NIR-Responsive Liquid Crystal Elastomers And Their Soft Actuators

Liquid crystal elastomer(LCE)is a kind of intelligent polymer material with high elasticity of rubber and anisotropy of crystal.It is widely used in the preparation of various actuators attributing to the outstanding responsive behaviors(such as contraction/expansion)to external stimuli(such as light,heat,magnetic and electric field).For photoresponsive LCE,the shape morphing mechanism generally include photochemical response and photothermal response.Currently,photochemical strategy mainly relies on UV to induce azobenzene group producing isomerization,but UV is not friendly to the environment and is harmful to human health,limiting practical applications.Near-infrared light(NIR)has low energy,showing good biosafety and being used in biomedical researches.As a light source,NIR has good photothermal effect,which is an ideal tool to drive LCE by photothermal effect.Therefore,this paper will focus on the NIR responsive LCEs to design and synthesis the NIR-absorbing monomer,and select an appropriate material chemistry method to prepare NIRresponsive LCE,and further design different LCE actuators through processing,exploring the various functions and applications of actuators.(1)Initially,we designed and synthesized a diacrylate modified croconium dye,which has strong NIR-absorbing intensity.Through the thiol-ene click reaction and the mechanical stretching method to prepare high-performance uniaxially oriented LCE film.We further characterized LCE film with UV-Vis spectrum,photothermal effect,tensile properties,photomechanical properties and 2D-XRD,etc.The results show that LCE film has high NIR-absorbing performance,thermal stability and mechanical properties.LCE exhibits ～40% actuation strain and 1.6 MPa actuation strength upon NIR.Under NIR exposure,LCE is able to pull a heavy weight that is 24456 times to LCE weight,showing the excellent responsive ability.(2)In addition,we create terrain-adaptive miniature soft robots that can be powered,guided,and maneuvered on challenging terrains by a versatile phototactic strategy.These robots,constructed with a rationally designed liquid crystal elastomer with powerful photomechanical actuation,enable self-actuation to generate autonomous and self-sustained rolling locomotion under constant near infrared light stimuli without any on-off switching.They exhibit exceptional terrain adaptability and outperform previous light-driven miniature robots in their capability to traverse a broad range of simple and complex terrains,and even hybrid terrains with varying topology,mechanics,and rheology.The terrain-adaptive robots can directionally leap over hurdlers,and even exert high jumping to overcome high wall obstacle.We envision that this proposed technique would enable the design of miniature mobile robots to accommodate varying terrains and fulfill multiple tasks in unpredictable environments.(3)For another,we show a facile strategy to photomanipulate shape morphing of soft millirobot made of a NIR-active liquid crystal elastomer by programming the direction and magnitude of strain gradient in the LCE via using the spatial and localized features of light stimuli.This photomanipulation strategy not only enables the millirobot to generate a wide range of morphing modes,but also allows local and reversible evolution between different morphing modes in a single soft robot.With this phototuable morphing capabilities,the millirobot is able to exert multimodal locomotive behaviors,including crawling,shifting,rotating,somersaulting,rolling,and even fast autonomous rocking driven by a self-oscillatory motion.In addition,the highly mobile millirobots show the multifunctions of not only navigating with controllable directions,but also escaping out from heavy weight,and even swimming through a small pipe.It is anticipated that this photoactive and reconfigurable material system would open exiting possibilities to develop intelligent soft robots with seamless integration of multimodal shape morphing and multifunctions.(4)Finally,we report a bioinspired robotic system that not only allows photomorphogenesis of on-demand 3D wavy morphologies,but also enables autonomous wave propagation in monolithic soft artificial muscle.This system employs a conceptually different design strategy based on a combination of two principles derived from plant morphogenesis and undulatory motion of ray fish.The former offers a shaping principle based on differential growth that enables morphing monolithic soft artificial muscle into target wavy configurations,while the latter inspires a driving principle that induces autonomous propagation of shaped waves by rhythmic motor patterns.This waving system can be used as adaptive “soft engines/motors” that enable directional locomotion,intelligent transportation of cargo,and autonomous propulsion.It even produces programmable,complex artificial peristaltic waves.Our design allows controllable formation of 3D wavy morphologies and autonomous wave behaviors in the soft robotic system would be useful for a variety of applications in adaptive,self-regulated mechanical systems for advanced robotics,soft machines,and energy harvest.