Jaesam Sim

A flexible hybrid strain energy harvester using piezoelectric and electrostatic conversion

A new design of flexible energy harvester to utilize piezoelectric and electrostatic energy conversion mechanisms simultaneously from a single mechanical energy source is proposed. This non-resonant type harvester enables low-frequency mechanical inputs to be converted to electricity, and the polymeric structures make the harvester mechanically flexible, allowing it to be applied to non-planar surfaces. The fabricated harvester generated peak- and average power densities of 159 and 1.79 μW cm−2 respectively by piezoelectric conversion, and 52.9 μW cm−2 and 1.59 nW cm−2 respectively by electrostatic conversion from an input force of 1.2 N at 3 Hz. Considering its flexibility and ability to harvest mechanical inputs at frequencies below 3 Hz, low-frequency human movements could be a potential energy source for the proposed hybrid harvester to exploit.

Piezoelectric energy harvester converting strain energy into kinetic energy for extremely low frequency operation

In this study, we developed a flexible energy harvester that uses the frequency up-conversion mechanism. The harvester is composed of a flexible piezoelectric cantilever and substrate, and it can scavenge energy from deformation or strain by converting it into a mechanical vibration of the cantilever. We found experimentally that the output voltage of the harvester not affected by an input frequency as long as the strain was large enough, and there was no lower limit of the input frequency. The critical strain, i.e., the threshold radius of curvature of the harvester, could be modulated by adjusting magnetic force; therefore, it is possible to optimally apply the harvester to various deformation ranges. The maximum and average power density at 0.5 Hz of input frequency was measured to be 320 μW/cm2 and 6.8 μW/cm2 for a resistive load of 10 MΩ.

Suspended GaN nanowires as NO2 sensor for high temperature applications

We propose a gas sensor operable over a wide temperature range and using suspended GaN nanowires functionalized with Pt-Pd. The sensor is batch-fabricated by directly integrating the GaN nanowires onto batch-processed silicon microelectrodes in parallel. The high thermal stability of the sensor originates from a large band gap of GaN nanowires that enables the detection of NO2 gas at an elevated temperature of up to 350 °C without a decrease in responsiveness. Exposed to NO2 at 100-1000 ppm at 350 °C, the sensor shows a linear increment in relative response with respect to the change in gas concentration. The sensor results in a two- to four-fold increase in responsiveness to NO2 at 100 ppm compared to NH3 at 100 ppm and CO2 at 1000 ppm. The nanowires suspended over a substrate provide increased surface area that could interact with gas molecules for enhanced responsiveness, and prevent any unnecessary interactions between the nanowires and the substrate.

An electrodynamic preconcentrator integrated thermoelectric biosensor chip for continuous monitoring of biochemical process

This paper proposes an integrated sensor chip for continuous monitoring of a biochemical process. It is composed of a preconcentrator and a thermoelectric biosensor. In the preconcentrator, the concentration of the injected biochemical sample is electrodynamically condensed. Then, in the downstream thermoelectric biosensor, the preconcentrated target molecules react with sequentially injected capture molecules and generate reaction heat. The reaction heat is detected based on the thermoelectric effect, and an integrated split-flow microchannel improves the sensor stability by providing ability to self-compensate thermal noise. These sequential preconcentration and detection processes are performed in completely label-free and continuous conditions and consequently enhance the sensor sensitivity. The performance of the integrated biosensor chip was evaluated at various flow rates and applied voltages. First, in order to verify characteristics of the fabricated preconcentrator, 10 ?m -diameter polystyrene (PS) particles were used. The particles were concentrated by applying ac voltage from 0 to 16 Vpp?at 3 MHz at various flow rates. In the experimental result, approximately 92.8% of concentration efficiency was achieved at a voltage over 16 Vpp?and at a flow rate below 100 ?l h?1. The downstream thermoelectric biosensor was characterized by measuring reaction heat of biotin?streptavidin interaction. The preconcentrated streptavidin-coated PS particles flow into the reaction chamber and react with titrated biotin. The measured output voltage was 288.2 ?V at a flow rate of 100 ?l h?1?without preconcentration. However, by using proposed preconcentrator, an output voltage of 812.3 ?V was achieved with a 16 Vpp-applied preconcentration in the same given sample and flow rate. According to these results, the proposed label-free biomolecular preconcentration and detection technique can be applied in continuous and high-throughput biochemical applications.

Frequency Tuning of Nanowire Resonator Using Electrostatic Spring Effect

We have demonstrated resonant frequency tuning of nanowire resonator operated in both atmospheric circumstance and high vacuum environment using electrostatic spring-softening effect. The nanowire is synthesized at any desired position by focused ion beam-chemical vapor deposition (FIB-CVD) on the sidewall of batch-processed micro electrode. The resonant frequency in a vacuum chamber of 2.5 times 10-4 Pa is 1.564 MHz under the driving voltage of 5 VDC plusmn5 VAC. When 30 VDC tuning bias is applied on tuning electrode, the resonant frequency is reduced to 1.529 MHz due to the electrostatic spring-softening effect while the driving bias is maintained. For the tuning bias of 60 V, the resonant frequency at atmospheric pressure has been tuned from 1.49 MHz to 1.41 MHz under the driving voltage of 30 VDC plusmn 10 VAC. The method demonstrated both in high vacuum and at atmospheric pressure is a simple and effective way to tune the deviated resonant frequency of nanowire resonator to the desired value without the alteration of the structure or post fabrication process.

Acid-sensitive pH sensor using electrolysis and a microfluidic channel for read-out amplification

We demonstrated an acid-sensitive pH sensor based on a novel and simple sensing mechanism. Electrolysis bubbles and a microfluidic channel are used for read-out amplification. The amount of hydrogen bubbles generated through electrolysis varies with the pH levels of a buffer solution. The change in volume with respect to pH levels is measured electrically through sensing electrodes. To maximize the change of the pushed liquid column length for a given bubble volume, a microfluidic channel with small cross-sectional area was fabricated and integrated onto the sensing components. For a stronger acid, the change in current increased with increasing liquid column length and higher liquid conductivity. The interplay of these two effects enhanced sensor output, increasing sensitivity at lower pH levels in our sensor.

An electrodynamic preconcentrator-integrated thermoelectric biosensor chip for continuous monitoring

An integrated thermoelectric biosensor chip is being proposed for continuous monitoring of biochemical process. It is composed of a preconcentrator and a thermoelectric biosensor. In the preconcentrator, the concentration of the biochemical sample is electrodynamically condensed, which results in enhancement of the sensitivity of the integrated sensor. Then, in the sensor, the preconcentrated target molecules interact with injected capture molecules and generate reaction heat. The reaction heat is detected based on thermoelectric effect with an integrated split-flow micro channel which provides self-compensation of thermal noise. In the fabricated biochip, 95.8% of the sample was concentrated to be 2.88-fold and detected with the sensitivity of 54.7V/molarity in biotin-streptavidin application