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TABLE OF CONTENTS: Click title to view topic
FUNDAMENTALS OF GAS PLASMA TECHNIQUE
FUNDAMENTALS OF GAS PLASMA TECHNIQUE
The process consists in an excitation of gaseous products at very low pressures and ambient temperature by radio-frequency (RF) energy. Although the electrons' temperature in a plasma can be in a range of thousands of degrees, because of the vacuum conditions, the bulk temperature of the gas in the chamber is essentially ambient, therefore the technique is called Cold Gas Plasma.
The plasma contains free electrons as well as other metastable particles, which upon collision with the polymeric surfaces of the parts placed in the plasma environment break chemical bonds. This creates free radicals on the surface. These free radicals then undergo additional reactions depending on the gases present in the plasma. The result is that these gas-radical reactions form a surface that is potentially very different from that of the starting bulk polymer.
The effect of a plasma on a polymer surface is determined by the gas chemistry and by the process parameters of the plasma system. Plasma processes cause changes only to the surface of a material, with the effects being confined to a few molecular layers on the surface.
The type of modification is dependent on the composition of the substrate, the type and the amount of the reactive gases employed, the vacuum in the chamber and the RF power (the excitation power is supplied by an RF generator at the industry standard of 13.56 Mhz frequency.
There are three ways to modify polymer surfaces by cold gas plasma:
Gas plasma technique may use any gas or gas mixture, within equipment limitations. In a few cases, reactant gases may require special handling. Each gas or mixture of gases produces a distinct plasma, making possible a very diverse range of chemical environments. For example:
- Plasma Treatment of functional groups at the surface, altering the surface (bio) chemical reactivity while the bulk properties remain unaffected. By varying gas and plasma conditions, specific characteristics (e.g. hydrophilicity or hydrophobicity) can be obtained.
- Plasma induced Grafting: A plasma of a noble gas, such as argon or helium, creates free radicals on the surface that subsequently can be reacted with other monomers, yielding surfaces with specialized properties.
- Plasma Deposition: Certain gases in a plasma may undergo polymerization, usually via a free radical initiation process. When a gas polymerizes and adheres to a dissimilar materials sharing its plasma environment, the process is called plasma deposition. Plasma polymerization creates new and unusual polymer properties that have only begun to be explored in biomedical applications.
- Oxidizing gases such as air, oxygen, water vapor or nitrous oxide.
- Reducing gases such as hydrogen, methane or a diluted mixture of these with nitrogen or argon
- Noble gases such as argon and helium.
- Active gases (e.g. ammonia).
- Fluorinated gases like carbon tetrafluoride, sulfur hexafluoride.
- Polymerizing gases. A few examples of plasma polymerizable gases are: acetylene, organosilanes, organosiloxanes, fluorohydrocarbons.
A diagram of the Gas Plasma Process
ADVANTAGES OF GAS PLASMA
Plasma is an increasingly appreciated alternative to other processes, since it is capable to modify the outermost layer (sometimes as thin as few Angstroms) of the substrate material, and even to grow on the surfaces totally new materials with customized characteristics, without sacrificing the desirable properties of the bulk material. A major advantage of plasma processing is that the gases employed in the process can reach places which are typically inaccessible to other techniques.
By using gas plasma modification technique, a separate and distinct surface property can be established, including a unique biological interface. This approach affords the greatest flexibility in research and development, product design and ultimately in manufacturing of biomedical products.
The following summary lists the most important advantages of cold gas plasma processes (treatment, grafting or deposition) for biomedical applications:
Conformal; Pinhole-free;Applicable on any substrate;Good adhesion to the substrate; Unique film chemistry; Barrier film; High purity; Simple to produce and repeatable; Good track record; Characterizable; Sterile; It does not affect the substrate material; It does not leave a residue; It is workplace and environmentally clean and safe.
BIOMEDICAL APPLICATIONS OF GAS PLASMA TECHNIQUE
The literature suggests many biomedical applications for plasma treatments and polymeric depositions, including the following:
Contact lenses; Neurological probes; Ocular prostheses; Ear implants & replacement; Maxillofacial implants; Cartilage replacement; Dental implants; Cardiac assist devices; Vascular prostheses Mammary prostheses; Synthetic skin; Urological catheters; IV catheters; Guide wires; Hip prostheses; Orthopedic implants; Ob/Gyn, Penile devices; Bone fixation devices; Knee prostheses; Tendon prostheses; Drainage catheters
Plasma treatment (etching) is mostly used for:
- Cleaning
- Sterilizing
- Crosslinking the molecules on the surface
- Activation: exposing polymers to a gas plasma results in formation of active groups that can change the surface characteristics, such as its wetting properties, adhesivity, etc.
Plasma treatment combined with plasma grafting and plasma polymerization and deposition are mostly used to:
- Form barrier film:
- Protective coating
- Insulating coating
- Reduce absorption from environment
- Reduce release rate of leachable materials
- Control drug delivery rate.
- Modify protein and cell interactions:
- Improve "biocompatibility"
- Promote selective protein adsorption
- Enhance cell adhesion
- Improve cell culture surfaces
- Provide non-fouling surfaces
- Reduce surface friction
- Provide reactive sites:
- Grafting or polymerizing films
- Immobilizing biomolecules.
EXAMPLES OF GAS PLASMA BIOMEDICAL APPLICATIONS
CLEANING
Prior to adhesive bonding, plasma is commonly used to clean away loosely held residues and to provide surface activation for adhesive bonding.
ADHESIVE PROMOTION
Good adhesion between two surfaces require strong interfacial forces via chemical compatibility and/or chemical bonding. Plasma surface treatment can help form functional groups to improve interfacial adhesion. Common applications include pretreatment for Catheters, Syringe components, Dialysis pump parts, Medical and Dental Implants and plastic film for blood and drug bags.
CHANGE THE WETTING CHARACTERISTICS
Plasma has been used to enhance or decrease the wetting ( typically measured by a decrease or an increase of the contact angle of various liquids including water) on a large variety of substrates.
If the surfaces are transformed to become Hydrophobic, they will reject water, so that when submerged in aqueous solutions, they no longer draw fluid by capillary action. In the case of Hydrophilic surfaces, they will attract water. Contact angles as low as two degrees have been demonstrated after only few minutes of plasma treatment.
LUBRICIOUS SURFACES
The instruments used for catheterization in urinary, tracheal and cardiovascular tracts or for endoscopy/laparoscopy procedures as well as the ophthalmologic materials, should have a surface that preferably becomes slippery upon contact with aqueous body liquids. Gas plasma can create such a low friction surface that would enable easy insertion and removal from a patient's body and will prevent mechanical injury to the mucous membranes and would minimize discomfort to the patient.
Medical Instruments Gas plasma has been used successfully, either by itself or in combination with other techniques, especially the Xylylene polymerization, in manufacturing of medical instruments used for various procedures, such as ophthalmology and videosurgery.
BIOABSORBABLE POLYMERS
Plasma treated bioabsorbable polymers could be used for wound closure sutures, controlled-release delivery systems and devices used in orthopedic, cardiovascular, neurological, urological and abdominal surgery.
BIOCOMPATIBILITY ENHANCEMENT
Functional groups attached by plasma treatment acts as "hooks" for the addition of an anticoagulant such as heparin, thereby decreasing thrombogenicity. Plasma deposited polymers on small diameter vascular grafts have demonstrated a dramatic improvement in patency and decreased embolization.
BONE INTERNAL FIXATION DEVICES
The literature references the use of gas plasma polymerization to enhance the adhesion between the reinforced absorbable calcium phosphate fibers and the absorbable polyglycolide acid matrix.
DIAGNOSTIC BIOSENSORS
For example, an active immobilized glucose oxidase membrane was obtained via plasma initiated polymerization that immobilized a biological component ( an enzyme) on the sensor surface.
CONTROLLED-RELEASE DELIVERY SYSTEMS
Plasma surface modification of known controlled-release delivery systems can overcome the problems of maintaining zero-order release kinetics as well as the tolerance of the human body.
OCULAR PROSTHESES
Contact lenses and intraocular lenses have been successfully modified by gas plasma to impart protein- and cell-repelling characteristics, decrease bacterial adhesion, improve wettability and enhance comfort.
CELL CULTURE SUBSTRATES
The surfaces of cell culture substrates, such as Petri dishes, Roller bottles, microcarriers and membranes, can be modified by plasma to greatly increase wetting (lower the contact angle).
SEPARATION MEMBRANES
Plasma processing is a proven technique for modifying membrane materials to enhance the diffusion selectivity.
STERILIZATION
Gas plasma is an ideal technique to sterilize biomedical devices. It is less capital intensive than electron-beam sterilization, it is less toxic than ethylene oxide sterilization and since it operates at ambient temperatures is less prone to thermal and hydrolytic attack than steam sterilization.
PLASMA PROCESS IN CONJUNCTION WITH PARYLENE COATING
Similarly to Gas Plasma process, the deposition of Xylylene (mostly known under commercial name Parylene) is also done in vacuum, from a pure monomer gas that polymerizes and deposits conformally over substrates as a uniform, pinhole free layer with impressive chemical, physical, electrical, and biochemical properties. Parylene coatings in conjunction with Gas Plasma treatment have found many biomedical applications, such as: Pressure sensors, Prosthetic components, Bone pins, Catheters and Stylettes, Stents, Bone growth stimulators, Brain probes, Needles, Feeder tubes, Cochlear and electronic implants, Orthodontics as well as in manufacture of Laparoscopic instruments, especially the instruments used in Electrosurgical procedures.
LAPAROSCOPIC INSTRUMENTS.
The videosurgery (also called laparoscopy) is growing fast in popularity due to the fact that the procedure is less painful and the patients recover faster than from the conventional open surgery. The doctor makes small incisions for tubes carrying camera's components, such as fiber optics and special surgical instruments. Guided by the small camera, sometimes called a laparoscope, the surgeon uses special instruments to perform surgery.
The Laparoscopic Electrosurgical instruments can cut, cauterize and coagulate tissue by means of electrical current at high frequency and voltage; their metal tube (which is also an electrode) must be coated with a very thin, pin-hole free, high dielectric material with low coefficient of friction. The best results have been obtained when plasma process was used in conjunction with deposition of Parylene.
The following table shows the number of videosurgery procedures done in the United States in a year.
OPERATION BY LAPAROSCOPY, # TOTAL, # PERCENTAGE, % Gallbladder 653,100 796,250 82 Hernia repairs 183,645 734,445 25 Hysterectomy/gynecology 133,712 460,112 29 Appendectomy 45,900 306,300 15 Lung/heart 27,350 70,990 39 Colon removal 26,900 288,680 9 Kidney/urology 11,100 47,126 24 Total 1,081,707 2,703,903 40
