Biomaterials Tutorial

Plasma Deposition

Winston Ciridon and Lan Cao
University of Washington Engineered Biomaterials

Plasma is a complex ionized state of gas sometimes called the 4th state of matter. Electrons, ions, free radicals, photons, gas atoms and molecules are present in various states and various energetic conditions in plasma [1, 2].

In the lab, we are usually interested in cold plasma or glow discharge, where the plasma state is sustained by applying electrical power (direct current, radio frequency or microwave) to a gas.  The weakly ionized gas consists of neutral particles and also a very low density of charged particles. In the cold plasma, gas temperature is near ambient values, and the gas phase lacks thermodynamic equilibrium. The charged particles can absorb energy from the electric field.  This small, but highly energetic, charged population within the bulk room temperature neutral particles allows high temperature reactions to occur at room temperature in the plasma. This is an advantage in treating heat sensitive materials such as polymers.

When the electrons acquire energy from the external electric field, they can redistribute the energy to neutral species by means of collision, which produces neutral excitations that fragment molecules into atoms, reactive free radicals, and charged particles.  The radicals generated from the feed gas (monomer) can interact with a substrate to clean or etch a surface, or to form a cross-linked thin film called plasma polymer

 [3].

The main characteristics and advantages of plasma deposition are:

• Sterile, conformal, pinhole-free, and highly cross-linked films can be deposited on complex geometric shapes;
• Plasma polymers can be deposited on almost any solid substrate (metals, polymers, etc.);
• Plasma films have good adhesion to the substrate;
• Multilayer films or films with gradient of chemical or physical characteristics are easily made by this process;
• Plasma polymers can be easily formed with varying film thickness of 10-1000 Angstroms;
• A wide range of chemistries can be achieved by plasma deposition, but not by conventional polymer methods;
• Plasma deposition is a solvent free process and hence environmentally benign;
• The process is rapid and low cost.

Because of the above useful properties, plasma deposited films have been used for electrode insulation, diffusion barriers [4], improving lubricity [5] and corrosion protection.

Biological systems (e.g., proteins and cells) are sensitive to the nature of a surface [6]. If  a biomedical material is not satisfactory in the biological environment, then we can selectively modify the surface of the material by plasma deposition without changing its bulk properties. In the biomedical field, plasma treatments and depositions are particularly useful for enhancing biostability of biomaterials [7], improving adhesion in the assembly of medical devices [8], altering material wettability and enhancing biocompatibility [9]. Plasma-deposited films are also used to introduce functional groups for immobilization of bioactive molecules.

References

1. d'Agostino R.  Plasma deposition, treatment, and etching of polymers. Boston: Academic Press, 1990.
2. Gombotz WR, Hoffman AS. Gas-discharge techniques for biomaterial modification. Crc Critical Reviews in Biocompatibility 1987; 4: 1-42.
3. Yasuda H. Plasma polymerization. Academic Press, 1985.
4. Vasquez-Borucki S, Achete CA, Jacob W. Hydrogen plasma treatment of poly(ethylene terephthalate) surfaces. Surf Coat Technol 2001; 138: 256-63.
5. Marmieri G, Pettenati M, Cassinelli C, Morra M. Evaluation of slipperiness of catheter surfaces. J Biomed Mater Res 1996; 33: 29-33.
6. Ratner BD. Plasma Deposition for Biomedical Applications—A Brief Review. J Biomater Sci-Polym Ed 1992; 4: 3-11.
7. Tang YW, Santerre JP, Labow RS, Taylor DG. Use of surface-modifying macromolecules to enhance the biostability of segmented polyurethanes. J Biomed Mater Res 1997; 35: 371-81.
8. Bhat NV, Upadhyay DJ. Adhesion enhancement and characterization of plasma polymerized 1,2-dichloroethane on polypropylene surface. Plasma Chem Plasma Process 2003; 23: 389-411.
9. Clarotti G, Schue F, Sledz J, Benaoumar AA, Geckeler KE, Orsetti A, Paleirac G. Modification of the biocompatible and haemocompatible properties of polymer substrates by plasma-deposited fluorocarbon coatings. Biomaterials 1992; 13: 832-40.