Improving Pinched Flow Fractionation through Hybrid Inlet Design for Label-Free Size-Based Separation of Microalgae
This study presents a hybrid-inlet pinched flow fractionation microfluidic chip that overcomes particle retention and focusing instability to achieve highly efficient, reproducible, and label-free size-based separation of microalgae with sorting rates exceeding 90% and purities above 92%.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
In the microscopic world of water, tiny plants called microalgae form the foundation of aquatic life. These single-celled organisms are vital for capturing carbon, cleaning wastewater, and potentially fueling the future, but studying them or using them for industry requires a difficult first step: separating specific types from a messy mixture. Natural water samples are crowded with cells of different sizes, shapes, and densities, along with debris that clogs traditional filters or damages delicate cells during spinning. Scientists have long sought a way to sort these tiny life forms without using labels, chemicals, or expensive machinery that requires heavy equipment. The challenge lies in creating a method that is gentle, continuous, and precise enough to tell a small cell from a large one just by how it moves through a fluid.
Researchers at National Tsing Hua University in Taiwan have developed a new approach to this problem using a technique called pinched flow fractionation. Imagine a narrow hallway where a crowd of people is being pushed against one wall by a fast-moving stream of air; the people standing closest to the wall are squeezed tight, while those standing a few steps back have more room. In this microfluidic device, a sample of water containing algae is squeezed against the side of a tiny channel by a faster stream of clean water. Because larger cells cannot get as close to the wall as smaller ones, they end up in slightly different lanes. When the channel suddenly widens, these lanes spread apart, allowing the different sizes to flow into separate exits. While this concept is not new, previous versions of these devices often struggled when handling real biological samples. The cells would get stuck at the entrance, or the flow would become unstable, causing the sorting to fail.
To solve these issues, the team designed a microfluidic chip with a modified entrance. They tested three different ways to open the inlet where the sample enters the device. One design left the opening partially blocked, which helped guide the cells but caused too many to get stuck. Another design left the opening fully wide, which reduced sticking but made the starting position of the cells too unpredictable. The researchers found that a "hybrid" design, which combined features of both, offered the best balance. This new entrance allowed the cells to enter smoothly without clogging while keeping them tightly focused in their lanes. They also adjusted the length of the tubes connected to the exits and added a tiny amount of a slippery substance to the water to prevent the cells from sticking to the sides of the device.
The team first tested their device using plastic beads of two distinct sizes, measuring 3 and 15 AU. By carefully adjusting the speed of the sample flow compared to the sheath flow, they were able to direct the small beads almost entirely to one exit and the large beads to the other. The device successfully sorted the small beads with a success rate of over 90 percent and recovered nearly all of the beads from the system, meaning very few were lost. This confirmed that the new entrance design worked as intended for simple, round objects.
Encouraged by these results, the researchers moved on to real biological samples. They selected two types of microalgae: Nannochloropsis sp., which are small and roughly spherical, measuring between 3 and 5 AU, and Thalassiosira weissflogii, which are larger and more irregular, measuring between 11 and 16. Unlike the uniform plastic beads, these living cells vary in shape and can rotate as they move, making them harder to predict. The researchers found that the optimal flow speed for the larger algae was slightly different from that of the plastic beads, requiring a specific ratio of 1 part sample to 9 parts sheath fluid. Under these conditions, the device successfully separated the two species. It directed 90.3 percent of the small Nannochloropsis cells to the correct outlet and 92.0 percent of the larger Thalassiosira cells to their own. The purity of the collected samples was high, with over 96 percent of the small cells and 92 percent of the large cells being the correct type.
Crucially, the researchers repeated the experiment four times to ensure the results were reliable. The performance remained consistent across all trials, with very little variation in the sorting success or the number of cells recovered. This repeatability suggests that the device is robust enough for practical use. The study demonstrates that by simply refining the geometry of the entrance and the flow conditions, it is possible to create a passive, label-free system that separates microalgae based on size with high efficiency. While the device currently relies on size differences and may struggle with species that are the same size, it offers a simple, low-cost, and gentle method for preparing biological samples, potentially aiding in environmental monitoring and the cultivation of algae for various industrial applications.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.