As Charles Darwin once said, it is not the strongest of the species that survives, nor the most intelligent; it is the one most adaptable to change 
. Through millions of years’ evolution, lives on the earth have developed basic and crucial surviving skills: environment adaptability. Having sensed different environmental signals (e.g., seasonal, daily, and instant change of temperature, illumination, and humidity), many animals can actively respond with actions of biological importance. For instance, birds, such as storks, turtle doves, and swallows, migrate between breeding and non-breeding grounds 
. Atlantic salmon, a well-known anadromous fish, returns to its home streams and rivers to spawn after a long migration in the ocean, ensuring the survival of its offspring 
Plants can demonstrate movements from a slow to fast, and even to furious speed, even though their roots are fixed to the ground 
. Different from the muscular movements in animals, movements in plants are often stimuli-responsive and can be generally categorized into two types: tropic movements and nastic movements 
. Tropic movements, associated with the growth of plants, are universal in plants. For instance, the phenomenon of roots growing toward the gravitational pull, while shoots growing against the gravitational pull is called gravitropism. Similarly, there are also phototropism and hydrotropism, which are both beneficial to the growth of plants. Unlike the universal and directional tropic movements, nastic movements of plants are more individualistic and independent of the direction of the stimulus 
. In response to changes in relative humidity, pinecone 
opens/closes its scales to release its seeds, while wheat awn 
does so by bending/unbending its seed dispersal units. Instead of responding to the humidity change, ice plant opens its protective valves to release the seeds only when the seed capsule is sufficiently hydrated with liquid water, which ensures that the seeds are dispersed under favorable conditions for germination 
. Another fascinating nastic movement of plants is the folding/unfolding response to external mechanical stimulus (e.g., touch or vibration). For instance, Venus flytrap can rapidly close its trap within 100 ms when its sensitive trigger hairs are stimulated by the struggle of insects and reopens its trap when the food is fully digested 
. Similar to Venus flytrap’s pray capturing technique, sundew uses its sticky glands and wrappable tentacles to trap the visiting insect and unwrap its tentacles for the next round of hunting 
. Another well-known plant responsive to the mechanical stimulus is mimosa 
; it folds its leaves inward to defend itself from potential harm when being touched or shaken and unfolds its leaves a few minutes later when the danger is vanished. In addition to the hydronastic (i.e., humidity-/water-responsive) movements and thigmonastic (i.e., mechano-responsive) movements in plants, there are some other types of nastic movements, like photonastic and thermonastic movements, in flowers 
. Notably, these nastic movements in plants are energy-efficient, fast, reversible, and robust, representing perfect modeling systems for the design of artificial soft robotics 
By combining bio-inspired designs with stimuli-responsive motifs, numerous intelligent soft robotic prototypes have been developed 
. These intelligent biomimetic soft systems are finding applications in shape morphing, actuation, and gripping/manipulation where mechanical forces are effectively generated from various energy inputs, such as heat, electricity, and illumination. Soft robotic systems are normally fabricated by compliant soft materials, such as silicone elastomers, polyurethanes, hydrogels, braided fabrics, hydraulic fluids, and gasses 
. Among these building materials, hydrogels composed of up to 90% of water embedded in polymeric networks are standing out because of the excellent integration of important properties, including being variable moduli-matching biological soft systems, stimuli-responsiveness available, stretchable and tough, non-toxic, conductive, and transparent 
. More importantly, fabrication of hydrogel-based soft robotics is greatly facilitated by the facile UV polymerization and 3D printing of hydrogels 
In this review article, we aim to summarize recent progress on the development of hydrogel-based soft robotics with plant-inspired designs and actuation mechanisms. The article is organized in the following way (). First, the two common types of nastic movements of plants, namely hydronastic movements and thigmonastic movements, are introduced in Section 2
. Then, the underlying actuation mechanism for each type of nastic movement and the derivation of the design principles for biomimetic soft robotics are discussed. Based on these bio-inspired design principles, representative prototypes with stimuli-responsive hydrogels as the building materials in terms of fabrication and actuation mechanism are introduced in Section 3
. Finally, some critical challenges hampering the development of hydrogel-based soft robotics are discussed, and the corresponding possible solutions are proposed in Section 4
. It is anticipated that this review article would spark broader interests in the investigation and advancement of biomimicry hydrogel-based soft robotics.
Figure 1. Plant-inspired soft robotics: nastic movements in plants and derived design principles for biomimetic soft robotics.
3. Plant-Inspired, Hydrogel-Based Soft Robotics
As discussed above, the various movements of plants are directly triggered by change of humidity and vibrational touch. Owing to the structural and compositional differences in the moveable units of plants, response divergence in terms of motion degree and sequence (timescale) is generated, resulting in macroscopic movements. Therefore, creating anisotropy structurally or compositionally can be one of the design fundamentals of artificial soft robotics. As shown in Scheme 1
, three representative design principles with build-in anisotropy can be derived to fabricate artificial soft robotics: bilayer structures, gradient structures and patterned structures 
. Incorporating these plant-inspired design principles into stimuli-responsive hydrogels has enabled the great development of hydrogel-based soft robotics. Importantly, these hydrogel-based soft robotics can respond to more external stimuli, such as thermal, photo, and pH. In this section, we discuss the recent progress on plant-inspired soft robotics composed of hydrogel building materials.
Scheme 1. Summary of the movements of nastic movements of plants and corresponding hydrogel-based biomimetics.
3.1. Bilayer Hydrogel-Based Soft Robotics
Traditionally, bilayer hydrogel-based soft robotics, consisting of two hydrogel sheets with different swelling rates or ratios, have been successfully developed to perform controllable deformations, such as bending and bucking, on the basis of asymmetrical responsive properties of the two parts. Several approaches, including layer-by-layer polymerization, assembly of different hydrogels via reversible switches, such as host guest interactions and hydrogen bonding, have been explored to construct bilayer hydrogel actuators.
Normally, engineers tend to utilize layer-by-layer polymerization for fabricating hydrogel-based soft robotics with bilayer structures 
. To be more specific, in the preparation process, the monomer solution of the second layer slightly penetrates into the first layer, leading to the formation of an interpenetrating-network at the interface, which acts as a junction layer to connect the two layers tightly. For instance, He et al. 
reported a bilayer poly (N-isopropylacrylamide)/graphene oxide (pNIPAm/GO) hydrogel capable of achieving tunable, fast and bidirectional bending under a thermal or near-infrared radiation (NIR) stimulus (a). By tuning the GO concentration and centrifugation speed, a transparent poly NIPAM layer and a dark-brown GO-rich layer are formed a(ii), where each layer displays distinct network structures and swelling behaviors.
Bilayer-based hydrogel soft robotics. (a
) Bilayer poly (N-isopropylacrylamide)/graphene oxide (pNIPAm/GO) hydrogel actuator (reproduced with permission from Reference 
) Temperature and pH responsive hydrogel actuator and its SEM image of the interface of the two layers (reproduced with permission from Reference 
) The microstructure and deformation mechanism of mimosa-inspired hydrogel actuator and SEM image showing the bilayer structure tightly joined by a 5μm interfacial layer (reproduced with permission from Reference 
) Schematic self-bending deformation of the pH responsive bilayer hydrogel actuator and the SEM image of the cross-section of the bilayer hydrogel (reproduced with permission from Reference 
Different from traditional hydrogel networks which are held together by covalent crosslinks, another series of hydrogels called interpenetrating polymer networks (IPNs) are formed by combining two polymers (with at least one being responsive) that physically interact with each other to hold the network together 
. Furthermore, there are semi-IPNs that are composed of a covalently crosslinked hydrogel infused with another linear polymer which is physically interpenetrated inside the network 
. Compared with single network hydrogels, hydrogels with IPNs are possible to produce “new materials” under the work of each component exhibiting new behavior that is not expected from the responses of the individual components 
. For example, by generating a poly(N-isopropylacrylamide)-based hydrogel in the presence of positively charged polyelectrolyte poly(diallyl dimethylammonium chloride) (pDADMAC) on a layer of gold-coated polydimethylsiloxane (PDMS), Li et al. 
fabricated a temperature and pH responsive semi-IPN hydrogel-based bilayer actuator (b). Notably, the image in b clearly shows the crosslinked hydrogel network attached to the PDMS surface, and the two layers do not separate, even after many rounds of bending and unbending.
In addition, the layer-by-layer polymerization can also be used to fabricate bilayer hydrogels with different functions. For example, Zhen et al. 
reported a mimosa-inspired bilayer hydrogel actuator which can function in multi-environment conditions (c). Featured with a reverse thermal responsive bilayer composite structure, this hydrogel-based actuator is composed of a layer derived from a polymer with a lower critical solution temperature (LCST) and a second layer with an upper critical solution temperature (UCST). After heating, this bilayer hydrogel actuator could transfer water molecules from the LCST layer to the UCST layer. After cooling, the opposite process takes place, allowing for the actuation even in non-aqueous environments. Naturally, hydrogel actuators meet the extreme demand for biomedical applications due to their excellent biocompatibility and biodegradation. Duan rt al. 
have presented a bilayer-based hydrogel actuator inspired by the bilayer structure of plant organs (d), which is fabricated by adding cellulose to the CS and cellulose/carboxymethylcellulose (C/CMC) pre-gel alkaline aqueous solutions. By strong electrostatic attraction and chemical crosslinking, this hydrogel actuator shows a remarkable adhesion between two layers, which is about 20 kPa in tensile test.
Though bilayer hydrogel-based soft robotics have been widely investigated, there are still some challenges. The main challenge is that they could only undergo simple shape deformations, such as bending, since the hydrogels are typically isotropic materials which usually exhibit uniform volumetric expansion and contraction in response to stimuli.
3.2. Gradient Hydrogel-Based Soft Robotics
Gradient hydrogel is another inhomogeneous structure to expand the application of hydrogels since it is efficient to produce complex shape deformations. One of the most common practices is to embed stimuli-responsive nanoparticles into the gradient hydrogel and utilize the migration of nanoparticles under external electric or magnetic fields during the polymerization process 
. For example, Yang et al. 
reported a series of gradient hydrogels inspired by the bilayer structures of plant organs (a). They used the facile electrophoresis method and NIR-induced fabrication method to successfully create gradient-based hydrogel actuators. Upon near-infrared light irradiation, the hydrogel exhibits comprehensive actuation performance as a result of directional bending deformation and high photothermal conversion efficiency of graphene oxide embedded in the poly (N-isopropylacrylamide) hydrogel. Furthermore, gradient hydrogel-based soft robotics can also be fabricated by an asymmetric distribution of polymer chains. For instance, Maeda et al. 
reported a self-walking hydrogel-based actuator without nanoparticles. In their study, two different surfaces of plates, namely a hydrophilic glass surface and a hydrophobic surface (i.e., Teflon), were used to induce uneven distribution of materials based on the difference in hydrophilicity and hydrophobicity during the polymerization.
Gradient-based hydrogel soft robotics. (a
) Photo/thermal-responsive PNIPAm/GO gradient hydrogel actuator (reproduced with permission from Reference 
) Thermal-assisted extrusion 3D printing of the biohybrid gradient scaffolds for repair of osteochondral defect and SEM images of the printed porous hydrogel scaffolds (reproduced with permission from Reference 
) Thermal-responsive pNIPAm/Laponite hydrogel actuator (reproduced with permission from Reference 
It is worth noting that the emerging 3D printing technique is another efficient method to fabricate gradient hydrogel-based soft robotics. For instance, Fei et al. 
presented a new bio-ink fabricated by one-step copolymerization of dual hydrogen bonding monomers, N-acryloyl glycinamide and N-[tris(hydroxymethyl)methyl] acrylamide. The as-printed hydrogel has excellent mechanical properties of high tensile strength (up to 0.41 MPa), large stretchability (up to 860%) and high compressive strength (up to 8.4 MPa). It is obvious that 3D printed gradient hydrogels are more likely to be applied to the repair of biological tissues due to its capability for accommodating tailoring structures 
. Besides, engineers prefer to fabricate hydrogels with shorter response time and higher mechanical properties to be suited to more specific scenarios. For instance, Yun et al. 
fabricated a temperature-response poly (N-isopropylacrylamide)/Laponite (pNIPAm/Laponite) gradient nanocomposite hydrogel actuator by using a facile electrophoresis method. The actuator exhibits a rapid (20 s response time) and reversible response, as well as large deformation (bending angle of 231°), which is due to the graded forces generated by the thermo-induced anisotropic shrinkage and extension of the gradient hydrogels.
3.3. Patterned Hydrogel-Based Soft Robotics
Patterned hydrogel is a result of the exploration of hydrogels with anisotropic structures in plane to create complex 3D structures, which achieves anisotropy by adding different patterns on the hydrogel sheet. For instance, Jasmin et al. 
reported an enzyme-triggered hydrogel which mimics the structure of Venus fly trap. In their study, a class of self-folding hydrogel actuators which can respond to specific stimuli, such as matrix and cells at a low concentration, was fabricated. Specifically, the actuator has a bilayer configuration with a bottom passive layer and a top active layer composed of two alternating layers. When the flat hydrogel actuator is put in water, the bottom layer tends to swell more than the top layer as it is more hydrophilic. At this point, the top layer is still stiff, restricting the premature deformation. Only when the collagenase enzyme is added to the water can the stiff top layer be softened, which facilitates the swollen C layer to fold over the top layer and results in the shape transformation from a sheet to a specific shape a(ii).
Furthermore, Wu et al. 
reported a patterned hydrogel-based sheet in which 3D shape transformation is achieved. This hydrogel-based sheet has periodic stripes with different compositions that are arranged at an oblique angle with respect to the long axis of the sheet. These different stripes have large differences in swelling/shrinking ratios and mechanical moduli, which is responsible for the transformation from a plant sheet to a helix when it is exposed to temperature stimuli.
Though fabricating hydrogels with versatile patterns by 3D printing has become an emerging method 
, the typical stepwise layer-by-layer process has severely limited the printing speed. Featured with a quick 2D to 3D transformation induced by the controllable uneven stress distribution, 4D printing has become a better alternative to fabricate pattern-based hydrogel soft robotics. For instance, Huang et al. 
fabricated a series of thermal-responsive hydrogel sheets with various patterns, which achieves complicated configuration change. They fabricated a flat hydrogel sheet which has concentric circular print layout with different exposure time of 12 and 24 s, respectively. Upon swelling by heating above 45 ℃, the printed flat sheet transforms into a 3D round cap, and the cap geometry can be precisely tuned by the radius ratio (r/R) in the original layout.
Similarly, by employing 4D printing, Ding et al. 
reported an approach to print composite polymers with 3D architectures which allows the object to transform into a new permanent shape by releasing the constraint on the strained elastomer of shaper memory polymer with time (the fourth dimension) after heating, which can then be reprogrammed into multiple subsequent shapes. They simplified the creation of high-resolution complex 3D reprogrammable structures by controlling the photopolymerization process during printing to enable 3D components with a complex geometric form at high spatial resolution. When removed from the build tray, those components exhibit high-fidelity features but with controlled built-in strains. The transformation step is also simply triggered by heating, and the shape remains stable in later variations in temperature (d).
Pattern-based hydrogel soft robotics. (a
) Fabrication of enzyme-responsive hydrogel actuator by UV-photolithography (reproduced with permission from Reference 
) Thermal responsive hydrogel actuator with periodic stripes (reproduced with permission from Reference 
) Experimental printing setup and process illustration from a planar sheet with patterned concentric circles swollen into a cap-shape 3D structure (reproduced with permission from Reference 
) The concept of direct 4D printing and its versatility in printing various configurations with a single material (reproduced with permission from Reference