Abstract
The response of soft actuators made of stimuli-responsive materials can be phenomenologically described by a stimulus-deformation curve, depicting the controllability and sensitivity of the actuator system. Manipulating such stimulus-deformation curve allows fabricating soft microrobots with reconfigurable actuation behavior, which is not easily achievable using conventional materials. Here, we report a light-driven actuator based on a liquid crystal polymer network containing diarylethene (DAE) photoswitches as cross-links, in which the stimulus-deformation curve under visible-light illumination is tuned with UV light. The tuning is brought about by the reversible electrocyclization of the DAE units. Because of the excellent thermal stability of the visible-absorbing closed-form DAEs, the absorbance of the actuator can be optically fixed to a desired value, which in turn dictates the efficiency of photothermally induced deformation. We employ the controllability in devising a logical AND gate with macroscopic output, i.e., an actuator that bends negligibly under UV or visible light irradiation, but with profound shape change when addressed to both simultaneously. The results provide design tools for reconfigurable microrobotics and polymer-based logic gating.
Highlights
Soft robotics is a research frontier dedicated to combine the flexibility of soft materials with the accurate control inherent to conventional rigid-bodied robots, anticipated to provide technological breakthroughs for human-friendly interfaces, controlled locomotion, and bioinspired robotic adaptation.[1]
An attractive alternative to trigger actuation is the use of the photothermal effect, i.e., the molecular disorder created by heat released during nonradiative relaxation of photoexcited moieties.[16−19] In the past decade, many photothermally fueled liquid crystal polymer networks (LCNs) robotic movements have been demonstrated, including 3D kirigami/origami devices,[20] light steering motion by walking[21] and swimming,[22] object manipulation through gripping,[23,24] and self-sustainable oscillation.[25−27]
The electrocyclization was studied with 1H NMR, which under 365 nm irradiation yielded a photostationary state (PSS) consisting ca. 56% of the closed-form DAE, the quantum yield (QY) for the o−c conversion being Φo→c = 0.54
Summary
Soft robotics is a research frontier dedicated to combine the flexibility of soft materials with the accurate control inherent to conventional rigid-bodied robots, anticipated to provide technological breakthroughs for human-friendly interfaces, controlled locomotion, and bioinspired robotic adaptation.[1] To apply the soft robots in single-cell manipulation, drug delivery/release, or microfluidics, their size has to be miniaturized.[2] For this, stimuli-responsive materials are often adopted,[3,4] allowing to activate robotic movements wirelessly using heat, magnetic or electric fields, humidity, or light.[5−9] Among the different classes of stimuli-responsive materials, light-driven liquid crystal polymer networks (LCNs) stand out due to their large shape-changes, versatile control over deformation (e.g., bending, coiling, and twisting), easy scalability from centimeter down to micrometer size, and high spatial and temporal resolution of the light activation.[10,11] Conventionally, the light-fueled actuation of LCNs is triggered by photoswitchable molecules incorporated into the polymer network.[12,13] Upon photon absorption, the photoswitches undergo reversible shape changes and induce disorder into the initially ordered polymer network, yielding photochemically induced macroscopic actuation.[14,15] An attractive alternative to trigger actuation is the use of the photothermal effect, i.e., the molecular disorder created by heat released during nonradiative relaxation of photoexcited moieties.[16−19] In the past decade, many photothermally fueled LCN robotic movements have been demonstrated, including 3D kirigami/origami devices,[20] light steering motion by walking[21] and swimming,[22] object manipulation through gripping,[23,24] and self-sustainable oscillation.[25−27]
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