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FUNDAMENTALS OF COATING ELECTROSURGICAL INSTRUMENTS
TABLE OF CONTENTS: Click title to view topic
INTRODUCTION
INTRODUCTION
One of the most important characteristics of Laparoscopy as well as Electrosurgery is the fact that they are technological intensive and very dependent on innovative ideas, including special coating materials and techniques.
In Laparoscopy (also called videosurgery) the surgeon uses a number of instruments, usually in combination, to perform the tasks of manipulation and dissection. These multipurpose tools can serve several functions in addition to dissection, e.g., retraction, aspiration, irrigation. The modern trend in the design of laparoscopic surgical instruments has been the incorporation of multiple functions in one instrument. These multi-function instruments incorporate several tips (as many as nine) and can be used for variety of purposes, including Electrosurgery.
The Electrosurgical instruments have become one of the useful tools available to the surgeon not only in conventional surgery but also in laparoscopy. Successful laparoscopic surgery requires not only meticulous hemostasis but also a virtually bloodless operative field and the Electrosurgery meets these demands. While initially many traditional surgical instruments have been adapted for laparoscopic use, recently new electrosurgical instruments have been specifically developed for laparoscopic procedures. The modern trend in the design of laparoscopic surgical instruments has been the incorporation of multiple functions in one instrument by providing many (as many as nine) tips to provide a variety of purposes, including electrosurgical.
Depending how the instruments use electrical energy, electrosurgical procedures can be categorized as:
- Electrosurgery: it utilizes the generation and delivery of radiofrequency current between active and dispersive electrodes to elevate tissue temperature to perform vaporization (cutting), fulguration, and desiccation.
- Electrocautery accomplishes the cutting of the tissue by concentrating the density of the electrical current and increasing the power (voltage). The electrosurgical knife, which has a small service area, serves as the active electrode.
- Electrocoagulation is accomplished by the same means, but the current is interrupted with "off times" from 50 to 150 microseconds, thereby generating heat which boils out the water in tissues and produces a zone of desiccated cells.
The SAFETY of the ELECTROSURGICAL Procedures and Instruments
Typical Hazards
Used in conjunction with an electrosurgical unit (ESU) that supplies the necessary power, electrosurgical devices can cut, cauterize, and coagulate tissue by means of radiofrequency (RF) energy. Typically, some form of insulation is needed to insure that the energy is directed at the target tissue.
Electrosurgical injuries occur during laparoscopic operations and are potentially serious. The overall incidence of recognized injuries is between one and two patients per 1,000 operations. Sometimes there are no immediate clinical evidence of the injury- the time from injury to onset of symptoms can vary from 18 hours to 14 days.
One potential hazard in the use of electrosurgical energy during laparoscopy stems from the fact that most laparoscopic accessories are approximately 35 centimeters long and the laparoscopic images viewed on the monitor typically show less than 5 cm of the distal end of the device.
Insulation Failures
Another very important risk and probably the greatest contribution to accidental injuries occurs from inappropriate, inadequate or defective insulation that covers the metal components of the devices.
The active electrode used to deliver the electrosurgery has an insulating covering, but 90 percent or more of this insulation is out of the viewing range. If a breakdown of the insulation occurs on the shaft of the electrode, the organs touching the electrode at this site can suffer severe burns that may not be noticed during surgery, but may result in severe postoperative complications. Also, defects in the insulation can allow the full amount of RF energy to be applied in unintended and unseen areas, causing burns or cuts.
Insulation failures were noticed most commonly in the clinical series by observing arcing to the liver and even jerking of the abdominal wall or diaphragm.
The hazard of insulation failure is dependent on the location of the point of failure.
Defects in the handle of the electrode (zone 4 in the figure) are generally result of poor engineering and pose little direct risk to the patient, unless an unexpected shock to the surgeon leads to a patient injury. Insulation failures occur most commonly in the distal portion of the electrode (zone 1) as a result of trauma during insertion through cannula valves and repeated heating during electrical activation.
The zone 2 represents the shaft of the electrode that is neither within the view of the laparoscope nor inside the cannula. There is no clue to the most careful surgeon that insulation failure and thermal injury have occurred in this zone. Insulation failure within the cannula (zone 3) is not detectable if a plastic cannula is used. If a metal cannula is used, there is often current flow between the metal of the electrode and the metal of the cannula, and the resulting arcing of the current creates a lower frequency current that may lead to neuromuscular stimulation with jerking of the abdominal wall or diaphragm as well as possible interference with the electrosurgical generator or video monitor.
TYPES of ELECTROSURGICAL Instruments
1) MONOPOLAR ELECTROSURGICAL INSTRUMENTS, in which the RF current passes from the active electrode to the passive electrode (‘grounding plate’) attached to the patient. They can be used for:
- Monopolar Cutting (vaporizing), which requires a relatively low-voltage, sinusoidal waveform of current at frequency of approximately 400 kHz; the sinusoids of the RF current occurs either continuously in the pure type of monopolar cutting or by recurring in bursts at approx. 30 kHz intervals in the blend type of monopolar cutting. The following are the typical current parameters used in monopolar cutting: voltage between 1200 and 3500 volts, rated load 300 ohms, maximum power 200-300 watts.
- Monopolar Coagulation, used to Desiccate, Fulgurate or Spray.
In the Desiccation process, a relatively low frequency RF current (typical 250 kHz) repeated at approx. 40 kHz intervals is sent through the coated electrode, which is placed in contact with the tissue. Since most of the electrosurgical energy is converted into heat within the tissue, the desiccation results in a deep, wide necrosis where the electrode makes surface contact. Typical parameters for the RF current are: voltage-3500 volts, rated load-500 ohms, 120 watts maximum power.
In the Fulguration process as well as in the Spraying process, the coagulation is obtained by means of spraying long electrical sparks to the tissue. In the Fulguration process, a high voltage, low-current, non-continuous waveform highly damped RF current of approximately 400 kHz is sent through the coated electrode with a repetition frequency of approx. 50 kHz mostly to heat up the atmosphere between the electrode and the tissue. With fulguration, a very superficial eschar is produced and the depth of necrosis is minimal. The RF current is applied at the following parameters: voltage of 7000 volts, rated load 500 ohms, maximum power 120 watts.
The Spray process is similar to Fulguration with the main exception of the maximum voltage, which is higher, e.g. 9000 volts. Typically, a RF current at approx. 400 kHz of damped sinusoidal bursts with a randomized repetition centered at approximately 30 kHz is applied at 500 ohms and 120 watts maximum power.
- Ablation Recently developed instruments have been able to provide an ablation process by accomplishing a complete removal of tissues by using a RF electrical current, instead of using conventional mechanical technique.
2) BIPOLAR ELECTROSURGICAL INSTRUMENTS contain both the active and the passive electrodes and the electrical current flows from the tip of one electrode, through the tissue, to the tip of the other electrode.
With the exception of the current’s frequency, which at approximately 500 kHz sinusoid is higher than in the Monopolar electrosurgery, the typical values of the RF current parameters for the Bipolar are much lower then in the Monopolar technique: rated loads of only 100 ohms and low maximum power of only 70 watts.
Based on maximum voltage of the RF current applied through the instrument, the bipolar technology can be categorized in the following techniques: Precise (low)-approx. 300 volts, Standard (medium)- approx. 400 volts and Macro bipolar which typically works at 700 volts.
REQUIREMENTS FOR the INSULATION of the ELECTROSURGICAL INSTRUMENTS
In order to function properly and safely, electrosurgical instruments, including those employed in laparoscopic procedures, must be coated with materials that have the following characteristics:
- Superior dielectric properties: the coating must provide electrical insulation to the metal substrates of the electrosurgical instruments, which operate at high voltage and very high frequency electrical current. Typical operating conditions are: a voltage of 3000 volts and a frequency of 500,000 Hz. However, recent trends (especially in Europe) are requiring significantly higher ESU standards: voltage as high as 13,000 volts and RF frequencies in range of Megahertz. The designer of insulating materials and coating technique must consider the following electrical characteristics of the potential coating candidates:
- Dielectric strength of the material. It is the electrical stress required to puncture the insulation, expressed in volts per thickness unit, e.g., V/mil
- Specific resistance of a material. The resistance of insulation to the path of electrical current is proportional to its length, inversely proportional to its cross-section and proportional to a specific property of the material called specific resistivity. The insulating material should also offer resistance to the potential tracking of the electrical current either throughout its volume (volume tracking) or on its surface (surface tracking).
- Dielectric constant or specific inductive capacity (SIC). It has a direct impact on the function of the insulation. One has to take in account the SIC value at the operating conditions, specifically at what temperature, what voltage, what current (how many amperes) is applied and especially at what RF frequencies the ESU is operating.
- Power factor. It measures the amount of energy given by the insulating material when it is subjected to electrical current.
- Power loss. A high power loss causes a temperature rise in the insulation that may result in a reduced life of the insulation and even catastrophic failure. A higher power factor and higher dielectric constant translates into a higher power loss.
- The dielectric constant and electrical power factor, and therefore the power loss factor should be as low as possible and also stable when immersed in body fluids and tissue; the electrical resistivity should be as high as possible, unaffected by wet environments and trace ionic impurities- notably sodium present in saline solutions.
- The coating must be tightly adhered to the metallic substrate without voids at the interface; the presence of air voids can adversely affect the performance of the electrosurgical instruments and even lead to insulation failure, especially since they work under very high voltage and frequency currents.
- The insulating coating should be uniform in thickness over all sections of the metallic substrate, especially over the areas with complicated shapes where the insulation failures occur most commonly, sometimes without being detected during the laparoscopy; otherwise the high voltage current will “ground out” laterally rather than reaching the target tissue and create severe injuries to the patient.
- The coating may be required to have a lubricious surface and low coefficient of friction since the devices have to be inserted and removed either through the body, or through cannula.
- The insulation also needs to be resistant to wear, abrasion and cut-through. Repeated use of instruments often lead to insulation failure as a result of physical abrasion, wear or cut that may occur during the handling of the instruments prior or during the surgery.
- The coating has to be pin-hole free in order to provide a defect free insulating layer able to resist high voltage/high frequency electrical current in environments that contain bodily fluids, water vapors and solutions such as saline, which contain migrating ions; otherwise electrical shocks and burns may occur.
- The coating is desired to seal the micro porosity of the electrode substrate which may otherwise trap and retain contaminants.
- The materials that compose the insulation have to be sterilizable - to survive the media and the conditions of the sterilization techniques, such steam autoclave, radiation, and ethylene oxide. In the case of reusable instruments they have to resist repeated sterilization cycles.
- Since they are part of invasive devices, the coating insulation has to be biocompatible and biostable (typically approved as a USP class VI material).
INDUSTRY STANDARDS
US Standards
The Association for the Advancement of Medical Instrumentation (AAMI) has established minimum safety and performance requirements for electrosurgical devices, which were adopted by the American National Institute (ANSI) and are detailed in a document, designated ANSI/AAMI HF8, which also includes the tests by which compliance with these requirements can be verified. Prior to the latest revision of this standard adopted in 1993, the experimental test methodology and protocol was based on HF18-1986.
The typical parameters under which an electrosurgical unit operates are 3000 volts at a frequency of 500 KHz. Compared to the previous HF18-1986 standard which required that the insulated surgical instruments pass a 30-second test at 4000 volts peak to peak at 1 MHz electrical current, the HF18-1993 revision increased the safety margin of the output voltage- the test requirement is 1.5 times the operating voltage, but left no safety margin in the case of frequency- it lowered the testing frequency to the level of the specified operating frequency of the ESU.
International Standards
Certain international standards, such as IEC-European standards for EC mark, are more demanding for the approval of ESUs. For instance, they require that the testing of the insulated instruments should be done at 1.5 times maximum output voltage that the electrical RF generator is capable. Therefore in many cases, the qualifying tests are required to be done at voltages as high as 13,000 volts.
VITEK COATING TECHNOLOGY VERSUS HEAT-SHRINK TECHNOLOGY
HEAT-SHRINK TECHNOLOGY
Technique
The heat-shrink technology uses a process where operators install heat-shrinkable materials in tubular form, previously manufactured by an extruding technology, over the substrates (mostly in tubular shape). By using heat generated typically by a heat-gun, the operators shrink the tubing onto steel substrates in order to make it tight to the substrate shape as well as to as to remove the air, which is entrapped between the insulating layers and the substrates that form the electrosurgical electrodes.
Materials
A technical article (6) has published the results of a study done by testing various materials applied by the Heat-shrink technique on electrosurgical instruments. The authors created a test apparatus and a test protocol based on the ANSI/AAMI HF18 standards published in 1986 and 1993 to evaluate six different insulating materials: polyvinylidene fluoride (PVDF), low-density polyethylene (LDPE), a blend of polyolefin and zinc or sodium partially neutralized ethylene acrylic acid copolymer (ionomer), high-density polyethylene (HDPE), fluorinated ethylene propylene (FEP), and polyvinyl chloride (PVC).
The test results indicate that only the electrosurgical instruments insulated with LDPE, HDPE, ionomer and FEP with a minimum wall thickness of 0.016 inches were able to pass the HF18-1986 requirements of 4000 Volts and 1 MHz electrical current for 30 seconds duration. Since the test frequency of the HF18-1993 test protocol has no safety margin versus the operating frequency; the authors of this study recommend considering using a higher-frequency test like the HF18-1986 standard, to screen candidate materials for insulating electrosurgical instruments.
From a technical aspect, the heat-shrink technology has several drawbacks. Few examples:
- Since the process of heat shrinking is subject to the operator’s skill, in principle there is no guarantee that the insulating material is consistently adhered to the substrate and there are no air voids at the interface. These air voids are detrimental because under high voltage and high frequency electrical current under which the typical electrosurgical units operate, a “corona discharge” phenomenon may occur in these air voids and negatively affect the safe operation of the electrosurgical instrument.
- It should be noted that certain sterilization technologies, such as steam autoclaving could dramatically increase the number of voids at the interface between heat-shrunk insulation and metallic substrates. This condition is exacerbated in the case of reusable surgical devices, because they are intended to be reused after repeated sterilization cycles.
- This technique is especially deficient in the case that the substrate does not have a simple shape. The tubing tend to shrink circularly and in the most cases the coated instrument will end up with many air gaps at most of the points at the interface between the tubing and the substrate.
- The heat-shrink technology has coating thickness limitation. In certain applications, where due to space limitation the coating cannot surpass a certain thickness, the heat-shrink tubing is virtually unusable.
- The heat-shrink technology is also limited in the choice of coating materials. This technique is limited only to few types of materials that can be used to manufacture those heat-shrinkable tubings. In addition, some of these materials are incapable to pass the requirements of the industry standards.
VITEK’S COATING TECHNOLOGY
Vitek’s main business is to provide high performance, functional custom coating services to the Medical Device and Instruments industry, including electrosurgical instruments, used in cardiology, arthroscopy, neurology, OBG-GYN, ear, nose and throat, urology, orthopedic and eye surgery, etc.
In the field of coating Medical Instruments and Devices, Vitek is uniquely positioned due to our versatility. Compared to any other company, we have the expertise to apply almost ANY ORGANIC COATING MATERIAL, over most substrates, including ferrous or nonferrous metals, plastics, rubbers, ceramics and glass.
The following are some of the coating materials that we have the capability to apply: Fluoropolymers (Teflon, Halar, Kynar types), Parylene (type N, C and D), Nylon, Epoxy, Polyamideimide, Polyimide, Polyamide, Urethane, Acrylic, Polysulfide, Polysulfone, Silicone, Vinyl, etc. If necessary, we can develop in our laboratory proprietary coating materials (under Vicoat trade name), customized to special requirements.
Our technical personnel have also developed a rather special technique that allows us to apply a COMPOSITE coating composed of different materials. These are especially useful in applications where a single coating material will not satisfy all requirements. For instance, we can apply a Parylene coating layer to provide electrical insulation and apply above it other coating materials specially selected to provide additional features, such as: certain color, resistance to abrasion, resistance to cutting/scratch, resistance to multiple sterilizations, etc.
Vitek utilizes the latest application equipment, including electrostatic powder coating, liquid spray and gas vacuum deposition. We also employ state-of-the-art gas plasma equipment to prepare and pre-treat all substrates prior to coating to insure the optimum adhesion between the coating and the substrate and to prevent the voids at the interface.
Since each electrosurgical device operates under different conditions and has varying requirements for its insulating material, Vitek technical personnel will work with the customer to select the optimum coating materials and coating processes in order to provide the best functional benefits for the application. A few examples of special features that we can provide to the coating:
Electrosurgical instruments coated at Vitek have been used in all types of electrosurgical processes, such as Monopolar and Bipolar instruments, including Ablative instruments. The excellent field record of the medical devices and instruments coated by Vitek, including the laparoscopic and electrosurgical types, has demonstrated their superiority to similar devices coated with other technologies, including the Heat-shrink technique.
- pin-hole free dielectric insulation able to sustain very high voltage
- no voids at the interface with the substrate
- dry or wet lubrication
- impact/abrasion/cut and scratch resistance
- resistance to chemically corrosive environments
- barrier to various environments
- hydrophobic or hydrophilic surface properties
- special adhesive or release properties
- customized color
- resistance to high temperature environments
- resistance to various sterilization media and repeated sterilization cycles
The data in the following table presents the number of laparoscopic procedures as a percentage of the total surgical procedures done in the United States per year based on recent statistics published in a New York Times article. It confirms the fact that Laparoscopy/Electrosurgery are fast growing in popularity due to the fact that it is less painful, patients recover faster, requires less hospitalization time and therefore it is less expensive than the conventional open surgery.
The data in the following table presents the number of laparoscopic procedures as a percentage of the total surgical procedures done in the United States per year based on recent statistics published in the New York Times. It confirms the fact that laparoscopy is growing fast in popularity due to the fact that it is less painful, patients recover faster and require less hospitalization time and therefore it is less expensive than the conventional open surgery.
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
REFERENCES
1. Ballantine G.H., “Laparoscopic Surgery”, W.B.Saunders Co., Philadelphia, 1994.
2. Odell R.C., et al., “Laparoscopic Electrosurgery”, in “Minimally Invasive Surgery”, New York, 1993
3. Voyles C.R., et al., “Education and Engineering solutions for Potential problems with Laparoscopic Monopolar Electrosurgery”, The American Journal of Surgery, vol 164, July 1992.
4. Voyles C.R., et al., “Essentials of Monopolar Electrosurgery for Laparoscopy”, Electrosurgical Concepts, 1992.
5. Nduka C.C, et al., “Cause and Prevention of Electrosurgical Injuries in Laparoscopy”, Journal of American College of Surgeons, vol. 179, August 1994.
6. Kleinhenz P. and Vogdes C., “Comparing Insulating Materials for Electrosurgical Instruments”, Medical Device and Diagnostic Industry, February 1996.
7. Para Tech Co., Galxyl- Parylene Coating”.
8. Humphrey B., “Using Parylene for Medical Substrate Coating”, Medical Plastics and Biomaterials, January/February 1996.
9. S.C.S. Co., “Parylene, a biostable coating for medical applications”.
10. Electrosurgical devices, ANSI/AAMI, HF18-1986, Arlington, VA. Association for the Advancement of Medical Instrumentation (AAMI), 1986.
11. Electrosurgical devices, ANSI/AAMI, HF18-1993, Arlington, VA. Association for the Advancement of Medical Instrumentation (AAMI), 1993.
12. Loh I.H and Hudson D.M., Advance Surface Technology Inc. “Coatings for implantable electronics”
13. Hahn A.W. and York D. H., “Biocompatibility of glow-discharged polymerized films and vacuum-deposited Parylene”, Journal of Applied Polymer Science: Applied Polymer Symposium, 38, 1984.
14. Beach W.F., Lee C., et al., “Xylylene polymers”, Encyclopedia of Polymer Science and Engineering, volume 17, J.Wiley & Sons Inc., 1989.
15. Licari J.J., Hughes L.A., Handbook of Polymer Coatings for Electronics, Noyes Publications, 1990
16. Moscovici A., “Electrical Insulation”, chapter of Kirk-Othmer Encyclopedia of Chemical Technology fourth edition, volume 14, J.Wiley & Sons Inc., 1995
17. Moscovici A., “Insulation, Electric”, chapter of Encyclopedia of Energy Technology and Environment, volume 3, J.Wiley & Sons Inc., 1995
