Power/Performance Bits: Sept. 28

Pneumatic memory; nanowire recycling.


Pneumatic memory
Engineers at the University of California Riverside developed a pneumatic memory that can be used to control soft robots.

Pneumatic soft robots use pressurized air to move soft, rubbery limbs and grippers, making them ideal for delicate tasks as well as safer to be around. However, they still require electronic valves and computers to control and maintain positions.

The researchers note: “Pneumatic logic predates electronic computers and once provided advanced levels of control in a variety of products, from thermostats and other components of climate control systems to player pianos in the early 1900s. In pneumatic logic, air, not electricity, flows through circuits or channels and air pressure is used to represent on/off or true/false. In modern computers, these logical states are represented by 1 and 0 in code to trigger or end electrical charges.”

To enable soft robots to remember and maintain position, the team proposed pneumatic logic memory. The pneumatic RAM chip uses microfluidic valves instead of electronic transistors. The microfluidic valves were originally designed to control the flow of liquids on microfluidic chips, but they can also control the flow of air. The valves remain sealed against a pressure differential even when disconnected from an air supply line, creating trapped pressure differentials that function as memories and maintain the states of a robot’s actuators. Dense arrays of these valves can perform advanced operations and reduce the electronic hardware typically used to control pneumatic robots.

After modifying the microfluidic valves to handle larger air flow rates, the team produced an 8-bit pneumatic RAM chip able to control larger and faster-moving soft robots and incorporated it into a pair of 3D-printed rubber hands. The pneumatic RAM uses atmospheric-pressure air to represent a “0” or FALSE value, and vacuum to represent a “1” or TRUE value. The soft robotic fingers are extended when connected to atmospheric pressure and contracted when connected to vacuum.

As a proof of concept, the researchers used the soft robotic hand to play songs on a piano. The system could be used to operate other robots without any electronic hardware and only a battery-powered pump to create a vacuum. The researchers note that without positive pressure anywhere in the system, only normal atmospheric air pressure,  there is no risk of accidental overpressurization and violent failure of the robot or its control system.

Nanowire recycling
Researchers at North Carolina State University propose a way to construct recyclable wearable sensors.

“There is a lot of interest in recycling electronic materials because we want to both reduce electronic waste and maximize the use we get out of rare or costly materials,” said Yuxuan Liu, a Ph.D. student at NC State. “We’ve demonstrated an approach that allows us to recycle nanowires, and that we think could be extended to other nanomaterials — including nanomaterials containing noble and rare-earth elements.”

“Our recycling technique differs from conventional recycling,” said Yong Zhu, Professor of Mechanical and Aerospace Engineering at NC State. “When you think about recycling a glass bottle, it is completely melted down before being used to create another glass object. In our approach, a silver nanowire network is separated from the rest of the materials in a device. That network is then disassembled into a collection of separate silver nanowires in solution. Those nanowires can then be used to create a new network and incorporated into a new sensor or other devices.”

Key is using polymers that are soluble in solvents that will not also dissolve the nanowires. Once a device has been used, the polymer matrix containing the silver nanowires is dissolved, leaving behind the nanowire network. The network is then placed in a separate solvent and hit with ultrasound. This disperses the nanowires, separating them out of the network.

To test the idea, the team created a wearable health sensor patch capable of monitoring temperature and hydration using a silver nanowire network embedded in a polymer. After testing to make sure the sensor worked, it was dissolved in water. The nanowire network was broken down and then used to create a new wearable sensor. There was some degradation in the nanowire network, but it was able to be reused four times without harming performance.

“Using our approach, you get far more use from the nanowires,” Zhu said. “And even after the nanowires have broken down many times, to the point where they can’t be reused, we can still use them as feedstock for conventional recycling. It’s a tremendous reduction in waste.”

The concept works for other nanomaterials as well, Zhu said. “The approach we’ve demonstrated here could be used to recycle other nanomaterials — such as nanoparticles, carbon nanotubes, other types of nanowires, and two-dimensional materials — as long as they are used in the form of a network.”

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