Conventional methods of applying agrochemicals in agriculture, such as crop spraying, face a serious problem of inefficiency. It is estimated that a significant proportion, between 30 and 50 percent, of applied pesticides, herbicides, or nutrients do not reach the target plants at all. Instead, they are dispersed into the air, settle on the soil, or are washed away by rain, leading to environmental pollution, potential harm to non-target organisms, including beneficial insects and soil microorganisms, and represent a significant financial loss for farmers. This wastage not only reduces the effectiveness of crop protection and fertilization but also creates ecological risks by contaminating watercourses and groundwater.

In search of more precise and sustainable solutions, a team of researchers from the Massachusetts Institute of Technology (MIT) and their collaborators from Singapore have developed an innovative technology that promises to revolutionize the way necessary substances are delivered to plants. Their approach uses tiny needles, known as microneedles, made from silk protein. These microneedles allow for the direct and targeted injection of agrochemicals, vitamins, and other compounds directly into plant tissues, bypassing the problems associated with external application.

Revolutionary precision using silk microneedles

The results of this research, published in the prestigious scientific journal Nature Nanotechnology at the end of April 2024, represent a significant step forward in agricultural technology. The developed method allows for the production of hollow silk microneedles in large quantities, which is crucial for potential application in commercial agriculture. These hollow structures can carry significantly larger amounts of active substances compared to earlier, solid microneedles, thereby opening up the possibility for more effective application.

Benedetto Marelli, an associate professor of civil and environmental engineering at MIT and the lead researcher, emphasizes the need for increased efficiency in agriculture. "Agrochemicals are important to support our food system, but they are also expensive and carry negative environmental consequences. Therefore, there is a great need for their precise delivery," explains Marelli. Microneedle technology, originally developed for medical purposes such as vaccine delivery to humans, has now been adapted for use with plants, offering a solution to the long-standing problems of inefficiency and the environmental impact of traditional methods.

The research was led by Yunteng Cao, currently a postdoctoral fellow at Yale University, and Doyoon Kim, a former postdoctoral fellow in Marelli's lab. The project also involved collaboration with the interdisciplinary research group Disruptive and Sustainable Technologies for Agricultural Precision (DiSTAP) at the Singapore-MIT Alliance for Research and Technology (SMART), MIT's research center in Singapore. The DiSTAP group focuses specifically on developing advanced technologies to solve key problems in global food production.

Simple fabrication, great potential

One of the key aspects of this innovation is the surprisingly simple process for manufacturing hollow microneedles. Unlike many nanotechnological processes that require expensive clean rooms and specialized equipment, Cao and Kim developed a method that can be performed under significantly less controlled conditions.

The process begins by combining silk fibroin protein (the key structural protein in silk) with a saline solution inside tiny cone-shaped molds. As the water evaporates from the solution, the silk solidifies within the mold, taking its shape. Simultaneously, the salt forms crystalline structures within the future needle. Once the silk has completely hardened, the salt is washed away, leaving behind a hollow interior or, depending on the salt concentration and phase separation, a network of tiny pores within the needle. This cavity or porosity is crucial for the microneedle's ability to carry and deliver liquid substances.

"It's a fairly simple fabrication process. It can be done outside a clean room – you could do it in your kitchen if you wanted to," points out Kim. "It doesn't require any expensive machinery." This ease of production opens the door to wider availability and potentially lower implementation costs for the technology in the future.

Treating plants and enriching crops

To demonstrate the effectiveness of their technology, the researchers conducted a series of experiments on tomato plants. One focus was the treatment of chlorosis, a disease caused by iron deficiency in plants. Chlorosis manifests as yellowing of the leaves due to the plant's inability to produce enough chlorophyll, which can significantly reduce yields. The problem is often pronounced in soils with high pH (alkaline soils, such as those rich in limestone) or in heavy, poorly drained clay soils, where iron, although present, is not in a form available to plants. Traditional treatment by spraying with iron sulfate or chelates is often ineffective and short-lived, as the substance is poorly absorbed through the leaves or quickly washed away.

Using their hollow silk microneedles, the team successfully applied iron directly into the stems of tomatoes suffering from chlorosis. The results showed that this method allows for sustained and longer-lasting delivery of iron directly into the plant's vascular system, without causing significant damage. Study co-author Daisuke Urano, a plant scientist from DiSTAP, confirmed that detailed assessments showed minimal negative effects from the microneedle injections, with no observed short-term or long-term consequences for plant health.

Another significant experiment focused on biofortification – the process of increasing the nutritional value of crops during their growth. While previous efforts have mainly focused on minerals like zinc or iron, adding vitamins, especially those not naturally found in plants, posed a greater challenge. Vitamin B12, essential for human health and primarily present in animal products, is often lacking in vegan and vegetarian diets.

The researchers used microneedles to inject vitamin B12 into the stems of growing tomato plants. Analyses later confirmed that the vitamin was successfully transported throughout the plant and accumulated in the tomato fruits before harvest. This discovery opens up entirely new possibilities for enriching various types of fruits and vegetables with vitamins during cultivation, making them nutritionally more valuable. "This new delivery mechanism opens up many potential applications, so we wanted to do something no one has done before," explains Marelli.

Real-time plant health monitoring

In addition to substance delivery, silk microneedles have also proven useful for monitoring plant health. Current advanced monitoring techniques, such as leaf color analysis (colorimetry) or hyperspectral analysis, can often detect problems only when plant growth is already compromised. Other methods, like plant sap sampling, can be invasive and time-consuming.

The research team explored the possibility of using microneedles to detect heavy metals in plants. They grew tomatoes in hydroponic solutions contaminated with cadmium, a toxic metal often found in soils near industrial and mining areas. After injecting microneedles into the tomato stems, it was shown that the needles absorbed cadmium from the plant sap within just 15 minutes. This suggests that microneedles could be used as a rapid diagnostic tool for detecting contaminants.

Furthermore, microneedles allow for the collection of small plant sap samples for continuous chemical analysis. The team demonstrated the ability to monitor cadmium levels in tomatoes over 18 hours, indicating the potential for developing systems for real-time monitoring of plant health and environmental conditions. Such systems could provide early warning signals for stress caused by drought, nutrient deficiency, or the presence of toxins, enabling timely intervention.

New horizons for sustainable agriculture

Although the microneedles were applied manually in the study, Marelli and his team envision a future where this process could be automated. Integration with existing agricultural machinery, such as autonomous vehicles, tractors, or even drones (although drone application still presents technical challenges), could enable application over large agricultural areas.

The researchers believe that silk microneedle technology could complement existing agricultural practices, offering a more precise and environmentally friendly alternative or addition to spraying. By reducing the amount of agrochemicals used and delivering them targetedly, the risk of environmental pollution is reduced, money is saved, and the efficiency of treatments and fertilization is potentially increased.

It is important to note that the potential of this technology extends beyond agriculture. "This new fabrication technique for polymer microneedles could also benefit research in transdermal and intradermal drug delivery and health monitoring in humans," says Cao. The properties of silk as a biocompatible and biodegradable material make it attractive for various biomedical applications.

However, the primary focus currently remains on transforming agriculture towards more sustainable practices. "We want to maximize plant growth without negatively impacting the health of the farm or the biodiversity of surrounding ecosystems," concludes Marelli. "There should be no trade-off between the agricultural industry and the environment. They should work together." The development of precision technologies like silk microneedles represents a crucial step towards realizing this vision, where productivity and environmental responsibility support each other.

Source: Massachusetts Institute of Technology