A platform for new interfaces, new surfaces, and new nanomaterials
Plasma is the fourth state of matter after solids, liquids and gases. It is actually partially ionized gas and consists of electrons, ions, free radicals, UV photons, and neutral particles. Much of the universe and most of the solar system is made of plasma. Solar winds, a flow of plasma, blow out from the sun and spread all the way across the solar system. The Aurora, lightning, and the outer layer of our earth's atmosphere are plasma. The plasma helps to protect our environment and health.
Plasmas can be generated in gases at either atmospheric pressure or low pressure (i.e. under vacuum) using alternating electromagnetic fields. We know plasma TV and are familiar with fluorescent lights. In this laboratory the plasmas being generated are generally referred to as cold plasmas because most of the energy is carried by the light electrons and the ions move at relatively slow speeds and so the material being treated can remain near room temperature, if desired.
The interaction between the reactive species in a plasma and a substrate can be controlled by external parameters (such as frequency, power, and pressure) which determine internal bulk plasma characteristics (such as electron temperature and density). Pulsed plasmas have been shown to be particularly useful because the photons and charged particles disappear quickly in the off period allowing the neutral radicals to catalyse many reactions. Thus, plasma is a general tool that can remove or alter surface properties or add (many) new surface layers to a very wide range of materials without altering the bulk properties, and that can be used for fabricating nanomaterials at lower temperature and for doping of nanomaterials to change their electronic and magnetic properties. Moreover, it is a dry process and a number of sequential operations can be carried out within the one reaction chamber by changing the gases/monomers/elements and plasma conditions.
Details of the research can be found under the two main research themes:
The plasma facilities
Novel technologies including improved pulsed plasma, continuous wave plus pulsed (CW+P) plasma mode, separation of reactive species, and the ability to operate with different gases and/or monomers at low or atmospheric pressure in specialized reactors means we have a platform that can provide controllable and selectable reactive species and functionality. A number of specific plasma reaction chambers have been designed and assembled with different power supplies and accessories, and these are being used in a diverse range of multi-disciplinary projects and collaborations. The key facilities that we are establishing can be divided into four categories:
- Stirring & rotation plasma systems
- Pulsed plasma polymerization systems
- Plasma-plus-furnace (or heating) systems
- Nanosecond-pulsed APGD systems
Stirring or rotation plasma systems
We are aiming at surface functionalization of nanopowders or other forms of nano-materials (nanotubes etc.). These systems address the challenges of achieving uniform treatment with a high functional group density and easy handling, and enable uniform surface functionalization of nano-powders (tubes, wires, belts, particles etc.).
Pulsed plasma polymerization systems
This electrode-less reactor enables the controllable and selectable functionality of surfaces or membranes with high stability, on larger samples of any shape. The choice of gases/monomers, pressure, power and duty cycle allows a great degree of control over the polymer and surface functional groups.
Plasma-plus-furnace (i.e. plasma plus heating) system
These new systems are aimed at fabricating and doping of nano-semiconductors. The equipment combines heating (or furnace) with plasma input. The plasma allows the production of reactive species and so selected reactions can be carried out at elevated temperature but at a lower temperature than without the plasma. This allows fabrication and doping of nano-semiconductors, with the potential to alter band-gaps, increase photo-activity, and change electronic and magnetic properties.
Nanosecond pulsed atmospheric pressure glow plasma systems
These nanosecond pulsed atmospheric pressure glow plasma systems are aimed at biomedical applications and micro/nano-devices. Several electrodes, including shower, flat, mesh, and cylindrical, have been developed to ensure that a glow discharge occurs which gives a gentle even treatment and fabrication (called APGD: Atmospheric Pressure Glow Discharge).
We are particularly interested in producing:
- Nano-structured electrodes for solar cells, batteries, super-capacitors, fuel cells
- Electro-optic devices
using plasma enhanced chemical vapour deposition (bottom up) and plasma pattern etching (top down). Doping different elements into nano-semiconductors can reduce band-gap and increase photoactivity, and obtain novel magnetic and electric properties. The materials that we are focused on so far are BNNTs, CNTs, SnO2 , SiO2 , TiO2 , ZnO, V2O5, etc.
We are mainly focused on:
- Bio-surfaces/interfaces for implants, tissue scaffolds, drug release;
- Novel composites for aerospace and weight saving;
- Strong adhesion for metals and ceramics.
Controlled Amine Functionalization and Hydrophilicity of a Poly(lactic acid) Fabric
X. J. Dai*, J. du Plessis, I. L. Kyratzis, G. Maurdev, M. G. Huson, C. Coombs
Plasma Process. Polym. 2009, 6(8), 490 (Cover page)
Abstract: This first paper in the series demonstrates that the density of primary amines (-NH2) on a degradable polymer can be controlled by selection of pulsed plasma conditions using the plasma polymerization of heptylamine (PPHA). Primary amines are an important functional group for bonding biological molecules so this was a key step towards improved bio-interfaces. The amount of NH2 groups was quantified by chemical derivatization. A sufficient level of primary amine functionality (3.5%) for practical applications was achieved. The duty cycle and the average RF power were the key parameters for achieving both a higher density of primary amines and increased hydrophilicity.
The pulsed plasma functionalization approach can give nanostructured coatings or introduce various chemical functional groups (-NH2, -COOH, -SH, etc.) onto metals, semiconductors, ceramics, polymers, fibres, CNTs, and nanoparticles. The functional groups can be well controlled to match the requirements of the new surface, especially for improved biocompatibility of implants.
Combined Continuous Wave and Pulsed Plasma Modes: for More Stable Interfaces with Higher Functionality on Metal and Semiconductor Surfaces
L. Li, X. J. Dai*, H. S. Xu, J. H. Zhao, P. Yang, G. Maurdev, J. du Plessis, P. R. Lamb, B. L. Fox, W. P. Michalski
Plasma Process. Polym. 2009, 6(10), 615 (Cover page)
Abstract: The second paper is the first report of the novel approach of combining continuous wave (CW) and pulsed plasma modes. This has enabled the generation of stable interfaces with a higher density of -NH2 on metals, ceramics and semiconductors. The thin CW plasma polymerized heptylamine layer provides strong cross-linking and attachment to the metal or semiconductor surface and a good foundation for better bonding of a pulsed PPHA layer. This top layer has higher levels of functional groups because it retains more of the monomer structure. The combined mode provides the pulsed mode advantage of a 3-fold higher density of -NH2 while retaining much of the markedly higher stability in aqueous solutions, or during sterilization, of the continuous mode.
Controlling Cell Growth on Titanium by Surface Functionalization of Heptylamine using a Novel Combined Plasma Polymerization Mode
J. H. Zhao, W. P. Michalski, C. Williams, L. Li, H. S. Xu, P. R. Lamb, S. Jones, Y. M. Zhou and X. J. Dai*
J. Biomed. Res. Part A (accepted).
Abstract: This is the third in a series of papers that establishes that the bio-compatibility of titanium implants can be greatly improved by appropriate plasma treatment. A novel bio-interface, produced by a combined plasma polymerization mode on a titanium (Ti) surface, was shown to enhance osteoblast growth and reduce fibroblast cell growth. This new method can securely attach a tailored interface to difficult materials such as Ti or ceramics. Here a more stable and higher density of -NH2 functional groups is able to withstand sterilization in ethanol. The biocompatibility, in terms of cell attachment and actin cytoskeleton development, was markedly improved in vitro, compared with untreated Ti surfaces and samples treated by other plasma modes. It gave a boosted (~ 6 times higher) cellular response of osteoblasts in their initial adhesion stage. These factors should increase the formation of new bone around implants (reducing healing time), promoting osseointegration and thereby increasing implantation success rates.
Pulsed Plasma Polymerization of Hexamethyldisiloxane onto Wool: Control of Moisture Vapor Transmission Rate and Surface Adhesion
X. J. Dai*, J. S. Church, M. G. Huson
Plasma Process. Polym. 2009, 6, 139
Abstract: A new fabric with potential in medical textiles was developed by application of a surface coating on wool using pulsed plasma polymerization of HMDSO. This coating enabled a controllable MVTR and surface adhesion. MVTR in the range recommended for optimum wound healing was obtained by varying frequency, monomer pressure and deposition time. Lower surface adhesion was achieved. Peeling tests, contact angle measurements, SPM force curves and ATR FT-IR were used to characterize the surfaces for both wool and a polyethylene model substrate. All the results were consistent with a decrease in surface energy after PP-HMDSO treatment. ATR FT-IR results showed a siloxane film with less organic Si-(CH3)n groups and more Si-O-Si cross-links.
Poly(L-lactide) Crystallization Induced by Multi-wall Carbon Nanotubes at Very Low Loading
H. S. Xu, X. J. Dai, P. R. Lamb, Z. M. Li
Journal of Polymer Science Part B: Polymer Physics 2009, 47, 2341
Abstract: Composites of a biodegradable polymer (PLLA) and multi-wall carbon nanotubes (MWNTs) were prepared as possible bio-substrates with greatly enhanced properties. Only very low concentrations of MWNTs (<0.08 wt %) were added in the composites. Isothermal and non-isothermal crystalline measurements were carried out on PLLA/MWNT composites to investigate the effect of MWNTs on PLLA crystalline behavior. Differential scanning calorimetry was used to investigate the effects on crystallinity. The results showed that the incorporation of MWNTs significantly shortened the crystalline induction time and increased the final crystallinity of the composite, which indicated MWNTs have a strong nucleation effect even at very low concentrations. It was concluded that the double melting peak (see picture) is caused by a disorder-order crystal phase transition.
Improved Biocompatibility of PLLA/MWCNT Composites
H. S. Xu, X. J. Dai*, L. Li, J.H. Zhao, P. Yang, P. R. Lamb, B. L. Fox, W. P. Michalski, Z. M. Li
19th International Symposium on Plasma Chemistry, Bochum, Germany, July 26th - 31st, 2009
Abstract: Multi-walled carbon nanotube (MWCNT) reinforced PLLA composites were investigated as a potential new-generation implant material. The surface of the composite was functionalized with -NH2 groups by plasma polymerization of heptylamine. A -NH2 density of about 6% was achieved giving remarkably improved hydrophilicity. Biocompatibility was significantly improved, shown by cell growth, due to the changes in roughness and surface chemistry.