Water and Moisture Uptake by Plasma Polymerized Thermoresponsive Hydrogel Films

Multitechnique investigation into the aqueous behavior of plasma polymers

Plasma polymers are often used in applications requiring aqueous immersion; therefore, it is important to understand how this exposure affects the physical and chemical properties of the films. Three different plasma polymer films were deposited at different distances from the electrode, and the film properties were characterized using contact angle, ellipsometry, and x-ray photoelectron spectroscopy. The film behaviors in aqueous solutions were studied via quartz crystal microbalance with dissipation (QCM-D). Exposure to buffer solutions produced significant swelling of the plasma polymerized acrylic acid films, with swelling increasing with distance from the powered electrode, results that could be correlated with changes in film chemistry. Plasma polymerized octadiene and allylamine exhibited little swelling. These films exhibited changes in thickness and contact angle with respect to distance from the electrode, but this had little influence on their behavior in aqueous solution. By combining QCM-D with the more traditional surface chemical analysis techniques, the authors have been able to explore both swelling behavior and the effect that sample position and thus deposition parameters have on film properties and aqueous behavior. This approach gives the authors the basis to define deposition parameters to assist the engineering of thin films for applications such as biosensing and tissue engineering applications where specific chemistries and film behaviors are desired.

Thin hydrogel films for optical biosensor applications

Hydrogel materials consisting of water-swollen polymer networks exhibit a large number of specific properties highly attractive for a variety of optical biosensor applications. This properties profile embraces the aqueous swelling medium as the basis of biocompatibility, non-fouling behavior, and being not cell toxic, while providing high optical quality and transparency. The present review focuses on some of the most interesting aspects of surface-attached hydrogel films as active binding matrices in optical biosensors based on surface plasmon resonance and optical waveguide mode spectroscopy. In particular, the chemical nature, specific properties, and applications of such hydrogel surface architectures for highly sensitive affinity biosensors based on evanescent wave optics are discussed. The specific class of responsive hydrogel systems, which can change their physical state in response to externally applied stimuli, have found large interest as sophisticated materials that provide a complex behavior to hydrogel-based sensing devices.

Plasma-enhanced copolymerization of amino acid and synthetic monomers

In this paper we report the use of plasma-enhanced chemical vapor deposition (PECVD) for the simultaneous deposition and copolymerization of an amino acid with other organic and inorganic monomers. We investigate the fundamental effects of plasma-enhanced copolymerization on different material chemistries in stable ultrathin coatings of mixed composition with an amino acid component. This study serves to determine the feasibility of a direct, facile method for integrating biocompatible/active materials into robust polymerized coatings with the ability to plasma copolymerize a biological molecule (L-tyrosine) with different synthetic materials in a dry, one-step process to form ultrathin coatings of mixed composition. This process may lead to a method of interfacing biologic systems with synthetic materials as a way to enhance the biomaterial-tissue interface and preserve biological activity within composite films.

CVD of polymeric thin films: applications in sensors, biotechnology, microelectronics/organic electronics, microfluidics, MEMS, composites and membranes

Polymers with their tunable functionalities offer the ability to rationally design micro- and nano-engineered materials. Their synthesis as thin films have significant advantages due to the reduced amounts of materials used, faster processing times and the ability to modify the surface while preserving the structural properties of the bulk. Furthermore, their low cost, ease of fabrication and the ability to be easily integrated into processing lines, make them attractive alternatives to their inorganic thin film counterparts. Chemical vapor deposition (CVD) as a polymer thin-film deposition technique offers a versatile platform for fabrication of a wide range of polymer thin films preserving all the functionalities. Solventless, vapor-phase deposition enable the integration of polymer thin films or nanostructures into micro- and nanodevices for improved performance. In this review, CVD of functional polymer thin films and the polymerization mechanisms are introduced. The properties of the polymer thin films that determine their behavior are discussed and their technological advances and applications are reviewed.

Sharp Hydrophilicity Switching and Conformality on Nanostructured Surfaces Prepared via Initiated Chemical Vapor Deposition (iCVD) of a Novel Thermally Responsive Copolymer

A novel thermally responsive copolymer p(NIPAAm-co-DEGDVE) is synthesized using the substrate independent method of iCVD and exhibits a sharp lower critical solution temperature (LCST) transition centered at ≈28.5 ± 0.3 °C determined via quartz crystal microbalance measurements with dissipation monitoring (QCM-D). Swelling with water below the LCST produces a reversible change of ≈3× in film thickness. The layer is conformal on nanostructured surfaces including MWCNT forests and electrospun nanofiber mats. Modified planar substrates exhibit ≈30°change in static contact angle over the LCST, while through conformal coating on nanostructured substrates changes in static contact angle up to 135° are achieved. Additionally, coated surfaces exhibit temperature sensitive BSA adsorption measured by QCM-D and is reversible as shown through fluorescence imaging of a coated electrospun nanofiber mat.

Nanoscale characterization of the equilibrium and kinetic response of hydrogel structures

The use of hydrogel nanostructured patterns and films in biomedical micro- and nanodevices requires the ability to analyze and understand their response properties at the nanoscale. Herein, the thermoresponse behavior of atom transfer radical polymerization (ATRP) grown poly(ethylene glycol) n dimethacrylate (PEGnDMA) cross-linked poly(N-isopropyl acrylamide) (PNIPAAm) hydrogel thin films over gold was studied. By controlling the mesh size of the hydrogel matrix through tuning the cross-linking density (i.e., using different molecular weight cross-linker and/or various amounts of cross-linker), the hydrogel volume swelling ratio was tailored for response applications. Thermoresponsive patterns exhibited a broad lower critical solution temperature (LCST) swelling transition, while rms roughness analysis of the hydrogel surface showed a sharp LCST transition. Mass and viscoelastic property changes were monitored using quartz crystal microbalance with dissipation (QCM-D), and the rapid response behavior of the thin hydrogel films was observed. The tunable response behavior along with the controlled growth of the hydrogel achieved via ATRP at the nanoscale make them applicable as functional components in diagnostic and therapeutic devices.

Chemical vapor deposition of conformal, functional, and responsive polymer films

Chemical vapor deposition (CVD) polymerization utilizes the delivery of vapor-phase monomers to form chemically well-defined polymeric films directly on the surface of a substrate. CVD polymers are desirable as conformal surface modification layers exhibiting strong retention of organic functional groups, and, in some cases, are responsive to external stimuli. Traditional wet-chemical chain- and step-growth mechanisms guide the development of new heterogeneous CVD polymerization techniques. Commonality with inorganic CVD methods facilitates the fabrication of hybrid devices. CVD polymers bridge microfabrication technology with chemical, biological, and nanoparticle systems and assembly. Robust interfaces can be achieved through covalent grafting enabling high-resolution (60 nm) patterning, even on flexible substrates. Utilizing only low-energy input to drive selective chemistry, modest vacuum, and room-temperature substrates, CVD polymerization is compatible with thermally sensitive substrates, such as paper, textiles, and plastics. CVD methods are particularly valuable for insoluble and infusible films, including fluoropolymers, electrically conductive polymers, and controllably crosslinked networks and for the potential to reduce environmental, health, and safety impacts associated with solvents. Quantitative models aid the development of large-area and roll-to-roll CVD polymer reactors. Relevant background, fundamental principles, and selected applications are reviewed.