Different Response Kinetics to Temperature and Water Vapor of Acrylamide Polymers Obtained by Initiated Chemical Vapor Deposition

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.

Humidity-Controlled Tunable Emission in a Dye-Incorporated Metal-Hydrogel-Metal Cavity

Actively controllable photoluminescence is potent for a wide variety of applications from biosensing and imaging to optoelectronic components. Traditionally, methods to achieve active emission control are limited due to complex fabrication processes or irreversible tuning. Here, we demonstrate active emission tuning, achieved by changing the ambient humidity in a fluorescent dye-containing hydrogel integrated into a metal-insulator-metal (MIM) system. Altering the overlapping region of the MIM cavity resonance and the absorption and emission spectra of the dye used is the underlying principle to achieving tunability of the emission. We first verify this by passive tuning of cavity resonance and further experimentally demonstrate active tuning in both air and aqueous environments. The proposed approach is reversible, easy to integrate, and spectrally scalable, thus providing opportunities for developing tunable photonic devices.

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

The response time of state of the art humidity sensors is approximately 8 s. A faster tracking of humidity change is especially required for health care devices. We focused our research on the direct nanostructuring of a humidity-sensitive polymer thin film and combined it with an optical read-out method. Our goal was 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 were fabricated by nanoimprint lithography on poly(2-hydroxyethyl methacrylate) thin films. Those thin films were made by initiated chemical vapor deposition (iCVD). To the author's knowledge, this is the first time nanoimprint lithography was applied on iCVD polymer thin films. With the imprinting process we developed a diffraction grating in the visible wavelength regime. The optical and physicochemical behavior of the nanostructures was modeled with multi-physic simulations. After successful modeling and fabrication a first proof of concept showed that humidity dependency by using an optical detection of the first diffraction order peak is observable. The response time of the structured thin film resulted to be at least three times faster compared to commercial sensors. This article is protected by copyright. All rights reserved.

Humidity- and Temperature-Tunable Metal-Hydrogel-Metal Reflective Filters

A tunable reflectance filter based on a metal-hydrogel-metal structure responsive to humidity and temperature is reported. The filter employs a poly(N-isopropylacrylamide)-acrylamidobenzophenone (PNIPAm-BP) hydrogel as an insulator layer in the metal-insulator-metal (MIM) assembly. The optical resonance of the structure is tunable by water immersion across the visible and near-infrared range. Swelling/deswelling and the volume phase transition of the hydrogel allow continuous reversible humidity- and/or temperature-induced tuning of the optical resonance. This work paves the way toward low-cost large-area fabrication of actively tunable reversible photonic devices.

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.

Interlink between Tunable Material Properties and Thermoresponsiveness of Cross-Linked Poly(N-vinylcaprolactam) Thin Films Deposited by Initiated Chemical Vapor Deposition

In this contribution, we report on the thin-film synthesis of a novel thermoresponsive polymer, namely, poly(N-vinylcaprolactam) cross-linked by di(ethylene glycol)divinyl ether [p(NVCL-co-DEGDVE)] by initiated chemical vapor deposition (iCVD). Its transition between swollen and shrunken states in film thickness and the corresponding lower critical solution temperature (LCST) was investigated by spectroscopic ellipsometry in water. Water contact angle measurements and nano-indentation experiments reveal that the transition is accompanied by a change in wettability and elastic modulus. The amount of cross-linking was used to tune the thermoresponsive behavior of the thin films, resulting in higher swelling and LCST, increased surface rearrangement, and lower stiffness for less cross-linked polymers. For the first time, the filament temperature during iCVD synthesis was used to vary the chain length of the resulting polymeric systems and, thus, the position of their thermoresponsive transition. With that, swelling of up to 250% compared to the dry thickness and transition temperatures ranging from 16 to 40 °C could be achieved.

On the transformation of "zincone"-like into porous ZnO thin films from sub-saturated plasma enhanced atomic layer deposition

The synthesis of nanoporous ZnO thin films is achieved through annealing of zinc-alkoxide ("zincone"-like) layers obtained by plasma-enhanced atomic layer deposition (PE-ALD). The zincone-like layers are deposited through sub-saturated PE-ALD adopting diethylzinc and O₂ plasma with doses below self-limiting values. Nanoporous ZnO thin films were subsequently obtained by calcination of the zincone-like layers between 100-600 °C. Spectroscopic ellipsometry (SE) and X-ray diffraction (XRD) were adopted in situ during calcination to investigate the removal of carbon impurities, development of controlled porosity, and formation and growth of ZnO crystallites. The layers developed controlled nanoporosity in the range of 1-5%, with pore sizes between 0.27 and 2.00 nm as measured with ellipsometric porosimetry (EP), as a function of the plasma dose and post-annealing temperature. Moreover, the crystallinity and crystallite orientation could be tuned, ranging from a powder-like to a (100) preferential growth in the out-of-plane direction, as measured by synchrotron-radiation grazing incidence XRD. Calcination temperature ranges were identified in which pore formation and subsequent crystal growth occurred, giving insights in the manufacturing of nanoporous ZnO from Zn-based hybrid materials.

Fast Optical Humidity Sensor Based on Hydrogel Thin Film Expansion for Harsh Environment

With the application of a recently developed deposition method called initiated chemical vapor deposition (iCVD), responsive hydrogel thin films in the order of a few hundred nanometers were created. When in contact with humid air, the hydrogel layer increases its thickness considerably. The measurement of the thickness change was realized interferometrically with a laser and a broadband light source in two different implementations. The relative change in thickness with respect to humidity can be described with the Flory⁻Huggins theory. The required Flory⁻Huggins interaction parameter was determined for the actual hydrogel composition. The setup was designed without electric components in the vicinity of the active sensor layer and is therefore applicable in harsh environments such as explosive or corrosive ones. The implemented sensor prototype delivered reproducible relative humidity ( R H ) values and the achieved response time for an abrupt change of the humidity τ 63 ≤ 2.5 s was about three times faster compared to one of the fastest commercially available sensors on the market.

Drug release from thin films encapsulated by a temperature-responsive hydrogel

Control over drug delivery may be interestingly achieved by using temperature responsive encapsulants, which change their thickness and mesh size with temperature. The prototype N-isopropylacrylamide hydrogel cross-linked with di(ethylene glycol) divinyl ether p(NIPAAm-co-DEGDVE) swells at low temperature and collapses above the lower critical solution temperature (LCST), ∼29 °C in a buffer. It might be expected that drug release from such encapsulation is always favored below the LCST, due to the larger free volume present in the swollen polymer film. Recent results show contradicting behavior where some cases behave as expected and others release much less when the polymer layer is swollen. In this study, layers of the drugs phenytoin, clotrimazole and indomethacin were drop cast on glass and p(NIPAAM-co-DEGDVE) layers were then synthesized directly on top of these drug layers via initiated chemical vapor deposition (iCVD), a solvent-free and gentle polymerization technique. Dissolution experiments were then performed, in which the drug release through the hindrance of the hydrogel was measured at different pH values. The results show that not only the swelling but also the permeate (drug in this case)-polymer interaction plays an important role in the release.

Mesoporous ZnO thin films obtained from molecular layer deposited “zincones”

The synthesis of MLD-derived mesoporous ZnO with 20% of porosity is demonstrated and studied by advanced in situ characterization techniques.

Thickness-Dependent Swelling Behavior of Vapor-Deposited Smart Polymer Thin Films

In this contribution, the temperature-dependent swelling behavior of vapor-deposited smart polymer thin films is shown to depend on cross-linking and deposited film thickness. Smart polymers find application in sensor and actuator setups and are mostly fabricated on delicate substrates with complex nanostructures that need to be conformally coated. As initiated chemical vapor deposition (iCVD) meets these specific requirements, the present work concentrates on temperature-dependent swelling behavior of iCVD poly(N-isopropylacrylamide) thin films. The transition between swollen and shrunken state and the corresponding lower critical solution temperature (LCST) was investigated by spectroscopic ellipsometry in water. The films' density in the dry state evaluated from X-ray reflectivity could be successfully correlated to the position of the LCST in water and was found to vary between 1.1 and 1.3 g/cm³ in the thickness range 30-330 nm. This work emphasizes the importance of insights in both the deposition process and mechanisms during swelling of smart polymeric structures.