Researchers in the US have developed tiny liquid pistons that could be used in drug delivery devices or as low-energy camera lenses. The technology uses drops of liquid filled with metal nanoparticles (ferrofluid) that move back and forth in the presence of an electromagnet and can displace a surrounding liquid.


This motion can be used to pump small volumes of liquid, for example in a microfluidic lab-on-a-chip system such as those used to analyse blood samples. The pistons are tuneable and scalable, and don’t degrade because they lack any solid moving parts.


When light is passed through the liquid they can also act as a lens that changes focus as the droplets moves.


‘It is possible to make mechanical pumps that are small enough for use in lab-on-a-chip applications, but it’s a very complex, expensive proposition,’ said research leader Professor Amir H Hirsa of the Rensselaer Polytechnic Institute, New York.


‘Our electromagnetic liquid pistons present a new strategy for tackling the challenge of microscale liquid pumping. Additionally, we have shown how these pistons are well-suited for chip-level, fast-acting adaptive liquid lenses.’


The piston is comprised of two ferrofluid droplets, each positioned in a hole in a substrate about the size of a piece of chewing gum.


Pulses from an electromagnet cause one of the ferrofluid droplets, the driver, to vibrate back and forth. This vibration prompts a combination of magnetic, capillary and inertial forces that cause the second droplet to vibrate in an inverted pattern.


The two droplets create a piston, resonating back and forth with great speed and a spring-like force. Researchers can finely control the strength and speed of these vibrations by exposing the driver ferrofluid to different magnetic fields.


In this way, the droplets become a liquid resonator, capable of moving the surrounding liquid back and forth from one chamber to another. Similarly, the liquid piston can also function as a pump.


The pistons could be used in micro displacement pumps and liquid switches, or even integrated into an implantable device that very accurately releases tiny, timed doses of drugs into the body of a patient.


The team have also used the droplets to create a miniature camera lens that changes focus as the liquid moves and that could reduce the energy needed by modern digital camera lenses.


As the droplets move and the lens continues to change focus, software can be used to edit out unfocused frames, leaving the user with a stream of clear video about the quality of a typical computer web cam.


But still-focused lenses can also be created using pistons with one drop larger than the other to make them behave like switches.


‘By toggling a desired number of the bi-stable ferrofluid switches we force the liquid lens to take the desired shape,’ Hirsa told The Engineer. ‘Once toggled, the switches being bi-stable keep their new state until the next time they are toggled.’


Hirsa added that this technology could even one day lead to replacement eye lenses that can be fine-tuned using only high-powered magnets.

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