A food grade microneedle sensor made from gelatin and natural pigments detects fish spoilage through clear color changes and smartphone interpretation, offering a simple way to assess freshness inside sealed packaging.
(Nanowerk Spotlight) Judging the safety of packaged meat, fish or dairy products often relies on what the label reports about storage and shelf life. These labels offer helpful guidance, but they cannot account for how food has been handled during transport, on store shelves or in home kitchens. A product can spoil ahead of schedule if temperature control breaks down, while other items remain edible well beyond their date but still end up discarded. This mismatch between estimated shelf life and actual conditions has encouraged efforts to create simple tools that track the real state of packaged food.
Engineers and food scientists have explored a wide range of methods to add more insight to packaging. Electronic tags can record temperature history. Chemical strips can react to compounds formed during microbial activity. Films can contain dyes that shift color when acidity changes. Some approaches work well in controlled tests but remain too costly or difficult to incorporate into commercial packaging. Others depend on electronics, batteries or specialized materials that raise practical and environmental concerns. Moisture permeable films, while useful for sensing, can weaken the protective barrier that keeps food fresh.
Progress in microneedle technology pointed to another possibility. Microneedles are tiny, pointed structures that pierce a surface while causing little damage. They first attracted attention in medicine because they can deliver drugs through the skin with minimal discomfort. Researchers then began adapting them to take samples or interact with materials by lightly penetrating them. This opened the door to a simple device that could pierce plastic film without opening a package and respond to the chemical state of the food underneath.
These developments created a foundation for a materials-based sensor that uses only food safe components and needs no electronics. A study in Advanced Science (“Food‐Activated Microneedle Sensor for Real‐Time, Colorimetric Spoilage Monitoring of Pre‐Packaged Food”) builds directly on this direction and describes a microneedle patch that becomes active only when it touches moist food. The patch combines dehydrated gelatin with anthocyanins extracted from red cabbage. Anthocyanins are natural pigments that change color with acidity. When fish spoils, molecules produced during microbial activity tend to raise the pH of the flesh. The pigments respond by shifting from purple to blue, creating a visible signal.
Overview of gelatin-anthocyanin fish spoilage sensor. a) Schematic illustration of the developed sensor applied to sealed fish for real-time spoilage monitoring. b) Treatment-induced dehydration for mechanical integrity and ambient stability, followed by test fluid-induced rehydration for sensing. c) Food-mediated hydration of the sensing microneedles, which subsequently exhibit pH-induced color change from purple to blue as the food product spoils. d) Proof-of-concept application showing colorimetric shift from purple to blue across fish product lifespan. Scale bars depict 20 mm (left) and 10 mm (middle, right). (Image: Reproduced from DOI:10.1002/advs.202512602, CC BY) (click on image to enlarge)
The device depends on a change in the mechanical properties of gelatin. Gelatin in its normal hydrated form behaves as a soft gel, which is not suitable for piercing packaging. The researchers cast gelatin solutions into silicone molds shaped like microneedles, let the mixture solidify and then strengthened it through a freeze and dehydration cycle. When the gelatin freezes at −20 °C, the water inside forms ice grains that squeeze the protein chains closer together. This creates more physical links between the chains and makes the structure stronger. During a later drying step at 30 °C, water evaporates and leaves behind a stiffer, more absorbent material.
The team tested gelatin concentrations of 5, 10, 15 and 20 % w/v. They measured how much the needles shortened under applied force from 100 to 600 gram force (gf). Lower height reduction meant better performance. Needles made from 5 % gelatin deformed too easily. Samples made from 20 % gelatin were stiff but brittle. The 15 % formulation provided the best balance, with about 12.8 % height reduction at 100 gf and about 55.1 % at 600 gf after 24 hours of dehydration.
They also compared the gelled, frozen and dehydrated states directly. Gelled needles collapsed under moderate force. Frozen needles stiffened only briefly before weakening as they thawed. Dehydrated needles kept their structure at room temperature and resisted force more effectively across the full range. Electron microscopy revealed slimmer needles and a textured surface after dehydration, consistent with the removal of water and tighter packing of the gelatin network.
The authors evaluated the ability of the needles to pierce common packaging. Dehydrated needles penetrated polyethylene films that measured 14, 38 and 76 µm thick. When they used the same patch ten times on 14 µm film at 500 gf, penetration remained above 93 % and all microneedles stayed intact. Thicker films reduced penetration and increased the chance of breakage during repeated use, but a real product would only require one insertion. Tests on bare fish and on fish beneath packaging produced similar results.
To give the microneedles the ability to signal spoilage, the researchers mixed in anthocyanins extracted from red cabbage. These pigments appear purple at mildly acidic pH and shift to blue as acidity decreases. Because fish spoilage typically involves nitrogen containing compounds that increase pH, anthocyanins are a suitable choice for tracking freshness.
They began by testing how simple anthocyanin solutions behaved when exposed to haddock samples stored for several days. They prepared 0.1 %, 0.5 % and 0.9 % solutions and tracked changes in absorbance at several wavelengths after 30-minute incubations with fish as its pH increased. The correlation between fish pH and absorbance was strongest for 0.5 % and 0.9 % solutions at 300 and 350 nm, with correlation coefficients above 0.95. These findings showed that the pigment responded in a predictable way as the fish moved from fresh to spoiled states.
Embedded in dehydrated gelatin microneedles, the pigments behaved in the same way. The dry patch appeared faint purple. When placed in solutions at pH 6.0, 8.0 and 10.0, the patch turned darker purple, then blue, then green in less than five minutes. This confirmed that rehydration activated the hydrogel state and exposed the pigments to the surrounding fluid. The 0.5 % concentration produced clear and consistent color changes, especially between pH 6.0 and 8.0, the most important range for fish spoilage. Higher concentrations risked retaining too much moisture and did not perform as well when little fluid was available.
Tests showed that adding anthocyanin did not weaken the needles. Samples with 0.1 %, 0.5 % and 0.9 % pigment showed height reductions within about 14 % of each other and matched pigment free controls closely. Needles stored for up to 21 days under ambient conditions kept both their mechanical performance and their ability to turn blue at pH 8.0.
Storage at −20, 4, 25 and 37 °C for seven days also left these properties largely unchanged. Higher temperatures and different humidity levels did not cause noticeable damage. Leaching tests in water over 60 minutes released little pigment, far less than a fully dissolved control sample.
With the materials and chemistry validated, the researchers moved to tests on real fish. They wrapped 5 g portions of haddock in polyethylene film and pressed a microneedle patch containing 0.5 % anthocyanin through the surface. A second layer of film covered the patch. They then stored the samples in a refrigerator and photographed the patch each day while measuring pH on separate portions.
Fish remained below pH 7.0 on day 1 and day 2, and the microneedle patch stayed purple. On day 3 the pH rose above 7.0 and the patch shifted toward blue. The color deepened on day 4. Red channel intensity in the images dropped from about 99.6 arbitrary units on day 1 to about 57.1 on day 4. Patches containing 0.9 % anthocyanin did not show reliable color changes in this setup, likely because the available moisture was not enough to rehydrate the denser pigment matrix.
The team also assessed rapid testing on spoiled fish that had already been opened. They inserted the sensor directly into the samples and recorded images every 15 minutes. Red intensity fell from about 96.0 at the start to about 67.8 at 45 minutes. The drop at each time point was statistically significant and matched the color level seen in sealed samples once they passed the spoilage threshold.
Flat dehydrated gelatin films without microneedles did not show a clear color shift in this timeframe, which highlights how the microneedles increase surface contact and speed up sensing. Tests on cod and tilapia produced similar results.
To support easier reading and improve accessibility, the authors built a simple classifier using a convolutional neural network. They trained it on 30 smartphone images of patches on fish, then tested it on a separate set of 25 images. The model correctly labeled all samples as fresh or spoiled with no false results. The dataset was small but suggested that a consumer could take a picture of the patch and receive an automatic interpretation.
The work demonstrates that a simple patch made from common food grade materials can track chemical changes tied to spoilage. The study focuses on fish, but the approach offers a basis for future sensors that give shoppers clearer information about the state of foods they store at home.
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