Yuya
Oaki
*a and
Syuji
Fujii
*b
aDepartment of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan. E-mail: oakiyuya@applc.keio.ac.jp
bDepartment of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology, 5-16-1 Omiya, Asahi-ku, Osaka 535-8585, Japan. E-mail: syuji.fujii@oit.ac.jp
First published on 19th July 2024
Responsiveness to stimuli is important in daily life: natural biological activity is governed by continuous stimulus responsiveness. The design of stimuli-responsive materials is required for the development of advanced sensing systems. Although fully controlled stimuli-responsive systems have been constructed in nature, artificial systems remain a challenge. Conventional stimuli-responsive materials show direct responsiveness to an applied stimulus (Stimulus 1), with structural changes in their molecules and organized states. This feature article focuses on cascading responses as a new concept for integrating stimuli-responsive material design. In cascading responses, an original stimulus (Stimulus 1) is converted into other stimuli (Stimulus 2, 3, …, N) through successive conversions. Stimulus N provides the eventual output response. Integration of multiple stimuli-responsive materials is required to achieve cascading responses. Although cascade, domino, and tandem chemical reactions have been reported at the molecular level, they are not used for materials with higher organized structures. In this article, we introduce functional carriers and sensors based on cascading responses as model cases. The concept of cascading responses enables the achievement of transscale responsivity and sensitivity, which are not directly induced by the original stimulus or its responsive material, for the development of advanced dynamic functional materials.
Living organisms exhibit responsiveness to various stimuli at multiple time and length scales. Cascading responses are commonly found in in vivo reactions, such as metabolic pathways. Inspired by nature, “cascade, domino, and tandem” reactions have been studied in chemistry and biology.16–22 These molecular-level reactions are illustrated by the engagement of gears (Fig. 1b): the initial reaction (or stimulus) triggers multiple subsequent reactions. The terms cascade, domino, and tandem are generally used in almost the same meaning. However, such concepts are commonly used in reactions at the molecular level but are not fully applied to stimuli-responsive materials, systems, and devices with the higher organized structures at the macroscopic scale. In materials chemistry and science, such concepts are significant for designing the functions. Here, we propose cascading responses for designing materials and devices that are responsive to external stimuli such as heat, light, and force. For example, a stimulus-responsive material (Material 1) exhibits chemical stress (Response 1) in response to the originally applied mechanical stress (Stimulus 1). If another stimulus-responsive material (Material 2) with responsiveness to chemical stress (Stimulus 2 = Response 1) is integrated into the same system (or device), Response 2 is successfully observed as an output signal that is not directly induced by Stimulus 1. This process enables the detection of an external stimuli that cannot be detected using conventional sensing materials. In other words, the detectable types and ranges of external stimuli are expanded by cascading responses through the integration of stimuli-responsive materials into a system. Stimuli-responsive materials exhibit various conversion capabilities, such as light to heat, force to color, and pH to structural change. For example, a stimulus-responsive material with light-to-heat conversion is combined with another stimulus-responsive material with heat-to-color conversion in the composite. The generated heat as the initial response is automatically converted to the color by the cascading response. The responsivity including the type, range, and scale can be tuned by the design of molecules and materials. Appropriate combinations of multiple stimuli-responsive materials integrated in the composites and devices enables cascading responses.
This article summarizes our recent works on the application of the cascading-response concept to materials systems (Fig. 2). Section 2 summarizes the cascading responses of soft active materials used as functional carriers (Fig. 2a). Based on the concept of cascading responses, functional carriers as integrated systems of stimuli-responsive materials are designed to achieve a variety of motions for delivery and release triggered by external stimuli. Section 3 presents the cascading responses of soft layered materials as sensors (Fig. 2b). Cascading responses induce color changes in the layered conjugated polymers in response to light and mechanical stresses that are not directly detected. The integration of stimulus responsive materials has great potential for the design of cascading responses to develop advanced functional materials.
Fig. 2 Cascading responses in this article. (a) Soft active material as a functional carrier (Section 2). Reproduced from ref. 23–25 with permission. Copyright 2017, 2009, 2022 American Chemical Society. (b) Soft layered material as a sensor (Section 3). Reproduced from ref. 26 with permission from the Royal Society of Chemistry. Reprinted with permission from ref. 27. Copyright 2023 American Chemical Society. |
The cascading responses presented in this section are multiscale systems, and all spatial scales are intricately coupled, which determine the motion and stability of the LMs. At the molecular/nanometer scale, the interface chemistry determines hydrophilic–hydrophobic balance of the particulate LM stabilizer, which affects the wetting phenomena of the particulate LM stabilizers at a gas–liquid interface, and the intermolecular interaction impacts the phases of the particulate materials (solid or liquid). The adsorption/desorption of the particles to/from the gas–liquid interface and the melting of the particles are significant events at the submicrometer and micrometer scales. At and above the micrometer scale, changes in the chemistry and physics of the liquid and gas phases affect the properties of the liquid–gas interfaces and the LM stabilizers, resulting in motion of LMs and release of materials from LMs via their disintegration.
Fig. 3 Motions of a liquid marble (LM) on a planar air–water interface driven by site-selective near infrared (NIR)-laser light irradiation. Reproduced from ref. 52 with permission. Copyright 2022 American Chemical Society. |
Using light-transmitting particles to cover the upper surfaces of the LM, a delivery system has been realized in which the LM moves both in the direction same and opposite to the light irradiation (negative and positive phototaxis) (Fig. 4).53
Fig. 4 Negative (a) and positive (b) phototaxis of an LM on a planar air–water interface driven by site-selective NIR laser light irradiation. The LM is coated by one polymer plate with photothermal property and three polymer plates with highly transparent property at the bottom and the upper parts of the LM, respectively. Reproduced from ref. 53 with permission. Copyright 2021 American Chemical Society. |
Newton's equation of motion can be utilized to describe the motion of the LM at the air–water interface (eqn (1)).23
m(d2x/dt2) = Δγw − ςv | (1) |
Fig. 5 (a) Motions of a plastic boat driven by focused sunlight irradiation. Inset is a magnified image of the boat carrying two dyed water droplets on it. Non-linear motions can be realized using two LMs docked to the boat. (b) The relationship between the tilt angle of the boat and the time during the motions. Reproduced from ref. 45 with permission from Wiley. |
Notably, the LMs show amphibious motions.23 The LMs placed on the planar water film can move down the meniscus at the rim of the water film in a sliding manner to be transferred onto the solid substrate (Fig. 6). The motion from air–water interface (fluidic field) to air–solid interface (elastic field) could be realized by both light-induced Marangoni flow and air blow. The LM can move on a solid substrate in a rolling manner because of its near-spherical shape. Here the potential energy at the air–water interface is converted to kinetic energy for motion on the solid substrates. Interestingly, the LM on the solid substrate can return to the air–water interface of the water film with the aid of an airflow. The motion from air–solid interface to air–water interface could not be realized by light irradiation. This is because the Marangoni flow could not be generated on elastic air–solid interface.
Fig. 6 Transfer of an LM from the air–water interface (fluid field) to the air–solid interface (elastic field). The LM driven by NIR laser-induced Marangoni propulsion escaped the water meniscus and is transferred to the air–solid interface. Application of mechanical stress to the LM using a cover glass leads to disintegration of LM, followed by release of the inner liquid. Reproduced from ref. 23 with permission. Copyright 2017 American Chemical Society. |
The particulate stabilizers of LMs for material delivery are not limited to PPy. Other conjugated polymers, such as polyaniline,46 poly(3,4-ethylenedioxythiophene),47 poly(ethylpyrrole),49,52 poly(alkylaniline),50,51 and poly(3-hexylthiophene),48 can also be used. The performance and longevity of LMs in motion and material delivery task should depend on the photothermal property, size/shape and hydrophilicity–hydrophobicity balance of the LM stabilizer. One can choose LM stabilizer depending on the purpose and conditions. Non-aqueous liquids can also be applied as the internal liquids, including glycerol, tetrabromoethane and diiodomethane.51 The internal liquids are another important parameter which determines performance of LM motions. The LMs containing liquids with lower specific heat and thermal conductivity showed longer path length per one light irradiation shot and longer decay time. This is because heat generated at LM stabilizer adsorbed at droplet surface could be efficiently transferred to the water surface around the LM, rather than consumed to increase the temperature of internal liquid. The density of internal liquid also plays a role in motion of LMs. Gravity works strongly on the LM containing the inner liquid with higher density, resulting in pushing the LM placed on the water surface downward and bending the water surface largely. This caused increases of the contact area between the LM and the water surface. Thereby, the heat generated at LM stabilizer adsorbed on the LM surface could be transferred to the water surface more efficiently, which can generate a stronger Marangoni convection.51 The concept of the cascading response-based material delivery can be applied to the solid particle-stabilized emulsions56 and bubbles57,58 coated by conjugated polymer shells and conjugated polymer-coated solid objects.59 In these systems, oil, gaseous and solid materials encapsulated by the particle shell may be delivered.
To enhance the motion control accuracy, the smaller size of light irradiation area might be preferable: the laser with smaller light irradiation area should realize irradiation in more precise site-selective manner, thereby resulting in well-controlled motion directions. Accurate laser positioning on the small objects including LMs, emulsions and bubbles is somewhat difficult in a manual manner, which should cause fluctuation of direction and velocity of their motions. To address this point, development of robotic systems that can automate the light-induced motions based on image analysis is one of promising ways. Development of systems that can control the motions of small objects by multiple external stimuli (e.g. magnetic and electric fields and light) can be another direction to which the research will proceed. This system should be realized using organic–inorganic composite particles that possess the properties of both organic and inorganic components as a particulate stabilizer.
The challenge in scaling up the light-induced material delivery system is a significant barrier against their practical application for the moment. To address this challenge, several points should be considered. (1) Fabrication of large LMs/emulsions/bubbles with high stability against mechanical stress and light irradiation on both solid and liquid surfaces. (2) Development of particulate stabilizer with high light-to-heat transducing ability. (3) Development of light source with high output power.
Fig. 7 Stimulus-induced disintegration of an LM placed at a planar air–water interface. The LMs are stable in the long term on the planar air–water interface under conditions where the particle surfaces are hydrophobic enough for the particles to adsorb at air–water interface. Conversely, the application of stimulus induces LM disintegration and spontaneous dispersal of the particles into aqueous media because of hydrophilization of particle surface. Reproduced from ref. 24 with permission. Copyright 2009, American Chemical Society. |
LMs stabilized with solid particles carrying basic groups can be disrupted by the addition of acids. Specifically, polystyrene (PS) particles carrying a poly[2-(diethylamino)ethyl methacrylate] (PDEA) steric stabilizer (PDEA-PS particles) can serve as a pH-responsive LM stabilizer.24,60 PDEA has a pKa value of approximately 7 and exists in a hydrophobic and water-insoluble state under basic pH conditions. In contrast, PDEA is hydrophilic and water-soluble owing to protonation and exhibits positive charges under acidic pH conditions. These PDEA-PS particles exhibit pH-responsive behavior in the LM system, where the LMs disintegrate rapidly (20 s) after exposure to acid, followed by the release of their inner liquids, while remaining stable against exposure to base (>1 h). Upon protonation at molecular scale, the PDEA-PS particles exhibit a high degree of hydrophilicity, resulting in a decrease in the energy required for the particles to detach from the air–water interface at submicrometer and micrometer scales. This process causes the particles to desorb from the droplet surface and renders the LMs to disintegrate quickly above micrometer scale. The stability time should be varied depending on various parameters including humidity, temperature and volume of internal liquid of the LMs. High humidity should increase the stability time, because the evaporation of internal water could be suppressed. Low temperature should increase the stability time, because the surface tension of water increases, which leads to less wetting of water to the LM stabilizer. Small volume of internal liquid should increase the stability time on planar air–water interface, because the gravity effect decreases. A recent report demonstrated that LMs stabilized with silica particles carrying poly[2-(diisopropylamino)ethyl methacrylate] (PDPA) hairs disintegrate upon exposure to HCl gas due to hydrophilization of PDPA hairs via protonation.61 Similarly, the LM stabilized with pH-responsive poly(2-vinylpyrridine) particles62 and hydrophobically surface-modified chitosan particles63 are disrupted by addition of acid via particle-to-microgel/polyelectrolyte solution transition on protonation.
Base-induced disintegration systems are realized by expanding the concept of acid-induced disintegration. The use of acid group-carrying particles as LM stabilizers allows for the attainment of LMs with complementary characteristics that are destabilized by the addition of bases. Silica particles carrying poly[6-(acrylamido)hexanoic acid] (PAaH) hairs (PAaH–silica particles) on their surfaces work as pH-responsive LM stabilizers.64 On the planar air–water interface of bulk water with neutral or acidic pH, the LM stabilized with PAaH–silica particles can maintain long-term stability. Once the pH of the bulk water increases with the addition of an aqueous alkaline solution, the LM immediately disintegrates and the inner liquid is released. The addition of a basic aqueous solution renders the PAaH hairs on the silica particle surfaces deprotonated and hydrophilic, leading to the desorption of the silica particles from the air–water interface, followed by disintegration. LMs that can be disrupted by the addition of bases have also been prepared using succinic anhydride-esterified poly(2-hydroxypropyl methacrylate)65 and poly(styrene-co-acrylic acid-co-2,2,3,4,4,4-hexafluorobutyl methacrylate).66
As a temperature triggered system, we developed poly(N-isopropylacrylamide) (PNIPAAm) particle-stabilized LMs.67 PNIPAAm are temperature-responsive polymers, exhibiting lower critical solution temperature (LCST) in aqueous solutions. Below the LCST (approximately 32 °C), water molecules and pendant amide groups of PNIPAAm form hydrogen bonds, making PNIPAAm hydrophilic and water-soluble. In contrast, PNIPAAm precipitates in the aqueous phase above the LCST because of the destruction of hydrogen bonds. This temperature-dependent behavior suggests that PNIPAAm exhibits hydrophilic and hydrophobic characteristics below and above the LCST, respectively. The LMs are fabricated at room temperature using an aqueous salt solution (Na2SO4) as the inner liquid and PNIPAAm particles as the stabilizers. Here, the salt was added to decrease the LCST of PNIPAAm, which is hydrophobic at room temperature. The LMs are stable for more than 24 h at room temperature on the planar air–water interface of the bulk water containing salt. Once the temperature of the bulk aqueous solution decreases, the LMs disintegrate because of an increase in the wettability of the PNIPAAm particles. The concept of cascading response-based material release can also be applied to solid particle-stabilized emulsion droplet68–71 and bubble57,58,72–79 systems.
The disintegration of the LMs can also be realized by the application of external stimuli, inducing a phase change of the LM stabilizer from solid to liquid. In this system, phase-change materials (PCMs), which show a solid-to-liquid phase transition upon increasing the temperature,80 are used as a base material for the LM stabilizer. Natural fatty acids (FAs) are the most widely studied PCMs. The melting points of the FAs depend on their alkyl chain length at molecular scale and vary in the range of 20–80 °C.81 Our group developed LMs that can be disrupted by light irradiation utilizing the FA core/PPy shell composite particles (FA/PPy particles) as an LM stabilizer (Fig. 8).25,82 The disintegration and release of the inner liquid of the LMs can be realized in a cascade manner. By hybridizing PPy as a photothermal converter (Material 1) and the FAs as a PCM (Material 2), NIR light (Stimulus 1) can be converted to heat (Response 1 = Stimulus 2) because of the photothermal properties of PPy, leading to a solid-to-liquid phase change of the FA core (Response 2 = Stimulus 3) at the submicrometer and micrometer scales. The contact of the inner liquid with the supporting substrate via penetration through the liquefied thin FAs film resulted in the disintegration of the LMs at and above micrometer scale, followed by release of the inner liquid (Response 3).
Fig. 8 Disintegration of LM stabilized with fatty acids (FAs) particles coated by PPy overlayer induced by NIR laser light irradiation. Reproduced from ref. 25 with permission. Copyright 2022, American Chemical Society. |
There is a close correlation between the melting point of the FAs and the light irradiation time required for LM disruption (disintegration time). Light power is another that can control the disintegration time. The tuning of these parameters makes it possible to control the disintegration time between instantaneous and approximately 10 s; the disintegration time decreases with the decreasing melting point of the FA component and increasing light power. Note that the LMs could be stable without any detectable shrinkage and disintegration for >10 min at 25 °C under no light irradiation. Notably, natural sunlight can lead to the LMs becoming disrupted.
The disintegration of LMs induced by external stimuli serves as a visual cue for the environmental changes surrounding the LMs. The liquid materials released from the LMs can be used as environmental monitoring indicators, which could potentially lead to the development of sensing technologies (Fig. 9). Furthermore, the disintegration of the LMs leads to the development of a microreactor. The coalescence of two LMs in contact with each other by NIR light irradiation can trigger a chemical reaction (Fig. 10). The NIR light irradiation of the contact area of the LMs leads to the melting of the FA component of the FA/PPy particles via heat generation due to the PPy shell and weakens the particle shell protection. Once the inner liquids of the LMs are in contact with each other (Response 3 = Stimulus 4), the coalescence of the LMs immediately starts chemical reaction by mixing the two inner liquids (Response 4).
Fig. 9 Motions of LM stabilized with FA particles covered by PPy overlayer on the filter paper by air blowing. Disintegration of the LM induced by the NIR-laser light irradiation, followed by release of inner liquid (aqueous solution of methyl orange) on (i) filter paper containing citric acid and (ii) pristine filter paper. Reproduced from ref. 25 with permission. Copyright 2022, American Chemical Society. |
Fig. 10 NIR laser light-induced coalescence of two LMs containing NaCl and AgNO3 aqueous solutions, respectively. Precipitation reaction of AgCl is induced by mixing two inner liquids. The LMs are stabilized with FA particles coated by PPy overlayer. Reproduced from ref. 25 with permission. Copyright 2022, American Chemical Society. |
Fig. 11 Schematic illustration of the layered PDA. (a) DA monomer, its topochemical polymerization, and molecular motions with the application of external stimuli. Reprinted from ref. 92 with permission from the Royal Society of Chemistry. (b) Intercalation route (i) and self-organization route (ii) for preparation of the layered composites based on PDA with topochemical polymerization. Reprinted from ref. 102 with permission from Elsevier and Wiley-VCH, respectively. |
Fig. 12 Visualization and quantitative detection of NIR using PPy/PDA-coated paper device. (a) Structure of the device consisting of layered PDA with PPy coating and its NIR visualization mechanisms with the irradiation (i), photothermal generation (ii), and color change (iii). (b)–(d) Photographs of PDA-coated paper (b), after the PPy coating (c), and after writing the characters with irradiation using a NIR laser pointer (808 nm) (d). (e) Time-dependent changes of the Δx value with irradiation of 50 mW (red circle), 100 mW (blue square), and 200 mW (green triangle) NIR light. Reproduced from ref. 111 with permission from the Royal Society of Chemistry. |
Fig. 13 Synthesis, structure, and application of PNIPAAm/PDA-VBA thermo-responsive color-change gel. (a) Layered composite of PCDA and VBA. (b) Grafting and polymerization of NIPAAm to the interlayer VBA in the PCDA–VBA. (c) Polymerization of PNIPAAm and topochemical polymerization of PCDA. (d) and (e) PNIPAAm/PCDA–VBA gel and its after the color change (e). (f) Relationship between the P and tf. Reproduced with permission from ref. 112. Copyright 2020 American Chemical Society. |
A thin layer of conjugated polymer is coated on the layered PDA (Fig. 12a–c).111 As mentioned in Section 2, conjugated macromolecules, such as conductive polymers and graphitic carbons, have photothermal conversion properties. For example, PPy generates heat upon irradiation with NIR light.43,44 If PPy (Material 1) is combined with layered PDA (Material 2), the NIR light (Stimulus 1) can be converted into thermal stress (Response 1 = Stimulus 2). The layered PDA exhibits the color change (Response 2) upon heating. In our previous work, a PPy thin-layer coating was achieved by supplying monomers from the vapor phase.113,114 The layered PDA is coated on a filter paper (Fig. 12b). The original blue color of PDA changes to blue-black after PPy coating (Fig. 12c). When NIR light (λ = 808 nm) is irradiated to the PPy/PDA paper, the color change to red is observed with increasing irradiation time (Fig. 12d). When the power of the irradiated NIR light increased, a higher temperature is achieved by irradiation. Layered PDA with intercalated zinc ion (Zn2+) is used to control the responsivity in the higher temperature range. PDA–Zn2+ shows responsiveness to irradiation power (Fig. 12e). In this manner, the visualization and quantification of NIR are achieved using PPy/PDA or PPy/PDA–Zn2+ based on the cascading response.
Another example of a light-responsive color change is the imaging and measurement of invisible microwaves (Fig. 13).112 Layered PDA is used as a cross linker of PNIPAAm as a typical thermoresponsive hydrogel (Fig. 13a–c). Layered PCDA intercalated with 4-vinylbenzylamine (VBA) acts as a crosslinker for the hydrogel. When PDA is dispersed in PNIPAAm with a typical organic crosslinker, N,N′-methylenebisacrylamide (MBAAm), the PNIPAAm/PDA/MBAAm hydrogel is easily broken upon stretching. In contrast, the PNIPAAm hydrogel crosslinked with PDA-VBA is durable during stretching (Fig. 13d). If the thermoresponsive hydrogel (Material 1) is combined with layered PDA (Material 2), microwave irradiation can induce heat generation and volume changes (Response 1 = Stimulus 2). Color change (Response 2) is achieved by thermal and mechanical stresses. The PNIPAAm/PDA-VBA hydrogel exhibits gradual color change with increasing temperature. As the changes in the red-color intensity and sample volume are correlated, the color changes are coupled with the temperature-responsive volume shrinkage of the PNIPAAm gel. The motion of the PNIPAAm chains initiate that of the layered PDA, leading to a color change. The PNIPAAm/PDA-VBA hydrogel exhibits a color change upon microwave irradiation (Fig. 13e). The red-color intensity increases with increasing irradiation time and microwave power (Fig. 13f). Visualization and quantification of microwave are achieved using the PNIPAAm/PDA-VBA hydrogel. In this manner, based on cascading responses, the layered PDA can exhibit color changes in response to stresses that are not directly detected by the layered PDA through the conversion of the original stimuli.
Fig. 14 Layered PDA with the intercalation of PEG–PEI embedded in PU matrix. (a) Schematic illustration of PU/PDA-(PEG–PEI) (upper) and its magnified image of the intercalated PEG–PEI with the interlayer interaction with the layered PDA (red) and outerlayer interaction with PU (green). (b) Photographs of the film before and after stretching. (c) Photographs (upper) and 2D mapping image (lower) of the PU/PDA-(PEG–PEI) film with stretching in the diagonal line at nt = 10 and 35. Reproduced with permission from ref. 27. Copyright 2023 American Chemical Society. |
In our previous works, friction forces are visualized and quantified using layered PDA coated on a paper substrate.102,132,133 The responsivity, which is the detection range of the strength, is controlled by the intercalated guests. For example, improved sensitivity, i.e. the detection of a weaker friction force, can be achieved by the intercalation of PEI. The macromolecular guest enhances the flexibility of the layered structure compared to low-molecular-weight guests. As the low-molecular-weight guests are self-assembled in the interlayer space, the structural flexibility is lowered by the densely packed layered structure with the higher crystallinity. In contrast, the polymeric guests are loosely accommodated in the interlayer space. The mobile polymer guests provide the more flexible layered structure. The layered PDA and PDA–PEI visualize and quantify the friction forces in the ranges of 7.6–23 and 0.2–6.4 N as observed in the writing and toothbrushing motions, respectively (Fig. 15a).132,133 However, the detection of the weaker friction forces is not achieved using the layered PDA. Moreover, other stresses, such as tensile and compressive stresses, in any strength range cannot be detected using PDA alone. This implies that the molecular motion of the layered structure that leads to color changes is not induced by weaker friction or other stresses. If a more dynamic movable structure is designed to achieve high sensitivity, the torsion of the PDA main chain yielding the color change can be caused by thermal stresses under ambient conditions. Therefore, specific designs of materials and devices are required to achieve high sensitivity. In particular, responsivity to weak compression stresses has not been reported, except in one 2022 report (Fig. 15b).134
Fig. 15 Cascading responses for mechanoresponsiveness. (a) and (b) Detection range of friction forces (a) and compression stresses (b) in the previous and our works. (c) PDA-coated paper. (d) DL consisting of PEI aq. and SiO2 nanoparticles as the core and shell, respectively. (e) DL/PDA integrated device. Reprinted from ref. 135 with permission from Wiley-VCH. |
The cascading response is introduced by combining dry liquid (DL) as a stimuli-responsive capsule material and layered PDA (Fig. 15c–e). A DL, a liquid droplet stabilized by solid particles, is a promising liquid material for storage and carriers.136,137 In our study, the interior liquid is an aqueous solution containing PEI. The shell part comprises silica nanoparticles with a surface modification by polydimethylsiloxane (PDMS) (Fig. 15d). As the fragile droplet shows mechanoresponsiveness, a scheme of cascading responses can be designed by combining it with layered PDA (Fig. 15e).26,135,138,139 When mechanical stresses (Stimuli 1) are applied to the DL (Material 1), the disruption induces the outflow of the interior liquid as a chemical stress (Response 1 = Stimuli 2). The PEI in the interior liquid is intercalated into layered PDA (Material 2). Torsion of the conjugated main chain induces a color change (Response 2). The DL is disrupted by multiple weak mechanical stresses, such as compression and friction. Therefore, the cascading response enables responsiveness to the type and strength of mechanical stresses that are not directly detected by the layered PDA. In particular, responsiveness to compressive stresses has not been reported for PDA in previous works. The device is prepared by just dispersion of DL on the layered PDA coated on a paper substrate (Fig. 15e).135 The weak friction and compression stresses are visualized and quantified by the device integrating PDA and DL (Fig. 16 and 17).26,138 The weaker friction stresses in the range of 0.006–0.08 N are also detected by DL/PDA (Fig. 16a–c). This range is weaker than that of the tooth-brushing motion (Fig. 15a). The device visualizes the pressure distribution in calligraphic writing (Fig. 16d–f). The pressure distribution is clearly different for the expert experience, and beginner.
Fig. 16 PDA/DL device in response to weak friction force. (a) and (b) Photographs of PDA/DL (h) and PDA (i) before (left) and after (right) the application of weak friction force. (j) Photographs and their Δx with the application of weak friction forces. (c) Relationship between F and Δx as the standard curve (open circles) and estimation of simulated unknown F (green and purple circles). (d)–(f) Photographs (upper) and force distribution mapping (lower) of the handwritten character on a paper using ink (left) and on the PDA/DL device without ink (right) by the expert (d), practician (e), and beginner (f). Reproduced from ref. 26 with permission from the Royal Society of Chemistry. |
Fig. 17 PDA/DL paper device for compression-stress imaging. (a) PDA/DL-integrated device on a paper exhibiting the color changes depending on the strength of the applied compression stresses. (b) Relationship between P and Δx. (c) Two different settings (the setups (i) and (ii)) to apply the different compression stresses. (d) Photographs of the PDA/DL paper after the application of the compression stress by the setups (i) and (ii) and their measured and estimated values. (e) Color-change properties and values with the application of compression stresses originating from strong (i) and weak (ii) expiratory pressure. Reproduced from ref. 135 with permission from Wiley-VCH. |
The PDA/DL paper-based device shows color changes in response to the compression stresses (P/kPa) in the range of 3.9 Pa–4.9 kPa that have not been achieved in the earlier works (Fig. 17).135 The applied P is colorimetrically measured by Δx using the relationship in the standard curve (Fig. 17a and b). Moreover, the applied P is visualized by the red-color intensity (Fig. 17c and d). A weak compression stress originating from expiration is imaged and colorimetrically measured using this device (Fig. 17e). Moreover, the spatial distribution of the compressive stresses is visualized by coating the layered PDA inside the melamine sponge (Fig. 18). After the dispersion of DL on the top face, the compression stress is applied to the sponge containing the layered PDA (Fig. 18a–c).138 The compression stress in the range of 3.50–675 kPa is detected using the sponge device. This range is higher than that of paper-based devices owing to the elasticity of the sponge. A cascading response is achieved in the sponge device. The cross-section of the device shows a red color distribution depending on the locally applied compression stress. In other words, the compression-stress distribution is visualized by color. For example, compression stress is applied to the sponge device using the plastic ornament of a giant panda with its feet in different contact states (Fig. 18d). The different stress distributions are visualized and quantified by the red-color distribution on the cross section (Fig. 18e and f). The DL/PDA device is used to visualize the distribution of the compression stresses originating from the circular stapler for anastomosis of the intestinal tract, which is a surgical device (Fig. 18g–k).
Fig. 18 PDA/DL sponge device for compression-stress imaging. (a) and (b) Schematic illustrations (a) and X-ray computerized tomography stereoscopic images and photographs (inset) (b) of the PDA/DL sponge device before (left) and after (right) the application of compression stress. (c) Relationship between P and Δx (black circles, left axis) with the colorimetric estimation of unknown compression stresses (red triangles) and stress–strain curve of the sponge device (blue, right axis). (d) Photograph of a plastic ornament of giant panda. (e) and (f) Photographs of the sponge devices (e) and colorimetrically estimated P (f) on the parts A–D with the compression in the direction K–M in the panel (d). (j) Cross-sectional photographs with the compression for t = 10 and 120 s. (k) Relationship between L and average Δx representing the stress distribution of the purple type for t = 10, 60, and 120 s. Reproduced from ref. 138 with permission from Wiley-VCH. |
The cascading response using the sponge enables spatiotemporal imaging of the compression stress (Fig. 19).139 The red-color intensity increases with increasing application time of the compression stresses (Fig. 19b and c). This means that the applied impact, i.e. the product of force (F/N) and time (t/s), can be visualized and quantified using the device. When the nickel ion (Ni2+) is intercalated in the interlayer space of the layered PDA, the elapsed time after the application of the compression stress (τ/min) is visualized using the same device (Fig. 19d and e). Intercalation of PEI flowing out of the disrupted DL is delayed in the presence of Ni2+ in the interlayer space. The delayed color change enables the imaging of τ.
Fig. 19 PDA/DL sponge device for spatiotemporal compression-stress imaging. (a) A plastic ornament of giant panda with four feet A–D contacting on the ground in the different states. (b) Cross-sectional photographs of the sponge devices set under the feet A–D for t = 10 and 30 s. (c) Relationship between the positions, Δx (left axis) and estimated maximum possible I (right axis). (d) Cross-sectional photographs of the PDA-Ni2+/DL sponge devices after the elapsed time (τ =) 5–180 min of the compression stress. (e) Relationship between the positions A–D and Δx. Reprinted from ref. 139 with permission from Wiley-VCH. |
In our previous works, we have mainly used a commercial DA monomer, PCDA, to develop sensing materials. Based on the insights, the newly designed DA molecules expand the potentials of development of advanced sensing materials. The more versatile combinations of stimuli-responsive materials can be designed for further development of cascading responses. In the applications, visualization of invisible forces is an important challenge in various fields, such as robotics, medicine, arts, and sports. Advanced PDA-based sensors contribute to visualization and quantification of forces with various types, ranges, and scales.
Cascading responses have been applied for the further development of functional carriers and sensors. Research on functional carriers could move toward autonomous motion and many-body systems, where carriers and other objects in the field of motion interact in both attractive and repulsive manners. To realize such systems and gain insight into the underlying cascading response mechanisms, the crucial challenges are the precise design and synthesis of particulate materials to fabricate functional carriers and the chemical and physical evaluation of the kinetic field of the carrier. Mechanoresponsive materials based on cascading responses are applied to analyze, understand, and reproduce motions. Such data are used to transmit the professional motions of artists, craftspeople, and physicians to the next generation. The cascading response is helpful for tuning the responsivity, such as the type, strength, and length of the applied stimuli. Chemists can design and synthesize stimulus-responsive molecules and materials. Their refined integration realizes tailored cascading responses, as observed in living organisms. In addition to development of unit materials, their integration into a system is an important challenge for cascading response applications. New dynamic functional materials, such as carriers, sensors, actuators, and reparative agents, with designed responsiveness and responsivity can be developed by integrating stimulus-responsive materials. Furthermore, the integration of cascading responsive systems, such as delivery, release, and sensing, contributes to the development of microbots and soft robots, and the understanding of animal signalling and locomotion mechanisms.
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