○  In this first period of our project, we desinged and built the cold plasma reactor which will be used in our surface engineering studies. Figures 1,2,3 and 4 present the plasma reactor chamber in detail. In figures 3 and 4 you can observe the reactor chamber with generated plasma discharge working in Helium at a pressure of 5 milibars, an input power of 60 W and a frequency of 1 MHz.

  ○  The generated cold plasma was diagnosed by means of Optical Emission Spectroscopy (OES) which helped us to identify the active molecular species that are present in the plasma discharge as a function of input power and process gas composition.
In order to find the most stable working regimes of the discharge (high homogenity and maximum emission of the important active species), we plotted a series of stability diagrams for the plasma working in the following gases: air, synthetic air, argon and helium.
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 ○  The kinetic temperature of the plasma (also known as "gas temperature") was measured directly using a regular alcohol thermometer which was introduced in the reactor chamber at atmospheric pressure and then the reading was performed through the frontal window of the reactor chamber when the plasma was ignited. (special thanks goes to Prof. Dr. Sorin Dan Anghel for providing us the ideea). The gas temperature was found to be around 35...36 deg. Celsius for an input power of 60 W. This means that we are dealing with a true cold plasma that will allow us to activate the surfaces of even the most thermosensitive materials (i.e biomaterials).

Figure 1. The plasma reactor chamber - lateral view.
Figure 2. The plasma reactor chamber - frontal view.
Figure 3. The plasma reactor chamber with ignited plasma at 5 milibars, 60 W input power and at a frequency of 1 MHz. You can observe the discharge in the middle of the chamber.
Figure 4. Frontal view of the plasma reactor chamber with ignited plasma at a working pressure of 5 milibars, 60 W input power and a frequency of 1 MHz.

 ○ Our first surface activation study using the cold plasma reactor was performed on polyethylene terephtalate (PET). The ideea was to improve the mechanical properties of bonded polyethylene structures by activating the material's surfaces prior to bonding. The aim here is to develop a performant activation method which will allow manufacturers to build polyethylene orthopaedic implants with superior mechanical properties.
In our tests, we prepared a series of PET samples that underwent the plasma activation process before being bonded with epoxy resin. After curing, we broke the assembled samples, noting the tensile strength values. We compared these values with the values obtained from the samples that had no plasma activation. The cleaning effect of the high frequency cold plasma on the polyethylene surfaces was studied by performing contact angle measurements on water droplets deposited on activated surfaces. The preliminary results are being published in a paper entitled: "Improving the mechanical properties of polyethylene orthopaedic implants by high frequency cold plasma surface activation", authors: Cristian D. Tudoran, Iulia. E. Vlad, Dorin N. Dădârlat and Sorin D. Anghel. (the paper is under review and it will be published at A.I.P, conference series, for the International conference "Processes in Isotopes and Molecules" PIM 2013, September 25...27, 2013).