Microneedles and hormones against food waste

A leaf of chard rots silently. In Asian markets or in Brazilian countryside markets, time moves quickly. The loss happens before the pot. Now, a group of scientists from the Massachusetts Institute of Technology (MIT) and the Smart Alliance in Singapore promises to slow down this process. The proposal is simple in form but ingenious in content: using biodegradable microneedles made of silk to inject melatonin directly into plant tissue. The result? Greener leaves for longer. No need for a cold chain.

The experiment focused on pak choy, a vegetable common in Asia that is highly perishable. After applying the melatonin needles, the vegetable lasted four days longer at room temperature. When refrigerated, it lasted up to ten extra days.

The change begins with the cut. When separated from the root, the plant begins a natural aging process — called senescence. Melatonin, a hormone also present in plants, acts as a moderator of this process.

“We wanted to see if we could control post-harvest physiology,” says Benedetto Marelli, an MIT professor and lead author of the study.

Innovation is a response to a global problem. Around 30% of harvested food is lost before consumption. Most of it spoils during transport and storage. Refrigeration helps, but it requires energy, logistics and money. In many countries, especially in Africa and Southeast Asia, these resources are scarce. Silk needles are emerging as an alternative.

The material chosen was not by chance. The silk used in the needles comes from fibroin, a natural, biodegradable and non-toxic protein. The shape allows the plant to pierce its waxy skin without causing visible injuries. The injected substance is distributed throughout the plant's internal vessels, reaching its vital parts.

Before testing melatonin, the researchers applied a fluorescent dye to the pak choy plants to see if their vascular system was working as intended. Once the distribution was confirmed, they tried two conventional approaches: spraying and dipping the plants in melatonin. Neither had any effect. Uptake was minimal. Only direct injection worked.

The treated leaves remained green until the fifth day, while the control group turned yellow on the third day. The scientists monitored weight loss, coloration and chlorophyll content. The difference was clear. On average, the injected leaves remained saleable until the eighth day at room temperature.

Under refrigeration, the benefit increased even further. The treated vegetables survived until the 25th day with good appearance. Laboratory analysis showed greater antioxidant activity. Gene expression also changed: the hormone activated mechanisms that protect the pigments and delay cell death.

The idea of ​​using silk and hormones on crops is not new to Marelli’s lab. His team has previously tested microneedles to deliver nutrients and monitor crop vital signs. The current challenge is to scale up the technique. In the experiment, the needles were applied manually. In the field, the application would need to be adapted to agricultural machinery, tractors, and drones.

“To adopt it on a large scale, you have to make the technology cheap,” Marelli admits. “But the potential is huge.”

More information at doi.org/10.1021/acs.nanolett.5c00487