Kenneth Cheng

Co-immobilization of biomolecules on ultrathin reactive chemical vapor deposition coatings using multiple click chemistry strategies.

Immobilization of biomolecules, such as proteins or sugars, is a key issue in biotechnology because it enables the understanding of cellular behavior in more biological relevant environment. Here, poly(4-ethynyl-p-xylylene-co-p-xylylene) coatings have been fabricated by chemical vapor deposition (CVD) polymerization in order to bind bioactive molecules onto the surface of the material. The control of the thickness of the CVD films has been achieved by tuning the amount of precursor used for deposition. Copper-catalyzed Huisgen cycloaddition has then been performed via microcontact printing to immobilize various biomolecules on the reactive coatings. The selectivity of this click chemistry reaction has been confirmed by spatially controlled conjugation of fluorescent sugar recognizing molecules (lectins) as well as cell adhesion onto the peptide pattern. In addition, a microstructured coating that may undergo multiple click chemistry reactions has been developed by two sequential CVD steps. Poly(4-ethynyl-p-xylylene-co-p-xylylene) and poly(4-formyl-p-xylylene-co-p-xylylene) have been patterned via vapor-assisted micropatterning in replica structures (VAMPIR). A combination of Huisgen cycloaddition and carbonyl-hydrazide coupling was used to spatially direct the immobilization of sugars on a patterned substrate. This work opens new perspectives in tailoring microstructured, multireactive interfaces that can be decorated via bio-orthogonal chemistry for use as mimicking the biological environment of cells.

Selective and Reversible Binding of Thiol-Functionalized Biomolecules on Polymers Prepared via Chemical Vapor Deposition Polymerization

We use chemical vapor deposition polymerization to prepare a novel dibromomaleimide-functionalized polymer for selective and reversible binding of thiol-containing biomolecules on a broad range of substrates. We report the synthesis and CVD polymerization of 4-(3,4-dibromomaleimide)[2.2]paracyclophane to yield nanometer thick polymer coatings. Fourier transformed infrared spectroscopy and X-ray photoelectron spectroscopy confirmed the chemical composition of the polymer coating. The reactivity of the polymer coating toward thiol-functionalized molecules was confirmed using fluorescent ligands. As a proof of concept, the binding and subsequent release of cysteine-modified peptides from the polymer coating were also demonstrated via sum frequency generation spectroscopy. This reactive polymer coating provides a flexible surface modification approach to selectively and reversibly bind biomolecules on a broad range of materials, which could open up new opportunities in many biomedical sensing and diagnostic applications where specific binding and release of target analytes are desired.

Backbone-Degradable Polymers Prepared by Chemical Vapor Deposition

Polymers prepared by chemical vapor deposition (CVD) polymerization have found broad acceptance in research and industrial applications. However, their intrinsic lack of degradability has limited wider applicability in many areas, such as biomedical devices or regenerative medicine. Herein, we demonstrate, for the first time, a backbone-degradable polymer directly synthesized via CVD. The CVD co-polymerization of [2.2]para-cyclophanes with cyclic ketene acetals, specifically 5,6-benzo-2-methylene-1,3-dioxepane (BMDO), results in well-defined, hydrolytically degradable polymers, as confirmed by FTIR spectroscopy and ellipsometry. The degradation kinetics are dependent on the ratio of ketene acetals to [2.2]para-cyclophanes as well as the hydrophobicity of the films. These coatings address an unmet need in the biomedical polymer field, as they provide access to a wide range of reactive polymer coatings that combine interfacial multifunctionality with degradability.

Multigrowth Factor Delivery via Immobilization of Gene Therapy Vectors

Molecules can be immobilized onto biomaterials by a chemical vapor deposition (CVD) coating strategy. Pentafluorophenolester groups react with amine side chains on antibodies, which can selectively immobilize adenoviral vectors for gene delivery of growth factors. These vectors can produce functional proteins within defined regions of biomaterials to produce customizable structures for targeted tissue regeneration.

Examining Nanoparticle Adsorption on Electrostatically “Patchy” Glycopolymer Brushes Using Real-Time ζ-Potential Measurements

Biomaterial surfaces can possess chemical, topographical, or electrostatic heterogeneity, which can profoundly influence their performance. By developing experimental models that reliably simulate this nanoscale heterogeneity, we can predict how heterogeneous surfaces are transformed by their interactions with the dynamic physiological environment. In this work, we present a model surface where well-defined glycopolymer brushes are interspersed with positively charged binding sites, giving rise to an interface presenting a mixture of repulsive and adhesive cues to an approaching virus particle. We show that the density of the affinity sites relative to the glycopolymer brushes can be tuned precisely by modifying the chemical vapor deposition (CVD) copolymerization conditions. Further, we examined the effects of binding site density and glycopolymer brush architecture on the adsorption kinetics of virus-like nanoparticles through a novel approach employing time-resolved ζ-potential measurements. Most materials have charge-bearing, dynamic surfaces that are sensitive to electrostatic effects. Hence, adsorption-triggered changes in ζ-potential measurements can be captured in real time to monitor interfacial events. Real-time ζ-potential measurements present an interesting platform to probe the structure and function of chemically and electrostatically heterogeneous polymer interfaces. To validate this electrokinetic method, we examined the effect of neutravidin concentration on its rate of binding to biotinylated surfaces using ζ-potential and compared our results with QCM studies. By applying electrokinetic methods to examine the roles of glycopolymer brush architecture and surface charge of these tunable glycopolymer coatings, we can enhance our understanding of the interactions of viruses with heterogeneous biomaterial interfaces.

Polylutidines: Multifunctional Surfaces through Vapor-Based Polymerization of Substituted Pyridinophanes

We report a new class of functionalized polylutidine polymers that are prepared by chemical vapor deposition polymerization of substituted [2](1,4)benzeno[2](2,5)pyridinophanes. To prepare sufficient amounts of monomer for CVD polymerization, a new synthesis route for ethynylpyridinophane has been developed in three steps with an overall yield of 59 %. Subsequent CVD polymerization yielded well-defined films of poly(2,5-lutidinylene-co-p-xylylene) and poly(4-ethynyl-2,5-lutidinylene-co-p-xylylene). All polymers were characterized by infrared reflection-absorption spectroscopy, ellipsometry, contact angle studies, and X-ray photoelectron spectroscopy. Moreover, ζ-potential measurements revealed that polylutidine films have higher isoelectric points than the corresponding poly-xylylene surfaces owing to the nitrogen atoms in the polymer backbone. The availability of reactive alkyne groups on the surface of poly(4-ethynyl-2,5-lutidinylene-co-p-xylylene) coatings was confirmed by spatially controlled surface modification by means of Huisgen 1,3-dipolar cycloaddition. Compared to the more hydrophobic poly-p-xylylyenes, the presence of the heteroatom in the polymer backbone of polylutidine polymers resulted in surfaces that supported an increased adhesion of primary human umbilical vein endothelial cells (HUVECs). Vapor-based polylutidine coatings are a new class of polymers that feature increased hydrophilicity and increased cell adhesion without limiting the flexibility in selecting appropriate functional side groups.

Carbohydrate-Based Polymer Brushes Prevent Viral Adsorption on Electrostatically Heterogeneous Interfaces

Chemical heterogeneity on biomaterial surfaces can transform its interfacial properties, rendering nanoscale heterogeneity profoundly consequential during bioadhesion. To examine the role played by chemical heterogeneity in the adsorption of viruses on synthetic surfaces, a range of novel coatings is developed wherein a tunable mixture of electrostatic tethers for viral binding, and carbohydrate brushes, bearing pendant α-mannose, β-galactose, or β-glucose groups, is incorporated. The effects of binding site density, brush composition, and brush architecture on viral adsorption, with the goal of formulating design specifications for virus-resistant coatings are experimentally evaluated. It is concluded that virus-coating interactions are shaped by the interplay between brush architecture and binding site density, after quantifying the adsorption of adenoviruses, influenza, and fibrinogen on a library of carbohydrate brushes co-immobilized with different ratios of binding sites. These insights will be of utility in guiding the design of polymer coatings in realistic settings where they will be populated with defects.

Micropatterned Scaffolds with Immobilized Growth Factor Genes Regenerate Bone and Periodontal Ligament-Like Tissues

Periodontal disease destroys supporting structures of teeth. However, tissue engineering strategies offer potential to enhance regeneration. Here, the strategies of patterned topography, spatiotemporally controlled growth factor gene delivery, and cell-based therapy to repair bone-periodontal ligament (PDL) interfaces are combined. Micropatterned scaffolds are fabricated for the ligament regions using polycaprolactone (PCL)/polylactic-co-glycolic acid and combined with amorphous PCL scaffolds for the bone region. Scaffolds are modified using chemical vapor deposition, followed by spatially controlled immobilization of vectors encoding either platelet-derived growth factor-BB or bone morphogenetic protein-7, respectively. The scaffolds are seeded with human cells and delivered to large alveolar bone defects in athymic rats. The effects of dual and single gene delivery with and without micropatterning are assessed after 3, 6, and 9 weeks. Gene delivery results in greater bone formation at three weeks. Micropatterning results in regenerated ligamentous tissues similar to native PDL. The combination results in more mature expression of collagen III and periostin, and with elastic moduli of regenerated tissues that are statistically indistinguishable from those of native tissue, while controls are less stiff than native tissues. Thus, controlled scaffold microtopography combined with localized growth factor gene delivery improves the regeneration of periodontal bone-PDL interfaces.