Microscale acoustic “rockets” navigate the human body


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Engineers turn bubbles into motors to propel minute vessels through landscapes of cells and particles suspended in fluid

Ever since nanotechnology became a real branch of engineering, its practitioners have been trying to design tiny structures that can work like submarines to navigate through the human body.

One stumbling block towards this goal has been what fuels and motor analogues could be used to propel and steer such nano-vessels around and inside blood vessels and organs without causing harm.

Researchers at Pennsylvania State University and the University of San Diego hit a wall with their research, because they were using toxic materials like hydrogen peroxide as fuel. A fortuitous discovery about the behaviour of bubbles has opened up a new avenue for their research, as they describe in Science Advances.

Working with material scientists at the Harbin Institute of technology in Shenzhen and surgeons at University of Michigan, Thomas Mallouk of the Department of Chemistry at Penn State was trying to move nano-vessels with acoustic levitation, a technique used to lift particles off microscope slides. Unexpectedly, he found that high-frequency sound waves made the vessels move at very fast speeds. Investigating this phenomenon further, Mallouk and his team designed microscale “rockets” that can use acoustics to zip around and steer in a liquid medium.

microrocket

The rockets are not rocket-shaped. They resemble a round-bottomed cup 10µm in length and 5µm wide, 3D printed from a polymer and coated with a 10nm-thick layer of nickel and a 40nm-thick layer of gold.

The inside of the cup is then coated with trichlorosilane, which repels water. When submerged in fluid, an air bubble spontaneously forms inside the cup. When bombarded with ultrasound waves, the bubble vibrates, turning it into a motor and propelling it through the fluid. The vessel can be steered with precision by manipulating an external magnetic field. Each rocket has a characteristic resonant frequency, so individual vessels can be driven independently.

Steering of the vessels is so precise that Mallouk’s team made them move up microscopic staircase structures. The addition of fins to the cup structures allows them to be steered freely in three dimensions.

Moreover, the team describes using the vessels to push other particles or cells around, or tow them with precision through a crowded environment. The key to this is the small size of the vessels, Mallouk claims.

“This wasn’t available on a larger scale,” he said. “There’s a lot of control you can do at this length scale. At this particular length scale, we’re right at the crossover point between when the power is enough to affect other particles.”

Changing the acoustic stimulation adjusts the speed of the vessels. “If I want it to go slow, I can turn the power down, and if I want it to go really fast, I can turn the power up,” explains Jeff McNeill, a graduate student who works on nano-and microscale motor projects. “That’s a really useful tool.”

Mallouk is working with engineers and roboticists at Penn to equip the vessels with computer chips and sensors to give them autonomy and intelligence, which would allow them to be used for tasks including imaging and even surgery. “We’d like to have controllable robots that can do tasks inside the body: delivering medicine, diagnostic snooping,” he said.

Penn State U. – New ‘Flow-Cell’ Battery Recharged with Carbon Dioxide – Capturing CO2 Emissions for an Untapped Source of Energy


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The pH-gradient flow cell has two channels: one containing an aqueous solution sparged with carbon dioxide (low pH) and the other containing an aqueous solution sparged with ambient air (high pH). The pH gradient causes ions to flow across …more

Researchers have developed a type of rechargeable battery called a flow cell that can be recharged with a water-based solution containing dissolved carbon dioxide (CO2) emitted from fossil fuel power plants. The device works by taking advantage of the CO2 concentration difference between CO2 emissions and ambient air, which can ultimately be used to generate electricity.

The new flow cell produces an average power density of 0.82 W/m2, which is almost 200 times higher than values obtained using previous similar methods. Although it is not yet clear whether the process could be economically viable on a large scale, the early results appear promising and could be further improved with future research.

The scientists, Taeyong Kim, Bruce E. Logan, and Christopher A. Gorski at The Pennsylvania State University, have published a paper on the new method of CO2-to-electricity conversion in a recent issue of Environmental Science & Technology Letters.

“This work offers an alternative, simpler means to capturing energy from CO2 emissions compared to existing technologies that require expensive catalyst materials and very high temperatures to convert CO2 into useful fuels,” said Gorski.

While the contrast of gray-white smoke against a blue sky illustrates the adverse environmental impact of burning , the large difference in CO2 concentration between the two gases is also what provides an untapped energy source for generating electricity.fossil-fuels-co2-to-green-images

In order to harness the potential energy in this concentration difference, the researchers first dissolved CO2 gas and in separate containers of an aqueous solution, in a process called sparging. At the end of this process, the CO2-sparged solution forms bicarbonate ions, which give it a lower pH of 7.7 compared to the air-sparged solution, which has a pH of 9.4.

After sparging, the researchers injected each solution into one of two channels in a flow cell, creating a pH gradient in the cell. The flow cell has electrodes on opposite sides of the two channels, along with a semi-porous membrane between the two channels that prevents instant mixing while still allowing ions to pass through. Due to the pH difference between the two solutions, various ions pass through the membrane, creating a voltage difference between the two electrodes and causing electrons to flow along a wire connecting the electrodes.

After the flow cell is discharged, it can be recharged again by switching the channels that the solutions flow through. By switching the solution that flows over each electrode, the charging mechanism is reversed so that the electrons flow in the opposite direction. Tests showed that the cell maintains its performance over 50 cycles of alternating solutions.

The results also showed that, the higher the pH difference between the two channels, the higher the average power density. Although the pH-gradient flow cell achieves a power density that is high compared to similar cells that convert waste CO2 to electricity, it is still much lower than the power densities of fuel cell systems that combine CO2 with other fuels, such as H2.

However, the new flow cell has certain advantages over these other devices, such as its use of inexpensive materials and room-temperature operation. These features make the flow cell attractive for practical applications at existing .

“A system containing numerous identical flow cells would be installed at power plants that combust fossil fuels,” Gorski said. “The flue gas emitted from fossil fuel combustion would need to be pre-cooled, then bubbled through a reservoir of water that can be pumped through the flow cells.”

In the future, the researchers plan to further improve the flow cell performance.

“We are currently looking to see how the solution conditions can be optimized to maximize the amount of energy produced,” Gorski said. “We are also investigating if we can dissolve chemicals in the water that exhibit pH-dependent redox properties, thus allowing us to increase the amount of energy that can be recovered. The latter approach would be analogous to a flow battery, which reduces and oxidizes dissolved chemicals in aqueous solutions, except we are causing them to be reduced and oxidized here by changing the solution pH with CO2.”

Explore further: Chemists present an innovative redox-flow battery based on organic polymers and water

More information: Taeyoung Kim et al. “A pH-Gradient Flow Cell for Converting Waste CO2 into Electricity.” Environmental Science & Technology Letters. DOI: 10.1021/acs.estlett.6b00467