Q&A: Microscope “Traffic Jam” Solutions Stimulate New Insights on Particle Movement and Drug Delivery

From microscope robots that can carry drugs within the human body to small particles that can detect and decompose microplastics, a new field called active matter is turning to microscales to solve some of the world’s biggest problems.

Stewart Mallory, an assistant professor of chemical and chemical engineering in Pennsylvania, leads a research group studying active problems, particularly collective behavior of self-administered microscopic particles. The group’s goal is to develop theoretical and computational tools to control the behavior of materials at a microscale, and ultimately design new materials and devices.

In the Journal of Chemical Physics, the team published a paper describing solutions to common problems in microengineering, an area focused on the design and creation of small machines and devices. Mallory talked about his research and field more generally in the Q&A below.

What were the microengineering problems and what was your solution?

A major issue when designing things that move, large and small, is how they change when placed in a trapped environment. Essentially, I want to know if an object starts at its initial position and how far it travels at a certain time interval. I’m interested in the problem when objects are limited to narrow channels and cannot be passed to each other. If something starts to move and you need to know how far you are later, then you need to be able to solve this problem.

This is a very old problem in statistical physics called single file dynamics, and in fact it appears in many places outside chemistry and physics. When you’re in a line or get caught in traffic, you can’t pass on someone next door, you’re moving to a single file and asking yourself how long it will take you to get where you want to go. That’s the problem we focused on solving.

When we’re talking about small particle sized robots, they are used in limited environments, such as supplying medicines to the bloodstream and various locations within the body. Before deploying these systems, simulations must first be performed to understand how these microscope swimmers behave in complex environments. You need to predict where they will travel and how long it will take them to get there. If they encounter a single file situation, they need to be considered at that time, so we have derived an equation to convey it.

Has this microscopic advancement changed anything about how we understand the world of human scale?

It definitely changed my perspective as a driver. Driving on two-lane roads is a good example of the dynamics of a single file, as cars cannot pass through each other. If you’ve driven your car and it appears to be stopped for no reason, it’s called a “phantom traffic jam.” The deceleration of these traffic occurs spontaneously and is usually caused by small variations in the speed or interval of the vehicle that amplifies over time due to human reaction times and delayed braking or acceleration.

In the work on active particles moving in narrow channels, a similar behavior was observed that led to particles clustering together and slowing down. Yes, this paper made me think more about traffic.

Before tackling this single file issue, we have published a paper showing potential ways to tune Janus particles that you care about. What are they, why are they important, why do you want to adjust them?

About 20 years ago, a team of Pennsylvania scientists invented these spontaneous nanoparticles, calling them Horetic Janus particles. They are these small particles, usually micron size or 100 times smaller than the width of a human hair, allowing them to propel themselves through the liquid.

Their surfaces are made up of two chemically distinct regions, and therefore have the name “Janus,” the Roman god of duality and transition. Its duality allows you to create and maintain chemical gradients around you in a way that allows for self-plants. Imagine a small submarine with two sides. One pushes out the water and the other pulls in. This creates a flow that propels the submarine forward. This is similar to how these particles work. By adjusting them, you can control how and where it moves according to the chemical signal.

Why do you want to concentrate on these particles and what have you discovered about them?

Firstly, it is interesting to me that they were discovered and designed here in Pennsylvania, and are now being studied all over the world. It also makes sense to focus on them as I am inherently interested in particles that can swim. These small microswimmers have a wide range of applications. In cases like targeted drug delivery, it can work within the body or clean up environmental issues to break down harmful chemicals, bacteria, or microplastics.

Because they are relatively new tools, our group is working to understand how these particles behave, how they self-machine, what fuels they use and what fuels will change the dynamics.

In general, these particles are ideal for applications that require targeted microscopic movement. Also, unlike passive particles that rely on external forces, the Phoretic Janus particles produce their own movements. This means that you can understand how to “drive” them by adjusting the chemical composition of the two surface regions of the particles.

In line with the ratio phosphor of its operation, what fuel does the particles use?

Different fuel sources move depending on the composition of the particles. For example, hydrogen peroxide can be used as a fuel for particles with metallic regions, while other enzyme-coated particles can use bio-based fuels such as glucose.

However, there are also interactions between particles that can affect movement. Therefore, our work focuses on understanding particle behavior at two levels: individual and group. We discussed individual levels that aim to control and accurately simulate the behavior of a single Janus particle using advanced calculation methods.

At the population level, we study how behavior changes when many particles interact and explore the dynamics of group behavior. Ultimately, our goal is to integrate these approaches and develop highly accurate simulations that capture the interactions of many particles in complex systems.

What are some potential applications for your research?

I’ll give you something very specific. There are nanoparticles made of calcium carbonate that respond to pH gradients produced by cancer cells, allowing them to swim towards cancer cells. Accurate particle design allows us to build things like molecules released by cancer cells that are essentially microscopic robots that can sense and move towards specific biological signals. At some point in the not too distant future, these particles were able to use them to have a payload of drugs and target harmful cells like cancer.

This concept can be extended to other applications, such as using particles to detect and collect microplastics, providing potential solutions for environmental cleanup.

You are also studying material research, so how does your research apply to that field?

This relates to aspects of collective behavior in our work. Nanoparticles are self-assembled. This is how nature builds structures, creating a larger part.

Our research shows that spontaneous particles enhance this process and that self-assembly becomes a more effective tool for building on a microscale. The idea is that building blocks can be designed and hanged into solutions containing spontaneous particles, ideally they will spontaneously form the structure of interest.

What is your lab currently working on? What is your next step?

We are developing theory and calculation modes to better understand how these particles behave in different environments. This is necessary when developing microscale devices for applications such as chemical and drug delivery.

As the work we do with Janus particles contributes to a broader field of research focusing on systems consisting of self-processed particles and their collective actions, the advances in group discovery have an impact across active issues. The first step we take is to understand and manipulate problems on a microscale.