# 3D and 4D Printing of Multistable Structures
## 4 Conclusions and Future Prospects  
Lastly, in this section, we introduce a few more recent works that utilized multistable structures for actuation and reconfiguration. Although they are not yet fully 3D-printed, these works provide interesting perspectives on multistability. As multi-material 3D-printing technologies are developing rapidly, we expect that similar structures could also be realized via 3D printing in the near future. [PERSON] et al. [64] developed a bistable spine mechanism for soft-legged robots (Figure 11a-c). They demonstrated high-speed yet energy-efficient spine flexion and extension with insights from quadrupedal mammals. High-speed locomotion requires the rapid storage and release of large mechanical energy as well as high force output. However, most soft robots have slow response time and low energy exertion due to material softness and structural compliance. Motivated by galloping cheetahs, a bistable hybrid soft bending actuator was proposed to overcome this limitation. It was built by joining 3D-printed, spring-based bistable linkages (\"spine\") to soft pneumatic bending actuators (\"muscles\") (Figure 11a,b). They demonstrated a high-speed soft crawler (Figure 11c) using a bistable spine mechanism, which is over 2.5 times faster and still requires less input energy for operation than high energy density dielectric crawlers. Figure 11: **(a)** Schematic of a bistable hybrid soft bending actuator (BH-SBA). It consists of two soft air-bending actuators, 3D-printed bistable linkages, and a preloaded spring that stores potential energy and releases when the air channel is pressurized; **(b)** Energy diagram of the bistable actuator. The axially pre-tensioned spring makes maximum energy at the zero bending angle. As the spring releases the energy with bending, the whole energy decreases and finally reaches the energy minima (\(\mathrm{\theta_{eq}}\)); **(c)** Fabricated bio-inspired crawler with the spine actuation. The spine bends upward to store energy and downward to release energy (Adapted from [64]. Copyright (2020) AAAS); **(d)** Fabricated soft actuator that can jump when the inner spherical cap flips downward during inflation; **(e)** Pressure-volume curve of the soft actuator. Isochoric snapping can occur and the inner spherical cap flips downward. During the flipping, the volume of cavity is maintained but the pressure drops. \(\mathrm{\Delta E}\) is the amount of energy releasing (Adapted from [65]. Copyright (2020) AAAS). [PERSON] et al. introduced a fluidic soft actuator utilizing the snapping mechanism of a spherical cap, which turns a slow input signal into a remarkably swift jump (Figure 11d,e) [65]. The fluidic soft actuator consists of elastomeric inner and outer spherical caps that are connected at their bottom to form a cavity, where incompressible fluid can flow. Isochoric snapping instability leads to a sudden release of substantial elastic energy, the amount of which is given by the enclosed domain between the upper limit point and the equivalent isochoric point on the lower in the pressure-volume curve. A rapid inversion of the inner cap occurs, resulting in ultimately jumping. The amount of energy release can be modulated by the geometry, stiffness, and the boundary conditions of the caps, so that the response of the actuators can be enhanced even though the inflation rate is tardy. Multistability can also be utilized to enable reconfigurable microelectronic devices. For example, [PERSON] et al. fabricated morphable 3D mesostructures and microelectronic devices using multistable structures based on pre-strained elastomers and 2D precursors [66]. Reversible reconfiguration is obtained by strategically organizing the time sequence of the strain release and by engineering precursor designs. They demonstrated more than 20 examples for MOSFET, LED, and RF circuit integrated devices. In this paper, we have briefly introduced recent developments in 3D and 4D printing of mechanical multistable structures.