A recent article in Microsystems & Nanoengineering introduces a low-cost method for optical sensing in microfluidic systems. The researchers used light-dependent resistors (LDRs) as optical detectors in centrifugal Lab-on-a-Disc (eLOD) platforms. Their approach aims to support a range of tasks, including droplet and particle detection, measuring fluid speed, and tracking sample volume in real time.
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Background
Microfluidic systems often rely on optical detection methods like light sensors and spectrophotometers. While these tools are accurate, they tend to be expensive and bulky, making them hard to use in compact or low-cost devices. To address this, researchers have been exploring simpler and cheaper alternatives.
This study focuses on using basic components—LEDs as light sources and LDRs as detectors—to build a sensitive, low-cost optical detection system. LDRs, typically made from cadmium sulfide (CdS), change resistance when exposed to different levels of light. This change can be used to measure how fluids behave as they pass through a microchannel.
Previous studies have used LDR-LED pairs for chemical sensing and medical testing. However, most setups using LDRs struggle with limited resolution due to their size and design. This research aims to improve on those issues.
The Current Study
The researchers developed a new type of optical sensor using LDRs built into an eLOD device. To improve accuracy, they added custom waveguides that direct light through small openings onto the LDRs. These waveguides help align the light path, making the sensors more precise and easier to reuse.
In the setup, different colored fluids pass over the small apertures in the waveguides. As the fluids absorb light, they cast shadows that reduce the light reaching the LDRs. These changes in light levels allow the system to measure things like fluid position and movement.
The team tested this setup using different types of fluids, droplet sizes, and disc rotation speeds up to 50 rad/s. One key use case was measuring red blood cell (RBC) deformability during centrifugation. Since the shape and behavior of RBCs can reveal medical conditions such as sickle cell anemia, this test showed how the system could support simple, real-time diagnostics.
Results and Discussion
The system showed improved performance across several applications. It successfully detected and counted dyed droplets, monitored fluid interfaces in two-phase flows, and tracked how long it took for blood plasma and RBCs to separate during spinning.
In one test, the setup measured the movement of dyed oil across the sensor area, allowing real-time volume tracking based on light intensity changes. This is useful in fast-moving fluid environments like those in centrifugal microfluidics.
The color differences between plasma and RBCs also allowed the device to monitor sedimentation as it happened. This could help in blood tests where timing and visual contrast matter.
Beyond basic detection, the sensors could also measure how flexible RBCs were by noting how light readings changed during spinning. This has potential for low-cost testing of conditions such as thalassemia, malaria, or diabetes, where changes in blood cell shape and movement are common.
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Conclusion
This study outlines a practical way to improve optical sensing in microfluidic devices using LDRs and specially designed waveguides. The approach is simple, low-cost, and flexible, with strong potential for real-time monitoring in small-scale systems like Lab-on-a-Disc devices.
By improving resolution and keeping hardware affordable, this system could be useful in many areas, especially medical diagnostics. It also opens the door for more accessible and compact tools in biomedical research and testing.
Journal Reference
Kordzadeh-Kermani V., et al. (2025). Low-cost optical sensors in electrified lab-on-a-disc platforms: liquid-phase boundary detection and automated diagnostics. Microsystems & Nanoengineering 11, 61. DOI: 10.1038/s41378-025-00896-5, https://www.nature.com/articles/s41378-025-00896-5