Jason D. Whittle

Nanoscale deposition of chemically functionalised films via plasma polymerisation

Plasma polymerisation is a technologically important surface engineering process capable of depositing ultra-thin functionalised films for a variety of purposes. It has many advantages over other surface engineering processes, including that it is completely dry, can be used for complex geometries, and the physico-chemical properties of the film can be tailored through judicious choice of processing conditions. Despite this, the mechanisms of film growth are largely unknown, and current models are based on purely chemical arguments. Consideration of some basic plasma physics shows that some species can arrive at surfaces with energies greater than 1000 kJ mol−1 (>10 eV), and thus open a range of surface reactions that have not been considered previously. This review aims to close the gap between the physics and chemistry of reactive plasma systems.

On the effect of monomer chemistry on growth mechanisms of nonfouling PEG-like plasma polymers.

It has been shown that both ions and neutral species may contribute to plasma polymer growth. However, the relative contribution from these mechanisms remains unclear. We present data elucidating the importance of considering monomer structure with respect to which the growth mechanism dominates for nonfouling PEG-like plasma polymers. The deposition rate for saturated monomers is directly linked with ion flux to the substrate. For unsaturated monomers, the neutral flux also plays a role, particularly at low power. Increased fragmentation of the monomer at high power reduces the ability of unsaturated monomers to grow via neutral grafting. Chemical characterization by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) confirm the role that plasma phase fragmentation plays in determining the deposition rate and surface chemistry of the deposited film. The simple experimental method used here may also be used to determine which mechanisms dominate plasma deposition for other monomers. This knowledge may enable significant improvement in future reactor design and process control.

Immobilization of heparan sulfate on electrospun meshes to support embryonic stem cell culture and differentiation

Background: Glycosaminoglycans influence stem cell fate but their combination with biomaterials remains to be optimized. Results: GAG bound to scaffolds presented essential sulfation epitopes and proved biologically active. Conclusion: Use of plasma polymerized allylamine proved effective in functionalizing a fibrous extracellular matrix mimic. Significance: The biomaterial has broad applicability to stem cell culture and has potential future applications in regenerative medicine. As our understanding of what guides the behavior of multi- and pluripotent stem cells deepens, so too does our ability to utilize certain cues to manipulate their behavior and maximize their therapeutic potential. Engineered, biologically functionalized materials have the capacity to influence stem cell behavior through a powerful combination of biological, mechanical, and topographical cues. Here, we present the development of a novel electrospun scaffold, functionalized with glycosaminoglycans (GAGs) ionically immobilized onto the fiber surface. Bound GAGs retained the ability to interact with GAG-binding molecules and, crucially, presented GAG sulfation motifs fundamental to mediating stem cell behavior. Bound GAG proved to be biologically active, rescuing the neural differentiation capacity of heparan sulfate-deficient mouse embryonic stem cells and functioning in concert with FGF4 to facilitate the formation of extensive neural processes across the scaffold surface. The combination of GAGs with electrospun scaffolds creates a biomaterial with potent applicability for the propagation and effective differentiation of pluripotent stem cells.

The link between mechanisms of deposition and the physico-chemical properties of plasma polymer films

Film thickness and functional group retention are routinely measured parameters for plasma polymers. It is known that other parameters such as density, solubility and mechanical properties can affect the performance of the plasma polymer film, however such parameters are not often measured; nor is there any understanding of the link between the mechanisms of film growth and these properties. In this investigation we produced thin films from three classes of commonly used plasma polymers (hydrocarbons, glymes and carboxylic acids). By choosing the monomer structure and applied RF power, the dominant mechanism of film growth was varied between ionic deposition and neutral grafting. The density, solubility and modulus of the resulting films were then measured by atomic force microscopy. Films grown from saturated monomers had higher moduli, were less soluble, and surprisingly had lower density compared to their unsaturated analogues. The results demonstrate that cognizance of the mechanism of film growth allows the physical properties of the film to be tailored for specific applications.

Defining plasma polymerization: New insight into what we should be measuring

External parameters (RF power and precursor flow rate) are typically quoted to define plasma polymerization experiments. Utilizing a parallel-plate electrode reactor with variable geometry, it is shown that these parameters cannot be transferred to reactors with different geometries in order to reproduce plasma polymer films using four precursors. Measurements of ion flux and power coupling efficiency confirm that intrinsic plasma properties vary greatly with reactor geometry at constant applied RF power. It is further demonstrated that controlling intrinsic parameters, in this case the ion flux, offers a more widely applicable method of defining plasma polymerization processes, particularly for saturated and allylic precursors.

Submillimeter-Scale Surface Gradients of Immobilized Protein Ligands

We describe a method to produce antibody-captured ligand gradients over biologically relevant distances (hundreds of micrometers) whereby the ligand density and gradient shape may be tailored. Separation of the ligand from the solid-phase surface ensures that the biological activity of the ligand remains unaffected by immobilization. Our method involves the use of a plasma-masking method to generate a surface chemical gradient on a glass substrate to which the 9E10 antibody is covalently coupled. This antibody captures myc-tagged biomolecules. In our example, the antibody is then used to immobilize a gradient of the intercellular signaling molecule delta-like-1 (Dll1). To visualize the gradient of Dll1, we have used the multistep approach of binding with rabbit anti-Dll1 primary antibody and then adding colloidal-gold-conjugated secondary antibody.

Development of a surface to enhance the effectiveness of fibroblast growth factor 2 (FGF-2)

Growth factors (GFs) play an important role in biological processes such as cell proliferation, differentiation and angiogenesis. GFs are known to bind to glycosaminoglycans (GAGs) in the extracellular matrix, aiding projection from degradation and pooling the GFs for quick response to biological stimuli in vivo. GFs are typically expensive and have a relatively short half-life in culture media, requiring regular replenishment. Here the cooperative binding of GF to a plasma polymerised surface decorated with heparin, and the subsequent culture of primary human dermal fibroblasts (HDFs) is investigated. A simple one-step technique suitable for coating a wide range of different substrates was utilised. Substrates such as culture-ware, scaffolds, bandages and devices for implantation could be coated. The modified surface was compared to standard culture techniques of addition of GF to the media. Results demonstrate that surface bound heparin and FGF-2 have a greater effect on cell proliferation especially at reduced serum concentrations. With performance equivalent to supplementing the media achieved at as little as 1% total FGF-2 added. The protective cooperative effect of FGF-2-GAG bound to modified surface at the interface could lead to reduced costs by reduction of FGF-2 required. Furthermore, for applications such as chronic non-healing wounds, bandages can be produced modified by plasma and decorated with GAGs that could utilise and protect important GFs. This would effectively re-introduce important biomolecules which are protected by GAG binding into a harsh environment.

Attachment of Poly(l-lactide) Nanoparticles to Plasma-Treated Non-Woven Polymer Fabrics Using Inkjet Printing.

Active dressings that based on fabric materials are an area of interest for the treatment of wounds. Poly(l-lactide) nanoparticles containing the antimicrobial agent octenidine can be controllably lysed by toxins released by pathogenic bacteria thus releasing antimicrobial material in response to the presence of the bacterial toxins and so counteracting the infection. We developed an integrated engineering solution that allows for the stable immobilisation of nanoparticles on non-woven fabrics. The process involves coating nanoparticles on non-woven polymer surfaces by using an inkjet printing process. In order to improve the adhesion and retention of the nanoparticles on the fabric, surface pretreatment of the non-woven fabric using plasma jet treatment can be applied to increase its surface energy.

The importance of ions in low pressure PECVD plasmas

Plasma enhanced chemical vapour deposition (PECVD) can be used to fabricate surfaces with a wide range of physical and chemical properties and are used in a variety of applications. Despite this, the mechanisms by which PECVD films grow are not well understood. Moreover, the species which contribute to film growth can be considered quite differently depending on the process. Particularly for functionalized plasma polymer films, the growth mechanisms are considered with respect to the chemistry of the depositing species, ignoring the physics of plasmas. Here we analyse the role ions play in the deposition of three common classes of depositing plasmas, and how these closely related fields treat ions very differently.

Where physics meets chemistry: Thin film deposition from reactive plasmas

AbstractFunctionalising surfaces using polymeric thin films is an industrially important field. One technique for achieving nanoscale, controlled surface functionalization is plasma deposition. Plasma deposition has advantages over other surface engineering processes, including that it is solvent free, substrate and geometry independent, and the surface properties of the film can be designed by judicious choice of precursor and plasma conditions. Despite the utility of this method, the mechanisms of plasma polymer growth are generally unknown, and are usually described by chemical (i.e., radical) pathways. In this review, we aim to show that plasma physics drives the chemistry of the plasma phase, and surface-plasma interactions. For example, we show that ionic species can react in the plasma to form larger ions, and also arrive at surfaces with energies greater than 1000 kJ∙mol–1 (>10 eV) and thus facilitate surface reactions that have not been taken into account previously. Thus, improving thin film deposition processes requires an understanding of both physical and chemical processes in plasma.