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Activation of actuating hydrogels with WS nanosheets for biomimetic cellular structures and steerable prompt deformation Lu Zong, Mingjie Li, Jun You, Xiaochen Wu, and Chaoxu Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10348 • Publication Date (Web): 22 Aug 2017 Downloaded from http://pubs.acs.org on August 24, 2017

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ACS Applied Materials & Interfaces

Activation of actuating hydrogels with WS2 nanosheets for biomimetic cellular structures and steerable prompt deformation

Lu Zong†,‡, Mingjie Li†,*, Jun You†, Xiaochen Wu† and Chaoxu Li†,‡,* †

CAS Key Laboratory of Bio-based materials, Qingdao Institute of Bioenergy and Bioprocess

Technology, Chinese Academy of Sciences, Songling Road 189, Qingdao 266101, P. R. China ‡

University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, P. R. China

*Corresponding authors: Mingjie Li (Email: [email protected]) and Chaoxu Li (Email: [email protected]).

ACS Paragon Plus Environment


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ABSTRACT Macroscopic soft actuation is intrinsic to living organisms in nature, including slow deformation (e.g. contraction, bending, twisting and curling) of plants motivated by microscopic swelling/shrinking of cells, and rapid motion of animals (e.g. deformation of jellyfish) motivated by cooperative nanoscale movement of motor proteins. These actuation behaviours, with exceptional combination of tuneable speed and programmable deformation direction, inspire us to design artificial soft actuators for broad applications in artificial muscles, nanofabrication, chemical valve, microlenses, soft robotics etc. However, so far artificial soft actuators have been typically produced on the basis of poly(N-isopropylacrylamide)

(PNiPAM),

whose

deformation

is

motived

by

volumetric

shrinkage/swelling in analogue to plant cells, and exhibits sluggish actuation kinetics. In this study alginate-exfoliated WS2 nanosheets were incorporated into ice-template-polymerized PNiPAM hydrogels with mimic cellular microstructures of plant cells, yet prompt steerable actuation of animals. Because of the nanosheet reinforced pore walls in situ formed in freezing polymerization and reasonable hierarchical water channels, this cellular hybrid hydrogel achieves super deformation speed (the order of magnitude 100 s), controllable deformation direction and high near-infrared light responsiveness, offering an unprecedentedly platform of artificial muscles for various soft robotics and devices (e.g. rotator, microvalve, aquatic swimmer and water-lifting filter). KEYWORDS: biomimetics; actuating hydrogel; cellular structure; transition-metal dichalcogenide; prompt deformation


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INTRODUCTION In order to achieve macroscopic soft actuation with high deformation speed and controllable direction like living organisms.1-6 Smart actuating hydrogels, whose three-dimensional polymeric networks can uptake and release water in response to environmental stimuli (e.g. temperature, pH, chemicals, electricity, concentration and light etc.), have been pioneered to mimic shrinking/swelling behaviours of plant cells.7-9 For example, thermos-responsive hydrogels were frequently synthesized on

the

basis

of

poly(N-isopropylacrylamide)

(PNiPAM)

chemically

crosslinked

by

N,N-methylenebisacrylamide (BIS).8,10 However, because of slow water transport between the crosslinked networks and the circumstances, they typically exhibited sluggish actuation kinetics from minutes to hours at the lower critical solution temperature (LCST). Moreover, the inhomogeneity in their chemical structures and stimulus fields had to be precisely engineered to achieve controllable deformation directions.6 The instant and controllable actuation became even more challenging for their aquatic applications (e.g. in drug delivery, tissue engineering, medical devices, underwater devices etc.), since there lack efficient and remote methods to instantaneously vary the local temperature in vivo or underwater. With the purpose of diverse on-demand applications, intense endeavour have to be made to overcome the response bottleneck of smart hydrogels and accelerate their actuation behaviours to be comparable with the animal motions. According to the Tanaka-Fillmore theory of transformative hydrogels,11 the response rate is proportional to the transfer coefficient of water within the hydrogel, and inversely associated with the characteristic dimension of the hydrogel. Besides a variety of miniaturized hydrogels (e.g. nanospheres, thin films and microfibers),12-14 improving the transport kinetics of water has widely endeavoured to achieve faster responsive rates of macroscopic smart hydrogels. The initial effort



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included incorporation of hydrophilic polymers (e.g. polyvinyl alcohol and polyacrylic acid) into PNiPAM networks as water channels.15,16 Porous microstructures were also introduced with the help of emulsifiers, porogens and sacrificing templates.7,17-20 The response rates of the hydrogels, though being enhanced by these strategies to different extents, were still limited to tens of seconds. This is because porous microstructures normally deteriorated their elastic properties, thus slowing down their macroscopic contraction to release water. Exfoliated clay,21 macromolecular spheres22 and responsive nanoparticles23 as nanocross-linkers, in spite of being able to improve their elastic properties, reduced their stimuli-sensitivities, equilibrium swelling ratios and hereby actuation rates.13,19 For example, PNiPAM hydrogels could shrink within 6 min (>22 mm) by incorporating poly(NiPAM-co-MBA) nanogels, and over 10 min (cylindrical hydrogel with a diameter of 5.5 mm) by incorporating nanoclay.24 Meanwhile the control of deformation direction could in principle be achieved by inhomogeneous materials and inhomogeneous stimulus fields.25-29 Adoption of inhomogeneous materials, though being a predominant strategy for living organism to programme their actuation behaviours, generally requires delicate and complicated engineering procedures to fabricate. The typical examples include two layers or gradient distributions of chemical compositions,25,27,29 as well as preferential orientation of anisotropic nanofillers (e.g. aluminium oxide platelets, carbon nanotubes and titanate nanosheets).9,25,28 Inhomogeneous stimulus fields (e.g. gradient pH, temperature, ionic strength and concentrations), though existing extensively in nature, are difficult to control and maintain constant in many aquatic circumstances (e.g. in vivo and underwater). In this study, we showed that rapid and steerable actuation could be achieved by combining alginate-modified WS2 nanosheets and ice-templating in situ polymerization in PNiPAM hydrogels.


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WS2 nanosheets were first efficiently exfoliated and modified with marine alginate (e.g.

β-D-mannuronic acid).29 When incorporating into cellular PNiPAM hydrogels via ice-templating polymerization, these inorganic nanosheets with high moduli (e.g. 280 GPa) not only endowed exceptional near-infrared (NIR) photothermic responsiveness for remote controllability, but also reinforced the PNiPAM walls of cellular pores for powerful contraction to release water. Likewise, alginate on one hand assisted to efficiently exfoliate and enhance the compatibility of WS2 nanosheets. On the other hand, residual alginate also formed interpenetrating networks within PNiPAM networks through alginate-Ca2+ complex, thus promoting elastic properties of resultant hydrogels as well as increasing the transfer coefficient of water.30 Moreover, the polymerization of NiPAM were mainly carried out during and after freezing (e.g. −24 °C) at a higher temperature (i.e. 0 °C), which prevented the integrity and mechanical property of the porous hydrogel framework from puncturing by ice crystals, in contrast to the the reverse sequence of freezing after polymerization. Due to their unique microstructures, even in aquatic environments, the resultant hydrogels were capable of releasing water and fully recovering within couples of seconds under NIR stimulation. Though having homogeneous microstructures, the hydrogels could also demonstrate controllable macroscopic deformations by remotely manipulating the locations and intensity of NIR radiation. This prompt and controllable actuation could be further optimized through their pore sizes and compositions of WS2 nanosheets. Notably, NIR is known for its low absorption in water and biological tissues,31,32 favouring remote manipulation with high spatial and temporal resolutions in aquatic circumstances. Thus the use of NIR brought benefits to release technical hindrances in vivo and underwater like invasive wires or electrodes of smart devices.10 In brief, though mimicking



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shrinking/swelling of plant cells, the resultant hydrogels (i.e. Cellular Hybrid hydrogels) have the actuation behaviour comparable to some animals motions, e.g. gastropod larvae which retreats its body within seconds by releasing fluid from the internal haemal compartment.4,33 These exceptional microstructures and actuation may enable a novel biomimetic platform of smart hydrogels for various soft devices such as rotator, microvalve, aquatic swimmer and water-lifting filter. RESULTS AND DISCUSSION The Cellular Hybrid hydrogel was synthesized following a three-step procedure (Figure 1). (Step І) Freezing aqueous mixture of WS2 nanosheets and monomers to prepare ice templates: Owing to strong coordination (–C=O•••W•••O=C–) and H-bonding (–OH•••S–) between alginate molecules and WS2 nanosheets,29 WS2 nanosheets were exfoliated ultrasonically and stabilized in an aqueous dispersion of alginate (Figure S1). After adding these alginate-exfoliated nanosheets in a typical polymerizable system of PNiPAM hydrogels (i.e. Monomer: NiPAM; Initiator: ammonium persulfate (APS); Cross-linker: BIS), the homogeneous mixture was frozen gradually at −24 °C for 24 h to grow micrometric ice-crystals. (Step П) Ice-templating polymerization of PNiPAM networks: Ice-templating polymerization proceeded between the ice-crystals at 0 °C for 24 h. And thus PNiPAM networks were synthesized mainly due to the presence of chemical cross-linkers of BIS. (Step Ш) Ionically crosslinking alginate within molten PNiPAM networks: After melting the ice template at room temperature (RT), the PNiPAM hydrogels with micrometric cellular pores were further incubated in a CaCl2 solution for 6 h. And alginate molecules were cross-linked ionically into hydrophilic interpenetrating networks by Ca2+. The as-prepared Cellular Hybrid hydrogel was thoroughly washed with pure water before use. Conventional PNiPAM hydrogel (named as PNiPAM, produced without WS2 nanosheets and ice-templating), hydrogel produced with WS2 nanosheets but


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without micrometric pores (Hybrid) and hydrogel produced with micrometric pores but without WS2 nanosheets (Cellular PNiPAM) were also prepared as the control.

Figure 1. Pathway followed to synthesize Cellular Hybrid hydrogel: І Freezing aqueous mixture of alginate-exfoliated WS2 nanosheets and monomers to induce ice templates; П Ice-templating polymerization of PNiPAM networks at 0 °C; Ш Ionically crosslinking alginate within molten PNiPAM networks. Because of strong interactions of WS2 nanosheets with alginate and PNiPAM (e.g. H-bonding and –NH•••S–), WS2 nanosheets may also serve as the cross-linkers for both the PNiPAM networks and alginate networks (Figure S1 & S2). Due to the template of micrometric ice crystals, the resultant hydrogels showed cellular microstructures in analogue to plants (Figure 2A-2D), where the interpenetrating hybrid networks acted as the cellular membranes. An averaged pore size as large as 120-130 µm was achieved in this Cellular Hybrid hydrogel, which was much higher than those of PNiPAM, Hybrid and Cellular PNiPAM hydrogels (Figure 2E-2F & S3). And in sharp contrast to the



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pores in cellular hydrogels which were formed during the ice-templating polymerization, the pores in PNiPAM and Hybrid hydrogels were introduced by freezing-drying (Figure S3).

Figure 2. Structure and mechanic characterization of Cellular Hybrid hydrogel. (A) Visual observation. (B) Optical image. (C-D) Scanning electron microscopy (SEM) images of freezing-dried porous microstructure. (E) Pore histogram of Cellular Hybrid hydrogel. (F) Average pore size of as-prepared hydrogels. (G) Comparison of Cellular Hybrid hydrogel (Right) with PNiPAM hydrogel (Left) under compression. (H) Compression curves of as-prepared hydrogels. In spite of having micrometric pores, hybridization of alginate-exfoliated WS2 nanosheets and the formation of interpenetrating networks conspicuously enhanced mechanic properties of the PNiPAM hydrogels. For instance, the Cellular Hybrid hydrogel could withstand cyclic compression up to the ratio of 95% (Figure 2G, S4 & Video 1), in sharp contrast to conventional PNiPAM hydrogels (in Figure S2-S4), which fractured even at a lower compression ratio of 53%. Moreover,


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its stress at break increased >10 times up to 1.1 MPa (Figure 2H), being much higher than those of the Hybrid hydrogel and Cellular PNiPAM.7,19 And thus its higher elasticity was resulted predominantly from its stronger cellular membranes which were in situ polymerized between the ice-crystals. To be noted, many conventional porous hydrogels, which were produced by the reverse sequence of freezing and polymerization (i.e. freezing after polymerization), suffered from severe structural defects and hereby showed poor mechanic properties (Figure S3 & S4). When heating this Cellular Hybrid hydrogel across the LCST of PNiPAM (~32 °C), its cellular membranes would undergo a volumetric shrinkage due to the coil-globule transition of PNiPAM chains (Figure 3A-3B).34 By assuming a homogenous shrinking process, the volumetric variation is proportional to the variation of lateral length by an exponent of 3. As a result, its micrometric pores also shrunk and forced water out of this hydrogel. In principle, the entire volumetric shrinkage depended on both the net weight of cellular membranes and the size of cellular pores. As a matter of fact, by fixing a constant dry weight, the Cellular Hybrid hydrogel had the larger pore size as well as the larger water uptake (Figure 3C). Its Whydrogel/Wdry value (Whydrogel was the hydrogel weight and Wdry was the dry weight) achieved up to >3900 wt% below 32 °C, being much higher than those of other hydrogels. The Whydrogel/Wdry variation also achieved 3200 wt% upon heating from 12 to 40 °C. More remarkably, this Cellular Hybrid hydrogel showed much faster volumetric shrinkage and recovery than the other hydrogels during cyclic temperature alteration between 4 and 40 °C (Figure 3D-3E & Video S2). For example, a disk hydrogel (with the diameter of 10 mm and thickness of 3 mm) could shrink 88 % within 7 s, in contrast to 84 % within 25 s for Cellular PNiPAM, 91 % within 36 h for PNiPAM and 92 % within 2 h for Hybrid. Its entire shrinkage-recovery period could achieve as low as 12 s.



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Figure 3. Thermo-driven volumetric variation of Cellular Hybrid hydrogel. (A) Reversible volumetric variation between 4 °C and 40 °C. Disk diameter of 10 mm and thickness of 3 mm at 4 °C. (B) Shrinkage mechanism of cellular membrane and pore induced by coil-globule transition of PNiPAM. (C) Water-uptake evaluated by hydrogel weight (Whydrogel) vs. dry weight (Wdry). (D) Speed evaluation of volumetric shrinkage at 40 °C and subsequent recovery at 4 °C. (E) Repeatability of volumetric shrinkage and recovery by periodic alteration between 4 °C and 40 °C. Initial temperature: 4 °C. Further investigation revealed that the shrinking process followed a first-order kinetics model with a rate constant of 0.91 s−1 (Figure S5), being 2-4 orders of magnitude higher than that of the PNiPAM hydrogels in the literature.35,36 This was probably resulted from an exceptional combination of fast transfer coefficient of water within microscopic cellular membranes and strong shrinking force of cellular membranes to force water out of cellular pores. Firstly macroscopic hydrogels were


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miniaturized into cellular membranes with large specific area to shorten the tranfer pathway of water. And the transfer coefficient of water within the cellular membrane was further accelerated by the interpenerating hydrophilic alginate networks.29 Secondly, three types of cross-linkers (e.g. BIS, Ca2+ and WS2 nanosheets, detailed contributions of those crosslinkings on the mechanical rheological characteristics were shown in Figure S4E) endowed the cellular membrane with high mechanic properties, which could produce large shrinking forces as well as capillary forces. WS2 nanosheets, as the two dimensional semiconductor, have a band-gap of 1.50 eV,37 which ensures their strong NIR absorption (e.g. extinction coefficient as high as 23.8 Lg−1 cm−1)38 and photo-thermal effect under NIR radiation. The incorporation of WS2 nanosheets may enable the Cellular Hybrid hydrogel with a fast and photo-responsive actuating behavior for underwater applications. Indeed, only having the solid composition 2 wt% of WS2 nanosheets (Figure 4A), the Cellular Hybrid hydrogel could reach the LCST of PNiPAM (i.e. 32 °C) within 6 W cm−2) from RT. The rapid variation of local temperature enabled fast volumetric variation of the Cellular Hybrid hydrogel under NIR radiation. At RT, its volume shrank ~90 % within 6 s upon exposing to NIR radiation (~6 W cm−2) and fully recovered within 4 s upon removing NIR irridation (Figure 4B, S6 & Video S3). In contrast, there is no volumetric change for the hydrogels PNiPAM and Cellular PNiPAM (Video S4). And the Hybrid hydrogel required 40 h to reach the same volumetric variation (Figure 4C). To the best of our knowledge, this fast response is top listed among the macroscopic hydrogels in the literature (Figure 4D).7,13,15,19-21,30,39-46



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Figure 4. NIR-driven volumetric variation of Cellular Hybrid hydrogel. (A) Photothermal evaluation under different energy densities of radiation. Disk diameter of 4 mm and thickness of 2 mm at 4 °C. (B) Volumetric variation under radiation at RT. VRT is the volume at RT. Vt is the volume at the time t. (C) Speed evaluation of volumetric shrinkage under radiation and subsequent recovery when removing radiation at RT. (D) Speed comparison of volumetric variation with PNiPAM-based hydrogels under different conditions. Shrinking rate vshrink=

VRT Vshrunk × tshrink

, where Vshrunk is the equilibrium shrunk volume, and tshrink is the shrinking

time. Swelling rate vswell =

VRT Vshrunk × tSwell

, where tswell is the swelling time. (Semi)interpenetrating

(IPN) agents: Silk,19 SA,30 salecan (SC),40 and polyvinyl alcohol (PVA).15 Nanofillers: Au (NPs),13 TiO2,41 and clay.21 Copolymers: N,N-dimethylamino ethyl methacrylate (DMAEMA),45 and crown ether (CE).46 Porogens: NaCl,43 polyethylene oxide (PEO),42 oil,20 hydrothermal,7 metal mesh,39 and ice.44 (E) Effect of WS2 composition on speed of volumetric shrinkage under


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radiation and subsequent recovery at RT. (F) Effect of pre-freezing temperature on speed of volumetric shrinkage under radiation and subsequent recovery at RT. NIR radiation: 6 W cm−2. Furthermore, more rapid volumetric variation tended to occur with the higher solid compositions of WS2 nanosheets and larger pore size. When increasing 2 wt% WS2 nanosheets to 4 wt%, the enhanced photo-thermal effect and reinforced cellular membrane were both advantageous to accelerate water out of the hydrogel (Figure 4E & S7-S8). By progressively increasing the freezing temperature from −196 to −24 °C during Step І of the production procedure, the retarded water crystallization favored the formation of bigger ice-crystal templates and hereby larger cellular pores (Figure S9).47,48 These larger pores would be advantageous to water transfer and hereby faster volumetric variation of the hydrogel (Figure 4F). Without particular specification, 2 wt% WS2 nanosheets and the freezing temperature −24 °C were adopted to produce the Cellular Hybrid hydrogel for further experiments. This photo-driven, rapid and controllable volumetric variation may start a unique opportunity to mimic motions of a variety of aquatic animals, and produce smart actuating devices with remote controllability, super deformation speed and tunable deformation direction. When putting a rod (length of ~2 cm and diameter of ~2 mm) made from the Cellular Hybrid hydrogel underwater at RT, NIR radiation (8 W cm−2) produced a local volumetric shrink along the rod and thereby a bending deformation up to 7 mm within 2 s, which further fully recovered within 1 s upon switching off the radiation (Figure 5A, S10A & Video S5). Moreover, its deformation direction was able to be precisely tuned within 360º by altering the position of NIR radiation (Figure 5B & Video S5).



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Figure 5. NIR-driven actuating behavior of Cellular Hybrid hydrogel in water. (A) Deformation controlled by radiation (8 W cm−2). Rod diameter of 2 mm and length of 2 cm. (B) Deformation direction controlled by radiation position. (C) Comparison of actuation speed with PNiPAM-based actuators with different photothermal agents. Graphene,8,50,53 metal nanoparticle plasmons,13,23,52 polymers,7,49 carbon nanotubes,51,54 and transitional metal dichalcogenides (TMDs).55 (D) Schematic illustration of jellyfish-inspired swimmer with hydrogel coelom. (E) Swimming behavior driven by actuation of hydrogel coelom under periodic radiation (Energy density: 8 W cm−2; Illumintion time:~1.5 s; Interval: ~1.5 s).


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This unprecedented actuating behavior may offer a possibility of overcoming technical hindrances of artificial muscles for diverse actuating applications (Figure 5C).7,8,13,23,49-55 For example, a phototropic actuator was constructed from the Cellular Hybrid hydrogel to mimic apricus sunflower,56-58 which bended within 2 s under 0.5 W cm−2 NIR illumination (Figure S10B & Video S6), superior to the reported results.49-55 Besides plant motions, this fast actuating behavior could also be used to mimic dexterous movements of aquatic animals (Figure 5D-4E & S10C), e.g. jellyfish which propels their body based on fluid jettison from coelom contraction and paddling of tentacles.59 The propulsive force was invoked by exposing the analogous hydrogel coelom to NIR radiation (8 W cm−2), where the hydrogel shrinkage was rapid enough to drive elastic tentacles to stroke water. When removing the NIR radiation, the hydrogel coelom recovered to original state within 1 s. Thus this bioinspired aquatic swimmer could swim continuously at an average speed of 0.75 mm s−1 under periodic NIR radiation (Illumintion time:~1.5 s; Interval: ~1.5 s) (Video S7), which may serve diverse applications in vivo and underwater. When molding into a regular cylinder (diameter 10 mm and length of 5 mm), a photo-driven rotator could also be constructed from the Cellular Hybrid hydrogel (Figure 6A-6B). At RT, the NIR radiation (2 W cm−2) could shrink the hydrogel at the intersection of the rotator face and watery surface. This local volumetric shrinkage would change the center of gravity and drive the rotator forward at an average speed of 1 mm s−1 with persistent NIR irradiated at the the intersection. Its moving direction could be further steerred by altering the radiation positions (Video S8).



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Figure 6. Exampled applications based on NIR-driven volumetric variation of Cellular Hybrid hydrogel. (A) Cylindric rotator on watery surface driven by radiation (2 W cm−2). Cylinder diameter of 10 mm and length of 5 mm. (B) Displacement induced by volumetric shrinkage of cylindric rotator. (C) Wate-lifting filter under periodic radiation (0.11 W cm−2). The inset gives water-lifting mechanism. (D) Selective removal of metal ions with water-lifting filter. (E) Liquid microvalve controlled by volumetric variation under radiation. (F) Water flux and response time of microvalve under different energy densities of radiation. Through volumetric variation of the Cellular Hybrid hydrogel, water could also be lifted upwards in analogue to plant roots and trunks (Figure 6C & S11-12).33,60 The water lifter (length of 12.5 cm; diameter of 10 mm) was constructed by filling the hydrogel in a elastic tube of SEBS (with an elastic modulus ~60 kPa). When immersing in water under NIR radiation (0.11 W cm−2), the NIR-induced volumetric shrinkage would force water upwards out of the tube. When switching off


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the NIR radiation, the subsequent volumetric recovery would lift water upwards through the capillary force (Video S9 & S10). Thus this water pump was able to lift water up continuously by repeating on-off NIR radiation (Video S10) as well as sunlight (Figure S11 & Video S3, S11). The bionic water pump can lift ~ 1000 mg water to 10 cm high within 15 min (Figure 5C & Video S11). Considering the diminutive size of the bionic pump and the mild irradiation conditions (e.g. weak absorption of NIR in biological tissues, small irradiation area of 3.7 cm−2, low power density of 0.11 W cm−2), the performance of the bionic water pump was breathtaking. Moreover, due to high porosity as well as strong complexation of alginate molecules and WS2 nanosheets with diverse metal ions,61 the lifted water could be purified through this water-lifting process. For instance, heavy-metal ions such as Cr3+ and Pb2+ were preferentially removed (Figure 6D & S13). When trapping bactericidal Ag nanoparticles into this water pump (Figure S14), bacteria such as E. coli were also removed or sterilized (e.g. a high retention rate of 99.6% achieved when E. coli containing water was pumped through as shown in Figure S15). Considering health and enviromental risks of heavy-metals and bacteria,62 this bionic water-lifting filter may promise applications in water treatment. This rapid and controllable volumetric variation could be used to construct a photo-controlled microvalve of aqueous solutions, being applicable in microfluidic channels (Figure 6E-6F & S16). When blocking a tube with the Cellular Hybrid hydrogel, NIR radiation would shrink the hydrogel and allow water flow. And the water flux was able to be precisely tuned by the energy density of NIR. For instance, the fluid flow was as high as 18.5 ml min−1 when irradiated with a 4 W cm−2 NIR at 20 °C in sharp contrast to the 0.6 ml min−1 with a 0.3 W cm−2 NIR at 0 °C when the water pressure nearby the valve was kept constant of 3 kPa (Figure 6F). Its response time across one “On-Off” cycle



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was as low as

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