Physicochemical Properties of 20 Ionic Liquids Prepared by the Carbonate-Based IL (CBILS) Process.

Ionic liquids (ILs) are an emerging materials' class with applications in areas such as energy storage, catalysis, and biomass dissolution and processing. Their physicochemical properties including surface tension, viscosity, density and their interplay between cation and anion chemistry are decisive in these applications. For many commercially available ILs, a full set of physicochemical data is not available. Here, we extend the knowledge base by providing physicochemical properties such as density (20 and 25 °C), refractive index (20 and 25 °C), surface tension (23 °C, including polar and dispersive components), and shear viscosity (ambient atmosphere, shear rate 1-200 s-1), for 20 commercial ILs. A correlation between the crystal volume, dispersive surface tension, and shear viscosity is introduced as a predictive tool, allowing for viscosity estimation. Systematic exploration of cation/anion alkyl side chain lengths reveals the impact on the IL's physicochemical attributes. Increasing the anion's headgroup decreases surface tension up to 35.7% and consequently shear viscosity. We further demonstrate that the dispersive part of the surface tension linearly correlates with the refractive index of the ionic liquid. While we provide additional physicochemical data, the screening and modeling efforts will contribute to better structure property predictions enabling faster progress in design and applications of ILs.

Structural Colored Based Humidity Sensor Consisting of High Resolution 3D Printed Photonic Crystal Coated with Ultrathin Responsive Hydrogels.

Today, humidity sensors have become an integral part of the daily lives. In particular, humidity sensors using an electronic measuring principle have become the standard. Although these sensors have proven to be a stable measurement method, they have some disadvantages, such as their long response time or the danger of using them in explosive environments. This work introduces photonic crystals as an alternative optical measurement approach. The novel technology of ultra-fast two-photon polymerisation printing is combined with a thin-film deposition process, namely iCVD. This allows to print large area high-precision 3D templates, which are subsequently coated with a humidity responsive hydrogel thin film (p(HEMA) of 20 nm.The limits of 2PP technology are being pushed allowing the production ofs table and periodic large-area 3D structures. The flexible customization of hydrogels for ambient conditions make them exceptionally promising for a wide range of sensing applications. Additionally, optical methods for measuring humidity seem to be an excellent alternative to overcome the limitations for current state of the art humidity sensors. The optical detection of changes in ambient air humidity is achieved by observing color changes of the printed structure within the visible wavelength range.

Icephobic Gradient Polymer Coatings Deposited via iCVD: A Novel Approach for Icing Control and Mitigation.

Materials against ice formation and accretion are highly desirable for different industrial applications and daily activities affected by icing. Although several concepts have been proposed, no material has so far shown wide-ranging icephobic features, enabling durability and manufacturing on large scales. Herein, we present gradient polymers made of 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (V4D4) and 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) deposited in one step via initiated chemical vapor deposition (iCVD) as an effective coating to mitigate ice accretion and reduce ice adhesion. The gradient structures easily overcome adhesion, stability, and durability issues of traditional fluorinated coatings. The coatings show promising icephobic performance by reducing ice adhesion, depressing the freezing point, delaying drop freezing, and inhibiting ice nucleation and frost propagation. Icephobicity correlates with surface energy discontinuities at the surface plane resulting from the random orientation of the fluorinated groups of PFDA, as confirmed by grazing-incidence X-ray diffraction measurements. The icephobicity could be further improved by tuning the surface crystallinity rather than surface wetting, as samples with random crystal orientation show the lowest ice adhesion despite high contact angle hysteresis. The iCVD-manufactured coatings show promising results, indicating the potential for ice control on larger scales and various applications.

Single- and Multilayer Build-Up of an Antibacterial Temperature- and UV-Curing Sol-Gel System with Atmospheric Pressure Plasma.

The versatility of sol-gel systems makes them ideal for functional coatings in industry. However, existing coatings are either too thin or take too long to cure. To address these issues, this paper proposes using an atmospheric pressure plasma source to fully cure and functionalize thicker sol-gel coatings in a single step. The study explores coating various substrates with sol-gel layers to make them scratch-resistant, antibacterial, and antiadhesive. Microparticles like copper, zinc, or copper flakes are added to achieve antibacterial effects. The sol-gel system can be sprayed on and quickly functionalized on the substrate. The study focuses on introducing and anchoring particles in the sol-gel layer to achieve an excellent antibacterial effect by changing the penetration depth. Overall, this method offers a more efficient and effective approach to sol-gel coatings for industrial applications. In order to achieve a layer thickness of more than 100 µm, the second part of the study proposes a multilayer system comprising 15 to 30 µm thick monolayers that can be modified by introducing fillers (such as TiO2) or scratch-resistant chemicals like titanium isopropoxide. This system also allows for individual plasma functionalization of each sol-gel layer. For instance, the top layer can be introduced with antibacterial particles, while another layer can be enhanced with fillers to increase wear resistance. The study reveals the varying antibacterial effects of spherical particles versus flat flakes and the different scratch hardnesses induced by changes in pH, number of layers, and particle introduction.

Enhancement of the Sensing Performance of Devices based on Multistimuli-Responsive Hybrid Materials.

Capturing environmental stimuli is an essential aspect of electronic skin applications in robotics and prosthetics. Sensors made of temperature- and humidity-responsive hydrogel and piezoelectric zinc oxide (ZnO) core-shell nanorods have shown the necessary sensitivity. This is achieved by using highly conformal and substrate independent deposition methods for the ZnO and the hydrogel, i.e., plasma enhanced atomic layer deposition (PEALD) and initiated chemical vapor deposition (iCVD). In this work, we demonstrate that the use of a multichamber reactor enables performing PEALD and iCVD, sequentially, without breaking the vacuum. The sequential deposition of uniform as well as conformal thin films responsive to force, temperature, and humidity improved the deposition time and quality significantly. Proper interlayer adhesion could be achieved via in situ interface activation, a procedure easily realizable in this unique multichamber reactor. Beyond the fabrication method, also the mechanical properties of the template used to embed the core-shell nanorods and the cross-linker density in the hydrogel were optimized following the results of finite element models. Finally, galvanostatic electrochemical impedance spectroscopy measurements showed how temperature and humidity stimuli have different effects on the device impedance and phase, and these differences can be the basis for stimuli recognition.

Capillary-Driven Water Transport by Contrast Wettability-Based Durable Surfaces.

Controlling water transport and management is crucial for continuous and reliable system operation in harsh weather conditions. Passive strategies based on nonwetting surfaces are desirable, but so far, the implementation of superhydrophobic coatings into real-world applications has been limited by durability issues and, in some cases, lack of compliance with environmental regulations. Inspired by surface patterning observed on living organisms, in this study we have developed durable surfaces based on contrast wettability for capillary-driven water transport and management. The surface fabrication process combines a hydrophobic coating with hard-anodized aluminum patterning, using a scalable femtosecond laser microtexturing technique. The concept targets heavy-duty engineering applications; particularly in aggressive weather conditions where corrosion is prevalent and typically the anodic aluminum oxide-based coating is used to protect the surface from corrosion, the concept has been validated on anodic aluminum oxide coated aluminum alloy substrates. Such substrates with contrast wettable characteristics show long-term durability in both natural and lab-based artificial UV and corrosion tests where superhydrophobic coatings tend to degrade.

Measurements of Temperature and Humidity Responsive Swelling of Thin Hydrogel Films by Interferometry in an Environmental Chamber.

Thin film thermo-responsive hydrogels have become a huge interest in applications such as smart drug-delivery systems or sensor/actuator technology. So far, mostly, the response of such hydrogels has been measured only by varying the temperature in a liquid environment, but studies of the response towards humidity and temperature are rare because of experimental limitations. Often the swelling measurements are performed on samples placed on a stage that can be heated/cooled, while vapors enter the permeation chamber at their own temperature. This thermal difference leads to some uncertainties on the exact relative humidity to which the sample is exposed to. In this study, we explored the possibility of performing swelling measurements under thermal equilibrium by placing the sample and an interferometer, as a detector, in an environmental chamber and therefore exposing the smart hydrogel to adjustable temperatures and relative humidity conditions while measuring the hydrogel's thin film thickness changes. As a case study, we used thin films of the thermo-responsive hydrogel, poly N-vinylcaprolactam deposited by initiated chemical vapor deposition (iCVD). Similar thin films were previously characterized by in situ ellipsometry while the sample was heated on a stage and exposed to humid air produced at room temperature. The comparison between the two measurement methods showed that while measurements in the presence of thermal gradients are limited mostly to low humidity, measurements in thermal equilibrium are restricted only by the operation limits of the used environmental chamber.

Tuning the Porosity of Piezoelectric Zinc Oxide Thin Films Obtained from Molecular Layer-Deposited "Zincones".

Porous zinc oxide (ZnO) thin films were synthesized via the calcination of molecular layer-deposited (MLD) "zincone" layers. The effect of the MLD process temperature (110 °C, 125 °C) and of the calcination temperature (340 °C, 400 °C, 500 °C) on the chemical, morphological, and crystallographic properties of the resulting ZnO was thoroughly investigated. Spectroscopic ellipsometry reveals that the thickness of the calcinated layers depends on the MLD temperature, resulting in 38-43% and 52-56% of remaining thickness for the 110 °C and 125 °C samples, respectively. Ellipsometric porosimetry shows that the open porosity of the ZnO thin films depends on the calcination temperature as well as on the MLD process temperature. The maximum open porosity of ZnO derived from zincone deposited at 110 °C ranges from 14.5% to 24%, rising with increasing calcination temperature. Compared with the 110 °C samples, the ZnO obtained from 125 °C zincone yields a higher porosity for low calcination temperatures, namely 18% for calcination at 340 °C; and up to 24% for calcination at 500 °C. Additionally, the porous ZnO thin films were subjected to piezoelectric measurements. The piezoelectric coefficient, d33, was determined to be 2.8 pC/N, demonstrating the potential of the porous ZnO as an, e.g., piezoelectric sensor or energy harvester.

Glucose-Responsive Boronic Acid Hydrogel Thin Films Obtained via Initiated Chemical Vapor Deposition.

Glucose-responsive materials are of great importance in the field of monitoring the physiological glucose level or smart insulin management. This study presents the first vacuum-based deposition of a glucose-responsive hydrogel thin film. The successful vacuum-based synthesis of a glucose-responsive hydrogel may open the door to a vast variety of new applications, where, for example, the hydrogel thin film is applied on new possible substrates. In addition, vacuum-deposited films are free of leachables (e.g., plasticizers and residual solvents). Therefore, they are, in principle, safe for in-body applications. A hydrogel made of but-3-enylboronic acid units, a boronic acid compound, was synthesized via initiated chemical vapor deposition. The thin film was characterized in terms of chemical composition, surface morphology, and swelling response toward pH and sucrose, a glucose-fructose compound. The film was stable in aqueous solutions, consisting of polymerized boronic acid and the initiator unit, and had an undulating texture appearance (rms 2.1 nm). The hydrogel was in its shrunken state at pH 4-7 and swelled by increasing the pH to 9. The pKa was 8.2 ± 0.2. The response to glucose was observed at pH 10 and resulted in thickness shrinking.

Humidity Responsive Reflection Grating Made by Ultrafast Nanoimprinting of a Hydrogel Thin Film.

The response time of state-of-the-art humidity sensors is ≈8 s. A faster tracking of humidity change is especially required for health care devices. This research is focused on the direct nanostructuring of a humidity-sensitive polymer thin film and it is combined with an optical read-out method. The goal is to improve the response time by changing the surface-to-volume ratio of the thin film and to test a different measurement method compared to state-of-the-art sensors. Large and homogeneous nanostructured areas are fabricated by nanoimprint lithography on poly(2-hydroxyethyl methacrylate) thin films. Those thin films are made by initiated chemical vapor deposition (iCVD). To the author's knowledge, this is the first time nanoimprint lithography is applied on iCVD polymer thin films. With the imprinting process, a diffraction grating is developed in the visible wavelength regime. The optical and physicochemical behavior of the nanostructures is modeled with multi-physic simulations. After successful modeling and fabrication a first proof of concept shows that humidity dependency by using an optical detection of the first diffraction order peak is observable. The response time of the structured thin film results to be at least three times faster compared to commercial sensors.

Deep tissue localization and sensing using optical microcavity probes.

Optical microcavities and microlasers were recently introduced as probes inside living cells and tissues. Their main advantages are spectrally narrow emission lines and high sensitivity to the environment. Despite numerous novel methods for optical imaging in strongly scattering biological tissues, imaging at single-cell resolution beyond the ballistic light transport regime remains very challenging. Here, we show that optical microcavity probes embedded inside cells enable three-dimensional localization and tracking of individual cells over extended time periods, as well as sensing of their environment, at depths well beyond the light transport length. This is achieved by utilizing unique spectral features of the whispering-gallery modes, which are unaffected by tissue scattering, absorption, and autofluorescence. In addition, microcavities can be functionalized for simultaneous sensing of various parameters, such as temperature or pH value, which extends their versatility beyond the capabilities of standard fluorescent labels.

Multiresponsive Soft Actuators Based on a Thermoresponsive Hydrogel and Embedded Laser-Induced Graphene.

The method of converting insulating polymers into conducting 3D porous graphene structures, so-called laser-induced graphene (LIG) with a commercially available CO2 laser engraving system in an ambient atmosphere, resulted in several applications in sensing, actuation, and energy. In this paper, we demonstrate a combination of LIG and a smart hydrogel (poly(N-vinylcaprolactam)-pNVCL) for multiresponsive actuation in a humid environment. Initiated chemical vapor deposition (iCVD) was used to deposit a thin layer of the smart hydrogel onto a matrix of poly(dimethylsiloxane) (PDMS) and embedded LIG tracks. An intriguing property of smart hydrogels, such as pNVCL, is that the change of an external stimulus (temperature, pH, magnetic/electric fields) induces a reversible phase transition from a swollen to a collapsed state. While the active smart hydrogel layer had a thickness of only 300 nm (compared to the 500 times thicker actuator matrix), it was possible to induce a reversible bending of over 30° in the humid environment triggered by Joule heating. The properties of each material were investigated by means of scanning electron microscopy (SEM), Raman spectroscopy, tensile testing, and ellipsometry. The actuation performances of single-responsive versions were investigated by creating a thermoresponsive PDMS/LIG actuator and a humidity-responsive PDMS/pNVCL actuator. These results were used to tune the properties of the multiresponsive PDMS/LIG/pNVCL actuator. Furthermore, its self-sensing capabilities were investigated. By getting a feedback from the piezoresistive change of the PMDS/LIG composite, the bending angle could be tracked by measuring the change in resistance. To highlight the possibilities of the processing techniques and the combination of materials, a demonstrator in the shape of an octopus with four independently controllable arms was developed.

Study on Porosity in Zinc Oxide Ultrathin Films from Three-Step MLD Zn-Hybrid Polymers.

Deriving mesoporous ZnO from calcinated, molecular layer deposited (MLD) metal-organic hybrid thin films offers various advantages, e.g., tunable crystallinity and porosity, as well as great film conformality and thickness control. However, such methods have barely been investigated. In this contribution, zinc-organic hybrid layers were for the first time formed via a three-step MLD sequence, using diethylzinc, ethanolamine, and maleic anhydride. These zinc-organic hybrid films were then calcinated with the aim of enhancing the porosity of the obtained ZnO films. The saturation curves for the three-step MLD process were measured, showing a growth rate of 4.4 ± 0.2 Å/cycle. After initial degradation, the zinc-organic layers were found to be stable in ambient air. The transformation behavior of the zinc-organic layers, i.e., the evolution of the film thickness and refractive index as well as the pore formation upon heating to 400, 500, and 600 °C were investigated with the help of spectroscopic ellipsometry and ellipsometric porosimetry. The calculated pore size distribution showed open porosity values of 25%, for the sample calcinated at 400 °C. The corresponding expectation value for the pore radius obtained from this distribution was 2.8 nm.

Deposition of Ion-Conductive Membranes from Ionic Liquids via Initiated Chemical Vapor Deposition.

In this study, liquid droplets of 1-allyl-3-methylimidazolium dicyanamide have been processed by initiated chemical vapor deposition (iCVD) with a cross-linked polymer film consisting of (hydroxyethyl)methacrylate and ethylene glycol dimethacrylate to develop free-standing, ion-conductive membranes. We found that the obtained films are solids and have a conductivity of up to 18 ± 6 mS/cm, associated with the negatively charged counterion, indicating no loss of conductivity, compared to the ionic liquid in the liquid state. The membranes were conductive within a large process window and in air, thanks to the fact that the iCVD process does not affect the mobility of the anion in the ionic liquid. Furthermore, we demonstrate that varying the deposition conditions can influence the homogeneity and conductivity of the resulting membranes. The promising results of this study represent an important stepping stone on the way to novel ion-conductive membranes.

Conformal Coating of Powder by Initiated Chemical Vapor Deposition on Vibrating Substrate.

Encapsulation of pharmaceutical powders within thin functional polymer films is a powerful and versatile method to modify drug release properties. Conformal coating over the complete surface of the particle via chemical vapor deposition techniques is a challenging task due to the compromised gas-solid contact. In this study, an initiated chemical vapor deposition reactor was adapted with speakers and vibration of particles was achieved by playing AC/DC's song "Thunderstruck" to overcome the above-mentioned problem. To show the possibilities of this method, two types of powder of very different particle sizes were chosen, magnesium citrate (3-10 µm, cohesive powder) and aspirin (100-500 µm, good flowability), and coated with poly-ethylene-glycol-di-methacrylate. The release curve of coated magnesium citrate powder was retarded compared to uncoated powder. However, neither changing the thickness coating nor vibrating the powder during the deposition had influence on the release parameters, indicating, that cohesive powders cannot be coated conformally. The release of coated aspirin was as well retarded as compared to uncoated aspirin, especially in the case of the powder that vibrated during deposition. We attribute the enhancement of the retarded release to the formation of a conformal coating on the aspirin powder.

Screen-Printed Ferroelectric P(VDF-TrFE)-co-PbTiO3 and P(VDF-TrFE)-co-NaBiTi2O6 Nanocomposites for Selective Temperature and Pressure Sensing.

Piezo- and pyroelectricity is an intrinsically combined material property for all ferroelectric materials. While the pyroelectric coefficients of most ferroelectric ceramics and polymers have the same sign, their piezoelectric coefficients have opposite ones. On this basis, we can create a polymer-ceramic nanocomposite material where either the piezo- or the pyroelectric effect is suppressed by a selective poling of the single constituents, a concept that was shown for composite pellets in the late 1990s. Motivated by the current demand for lightweight and low-cost piezoelectric sensors with reduced cross-sensitivity to temperature variations, we have taken up this idea and formulated screen-printable nanocomposite pastes from poly(vinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) and lead titanate (PbTiO3, PT) or sodium bismuth titanate (NaBiTi2O6 or BNT) nanoparticles, respectively. We demonstrate that printed sensors on flexible substrates based on these materials can be conditioned by selective poling of the nanoparticles and the polymer matrix to show either only piezoelectric or only pyroelectric sensor response. We examined the degree of cross-talk between the thermal and pressure sensing channels and show a reduction of over 90% cross-sensitivity for the ferroelectric composites compared to pure P(VDF-TrFE) sensors.

Applicability of Vapor-Deposited Thermoresponsive Hydrogel Thin Films in Ultrafast Humidity Sensors/Actuators.

Thermoresponsive polymers reversibly react to changes in temperature and water content of their environment (i.e., relative humidity, RH). In the present contribution, the thermoresponsiveness of poly(N-vinylcaprolactam) thin films cross-linked by di(ethylene glycol) divinyl ether deposited by initiated chemical vapor deposition are investigated to assess their applicability to sensor and actuator setups. A lower critical solution temperature (LCST) is observed at around 16 °C in aqueous environment, associated with a dramatic change in film thickness (e.g., 200% increase at low temperatures) and refractive index, while only thermal expansion of the polymeric system is found, when ramping the temperature in dry atmosphere. In humid environment, we observed a significant response occurring in low RH (already below 5% RH), with the moisture swelling the thin film (up to 4%), but mainly replacing air in the polymeric structure up to ∼40% RH. Non-temperature-dependent swelling is observed up to 80% RH. Above that, thermoresponsive behavior is also demonstrated to be present in humid environment for the first time, whereas toward 100% RH, film thickness and index appear to approach the values obtained in water at the respective temperatures. The response times are similar in a large range of RH and are faster than the ones of the reference humidity sensor used (i.e., seconds). A sensor/actuator hygromorphic device was built by coating a thin flower-shaped poly(dimethylsiloxane) (PDMS) substrate with the thermoresponsive polymer. The large swelling due to water uptake upon exposure to humid environment at temperatures below the LCST caused the petals to bend, mimicking the capability of plants to respond to environmental stimuli via reversible mechanical motion.

Fabrication, characterization and cytocompatibility assessment of gelatin nanofibers coated with a polymer thin film by initiated chemical vapor deposition.

The presence of various functional groups in the structure of gelatin nanofibers (GNFs) has made it a suitable candidate for biomedical applications, yet its fast dissolution in aqueous media has been a real challenge for years. In the present work, we propose an efficient procedure to improve the durability of the GNFs. The electrospun GNFs were coated with poly(ethylene glycol dimethacrylate) (pEGDMA) using initiated chemical vapor deposition (iCVD) as a completely dry polymerization method. Morphological and chemical analysis revealed that an ultrathin layer formed around nanofibers (iCVD-GNFs) which has covalently bonded to gelatin chains. Against the instant dissolution of GNFs, the in vitro biodegradability test showed the iCVD-GNFs, to a large extent, preserve their morphology after 14 days of immersion and did not lose its integrity even after 31 days. In vitro cell culture studies, also, revealed cytocompatibility of the iCVD-GNFs for human fibroblast cells (hFC), as well as higher cell proliferation on the iCVD-GNFs compared to control made from tissue culture plate (TCP). Furthermore, contact angle measurements indicated that the hydrophilic GNFs became hydrophobic after the iCVD, yet FE-SEM images of cell-seeded iCVD-GNFs showed satisfactory cell adhesion. Taken together, the proposed method paves a promising way for the production of water-resistant GNFs utilized in biomedical applications; for instance, tissue engineering scaffolds and wound dressings.

Initiated Chemical Vapor Deposition of Crosslinked Organic Coatings for Controlling Gentamicin Delivery.

A coating consisting of a copolymer of methacrylic acid and ethylene glycol dimethacrylate was deposited over a gentamicin film by initiated chemical vapor deposition with the aim of controlling the drug release. Gentamicin release in water was monitored by means of conductance measurements and of UV-vis Fluorescence Spectroscopy. The influence of the polymer chemical composition, specifically of its crosslinking density, has been investigated as a tool to control the swelling behavior of the initiated chemical vapor deposition (iCVD) coating in water, and therefore its ability to release the drug. Agar diffusion test and microbroth dilution assays against Staphylococcus aureus and Pseudomonas aeruginosa on cellulose coated substrates confirmed that the antibacterial activity of the drug released by the coating was retained, though the release of gentamicin was not complete.