Plants that harvest energy from nature through photosynthesis accomplish perception and response to changing conditions without brain control. They provide possibilities for constructing advanced biomimetic structures that rely on adaptive material systems without control centers. The sense and response of a material system to changing environmental conditions are related to the corresponding biological reaction mechanisms. Therefore, plant-inspired biomimetic structures have focused on the achievement of plant functional principles, rather than simply imitating the behavior of plant movement in past decades [12]^[1]. These biomimetic structures, which appeared closely to the biological model, are to achieve self-growth and self-repair and respond correctly when sensing changes in ambient conditions. Since these strategies are still in their infancy stage, the existing Venus flytrap biomimetic structures are still not capable of self-growing and self-repairing. Therefore, the main biomimetic characteristics in terms of sensing, and actuation, as well as the rapid snap-trapping phenomenon of the biomimetic flytrap structures are mainly summarized here, with a focus on exploring the smart composite technology.

The function of sensing for biomimetic flytrap structure is usually achieved by integrating smart material-based sensors. To date, the sensing characteristics of ionic polymer metal composites (IPMCs) are the most popular to mimic the trigger hairs of flytrap leaves. IPMC is an intelligent material superior in terms of bidirectional motion, fast response, and low driving voltage. When it is bent by an external force, the solvent is replaced, and the resulting charge polarisation generates a voltage on both sides of the material. Based on this acting principle, the IPMC has been applied to mimetic the trigger hairs, i.e., bending sensor. Under the influence of an external electric field, the redistribution of water molecules in IPMC leads to its bending deformation, involving a series of energy conversions, including electrical energy, chemical energy and mechanical energy.

The sensing characteristics and mechanical bending of an IPMC system are similar to the sensing and actuation of the flytrap. The IPMC-based bristles initiate signals via an amplifying circuit when subjected to bending, and trigger the actuation circuit of the IPMC lobes, which are then bent rapidly and close within nearly 0.3 s. Although the sensing characteristics of IPMC related to internal ion transfers are similar to those of a real flytrap, the output of IPMC brush as a sensor within a biomimetic structure is rather weak and unstable, its bending is also limited. The infrared proximity sensors were then developed to replace the IPMC bristles [57]^[2]: in order to further improve the capability in sensing, number of proximity sensors can be increased; to obtain maximal shape-changing, three IPMC lobes were used to reduce gaps.

The sensing function of biomimetic structures is often achieved by using stimulus-responsive materials, which are sensitive to changes in environmental conditions and deformed accordingly. A notable feature of hydrogels is that their volume changes during a wide range of environmental conditions. However, the leaves of a flytrap can quickly close within a tenth of a second to catch insects, significantly different from the usual slow shape transition of hydrogels. Therefore, the ingenious combination of sensing external stimuli and improving responsive speed needs to be implemented in hydrogel-based biomimetic designs. The hydrogel leaf protrudes outward, and three microfluidic channels are embedded on its inner surface for solvent transportation. The expansion of hydrogel is controlled in the same way as in the deformation of a flytrap leaf, only the curvature in one direction is actively manipulated by changes in solvent through the microfluidic channels, and the curvature in the other direction remains passive. During the swelling of the hydrogel, bending–stretching coupling of the doubly-curved geometry stores elastic strain energy along the axis. With further expansion, the stored elastic potential energy is released instantaneously after passing through the energy barrier, leading to the snap-buckling of the hydrogel leaf. During the drying or de-swelling process, reverse movements occur and quickly return to the original shape of the leaf. The whole shape-changing cycle can be controlled within 5 s.

It is worth noting that soft materials in nature are inhomogeneous. They have multiple functional regions with different chemical and mechanical compositions. Therefore, the composition and distribution of different materials could also be learned for biomimetic response structural systems. The gels are constructed into a bilayer structure, which consists of two layers: a gel A/B layer is sandwiched above a layer of gel C. Two elliptical leaves made of gel A are connected by a hinge made of a mixed gel. Initially, when the biomimetic flytrap is placed in water, the gel C layer swells more than the gel A/B layer. Despite the mismatch in swelling speed, the hinge remains flat since the gel A/B layer is produced to be stiffer. In addition to the collagenase enzyme in the water, it cleaves the gelatin chains in gel B, which then reduces the stiffness of the A/B layer. The swollen gel C layer is now able to fold over the A/B layer, leading to the hinge being transformed into a specific shape. The reaction time for the leaf to be fully folded is about 50 min for 50 U/mL of enzyme. It should be noted that it is necessary to include energy storage and release mechanisms in order to improve the response period.

The composite hydrogel sheet has a double gradient along the thickness direction, i.e., chain density and cross-linking density gradient, which is able to accumulate elastic energy and release the stored energy rapidly through ultrafast snapping deformation. The composite gel is flat when immersed in 20 °C water. When immersed in 60 °C water, the composite sheet is bent along the longitudinal axis to the higher gradient side and transferred into a tubular structure, with stored energy. When replaced with the 20 °C water, the hydrogel sheet does not follow exactly the opposite path of the original shape transition, but a third state appears. During flattening, the tubular hydrogel snaps rapidly and flattens after curling. Trigger conditions for gel plates are complex and time-consuming, and the snapping velocity, angle, and location of the sheet can be tuned by modulating the magnitude and location of stored energy within the hydrogel.

It is obtained by bonding pre-stretched poly dimethyl siloxane (PDMS) layers prior to depositing electrospun polyethylene oxide nanofibers to induce hygroscopic bistability [62]^[3]. The moisture absorption capacity of the electrospun material is combined with the mechanical advantages of the preloaded structure to increase the actuating speed. When polyethylene oxide expands with increasing ambient humidity, its coupling with the passive layer causes the curvature of the artificial leaf to decrease until it snaps within 0.5 s. When the humidity is reduced, the initial state is restored.

Compared with the above hydrogel-based driving systems that can only perceive and react in a liquid environment, light-driven smart material has lower environmental requirements. The open-aligned LCE actuator is integrated with the fiber tip and leaves a window in the center for light injection. When the structure is subjected to light, the molecular alignment arrangement changes in LCE, sufficient optical feedback (reflected or scattered light) then generates strains, leading to expansion and shrinkage on different surfaces; the flytrap structure is then closed by the induced strain difference on the surfaces. It can be closed in low humidity levels and high light levels, and open in no light and high humidity levels. The sensing strategies of the light-driven structures are flexible and diverse, they are expected to be used in future adaptive and intelligent biomimetic structures.

The biomimetic strategies stated above mostly rely on the sensing capability to initiate snap-trapping. They lack elastic deformation-induced actuation, and the smart responses are usually in slow motion. Integration of the sensing capability into a bistable system may be more suitable for the biomechanics of the flytrap structure. The flytrap leaf is regarded as an elastic irregular curved shape. The macroscopic response of the flytrap after sensing the prey is that the leaves deform and pass through the energy barrier, and the stored elastic energy is instantly released and converted into kinetic energy, thereby forming a rapid snap-trapping action [65]^[4]. To date, the biomimetic structure close to this reaction is basically bistable. There are two ways to achieve the bistable characteristics of the biomimetic flytrap structure. These are not able to sense and are usually triggered by manual input. One strategy is to bind the bistable actuator between the artificial leaves, shape transitions between the two steady states of the actuator and drive the artificial leaves to morph. Although these structures have energy storage and release processes, they lack curvature changes as in a real leaf, and their reaction is similar to grasping rather than snap-trapping. The other strategy is to explore the curvature changes during the morphing of a bistable composite structure.

Therefore, the morphing process of an OSB- or ESB-based bistable structure can be designed to introduce elastic energy-driven mechanisms into the flytrap structure. The bistable structure is used to imitate the artificial leaves, providing similar shapes to a real flytrap leaf. The two stable configurations correspond to the open and curled closed shapes of the real leaf with different structural curvatures. The SMA coil spring is used to induce snap-through of artificial leaves after being electrically heated, similar to the active motion of a flytrap, which is embedded on both surfaces of the artificial leaves to be repeatable of the rapid actions. Since the SMA-based actuator requires relaxation time to cool down, the morphing frequency of the structure is limited. It is verified by experiment that this type of bistable biomimetic flytrap structure is able to close within 100 ms, and the macroscopic rapid snapping deformation mechanism is similar to that of real flytrap leaves. The performance of the bistable biomimetic structure is then improved by designing the bistable characteristics of artificial leaves and changing the geometry and locations of the embedded SMA [68]^[5].

In addition, attempts have also been explored by using ESB-based structures, which also depict the real curvature changes of the flytrap leaves. The bistability is derived by using the antisymmetric composite layup-induced geometry curvature effects [69]^[6]. On the upper side of an artificial leaf, the iron sheet is attached to the middle part of the outer curved edge to be attracted by magnets. The electromagnet is placed just above the iron patch in order to generate a suitable trigger force for the shape-changing activation. When the electromagnet is activated, the curve edge is subjected to magnetic force and produces a bending moment on the artificial leaves, which makes the biomimetic structure shift to the second stable state. This non-contact actuation simplifies the biomimetic structure and actuation design, with adjustable magnetic force through current control. A further improvement was then carried out to reduce the actuation force required to trigger the morphing action. The inner curved edges are clamped, and the snap-through of artificial leaves is constrained by a clamping device. Controlling the width of the clamping edge can effectively reduce the actuation force required to trigger the morphing actuation.

Although the shape-changing characteristics of the bistable artificial leaves are similar to those of the real flytrap leaves, the internal stress field of the real flytrap leaf is locally orthotropic and follows the geometric shape of the leaf as a whole. The microstructural changes are controlled by a magnetic field using locally oriented rigid anisotropic magnetic particles, in order to adjust the local prestrain and stiffness anisotropy of the composite. Compared with carbon fiber reinforced composites, the local residual strain of this biomimetic composite structure is more controllable, with a certain performance and shape gradient, and can achieve rapid shape-changing actuation, approaching the true shape transition mechanisms of the real flytrap.

A further strategy is considering multiple driving methods to mimic the biomechanics of the flytrap. The multi-responsive composites consist of a hydrogel layer and an architected particle-reinforced epoxy bilayer. The spatial distribution orientation of the magnetic responsive plates in each epoxy layer is achieved by using a magnetic field to induce in-plane mechanical properties and shrinkage. The epoxy double layer is used to adjust the prestress in the material, while the hydrogel layer controls the time response according to the hydration level. This smart composite-based biomimetic structure exhibits rapid and slow deformation in response to mechanical, magnetic, thermal and hydration stimuli. The integration of bistable and stimuli-responsive materials or smart materials can sense a variety of specific environmental conditions and react quickly.