The actuator “learns” to respond to the irradiation by bending after the association process, as depicted by the thermal camera images shown in Figure 3 . Before the association, the strip exhibits negligible photoactuation, with about 4° bending and maximum 5°C temperature increase upon 290 mW cmirradiation. The conditioning enables the material to evolve to a new state, in which the strip shows 25°C temperature increase and >90° bending under identical irradiation conditions. This corresponds to a 5-fold increase in the photogenerated heat and a 20-fold increase in the light-induced deformation as compared with the unconditioned sample ( Figure 2 E).

Light-induced deformation and photothermal heat generation are significantly enhanced through the association process. The temperature scale is chosen such that it properly enhances the image contrast. The exact temperature increase is given in Figure 2 E. Scale bars, 4 mm.

The light response can be promoted by diffusion of the dye molecules into the interior polymer network, thus providing a time scale for the association process, i.e., time-dependent memory. Driven by heat, the dye molecules initially confined to random positions on the planar sample surface start diffusing into the bulk network, leading to an increase in the overall light absorption. The diffusion flux J is given by the Fick's law,, where D is the dye diffusivity at temperature T, ϕ the molar concentration of the dye, andthe dye concentration gradient at the dye-polymer interface. Figure S6 presents J at different temperatures, showing that the diffusion remains negligible up to 80°C but rapidly increases above 100°C. This explains the fact that upon heating to 70°C or irradiation (photothermal heating <20°C; Figures S3 and S4 ), the dye diffusion is limited and the change in material absorption negligible ( Figure 2 C). More details on diffusion flux calculation are given in Experimental Procedures

The present actuator is based on a splay-aligned LCN film covered on the planar side with a dye, Disperse Blue 14 ( Figure 2 A), which acts as a light absorber. The splayed alignment ensures bending upon heating as the intrinsic (unconditioned) response, and heating can also be achieved via light absorption by the dyes and subsequent photothermal effects.The programmed response is shown in Figures 2 B–2E. As expected,heating above the glass transition temperature (T≈ 40°C, see Figure S1 ) leads to gradual bending toward the planar-oriented side of the film ( Figures 2 B and S2 ). Note that the LCN actuator shows an initial curvature toward the homeotropic side due to anisotropic thermal expansion during cooling from polymerization temperature to room temperature. Irradiation at 635 nm yields negligible bending ( Figure 2 C), as the dye particles are localized only at scattered spots on the surface, and most of the incident light (>80%) simply penetrates the sample or is scattered by the dye clusters. Therefore, the light absorbed by them is insufficient to induce bulk heating of the actuator. The moderate temperature enhancement is due to dyes being clustered on the surface, in combination with relatively low irradiation intensity ( Figure S3 ). Light-induced heating of the bulk LCN is dictated by the dye distribution, while sample dimensions yielding different thermal gradient conditions also affect the achievable temperature. Figures S4 and S5 present detailed photothermal characterization by infrared imaging.

(E) Light response after association. Left: light-induced heating and deformation in “conditioned” and original samples. Error bars (indicated by the widths of the lines connecting the measured points) denote standard deviations for n = 3 measurements. Insets: photographs showing the response of the “conditioned” sample under irradiation and the bending angle (α). Scale bars, 2 mm.

In LCNs, the interplay between the thermal expansion of the material and control over mesogen orientation within the self-assembled network classically allows macroscopic actuation in response to a variety of external stimuli.Alignment programming enables the design of actuators with versatile deformation modesand externally controlled devices with sophisticated properties.Recently, reconfigurable actuators based on, for example, dynamic covalent bondsor synergistic use of photochemical and photothermal effectshave been demonstrated to yield multiple deformation modes under identical illumination conditions, allowed by a programming step prior to the shape morphing. However, no actuators that can be activated by a new, initially neutral stimulus, to allow response upon conditioning, have been presented. The development of the LCN actuator with an associative memory (as opposed to other types of stimuli-responsive systems) is motivated by the following three reasons: (1) stimuli-driven actuation such as contraction or bending is reversible and easy to quantify, rendering changes in the material response straightforward to monitor; (2) LCNs are intrinsically thermoresponsive, and often sensitive also to other stimuli such as light, serving as a good basis for choosing the stimuli; (3) the field of soft robotics is drawing growing attention,and soft devices that “learn” could provide unforeseen opportunities for future microrobotics.

Soft Robots and Conditioning with Associative Memory

46 Hu W.

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Sitti M. Small-scale soft-bodied robot with multimodal locomotion. , 47 Kohlmeyer R.R.

Chen J. Wavelength-selective, IR light-driven hinges based on liquid crystalline elastomer composites. 48 Palagi S.

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et al. Structured light enables biomimetic swimming and versatile locomotion of photoresponsive soft microrobots. , 49 Rogóż M.

Zeng H.

Xuan C.

Wiersma D.S.

Wasylczyk P. Light-driven soft robot mimics caterpillar locomotion in natural scale. −1) on a surface with asymmetric friction ( Figure 4 Pavlov-Inspired Soft Robots Show full caption (A–E) Training flow of the walker with associative memory. An original LCN-based walker (A) deforms upon heating (B) and is insensitive to light (C). After association of the two stimuli, the absorption of the walker around 635 nm increases (D), which allows for efficient photoactuation upon red-light irradiation (E). (F) Superimposed images showing the “conditioned” walker translocating on a ratchet-structured surface under temporally modulated illumination. (G–K) Grippers that learn to respond to different irradiation wavelengths. (G) Photographs of grippers composed of original actuator (I), associated with red light (II), and associated with blue light (III). (H) Chemical structures and absorption spectra of the dyes used and the wavelengths used for the association process. DR1, Disperse Red 1; DR14, Disperse Blue 14. (I) An original gripper (I) and grippers associated with red (II) and blue light (III) that close upon heating to 70°C. (J) Only gripper III closes upon irradiation with blue light (488 nm, 300 mW) that is scattered by the white paper strip at room temperature (RT). (K) Only gripper II closes upon irradiation with red light (635 nm, 300 mW) at RT. Dashed arrows indicate the opening (white) or closing (green) of the grippers. Scale bars, 5 mm. As shown above, the process inspired by classical conditioning enables a thermoresponsive material to “learn” to respond to light, i.e., to show the response (bending) based on an initially neutral stimulus (light). We believe, more generally, that acquiring new stimuli for the responses by association processes becomes important for the emergent wireless soft microrobotics.Light is an attractive energy source for remotely controlled microrobotics due to its tunability (e.g., in wavelength and intensity) and the high degree of spatial and temporal control over the properties of light fields. Figures 4 A–4E illustrate the role of the association process in devising a locomotive robot that “learns” to walk by irradiation. The LCN-based robot is initially sensitive to heat to allow bending and locomotion only by thermal pulses, but is insensitive to light ( Figures 4 A–4C), yet becomes light-active after associating the two stimuli ( Figures 4 D and 4E) through the mechanism explicated in Figure 2 . Under temporally modulated irradiation, the “conditioned” soft robot starts to walk (velocity ∼1 mm s) on a surface with asymmetric friction ( Figures 4 F and S11 Video S1 ), which is beyond its capabilities before the conditioning process.

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50 Holland J.H. Adaptation in Natural and Artificial Systems. Biologically, the classical conditioning can take place in response to a variety of stimuli, thus enabling the animal to adapt to different changes in environmental conditions.For instance, the bell in the classical Pavlov's dog experiment could be replaced by an arbitrary stimulus the dog can perceive, for example, music or a flash of light. While the diversity of applicable neutral stimuli in our present artificial materials is limited though expandable by using multicomponent hybrids, we next demonstrate some degree of tunability in its choice, more specifically the differentiation between light colors. The associative memory in the present actuator relies on the time constants given by the dye diffusion from one surface to the bulk, leading to their rearrangement, and, due to the wealth of dyes with different spectral properties available, the absorption region can be easily tuned. This tunability is exemplified in Figure S12 where dyes absorbing near-UV (Disperse Orange 3), blue-green (Disperse Red 1), and red light (Disperse Blue 14) are used for the association process upon 405, 488, and 635 nm irradiation, respectively. Utilizing the spectral selectivity, we designed soft grippers that are tuned to recognize and respond to different colors of light after the association. Figure 4 G shows photos of the actuator described above based on Disperse Blue Blue 14 (I, unconditioned; II, conditioned), as well as the conditioned one based on Disperse Red 1 (III). The spectra of the photosensitive dyes and the corresponding light wavelengths used for association process are shown in Figure 4 H. All grippers are thermoresponsive, closing around an inserted object at sufficiently high temperature ( Figure 4 I). Once they are placed in front of a strongly scattering object (a white paper strip for demonstration), the “conditioned” grippers only close when the wavelength of the scattered light matches the absorption range of the dye (red light for gripper II, blue light for gripper III), while the original gripper remains indifferent to irradiation ( Figures 4 J and 4K). More details on the gripper realization and light actuation are presented in Figure S13