# Characterization and Theoretical Analysis of the Venus Flytrap Trigger Hair
## 3 Results and Discussion  
### The Characteristics of the Venus Flytrap Trigger Hair  
The B52 Venus flytrap is a typical carnivorous plant, known for its larger and more robust traps compared to other varieties. The trapping structure of the B52 Venus flytrap includes trigger hairs, midrib, leaf lobes and toothed tines, and there are three or four trigger sensory hairs distributed on the upper side of each lobe (Figure 1a). The outer surface of the trigger hair is red, as indicated in Figure 1a. Each trigger sensory hair is composed of two kinds of tissues: a flexible short basal podium and a stiff long hair lever. The length of the hair lever is 800-3000 \(\upmu\)m, and the length of the basal podium is about 80-120 \(\upmu\)m. The diameter of the lever increases gradually from the top to the base. And the maximum diameter of the lever is in the range of 60-200 \(\upmu\)m. There is a notch structure located at basal podium, as shown in Figure 1d(I). Many studies have revealed that the kidney-like shaped sensory cells are distributed in a circular array at the notch area (Figure 1b) [15,16]. Finite element analysis using ABAQUS2019a was carried out on the model of the trigger hair of the Venus flytrap to investigate the deformation response of the trigger sensory hair to the external tactile signal. The elastic moduli of the lever and podium of the model were set to 18.167 Mpa and 138.167 Kpa, respectively. The density of the model was uniformly set to \(\rho=1\) g/cm\({}^{3}\), and the Poisson's ratio was set to \(\
u=0.3\). The results clearly indicated that the podium, which is the strain concentration area of the trigger hair, experienced significant bending deformation, while the lever, which only deflected with the podium, did not undergo any bending deformation (Figure 1c). Obviously, the Venus flytrap can ultra-sensitively detect extremely tiny tactile signals utilizing the unique cantilever-like hair as a mechanosensor [9]. Under the action of external force, the stiff lever is only deflected by an angle \(\theta\), but there is no obvious bending deformation phenomenon. This design means that trigger hair amplifies forces and concentrates stresses to a flexible basal podium, and the maximum strain occurs on the contraction area where sensory cells are located. Subsequently, the unique material composition and the deflection characteristic determining the sensing performance of the tactile sensilla were further researched. Figure 1: Morphology of Venus flytrap: (**a**) composition of Venus flytrap leaves and enlarged view of external morphology of trigger hair; (**b**) computer rendering of the sensory hair (orange) and the sensory cell (green); (**c**) finite element analysis output; (**d**) SEM images of the trigger hair of Venus flytrap. (**I,II**) are enlarged views of the base podium and hair lever of the trigger hair of Venus flytrap, respectively. Image (**III**) shows a cross-sectional view of the trigger hair lever of a Venus flytrap. Scale bar is 20 \(\upmu\)m.  
Many studies have revealed that the mechanical properties of the materials have a great influence on the deformation of the engineered structure [17; 18; 19; 20]. Among many physical parameters, stiffness, as a measure of a material's resistance to elastic deformation, depends on the elastic modulus. To enhance the analysis of the mechanical properties of Venus flytrap trigger hairs, nanoindentation experiments were performed. First, fresh leaves of the Venus flytrap were fixed onto glass slides using glue. Then the trigger hairs were carefully excised using a blade under an optical microscope while ensuring the preservation of their complete morphological features, including an intact basal podium and hair lever. Subsequently, the excised trigger hairs were quickly adhered to the glue, as well as ensuring that most of the volume of the trigger hairs was not submerged in the glue. Once the glue was solidified, an uncontaminated sample was obtained (Figure 2a).