Kultananosauvapohjaiset Valomekaaniset Nestekide-Elastomeerit

Smart stimuli-responsive materials that can be externally triggered to undergo mechanical motions have a huge potential in applications ranging from artificial muscles and sensing to microrobotics. Among different classes of stimuli-responsive materials, photomechanical actuators based on liquid-crystal elastomers (LCEs) are of particular importance because they provide a route to control mechanical motions with light. Liquid-crystal elastomers exhibit a unique combination of anisotropic molecular order due to liquid crystallinity, and elasticity brought about by the polymer network. This coupling leads to the ability of free-standing LCE samples to change their shape reversibly after the application of external stimulus. Plasmonic nanoparticles, for example gold nano-rods, have the ability to absorb light strongly at different wavelengths, depending on their dimensions, which makes them a possible candidate to produce the light response needed to actuate LCEs. In the Thesis work three photoactuable liquid-crystal elastomers, which absorb light in different wavelengths, were studied. Infrared-absorbing gold nanorods, cross-linked UV-absorbing azobenzenes, and doped visible-absorbing azobenzenes, were used as light-absorbing moieties. The LCE that was used was acrylate-based and the sample films were prepared by applying well-developed liquid crystal-alignment technologies and photopolymerization. As a major part of this work, gold nanorods were synthesized and functionalized. Gold nanorods were synthesized with a seed-mediated growth method in water by using hexadecylcetyltrimethylammonium bromide as a surfactant and 5-bromosalisylic acid as an additive. Rods were also synthesized without the additive for comparison and it was found that nanorods synthesized with the additive had smaller size dispersion and better morphology. Gold nanorods were successfully functionalized and transferred to organic solutions with dodecanethiol when thiolated polyethylene glycol was used for pre-functionalization. The solubility of the functionalized gold nanorods into LCE mixture (comprising photopolymerizable monomer and cross-linker) was studied. However, functionalized gold nanorods were not soluble into the LCE matrix, whereas the two dyes showed a good solubility and polymerized films could be efficiently actuated with light. The main characterization methods used in this work were UV-Vis-NIR spectroscopy, transmission electron microscopy, and polarized optical microscopy

Reconfiguring Gaussian Curvature of Hydrogel Sheets with Photoswitchable Host–Guest Interactions

Photoinduced shape morphing has implications in fields ranging from soft robotics to biomedical devices. Despite considerable effort in this area, it remains a challenge to design materials that can be both rapidly deployed and reconfigured into multiple different three-dimensional forms, particularly in aqueous environments. In this work, we present a simple method to program and rewrite spatial variations in swelling and, therefore, Gaussian curvature in thin sheets of hydrogels using photoswitchable supramolecular complexation of azobenzene pendent groups with dissolved α-cyclodextrin. We show that the extent of swelling can be programmed via the proportion of azobenzene isomers, with a 60% decrease in areal swelling from the all trans to the predominantly cis state near room temperature. The use of thin gel sheets provides fast response times in the range of a few tens of seconds, while the shape change is persistent in the absence of light thanks to the slow rate of thermal cis–trans isomerization. Finally, we demonstrate that a single gel sheet can be programmed with a first swelling pattern via spatially defined illumination with ultraviolet light, then erased with white light, and finally redeployed with a different swelling pattern

Hydrogel Lasers Via Supramolecular Host-Guest Complexation

Peer reviewe

Humidity-Controlled Tunable Emission in a Dye-Incorporated Metal-Hydrogel-Metal Cavity

Actively controllable photoluminescence is potent for a wide variety of applications from biosensing and imaging to optoelectronic components. Traditionally, methods to achieve active emission control are limited due to complex fabrication processes or irreversible tuning. Here, we demonstrate active emission tuning, achieved by changing the ambient humidity in a fluorescent dye-containing hydrogel integrated into a metal-insulator-metal (MIM) system. Altering the overlapping region of the MIM cavity resonance and the absorption and emission spectra of the dye used is the underlying principle to achieving tunability of the emission. We first verify this by passive tuning of cavity resonance and further experimentally demonstrate active tuning in both air and aqueous environments. The proposed approach is reversible, easy to integrate, and spectrally scalable, thus providing opportunities for developing tunable photonic devices.publishedVersionPeer reviewe

Kultananosauvapohjaiset Valomekaaniset Nestekide-Elastomeerit

Smart stimuli-responsive materials that can be externally triggered to undergo mechanical motions have a huge potential in applications ranging from artificial muscles and sensing to microrobotics. Among different classes of stimuli-responsive materials, photomechanical actuators based on liquid-crystal elastomers (LCEs) are of particular importance because they provide a route to control mechanical motions with light. Liquid-crystal elastomers exhibit a unique combination of anisotropic molecular order due to liquid crystallinity, and elasticity brought about by the polymer network. This coupling leads to the ability of free-standing LCE samples to change their shape reversibly after the application of external stimulus. Plasmonic nanoparticles, for example gold nano-rods, have the ability to absorb light strongly at different wavelengths, depending on their dimensions, which makes them a possible candidate to produce the light response needed to actuate LCEs. In the Thesis work three photoactuable liquid-crystal elastomers, which absorb light in different wavelengths, were studied. Infrared-absorbing gold nanorods, cross-linked UV-absorbing azobenzenes, and doped visible-absorbing azobenzenes, were used as light-absorbing moieties. The LCE that was used was acrylate-based and the sample films were prepared by applying well-developed liquid crystal-alignment technologies and photopolymerization. As a major part of this work, gold nanorods were synthesized and functionalized. Gold nanorods were synthesized with a seed-mediated growth method in water by using hexadecylcetyltrimethylammonium bromide as a surfactant and 5-bromosalisylic acid as an additive. Rods were also synthesized without the additive for comparison and it was found that nanorods synthesized with the additive had smaller size dispersion and better morphology. Gold nanorods were successfully functionalized and transferred to organic solutions with dodecanethiol when thiolated polyethylene glycol was used for pre-functionalization. The solubility of the functionalized gold nanorods into LCE mixture (comprising photopolymerizable monomer and cross-linker) was studied. However, functionalized gold nanorods were not soluble into the LCE matrix, whereas the two dyes showed a good solubility and polymerized films could be efficiently actuated with light. The main characterization methods used in this work were UV-Vis-NIR spectroscopy, transmission electron microscopy, and polarized optical microscopy

Advanced Control Strategies of Light-Responsive Polymers for Soft Robotics

Pehmeä robotiikka on nopeasti kehittyvä tutkimusala, mikä pyrkii tuottamaan uusia pehmeisiin materiaaleihin pohjautuvia ratkaisuja nykyisin käytössä oleville metallikuorisille roboteille, joiden liikettä ohjataan sähköllä elektronisten komponenttien avulla. Lupaavimmat materiaalit pehmeiden robottien valmistuksessa ovat ”älykkäät”, ulkoisiin ärsykkeisiin reagoivat polymeerit, joiden liikettä voidaan ohjata käyttämällä polttoaineena valoenergiaa. Nämä niin sanotut valoaktuaattorit pystyvät absorboimaan valoenergiaa ja muuttamaan sen avulla muotoaan. Tyypillisen valoaktuaattorin toiminta perustuu valokemialliseen ilmiöön, eli ne sisältävät valoon reagoivia molekyylikytkimiä, joiden muotoa ja muita ominaisuuksia voidaan muuttaa valon avulla. Toinen tapa perustuu valotermiseen ilmiöön, jossa esimerkiksi orgaaniset molekyylit absorboivat valoa, muuttavat sen lämpöenergiaksi ja edelleen mekaaniseksi liikkeeksi. Tässä väitöskirjassa valoaktuaattorien ja pehmeiden robottien valmistuksessa käytetään kahta erilaista materiaalia: nestekidepolymeerejä ja hydrogeelejä. Nestekidepolymeerit mahdollistavat ennalta ohjelmoitavan muodonmuutoksen, jolloin saman materiaalin avulla pystytään tuottamaan erilaista liikettä. Hydrogeeleissä monimutkaiset muodonmuutokset voidaan ohjelmoida muuttamalla materiaalin ominaisuuksia paikallisesti polymeerin valmistuksen jälkeen. Molemmat materiaalit kykenevät reversiibeliin muodonmuutokseen valoenergian avulla ja ovat siksi erinomaisia materiaaleja pehmeässä robotiikassa. Hyödyntämällä sekä valotermistä että valokemiallista ilmiötä, tässä väitöskirjassa tutkitaan kolmea ohjausstrategiaa valoaktuaation aikaan saamiseksi: 1) itse ylläpidettyä liikettä kuten valomekaanista oskillaatiota, 2) moniväriaktuaatioita, jossa käytetään valon eri aallonpituuksia tuottamaan erilaista liikettä, ja 3) uudelleenohjausta, jossa aktuaattori voidaan valmistuksen jälkeen ohjelmoida liikkumaan eri tavoin. Tämän väitöskirjan tulokset syventävät ymmärrystä valotermisen ja valokemiallisen ilmiöiden roolista valoaktuaatiossa ja tarjoavat uusia ohjausstrategioita pehmeille roboteille.Soft robotics is a rapidly developing research field that has triggered a significant amount of research effort during the past few years. The field aims at providing new technical innovations to overcome the challenges encountered in conventional hard-bodied robotic systems constructed using rigid joints and links, such as lack of flexibility and adaptability. Among the most promising materials for the fabrication of soft robots are smart stimuli-responsive polymers that can be triggered with external energy sources to undergo various chemical and physical changes such as mechanical motions like contraction or bending. Among different classes of stimuli-responsive polymers, photomechanical actuators are of particular interest as they provide a route to harness light energy to remotely fuel mechanical motions. Today, most photochemical actuators are based on reversible photochemical switching of photochromic molecules between two forms with distinct structural and photochemical properties. On the other hand, photoactuation can also be driven photothermally using light absorption by organic dyes or inorganic moieties for heat generation, which stimulates the shape changes of the polymer. In this thesis we use liquid crystal networks and hydrogels as materials platforms to devise photoactuators and soft robots that can be controlled and powered remotely with light producing reversible shape changes. Liquid crystal networks enable pre-programmable shape changes and hence several actuation modes can be achieved within one material. In hydrogels, complex shape changes can be programmed by tuning materials properties locally after fabrication. By utilizing both photothermal and photochemical effects, we use three advanced light control strategies to power photomechanical actuation: self-sustained motion, multicolor functions, and reconfigurability. By using these strategies, we demonstrate sophisticated photoactuators exhibiting self-oscillation, non-reciprocal motions, logic gate actuation, reconfigurable gripping, and shape-morphing between Gaussian curvatures. The results of this thesis deepen the understanding on the role of photothermal and photochemical effects in controlling photomechanical actuation, and present new pathways and control strategies for soft micro-robotics

Reconfigurable photoactuator through synergistic use of photochemical and photothermal effects

A reconfigurable actuator is a stimuli-responsive structure that can be programmed to adapt different shapes under identical stimulus. Reconfigurable actuators that function without control circuitry and are fueled remotely are in great demand to devise adaptive soft robotic devices. Yet, obtaining fast and reliable reconfiguration remains a grand challenge. Here we report a facile fabrication pathway towards reconfigurability, through synergistic use of photochemical and photothermal responses in light-active liquid crystal polymer networks. We utilize azobenzene photoisomerization to locally control the cis-isomer content and to program the actuator response, while subsequent photothermal stimulus actuates the structure, leading to shape morphing. We demonstrate six different shapes reconfigured from one single actuator under identical illumination conditions, and a light-fueled smart gripper that can be commanded to either grip and release or grip and hold an object after ceasing the illumination. We anticipate this work to enable all-optical control over actuator performance, paving way towards reprogrammable soft micro-robotics.publishedVersionPeer reviewe

Design principles for non-reciprocal photomechanical actuation

Non-reciprocal motions are a sequence of movements exhibiting time-reversal asymmetry. Such movements are common among various natural species, being adopted as a typical strategy for achieving efficient locomotion. Generally, the realization of non-reciprocal motions in man-made robotic devices requires synchronous control of at least two individual actuators, hence posing challenges to soft micro-robotics where the miniaturization limits integration of different mechanical components and the possibility of using onboard batteries. Here, we introduce general concepts for achieving non-reciprocal movements in wirelessly controlled soft actuators made of photomechanically responsive liquid crystal networks. The monolithic actuators are composed of two segments that can be actuated photochemically and photothermally, and the non-reciprocal motion is obtained by a control sequence that temporally modulates light sources of different wavelengths. Through proper selection of photoactive compounds, the number of modulated light sources can be decreased, from three to two, and eventually to one. Finally, we demonstrate non-reciprocal self-oscillation by self-shadowing effect in a flexible strip under a constant light field with no temporal modulation. This study provides general guidelines to light-controlled non-reciprocal actuation, offering new strategies for the control of wireless soft micro-robotics.publishedVersionPeer reviewe

Tunable Photomechanics in Diarylethene-Driven Liquid Crystal Network Actuators

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. © 2020 American Chemical Society. All rights reserved

Humidity- And Temperature-Tunable Metal-Hydrogel-Metal Reflective Filters

A tunable reflectance filter based on a metal-hydrogel-metal structure responsive to humidity and temperature is reported. The filter employs a poly(N-isopropylacrylamide)-acrylamidobenzophenone (PNIPAm-BP) hydrogel as an insulator layer in the metal-insulator-metal (MIM) assembly. The optical resonance of the structure is tunable by water immersion across the visible and near-infrared range. Swelling/deswelling and the volume phase transition of the hydrogel allow continuous reversible humidity- and/or temperature-induced tuning of the optical resonance. This work paves the way toward low-cost large-area fabrication of actively tunable reversible photonic devices.publishedVersionPeer reviewe