Researchers have successfully extended the shelf life of certain vegetables using a simple, striking solution: minuscule microneedles made of silk. These delicate spindles inject the plants with melatonin, a sleep aid in humans that also happens to help plants stay fresher for longer. In experiments, the microneedle patches increased the longevity of Pak choy plants by between four and 10 days.
Previous research has looked at how dipping and spraying plants with melatonin can influence their freshness and slow down the post-harvest spoilage of crops, says Benedetto Marelli, an associate professor of civil and environmental engineering at the Massachusetts Institute of Technology (MIT), and the study’s lead author. “This is the first time that we could control the delivery of the hormone in physiological quantities to slow down decay outside refrigeration,” Marelli says.
The international team, who are part of an initiative called the Singapore-MIT Alliance for Research and Technology, worked with microneedles that had been developed in previous research. These are just 0.7mm long, and are made of a special silk protein called fibroin that is non-toxic and fully biodegradable, and so leaves no trace behind in the vegetables it injects. Stuck on strips about half the size of a postage stamp, the needles are barely visible when placed on plants.
To check, first of all, whether these silky microneedles could effectively spread fluid into a plant’s vascular tissues, the researchers started by using them to inject a traceable fluorescent dye into the base of Pak choy plants. This showed that within an hour, the dye had visibly filtered into the plant’s tissues and would therefore successfully be able to transport melatonin into its leaves.
Then the team got stuck into the heart of the experiment, which was to stick the tiny needle patches, pre-loaded with a dose of melatonin, into several Pak choy plants and then closely observe them over several days. The dose they supplied to the plants was low enough that the plants were able to metabolize it, leaving only trace amounts when the produce was ready to be consumed.
Plants were tested in two scenarios: one non-refrigerated situation where Pak choys were stored at 25 °C, and another where they were kept cool at 4 °C. In each case, the performance of the melatonin-dosed plants was compared with plants that had not received a melatonin dose.
By the end of the days’-long experiment, the unrefrigerated plants turned out to be much more resilient when they had received a melatonin treatment. Compared to the untreated plants, these Pak choys staved off yellowing, remaining green for eight days, and experienced 51% less weight loss. Untreated plants, meanwhile, were rapidly yellowing by day four, which made them unsaleable at that stage. Overall, the melatonin injection extended the shelf life of treated, refrigerated plants by four days, compared to their untreated counterparts.
With refrigeration, the benefits were even stronger. Treated Pak choys that had been consistently cooled stayed relatively green until day 25, and showed significantly less weight loss. The untreated plants, however, had fully yellowed by day 15. So the melatonin-dosed lasted a full 10 days longer.
Melatonin, which is a naturally-occurring hormone in plants, has been shown to have an antioxidative effect in plants, reducing their stress response and delaying the ‘senescence’, or aging, of plant tissues. This study reveals by just how much, in one of the world’s most popular and widely-consumed leafy greens.
It’s possible that the invention could take some pressure off carbon-intensive cold chains that guzzle huge amounts of energy to keep produce fresh. Marelli says that most crucially, it could also help to reduce food waste in regions where refrigeration is not available to store fruit and veg. Enough food is lost and wasted each year to feed a stagering 1.26 billion people. “Food waste and loss are a societal crisis and a massive inefficiency of resources and energy, which has a steep impact on the prosperity of many communities,” Marelli says. “With this technology, we provide a new means to control the post-harvest physiology of crops, reducing the need for the cold chain to preserve them.”
The team’s next steps are to look beyond melatonin to other molecules that can increase food preservation, and to test these out on a broader range of fruit and veg, Marelli says. They’re also looking at retrofitting existing farm technologies so that farmers can efficiently pop the patches onto produce, and lock in its freshness at source.
Marelli et. al. “Precise Delivery of Physiological Doses of Melatonin in Planta to Control Postharvest Physiology and Extend Shelf Life Outside the Cold Chain.” Nano Letters. 2025.
Image: Dr. Yangyang Han; MIT News
