Biomarkers are small molecules of interest to researchers, because they can indicate underlying diseases, often even before symptoms even appear. However, detecting these markers can be challenging as they are often present in very low quantities, especially in the early stages of a disease. Traditional detection methods, while effective, usually require expensive components like prisms, metal films, or optical objectives.

In a recent paper published in Applied Physics Letters, researchers at the University of Illinois Urbana-Champaign have unveiled a novel approach to detecting low concentrations of biomarkers that paves the way for biodetection technology that is simple to use, highly sensitive, and surprisingly affordable.

“The goal of this technology is early diagnostics, to be able to detect molecules associated with diseases at very low concentrations, sometimes a few molecules per millions, very early on,” said Seemesh Bhaskar, a postdoctoral researcher in the Cunningham lab and first author on the study. “Looking for very small concentrations of micro-RNA, circulating tumor DNA, and exosomes, for example, can help determine whether a patient will develop cancer one or two years down the line.”

Early detection of biomarkers is crucial for predicting and managing diseases effectively. There are many strategies for measuring the presence and concentration of biomarkers, but a common approach involves binding them with a fluorescent molecule, called a fluorophore, which emits fluorescence when excited with light.

Bhaskar noted that while there are technologies adept at detecting these low levels of fluorescent biomarkers, they are often bulky and expensive, limiting their accessibility in healthcare, particularly in resource-limited areas.

The approach encompasses a novel phenomenon for detecting light, called radiating guided mode resonance, which utilizes photonic crystals — thin pieces of glass with small gratings on the surface. These gratings help direct the photons, which are small particles of light, emitted from biomarkers along a pathway via a steering effect. This pathway is “tuned” to match the wavelength of the fluorescence emitted by the biomarkers, optimizing light collection and enhancing detection sensitivity.

Bhaskar likens this to a rhythmic dance of light energy within the crystal, where light is amplified while taking on the properties of the photonic crystal. One property of the crystal, called polarization selection, equalizes the polarization of the light, making for clearer and sharper detection of fluorescence. Together, this can result in an output that is 100 times stronger.

“To me, this is a whole new way of looking into the properties of light itself,” said Bhaskar. “The photons adapt, change, and evolve as they pass through the photonic crystal. The light picks up new characteristics without losing its essence. It's a testament to the adaptability and transformative power of light.”

Discovery of this new phenomenon sets the stage for future detection platforms that will be able to detect molecules at picomolar levels without relying on costly components, making biodetection technology more sensitive, accessible, and affordable.

While radiating guided mode resonance can theoretically be used to enhance detection of many different biomarkers, the Cunningham lab is particularly interested in early cancer detection. The new phenomenon holds promise for affordable technology that will be critical for populations in resource-limited settings, where early disease detection and treatment can make a profound difference.

One of the lab’s long-term goals after development of this technology is to make it compatible with smartphones, further enhancing its accessibility. The envisioned future product would be a simple fixture attached to a smartphone’s camera, allowing a photonic crystal to illuminate a test sample while the phone’s camera measures the fluorescence emission.
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“We are creating biosensing systems that are extremely sensitive while utilizing simple and inexpensive detection instruments,” said Brian Cunningham (CGD leader/MMG), a professor of electrical and computer engineering and program leader at the Cancer Center at Illinois. “This is what creates a path toward sophisticated health diagnostics making their way to our health clinics, farms, and homes.”

The study was funded by NIH, NSF, and the Cancer Center at Illinois. The paper can be found at https://doi.org/10.1063/5.0203999

In coastal arid regions where water sources are scarce, windborne fog droplets play a crucial role in sustaining life. Many plants collect droplets from the fog, serving as a vital water source for various organisms, but a few beetle species have evolved their own unique strategy for water collection - utilizing their bodies to intercept the droplets. However, the mechanics behind this process have long puzzled researchers due to the beetles' bulky morphology. In a recent study published in PNAS Nexus, scientists delved into how adaptations in the beetles' back enable them to gather enough water.

Two species of Namib desert beetles make use of a behavior called fog basketing to collect water droplets from the morning fog. They ascend dunes and balance on their front appendages, leaning their bodies into the wind. Microscopic droplets from the fog accumulate on the beetle’s back, and then trickle towards its mouth. Given that slender structures, akin to those observed in plants and human-made meshes used in arid regions, are best for fog droplet collection, questions arose regarding the feasibility of water collection with the beetles' morphology.

“In terms of fluid dynamics, water collection efficiency is way higher with thin structures like a mesh,” explained Hunter King, an assistant professor of physics at Rutgers University. “But if you’re a beetle, you’ve got a ball shaped body, and you don't have the freedom to change your geometry that wildly. We wanted to determine the strategy they use for a situation that's not ideal to begin with.”

The experiment comprised two main components: a computational phase and an experimental phase. The computation segment, led by Mattia Gazzola (M-CELS), an associate professor of mechanical science and engineering at the University of Illinois Urbana-Champaign, involved the creation of models to assess the impact of various surface geometric features on water collection. Utilizing data from these simulations, they then produced 3D printed models replicating different curvatures and surface properties derived from the models.

In the experimental phase, led by King, these 3D printed models, alongside deceased beetles, were employed to gauge their water collection capacity. The researchers constructed miniature tunnels to channel fog, where the models and beetles were individually placed at the tunnel's end connected to a sensitive load-bearing measure. This apparatus measured how much water accumulated on the target’s surface.

In addition to altering the 3D printed targets, the scientists manipulated the surface features and wettability (how hydrophobic or hydrophilic something is) of the beetles to investigate its effect on water accumulation. Given that the beetles' backs are inherently hydrophobic, the researchers coated beetles with a nanoscale-thin layer of gold to render them hydrophilic. To eliminate surface irregularities, an intriguing method was employed: applying nail polish to smooth out the beetle's backs. Some beetles received only gold or nail polish, while others were coated in both.

“We wanted to change the geometry of the beetles without changing the wettability, and the student working on this came up the elegant solution to use nail polish,” said King. “Just as nail polish makes a perfect layer, smoothing out all the details on a nail, it worked the same way for the beetles.”

They found that in both the 3D models and the beetles, surface bumps and a textured surface resulted in the most water accumulation. Similar to rain droplets colliding with the pavement, Gazzola explained that water particles that collide with a flat surface tend to be mostly deflected away. However, the introduction of bumps and slopes alters the trajectory of the colliding water, facilitating increased water retention.

“We show in our designs that if you add bumps on the back, resulting in these very sharp deflections in the back’s curvature, the water particles hit the bump, but then the flow continues to move along the back,” explained Gazzola. “And then it continues to flow over another bump and another, and the space between bumps then serve as traps to collect the water.”

Surprisingly, the researchers discovered that the wettability of the beetles did not substantially influence water collection; instead, only geometric features played a significant role. However, the researchers did observe that wettability influenced the directional movement of water down the beetle's back, supporting previous research underscoring the importance of wettability in guiding water towards the beetle’s mouth.

“Cumulatively from a very smooth surface, we found that adding microscopic geometric features plus a roughness on the surface gives you about a 400% increase in water collection,” said Gazzola. “Now this is still a very small amount of water, but the beetle is also very small, so it’s enough water for its needs.”

The team says these findings likely won’t be applicable for collecting water on the scale that humans need, but the results could inform material designs for when the opposite pattern is desired – avoiding water accumulation.

“We show that by changing the texture of something that wouldn't otherwise have caught very much moisture, now it’s collecting more,” said King. “This is great from the beetle’s perspective, but this same effect could be disastrous if you don't want to collect droplets on something. There are aerodynamic elements for wind turbines or planes that involve changing the surface morphology so that drag is reduced, but that same thing might cause icing to be a much bigger issue.”

This experiment offers a biological insight into how the beetles, despite their shape, manage to gather enough droplets by fog basketing. Moving forward, the team intends to delve further into the adaptations of organisms with diverse masses and shapes in managing water and heat exchange.
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This study was funded by NSF, and can be found online at https://doi.org/10.1093/pnasnexus/pgae077