers of nanorods are a promising nanotechnology for preventing mammalian cell fouling of biomaterials.

Enzyme-based lactic acid detection using AlGaN∕GaN high electron mobility transistors with ZnO nanorods grown on the gate region

The detection of lactic acid with ZnO nanorod-gated AlGaN∕GaN high electron mobility transistors (HEMTs) was demonstrated. The array of ZnO nanorods provided a large effective surface area with a high surface-to-volume ratio and a favorable environment for the immobilization of lactate oxidase. The HEMT drain-source current showed a rapid response when various concentrations of lactic acid solutions were introduced to the gate area of the HEMT sensor. The HEMT could detect lactic acid concentrations from 167nM to 139μM. Our results show that portable, fast response, and wireless-based lactic acid detectors can be realized with AlGaN∕GaN HEMT based sensors.

Effect of Coated Platinum Thickness on Hydrogen Detection Sensitivity of Graphene-Based Sensors

The effect of Pt metal thickness on the hydrogen sensing sensitivity of Pt-coated, multi-layered graphene grown by chemical vapor deposition on Si-polar 4H-SiC, was investigated. As-grown graphene samples and graphene samples coated with 1 or 4 nm thick Pt films were used in this study. Compared to graphene without platinum, significantly improved hydrogen detection sensitivity was observed with the addition of platinum films. The highest hydrogen sensitivity was observed with the graphene sensor with 1 nm platinum coating. The platinum coated graphene sensor also showed good selectivity for hydrogen detection over methane, ammonia, oxygen, and nitrogen oxide.

Botulinum toxin detection using AlGaN∕GaN high electron mobility transistors

Antibody-functionalized, Au-gated AlGaN∕GaN high electron mobility transistors (HEMTs) were used to detect botulinum toxin. The antibody was anchored to the gate area through immobilized thioglycolic acid. The AlGaN∕GaN HEMT drain-source current showed a rapid response of less than 5s when the target toxin in a buffer was added to the antibody-immobilized surface. We could detect a range of concentrations from 1to10ng∕ml. These results clearly demonstrate the promise of field-deployable electronic biological sensors based on AlGaN∕GaN HEMTs for botulinum toxin detection.

Effect of humidity on hydrogen sensitivity of Pt-gated AlGaN/GaN high electron mobility transistor based sensors

The effects of relative humidity on sensing characteristics of Pt-gated AlGaN/GaN high electron mobility transistor diode based hydrogen sensors were investigated. The absorbed water and oxygen molecules blocked available Pt surface adsorption sites for H2 absorption and reduced the hydrogen sensing sensitivity compared to low humidity conditions. The hydrogen sensing sensitivity decreased proportional to the relative humidity. However, the presence of humidity improved the sensor recovery characteristics after exposure to the hydrogen ambient.

Electric-Field-Driven Degradation in off-State Step-Stressed AlGaN/GaN High-Electron Mobility Transistors

The critical degradation voltage of AlGaN/GaN high-electron mobility transistors during off-state electrical stress was determined as a function of Ni/Au gate dimensions (0.1-0.17 μm), drain bias voltage, and source/drain-gate contact distance. Devices with different gate lengths and gate-drain distances were found to exhibit the onset of degradation at different source-drain biases but similar electric field strengths, showing that the degradation mechanism is primarily field driven. The degradation field was calculated to be ~ 1.8 MV/cm by Automatically Tuned Linear Algebra Software simulations. Transmission electron microscopy imaging showed creation of defects under the gate after dc stress.

Fast detection of a protozoan pathogen, Perkinsus marinus, using AlGaN/GaN high electron mobility transistors

Antibody-functionalized, Au-gated AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect Perkinsus marinus. The antibody was anchored to the gate area through immobilized thioglycolic acid. The AlGaN/GaN HEMT drain-source current showed a rapid response of less than 5 s when the infected solution was added to the antibody-immobilized surface. The sensor can be recycled with a phosphate buffered saline wash. These results clearly demonstrate the promise of field-deployable electronic biological sensors based on AlGaN/GaN HEMTs for Perkinsus marinus detection.

Wireless Detection System for Glucose and pH Sensing in Exhaled Breath Condensate Using AlGaN/GaN High Electron Mobility Transistors

Peltier element cooling of ungated AlGaN/GaN high electron mobility transistors (HEMTs) is shown to be an effective method for condensing exhaled breath, enabling the measurement of the pH and glucose of the exhaled breath condensate (EBC). By comparison with standard solutions, the current change measured in the HEMTs with EBC shows that the sensitivity of the glucose detection is lower than the glucose concentration in the EBC of healthy human subjects and the pH of the condensate from the exhaled breath is within the range of 7-8, typical of that for human blood. The HEMT sensors can be integrated into a wireless data transmission system that allows for remote monitoring. Details of the transmitter and receiver design for the transmission system are given. Our work demonstrates the possibility of using AlGaN/GaN HEMTs for extended investigations of airway pathology without the need for clinical visits.