The relative RGB values of the thermoresponsive colorimetric patch under (a) the different ambient temperatures, (b) the different air flow rate (the distance between sample and flowmeter is 10 cm), and (c) the different relative humidity at the constant temperature of the thermal plate (30, 35 and 40 °C). Cross-sectional SEM images of the porous SMP in (c) original and (d) deformed states for (i) microporous and (ii) nanoporous regions.
Introduction
Stimuli-responsive systems in nature
In particular, humans can control body movements in terms of position, force, and speed as they think because skeletal muscle, which is attached to bone, contracts and relaxes in response to nerve stimulus.25 Ion channels in the human body open and close depending on differences in charges between two sides of the cell membrane, which induces ion flow through the channels.26, 27 The flux of ions plays an important role in many reactions in the human body, such as signal transduction, muscle contraction, and secretion of neurotransmitter. In addition, pine cones open and close depending on humidity conditions to germinate the seeds in a suitable environment because the seeds may be washed away on a rainy day.33, 34 The moisture-sensitive activation is possible because pine cone husks consist of two layers with different hygroscopic expansion coefficients.
Nature-inspired stimuli-responsive devices
Nature-inspired physical and chemical sensors. a) Chameleon-inspired pressure sensor exhibiting pressure-dependent color changes (H.-H.. b) Human skin-inspired multifunctional sensor based on the ferroelectric material (J. Park et al. Inspired by adhesive proteins in mussels, the coating of polydopamine in mesoporous silica nanoparticles in order to inhibit the metal surface from corrosion.48 a) Mussel-inspired hydrogel actuator (B. P. Lee et al.
Stimuli-responsive polymers with micro/nanostructures
It also responds to the mechanical force, therefore it can be used as the color changing voltage sensors. Compared to the responsive polymer brushes, the film thickness of LbL films can be simply controlled by changing the number of layers.
Application of responsive polymer-based smart system
The resulting surface can change wettability in response to temperature, pH, and electric field.88 The stimuli-responsive surface wettability can be used for oil-water separation, self-cleaning, and smart coating for textiles. By integrating the gripper with pressure sensors, the grip force can be controlled by pressure sensing and feedback.
Challenges of current stimuli-responsive devices
In Chapter 2, we introduce wearable colorimetric temperature sensors by integrating responsive polymer microgels and plasmonic gold nanoparticles (AuNPs). In this thesis, we propose responsive polymer-based wearable sensors and soft actuators with high sensitivity and fast response.
Colorimetric temperature sensor with thermoresponsive polymer
Introduction
Then, the color changes of the colorimetric strip part were analyzed on the thermal plate 5 s after reaching the temperature (Figure 2.13e). Photos of the color group patch attached to (a) the neck, (b) the hand, and (c) the glass filled with water. The current change of the artificial tongue was measured by a semiconductor characterization system (4200, Keithley Instruments) and a probe station (8000, MS-Tech).
Sensor performance of the artificial tongue. a) Current changes (ΔI/Io) of the artificial tongue as a function of TA concentration. As a result, the compressive modulus of the porous SMP increases with increasing concentration of PLA compared to TPU as shown in Figure 4.9b. On the other hand, the recovery ratios are more than 90% regardless of the PLA concentration (Figure 4.9c).
From the SEM images of the cryo-fractured samples in Figure 4.11, it can be seen that the micro/nanoporous structure exists both in the pillars and in the substrate. As shown in Figure 4.18, the Joule heating film is attached to the flat side of the pillar-patterned porous SMP.
Experimental Details
Results and Discussion
Principle of operation of thermoreactive colorimetric sensor. a) Schematic illustration of plasmonic microgels in PAAm hydrogel in swollen and contracted states. Thermoresponsive color shifts of a plasmonic microgel film. a) Schematic illustration of plasmonic microgel films under swollen and shrunken states encapsulated between PDMS films. The thermoresponsive color shift is recognizable even after 1 s of contact, and the size of the color shift increases with contact time (Figure 2.8b).
Localized temperature mapping on a thermoresponsive colorimetric array. a) Images of a dot-matrix film consisting of dots with a diameter of 2 mm. For thermoresponsive colorimetric array analysis, plasmonic microgel films with different transition temperatures (N2, N1, Ref, S1, and S2) were assembled into a single thermoresponsive array by sandwiching PDMS thin films. Surface changes of co-PNIPAM films (1 mm thick) with different amounts of comonomers as a function of temperature.
Conclusions
Soft astringency sensor with ion-conductive polymer
Introduction
Characterization: The cross-sectional morphology of the artificial tongue was examined by a field-emission scanning electron microscope (S-4800, Hitach). Applications of the artificial tongue. a) Schematic illustration of sweep-and-detect of the human tongue and artificial tongue. Characterization: The morphology of the porous SMP was observed by scanning electron microscopy (S-4800, Hitachi, Japan) with the cryofracture surface.
The T values of the porous SMP are stable for 30 minutes, which is attributed to the stable thermal insulation of the porous SMP regardless of the PLA concentration (Figure 4.7). Thermal insulation properties of porous SMP. a) Plot of ΔT/thickness as a function of plate temperature, (b) Photograph and (c) IR thermometer image of porous SMP with different PLA concentrations. Using the tunable thermal insulation property of pillar-patterned porous SMP. a) Photograph of the flexible shape of the pillar-patterned porous SMP.
Experimental Details
Results and Discussion
The working principle of the astringency detectable sensor. a) Schematic illustration of contraction principle of the human tongue. Plot of the changes in thickness of human tongue simulated hydrogel before and after TA treatment. To investigate the sensing performance of the artificial tongue, the relative current changes (ΔI/Io) under various concentrations of TAs are monitored (Figure 3.8).
While the artificial tongue is highly sensitive to TA, mucin-free PAAm ionic hydrogel shows poor sensitivity to TA (Figure 3.9a) due to the lack of hydrophobic nanopore structures induced by the mucin-TA complex. Sensing performance (ΔI/Io) of bare PAAm hydrogel under different TA concentrations. a) Current changes of mucin-free PAAm hydrogel over time. Artificial language sensitive performance. a) Current changes (ΔI/Io) as a function of different TA concentrations using a logarithmic scale at 10, 30 and 60 s of TA treatment time.
Conclusions
Tunable thermal insulator with micro/nanoporous shape memory polymer
Introduction
Dynamically adaptive thermal insulation materials with a high degree of thermal insulation will require modulation of the 3D configuration of micro/nanoporous materials in response to heat-related environmental changes (temperature, humidity, light, etc.). Shape memory polymers (SMPs) have the ability to take a reconfigurable 3D shape in response to stimuli such as heat, moisture, light, solvents, or magnetic fields.190 For example, a porous carbon nanotube/ethylene-vinyl acetate (CNT/EVA) composite with reconfigurable shape was proposed for the fabrication of an underwater vibration sensor.191 The porous CNT/EVA composite can also be deformed to obtain different desired shapes and easily restored to its original structure by heating. Due to its shapeability, such as fixation and recovery, reconfigurable SMP has the ability to control thermal insulation by transforming the polymer into a desired shape.
Furthermore, the SMP-based thermal insulating composite can be used as a wearable sensor because its shape can be adapted to curved surfaces such as human skin.192 However, few studies have been reported on the use of SMPs to develop porous materials with tunable thermal insulation. performance without continuous mechanical stimulus. Here, we demonstrate a smart thermal insulation film based on a hairy structure of porous SMP, which can control and maintain the degree of thermal insulation. The high twisted structure of the hierarchical micro/nanoporous structure enables the high thermal insulation ability.186 Moreover, inspired by animal hair, the hairy patterned SMP exhibited both great thermal insulation and tunability, enabling the application in the wearable thermal insulator and tunable encryption of thermal information .
Experimental Details
Results and Discussion
Raman spectra and mapping images of the porous SMP. a) Raman spectra of microporous and nanoporous walls in the same sample Optical and mapping images of (b) aromatic and (c) C-COO bands. This result indicates that the mechanical stability and shape memory performance of the porous SMP is high enough to undertake the repeated deformation and recovery. Mechanical and shape memory properties of the porous SMP. a) Photographs of the shape-programmed porous SMP films with knotted and twisted shape.
The pillar dimension is 500 µm diameter and aspect ratio 5 (diameter/height ratio 500/5), which was optimized in terms of thermal insulation during the shape memory test. Shape memory-related thermal insulation properties of pillar-patterned porous SMP. a) Photographs and images of an IR thermometer of a porous PLA/TPU/GO pillar pattern after each step of the shape memory test. The temperature difference between the heating and insulating sides of the flat porous SMP and the pillar-patterned porous SMP shows a higher thermal insulation of the pillar-patterned sample than the flat one.
Conclusions
Summary and Future perspective
Summary
In Chapter 4, we reported on the dynamic control of thermal insulation with a hairy micro/nanoporous SMP structure. Inspired by the hairy structure of animals, the SMP hairy structure has been developed for efficient and adjustable thermal insulation. The randomly distributed micro/nanoporous structure greatly improved the thermal insulation performance; the hairy structure formed a dynamic thermal insulation with shape memory properties.
Thanks to its adjustable thermal insulation performance and high flexibility, the hairy SMP can be applied to portable thermal insulators and IR coding films. In summary, this thesis presents wearable sensors and actuators based on various types of responsive polymer micro/nanostructures. Specifically, the microgels increased the change in plasmonic colors, and the hierarchical micro/nanoporous structure improved the ionic conductivity of hydrogels and the thermal insulation of porous polymers.
Future perspective
Ayoung Choe†, Jeonghee Yeom†, Ravi Shanker, Minsoo P Kim, Saewon Kang and Hyunhyub Ko, Stretchable and wearable colorimetric patches based on thermoresponsive plasmonic microgels embedded in a hydrogel film, NPG Asia Mater. Ayoung Choe, Jeonghee Yeom, Ravi Shanker, Minsoo Kim and Hyunhyub Ko, “Smart colorimetric patches based on thermoresponsive polymer microgels and plasmonic nanoparticles,” The Polymer Society of Korea 2018, Korea. Ayoung Choe, Jeonghee Yeom, Minsoo Kim and Hyunhyub Ko, “Thermoresponsive smart colorimetric patch based on responsive polymer and plasmonic nanoparticles,” The Polymer Society of Korea 2017, Korea.
Ayoung Choe, Jeonghee Yeom, Minsoo Kim, and Hyunhyub Ko, “Smart colorimetric patches based on plasmonic nanoparticle-decorated thermoresponsive microgels,” Spring ACS Meeting 2017, San Francisco, USA. Ayoung Choe, Sehee Ahn, Jonghwa Park and Hyunhyub Ko, “Carbon Nanotube Nanomesh Films for Transparent and Stretchable Electrodes,” The Korean Institute of Chemical Engineers 2015, Korea. Ayoung Choe, Sehee Ahn and Hyunhyub Ko, “Stretchable and Transparent Electrodes Based on Self-assembled Carbon Nanotube Nanomesh Structures,” The Polymer Society of Korea 2014, Korea.
