Silicones can do many things; They improve the aesthetics of skin and hair, and now all they can do is help keep mosquitos at bay! Kao Corporation’s Personal Health Products Research Laboratory and Materials Science Research Laboratory report that applying a low-viscosity silicone oil to human skin can prevent mosquitoes from sticking to the skin and prevent them from starting blood-feeding. Unlike traditional insect repellants, this mosquito repellent technology focuses on the microscopic structure of mosquito legs, a new approach that alters the surface of the skin to have properties that mosquitoes don’t like and prevents it from staying on the skin, so that Companies.
Kao notes that mosquitoes, vectors of infectious diseases, have killed more people than any other creature on earth. Kao hopes to use this technology to protect people from mosquito-borne infectious diseases. This research was published in Scientific Reports 1, an electronic journal from Nature Research.
The company notes that in recent years the incidence of mosquito-borne infectious diseases has increased dramatically, due to global warming and the development of transportation networks that have expanded the mosquito habitat. According to an article published in Nature in 2013, an estimated 390 million people annually are infected with dengue fever after being bitten by the Aedes mosquito that carries the pathogen.2 WHO also notes that 40% of the world’s population live in areas at risk of dengue fever.
Fig. 1: Droplets of glycerine and low-viscosity silicone oil, which are deposited on a surface that simulates a mosquito leg. However, no effective vaccine or treatment for dengue fever is currently established, and symptomatic therapy is the only treatment available. For this reason, preventing mosquito bites is an important way to prevent infections as well. In infected areas, regular use of insect repellant along with clothing that reduces skin exposure is recommended. However, research by Kao has found that in Indonesia, Thailand, and Vietnam, where mosquito-borne infections occur year-round, 80% of the population is bitten by mosquitoes almost every day
Fig. 2: Force values in relation to the immersion depth of the front legs.
When landing on the skin, mosquitoes stabilize their front legs with their front legs before they begin blood-feeding behavior. Therefore, if the mosquito retreats within a few seconds of landing on the skin, there is no blood nourishment.
Kao therefore began examining surfaces that mosquitoes dislike, using a high-speed camera to watch mosquitoes land on a variety of surfaces. The results showed that mosquitoes do not stay on surfaces that have a certain hydrophobic oil applied to them and fly away quickly. The researchers also observed that afterward, the mosquitoes would rub their legs together to wipe off the oil that had stuck to them.
Kao focused on this phenomenon, carefully considering what would happen if the mosquito’s legs came into contact with a liquid. If this liquid was water or glycerin, it stayed in the form of droplets and did not spread to the mosquito’s legs. This is due to the unique microstructure of the legs, which are very water-repellent. On the other hand, the researchers also learned that squalane – which is also used to formulate skin care products – and low-viscosity silicone oil form droplets that spread to the legs within a short time after contact (Figure 1).
Fig. 3: Contact time of the mosquito legs on liquid-coated glass substrates. When this wetting phenomenon occurs, a force is exerted on the mosquito’s legs in a short time, pulling it into the liquid. Kao’s researchers used a force tensiometer to measure the force generated when the mosquito’s legs were brought into contact with the liquid. The results showed that when wetting with a low-viscosity silicone oil spread out on the legs, a capillary force (the force generated by the wetting behavior of a liquid) of approximately 5 μN was generated (Figure 2). This force poses a threat to small, light insects such as mosquitoes and is said to trigger escape behavior.
Slippery when wet
Kao conducted further experiments in which mosquitoes landed on glass substrates to which liquids with varying degrees of wettability with respect to mosquito legs had been applied. The measurement of the time during which the mosquito’s legs were in contact with the glass substrate and the subsequent flight of the mosquito showed that the mosquitoes would not stay longer than three seconds on a substrate to which low-viscosity silicone oil had been applied (Figure 3 ).
In tests on human skin, an average of 85% of the female mosquitoes that landed on uncoated skin showed a blood-feeding behavior, while on average only 4% showed such behavior on skin to which low-viscosity silicone oil had been applied (FIG. 4).
Kao believed that there might be examples of insect repellants in the natural world that use this wetting
Fig. 4: Mosquito bites on liquid-coated forearms. Phenomenon. Several animals are known to secrete sweat and other substances. The researchers speculated that the “red sweat” secreted on the hippo’s skin not only provides UV protection and moisture, but could also protect the animal from mosquitos.
Hippo secretions were provided by Adventure World in Wakayama Prefecture, and a silicone oil with properties similar to these secretions was made. Kao researchers conducted an experiment in which mosquitoes landed on substrates to which each of the substances had been applied. The results showed that both fluids were effective in reducing mosquito landing (Figure 5).
This finding suggests that hippopotamus secretions can protect the skin from mosquitos.
Kao uses the wetting phenomenon and has developed a method to protect against mosquito bites. This is a new technology with a mechanism of action different from traditional insect repellants such as DEET and Picaridin, which are based on volatile agents. In the future, Kao plans to use the knowledge gained from this study to develop products to protect the skin from mosquitoes and to help protect people from mosquito-borne infectious diseases.
Fig. 5: Contact time of mosquitoes on substrates coated with hippopotamus secretion and water.
1. Iikura, H., Takizawa, H., Ozawa, S., Nakagawa, T., Matsui, Y., Nambu, H. Mosquito repellants induced by tarsal contact with hydrophobic fluids. Sci. Rep. 10, 14480 (2020).
2. Bhatt S., Gathing PW., Brady JO., Messina JP., Farlow AW., Et al. The global spread and burden of dengue fever. Nature. 496, 504- 507 (2013)
3rd survey conducted in 2020 (1,000 people aged 18 to 59 in each country)