Feature Story
Training micro-robots to swim in complex fluids
Published on 12 May 2021

Fluid mechanics scientists are realising targeted drug delivery by designing tiny robots that can swim through biological fluids.

The field of fluid mechanics is most often associated with how large vehicles such as aeroplanes or submarines move through air or water. But for Dr On Shun Pak (Croucher Scholarship 2010 and Fellowship 2013), it involves working at the microscopic level.

His aim is to understand how microorganisms, such as bacteria or sperm cells, move in fluids, then apply that knowledge to developing micro-robots that can mimic these swimming microorganisms. Such artificial micro-swimmers offer exciting potential for biomedical applications.

The way that microorganisms “swim” differs from a human approach because there is no inertia in their microscopic world where viscous force dominates. “It’s more like trying to swim in a pool of honey,” said Pak, a graduate of the University of Hong Kong who is now assistant professor in the Department of Mechanical Engineering at Santa Clara University, California, USA.

Microorganisms have evolved strategies to swim in such a viscous environment. Pak explained that scientists are now trying to understand the physics governing these strategies and apply it to our understanding of biological processes such as reproduction and bacterial infections.

Engineers can also apply the knowledge to design micro-robots that can swim effectively, just like microorganisms. Potential applications include the use of these micro-robots for drug delivery in cancer treatment, which is particularly meaningful for Pak, who lost his brother to the disease.

Currently, chemotherapy is often used to kill cancerous cells, but its broad reach destroys non-cancerous cells as well, Pak said. Micro-robots could transport cancer drugs directly to the cancerous cells, without impacting healthy cells. Such targeted drug delivery could potentially reduce treatment side effects and enhance its efficacy, Pak believes.

He identified another potential application in microsurgeries, where micro-robots enable access to locations in the human body that are difficult or impossible to reach. For instance, doctors could insert and direct micro-robots to break up blood clots.

One challenge for such applications is to design micro-robots with strong enough propulsion in thick biological fluids. Despite progress over the past decades in the development of micro-swimmers, previous studies have assumed the body’s fluids behaved the same as simple Newtonian fluids, such as water and air. Yet, biological fluids, such as blood and mucus, contain suspended blood cells or polymers, resulting in complex (non-Newtonian) fluid behaviours.

In order to design micro-robots to propel effectively in realistic biological fluids, scientists must first understand how these complex fluid properties affect propulsion. This is the challenge that Pak is tackling.

Pak and his research partner, Wei Gao, of California Institute of Technology, have recently received a grant from the US National Science Foundation for their combined theoretical and experimental investigation of swimming in complex fluids. The team previously developed micro-swimmers with propulsion speeds comparable to naturally-occurring bacteria, and demonstrated the use of these swimmers in drug delivery.

Building on that successful collaboration, the team will blend expertise in fluid mechanics, nanoengineering, and applied mathematics to develop the next generation of micro-robots with locomotive capabilities strong enough to swim in realistic biological fluids.

Pak says that the most interesting part of his work is seeing how well mathematics and different laws of physics can describe the world around us at both macro and micro levels. “An exciting moment is when you see a mathematical model – what you do by hand with a pencil – actually predicts the experimental results in the laboratory,” he said.

His time at Santa Clara University is split between his research and teaching. “You get to work closely with students, see them grow and advance their career after SCU. That’s almost the best part of this job,” he said.

On Shun Pak obtained a BEng in Mechanical Engineering from the University of Hong Kong in 2008 and a PhD in Mechanical Engineering from the University of California, San Diego, in 2013. After his PhD studies, he continued his research in fluid mechanics as a Postdoctoral Research Fellow in the Department of Mechanical and Aerospace Engineering at Princeton University from 2013 to 2014, before joining the Department of Mechanical Engineering at Santa Clara University in California. He received the Siebel Scholar Award in Bioengineering in 2013, and a Croucher Scholarship in 2010 and a Croucher Fellowship in 2013.

Extended Reading:

  1. Dr Pak‘ personal page (The Croucher Foundation): https://scholars.croucher.org.hk/scholars/pak-on-shun/
  2. Dr Pak’s personal page (Santa Clara University in California): https://www.scu.edu/engineering/faculty/pak-on-shun/