Lessons from Mosquitoes’ Painless Piercing

Dev Gurera¹, Bharat Bhushan¹, and Navin Kumar²

¹Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics (NLBB),

The Ohio State University, 201 W. 19th Avenue, Columbus, OH 43210-1142, USA

²School of Mechanical, Materials and Energy Engineering,

Indian Institute of Technology (IIT) Ropar, Nangal Road, Rupnagar, Punjab 140001, India

Introduction: It has been reported that there are about 7 to 9 million identified animals on planet earth ¹. Out of all the animals, mosquitoes trump the number of deaths caused in humans in a year ³. On the positive side, mosquitoes can painlessly pierce. To pierce, mosquitoes secrete a numbing agent and use vibratory motion to pierce its serrated-designed mouthpart. The objective of this study is to elucidate lessons from the mosquitoes’ painless piercing ⁴. First, microanatomy of mosquitoes’ mouthparts and their feeding process is described. Later, based on the shown mechanical characterization data of mosquitoes’ labrum, the mechanism behind the painless piercing is presented. Finally, based upon the understanding, a schematic of a mosquito-inspired microneedle has also been proposed.

Mosquitoes’ Painless Piercing: A mosquito feeds on its host using its mouthpart, known as the proboscis. The proboscis is a collective term used for mosquito’s fascicle and its retractable, thick, outer cover, known as labium. The fascicle, being sharper, only pierces the host. A fascicle is a term for the bundle of six subparts. The largest of them all is a pointed and hollow subpart, the labrum, which sucks the blood. The other subparts include a pair each of sharp and serrated-shaped, mandibles and maxilla, respectively, which are known to pierce the host first. The labrum follows the path formed by these piercing initiators and helps the entire fascicle pierce in. In addition, the maxilla, having serrated edges, which also helps anchor the fascicle in place once inside the host. The final subpart of the fascicle is the hypopharynx which releases saliva.

The piercing and sucking process is described as follows. The tip of the labium is used to search for a suitable location on the host. Once the location is found, the hypopharynx pierces in and the saliva is released which acts as a numbing agent. Next, a vibratory motion is used to pierce sharp mandible and serrated maxilla inside the host. The serration in the maxilla also helps in keeping the parts anchored to the host’s tissues. The wider labrum follows the path formed by those two, and helps the entire fascicle insert in. Once the labrum is in, mosquitoes lower their vibration frequency and start looking for a blood vessel using the sensor located at tip of the labrum. After locating a blood vessel, the labrum pierces in and start sucking in blood, until repletion. Next, the saliva is released again, which acts as an anti-blood-clotting agent. Finally, the fascicle is taken out and it goes back inside the labium.

Study of the Painless Piercing Mechanism: To further understand the piercing process, material properties of mosquitoes’ mouthparts are needed. During piercing, the thin mandible and maxilla initiate the piercing, but it is the widest labrum, which helps in the insertion of the entire fascicle. Therefore, the labrum’s tip has been chosen for mechanical characterization. The tip was characterized in static and dynamic conditions, since it is a viscoelastic material and properties change with the frequency. The depth-sensing nanoindentation technique was used to measure elastic modulus and hardness under static and dynamic conditions. Mosquitoes use the insertion load of 10–20 µN while piercing a prey and the frequency of 5–15 Hz. Therefore, a static load of 10 µN and an oscillating load 5–200 Hz was chosen.

Static and dynamic characterization data of the labrum’s tip is presented in Fig. 1. The top shows elastic modulus map and the 

bottom shows storage modulus of the labrum’s tip. The maps of nanohardness and tan δ are not shown. It is noted that elastic modulus and nanohardness decrease along the labrum, with end of the tip being most compliant and softest. Across the labrum, the properties are lowest in the middle. It is also noted that the labrum gets stiffer at higher frequencies. It is believed that the more compliant and softer end of the tip may cause lesser pain while piercing the host, because it will deform the skin lesser. The lesser deformation sensed by the nerve endings in the skin, will cause lesser pain. The end of the tip may also be compliant and soft because of the fact that it is used for sensing blood vessels inside the host. Furthermore, the stiffer and harder sides of the tip will also help the labrum to resist bending/buckling during piercing. Finally, increase in stiffness at the high frequency may maintain lower deformation at higher stresses.

Figure 1: Mechanical property maps of the labrum’s tip. The top presents elastic modulus and the bottom presents storage modulus ⁴.

Proposed Schematic of Microneedle: It is believed that the combination of the pre-piercing numbing, the fascicle’s serrated design, the vibratory actuation, and the graded and frequency dependent mechanical properties of the labrum’s tip is the mechanism behind the painless piercing. The serrated design and vibratory actuation are believed to lower the insertion force, which along with the pre-piercing numbing and the mechanical properties gradient along the tip of the labrum are believed to reduce the pain.

Based on this mechanism, a mosquito-inspired, vibrating, viscoelastic microneedle with serrated design and material gradient has also been proposed. In the proposed design, the microneedle has a channel for fluid passage and serrated design, similar to the labrum and the maxilla. The microneedle is made up of a viscoelastic material having material gradients similar to that of the labrum. The microneedle is also accompanied by a numbing agent secretor, which together are enclosed within a thick covering, similar to the hypopharynx and the labium, respectively.

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