Graft Polymerization of Anti-Fouling PEO Surfaces by Liquid-Free Initiated Chemical Vapor Deposition

Enhancing the Kinetics of Vapor-based Polymerization by Pulsed Filament Approach

Initiated chemical vapor deposition is a versatile technique for synthesizing conformal polymer films on both planar and porous surfaces. It can retain functional groups and avoid undesired cross-linking. However, there is still room for enhancing its performance without altering the feed parameters. Here, we investigate a pulsed iCVD approach to improve the deposition process, achieved by switching on and off the resistively heated filament periodically. By strategically switching off the filament, a shortage of thermally activated primary radicals was created, which allowed uninterrupted chain propagation with fewer termination reactions and potentially increased monomer conversion rates. This has caused significantly faster deposition kinetics with a higher molecular weight and longer chain length for poly(glycidyl methacrylate) compared to continuous deposition. Spectra analyses confirmed that the functionality and stoichiometry ratios remained intact throughout the pulsed deposition process. The pulsed iCVD method is therefore a competitive and sustainable tool, demonstrating fast deposition kinetics and a well-preserved functionality.

Controlled Release Utilizing Initiated Chemical Vapor Deposited (iCVD) of Polymeric Nanolayers

This review will focus on the controlled release of pharmaceuticals and other organic molecules utilizing polymeric nanolayers grown by initiated chemical vapor deposited (iCVD). The iCVD layers are able conform to the geometry of the underlying substrate, facilitating release from one- and two-dimensional nanostructures with high surface area. The reactors for iCVD film growth can be customized for specific substrate geometries and scaled to large overall dimensions. The absence of surface tension in vapor deposition processes allows the synthesis of pinhole-free layers, even for iCVD layers <10 nm thick. Such ultrathin layers also provide rapid transport of the drug across the polymeric layer. The mild conditions of the iCVD process avoid damage to the drug which is being encapsulated. Smart release is enabled by iCVD hydrogels which are responsive to pH, temperature, or light. Biodegradable iCVD layers have also be demonstrated for drug release.

Vapor-deposited functional polymer thin films in biological applications

Functional polymer coatings have become ubiquitous in biological applications, ranging from biomaterials and drug delivery to manufacturing-scale separation of biomolecules using functional membranes. Recent advances in the technology of chemical vapor deposition (CVD) have enabled precise control of the polymer chemistry, coating thickness, and conformality. That comprehensive control of surface properties has been used to elicit desirable interactions at the interface between synthetic materials and living organisms, making vapor-deposited functional polymers uniquely suitable for biological applications. This review captures the recent technological development in vapor-deposited functional polymer coatings, highlighting their biological applications, including membrane-based bio-separations, biosensing and bio-MEMS, drug delivery, and tissue engineering. The conformal nature of vapor-deposited coatings ensures uniform coverage over micro- and nano-structured surfaces, allowing the independent optimization of surface and bulk properties. The substrate-independence of CVD techniques enables facile transfer of surface characteristics among different applications. The vapor-deposited functional polymer thin films tend to be biocompatible because they are free of remnant toxic solvents and precursor molecules, potentially lowering the barrier to clinical success.

Vapor-Deposited Biointerfaces and Bacteria: An Evolving Conversation

At the biointerface where materials and microorganisms meet, the organic and synthetic worlds merge into a new science that directs the design and safe use of synthetic materials for biological applications. Vapor deposition techniques provide an effective way to control the material properties of these biointerfaces with molecular-level precision that is important for biomaterials to interface with bacteria. In recent years, biointerface research that focuses on bacteria-surface interactions has been primarily driven by the goals of killing bacteria (antimicrobial) and fouling prevention (antifouling). Nevertheless, vapor deposition techniques have the potential to create biointerfaces with features that can manipulate and dictate the behavior of bacteria rather than killing or deterring them. In this review, we focus on recent advances in antimicrobial and antifouling biointerfaces produced through vapor deposition and provide an outlook on opportunities to capitalize on the features of these techniques to find unexplored connections between surface features and microbial behavior.

Reduced cell attachment to poly(2-hydroxyethyl methacrylate)-coated ventricular catheters in vitro

The majority of patients with hydrocephalus are dependent on ventriculoperitoneal shunts for diversion of excess cerebrospinal fluid. Unfortunately, these shunts are failure-prone and over half of all life-threatening pediatric failures are caused by obstruction of the ventricular catheter by the brain's resident immune cells, reactive microglia and astrocytes. Poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogels are widely used for biomedical implants. The extreme hydrophilicity of PHEMA confers resistance to protein fouling, making it a strong candidate coating for ventricular catheters. With the advent of initiated chemical vapor deposition (iCVD), a solvent-free coating technology that creates a polymer in thin film form on a substrate surface by introducing gaseous reactant species into a vacuum reactor, it is now possible to apply uniform polymer coatings on complex three-dimensional substrate surfaces. iCVD was utilized to coat commercially available ventricular catheters with PHEMA. The chemical structure was confirmed on catheter surfaces using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. PHEMA coating morphology was characterized by scanning electron microscopy. Testing PHEMA-coated catheters against uncoated clinical-grade catheters in an in vitro hydrocephalus catheter bioreactor containing co-cultured astrocytes and microglia revealed significant reductions in cell attachment to PHEMA-coated catheters at both 17-day and 6-week time points. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1268-1279, 2018.

Polymer Thin Films and Surface Modification by Chemical Vapor Deposition: Recent Progress

Chemical vapor deposition (CVD) polymerization uses vapor phase monomeric reactants to synthesize organic thin films directly on substrates. These thin films are desirable as conformal surface engineering materials and functional layers. The facile tunability of the films and their surface properties allow successful integration of CVD thin films into prototypes for applications in surface modification, device fabrication, and protective films. CVD polymers also bridge microfabrication technology with chemical and biological systems. Robust coatings can be achieved via CVD methods as antifouling, anti-icing, and antihydrate surfaces, as well as stimuli-responsive or biocompatible polymers and novel nanostructures. Use of low-energy input, modest vacuum, and room-temperature substrates renders CVD polymerization compatible with thermally sensitive substrates and devices. Compared with solution-based methods, CVD is particularly useful for insoluble materials, such as electrically conductive polymers and controllably crosslinked networks, and has the potential to reduce environmental, health, and safety impacts associated with solvents. This review discusses the relevant background and selected applications of recent advances by two methods that display and use the high retention of the organic functional groups from their respective monomers, initiated CVD (iCVD) and oxidative CVD (oCVD) polymerization.

Fabricating Polymer Canopies onto Structured Surfaces Using Liquid Scaffolds

In this work, we study the use of initiated chemical vapor deposition in conjunction with liquid scaffolds to deposit polymer canopies onto structured surfaces. Liquid is applied to micropillar and microstructure surfaces to act as a scaffolding template such that the deposited polymer films take the shape of the liquid surface. Two methods for directing the location of the scaffolding liquid were examined. In the first method, high surface tension liquids rest in a Cassie-Baxter state over the structured surfaces, allowing for control over the canopy location and size by varying the position and volume of the liquid. In the second method, the structured surfaces are inverted onto a thin layer of low surface tension liquid, allowing the coverage and height of the canopy to be controlled by varying the area and thickness of the liquid layer. Although the canopies demonstrated in this study were fabricated using initiated chemical vapor deposition, the generality of our scaffolding method can easily be translated to other vapor deposition processes.

Amplified electron transfer at poly-ethylene-glycol (PEG) grafted electrodes

Electron transfer at pegylated electrode surfaces is suppressed for Fe(CN)₆^3−/4− and then recovered in the presence of ferrocene-dimethanol.

Surface-initiated hyperbranched polyglycerol as an ultralow-fouling coating on glass, silicon, and porous silicon substrates

Anionic ring-opening polymerization of glycidol was initiated from activated glass, silicon, and porous silicon substrates to yield thin, ultralow-fouling hyperbranched polyglycerol (HPG) graft polymer coatings. Substrates were activated by deprotonation of surface-bound silanol functionalities. HPG polymerization was initiated upon the addition of freshly distilled glycidol to yield films in the nanometer thickness range. X-ray photoelectron spectroscopy, contact angle measurements, and ellipsometry were used to characterize the resulting coatings. The antifouling properties of HPG-coated surfaces were evaluated in terms of protein adsorption and the attachment of mammalian cells. The adsorption of bovine serum albumin and collagen type I was found to be reduced by as much as 97 and 91%, respectively, in comparison to untreated surfaces. Human glioblastoma and mouse fibroblast attachment was reduced by 99 and 98%, respectively. HPG-grafted substrates outperformed polyethylene glycol (PEG) grafted substrates of comparable thickness under the same incubation conditions. Our results demonstrate the effectiveness of antifouling HPG graft polymer coatings on a selected range of substrate materials and open the door for their use in biomedical applications.

Ultra-low-cost and flexible paper-based microplasma generation devices for maskless patterning of poly(ethylene oxide)-like films

This work presents the use of an ultra-low-cost and flexible paper-based microplasma array to perform maskless patterning of poly(ethylene oxide)-like (PEO-like) thin films with a feature size down to submillimeter scale. In this process, the liquid precursor was directly applied to the paper substrate, gradually vaporized, and dissociated in the microplasma cavity, which leads to plasma polymerization. The FTIR and XPS spectra of the deposited film confirm the PEO-like structures. The protein adsorption test using the absorption of fluorescence-labeled fibrinogen conjugates on the treated surface shows the deposited films possessed the antifouling property with decent pattern transfer fidelity defined by the geometry of the microplasma array.

Full-field dynamic characterization of superhydrophobic condensation on biotemplated nanostructured surfaces

While superhydrophobic nanostructured surfaces have been shown to promote condensation heat transfer, the successful implementation of these coatings relies on the development of scalable manufacturing strategies as well as continued research into the fundamental physical mechanisms of enhancement. This work demonstrates the fabrication and characterization of superhydrophobic coatings using a simple scalable nanofabrication technique based on self-assembly of the Tobacco mosaic virus (TMV) combined with initiated chemical vapor deposition. TMV biotemplating is compatible with a wide range of surface materials and applicable over large areas and complex geometries without the use of any power or heat. The virus-structured coatings fabricated here are macroscopically superhydrophobic (contact angle >170°) and have been characterized using environmental electron scanning microscopy showing sustained and robust coalescence-induced ejection of condensate droplets. Additionally, full-field dynamic characterization of these surfaces during condensation in the presence of noncondensable gases is reported. This technique uses optical microscopy combined with image processing algorithms to track the wetting and growth dynamics of 100s to 1000s of microscale condensate droplets simultaneously. Using this approach, over 3 million independent measurements of droplet size have been used to characterize global heat transfer performance as a function of nucleation site density, coalescence length, and the apparent wetted surface area during dynamic loading. Additionally, the history and behavior of individual nucleation sites, including coalescence events, has been characterized. This work elucidates the nature of superhydrophobic condensation and its enhancement, including the role of nucleation site density during transient operation.

25th anniversary article: CVD polymers: a new paradigm for surface modification and device fabrication

Well-adhered, conformal, thin (<100 nm) coatings can easily be obtained by chemical vapor deposition (CVD) for a variety of technological applications. Room temperature modification with functional polymers can be achieved on virtually any substrate: organic, inorganic, rigid, flexible, planar, three-dimensional, dense, or porous. In CVD polymerization, the monomer(s) are delivered to the surface through the vapor phase and then undergo simultaneous polymerization and thin film formation. By eliminating the need to dissolve macromolecules, CVD enables insoluble polymers to be coated and prevents solvent damage to the substrate. CVD film growth proceeds from the substrate up, allowing for interfacial engineering, real-time monitoring, and thickness control. Initiated-CVD shows successful results in terms of rationally designed micro- and nanoengineered materials to control molecular interactions at material surfaces. The success of oxidative-CVD is mainly demonstrated for the deposition of organic conducting and semiconducting polymers.