Hygroscopic Polymer Research Articles

4D Printing of Humidity-Driven Seed Inspired Soft Robots.

Geraniaceae seeds represent a role model in soft robotics thanks to their ability to move autonomously across and into the soil driven by humidity changes. The secret behind their mobility and adaptivity is embodied in the hierarchical structures and anatomical features of the biological hygroscopic tissues, geometrically designed to be selectively responsive to environmental humidity. Following a bioinspired approach, the internal structure and biomechanics of Pelargonium appendiculatum (L.f.) Willd seeds are investigated to develop a model for the design of a soft robot. The authors exploit the re-shaping ability of 4D printed materials to fabricate a seed-like soft robot, according to the natural specifications and model, and using biodegradable and hygroscopic polymers. The robot mimics the movement and performances of the natural seed, reaching a torque value of ≈30µN m, an extensional force of ≈2.5 mN and it is capable to lift ≈100 times its own weight. Driven by environmental humidity changes, the artificial seed is able to explore a sample soil, adapting its morphology to interact with soil roughness and cracks.

Stretchable skin hydration sensor based on hygroscopic and ion conductive polymer composites

Transepidermal water loss (TEWL) is water emission from the skin induced by the diffusion of water molecules. Because TEWL is sensitive to damage on the outermost skin layer, it has been utilized as an indicator of skin barrier function. Here, we demonstrate a stretchable hydration sensor to monitor TEWL based on a porous thermoplastic polyurethane (TPU) film coated with ion conductive polyvinyl alcohol (PVA)/lithium chloride (LiCl) composite layers. The stretchable hydration sensor with the large surface area and the hygroscopic PVA/LiCl layer coated on the porous structure exhibits a remarkable relative current change (ΔI/I0 (%) ∼ 107) under a wide humidity range (10–95 % relative humidity (RH)). Moreover, the highly deformable characteristics of the device up to 50 % strain makes it an appropriate tool for detecting water emission from a curved skin surface. For a proof-of-concept demonstration of monitoring skin barrier function, the hydration sensor can precisely perceive the change in TEWL in response to various external treatments (heat, damage, oil treatment, and applying cosmetics). Finally, the stretchable hydration sensor array enables non-contact finger motion perception through water loss detection, making it suitable for use in touchless human–machine interfaces.

Facile Fabrication of a Bio-Inspired Leaf Vein-Based Ultra-Sensitive Humidity Sensor with a Hygroscopic Polymer.

Bio-inspired materials have received significant interest in the development of flexible electronics due to their natural grid structures, especially natural leaf vein networks. In this work, a bio-inspired leaf vein-based flexible humidity sensor is demonstrated. The proposed sensor is composed of a leaf/Al/glycerin/Ag paste. The Al-deposited leaf vein networks are used as a bottom electrode with a resistance of around 100 Ω. The humidity sensor responds well to relative humidity (RH) levels ranging from 15% to 70% at room temperature. The fabricated humidity sensor exhibits an ultra-sensitive response to different humidity conditions due to the biodegradable insulating hygroscopic polymer (glycerin), specifically the ionic conductivity reaction. To further verify the presence of ionic conduction, the device performance is tested by doping NaCl salt into the hygroscopic polymer sensing layer. In addition, both the repeatability and flexibility of the sensor are tested under different bending angles (0°, 90°, 180°, and 360°). The bioinspired ultrasensitive humidity sensor with a biocompatible and biodegradable sensing layer holds great potential, especially for health care applications (e.g., respiratory monitoring) without causing any body harm.

A novel method for measuring viscoelastic parameters of epoxy resins under water bath environments using embedded strain gauges

Epoxy resins are a class of viscoelastic hygroscopic polymers widely used in plastic packaged devices. Stress relaxation and yielding may occur in plastic packaged devices under thermal-hygro environments, which seriously affect the reliability of the devices. It is important to obtain accurate viscoelastic parameters of the hygroscopic materials for analyzing the stresses and strains of plastic packaged devices. In this paper, a novel method for measuring the viscoelastic parameters of epoxy resins under water bath environments by using embedded strain gauges is proposed. A kind of epoxy resin-copper bi-layer plate is fabricated, and the biaxial strain gauges are embedded between the epoxy resin layer and the copper layer to monitor the deformations. The interfacial strains of the two test pieces at 85 °C/100% relative humidity are continuously measured up to 144 h. Combine the experimental strain curves and the analytical representation, the viscoelastic parameters of the epoxy resins are obtained by curve fitting with MATLAB and genetic algorithm. It is found that the viscoelasticity of epoxy resins could be described by an exponential function of order five shear modulus and relaxation times, and the interfacial strain curves obtained by genetic algorithm fit well with the experimental ones. The proposed testing and solving methods provide a valuable reference for getting the time-dependent viscoelasticities of epoxy resins under thermal-hygro environments.

Fabry-Perot interferometers for highly-sensitive multi-point relative humidity sensing based on Vernier effect and digital signal processing.

A highly sensitive relative humidity (RH) sensor based on Fabry-Perot interferometers (FPI) is proposed and experimentally demonstrated. The sensor is fabricated by splicing a segment of hollow core Bragg fiber (HCBF) with single mode fiber (SMF) and functionalized with chitosan and ultraviolet optical adhesive (UVOA) composite at the end of HCBF to form a hygroscopic polymer film. The reflection beams from the splicing point and the two surfaces of the polymer film generate the Vernier effect in the reflection spectrum, which significantly improves the humidity sensitivity of the sensor. To demodulate the envelope based on the Vernier effect and realize multi-point sensing, a digital signal processing (DSP) algorithm is proposed to process the reflection spectrum. The performance of the DSP algorithm is theoretically analyzed and experimentally verified. The proposed sensor demonstrates a high sensitivity of 1.45 nm/% RH for RH ranging from 45% RH to 90% RH. The compact size, high sensitivity and multiplexing capability make this sensor a promising candidate for RH monitoring. Furthermore, the proposed DSP can potentially be applied to other sensors based on the Vernier effect to analyze and extract valuable information from the interference spectrum.

Hygroscopic Porous Polymer for Sorption-Based Atmospheric Water Harvesting.

Sorption-based atmospheric water harvesting (SAWH) holds huge potential due to its freshwater capabilities for alleviating water scarcity stress. The two essential parts, sorbent material and system structure, dominate the water sorption-desorption performance and the total water productivity for SAWH system together. Attributed to the superiorities in aspects of sorption-desorption performance, scalability, and compatibility in practical SAWH devices, hygroscopic porous polymers (HPPs) as next-generation sorbents are recently going through a vast surge. However, as HPPs' sorption mechanism, performance, and applied potential lack comprehensive and accurate guidelines, SAWH's subsequent development is restricted. To address the aforementioned problems, this review introduces HPPs' recent development related to mechanism, performance, and application. Furthermore, corresponding optimized strategies for both HPP-based sorbent bed and coupling structural design are proposed. Finally, original research routes are directed to develop next-generation HPP-based SAWH systems. The presented guidelines and insights can influence and inspire the future development of SAWH technology, further achieving SAWH's practical applications.

Synthesis, characterization, thermal stability, and in vitro and in vivo degradation study of polycaprolactone and polyglycolide block copolymers

Biodegradable polymer materials are attractive in various biomedical applications. Herein, we report a block aliphatic copolymer, polycaprolactone-block-polyglycolide (PCL-b-PGA, denoted as PCLGA), and explored the polymer composition effect on the thermal and degradation behaviors. Three copolymers with different PCL/PGA weight ratios were obtained by a two-step ring opening polymerization. The chemical structures and compositions of copolymers were studied by NMR and FT-IR. The thermal behaviors were investigated by DSC and TGA. The in vitro degradation in phosphate buffer saline and in vivo degradation as an implant in rats were evaluated and the hygroscopic rate and polymer weight loss changes were monitored in an eight-week period. PCLGA with a higher PCL composition showed a slower degradation in vitro and in vivo. These PCLGAs with different degradation rates could be used as biomedical implants for potential application in drug delivery and tissue repair.

Effect of fire retardants on germination of agricultural seeds

The occurrence of fires in agricultural areas grown with winter maize has become recurrent in the Central-West region of Brazil, since the harvest occurs in the dry season in the region. Fire retardants can be used as an indirect control of fire; they consisted of chemical products that can remain in the straws or on the soil after the fire extinction. The objective of this work was to evaluate the effect of two short-term fire retardants (Phos-Chek WD881® and Hold Fire®) and a hydrogel (alternative fire retardant) on germination of rice (Oryza sativa), common bean (Phaseolus vulgaris), millet (Pennisetum glaucum), maize (Zea mays), and soybean (Glycine max) seeds. The control treatment consisted of only distilled water. The seed germination percentage and speed index, mean time for germination, and seedling growth (shoot and root length and dry weight) were evaluated. A completely randomized experimental design was used, with 4 replications per treatment; each experimental unit consisted of 100 seeds maintained in germination chambers (30 °C and photoperiod of 12 hours). The effects of the fire retardants and hygroscopic polymer on the seed germination and seedling initial growth were specific, and no significant effect caused by the variations in the concentrations of the fire retardants was found.

High-yield and scalable water harvesting of honeycomb hygroscopic polymer driven by natural sunlight

Many materials have been tailored for sorption-based solar-thermal-driven atmospheric water harvesting (AWH), but AWH devices are still inefficient and unscalable. To close the large gap between materials research and practical application, a hydrogel-based composite sorbent (PCLG) with high adsorption capacity per unit volume has been developed with an optimized honeycomb structure, which has potential applications in global water harvesting. The PCLG sorbent achieves 1.8 g/cm 3 water harvested under 30°C and 30% relative humidity (RH). The optimized honeycomb structure enables 50% water absorption improvement due to better heat and mass transfer performance. In outdoor experiments, our water harvester realizes 2.9 L/m 2 /day of water collected per cycle under natural sunlight. Based on PCLG properties and worldwide weather data, our device can yield up to 6 L/m 2 /day of water. The high adsorption/desorption hydrogel sorbent and scalable atmospheric water harvester offer a solution for efficiently extracting water from the air to relieve the thirsty world. • PCLG is a high-efficiency sorbent suitable for AWH in a wide humidity range • Per unit volume adsorption capacity is proposed as an important indicator for AWH • An integrated device with honeycomb PCLG is designed to improve AWH efficiency • Water production of PCLG-based AWH is predicted to validate its global potential There remains a large gap between atmospheric water harvesting materials development and their practical application. Here, Wang et al. report a hydrogel-based composite sorbent with high adsorption capacity per unit volume, demonstrated to harvest 2.9 L/m 2 /day outdoors under natural sunlight as part of an integrated device with a honeycomb structure.

Scalable super hygroscopic polymer films for sustainable moisture harvesting in arid environments

Extracting ubiquitous atmospheric water is a sustainable strategy to enable decentralized access to safely managed water but remains challenging due to its limited daily water output at low relative humidity (≤30% RH). Here, we report super hygroscopic polymer films (SHPFs) composed of renewable biomasses and hygroscopic salt, exhibiting high water uptake of 0.64–0.96 g g−1 at 15–30% RH. Konjac glucomannan facilitates the highly porous structures with enlarged air-polymer interfaces for active moisture capture and water vapor transport. Thermoresponsive hydroxypropyl cellulose enables phase transition at a low temperature to assist the release of collected water via hydrophobic interactions. With rapid sorption-desorption kinetics, SHPFs operate 14–24 cycles per day in arid environments, equivalent to a water yield of 5.8–13.3 L kg−1. Synthesized via a simple casting method using sustainable raw materials, SHPFs highlight the potential for low-cost and scalable atmospheric water harvesting technology to mitigate the global water crisis.

How rotational speed of planetary ball mill and polymer load influence the performance and water vapor sorption in solid dispersions composed of tadalafil and soluplus

The presence of residual water may deteriorate the performance of amorphous solid dispersions prepared by ball milling, affecting molecular mobility, crystallinity, particle size and finally, the drug dissolution rate. As the stability of these metastable systems depend on both formulation and process variables, the aim of this study was to assess for the first time, the impact that the polymer load and the rotational speed applied upon high energy ball milling could have on the performance of binary co-milled solid dispersions composed of tadalafil (a hydrophobic crystalline drug) and Soluplus (an amphiphilic, hygroscopic amorphous polymer). Each of these variables was tested at three levels. Scanning electron microscopy, laser diffraction and X-ray powder diffraction were used to analyze morphology, particle size distribution and crystallinity of ball milled formulations respectively. Dissolution studies were also carried out. Advanced tools of applied physics, namely solid state 1H NMR and relaxometry were used to assess the structure and water mobility upon gaseous phase hydration on storage. It was shown that both tested variables determined the particle size of the formulation. When the rotational speed of 400 rpm was used, all solid dispersion were XRD-amorphous, but to ensure the immediate release of tadalafil its micellar solubilization in Soluplus was necessary. While the formulation was exposed to water vapor, the hydration level increased with an increasing polymer load as well. Hence, the rotational speed governed the space available for the adsorption of water molecules and their organization in a monolayer or multilayers. Such behavior may have impact on the kinetics of the amorphous drug recrystallization, and finally deteriorate its dissolution.

Bioinspired Color-Changing Photonic Polymer Coatings Based on Three-Dimensional Blue Phase Liquid Crystal Networks.

Photonic polymer coatings that can adaptively respond to the constant changes of surrounding environments are of profound significance for diverse applications such as optical sensors, information encryption, and adaptive camouflage. Here, we report the fabrication of humidity-driven color-changing photonic polymer coatings on the basis of judiciously designed hydrogen-bonded three-dimensional (3D) blue phase liquid crystal networks. Thanks to the inherent self-assembled 3D photonic nanostructures and tough covalent bonding between the polymers and substrate surfaces, the resulting polymer coatings are found to exhibit vivid structural colors, and humidity-driven reversible color changes across the visible spectrum of light can be achieved upon breaking the hydrogen bonds and subsequent conversion into a hygroscopic polymer coating. As the proof-of-concept applications, we demonstrate the information encryption, inkjet-printable photonic patterns, bioinspired adaptive camouflage, and colorimetric humidity sensor with such promising humidity-driven color-changing photonic polymer coatings. The results disclosed herein are expected to provide new insights into the development of stimuli-responsive advanced functional materials with tailorable 3D photonic nanostructures toward technological applications ranging from sensing, display, anticounterfeiting, and biomimetic camouflage.

Moisture absorption measurement and modelling of a cellulose acetate

With a view toward the application of highly hygroscopic polymers as a humidity responsive self-actuator, the evaluation of the real time moisture concentration in the material becomes a priority. In this paper, the moisture diffusion process in a cellulose acetate (53.3% of acetylation) has been studied. Membranes of cellulose acetate (thickness within the range 66–200 µm) have been prepared, and the moisture absorption at room temperature and at a different relative humidity (RH within the range 21–53%) has been monitored. An analytical model has been used to describe the observed non-Fickian sigmoidal behavior of moisture diffusion. A relaxation factor (β) of about 0.026 s−1 and a moisture diffusion coefficient (D) of 3.35 × 10–6 mm2/s have been determined. At constant room temperature, the moisture concentration at saturation (Csat) has shown a linear relation with relative humidity. The identified values β, D and Csat of the analytical model have been used as input for the finite element simulation of the non-Fickian diffusion. The reliability of the finite element simulations has been confirmed with a second set of experiments.

Improving the Kumada Catalyst Transfer Polymerization with Water-Scavenging Grignard Reagents.

Conjugated polymers have received widespread interest as optoelectronic materials. Recently, these macromolecules have been adopted for biologically relevant applications, such as sensors, imaging agents, and drug delivery vectors. A major limitation of the chemistry used to prepare these classes of compounds is that the resultant polymers themselves are not tolerant to water or are not inherently water-soluble. For example, the most controlled method of conjugated polymer synthesis, the Kumada catalyst transfer polymerization (KCTP), requires stringent drying of monomers, catalysts, and other reagents. Here, we describe an approach to use a water-scavenging Grignard reagent to alleviate many of the shortcomings that currently hinder the synthesis of water-soluble conjugated polymers. This method shows improved polymerization performance in both traditional conjugated polymer synthesis as well as more challenging syntheses of polar hygroscopic polymers that are of interest for biological applications.

A Roadmap to Sorption-Based Atmospheric Water Harvesting: From Molecular Sorption Mechanism to Sorbent Design and System Optimization.

Sorption-based atmospheric water harvesting (SAWH), which uses sorbents to capture water vapor from the air and low-grade energy to produce fresh liquid water, has been recognized as a promising strategy for decentralized water supply in arid areas. This review aims to summarize the latest progress in this field and provide perspectives for the further development of SAWH, focusing on the design of sorbent materials and the optimization of the entire system. We first introduce the water sorption mechanisms on different sorbent materials. Next, we discuss the properties and performances of various sorbents developed for SAWH by categorizing them into specific groups: nanoporous solids, hygroscopic polymers, salt-based composites, and liquid sorbents; for each type of sorbent materials, we have analyzed its advantages and limitations, as well as design strategies. In addition, we discuss the influences of the mass and heat transport of the SAWH system on its overall performance in actual operations, and introduce different types of water harvesters developed for SAWH. In the last section, we outline the challenges in this field from fundamental research and practical application aspects, and describe roadmaps for the future development of this technology.

The use of polymer blends to improve stability and performance of electrospun solid dispersions: The role of miscibility and phase separation

Solid dispersion-based nanofiber formulations of poorly soluble drugs prepared by electrospinning (ES) with a water-soluble polymer, can offer significant improvements in drug dissolution for oral drug administration. However, when hygroscopic polymers, such as polyvinylpyrrolidone (PVP) are used, environmental moisture sorption can lead to poor physical stability on storage. This study investigated the use of polymer blends to modify PVP-based ES formulations of a model poorly soluble drug, fenofibrate (FF), to improve its physical stability without compromising dissolution enhancement. FF-PVP ES dispersions demonstrated clear dissolution enhancement, but poor storage stability against high humidity. Polymer blends of PVP with Eudragit E, Soluplus and hypromellose acetate succinate (HPMCAS), were selected because of the low intrinsic moisture sorption of these polymers. The drug-polymer and polymer-polymer miscibility study revealed that FF was more miscible with Eudragit E and Soluplus than with PVP and HPMCAS, and that PVP was more miscible with HPMCAS than Eudragit E and Soluplus. This led to different configurations of phase separation in the placebo and drug-loaded fibres. The in vitro drug release data confirmed that the use of PVP-Eudragit E retained the dissolution enhancement of the PVP formulation, whereas PVP-Soluplus reduced the drug release rate in comparison to FF-PVP formulations. The moisture sorption results confirmed that moisture uptake by the polymer blends was reduced, but formulation deformation occurred to phase-separated blend formulations. The data revealed the importance of miscibility and phase separation in understanding the physical stability of the ES fibre mats. The findings provide insight into the design of formulations that can provide dissolution enhancement balanced with improved storage stability.

A highly sensitive humidity sensor based on an aggregation-induced emission luminogen-appended hygroscopic polymer microresonator

A self-assembled microsphere resonator, comprising a hygroscopic polymer with aggregation-induced emission luminogen pendants, can sense humidity with high sensitivity and repeatability.

Electronic nano sponge breath ammonia sensors using hygroscopic polymers on vertical channel nano-porous structure

With the low-cost hygroscopic polymers as the sensing layer of the nano-porous devices, the “Nano Sponge sensors” enable efficient and reversible ammonia gas absorption to realize highly sensitive and stable ammonia detection.

High humidity sensing by ‘hygromorphic’ dielectric elastomer actuator

Many capacitive or resistive humidity sensors cannot accurately measure high humidity because of surface water condensation and saturation of hygrosopic material in the sensors. Here, we propose a novel method of high humidity measurement using a ‘hydromorphic’ dielectric elastomer actuator (DEA). This novel humidity sensor consists of a ‘relatively water-inert’ acrylic dielectric elastomer layer (3M VHB 4910) sandwiched by a pair of hygroscopic polymer electrodes. The hygroscopic polymer electrodes are made of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS); they become softer upon moisture absorption while enabling moisture permeation into the dielectric elastomer substrate during electrical activation and read-out. While the dielectric elastomeric actuation (under constant voltage) enlarged with increasing humidity, the current leaked through it rose more significantly. This suggests a strong humidity dependence of dielectric resistivity. Experimental correlation showed the dielectric resistance be a piecewise linear function of humidity. Further, this DEA-based humidity sensor remained working when both its electrodes were sprayed with a mist of water droplets.

Interdependence of Hygroscopic Polymer Characteristics and Drying Kinetics during Desiccant Drying and Microwave Supported Drying

Abstract Hygroscopic polymers absorb and bind water. If not dried properly, the residual moisture can cause major problems for converting and affect the product quality significantly. Therefore, an effective drying down to an acceptable moisture level is essential for a successful production. Though data sheets give recommendations for drying parameters, these do not consider the actual current moisture up-take of the plastic pellets or the current ambient conditions. This paper investigates the interdependencies of polymer characteristics, like thermal properties and molecular structure, and the drying kinetics of the respective polymers. Tests are carried out with five different polymers on a State-of-the-Art desiccant dryer. The results show that distinctive drying behaviors can be attributed to the molecular structure of the respective plastic. This is reflected by the activation energy according to Arrhenius, the diffusion coefficient and the Flory-Huggins-Parameter, all showing a positive correlation with drying speed and therefore can be used as indicators to estimate drying times. Also, an industrial scale prototype of a microwave enhanced drying system was used to investigate the effect of microwave application on the drying kinetics. Experimental results show potential for reducing the drying times needed, especially for lower temperatures of the drying air and highly hydrophilic plastics. For higher temperatures, however, the prototype could not compete with the state-of-the-art desiccant dryer, due to heat losses and inefficient tubing of the prototype. Considering this, the benefits of microwave application could be shown representatively for polyamide 6 also at higher temperatures.