Usually, soft robotic movement, like locomotion, needs cyclic actuation of the light-responsive material, which is attained with spatially and/or temporally modulated light source. Recent studies have suggested a more advanced strategy, where the cyclic shape change is self-sustained and the structure can maintain its motion without a need to control the properties or spatial location of the power source.
Coincidentally, such self-sustained motion is also ubiquitous in nature, for instance, heartbeat, pulsation, and cell cycling being some examples.
The self-sustained motion, most widely studied in LCNs, is based on photomechanical-bending-induced oscillation, which was firstly observed in azobenzene-containing cantilevers.155,234 Those cantilevers were irradiated with a focused laser beam, triggering the bending motion towards the light source due to combination of photothermal and photochemical effects. Bending of the film exposed one of the surfaces to light, which subsequently caused bending to opposite direction and exposure of the other surface, as schematically shown in Fig. 4.1a. This caused alternated activation of both cantilever surfaces and oscillation at high frequency (up to 270 Hz). By adjusting parameters such as cantilever dimensions, laser intensity and polarization direction, different oscillation amplitudes and frequencies could be obtained. Also, by tuning the angle between the director n and the cantilever long axis, out‐of‐plane twisting was obtained.235 Bending-type oscillation can also be achieved by self-shadowing effects as schematically illustrated in Fig. 4.1b.131,144,209,236 In this case, LCN or hydrogel cantilevers comprise photothermal agents that cause light-induced heating and yield photothermal bending from specific position defined as a hinge. Once the cantilever bends, the structure itself sweeps across the beam path and shadows the hinge. Finally, the hinge cools down, cantilever relaxes and simultaneously exposes the hinge to light, and the whole process repeats.
Self-sustained motion has been used to realize LCN-based micro-devices, in which the light energy is converted to the mechanical energy to provide work output.162,164,180,237,238 For example, a self-sustained plastic motor can rotate upon simultaneous irradiation from two sides using different wavelengths (Fig. 4.2a).162 Self-sustained walker was fabricated by confining an LCN cantilever into a rigid support structure, and photothermal wave-like oscillator was used to propel locomotion of the structure (Fig. 4.2b).177 In hydrogels, a pillar structure capable of 0.7 Hz oscillation upon continuous irradiation was demonstrated, and such
self-sustained motion was further developed into a swimming protype on top of water surface (Fig. 4.2c).209
Figure 4.1 Self-sustained oscillation based on bending-induced exposure of different sides of the cantilever to light (a)155, and self-shadowing (b)131. Figures reproduced with permission: a) Copyright 2005, RCS. b) Copyright 2017, John Wiley and Sons.
Figure 4.2 Self-sustained locomotion under light illumination. a) LCN motor162, b) LCN wave-machine177, c) hydrogel swimmer209. Figures reproduced with permission: a) Copyright 2008, John Wiley and sons. b) Copyright 2017, Springer Nature. c) Copyright 2019, AAAS.
Being responsive or even responding autonomously to a constant stimulus is just the beginning of the advanced photo-control of smart soft actuators. An advanced robot should be capable of reacting to multiple stimuli from the environment. This is often inspired by nature, where numerous species can adapt to the environmental changes including temperature, light, humidity, chemicals, etc. However, obtaining this goal in synthetic material systems is still challenging. Usually, most LCN or hydrogel photoactuators are based on a single actuation mode upon irradiation with one specific light wavelength. To extend the actuating modes, multicolor actuation can be used, which enables orthogonal wavelength control through incorporation of photoactive molecules responding to different parts of the light spectrum. For example, splay-oriented LCNs having two regions with different absorbing azobenzenes have been fabricated with inkjet printing.239 By controlling bending of each stripe in a sequence of different light pulses, non-reciprocal motion was produced mimicking the motion of natural cilia (Fig. 4.3a). An LCN actuator assembly was reported to exhibit robotic locomotion in air240,241 (Fig. 4.3b) and under water, where the movement direction and other functions, like gripping, could be steered with different wavelengths.242 PNIMPAm hydrogel with altering layers of nanospheres (absorbing at 546 nm) and nanoshells (absorbing at 785 nm) was reported to show different swelling behavior upon different light fields (Fig. 4.3c).243 Through different chemical composition, one can include different controllability into the material. However, the above-listed examples are still restricted by the limited form of actuation that has been dictated during the fabrication, for instance, the fixation of LC molecular alignment.
In nature, many species possess various degrees of freedom in movement – they can change their body in many ways by reacting to one environmental stimulus – having a reconfigurable response. In artificial materials, to realize this kind of smartness has been one of the grand challenges. To achieve reconfigurability in photoactuation, one must be able to tune the material behavior after fabrication and re-program the actuation on demand for different applications.
This issue has been raised in some recent studies, and one extensively used approach is to adjust crosslinking density after sample fabrication by utilizing dynamic covalent chemistry. In this chemical architecture, a reconfigurable polymer system should contain elements that can reversibly form/cleave covalent bonds under specific stimuli, for instance, transesterification244 or Diels-alder reactions245 in LCN matrix. Another strategy is to utilize light-sensitive reactions based on disulfide metathesis246,247 or reaction with allyl sulfide248, to induce patterns of the bond-cleavage/forming across sample area. Recently, Zhao and co-workers
demonstrated reconfigurable photoactuator based on selective de-crosslinking of anthracene dimers controlled by light.249 The LCN was crosslinked via anthracene dimers, but upon irradiation with UV light, anthracene undergoes photocleavage and decrosslinking occurs. Thus, the sample could be patterned with crosslinked (actuation domains) and uncrosslinked (non-actuation domains) regions. Upon heating the polymer directly or phothermally by activation of NIR absorbing dye, it undergoes reversible shape-morphing determined by the photopatterned regions (Fig. 4.4a).
Figure 4.3 Multicolor actuation. a) Artificial LCN cilia producing non-reciprocal motion under distinct light illumination.239 b) A soft robot made by LCN assemblies able to move and grip the cargo.178 c) Wavelength-selective shaping of a PNIPAm structure containing gold nanospheres and -shells in alternating layers.243 Figures reproduced with permission: a) Copyright 2009, Springer Nature. b) under terms of CC BY 3.0, Copyright 2020, The Authors. c) Copyright 2012, ACS.
Without influencing the crosslinking density, reconfigurability can be obtained by adjusting the properties of the mesogens locally. For example, LCNs containing azomerocyanine photoswitch, which can be converted to hydroxyazopyridinium-form locally with acid treatment (vice versa, with a base), have been used.250 The two
forms have distinct absorption spectra and actuator cantilever can be designed to have hinges responding to specific wavelengths of light depending on the spatial distribution of state of the switches. The same LCN sample can be written, erased, and rewritten with different patterns, yielding different shape-morphing under identical illumination conditions (Fig. 4.4b).250 Another study utilized a bilayer structure combing SMP and azo-LCN. Specific shapes could be programmed by processing the SMP above its Tg, while reversible bi-shape morphing was achieved by activating the azo-LCN layer with UV/blue light at room temperature.165 In hydrogels, re-writable shape changes are also reported. For instance, activation of the photothermal agent locally using patterned light206 or (re-)patterning photothermal agents locally via stamping approach can be used for distinct light actuation (Fig. 4.4c).251
In this thesis, approaches to all the above-mentioned challenges for controlling photoactuation have been proposed and are elaborated in detail in the next three sections.
Figure 4.4 Reconfigurable photoactuators. a) Controlling dimerization of anthracene with UV light, LCN film can be patterned with light active (purple) and inert (blue) regions. Actuation is then possible by heat or light stimulus.249b) LCN actuator where light-active areas can be patterned with an acid.250c) A reconfigurable shape morphing of hydrogel thin film upon irradiation by varying the shape of the nanoparticle patterns (black areas).251 Figures reproduced with permission: a) Copyright 2019, John Wiley and Sons, b) Copyright 2017, John Wiley and Sons. c) Copyright 2019, ACS